Silver, mercury and ruthenium complexes of

N-heterocyclic carbene linked cyclophanes

by

Rosenani S. M. Anwarul Haque B.Sc., M.Sc.

This thesis is presented for the degree of Doctor of Philosophy

to: The University of Western Australia,

Department of Chemistry

February 2007

The work recorded in this thesis was carried out in the Department of

Chemistry at The University of Western Australia under the supervision of

Assoc. Prof. Murray V. Baker.

_________________

Rosenani S. M. Anwarul Haque

ii

Preface

Portions of this work have been published elsewhere.

Baker, M. V.; Brown, D. H.; Haque, R. A.; Skelton, B. W.; White, A. H.

“Dinuclear N-heterocyclic carbene complexes of silver(I), derived from

imidazolium-linked cyclophanes" Dalton Trans. 2004, 3756-3764.

Portions of this work have also been discussed at scientific meetings.

37th International Conference of Coordination Chemistry, 13-18 August 2006,

Cape Town South Africa.

Poster entitled: "Silver and Mercury Complexes of Imidazolium-linked

cyclophanes".

iii

Abstract

This thesis describes the synthesis and isolation of silver, mercury,

ruthenium and palladium complexes of bidentate N-heterocyclic carbenes

(NHCs), derived from imidazolium-linked cyclophanes and related bis-

imidazolium salts. The cyclophane structures contain two imidazolyl links

between ortho- and meta- substituted aromatic rings and the related structures

are ortho-, meta- and para-xylyl linked bis-imidazolium salts. The complexes

have been characterised by NMR spectroscopy and X-ray crystallography.

The synthesis of five new silver complexes has been achieved via a

simple complexation reaction of the cyclophane and bis-imidazolium salts with

the basic metal source Ag2O. The new silver carbene systems are thermally

stable. Three of the complexes are dinuclear, cationic complexes, while two are

mononuclear complexes, one cationic and one neutral.

A number of mono- and di-nuclear mercury(II)-NHC complexes have

been synthesised from the ortho- and meta-linked cyclophanes and the related

meta-linked bis-imidazolium salts. The mercury complexes were prepared by

direct mercuration method using mercury(II) acetate. The syntheses were

perfomed in air and the complexes are stable to air and moisture. Mercury

complexes I and II represent the first example of mononuclear metal

complexes derived from meta-substituted imidazolium-linked cyclophanes.

iv

N

N N

NHg

2+

2PF6-

N

N

N

N

Hg

2+

2PF6-

I II

NHC-ligand transfer reactions from NHC-silver complexes and NHC-

mercury complexes are described. An ortho-cyclophane ligand was

successfully transfered from a silver complex to its palladium counterpart.

Furthermore, palladium complex III, bearing a para-xylyl linked bis-NHC

ligand, was made by transmetallation from both a silver and mercury complex.

This is the first reported NHC-palladium complex of a para-xylyl linked bis-

NHC ligand. A new redox transmetallation method for NHC ligand transfer,

using a mercury complex, is presented. A palladium complex was made via

redox transmetallation using a mercury complex of an ortho-NHC-cyclophane.

N

NN

NPdCl

NCMeCl

PdCl

ClMeCN

III

A ruthenium(II)-NHC complex, IV containing an ortho-cyclophane

ligand has been prepared via silver transmetallation and in situ complexation

methods. In the transmetallation route, a silver complex of an ortho-

cyclophane was treated with RuCl2(PPh3)3 to form IV. This complex represents

the first example of a ruthenium complex bearing an NHC-cyclophane ligand,

v

and is also the first example of a metal complex derived from an imidazolium-

linked cyclophane where the arene unit of the cyclophane is also involved in

bonding to the metal centre.

N

N

N

NRu

Cl

+

IV

vi

Table of Contents

Preface ii

Abstract iii

Table of Contents vi

Acknowledgements viii

Abbreviations ix

1 Introduction 1

1.1 Carbenes and N-heterocyclic carbenes 1

1.2 Comparing phosphine and N-heterocyclic carbene catalysts 4

1.3 Interest in metal complexes of NHC 6

1.4 Methods of preparation for NHC ligands and complexes 9

1.5 Cyclophanes and imidazolium-linked cyclophanes 15

1.6 Proposed work 17

2 Imidazolium-linked cyclophanes and related 24

bis-imidazolium salts

2.1 Introduction 24

2.2 Synthesis of the cyclophane salts 25

2.3 Properties of the cyclophane salts 26

2.4 Synthesis of the bis-imidazolium cyclophane analogues 27

2.5 Solubility properties of the non-cyclophane 29

bis-imidazolium systems

2.6 NMR studies of the cyclophane salts and related 30

bis-imidazolium systems

2.7 Structural studies of the bis-imidazolium systems 37

3 NHC complexes of silver 39

3.1 Introduction 39

3.2 Results and discussion 53

3.3 Structural studies 60

3.4 NMR studies 72

vii

4 Cyclophane NHC complexes of Mercury 80

4.1 Introduction 80

4.2 Results and discussion 83

4.3 NMR and Mass Spectrometry studies of 88

complexes 62, and 67 - 69

4.4 Structural studies 93

5 Ligand transfer reactions from silver and 100

mercury complexes

5.1 Introduction 100

5.2 Results and discussion 107

5.3 Structural Studies 122

6 Ruthenium-NHC Complexes 128

6.1 Introduction 128

6.2 Results and discussion 132

6.3 NMR Studies 139

6.4 Structural Studies 143

7 Conclusions and suggestions for future works 147

8 Experimental 151

8.1 General procedures 151

8.2 Materials 152

8.3 Synthesis of metal reagents 153

8.4 Synthesis of imidazolium salts 153

8.5 Synthesis of the silver complexes 158

8.6 Synthesis of the mercury complexes 165

8.7 Transmetallation reactions 169

8.8 Synthesis of ruthenium complex 174

References 177

Appendix 197

viii

Acknowledgements

After four years of doctorate study and training, it is finally time to finish

my thesis and have the chance to acknowledge and thank the people who have

helped me in all kinds of ways.

First and foremost, I would like to thank University Sains Malaysia for

the financial support in making it possible for me to achieve my dream.

Many thanks go to my supervisor, Assoc. Prof. Murray Baker for his

invaluable support, guidance and inspiration. Thank you so much for giving me

this opportunity. My gratitude also goes to Dr David H. Brown for his

assistance in the lab and advice in writing this thesis. Without your help, this

thesis would not have been finish on time.

I would also like to extend my thanks to Professor Allan White and Dr.

Brian Skelton for their brilliant work on the crystal structures. Thanks also to

Brian for the discussions on the crystals data and to Allan for proof-reading

this thesis. Thank you also goes to Dr Lindsay Byrne for all his assistance and

advice with everything NMR and for going out of his way to help me. Dr Tony

Reeder, thanks for the help regarding mass spectroscopy.

To everyone in the Baker group, past and present, and to all my dear

friends, thank you for all your help and support.

Last but not least, to my family in Malaysia, my dearest husband "abah"

and my wonderful children Dinie, Qia and Aesyah, thank you so much for your

love, support, patience and the spirit of "understanding mom ok!" You have

given me the determination to complete this doctoral study. This is 'OUR PhD.'

" Dinie, Qia, and Aesyah, finally, mom has completed our PhD!! ".

ix

Abbreviations

DMF N,N-dimethylformamide

DMSO Dimethylsulfoxide

THF Tetrahydrofuran

h Hours

NHC N-heterocyclic carbene

L Ligand

t-BuOK Potassium tert-butoxide

NEt3 Triethylamine

KN(TMS)2 Potassium bis(trimethylsilyl)amide

DBU 1,8-diazabicycloundeca-7-ene

PPh3 Triphenylphosphine

OAc Acetate

Ph Phenyl

Cy Cyclohexyl

HSQC Heteronuclear single quantum correlation

COSY Correlation spectroscopy

Wh/2 Width at half height

Ar Arene

1

1 Introduction

1.1 Carbenes and N-heterocyclic carbenes

A carbene is defined as a neutral divalent carbon atom with two

nonbonding electrons,1 for example 1, 2 and 3.

CH2 CCl2

dichlorocarbene (2) cyclohexylidene (3)methylene (1)

The nonbonding electrons in a carbene carbon can either be in a singlet or a

triplet state. N-Heterocyclic carbenes (NHCs) are usually in the singlet state.

NHCs can be formed by removing an acidic proton from a heterocyclic system

such as an imidazolium ion (Scheme 1.1).

N

N

deprotonation

an imidazolium ion an imidazol-2-ylidene

R

R

HN

N

R

R

base

Scheme 1.1

A singlet carbene in the ground state has the two nonbonding electrons paired

in an sp2 orbital, leaving a vacant 2p orbital (Figure 1.1).

2

Carbene sp2-hybridised

vacant p-orbital

two unshared electrons

Figure 1.1: Orbital picture of a singlet carbene (sp2-hybridised carbon).

Electronic and steric factors affect the stability of a singlet carbene. The

electronic contribution is generally believed to be the main stabilising factor,

and involves electron donation from groups such as N, S, or O heteroatoms to

the vacant p orbital (e.g., Figure 1.2).2-8

N

N

N

N+

-

N

NN

orbital view

-

electronic stabilisation of NHC carbenes

N

R

R

R

R

R

R

+

Figure 1.2: Orbital and resonance representations of electronic stabilisation in

imidazol-2-ylidenes.

