Homoleptic complexes of divalent metals bearing N,O-bidentate imidazo[1,5-a]pyridine ligands: Synthesis, X-ray characterization and catalytic activity in the Heck reaction

Two imidazo[1,5-a]pyridine-based ligands, namely 2-(1-phenylimidazo[1,5-a]pyridin-3-yl)phenol (N,OH-LPh) and 2-(1-methylimidazo[1,5-a]pyridin-3-yl)phenol (N,OH-LMe) have been reacted with different divalent metals (M = Co, Cu, Ni, Pd) to obtain the following complexes [Co(N,O-LPh)2] (1), [Cu(N,O-LPh)2] (2), [Ni(N,O-LPh)2] (3), [Pd(N,O-LPh)2] (4), [Co(N,O-LMe)2] (5), [Cu(N,O-LMe)2] (6), [Ni(N,O-LMe)2] (7) and [Pd(N,O-LMe)2] (8). These homoleptic complexes showed the monoanionic ligands to be coordinated in a N,O-bidentate mode via the pyridine-like nitrogen of the imidazo[1,5-a]pyridine skeleton and the phenolic oxygen. The coordination mode of the ligands was confirmed by the single-crystal X-ray structure analysis of [Co(N,O-LPh)2] (1) and [Pd(N,O-LMe)2] (8). The palladium compounds [Pd(N,O-LPh)2] (4) and [Pd(N,O-LMe)2] (8) proved to be good catalysts in the Heck reaction between iodobenzene and different olefins.