Ruthenium (II) and iridium (III) complexes of N-heterocyclic carbene and pyridinol derived bidentate chelates: Synthesis, characterization, and reactivity

Abstract

We report the synthesis and characterization of new ruthenium(II) and iridium(III) complexes of a new bidentate chelate, NHCR′-pyOR (OR=OMe, OtBu, OH and R′=Me, Et). Synthesis and characterization studies were done on the following compounds: four ligand precursors (1–4); two silver complexes of these NHCR′-pyOR ligands (5–7); six ruthenium complexes of the type [η6-(p-cymene)Ru(NHCR′-pyOR)Cl]X with R′=Me, Et and R=Me, tBu, H and X=OTf−, PF6− and PO2F2− (8–13); and two iridium complexes, [Cp∗Ir(NHCMe-pyOtBu)Cl]PF6 (14) and [Cp∗Ir(NHCMe-pyOH)Cl]PO2F2 (15). The complexes are air stable and were isolated in moderate yield. However, for the PF6− salts, hydrolysis of the PF6− counter anion to PO2F2− during t-butyl ether deprotection was observed. Most of the complexes were characterized by 1H and 13C NMR, MS, IR, and X-ray diffraction. The ruthenium complexes [η6-(p-cymene)Ru(NHCMe-pyOR)Cl]OTf (R=Me (8) and tBu (9)) were tested for their ability to accelerate CO2 hydrogenation and formic acid dehydrogenation. However, our studies show that the complexes transform during the reaction and these complexes are best thought of as pre-catalysts.