It is a common belief that the steric factor is small or unimportant in

contributing to the stability of a carbene.2, 3, 9, 10 However, that belief is fast

changing based on newly emerging results.11-14 Some researchers believe steric

factors are now as important as electronic factors in determining the stability of

an NHC and its complexes.15, 16 In some instances, changing the steric bulk of

the N-substituents on the NHC completely changes the stability of complexes

formed.16, 17

Carbenes are important transient intermediates but due to their high

3

reactivity, they are not usually isolable. Although isolation of free N-

heterocyclic carbenes was attempted in the 1960's,18 success only came in 1991

with the isolation of the first stable crystalline NHC by Arduengo and co-

workers.19 That NHC, 1,3-di-(1-adamantyl)imidazol-2-ylidene 4, has become a

significant ligand in organometallic chemistry. Since the first isolation of 4,

various related NHCs have been isolated and characterised.9, 20-22 For example,

the tris(NHC) 5 was isolated and later became useful as a ligand in preparing

various metal complexes.23-26

4

N

N

NN

N

N

5

N

N

t-Bu

t-Bu

t-Bu

Although the isolation of a free carbene was not achieved until 1991,

metal complexes of carbenes were made much earlier. The first metal

complexes of N-heterocyclic carbenes, complexes 6 and 7 (equations 1 and 2,

Scheme 1.2), were synthesized from imidazolium salts and appropriate metal

sources by Wanzlick and co-workers27 and Öfele28 in 1968.

4

N

N

N

N

N

N

2+

- 2AcOH

2ClO4-

HgHg(OAc)2, heat

eq. 1

6ClO4-

2

Ph

Ph

Ph

Ph

Ph

Ph

N

N

- H2Cr(CO)5

HCr(CO)5-

heateq. 2

7

N

N

Ph

Ph

Ph

Ph

Scheme 1.2

In these syntheses, the mercuric acetate and pentacarbonylhydridochromium

each served as sources of both metal ions and bases. A carbene intermediate

was believed to be formed in situ, by removal of the acidic C2-proton of the

imidazolium salt, and subsequent trapping of the NHC by the metal ions led to

the NHC complexes. This simple and efficient method for preparing NHC

complexes has become widely used by other researchers.

1.2 Comparing phosphine and N-heterocyclic carbene catalysts

NHCs were first recognised as phosphine mimics by Herrmann and co-

workers around 1992-93.29 They had noticed similar ligand properties and

metal coordination chemistry when comparing organophosphines (PRR3) and

NHCs. Both ligands are good electron donors and are compatible with many

metals in various oxidation states.

5

Formation of a carbon-carbon bond is an important step in many

industrial processes, including synthesis of pharmaceuticals30 and

agrochemicals,30 fine organic chemicals,31 and natural products.31 Many of

these processes employ metal-phosphine catalysts.32, 33 Examples include

Wilkinson’s catalyst 8, developed by Wilkinson and co-workers,34 used in

reductive coupling of olefins,35 and palladium catalysts36-40 (eg: 9)41 used in

Heck and Suzuki coupling reactions.

Rh

Cl

Ph3P PPh3

PPh3

Pd

Ph3P

Ph3PPPh3

PPh38 9

Phosphines are good σ donors. They stabilise metal centres in various

oxidation states, especially low oxidation states,42 and also prevent metal

centres from aggregating (eg: formation of colloids).43, 44 These features make

them good ligands for metal complexes in catalysis. However, metal phosphine

complexes are susceptible to decomposition via P-C bond cleavage at elevated

temperatures,45-50 leading to deactivation of the catalysts. Furthermore,

phosphines and metal-phosphine complexes are frequently air sensitive,44, 51, 52

and phosphine ligands are expensive, toxic and unrecoverable.42

As ligands, NHCs also can stabilise metal centres in a variety of

oxidation states, and are more potent σ-donors than phosphines.43, 53-55 This

strong σ-donor ability is attributed to electron donation from the nitrogen

atoms to the carbene carbon (the electronic stabilisation discussed earlier),9, 56

compensating the carbene carbon for electrons used in the M-L bond,57 thus

giving better coordination and stability to the M-L bonds. Studies done by

6

Cavallo and co-workers on bond dissociation energy (BDE) of metal-NHC and

metal-PR3 bonds reveal stronger bonding to metals (higher BDE) of metal-

NHC as compared to metal-PR3 bonds.16 This stronger bonding in turn gives

NHC complexes better thermal stability and better protection against

degradation by M-donor atom bond cleavage, compared to PR3 complexes.

Another advantage of NHC complexes is ease of handling. Imidazolium

salts, the precursors to NHCs, are not air sensitive, and NHC complexes are

generally less air sensitive than metal-phosphine complexes.48, 58 In many

instances, where phosphine catalysts show inferior catalytic performance, NHC

analogues excel in terms of stability and catalytic activities.45, 46, 53, 59-63

Although the binding of NHCs to metals is generally understood to be

through σ donation only, with negligible contribution from metal to NHC π-

back-bonding,2, 3, 64-67 more recent studies have refuted this idea.16 In a study

conducted by Hu and co-workers on some diaminocarbene metal complexes,

they found that not only σ interactions involve metal-carbene bonds, but metal-

to-ligand π−back bonding interactions contribute about 15-30% of the

complexes overall orbital interaction energies.

1.3 Interest in metal complexes of NHC

Interest in metal complexes of NHCs heightened after various reports of

their catalytic activity. To date, almost all transition metals and main group

metals have been complexed with NHCs. Numerous review papers have been

written on this subject.43, 53, 56, 68-80

The applications of the new NHC complexes in catalysis are widespread.

One of the most significant and extensively researched areas concerns

7

ruthenium complexes of NHCs, for use in olefin metathesis reactions. In 1996,

Grubbs and co-workers isolated ruthenium alkylidene diphosphine complexes

of the type L2X2Ru=CHR such as 10 and 11.81-83 These complexes, the first

generation Grubbs catalysts, quickly gained popularity as they were found to

be stable, catalytically active in olefin metathesis reactions and highly tolerant

to a broad range of functional groups.

RuCl

PPh3

Cl

PPh3

Ph

Ph

RuCl

PCy3

Cl

PCy3

Ph

Ph

10 11

Subsequently, Grubbs and co-workers replaced a phosphine by an NHC

ligand to develop "second generation Grubbs catalysts" (e.g., 12) for alkene

metathesis,84, 85 after which research in this area accelerated. More ruthenium-

NHC complexes were developed, showing increased activity, and selectivity in

olefin metathesis reactions.45, 77, 78, 86-93

Palladium NHC complexes (e.g., 13) are another successful story with

widespread applications in C-C bond-forming reactions such as Heck and

Suzuki couplings.46-48, 57, 94-106 Other applications of NHC complexes include

iridium catalysed hydrogenation and hydrogen transfer,107-109 platinum

catalysed hydrosilation,59 nickel catalysed polymerisation reactions,110-114 and

rhodium catalysed hydroboration.115 To date, ruthenium, palladium and nickel

complexes appear to be the most successful catalysts for these metathesis and

coupling reactions.

8

Current research116-118 in NHC metal complexes includes seeking for

better and more cost-effective preparations of the complexes. Recently, a

synthesis of complex 14 on a large scale while maintaining low cost was

reported.119 Complex 14 was found to be an active catalyst for Suzuki-Miyaura

cross-coupling reactions and has great potential as a catalyst for other cross-

coupling reactions.

N

N N

NPd

NN

RuPh

Cl

ClPCy3

12

N N

Pd

N

Cl

14

13

ClCl

Other work in this area includes studies of new NHC metal complexes

that offer greater stability and catalytic ability compared to those of established

phosphine-based catalysts. For example, Crabtree and co-workers anticipated

that the stability of carbene complexes could be enhanced by having pincer

(tridentate) ligand frameworks. They reported the pincer-type complex 15 to be

air and heat stable.120 Complex 15 catalysed Heck olefination of aryl chlorides,

9

rarely used substrates42, 47, 103 compared to the more commonly used aryl

bromides and iodides.

Systems that combined the stability of N-heterocyclic carbenes and high

catalyst activity of phosphines have also been explored.99 Preliminary results

show complexes of the type (NHC)Pd(PR3)I2 (eg: 16) are efficient catalysts

for Suzuki and Stille cross-coupling reactions.

N

NPd

E

Br

N

N

CH

N

N

CH3

CH CH3

Pd PR3

I

I

E = N, n = 1E = C, n = 0

+ nBr-

15 16R=Cy, Ph

1.4 Methods of preparation for NHC ligands and complexes

There are various methods that can be used to synthesise NHC ligands

and to attach substituents and functional groups to the ligands, and this area has

been reviewed extensively.43, 56, 72, 75, 121 These ligands can be complexed to a

metal via three major routes:

(i) The in situ deprotonation method: The reaction of azolium salts

with base in the presence of metal ions, with NHCs as presumed intermediates

that are quickly trapped by the metals. Possible bases includes potassium or

lithium t-butoxide (Scheme 1.3),122, 123 triethylamine (Scheme 1.4),124 a

phosphazene125 and even dilute NaOH with a phase-transfer catalyst such as

10

tetrabutylammonium bromide.126 Alternative bases such as sodium acetate or

basic metal sources such as palladium acetate or silver oxide have also been

used (Scheme 1.5).98, 120, 127-132 This method, pioneered by Wanzlick27 and

Öfele,28 is popular with researchers and is normally performed as a one-pot

synthesis.

