Gold(I) and Gold(III) Complexes of Cyclic (Alkyl)(amino)carbenes

The chemistry of Au(I) complexes with two types of cyclic (alkyl)(amino)carbene (CAAC) ligands has been explored, using the sterically less demanding dimethyl derivative Me2 CAAC and the 2-adamantyl ligand Ad CAAC. The conversion of ( Ad CAAC)AuCl into ( Ad CAAC)AuOH by treatment with KOH is significantly accelerated by the addition of t BuOH. ( Ad CAAC)AuOH is a convenient starting material for the high-yield syntheses of ( Ad CAAC)AuX complexes by acid/base and C-H activation reactions (X = OAryl, CF 3 CO 2 , N(Tf) 2 , C 2 Ph, C 6 F 5 , C 6 HF 4 , C 6 H 2 F 3 , CH 2 C(O)C 6 H 4 OMe, CH(Ph)C(O)Ph, CH 2 SO 2 Ph), while the cationic complexes [( Ad CAAC)AuL] + (L = CO, CN t Bu) and ( Ad CAAC)AuCN were obtained by chloride substitution from ( Ad CAAC)AuCl. The reactivity toward variously substituted fluoroarenes suggests that ( Ad CAAC)AuOH is able to react with C-H bonds with p K a values lower than about 31.5. This, together with the spectroscopic data, confirm the somewhat stronger electron-donor properties of CAAC ligands in comparison to imidazolylidene-type N-heterocyclic carbenes (NHCs). In spite of this, the oxidation of Me2 CAAC and Ad CAAC gold compounds is much less facile. Oxidations proceed with C-Au cleavage by halogens unless light is strictly excluded. The oxidation of ( Ad CAAC)AuCl with PhICl 2 in the dark gives near-quantitative yields of ( Ad CAAC)AuCl 3 , while [Au( Me2 CAAC) 2 ]Cl leads to trans -[AuCl 2 ( Me2 CAAC) 2 ]Cl. In contrast to the chemistry of imidazolylidene-type gold NHC complexes, oxidation products containing Au-Br or Au-I bonds could not be obtained; whereas the reaction with CsBr 3 cleaves the Au-C bond to give mixtures of [ Ad CAAC-Br] + [AuBr 2 ] - and [( Ad CAAC-Br)] + [AuBr 4 ] - , the oxidation of ( Ad CAAC)AuI with I 2 leads to the adduct ( Ad CAAC)AuI·I 2 . Irrespective of the steric demands of the CAAC ligands, their gold complexes proved more resistant to oxidation and more prone to halogen cleavage of the Au-C bonds than gold(I) complexes of imidazole-based NHC ligands.