Preparation of tris(azolyl)phosphine gold(i) complexes: digold(i) coordination and variation in solid state intermolecular interactions

Abstract

Various tris(azolyl)phosphines R₃P (R = 1-methylimidazol-2-yl, thiazol-2-yl, 4-methylthiazol-2-yl or 4,5-dimethylthiazol-2-yl) (1a–d) were utilised to prepare complexes of the type R₃PAuCl (2a–d). The donor strength of the nitrogen atoms was assessed with natural-abundance ¹⁵N{¹H} NMR of 1a–c and 2a–c. The chloride of 2c could be successfully substituted by the anions BzS^– and NCS^–. Further utilisation of the imine nitrogens of the tris(azole)phosphines to coordinate additional Au^I centres was successful only for 2a where treatment with 3 mole equivalents of C₆F₅Au(tht) (tht = tetrahydrothiophene) afforded bis(pentafluorophenyl)-µ-[tris(1-methylimidazol-2-yl)phosphine-κ²P,N]digold(I) (4). A hydrolysis product consisting of two bis(1-methylimidazol-2-yl)phosphinite ligands bridging a Au₂⁴⁺ centre and further coordination to two AuC₆F₅ moieties (5) was formed during this reaction. The crystal and molecular structures were determined of compounds 1d, 2a–d and 3b. Intriguingly, 2b and 2c crystallise in a total of seven polymorphs and solvates exhibiting different modes of intermolecular association. Compound 2b crystallises in three polymorphs; two of them and the solvate 2b·0.5CH₂Cl₂ exhibit aurophilic interaction while the third one is stabilised by a short Au⋯Cl interaction of 3.2660(9) Å. In 2c, one polymorph exhibits a strong aurophilic interaction of 3.0393(4) Å, but the other, as well as the solvate 2c·thf, lack such contacts. Product 2b is the first simple gold compound known to have both Au⋯Au or Au⋯Cl contacts in different crystals. Calculations at the B3LYP and MP2 levels of theory using quasi-relativistic basis sets show that for 2b₂ the Au⋯Cl interaction is between 2.6 and 12.2 kJ mol^–1 greater than the Au⋯Au interaction, depending on the level of theory and basis set. This contrasts with a model (PH₃AuCl)₂ dimer, where the Au⋯Au interaction is found to be stronger.