cyclononanes: the extensive chemistry of fundamentally simple …378128/uq378128... · 2019. 10....

Accepted Manuscript

Title: Cyclononanes: The extensive chemistry offundamentally simple ligands

Author: Lawrence R. Gahan

PII: S0010-8545(15)30035-7DOI: http://dx.doi.org/doi:10.1016/j.ccr.2015.11.011Reference: CCR 112170

To appear in: Coordination Chemistry Reviews

Received date: 21-8-2015Revised date: 12-11-2015Accepted date: 13-11-2015

Please cite this article as: L.R. Gahan, Cyclononanes: the extensive chemistryof fundamentally simple ligands, Coordination Chemistry Reviews (2015),http://dx.doi.org/10.1016/j.ccr.2015.11.011

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http://dx.doi.org/doi:10.1016/j.ccr.2015.11.011http://dx.doi.org/10.1016/j.ccr.2015.11.011

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Cyclononanes: the extensive chemistry of fundamentally simple ligands

Lawrence R Gahan

School of Chemistry and Molecular Biosciences,

The University of Queensland,

Brisbane 4072, Australia

Keywords:

Cyclononane

Ligand

Complex

Synthesis

Structure

Spectroscopy

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Contents

1. Introduction

1.1 Scope of the review

1.2 Nomenclature

1.3 Synthetic methodology

1.4 Donor Properties

1.5 Ligand Conformation

1.6 Geometric Isomers

2. 1-thia-4,7-diazacyclononane (1,4,7-thiadiazonane; [9]aneN2S) and N,N'-dimethyl-1-

thia-4,7-diazacyclononane (4,7-dimethyl-1,4,7-thiadiazonane; Me2[9]aneN2S)

2.1 Synthesis and properties

2.2 X-ray crystal structure of [9]aneN2S

2.3 Metal Complexes

2.3.1 Titanium

3.3.2 Vanadium(IV)

2.3.3 Chromium, Molybdenum and Tungsten

2.3.4 Manganese(II), Rhenium(I)

2.3.5 Iron(II/III), Ruthenium(II/III)

2.3.6 Cobalt(III), Rhodium(III),

3.3.7 Nickel(II/III), Platinum(II), Palladium(II)

2.3.7 Copper(II), Silver and Gold(III)

2.3.8 Zinc(II), Mercury(II) and Cadmium(II)

2.3.9 Indium(III), Thallium(I,III)

2.3.10 Lead(II)

2.3.11 Lithium

2.3.12 Stability constants

3. 1-Thia-4,7-diazacycyclononane-S-oxide (1,4,7-thiadiazonane 1-oxide; [9]aneN2S(O))

3.1 Synthesis

3.2 Metal Complexes

3.2.1 Cobalt(III)

3.2.2 Nickel(II), Platinum(II), Palladium(II)

3.2.3 Copper(II)

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4. 1,4-dithia-7-azacyclononane (1,4,7-dithiazonane ; [9]aneNS2) and N-methyl-1,4-

dithia-7-azacyclononane (7-methyl-1,4,7-dithiazonane; Me[9]aneNS2)

4.1 Synthesis

4.2 Metal Complexes

4.2.1 Iron(II), Ruthenium(II)

4.2.2 Ruthenium(II)

4.2.3 Rhodium(III)

4.2.4 Nickel(II/III), Palladium(II/III)

4.2.5 Copper(II), Silver(I), Gold(I)

4.2.6 Chemosensor Applications

5. 4,7-diaza-1-selenocyclononane (1,4,7-selenadiazonane; [9]aneN2Se)

5.1 Synthesis

6. 1-aza-4-oxa-7-thiacyclononane (1,4,7-oxathiazonane; [9]aneNOS)

6.1 Synthesis

6.2 Metal Complexes

6.2.1 Nickel(II)

7. 1-oxa-4,7-diazacyclononane (1,4,7-oxadiazonane; [9]aneN2O) and N,N'-dimethyl-1-

oxa-4,7-diazacyclononane (4,7-dimethyl-1,4,7-oxadiazonane; Me2[9]aneN2O)

7.1 Ligand Synthesis and Properties

7.2 X-ray crystal structure of [9]aneN2O.2HBr

7.3 Metal Complexes

7.3.1 Lithium(I)

7.3.2 Titanium(IV)

7.3.3 Vanadium(IV)

7.3.4 Manganese(II/III)

7.3.5 Iron (III)

7.3.6 Cobalt(II)

7.3.7 Nickel(II), Palladium(II)

7.3.8 Copper(II)

7.3.9 Zinc(II)

7.4 Lanthanide Complexes

7.5 Stability constants

8. 1,4-dioxa-7-azacyclononane (1,4,7-dioxazonane; [9]aneNO2)

8.1 Synthesis

9. 1-oxa-4,7-dithiacyclononane (1,4,7-oxadithionane; [9]aneOS2)

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9.1 Synthesis

9.2 Metal Complexes

9.2.1 Vanadium(III)

9.2.2 Chromium(III)

9.2.3 Cobalt(II/III)

9.2.4 Nickel(II), Platinum(II) and Palladium(II)

9.2.5 Copper(I/II), Silver(I), Gold(I/II/III)

9.2.6 Cadmium(II), Mercury(II)

9.2.7 Antimony(III)

10. 1-thia-4,7-dioxacyclononane (1,4,7-dioxathionane; [9]aneO2S)

10.1 Synthesis

11. 1,4-dithia-7-telluracyclononane (1,4,7-dithiatelluronane; [9]aneS2Te)

11.1 Synthesis

11.2 Metal Complexes

11.2.1 Manganese(I)

11.2.2 Rhodium(III)

12. 1-tellura-4,7-dioxoacyclononane (1,4,7-dioxatelluronane; [9]aneO2Te)

12.1 Synthesis


12.2.1 Palladium(II) and Platinum(II)

13. 1-selena-4,7-dioxacyclononane (1,4,7-dioxaselenonane; [9]aneO2Se)

13.1 Synthesis

14. 1,4,7-triphenyl-1,4,7-triphosphonane (based on [9]aneP3) and 5,10,15-tris(2-

fluorophenyl)-10,15-dihydro-5H-tribenzo[b,e,h][1,4,7]triarsonine

(tribenzo[9]aneAs3Ph,PhF2)

14.1 Syntheses and properties of metal complexes

15. 4-benzyldodecahydro-1H-benzo[b][1,4,7]diazaphosphonine ([9]aneN2P)

15.1 Synthesis and metal complexes

16. 1-Phenyl-1-phospha-4,7-dithiacyclononane (7-phenyl-1,4,7-dithiaphosphonane;

[9]aneP(Ph)S2)

16.1 Synthesis


16.2.1 Molybdenum

16.2.2 Iron(II)

16.2.3 Nickel(II)

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16.2.4 Copper(I)

16.2.5 Mercury(II)

17. Conclusions

Abstract

The cyclononane ligands are fundamental examples of the macrocyclic ligand. With three

donor atoms in a nine-membered ring they represent the simplest of the macrocyclic class.

However, this simplicity is deceptive, for within this class the variations are extensive and

perhaps hitherto unrecognized. As well as the relatively familiar 1,4,7-triaza- ([9]aneN3) and

1,4,7-trithia-([9]aneS3) cyclononanes whole families of cyclononane ligands are known with

thioether/nitrogen, nitrogen/oxygen, selenium, tellurium as well as phosphorus and arsenic

donors. The chemistry of some of these ligands, for example [9]aneN2S, [9]aneNS2 and

[9]aneN2O has been reported extensively, and some examples have simply been reported as

having been synthesised (for example [9]aneO2S and [9]aneO2Se) with little other

information. This review attempts to introduce the extensive chemistry of what are

fundamentally the simplest examples of macrocyclic ligands.

1. Introduction

Macrocyclic ligands are defined as polydentate ligands containing the donor atoms

incorporated in a cyclic backbone and containing at least three donor atoms, the macrocyclic

ring consisting of a minimum of nine atoms [1]. As such, the cyclononane ligands are

representatives of the ultimate definition of the macrocyclic ligand. Of the cyclononanes the

chemistry of 1,4,7-triazacyclononane ([9]aneN3) and 1,4,7-trithiacyclononane ([9]aneS3) has

been widely reported and the contribution of these ligands to our understanding of all areas of

coordination chemistry has been significant. The chemistry and applications of [9]aneN3 have

been reviewed extensively [2-38], the [9]aneS3 ligand less so [39].

These cyclononane ligands are of interest for a number of reasons, including:- (i) the

small cyclononane ring invariably means that complexation with a metal ion occurs on one

face, rather than the more traditional macrocyclic coordination mode of the metal ions placed

within the macrocyclic ring; (ii) within this facial arrangement, cyclononane ligands may

adopt an endodentate (the donor electron pairs point out of the cavity of the macrocycle,

essentially away from the metal ion) or exodentate conformation of the donors (the electrons

on the donor atom point into the cavity of the macrocycle towards the metal ion), depending

on the nature of the donors and the metal ion; (iii) a variety of donor atoms are represented –

examples containing secondary and tertiary amines, thiaether, oxygen, phosphorus, arsenic,

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selenium and tellurium donors have been synthesised; and, (iv) given the scope of donor

atoms available, complexation to many of the metal ions in the periodic table is possible.

1.1 Scope of the review

This review focuses on the cyclononane macrocycles containing mixed donors and their

transition metal complexes; the chemistry of the oxygen analogue [9]aneO3 is not considered

[40-42, 43 , 44 , 45, 46 , 47-56]. An excellent review, published in 1998, discusses the

chemistry of the mixed nitrogen and sulfur donor cyclononane and cyclodecane macrocycles

[57]. That review covered the chemistry of the [9]aneN2S, [9]aneNS2, [10]aneN2S and

[10]aneNS2 ligands, as well as the known analogues of these systems [57]. In order to allow

full appreciation of the extensive chemistry of these nitrogen- and sulfur-tridentate ligands,

and the subsequent extensions of the chemistry of cyclononane ligands, it has been necessary

to recap some of the examples in that earlier report in order to allow a more complete

perspective of the developments in this chemistry post-1998. Where figures depicting the

structures of metal complexes under discussion have been included in the previous review,

these figures are not duplicated here. Due acknowledgement of the content of that earlier

review is given at his point [57].

1.2 Nomenclature

As the chemistry of the cyclononane ligands developed the nomenclature employed to

describe them has been systematised, to some extent.

Initial papers reported the 1,4,7-triazacyclonane ligand as “tacn” [7] although the

nomenclature [9]aneN3 has now mostly replaced this initial name. The IUPAC nomenclature,

1,4,7-triazonane, has been employed in some instances [58-67]. The initial papers describing

1-dithia-4,7-azacyclononane abbreviated the name to “tasn” [68-70] in line with the “tacn”

nomenclature and this nomenclature is still in use in some reports [71] although the

abbreviation [9]aneN2S is common, although not strictly correct in relation to the IUPAC

nomenclature, 1,4,7-thiadiazonane. In this work, as much as is practicable, the [9]aneXYZ

nomenclature will be applied, except in instances where the report details a more complex

synthetically elaborated form of the ligand and in that case the nomenclature employed in the

report will be used. Where possible, the IUPAC ligand name will be introduced.

