Facile oxidation of NHC-Au(I) to NHC-Au(III) complexes by CsBr3

CsBr3 was investigated as a new and convenient oxidant for NHC-Au(I) complexes (NHC = imidazo[1,5-a]pyridin-3-ylidene) for the preparation of the respective Au(III) complexes. The Au(I) complexes were synthesized by the silver salt method using [(NHC)2Ag]PF6 and (tht)AuBr. Unexpectedly, the reactions yielded both neutral (NHC)AuBr and ionic [(NHC)2Au]PF6, depending on the N-substituent of the NHC ligand. Oxidation with CsBr3 gave the complexes (NHC)AuBr3 and [(NHC)2AuBr2]PF6 in high yields and purity, which proves the suitability of this reagent. The complexes were further characterised by X-ray diffraction and electronic absorption and emission spectroscopy. The Au(I) complexes exhibit a dual emission attributable to intraligand fluorescence and phosphorescence at both room temperature and 77 K. Upon irradiation with polychromatic light (λ > 305 nm), the Au(III) complexes are cleanly photo-reduced to the Au(I) congener.