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Doctoral Thesis

New Trop-based Ligands and their Application in Catalysis

Author(s): Sacchetti, Vittorio

Publication Date: 2016

Permanent Link: https://doi.org/10.3929/ethz-a-010652044

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DISS. ETH NO. 23218

New Trop-based Ligands and their Application in

Catalysis

A thesis submitted to attain the degree of

DOCTOR OF SCIENCES of ETH ZURICH

(Dr. sc. ETH Zurich)

presented by

VITTORIO SACCHETTI

MSc Chemistry, ETH Zürich

born on 22.08.1984

citizen of Zürich (ZH) and Italy

accepted on the recommendation of

Prof. Dr. Hansjörg Grützmacher

Prof. Dr. Antonio Togni

2016

La sapienza è figliola della sperienzia.

(Codice Forster III)

L’acqua che tocchi de’ fiumi è l’ultima di quelle che andò e la prima di quella che viene.

Così il tempo presente.

(Codice Trivulziano)

Leonardo da Vinci (1452-1519)

Ai miei genitori.

I

Acknowledgements

First and foremost, I would like to thank Prof. Dr. Hansjörg Grützmacher for the opportunity

he gave me to be part of his group and for the occasions to learn from him. His patience

and goodwill are two characteristics which were very much appreciated. Thank you

Hansjörg.

Prof. Dr. Antonio Togni is thanked for being the co-examiner of this thesis. His influence as

a teacher during the studies at ETH Zürich surely influenced my “development” and without

doubt parts of this thesis.

Ms. Christine Rüegg – a really big THANK YOU! - is the heart and the good spirit every lab

needs. Thank you for being there and for your countless support.

Dr. Hartmut Schönberg is a master when something has to be fixed and when old-school

ingenious ideas are needed. Thank you for your support during the years.

Dr. René Verel, Dr. Aitor Moreno and Dr. Barbara Czarniecki are thanked for the valuable

time spent at the NMR spectrometers - and not only.

Dr. Matthias Vogt, Dr. Amos Rosenthal, Dr. Gustavo Santiso, Riccardo Suter and Bruno

Pribanic are thanked for their help with X-ray diffraction and the solving of crystal

structures.

All the Grützi group members are thanked who throughout the years were there and shared

a part of their life and experience in the same labs and environment. From the old school

special thanks go to: Dr. Fritzi Tewes – for the introduction to oxidation catalysis, Dr.

Matthias Vogt and Dr. Monica Trincado.

Among all the people who were working in the Grützi group the “poor guys” who had to

share the same lab are especially thanked: Dominikus Heift, Xiaodan Chen, Mark

Bispinghoff. Thank you for standing the loud music, the long nights in the lab and the talks

about this and that. Special thanks also go to the chosen one, Amos Rosenthal, for the

unlimited amount of discussions.

Dr. Aaron Tondreau is thanked for the introduction to the iron chemistry and

hydrosilylation. Dr. Zoltan Benkö is thanked for many ideas shared.

Acknowledgements

II

I had the privilege to assist and supervise many students during their semester theses: Their

influence and research are part of this thesis. Thank you: Benjamin – P-ligands, Deborah –

oxidations, Sandro – oxidations, Kathrin, Theresa – Nickel Project (not in this thesis).

The original mafia lunch people are thanked for insights into other fields of chemistry and

further distraction during Thursday’s lunch. It was good to hear about other projects going

on throughout the laboratories at ETHZ. Thank you: Ivano, Luca, Paolo, Malù, Stefano and

Carlotta.

A thesis would not be an experience without its ups and downs. And especially in moments

where the chemistry is not doing what you want, people outside the lab are the ones that

bring you back to reality. Therefore, some special thanks are reserved for Römu and Sarah,

Role, Ricci, Daniel, Laurie and all of you out there.

Thanks go to my roommates who made living over the years in Zürich so stress-less: Eva,

Bruna, Glen and Lara. I won’t forget the evenings together.

My huge gratitude goes to Raphaël Rochat, for being a fun roommate, a great support and

most important such a good friend! THANK YOU.

In the last two years the team of Willi Möller AG, in Zürich was a big support. Especially

Claudio Adda is thanked for his insightful help during the time of finishing the thesis.

The biggest thank goes out to the one and only lady who went all the way with me. From

the beginning to the end. I am proud and feel lucky to have you at my side. Thank you for

all you give me day by day and for the continuous support. It would have been so different

without you. Grazie, Diana.

Last but not least my family is thanked. Soprattutto vorrei ringraziare i miei genitori –a cui

e dedicata questa tesi - per tutti i sacrifici che hanno fatto solo per realizzarmi un sogno –

Grazie! Tonee, Danke für aui die Mau wo es guets Bispiu bisch. Me cha zu dir ufeluege und

stouz si. Marco, wie chönnti ohni di nur dert si woni jitz bi? Au die Gspräch wo nur Du

chasch verstah! Gang witerhin diner Tröim nache und blib wie du bisch. MERCI!

III

Abstract

In the first part of the thesis the catalytic oxidation of primary alcohols with

[Ir(trop2DAD)]OTf 4 complex as highly active catalyst is discussed. The problems of an

environmentally friendly oxidation process, the possibilities and drawbacks of the systems,

with electron transfer mediator (ETM) and regenerative vitamin K3 7 as co-oxidant are

discussed (Scheme 1).

Scheme 1: Oxidation of non-activated alcohols by [Ir(trop2DAD)]OTf and vitamin K3 as co-oxidant.

The second and third part deal with the search of new redox non-innocent tridentate trop

ligands for the general application in catalysis and as an alternative in the

transferhydrogenation of small molecules. The newly synthesized ligands are introduced

and their properties described.

Additionally, to the tropPicol 8, a special case of a tridentate trop based ligand a

bis(trop)phospholidine 16 ligand and its rhodium complex are discussed.

Figure 1: Newly synthesized ligands with redox active properties.

The fourth part handles with new iron tropPDI and iron tropPicol complexes and their

catalytic applicability as catalysts in the hydrosilylation and hydroboration of non-activated

substrates.

IV

Zusammenfassung

Im ersten Teil der Arbeit wird die katalytische Oxidation von primären Alkoholen mit Hilfe

des [Ir(trop2DAD)]OTf 4 Komplexes untersucht. Die bekannten Nachteile des Systems

werden besprochen und Alternativen vorgestellt. Spezieller Fokus wird auf das Vitamin K3

(7) als stöchiometrisches Oxidationsmittel gelegt, vor allem um das umweltschädliche

Benzochinon zu ersetzen. Wie in Schema 1 beschrieben, können nicht aktivierte

aliphatische Alkohole oxidiert werden.

Schema 1: Iridium katalysierte Oxidation von n-Octanol mit Vitamin K3 als Oxidationsmittel.

Der zweite und dritte Abschnitt, widmet sich der Suche neuer chelatisierenden trop-

Liganden mit redoxaktiven Eigenschaften. Diese sollten für die allgemeine Anwendung in

der Katalyse und als Alternative für die Transferhydrierungen von Molekülen eingesetzt

werden können. Die neu synthetisierten Liganden werden eingeführt und ihre

Charakterisierung beschrieben.

Neben dem Ligand tropPicol 8 wird ein Spezialfall eines chelierenden, tropbasierten

Bis(trop)phospholidine 16 Ligand behandelt. Beide Liganden wurden unter anderem als

deren Iridium oder Rhodium Komplexe studiert.

Abbildung 1: Neu synthetisierte Liganden: tropPicol 8 und bis(trop)phospholidine 16.

Im letzten Teil der Arbeit werden Eisen Komplexe mit tropPDI und tropPicol (8) untersucht.

Ihre katalytische Anwendbarkeit in der Hydrosilylierung und Hydroborierung von nicht

aktivierten Substraten werden getestet.

V

Table of contents

Acknowledgements I

Abstract III

Zusammenfassung IV

Table of contents V

Introduction 1

Trop chemistry 2

Redox active ligands 6

Oxidations 7

Iron in catalysis 9

Oxidations 11

Oxidation of Alcohols 12

Oxidation of Hydroquinone to Benzoquinone 18

Alternative Oxidants 20

Vitamin K3 as oxidant 21

Proof of concept 29

Concluding remarks 30

New trop Based Ligands 31

Introduction 32

TropPicol 38

Synthesis of the ligand 38

Synthesis of [Ir(tropPicol)Cl]2 39

Synthesis of [Rh(tropPicol)Cl]2 39

Discussion 40

Synthesis of [Ru(tropPicol)(cymene)Cl] and [Cu(tropPicol)(ACN)]PF6 41

tropNacAc 44

Synthesis of [Pd(tropNacAc)]OAc 45


Phosphorus Ligands 47

VI

Bis(trop)-1,3,2-Diazaphospholidine 48

Introduction 48

Synthesis of tricoordinative phosphorus trop ligands 50

Results 53

Attempted synthesis of the bis(trop)diazaphospholene as a precursor 53

Alternative synthesis of the bis(trop)diazaphospholene 54

Reaction of trop2DAD with PI3 55

Synthesis of diazaphospholidines 56

Coordination to Rhodium 59

Discussion 63

Diazaphosphole 63

Diazaphospholidine 63

Diazaphospholidinium 66

NMR analysis of Diazaphospholidine and Diazaphospholidinium 67

Synthesis of [Rh(trop2sNHPCl)Cl] 69

Crystal analysis of [Rh(trop2sNHPCl)Cl] 69


Iron Chemistry 72

Introduction 73

Synthesis of the ligands 75

Synthesis of tropPicol 75

Synthesis of tropPDI ligands 75

Results 77

Coordination of the tropPDI ligands to Fe 79

Synthesis of [Fe(tropPDIdipp)Br2] 79

Reduction of [Fe(tropPDIdipp)Br2] 80

Synthesis of [Fe(tropPDIsubst)Br2] 83

Synthesis of tropPicol iron complexes 83

Reduction of [Fe(tropPicol)Br2] 85

Catalytic activity of Iron complexes 87


VII

Conclusion 90

Outlook 92

Experimental Part 93

General Techniques 94

Analytical Techniques and Instruments 95

Oxidations 96

Experimental 96

Ligands 96

TropPicol 96

TropNacAc 97

[Ir(tropPicol]Cl]2 98

[Rh(tropPicol)Cl]2 98

[Ru(tropPicol)(cymene)Cl]OTf 99

[Cu(tropPicol)ACN]PF6 100

[Pd(tropNacAc)]OAc 100

Phosphorus Ligands 101

Chlorobis(trop)diazaphospholidine, trop2sNHP 101

Chlorobis(trop)diazaphospholidinium Triflate 101

[Rh(trop2sNHP)Cl2] 102

Iron Chemistry 103

2,6-diacetylpyridine 103

1-{6-[(2,6-Diisopropylphenyl)ethanimidoyl]-2-pyridinyl}-1-ethanone 103

1-(6-(1-((2,6-Dimethylphenyl)imino)ethyl)pyridin-2-yl)ethanone 104

1-(6-(1-(mesitylimino)ethyl)pyridin-2-yl)ethanone 105

TropPDIDipp 105

N-(1-(6-(1-((2,6-diisopropylphenyl)imino)ethyl)pyridin-2-yl)ethylidene)-5H-

dibenzo[a,d][7]annulen-5-amine 105

TropPDIMe 106

TropPDIMes 107

saturTropPDIDipp 108

[Fe(tropPDIDipp)Br2] 109

VIII

[Fe(tropPDIMe)Br2] 109

TropSatFeBr2 110

List of Abbreviations 111

Compounds 114

Crystallographic Data 116

Literature 130

Curriculum Vitae

1

Introduction

Introduction

2

Trop chemistry

Throughout this work the 5H-dibenzo[a,d]cyclohepten-5-yl frame will be named trop,

derived from the tropone (cyclohepta-2,4,6-trien-1-one). As depicted in Figure 2 the trop

scaffold has a very special conformation with a concave form, which makes it unique in its

binding characteristics.

Figure 2: Different depictions of the trop scaffold used throughout this thesis.

The 5H-dibenzo[a,d]cyclohepten-5-yl unit has proven to be a wonderful tool when utilized

as ligand. Many new reaction patterns were explored and stable transition metal complexes

synthesized thanks to the unique properties of the trop ligand. All started in 1999 when

Grützmacher discovered the fascinating geometry and recognized the possibilities such a

ligand can offer.[1] Since then a plethora of different ligands and complexes containing the

trop unit were synthesized and described.[2]

The trop ligand unifies two important concepts of coordination chemistry. One is directly

coupled with the space such a ligand claims. Trop ligands can be seen and deployed as

bulky ligands and thus influence the steric surrounding of a transition metal when it is

coordinated by it. The two benzyl rings on the side force the cycloheptyl unit in a precise

concave form, which forms a pocket-like structure for further coordination while blocking

the sides. This introduces the second characteristic of the trop ligands. The double bond of

the cycloheptenyl is now forced out of plane and thus is ready for coordination through

the olefinic part.

In 1831 Zeise described the first organometallic complex containing a coordinated olefin.[3]

The yellow crystals of K[PtCl3(C2H4)]·H2O were synthesized by boiling tetrachloroplatinate

[PtCl4]2- in ethanol. The structure of the molecule was not fully understood at the time and

it was not until Birnbaum synthesized the same molecule by using ethene gas that the

coordination of ethene as a ligand was demonstrated.[4] The molecular coordination was

definitively proven when the crystal structure was elucidated.[5] Zeise’s salt can be seen as

an example for the explanation of the Dewar-Chatt-Duncanson model, describing the

Introduction

3

bonding of olefins to transition metals. According to this model, the bonding is divided in

a and a component.[6] The bonding between the ethene and the platinum can be

described as a coordinative bond between the orbitals of the ligand and an unfilled

orbital of the metal. Additionally a back-bonding of the filled d-orbitals of the metal to the

* orbitals of the ligand can be assumed (Figure 3).[7]

Figure 3: Metal-olefin bonding model according to Dewar-Chatt-Duncanson. (ligand)-metal -bond and

back donation of the d(metal)-*(ligand) orbitals.

The bonding properties in an olefin-metal bond are substantially influenced by the metal

and the substituents on the olefin ligand. The influence of the back bonding from the metal

to the ligand has a big effect on the stability of the bond between metal and olefin ligand.

Therefore, metals in low oxidation states with filled d-orbitals allow a strong back-donation.

The effect of the back-donation into the empty * orbitals of the ligands can be that strong

that the bond length of the olefin increases and the substituents of the olefin turn out of

plane. In extreme cases, when the interaction between the metal and the olefin is strong,

the bonding can then be described as metallacyclopropane (Figure 4). [7]

Figure 4: Metal-olefin bonding representation. -bonding vs extreme case of d back-bonding and

description as metallacyclopropane.

This elongation of the bond length is not only reflected in the molecular structure analysis

but also in the 13C NMR chemical shift of the carbons, which are shifted towards lower

frequencies, compared to the free ligand.[7]

The unique properties of the olefin ligands, i.e. their kinetic lability as a ligand and the

tuneability by adding any substituent to the olefin and thus change its binding property,

Introduction

4

made them one of the most used ligands in late transition metal chemistry. Among the

best known transition metal precursors are the cod and coe complexes such as [Ir(coe)2Cl]2,

[Rh(cod)Cl]2, [Ni(cod)] or [Pt(cod)Cl2], where the labile olefins can be replaced by stronger

ligands. In other cases, the olefins can act as stabilizing ligands for transition metal catalysts,

which are used in an industrial process. In addition, coordinated olefins are found in

industry as starting materials in reactions where a catalytic process is used to transform

these olefins. As in polymerization, a transition metal complex is used as catalyst where the

olefin binds to the catalyst and is reacted further.

The use of olefin ligands as steering ligands for metal complexes was established in the last

decade.[2a, 2b, 8] Grützmacher recognized the potential of such ligands due to their reactivity

and kinetic stability. Many other examples were described through the last few years where

olefin ligands are crucial for organic group transformations.[9]

The trop ligand with its fixed rigid concave conformation made it possible to synthesize an

assemblage of transition metal complexes. Among these structures, many new stable metal

complexes in low-oxidation state were described.[8a, 10] Other complexes act as active pre-

catalyst for reactions such as oxidation of alcohols, transfer hydrogenation,

dehydrogenative coupling of amine-boranes, dehydrogenative coupling of amines,

hydrogenations of functionalized and non-functionalized olefins, clean hydrogen

production from methanol and OMFC (OrganoMetallic Fuel Cells).[2g, 11]

All these interesting complexes bear at least one trop unit as active ligand. As described in

chapter 3.1 many new ligands were synthesized containing phosphorus, nitrogen, oxygen

or even carbenes as co-active ligands such as: trop2DAD (bis(trop)DiAzaDiene, 3), trop2DAE

(bis(trop)DiAminoEthane), trop2DAP (bis(trop)DiAminoPropane) trop2NH (bis(trop)Amine),

tropNH2 (tropAmine, 2), trop2PX (bis(trop)PhosphaneX), trop3P (tris(trop)Phosphane), NHC-

trop (tropN-HeterocycloCarbene), tropindene to name some.[12]

The cheap tropone (5H-dibenzo[a,d][7]annulen-5-one) can be used as starting material for

most ligands, which is then reacted to the tropChloride 1 or tropAmine 2. These starting

compounds are very interesting and versatile, and were used as starting point for new

materials synthesized in this work.

Introduction

5

The trop ligands proved to be a very interesting species with which special oxidation states

of transition metals can be stabilized. In the last years it was shown by this group that Ir

and Rh complexes can be stabilized by the tropPPh2 ligand in the oxidation states (0) and

(-I). In addition, in the following years the Grützmacher group has shown that many redox

analog complexes to the [Ir(tropPPh2)2] can be synthesized and stabilized in the formal

oxidation states -I, 0, and +I thanks to the good -accepting properties of the trop unit.[2b,

13] However, not only the metal benefits from the properties of the ligand but also the

ligand itself can have special electronic properties. As example bis(trop)Amine complexes

of rhodium can be deprotonated resulting in the amide complex [Rh(trop2N)(Lig)] (“Lig”

being a generic ligand as 2,2’-bipyridine, PPh3…). Interestingly this complex does not show

the normal planar structure but rather adopts a butterfly geometry.[2d, 11a, 11b] The

[Rh(trop2N)(PPh3)] proved to be a very active catalyst for transfer hydrogenation. This is

partly due to its electronic configuration where the HOMO is located on the ligand whilst

the LUMO of the complex is found on the transition metal. Further examples of amido

ligands stabilized by the trop unit are the trop2DAE complexes of rhodium(I), cobalt (0) and

nickel (0). Positioning the trop olefin trans to the amide in these complexes, they help to

stabilize the complex by the formation of a 4-electron-3 center system. Normally, the

interaction between the electrons of the filled amide with the filled d-orbitals of the

transition metal center would result in a repulsive interaction leading to a 4-electron-2

center situation[12c].

The ability of the ligand to influence the electronic properties is of crucial importance for

many complexes as the properties of a metal complex are the result of the interplay

between the metal and the ligand. [14] In 1966 the term of innocent ligands was coined by

Jørgensen,[15] describing a ligand, which allows the determination of the oxidation state of

the metal. The non-innocent ligands, thus the ligands with redox active behavior have

become more and more important in the last decades for catalytic activity. Many

cooperative, non-innocent ligands usually exhibit a -delocalized system whose interaction

with the transition metal does not allow to easily determine its oxidation state. Especially

in biomimetic systems, the synergy of the ligand and the metal is noticeable. The ligand

cooperates in the activation of bonds by undergoing reversible chemical transformation in

the interplay with the metal. As an example, the copper-based galactose oxidase can be

named. Here the oxidation of alcohols is promoted by the interplay between the copper

Introduction

6

metal and the tyrosine ligand,[16] taking up and releasing electrons. This example shows one

of the possible interaction that non-innocent ligands can have. Generally, the term non-

innocent can mean that the redox active ligands can act as reactive intermediates (e.g.

radicals), as substrates in the reactions (e.g. oxygen in oxygenations) or can just be electron

reservoirs ( systems).

Redox active ligands

One of the earliest non-innocent ligands that was determined was the diazadiene scaffold

(DAD, ’-diimine, R’N=C(R)-C(R)=NR’). This scaffold can easily be tuned by simple

variation of the substituents at position R and R’. The donor and acceptor capacities of

the ligand can be varied just by replacing the substituents. A huge amount of ligands

containing the DAD backbone were synthesized in the last years and found wide

application in catalysis. The diazadiene ligand can actively contribute to the redox state of

the metal center through its -acceptor properties. In combination with sterically

demanding substituents at the imine, the DAD ligand is able to stabilize metal centers with

low oxidation states.[17] The non-innocent behavior leads to the formation of redox isomeric

structures Scheme 2:

Scheme 2: Resonance structures of a metal complex with the non-innocent ligand DAD. Left: neutral closed-

shell diamine, middle: open-shell p-radical anion species, right: closed-shell dianionic diamido species.

Diazadiene backbones can therefore act as an electron reservoir relieving the metals of

excess electron density or pumping the electrons back when they are needed. The control

of the redox activity through the substituents of the ligands was shown by Kaim et al. [18]

The trop2DAD 3 proved to be a redox active ligand. In a series of reaction, Tewes showed

that the complex [Ir(trop2DAD)]OTf 4 can subsequently be reduced to the radical complex

and further to the enediamido complex (Scheme 3).

Introduction

7

Scheme 3: Schematic representation of the redox process of [Ir(trop2DAD)]OTf 4 described by Tewes.[12b]

This finding proved to be crucial for the catalytic dehydrogenation of alcohols by

[Ir(trop2DAD)]OTf 4. In the proposed mechanism, the diazadiene backbone acts as highly

redox active ligand going through the reduction states described in Scheme 2.

Oxidations

Oxidations are still a hot topic in chemistry. Even though the transformation of alcohols to

aldehydes or ketones is a routine reaction in organic chemistry, there is still a lot of interest

in better controlling this reaction. Nature has optimized and controlled the mechanism of

oxidation over time and has developed the ultimate environmental friendly oxidation

process using O2 as final oxidant. As seen in Chapter 1.1 the interaction of a redox active

ligand and the metal is also found in nature (e.g. copper containing enzyme GOase) and is

the key step for a green oxidation cycle. Nowadays, most oxidants in large-scale processes

and in fine chemicals manufacture are still required in stoichiometric amounts and are toxic

most of the time.[19] The work-up of oxidations and purification of the product is often a

demanding task and, therefore, clean catalytic oxidation processes are of great interest.