The ligands discussed in this review include, 1-thia-4,7-diazazcyclononane

([9]aneN2S) and 1,4-dithia-7-azacyclononane ([9]aneNS2), and in more detail the perhaps

less well known examples of the cyclononane ligands including, 1-oxa-4,7-

diazacyclononane ([9]aneN2O), 1,4-dioxa-7-azacyclononane ([9]aneNO2), 1-oxa-4,7-

dithiacyclononane ([9]aneOS2), 4,7-diaza-1-selenocyclononane ([9]aneN2Se), 1-aza-4-oxa-

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7-thiacyclononane ([9]aneNOS), 1-thia-4,7-dioxacyclononane ([9]aneO2S), 1,4-dithia-7-

telluracyclononane ([9]aneS2Te), 1-tellura-4,7-dioxoacyclononane ([9]aneO2Te), 1-selena-

4,7-dioxacyclononane ([9]aneO2Se), [9]aneP3, [9]aneN2P, and [9]aneAs3.

In addition, examples of structurally enhanced cyclononane ligands including 1-thia-

4,7-diazacyclononane-N,N’-diacetic acid [72], dimethyl 1-thia-4,7-diaza-4,7-

cyclononanediacetate, [73] 4,7-bis(2-methylpyridyl)-1-thia-4,7-diazacyclononane [74], 4,7-

bis(hydroxyethyl)-1-thia-4,7-diazacyclononane [75], 4,7-bis(2-hydroxy-2-methylpropyl)-1-

thia-4,7-diazacyclononane [75], 4,7-bis(2-cyclohexyl-2-hydroxymethyl)-1-thia-4,7-

diazacyclononane [75], 4,7-bis(2-cyanoethyl)-1-thia-4,7-diazacyclononane [76-78], 15-thia-

1,5,8,12-tetraazabicyclo[10.5.2]nonadecane [77], 4,4'-((4,5-dihydro-1H-pyrazole-3,5-

diyl)bis(methylene))bis(1,4,7-thiadiazonane) [79] and 4,7-bis(3-aminopropyl)-1-thia-4,7-

diazacyclononane [77, 78, 80] and their metal complexes are discussed throughout. In these

cases the literature reports employ numerous nomenclature systems, (for example the

abbreviations (py2[9]aneN2S; bmmpTASN; L1, L2, etc), and in these cases the

nomenclature employed in the actual report has been used, recognising that this may be

repetitive and/or ambiguous in some cases.

In cases where the X-ray crystal structures of metal complex cations or anions are

shown, the hydrogen atoms and counter ion have been removed for clarity. The structures

were obtained through ConQuest V1.17 based on data deposited at the Cambridge

Crystallographic Data Centre [81, 82].

1.3 Synthetic methodology

The synthetic methodologies employed for the cyclononane ligands are generally

relatively straightforward and in the majority of cases in what follows a brief description of

the synthetic approach is given but not a synthetic scheme; there are some exceptions. The

syntheses usually involve the cyclic coupling of two linear moieties in the presence of a base

and most often in dimethylformamide (DMF) solution. Tosylated oligomeric glycols are

commonly utilized as one component, the synthetic challenge usually being the form of the

second fragment containing the heteroatoms; for example, in the cases of the syntheses of

[9]aneO2Te and [9]aneO2Se. The synthetic approaches to the [9]aneP3, [9]aneN2P, and

[9]aneAs3 ligands are more elaborate and are considered in some detail.

1.4 Donor Properties

Nitrogen donors, in contrast to thioethers, are considered to be hard bases, having strong

affinity for hard acids like Co(III), Fe(III) and Cr(III) [83] as well as alkali and alkali earth

cations [84]. The lone pair of electrons for amine ligands allows for σ-donation to metal ions

but not π-interactions. In comparison to donors like nitrogen and phosphorus [85, 86]

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thiaether donors bind transition metals relatively weakly [85-87], the weak bonding attributed

to the relatively low σ-donor and π-acceptor abilities of thiaethers. The sulfur donor can also

act as a π-acceptor where its empty d-orbitals can take part in metal to sulfur π-back donation

if the symmetry is suitable [85]; there is little evidence for thiaethers acting as π-donors [85].

There are a number of characteristics of thiaether complexes which can be rationalised by the

π-acceptor ability of sulfur donors (i) they have a tendency to stabilise lower oxidation states

of metal ions, in comparison to amine donors [86, 88]; (ii) they usually enforce low spin

conditions on transition metal states that commonly exhibit high spin behaviour [89-97], a

property believed to be due to the delocalisation of t2g electron density into ligand π* orbitals

by π-back donation, thus lowering electron-electron repulsion causing the reduction of the

spin pairing energy [88]; and, (iii) thioether donors are found lower in the nephelauxetic

series relative to aqua and amine ligands [98]. Phosphorus-containing ligands such as 1-

phenyl-1-phospha-4,7-dithiacyclononane are suggested to exert a stronger ligand field than

[9]aneS3 [99]. The donor properties of selenium and tellurium have not been extensively

discussed [100, 101].

1.5 Ligand Conformation

The classification scheme employed in this review to describe the conformations of the

chelate rings in the nine-membered macrocycles is largely that of Dale [102-104]; there are

a number of reports containing excellent explanations of the classification scheme [105,

106]. Briefly, the classification is based on the torsion-angle sequences between the anti

and gauche bonds in the cyclononane ring, although it is applicable to larger macrocyclic

rings. A series of numbers is used to define the conformation, each number designating the

number of chemical bonds between what are called consecutive genuine corners. A genuine

corner occurs when the central atom of an anti–gauche–gauche–anti bond has both

consecutive gauche torsion angles of the same sign; a pseudo corner occurs when they are of

opposite sign. The starting point and direction followed around the ring are chosen so as to

give the smallest number [102-106]. For cyclononane rings the three lowest energy

conformations are [333] , [234] and [12222]; these are illustrated in Figure 1 [102-104].

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Figure 1: Conformations of cyclononane ligands [102-104]

Molecular mechanics calculations indicate that the strain energy for complexes of

cyclononane macrocycles is always less for the [333] conformer than for the [234]

conformer [107]. For example, for the [Ni([9]aneN3)2]2+ and [Ni([9]aneS3)2]

2+ complexes

the energy difference between the [234] and [333] conformations was considerable (35.7

and 28.1 kcal mol-1 and 21.4 and 12.8 kcal mol-1, respectively) suggesting why the [333]

conformer was prevalent in these complexes and suggesting a reason for the rarity of the

[234] conformer [107]. In the case of the mixed donor systems the differences are much

smaller. For example for [Ni([9]aneN2S)2]2+ calculations show that for the [234] conformer

the strain energy is 28.6 kcal mol-1 whereas for the [333] conformer it is 25.1 kcal mol-1; for

the [Ni([9]aneNS2)2]2+ the calculated difference is also small (27.8 compared with 19.2 kcal

mol-1) [107]. The [234] conformer would therefore be expected to be more common in the

mixed donor systems. An extensive list of ring conformations for complexes with [9]aneN3

and [9]aneS3 ligands, in addition to a limited number of examples of [9]aneN2S, and

[9]aneN2O complexes has been reported [106].

1.6 Geometric Isomers

For metal complexes of the [9]aneN3 and [9]aneS3 (and [9]aneP3 and [9]aneAs3 analogues,

vide infra) ligands, facial η3 coordination of the ligands to the metal ion is commonly, but

not exclusively, found. In the case of the [9]aneX2Y type ligands complexes with both cis-

and trans-diastereoisomers are possible, and in some instances have been isolated. The

trans-, or meso-, isomer has C2h symmetry, whereas the cis-isomer displays C2 symmetry.

The possible geometric forms are shown diagrammatically in Figure 2.

-135+55

+55

-135

+55+55

-135

+55

+55+65 -85

-45

+150

-55

-40+155

-140

+50

[333] [234]

+110

-45

-85

+75

-85

-45

+110

[12222]

-100

+75

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Figure 2. Possible geometric isomers for [M([9]aneX2Y)2]n+ complexes [108, 109].

2. 1-thia-4,7-diazacyclononane (1,4,7-thiadiazonane; [9]aneN2S) and N,N'-dimethyl-

1-thia-4,7-diazacyclononane (4,7-dimethyl-1,4,7-thiadiazonane; Me2[9]aneN2S)

2.1 Synthesis and properties

The synthesis of [9]aneN2S (the ligand was named “tasn” in that paper) was reported

in 1982; at the time no synthetic details were given, the method described as similar to that

employed for the [9]aneN3 analogue using the Richman-Atkins procedure [68, 110]. The

manuscript described the preparation and isolation of the [Co([9]aneN2S)2](ClO4)3 complex

[68].

Subsequently, the syntheses of the [9]aneN2S ligand, and derivatives, have been

reported a number of times [75, 111-117]. The synthetic strategy typically involved the use of

bis(2-aminoethyl)sulfide, the synthesis of which traditionally employed the reaction of

ethyleneimine and H2S [118-121]. Other syntheses of bis(2-aminoethyl)sulfide have

employed NH2CH2CH2OSO3H with NaSH and S or Na2S and S [122], and the treatment of

cysteamine hydrochloride with chloroethylamine hydrochloride in the presence of NaOH

[123] or reaction of 2-bromoethylamine hydrobromide with sodium hydroxide and sodium

sulfide nonahydrate [124]. Reaction of the bis(2-aminoethyl)sulfide with toluenesulfonyl

chloride, resulting in N,N'-(thiobis(ethane-2,1-diyl))bis(4-methylbenzenesulfonamide), and

subsequent reaction of the disodium salt with ethane-1,2-diyl bis(4-methylbenzenesulfonate)

in dimethylformamide resulted in 4,7-bis(tolyl-p-sulfonyl)-1-thia-4,7-diazacyclononane.

Removal of the protecting groups with, for example, hydrobromic acid/acetic acid [68, 75],

X Y X

XYX

X Y X

YXX

M M

trans- or meso-diastereoisomer cis-diastereoisomer

S

NH

HN

[9]aneN2SS

N

N

Me2[9]aneN2S

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or lithium in ammonia [75], resulted in the desired ligand, in varying yields. The ligand

displays two pKa values of 9.67(2) and 3.98(2) [125]. The analogue Me2[9]aneN2S was

prepared by refluxing [9]aneN2S.2HBr with formic acid and formaldehyde [126].

Analogues such as N,N'-(((1,4,7-thiadiazonane-4,7-diyl)bis(methylene))bis(1-methyl-

1H-imidazole-2,4-diyl))bis(2,2-dimethylpropanamide) and 4,7-bis(2-thiophenoyl)-1-thia-4,7-

diazacyclononane have been prepared by reaction of the [9]aneN2S with N-(2-formyl-1-

methyl-1H-imidazol-4-yl)pivalamide [127], and 2-chlorocarbonyl)thiophene [128],

respectively (Figure 3).

(a) (b)

Figure 3. (a) N,N'-(((1,4,7-thiadiazonane-4,7-diyl)bis(methylene))bis(1-methyl-1H-

imidazole-2,4-diyl))bis(2,2-dimethylpropanamide); (b) 4,7-bis(2-thiophenoyl)-1-thia-4,7-

diazacyclononane

The salt 4-(2-bromoacetyl)-8-oxo-1-thionia-4,7-diazabicyclo[5.2.2]undecane bromide

resulted after reaction of [9]aneN2S with bromoacetyl bromide in chloroform (Figure 4). The

crystal structure shows that two salt moieties are linked by S···Br contacts about a

crystallographic inversion centre, forming dimers linked by halide-halide contacts into

extended ribbons [129].