X

N

N

R

X

N

N

R

NaI, t-BuOK

0.5 eq Pd(OAc)2

2

PdI2

1. X = CH, R = (S)-1-phenylethyl2. X = N, R = phenyl

MePh MePhClO4-

Scheme 1.3

N

N

N

R2

R1

NEt3, THF

[Rh(COD)Cl]2

ClO4-

N

N

N

R2

R1

RhCl

R1 = (R)-1-phenylethylR2 = Me, Ph, t-Bu

Scheme 1.4

11

Pd(OAc)2

DMSO50 °C, 4 h

DMSOreflux20 min

-HOAc

N

N

N

N

N

N

t-Bu

t-Bu

N

N

N

N

t-Bu

t-Bu

Pd

Br

Br

OAc-

N N

Pd

Br

Br

t-Bu

t-Bu

Scheme 1.5

(ii) The free carbene method: Treatment of azolium salts with strong

bases to make a free NHCs, to which metal sources are then added. In this

route, the NHC ligand precursor is first deprotonated to form a free carbene,

which is subsequently treated with a metal source to produce the NHC

complex. The free carbene can be isolated or used without isolation. Free

carbenes can be generated using strong base such as NaH, KN(SiMe3)2

(Scheme 1.6),133 potassium t-butoxide (t-BuOK) (Scheme 1.7),9 and the dimsyl

anion (CH3SOCH2-) in THF, or NaH in a mixture of liquid ammonia and

THF.19, 56, 134, 135 Use of the free carbene method can, however, pose problems

such as inconvenience, NHC ligand decomposition, or sometimes dimerisation

of the free NHC by Wanzlick equilibrium (Scheme 1.8).136

12

N

N

N

N

N

Ar

Ar

KN(SiMe3)2

THF, -10 °C

RuCl2(PPh3)3

THF

N

N

N

N N

RuCl

PPh3

N

N

N

N

N

Ar

Ar

Cl

Ar

Ar

Ar = 2,6-i-Pr2C6H3

2+

2Br-

Scheme 1.6

N

N

Cl

t-BuOK

THF

Cl

N

N

Cl

Cl

Scheme 1.7

N

N

N

N

R R

R R

N

N

R

R

Scheme 1.8, The Wanzlick equilibrium

13

(iii) The silver-carbene transfer method is a more sophisticated method,

for preparing carbene complexes that are hard to prepare by conventional

methods. This method was developed by Wang and Lin.137 In this route, the

NHC ligand is first complexed with silver, usually via a reaction between the

appropriate azolium salt and a silver source such as silver oxide or silver nitrate

with a base. Then the NHC ligand is transferred to another metal of choice.

The silver complexation method works well in the presence of air or moisture,

is tolerant of a wide range of solvents including water, and has become the

method of choice for many researchers.54, 138-145 Lin and co-workers used Ag(I)

carbene complexes (prepared by reaction of imidazolium salts with Ag2O) as

transfer agents to prepare palladium and gold complexes (Scheme 1.9).137

Coleman and co-workers used this method to prepare NHC-palladium and

NHC-rhodium complexes (Scheme 1.10).146

The carbene transfer method avoids generation of free NHC and thus

problems such as the potential for NHC decomposition. With this method,

however, there have been reports of low yield and failure to

transmetallate.128, 147 Sometimes after a transfer process, a potentially chelating

NHC ligand has failed to form a chelate complex with the target metal, as

reported by Mata and co-workers (Scheme 1.11),148 or mixtures of both chelate

and non-chelate products can be obtained.128 The failure to chelate is

considered a serious deficiency of this method because it prevents the

formation of chelate and pincer NHC complexes, analogues of which have

been so useful in phosphine chemistry.75

14

Et

N

NEt

2

Pd(MeCN)2Cl2

Ag2O

Et

N

NEt Et

N

NEt

Ag

Et

N

NEt Et

N

NEt

Au

Et

N

NEt Et

N

NEt

Pd

Au(SMe2)Cl

Cl

Cl

PF6-

PF6-

+

PF6-

+

Scheme 1.9

N

N

N

Br-

N

N

N

N

N

N

Ag

AgBr2

PdCl2(MeCN)2DCM

N N

NHPd

Cl

Cl

Rh2Cl2(COD)2AgBF4THF, 24 h

N

N

NH

Rh

+

BF4-

Ag2O

DCM, 48 h

Scheme 1.10

15

2Br-

2. [Rh(COD)Cl)2

1. Ag2O, DCM, RT, 90 minN

N

N

N

n-Bu

n-Bu

N

N

N

N

n-Bu

n-Bu

RhCl(COD)

RhCl(COD)

Scheme 1.11

1.5 Cyclophanes and imidazolium-linked cyclophanes

Cyclophanes are bridged aromatic systems in which two benzene rings

are linked together by two or more bridging groups. In 1997, Bosnich149, 150

synthesised a variety of imidazolium-linked cyclophanes (e.g., 17). The project

was extended further by Williams, with synthesis and detailed structural

studies of imidazolium-linked cyclophanes and related systems.151 Williams

converted some of these cyclophanes into metal carbene complexes, such as

those of Pd, Pt and Ni, where NHCs are part of the cyclophane framework

(e.g., 18 and 19). This work was later continued with the synthesis being

extended to rhodium, iridium and gold cyclophane complexes (e.g., 20).152-155

16

N

N

N

N

N

N

3Br-N

N

N

N MXX

M = Pd,X = I, BrM = Ni, X = ClM = Pt, X = Cl

N

N N

N N

N

Ni

2Br-

N

N

N

N

PF6-

17 18

19 20

Rh

+2+

The above complexes were made by adaptations of the methods of

Wanzlick27 and Öfele28 using either a basic metal source or metal halides in the

presence of a base such as sodium acetate. No free carbenes were isolated

when synthesising the cyclophane complexes, but it was presumed that the

carbenes were formed in situ as intermediates.

Preliminary catalytic studies on these complexes have given encouraging

results. Complex 21, for example, was found to be highly stable in air and

catalysed Heck and Suzuki couplings with turnover numbers approaching 107

for the Heck reaction, at the time, the highest recorded for NHC-based

catalysts.128, 152

In this respect, complex 21 is superior to Herrmann’s96 complex 22. The

superior catalytic activity of 21 may be due to the cyclophane skeleton in 21

orienting the imidazolyl groups out of the coordination plane of the Pd centre.

17

By contrast, the imidazolyl groups in 22 are oriented approximately in the

coordination plane of the Pd atom, and may hinder approach of groups to the

remaining coordination sites during catalysis.

Gold complexes of cyclophanes such as 23 have been tested for

biological applications and so far have shown interesting antimitochondrial

activity, with some selectivity for mitochondria in tumour cells over normal

cells.156

N

N

N

NN

N

N

NAu

Au

2+

2Br-23

N

NCH3

Pd

N

N

CH2

CH3

I

I

22

N

N

N

N Pd

21

Br

Br

1.6 Proposed work

The objective of this project is to synthesise complexes of N-heterocyclic

carbenes based on imidazolium-linked cyclophanes and related bis-

imidazolium structures. At the commencement of this project, a number of

metal complexes of NHC-cyclophanes had been prepared in this laboratory but

their scope was limited largely to the complexes of an ortho-cyclophane NHC

ligands.152-155 Other than that, Cavell and co-workers128 had published a

18

palladium complex of a meta-cyclophane, and Youngs and co-workers have

reported several silver complexes,157-159 synthesised from reactions of a

'pyridinophane' ligand with Ag2O in different combinations of stoichiometries

and solvents. Well-defined complexes of meta-cyclophanes remain elusive

(although Cavell and co-workers128 reported the palladium complex of a meta-

cyclophane, the complex was not fully characterised). These complexes are of

interest because they will have different C-M-C geometries, and perhaps

unusual catalytic properties and applications compared to their ortho-

analogues.

1.6.1 Target cyclophane and related bis-imidazolium salts

For this project, the target cyclophanes and related bis-imidazolium

compounds of interest are the ortho, meta and para varieties, i.e. compounds I-

VI as their Br-, Cl-, PF6- and BPh4

- salts.

N N

N N

N N

N N

N N

N N

I II III

N N

N N

N

NN

N

IV V

N

N

N

N

VI

19

1.6.2 Target metals

With the above imidazolium-linked cyclophanes as NHC ligand

precursors, the aim is to explore routes to cyclophane complexes of other

metals, in particular silver, mercury, and ruthenium.

Ruthenium complexes were of primary interest, due to their potential

catalytic activity. Ruthenium complexes of NHC-cyclophanes have no

precedent and it is of interest to develop a method of synthesising them. Since

a palladium complex of an ortho-cyclophane ligand (21) was found to be a

good catalyst for C-C coupling reactions,152 related ruthenium complexes may

also have interesting applications.

Silver and mercury complexes became a focus in this work because

carbene complexes of these metals are generally easy to prepare, starting from

Ag2O and Hg(OAc)2. Ease of preparation is an important consideration for

new NHC-cyclophane complexes and related systems with unusual geometries,

given that so far it has only been possible to prepare well-defined ortho-

cyclophane complexes.

The applications of mercury NHC complexes are not very well

documented in the literature, so the behaviour of these complexes was of

interest. Some organomercury complexes are known to serve as ligand transfer

agents to other metals and ligand transfer from an organomercury complex to a

different metal centre is a well-established route in organometallic synthesis. A

series of lanthanoid(II) and -(III) complexes as well as Sn(IV) complexes were

reported recently, prepared by redox transmetallation using metal(0) sources

and mercury(II) complexes.160-163 Similar behaviour of mercury NHC-

complexes could provide new synthetically useful routes in NHC chemistry.

20

Silver carbene complexes are known to be useful carbene transfer

reagents, and silver meta- or para-cyclophane complexes may serve as

synthetic precursors for meta- and para-cyclophane complexes of ruthenium,

palladium, and other metals.

1.6.3 Methods

The complexes in this project were prepared via three methods:

I. The in situ deprotonation method

For silver and mercury complexes, the in situ deprotonation method was

employed, using the basic metal sources Ag2O and Hg(OAc)2; Scheme 1.12

and 1.13 (general).

Hg(OAc)2+ 2HOAc

N N

N N polar solvent

N

N

N

NN

N

N

NHg

Hg

4+

4PF6-2PF6-

Scheme 1.12

Ag2O

+ H2ON N

N N polar solvent

N

N

N

NN

N

N

NAg

Ag

2+

2PF6-2PF6-

Scheme 1.13

21

For ruthenium complexes the synthesis was attempted by the in situ

deprotonation method (Scheme 1.14) and the free carbene method (Scheme

1.15, Section II). RuCl2(PPh3)3, RuCl2(DMSO)4 and [RuCl2(p-cymene)]2

were tested as ruthenium sources and using bases such as NaH/potassium t-

butoxide, 1,8-diazabicycloundeca-7-ene (DBU) and lithium

bis(trimethylsilyl)amide LiN(SiMe3)2.

