Synthetic and structural studies on dinuclear rhodium (ii) compounds

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on

Dinuclear Rhodium(II) Compounds.

A thesis presented to the University o f London

in partial fulfilment o f the requirements for the degree

o f Doctor o f Philosophy in the faculty o f Science.

by

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This thesis describes synthetic and structural studies on dinuclear rhodium(II)

compounds. The emphasis is on derivatives in which only two ligands bridge the Rh- Rh bond.

The removal of one o f the axial pyridine molecules from the compound

[Rh2(02CCH3)2(CF3C0CHC0CF3)2(NC5H5)2] is described in chapter two. The resulting compound [Rh2(02CCH3)2(CF3C0CHC0CF3)2(NCgH3)] has been structurally

characterised using ‘H NMR and NMR spectroscopy, and X-ray crystallography. The X-ray diffraction study reveals a coupling between two adjacent molecules via Rh- O(acetate) interactions in the unit cell. The removal o f the remaining pyridine

molecule is also described.

Chapter three describes the preparation of the novel compounds [Rh2(O2CCH3)2(l,10- phen)2Cl2] and [Rh2(HNOCCF3)2(l,10-phen)2Cl2] and their corresponding dicationic pyridine adducts, which have been fully characterised using ‘H NMR spectroscopy. The crystal structures of the pyridine adducts [Rh2(02CCH3)2( 1, 10-phen)2(NC3Hg)2]-

[PFJ2 and [Rh2(HNOCCF3)2(l,10-phen)2(NC5H5)2][PFj2 are reported. Attempts to remove the axial pyridine molecules using a variety o f methods are described.

The reactions o f 2-phenylpyridine (Hppy) and 6-phenyl-2,2’-bipyridine (Hpbipy) with [Rh2(02CCH3)^] are described in chapter four. Cyclometallation was attempted using the ligands as their mercury salts, [Hg(ppy)Cl] and [Hg(pbipy)Cl]. The structures of the axial adducts [Rh2(02CCH3)4(Hppy)2] and [Rh2(02CCH3)4(Hbipy)2] are reported. An interesting cyclometallated monomer [Rh(02CCH3)(ppy)2] which is formed in acetic acid is described and its crystal structure reported.

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Abstract 2

Contents 3

Acknowledgements 6

Chapter One 7

Introduction 8

1.1 Historical Recognition o f Multiple Metal-Metal Bonding 8

1.2 Bonding in Multiple Metal-Metal Bonded Compounds 12

1.3 Dirhodium (ll) Compounds 15

1.4 The Chemistry o f Dirhodium (ll) Compounds 19

1.5 The Chemistry of [Rh2(02CR)J Molecules 26

1.6 Dirhodium Compounds Containing Four Alternative Bridging

Ligands 30

1.7 Mixed Sets o f Bridging Ligands 44

1.8 Compounds With Only Two Carhoxylate Bridging Ligands 52

1.9 Compounds Containing Unsupported R hodium (ll)-R hodium (ll)

Bonds 56

Chapter Two 63

D irhodium (ll) Compounds Containing

[CF3COCHCOCF3]

Ligands

2.1 Introduction 64

2.2 Preparation and Characterisation o f the Compound

[Rh2(02CCH3)2(CF3C0CHC0CF3)2(py)] 72

2.3 Structural Investigation hy X-ray Diffraction 77

2.4 Conclusion 86

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[Rh2(02CCH3)2(CF3C0CHC0CF3)2(py)] 93

Chapter Three

Dirhodium(II) Compounds Containing 1,10-Phenanthroline Ligands 99

3.1 Introduction 100

3.2 Preparation and Characterisation o f the Compounds

[Rh2(HNOCCF3)2(phen)2Cl2], [Rh2(02CCH3)2(phen)2Cl2] And Their

Pyridine Adducts 108

3.3 Structural Investigation by X-ray Diffraction 118

3.4 Removal o f The Axial Ligands 123

3.5 Electrochemical Investigation 124

3.6 Experimental 126

3.7 The Crystal Structure Determination o f the Compounds 130

[Rh2(HN0CCF3)2(phen)2(py)2][PFJ,(C2H5)20 And

[Rh2(02CCH3)2(phen)2(py)2][PF,yCH3)2C0

Chapter Four

Complexes o f the Ligands 2-phenylpyridine and 6-phenyl-2,2 ’-bipyridine 144

4.1 Introduction to the Chemistry o f the Ligands Hppy and Hpbipy 145

4.2 Preparation and Characterisation o f [Rh2(02CCH3)4(Hppy)2],

[Rh(02CCH3)(ppy)2] and [Rh2(02CCH3)4(Hpbipy)2] 152

4.3 Structural Investigation by X-ray Diffraction 163

4.4 Experimental 171

4.5 Crystal Structure Determination o f the Compounds

[Rh2(02CCH3)4(Hpbipy)2], [Rh2(02CCH3)4(Hppy)2] and

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Dirhodium(II) Compounds Containing Dimethylglyoxime Ligands 184

5.1 Introduction 185

5.2 Preparation and Characterisation o f the compounds

[Rh2(dmg%(py)j, [Rhj(0 2CCH3)2(dmg)2(py)2] and

[Rh2(02CCH3)2(dmg)2(EPh3)2] (E = As,Sb,P) 188

5.3 Structural Investigation by X-ray Diffraction 193

5.4 Electrochemical Investigation 201

5.5 Experimental 204

5.6 Crystal Structure Determination o f the Compounds

[Rh2(dmg)4(py)2], and [Rh2(02CCH3)2(dmg)2(EPh3)] (E = As, Sb, P) 207

List o f Figures and Tables 233

Abréviations 240

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I would like to thank my supervisor Dr. D. A. Tocher for his constant encouragement, and advice throughout the course o f this work, and his guidance and patience in the writing up of this thesis.

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It was not until 1963 that recognition was given to the chemistry of compounds with direct metal-metal bonds. Before this transition metal compounds were only considered in terms of one centre coordination chemistry. Although there are examples of compounds known since 1844*’^ which were not one centre

coordination compounds, none of these compounds were assigned structures with metal-metal bonds until the I960’s. With the discovery and utilisation of X-ray crystallography the structures of multi-centre metal-metal bonded compounds could at last be shown in detail. The first of these structures to be disclosed, in 1946, was that of the lower chlorides of molybdenum.^ These have Mo-Mo bond distances of ~ 2.6 Â, which were shorter than those of metallic Mo (2.725 Â ). Four years later, in 1950, it was shown that Ta6Cl,4.7H20,'‘ its bromide analogue, and the

corresponding niobium compounds, all contain octahedral groups of metal atoms. The metal-metal distances in these compounds were ~ 2.8 Â. It was not until 1963, with the structural elucidation of Cs[ReClJ^'^ (Figure 1.1), which was shown to contain the anion [RegCl;2]^ , that the first metal-metal double bond was identified. This anion had three Re ions arranged in a triangle with each metal bound by two bridging and three axial chloride ions, and double Re-Re bonds, with a length of

2.477(3) Â. This discovery was followed one year later by the recognition o f the first quadruple bond,^ * in the ion [Re2Clg],^' Figure 1.2. The compounds

[Rc2(02CR)4X2], which were reported at around the same time, were also later shown to contain Re-Re quadruple bonds.^ ‘° The molecular structure of

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recognised . This type of bond, with an electronic configuration of a^Tc'^ô^ô*, was found in the paramagnetic compound (NH^)3[Tc2Clg.2H20], ' which contains the anion [Tc2Clg]^‘, with a Tc-Tc bond length o f 2.13(1) Â, and a structure similar to that of [Re2Clg]^‘.

Figure 1.1 The Structure o f

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Figure 1,2 The Structure o f fReXUV

In each following years new advances were made in this rapidly growing field. These included the first diruthenium structure, of [Ru2(02CC3Hy)4Cl]' in 1969, and the first mixed metal multiple bond, in the compound [C rM o (0 2 C C H ))J in 1974. These were followed in the next ten years by the synthesis and study of a variety of compounds with multiple metal-metal bonds between Itungsten, vanadium and

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Figure 1.3 The Structure of [M o,(0,C C H ,)J

Figure 1.4 The Structure of rRe,Ch(dth),1

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1.2 Bonding in M ultiple M etal-M etal Bonded Com pounds

The dinuclear Rh(II) com pounds which will be discussed in this thesis all contain metal-metal bonds albeit single ones. These metal-metal bonds affect not only the geometry of these com pounds but also their reactivity. It is therefore important to give a general outline of the theoretical framework for metal-metal bonding.

1.2.1 The M olecular O rbital Model F or M etal-M etal Bonding.

The m etal-metal bonds found in the com pounds discussed in this thesis can be understood by considering the overlap o f the d-orbitals on the two metal ions. As the two metal atoms approach each other along the z axis, Figure 1.5, overlap

Figure 1.5 Bonding Interactions Between d-O rbitals

a)

C O

d,.

