Insights on the Anion Effect in N-heterocyclic Carbene Based Dinuclear Gold(I) Catalysts

Dinuclear bisNHC (bis(N-heterocyclic carbene)) gold(I) complexes 3 a and 4 a of general formula [Au2Br2(bisNHC)] were tested as catalysts in the cycloisomerization of N-(prop-2-yn-1-yl)benzamide and in the hydromethoxylation of 3-hexyne in the presence of silver(I) activators bearing different counteranions. The catalytic performance of mononuclear NHC complexes (1 a, 2 a) in the same reactions was also studied. The results highlighted the fundamental role of both NHC ligand and counterion in the catalytic cycles and activation process: dinuclear catalysts exhibit higher initial activity even under milder conditions but suffer in terms of stability with respect to mono NHCs. Furthermore, a new dinuclear bisNHC gold(I) complex 4 b of general formula [Au2(OTs)2(bisNHC)] (OTs=p-toluenesulfonate) was successfully synthesized and characterized by means of NMR and ESI-MS analyses.The catalytic activity of dinuclear gold(I) complexes depends on the nature of the counterion and on the linker length. Both factors influence two main important steps in hydromethoxylation of 3-hexyne and cycloisomerization of N-(prop-2-yn-1-yl)benzamide: deactivation and pre-equilibrium (formation of active intimate OSIP ion pairs)+imag