N

NaH, t-BuOH, DMFN

N N RuRuCl2( PPh3)3

Ru = Ru and ancillary ligands

N N

N N

Scheme 1.14

II. The free carbene method

In the free carbene method, RuCl2(PPh3)3 was reacted with an NHC

ligand, generated, for example, from an ortho-cyclophane salt in the presence

of NaH in DMF. Dissociation of PPh3 from RuCl2(PPh3)3 in solution is known

to occur readily,164, 165 and with this information in mind, it was expected that

carbenes generated in situ would be trapped by ruthenium as per Scheme 1.15.

22

Ru

DMF

DMF

Cl

Cl

DMFDMF

t-BuONa

N

N

N

N Ru

Ru = Ru and ancillary ligands

Ru

PPh3

Ph3P

Cl

Cl

PPh3

N N

N N

N N

N N

Scheme 1.15

Exploration of the free carbene method was prompted by similar works

(although with different imidazolium salts) by other groups, in particular those

of Herrmann and Grubbs (e.g., Scheme 1.16).87-89, 166

NaH

NNR R Liq NH3/THF-40 oC, 2 h

NNR R

NN

RuPhCl

PCy3

RR

RT, 1.5 hRu

PhCl

PCy3

PCy3

ClCl

Cl-

+

NNR R+

toluene

R = C6H2-2,4,6-(CH3)3

Scheme 1.16

23

III. Ligand transfer reaction

Complexes of ruthenium and palladium in this project were also prepared by

ligand-transfer methods. Silver and mercury complexes made in this study

were used as NHC transfer agents to ruthenium and palladium. The syntheses

were done in one-pot reactions, using appropriate palladium sources such as

palladium chloride and iodide and ruthenium metal sources such as

RuCl2(PPh3)3, dichloro(p-cymene)ruthenium(II) dimer [RuCl2(p-cymene)]2,

and RuCl2(DMSO)4 (Scheme 1.17 for palladium from an ortho-cyclophane

silver complex is shown).

2 PdCl2

N

N

N

NN

N

N

NAg

Ag

2+

2PF6-

N

N

N N PdCl2

Cl

+ 2 AgPF6

Scheme 1.17

24

2 Imidazolium-linked cyclophanes and related

bis-imidazolium salts

2.1 Introduction

This chapter describes the synthesis and characterisation of three

imidazolium-linked cyclophanes and three related bis-imidazolium systems,

used as ligand precursors for making the silver, mercury, ruthenium and

palladium complexes in this project. The cyclophane salts are based on an

ortho-cyclophane cation (I) and two meta-cyclophane cations (II and III). The

related bis-imidazolium systems are based on the para-, meta- and ortho-linked

cations IV - VI respectively. X-ray structures are presented for the salts

IV.2PF6 and V.2PF6.

N N

N N

N N

N N

N N

N N

I II III

N N

N N

N

NN

N

IV V

N

N

N

N

VI

25

2.2 Synthesis of the cyclophane salts

The cyclophane salts I.2Br, II.2Br and III.2Br used in this project were

prepared by the reaction of an appropriate bromo-methylated benzene with a

suitable imidazol-1-ylmethylbenzene moiety, in acetone, following published

procedures.128, 152 A typical reaction is shown for the synthesis of the ortho-

cyclophane salt I.2Br (Scheme 2.1).

acetonereflux+

N NBr N N

N NN N

Br

2Br-

I.2Br Scheme 2.1

The cyclophanes precipitated from acetone as white powders in reasonably

good yield (the ortho-cyclophane salt I.2Br: 97%; the non-methylated meta-

cyclophane salt II.2Br: 65%; and the methylated meta-cyclophane salt III.2Br:

73%). The cyclophanes were obtained sufficiently pure for further use after

washing with acetone (I.2Br and II.2Br) or recrystallisation from methanol

(III.2Br).

N N

N N

N N

N N

II.2Br III.2Br

2Br-2Br-

26

2.3 Properties of the cyclophane salts

Solubility issues frequently make these cyclophanes difficult to work

with. As their dibromide salts, the cyclophanes are insoluble in most organic

solvents and only soluble in polar solvents such as water, methanol, DMSO

and DMF (sometimes elevated temperatures are required for adequate

solubility).167 The syntheses of silver, mercury, ruthenium, and palladium

complexes in the high-boiling solvents DMSO and DMF work well, but

complete removal of these solvents after workup can be difficult to achieve.

The growth of crystals of the dibromide salts from these high boiling solvents

for X-ray study can also be a challenge, as the solvents do not evaporate easily

at low temperature.

Furthermore, the halide counterion can sometimes cause complications in

reactions of metal ions with carbenes, as reported by some research

groups.137, 140, 168-172 For better solubility in organic solvents, and to avoid

complications associated with bromide counterions, the bromide salts were

converted to their respective hexafluorophosphate or tetraphenylborate salts.

These conversions extend the choice of solvents suitable for further work to

include organic solvents such as acetone and acetonitrile, which are much more

convenient than high boiling DMSO and DMF.

The hexafluorophosphate salts were prepared by addition of an aqueous

solution of KPF6 to an aqueous solution of the appropriate cyclophane halide

salt (e.g., Scheme 2.2 for the ortho-cyclophane salt I.2PF6). The cyclophane

hexafluorophosphate salts precipitated from water and were collected by

filtration and purified by several washings with water.

27

N N

N N

2KPF6water, RT

N N

N N+ 2KBr(aq)

I.2Br I.2PF6

2Br-(aq) 2PF6-(s)

Scheme 2.2

The tetraphenylborate salt I.2BPh4 was prepared similarly, from I.2Br

and NaBPh4 in methanol. NMR showed that the cyclophane

hexafluorophosphate and tetraphenylborate salts were essentially pure and

displayed similar chemical shifts to their bromide counterparts. The yields

were reasonable: I.2PF6, 52%; I.2BPh4, 65%; II.2PF6, 63%; and III.2PF6,

56%.

2.4 Synthesis of the bis-imidazolium cyclophane analogues

The para-, meta- and ortho-bis-imidazolium salts IV.2Cl, V.2Br and

VI.2Br were made following the methods developed by Williams,151 Dias and

Jin,23 and Nguyen.173

N

NN

N

N N

N N

N N

N N

VI.2BrV.2BrIV.2Cl

2Cl- 2Br- 2Br-

28

The para-linked bis-imidazolium dichloride salt IV.2Cl, was obtained by

the reaction of α,α'-dichloro-p-xylene with two equivalents of N-

methylimidazole in refluxing dioxane for 24 h, in 81% yield (Scheme 2.3). The

crude product precipitated as a yellowish solid, which was recrystallised from

ethanol-diethyl ether to give the pure product.

N

N+ 2

dioxane

reflux, 24 hClCl N

NN

N2Cl-

IV.2Cl

Scheme 2.3

The "methylated meta- " bis-imidazolium salt V.2Br was obtained from a

previous student, Anu Sharma, and was synthesised in similar fashion to

IV.2Br, by the reaction of 1,3-bis(bromomethyl)mesitylene (made according to

the methods developed by Williams151) with two equivalents of N-

methylimidazole in refluxing dioxane (Scheme 2.4). The product precipitated

from the reaction mixture, and was purified by recrystallisation from methanol.

+

IV.2Br

N N

N N

N

N2

dioxane

reflux, 24 h

Br

Br2Br-

Scheme 2.4

29

The ortho-linked bis-imidazolium salt VI.2Br was prepared following

the methods of Dias and Jin23 and Nguyen.173 α,α'-Dibromo-o-xylene was

reacted with two equivalents of N-butylimidazole in refluxing dioxane for 24 h,

to afford the desired salt in 89% yield (Scheme 2.5). The salt was obtained

pure after several washings with dioxane and followed by washing with diethyl

ether.

N NN

N+ 2

N N

Br dioxane

reflux, 24 hBr

VI.2Br

2Br-

moisture.

Scheme 2.5

2.5 Solubility properties of the non-cyclophane bis-imidazolium systems

Similar to the cyclophane halide salts discussed previously, the para- and

meta-linked bis-imidazolium salts IV.2Cl and V.2Br are very soluble in polar

solvents such as ethanol, methanol, DMSO, DMF and water, but are insoluble

in less polar solvents such as dichloromethane, acetone, acetonitrile and diethyl

ether. Conversions of these salts to their PF6- counterparts were performed by

salt metathesis in water or methanol using KPF6, similar to the method used

for the cyclophanes, to afford IV.2PF6 in 71% yield and V.2PF6 in 85% yield.

The salt IV.2Cl was found to be slightly hygroscopic (it became slightly wet

after exposure to air for a few months) but IV.2PF6 can be kept in air

indefinitely without showing any signs of absorption of

30

As expected (due to its butyl substituents) the ortho-linked bis-

imidazolium salt VI.2Br is soluble in most organic solvents, including

dichloromethane and acetonitrile. Unfortunately, however, this salt was not

soluble in THF, a polar aprotic solvent used in the free carbene method for

making ruthenium complexes.88 Thus, VI.2Br was also converted to the PF6-

salt by a metathesis reaction in water using KPF6, similar to the method

described previously for the other salts in this project. The salt VI.2PF6 was

obtained in 60% yield and proved to be very soluble in THF and other organic

solvents such as dichloromethane, acetone and acetonitrile.