C O

e )

b)

^

"

f

5

?

d.'-i-030

G

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o f the dp. orbitals will create bonding and anti-bonding sigma (a , a * ) orbitals. Overlap o f the d^^ orbitals and overlap o f the d^y orbitals creates a set o f doubly degenerate bonding and anti-bonding pi ( K, k* ) orbitals. A set of doubly

degenerate bonding and anti-bonding delta ( 6, Ô* ) orbitals is formed from the

overlap o f d^2.y2 and from the overlap o f d^y orbitals, on the two metal atoms.

A ssum ing the basic Huckel concept, which states that when the orbital types are similar, the orbital energies are proportional to the overlap integrals, the ordering of orbitals according to energy, is a<7i«ô<ô%<7i ’<a*. Figure 1.6. the most stable

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being the sigma bonding orbital and the least stable the sigma anti­

bonding orbital. When this model is considered in the context of a set of eight ligands being introduced, the degeneracy of the a and

n

orbitals is not affected but the degeneracy of the 5 orbitals is lifted. This is due to the d^2.y2 orbitals pointing in the direction of the incoming ligands, whereas the dxy orbitals point between the incoming ligands. Hence the d^2.y2 orbitals are used in metal-ligand bonding and the d^y orbitals are used in metal-metal bonding. Therefore there are a total of four bonding and four anti-bonding orbitals for use in metal-metal bonding.

1.2.2 The Valence Bond Model For Metal-Metal Bonding.

The hybridisation of atomic orbitals can also be invoked to describe metal- metal bonds. The nine valence shell orbitals on a transition metal give rise to three types of hybrid orbitals. Figure 1.7.

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Type A, of which there is only one, points away from the other metal atom, and is used in additional axial ligand bonding. Type B, o f which there are four, point in a direction perpendicular to the metal-metal bond and are used in metal-ligand bonding. Type C, of which there are also four, are available for interaction with the type C orbitals on the adjacent metal. This interaction results in the formation o f four extra, single, bent bonds. These single bent bonds can be thought of as being along the arc of a circle. The distance between the two metal atoms can be

calculated assuming the length of the arc to be twice the radius of the single bond, at angles that are equal to the optimum spd bond angles. These calculated bond lengths agreed quite well with experimentally found bond lengths. However, the use of valence bond theory in this area is limited, since it assumes that all the hybrid orbitals are o f equal energy, and does not explain the need for a quadruple bond to have an eclipsed configuration o f ligands.

1.3 Dirhodium (II) Compounds.

The first dirhodium(II) compound was reported in 1960.^° The compound of supposed composition [HRh(O2CH)20.5H2O], which was formulated as a Rh(I) compound initially, was shown to be [Rh2(02CCH)4(H20)2].^* Two years later, structural data based on two dimensional electron density projections gave a Rh-Rh bond distance o f 2.45 Â for this compound. An accurate crystal structure

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undisputed Rh-Rh single bonds were typically 2.7-2.8 Â.23

Figure 1.8 The Crystal Structure of rR h,(0,C C H dJ

C(2)

C (1)

0(1)

0(2)

0(4) 0(5)

R h 0(3)

0 (3')

R h ' 0(4')

0 (5')

0(2 ) 0(1')

The only other structural report on a dirhodium(II) compound at that time was of [Rh2(dmg)4(PPh3)2],^'‘’^^ which was reported to have an Rh-Rh bond length of

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6

*^

and a bond order of 3.0, Figure 1.9. This difference of opinion

continued until 1977 when SCF-X^-SW^^ calculations demonstrated in a

sophisticated molecular orbital model that the non-bonding Rh 5s and 5p^ orbitals were of much higher energy than initially thought, and the Rh-Rh bond was indeed single.

Figure 1.9 Energy Level Diagram For The Configuration 5*^

4 t

---4

b„(4)

eu(TT)

1.3.1 A M olecular Orbital Scheme For Dirhodium Compounds.

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H ow ever, in these com pounds, interaction between the 7i-orbital of the carhoxylate

oxygen and d,y orbitals causes the k anti-bonding orbitals to be more stable than the Ô* anti-bonding orbitals. This makes the correct electronic configuration a^TC‘^ô^7C*‘’Ô*^, Figure 1.10, giving a net single bond between the metal atoms, and

Figure 1.10 O rbital Energy D iagram For Dinuclear Rhodium (II) Com pounds of The rR h,(0 ,C R )d Type.

K,0

-OJO

.80,

I

4o

2

Occupied -020

>-g 2b _ — 8'

- 0 3 0

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1.4. T h e C hem istry of D irhodium (II) C om pounds.

This survey o f the chemistry of dirhodium(II) compounds will concentrate substantially on synthetic and structural studies.

1.4.1 Dirhodium Tetracarboxylates.

The compound [Rh2(02CH)4(H20)2]^* was first prepared in the early 1960’s by refluxing salts o f [RhCl^^ ] in aqueous formic acid. The compound

[Rh2(02CCH3)4(H20)2l was prepared from a mixture o f acetic acid and an alcohol refluxed with [Rh(0H)3H20]. The standard preparative procedure for

tetra(carboxylato) dirhodium(II) compounds was designed in 1970/^^^ and is shown in Equation 1.1. This procedure involves heating sodium acetate with rhodium trichloride trihydrate in a mixture o f acetic acid and ethanol. The green/blue coloured crude product can be recrystallised from methanol to give crystals of [Rh2(02CCH3)4(CH30H)2]. The axial methanol molecules may be subsequently removed by heating the compound for 30 minutes, under vacuum, above 100°C. The compound [Rh2(02CCH3)4] is used extensively in the preparation of other tetra(carboxylato)dirhodium(II) compounds,” '^^ by a ligand exchange procedure. An excess o f the carboxylic acid to be introduced is heated with [Rh2(02CCH3)4] at a E q u a tio n 1.1

CH3C02Na

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temperature just above its boiling or melting point, for several hours. The excess acid is removed under vacuum, leaving the required tetra(carboxylato)dirhodium(II) compound. A large number of tetra(carboxylato)-dirhodium(II) compounds have been prepared, R = CH3, CF3, C2H5, nC3H7, n-C3H7, CMe3, QHg, QF5, linear chain

n-alkanoates; n = (C H ^ jfh (n = 2,3/ ' " \

C P h ^', (2- P h ) Q H / \ 2,4,6-(p-tol)3QH2‘'\ (2-H0)QH4/^'"^ Complexes using optically active carhoxylate ligands, (O2CCHRR’, R = OH, R’ = and R =

OMe, R’ = Ph)^” are also known. Amino acid ligands such as CH3CH(NH2)C02H (a-alanine)^', and NH2(CH2)2C02H (p-alanine)^^’^^ and pyrrolidine-2-carboxylic^'‘ acid, have also been used as bridging ligands. All of these compounds have a similar structure to that of [Rh2(02CCH3)4(H20)2]. In each compound there are two metal ions bridged by four carhoxylate groups lying in two mutually perpendicular planes, while a variety of donor ligands may occupy the two axial positions, Figure

1.1 1.

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The 1:1 adducts [Rh2(0 2C R )4L], R = C H3, L = A A M P (4-am ino-5-(aminomethyl)-

2-methylpyrimidine)^^, R = CF^, L = IM M e (2,4,4,5,5-pentam ethyl-4,5-dihydro-l//- im id az o ly l-1 -oxy)^^, N IT M e (2,4,4,5,5-pentam ethyl-4,5-dihydro-1 //- im id a z o ly l- 1 - oxy 3-oxide)^^ R = C2H5, L = DD A (durenediamine)^^ PH Z (phenazine)^’ also

possess this structure, with the [Rh2(0 2C R )4L] units linked via the ligands L, into infinite chains. The only dirhodium carhoxylate to be structurally characterised in the solid state as having no formal axial co-ordination is [Rh2(0 2C C3H-7)4]'‘^^ which is arranged in infinite chains through intermolecular Rh-O axial interactions

(2.34(1) A). The only dirhodium carhoxylate that does not have a structure similar to that o f [Rh2(0 2C C H3)4(H2 0)2] is [Rh2(0 2C C H3)4(hpy)]^\ Figure 1.12

Figure 1.12 The S tru ctu re of lR hTO .C C H O .(bpv),1

C11

C

26

017

^ ^ C 2 0

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This contains three bridging and one chelating acetate group, as well as a

chelating 2,2-bipyridine ligand. This results in the two Rh atoms being in different environments, and the Rh-L distances being very different. (Rh-N (terminal)

2.120(10) Â, Rh-O (terminal) 2.466(8) Â).