The combined use of metal salts of elements (i.e. vanadium, molybdenum, ruthenium and

cobalt) and stoichiometric amounts of oxidants (i.e. tBuOOH, PhIO, NaOCl, NMO or H2O2)

has attained more and more influence and is widely used nowadays.[20] Nevertheless,

catalytic oxidations still need large amounts of expensive transition metals and thus are

being avoided whenever possible.[19] The use of cheap and environmentally friendly

oxidants, such as oxygen as terminal oxidant, is one of main goals, which should be pursued

in the future. Therefore, such a process is not only environmentally desirable but also

economically very attractive as some of the main drawbacks of this kind of reaction would

be eliminated. The use of O2 is also very important from an economical point of view.

Therefore, catalytic systems employing molecular oxygen or air are attractive.

Introduction

8

Catalytic oxidations which use a small amount of precious metals are interesting for

industrial purposes. To get a step closer to environmentally friendly use of resources a

highly active catalyst is a prerequisite. Our group has developed an iridium catalyst for the

oxidation of primary alcohols, which was inspired by the galactose oxidase. The first

generation trop2DACH complex showed to be an active ligand but did not induce any

stereochemistry to the final product.[8b] Tewes found that [Ir(trop2DAD)]OTf 4, the 2nd

generation iridium based catalyst, is very specific for the conversion of primary alcohols to

the corresponding aldehydes with 1,4-benzoquinone used as a co-oxidant (Scheme 4).[12b]

Scheme 4: Simplified mechanism for the dehydrogenation of primary alcohols by [Ir(trop2DAD)]OTf 4 as

describe by Tewes.[12b]

A wide range of substrates could successfully be converted. The mild and almost base free

conditions and low catalyst loadings (0.01 mol%) make the catalytic process very interesting

for large scale applications. A drawback is the use of 1,4-benzoquinone as co-oxidant. The

use of an environmentally friendlier co-oxidant would be worthwhile. The work of

Bäckvall[21] with electron transfer mediators (ETMs) between a benign oxidant and the co-

oxidant in the catalytic cycle takes another step towards green catalysis (Scheme 5). The

application of ETMs in the catalysis is eligible in order to avoid big amounts of waste in the

process and to profit from the benefit of the low catalyst loads needed in the catalytic cycle.

Introduction

9

Scheme 5: Simplified electron transfer facilitated by ETM as proposed by Bäckvall [21]

Iron in catalysis

The increasing demand for environmentally friendly and sustainable methods in chemistry

has raised the interest for the use of iron in catalysis. Being the third most abundant metal

on earth makes iron a perfect and cheap alternative to the mostly expensive precious

metals. Iron is found in its oxidized form as salt, since it readily oxidizes in the presence of

oxygen and moisture.[22] The preferred oxidation state is +2 or +3 although oxidation states

of -2, -1, 0, +1, +4 +5 and +6 are possible.[23] Since the introduction of the HSAB principle

in 1963 by Pearson,[24] Fe(III) has often been used as a classical hard acid in reactions in

stoichiometric and catalytic amounts. Due to the fact that iron has such a broad range of

oxidation states it is found in many biochemical transformations. This lead to an immense

research of catalytic reaction scope for iron complexes making it one of the most versatile

metal for catalytic applications.[23] Inspired by nature many catalysts for oxygen-transfer

reactions were developed.[22] But also other reaction types can be catalyzed by iron

complexes and are being studied. Among many others the classic 1,4-addition, ring

opening, hydroamination, dehydrogenation, polymerization, cyclization, oxidation,

deprotection of alcohols, allylic substitutions, cross couplings, reductions reactions and

many more were described.[25] Many reactions were inspired by biological systems. Lately

many groups started to prepare iron complexes for catalysis. The groups of Morris[26],

Milstein,[27] Beller[28] and Chirik[29] are to be mentioned among others.[25] Their combination

of a redox-active ligand, with an iron lead to catalysts, which are highly active for the

hydrogenation (Figure 5).

The interaction of a non-innocent ligand with iron proved to be very effective in catalysis

for many reactions. Among others the selective anti-Markovnikov alkene hydrosilylation

Introduction

10

described by Tondreau et al.[30] drew a lot of interest. The catalytic hydrosilylation is a

versatile synthetic method for obtaining organosilicon compounds. The use of precious

metals as catalysts such as the Karstedt’s catalyst Pt2{[(CH2=CH)SiMe2]2O}3 do not lead to

pure organosilicon compounds and the isomerization side reaction often needs

subsequent purification steps which increase the costs and the inefficacy of the reaction.[30]

The discovery of the early Speier hydrosilylation catalyst [H2PtCl6]·6H2O/iPrOH was a major

breakthrough in silicon chemistry. Still, the poor regiocontrol and side reactions (alkene

isomerization) and the classical thermal and radical-initiated reactions, which often lead to

oligomers, call for catalysts to control the hydrosilylation of alkenes.[25]

Figure 5: Examples of iron hydrogenation complexes.

Oxidations

11

Oxidations

Oxidations

12

Oxidation of Alcohols

Oxidation of alcohols to carboxylic acids, ketones or aldehydes is of paramount importance

in chemistry and is one of the most investigated reactions. The amount of publications over

the last decades emphasizes its importance and proves that oxidations are still an active

field of research.[19, 31] A subclass of particular interest in the field of oxidations is the

selective oxidation of primary alcohols. The reactivity of aldehydes is completely different

from the one of carboxylic acids. Many known oxidation reactions do not stop at the

aldehyde but rather oxidize further to the carboxylic acid.[19] In order to have a clean

oxidation which ends at the aldehyde and does not react further to the corresponding

carboxylic acid, mild conditions are required. Furthermore, functional group tolerance and

selective reaction control is still a big topic. For small scale reactions the common oxidation

method used in organic chemistry is still the Swern-Oxidation which most of the times leads

to a pure product, but has the known side effect of an unpleasant smell.[32] Oxidation

methods with stoichiometric amounts of heavy and toxic metals such as manganese (e.g.

MnO2), chromium reagents (e.g. PCC/PDC) or vanadates (V2O5) can also be used. The

environmental imprint of these chemicals is enormous and thus their use should be

avoided. Chemical research should lead to the breakthrough of environmental friendly

reactions, which do not influence the environmental equilibrium in an extraordinary scale.

Through the research which has been done on oxidation chemistry, many new alternatives

to the heavy metal methods have been found.[19] Catalytic processes proved to be amongst

the most promising prospects. Palladium, rhodium, iridium and other transition metal

catalysts were developed, of which many found their way from research laboratories to

large-scale process applications. The main drawback with the most commonly used

catalysts is that the amounts of catalyst needed are in a submolar range compared to the

reactant and thus make the oxidation process very expensive. Furthermore, the reaction

times are long (>h) and turnover frequencies (activity) are low. These problems have been

partly overcome in industrial processes by the introduction of solid phase anchored

catalysts in continuous flow reactors. Even though the amount of catalysts used is lowered,

the amount and use of oxidant and co-catalyst is still a big issue. Additionally, economic

and environmental aspects are not to be underestimated.

Oxidations

13

There is a big interest in applied chemistry to keep the amount of waste as low as possible.

That is the reason why in the last few years research focused on the utilization of “green”

reagents with a low ecological impact in oxidation chemistry.[33] The idea is to replace all of

the old-fashioned stoichiometric oxidants, such as inorganic salts, which yield a big amount

of waste (which even generates more costs through its disposal) with low-cost recyclable

oxidants. These “benign” oxidation reagents are usually oxygen, hydrogen peroxide or

reactants that can easily be reused in a further process in order to avoid waste and be

reintroduced in the lifecycle of a chemical product (Scheme 7).[21] Another way to an atom-

efficient oxidation is inspired by biological processes where the oxidation of small

molecules runs down electron mediator chains with well-defined reduction potentials in

order to overcome the high kinetic barrier of oxygen. The specific mediator can be

regenerated in every step, meaning that the oxidant is regenerated by another complex

that transfers electrons from a benign oxidant to an oxidant with lower potential acting as

an electron transfer mediator (ETM). This system is found everywhere in living processes,

e.g. cellular oxidation and photolysis (Scheme 6).[21]

The group of Bäckvall has been very efficient in working with ETM for the oxidation of small

molecules. However, the main drawback of these systems still is the turnover frequency of

a catalyst and the difficulty to find a working system which is environmentally and

economically interesting.[21, 34]

Scheme 6: Schematic representation of an oxidation process involving an ETM (electron transfer mediator).

Oxidations

14

In the last years our group has developed a catalyst system based on the iridium complex

[Ir(trop2DAD)]OTf 4 which is not only the fastest of its type, but also oxidizes non-activated

alcohols to the desired aldehydes with high activity and efficiency (Scheme 7).[12b]

Scheme 7: Catalytic dehydrogenation of alcohols described by Tewes.[12b]

The drawback of this system is the use of benzoquinone and the high amount of waste

generated in this process. A further disadvantage for the use in large-scale processes is the

high turnover frequency and the exothermic reaction process, which does not allow to

control the reaction easily. In order to optimize the catalytic process described, an

environmentally friendlier co-oxidant is needed and the reaction rate should become

controllable.

The redox-active DAD backbone significantly influences the reaction mechanism. It was

proven that the backbone has to be activated by a small amount of base. Subsequent

oxidation of the trop2MIMA 5 by benzoquinone yields the active iridium trop catalyst.[12b]

This specific pathway proves that the trop2MIMA 5 is of immense importance as a reactive

intermediate. The interplay between 4 and the co-oxidant is a crucial balance for the fast

oxidation of the alcohol. The potential differences in the reaction cycle displays itself in an

exothermic reaction. The reaction heats up from room temperature to 80 °C in less than

one minute. The catalyst is with a TOF >100’000 s-1 one of the most efficient of its type in

oxidation processes.[12b] The fact, that the catalysis works perfectly well for non-activated

Oxidations

15

alcohols proves that the reaction is not only driven by the energy gain in the oxidation but

also by the energy gain in the reduction step of the co-oxidant, here from benzoquinone

to the stable hydroquinone. As shown by Tewes, the [Ir(trop2DAD)]OTf 4 is bulky due to the

trop units but has a high symmetry. Therefore, it is easy to understand why the catalyst is

specific for primary alcohols and shows higher TOF for small molecules. This reaction

remains especially interesting for bulk chemistry transformations in industry.

Unfortunately, the use of stoichiometric amounts of benzoquinone as oxidant leads to the

production of big amounts of toxic by-product. The hydroquinone formed in the reaction

is not easily separated from the aldehyde. Inspired by the aerobic respiratory chain (Scheme

8), a system that overcomes the different reduction potentials by using ETM was tested for

the above mentioned catalytic system.

Scheme 8: Aerobic respiratory chain (electron transfer process) described by Bäckvall et al. [21]

As every oxidation system has to be tailored according to the desired degree of oxidation,

the oxidation of alcohols can be stopped at the stage of the aldehyde, ketone or the

carboxylic acid. In order to control the oxidation process, nature has developed a

sophisticated system in living systems like the respiratory chain where the energy of the

oxidation process with O2 is transferred in an electron transport chain consisting of different

complexes in a cell membrane, which make use of the different reduction potentials along

Oxidations

16

the membrane to gain energy. Therefore, the electrons are transported from electron

donors to electron acceptors through redox reactions. The transfer of the protons is

coupled to this process and follows the chain transfer causing a gradient in the cell

membrane. At the end of the electron chain stands O2 which is reduced and equally acts as

the cleanest oxidant. The electron transport chain is the result of millions of years of

evolution, which finally has made life on earth possible.

Based on this principle, an electron chain transfer was established. As shown in Scheme 9,

the electron transfer mediator is used as a redox active complex, which is able to transfer

electrons from a benign oxidant to [Ir(trop2MIMA)]OTf 6 In this way the amount of

benzoquinone can be reduced.

Scheme 9: An ETM was introduced in order to transfer electrons from a benign oxidant to the [Ir] catalyst.

The use of ETMs, has already been successfully applied by Bäckvall and co-workers to

palladium catalyzed oxidations of dienes using SALEN based complexes as ETM (Scheme

10).[21]

The process is successful due to the fact that the high energy barrier between O2 and the

product is overcome by a stepwise low-energy electron transfer processes which has

lowered redox potentials.

Oxidations

17

Scheme 10: Influence of ETMs on the energy barrier in the oxidation of diene by palladium catalysts.[21]

Based on these results, different ETMs were synthesized for the clean oxidation of alcohols

with ETM as co-oxidant copper SALEN, manganese SALEN and VO(acac)2 were synthesized

and tested. In order to get an understanding of the mechanism and to see if the ETMs can

act as direct oxidant for the [Ir(trop)2DAD]OTf catalyst, the oxidation of [Ir(trop2MIMA)]OTf

6 to [Ir(trop2DAD)]OTf 4 was undertaken. This reaction was identified to be of importance

in the catalytic process in the oxidation of primary alcohols to aldehydes[12b] and thus was

investigated. All test reactions were effectuated under the same conditions: The reduced

[Ir(trop2MIMA)]OTf complex 6 was dissolved in toluene or thf, one equivalent of ETM was

added and stirred under an atmosphere of oxygen by bubbling pure O2 through the

reaction mixture. In order to compare the results, the regeneration was also conducted

under normal atmospheric conditions with a lower concentration of O2. The results for the

different ETMs are given in Table 1.

Table 1: Re-oxidation of [Ir(trop2MIMA)]OTf 6 to [Ir(trop2DAD)]OTf 4 in toluene at room temperature.

ETM Reaction time [h] Conversion [%]

- 48 79

[Mn(III)(salen)]Cl 48 no improvement

Cu(II)(salen) 48 no improvement

Cu(II)salpn 48 no improvement

Cu(II)saldien 48 no improvement

VO(acac)2 48 no improvement

Oxidations

18

Unfortunately, none of the tested complexes acted as an ETM for the oxidation of

[Ir(trop2MIMA)]OTf 6.

The electron transfer from the oxygen to the ETM seemed to be too slow for a relevant

catalytic process and thus the electron transfer of the ETM to 6 was not successful.

Therefore, the next step in avoiding benzoquinone was to find a way to re-oxidize the

hydroquinone in a catalytic way.

Oxidation of Hydroquinone to Benzoquinone

The in situ oxidation of the hydroquinone (HQ) to benzoquinone (1,4-BQ) with an ETM or

directly, is a different strategy to improve the reaction conditions. Therefore, the electron

transport chain is prolonged by one step as shown in Scheme 11.

Scheme 11: ETM (E) is used in order to oxidize HQ to 1,4-BQ.

The oxidation of HQ to 1,4-BQ has been described in many papers[35] and has proven to be

more difficult than it first appears. The high aromatic stabilization energy of the

hydroquinone is difficult to overcome. Three different approaches to re-oxidize

hydroquinone were followed.

As described by Sikawar,[36] the Cu(saldien) complex can be used as a HQ oxidation ETM in

combination with H2O2. In order to have a possibility to remove the copper metal from the

reaction mixture, a polystyrene anchored Cu(saldien) complex was synthesized as described

by Maurya.[36] Furthermore, the oxidation of HQ to 1,4-BQ with O2, N2O and air was

followed. The outcome of the test did not prove to be successful.

Oxidations

19

The re-oxidation with the polystyrene anchored Cu(saldien) with H2O2 is described to reach

a conversion rate of >90%. This result could not be confirmed. The re-oxidation with

Cu(salen) as described by Bäckvall, works but is too slow for our reaction pathway. The re-

oxidation of HQ to 1,4-BQ is the rate determining step and occurs so slowly that it does

not make sense to use copper as a ETM. The oxidation by the cheap N2O did not meet the

demands of the iridium catalyzed reaction. The reaction kinetics is slowed down by the re-

oxidation so that it is not useful.

Since this approach did not lead to an improvement of the reaction, the search of a new

co-oxidant was started.

Oxidations

20

Alternative Oxidants

To find a simple way to replace the benzoquinone, different approaches were taken into

account. Since the unique structural properties of benzoquinone seemed to be of interest,

a variety of quinoide structures were tested in earlier experiments.[12b] In addition, further

inorganic and organic oxidants were reviewed and a variety of non-quinoide oxidants were

tested such as flavones, flavonoids, hypochlorite, flavines, TEMPO, dehydroascorbic acid,

gallic acid, uric acid, different peroxides, CAN, further quinones and other metal-organic

reactants. Since the reduction potential of 1,4-BQ/HQ pair is well defined, the co-oxidants

with similar reduction potential differences were analyzed more thoroughly. The widely

described TEMPO was not successful and neither was CAN, H2O2 nor the Oppenauer style

oxidation with acetone.

An overview of the tested oxidants is shown in Table 2.

Table 2: Overview of different oxidants used for the catalytic oxidation with 4.

Entry Catalyst Base Co-Oxidant Conversion Comment

1 - - TEMPO - after 3.5 h, air

2 - NaOtBu TEMPO - after 3.5 h, air

3 [Ir] NaOtBu TEMPO 5 % (5 d) 60 °C, decomp.

4 [Ir] NaOtBu TEMPO 25 % (1 h) air, no SM after

24 h (side-prod)

5 [Ir] NaOtBu Vitamin C - -

6 - NaOtBu CAN - -

7 [Ir] NaOtBu gallic acid - 2 d

8 [Ir] NaOtBu Vitamin K3 > 95 % (1 h) -

9 [Ir] NaOtBu H2O2 observable side products

10 [Ir] NaOtBu acetone - -

Oxidations

21

Vitamin K3 (menadione, 7) acted as a good alternative oxidant. Considering the similar

redox potential difference of vitamin K3 7 to benzoquinone, it is not surprising that it turned

out to be the most promising candidate. Therefore, further research was focused on vitamin

K3.

Vitamin K3 as oxidant

The term vitamin K is used as a collective for several related chemical compounds. All those

compounds are based on a 2-methyl-1,4-naphthoquinone backbone structure, but have

different side chains at C3 position and are represented in Figure 6. There are three main

representatives of vitamin K: vitamin K1 (phylloquinone), which possesses a mostly

saturated C20 side chain and is found in cyanobacteria; vitamin K2 (menaquinone), which

represents a group of compounds characterized by a partly unsaturated, predominantly

C40 side chain, and is found in microbial organisms; and vitamin K3 (menadione, 7), an

artificial quinone with a methyl sidearm. Vitamin K is named so, since it is found to be a co-

factor in the blood coagulation process and the naturally occurring vitamin K molecules in

organisms are involved in electron transport processes. Phylloquinone is found in the

chloroplasts of plants where it acts as electron transfer cofactor QK-A and QK-B in

photosystem I. Menaquinone plays a role in several microbial electron transport systems

and is a co-factor in the -glutamylcarboxylation. [37]

Figure 6: Different structures of the vitamin K family.[38]

Oxidations

22

Interestingly, natural vitamin K and synthetic vitamin K3 7 have similar redox potentials as

benzoquinone. The closest is found in vitamin K3 7. As depicted in Figure 7, the difference

of the potential between benzoquinone, methyl-benzoquinone and menadione are given.

In preliminary studies, menadione 7 proved to be a very active co-oxidant and acts milder

compared to 1,4-BQ. Methyl-BQ proved to be no alternative to 1,4-BQ in earlier tests.[12b]

Figure 7: Difference of the redox potential between a) 1,4-BQ: E1-0.817; E2-1.447 V E) -560 mV, methyl

1,4-BQ: E1 -0.887; E2-1.407 E) -520 mV, Vitk3: E1 -0.635; E2 -1.146 V E) -511 mV. All reduction

potentials were measured vs Fc+/Fc.

The difference of the reduction potentials between the three quinoide structures are

between 560 and 511 mV. The half wave reduction potentials for [Ir(trop2DAD)]OTf were

determined (vs Fc+/Fc) to be E1 -0.70 V and E2 -1.45 V (E) 0.74 V).[12b] The best results

in catalysis with 4 are found with 1,4-BQ which has a very similar reduction potential.

Menadione has the first reduction potential very close to the one of benzoquinone and

thus may be a reason for the incorporation in the catalytic process.

In Figure 8 it is shown that the benchmark reaction with benzoquinone reaches full

conversion to the aldehyde in less than 5 minutes. This fast reaction is accelerated by the

fact that the energy gain by the stabilization energy of the formation of the hydroquinone

is so high that it results in an exothermic reaction, heating the reaction mixture up to 80

°C. In order to compare the two reactions, the catalysis with 7 was carried out at three

representative temperatures: room temperature, 40 °C and 80 °C. The reaction at room

temperature reaches full conversion of the alcohol to the aldehyde after nearly 70 minutes.

The reaction at 40 °C has a steeper and therefore faster conversion rate at the beginning

and reaches a conversion rate >90% after 60 minutes. The comparable reaction at 80 °C

reaches full conversion after 30 minutes.

Oxidations

23

Figure 8: Conversion of n-octanol to 1-octanal at different temperatures with vitamin K3 7 compared to

oxidation with p-benzoquinone as co-oxidant (2.5 min, 99%). Conversion determined by GC-FID. Conditions

0.01 mol% 4, 0.01 mol% base, 1.1 eq of co-oxidant in thf.

This result is interesting since it shows that the catalysis can be controlled through the

reaction conditions when using menadione 7 as oxidant.

The influence of further reaction conditions was analyzed proving that neither the quality

of the solvent nor the quality of the n-octanol significantly influence the outcome of the

reaction. The concentration, at which the catalysis is run, does not influence the outcome.

Following experimental conditions were used to prove the concept 0.01% 4, 0.01 mol%

base, 1.1 eq menadione 7 and thf as solvent. Further investigation showed that the

minimum amount of catalyst, which can be used, is 0.001 mol%. If less is used the reaction

does not start. If a tenfold amount of catalyst is used, the reaction shows a faster conversion

of the alcohol.

As depicted in Figure 9, the base has a crucial influence in the reaction process as activating

agent of the catalyst. When 0.001 mol% 4 are used without any base meaning no activation,

only 40% conversion were observed after 60 minutes.

0

20

40

60

80

100

120

0 20 40 60 80 100

conve

rsio

n [

%]

time [min]

benchmark with BQ

room temperature

40 °C

80 °C

Oxidations

24

Figure 9: Effect of used amount of base in the catalytic dehydrogenation of n-octanol with benzoquinone

and menadione as co-oxidant. Blue square 0.01 mol% catalyst, red dots 0.001 mol% catalyst. Further conditions

1 eq n-octanol, 1.1 eq menadione, thf at room temperature.

With those tools in hand it was decided to monitor the reaction progress by NMR

spectroscopy. Especially the change of the C1 proton of the alcohol to the aldehyde and

the change of the methyl substituents and the –OH formation of the reduction in the

menadione can be observed by 1H NMR studies. Time resolved stacked 1H-NMR spectra

clearly show the evolution of the reaction:

Figure 10: Time resolved 1H-NMR spectroscopy of the catalytic reaction.

