David I. Gilmour

Molecular orbital calculations on platinum-gold heterometallic clusters

Abstract The bonding in some platinum—gold cluster compounds has been analysed using semi-empirical molecular orbital calculations. In (PtAu 2 (PR 3 ) 4 Cl] + (I) and [Pt 2 Au 2 (PR 3 ) 4 (CNR′) 4 ] 2+ (II) the platinum—gold bonding involves three-centre two-electron bonds between the bonding orbital of the Au 2 (PR 3 ) 2 dimer and the frontier orbitals of the T-shaped platinum fragments. The observed distortions in II have been satisfactorily rationalised. In [Pt 3 Au(μ-CO) 3 (PR 3 ) 4 ] + (III) and [Pt 3 Au(μ-SO 2 ) 2 Cl(PR 3 ) 4 ] (IV) the AuPR 3 + fragments cap the Pt 3 triangles by utilising the acceptor orbital of a 1 symmetry localised on gold. The observed differences in bond lengths for these 54 and 56 electron clusters have been accounted for on the basis of the reduced overlap populations.

Molecular orbital analysis of the bonding in high nuclearity gold cluster compounds

Abstract Extended Huckel molecular orbital calculations on high nuclearity gold clusters of the general type [Au(AuPH 3 ) n ] x+ have demonstrated that they can be classified into two broad topological classes according to the three-dimensional disposition of the peripheral gold atoms. If they lie approximately on a sphere they are characterised by a total of 12 n + 18 valence electrons, but if they adopt a toroidal or eliptical arrangement the total electron count is 12 n + 16. The computed energy differences between alternative polyhedral geometries is generally small and accounts for the stereochemical non-rigidity of the gold cluster compounds in solution. Detailed aspects of the structures of the high nuclearity gold cluster compounds have been interpreted in terms of molecular orbital calculations on clusters derived from the centred chair [Au 7 (PH 3 ) 6 ] + by edge- and face-capping with Au(PH 3 ) + fragments.

Chemistry of platinum sulphido-complexes. Part 5. Synthesis and crystal and molecular structure of 3-(η-cyclo-octa-1,5-diene)bis(µ3-sulphido)-1,1,2,2-tetrakis(triphenylphosphine)diplatinum(II)rhodium(I) hexafluorophosphate–dichloromethane (1/1), [Pt2Rh(µ3-S)2(PPh3)4(η-C8H12)]PF6·CH2Cl2

The reaction between [Pt2(µ-S)2(PPh3)4] and [{RhCl(cod)}2](cod = cycle-octa-1,5-diene) gives [Pt2Rh(µ3-S)2(PPh3)4(cod)]Y (Y = Cl, PF6, or BPh4). The molecular structure of [Pt3Rh(µ3-S)2(PPh3)4(cod)]PF6·CH2Cl2 has been determined by single-crystal X-ray techniques using diffractometer data. The compound crystallises in the monoclinic space group P21/c with four formula units in a cell of dimensions a= 16.681(2), b= 17.181(5), c= 27.476(2)A, and β= 103.58(4)°. Least-squares refinement of the structure has led to a final R value of 0.043 using 6 349 observed intensities [I 3σ(I)]. The structure shows an approximately equilateral triangular arrangement of non-bonded metal atoms. The geometries at individual metal sites are approximately square planar.

The synthesis, characterisation and crystal structure of [Pt3(PPh3)2(CNC6H3Me2)6](PF6)2; a linear trinuclear cluster of platinum

[Pt 2 (PPh 3 ) 2 (CN-xylyl) 4 ] 2+ (CN-xylyl = 2,6-dimethylphenyl isocyanide) and [Pt 3 (PPh 3 ) 2 (CN-xylyl) 6 ] 2+ have been synthesized by reaction of [Pt(PPh 3 ) 2 (C 2 H 4 )] with either [Pt(PPh 3 ) 2 Cl 2 ] and CN-xylyl or [Pt(CN-xylyl) 4 ] 2+ . The products have been characterised by 31 P{ 1 H} and 195 Pt{ 1 H} NMR spectroscopy, and a single crystal X-ray diffraction study of the trinuclear compound has demonstrated that the skeletal atoms are linear.

Synthesis and structural characterisation of [Pt2Au2(PPh3)4(CNC6H3Me2-2,6)4][PF6]2; a platinum–gold cluster with a distorted butterfly geometry

The complex [Pt2Au2(PPh3)4(CNC6H3Me2-2,6)4][PF6]2 has been synthesised from [Pt(PPh3)2(C2H4)] and [Au (CNC6H3Me2-2,6)2]+ in acetone. The cluster crystallises in the triclinic space group P with two formula units and the unit cell of dimensions a= 13.882(4), b= 15.460(2), c= 27.578(5)A, α= 77.99(5), β= 88.95(2), and γ= 67.41(5)°. The metal atoms define a distorted flattened butterfly with the gold atoms occupying the higher connectivity sites and forming a short gold–gold bond of length 2.593(2)A. The platinum–gold distances lie in the range 2.712(2)–3.028(2)A reflecting a distortion of the skeletal geometry from D2h to C2v. 31Pt-{1H} and 195Pt-{1H} n.m.r. studies suggest that in solution a symmetric D2h structure is adopted in which the gold atoms are equivalent.

Synthesis and structural characterisation of [Au2Pt2(PPh3)4(CN-xylyl)4](PF6)2: A platinum—gold cluster with a flattened butterfly geometry

[Au2Pt2(PPh3)4(CN-xylyl)4](PF6)2 (CN-xylyl = 2,6-dimethylphenylisocyanide) has been synthesised from [Pt(C2H4)(PPh3)2] and [Au(CN-xylyl)2]+ in CH2Cl2 and in the presence of an excess of CN-xylyl. A single crystal X-ray diffraction study has demonstrated that the metal atoms define a flattened butterfly with the gold atoms occupying the higher connectivity sites and forming a short bond of length 2.590(2) A. The platinum—gold distances lie in the range 2.710(2)–3.026(2) A.