1.4.2 The Influence o f Axial Ligands on Rh-Rh Bond Lengths.

The compounds [Rh2(02CR)4] form a wide range o f diaxial adducts [Rh2(02CR)4L2] (L = py, PPh^, M c2S0^’^'

etc,)

These are of a variety of colours and that colour is closely related to which atom is bound to the rhodium ion. Green or blue coloured adducts tend to be formed by oxygen donors. Pink, red or violet coloured compounds frequently have a nitrogen atom bound to the metal ion. In contrast compounds which are orange to deep red in colour tend to be observed for sulphur donors. Crystallographic data on a selection of those that have been studied by X-ray diffraction is presented in Table 1.1. The Rh-Rh bond distances vary between 2.486(1) Â for [Rh2(02CCF3)4(PPh3)2] to 2.371(2) Â for compounds [Rh2(02CCMc3)4(H20)2] and [Rh2 {02C( 1 -adamantyl)41 (MeOH)2].

1.4.3 Nitrogen Donor Ligands

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T A B L E 1.1 C ry stallo g rap h ic D ata on Selected rR h -,(O X R )X^1 C om pounds.

C om pound R h-R h

(Â)

R h-L

(A)

Rh2(02CCMe3)4(H20)2 2.371(1) 2.295(2)

Rh2(02CH)4(H20)2 2.38 2.45

Rh2(02CCH3)4(H20)2 2.3855(5) 2.310(3) Rh2(02CC2Hg)4(DDA)2 2.387(1) 2.324(6) Rh2(02CCH3)4(caffeine)2 2.395(1) 2.315(9) Rh2(02CCH3)4(Py)2 2.3963(2) 2.227(3) Rh2(02CCH3)4(PhCH2SH)2 2.4020(3) 2.551(2) Rh2(02CCH3)4(EtNH)2 2.4020(7) 2.301(5) Rh2(02CPh)4(Py)2 2.402(1) 2.247(4) Rh2(02CCH3)4(Me2S0)2 2.406(1) 2.451(2)

Rh

2

(

02

CC

2

Hs)

4

(Me

2

S

0)2

2.407(1) 2.449(1)

Rh

2

(

02

CC

2

H

5

)

4

(PHZ)

2.409(1) 2.362(4)

Rh2(02CCH3)4(tp)2 2.412(6) 2.23(3) Rh2(02CCH3)4(THT)2 2.413(1) 2.517(1)

Rh

2

(

02

CC

2

Hs)

4

(ACR

)2

2.417(1) 2.413(3)

Rh2(02CCH3)4(C0)2 2.419(3) 2.095(2)

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[Rh2(02CCH3)4(N0)(N02)].^^ Ammonia, aliphatic, alicyclic and aromatic amines including polyfunctional amines such as ethylenediamine, o-phenylene-diamine, hydrazine, phenazine (PHZ), durene diamine (DDA), guanidine and its derivatives, diaryItriazenes, and cyclam, as well as heterocyclic nitrogen containing ligands such acridines, 2,2’-bipyridine, acetonitrile, nitric oxide, nitrite and derivatives of pyrimidine all form adducts with [Rh2(02CR)4], in which N atom coordination takes place.

1.4.4 Oxygen Donor Ligands

Since the first dimethyl sulphoxide adduct, [Rh2(02CCH3)4(Me2S0)2],^ ( Rh-Rh = 2.406(1) Â and Rh-O = 2.451(2) Â) was reported, several other

adducts with dimethyl sulphoxide have been prepared and structurally characterised. Other oxygen containing donor ligands include MeOH, EtOH, THF, and a few organic nitroxide radicals readily form adducts. Oxygen coordination also takes place in the urea and 1:2 DMF adducts of the acetate and trifluoroacetate complexes.

1.4.5 Carbon Donor Ligands

Ligands which form axial adducts with [Rh2(02CR)4] in which carbon co­ ordination takes place include CO, C N , RNC, and olefins. The compound [Rh2(02CCH3)4(C0)2]^^^^ must be prepared at -20°C. Complexes such as

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1:1 adducts with [Rh2(02CR)4], (R = CH3, QH5, C3H7, CF3).

1.4.6 Phosphorus Donor Ligands

Since the 1960’s several tertiary phosphine, PR3, adducts^’’^"^ have been prepared. X-ray studies have been reported for [Rh2(02CCH3)4(PX3)2] ( X = OPh,"" Ph"" and OMe""'"") and for [Rh2(02CCF3)4(PX3)2] ( X = Ph, OPh).^ The Rh-Rh bond lengths are the longest found in [Rh2(02CR)4L2] compounds, with the noted exception o f [Rh2(02CCH3)4(NO)(NO2)], with values ranging from 2.4215(6) Â, in [Rh2(02CCH3)4(PF3)4], to 2.486(1) Â, in [Rh2(02CCF3)4(PPh3)2]. The axial bond lengths are also unusually long. These vary from 2.494(2) Â in

[Rh2(02CCF3)4(PF3)2] to 2.340(2) Â in [Rh2(02CCF3)4(PPh3)2]. The Rh-PPh, bonds are found to be longer than Rh-P(OPh)3 bonds by about 0.07 Â in otherwise comparable compounds. This was somewhat surprising since PPh3 is a better electron donor than P(0Ph)3, but could be due to the fact that the phosphorus atom has a smaller covalent radius in P(0Ph)3 ( 1.83 Â vs 1.87 A). The compound [Rh2(02CCH3)4] reacts with phosphine and phosphite ligands in acetonitrile and water to give the adducts [Rh2(02CCH3)4(X)(PR3)]^^’^*( X = NCCH3, PR3= PPhj, P(CH2CH2CN)3, P(OMe)3, P(0Ph)3, Ph2P(CH2)„PPh2( n = 1-4), and X = H2O, PR3 =

[Ph

2

P(m-S

0 3

Ph)]-, [P(CH2CH2C02)3]^-, [P2PCH2CH2NH(Me)2]\

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1.4.7 Other Donor Ligands.

Sulphur bound adducts are formed by the reaction of the compound

[Rh2(02CR)4] with the ligands DMSG, dibenzylsulphide, tetrathiofulvalene, and the macrocycles 1,4,7-trithiacyclononane. The ligand (€113)280 co-ordinates through the sulphur atom with the compounds [Rh2(02CCH3)J [Rh2(02CC2Hg)4]^' and [Rh2(02CC6H5)4]^‘, with respective Rh-S distances of 2.451(2) Â, 2.449(4) A and

2.448(6) Â. However with [Rh2(02CCF3)4] where the metal atoms have a lower charge density the preferred donor site o f (CH3)2SO is the oxygen atom,( Rh-O

2.236(3) Â^‘). The complex [Rh2(02CCH2NMe3)4Cl2].Cl2.4H20^^ contains axial chloride ligands, and has a Rh-Rh bond length of 2.413(1) Â and Rh-Cl distance of 2.557(1) Â. Other halides form corresponding dianionic adducts with

[Rh2(02CCH3)4]^l

1.5 The Chemistry o f rR h,(O X R )J Molecules.

1.5.1 Oxidation and Reduction.

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addition of an electron to a bonding orbital will increase the bond order and

strengthen the metal-metal bond. Whereas addition of an electron to an antibonding orbital will decrease the bond order and weaken the metal-metal bond.

The compounds [Rh2(02CCH3)4] can be chemically oxidised in H2O using CI2 , lead dioxide, or eerie ion, to give [Rh2(02CCH3)4]'^ which is reduced back to the neutral complex in air. The electrochemical oxidation of [Rh2(02CCH3)4] in 0.1 M sulphuric acid is reversible, and occurs at a potential of 1.225(5) V (vs

Similar oxidations occur for essentially all other [Rh2(02CR)4] compounds. However these compounds undergo only irreversible electro-reduction to [Rh2(02CR)4]' ions, which are unstable and rapidly decompose.^^

The oxidation potential of an [Rh2(02CR)4L2] compound is affected by the identity of the axial ligand, with the tendency of the compounds to oxidise to the mono cation increasing with increasing donor ability of that ligand.^^’^^ For example the compound [Rh2(02CC2H5)4(PPh3)2] is oxidised at E„2= 0.61 V (vs SCE

whereas the compound [Rh2(02CC2Hg)4(H20)2] undergoes oxidation at E ,,2 = 0.99

The relative contribution of the a and

n

bonding ability o f the ligand also affects the ease o f oxidation. The Rh-L bond, which is mostly due to sigma

interaction, is strengthened by the

n

acceptor ability of many ligands. The stronger the interaction between the ligand and the Rh atom the easier the oxidation

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compounds. Electron donating groups on the carboxylate contribute to the stability o f the higher oxidation state of the metal. Conversely when R is an electron withdrawing group the stability of the higher oxidation state of the metal is reduced. Experimental evidence for these arguments has been obtained by cyclic voltammetric investigations.^^ When the substituent group R is CMe^, oxidation of [Rh2(02CR)^] to [Rh2(02CR)4]^ takes place at E1/2 = 0.92V (in DMF vs SCE ), while if it is CHCICH3, oxidation takes place at the higher potential of

E|/2 = 127 V.