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100

conve

rsio

n[%

]

time [min]

0.01 mol%

0.001 mol%

Oxidations

25

Figure 11 represents a magnification of the spectra from 4.5 to 10 ppm. At 9.6 ppm, the

signal of the aldehyde proton is clearly increasing. The signal at 8.5 ppm matches the shift

towards higher frequencies of the aromatic substituents, consistent with aromatization of

the reduced menadione 7. Also, the change of shift from 6.9 ppm to 6.5 ppm of the

menadione 7 proton at position 2 can clearly be seen. This shift towards lower frequencies

is corresponding to the aromatization of the molecule. The formation of the -OH groups

on the reduced vitamin K3 are observable at around 4.7 ppm.

Figure 11: Magnification of 1H NMR reaction survey of the oxidation of alcohols.

Oxidations

26

2.1.3.1 Re-oxidation of vitamin K3 7

One advantage of vitamin K3 7 compared to 1,4-BQ is that the reaction can be followed

optically, since menadione 7 acts as a color indicator and changes its color from green to

blue when it gets reduced. Moreover, the reduced form of 7 is less soluble than

hydroquinone and can be filtered off from the reaction mixture. A further and crucial

advantage of 7 is that it can be re-oxidized by simple addition of 35% H2O2 solution. The

re-oxidation takes at least 24 h at room temperature but the regenerated vitamin K3 7 can

be used again in a next oxidation cycle.

2.1.3.2 Substrate Scope

Tewes demonstrated that the oxidation with [Ir(trop2DAD)]OTf 4 is very specific for primary

alcohols.[12b] Secondary alcohols are not oxidized. The non-activated aliphatic n-octanol

acts as model molecule throughout the reaction optimizations. In a further experiment the

substrate scope was examined and is represented in Table 3.

Table 3: Oxidation products of the catalytic dehydrogenation with menadione 7 as oxidant. The given

conversions are determined by GC-FID and 1H NMR spectroscopy. The catalysis was carried out by standard

procedure unless otherwise stated.

Conditions: 0.01 mol% [Ir(trop2DAD)]OTf 4, 0.01 mol% NaOtBu, 1.1 eq co-ox in thf at rt. a) toluene as solvent;

b) 0.1 mol% catalyst and base were used; c) reaction temperature 80 °C; d) 0.5 mol% catalyst and base were

used.

Entry Reactant Product Time

[min]

Conversion

[%]

1

30

60

30

>95a)

89b)

>97c)

2 60 >99c)

3

3.5 h 83c)

4

90 10c)d)

Oxidations

27

5

150

8 h

48

74c)

6

60 31c)

7

150 <4c)

8

150 4b)

9 150 <2b)

8

120 black polymer

9 150 <2b)

The different reactions with alcohols lead to similar results as found with 1,4-BQ as co-

oxidant. No secondary alcohols were oxidized. Only primary alcohols did undergo a

dehydrogenation reaction. Further examination of functional group tolerance showed that

with menadione 7 as co-oxidant the oxidation is more sensitive to functional groups. Even

though a new co-oxidant is found, which is environmentally acceptable, the problem of the

substrate scope remains an issue for this catalytic process which is probably due to the

bulkiness of the [Ir(trop2DAD)]OTf 4 catalyst.

The reactions with the substrates showed a slower reaction rate, which led to the idea that

intramolecular lactonization could be achieved. The reaction of diols and polyols to the

more interesting lactones was tested. In order to compare the results, the reaction

conditions were summarized as follows: Direct addition of menadione 7 (Vit K3), direct

addition of 1,4-benzquinone (1,4-BQ) and dropwise addition of a solution of 1,4-

Oxidations

28

benzoquinone over a timespan of 1 hour (1,4-BQ*). The results and descriptions are

summed up in Table 4.

Table 4: Oxidation of diols with 0.1 mol% [Ir(trop2DAD)]OTf 4, 0.1 mol% NatBuO and 1,4-Benzoquinone

(1,4-BQ) or menadione (Vit K3, 7) as oxidant. The given conversions are determined by GC-FID and NMR

spectroscopy. The catalyses were carried out by standard procedure with 2.2 equivalents of oxidant unless

otherwise stated.

Conditions: *) 1,4-BQ* stands for the dropwise addition of the oxidant over a time period of 1 h. The oxidant was

therefore previously dissolved in thf and added by syringe pump. The yields are given for the time after the

total addition of the oxidant; a) the same result is observed with as less as 0.01 mol% catalyst; b) conversion

given after 1h reaction time; c) 0.03 mol% catalyst.

Entry Reactant Oxidant Product Conditions

1

a) 1,4-BQ >85% - - <8%

b) 1,4-BQ* >98% - - - a)

c) Vit K3 >98% - - - a) b)

2

a) 1,4-BQ 90% 10% - - a)

b) 1,4-BQ* >95% - - - a)

c) Vit K3 >96% - - - a) b)

3

a) 1,4-BQ - - >98% - a)

b) 1,4-BQ* <6% - <80% >10% a)

c) Vit K3 mixture mixture mixture mixture a) c)

4

a) 1,4-BQ* >95% - -

b) Vit K3 - <5% - b)

Oxidations

29

The products yielded from the catalysis were characterized by 1H and 13C NMR

spectroscopy in addition to GC-FID. It is shown that vitamin K3 7 can act as a co-oxidant

even in the intramolecular lactonization reaction. 1,4-Benzoquinone did act as a good

oxidant, but needed special conditions – thus the addition over 1 h by syringe pump – in

order to have the same result. Unfortunately, when diols bearing free amines or amino

groups in the molecule were tested the product outcome was not easily identifiable. It

resulted in a black polymer which was not further identified. In the case where hexane-1,6-

diol was used, the thermodynamic favorable lactonization did not occur but rather a double

oxidation of the alcohols was observed.

When menadione 7 was used, a mixture of non-reacted to fully oxidized molecule was

observed.

Proof of concept

The results are given in Table 5.

Table 5: Examples of applied Iridium catalyzed oxidations on molecules of industrial/synthetic relevance

with 1,4-BQ as oxidant. Reaction control was followed by DC for Entry 1 and with GC/MS for Entry 2.

# unpublished results, Ivano Pusterla, ETH Zürich.

The oxidation 4 can be used with molecules that are synthesized in bigger scale in industrial

laboratories.

Entry Reactant Product Time

[min]

Conversion

[%]

1

Fmoc-Arg(Z)2-alcohol Fmoc-Arg(Z)2-aldehyde

90

>90

2 # #

30

>80

Oxidations

30

Concluding remarks

The results show that menadione 7 is a valuable alternative to 1,4-BQ as co-oxidant. It slows

down the reaction which allows a softer and gentler reaction procedure and a higher

degree of control. There is no noteworthy heat production and the reaction can be

accelerated by warming up the reaction mixture. The slower process of the reaction allows

the conversion of certain small diols to the corresponding lactones in quantitative yields,

whilst as soon as larger molecules are involved a mixture of oxidation products is observed.

Non-activated aliphatic alcohols react completely to the corresponding aldehydes at good

rates. On the other side, it was shown that sterically demanding substrates do not show

satisfying conversion rates when vitamin K3 7 is used as an oxidant.

This proves that the [Ir(trop2DAD)]OTf 4 catalyst is very specific for sterically non-

demanding reactants and has a perfect balance with 1,4-benzoquinone as co-oxidant.

Whit this in mind a sterically less demanding square-planar ligand for the coordination of

iridium and its application as an oxidation catalyst was considered.

New trop Based Ligands

31



32

Introduction

Since the 5H-dibenzo[a,d][7]annulene scaffold (trop) was first used as a ligand for transition

metals in 1999,[13b] it proved to be a rather interesting and useful ligand in catalytic

processes.[9, 11b, 11c, 39] Its characteristic structure with the unique symbiosis of a bulky and

rather soft olefin ligand has been its key to success. The 5H-dibenzo[a,d][7]annulene

scaffold has not only been proven to be a steering ligand when coordinated to transition

metals such as nickel, cobalt, iridium, rhodium and iron but is also crucial for the complexes

and the resulting catalytic activities.[12d] The tropylidene ligand stabilizes its metal

complexes through electronic and steric effects, which are best shown in stable low

oxidation state late transition metal aminyl radical complexes.[10c, 39] In these complexes the

bis(trop)amine ligands coordinate the metal in a butterfly coordination in which the

complex is stabilized in a cooperation between a redox non-innocent backbone, its bulky

trop units and the transition metals. This interplay also improves the catalytic activity of the

transition metal complexes, by perfectly matching the electronic balance between the

metal and the radical complex. Furthermore the similar bis(trop)phosphane ligand showed

interesting properties,[40] where the tropylidene unit is of paramount importance in the

stabilization of the metal radical. But the interplay of the bulky trop unit and the redox

active backbone combined in a ligand is not only limited to bis(trop)amine or

bis(trop)phosphane ligands.

Many classes of redox-active ligands with trop substituents are known. The most prominent

group are the tetradentate ligands with diazadiene backbones. These ligands have a co-

operative backbone, which means that the diazadiene scaffold of the backbone takes part

in the catalytic process by showing a high grade of redox activity. In an interplay with the

coordinated metal, this interaction allows fast and clean catalytic transformations such as

C-H-activation, hydrogenation, small molecule activation, the use in ruthenium catalyzed

hydrogen generation or high conversion in the production of bulk chemicals.[2g]

In the last decades, the Grützmacher group has developed many different trop-based

ligands, which were of crucial importance for stabilizing unusual, novel transition metal

oxidation states, for developing exceptional catalytic processes or just to be able to

synthesize complexes of pure academic interest. These tropylidene based ligands are tetra,


33

tri-, or bidentate, depending on the coordinated metal, the steric and electronic

environment, the co-ligands, and, most importantly, the purpose of the complex.

The tropylidene scaffold has C2v symmetry, which simplifies the characterization of the

complexes, especially the analysis of the 1H NMR spectra. Due to the fact that the chemical

shift of the trop olefin changes upon coordination to the metal, this process can easily be

monitored by NMR. Therefore, a fast first interpretation of the coordination sphere can be

carried out by common NMR analysis. This symmetry is also of crucial importance for the

stabilization of metal centers. The benzannulated rings on the seven-membered central

ring are powerful steric shields for the coordinated metal. The flexibility and geometry of

the cycloheptene ring form a concave shield for the metal center. The ligand “embraces”

the metal and keeps it fixed and stable. The advantageous symmetry was explored widely

in earlier projects resulting in new ligands.

The scope of ligand design remains the exploration of outstanding possibilities when

coordinated to a metal. Throughout the history of chemistry, many examples can be found

where the synthesis of a new ligand led to a vast number of new types of reaction

possibilities. Ligands can stabilize, or even force metals in oxidation states which do

normally not occur. Through ligand tuning, new catalytic processes are “made possible”,[41]

or systems are developed which lower the costs of production for bulk chemicals, and are

therefore of paramount importance for our environment.[33] Coordination and

organometallic chemistry became valuable tools for synthetic chemists. This source of

innovation has still not been exploited to its full capacity. The search for new, better

catalytic reactions is a growing and active field in chemistry and cannot be covered without

synthesis of new ligands and proof of new reaction types.

Some novel aspects and inputs in coordination chemistry and new highly efficient transition

metal complexes are known today which are trop-based systems developed in the last

years by Grützmacher and co-workers.

The design of new ligands where the highly symmetric trop unit is integrated can lead to

interesting new coordination of metals. The incorporation of the tropylidene scaffold in

different ligands integrates a higher degree of symmetry. The bulky ligand being highly

symmetric itself can be used as a tool in order to enlarge the symmetry of the whole ligand

after coordination to a metal e.g. bis(trop)amine, trop2DAD.


34

With this in mind, the design of a tridentate ligand which can be easily tuned by its

substituents was considered.

The trop2DAD 3 ligand has proven to be an especially interesting ligand. Its redox

cooperativity with the substrate in the catalytic oxidation of primary alcohols using the

[Ir(trop2DAD)]OTf 4 complex showed to be of high importance. But also the interplay of

ligand and metal must be taken into account, due to the fact that even though a ligand

may be redox active, it does not guarantee a catalytic process. As described by Vogt,[12d]

the analogous rhodium trop2DAD complex does not show any cooperative interaction

between the ligand and the metal and thus leads to a low catalytic activity. However, when

coordinated to other transition metals such as ruthenium, nickel, or iron, the redox ability

of trop2DAD showed to be of a vast importance for the catalytic performance of these

complexes.[12d] The stabilizing effect of the trop ligand in the case of lower oxidation state

metals demonstrates the applicability of these ligands in synthetically challenging

complexes. Over the years, the research in the Grützmacher group has shown that the

combination of the trop unit with redox-active backbones can lead to very effective

catalysts.[10b] These catalysts are of immense importance for the energy efficient

transformation of crude bulk chemicals to specialty chemicals. The transformation of crude

chemicals into higher raffinated intermediates for chemical processes is of interest in regard

of the energy consuming lifestyle of modern times. For these processes, the catalytic

approach is an interesting alternative to classical energy consuming reactions, where atom

efficiency is also a key point to be taken into account. For this purpose, the design of

defined complexes for a specific reaction is of paramount interest. The classical way of

catalyst design is to tune the steric and electronic effects of a ligand backbone in a manner

that the catalyst meets the desired performance. A possibility of ligand design is based on

cooperative ligands, such as our trop2DAD, which already proved its efficiency.[42] Another

way is that the ligand is redox inactive and the catalytic process is SET (single electron

transfer) based. This can be illustrated by an example from our research group: In the case

of Bis(trop)Amino and Bis(trop)Phosphane where the metal is coordinated by two trop units

and one amino or phosphane unit, respectively, the metal has a tricoordinate sphere. The

consecutive reduction of the rhodium Bis(trop)Amino complex leads to a highly reactive

catalyst with the formation of an amido concerted radical resulting in an open shell radical

system concentrated predominantly on the ligand. A further one-electron reduction leads


35

to an amido complex which has shown to be of great interest for the catalytic activity in

transfer hydrogenation and dihydrogen activation. The active and cooperative part in this

case is caused by the reactive center of the amido branch.

With the concept of unifying a redox-active backbone with the unique characteristics of

the tropylidene, the design of a new generation of tethered trop ligands was undertaken.

The main interest was to create a new ligand that may be active in the

dehydrogenation/hydrogenation of small molecules. The main interest was focused on a

new generation of iridium catalysts for the oxidation of alcohols. The trop2DAD ligand with

its symmetric and planar structure was taken as model. Because the tripodal coordination

of the Bis(trop)Amine ligand has proven to be a powerful instrument for catalytic systems,

an approach towards a tridentate ligand design was attempted. The planar coordination

sphere of the metal proved to be of value to the catalytic performance in the oxidation of

alcohols, therefore a planar design was followed. The ligand should ideally be non-innocent

– i.e. be active in the catalytic process of the oxidation of alcohols as described in chapter

“Oxidations”, sterically shielding the metal center in order to allow a selection of alcohols

to be oxidized and thus be a highly selective catalyst for primary alcohols. The idea behind

a tridentate ligand tropDADR is the potential availability of a free coordination site. This can

be taken by an additional ligand (Lig) in order to control the electronic and steric structure

of the catalyst (Figure 12). The substrate molecule likely binds above the plane defined by

tropDADR and the ligand (Lig).

Figure 12: Schematic representation of an idealized ligand for the oxidation of primary alcohols.

The butterfly coordination sphere in the Bis(trop)Amine coordination of metals proved to

be crucial for the efficiency of the catalytic reaction. As shown by this group, the reduction

of the amino unit changes the character of the hybridization of the nitrogen atom. In this

new design, the ligand is arranged in a way that the trop unit can chelate the metal in a

trigonal planar fashion. This is a further reason why a planar environment around the metal

is desired.


36

The transformation of substrates that are difficult to activate often take place in catalytic

redox-coupled reactions. The ligand, which was to be designed, in this case should be

cooperative, i.e. redox-active, meaning that the ligand may act as charge carrier for the

redox events in a catalytic transformation. In the past years, many new classes of ligands

were designed in order to fulfil such properties, which opened an enormous potential in

catalytic reactions and the area evolved to produce many new synthetic approaches

towards new molecules.

Possible ligand candidates for a library of catalysts with similar reactivity as trop2DAD

complexes were all designed with an alpha diimine backbone and soft atoms such as

nitrogen and phosphorus as coordination partners. Following ligands were created by

combining soft atoms and the trop unit – except for one case where a hard oxygen atom

is present:

Figure 13: Possible new redox-active ligands.

With the desired sterical changes in mind, a new family of ligands based on alpha-

iminopyridine was considered. Described by van Koten and Vrieze in 1983,[17b] those ligands

have shown to be involved in electron-transfer (ET) processes to and from catalytically

active transition metals. This intrigued our interest for the design of these ligands. A lot of

work has been done in the field of redox-active ligands in general and specifically with

alpha-iminopyridine based ligands. They have been reported in the closed-shell neutral

coordination form (L0), the monoanionic radical form (L.-), and in the closed-shell dianionic

form (L2-).

In order to compare the well-established trop2DAD described in Chapter 2, ligand tropPicol

8 was synthesized in a straightforward route. The trop moiety does not only act as a

cooperative hemi-labile ligand but rather as a steric placeholder in order to have a

comparison with the trop2DAD complex.


37

In the alcohol oxidation process the coordination of the ligand to the metal and the redox-

activity of the ligand are prerequisites for the reaction to take place. It was shown that the

oxidation proceeds well as long as the alcohols are not sterically demanding. This fact leads

to the question on how this problem can be solved. The tropPicol 8 ligand acts as perfect

model ligand when coordinated to the desired metal for the comparison of the two

systems. In addition to the redox-active backbone, the free coordination site allows the

tuning of the electronic environment around the metal. Given the right choice of ligand, a

fine-tuning can be pursued. This option gives two handy tools, which influence the

reactivity of the complex.

Figure 14: Selected ligand design. The electronic properties of the complex can be tuned by the right choice

of the co-ligand. E.g. an electron rich transition metal can be “pumped” with more electrons with the right

ligand.

Once the ligand is coordinated to a metal, one could “play” with its electronic properties.

The electrochemistry of the backbone is similar to the active one found in the trop2DAD

and thus reduction of the backbone should lead to a cooperating moiety.

Scheme 12: Schematic electron pumping in a redox active ligand.


38

TropPicol

Synthesis of the ligand

The synthesis of the trop-based ligand tropPicol 8 is straightforward and depicted in

Scheme 13. Picolinaldehyde reacts with tropAmine in the presence of catalytic amounts of

triflouroacetic acid in methanol to give the desired (E)-N-(5H-dibenzo[a,d][7]annulen-5-yl)-

1-(pyridine-2-yl)methamine (tropPicol, 8) ligand in a condensation reaction.

Scheme 13: Synthesis of tropPicol 8 from picolinaldehyde and tropAmine 2.

Single crystals of 8 were obtained from slow diffusion of a concentrated DCM solution of

8 overlaid with hexanes. The ORTEP of the molecular structure is depicted in Figure 15

Selected bond lengths are given in the caption.

Figure 15: ORTEP of TropPicol 8 at 50% ellipsoid probability. Hydrogen atoms omitted for clarity. Selected

bond lengths [Å]: N2-C17 1.340(4), C17-C16 1.474(6), C16-N1 1.257(4), N1-C1 1.480(5), CB-C5 1.341(4).

In order to have possible oxidation catalysts, different transition metal complexes with

tropPicol were synthesized.

N2

N1

C17

C16C1

C4

C5


39

Synthesis of [Ir(tropPicol)Cl]2 9

To a solution of tropPicol 8 in DCM half an equivalent of [Ir(coe)2Cl]2 dissolved in DCM was

added. The reaction mixture was stirred at room temperature whereby the reaction color

turned deep green. After concentration under reduced pressure the solution was overlaid

with n-hexanes. The deep green needles were filtered off and washed with n-hexanes

yielding the pure complex [Ir(tropPicol)Cl]2 9. The ORTEP is depicted in Figure 16.

Figure 16: ORTEP of [Ir(tropPicol)Cl]2 9 at 50% ellipsoid probability. Hydrogen atoms omitted for clarity.

Selected bond lengths [Å]: C17-N2 1.365(6), C16-C17 1.471(5), N1-C16 1.428(6), N1-C1 1.482(4), C4-C5 1.438(5),

Ct(C4-C5)-Ir1 1.987, Ir-N1 2.038(3), Ir-N2 2.107(3), Ir1-Cl 2.378(4), Ct(N=C)-Ir1 1.970.

Synthesis of [Rh(tropPicol)Cl]2 10

Complex 10 was synthesized in analogy to complex 9. To a solution of tropPicol 8 half an

equivalent of rhodium precursor [Rh(cod)Cl]2 was added. The red solution was stirred for 2

h at room temperature before being concentrated in vacuo. The concentrated solution was

overlaid with n-hexanes. Deep red, long needles formed and were of good quality for single

crystal structure analysis. The ORTEP is depicted in Figure 17.


40

Figure 17: ORTEP of [Rh(tropPicol)Cl]2 10 at 50% ellipsoid probability. Hydrogen atoms omitted for clarity.

Selected bond lengths [Å]: C17-N2 1.351(4), C16-C17 1.464(3), N1-C16 1.404(3), C1-N1 1.492(3), C4-C5 1.421(3),

Ct(C4-C5)-Rh1 2.020, N1-Rh1 2.023(2), Cl1-Rh1 2.3682(7), ct(C=N)-Rh1 1.976.

Discussion

As depicted in Figure 16 and Figure 17 the coordination of the transition metal results in a

dimeric complex. The imine bond of the ligand acts as fifth coordinating site resulting in a

trigonal-bipyramidal environment of the metal with the sum of angles being 357° around

the iridium and 358° around the rhodium complex. The nitrogen of the imine and the

chlorine ligand are cis to each other whilst the trop unit, the C=N imine bond of the second

ligand unit and the nitrogen of the aromatic ring lie in a plane. This coordination sphere is

not unusual for rhodium(I) complexes.

In Table 6, selected bond lengths and angles of the free ligand tropPicol 8, the iridium

complex 9, and rhodium complex 10 are compared.


41

Table 6: Comparison of selected bond lengths and angles between 8, 9 and 10.