2.6 NMR studies of the cyclophane salts and related bis-imidazolium

systems

In her thesis,151 Williams discussed in detail the NMR features of an

assorted collection of cyclophane salts including I.2Br, II.2Br and III.2Br, and

a paper has also been published on this subject,167 so only a brief discussion

will be presented here.

Williams found that ortho- and meta-cyclophanes can exist in various syn

and anti conformations (e.g., Scheme 2.6) and that rapid interconversion

between these conformations can occur at rates comparable to the NMR

timescale.167 The ortho-cyclophane salt I.2PF6 shows such behaviour. The 1H

NMR spectra of d6-DMSO solutions of I.2PF6 display broad signals (Figure

2.1(a)), which can be explained in terms of exchange between syn and anti

conformations (and between equivalent syn conformations and between

equivalent anti conformations), occurring at rates comparable to the NMR

timescale. The 1H NMR spectra of solutions of II.2PF6 are sharp and show

31

resonances consistent with exchange between conformations being rapid on the

NMR timescale (e.g., Figure 2.1(b)). The NMR data for I.2PF6 and II.2PF6 are

summarised in Table 2.1.

N N

N N

N N

N N

I II

N

N N

N+

+N

N N

N+

+

syn anti

Scheme 2.6: The interconversion between a syn and an anti conformation of a

cyclophane via "benzene ring flip".

32

Figure 2.1: 1H NMR spectrum for (a) the ortho-cyclophane salt I.2PF6

(300.13 MHz, d6-DMSO, ambient temperature); and (b) the non-methylated

meta-cyclophane salt II.2PF6 (500.13 MHz, CD3CN, ambient temperature).

33

Insert Table 2

34

For the methylated meta-cyclophane cation III, both the Br- and PF6-

salts displayed different NMR behaviour to that of I and II.

N N

N N

III

In spectra recorded at ambient temperature in d6-DMSO, the protons display

relatively sharp signals (Table 2.1 and Figure 2.2). The benzylic protons appear

as AB multiplets centred at δ 5.32 and 5.44. The arene and imidazolium H4/H5

protons appear as singlets, at δ 7.99 and 7.01 respectively. The imidazolium-

H2 protons also appear as a singlet, but at the relatively upfield shift of δ 7.21.

These results are consistent with the cyclophane being a rigid structure in

which the arene groups are mutually syn, with the imidazolium groups oriented

so that their H2 protons lie in the region of shielding by the ring currents of the

arene rings.

Variable temperature studies of this salt from 298 to 398 K show that, at

higher temperatures, the cyclophane begins to display fluxional behaviour. At

338 K, the two benzylic peaks began to collapse, and later to coalesce into a

single sharp peak (Figure 2.2 (c - e)). At the same time, the signal due to the

imidazolium H4/H5 protons broadens but becomes sharp again at high

temperature, as does the signal due to the imidazolium H2 protons. With the

35

exception of the signal for the imidazolium H2 protons and the benzylic

protons, there is negligible change in chemical shift with temperature. This

result suggests that over the temperature range examined, only syn

conformations have a significant population (concentration) within the system,

and that fluxionality evident in the NMR spectra arises from interconversion of

two equivalent syn conformations, presumably via trace amounts of short-lived

anti conformations (Scheme 2.7). As the temperature is increased, the signal

due to the imidazolium H2 protons moves upfield, perhaps due to changes in

the degree of hydrogen-bonding with solvent molecules or due to rapid H-H

exchange with H2O. Participation of imidazolium H2 protons in the syn

conformation of III in hydrogen-bonding with H2O has been observed in an X-

ray study of III.2Br.4H2O,167 but fluxionality of III has not been described

previously.

N

N

N

N ++ N

N

N

N++

N

N N

N+

+N

N N

N+

+

syn anti anti syn

imidazoliumring rotation

benzenering flip

benzenering flip

Scheme 2.7: The interconversion between two equivalent syn and anti (less

populated) conformations of cation III in solution.

36

Figure 2.2: Variable temperature 1H NMR spectra (500.1 MHz, d6-DMSO) for

the methylated meta-cyclophane salt III.2PF6.

The NMR spectra for the bis-imidazolium systems IV, V and VI, both as

halides and hexafluorophosphate salts, display all the expected signals (Table

2.1), and are consistent with the spectra reported for related imidazolium

salts.19, 128, 168, 169, 174-176

37

2.7 Structural studies of the bis-imidazolium systems†

Crystals of VI.2PF6 and V.2PF6 suitable for X-ray diffraction studies

were grown from concentrated solutions of the salts in acetonitrile at 4 °C. The

structures were consistent with the formulations given above, and bond

distances and angles were unexceptional. The structure of the cation IV is

shown in Figure 2.3 and that of cation V is shown in Figure 2.4.

N

NN

N

Figure 2.3: Structure of the cation in IV.2PF6.

† Throughout this thesis, in the figures of structures from crystallographic determinations, the

non-hydrogen atomic displacement ellipsoids are drawn at 20% (300 K) or 50% (150 K)

probability amplitude levels with hydrogen atoms shown with arbitrary radii of 0.1 Å.

Hydrogen atoms are omitted from unit cell projections.

38

N N

N N

Figure 2.4: Structure of the cation in IV.2PF6.

39

3 NHC complexes of silver

3.1 Introduction

A number of excellent reviews on NHC-silver complexes have recently

been published.79, 177-179 While specific examples are included in this

introduction, readers are referred to these reviews for a more expansive

coverage of the literature.

3.1.1 History

The first silver NHC complex was reported in 1993 by Arduengo and co-

workers,180 following their success in isolating free carbenes such as 4 and

24.9, 19 The silver complex 25 was obtained in 80% yield from the reaction of

silver(I) triflate with two equivalents of the NHC 24, in THF under a dry

nitrogen atmosphere (Scheme 3.1).180

4

NN

24

NN

N

NR

R

R = mesityl

+ AgOSO2CF3N

NR

R

N

NR

R

Ag

CF3SO3-

24

2

25

THF

+

Scheme 3.1

40

3.1.2 Synthetic methods

Following the reported synthesis of 25, the free carbene method was used

to prepare a number of silver NHC complexes.135, 181, 182 For example,

Caballero and co-workers prepared the double helical silver complex 26 by

first preparing a free carbene by the reaction of a pyridyl-bridged imidazolium

salt with t-BuOK in THF at 0 °C, and then treatment of the resulting carbene

with one equivalent of silver(I) triflate (Scheme 3.2).182

2+

t-BuOK

THF, 0 °C

2Br-

2CF3SO3-

NN N

N N

R R

NN N

N N

R R

N

N

N

N

N

R

R

R = CH2Ph

NN

N

N

N

R

R

26

+ +

AgOSO2CF3

2+

AgAg

Scheme 3.2

Although the isolation of the silver complex 25 heralded the start of what

would become a significant research field within the area of NHC-metal

complexes, the use of the free carbene method for the synthesis of silver-NHC

complexes has not gained widespread popularity.177, 178 The free carbene

41

method involves the treatment of a NHC precursor with a strong base under

strictly anhydrous conditions for the generation of the free carbene.48, 67, 181-183

In general, NHC carbenes are sensitive to air and moisture, and are often

thermally sensitive, and may decompose before they can react with a metal

source.181, 182 The requirements for the free carbene method have proven to be

tedious, inconvenient and harsh to some ligand systems.95, 184-186

Furthermore, when an imidazolium salt is treated with strong bases such

as NaH or t-BuOK, deprotonation of acidic protons other than the

imidazolium-H2 ring proton can occur,79 which can lead to the decomposition

of the NHC precursor.95, 111, 168, 174, 182, 184, 186 Imidazolium salts that have

methylene groups linked to the nitrogen atoms on the N-heterocyclic carbenes

are affected by such reactivity.79, 111, 174 For example, Cavell and co-workers

attempted to generate free carbene from the imidazolium salt 27 by treating it

with NaH in liquid NH3 at -78 °C (Scheme 3.3),174 but the carbene showed

poor stability, which they attributed to the acidity of the methylene protons

linked to the NHC nitrogens. Similar problems were reported by Wang and co-

workers in their attempts to deprotonate imidazolium salt 28 to form the

corresponding carbene (Scheme 3.4).111 The attempt failed due to the high

acidity of the methylene protons associated with the carbene precursor.