Figure 4. 4-(2-bromoacetyl)-8-oxo-1-thionia-4,7-diazabicyclo[5.2.2]undecane bromide

Further examples of derivatised [9]aneN2S ligands and the metal complexes formed

from them, are discussed in the following text.

N

N

OHN

S

NN

NHO

N

N

S

S

S

N

N

S+

N

N OO

Br

Br-

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2.2 X-ray crystal structure of [9]aneN2S

The X-ray crystal structure of [9]aneN2S [130] has been reported. The hydrogen-

bond networks in mono- and diprotonated [9]aneN2S have been investigated [130]. The

structure of [9]aneN2S·HCl consists of a monoprotonated [9]aneN2S ring and a chloride anion

(Figure 5).

Figure 5. Crystal structure of [9]aneN2S·HCl [130]

The protonated nitrogen serves as a hydrogen-bond donor to the free amine via an

intramolecular interaction. In addition to the intramolecular H-bonding interaction, each

[9]aneN2S·HCl unit serves as both an H-bond donor and an H-bond acceptor via

intermolecular interactions [130]. The stronger of the two interactions occurs between the

protonated nitrogen and chloride. The chloride anions of [9]aneN2S⋅HCl align in interlocking

columns in the a- and c-direction. The structure of [9]aneN2S·2HBr contains a doubly

protonated [9]aneN2S ring and two bromide anions. The diprotonated [9]aneN2S⋅2HBr also

exhibits the ribbon-like network with strong intermolecular and weak intramolecular N-

H⋅⋅⋅Br hydrogen bonds. The intramolecular and intermolecular interactions result in an

extended array in the b-direction. These ribbons align in the crystal in an interlocking or

“zipper”-like arrangement, although no notable inter-strand contacts apparent. The extended

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H-bonding network yields a columnar array in the b-direction. The bromine columns fill a

channel that runs through the crystal along the c-direction [130].

2.3 Metal Complexes

2.3.1 Titanium

Reaction of [Ti(NBut)Cl2(py)3] with Me2[9]aneN2S in dichloromethane at room temperature

resulted in the air and moisture sensitive orange complex [Ti(NBut)Cl2(Me2[9]aneN2S)] [131,

132]. The Ti(IV) ion is coordinated by the facially arranged Me2[9]aneN2S macrocycle, two

mutually cis-chlorido ligands and the multiply bonded tert-butylimido ligand (Figure 6).

Figure 6. Crystal structure of [Ti(NBut)Cl2(Me2[9]aneN2S)] [131,

132]

The Ti-N(cyclononane) bonds cis- (2.285(9) Ǻ) and trans- (2.498(8) Ǻ) show the trans-effect of

the tert-butylimido ligand with the Ti-Nimide 1.708(8) Ǻ; the Ti-S bond distance is 2.561(4) Ǻ

and Ti-Cl distances 2.379(4) and 2.397(4) [131, 132]. The 13C NMR of the complex

displayed a ten line spectrum consistent with a C1 symmetric arrangement of the macrocycle

with the thiaether donor cis to the imido group; the sharp 13C and 1H NMR resonances

suggested that the complex was conformationally rigid in solution [132].

3.3.2 Vanadium(IV)

The light blue vanadium(IV) complex [VOCl2([9]aneN2S)].CH3CN was obtained after

reaction of VCl3 and [9]aneN2S in refluxing acetonitrile under nitrogen, the oxidation to

V(IV) attributed to trace amounts of oxygen during crystallisation of the complex [133].

The X-ray structure shows that in [VOCl2([9]aneN2S)].CH3CN the metal ion is coordinated

facially by cyclononane ligand, the thiaether and the vanadyl oxygen arranged trans to each

other, the chloride ligands mutually cis and the metal ion is six coordinate. The metal ion

sits 0.32 Å above the plane of the nitrogen and chloride donors. The V-O bond is short

(1.632(2) Ǻ) and the V-S bond trans is elongated (2.689(1) Ǻ) suggesting a strong trans

influence; the same effect is seen in the [VOCl2([9]aneS3)] analogue [134]. The V-N bond

lengths (2.153(2) Ǻ, 2.150(2) Ǻ) are similar to those observed for other oxovanadium

complexes, for example [V2O2(µ-OH)2([9]aneN3)2]Br2 (2.151(5)-2.303(6) Ǻ) [135]. The V-

Cl bond lengths (2.346(1) Ǻ, 2.337(1) Å) are longer than in the related [VOCl2([9]aneS3)]

complex (2.295(5)average Ǻ) [134]. The IR spectrum of the complex shows sharp ν(NH)

absorptions at 3240 and 3200 cm-1, the ν(VO) vibration shifted to 985 cm-1 as compared with

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the [VOCl2([9]aneS3)] complex (962 cm-1) as a result of the weak bonding of the thiaether in

[VOCl2([9]aneN2S)].CH3CN [133, 134]. For the d1 system, three vanadyl(IV) d–d

transitions at 27624 cm-1 (ε = 67 M-1 cm-1), 19800 cm-1 (ε = 21 M-1 cm-1) and 16474 cm-1 (ε

= 63 M-1 cm-1) were observed with Dq = 19 800 cm-1 [133]. The higher than expected

ligand field exerted by [9]aneN2S was confirmed by the EPR parameters, with 51V

hyperfine coupling constants Aiso = 91.2 × 10-4 cm-1 at g = 1.986 and A|| = 161.1 × 10

-4

cm-1 at g|| = 1.965 [133].

2.3.3 Chromium, Molybdenum and Tungsten

The chromium(III) complex [Cr2(OH)(O2CMe)2(Me2[9]aneN2S)2](ClO4)3 was

prepared following the procedure employed for the analogous

[Cr2(OH)(O2CMe)2(Me3[9]aneN3)2](ClO4)3 complex [136, 137]. The X-ray crystal structure

of [Cr2(OH)(O2CMe)2(Me2[9]aneN2S)2](ClO4)3 shows Cr-N bonds lengths (2.15(2) Ǻ) longer

than those for the Me3[9]aneN3 analogue (2.102(6) Ǻ) [136, 137]. Fitting of the magnetic

susceptibility data (4.2 – 300 K) for exchange coupled pairs of chromium(III) ions (S1 =3/2,

S2 =3/2; H = -2JS1·S2) for [Cr2(OH)(O2CMe)2(Me2[9]aneN2S)2](ClO4)3 showed that the metal

ions were antiferromagnetically coupled (J = -15 cm-1), the coupling very similar to that

observed for the Me3[9]aneN3 analogue (J = -15.5 cm-1) [137].

Reaction of Mo(CO)6 with [9]aneN2S in ethanol resulted in isolation of

[Mo([9]aneN2S)(CO)3]; a similar approach resulted in [W([9]aneN2S)(CO)3] [113]. The

infrared spectrum of the molybdenum complex shows bands at 1913, 1783 and 1713 cm-1

assigned to the carbonyl group. The X-ray crystal structure of [Mo([9]aneN2S)(CO)3] shows

the facial arrangement of the cyclononane macrocycle [113], the ring having the [333]

conformation [102] with Mo-N distances of 2.317(5) Ǻ and 2.292(5) Ǻ and Mo-S 2.526(2) Ǻ

[113]. The three Mo-C distances were 1.924(6), 1.926(6) and 1.952(6) Ǻ, the shorter bond

trans to the thiaether donor [113]. Both [Mo([9]aneN2S)(CO)3] and [W([9]aneN2S)(CO)3]

were employed to prepare nitrosyl-, halogeno, and for the former, oxomolybdenum

complexes [138]. Complexes with M = Mo or W were obtained commencing with the

appropriate [M([9]aneN2S)(CO)3] complex. Thus, reaction of [M([9]aneN2S)(CO)3] with an

aqueous solution of sodium nitrite, with subsequent reaction with HCl, resulted in isolation of

[M([9]aneN2S)(CO)2(NO)](PF6). When [M([9]aneN2S)(CO)3] was dissolved in 1 M HCl

and reacted with aqueous sodium nitrite the product obtained was

[M([9]aneN2S)(NO)2Cl](PF6) (M = Mo, W); a similar approach with HBr resulted in the

analogous [W([9]aneN2S)(NO)2Br](PF6) complex [138]. The infrared spectrum of

[Mo([9]aneN2S)(CO)2(NO)](PF6) showed carbonyl stretching frequencies at 2010 and 1940

cm-1 and an NO stretch at 1670 cm-1; the nitrosyl stretching frequencies for

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[Mo([9]aneN2S)(NO)2Cl](PF6) were observed at 1790 and 1690 cm-1 reportedly typical for

dinitrosyl compounds with linear M-N-O groups. The complex [Mo([9]aneN2S)Cl3] was

prepared after reaction of [Mo([9]aneN2S)(CO)3] with concentrated HCl [138]. Reaction of

[Mo([9]aneN2S)(CO)3] in trichloromethane with bromine resulted in the isolation of

[Mo([9]aneN2S)(CO)3Br](PF6); the analogous tungsten complex was not reported [138].

[Mo2([9]aneN2S)2O4](ClO4)2 was isolated after reaction of [Mo([9]aneN2S)(CO)3] with

HClO4; reaction of this product with HCl resulted in the green complex

[Mo2([9]aneN2S)2O3Cl2](ClO4)2 [138]. The [Mo2([9]aneN2S)2O4](ClO4)2 complex

displayed infrared and Raman bands at 965, 940, 735, 715, 450 and 445 cm-1 typical of a syn-

Mo2O4 structure [138]. For [Mo2([9]aneN2S)2O3Cl2](ClO4)2 the infrared spectrum displayed

bands at 905 and 775 cm-1 assigned to υMo=O and υas(Mo-O-Mo) of the Mo2O3 core [139, 140].

The X-ray structures of the complexes [Mo([9]aneN2S)Cl3], [Mo([9]aneN2S)(NO)2Cl](PF6),

[Mo([9]aneN2S)(CO)2(NO)](PF6) and [Mo2([9]aneN2S)2O4](ZnCl4).H2O have been reported

[138]. The structures of [Mo([9]aneN2S)(CO)2(NO)](PF6) and

[Mo([9]aneN2S)(NO)2Cl](PF6) [138] show that in both complexes the metal ions are six-

coordinate with the cyclononane ligand coordinated facially. In the former complex ion, the

strong π-acid ligand NO is trans to a nitrogen donor of the macrocycle, underlining the π-

acceptor properties of the thiaether. In [Mo([9]aneN2S)(NO)2Cl](PF6) the NO ligands are

again trans to the cyclononane amine donor, with the thiaether then trans to the weakly π-

accepting chlorido ligand. In the two complexes the Mo-NO bond distances differ

considerably ([Mo([9]aneN2S)(CO)2(NO)](PF6), 1.874(3) Ǻ;

[Mo([9]aneN2S)(NO)2Cl](PF6), 1.819(7) Ǻ) the difference attributed to the different number

of strong π-acid ligands, which share the back-donation [138]. The mixed donor sets on the

[Mo([9]aneN2S)(CO)2(NO)](PF6) and [Mo([9]aneN2S)(NO)2Cl](PF6) complexes offer the

possibly of isomeric forms. Based on the spectroscopic properties and the structural

parameters for the [Mo([9]aneN2S)(CO)3] complex, the CO ligand trans to the thiaether is

less strongly bound than the CO ligands in the cis positions [138]. The expectation would

have been therefore that the former CO group would be replaced by the NO ligand. In

contrast, the entering ligand NO is a better π-acceptor than CO and may preferentially bind

trans to the N donor atoms, if the thiaether sulfur atom possesses some π-acidity.