Bond length [Å] free ligand

8

[Ir(tropPicol)Cl]2

9

[Rh(tropPicol)Cl]2

10

C=C 1.341(4) 1.438(5) 1.421(3)

C=N imine 1.257(4) 1.428(6) 1.404(3)

C-N backbone 1.480(5) 1.484(4) 1.492(3)

C=N aromatic 1.340(4) 1.365(6) 1.351

M-ct - 1.987 2.020

M-N pyridine - 2.107 1.976

M-N imine - 2.038(3) 2.023(2)

M-Cl - 2.378(4) 2.368(2)

As expected, the most striking difference between the free and the coordinated ligand is

found in the olefinic part of the tropylidene unit and the coordinated imine unit. In both

cases, the trop double bond is elongated when coordinated to the metal. This is the result

of the interaction of the olefin with the metal showing a higher degree of

metallacyclopropane content in the bond formation. The elongation is 0.097 Å for the

coordinated iridium and 0.080 Å for the rhodium. Whilst the C-N bond length of the

backbone (C1-N1) does not change significantly and thereby showing that there is not a

strong steric interaction in the coordination process, the imine C=N bond is elongated by

0.171 Å and 0.147 Å when coordinated to iridium and rhodium, respectively. This is the

direct result of the coordination of the metal to the imine. The character of the imine

changes from a sp2 nitrogen to a sp3 hybridized center with a large contribution from back

bonding.

The newly synthesized complexes could not be used as catalysts. The surrounding of the

transition metal did not allow any catalytically active site for the oxidation of primary

alcohols. Therefore, new potential catalytically active complexes were synthesized.

Synthesis of [Ru(tropPicol)(cymene)Cl] and [Cu(tropPicol)(ACN)]PF6

The reaction of the ligand 8 with further interesting metals for oxidation reactions yielded

two new complexes. [Ru(cymene)(tropPicol)Cl] 11 and [Cu(ACN)(tropPicol)]PF6 12.


42

3.2.5.1 [Ru(tropPicol)(cymene)Cl] 11

To a solution of tropPicol 8 in thf, 0.5 eq of [Ru(cymene)Cl2]2 precursor was added. The

solution turned immediately red and was stirred further for 10 h at 80 °C. After cooling

down to room temperature, the solution was concentrated under reduced pressure.

Crystals of good quality for X-ray were grown from a DCM:n-hexanes solution. The ORTEP

is depicted in Figure 18.

Figure 18: ORTEP of [Ru(tropPicol)(cymene)Cl] 11 at 50% ellipsoid probability. Hydrogen atoms omitted for

clarity. Selected bond lengths [Å]: C1-N1 1.505(2), N1-C16 1.278(2), C16-C17 1.455(3), C17-N2 1.353(3), C4-C5

1.345(3), Ru1-N1 2.116(2), Ru1-N2 2.077(1), Ru1-Cl1 2.3909(7), Ru-Cym 1.682.

3.2.5.2 [Cu(tropPicol)(ACN)]PF6 12

[Cu(ACN)6]PF6 is added to a solution of tropPicol. Immediately, a color change of the

solution to a dark red is observed. The NMR analysis shows the coordination of the

backbone but not the coordination of the trop scaffold.

Crystals for X-ray analysis were grown from a solution from DCM:hexanes.

Cl1

RuN2 C17

C16N1

C1

C4C5


43

Figure 19: ORTEP of [Cu(tropPicol)(ACN)]PF6 12 at 50% ellipsoid probability. Hydrogen atoms and counter

ion (PF6-) omitted for clarity. Selected bond lengths [Å]: C17 N2 1.353(3), C16 C17 1.475(4), N1 C16 1.271(3), C1

N1 1.492(3), C4 C5 1.335(5), Cu1-N1 2.025(2), Cu1-N2 2.051(2), Cu-N3 (CH3CN) 1.859(2).

3.2.5.3 Discussion

The summarized representation of the bond lengths of the free ligand 8, the ruthenium

complex 11 and the copper complex 12 are shown in Table 1Table 7.

Table 7: Comparison of selected bond lengths and angles between 8, 11 and 12.

Bond length

[Å]

tropPicol

8

[Ru(tropPicol)(cymene)Cl]

11

[Cu(tropPicol)ACN]

12

C=C 1.341(4) 1.345(3) 1.335(5)

C=N imine 1.257(4) 1.278(2) 1.271(3)

C-N

backbone

1.480(5) 1.505(2) 1.492(3)

C=N

aromatic

1.340(4) 1.353(3) 1.353(3)

M-N pyridine - 2.116(2) 2.025(2)

M-N imine - 2.077(1) 2.051(2)

M-Cl - 2.391(7) -

M-additional

ligand

- 1.682 1.859(2)

Cu

N1

N2

N3

C4 C5

C16

C17


44

The X-ray analysis of complex 11 and 12 shows that the ligand scaffold is not remarkably

influenced by the coordination to the metal.

tropNacAc

Based on the same findings a further tri-coordinate ligand was synthesized. The tropNacAc

13 ligand combines the advantages of the NacAc ligands and the trop unit. The synthesis

of tropNacAc 13 is depicted in Scheme 14.

The synthesis of tropNacAc 13 is catalyzed by a strong acid. As a side reaction, a SN2

reaction occurred at C3, resulting in a C3-trop substituted AcAc backbone. Even though

the major product is the tropNacAc 13 with more than 60% yield.

Scheme 14: Synthesis of tropNacAc ligand.

The products can be separated by flash column chromatography and further purified by

crystallization.

Crystals for solid state analysis of 13 were grown from a concentrated DCM:DME solution.

The ORTEP of the ligand is shown in Figure 20.

Figure 20: ORTEP of tropNacAc 13 at 50% ellipsoid probability. Hydrogen atoms omitted for clarity. Selected

bond lengths [Å]: O1-C2 1.242(2), C2-C3 1.409(2), C3-C4 1.376(2), C4-N1 1.331(2), N1-C6 1.452(2), C9-C10

1.330(5).


45

The crystal structure of 13 shows a conformation where the trop unit is turned away from

the methyl group at C4 thus minimizing the interactions with the NacAc backbone. An

interaction between the NH -O1, where the distance of the H-O1 is 1.90(2) Å. This hydrogen

bridge turns the NacAc backbone towards a position where a metal can be coordinated

through the heteroatoms N1 and O1.

Synthesis of [Pd(tropNacAc)]OAc 14

In order to understand the new ligand, tropNacAc 13 was coordinated to a Pd(II) center.

Palladium is one of the most used elements in catalysis and known to act as a good catalyst

for many reactions. Among them coupling reactions, heterolytic small molecule activations,

introduction reactions and oxidations.[43] The free NacAc ligand was coordinated to

Pd(OAc)2 in toluene. The crystals were grown from thf and showed a planar coordination

sphere around the palladium metal. The sum of angles being 359.89 °. The angle of the

centroid (C3-C4)-Pd1-N1 is 92.05°, whilst the angle O1-Pd1-N1 is slightly larger with 94.16°.

The complex crystallizes in a square planar conformation.

Figure 21: ORTEP of [Pd(tropNacAc)OAc] 14 at 50% ellipsoid probability. Hydrogen atoms omitted for clarity.

Selected bond lengths [Å] and angles [°]: O1-C19 1.294(7), C19-C18 1.364(7), C18-C17 1.410(7), C17-N1

1.320(7), N1-C1 1.500(6), C4-C5 1.397(7), Pd1-O1 2.003(3), Pd1-O2 2.046(4), Pd1-Ct(C4-C5) 2.029, Pd1-N1

2.009(4), O1-Pd1-O2 88.11, O1-Pd1-N1 94.16, N1-Pd1-Ct 92.05, Ct-Pd1-O2 85.57.

Through the coordination of the Pd the C1-N1 bond distance is elongated compared to

the free ligand. The coordination of the trop unit to the Pd causes the elongation of its

O2

Pd

O1

N1

C19

C18

C17C1

C5

C4


46

double bond, clearly showing an interaction. With 2.029 Å the bond length between the Pd

and the ct(C4-C5) is slightly longer than the bond lengths of the metal with the

coordinating heteroatoms of the NacAc backbone N1, O1 and the thf. Whilst the bond

distances throughout the NacAc backbone are shortened compared to the free ligand.

Concluding remarks

Two potentially redox active and interesting ligands were synthesized and their

coordination to several transition metals were shown. TropNacAc 13 resulted to be an

interesting ligand for square planar palladium complexes which can be tested for oxidation

reactions.

TropPicol 8 was planned as an alternative ligand to the trop2DAD but its complexation to

the [Ir(cod)Cl]2 resulted in a dimer, which proved to be too stable to act as catalyst. But, its

coordination to other metals seemed readily available and thus the complexation to iron

and its catalytic activity will be described in the Chapter “Iron Chemistry”.

Phosphorus Ligands

47

Phosphorus Ligands

Phosphorus Ligands

48

Bis(trop)-1,3,2-Diazaphospholidine

Introduction

Carbenes have prominently been represented in the chemistry of the last few decades.[44]

Since their discovery in the 1990’s, carbenes have been subject of a vast interest due to the

fact that they can be seen as substituent of the omnipresent phosphane ligands. The strong

donor and weak acceptor qualities make them especially interesting as ligands in

transition metal chemistry. Due to their unique properties, carbenes gained much attention

as valuable tools for catalysis.[45] In our group several carbene-trop ligands were

synthesized[2c] and their Nickel complexes described.[12d] The huge success of N-

heterocyclic carbenes (NHC) inspired the search for a group of similar ligands where the

divalent carbene carbon atom unit is replaced by group 13-16 elements. These formal

isoelectronic molecules attained a lot of interest (Figure 22). Of all the possible substitute

ligands, the most investigated are the diazaphospholenes, abbreviated NHP, in analogy to

the N-heterocyclic carbene (NHC).

Figure 22: Generic description of a N-heterocyclic carbene (left) and group 13-16 analog (right).

The neutral 1,3,2-diazaphospholenes (Figure 23) are being applied as ligands or as organic

catalysts for the hydrogenation of N=N bonds with ammonia-borane.[46] An intriguing

difference compared to the carbene is the possibility to act as a strong hydride donor if the

third substituent on the phosphorus is a hydrogen, due to the peculiar P-H bond

properties.[47] NHPs are also good precursors to the interesting isoelectronic equivalent to

the NHC species, the phospholenium cations.

Figure 23: Representation of different 1,3,2-diazaphospholenes.

Phosphorus Ligands

49

The vacant orbital and the lone pair electrons of the phosphorus atom in the cationic

NHP are similar to a singlet carbene and, therefore, a phospholenium ion can act as -

donor (LB) and -acceptor (LA) Figure 24

.

Figure 24: interaction between the diazaphospholenium and a metal.

Although the phospholenium ions were already isolated in 1972[48], they regained a lot of

interest and became one of the most studied and described species in the field of hetero

nuclear ring structures and were widely discussed in the literature due to the similarity with

Fischer carbenes.[45]

Even though the phospholenium ion has inverted electronic properties compared to the

carbene, it has parallels in its reactivity. Through its unsaturated backbone and

delocalization of the electrons over the ring system the NHP-cation possesses aromatic

character, as proven in agreement with different studies including DFT calculations and

ELF.[45, 49] It was shown by calculation and proven experimentally that the cationic NHPs

have planar geometries and bond lengths which are relatively close to the values found in

aromatic compounds, suggesting a delocalization of the -electrons.[49]

The stability of such cationic ligands is not only given through the delocalization of the

electrons in the unsaturated backbone but rather through the interaction of the -electron

conjugation between the nitrogen-lone pair and the empty p() orbital.

It is this interaction that allows the synthesis of stable phospholidine cations (Figure 24)

with saturated backbones, even though the aromatic stabilization energy is missing. The

structural differences between NHPs and saturated phospholidines in the backbone are

obvious. NHPs have short bonds throughout the backbone (average bond length is 1.43 Å)

[50] and relatively short NPN bond lengths (1.44 Å) in agreement with an aromatic character

dispersed over the entire ring system. The symmetry of the NHP molecules is given by the

conjugation of the heteroatoms and is planar. Examples can be found in the literature.[45]

In contrast to NHPs, the diazaphospholidines described in the literature are not planar but

Phosphorus Ligands

50

have a twisted conformation, with longer bonds between the phosphorus and nitrogen

atoms (1.656 Å).[51] Further, in this conformation, the hydrogen atoms of the saturated

backbone are found in trans position in order to minimize interactions and thus stabilize

the molecule.

Scheme 15: Overview of the different nomenclatures used herein.

Even though phospholidines have no aromatic stabilization, they can exist as cationic

species, due to the strong interaction between the nitrogen and the phosphorus atom. As

neutral molecules, phospholidines are mainly described as chiral phosphine ligands in

coordination chemistry. Compared to the phospholenium, the cationic phospholidine has

a more electrophilic character. On the other hand, when coordinated, the NHP cation shows

a stronger metal-to-ligand charge-transfer, due to the fact that the aromatic backbone can

take up more electron charge from a basic metal compared to the saturated backbone

of the phospholidine.[45] The NHPs as the phospholidines are, in combination with the two

trop units, of big interest due to their redox active backbones.

Synthesis of tricoordinative phosphorus trop ligands

Phospholidines can be accessed via hydrogenation of 1,3,2-diazaphospholenes, which are

in turn prepared in a straight-forward manner based on the interaction of the two

nucleophilic nitrogen atoms with an electrophilic phosphorus building block. Further

functionalization of the neutral phospholidine ligands opens the door to new reactive

phosphorus species. Both neutral heterocyclic rings can act as precursors for the very

reactive cationic species.

Since NHPs are good starting materials for different targets, many ways to synthesize these

molecules were investigated and described, e.g. the NHP can be transformed to the cationic

precursor in straightforward steps. Starting from the reduction of an alpha diimine

(diazadiene) with alkaline metal followed by a condensation with PCl3 or PCl2R

(chlorophosphines) the reaction results in a halogen- or alkylphospholene. Alternatively,

Phosphorus Ligands

51

the reduced diazadiene can be reacted with GeCl2 or SiCl4 leading to the corresponding

germanele or silylene, followed by a metathesis reaction with the dihalogenphosphine

yielding the desired diazaphospholene (Scheme 16).[52]

Scheme 16: Route for the synthesis of diazaphospholenes through direct condensation of PCl3, or by

metathetical reaction where E represents Ge or Si.

A further way consists of quenching the diamide with triethylammonium chloride in order

to produce an α-aminoimine followed by condensation with PCl3. The advantage of this

reaction is that triethylamine acts as base in the following condensation, scavenging the

liberated hydrogen chloride.[53]

Scheme 17: Synthesis of the diazaphospholene with H+ scavenger.

The synthesis of the saturated diazaphospholidines can be carried out in a similar way

(Scheme 18). Yet another alternative is the synthesis through a simple deprotonation of the

amine group followed by substitution with PCl3. These diazachlorophospha heterocycles

are the ideal precursors for the synthesis of diazaphospholenium ions and

diazaphospholidine cations.

Phosphorus Ligands

52

Scheme 18: Generic representation for the synthesis of diazaphospholidines. Diazadiene backbones can be

reduced (e.g. by LAH) to the corresponding amines. The diamine is then reacted with a phosphorus source (e.g.

PCl3) yielding the 1,3,2-diaza-1-chlorophospholidine, which can further be reacted with e.g.

triphenylphosphonium salts resulting in a P-P bonding.

Historically diazaphospholenium ions were synthesized by chloride abstraction from 1,3,2-

diazachlorophospholenes with Lewis acids, cleavage of the P-N bond in an amino

substituted phospholene with a strong acid, or direct coordination to a transition metal

after a heterolytic P-Cl cleavage.[52d, 54] For suitable substituents, a direct conversion to the

cationic species was described via [4+1] cycloaddition of the diazabutadiene with P(I)

cations. These one-step redox reactions are either generated by the disproportionation of

PI3 [55] or by reduction of PCl3 with SnCl2 in presence of a diimine Scheme 19.[56]

Scheme 19: Direct conversion to yield the phospholenium cation.

As described in Scheme 20, diazaphospholidinium ions are synthesized in a similar way.

The reaction of 1-chloro-1,3,2-diazaphospholidines with the appropriate Lewis acids, silver

salts, or TMS triflate can yield the desired cation.[52a, 57]

Scheme 20: Synthetic pathway for the achievement of diazaphospholidinium salts.

Phosphorus Ligands

53

Results

Attempted synthesis of the bis(trop)diazaphospholene as a precursor

The first step in the synthesis of the bis(trop)diazaphospholene is the reduction of the

diazadiene backbone with quantitative amounts of alkali metals followed by a metathesis

with quantitative amounts of PCl3. This step was first described in the late 1970s and proved

to work well for many diazadiene backbones in the synthesis of precursors for NHPs.[45]

Following this methodology, it was tried to reduce trop2DAD with 2 eq Li or Na and to

readily react it with PCl3 in order to produce the 2-chloro-1,3-bis(5H-

dibenzo[a,d][7]annulen-5-yl)-2,3-dihydro-1H-1,3,2-diazaphospholene 15.

Scheme 21: Planned synthesis of bistrop-1,3,2-diazaphospholene (trop2NHP) 15.

To a solution containing trop2DAD in thf or toluene two equivalents of reducing agent were

added. After addition of the alkali metal, the colorless solution turned deep red

immediately. The outcome showed to be the product of a reduction of the backbone with

an extended conjugated system throughout the trop2DAD system, resulting in the change

of color. The double amount of alkali metal did not only reduce the backbone but rather

reacted with the trop unit by reducing it.

In order to understand the mechanism of this reaction, a simple reduction with one

equivalent of Li was carried out. Crystals for X-ray analysis of the outcome were grown from

thf. The red crystals proved to be the reduced system of the diazadiene backbone. This is

represented in the shortage of the bond length between the two C-C bonds. As shown in

Figure 25, the two alkali metal ions are each coordinated to a nitrogen atom when 2

equivalents of reductant were used.

Phosphorus Ligands

54

Figure 25: Schematic depiction of the reduced trop2DAD scaffold.

The trop unit is also reduced. This further reduction of the tropylidene results in a deeper

red color which is due to the extended conjugated -system. Therefore, this route is not

suitable for our purpose.

Scheme 22: Attempted reduction of the diazadiene backbone of trop2DAD.

Alternative synthesis of the bis(trop)diazaphospholene

Since the one pot synthesis of the diazaphospholene by reduction and subsequent addition

of a dihalophosphane did not lead to success, the strategy was changed. In order to obtain

the desired product, a stepwise reaction with lithium and Et3NHCl as proton

donor/scavenger was followed.[45] The optimization of this synthetic approach was achieved

in the last decade and adapted to different diazadiene backbones. Of all the described

pathways, the most promising one is the reaction in which a phosphorus precursor is added

to a diazadiene precursor at -78 °C and the reduction of the backbone is formed. This can

be achieved in a stepwise reaction through controlled reduction of the imine of the DAD

unit through alkali metals and by addition of triethyl ammonium chloride as a proton

donor, whereby the chlorine salt of the alkali metal can be filtered off allowing the isolation

Phosphorus Ligands

55

and purification of the intermediate. The remaining Et3N in the reaction mixture serves as

a proton scavenger upon addition of PCl3 as phosphorus source, forming back the starting

NEt3HCl. For easier handling of the intermediates, additional base can be added which

facilitates the reaction with PCl3 (Scheme 23).

Scheme 23: Reaction pathway with Et3N as H+ scavenger.

The reaction with Et3NHCl did not lead to success. The reduction of trop2DAD proved to be

not easily accessible. The intermediate was observed in a mixture of different products.

Upon addition of PCl3, the reaction outcome was a mixture of different adducts. The 31P

NMR analysis showed many different products Extra addition of a small quantity of Et3N

did not improve the outcome. Further methods for achieving the NHP were taken into

account.

Reaction of trop2DAD with PI3

A further interesting alternative is the reaction of PI3 with a diazadiene backbone where PI3

acts as both a reducing agent and a phosphorous source.

In this reaction pathway, the diazadiene is reduced in situ by the iodide, which is oxidized

to I2. The challenge in this reaction is the difficulty to separate the I2 from the reaction

mixture and to avoid further reaction of the I2 (Scheme 24).

Phosphorus Ligands

56

Scheme 24: Reaction with PI3, where the I3- disproportionates to I2 and the active counter ion yielding 15.

The reaction with phosphorus triiodide resulted in an interesting reaction profile. After

addition of PI3 to a solution of trop2DAD in thf at -78 °C the reaction mixture immediately

turned red and after 10 minutes at room temperature, the solution turned brownish.

Reaction control with 31P-NMR showed a clear reaction progression and formation of the

desired target molecule. The 31P-NMR shift is found at 148 ppm which is in agreement with

similar compounds. This reaction proved that the redox reaction proceeded with our bulky

system. Since the in situ formation of the phospholene was observed, the abstraction of

the iodide was pursued. Me3SiOTf was added to the solution and a change of color was

observed. After exchange of the iodide with Me3SiOTf, a shift of the 31P-NMR signal towards

higher frequencies was observed. The NMR signal at 218 ppm is concordant with published

values found for phospholenium ions. Isolation of the diazatropphospholenium salt failed.

Its formation is evidenced by the distinct change of color from brownish to red and the in

situ 31P NMR spectroscopy. Attempts to crystallize the product resulted in a non-soluble

powdery substance.

A possible further reaction of the phosphorus salt with transition metal complex precursors

as trapping reagents for easier isolation did not result in any coordination. Further attempts

were abandoned.

Synthesis of diazaphospholidines

Since the bis(trop)diazaphospholene could not be isolated, the alternative molecule with a

saturated backbone was taken into account as a chelating molecule.

The idea to synthesize saturated NHP (sNHP) was the obvious following step in the search

of a new tridentate ligand. sNHP’s are known as asymmetric phosphane ligands for

transition metals in organic synthesis, but not necessarily as tridentate transition metal

Phosphorus Ligands

57

chelates.[57d] The connection of a phosphane ligand with the soft trop ligand system would

yield in an interesting interplay of properties and therefore yield a promising ligand for

transition metals. The innocent phosphane tropdiazaphospholidine acts as a precursor to

the cationic phospholidinium ligand which has potentially interesting cooperative

capacities when coordinated to a metal. The chemical properties of the phosphorus center

change drastically. In the phosphane ligand the phosphorus atom is a “spectator” whereas

in the phospholidinium the phosphorus becomes a positively charged, hard ligand which

actively interacts with the metal center.

Several synthetic routes are described in the literature.[45, 52a, 52c, 54b, 55] Based on the gained

experience for the synthesis from the bis(trop)phospholene, different routes to synthesize

the bis(trop)sNHP were examined.

Scheme 25: Synthetic possibilities to access tropPhospholidine.