42

N

N N

N N

2+

2PF6-

NaHLiq NH3, THF-78 °C

N

N N

N N

poor stability27

Scheme 3.3

NN

NN

N

N

+I-

28

Scheme 3.4

Due to the above-mentioned limitations associated with the free carbene

method, alternative synthetic methods for the synthesis of silver-NHC

complexes have been developed. One of the most popular methods is the in

situ deprotonation method using a NHC precursor and a basic silver source. In

this method, the base from the metal source removes an acidic proton from a

carbene precursor (most commonly an imidazolium salt), and the resulting

carbene is trapped by silver to form a complex. A number of basic silver

sources have been reported in the literature. Bertrand and co-workers reported

the use of silver acetate in 1997,187 and Danopoulos and co-workers reported

the use of silver carbonate in 2000.169 However, the turning point for the

synthesis of silver-NHC complexes was the use of silver oxide as a basic silver

source, reported by Wang and Lin in 1998.137 The use of silver oxide for the

preparation of silver NHC complex is now prevalent.79, 179

43

The silver complex 29 was prepared by reacting 1,3-diethyl-

benzimidazolium dibromide with silver oxide in dichloromethane at RT, and

was isolated in 89% yield (Scheme 3.5).137 A single crystal X-ray study

showed that, in the solid state, the cation of 29 formed an ion pair with

[AgBr2]- through a weak Ag...Ag interaction. However, in solution, an

equilibrium is thought to exist, involving transfer of a NHC ligand and a

bromide, between the ion pair to produce two neutral species (Scheme 3.6).137

Reaction of 1,3-diethylbenzimidazolium bis(hexafluorophosphate) with Ag2O

in the presence of a base and a phase transfer catalyst {[Bu4N]PF6/NaOH} in

dichloromethane affords 30, a similar product to 29 but free of the [AgBr2]-

anion and the associated equilibrium behaviour (Scheme 3.5).137

N

NEt

EtMol. sieves N

NEt

EtN

NEt

Et

+

X- = Br-

Ag

Ag BrBr

29

N

NEt

EtN

NEt

Et

Ag

30

2

Ag2O, NaOH

X- = PF6-

PF6-

+

dichloromethaneAg2O

dichloromethane [Bu4N]PF6

X-

Scheme 3.5

44

Et N N Et

EtNNEt

Ag AgBr

Br

EtN

NEt

Ag

Et NN Et

Br

Ag Br

EtN

NEt

Ag2 Br

Scheme 3.6: The equilibrium between a bis-NHC silver complex and a NHC-

silver halide complex, including the proposed transition state.137

NHC-silver complex formation using Ag2O produces water as a by-

product, which can be removed by the addition of 4Å molecular sieves to the

reaction mixture, thus improving the yield of the NHC-silver complex.79, 177

The synthesis of NHC-silver complexes using Ag2O has been reported in a

wide range of solvents79 including DMSO, DMF, THF, toluene, acetonitrile,

acetone, dichloromethane and dichloroethane, and protic solvents including

methanol188 and water!157, 189

The reactions can be carried out in air at ambient temperature, as well as

elevated temperatures,79 and in the most part reactions are typically performed

in the absence of light,175, 190-192 as silver complexes tend to be light sensitive.

The Ag2O method has been used for the synthesis of silver complexes bearing

a diverse range of NHC ligands, including NHCs bearing a variety of

functional groups, for which this method displays excellent tolerance.79, 178, 179

45

3.1.3 Applications

Silver-NHC complexes have attracted immense interest due to their

usefulness in several applications. By far their most significant application is as

carbene transfer reagents. Wang and Lin137 first reported their use to prepare

NHC-gold (31) and -palladium (32) complexes via carbene transfer (Scheme

3.7). In this method, a NHC ligand is transferred from the silver centre to

another metal centre. Since the report by Wang and Lin, the use of NHC-silver

complexes instead of free NHCs has become increasingly popular and the

carbene transfer method has been used to prepare NHC complexes of

numerous transition metals.79, 178, 179 A more in-depth discussion of this

transmetallation route is provided in Chapter 5.

Pd(MeCN)2Cl2

Et

N

NEt Et

N

NEt

Ag

Et

N

NEt Et

N

NEt

Au

Et

N

NEt Et

N

NEt

Pd

Au(SMe2)Cl

Cl

Cl

PF6-

PF6-

31 32

+

+

Scheme 3.7

46

Other applications of NHC-silver complexes includes their potential use

as antimicrobial agents.188, 193 Two NHC-silver complexes (33 and 34) have

been encapsulated in fiber mats and tested on some clinically important

bacteria and have been found to be effective antimicrobial agents. Although

silver and its salts have been used in the treatment of wounds to prevent

infections since the seventeenth century, this is the first time NHC-silver

complexes have been applied for this purpose.79 A review has been published

recently on silver(I)-NHC complexes and their application as antimicrobials.194

N

N

N

N

N

N

N

N

HO OH

N

N

N

N

N

OHHO

Ag Ag

2OH-

N

N

HOHO

N

N

N

N

N

HOHO

Ag

2+

n 2OH-

33.2OH

34.2OH

2+

47

Silver-NHC complexes have also been used in catalysis, though the area is new

and remains largely unexplored. Peris and co-workers first reported the use of

silver(I)-NHC complexes as catalysts. They reported the use of NHC-silver

complex 35 as a catalyst in diboration reactions of internal and terminal

alkenes.195 Silver complexes 36, 37 and 38 have been investigated by

Waymouth and co-workers as catalysts in the ring-opening polymerisation of

L-lactide and transesterification of methyl benzoates.196 Complex 36 was found

to catalyse the polymerisation of L-lactide with 90% conversion whereas

complex 37 has similar conversion although the reaction was much slower.

Complex 36 catalysed the transesterification of methyl benzoate with primary,

secondary and tertiary alcohols, though the highest activity was reported with

primary alcohols.196

N

N N

NAg

AgCl2-

36

N

N

R1 R1

R1 R1

37: R1=R2=Me

38: R1=isopropyl,R2=H

R2

R2

+

R1 R1

R1 R1

R2

R2

N

NAg

AgCl2-

+NN

N N

Ag

35

AgCl2-

O

O

48

3.1.4 Structural types

Silver-NHC complexes with a diverse variety of structural geometries

have been isolated, including mononuclear and dinuclear complexes, and have

been crystallographically characterised. This diversity is comprehensively

covered in recent reviews.79, 177, 178, 197 In general the typical NHC-silver

bonding motifs are LAgX and [L2Ag]+. However, depending on whether the

counterions are coordinating, such as halides, or non-coordinating, such as

PF6-, BF4

-, CF3SO3-, and BPh4

-, as well as factors such as the steric influence

of the substituents on the NHC rings, the solid-state structures can show

significant diversity. Examples of such diversity are depicted below for

complexes 39 - 45.11, 142, 169, 171, 174, 186, 198 When the counterions are halides,

common bonding motifs observed in the solid state include bridging AgX

species and [AgX2]nn- counterions, which may or may not be weakly

coordinated to silver centre of a [L2X]+ cation.

3940

N

Br

N

N

N

N

Ag

Cl

Ag

Cl

N N

N N

N

N Ag

B

Ag

r

N NO

NAgBr

41

N

N

N

NAg

BrAg

Br

42

49

N

N

N

NAg

44.2BPh4

NN

N N

Ag

NN

N N

AgAg

I

I

Ag

I

43

N

N

N

NAg

N

N

2+

2BPh4-

I

3+

3PF6-

NN

NN

N

NN

N

Ag

NN

NN

AgAgC

C

45.3PF6

3.1.5 Silver complexes of bis(NHC)-ligands containing a cyclophane

structure

Although numerous examples of NHC-silver complexes exist, reports of

NHC-silver complexes derived from imidazolium-based cyclophanes are rare.

Youngs and co-workers reported the first examples of silver complexes bearing

a cyclophane-NHC ligand which was derived from an imidazolium-linked

'pyridinophane'.157-159 The silver complexes 47 and 48 were synthesised by the

reaction of the pyridinophane cation 46, as the Br- or PF6- salt, with Ag2O. The

reaction of the pyridinophane salt 46.2PF6 and Ag2O in a 1:2 (46 : Ag2O)

molar ratio, in DMSO, afforded the dinuclear complex 47.158 However, the

reaction of 46.2PF6 with Ag2O in a 1:4 (46 : Ag2O) molar ratio, in DMSO,

afforded the tetranuclear silver complex 49.4PF6.157 Interestingly, the solid-

50

state structure of 49.4PF6 suggests that each NHC moiety is bound to two

silver atoms! The reaction of 46.2Br with Ag2O in a 1:4 (46 : Ag2O) molar

ratio, in water, affords only the dinuclear complex 48.157

N

N

N

N

NN

N

NN

NAg

Ag

N

N

2+

2X-

N

N N

N N

N

47, X = PF648, X = Br46

N

N

N

N

NN

N

NN

NAg

Ag

N

N

4+

4PF6-

Ag Ag

49.4PF6

Ag

Ag

Ag

Ag

NHC

NHC NHC

NHC

The bonding motif in 49

3.1.6 Silver complexes of noncyclic xylyl-linked bis(NHC) ligands

Silver complexes derived from noncyclic xylyl-linked bis(NHC) ligands

have been reported by several groups.132, 142, 168, 169, 174, 188, 193 In all cases, the

bis(NHC) ligands are linked by an ortho- or meta-xylyl, or xylyl-type ring (as

in the cases of the pyridyl derivatives below). These complexes display

interesting structural features including both mononuclear and dinuclear

51

complexes.79, 178, 197 Tulloch and co-workers reported complex 50 derived from

an ortho-xylyl linked bis-imidazolium salt, where each NHC unit is bound to

an independent AgCl moiety.169 Matsumoto and co-workers have reported a

similar silver complex derived from the meta-xylyl linked bis-imidazolium salt

(51).168 Analogues of 51, containing a meta substituted pyridyl ring, instead of

a phenyl ring, have also been prepared (52 and 53).132, 142 Using a similar

bis(NHC) ligand containing the meta-substituted-pyridyl core, the dinuclear

silver complex 44 has been isolated.174 None of the silver complexes reported

to date have been derived from a para-xylyl linked bis-imidazolium salt.

NN N

N N

AgClAgCl

51

N N

N NAg Cl

AgCl

N

N

N

N

50

AgCl

AgCl

52 53

NN N

N NArAr

AgCl

AgCl

Ar = 2,6-i-Pr2C6H3; Mesityl

N

N

N

NAg

44.2BPh4

N

N

N

NAg

N

N

2+

2BPh4-

52

3.1.7 Aims

In this thesis, the synthesis and characterisation of silver-NHC complexes

derived from ortho- (I) and meta- (II and III) imidazolium-linked cyclophanes

and the para- (IV) and meta-xylyl (V) linked bis-imidazolium salts, are

presented. These complexes have been synthesised in view of their potential

use as carbene transfer reagents. The ability to act as NHC transfer agents is

discussed in Chapters 5 and 6.

