Co-ordination and conformational isomerism in bis(tricyclohexylphosphine) gold(I) halides

Abstract

Solid state complexes of bis(tricyclohexylphosphine)gold(I) with halide counter ions, Au(PCy3)2X (X = Cl, Br or I), have been crystallised from solutions of [NBu4][AuX2] and PCy3 in dimethylformamide and characterised by single crystal structure determinations, far-infrared spectroscopy and solid state 31P CP MAS NMR spectroscopy. The results show the complexes crystallise as a unique array of co-ordination and conformational isomers which reflect the conformational flexibility of the tricyclohexylphosphine ligand and the relative preferences of the halide anions for C–H· · ·X–versus Au+· · ·X– bonding interactions. The chloride crystallises as a single ionic complex, [Au(PCy3)2]+Cl–, with the gold two-co-ordinate and the cations and anions well separated in the crystal lattice. The bromide crystallises as three different polymorphs with four distinct molecular structures. The first (α) form crystallises as ionic [Au(PCy3)2]+Br– with the bromide adjacent to but not co-ordinated to the gold. The second (β) form presents two crystallographically independent [Au(PCy3)2Br] molecules with semi-co-ordinated bromide and significant differences in the conformational dispositions of the PCy3 ligands. The third (γ) form crystallises as [Au(PCy3)2Br] with fully co-ordinated bromide. The iodide crystallises with semi-co-ordinated and co-ordinated iodide in [Au(PCy3)2I]·PCy3 and [Au(PCy3)2I] complexes respectively. The study provides an unusual series of molecular ‘vignettes’ in which the response of the [Au(PCy3)2]+ cation to halide anion can be monitored as it passes through the ligand substituents and co-ordinates to the central gold atom.