The synthesis of sNHP was based on the assumption that a simple deprotonation of

trop2DAE followed by addition of a halophosphane would lead to the target molecule

(Scheme 25).

In order to get a clean reaction without any undesired byproducts, a base with specific

requirements had to be chosen. It should be either simple to be removed (e.g. by filtration

or crystallization) or not to get involved in the subsequent reaction after deprotonation.

The choice fell for the strong base n-BuLi on the basis of the facts that butane leaves the

reaction mixture readily as a gas and that after metathetical ion exchange of the PCl3, the

salt LiCl can be filtered off from the reaction mixture. To a solution of trop2DAE in thf at -

78 °C were added dropwise 2 equivalents of n-BuLi. The white turbid solution turned red

immediately. Based on the observation of the change of color, the deprotonation seemed

to be successful. After deprotonation, PCl3 was added dropwise followed by an immediate

change of color from red to white. The solution was stirred for 30 minutes allowing to warm

to room temperature. Formation of a white solid product was observed. After filtration, 31P

16

Phosphorus Ligands

58

NMR spectroscopy showed that the entire PCl3 was consumed and a new species with a

signal at 162 ppm was formed which corresponds to the shift of known sNHP. On the other

hand, 1H NMR analysis showed new products which were related to the NHP, but were

identified as side products. Therefore, this synthesis route was abandoned for an improved

protocol. In order to avoid the problem of the side reactions, the strategy was changed and

a step in the synthetic route added. The idea was to add TMSCl after the deprotonation

and isolate the intermediate before adding the phosphorustrichloride. The TMS adduct was

observed by NMR spectroscopy after full conversion. Unfortunately, the formation of

byproducts was observed during the deprotonation. After addition of PCl3, the control by

31P NMR spectroscopy showed that phosphorustrichloride was consumed, but also a

considerable amount of side products were formed. The side products were proven to be

doubly deprotonated trop2DAE fragments and double addition products of PCl3 with

trop2DAE.

As alternative, a base which can directly act as a chloride scavenger was used. The easiest

reactant in this case is Et3N. As described by Abrams,[52a] the base/reactant ratio is crucial.

It was found that for our purpose, the deprotonation of trop2DAE with trimethylamine

required an eightfold excess of base and one equivalent of PCl3. To trop2DAE in thf 8

equivalents of Et3N were added and the solution was cooled to 0 °C. Upon slow addition

of 1 equivalent of PCl3, immediately a white precipitate was formed. After removal of the

solvent, the yellowish precipitate was washed twice with Et2O and dried under reduced

pressure. The solid was dissolved in toluene in order to filter off the insoluble Et3NHCl. The

mother liquor was dried under reduced pressure resulting in a white powdery product. The

analysis of the white powder proved to be the desired target molecule in a high degree of

purity. Crystals of good quality for X-ray diffraction were grown from a DCM:hexanes

solution (Scheme 26).

The newly synthesized phospholidine 16 can act as a precursor to the

diazaphospholidinium cation after abstraction of the halogen by a scavenger. Different

halogen scavengers were examined in order to obtain the desired product.

Phosphorus Ligands

59

Scheme 26: Planned synthesis of a bistrop-1,3,2-diaza-1-phospholidinium salt (trop2PhosphoniumX-) 17.

Among the scavengers used were the silver salts AgPF6, AgOTf, AgBF4, the bulky NaBArF, and TMSOTf.

To solutions of 16 in DCM 1.05 eq of halogen scavenger were added. when AgOTf was

used the milky white solution turned pink immediately and a white precipitate was formed.

After filtration, NMR analysis of the solution revealed full conversion of 16 to 17.

Unfortunately, we were not able to isolate the highly reactive cationic species.

In order to get an overview of the coordination properties of phospholidinium ligands,

coordination to an electron-rich metal was undertaken.

Coordination to Rhodium

In order to compare the new phospholidine ligand and its cationic species with the non-

innocent bis(trop)amine ligand, it had to be coordinated to a metal. The metals of choice

for a phosphane ligand are electron-rich first row-transition metals such as Pd or Rh.

Addition of half an equivalent of [RhCl(C2H4)2]2 metal precursor to a solution of 16 in DCM

resulted in the first synthesis of complex 18. To a solution of 16 in d-DCM was added 0.5

equivalents of [RhCl(C2H4)2]2 in d-DCM. The clear solution immediately turned deep dark

red and gas evolution was observed. After 10 min of stirring at room temperature deep

dark microcrystals were formed. The crystals were filtered off, washed with pentane and

dried in vacuo.

NMR analysis of the crystals showed a pure form of 18 (Scheme 27). The crystals were of

good quality suitable for X-ray diffraction.

16 17

Phosphorus Ligands

60

Scheme 27: This reaction pattern allows a clean formation of complex 18 in good yields. Crystals of good

quality suitable for X-ray diffraction were grown from the mother liquor.

Intrigued by the fact that the Rh precursor is easily coordinated by 16, the synthesis of the

cationic species was aimed at.

The synthesis of the cationic metal species was attempted in two ways. As depicted in

Scheme 28, ligand 16 was firstly dehalogenated with 1.1 equivalent of AgOTf in DCM and

then the metal precursor was added. The reaction evolution was followed by NMR

spectroscopy. The pink solution described as 17 turned deep orange immediately after the

addition of the metal precursor. After a few minutes at room temperature the color

changed to deep pale red and a solid precipitate was observed. The 31P NMR signal was

lost during this reaction. The solid was filtrated and dried in vacuo. Unfortunately, complex

19 could not be isolated.

Alternatively, a reaction pathway to complex 19 through complex 18 was chosen. To a

solution of 18 in DCM was added 1.01 eq of AgOTf. The reaction was followed by NMR

spectroscopy but did not result in the desired product.

The abstraction of the halogen from the P-Cl bond was not successful. Again, different

halogen scavengers were scrutinized.

16 18

Phosphorus Ligands

61

Scheme 28: Attempted synthesis of a rhodium(I) complex 19. ScavX stands for a generic halogen scavenger.

As example AgOTf may be used (vide supra).

In order to overcome the halogen abstraction problem, the target molecule needed

substituents on the P-atom other than halogens.

As depicted in Scheme 29, instead of using halogenphosphanes as phosphorus source,

PX2Ph was used. Similar as described for the trichlorophosphane, trop2DAE was

deprotonated in thf at -78 °C with 2.1 eq of a 1.6 M n-BuLi hexane solution before 1.1 eq

PCl2Ph was added dropwise. The solution was stirred to warm up to room temperature for

12 h. The white precipitate was filtered off and recrystallized from a DCM:hexanes mixture.

Scheme 29: Attempted synthesis of a phenyl substituted phospholidine reacted with PCl2Ph.

16 17 19

16 18 19

Phosphorus Ligands

62

The crystals were of good quality for X-ray diffraction and the resolved structure is shown

in Figure 26. The molecule resulted in a doubly substituted aza-PClPh trop2DAE instead of

the desired product.

This reaction would have been of interest in order to compare the P-P interaction and its

properties.

Figure 26: ORTEP of 20 at 50% ellipsoid probability. Hydrogen atoms were omitted for clarity. Selected bond

lengths [Å]: Cl1-P1 2.129(6), P1-N1 1.671(4), N1-C16 1.479(4).

Phosphorus Ligands

63

Discussion

Diazaphosphole

The attempted synthesis of diazatropphospholene 15 did not lead to the desired results.

The main drawback in the synthetic strategy was the reduction of the diimine. The reaction

with alkali metals reduced the trop2DAD system instead of the diimine, resulting in a highly

delocalized π-system. Even though the product of the reduction was isolable and the crystal

structure was determined, it did not react as predicted with PCl3. The redox reaction of the

bis(trop)diimine skeleton with PI3 proceeded smoothly and did yield a promising

intermediate. The clear trop2DAD thf solution turned deep red immediately after the

dropwise addition of PI3 at -78 °C. Unfortunately, the target molecule was only detectable

by in situ NMR spectroscopy. The NMR data, especially the 31P NMR at 218 ppm shows that

the diazaphospholenium ion is formed when compared to phospholenium cations

described in the literature. An interaction between the counter ion OTf- and the

phospholenium ion is not observable. And as expected after drying in vacuo the TMSCl is

not observable by NMR. Unfortunately, all attempts to crystalize the phospholenium ion

failed. Reaction of the diazaphospholium with a trapping reagent was not successful and

all attempts to isolate the product failed.

Diazaphospholidine

The synthesis of the diazaphospholidine proceeded straight-forward and was confirmed by

NMR and X-ray analysis. The smooth crystallization of the ligand from a DCM:hexanes

solution resulted in crystals of good quality for X-ray diffraction. During the synthetic

description of the reaction there are two pivots that have to be taken. In the synthesis from

trop2DAE the deprotonation step and the phosphorus source need closer attention. The

base for the deprotonation was chosen on the motivation of being the driving force and it

has to be non-interacting with PCl3. Triethylamine proved to be the best choice. After

deprotonation of the trop2DAE, PCl3 was selected as phosphorus source. The reaction

worked very well and the ligand could be synthesized and easily recrystallized from

DCM:hexanes with yields up to >80%.

Phosphorus Ligands

64

The choice of the base has proven to be of greatest importance. When n-BuLi was used,

many side reactions occurred, which were identified as different deprotonation

intermediates such as double deprotonation, deprotonation of the backbone and benzylic

H elimination. When using Et3N, the ratio of base/reactant was of crucial importance. As

described by Abrams,[52a] it is important to find the right ratio in order to have enough base

and enough scavenger. In the case with trop2DAE, successful deprotonation and

subsequent addition of PCl3 was only efficient after a base/reactant ratio of 8:1 was reached.

The best results were found with ratios between 8:1 and 9:1.

4.4.2.1 NMR analysis

NMR spectroscopy has proven to be a powerful tool in the analysis of the synthesis of 16.

The reaction can be followed by 31P NMR spectroscopy. Detailed NMR spectra information

is given in chapter 4.4.

4.4.2.2 Crystal structure analysis

The crystal structure confirmed the results found by NMR analysis. The molecule

crystallized in a very symmetric form and has a mirror plane going through the P-Cl bond

(see Figure 27). Interestingly, the trop units are both situated trans to the chlorine atom

thus minimizing the trop - Cl interaction and forcing the entire molecule to a symmetrical

conformation. The eclipsed conformation of the H-atoms of the C32 and C31 atom is clearly

energetically less relevant than the interactions caused by the big trop substituents. This

conformation shows how important the substituents at the nitrogen atom are. In this case,

the substituents arrange the diazaphospholidine skeleton to a slightly planar backbone

with the phosphorus and the two trop units arranged like a pocket ready for coordination

of a metal by the phosphane and the trop substituents.

Interestingly, the diazaphospholidines described in the literature have a twist conformation

minimizing the H-H repulsions of the backbone and the interactions between the chlorine

and the substituents on the nitrogen atoms.[51]

Compared to most diazaphospholidines found in the literature the

bis(trop)diazaphospholidine 16 does not show the twisting of the backbone as normally

Phosphorus Ligands

65

found (Scheme 30). Furthermore, the trop units in 16 are syn and not trans to each other

representing a novelty compared to the diazaphospholidines described in the literature.[51]

Scheme 30: Twisted backbone found in most tropylidene vs. flat backbone in the bis(trop)phospholidine.

The trop units are big enough to force 16 to a more planar conformation which can be

described by a plane through the N1-C31-C32-N2 atoms and a plane through the N2-P1-

N1 atoms. The angle between these two planes is 26.66 °.

Figure 27: Front view ORTEP of 16 at 50% ellipsoid probability. Hydrogen atoms were omitted for clarity.

Selected bond lengths [Å] and angles [°]: Cl1-P1 2.253(4), P1-N1 1.672(1), P1-N2 1.665(1), N1-C31 1.474(2), N2-

C32 1.472(2), C31-C32 1.534(4), C4-C5 1.340(2), C19-C20 1.340(2); N1-P1-C31 106.75(1), N2-P1-C32 112.90(9),

N1-P1-N2 91.76(6), N1-C31-C32 106.75(1), N2-C32-C31 107.22(1).

As described in Figure 28, the nitrogen atoms are forced to a more planar hybridization

which is also represented in the sums of angles for N1 and N2 being 345.9 ° and 347.7 °,

respectively. This slight difference in the angular sum is also represented in the distances

between the nitrogen atoms and phosphorus.

Cl1

P1 N1N2

C1C16

C19 C5C4C20

C31C32

Phosphorus Ligands

66

Figure 28: Top view of 16. The angle of P1-Cl1 with the plane N1-P1-N2 is relatively small with 91.75°

compared to the angles of the nitrogen atoms. The sums of angles for N1 sums up to 345.9° compared to the

slightly bigger 347.7° of N2. This shows that both nitrogen atoms are forced to be quite planar.

The distances in the ring system are slightly different between the two P-N atoms. The P1-

N1 bond with 1.672(1) Å is slightly longer compared to the P1-N2 bond (1.665(1) Å). The

distances between the two nitrogen and carbon atoms of the backbone are nearly the

same. Interestingly, the P1-Cl1 bond length with 2.253(4) Å is significantly longer than the

average P-Cl (2.13 +/- 0.06 Å) bond lengths reported up to date.[51] This can be seen as a

result of the big trop substituents forcing an elongation of the bond and the contribution

of the n(N)-s*(P-Cl) hyperconjugation.[51]

Diazaphospholidinium

The synthesis of 17 from 16 is a straightforward reaction consisting in scavenging the

chloride (Scheme 26). A screening of different halogen scavengers showed that AgOTf was

suitable without interacting with the target molecule. The reaction with AgOTf was

accompanied by a notable change of color from white (16) to pink (17). Therefore, the color

change is a simple qualitative indicator for the scavenging reaction allowing a first verdict

at sight. The amount of AgOTf used must be exact in order to have a clean reaction. When

2 eq of AgOTf were used a recombination of the ligand was observed. NMR analysis

Phosphorus Ligands

67

showed that a new compound is formed. The white crystalline compound was determined

to be bis(trop). Two trop units bound through the benzylic carbon of the seven-membered

ring.

In order to avoid this phenomenon, other scavengers were tested: AgPF6 acted as medium-

good scavenger. The reaction did not proceed to completion. Using NaBArF turned out to

be effective just to a point. The color change was observable but the solution turned turbid.

NMR analysis showed an incomplete conversion. The use of Me3SiOTf showed a

dehalogenation of the phosphorus, but resulted in an interaction between the P atom and

OTf. The 31P NMR spectrum showed a signal at ~80 ppm being the interaction of the

triflate-P.

Therefore, the synthesis of 17 was carried out with AgOTf as scavenger.

Attempts to isolate the highly reactive species 17 were unsuccessful. The decomposition

and reaction with other molecules was readily visible. The NMR data doubtlessly confirmed

the formation of 17.

NMR analysis of Diazaphospholidine and Diazaphospholidinium

As described earlier, NMR spectroscopy turned out to be an ideal tool to follow the reaction

progress. The following table sums up the most important NMR signals for the

phospholidine and the phospholidinium cation.

Table 8: Comparison of NMR data between 16 and 17.

1H NMR

(400 MHz, CD2Cl2):

Bis(trop)diazaphospholidine

16

Bis(trop)diazaphospholidinium

17

CHCH2CH2CH 2.27-2.44 (m, 2H) 3.25 d, (4H)

CHCH2CH2CH 2.81-2.93 (t, 2H) -

Benzylic CH 5.23 (d, 2H) 5.73 (d, 2H)

Olefinic CH 6.95 (s, 4H) 7.1 (s, 4H)

31P NMR:

NPN 161.7 (s) 261.9 (s)

Phosphorus Ligands

68

The most striking evidence for 17 is the difference in 31P NMR shift from 162.8 ppm in 16

to 263.2 ppm. This shift towards higher frequencies is an indication of the deshielding of

the phosphorus atom. Compared to known diazaphospholidines and

diazaphospholidinium cations, the shifts reported herein are in perfect agreement with

reference values.

The 1H NMR spectrum is of the same importance in this case. As shown in Figure 29, the

change from a neutral to a cationic species is relevant. The cationic species has a higher

symmetry which is clearly represented by the NMR data.

Figure 29: Comparison of significant 1H NMR shifts of diazaphospholidine 16 (O) vs diazaphospholidinium

17 (+). X represents impurities of the backbone

The 1H NMR signals of the heterocyclic ring change significantly and are shifted to higher

frequencies for the cationic species. This can be explained by the positive charge of the

phosphorus. Hyperconjugation of the N-P changes and affects the electronics of the entire

backbone and the benzylic protons. The symmetry of the five membered ring is higher. The

gem and syn hydrogen atoms of the NCH2CH2N backbone in give raise to doublets and

X

o

o

o

x x x

I

I

X X

+ o

+

o o

Phosphorus Ligands

69

thus just interact with the geminal proton. The change of charge through the ring may also

explain the bright pink color which follows the dehalogenation step.

Synthesis of [Rh(trop2sNHPCl)Cl] 18

The synthesis of 18 was carried out in DCM and at room temperature. After the addition of

0.5 equivalent of [RhCl(C2H4)2]2 to the colorless solution of 16, the reaction mixture turned

deep orange immediately. Small crystals started to precipitate from the solution.

Since the [RhCl(C2H4)2]2 proved to be such a good precursor for the coordination of the

ligand, its corresponding iridium precursor was tried as well. Unfortunately, the reaction

with the iridium precursor was not as clean as the reaction with the rhodium precursor.

Crystal analysis of [Rh(trop2sNHPCl)Cl]

The inspection of the molecular structure of [Rh(trop2sNHPCl)Cl] 18 revealed a geometry

around the rhodium metal where the symmetry of the molecule is not given by a h as in

compound 16. The NPN ring and the trop units do not lie in a symmetrical flat environment

but are rather distorted. Through the coordination of the rhodium metal by the trop units,

the nitrogen atoms in the five-membered ring were forced to get a flatter conformation.

This flattening is proven by the larger sum of the angles around the nitrogen (N1 353.57 °,

N2 355.33°). This is even more obvious when compared to the free ligand (N1 7.67°, N2

9.63°). Through this flattened conformation of the nitrogen atoms, the backbone of the

ligand got twisted by 18.49°, thus losing the high symmetry found in the free ligand.

Phosphorus Ligands

70


lengths [Å] and angles [°]: Cl2-P1 2.145(1), P1-N1 1.650(4), P1-N2 1.636(4), N1-C31 1.472(6), N2-C32 1.475(7),

C31-C32 1.514(7), C4-C5 1.408(7), C19-C20 1.385(8), Rh-Ct(C4-C5) 2.143, Rh-Ct(C19-C20) 2.082, Rh-P1 2.159,

Rh-Cl1 2.426; N1-P1-C31 113.2(3), N2-P1-C32 111.5(3), N1-P1-N2 94.16(2); sum of angles N1 353.57°, N2

355.33°.

As can be seen in Figure 30, the loss of symmetry does not only affect the backbone of the

ligand but also the trop ligands. The bond length of the coordinating double bond in the

trop unit is different between the two trop units, and both are elongated compared to the

free ligand (1.340(2) Å). The bond length between C4=C5 is 1.408 Å which already shows a

slight degree of metallo-cyclopropane character, while the bond length of C19=C20 is

0.023 Å shorter (1.385 Å). This difference is also represented in the interaction with the

rhodium. Whilst the Rh-Ct(C4-C5) is 2.143 Å, the interaction with Ct(C19-C20) is 2.082 Å.

Even the shape of the trop scaffold is affected in the sterically demanding situation. As

introduced by Vogt[12d] the angles and are introduced in the trop unity in order to

describe the overall conformation of the coordinating trop ligand (Figure 31).

Figure 31: Plane angles and of the tropylidene backbone.

Remarkably, the angles of the trop scaffold have differently flattened angles in 18 when

coordinated to rhodium. Compared to the free ligand, both trop units in 18 open up their

relative angle in the trop scaffold. Trop unit A has angles of = 46.70 ° and = 19.84 °

whilst trop unit B has angles of = 44.19 ° and = 15.11 °. This angles describe a relatively

Cl1

Cl2

Rh

N1N2

C32 C31C1C16

C19

C20 C4

C5

P

Phosphorus Ligands

71

large flattening of the trop unit when compared to the angles = 51.27 ° and = 24.16 °

in the free ligand.

The geometry around the phosphorus is also of interest. All bond lengths around the

central phosphorus atom shortened compared to the free ligand. The bond lengths of the

surrounding are P1-Cl2 2.145(1) Å ( = 0.108 Å), P1-N1 1.650(0) ( = 0.022 Å) and P1-N2

1.636 Å ( = 0.029 Å). The N-P-N angle remained nearly the same (94.16 ° in 18 vs 91.76 °

in 16) being only minimally enlarged.

The Cl2 atom has angle of 107.37(7)° to the P-Rh-Cl1. The rhodium is coordinated in a

square planar manner – as expected for Rh(I) (d8) complexes - where the two trop units are

trans to each other and the phosphane coordinates the Rh atom trans to the chlorine (Cl2).

The bond lengths are 2.426 Å for Rh-Cl1, 2.159 Å for Rh-P1, 2.143 Å for Rh-Ct(c4-C5) and

2.082 Å for Rh-Ct(C19-C20) showing different bond lengths for every coordination unit.

Concluding remarks

A new diazaphospholidine ligand was synthesized and characterized by NMR and

crystallography. The corresponding rhodium complex [Rh(trop2PhosCl)Cl] 18 was

successfully described as red crystalline product and its synthesis shown. Unfortunately, the

corresponding iridium complex was not formed without by-products.