N N

N N

N N

N N

N N

N N

I II III

N N

N NN

NN

N

IV V

53

3.2 Results and discussion

3.2.1 Synthesis of silver complexes from imidazolium-linked cyclophanes

The silver complexes 54 - 56 were prepared by the reaction of Ag2O

with the corresponding cyclophane salts under an exclusion of light, based on

the procedures developed by Wang and Lin.137

N

N

N

NN

N

N

NAg

Ag

2+

54

N

N N

N N

NN

NAg

Ag

2+

55

N

N

N

NAg

Ag BrBr

56

Reaction of I.2PF6 with three equivalents of Ag2O in hot acetonitrile

afforded 54.2PF6, which was isolated as colourless crystals in 43% yield after

recrystallisation from acetonitrile (Scheme 3.8). The by-product in this

reaction, AgPF6, proved difficult to remove by recrystallisation from organic

solvents. However, if the reaction mixture is poured into water 54.2PF6

precipitates and can be easily isolated by filtration, while AgPF6 remains in

solution. The tetraphenylborate salt 54.2BPh4, was prepared similarly, by the

reaction of I.2BPh4 with a slight excess of Ag2O, and was isolated in 38%

yield following precipitation from water.

54

N

N

N

NN

N

N

NAg

Ag

2+

N N

N N

I.2PF6

+ 3Ag2O

50 °C, 24 h

54.2PF6

acetonitrile

2PF6- 2PF6

-

Scheme 3.8

The dibromide salt 55.2Br was prepared by the reaction of III.2Br with

two equivalents of Ag2O in DMF (Scheme 3.9). However, analytically pure

samples of this material were difficult to obtain, possibly because of

contamination of the product with residual silver bromide species.

Interestingly, during attempts to purify 55.2Br by recrystallisation from DMF,

crystals of 55.(Ag2Br4) were obtained (see Structural Studies). Analytically

pure samples of complex 55 were obtained as the hexafluorophosphate salt

55.2PF6. The addition of an aqueous solution of 55.2Br to an aqueous solution

of KPF6 resulted in a white precipitate, which was then recrystallised from hot

water affording 55.2PF6.

N

N N

N N

NN

NAg

Ag

2+

55.2Br

N N

N N

II.2Br

90 °C, 24 h

DMF+ 2Ag2O

2Br-2Br-

Scheme 3.9

55

The synthesis of 56 was similar to those of complexes 54 and 55, except

that in this case III.2Br was allowed to react with only one equivalent of

Ag2O, in DMSO (Scheme 3.10). This reaction, which was monitored by NMR

spectroscopy, was much slower than those leading to complexes 54 and 55, in

this case taking four days to reach completion. Presumably this slowness is a

consequence of the rigid conformation of III;151, 167 where the mesitylene units

are mutually syn and may hinder access to the imidazolium-H2 protons (Figure

3.1).151, 167, 199 The complex 56 was obtained as a grey powder in 43% yield,

after recrystallisation from DMF.

N

N

N

NAg

Ag BrBr

56

N N

N N+ Ag2O

80 °C, 4 days

DMSO

2Br-

III.2Br

Scheme 3.10

N

N

N

N H+

+H

Figure 3.1: The structure of the cation III; the imidazolium rings are directed

into the cavity of the mutually syn mesitylene units.

56

During the syntheses of complexes 54 - 56, it was found that, in general,

using excess Ag2O in the reaction affords products with higher purities. The

insoluble nature of Ag2O enables easy removal of any excess of the reagent at

the completion of the reaction. However, the requirement for excess Ag2O was

somewhat solvent dependent. Generally, when using DMSO as the reaction

solvent, an excess of Ag2O was not required, but when the reactions were

performed in DMF, acetonitrile or methanol an excess of Ag2O proved

beneficial.

The salts 54.2PF6 and 55.2PF6, containing the cationic binuclear

complexes, are soluble in DMSO, DMF and CH3CN, while the neutral

complex 56 is insoluble in most common organic solvents and only sparingly

soluble in DMF and DMSO at RT. The syntheses of complexes 54 - 56 were

conducted with the exclusion of light, to prevent the formation of black

precipitates. However, once isolated, the complexes are generally stable to

moisture, light and heat. Solutions of 54.2PF6, 55.2PF6 and 56 in d6-DMSO

did not exhibit decomposition after eight days of exposure to light and

moisture, or heating at 100 °C in air.

The structures of the cationic binuclear complexes 54 and 55 were

established by a combination of NMR spectroscopy, X-ray diffraction studies,

and elemental analysis. For complex 56, elemental analysis and NMR studies

were consistent with complex 56 having the stoichiometry and the monomeric

structure [(cyclophane)(AgBr)2] (Figure 3.2). However, a single crystal X-ray

study performed on a crystal grown from a DMSO solution of 56 revealed the

more complex dimeric structure 57, where two molecules of 56 are bridged

with [AgBr]2. Bridging halides are commonly seen in X-ray studies of NHC-

57

silver complexes though evidence of these bridged structures in solution has

yet to be reported.140, 169-172 In this case, 57 presumably results from residual

AgBr, possibly in the original sample of 56 or from the minor decomposition

of 56 during the preparation of the crystal-growth solution. Further details of

NMR spectroscopic and single crystal X-ray diffraction studies for complexes

54 - 57 are discussed later in this Chapter.

N

N

N

NAg

Ag BrBr

56

N

N

N

NAg

Ag BrBr

N

N

N

NAg

AgBrBr

Ag BrBr Ag

57

Figure 3.2: Complex 56, as determined by NMR spectroscopy and

microanalysis, and complex 57, determined from a single-crystal X-ray study.

3.2.2 Synthesis of silver complexes from non-cyclic bis-imidazolium salts

The silver complexes 58 and 59 were made using the same Ag2O one pot

reaction method discussed above. Perhaps due to the non-cyclic nature of the

bis-imidazolium salts, the reactions took less time to complete (2 h compared

to a minimum 24 h for the cyclophanes). However, in general, the complexes

were more light sensitive and darkened if left uncovered in the solid state.

58

N NNN

N NNN

Ag Ag

2+

58

N

N

N

NAg

+

59

Reaction of IV.2Cl with an excess of Ag2O in methanol (Scheme 3.11)

afforded the salt 58.2Cl in 52% yield after recrystallisation from hot methanol.

The reaction also proceeds well in DMF or DMSO. The salt 58.2Cl is soluble

in DMF, DMSO and ethanol, and sparingly soluble in methanol. Slow

evaporation of a concentrated solution of 58.2Cl in methanol produced crystals

suitable for an X-ray study, which confirmed the dinuclear structure in the

solid state.

N

NN

N

IV.2Cl

+ Ag2O60 °C, 2 h

methanolN NNN

N NNN

Ag Ag

2+

58.2Cl

2Cl-

2Cl-

Scheme 3.11

The PF6- salt 58.2PF6 was prepared similarly, by reacting IV.2PF6 with

Ag2O in acetonitrile, and was isolated in 77% yield. Again, removal of the

associated AgPF6 by-product proved problematic, since, in this case,

precipitation of the product into water was not feasible as the material

59

darkened immediately in water. Careful washing of the reaction product with

dichloromethane, followed by recrystallisation from acetonitrile/diethyl ether

affords a pure material. Concentrated acetonitrile solutions of 58.2PF6, allowed

to stand at 4 °C afforded colourless needles, which, however, proved

unsuitable for X-ray studies.

Reaction of V.2Br with Ag2O in methanol afforded complex 59.Br

(Scheme 3.12), which was purified by salt metathesis in water. Addition of an

aqueous solution of 59.Br to an aqueous solution of KPF6 affords 59.PF6 as a

grey precipitate, which was isolated in an overall yield of 76%. Complex 59, as

the PF6- salt, was characterised by NMR spectroscopy, elemental analysis, and

structurally characterised by a single crystal X-ray study. The X-ray studies

confirmed the mononuclear structure of the silver(I) NHC in the solid state.

Similar to the bromide and hexafluorophosphate salts of 58, 59.Br dissolves

well in DMF, DMSO and water while 59.PF6 dissolves well in acetonitrile and

acetone at RT.

+

+ Ag2O50 °C, 2 h

methanolN N

N N

V.2Br

N

N

N

NAg

59.Br2Br- Br-

Scheme 3.12

Complexes 58 and 59, as the hexafluorophosphate salts, showed

excellent stability to moisture and heat. The solutions of 58.2PF6 and 59.PF6 in

d6-DMSO exposed to moisture at ambient temperature for a week, showed no

60

decomposition (determined by 1H NMR spectroscopy). Similarly, no

decomposition was observed (visually or by 1H NMR spectroscopy) when

solutions of the complexes in d6-DMSO were heated for 2 days at 100 °C.

3.3 Structural studies

3.3.1 The silver(I)-cyclophane complexes

I. The ortho-cyclophane complex 54

Crystals suitable for X-ray diffraction studies of the ortho-cyclophane

silver complexes 54.2PF6 and 54.2BPh4 were grown in each case by vapour

diffusion between neat diethyl ether and a concentrated solution of the complex

in acetonitrile. The structure of 54.2PF6 is shown in Figure 3.3 as a

representative example. A figure of the cation from the structure determination

of 54.2BPh4 is provided in the Appendix. In both cases, 54.2PF6 and

54.2BPh4, the structures contain acetonitrile molecules in the unit cell. As

expected, the cations within each structure are similar. The cations are each

symmetrical about an inversion centre, the imidazolium planes are

approximately parallel, and the cyclophane units have the o-xylyl groups

mutually syn, and oriented anti to the Ag atoms. Each of the carbene carbons is

connected to one silver atom, which is in turn connected to a carbene carbon of

a different cyclophane unit. The carbene-silver-carbene moiety approaches

linearity and the Ag-C bond lengths are similar in each case. Table 3.1 lists

selected bond lengths and angles for the structure determinations of 54.2PF6

and 54.2BPh4. The differences in the bond lengths and angles for the cation 54

in each of the structures are attributed to crystal packing effects. Silver-NHC

61

complexes reported in the literature137, 144, 158, 195, 200, 201 also have linear

geometries about silver and display similar Ag-C bond angles and distances.

