Synthesis and structural characterisation of hexakis(triphenyl phosphine)hexagold(2+) nitrate, [Au6(PPh3)6][NO3]2, and related clusters with edgesharing bitetrahedral geometries

Abstract

[Au6(PPh3)6][NO3]2 has been synthesised in low yield from [Au8(PPh3)8][NO3]2 and K[Ag(CN)2] and characterised by a single-crystal X-ray diffraction study. The compound crystallises as [Au6(PPh3)6][NO3]2·3CH2Cl2 in the monoclinic space group P21/a with four formula units in a cell of dimensions a= 25.674(5), b= 15.843(10), c= 26.368(4)A, and β= 91.76(2)°. Least-squares refinement of the structure has led to a final R value of 0.049 using 4 430 unique absorption corrected reflections with I 3σ(I). The gold atoms in the structure define a pair of tetrahedra which share a common edge and therefore the structure is significantly different from the octahedral geometry reported for the related cluster [Au6{P(C6H4Me-p)3}6][BPh4]2. Nevertheless, the observed structure supports an earlier theoretical prediction which suggested that the edge-bridged bitetrahedral structure was electronically preferred for [Au6(PR3)6]2+ clusters. The Au–Au bond lengths fall in the range 2.651(2)–2.839(2)A. The related cluster compounds [Au6{P(C6H4Me-o)-Ph2}6]Y2(Y = NO3, ClO4, or BF4) and [Au6{P(C6H11) Ph2}6][NO3]2(C6H11= cyclohexyl) have been subsequently synthesised in good yield from the corresponding mononuclear compounds and either NaBH4 or [Ti(η-C6HBMe)2]. These compounds have been characterised on the basis of their 31P-{1H} n.m.r. and electronic spectral characteristics. Only at low temperatures are the 31P-{1H} n.m.r. data consistent with the solid-state structure and at room temperature only an average single resonance is observed. Some reactions of the hexanuclear clusters are also described.