Iron Chemistry

72

Iron Chemistry

Iron Chemistry

73

Introduction

Iron is a perfect candidate to fulfill many aspects of a new generation of catalysts for

modern catalytic organic transformations. The development of environmentally friendly

and reliable transition metal-catalyzed methods is of high value in chemical industry and

research chemistry. Especially, if one keeps in mind that nearly 80% of all chemical and

pharmaceutical products synthesized in the chemical industry are based on catalytic

processes. In the last decades most of them were based on precious metal catalysts such

as rhodium, palladium and iridium, this emphasizes the importance of the development of

new less expensive catalysts for industrial processes. Not only the economic factors are

important, but also the environmental argument must be considered when using such

precious metals. Therefore, the search for first row transition metal catalysts such as zinc,

manganese, iron and copper is a growing field in chemistry.[58] Given its high abundance in

the earth crust (4.7 wt%), iron is an ideal transition metal for new applications, with its low

price, non-toxicity, and the environmentally benign character.[58] Iron is generally found in

its oxidized form Fe(III) and Fe(II). In air, most iron(II) compounds are readily oxidized to

their most stable and widespread iron(III) analogs. Of great interest for organometallic

chemistry and for iron-catalyzed reactions are complexes which stabilize iron in low

oxidation states.[59] These complexes can form more reactive complexes than their Fe(II)

and Fe(III) counterparts. Iron(0) and iron(-II) are of immense interest for their catalytic

activity. In an interesting review, Bolm described the big number and different reactions

catalyzed by iron complexes in different oxidation processes.[60] It is the facile change of

oxidation state and its Lewis acidity (Fe(III) is a harder Lewis acid than Fe(II)) that allow this

broad range of synthetic reactions. Carbon-heteroatom and heteroatom-heteroatom bond

forming processes are described. Among them substitutions, additions, cycloadditions,

hydrogenations, reductions, oxidations, coupling reactions, isomerizations,

rearrangements, hydroborations and hydrosilylations.[60a] In the last years the group of

Chirik synthesized redox active ligands that yielded highly active complexes upon

coordination to low-valent iron.[29, 61] One of the most elegant applications is the reduction

of the iron complex shown in Scheme 34 and its use as a catalyst in the hydrosilylation

reaction.[30] Chirik and co-workers have synthesized different PDI (PDI = bis(imino)pyridine)

ligands and were able to stabilize the iron in different oxidation states. The PDI ligand

Iron Chemistry

74

proved to be a redox active ligand and the electrons are transferred from the metal center

to the ligand.

Scheme 31: Controlled reduction of (iPrPDI)Fe(X)2 to Fe(I) and Fe (0) complexes. (iPrPDI)Fe(N2)2 was described

to be an active catalyst for hydrogenation of alkenes and active in the hydrosilylation. [29, 62]

Scheme 32: Nitrogen bridging Fe(PDI) complex when substituents at the nitrogen are small.[62]

The PDI complexes described by Chirik proved to be highly active catalysts for the

hydrogenation and more importantly hydrosilylation reactions. Depending on the aromatic

substituent on the PDI scaffold, different nitrogen complexes could be isolated (Scheme

32).

Iron Chemistry

75

We were especially intrigued by the stabilized low-oxidation states of the iron. Since Fe(0)

is mostly coordinated by five or six ligands with trigonal bipyramidal or octahedral

geometry and Fe(-II) is tetrahedrally coordinated, a PDI ligand with trop unit was designed

and coordinated to Fe(II) as precursor. The in situ reduction leads to a low valent iron

product. This complex is compared with an analogous [Fe(tropPicol)2] complex and both

are tested in the reductive hydroboration and hydrosilylation reaction of small non

activated molecules.

The nomenclature of the tropPDI ligands is given as follows: tropPDIDipp describes the two

side branches of the ligand around the PDI center. In this case a dipp and a trop unit. PDI

stands as defined by Chirik and co-workers for the diiminopyridine (2,6-(ArN=CMe)2C5H3N,

with Ar = dipp, trop, 2,6-Et2-C6H3N…) unit.

Synthesis of the ligands

Synthesis of tropPicol

The synthesis of the tropPicol is described in Chapter 3 and will not be discussed further

in this chapter. The coordination properties of the ligand towards iron will be discussed

later in this chapter.

Synthesis of tropPDI ligands

The synthesis of the tropPDI ligands consists of three steps and is essentially carried out in

the same way for each sidearm.

As depicted in Scheme 33, firstly the dicarboxylic acid was reduced in a decarboxylation

reaction to the diacetyl pyridine. After a simple condensation with aniline the half-acetal

was obtained, which was reacted further with tropAmine 2 leading to the desired product.

Iron Chemistry

76

Scheme 33: Reaction Scheme of the Synthesis of tropPDIDipp 21. Alternatively, to the diisopropyl-aniline in

step 2, other aromatic amines can be used.

The ligand was dissolved in a minimal amount of hot MeOH or toluene and kept at -37 °C

for recrystallization. After purification the crystals were filtered and washed with a non-

polar solvent.

The same procedure was followed to create a small tropPDI library. As shown in Figure 32

the second substituent on the PDI can be alternated, yielding the following ligands:

Iron Chemistry

77

Figure 32: Library of synthesized tropPDI ligands.

The synthesis of the ligands in the library differs only in the time needed for the

condensation step. The yields of the synthesis vary between 30-80%. This is mainly due to

the recrystallization process. Interestingly, the bigger the substituents on the imine the

higher the crystallinity of the obtained product and the higher the isolated yields of the

product. The difference of the crystallization product between 21 and 22 is remarkable. In

the case where the trop ligand is fixed as a substituent on the PDI, the change of the second

substituent results in an optically change of color for the ligand. Whilst 21 is a purely clear

white crystalline compound, 22 results in long yellow needles.

As mentioned in the introduction chapter, transition metals in low-oxidation states are

stabilized through olefin coordination. Therefore, the different ligands described in Figure

32 were synthesized with the idea of stabilizing iron complexes with extraordinary oxidation

states.

Results

The ligand library was synthesized according to the procedure described in chapter 5.2.2.

The yield of the ligands is depended on the time of the condensation reaction. With bigger

substituents, this reaction step was improved and less by-products were observed.

Iron Chemistry

78

Crystals of good quality for X-ray diffraction were grown from a concentrated toluene

solution for tropPDIDipp 21 and tropPDItrop25. The ORTEP are depicted in Figure 33 and

Figure 34, respectively.

The other ligands crystallized as needles or as microcrystalline powder, which were

impossible to measure with X-ray analysis.


lengths [Å]: N3-C8 1.274(2), C8-C7 1.504(2), C7-N2 1.343(2), N2-C3 1.347(2), C3-C2 1.500(2), C2-N1 1.280(2),

C26-C25 1.340(3).


lengths [Å]: C5-C4 1.342(4), C1-N1 1.457(4), N1-C32 1.265(3), C32-C33 1.508(3), C33-N2 1.331(3), N2-C37

1.343(3), C37-C38 1.495(3), C38-N3 1.270(3), N3-C16 1.455(3), C19-C20 1.332(4).

The bond distances for the two different free ligands are in the expected range. A difference

of the C=N imine bonds is observable in 21. The bond length between C8-N3 is 1.274(2) Å

and 1.280(2) Å between C2-N1. The difference between the two imine bonds is minimal

even though the steric demand of the substituents is quite different. In the case of the

N3

C8

C7

N2

C3

C2

N1

N2

N3

N1C37

C33C38

C32

C1

C5

C4

C16

C19

C20

Iron Chemistry

79

doubly substituted tropPDItrop, the bond lengths through the ligand are quite constant as

expected for a ligand with the same substituents.

Coordination of the tropPDI ligands to Fe

Synthesis of [Fe(tropPDIdipp)Br2] 26

Inspired by the results of Chirik the new ligand 21 was coordinated to an iron(II) precursor

and test reactions were carried out with it. As depicted in Scheme 34 ligand 21 was added

to a solution of dry FeBr2(thf)2 and stirred overnight. A deep blue color of the solution and

a blue solid formed immediately, which were an optically hint for a clean coordination of

the ligand to the iron. The solvent was pumped off and the blue solid washed with a non-

polar solvent as hexanes or pentanes. After filtration and drying under reduced pressure

quantitative amounts of [Fe(tropPDIdipp)Br2] 26 were obtained.

Scheme 34: Synthesis of the [Fe(tropPDIdipp)Br2] 26.

Crystals of good quality for X-ray diffraction were grown from a concentrated thf solution

at -37 °C after one week. An ORTEP of the coordinated ligand is depicted in Figure 35.

21 26

Iron Chemistry

80


lengths [Å] and angles [°]: N1-Fe 2.191(6), N2-Fe 2.064(6), N3-Fe 2.205(5), N3-C16 1.285(9), C16-C17 1.491(1),

C17-N2 1.349(8), C21-N2 1.331(9), C21-C22 1.483(9), C22-N1 1.292(9), N1-C1 1.500(9), Fe-Br1 2.472(1), Fe-Br2

2.418(1), Br1-Fe-Br2 117.01(5), Fe1-N2-C22 116.8(5), C4-C5 1.35(1), N3-Fe-N2 73.5(2), N1-Fe-N2 73.70, Br1-Fe-

N2 95.6(2). N1-Fe-Br2 100.41(1), N3-Fe-Br2 99.20(1).

Analysis of the X-ray structure shows that the distances between the three coordinated

nitrogen atoms differ between the two lateral nitrogens and the central pincer nitrogen.

The distance between N2-Fe is nearly 13.4 pm shorter than the N1-Fe and N3-Fe distances,

respectively. The bond distances throughout the backbone are clearly divided between the

shorter imine bonds and the bonds in the pyridine. The bond lengths between C16-C17

and C21-C22 are slightly longer. The angles of the coordinating nitrogen with the iron

center are relatively small with 73.7 ° for N1-Fe-N2 and 73.5(2) ° for N3-Fe-N1, whilst the

angles between N1-Fe-Br2 (100.41(1) °) and N3-Fe-Br2 (99.20(1) °) are larger.

Reduction of [Fe(tropPDIdipp)Br2] 27 and 28

26 was reduced with following reduction methods: sodium metal, sodium amalgamate,

zinc or NaBEt3H (Scheme 35) as described in the literature in order to get iron in low

oxidation states. The group of Chirik showed that different reduction approaches result in

several different reduced iron species. Thus for the PDI systems used in their group different

reduction methods resulting in the iron oxidation states (I), (0) or (-I) are described.[63] [64]

Fe

Br2

Br1

N1

N3

N2

C1

C22

C21

C17C16

C4

C5

Iron Chemistry

81

Scheme 35: Reduction of FePDI ligands described by Chirik.[62]

Several reduction methods in order to obtain Fe(I), Fe(0) and possibly Fe(-I) tropPDIdipp

complexes were tested. 26 was dissolved in Et2O, thf, dioxane, dme or iPr2O and the

reductant was added. The reaction condition tested for the reduction of [Fe(tropPDIdipp)Br2]

under argon atmosphere are summarized as follows:

To the complex in the solvent, a defined amount of reductant was added. As reductant

were used: Na, Na(powder), Na/18c6, Na/naphthalene, Li, Zn and LiBEt3H.

The blue solution turned red upon addition of the reducing agent. The reduction was

carried out at room temperature. After 24 h the solution was filtered over dried Celite and

the solvent concentrated under reduced pressure.

Unfortunately, the Fe(I) species could not be isolated. Comproportionation reactions of

Fe(II) and Fe (0) species after the reduction did not lead to the desired Fe(I) product.

A controlled reduction with Na/18c6 in thf at room temperature led to the new species 27.

The deep red solution was concentrated under reduced pressure, washed with a non-polar

solvent and dissolved in a minimal amount of thf.

Crystals of [Fe(tropPDIdipp)(thf)] 27 were grown from a concentrated thf solution at – 37 °C.

The ORTEP of the deep red crystals is shown in Figure 36. Chosen bond lengths and angles

are given in the caption.

Iron Chemistry

82


lengths [Å] and angles [°]: N1-Fe 1.899(3), N2-Fe 1.849(3), N3-Fe 1.926(3), Fe-ct(C4-C5) 1.904, N1-C28 1.334(6),

N3-C34 1.352(5), N2-C29 1.380(5), N2-C33 1.372(5), C28-C29 1.433(6), C33-C34 1.427(5), C4-C5 1.442(6), Fe-

O(thf) 2.205(3), N3-Fe-N2 80.6(1), N2-Fe-N1 80.7(1), N1-Fe-ct 92.94, ct-Fe-N3 100.00, N2-Fe-O 89.03(1), N3-

Fe-O 94.4(1), N1-Fe-O 94.1(1).

The Fe(0) species shows differences in its crystal structure as compared to 26. The trop unit

is bent towards the iron center, coordinating the metal through its double bond (1.904 Å),

which is also represented in the longer C4-C5 bond length 1.442(6) Å versus 1.346(1) Å in

the free ligand. The Fe-N bond distances are shorter than in 26. N1-Fe is 0.292 Å, N2-Fe is

0.215 Å and the bond N3-Fe is 0.279 Å shorter. Whilst the bond distances of the backbone

are slightly elongated.

27 crystallizes with a thf molecule which is coordinated to the meal through its O (2.205(3)

Å). A remarkable enlargement of the angles between N1-Fe-N2 and N3-Fe-N2 is observed.

The N1-Fe-N2 angle is widened by 7.0 ° and the N3-Fe-N2 by 7.1 ° compared to 26.

Crystals of the anionic type of the reduction of 26 with excess amounts of Na/18c6 as

reducing agent were obtained from a concentrated thf:DME solution. Even though the

crystals were of good quality for X-ray diffraction, the structure could not be clearly

elucidated due to undefined solvent molecules in the crystal lattice. Therefore, a ball-and-

stick representation of the anionic [Fe(tropPDIdipp)][Na18c6(dme)2] 28 is shown in Figure

37.

Iron Chemistry

83

Figure 37: Ball and stick representation of 28. The counter ion and solvent molecules are omitted for clarity.

Synthesis of [Fe(tropPDIsubst)Br2]

The Ligands 22, 23, 24 and 25 were also reacted with FeBr2(thf)2 under the same conditions

described in 5.3.1 and resulted in colorful complexes. The reaction of 22 with the iron

precursor yielded in a purple solid, while the reaction with the ligands 23-25 yielded

complexes in different shades of blue.

Unfortunately, crystals for X-ray analysis for the other substituted tropPDI ligands could

not be obtained. Neither did the reduction result in interesting products.

Synthesis of tropPicol iron complexes

In order to be able to compare the PDI ligand to other ligands, 8 was reacted with

FeBr2(thf)2 in thf (Scheme 36), resulting in a brownish solution with formation of a rust-red

solid. After drying under reduced pressure the complex was washed with small amounts of

ether and filtered upon repeated drying.

Fe N2

N1

N3

C4

C5

C28C29

C30C31

C32

C33C34

Iron Chemistry

84

Scheme 36: Synthesis of [Fe(tropPicol)Br2] 29.

The solid state analysis was accomplished from crystals which were grown from a

concentrated thf solution. [Fe(tropPicol)Br2] 29 crystallizes in an interesting manner,

depicted in Figure 38, showing the trop unit slightly coordinated to the iron atom (right in

Figure 38) pushing the bromides out of plane in one molecule and in the other completely

turned away from the iron (left in Figure 38).

Figure 38: ORTEP representation of 29 at 50% ellipsoid probability. Hydrogen atoms were omitted for clarity.

Selected bond lengths [Å] and angles [°] for the left conformer A: N1-Fe 2.119(9), N2-Fe 2.110(2), N1-C16

1.269(3), N2-C17 1.358(3), C16-C17 1.477(3), Fe-Br1 2.374(5), Fe-Br2 2.3887(5), C4-C5 1.346(5), N1-Fe-N2

77.51(8), Br1-Fe-Br2 116.39(2). Right conformer B: N1-Fe 2.173(2), N2-Fe 2.166(2), N1-C16 1.274(3), N2-C17

1.351(3), C16-C17 1.463(3), Fe-Br1 2.450(4), Fe-Br2 2.441(3), C4-C5 1.346, Fe-ct(C4-C5) 2.464, N1-Fe-N2

74.98(7), Br1-Fe-Br2, 103.87(1).

Interestingly, the bond lengths do not dramatically change between conformation A and

conformation B, even though the coordination around the Fe center is different. The angle

around the iron N1-Fe-N2 in B gets smaller (77.51(8) °) compared to 74.98(7) ° in A. The

8

29

Iron Chemistry

85

coordination of the trop influences the bromides and with them the angle to the iron Br1-

Fe-Br2. Pushing the bromides out of plane makes the angle in conformation B smaller by

12.52 °.

Reduction of [Fe(tropPicol)Br2] 30

After the reduction with 1 equivalent of reducing agent the expected Fe(I) complex could

not be synthesized (Scheme 37). The reduction of 29 in dme directly yielded a Fe(0) species.

Scheme 37: Unsuccessful try of the synthesis of [Fe(tropPicol)Br].

The outcome of the reaction was dried and washed with little amounts of hexanes. Crystals

of deep red color were grown from a concentrated dme solution and were of good quality

for X-ray analysis. An ORTEP representation is shown in Figure 39:

29

Iron Chemistry

86

Figure 39: ORTEP of 30 at 50% ellipsoid probability. Hydrogen atoms were omitted for clarity. Selected

bond lengths [Å] and angles [°]: N1A-Fe 1.883(2), N2A-Fe 1.954(2), Fe-ct(C4A-C5A) 1.918, N1A-C16A 1.324(3),

N2A-C17A 1.382(3), C16A-C17A 1.404(3), C4A-C5A 1.448(3), N1B-Fe 1.972(2), N2B-Fe 1.930(2), N1B-C16B

1.428(3), N2B-C17B 1.366(3), C16B-C17B 1.428(3), C4B-C5B 1.336(3). N1A-Fe-N2A 80.64(8), N1B-Fe-N2B

80.55(8), N2B-Fe-N2 94.91(8).

The reduced form of 29 shows a Fe(0) species which is coordinated by two tropPicol

molecules, whereas one trop moiety of a tropPicol coordinates to the metal whilst the other

trop is turned away. The bite-angles N1A-Fe-N2A and N1B-Fe-N2B of the nitrogens to the

iron are similar for both units (80.64(8) ° and 80.55(8) °, respectively). The bond lengths

N1A/B-C16A/B (1.324(3) Å for N1A-C16A and 1.428(3) for N1B-C16B) of both coordinating

of tropPicol units are remarkably longer than in the free ligand 1.257 Å. The bond N1B-C16

where the non-coordinated trop unit is bond is slightly longer than the same bond in the

tropPicol where the trop unit is bonding. The same effect is observable for the distances of

the nitrogen to iron. For N1A-Fe the bond length is 1.883(2) Å, whilst the bond length N1B-

Fe is 1.972(2) Å, clearly showing how the trop unity influences the molecule. In this molecule

the difference between the coordinating and the non-coordinated trop unit is very well

elucidated. The double bond C4B-C5B of the non-coordinated trop moiety is 1.336(3) Å

compared to the coordinating bond C4A-C5A 1.448(3) Å where the interaction with the

metal center is elucidated.

In order to test the possible catalytic activity of the synthesized complexes, two reactions

were tested and are described.

Fe

N1A

N2A

N2B

N1B

C4A

C5A

C16A

C17A

C16B

C17B

C5B

C4B

C1A

C1B

Iron Chemistry

87

Catalytic activity of Iron complexes

The synthesized iron complexes were tested for their catalytic activity. In organic chemistry

iron complexes are known in many different reactions. Since Tondreau et al.[30]

demonstrated that the iron PDI complexes are interesting catalysts for the hydrosilylation

of alkenes, the tropPDI and the tropPicol iron complexes were tested for the in situ

hydrosilylation of alkenes.

In order to have comparable results to the benchmark reaction of Chirik[30] 1-octene was

chosen as a model for the alkene and (TMSO)2SiHMe as silane source.

Different approaches were followed for the in situ reduction and thus activation of the iron

catalyst. In a standard reaction, a vial was charged in an argon filled glovebox with the

alkene and the iron complex. The reducing agent was added and the reaction was stirred

1 minute before the silane was added. After addition of the reactants, the reaction was

stirred for the desired amount of time and aliquots were taken form the solution. Upon

exposure to air the quenched product was analyzed by GC and NMR spectroscopy. In the

search for the best reaction conditions the following parameters were examined: Amount

of catalyst, reducing agent, solvent effect and silane source.

An overview of the tested conditions is given Table 9:

Table 9: Reaction conditions for the hydrosilylation reaction with iron tropPDI complexes.

Entry: Reaction

1

2

3

Iron Chemistry

88

4

5

6

26 did not prove to be a good catalyst for the hydrosilylation of octene compared to the

benchmark [(iPrPDI)Fe(N2)2] catalyst of Tondreau. [30] The best results were achieved with

Na/naphthalene as reducing agent, in neat reactants. After 72 h of reaction at room

temperature 80% of conversion to the product was observed. If the order of addition of

reactants is changed the conversion is negatively influenced. Only 37% conversion was

observed if the silane is added before the octene in the reaction mixture. Changing the

reducing agent to Zn or to LiBEt3H did not improve the outcome. Nor did the temperature

increase speed up the reaction. Changing the silane or the alkene source resulted in a

drawback. Nearly no conversion was observed with other alkenes or silane sources.

Complex 26 cannot be used as highly active catalyst for hydrosilylation reactions.

Therefore, the smaller complex 29 was tested with the same conditions as 26.

Unfortunately, 29 proved to be less reactive than the [Fe(tropPDIDipp)Br2].

As a further catalytic approach the hydroboration was tested as summarized in Table 10.

Iron Chemistry

89

Table 10: Hydroboration of alkenes with 29.

Entry: Reaction

1

2

Hydroboration is a highly interesting reaction for Fe catalyst. 29 did not catalyze the

reaction in very effective manner. The reaction conditions were chosen the same as for the

hydrosilylation reaction.

Concluding remarks

New iron complexes of tropPDI and tropPicol ligands were synthesized, characterized by

NMR spectroscopy and tested towards catalytic activity. Unfortunately, the complexes did

not prove to be efficient catalysts for the tested reactions. Nonetheless, the newly

synthesized complexes are of interest. The electronic properties can be investigated and

their understanding opens up other doors for further new iron trop complexes, which may

be catalytically highly active.

Conclusion

90

Conclusion

The Grützmacher group has developed a catalyst system based on the iridium complex

[Ir(trop2DAD)]OTf 4 which is not only the fastest of its type, but also oxidizes non-activated

alcohols to the desired aldehydes with high activity and efficiency. To overcome two main

disadvantages of the system, namely the uncontrollable speed of the reaction and the

stoichiometric amount of benzoquinone waste produced during the catalysis, a number of

co-oxidants were screened. Menadione 7 was identified as valuable alternative to

benzoquinone as it slows down the reaction and, therefore, allows a softer and gentler

reaction procedure and a higher degree of control. In contrast to the reaction with

benzoquinone, there is no noteworthy heat production and the reaction can be accelerated

by warming up. Additionally, the slower process of the reaction also widens the substrate

scope and now the conversion of diols to the corresponding lactones in quantitative yields

is possible. Non-activated aliphatic alcohols still react completely to the corresponding

aldehydes at good rates.

However, sterically demanding substrates remain a problem and are not satisfyingly

converted when vitamin K3 7 is used as an oxidant. Therefore, this study shows that the

[Ir(trop2DAD)]OTf 4 catalyst is very specific for sterically non-demanding reactants and has

a perfect balance with 1,4-benzoquinone as co-oxidant.

