Chloro-ruthenium complexes with carbonyl and N-(aryl)pyridine-2-aldimines as ancillary ligands. Synthesis, characterization and catalytic application in C–C cross-coupling of arylaldehydes with arylboronic acids

Reaction of N-(aryl)pyridine-2-aldimines (L–R, R = OCH3, CH3, H, Cl and NO2) with [Ru(CO)2Cl2]n in refluxing ethanol affords a group of complexes of type [Ru(L–R)(CO)2Cl2]. In these complexes the diimine ligands (L–R) are coordinated to the metal center as NN-donors forming five-membered chelate rings, the carbonyls are mutually cis and the two chlorides are trans. Crystal structure of [Ru(L–OCH3)(CO)2Cl2] has been determined. All the complexes show characteristic 1H NMR signals, and in dichloromethane solution they display intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on the complexes shows an irreversible oxidation of the metal center within 1.15–1.23 V vs SCE, and reduction(s) of the diimine ligand within −0.70 to −0.96 V vs SCE. The [Ru(L–R)(CO)2Cl2] complexes efficiently catalyze cross-coupling of arylaldehydes with arylboronic acids yielding diaryl ketones.