The molecule [Mo([9]aneN2S)C13] is chiral; the five-membered chelate rings formed

by the cyclononane ligand have either the δδδ or λλλ conformation. The complex crystallises

in an acentric space group (Pn21a) which contains both enantiomers [138]. The Mo-N bond

distances are short (2.193(3) and 2.209(4) Ǻ) reflecting the 3+ oxidation state of the Mo. The

Mo-S bond (2.467(1) Ǻ) is longer than in [Mo([9]aneN2S)(NO)2Cl](PF6) (2.447(2) Ǻ) and the

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trans Mo-Cl bond in the latter complex (2.440(2) Ǻ) is longer than those in

[Mo([9]aneN2S)C13] (2.425(1), 2.408(1), 2.426(1) Ǻ), the difference related to π-acceptor

properties of the thiaether group. Interestingly, reaction of [Mo([9]aneN2S)C13] with nitric

acid led to isolation of a colourless solid, the infrared spectrum of which contained a strong

band at 1150 cm-1, assigned as an S=O stretching frequency [138]. Previously it has been

reported that reaction of [9]aneN2S with sodium bromite resulted in clean oxidation of the

thiaether to a sulfoxide with formation of 1-thia-4,7-diazacyclononane-S-oxide [115, 141].

The X-ray crystal structure of the orange [Mo2([9]aneN2S)2O4]2+ complex consists of

two [(1-thia-4,7-diazacyclononane)oxomolybdenum(V)] cations linked through a bis(µ-oxo)

moiety, the Mo=O groups in the syn position, and the Mo-Mo distance 2.549(1) Ǻ [138].

The [9]aneN2S ligands were rotationally disordered at both molybdenum sites such that each

macrocycle was attached to the metal atom statistically in two different orientations. This

rotational disorder had the effect that only one coordination site in the equatorial planes at the

two molybdenum atoms is occupied exclusively by nitrogen. The other equatorial sites and

the axial sites trans to the terminal oxo ligands are occupied statistically by the thiaether and

nitrogen donor atoms. The site occupation factors at the metal atoms showed that the trans

position was preferred by the thiaether group with a probability of 70-80% [138]. The

conformation of the five-membered chelate rings in [Mo2([9]aneN2S)2O4]2+ was δλλ at one

Mo site and λδδ at the other [138].

2.3.4 Manganese(II), Rhenium(I)

Reaction of manganese(II) perchlorate and [9]aneN2S in methanol solution resulted in

isolation of [Mn([9]aneN2S)2](ClO4)2 [124, 133]. The X-ray crystal structure shows that all

donors are coordinated to the metal ion, the thiaether donors are trans and the nitrogen donors

in the equatorial plane of the molecule. At 298 K the magnetic moment for the complex was

5.82 µB, indicative of a high spin (S = 5/2) configuration. The magnetic moment remained

constant over the range 300 to 80 K and upon lowering the temperature to 4.2 K the magnetic

moment was reduced to 5.65 µB. The decrease in magnetic moment was ascribed to second

order spin orbit coupling, giving rise to zero-field splitting of the 6A1g state [124]. Computer

simulation of the Q-band EPR spectrum for the complex yielded g = 1.99±0.01, ׀D׀ =

0.19±0.005 cm-1 and E/D = 0.04±0.02 [124]. These values were compared with those for the

analogous [Mn([9]aneN3)2](ClO4)2 complex, with g = 1.98±0.01, ׀D0.090±0.003 = ׀ cm-1 and

E/D = 0.10±0.01, the larger value of D for the former complex reflecting the larger axial

distortion induced by the axial thiaether donors [124].

The bimetallic complex [Mn2(Me2[9]aneN2S)2(µ-OH)(µ-CH3COO)2](ClO4) has been

prepared but not structurally characterised [126]. Variable temperature (300-2K) magnetic

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susceptibility of a powered sample exhibited a decrease in the effective magnetic moment

from 7.63 µB/molecule at 300 K to 1.36 µB/molecule at 4.2 K, indicative of antiferromagnetic

coupling between the two manganese(II) sites. A least squares fit of the molar

susceptibilities using the general isotropic exchange Hamiltonian H = -2JS1·S2, where S1 = S2

= 5/2 produced J = -3.6 cm-1 with g = 1.92, Ɵ = -0.87 K and p = 1.10 (percent monomeric

paramagnetic impurity) [126]. In solution the magnetic moment was determined to be 7.4

µB/molecule, in agreement with the solid state measurement at 300 K indicating that the

complex retained its integrity in solution [126]. For the [Mn2(Me3[9]aneN3)2(µ-OH)(µ-

CH3COO)2](ClO4) analogue, J = -9 cm-1 with g = 1.98 [142]. The solid state X-band EPR

spectrum of [Mn2(Me2[9]aneN2S)2(µ-OH)(µ-CH3COO)2](ClO4) at 130 K displayed a group

of intense transitions at g = 2, a group at half field showing fine structure at g = 4 and a broad

peak at g = 16. The hyperfine structure observed on the intense peak and the peak at half

field (0.0570 T) exhibited a hyperfine coupling constant of A = 42 x 10-4 cm-1, a value half

that for mononuclear Mn(II) complexes (typically A = 90 x 10-4 cm-1) and confirming that the

complex is dinuclear. The intense transitions at g = 2 were assigned to ∆Ms = ±1 transitions

and the weaker half field transitions at g = 4 to the ∆Ms = ±2 transitions. The presence of the

∆Ms = ±2 transitions indicate coupling of the two Mn(II) ions. The broad peak at g = 16 was

tentatively assigned to the ∆Ms = ±4 transitions [126]. Variable temperature (110-10 K)

studies on the solid and frozen solution (acetonitrile:toluene glass) were used in order to

corroborate the magnetic susceptibility studies. Analysis of the data fitted to an excited S = 2

manifold resulted in a coupling of -3.2 to -3.4 cm-1, in good agreement with the data obtained

from the solid state susceptibility measurements [126]. Analysis of the axial field splitting,

cm-1, in terms of the magnetic dipole interaction between the two Mn(II) ions 0.073 = ׀D׀

resulted in a calculated metal-metal separation of 3.28 Ǻ [126].

Reaction of [9]aneN2S with picolyl chloride hydrochloride in the presence of base

resulted in isolation of the pentadentate ligand 4,7-bis(2-pyridylmethyl)-1-thia-4,7-

diazacyclononane (py2[9]aneN2S) (Figure 7(a)) [117]. The Mn(II) complex

[Mn(py2[9]aneN2S)Cl](CF3SO3) was isolated and structurally characterised (Figure 7(b)).

N

N

S

N

N

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(a) (b)

Figure 7. (a) 4,7-bis(2-pyridylmethyl)-1-thia-4,7-diazacyclononane (py2[9]aneN2S), and (b)

Crystal structure of [Mn(py2[9]aneN2S)Cl]+ [117]

Of particular interest were the relative conformations of the pendant arms on the

diazacyclononane ring. Two coordinating conformers are possible, one with the pendant

arms opposite to (α) or on the same side (β) of the metal ion. The β conformer leads to a

trigonal prismatic geometry at the metal centre whereas the α conformer leads to a distorted

octahedral geometry. The [Mn([9]N2Spy2)Cl](CF3SO3) complex displayed a coordination

geometry closer to trigonal prismatic. The complex exhibited a magnetic moment of 5.90 µB,

indicative of a high spin d5 configuration. The cyclic voltammogram displayed a Mn(II/III)

couple at 0.92 V (NHE) with a peak separation of 80 mV [117].

The rhenium complex [Re([9]aneN2S)(CO)3]Br has been isolated after reaction of the

ligand with [Re(CO)5Br] in DMF [113]. The complex shows IR bands at 2030, and 1980-

1860 cm-1 assigned to carbonyl vibrational modes. The X-ray crystal structure of the

complex shows Re-N distances of 2.209(6) Ǻ, 2.196(6) Ǻ and 2.441(2) Ǻ, shorter than the

Mo analogue (2.317(5) Ǻ and 2.292(5) Ǻ and Mo-S 2.526(2) Ǻ) [113]. The three Re-C

distances 1.900(7) Ǻ, 1.927(7) Ǻ and 1.912(7) Ǻ, do not follow the trend seen for the Mo

analogue, with the bond trans to the thiaether donor (1.912(7) Ǻ) being in the middle of the

range seen [113].

Reaction of dirhenium heptaoxide with [9]aneN2S in tetrahydrofuran resulted in a

rhenium(VII) half-sandwich complex formulated as [ReO3([9]aneN2S)][ReO4] [133]. The

Raman spectrum showed two strong, well resolved bands of equal intensity at 972 and 964

cm-1 assigned to the symmetric stretching vibrations of ReO3+ and ReO4

-. A vibration at 910

cm-1 and a shoulder at 930 cm-1 in the IR spectrum were assigned to the asymmetric

stretching vibrations of ReO3+ and ReO4

- after comparison with similar IR bands seen for

the analogous complexes[ReO3([9]aneN3)][ReO4] and [ReO3([9]aneS3)][ReO4] (935 and 909,

921 and 912 cm-1, respectively) [133, 143, 144].

2.3.5 Iron(II/III), Ruthenium(II/III)

The synthesis and characterisation of the iron(II) complex [Fe([9]aneN2S)2](ClO4)2 has been

reported [94, 133]. The metal ion has the trans-N4S2 coordination although only one of the

possible geometric isomers has been characterised. At 81 K the complex crystallised in the

monoclinic space group P21/c with the Fe atom on an inversion centre [133]. At room

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temperature the orthorhombic space group Pmcb was found and the complex was disordered

in order to accommodate the symmetry [94]. The Fe-S and Fe-N bond lengths in both

structures were indistinguishable [94, 133]. The structure of the corresponding iron(III)

complex [Fe([9]aneN2S)2](ClO4)3 shows a centrosymmetric cation, the iron lying on a 2/m

symmetry site, and the two [9]aneN2S ligands facially coordinated [94]. The thiaether donors

were in axial positions (S-Fe-S 180(1)°) with the amine donors in the equatorial positions

completing the coordination sphere [94]. Comparison of the reported structures of

[Fe([9]aneN2S)2](ClO4)2 and [Fe([9]aneN2S)2](ClO4)3 shows a contraction in the Fe-N

bond lengths (Fe(II), 2.072(2), 2.063(7); Fe(III) 2.006(3) Å), also observed in the

complexes [Fe([9]aneN3)2]Cl2 and [Fe([9]aneN3)2]Cl3 (2.03(1) and 1.99(2) Å,

respectively), and expected on the basis of the decrease in ionic radius from Fe(II) to

Fe(III). The Fe-S bond distance in [Fe([9]aneN2S)2](ClO4)2 is also longer than that

observed for [Fe([9]aneN2S)2](ClO4)3 (Fe(II)-S 2.337(1) Å, Fe(III)-S 2.272(1) Å). This

difference has been advanced as structural evidence for the π-acceptor properties of the

thiaether donor atoms [145].

