Synthesis, structures and spectroscopic properties of 1:1 complexes of gold(I) halides with tricyclohexylphosphine, [Au(PCy3)X], X = Cl, Br and I

Two-coordinate gold(I) complexes, [Au(PCy3)X] (PCy3 = tricyclohexylphosphine, X = Cl, Br and I), have been prepared by reaction of stoichiometric quantities of [NBu4] [AuX2] and PCy3 in dimethylformamide and, for X = Cl and Br, by anodic dissolution of metallic gold in a solution of aqueous HX and PCy3 in acetonitrile. The complexes were characterized by solution and solid-state 31P n.m.r. spectroscopy, far-infrared spectroscopy and single-crystal X-ray structure determinations. The chloride, bromide and iodide complexes form an isomorphous series, crystallizing in the triclinic space group P 1̄ (a ≈ 9.3, b ≈10.3, c ≈ 10.9 Å, α ≈ 88, β ≈ 80, γ ≈ 77°) as discrete molecules which stack in parallel head-to-tail mode to form a zigzag chain of gold atoms along the crystallographic c axis. Au⋯Au separations are 5.71, 6.20 Å for X = Cl, 5.72, 6.17 Å for X = Br and 5.74, 6.20 Å for X = I. The iodide also crystallizes as an orthorhombic form in space group Pnma (a 16.809(4), b 14.373(5), c 8.623(3) Å) with a different conformational structure for the PCy3 ligand and loss of the zigzag chain structure. Far-infrared spectra of the complexes show zx(AuX) at 332, 324 cm-1 for X = Cl and 232 cm-1 for X = Br with multiple bands in the region 150-200 cm-1 for both iodide complexes, precluding definitive assignment of v(AuI). Solution 31P n.m.r. spectra in chloroform give sharp single peaks with chemical shifts of 54.5, 56.6 and 59.9 ppm for X = Cl, Br and I respectively. The solid-state CPMAS 31P n.m.r. spectra also yield single peaks with chemical shifts of 55 (Cl), 58 (Br) and 63 ppm (I) for the triclinic complexes and 57 ppm for the orthorhombic iodide. The chemical shift differences between the two forms of the iodide and between the complexes in the solution and solid states are ascribed to variations in the conformational structure of the phosphine ligands.