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)Å, 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)Å. 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.