Variable-temperature susceptibility measurements for [Fe([9]aneN2S)2](ClO4)2 in the

range 2–300 K revealed a temperature-dependent magnetic moment [94]. Between 4.2 and

150 K the magnetic moment was 0.5 µB and this gradually increased to 2.95 µB at 300 K. For

a high spin d6 configuration the expected spin-only magnetic moment is 4.9 µB while the low

spin state would be diamagnetic. The behaviour exhibited by this complex was typical of a

thermally induced high-low spin transition between the high spin 5T2g and low-spin 1A1g

ground states, although the data indicate that the transition is not complete at 300 K. In

solution, [Fe([9]aneN2S)2]2+ displays the same temperature magnetic behaviour. Assuming

spin-only values of 4.90 and 0 µB, and from plots of lnKeq versus 1/T, the variable

temperature solution susceptibility data were fitted such that ∆H 0 = 20 kJ mol–1 and ∆S 0 =

53 J mol–1 K–1; these data were similar to those reported for analogous thermally induced

high/low spin transitions in iron(II) complexes [146]. For the iron(III) complex,

[Fe([9]aneN2S)2](ClO4)3 the solid state susceptibility data followed a Curie-Weiss law in the

range 300 K (2.3 µB) – 4.2 K (1.9 µB). The solid-state X-band EPR spectrum of

[Fe([9]aneN2S)2](ClO4)3 revealed axial symmetry (g⊥ = 2.607, g|| = 1.599). At Q-band

frequencies the perpendicular resonance was resolved into two components characteristic of a

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rhombically distorted low spin Fe(III) centre. Computer simulation gave gx = 2.687, gy =

2.526 and gz = 1.599. Based on the EPR analysis, the susceptibility data were fit to a simple

model (H = –λL·S + µLz) to give the spin–orbit coupling constant (λ) of –260 ± 10 cm–1 and

the axial ligand field parameter µ ~ 760 cm-1. The Mössbauer spectra (isomer shift/mm s–1,

quadrupole splitting/mm s–1, 4.2 K) for [Fe([9]aneN2S)2][ClO4]2 (0.52, 0.57),

[Fe([9]aneN2S)2][ClO4]3 (0.25, 2.72) and [Fe([9]aneNS2)2][ClO4]2 (0.43, 0.28) were typical

for iron(II) and iron(III) complexes [94].

The low-temperature single-crystal absorption spectra of [Fe([9]aneN2S)2](ClO4)2

exhibited additional bands which resembled pseudo-tetragonal low-symmetry splitting of the

parent octahedral 1A1g → 1T2g and 1A1g → 1T1g transitions [94]. However, the magnitude of

this splitting was too large, requiring 10Dq for the thiaether donors to be significantly larger

than for the amine donors. Instead, these bands were tentatively assigned to weak, low-energy

S → FeII charge-transfer transitions. Above 200 K, thermal occupation of the high-spin 5T2g

ground state resulted in observation of the 5T2g → 5Eg transition in the crystal spectrum of

[Fe([9]aneN2S)2][ClO4]2. From a temperature-dependence study, the separation of the low-

spin 1A1g and high-spin 5T2g ground states was approximately 1700 cm

-1. The spectrum of

the iron(III) complex [Fe([9]aneN2S)2][ClO4]3 was consistent with a low-spin d5

configuration.

Reaction of [9]aneN2S with an aqueous mixture of formic acid and formaldehyde

resulted in the isolation in high yield of the N-methylated ligand N,N ′-dimethyl-1,4-diaza-7-

thiacyclononane (Me2[9]aneN2S) [147]. Reaction of this ligand with ferric chloride in

methanol resulted in the isolation of [Fe(Me2[9]aneN2S)Cl3]; a similar procedure with

[9]aneN2S resulted in [Fe([9]aneN2S)Cl3]. Both complexes were characterised with X-ray

crystallography. The addition of the N-methyl substituents made very little difference to the

N-Fe-N and N-Fe-S bond angles with a small elongation observed (0.08 Ǻ) in the Fe-N bond

length on going from [Fe([9]aneN2S)Cl3] to [Fe(Me2[9]aneN2S)Cl3] (Figures 8(a) and 8(b))

[147].

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(a) (b)

Figure 8. Crystal structures of (a) [Fe([9]aneN2S)Cl3] and (b) [Fe(Me2[9]aneN2S)Cl3] [147]

The µ-oxo-bis(µ-acetato)diiron(III) complex [Fe2O(O2CMe)2([9]aneN2S)2](PF6)2 was

prepared by reaction of [Fe([9]aneN2S)Cl3] with sodium acetate and addition of ammonium

hexafluorophosphate; an analogous reaction with [Fe(Me2[9]aneN2S)Cl3] gave

[Fe2O(O2CMe)2(Me2[9]aneN2S)2](PF6)2 [147]. The iron(II) dimer

[Fe2(OH)(O2CMe)2(Me2[9]aneN2S)2](ClO4) was prepared under anaerobic conditions. The

two iron(III) complexes were characterised by crystal structural studies (Figures 9(a) and

9(b)).

(a) (b)

Figure 9. Crystal structures of (a) [Fe2O(O2CMe)2([9]aneN2S)2]2+ and (b)

[Fe2O(O2CMe)2(Me2[9]aneN2S)2]2+ [147]

In these dimer complexes the Me2[9]aneN2S ligand offered less steric repulsion than the

Me3[9]aneN3 analogue. A slight lengthening (~0.02 Ǻ) of the Fe…Fe distances between

[Fe2O(O2CMe)2([9]aneN2S)2](PF6)2 and [Fe2O(O2CMe)2(Me2[9]aneN2S)2](PF6)2 was

observed, reflecting an effect of addition of the N-methyl groups. The effect was not as

pronounced as for the [9]aneN3 analogues where the increase in Fe….Fe distance

between [Fe2O(O2CMe)2([9]aneN3)2])(PF6)2 and [Fe2O(O2CMe)2(Me3[9]aneN3)2](PF6)2

was approximately 0.06 Å. As well, a potential distribution of products with the mixed

sulfur–nitrogen ligands was possible, with the thioether donors being cis and/or trans

with respect to the bridging oxo moiety, and in a gauche, anti or syn arrangement with

respect to the Fe-O-Fe projection. However, for the [Fe2O(O2CMe)2([9]aneN2S)2]2+

complex the crystal structure indicated that the product isolated displayed the

thiaethers trans with respect to the bridging oxo unit, with S-Fe-O bond angles of 175.5(2)

and 178.2(2)° and in a syn configuration with respect to the Fe-O-Fe plane. For the

analogous [Fe2O(O2CMe)2(Me2[9]aneN2S)2]2+ complex, however, the structural analysis

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indicated that the thiaethers were located cis to the bridging oxo group, and in a gauche

configuration with respect to the Fe-O-Fe plane. The solid-state structure, of course, does

not necessarily reflect that which exists in solution. The Mössbauer spectra of the two

Fe(III) complexes [Fe2O(O2CMe)2([9]aneN2S)2](PF6)2 and [Fe2O(O2CMe)2(Me2[9]aneN2

S)2](PF6)2 at 4.2 K and zero field consisted of a symmetric quadrupole doublet with isomer

shifts of 0.48 and 0.49 mm s-1, respectively; the quadrupole splittings were 1.23 and 1.52

mm s-1 and these data are consistent with similar high spin Fe-O-Fe type complexes. For the

Fe(II) complex [Fe2(OH)(O2CMe)2(Me2[9]aneN2S)2](ClO4) the isomer shift (1.19 mm s-1)

and quadrupole splitting (2.67 mm s-1) were consistent with other high spin binuclear Fe(II)

complexes. The temperature dependence of the magnetic susceptibility, measured from

300–4.2 K, for [Fe2O(O2CMe)2([9]aneN2S)2](PF6)2 (H = -2JS1·S2) indicated that the iron(III)

sites were antiferromagnetically coupled with J = -125 cm-1 and g = 2.078; the exchange

coupling for this complex has also been explored using density functional theory [148]. For

the iron(II) complex [Fe2(OH)(O2CMe)2(Me2[9]aneN2S)2](ClO4) the least squares fit to the

susceptibility data (H = -2JS1·S2) gave J = -7.4 cm-1 with g = 2.23 [147]. For

[Fe2(OH)(O2CMe)2(Me3[9]aneN3)2](ClO4) J = -13 cm-1 [149].

The Fe(II) complex of py2[9]aneN2S, [Fe(py2[9]aneN2S)Cl](CF3SO3) has been

reported (Figure 10) [117].

Figure 10. Crystal structure of the [Fe(py2[9]aneN2S)Cl]+ complex [117]

The iron(II) complex, like the Mn(II) analogue, adopted the β-conformation and the complex

was high spin with 5.17 µB, slightly higher than the spin only value of 4.90 µB [117]. The

electronic spectrum in acetonitrile displayed a band at 25000 cm-1 (ε = 1710 M-1 cm-1)

assigned to a metal-ligand charge transfer; on dissolution in water this band shifted to 27027

cm-1 (ε 1460 M-1 cm-1), the species in solution proposed to be [Fe(py2[9]aneN2S)(H2O)]2+.

Addition of base to form a complex presumed to be [Fe(py2[9]aneN2S)(OH)]+ and resulted in

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a redshift of the absorption band to 24390 cm-1 (ε 1660 M-1 cm-1). A corresponding shift in

the Fe(II/III) redox couple was observed with [Fe(py2[9]aneN2S)Cl]+/2+ E1/2 = 0.66 V,

[Fe(py2[9]aneN2S)(H2O)]2+/3+ E1/2 = 0.70 V and [Fe(py2[9]aneN2S)(OH)]

+/2+ E1/2 = 0.37 V

(NHE) [117].

The substituted [9]aneN2S ligand, 4,7-bis(2’-methyl-2’-mercaptopropyl)-1-thia-4,7-

diazacyclononane (bmmpTASN) (Figure 11) as its iron complex, has been investigated

extensively as a model NO-inactivated iron containing nitrile hydratase (NHases), the first

non-heme bacterial enzyme characterised by a low spin Fe(III) state [71, 150-154].

Figure 11. 4,7-bis(2’-methyl-2’-mercaptopropyl)-1-thia-4,7-diazacyclononane (bmmpTASN)

NHases catalyse the hydration of nitriles to the corresponding amides; they exhibit a

protein absorption band at 35700 cm-1 in the UV/visible spectrum with a shoulder at 25000

cm-1 and a less intense band around 14280 cm-1, assigned tentatively as a thiolate- iron charge

transfer band [155, 156]. The first X-ray structure revealed that NHase consisted of two

subunits (α and β) with a basic stoichiometry of α1β1M1 (M = Fe(III)) with a N2S3(O) iron

coordination sphere, the ligands being part of a small peptide, CysXYCysSerCys, the two

nitrogen donors arising from the amides of the peptide main chain, and the three thiolates

from the cysteines (Figure 12) [157].

