Gold–boron chemistry. Part 3. The synthesis, characterisation, and molecular structure of [(H11C6)3PAu]2B8H10. Comments on the ‘anomalous’ structure of B8H12

Abstract

The reaction between [AuMe{P(C₆H₁₁)₃}] and B₁₀H₁₂(PPh₃)₂ yields an unexpected product, [(H₁₁C₆)₃PAu]₂B₈H₁₀ This complex was identified by multinuclear n.m.r. spectroscopy, and by a single-crystal X-ray diffraction study. Four molecules of [(H₁₁C₆)₃PAu]₂B₈H₁₀ and eight of water cocrystallise in space group C2/c with a= 1 7.506(7), b= 1 4.064(3), and c= 20.179(3)Å, and β= 105.012(22)°; R= 0.0469 for 2 512 observed data. The gold–borane complex has crystallographically imposed C₂ molecular symmetry, the {Au₂B₈H₁₀} fragment having effective C_2v symmetry. The complex is described as an eight-vertex borane cluster with two µ₃ gold phosphine units, and an analogy is drawn between the structure of [(H₁₁C₆)₃PAu]₂B₈H₁₀ and that of B₈H₁₂, a species whose structure is anomalous in terms of established electron-counting rules. Molecular orbital calculations show that, given the structures they have, the observed electron counts for [(H₁₁C₆)₃PAu]₂B₈H₁₀ and B₈H₁₂ afford thermodynamically stable compounds.