Ruthenium (II) and Iridium (III) Complexes of N-Heterocyclic Carbene and Pyridinol Derived Bidentate Chelates: Synthesis, Characterization, and Reactivity

We report the synthesis and characterization of new ruthenium(II) and iridium(III) complexes of a new bidentate chelate, NHC^R'-py^OR (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 NHC^R'-py^OR ligands (5-7); six ruthenium complexes of the type [η⁶-(p-cymene)Ru(NHC^R'-py^OR)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(NHC^Me-py^OtBu)Cl]PF₆ (14) and [Cp*Ir(NHC^Me-py^OH)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(NHC^Me-py^OR)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.