Figure 12. Active site structure of NO-inactivated iron containing nitrile hydratase; adapted

from [157]

S

N

N

SH

SH

Fe

SN

SN

O

O

O

O

OOH

O

S

Cys109

Henzyme

enzyme

Cys112

Cys114

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The active form of the enzyme NHaselight reacts with endogenous nitric oxide to yield an

inactive form NHasedark, The nomenclature employed for this, [Fe-NO]6, and similar types of

MNO complexes, is derived from the number of d electrons present [158]. In terms of the

molecular orbital diagram for a six-coordinate complex with linear MNO groups, the electron

configuration (3a1)2(2e)4(1b2)

2 would lead to the [MNO]6 nomenclature [158]. The core of the

NHasedark form exhibits the N2S3Fe-NO coordination with two amido nitrogen donors and

three sulfur donors from cysteine-[159]. NHasedark is EPR silent and its UV-vis spectrum

shows the two absorptions at 35700 and 27000 cm-1; however, the broad band at 14280

cm-1 was absent, suggesting that the presence of this band is directly correlated to the

enzyme activity. Resonance Raman studies showed that NO was released upon photo-

irradiation [159].

The complex [Fe(bmmpTASN)Cl], reacts with NO to give

[Fe(bmmpTASN)(NO)](BPh4); the X-ray structure of the complex shows that the Fe(III) is

pseudo-octahedral with the cyclononane macrocycle occupying one face, the two thiolate

donors cis, and cis to the NO ligand, the arrangement mimicking the coordination of the

cysteine donors in NHasedark (Figure 13) [71]. The complex was reported as an accurate

structural model of the enzyme itself. The Fe-N(tertiary) bond distances (average 2.043(7) Ǻ)

reproduce those seen in the NHasedark enzyme (2.07 Ǻ), as do the Fe-S distances (2.287(3) Ǻ

model, 2.30 Ǻ enzyme) [71].

Figure 13. Crystal structure of [Fe(bmmpTASN)(NO)]+ [71]

The complex displays a υNO stretching frequency in the infrared spectrum at 1856 cm-1,

whereas the NHasedark displays a band at 1852 cm-1 [71, 155, 156]. In addition, the

Mössbauer spectra of the model and NHasedark are similar (isomer shift: 0.06 mm s-1, 0.03

mm s-1, respectively; quadrupole splitting: 1.75 mm s-1, 1.47 mm s-1, respectively) [71, 160].

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Reaction of [Fe(bmmpTASN)Cl] with Et4NCN results in [Fe(bmmpTASN)(CN)]

[150, 151] (Figure 14(a)) which displays a υCN infrared stretch at 2083 cm-1; addition of base

to [Fe(bmmpTASN)Cl] resulted in isolation of [Fe(bmmpTASN)(O)]2 (Figure 14(b)) with an

infrared frequency at 799 cm-1 typical of a µ-oxo diiron complex [150].

(a) (b)

Figure 14. Crystal structures of (a) [Fe(bmmpTASN)(CN)] [151] and (b)

[Fe(bmmpTASN)(O)]2 [150]

The EPR spectra of [Fe(bmmpTASN)Cl] displayed a single line at g = 4.28, consistent with a

high spin Fe(III) (S = 5/2) ground state in a rhombic environment; [Fe(bmmpTASN)(CN)]

displayed a rhombic signal (g1 = 2.31, g2 = 2.16, g3 = 1.96) consistent with an S = ½, low

spin, ground state [150]. The [Fe(bmmpTASN)(NO)](BPh4), NHasedark, mimic had an S = 0

ground state [71, 158]. The authors concluded that the nature of the ligand X in the complex

[Fe(bmmpTASN)(X)], rather than the presence of the two cis thiolato ligands, determined the

high- or low-spin behaviour with π-accepting ligands promoting low-spin and the π-donating

ligands high-spin [150]. The visible spectra of the four complexes were analysed in terms of

their low spin ([Fe(bmmpTASN)(CN)], [Fe(bmmpTASN)(NO)](BPh4)) or high spin

([Fe(bmmpTASN)Cl] and [Fe(bmmpTASN)(O)]2) properties. The low spin complexes

display a Fe(III)-thiolate charge transfer band at 15200 cm-1 and 15400 cm-1, respectively,

whereas for the high-spin complexes the band occurs at 16000 cm-1 and 19010 cm-1,

respectively [150]. The cyclic voltammograms of the four complexes display an irreversible

oxidation ranging from +0.29 to +0.96 V (Ag/AgCl) assigned to a thiolate to thiyl oxidation

[150]; in addition, the high spin complexes display a quasi-reversible reduction around -0.66

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V, assigned as the Fe(III/II) couple. The low spin cyanide complex was, however, more

difficult to reduce (-0.88 V) [150].

Reaction of [Fe(bmmpTASN)Cl] with thallium triflate resulted in the isolation of

[Fe(bmmpTASN)](CF3SO3) [152]. The complex displayed an S = ½ ground state in non-

coordinating solvents with the room temperature µeff = 1.78 µΒ. The mass spectrum

displayed m/z = 376.08 as expected for [Fe(bmmpTASN)]+ with no evidence of anion or

solvent coordination under mass spectrometry conditions. The EPR spectrum in acetonitrile

(77 K) confirmed binding of solvent with, in addition to a sharp axial signal of

[Fe(bmmpTASN)]+ (g1 = 2.04, g2 = 2.02 and g3 = 2.01), a rhombic signal (g1 = 2.27, g2 =

2.18 and g3 = 1.98) attributed to [Fe(bmmpTASN)(CH3CN)]+, the g values typical of low-

spin iron(III). [Fe(bmmpTASN)(CH3CN)]+ was in equilibrium with [Fe(bmmpTASN)]+

with a binding constant of Keq = 4.7 at room temperature [152]. [Fe(bmmpTASN)]+ was

found also to coordinate a variety of solvents resulting in six-coordinate complexes of the

form [Fe(bmmpTASN)(solvent)]+ (solvent = H2O, DMF, methanol, DMSO, and pyridine) to

form high-spin six-coordinate complexes, the EPR spectra of which display significant strain

in the rhombic zero-field splitting term E/D. Addition of triflic acid to

[Fe(bmmpTASN)(O)]2 resulted in the formation of [(Fe(bmmpTASN))2OH](CF3SO3), the

complex characterised by X-ray crystallography (Figure 15).

Figure 15. Crystal structure of [(Fe(bmmpTASN))2OH]+ [152]

In aqueous solution three distinct species were formed depending on solution pH:

[Fe(bmmpTASN)(H2O)]+ (pKa = 5.04±0.1), [(Fe(bmmpTASN))2(OH)]

+ (pKa = 6.52 ±0.05)

and [(Fe(bmmpTASN))2O] [152].

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The crystal structure of NHasedark, showed that αCys114 had been post-

translationally modified to a cysteine sulfenic acid (Cys–SOH) [159]. Thus, NHase requires

oxygen addition to sulfur for modification of the cysteine residues to sulfur oxygenates. In

order to model this, the oxygen sensitivity of [Fe(bmmpTASN)(CN)] and

[Fe(bmmpTASN)Cl] has been examined [151]. Oxygen exposure of the low-spin complex

[Fe(bmmpTASN)(CN)] over a period of several days resulted in the disulfonate complex

[Fe(bmmp-O6-TASN)(CN)] as an olive-green solid (Figure 16) [151].

Figure 16. [Fe(bmmp-O6-TASN)(CN)]; adapted from [151]

The complex displayed characteristic peaks in the IR spectrum at 2062 cm-1 assigned

to a υCN asymmetric stretch as well as bands consistent with the presence of sulfur-

oxygenates, a result confirmed after 18O substitution [151]. Oxygen exposure of the high-

spin complex [Fe(bmmpTASN)Cl] results in disulfide formation and decomplexation of the

metal with subsequent iron-oxo cluster formation [151]. A natural bond order/natural

localized molecular orbital covalency analysis revealed that the low-spin complex

[Fe(bmmpTASN)CN] contained Fe–Sthiolate bonds with calculated covalency of 75 and 86%,

while for the high-spin complex [Fe(bmmpTASN)Cl] the calculated covalencies were 11 and

40% [151]. The results indicate the degree of covalency of the Fe–S bonds plays a major role

in determining the reaction pathway associated with oxygen exposure of iron thiolates [151].

The iron complex of the tetradentate ligand 4-((1-methyl-1H-imidazol-2-yl)methyl)-

1-thia-4,7-diazacyclononane (mimTASN) [Fe(mimTASN)Cl2](FeCl4) has been

characterised by X-ray crystallography (Figure 17) [154].

S

N

HN N

N

Fe

NN

CNO

S

OS

O O SO

O

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(a) (b)

Figure 17. (a) 4-((1-methyl-1H-imidazol-2-yl)methyl)-1-thia-4,7-

diazacyclononane (mimTASN) and (b) Crystal structure of [Fe(mimTASN)Cl2]2+ [154]

The structure revealed that the iron(III) exhibited a pseudo-octahedral environment with the

three nitrogen donors of the ligand coordinated facially. Replacement of the

tetrachloridoferrate(III) anion with PF6- resulted in [Fe(mimTASN)Cl2](PF6) the structure of

which revealed meridional coordination of the three nitrogen donors of the ligand [154].

Cyclic voltammetry of [Fe(mimTASN)Cl2](PF6) in acetonitrile revealed a single Fe(III)/(II)

reduction (-280 mV (versus Fc+/Fc). In methanol solution the cyclic voltammogram revealed

a broad cathodic wave due to partial exchange of one chloride for methoxide with half-

potentials of -170 mV and -440 mV for [Fe(mimTASN)Cl2]+/0 and

[Fe(mimTASN)(OCH3)Cl2]+/0 with K(chloride exchange) = 7 x 10

-4 M-1 for Fe(III) and 2 x 10-8

M-1 for Fe(II). In aqueous solutions, and as a function of pH, three complexes are available

after chloride exchange: in strongly acidic conditions the aqua complex

[Fe(mimTASN)Cl(H2O)]2+ (pKa = 3.8 ±0.1), in mildly acidic conditions, the µ-OH complex

[(Fe(mimTASN)Cl)2(OH)]3+ (pKa = 6.1 ± 0.3) and the µ-oxo complex

[(Fe(mimTASN)Cl)2(O)]2+ under basic conditions [154].

