Synthesis and catalytic activity of N-heterocyclic carbene silver complexes derived from 1-[2-(pyrazol-1-yl)phenyl]imidazole

Abstract

Six mono- and dinuclear N-heterocyclic carbene (NHC) silver complexes based on 1-[2-(pyrazol-1-yl)phenyl]imidazole have been synthesized and characterized by elemental analysis and NMR spectroscopy, and their structures have been confirmed by single crystal X-ray diffraction. The N-functionalized carbene ligands exhibit versatile coordination modes in these silver complexes. N-[2-(3,5-Dimethylpyrazol-1-yl)phenyl]-N-benzylimidazol-2-ylidene (L) acts as a monodentate ligand through the carbene carbon in mononuclear LAgCl. While in dinuclear L2Ag2(PF6)2 and L′2Ag2(PF6)2 (L′ = N-[2-(pyrazol-1-yl)phenyl]-N-benzylimidazol-2-ylidene), L and L′ act as bridging bidentate ligands through the pyrazolyl nitrogen and the carbene carbon atoms to two silver atoms. Though, these two silver atoms have different coordination environments. In the former, one silver atom coordinates with two carbene carbons, the other coordinates with two pyrazolyl nitrogen atoms. In the latter, each silver atom coordinates with one carbene carbon and one pyrazolyl nitrogen atom, respectively. A dinuclear macrocyclic structure is observed in m-xylyl bridging tetradentate bis-NHC complexes L2Ag2(BF4)2 and (L)CH2C6H4CH2(L)Ag2(BF4)2, in which the coordination mode of carbene ligands is similar with that in L2Ag2(PF6)2. Preliminary catalytic tests show that all these complexes exhibit highly effective catalytic activity in the three-component coupling reaction of alkyne, aldehyde and amine forming propargylamines.