Dinuclear silver-bis(N-heterocyclic carbene) complexes: Synthesis, catalytic activity in propargylamine formation and computational studies

Preparation of a series of 1,1 '-dialkyl-3,3 '-(2-methylenepropane-1,3-diyl)dibenzimidazolium salts (alkyl = ethyl, propyl, butyl) (2a-2c) and their dinuclear bis(N-heterocyclic carbene) silver complexes (3a-3c) is reported. The silver complexes were synthesized via in situ deprotonation with Ag2O in 67-74% yields. The structural definitions of the synthesized compounds were performed by H-1 NMR, C-13 NMR, elemental analysis, FT-IR and also LC-MSMS techniques (only for 3a-3c). The silver complexes were utilized for three-component coupling reactions of aldehydes, amines and alkynes (A(3)-coupling). It was found that increasing the chain length of the alkyl substituent on the N-heterocyclic carbene ligand led to increase in catalytic performance. 3c showed the highest activity with 81% yield and with TON of 27 for N,N-diethyl-3-phenylprop-2-yn-1-amine formation. Geometry optimization was performed and geometric parameters were calculated for the compounds. The theoretical harmonic vibration frequencies of molecules were computed by DFT/B3LYP methods for their optimized geometries