In addition to studies of the iron complex of bmmpTASN, as a model NO-inactivated

iron-containing nitrile hydratase (NHases) [71, 150-154], the ruthenium complexes have also

been investigated. In a series of papers the reactions of bmmpTASN, in addition to 4-(2′-

methyl-2′-sulfinatopropyl)-7-(2′-methyl-2′-mercapto-propyl)-1-thia-4,7-diazacyclononane

(bmmpO2TASN), and 4-(2′-methyl-2′-sulfinatopropyl)-7-(2′-methyl-2′-sulfenato-propyl)-1-

thia-4,7-diazacyclononane) (bmmpO3TASN), in the form of their respective ruthenium(II)

complexes, [Ru (bmmpOnTASN) PPh3] and [Ru(bmmpOnTASN) (PPh2CH3)] (n = 1–3), have

been used to investigate how these complexes mimic the reaction of nitrile hydratase [161-

166]. Under O2 under limiting conditions, the complex [Ru(bmmpTASN)(PPh3)] reacted to

yield a mixed sulfenato/sulfinato product [Ru (bmmpO3TASN)(PPh3)] [162]. The complex

prepared with 16O2 displayed intense bands at 1140 and 1020 cm−1 in the IR spectrum

attributed to the asymmetric and symmetric S═O stretches, respectively, of the sulfinato

donor. Reaction with 18O2 resulted in a shift of these bands to 1095 and 982 cm−1 for the

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labelled [Ru(bmmpO3TASN)(PPh3)] complex. In addition, positive ion ESI-MS of

[Ru(bmmpO3TASN)(PPh3)] prepared with 16O2, displayed a parent peak at m/z 731.1138

which shifted to m/z 737.1267 for the complex prepared with 18O2 [162]. The crystal

structure of [Ru(bmmpO3TASN)(PPh3)] (Figure 18) [162] showed the two oxygen donors of

the sulfinato donor are directed along the S1−Ru−S3 bond axis, while the sulfenato oxygen is

oriented toward a nitrogen donor along the P1−Ru−N1 axis. The triphenylphosphine ligand

appears to impede access to the remaining potential thiaether oxygenation site, possibly

retarding the rate of further oxygenation under limited O2 [162].

Figure 18. Crystal structure of

[Ru(bmmpO3TASN)(PPh3)] [162]

A subsequent report explored the same chemistry of [Ru(bmmpTASN)(PPh3)] but

showed that it was possible to isolate a number of sulfur oxygenated derivatives based on

reaction time [165]. Thus, addition of five equivalents of O2 yielded the thiolato/sulfinato

complex [Ru(bmmpO2TASN)(PPh3)] (Figure 19) within 5 minutes whereas a reaction time

of 12 hours gave the sulfenato/sulfinato derivative [Ru(bmmpO3TASN)(PPh3)]. The bis-

sulfinato complex [Ru(bmmpO4TASN)(PPh3)] was formed with either longer reaction times

or additional O2; in all cases the oxidation state of the metal ion remained Ru(II) [165].

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Figure 19. Crystal structure of [Ru(bmmpO2TASN)(PPh3)] [165]

Stoichiometric hydrolysis of acetonitrile to acetamide by these complexes was investigated

[165]. The hydrolysis reaction would require either dissociation of the phosphine leading to

an open coordination site for substrate binding or direct participation of the S-oxygenate

moieties. The elongation of the Ru−P bond distance as a function of the state of sulfur

oxygenation suggested a transient five-coordinate intermediate after dissociation of the

triphenylphosphine ligand. Solutions of [Ru(bmmpO2TASN)(PPh3)] and

[Ru(bmmpO3TASN)(PPh3)] in a mixed solvent system of acetonitrile, methanol, and buffer

(PIPES, pH = 7.0) gave small quantities of acetamide after 5 days; [Ru(bmmpTASN)

(PPh3)] gave no product under the same reaction conditions [165].

The precatalyst complexes [Ru(Ln)(PPh3)] (n = 1–3; Ln = bmmpTASN,

bmmpO2TASN or bmmpO3TASN) have been used to study the rate of benzonitrile hydration,

the study inspired by the metalloenzyme nitrile hydratase [71, 150-154, 161]. Previous

studies had suggested that dissociation of the triphenylphosphine ligand was facile such that

the open coordination site could be occupied by substrate, in this case the benzonitrile [165].

The kinetic data were consistent with a mechanism involving initial activation by complete

PPh3 dissociation to give the aqua complex [Ru(Ln)(OH2)] , which is in equilibrium with the

nitrile complex [Ru(Ln)(NCR)]. Subsequent hydration of this nitrile complex through an

activated water molecule resulted in [Ru(Ln)(NH2C(O)R)], the product complex. For the

different ligands the hydration rate constants were reported to be 0.37 ± 0.01, 0.82 ± 0.07,

and 1.59 ± 0.12 M–1 h–1 for L1 to L3, respectively [161]. Substitution of the amide by water

completes the catalytic cycle [161].

In the course of these studies the ruthenium(II) dimer [Ru2(bmmpTASN)2] was

isolated and characterised (Figure 20) [166]. The authors reported that repeated attempts to

reproduce the synthesis of this complex were

unsuccessful [166].

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Figure 20. Crystal structure of [Ru2(bmmpTASN)2] [166]

Reaction of [(RuCl2(cym))2] and [9]aneN2S in methanol resulted in the mixed-

sandwich Ru(III) complex [Ru(cym)([9]aneN2S)](BPh4)Cl2·MeCN (Figure 21) [133]. The

structure shows the cyclononane coordinated facially through the two nitrogen and the

thiaether donors, the macrocyclic ligand displaying λλλ (or δδδ) configuration. The p-

cymene is η6 coordinated to the ruthenium(III) with Ru-C bonds in the range 2.196(7)–

2.233(8) Å. The Ru(III)-N bond lengths were 2.120(7) Ǻ and 2.105(7) Ǻ with the Ru-S bond

length being 2.324(2) Ǻ [133].

Figure 21. Crystal structure of

[Ru(cym)([9]aneN2S)]3+ [133]

2.3.6 Cobalt(III), Rhodium(III),

The synthesis of [Co([9]aneN2S)2](ClO4)3 was reported in a manuscript which first

mentioned the synthesis of the [9]aneN2S ligand itself but concentrated on the chemistry of

cobalt(III) complexes of two quinquidentate ligands, 4,7-bis(2-aminoethyl)-1-thia-4,7-

diazacyclononane (dats) and 1,4-bis(2-aminoethyl)-1,4,7-triazacyclononane (datn) (Figure

22), derived from [9]aneN2S and [9]aneN3, respectively [68]. The 13C NMR of

[Co([9]aneN2S)2](ClO4)3 was reported to display eight resonances and it was predicted that in

solution the complex existed as a mixture of cis- and trans-isomers; attempts to separate the

isomers with chromatographic methods were reportedly unsuccessful [68]. Crystallisation of

the complex from aqueous ethanol did result in isolation and characterisation by X-ray

crystallography of the trans-S isomer of [Co([9]aneN2S)2](ClO4)3 [70] The synthesis of the

complex was reported in a later paper, and again the authors reported the presence of

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resonances in the 13C NMR indicative of the presence of cis- and trans-isomers, and the lack

of success in their separation [114]. The two geometric isomers of [Co([9]aneN2S)2]3+ were

ultimately separated by fractional crystallization and cation-exchange chromatography on SP-

Sephadex [109]. Attempts to separate the enantiomers of the dissymmetric cis-complex were

unsuccessful [109]. The isomers underwent a facile base-catalysed isomerisation to a

cis/trans (4/1) mixture at room temperature with the rate being significant down to pH 5, the

successful chromatographic separation requiring acidic conditions [109]. The X-ray crystal

structures of both the cis and trans isomers of [Co([9]aneN2S)2]3+ were subsequently reported

[167].

The base hydrolysis of [Co(datn)Cl](ClO4)2 and [Co(dats)Cl](ClO4)2 proceeded for

both complexes with two consecutive steps, the first reaction in each case being chloride

hydrolysis (kOH(dats) = 3.6 x 104 M-1 s-1; kOH(datn) = 2.85 x 10

3 M-1 s-1), the second reaction for

the dats complex was proposed to be base-catalyzed dissociation of the thiaether resulting in

the cis-[Co(dats)(OH)2]+ complex whilst for the datn complex the reaction was proposed to

be base catalysed terminal ring opening (Co-N cleavage) [68].

(a)

(b)

Figure 22. (a) 1,4,7-tris(2-aminoethyl)-1,4,7-

triazacyclononane (datn) and (b) 1,4-bis(2-aminoethyl)-1-thia-4,7-diazacyclononane (dats)

[Rh([9]aneN2S)Cl3].H2O was prepared by reaction of RhCl3·H2O and [9]aneN2S in

refluxing ethanol [133]. The X-ray crystal structure showed that the six coordinate metal ion

was coordinated by the cyclononane and three chlorido ligands in a facial arrangement

[133]. The three five-membered chelate rings of [9]aneN2S had the λλλ (or δδδ)

conformation. The Rh-N bond lengths (2.036(3) Ǻ and 2.040(3) Ǻ) are shorter than in

[Rh([9]aneN3)2]Br3 (2.061 Ǻ) [168] and the Rh-S bond (2.246(1) Å) was shorter than in the

[Rh([9]aneS3)2]3+ complex (2.331(2)–2.348(2) Å) [169, 170] suggesting Rh→S back

donation. Of the three Rh-Cl bonds, one (2.396(1) Ǻ compared with 2.368(1) Ǻ and 2.358(1)

Ǻ) shows a slight structural influence from the trans thiaether donor [133].

3.3.7 Nickel(II/III), Platinum(II), Palladium(II)

N

S

N

H2N NH2

N

N

N

H2N

H2N NH2

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The complex [Ni([9]aneN2S)2](NO3)2 was prepared from reaction of nickel(II) nitrate and

the ligand in an ethanol water mixture; purple crystals were obtained on standing [125]. The

X-ray crystal structure of [Ni([9]aneN2S)2]2+ showed that the thiaether donors were trans,

[125] whereas in the analogue [Ni(daes)2]2+ the sulfur donors were cis [171]. The Ni-N

distances were 2.122(2) Ǻ and 2.108(2) Ǻ, close to the strain-free distance, while the Ni-S

distance (2.418(1) Ǻ) was shorter than that seen for the [Ni(daes)2]2+ analogue (2.455 Ǻ)

[171]. The coordination geometry around the metal ion in [Ni([9]aneN2S)2]2+ was distorted

from octahedral with the angles around the metal ion significantly different from 90° [125].

Potentiometric studies revealed that the Ni(II) binds the two ligands strongly with logK1 =

10.45(2) and logK2 = 9.60(2) [125].

Two analogues of the [9]aneN2S ligand, 15-thia-1,5,8,11-

tetraazabicyclo[10.5.2]nonadecane (L1) and 1,11-dithia-4,8,14,18-tetraaza[5.2.2.5]eicosane

(L2) have been reported (Figure

23) [77, 172-174].

(a)

(b)

Figure 23. (a) 15-thia-1,5,8,11-tetraazabicyclo[10.5.2]nonadecane (L1) and (b) 1,11-dithia-

4,8,14,18-tetraaza[5.2.2.5]eicosane (L2) (syn- and anti-isomers)

Both ligands were synthesised from a ligand base of 1,4,8,11-tetraazacyclononane (cyclam);

1,11-dithia-4,8,14,18-tetraaza[5.2.2.5]eicosane was synthesised by reaction of 1,4,8,11-

tetraazacyclononane with chloroacetyl chloride and subsequent reaction with sodium sulfide

and reduction of the product with borane; anti- and syn-isomers of the ligand were isolated

[172-174]. The ligand 15-thia-1,5,8,11-tetraazabicyclo[10.5.2]nonadecane was prepared

after complexation of 4,7-bis(3-aminopropyl)-1-thia-4,7-diazacyclononane (Figure 24) with

Cu(II), template addition of glyoxal, reduction with sodium borohydride and subsequent

isolation of the free ligand [77].