1.5.2 The Use o f [Rh2(02CR)J Compounds as Catalysts.

A variety of [Rh2(02CR)J compounds are useful catalysts in a number of reactions. A few representative examples are mentioned below

a) oxidation, hydroformylation, and hydrosilylation reactions:

The oxidation o f cyclohexene to 1,2-epoxycyclohexene-3-ol is catalysed by the use o f [Rh2(02CR)4]^’ compounds in conjunction with vanadium or molybdenum compounds at 55° C under 1 atm. pressure o f oxygen. The compound

[Rh2(02CCH3)4] can be reduced to Rh(I) complexes which are used in olefin hydroformylations.*® Additionally [Rh2(02CCH3)4] will also catalyse the

hyrosilylation o f terminal olefins, dienes, cyclic ketones and terminal acetylenes. b) hydrogenation reactions:

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not take place with internal olefins, trans olefins or ketones, whereas terminal olefins undergo double bond isomérisation,

c) carbene reactions:

Carbenes, R2C: , are highly reactive species and can be generated by the decomposition of diazoester, eg CH302CCH=N=N -> CH3O2CCH + N2. [Rh2(02CR)4] complexes will catalyse diazoester decompositions giving the

appropriate carbenes in high yield. Carbenes will undergo insertion reactions into polar bonds, catalysed by [Rh2(02CCH3)4], eg. carbomethoxycarbene CHCO2CH3 inserts into X-H bonds (X = O, S, NH) to give RXCH2CO2CH3. Likewise ethyl diazoacetate undergoes insertion into ROH bonds to give ROCH2CO2CH2CH3 (R = Et, Pd, Bu‘, H, CH3CO The compound [Rh2(02CCH3)4] is also used in the cycloaddition o f diazoesters to alkenes and alkynes giving cyclopropanes and cyclopropenes. The cyclisation of X2C=CHCH=CMe2 ( X = Cl , Br) with ethyl diazoacetate to give cyclopropane carboxylic acid esters is catalysed by

[Rh2(02CCH3)4].*'‘ The compound [Rh2(02CCH3)4] can also be used as a catalyst in the production o f thiophenium ylides from the addition of dimethyl diazomalonate to thiophene.®^

1.5.3 Activity as an Anticancer Agent

In 1972 it was reported that the [Rh2(02CR)4] compounds are effective against certain tumours in mice. The effectiveness o f the reagents can be increased by the use of the polyadenylic acid adduct o f [Rh2(02CC2Hg)4], and use of

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drawbacks to the use o f this type of compound in chemotherapy, namely

a) the instability of [Rh2(02CR)J

in vivo,

where it decomposes within hours to give CO2, acetate ions and Rh metal,

b) the effectiveness is limited since almost no effect was observed against melanoma B16 or Leukaemia L1210 tumours in mice, which are the most commonly used predictors of effectiveness against a range of human cancers, c) the inherent toxicity of the compounds.

1.6 Dirhodium Compounds Containing Four Alternative Bridging Ligands.

As we have seen, there is a large variety of ligands that form axial adducts with [Rh2(02CR)4l compounds. The complexes o f Rh2^^^ have the most diverse range o f axial ligand adducts in comparison to the complexes o f Cr2'^‘^, Mo2^^^, Tc2^'^, Ru2^^, Re2^^, and Os2^^. The number of non-carboxylate ligands that form bridges across Rh2^^^ units, was in the past limited, but has been expanding over the last decade.

1.6.1 Oxyanion Bridging Ligands

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the compound [(NH4)Rh2(S04)J.4.5H20]^^ was isolated, and it was proposed that this compound contained bridging sulphate ligands. Eight years later, the reaction of concentrated sulphuric acid with [Rh2(02CH)4] was observed to result in the formation of Cs4[Rh2(S04)4(H20)2].2H20*^ which led to the suggestion that the compounds synthesised in 1966, did in fact contain sulphate bridges. Since that time several complexes with sulphate bridges have been prepared and structurally characterised.^’^'

The compound with an empirical formula [C(NH2)3]2Rh(C03)2 was first reported in 1967^^, as was [Rh2(C03)4]''‘ which was prepared by the reaction of

[Rh2(02CCH3)4] with concentrated aqueous solution of the alkali metal carbonate. However it was not until 1980, that the structures of Cs4[Rh2(CQ)4(H20)2].6H2O and Cs4Na2[Rh2(C03)4Cl2].8H20, with Rh-Rh bond lengths of 2.378(1)

A

and

2.380(2) Â respectively, were structurally characterised.^^ The reaction of

orthophosphoric acid with [Rh2(02CCH3)4] gave [Rh2(H2P04)4(H20)2]^\ which was structurally analysed by X-ray diffraction. Figure 1.13.

Figure 1.13 Crystal Structure o f rRh,(H,POjH(H,0),l

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The results of the diffraction study confirmed four bridging [H2PO4] groups around a Rh2'^^ unit with an Rh-Rh bond distance of 2.487(1) much longer than that observed in related [Rh2(02CR)J compounds.

1.6.2 Hydroxypyridine and Aminopyridine Ligands.

Dirhodium compounds with hydroxypyridinate and aminopyridinate bridging ligands have been prepared by the reaction o f [Rh2(02CCH3)4] with an excess of the molten ligand under an inert atmosphere for 60 hours.^^ The excess ligand is removed by vacuum sublimation, and the procedure repeated with a fresh batch of ligand. A range of Rh-Rh bond lengths are observed in compounds containing these two ligands, and are given in Table 1.2. Six crystallographic studies on compounds with four bridging 6-methyl-2-hyroxypyridine ligands have been reported.^^'^ Structures containing four bridging 6-chloro-2-hydroxypyridine ligands and 6-fluoro-2-hydroxypyridine ligands have also been reported.^^ '°°The

compounds [Rh2(6-CH3C5H3NO)4],^' [Rh2(6-CH3QH3N0)4 (H2O)]"", and [Rh2(6- ClCgH3N0)4]^^ all adopt a 2:2 transoid arrangement of ligands about the metal- metal bond, all have no axial ligands, and the pendant substituents on the aromatic rings block the axial sites. The Rh-Rh bond distances are all short, 2.359(1) Â,

2.367(1) Â and 2.379(1) respectively. The compounds [Rh2(6-CH3CgH3N0)4(6- CH3QH3NOH)]"\ [Rh2(6-CH3qH3NO)4(C3H4N2)0.5CH3CN],"" and [Rh2(6-

CH3CgH3N0)4(CH3CN)]^^ all adopted a 3:1 arrangement o f bridging ligands. Figure

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Table 1.2 Rh-Rh Bond Lengths For a Range of Dirhodium Compounds.

Compound Rh-Rh(A)

[Rh;(CH,C;H]NO)J 2.359(1:

[Rh2(CH3C;H3NO),(CH;C5H3NOH)C6H;CH;] 2.383(1

[Rhj(CH3C5H3NO)4CHjCl2] 2.369(1

[Rh2(CH3C;H;N0)4H20] 2.367(1

[Rh2(CH3C5H3NO)4CH3CN] 2.372(1

[Rh3(CH3C;H3NO)4(C3H4N3).0.5CH3CN] 2.384(1

[Rh3(CH3C5H3N0)j(02CCH3)2(C3H4N2)] 2.388(2

[Rh2(CH3C5H3NO)2(OjCCH3)3(C3H4N2)(CH2Cl2)2] 2.388(1

[Rh3(ClC;H3N0)4] 2.379(1

[Rhj(ClC5H3NO)4(C3H4N3)(HP)J 2.385(1

[Rh;(FC;H3NO)4(DMSO)] 2.410(1

[R h3(C ,H 4N N C A )4(C A C N )] 2.412(1

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compounds to have only one axial ligand, and the first to show twisting (-20°) of the bridging ligands about the Rh-Rh bond. The compound [Rh2(6-CH3QH3N0)4- (6-CH3C5H3NOH)] has an affinity for the 3:1 orientation of the ligands, so much so that once the [6-CH3C5H3NOH] ligand has coordinated to the axial site of the [Rh2(02CCH3)J molecule, exchange o f the acetate ligands takes place to give only the 3:1 orientation, instead of the 2:2 orientation.

Figure 1.14 The Structure of rRh.(6-CHXcH,NO).(CH,CN)1

c 1 4 C 2 3

C 2 6

C 3 3

C 4 6

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atom from a bridging ligand o f another molecule. Thus in the crystal adjacent units were found to be linked together

via

intermolecular Rh-O bonds (2.236(3) Â), Figure 1.15.

