Synthesis and catalytic activity of silver N-heterocyclic carbene complexes based on bis(3,5-dimethylpyrazol-1-yl)methyl-substituted pyridylimidazole

A series of N-heterocyclic carbene (NHC) silver complexes derived from 6-[bis(3,5-dimethylpyrazol-1-yl)methyl]-2-pyridylimidazole have been synthesized and characterized by physico-chemical and spectroscopic methods, and the structures of four complexes have been confirmed by X-ray structural analyses. The functionalized NHC is a monodentate ligand in (NHC)AgCl through the carbene carbon atom, and a bridging [C,N,N] tridentate ligand in (NHC)(2)Ag-2(BF4)(2) and (NHC)(2)Ag-2(PF6)(2) through two pyrazolyl nitrogens and the carbene carbon to form a metallomacrocycle, in which a head (carbene)- to tail (nitrogen) coordination mode to two silver atoms is observed. Preliminary catalytic tests show that these newly synthesized Ag-NHC complexes exhibit excellent catalytic activity at a low catalyst loading (0.1-1 mol%) in the aldehyde-amine-alkyne coupling reaction.