Synthesis of a new bidentate NHC-Ag(I) complex and its unanticipated reaction with the Hoveyda-Grubbs first generation catalyst

A new sterically demanding bidentate imidazolium bromide has been prepared and used as ligand precursor for the synthesis of the corresponding NHC-silver(I) complex. The X-ray analysis of the silver(I) complex revealed a rare Ag₄O₄ core cubane cluster. The silver(I) complex reacts readily with the Grubbs first generation catalyst providing a labile alkylidene complex. When the transmetallation was attempted with Hoveyda-Grubbs first generation catalyst in the presence of THF as solvent, two very stable phosphine free bis-bidentate N-heterocyclic carbene complexes, one green and one orange, were formed. Notably, one of these complexes is the first observation of a metal alkylidene group substituted by a NHC ligand, a surprising result since the new complex is formally derived from a nucleophile substitution of a hydride by a NHC ligand on the alkylidene carbon. A proposal for the reaction mechanism is elaborated.