Having these results in hand a sterically less demanding square-planar ligand for the

coordination of iridium and its application as an oxidation catalyst was designed.

For this purpose, new trop-based as well as phosphorous containing ligands were

designed. Two potentially redox active and interesting trop ligands were synthesized and

their coordination shown. TropNacAc 13 resulted to be an interesting ligand for square

planar palladium complexes, which can be tested for oxidation reactions.

Conclusion

91

TropPicol 8 was planned as an alternative ligand to the trop2DAD but its complexation to

the [Ir(cod)Cl]2 resulted in a dimer, which proved to be too stable to act as catalyst. But, its

coordination to other metals seemed readily available and thus the complexation to iron

and its catalytic activity was tested.

Additionally, a diazaphospholidine ligand was synthesized and characterized by NMR and

crystallography. The corresponding rhodium complex [Rh(trop2PhosCl)Cl] 18 synthesis was

successful and symmetrical crystals were isolated and analyzed. Unfortunately, the

corresponding iridium complex was not formed without by-products and, therefore, could

not be used in further catalytic steps.

Further redox active ligands were synthesized. The combination of the trop unit with the

known PDI ligand were synthesized for the stabilization of iron in low-oxidation states. The

complex was tested for hydrosilylation of n-octene but did not result as an active catalyst

when compared to the dippPDI ligand described by Chirik. Alternatively, the tropPicol iron

complexes were synthesized and their reduced complexes were described by crystal

structure. Unfortunately, the complex was not active as a HS-catalyst. The use of the same

complex for a hydroboration yielded only small conversions.

Three different iron tropPDI complexes with Fe in the oxidation state (+II), (0), and the

anionic structure were synthesized and described by X-ray crystallography. The homologue

tropPicol Fe complexes were isolated in the oxidation states (+II) and (0).

Outlook

92

Outlook

The electronic properties and the tuning of the described ligands were not further elucidated

in this work. It would be of high interest to perform reactions on the ligand and further to

coordinate these ligands to transition metals.

The search for cheap but effective metals for catalytic applications will bring iron more and

more into focus of the chemistry community. Therefore, the understanding of the interactions

between iron and the co-operating ligand is of paramount importance. It would be of great

interest to further investigate the electrochemistry of the presented Fe complexes. The

interplay of the non-innocent ligands and the different oxidation states of iron may lighten up

many reaction possibilities after deepen into the subject.

The key to interesting catalytic reaction goes through the discovery of complexes, which have

an optimal cooperativity with the supporting ligand.

Therefore, further interesting iron complexes can be synthesized being supported by other

known trop ligands, such as the trop2DAD, the bis(trop)Amine, trop2DAE or tropAmide. The

unique ability of the trop ligand to stabilize and shield a complex would be of interest.

Figure 40: Possible future ligands to stabilize iron in low oxidation states.

Their synthesis and application in catalytic processes as hydrosilylation, hydrogenation or

transferhydrogenation may be of big interest.

Experimental Part

93

Experimental Part

Experimental Part

94

General Techniques

All syntheses with air and/or moisture sensitive products were performed under argon

atmosphere using standard Schlenk techniques or in a glove box (MBraun MB 150B-G

system). The glassware was flame dried under high vacuum with a heat gun or preheated

at 120 °C in an oven overnight prior to use. Solvents were - when needed anhydrous -

freshly distilled under argon from sodium/benzophenone (toluene, thf, Et2O, DME),

sodium/benzophenone/tetraglyme (hexanes) or calcium hydride (DCM) and stored over 3

Å molecular sieves. Solvents used for synthetic and recrystallization purposes were of

“puriss. p.a.” (Riedel-de-Haen, Sigma-Aldrich, Carlo Erba S.A., J.T. Baker or Merck) grade.

Solvents of technical grade were used for column chromatography and TLC.

Deuterated solvents were purified by bulb-to-bulb distillation from sodium/benzophenone

(d8-thf, C6D6) or CaH2 (CD2Cl2) and degassed with three freeze-pump-thaw cycles and

stored in Young-Schlenk tubes over 3A molecular sieves or used as ordered in the case of

non-sensitive substances.

Chemicals

Commercially available chemicals were purchased from ABCR, Acros, Sigma-Aldrich,

Lancaster or STREM and used without further purification. Sodium tert-butoxide was

purified by sublimation and stored under argon in a glove box. Benzoquinone, menadione

(Vitamin K3, 7), phylloquinone, menaquinone were always freshly sublimed before use.

Precious metals were purchased from Johnson & Matthey, Sigma-Aldrich or Precious

Metals Online and used without further purification.

The following compounds were prepared by literature methods: [Ir2(-Cl)2(coe)4], [Ir2(-

Cl)2(cod)2], tropNH2 2, trop2DAD 3.

Experimental Part

95

Analytical Techniques and Instruments

NMR Spectroscopy

1H NMR, 13C NMR, and 31P NMR spectra were recorded on Bruker 700 MHz Avance, 500

MHz DPX Avance, 400 MHz DPX Avance, 300 MHz DPX Avance, 250 MHz DPX Avance or

200 MHz DPX Avance spectrometer at room temperature (unless stated otherwise). The

chemical shifts () are reported in ppm relative to the residual solvent peaks. Coupling

constants J are given in Hertz (Hz). The multiplicities are reported as follows: s = singlet, br

= broad singlet, d = doublet, t = triplet, q = quartet, quint. = quintet, m = multiplet.

Thin layer chromatography (TLC) was carried out on pre-coated Merck silica gel 60 F254

plates visualized by UV light at 254 nm or stained with appropriate stain. Flash column

chromatography was performed using Fluka silica gel 60 (230-400 mesh). Solvent

concentration in vacuo was done at ~10 mbar and 40 °C, drying at ~10-3 mbar at room

temperature.

Melting points were measured on a Büchi M560 melting point apparatus and were not

corrected.

IR Spectroscopy was performed on a Perkin-Elmer-Spectrum 2000 FT-IR-Raman

spectrometer with KBr beam splitter and reported in cm-1. The intensities of the bands are

reported as: w = weak, m = medium, s = strong.

UV/Vis spectra were recorded with a Perkin-Elmer Lambda 19 spectrometer in quartz

cuvettes.

Gas chromatography was performed on a Hewlett Packard HP 6890 Series GC System

equipped with an EPC split injector, on a HP-5 cross-linked 5% phenyl methyl siloxane

column (30 m x 0.32 mm, film thickness 0.25 m) at an inlet pressure of 4.88 psi, with a split

flow of 108 mL/min and a H2 carrier gas at a flow rate of 27.2 mL/min. The initial

temperature of 80 °C was hold for 1 min before increasing to 180 °C at a rate of 4 °C/min.

High-resolution mass spectra (HR EI, ESI and MALDI MS) were performed by the MS

Service of the Laboratory of Organic Chemistry, ETH Zürich.

Experimental Part

96

Elemental analyses (EA) were performed by the Micro-Laboratory at the Laboratory of

Organic Chemistry, ETH Zürich.

Oxidations

Experimental

In a typical oxidation reaction, a flask was charged with 7.8 mg (0.01 mmol)

[Ir(trop2DAD)]OTf 4, 0.96 mg (0.01 mmol) NaOtBu and 1 mmol substrate in thf (5 mL). The

reddish mixture was stirred for a minute before 1.1 mmol of oxidant were added.

In order to follow the reaction proceedings, aliquots were taken at regular time intervals

from the reaction mixture and analyzed by GC-FID.

Isolation of the product was afforded by pouring n-pentane into the reaction mixture,

filtration over Celite and aqueous work up.

Removal of the solvent under reduced pressure yielded the products mostly as colorless

oils.

Ligands

TropPicol 8

(E)-N-(5H-dibenzo[a,d][7]annulen-5-yl)-1-(pyridine-2-yl)methanimine

C21H16N2

296.37 g mol-1

In a 50 mL round bottom flask, 1.00 g (9.34 mmol) of 2-pyridinecarboxaldehyde and 2.00 g

(9.65 mmol) of tropAmine 2 were added to 30 mL of methanol. Two drops of acetic acid

were added to the stirring solution. After several minutes a white precipitate began to form.

The reaction was stirred at room temperature overnight. The precipitate was filtered on a

Experimental Part

97

glass frit, washed twice with methanol, and dried under reduced pressure to yield 1.65 g

(59 %) of the desired imine.

1H NMR (250 MHz, CD2Cl2): δ [ppm] = 8.65 (d, J = 4.8 Hz, 1H, NCH, py), 8.42 (s, 1H, NCH),

7.86 (m, 1H, CHar, py), 7.72 (m, 2H, CHar, py), 7.42 (m, 6H, CHar), 7.35 (m, 2H, CHar), 7.19 (s,

2H, CHolef), 5.22 (br, 1H, CHbenzyl).

13C {1H} NMR (63 MHz, CD2Cl2) δ [ppm] = 162.70 (s, 1C, Cquart), 154.74 (s, 1C, NCH), 149.42

(s, 1C, CHpy), 141.16 (br, 2C, Cquart), 136.47 (s, 1C, CHpy), 133.56 (br, 2C, Cquart), 131.10 (s, 2C,

CHolef), 128.53 (s, 2C, CHar), 128.53-128.24 (4, 2C, CHar), 126.33 (s, 1C, CH), 124.95 (s, 2C,

CHar), 121.18 (s, 1C, CHpy), 50.30 (s, 1C, CHbenzyl).

EA: Calcd. for C21H16N2, (296.37 g/mol): C 85.11, H 5.44, N 9.45 ; Found: C 84.93, H 5.59, N

9.39. Air stable.

MS (MALDI FT-ICR 3-HPA, m/z): 297.1368 (found), 297.1368 (calc.) (error +0.2 ppm).

MP: 200 °C decomposition.

TropNacAc 13

(E)-4-((5H-dibenzo[a,d][7]annulen-5-yl)imino)pentan-2-one

C20H19NO

289.38 g mol-1

2,4-Pentadione (0.9 mL, 8.5 mmol, 1.2 eq.) and tropAmine 2 (1.47 g, 7.1 mmol, 1 eq.) were

dissolved in 100 mL toluene. The clear solution was then heated to 130 ºC and stirred for

24 hours. Toluene was removed under reduced pressure and the white residue was purified

by column chromatography (silica gel, 3:1 n-hexanes:ethyl acetate).

1H-NMR (250 MHz; CDCl3): δ [ppm] = 1.86-2.14 (m, 6H), 4.87-5.71 (m, 2H), 7.40-7.07 (m,

10H).

Experimental Part

98

EA: Anal. Calcd. for C20H18O2 (290.36 g/mol): C 82.73; H 6.25; O 11.02%. Found: C 82.46; H

6.30; O 10.97%

MS (EI, m/z): 290.1306 (found), 290.1302 (calculated), error 1.38 ppm.

MP: 176 ºC.

[Ir(tropPicol]Cl]2 9

C42H32Cl2Ir2N4

1048.08 g mol-1

To a solution of 8 (200 mg, 0.674 mmol, 2 eq) in 15 mL thf was added [Ir(coe)2Cl]2 (293.3

mg, 0.337 mmol, 1 eq). The solution turned green immediately. After stirring for 4 h at room

temperature the solvent was removed under reduced pressure. The solid was washed with

hexane and dried in vacuo. The solid was dissolved in a minimal amount of DCM and

overlaid with n-hexanes. Crystals of good quality for X-ray diffraction were grown.

1H NMR (300 MHz, CDCl3) [ppm] = 8.80 – 8.52 (m, 2H), 7.28 – 7.01 (m, 8H), 7.01 – 6.85 (m,

10H), 6.71 (td, J=7.6, 1.1, 2H), 6.58 (dd, J=7.6, 1.4, 2H), 5.63 (d, J=8.7, 2H), 5.55 (s, 2H), 4.95

(d, J=8.7, 2H), 4.23 (s, 2H).

13C{1H} NMR (75 MHz, CDCl3) [ppm] = 166.18, 149.78, 139.65, 139.58, 137.97, 136.95,

136.64, 129.77, 129.01, 128.43, 128.01, 127.51, 125.61, 125.34, 124.78, 120.73, 119.57, 77.18,

68.13, 58.10, 55.40, 52.91, 31.57, 22.64, 14.09.

[Rh(tropPicol)Cl]2 10

C42H32Cl2N4Rh2

869.46 g mol-1

Experimental Part

99

[Rh(cod)Cl]2 (166.2 mg, 0.337 mmol, 1 eq) was added to a solution of 8 (200 mg, 0.675

mmol, 2 eq) in 10 mL ACN. The solution turned deep red immediately. After 2 h of stirring

at room temperature a red precipitate is formed. The precipitate is filtered off, dissolved in

a small amount of thf and overlaid with n-hexanes. Crystals of good quality for X-ray

diffraction were grown after 2 weeks at room temperature.

[Ru(tropPicol)(cymene)Cl]OTf 11

C31H30ClN2Ru

567.12 g mol-1

To tropPicol 8 in 5 mL EtOH (148.2 mg, 0.5 mmol, 2 eq) was added the ruthenium precursor

[Ru(p-cymene)Cl2]2 (153.1 mg, 0.25 mmol, 1 eq). The solution was stirred for 10 h at 80 °C.

After cooling to room temperature the solvent was removed under reduced pressure. The

remaining solid is dissolved in DCM and filtered over alumina N. The red solution is

concentrated under reduced pressure and overlaid with n-hexanes. Crystals of good quality

for X-ray analysis were grown from the solution.

1H NMR (300 MHz, CDCl3) [ppm] = 10.16 (s, 1H), 8.15 – 7.91 (m, 3H), 7.88 – 7.74 (m, 3H),

7.74 – 7.45 (m, 10H), 7.33 (s, 1H), 7.21 – 6.95 (m, 2H), 6.77 (d, J=2.3, 1H), 5.76 (s, 1H), 5.02

(s, 2H), 2.14 (s, 4H), 1.04 (m, 6H).

13C{1H} NMR (75 MHz, CDCl3) [ppm] = 163.56, 158.45, 153.41, 138.93, 135.52, 135.31,

134.87, 134.32, 132.09, 131.82, 130.61, 130.19, 130.06, 129.58, 129.47, 129.25, 129.12, 80.54,

65.86, 50.61, 31.06, 22.43, 19.28, 15.29

Experimental Part

100

[Cu(tropPicol)ACN]PF6 12

C23H19CuF6N3P

545.94 g mol-1

To tropPicol 8 (500 mg, 1.676 mmol, 1 eq) in 6 mL ACN was added [Cu(ACN)4]PF6 (630.8

mg, 1.69 mmol, 1.01 eq). An immediate color change of the solution from colorless to deep

red is observed. After stirring for 8 h at room temperature the solvent was removed under

reduced pressure. The reddish powder was dissolved in a small amount of DCM and

overlaid with n-hexanes. Orange crystals of good quality for X-ray diffraction were grown

yielding 93%.


C22H21NO3Pd

453.83 g mol-1

TropNacAc 13 (38.5 mg, 0.133 mmol, 1 eq) in toluene (5 mL) was deprotonated with n-BuLi

(0.08 ml, 1 eq) at – 78 °C. After allowing to warm to room temperature the resulting deep

yellow solution was added to a pre-stirred solution of Pd(OAc)2 in toluene. Upon addition

the color of the solution turns deep dark red and was stirred for 8 h at room temperature.

The mixture was filtered over Celite and the clear red filtrate dried in vacuo. Crystals of the

product were grown form a concentrated DCM solution overlaid with hexanes.

Experimental Part

101

Phosphorus Ligands

Chlorobis(trop)diazaphospholidine, trop2sNHP 16

2-chloro-1,3-bis(5H-dibenzo[a,d][7]annulen-5-yl)-1,3,2-diazaphospholidine

C32H26CN2P

505.00 g mol-1

Trop2DAE (0.7 g, 1.6 mmol, 1 eq.) and NEt3 (2.1 mL, 14.9 mmol, 9.3 eq.) were dissolved in

20 mL thf. The reaction mixture was cooled to 0 ºC and PCl3 (0.14 mL, 1.6 mmol, 1 eq.) was

slowly added. A white precipitate instantly formed. The reaction solution was stirred at 0

ºC for 30 min and for another 95 min at room temperature. The solvent was removed under

reduced pressure and the yellow solid was washed with dry 15 mL diethyl ether. The

yellowish solid was dissolved in toluene, filtered through a pad of Celite and concentrated

under reduced pressure yielding 348.9 mg (43%) tropPhosphilidine, as a white powder.

Crystals for X-ray diffraction were grown from DCM:hexane.

1H-NMR (250 MHz; CD2Cl2): δ 2.44-2.27 (m, 3H), 2.93-2.81 (m, 2H), 5.23 (d, J = 6.1 Hz, 2H),

7.15-6.93 (m, 5H), 7.61-7.15 (m, 16H).

31P-NMR (101 MHz; CD2Cl2): δ 164.81 (s, 1P).

Chlorobis(trop)diazaphospholidinium Triflate 17

[(1,3-bis(5H-dibenzo[a,d][7]annulen-5-yl)-1,3,2-diazaphosphonia)] Triflate

C33H26F3N2O3PS

618.14 g mol-1

Experimental Part

102

Tropphospholidine (10 mg, 0.02 mmol, 1 eq.) was dissolved in 0.4 mL CD2Cl2 in a NMR tube

and AgOTf (5.1 mg, 0.02 mmol, 1 eq.) was added. The solution turned pink.

No further isolation was achieved.

1H-NMR (250 MHz; CDCl3): δ 3.25 (d, J = 4.7 Hz, 4H), 5.73 (d, J = 6.4 Hz, 2H), 7.11 (s, 4H),

7.59-7.41 (m, 16H).

31P-NMR (101 MHz; CD2Cl2): δ 260.16 (s, 1P).

[Rh(trop2sNHP)Cl2] 18

(2-chloro-1,3-bis(5H-dibenzo[a,d][7]annulen-5-yl)-1,3,2l5-

diazaphospholidin-2-yl)rhodium(II) chloride

C32H26Cl2N2PRh

643.35 g mol-1

16 (50 mg, 0.01 mmol, 2 eq.) was dissolved in 5 mL DCM and [RhCl(ethene)2]2 (19 mg, 0.005

mmol, 1 eq.) added. The solution turned immediately dark red and gas formation was

observed. Crystallisation of the product was observed after 10 min. The solution was

allowed to stand overnight. The deep red crystals were washed with hexanes and dried in

vacuo. Further concentration and layering with hexanes of the mother liquor yielded a

second batch of product. The combined yield summed to 45 mg (70%).

Crystals for X-ray were grown from DCM:hex (1:1).

1H-NMR (500 MHz; CD2Cl2): δ 7.85 (d, J=7.7, 2H), 7.68 (ddd, J=15.8, 8.3, 2.7, 4H), 7.48 (t,

J=7.4, 2H), 7.42 – 7.32 (m, 6H), 7.27 (d, J=7.4, 2H), 7.04 (dd, J=9.8, 2.7, 2H), 5.04 (d, J=20.8,

2H), 3.43 – 3.32 (m, 2H), 2.90 – 2.76 (m, 2H).

31P-NMR (202.5 MHz; CD2Cl2): δ 114.74 (d, J=222.1).

Experimental Part

103

Iron Chemistry

2,6-diacetylpyridine

C9H9NO2

163.17 g mol-1

A suspension of 15.52 g (0.079 mol) pyridine-2,6-dicarboxylicacid dimethyl ester in 150 mL

EtOAc was slowly added into a 1 L round-bottomed flask charged with 27.3 g (0.401 mol)

NaOEt in 100 mL EtOAc and refluxed for 20 h. After cooling to room temperature 175 mL

fuming HCl (≥37%) was added to the slurry and stirred for 20 h at 90 °C. The mixture was

allowed to warm to room temperature before being quenched with H2O (500 mL). The

aqueous phase was extracted with EtOAc (150 mL) and multiple times with CH2Cl2. The

combined organic phases were washed with a saturated Na2CO3 solution, dried over

Na2SO4, filtered and evaporated to dryness. The residue was dissolved in a minimum

amount of CH2Cl2. Recrystallization from n-pentane at -78 °C yielded quantitatively bright

brown product (12.64 g, 97%).

Characterization is in agreement with published data.[65]

1-{6-[(2,6-Diisopropylphenyl)ethanimidoyl]-2-pyridinyl}-1-ethanone X1

C21H26N2O

322.44 g mol-1

To a mixture of 1.63 g (10 mmol) 2,6-diacetylpyridine and 1.60 g (9 mmol) 2,6-

diisopropylamine in 15 mL MeOH at 0 °C was added a drop of formic acid. The reaction

mixture was stirred for 5 min and allowed to warm to room temperature without stirring

for 24 h. The resulting yellowish solid was filtered off and washed with ice cold MeOH, re-

Experimental Part

104

dissolved was in warm EtOH and filtered while hot. Removal of the solvent under reduced

pressure resulted in a yellow crystalline product in 72% yield (2.1 g, 6.5 mmol).

MS (MALDI-3HPA +H, m/z): 323.2118 (found), 323.21 (calculated), error 0.8 ppm

Characterization of the product matches the published data.[66]

1-(6-(1-((2,6-Dimethylphenyl)imino)ethyl)pyridin-2-yl)ethanone

C17H18N2O

266.34 g mol-1

To a stirred mixture of 2,6-diacetylpyridine (0.49 g, 3 mmol) and 2,6-dimethylaniline (0.33

mL, 0.327 mg, 2.7 mol) in MeOH at 0 °C was added a drop of formic acid. After stirring was

stopped the solution was kept in the freezer at -15 °C for 48 h. The yellow crystalline

product was filtered off and washed with ice cold MeOH. The mother liquor was kept again

at -15 °C resulting in a second crop of yellow crystals after further 48 h. Yield, 85% (611 mg,

2.3 mmol).

Characterization of the product matched the published data.[66]

1H NMR (300 MHz, CDCl3) δ [ppm] = 8.64 (dd, J=7.9, 1.2, 1H), 8.21 (dd, J=7.7, 1.2, 1H), 8.01

(t, J=7.8, 1H), 7.15 (d, J=7.5, 2H), 7.06 – 6.98 (m, 1H), 2.86 (s, 3H), 2.31 (s, 3H), 2.12 (d, J=4.8,

6H).