S

N N

HNNH

S

S

N N

NN

S

S

N N

NN

anti isomer

syn isomer

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Figure 24. 4,7-bis(3-aminopropyl)-1-thia-4,7-diazacyclononane

Upon complexation the cyclam backbone of 15-thia-1,5,8,11-

tetraazabicyclo[10.5.2]nonadecane offers the potential for a number of stereochemical

conformations, including five possible trans isomers (trans-I – trans-V) [175-180] as well as

a cis isomer; incorporation of the [9]aneN2S fragment adds both anti- and syn-possibilities.

The cis-[Ni( trans-I,syn-L2)]2+ and the trans-[Ni( trans-IV,anti-L2)]2+ complexes

have been structurally characterised (Figure 25) [172].

(a) (b)

Figure 25. Crystal structures of (a) cis-[Ni( trans-I, syn-L2)]2+ and (b) trans-[Ni( trans-IV,

anti-L2)]2+ complexes [172]

The equatorial plane for the cis-[Ni( trans-I, syn-L2)]2+ ion is defined by two N-donors and

two S-donors, the other two N-donors occupying the axial positions. The equatorial Ni–N

distances are 2.164(3) and 2.179(3) Ǻ with the Ni–S distances being 2.4265(11) Ǻ and

2.4512(11) Ǻ [172]. As is common in Ni(II) complexes with the cis-coordinated cyclam

ligand the geometry around the nickel(II) centre is distorted. In the trans-[Ni( trans-IV, anti-

L2)]2+ ion, the nickel sits at an inversion centre and the N-donors lie in the equatorial plane;

the average Ni–N distance was 2.125(4) Ǻ with Ni-S distances of 2.5321(11) Ǻ - the S-

S

N

N

NH2

H2N

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donors occupy the axial positions. The cation exhibits D4h symmetry and the trans-IV

configuration, the latter imposed by the anti-orientation of the [9]aneN2S rings. This trans-IV

configuration has the highest strain energy and was the least favoured among the various

possible stereochemical arrangements (trans-I – trans-V) adopted by the cyclam ring [172].

The geometry imposed by the syn- and the anti-L2 is reflected in the spectroscopic

data for the Ni(II) complexes [172]. The cis-[Ni(syn, trans-I, L2)]2+ ion shows absorption

bands at 10020 cm-1 (ε = 26 M-1 cm-1 ) and 18590 cm-1 (ε = 16 M-1 cm-1 ) whilst the trans-

[Ni(anti, trans-IV, L2)] 2+ ion has absorption bands at 10040 cm-1 (ε = 10 M-1 cm-1 ) and

19160 cm-1 (ε = 11 M-1 cm-1 ) with 10Dq values of 10485 cm-1 for the cis-[Ni( trans-I, syn-

L)] 2+ ion , and 16690 cm-1 for trans-[Ni( trans-IV anti-L2)]2+ ion, a value considered to be

abnormally high [172]. The absorption spectra of Ni(II) complexes and the problems

associated with assignment of the 10Dq value and the origins of the transitions is addressed

later in this review [171, 172, 181, 182].

Further studies were undertaken using cyclic voltammetry and EPR [172]. The

trans-[Ni( trans-IV , anti-L2)]2+ ion showed a reversible wave (E1/2 = 0.91 V) and an axial

EPR spectrum with g┴ = 2.18 and g║ = 2.01, consistent with D4h symmetry. The cis-

[Ni( trans-I, syn-L2)]2+ complex exhibited more complicated and scan rate dependent

electrochemical behaviour. At 1000 mV s-1, the redox cycle for the Ni2+/3+ couple showed a

quasi-reversible wave with Ep,c = 1.35 V, Ep,a =1.45 V; lowering the scan rate to

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complexes. The five-membered chelate rings adopt the gauche configuration with the six-

membered rings in the chair form [172] .

Figure 26. Crystal structure of [Ni(Ll)(ClO4)]+ [172]

The UV-visible spectrum for [Ni(II)(L1)]2+ corresponded to that expected for a six-

coordinate complex. The 3A2g → 3T1g(F) transition observed at 19840 cm-1 is at high energy

for a high-spin Ni(II) system suggesting that the bicyclic ligand system exerts a strong

ligand field on the metal centre. Cyclic voltammetry of the Ni(II) complex showed a

reversible wave in aqueous solution for the Ni3+/2+ couple (0.765 V vs SCE) and two

reversible waves in CH3CN corresponding to the Ni2+/+ and Ni3+/ 2+ couples (-1.675 V

and 0.775 V vs Fc+/Fc, respectively). It was proposed that the additional six-membered

chelate ring imparted an extra stabilisation on the Ni(III) complex attributed to (i) the

cryptate effect from the additional chelate ring, and (ii) the formation of the 14-

membered N4 ring which results in a stronger in-plane interaction, thus raising the

energy of the eg orbitals and making it easier to remove an electron from the metal

centre. The [Ni(III)(Ll)(H2O)]3+ complex ion exhibited an EPR spectrum characteristic of a

low-spin d7 ion in a distorted octahedral environment with g┴ = 2.169 and g║ = 2.025

(frozen solution 77 K) [172].

The Ni(II) complex with py2[9]aneN2S crystallised with all five of the ligand donors

bound; a water molecule occupied the sixth coordination site, and the pyridyl groups were

situated cis (Figure 27) [183].

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Figure 27. Crystal structure of [Ni(py2[9]aneN2S)(H2O)]2+ [183]

The olive-green complex [Ni(bmmpTASN)] was prepared from reaction of

NiCl2·6H2O with H2(bmmpTASN) in ethanol in the presence of KOH [151]. The complex

reacted rapidly with hydrogen peroxide in acetonitrile, producing [Ni(bmmp-O6-TASN)].

The X-ray crystal structures of both complexes have been determined [151]. For

[Ni(bmmpTASN)] the metal ion is in a square-planar environment with an N2S2 donor set.

The Ni–S(thiolate) and Ni–N bond distances (2.1775(7) Ǻ and 2.1925(7) Ǻ; 1.957(2) Ǻ and

1.982(2) Ǻ, respectively) are typical for similar square-planar Ni(II) complexes. The axial

thiaether sulfur of the [9]aneN2S ligand is essentially non-bonding (2.824 Ǻ). The structure

of [Ni(bmmp-O6-TASN)] has the metal ion in a pseudo-octahedral coordination

environment, the donor set consisting of the two amine donors (2.103(4) Ǻ and 2.139(5) Ǻ) ,

the thiaether sulfur (2.3847(14) Ǻ) and the η1 (1.991(4) Ǻ) and η2 sulfonate donors (2.134(4)

Ǻ and 2.283(4) Ǻ) [151].

The complexes [Pd([9]aneN2S)2](PF6)2 and [Pt([9]aneN2S)2]Br2.H2O were prepared

by reaction of the ligand with Pd(II) acetate and K2PtCl4, respectively [184]. The X-ray

crystal structures of both complexes have been reported. Under different solvent (3:1

nitromethane and dichloromethane) and temperature conditions, reaction of PdCl2 and

[9]aneN2S resulted in [Pd([9]aneN2S)2]Cl2.H2O at room temperature and cis-

[Pd([9]aneN2S)Cl2] at 80oC. Attempts to produce X-ray quality crystals from cis-

[Pd([9]aneN2S)Cl2] resulted in the dark red [Pd3([9]aneN2S)4Cl2]Cl4.2H2O complex [185].

The syntheses of the complexes [Pt([9]aneN2S)2](PF6)2 and [Pt([9]aneN2S)2]Br2 from

reaction with K2PtCl4 and ligand in aqueous solution have also been reported, the bromide

anion in the latter arising apparently from the use of the hydrobromide salt of the

cyclononane ligand [184, 185].

The structures of the [Pd([9]aneN2S)2](PF6)2 and [Pd([9]aneN2S)2]Cl2·H2O

complexes differ with respect to both the conformation of the cyclononane ligand and as a

result the coordination of the thioether group [184, 185]. The [333] type conformation in the

latter means that the thiaether donor is directed away from the metal ion, whereas the [234]

conformation in the former leads to a weak interaction with the metal ion. In the case of the

platinum complexes [Pt([9]aneN2S)2](PF6) and [Pt([9]aneN2S)]Br2 both adopt a [333]

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conformation where the thiaether is non-coordinating [184, 185]. Based on these

observations, the fluxional behaviour observed previously in solution for the

[Pd([9]aneN3)2]2+ analogue was speculated to be a result of an isomerisation process

between the [234] and [333] conformation of the ligand [185, 186] .

For [Pd([9]aneN2S)2](PF6)2 the metal atom exhibits a distorted octahedral geometry

made up of the two nitrogen and one thiaether donor of each facially coordinated

cyclononane macrocycle [184]. The four nitrogen donors (two from each ligand) form a

square plane around the metal ion with Pd-N distances of 2.054(3) Ǻ and 2.064(3) Å [184].

The interaction with the axial thiaether donors is weak, with a Pd-distance of 3.034(1) Å.

The [234] conformation of the ligand is similar to that reported for the gold(III) complex

[Au([9]aneN2S)Cl2][AuCl 4] where again the thiaether donor exhibits a long apical interaction

(2.973(3) Ǻ) [112]. The structure of the platinum(II) complex [Pt([9]aneN2S)2]Br2 complex

is different [184]. Again, the metal ion is coordinated to four of the nitrogen atoms of both

cyclononane ligands in a square planar geometry but the thiaethers are folded out and away

from the central atom, the ligand having a [333] type conformation. The Pt-N bond lengths

(2.04(2) Ǻ and 2.04(1) Ǻ) are shorter than those for the Pd(II) analogue [184]. Hydrogen

bonding interactions, average length 3.31-3.41 Å, involving the bromide anions and the

nitrogen atoms and the water molecule are evident in the structure [184]. The cyclic

voltammogram of [Pd([9]aneN2S)2](PF6)2 in acetonitrile exhibited a quasi-reversible single

electron oxidation, E1/2 = +0.30 V (Fc+/Fc) assigned to a Pd(II)/Pd(III) redox pair [184].

The complex [Pd3([9]aneN2S)4Cl2]+ resulted from attempts to obtain crystals of cis-

[Pd([9]aneN2S)Cl2] [185]. The structure consists of a central [Pd([9]aneN2S)2]2+ cation

bridged through its thiaether donor atoms to two [Pd([9]aneN2S)Cl] moieties; there are

additional Cl- ions acting as counter-ions. The central Pd(II) is coordinated to four nitrogen

donors of two [9]aneN2S ligands in a square-planar configuration, the thiaether donors from

each cyclononane ligand of the central [Pd([9]aneN2S)2]2+ cation displaying long apical

interactions (3.008(2) Ǻ) with one halide from the [Pd([9]aneN2S)Cl] moieties [185].

Reaction of py2[9]aneN2S with [Pd(CH3CN)4](BF4)2 resulted in

[Pd(py2[9]aneN2S)](BF4)2 [187]; a separate study reported the same complex as the

hexafluorophosphate salt [183]. The X-ray structure of the tetrafluoroborate salt showed that

the

cyclononanes: the extensive chemistry of fundamentally simple …378128/uq378128... · 2019. 10....

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