Synthesis and structural characterization of [Au9{P(p-C6H4OMe)3}8](BF4)3; a cluster with a centred crown of gold atoms

The single crystal X-ray analysis of [Au9{P(p-C6H4OMe)3}8]-(BF4)3; has provided the first example of a cluster compound with a metal atom incorporated into the centre of a puckered ring of like metal atoms.

Chemistry of platinum sulphido-complexes. Part 6. Reactions of [Pt2(-S)2(PPh3)4] with the rhodium complexes [(RhL2Cl)2](L = CO or C2H4) and [Rh(2,6-Me2C6H3NC)4]PF6. Crystal and molecular structures of [Pt2Rh(-S)2(PPh3)4(CO)2][Rh(CO)2Cl2]C3H6O and [Pt2Rh(-S)2(PPh3)4(2,6-Me2C6H3NC)2]PF6

The compound [Pt2(µ-S)2(PPh3)4] reacts with the chloride-bridged rhodium dimers [{Rh(CO)2Cl}2] and [{Rh(C2H4)2Cl}2] to give [Pt2Rh(µ-S)2(PPh3)4(CO)2][Rh(CO)2Cl2](1) and [Pt2Rh(µ-S)2(PPh3)4(C2H4)2]Cl (2a) respectively. The reactions of compound (1) and the PF6– derivative of (2a) with the chelating diphosphine Ph2PCH2CH2PPh2 have also been studied and the products characterised on the basis of n.m.r. data. The reaction of [Pt2(µ-S)2(PPh3)4] with [Rh(2,6-Me2C6H3NC)4]PF6 results in the formation of [Pt2Rh(µ-S)2(PPh3)4(2,6-Me2C6H3NC)2]PF6(3). The molecular structures of compounds (1) and (3) have been determined by single-crystal X-ray techniques using diffractometer data. Compound (1) crystallises in the monoclinic space group P21/c with four units of formula [Pt2Rh(µ-S)2(PPh3)4(CO)2][Rh(CO)2Cl2][Rh(CO)2Cl2]·C3H6O in a cell of dimensions a= 16.813(3), b= 16.068(4), c= 28.342(5)A, and β= 99.24(2)°, Least-squares refinement of the structure gave a final R value of 0.033 using 5 406 observed intensities [I 3σ(I)]. Compound (3) crystallises in the monoclinic space group C2/c with four units of formula [Pt2Rh(µ-S)2(PPh3)4(2,6-Me2C6H3NC)2]PF6 in a cell of dimensions a= 22.089(8), b= 18.141(7), c= 21.363(11)A, and β= 105.35(4)°. Least-squares refinement of the structure led to a final R value of 0.036 using 4 867 observed intensities [I⩽ 3σ(I)]. The structures of compounds (1) and (3) both show an approximately equilateral triangular arrangement of the platinum and rhodium atoms capped by two µ3-sulphido-ligands. The metal–metal distance lie in the range 3.034(1)–3.291(1)A which is greater than those normally associated with Pt–Pt and Pt–Rh bonding distances. The efficiency of these heterometallic platinum–rhodium compounds as homogeneous hydrogenation catalysts has also been studied.

Synthesis and structural characterisation of some triangulo-platinum clusters containing lsocyanide ligands

The compound [Pt3(µ-CO)3{P(C6H11)3}3] reacts with 3 and 5 mol of 2,6-xylyl isocyanide, CNC8H9, to give [Pt3(µ-CO)(µ-CNC8H9)2(CNC8H9){P(C6H11)3}2](1) and [Pt3(µ-CNC8H9)3(CNC8H9)2{P(C6H11)3}](2) respectively. The molecular structures of both compounds have been determined by single-crystal X-ray techniques using diffractometer data. Compound (1) crystallises in the monoclinic space group P21/n with four units of formula [Pt3(µ-CO)(µ-CNC8H9)2(CNC8H9)-{P(C6H11)3}2]·0.5C6H6in a cell of dimensions a= 15.431(5), b= 15.810(3), c= 28.489(19)A, and β= 95.22(4)° and (2) in the monoclinic space group P21/a with four formula units in a cell of dimensions a= 16.412(2), b= 18.444(2), c= 24.383(2)A, and β= 90.155(9)°. The structures of both molecules are approximately equilateral triangles of Pt atoms with Pt–Pt distances in the range 2.618(1)–2.654(1)A. The bonded ligand atoms are approximately coplanar with the metal triangles and 31P-{1H} and 195Pt-{1H} n.m.r. studies suggest that both solid-state structures are retained in solution. When intermediate mol ratios of isocyanide were added to [Pt3(µ-CO)3{P(C6H11)3}3] then mixtures of (1), (2), and starting material only were obtained. No evidence for the formation of symmetrical products of the type [Pt3(µ-CNC8H9)3{P(C6H11)3}3] was obtained. Examination of the molecular structures of (1) and (2) has formed a basis for rationalising the observed stoicheiometries. The bridging isocyanide ligands are severely bent about the nitrogen atoms and their steric requirements are incompatible with the presence of P(C6H11)3ligands on both of the metal atoms being bridged. Consequently, replacement of bridging CO by the isocyanide labilises an adjacent terminal P(C6H11)3ligand. Compound (2) has also been synthesised by sodium-amalgam reduction of [Pt(CO)Cl2{P(C6H11)3}] in the presence of an excess of isocyanide ligand.