Dinuclear bisNHC (bis(N-heterocyclic carbene)) gold(I) complexes 3 a and 4 a of general formula [Au2 Br2 (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.
Keywords: carbenes; counterion effect; dinuclear catalyst synthesis; gold; homogeneous catalysis; kinetics.
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