Figure 1.15 Two Adjacent Units o f rRh,(6-CH,CcH^NO)J

Linked by Rh-O Bonds

R h(2

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ligation takes place at the rhodium atom co-ordinated by four oxygen donors. Two crystallographic studies on compounds with four bridging aminopyridinate ligands have been reported. The compound [Rh2(QH4NNQH5)4(C6H5CN)]/°‘ was shown to adopt a 2:2 transoid arrangement of the bridging ligands while the other compound, [Rh2(QH4NNQH5)4Cl],*®‘ which contains a [Rh2]^^ unit, adopts a polar arrangement around the metal-metal bond.

1.6.3 Acetamide And Amidate Ligands

Amidate ions provide a bridging functionality which bonds through two different substituents to a metal. In theory, the replacement of the four bridging carboxylate ligands by the amidates can give up to twelve different species. Figure

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Figure 1.16 The Twelve Species G enerated By The Replacem ent of Carboxylates By Amidate Ligands.

" <

o / \

I R"

\

• " ' V X

IE m

!

e

/-I

' \

%

\

x i ~ /

%

X X

N

X X

v x

\ X „ .

X . X

X ,

z r Z M

b- iX

X

XI

\x"x

!

I T

-Xbb

X I

X / \ .

(39)

Reaction of [Rh2(02CCH3)^(CH30H)2] with a c e ta m id e g a v e six of the twelve possible species. A crystal structure determination of one o f these,

[Rh2(HN0CCH3%(H20)2.3H20] demonstrated structural type IX. The reaction of [Rh2(02CCH3)4(CH)OH)2] with benzamide‘°^ gave only one isomer. The crystal structures of the pyridine and triphenylstibene adducts were investigated by X-ray d if f r a c tio n ,F ig u r e 1.17. When N-phenylacetamide'°^ '°^ was reacted with

[Rh2(02CCH3)^(CH)0H)2] seven of the possible twelve species were isolated. The pyridine and triphenylstibene adducts of two of these were characterised by crystallography'®® and shown to be of types IX and XI. All the compounds which have been structurally characterised show pairs of

cis

nitrogen and

cis

oxygen

Figure 1.17 Crystal Structure o f rRh,(HNOCC^Hg)^ (CcHgN),1

CIS N2

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atoms arranged around each Rh atom, with Rh-Rh bond lengths ranging from

2.415(1)Â to 2.472(3) Â and Rh-L distances o f 2.227(7) Â to 2.681(1) Â. The triphenylstibene adducts of the compound' [Rh2(02CCH3)

-(HNOCCH))]] have been prepared and structurally c h a r a c te r is e d .A summary of some relevant structural data for these compounds is presented in Table 1.3.

Table 1.3 List of Rh-Rh Bond Lengths For rRh,(HNOCR)X->1 Compounds.

Compound Rh-Rh(A) Type

[Rh2(HNOCC6H5)4(C6H5N)2] 2.437(1) IX [Rh2(HNOCCF3)4( C A N )2] 2.472(3) IX [Rh2(HNOCC6H5)4(SbPh3)2.CH2Cl2] 2.463(1) IX [Rh2(HN0CCH3)4(H20)2.3H20] 2.415(1) IX [Rh2(C6H5NOCCH3)4(DMSO)2] 2.448(1) IX

[Rh2(C6H5NOCCH3)4(DMSO)] 2.397(1) XI

Electrochemical investigations were carried out on the compounds [Rh2(02CCH3)„- (HN0CCH3)4.„]^°^’^‘‘^ ( n = 0, 1, 2, 3, 4 ) in four different solvents. The results showed the redox potentials shifted to more cathodic values when carboxylate ligands were replaced by amidate ligands. For the compounds [Rh2(02CCH3)- (HNOCCH)))] and [Rh2(HN0CCH3)J a second one electron oxidation was

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the amidate ligands and on the solvents used. The electrochemically produced cations [Rh2(02CCHg)4„(HN0CCH3)J^, and their adducts have been analysed by ESR, electronic absorption and Raman s p e c t r o s c o p y . A one electron oxidation o f [Rh2(HNOCCF^)^] is also observed in a variety of non-aqueous solvents, where E„2(ox) varies between +0.91 V, and +1.08 V. Electrolysis of the

[Rh2(HNO-CCH3)4(SbPh3)2] in dichloromethane in the presence of an excess of SbPh3 cleaves the Rh-Rh bond and results in the formation and isolation of [Rh2(Ph)Cl2(Sb-Ph3)2(NCMe)]. ‘

1.6.4 T hiocarboxylate Ligands.

The compounds [Rh2(OSCR)4(P(C6Hs)3)2], (R = CH3, QH5, € ( ^ 3 ) 3 )

prepared by the reaction of the orthometallated compound [Rh2(02CCH3)2[(QH5)2P- (C6H4)]2(THF)2] with monothiocarboxylic acids (HO(S)CR R = CH3, QH5,

€(0113)3) have been fully ch aracterised .* T h e crystal structure determination of [Rh2(OSCC(CH3)3)4(P(C6H5)3)2] showed four cisoid bridging monothiopivalate groups with the triphenylphosphine molecules

trans

to the metal-metal bond, Figure

1.18. The Rh-Rh bond distance, 2.584(1) Â is greater than that observed earlier in the structure of [Rh2(OSCCH3)4(HOSCCH3)2], 2.550(3) and both of these bond lengths are longer than those observed in [Rh2(02CR)4LJ compounds. This observation is probably due to the large bite o f the RCSO ligand. The compounds [Rh2(OSCR)4(CgHgN)2] (R = CMc3, Ph)**^ have also been prepared and

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com pound [Rh2(0SCCMe3)^(C5H)N)2 ]. Subtle differences in bond lengths in related molecules of these general types have been attributed to the electronic effects o f the two different substituents on the carboxylate groups as mentioned earlier.

Electrochemical studies on solutions o f the com pounds [Rh2(0SCR)4(PPh^)2] ( R = CMC), CH3, Ph) in dichloromethane have shown potentials o f E„2= + 0.56 V, + 0.65 V, and + 0.99 V vs Ag/AgCl, respectively. The electronic, infrared, Raman and resonance Raman spectra of [Rh2(0SCCH3)^L2], L = PPh^, AsPh^, SbPh^, and CH3COSH, have been recorded and show"^, as earlier studies o f Rh2‘*'’ com pounds indicated, the G(Rh2) a*(Rh2) transition at -4 0 0 nm and o(R h-R h) frequencies in the range 226-251 cm '

Figure 1.18 The Structure of rR h,(O SCC(CH A .)TP(CH AQ ,l

CIO

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1.6.5 Formamidinate Ligands.

Reaction of [Rh2(02CCH3)4] with N ,N ’-diphenylformamidine (Hdpf) results in the substitution of all the bridging acetate ligands by formamidinate bridges to produce the compound [Rh2(dpf)j/^^ This compound readily reacts to form adducts with CH3CN and CO. The Rh-Rh bond length in [Rh2(dpf)4(CNCH3)]^‘® o f 2.459(1) Â is similar to that o f 2.457(1) Â in [Rh2(dpf)4]. The compound [Rh2(form)4]**^, which is prepared by the reaction of [Rh2(02CCF3)2(form)2(H20)2] with N ,N ’-di-p- tolylformamidine, is similar in structure to [Rli2(dpf)4] and exhibits a Rh-Rh bond distance of 2.4336(4) Â. The reaction of the Rh2^^ complex [Rh2(form)3(N03)2]'^° with an excess o f the ligands PPh3, py and Me2NH results in the formation of the compounds [Rh2(form)3(N03)L]. The structural characterisation of the adducts [Rh2(form)3(N03)L], (L = PPh3 and py)*^* show a structural rearrangement, with the L ligand equatorially bound to one rhodium atom and the nitrate group chelated to the other rhodium atom, with Rh-Rh distances of 2.498(2) Â and 2.476(1) Â, and Rh-L distances o f 2.199(16) Â and 2.280(6) Â respectively.