The Ag…Ag distance in the structure of 54.2PF6 is 2.962(6) Å and for

54.2BPh4 the Ag…Ag distance is 3.084(8) Å. In each case this would suggest

a possible metal-metal interaction, with an upper van der Waals limit202 of 3.44

Å. A similar metal-metal interaction is seen in the gold analogue of 54.154

Garrison and Youngs defined strong interactions to be metal-metal distances of

smaller than 3.0 Å while weaker interactions are those longer than 3.3 Å.79

62

N

N

N

NN

N

N

NAg

Ag

2

Figure 3.3: Structure of 54.2PF6 showing the cation and associated anions.

63

Table 3.1: Selected bond angles and distances for silver complexes 54.2PF ,

54.2BPh and 55.Ag Br . 6

4 2 4

N

N

N

NN

N

N

NAg

Ag

2+

.2PF6 .2BPh4

N

N

N

NN

N

N

NAg

Ag

2+

N

N N

N N

NN

NAg

Ag

2+

.(Ag2Br4)2-

54.2PF6 54.2BPh4 55.(Ag Br ) 2 4

Interatomic distances (Å)

Ag-carbene(C22) 2.095(3) 2.088(5) 2.095(4)

Ag-carbene(C42) 2.093(3) 2.090(5) 2.089(4)

Ag-Ag' 2.962(1) 3.084(8) 5.0267(5)

Bond angles (degrees)

C42-Ag-C22 176.3(1) 174.4(2) 174.8(1)

II. The non-methylated meta-cyclophane complex 55

The cation of 55.(Ag2Br4), in which the benzene rings are meta

substituted, adopts a similar structure to that in complex 54, two cyclophane

units being linked together by two silver metal atoms in a centrosymmetric

array. Ag-C bond lengths, C-Ag-C angles and the intra-cation Ag...Ag distance

are detailed in Table 3.1. In this case, the meta substitution pattern of the

benzene rings results in a substantially larger intra-cation Ag...Ag distance.

The asymmetric unit of the structure is made up of one half of the molecule.

One of the benzene rings in the cyclophane is inclined "endo" while the other

points outwards, "exo" (Figure 3.4). A pair of cations crystallise together with

bridging Ag2Br42- anions and two DMF solvent molecules, as shown in Figure

3.5.

64

N

N N

N N

NN

NAg

Ag

2+

Figure 3.4: Structure of the cation 55 in 55.Ag2Br4.

65

Figure 3.5: Structure of 55.(Ag2Br4) showing the complex with the bridging

Ag2Br42- anion and two DMF molecules.

The (Ag2Br4)2- anion packs between cations (55), and an interaction

exists between each of the silver atoms of the cation and a silver atom of the

anion (see Figure 3.5). The silver (cation)…silver (anion) distance is 2.899(4)

Å. Each silver atom in the anion is bound to three Br- ions with Ag-Br bond

distances of 2.653(1) Å, 2.515(1) Å and 2.652(1) Å. The Br-Ag-Br bond angles

are 135.2(2)°, 94.6(2)° and 123.8(2)°. Other groups have reported similar silver

halide bridging arrangements in silver-NHC complexes. For example, in the

solid-state structure of the silver complex 43, silver-NHC cations are bridged

by an [Ag2I4]2- anion, in a similar geometric arrangement as in 55.(Ag2Br4),

with Ag-I bond distances between 2.7 and 2.8 Å and weak silver (cation) -

66

silver (anion) interactions of 3.0424(14) Å.171

NN

N N

Ag

NN

N N

AgAg

I

I

Ag

I

43

I

III. The methylated meta-cyclophane complex 57

Attempts to grow a crystal of 56 suitable for an X-ray diffraction study

by layering a solution of 56 in d6-DMSO with acetonitrile afforded crystals of

57. The structure obtained from the X-ray study (Figure 3.6), although poorly

resolved compared to that of the structures for 54.2PF , 54.2BPh 6 4 and

55.(Ag2Br4), was still informative and interesting. The structure 57

approximates two molecules of 56 linked by an (AgBr)2 unit. The extra silver

bromide in this compound may have come from the minor decomposition of 56

during the crystal-growth experiment.

N

N

N

NAgAg Br

Br

56

N

N

N

NAg

Ag BrBr

N

N

N

NAg

AgBrBr

Ag BrBr Ag

57

67

The mesitylene groups of the cyclophane units are mutually syn, and also

syn to the silver atoms. This result is consistent with the conformation of the

cyclophane in 56, determined from 1H NMR spectral data. The Ag-C bond

distances in 57 [2.138(2) (C122-Ag11) and 2.104(2) Å (C142-Ag12)] are

similar to those of the ortho- and meta-cyclophane silver complexes 54 and 55.

The Ag...Ag distances for the carbene-bound silver atoms are 2.996(2) and

3.003(3). Presumably the steric influences of the mesitylene rings lying above

and below the silver centres prevent this system from adopting the

[(cyclophane)2Ag2]2+ structure observed in 54 and 55.

Figure 3.6: Structure of 57 with the bridging (AgBr)2 unit.

68

3.3.2 The silver(I)-non-cyclophane bis-imidazolium complexes

I. The meta-linked bis-imidazolium complex 59

An attempt to grow crystals of 59.Br, for an X-ray diffraction study, by

the very slow evaporation of a concentrated solution of 59.Br in acetonitrile at

4 °C afforded crystals with the stoichiometry 59.HCO3.H2O. The

hydrogencarbonate presumably arises from the absorption of carbon dioxide

into solution, from the atmosphere. The hydrogencarbonate salt in this case

being more insoluble than the bromide salt. In the crystal structure, the

hydrogencarbonate ions and the water molecules are hydrogen-bonded together

as a linear chain. The mononuclear cation 59, shown in Figure 3.7, consists of

one silver atom bound by two carbenes in a linear fashion [C-Ag-C 178.4(1)°].

The two NHC rings are virtually co-planar, and perpendicular [89.7(8)°] to the

plane of the mesitylene ring. The carbene-silver bond lengths [2.093(2) (Ag-

C22) and 2.092(2) Å (Ag-C42)] are consistent with other silver(I)-NHC

complexes.168, 169, 197 The structure of the cation 59 is different when compared

to the structures of silver complexes 44174 and 51,14 which both also contain a

bis(NHC) ligand where the NHCs are linked by a meta-"xylyl" type spacer.

The differences may be a consequence of ligand types as well as the method of

preparation.

69

Figure 3.7: Structure of the cation 59.

N

N

N

NAg

59

51

+

N

N

N

NAg

44

N

N

N

NAg

N

N

2+

N

N N

N

Ag AgClCl

70

II. The para-linked bis-imidazolium complex 58

An attempt to grow crystals of 58.2Cl followed the method used above

that afforded crystals of 59.HCO3.H2O. The very slow evaporation of a

concentrated solution of 58.2Cl in methanol at 4 °C afforded crystals with the

unexpected stoichiometry assigned as 58.2NO3.2CH3OH, as determined by a

single crystal X-ray diffraction study. In this case either chloride or

hydrogencarbonate counter anions were expected, based on the method of

crystal growth adopted. The source of the nitrate is unknown and the crystals

were not further analysed, by methods such as by IR or NMR spectroscopy,

which may have shed further light on the assignment of the counter ions as

nitrates.

The structure contains two independent cations (58) in the asymmetric

unit cell; as a representative example, one of the independent cations is shown

in Figure 3.8, though the other cation is not significantly different. The two

cations are packed in a zig-zag arrangement in the unit cell. A view of the unit

cell is shown in Figure 3.9. The nitrate oxygen atoms are quite distant from the

silver atoms at 3.471(5) Å and 3.418(5) Å. Each silver atom is bound to two

carbenes in a linear fashion [173.1(2)° (cation 1) and 175.2(2)° (cation 2)] and

the associated NHC rings are essentially co-planar. The dihedral angles

between the two NHC rings and the phenyl rings are 74.0(2)° and 75.4(2)° for

cation 1 and 81.1(2)° and 76.9(2)° for cation 2.

71

Figure 3.8: Structure of the cation of 58.

72

Figure 3.9: The unit cell for 58.2NO3.2CH3OH.

3.4 NMR studies

3.4.1 The silver complexes of cyclophanes

The 1H and 13C NMR spectral data of complexes 54 - 56 are summarised

in Tables 3.2 and 3.3, respectively. The 1H NMR data for 54 and 55 are similar

to those of the Au(I) analogues.154 In the 1H NMR spectrum, each complex

exhibits two sets of sharp doublets for the benzylic protons, being an AX

pattern for 54 and AB patterns for 55 and 56, which suggests that each

complex has a rigid conformation in solution.152 Figure 3.11 shows the AX

benzylic pattern for 54.2PF6 and the AB patterns for 55.2PF6 and 56. The 1H

NMR spectra for 54.2PF6 displays a signal at ca. δ 6.3 (see Figure 3.11), which

is assigned as the NHC ring protons. The up-field shift of this signal compared

to the corresponding signals for 55.2PF6 (δ 7.3) and 56 (δ 7.6) is because of

their proximity to the magnetically anisotropic phenyl rings in 54. In the 1H

NMR spectra of complexes containing a meta-substituted arene unit, 55 and

73

56, the signal due to the arene substituent lying between the imidazolyl groups

shows an upfield chemical shift, consistent with shielding of the substituent by

the magnetically anisotropic imidazolyl units. For example, in the spectrum of

55, the arene H2 proton resonates at δ 5.67, unusually upfield for an aryl

proton and significantly upfield compared to the other arene protons, which

resonate at δ 7.07-7.18 [Figure 3.10, see also Figure 3.11(b)]. Similarly, the

methyl group between the imidazolyl substituents in 56 resonates at a higher

field than the other methyl groups (δ1.78 vs δ 2.15) (Figure 3.10).

N

N N

N N

NN

NAg