Synthesis and Mechanism of Formation of Novel NHC-NAC Bis-Carbene Complexes of Gold(I)

Novel [Au(NHC)(DIC)]BF4 complexes (NHC=SIMes, IMes, SIPr, IPr;DIC=2,6-dimethylphenyl isocyanide) were synthesized and characterized. These substrates react with secondary amines at room temperature under mild experimental conditions to give new bis-carbene NHC-NAC derivatives. The reaction of formation of the latter complexes with piperidine, morpholine, and diethylamine was studied by NMR and UV-vis spectrometry, and a mechanism involving a concomitant attack of the amine at the metal center and at the coordinated isocyanide was proposed on the basis of a detailed kinetic study. The mechanistic network was completely resolved by means of an independent determination of the equilibrium constant of formation of the adduct [Au(NHC)- (DIC)(amine)]þ, Keq, calculated from the absorbance vs amine concentration data obtained from direct spectrophotometric titration of the starting complex [Au(NHC)(DIC)]þ with the amine. The [Au(NHC){C(NHAr)(NC5H10)}]BF4 bis-carbene complexes obtained by attack of piperidine at the [Au(NHC)(DIC)] derivatives were isolated and fully characterized. The solid-state structures of the new complexes [Au(IMes)(DIC)]BF4 and [Au(IMes){C(NHAr)(NC5H10) }]BF4 were resolved and reported.