1.6.6 Pyrazolato Ligands

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reform the compound [Rh2(3,5-Me2pz)4]. Other related compounds which have been prepared and structurally characterised are [Rh2(pz)4(NCCH3)2], [Rh2(pz)4](pz = pyrazolato) and [Rh2(|i-pz)2{fi-Ph2P(C6F3)Br}(CO)(PCBr)], (PCBr =

PPh2(QF4-o-1.6.7 Other Ligands

The compound [Rh2(NCgH4NPh)4] is prepared by the reaction of

RhCl3.xH20 with the sodium salt of 2-anilinopyridine. This compound can exist as four geometric isomers. One isomer, with a benzonitrile axial ligand, has been structurally characterised, Rh-Rh = 2.412(1) Â, Rh-NC^Hg = 2.189(10) Â. Reaction of [Rh2(02CCH3)4] with the ligands 2-pyrrolidinone (Hpyro), Ô- valerolactam(2-piperidinone, Hvall) and (o-thiocaprolactamate(tcl), produces the tetra bridged complexes. The compounds [Rh2(pyro)4(Hpyro)2] and

[Rh2(vall)4(Hvall)2] both have a cisoid arrangement of the N and O donor atoms about each Rh ion, with Rh-Rh bond distances of 2.445(1) Â and 2.392(1) Â, respectively.*^^ The compound [Rh2(tcl)4(Htcl)] (Htcl = co-thiocaprolactam) has four S atoms bound to one Rh and four N atoms bound to the other Rh atom. Axial co­ ordination takes place at the Rh atom with four S atoms, with a Rh-S axial bond of

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1.7 Mixed Sets o f Bridging Ligands

1.7.1 Hydroxypyridine Ligands.

The first mixed bridging ligand dirhodium compound to be characterised structurally was [Rh2(02CCH3)2(6-CH3CsH3N0)2(C3H^N2)]^^, Figure 1.19. The compound was obtained in an incomplete exchange reaction using the procedure

Figure 1.19 The Crystal Structure of rRh,(0,CCH,),(6-CH.CcH»NQ).(C,H.N,)1

C 1 0 N3

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[Rh2(CH3QH]N0)^-(C3H4N2)], and shorter by ~ 0.1 A than that o f a range o f [Rh2(02CCH3)4L2] com pounds, ( L = heterocyclic N donor ligand).

1.7.2 O rthom etallated Phosphine Ligands.

The com pounds [Rh2(02CCH3)2((C6H))2P(QH4))2L2]'^^ (L = CH3CO2H, NC5H5) were first reported in 1985, and contain orthometallated triphenylphosphine ligands. Crystallographic studies showed each structure to have two cisoid bridging acetate groups and two metallated triphenylphosphine ligands. Each rhodium atom is bonded to one phosphorus atom, a carbon, and two carboxylate oxygens, in addition to the axial ligand. The R h-Rh bond lengths are 2 .508( 1) À for

[Rh2(02CCH3)2[(C6H5)2(C6H4)]2(CH3C02H)2] and 2 .556(2) Â for [Rh2(02CCH3)2- {(C6H,)P(C,H4)}2(C,H3N)2], Figure 1.20.

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The compounds [Rh2{Ph2P(QHJ )2Cl2(PR3)2], (PR3 = PMe^, PPh^), and

[Rh2{Ph2P(QH4)}2Cl2(dmpm)], dmpm = Me2PCH2PMc2, have been prepared by the reaction of [Rh2(02CCH3)2{Ph2P(QH4))2] with PMe^, or PPh^ and dmpm, in the presence of Me^SiCL All three have a similar structure with two orthometallated phosphine bridging ligands and two chloride bridges arranged about the dimetal centre with Rh-Rh bond distances of 2.506(1)

A,

2.499(1) Â and 2.770(3) Â re s p e c tiv e ly .'^ ^ M a n y additional examples of orthometallated dirhodium(II) compounds with a variety of carboxylates and tertiary phosphine ligands have now been reported, including [Rh2(02CCMe3)2 {Mc2P(C6H4)}2(H20)2] Rh-Rh distance

2.492(1) À and [Rh2(02CCMe3)2 {PhMeP(C6H4)} 2] ' wi t h a Rh-Rh bond of

2.535(5) Â, Figure 1.21. Structural data for these are collected in Table 1.4.

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Table 1.4 Rh-Rh Bond Lengths o f Orthometallated Dirhodium dn Compounds

Compound Rh-Rh (Â)

[Rh2(O^CCH3)2((C,HJP(C,H;)2)2(C;H;N)j 2.556(2)

[Rh,(02C CH ,),((C «H JP(C ,H ;)2},(CH3C02H ) j 2.508(1) [Rh,(02CCH3)2{ (C ,H ,)P (o -C lC A )(C ,H ;)|- 2.558(1) | ( C A ) P ( C A ) J P ( C A ) ]

[Rh2(0,CCH3),((C ;H ;N )P(C ,H ;);)2CW 2.518(1) [Rh2(0,CC(CH3)3)2{(C«HJP(CH3)(C«H:)}2(C ;H ;N )J 2.535(5)

[Rh2{(CH]),PCH2P(CH3)j2((C6HJP(C«H;)j2Cl2] 2.770(3)

[Rh2(02CCH]),{Ph2P (C A )) (C H3C02H)2] 2.430(2) [Rh2(02CCM e;)2 {M e;P(C ,H J ) ^(H^O);] 2.492(1)

[RhjlOjCCHj)^! (m-MeC(,H,)2P(m-MeC6H3) I2- 2.502(3)

(CH;C02H)2].CH3C02H

[Rh2(02CCH3)3|PhP(C,HJ(C«F,Br)} (PPh2(CA Br)} 2.519(3) [Rh2(02CCH3)2{ P P h (C ,H J(C f,B r)}2(H20)].H20.CH2Cl2 2.485(1) [Rh2(02CCH3)2(P h P (C ^H J(C f,B r)}2].2CH2Cl2 2.475(1) [Rh2(Ph2P ( C A ) }2Cl2(PMe3)2].C2H :.C,H; 0 2.506(1)

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T w o different bridging metallated phosphine ligands are present in the com pound [ R h ; ( 0 ,C C H ,) 2 ( ( C A ) P ( Q H .,) 2 H ( Q H ,) P ( o - C I C ,H ,) ( Q H 5 ) |( P ( C ,H 5 ) ,) ] .'” One rhodium ion has both the phosphorus atoms, o f the bridging phosphines, c o ­ ordinated to it, while the other rhodium ion has the orthometallated carbon atoms and the axial triphenylphosphine ligand in its coordination sphere. This observation led to the suggestion that this com pound could be formulated as containing a Rh(I)/Rh(III) unit, rather than a Rh(II)/Rh(II) one.

Recently it has been reported that the reaction o f [Rh2(02CCH3)4(CH30H)2] with the phosphine ligand îris{2, 4 , 6-trimethoxyphenyl) phosphine (TMPP)

resulted in formation of the com pound [Rh2(02CCH3)3[( C^H2(OCH3)3}2P-

{QH2(0CH3)20}](CH30H)(CH3CH20H)].'^-^ ‘^^ a single crystal X-ray diffraction study confirmed that three acetate bridging ligands were present, and that one T M P P ligand formed two chelate rings with the rhodium atoms. Fig. 1.22

Figure 1.22 The S tructu re of [R hTO X C H O TTM PP)]

0 ( 1 4

C ( 2 3 )

C( 31 C ( 2 6 )

0( l2) ^ ^ C (27) \ ^ ^ ^ ^ ^ j C { 2 0 )

C ( 1 9 )

C ( 2 4 )

0(1 0)

C ( 2 2 ) ^ ( 3 2 ) y C ( 1 5 )

CmT! 0(2)

0 ( 1

%

0(1 6)

0(2 1)

0 ( 6 ) C ( 5 ) P C ( 6 )

O C ( 1 3 )

0 ( 3 3 )

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The phosphorus atom occupies an equatorial position on one metal, and one

o-methoxy group has demethylated to form an alkoxide linkage. The Rh-Rh bond distance is 2.4228(3) Â, and the metallated oxygen atom is bound more strongly to the rhodium ion (2.048(2) A), than the axial ether group (2.351(2) A).

1.7.3 N ap h th y rid in e Ligands.

Dirhodium(II) compounds containing a combination of acetate and 1,8- naphthyridine ligands were prepared by Ford and c o - w o r k e r s , a n d the crystal structure of one of them, [Rh2(02CCH2)3(C|gH,2N4)][PFJ was reported, Figure 1.23. One o f the acetate groups of [Rh2(02CCH3)^] is replaced by the tetradentate ligand

2,7-6/j(2-pyridyl)-1,8-naphthyridine, leaving a structure with three bridging acetate and one naphthyridine ligand.

Figure 1.23 The Structure o f lRh,(O.CCH,),(C,RHnN^)r

0 6 0 5

0 5 061

,04

0 4

,024 ,07

0 2 3 Rh2,

108

N4

02

,022 t2

09

,011

020 N2

021 N3

'CIO

1012

'019 016

018 ,015 ,013 014

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The com pounds [Rh2(02CCH3)3(bpnp)]-[PFJ.2H20, [Rh2(02CCH3)3(dinp)]-

[P F J.3 H 2 0 , [Rh2(0 2C C H3)3(pynp)][PFd, and [Rh2(0 2C C H3)2(pynp)2][P F J , [bpnp =

(2,7-^w (2-pyridyl)-l,8-naphthyridine), dinp = (5,6-dihydrodipyrido[2,3-^:3’,2’- y][l,10]-phenanthroline), pynp = '(2-(2-pyridyl)-l,8-naphthyridine)] each show a

one-electron reversible oxidation plus one or tw o reversible reductions in

acetonitrile.'^^’*^^ An anodic shift for both oxidations and reductions was observed for these com pounds in com parison to those observed for [Rh2(0 2 CCH3)^-

(CH3CN )2]. This led to the suggestion that the naphthyridine ligands are less

electron donating than the acetate ligands.

