New homoleptic gold carbene complexes via Ag–Au transmetalation: synthesis and application of [Au(diNHC)2]3+ cations as 1H-NMR and UV-vis halide sensors†
Three novel gold(III) complexes with bidentate di(N-heterocyclic carbene) (diNHC) ligands of the general formula [AuL2](PF6)3 (L = diNHC) were isolated by transmetalation of the carbene ligand from the corresponding silver(I) complexes [Ag2L2](PF6)2 to KAuBr4 in the presence of AgPF6. In this regard, the scope of the stereo-electronic properties of the ligand has been investigated. Over a number of seven tested ligands, three ligands led to the formation of the desired gold(III) complex, whereas in the other cases a gold(I) complex of the formula [Au2L2](PF6)2 was isolated as the main product. The anion sensing ability of the prepared gold(III) complexes towards the halides Cl−, Br− and I− has been studied in DMSO-d6 and D2O by means of 1H-NMR titration experiments and in one case also by UV-vis. Replacement of a methyl with a n-butyl wingtip substituent on the imidazole ring strongly favours the interaction with the larger iodide anion with respect to chloride. Introduction of an OH group in the bridge gives rise to metal- and ligand-centred interactions, which allow discrimination of the nature of the involved anion. The formation of the gold(III) complex-halide aggregates has been also analysed with X-ray crystallography studies.