Carboxylate-Functionalized Mesoionic Carbene Precursors: Decarboxylation, Ruthenium Bonding, and Catalytic Activity in Hydrogen Transfer Reactions

Metalation of a carboxylate-functionalized pyridyl-triazolium salt containing a N1-bound pyridyl substituent either by direct means with [RuCl2(cymene)]2 or via a transmetalation procedure involving Ag2O and [RuCl2(cymene)]2 induces rapid decarboxylation without concomitant metalation. Subsequent metalation of the formed C4- and C5-unsubstituted triazolium salt is selective and occurs at the C4 position, i.e. remote from the pyridyl substituent, when the reaction is under kinetic control and at the C5 position adjacent to the pyridyl group, when the reaction is performed under thermodynamic control. Preservation of the carboxylate functional group in the complex is achieved when the corresponding ester-functionalized pyridyl-triazolium salt is metalated first and then subjected to ester hydrolysis. The formed complex contains a N,C-bidentate chelating pyridyl-triazolylidene ligand with a pendant carboxylate unit that is not coordinating to the metal center. These new triazolylidene ruthenium complexes show modest catalytic activity in alcohol oxidation and better performance in the transfer hydrogenation of ketones. The data suggest that the presence of a pendant carboxylic acid or ester group is beneficial for enhancing the activity of the catalyst