1.7.4 Trifluoroacetate Ligands.

The com pound [Rh2(02CCF3)2 {[(p-tolN )2CH] } 2(H20)2 reacts w ith Ph2Ppy to form

the com pounds [Rh2(|i-0 2 CC F3){[(p-tolN )2CH] }2(li-Ph2P py)(0 2C C F3)] and [Rh2{[p-

tolN )2CH] } 2(|i-P h2Ppy)2(0 2C C F3)2], in w hich the axial sites are occupied by one or

two m onoligated trifluoroacetate groups r e s p e c t i v e l y .T h e structure o f the 1:2 com plex has been characterised by X -ray crystallography and shows a cisoid

arrangem ent o f the bridging ligands.

1.7.5 Other Ligands

The com pound [Rh2Cl4(dppm )2]'^^ is prepared by the reaction o f [Rh2(02CCH3)4]

w ith dppm and 4 equivalents o f M e3SiCl in refluxing benzene. The structure,

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compound readily reacts with CO to form [Rh2(CO)Cl2(dppm)2] which is oxidized

Figure 1.24 The S tru ctu re of [R h,C L(dppm ),l (phenyl groups om itted for clarity)

CK2)

iCKl)

Rh(l) Rh(2)

P ( 3 )

P(l) P (4)

P(2) w

cW

to [Rh2Cl6(dppm)2] with PhICl2, and reacts with Ag[PF6] to give

[Rh2(CO)Cl)(dppm)2][PFJ containing a long Rh-Rh bond, 3 .010(2) Â.''” The complex [Rh2(|i-pyS)2Cl2(CO)2(pySH)2], prepared by the reaction of

[Rh2Cl2(C0 )^] with pySH in chloroform, has nitrogen and sulphur co-ordinated in a

cis arrangement about the dimetal core. The Rh-Rh bond distance, 2.652(4) A, is an indication of the Ti-accepting behaviour of the two CO ligands.''*^

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1.8 Compounds With Only Two Carboxylate Bridging Ligands.

Although the most common dirhodium (II/II) compounds are those with four bridging carboxylate groups, there is a growing number of compounds with less than four bridging groups, and indeed there are some dirhodium(II/II)

compounds with no bridging ligands at all. Quite clearly, there is no intrinsic need for four bridging ligands, and we now briefly discuss some compounds in which there are only two bridging ligands.

1.8,1 p-Diketonate Containing Complexes.

Rhodium(II) dimers with two bridging carboxylates of the general formula [Rh2(02CR)2(R’C 0 C H C 0 R ” )2L2] (R = CH3, CH3CH2, and (CH3)3C; R ’, R ” =CF3, CH3; L=NCgH3), were first prepared by Cennini

et

in 1967. The appropriate dirhodium tetracarboxylate compound was refluxed with an excess of the p- diketonate ligand in aqueous or alcoholic/aqueous mixtures. The subsequent reaction of the residue with pyridine gave the corresponding

bis

adducts.

(54)

closely related structure. With unsymmetrical p-diketonate ligands such as

[CFjCOCHCOCH,]- two isomers of [Rh2(02CCH,)2(CF;COCHCOCH;)2(NC,H,)]'"" are formed and these will be discussed fully in Chapter 2.

Figure 1.25 The C rystal S tru ctu re of

rRh.(O,CCH0.(CF,COCHCOCF,),(NCcH.),1

C(2 3 )

Rh(2) r i7) F(8l

p

F;9 l F(10)

C n7 l

(55)

1.8.2 Phenanthroline And Bipyridine Containing Complexes.

A compound with two bridging formate ligands, and chelating 1,10- phenanthroline ligands, [Rh2(02CH)2(phen)2Cl2] was first prepared in 1976 by Pasternak and Pruchnik.''*^ Since then Calligaris

et al

have prepared a range of compounds o f general formula [Rh2(02CCH])2(chel)2Cl2l (chel - 1,10

phenanthroline, 4,7-Me2-phen, 3,4,7,8-Me^phen) by refluxing [Rh2(02CCH3)J with the chelating ligand and HCl in stoichiometric amounts. The crystal structures of the N-methylimidazole adducts of these compounds were determined and these will be discussed in detail in Chapter 3. The compounds [RhCl3(bpy)(SR2)], R = Me, Et and [RhCl2(2-methylallyl)]2 have both been used as starting materials to prepare [Rh2(02CH)2Cl2(bpy)2.4H20]'‘^^, Figure 1.26, which has been structurally

characterised. Subsequent reaction with KtMnO^] results in the formation of

[Rh2(02CH)2Cl2(bpy)2(H20)2][Mn0j2.'149

Figue 1.26 The Structure o f rRh,(0,CH).CE(bpvT.4H,01

(56)

1.8.3 Acetonitrile Containing Complexes.

Compounds o f the general formula [Rh2(02CCH^)2L6][X]2 (L = CH3CN, NC5H5; X = [BF4]', [P F J' and [CF3SO3]' have been prepared by reacting

[Rh2(02CCH3)4] with alkylating reagents or strong acids such as [Me30][BF4], in acetonitrile. Crystal structure determinations were carried out on the compounds

[Rh2(02CCH,MCH;CN)/NC,H;)J[BFJ2''"and [Rhj(02CCHj)2(CH,CN)J[BF,]2'“

and both structures have two bridging acetate ligands, four solvent molecules in the remaining equatorial positions, and further solvent or pyridine molecules in the axial sites. The two compounds have similar Rh-Rh bond lengths, 2.548(2) Â and

2.534(1) Â, respectively. Fig. 1.27

Figure 1.27 The Crystal Structure o f rRh,(OXCH,),(CH^CN)(NCcHg)-,12+

C(UL

C ( 5 4 )

(57)

1.8.4 Dimethylglyoximate Containing Complexes.

The compound [Rh2(02CCH3)2(dmg)2(H20)2] was prepared by the reaction of [Rh2(02CCHg)4(H20)2] with dimethylglyoxime in methanol. A crystal structure determination carried out in 1971, on the related tertiary phosphine adduct

[Rh2(02CCH3)2(dmg)2(PPh3)2]'^‘, shows two acetate ligands bridging in a cisoid arrangement, with one dimethylglyoximate anion chelating to each metal atom, and the PPh3 ligands occupying the axial positions. This structure and that of related compounds will be discussed fully in Chapter 5.

1.9 Compounds Containing Unsupported Rhodium(II)-Rhodium(II) Bonds.

The identity, type and number of bridging ligands are important factors that influence Rh-Rh bond lengths. However they are not a requirement for keeping the two rhodium atoms together, and this is exemplified in the growing number of compounds with unsupported Rh(II)-Rh(II) bonds. A brief survey o f the known compounds containing such unsupported bonds is presented below.

1.9.1 Dimethylglyoximate Containing Compounds.

One o f the earliest examples o f a dirhodium(II)/(II) compound with no bridging ligands was [Rh2(dmg)4(PPh3)2]‘^^ which was prepared by the reaction of

(58)

such as H2O, M cjSO , and pyridine are also known to occupy the axial sites. The crystal structure of [Rh2(dmg)^(PPh3)2]^^ was determined, Figure 1.28, and the unsupported Rh-Rh bond has a length of 2 .936(2) Â. A related com pound, of formula [Rh2(dmg)^(py)2] is described in C hapter 5. It has a dramatically shorter unsupported R h-Rh bond, of length o f 2.724(2) Â.

Figure 1.28 The Crystal S tru cture of [R h,(dm g)TPPh,),1

1.9.2 M acrocyclic And Porphyrin C ontaining Com pounds.

The condensation of acetylacetonate with o-phenylenediam ine gives the c om pound C22H24N4, (tetraaza(14)annulene).‘^'‘ The reaction o f [C22H22N4]^' with [Rh2(02CCH^)4(CH^OH)2] results in the formation o f a black air sensitive

(59)

one ligand occupying the equatorial plane of each metal atom. The Rh-Rh bond distance, 2.625(2) Â '” , is less than that of [Rh2(dmg)4(PPh^)2], (2.936(2) Â)."'*-^*' This difference was ascribed to the minimisation of the steric repulsions between the macrocyclic ligands in the staggered configuration, and the out-of-plane displacements of the Rh atoms toward one another, plus the absence o f axial ligands. The porphyrin ligands, tetraphenylporphyrin dianion (TPP^‘) and

Figure 1.29 The S tru c tu re of

N

N

octaethylporphyrin dianion (OEP^ ) have both been used to produce dinuclear rhodium(II) porphyrin compounds. The reaction of [Rh(OEP)Cl] with H2 in methanol solution gives the compound [Rh(OEP)H] which is converted thermally or photochemically'^^ to [Rh(0EP)]2. This compound is diamagnetic and is

presumed to have a similar structure to [Rh2(dmg)4(PPh;)2]. The dimer, [Rh(0EP)]2, reacts with NO in toluene solution to give [Rh(OEP)(NO)], and with oxygen at -

(60)

refluxing [Rh(CO)2Cl]2 with a solution of H2TPP in acetic acid was reported to give a mononuclear, paramagnetic compound thought to be [Rh(TPP)]. Later, it was proposed that this compound was actually [Rh(TPP)(02)] which on sublimation in vacuum gave a diamagnetic compound with an analysis consistent with

[Rh(TPP)]2, and which reacted with NO and O2 in a similar fashion to [Rh(0EP)2].