13C{1H} NMR (75 MHz, CDCl3) δ [ppm] = 200.0 (s, C(O)CH3), 166.7 (s, C(Nar)CH3), 155.6 (s, 1

C, Car), 152.52 (s, 1 C, Car), 148.57 (s, 1 C, Car), 137.34 (s, 1 C, CHar), 127.95 (d, 2 C, CHar), 125.34

(s, 2 C, Car), 124.57 (s, 1 C, CHar), 123.22 (s, 1 C, CHar), 122.66 (s, 1 C, CHar), 25.70 (s, 1 C,

C(O)CH3), 17.96 (s, 2 C, CH3), 16.38 (s, 1 C, C(Nar)CH3).

Experimental Part

105

1-(6-(1-(mesitylimino)ethyl)pyridin-2-yl)ethanone

C18H20N2O

280.36 g mol-1

To solution of 2,6-diacetylpyridine (740 mg, 4.53 mmol) in 10 mL MeOH were added 0.57

mL 2,4,6-trimethylaniline (551 mg, 4.07 mmol) and a drop of formic acid. The solution was

stirred for 15 min at room temperature and stored for 48 h at -15 °C. The product was

collected as bright yellow needles which were filtered off and washed with cold MeOH

yielding 24% (269 mg, 0.95 mmol).


(t, J=7.8, 1H), 6.96 (s, 2H), 2.87 (s, 3H), 2.32 (s, 3H), 2.10 (s, 3H), 2.08 (s, 6H).

13C NMR (75 MHz, CDCl3) δ [ppm] = 200.13 (s, C(O)CH3), 166.87 (s, C(Nar)CH3), 155.73 (s,

Cpy[C(Nar)]), 152.49 (s, Cpyr[C(O)]), 146.09 (s, Cmes), 137.31 (s, CHpyr), 132.44 (s, CHmes), 128.65

(s, Cmes), 125.19 (s Cmes), 124.57 (s, CHpyr), 122.59 (s, CHpyr), 25.71 (s, C(O)CH3), 17.90 (s,

C(Npyr)CH3), 16.33 (s, C(Nar)CH3).

Data corresponds to published patents.[67]

TropPDIDipp 21

N-(1-(6-(1-((2,6-diisopropylphenyl)imino)ethyl)pyridin-2-yl)ethylidene)-5H-

dibenzo[a,d][7]annulen-5-amine

C36H37N3

511.70 g mol-1

A 250 mL round-bottom flask was charged with X1 (7.17 g, 22.22 mmol), tropAmine 2 (5.06

g, 24.45 mmol) and 0.1 mL formic acid in EtOH (175 mL). The reaction mixture was stirred

at 80 °C for 1 h and after cooling to room temperature, allowed to stir further overnight.

The reaction mixture with the whitish precipitate was warmed up to 70 °C and filtered hot

Experimental Part

106

through a Büchi funnel. The white product was washed with cold EtOH and dried under

reduced pressure yielding 83 % (2.79 g, 5.45 mmol).


(t, J=7.8, 1H), 6.96 (s, 2H), 2.87 (s, 3H), 2.32 (s, 3H), 2.10 (s, 3H), 2.08 (s, 6H).





EA: Anal. Calcd. for C36H37N3 (511.71 g/mol): C 84.50; H 7.29; N 8.21%. Found: C 84.28; H

7.23; N 8.18%.

MS (MALDI, m/z): 512.3056 (found), 512.3060 (calculated), error 0.8 ppm.

MP: 194.6 °C

ATR-IR: 2957.23 (w), 1632.19, 1565.79, 1436.89, 1361.66, 1318.95, 1237.43, 1192.17.

1120.74, 1076.15, 1041.69, 792.89, 759.90, 736.84, 642.48.

TropPDIMe 22

N-(1-(6-(1-((2,6-dimethylphenyl)imino)ethyl)pyridin-2-yl)ethylidene)-5H-


C32H29N3

455.59 g mol-1

To a solution of X1 (0.8 g, 3 mmol) and tropAmine 2 (0.62 g, 3 mmol) in 15 mL MeOH, one

drop of formic acid was added. The reaction mixture was heated to reflux and stirred at

room temperature for 8 h. The yellowish precipitate was collected, washed with ice cold

MeOH and dried in vacuo. The mother liquor was collected and stored at -14 °C for several

days where more crystalline product was formed. Overall yield, 35% (478.4mg).

Experimental Part

107

1H NMR (300 MHz, CDCl3) δ [ppm] = 8.53 (d, J=7.7, 1H), 8.04 (t, J=7.8, 1H), 7.87 (d, J=7.8,

2H), 7.50 – 7.38 (m, 4H), 7.32 (d, J=4.3, 2H), 7.29 (d, J=2.8, 3H), 7.14 (d, J=7.5, 2H), 7.01 (dd,

J=8.2, 6.8, 1H), 5.35 (s, 1H), 2.35 (s, 3H), 2.28 (s, 3H), 2.12 (s, 6H).

13C NMR (75 MHz, CDCl3) δ [ppm] = 167.40 (s), 156.40 (s), 154.95 (s), 148.81 (s), 141.07 (s),

136.83 (s), 133.72 (s), 131.41 (s), 128.55 (s), 127.91 (s), 125.89 (s), 125.50 (s), 123.03 (s), 122.39

(s), 121.88 (s), 50.91 (s), 17.98 (s), 16.51 (s), 14.28 (s).


6.44; N 9.29%.

MS (MALDI, m/z): 456.2434 (found), 456.2436 (calculated), error -0.5 ppm.

MP: 180.8 °C

TropPDIMes 23

N-(1-(6-(1-(mesitylimino)ethyl)pyridin-2-yl)ethylidene)-5H-


C33H31N3

469.62 g mol-1

A 50 mL round bottomed flask was charged with X1 (849 mg, 3 mmol) and tropAmine 2

(684 mg, 3.3 mmol) in 10 mL MeOH. To the mixture was added one drop of formic acid and

stirred at reflux for 8 h. The yellowish solution was allowed to cool to room temperature

before being stored at -14 °C for three days. The yellow crystals were filtered off, washed

with ice cold MeOH and dried in vacuo. The mother liquor was stored further at -14 °C

allowing to grow a second batch of crystalline product. Resulting in an overall yield of 23%

(324 mg, 0.69 mmol).


(t, J=7.8, 1H), 6.96 (s, 2H), 2.87 (s, 3H), 2.32 (s, 3H), 2.10 (s, 3H), 2.08 (s, 6H).



Experimental Part

108



saturTropPDIDipp 25

N-(1-(6-(1-((2,6-diisopropylphenyl)imino)ethyl)pyridin-2-yl)ethylidene)-

10,11-dihydro-5H-dibenzo[a,d][7]annulen-5-amine

C36H39N3

513.71 g mol-1

A mixture of X1 (750 mg, 2.3 mmol), saturTropAmine (544 mg, 2.6 mmol) and one drop of

formic acid in 15 mL MeOH was refluxed over a period of 8 h. The reaction mixture was

cooled to 0 °C. The precipitated white product was separated, washed with ice cold MeOH

and dried in vacuo yielding 448 mg (0.87 mmol, 38 %).


(t, J=7.8, 1H), 6.96 (s, 2H), 2.87 (s, 3H), 2.32 (s, 3H), 2.10 (s, 3H), 2.08 (s, 6H).






8.41; N 8.54%.

MS (MALDI, m/z): 512.3056 (found), 512.3060 (calculated), error 0.8 ppm.

MP: 194.6 °C

Experimental Part

109

[Fe(tropPDIDipp)Br2] 26

C36H37N3FeBr2

727.36 g mol-1

To a solution of FeBr2(thf)2 in thf 1 equivalent of tropPDIDipp 21 was added. The reaction

mixture was stirred for 12 h at room temperature before the solvent was removed under

reduced pressure. The dark blue precipitate was washed with small amounts of ether and/or

hexanes, filtered and dried in vacuo resulting in quantitative amount of complex.

EA: Anal. Calcd. for C36H37N3FeBr2 (727.36 g/mol): C 59.45; H 5.13; N 5.78%. Found: C 58.00;

H 5.17; N 5.66%.

MS (MALDI, m/z) [FeBrC36H37N3+]: 646.1515 (found), 646.1517 (calculated), error 0.2 ppm.

MP: >178 °C decomposition.

[Fe(tropPDIMe)Br2] 31

C32H29N3FeBr2

671.26 g mol-1

The synthesis was analogous as described under 8.6.9

EA: Anal. Calcd. for C32H29N3FeBr2 (671.26 g/mol): C 57.26; H 4.35; N 6.26%. Found: C 57.21;

H 4.46; N 6.28%.

MS (MALDI, m/z) [FeBrC32H29N3+]: 590.0887 (found), 590.0891 (calculated), error 0.3 ppm.

MP: 242 °C decomposition.

Experimental Part

110

TropSatFeBr2

C36H39N3FeBr2

729.38 g mol-1

Analogous synthesis as described under 8.6.9, but with saturated trop.

Yield: 61%

EA: Anal. Calcd. for C36H39N3FeBr2 (729.38 g/mol): C 59.28; H 5.39; N 5.76%. Found: C

54.59; H 6.68; N 5.47%.

MS (MALDI FT-ICR 3-HPA, m/z): 297.1368 (found), 297.1368 (calc.) (error +0.2 ppm).

MP: >260 °C decomposition.

List of Abbreviations

111


Å Angström (1 Å = 10-10 m)

° Degree

°C Degree Celsius

ACAC Acetoacetate

ar Aromatic

asy Asymmetrical

ATR Attenuated Total Reflectance

br Broad

bz Benzylic

cat. Catalytic

cod 1,5-Cyclooctadiene

coe Cyclooctene

ct Centroid

CV Cyclic Voltammetry

DAD 1,4-Diazabuta-1,3-diene

DAE Diaminoethane

DCM Dichloromethane

DMSO Dimethylsulfoxide

EI Electron Ionization

ESI Electron Spray Ionization

ETM Electron Transfer Mediator

EtOAc Ethylacetate

FC Flash chromatography

Fc Ferrocene

Fc+ Ferrocenium Ion


112

GC Gas Chromatography

GOase Galactose Oxidase

h Hour

HRMS High Resolution Mass Spectroscopy

Hz Hertz

IR Infra-Red

J Coupling Constant [Hz]

K Kelvin

L Liter

lig Ligand

M Metal

Me Methyl

mg Milligram

MIMA Mono(imine)mono(amine)

min Minute

mL Milliliter

mmol Millimol

mol Mol

Mp Melting Point

MS Mass Spectroscopy

Mw Molecular Weight

NMO N-Methylmorpholine-N-Oxide

NMR Nuclear Magnetic Resonance

ol Olefin

OTf Triflate

ppm Parts Per Million

Rf Retention Factor

rt Room Temperature

s Second

sat. Saturated

st Stretching

sy Symmetrical


113

TEMPO 2,2,6,6-Tetramethylpiperidine-1-oxyl

tert Tertiary

thf Tetrahydrofuran

TLC Thin Layer Chromatography

TMS Trimethylsilane

TPAP Tetra-n-propylammonium Perruthenate

Trop 5H-dibenzo[a,d]cyclohepten-5-yl

TropCl 5-chloro-5H-dibenzo[a,d][7]annulene, 1

TropNH2 5H-dibenzo[a,d][7]annulen-5-amine 2

TropNacAc (E)-4-((5H-dibenzo[a,d][7]annulen-5-yl)amino)pent-3-en-2-one, 13

TropPicol (E)-N-(5H-dibenzo[a,d][7]annulen-5-yl)-1-(pyridin-2-

yl)methanimine, 8

TropPDI (E)-1-(6-((E)-1-((5H-dibenzo[a,d][7]annulen-5-

yl)imino)ethyl)pyridin-2-yl)-N-(2,6-diisopropylphenyl)ethan-1-

imine

UV/Vis Ultraviolet/visible

Wave Length

Compounds

114

Compounds

1 tropCl

2 tropNH2

3 trop2DAD

4 [Ir(trop2DAD)]OTf

5 trop2MIMA

6 [Ir(trop2MIMA)]

7 Vitamin K3 / menadione

8 tropPicol

9 [Ir(tropPicol)Cl]2

10 [Rh(tropPicol)Cl]2

11 [Ru(tropPicol)(cymene)Cl]OTf

12 [Cu(tropPicol)(ACN)]PF6

13 tropNacAc

14 [Pd(tropNacAc)]OAc

15 bistrop-1,3,2-diazaphospholene (trop2NHP)

16 Chloro-bis(trop)diazaphospholidine (trop2sNHPCl)

17 Bis(trop)diazaphospholidinium

18 [Rh(trop2NHP)Cl2]

19 [Rh(trop2NHP)Cl]X

20 aza-PClPh trop2DAE

21 tropPDIDipp

22 tropPDIMe

23 tropPDIMes

24 Saturated tropPDIDipp

25 tropPDItrop

Compounds

115

26 [Fe(tropPDIDipp)Br2]

27 [Fe(tropPDIDipp)(thf)]

28 [Fe(tropPDIDipp)][Na(18c6)(dme)2]

29

30

31

32

[Fe(tropPicol)Br2]

[Fe(tropPicol)]

[Fe(tropPDIMe)Br2]

[Fe(tropPDItrop)Br2]

Crystallographic Data

116



117

aza-PClPh trop2DAE

Identification code (tropnch2ch2ntrop)(pclph)2

Empirical formula C44H36Cl2N2P2

Formula weight 725.59

Temperature/K 100(2)

Crystal system N/A

Space group P-1

a/Å 8.806(2)

b/Å 10.352(3)

c/Å 11.392(3)

α/° 97.181(5)

β/° 108.781(5)

γ/° 110.605(5)

Volume/Å3 886.3(4)

Z 1

ρcalcg/cm3 1.359

μ/mm-1 0.310

F(000) 378.0

Crystal size/mm3 0.25 × 0.08 × 0.07

Radiation MoKα (λ = 0.71073)

2Θ range for data collection/° 3.92 to 52.74

Index ranges -10 ≤ h ≤ 10, -12 ≤ k ≤ 12, -14 ≤ l ≤ 14

Reflections collected 7866

Independent reflections 3586 [Rint = 0.0452, Rsigma = N/A]

Data/restraints/parameters 3586/0/278

Goodness-of-fit on F2 0.919

Final R indexes [I>=2σ (I)] R1 = 0.0390, wR2 = 0.0781

Final R indexes [all data] R1 = 0.0594, wR2 = 0.0833

Largest diff. peak/hole / e Å-3 0.40/-0.23


118

tropPicol 8

Identification code pn

Empirical formula C21H16N2



Crystal system N/A

Space group Pn

a/Å 10.1560(11)

b/Å 15.7518(17)

c/Å 10.1571(12)

α/° 90.00

β/° 106.700(2)

γ/° 90.00

Volume/Å3 1556.4(3)

Z 4

ρcalcg/cm3 1.265

μ/mm-1 0.075

F(000) 624.0

Crystal size/mm3 0.43 × 0.20 × 0.20



Index ranges -11 ≤ h ≤ 11, -18 ≤ k ≤ 16, -10 ≤ l ≤ 11








Flack parameter 2(3)


119

[Ru(tropPicol)(cymene)Cl] 11

Identification code Publishable

Empirical formula C35H34Cl2N2ORu



Crystal system N/A

Space group P-1

a/Å 9.7174(3)

b/Å 10.3185(3)

c/Å 17.2483(5)

α/° 104.7020(10)

β/° 95.4630(10)

γ/° 109.1250(10)

Volume/Å3 1550.12(8)

Z 2

ρcalcg/cm3 1.437

μ/mm-1 0.709

F(000) 688.0

Crystal size/mm3 0.45 × 0.25 × 0.15



Index ranges -14 ≤ h ≤ 14, -15 ≤ k ≤ 14, -24 ≤ l ≤ 25









120

[Cu(tropPicol)(ACN)]PF6 12

Identification code cu(troppicol)(ncch3)_pf6

Empirical formula C23H19CuF6.07N3P



Crystal system N/A

Space group P21/n

a/Å 11.2407(2)

b/Å 9.5111(2)

c/Å 20.7340(4)

α/° 90.00

β/° 98.4140(10)

γ/° 90.00

Volume/Å3 2192.84(7)

Z 4

ρcalcg/cm3 1.658

μ/mm-1 1.138

F(000) 1107.0

Crystal size/mm3 0.24 × 0.10 × 0.09



Index ranges -13 ≤ h ≤ 12, -11 ≤ k ≤ 11, -25 ≤ l ≤ 25









121

tropNacAc 13

Identification code tropNAcOAc

Empirical formula C53.33H50.67N2.67O2.67



Crystal system N/A

Space group Pbca

a/Å 10.4503(8)

b/Å 17.1833(14)

c/Å 17.7337(14)

α/° 90.00

β/° 90.00

γ/° 90.00

Volume/Å3 3184.5(4)

Z 3

ρcalcg/cm3 1.207

μ/mm-1 0.074

F(000) 1232.0

Crystal size/mm3 ? × ? × ?



Index ranges -13 ≤ h ≤ 13, -22 ≤ k ≤ 22, -23 ≤ l ≤ 23









122


Identification code pd(tropnc(ch3)chc(o)ch3)(oac)

Empirical formula C22H21NO3Pd



Crystal system N/A

Space group P21/c

a/Å 8.8201(12)

b/Å 11.8367(16)

c/Å 18.181(3)

α/° 90.00

β/° 102.514(2)

γ/° 90.00

Volume/Å3 1853.1(4)

Z 4

ρcalcg/cm3 1.627

μ/mm-1 1.024

F(000) 920.0

Crystal size/mm3 0.20 × 0.18 × 0.10



Index ranges -7 ≤ h ≤ 9, -13 ≤ k ≤ 13, -19 ≤ l ≤ 20









123

[Rh(trop2Phospholidine)Cl2] 18

Identification code rh(cl)(cyclop(cl)ntropch2ch2tropn)

Empirical formula C33H28Cl4N2PRh



Crystal system N/A

Space group P21/n

a/Å 9.235(3)

b/Å 18.604(5)

c/Å 17.406(5)

α/° 90.00

β/° 98.306(5)

γ/° 90.00

Volume/Å3 2959.2(15)

Z 4

ρcalcg/cm3 1.635

μ/mm-1 1.020

F(000) 1472.0

Crystal size/mm3 0.35 × 0.06 × 0.05



Index ranges -9 ≤ h ≤ 9, -20 ≤ k ≤ 20, -18 ≤ l ≤ 18









124

tropPDIDipp 21

Identification code trclinp-1



Temperature/K 100.15

Crystal system triclinic

Space group P-1

a/Å 11.3015(16)

b/Å 11.3475(14)

c/Å 12.531(2)

α/° 94.156(12)

β/° 94.020(13)

γ/° 115.196(13)

Volume/Å3 1441.2(4)

Z 1

ρcalcg/cm3 1.1791

μ/mm-1 0.069

F(000) 548.2

Crystal size/mm3 0.23 × 0.2 × 0.15

Radiation Mo Kα (λ = 0.71073)


Index ranges -16 ≤ h ≤ 15, -9 ≤ k ≤ 15, -16 ≤ l ≤ 17


Independent reflections 7506 [Rint = 0.0316, Rsigma = 0.0779]







125

tropPDItrop 25

Identification code sqzd




Crystal system N/A

Space group P21/n

a/Å 9.1546(12)

b/Å 13.8003(17)

c/Å 24.322(3)

α/° 90.00

β/° 91.527(2)

γ/° 90.00

Volume/Å3 3071.6(7)

Z 4

ρcalcg/cm3 1.171

μ/mm-1 0.069

F(000) 1144.0

Crystal size/mm3 0.40 × 0.25 × 0.10



Index ranges -11 ≤ h ≤ 11, 0 ≤ k ≤ 16, 0 ≤ l ≤ 29









126

[Fe(tropPDIdipp)Br2] 26

.

Identification code fe(br)2(trop-pyridine-di-imine)_ch2cl2

Empirical formula C37H39Br2Cl2FeN3



Crystal system N/A

Space group P21/c

a/Å 11.3458(16)

b/Å 16.120(2)

c/Å 19.012(3)

α/° 90.00

β/° 92.764(10)

γ/° 90.00

Volume/Å3 3473.0(8)

Z 4

ρcalcg/cm3 1.554

μ/mm-1 2.920

F(000) 1648.0

Crystal size/mm3 0.13 × 0.08 × 0.04



Index ranges -12 ≤ h ≤ 12, -16 ≤ k ≤ 17, -20 ≤ l ≤ 20









127

[Fe(tropPDIdipp)(thf)] 27

Identification code orthoAba2 - Copy

Empirical formula C40H45FeN3O



Crystal system N/A

Space group Aea2

a/Å 25.1678(16)

b/Å 29.823(2)

c/Å 17.5782(11)

α/° 90.00

β/° 90.00

γ/° 90.00

Volume/Å3 13193.9(16)

Z 16

ρcalcg/cm3 1.288

μ/mm-1 0.493

F(000) 5440.0

Crystal size/mm3 0.35 × 0.35 × 0.35



Index ranges -32 ≤ h ≤ 32, -37 ≤ k ≤ 38, -22 ≤ l ≤ 22








Flack parameter -0.022(14)


128

[Fe(tropPDIdipp)][Na18c6(dme)2] 28

Identification code VSPDIFeAn

Empirical formula C108Fe2N6Na2O18


Temperature/K N/A

Crystal system monoclinic

Space group Cc

a/Å 12.3247(9)

b/Å 22.7714(17)

c/Å 38.617(3)

α/° 90

β/° 96.261(5)

γ/° 90

Volume/Å3 10773.2(15)

Z 4

ρcalcg/cm3 1.1263

μ/mm-1 0.340

F(000) 3637.0

Crystal size/mm3 N/A × N/A × N/A

Radiation N/A (λ = 0.71073)


Index ranges -10 ≤ h ≤ 14, -24 ≤ k ≤ 24, -37 ≤ l ≤ 38


Independent reflections 7438 [Rint = 0.0827, Rsigma = 0.1011]



Final R indexes [I>=2σ (I)] R1 = 0.1135, wR2 = N/A



Flack parameter 0.39(5)


129

[Fe(tropPicol)Br2] 29

Identification code sadp21c

Empirical formula C21H16Br2FeN2



Crystal system N/A

Space group P21/c

a/Å 16.1520(3)

b/Å 14.3208(3)

c/Å 16.6477(3)

α/° 90.00

β/° 91.9400(10)

γ/° 90.00

Volume/Å3 3848.56(13)

Z 8

ρcalcg/cm3 1.767

μ/mm-1 4.943

F(000) 2016.0

Crystal size/mm3 0.50 × 0.28 × 0.19



Index ranges -23 ≤ h ≤ 22, -20 ≤ k ≤ 20, -23 ≤ l ≤ 22








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