1.9.3 Isocyanide Containing Compounds.

Isocyanide complexes of Rh(I) were known as early as 1959^^*, however Rh(II) species were only obtained in 1975*^^, by the reaction of [RliCCNR)^^ with [Rh(CNR)4X2]^ ( X = halogen, or SCF3) in solvents such as acetone and aceto­ nitrile. Later the reaction of I2 with [Rh(CNR)4]^ in a 1:2 mole ratio was used to obtain isocyanide complexes of Rhodium(II).^^'^^ The two structures which have been reported are those of [Rh2(p-CH3C6H4NC)gl2][PFJ2^^, (Rh-Rh = 2.785(2) Â, with a rotational conformation twisted 26° from eclipsed), and [Rh2(CNCH2C- H2CH2NC)4]Cl2.8H20]'^^ (with Rh-Rh = 2.837(1) Â, in an eclipsed conformation) Figure 1.30.

1.9.4 The Aqua Ion [Rh2(H20),o]'‘^

The ion [Rh2(H20)jo]'^'^ was first prepared in 1968 by Maspero

et a l}^

It was formed by the reduction o f [Rh(H20)gCl]^^ with [Cr(H20)6]^^. No crystal structure determination has been carried out and the formulation is based on the observations outlined

(61)

415 and 250 w hich w ere observed w hen the reactant solution ratio o f

Cr^^;RhCl^^= 1, the intensity o f these bands did not increase w ith increasing m ole ratio, but did decrease w hen the reagent ratio w as less than one,

b) the product solution m ade up w ith the reactants at equal concentrations was separated using a cation exchange resin. T he first species to be eluted was identified as [CrCl^^q)]^'' and the chrom atographic behaviour o f the second species indicated a charge greater than 4-3.

c) m agnetic susceptibility m easurem ents indicated a diam agnetic ion, w hich,

d) reacted readily w ith N a [0 2C C H3] to give a product w ith absorption bands in the

visible region at 445 and 587 nm (sim ilar to that o f [Rh2(02CH 3)/H 20)2] at 447 and 587 nm).

T hese observations led to the conclusion that the dom inant form o f the Rh(II) product w as a m etal-m etal bonded [Rh2]‘‘'' unit.

Figure 1.30 The S tructu re of The Cation lR h,(p-C H ,C H ,N C )«f+

C2B

C 2 A

C3B &

---N3 C3A CIA NI GIB

N4LX

Figure

Figure 1.1 The Structure of

Figure 1.1

The Structure of p.10
Figure 1,2 The Structure of fReXUV

Figure 1,2

The Structure of fReXUV p.11
Figure 1.3 The Structure of [M o,(0,CCH,)J

Figure 1.3

The Structure of [M o,(0,CCH,)J p.12
Figure 1.4 The Structure of rRe,Ch(dth),1

Figure 1.4

The Structure of rRe,Ch(dth),1 p.12
Figure 1.5 Bonding Interactions Between d-Orbitals

Figure 1.5

Bonding Interactions Between d-Orbitals p.13
Figure 1.7 The Three Types of Hybrid Orbital

Figure 1.7

The Three Types of Hybrid Orbital p.15
Figure 1.8 The Crystal Structure of rRh,(0,CCHdJ

Figure 1.8

The Crystal Structure of rRh,(0,CCHdJ p.17
Figure 1.10 Orbital Energy Diagram For Dinuclear

Figure 1.10

Orbital Energy Diagram For Dinuclear p.19
Figure 1.14 The Structure of rRh.(6-CHXcH,NO).(CH,CN)1

Figure 1.14

The Structure of rRh.(6-CHXcH,NO).(CH,CN)1 p.35
Figure 1.24 The Structure of [Rh,CL(dppm),l

Figure 1.24

The Structure of [Rh,CL(dppm),l p.52
Figure 1.27 The Crystal Structure of rRh,(OXCH,),(CH^CN)(NCcHg)-,12+

Figure 1.27

The Crystal Structure of rRh,(OXCH,),(CH^CN)(NCcHg)-,12+ p.56
Figure 1.28 The Crystal Structure of [Rh,(dmg)TPPh,),1

Figure 1.28

The Crystal Structure of [Rh,(dmg)TPPh,),1 p.58
Figure 1.29 The Structure of

Figure 1.29

The Structure of p.59
Figure 1.30 The Structure of The Cation lRh,(p-CH,CH,NC)«f+

Figure 1.30

The Structure of The Cation lRh,(p-CH,CH,NC)«f+ p.61
Figure 1.31 The Structure of The Cation [Rh,(CHXN),n1^'^

Figure 1.31

The Structure of The Cation [Rh,(CHXN),n1^'^ p.62
Figure 2.1 The Suggested Structures for rRh,(0 ,CCH,),(B-Diketonato)X^1

Figure 2.1

The Suggested Structures for rRh,(0 ,CCH,),(B-Diketonato)X^1 p.66
Figure 2.2 The X-ray Structure of rRh,(0,CCH,),(CFX0 CHC0CF,),(NCcH.),l

Figure 2.2

The X-ray Structure of rRh,(0,CCH,),(CFX0 CHC0CF,),(NCcH.),l p.68
Figure 2.4 The Cis-Isomer of rRh,(0 ,CCH,)-,(CF,C0 CHC0 CHQ,1

Figure 2.4

The Cis-Isomer of rRh,(0 ,CCH,)-,(CF,C0 CHC0 CHQ,1 p.70
Figure 2.3 The Trans-Isomer of rRh,(O,CCH,)-,(CF,COCHCOCH0,1

Figure 2.3

The Trans-Isomer of rRh,(O,CCH,)-,(CF,COCHCOCH0,1 p.70
Figure 2.5 The Rh-C Interactions Between Dinuclear Units of

Figure 2.5

The Rh-C Interactions Between Dinuclear Units of p.71
Figure 2.9 Structure of rRh,(O,CCH,)-,(CF^COCHCOCF^),(NC«;H01

Figure 2.9

Structure of rRh,(O,CCH,)-,(CF^COCHCOCF^),(NC«;H01 p.79
Figure 2.10 Two Dinuclear Units Coupled Via Rh And O Atoms

Figure 2.10

Two Dinuclear Units Coupled Via Rh And O Atoms p.80
Table 2.3 Atomic Coordinates (xlO'*) and Equivalent Isotropic Displacement

Table 2.3

Atomic Coordinates (xlO'*) and Equivalent Isotropic Displacement p.97
Figure 3.1 Structure of rRh,(0-,CCHa)^(2.2*-bpv)1

Figure 3.1

Structure of rRh,(0-,CCHa)^(2.2*-bpv)1 p.102
Figure 3.5 The Structure of rRh.(HNOCCF,).(NCcH.),l

Figure 3.5

The Structure of rRh.(HNOCCF,).(NCcH.),l p.108
Figure 3.6 'H NMR Spectrum of fRh,(HNOCCFQ,(phen)Xl.l

Figure 3.6

'H NMR Spectrum of fRh,(HNOCCFQ,(phen)Xl.l p.110
Figure 3.7 The Two Possible Isomers of [Rhi(HNOCCFi),(phen)?Cl7l

Figure 3.7

The Two Possible Isomers of [Rhi(HNOCCFi),(phen)?Cl7l p.111
Figure 3.8 H NMR Spectrum of One Isomer of rRh,(HNOCCF,),(phen),CM

Figure 3.8

H NMR Spectrum of One Isomer of rRh,(HNOCCF,),(phen),CM p.114
Figure 3.9 'H NMR Spectrum of lRh,(HNOCCF,l,(phenl,(NC,Hc),1lPFJ,

Figure 3.9

'H NMR Spectrum of lRh,(HNOCCF,l,(phenl,(NC,Hc),1lPFJ, p.115
Figure 3,10 *H NMR Decoupling Experiments on

Figure 3,10

*H NMR Decoupling Experiments on p.117

References