Synthesis, Structures, and Transfer Hydrogenation Catalysis of Bifunctional Iridium Complexes Bearing a C–N Chelate Oxime Ligand

Synthesis, Structures, and Transfer Hydrogenation Catalysis of Bifunctional Iridium Complexes Bearing a C–N Chelate Oxime Ligand:

Abstract

We have synthesized a series of organometallic oxime complexes as novel metal–ligand cooperating bifunctional catalysts. The reaction of [{Cp*IrCl(μ₂-Cl)}₂] with ketoximes in the presence of sodium acetate afforded the half-sandwich chlorido iridium complexes 6 bearing a C–N chelate oxime ligand with a protic OH group in the β-position to the metal. Complex 6a, derived from acetophenone oxime, reacted with silver triflate to give the triflate complex 7 and cationic nitrile complex 8 depending upon the reaction solvent. Complexes 6 also reacted with a base in dichloromethane to afford the oximato-bridged dinuclear complexes 9, which were converted back to the chlorido complexes 6 upon treatment with an amine hydrochloride. In contrast, dehydrochlorination of 6 in 2-propanol as well as the reaction of 9 with 2-propanol yielded the hydrido-bridged dinuclear oxime–oximato complexes 10. Crossover experiments revealed that 10 dissociates into the mononuclear hydrido–oxime complex 11 and unsaturated oximato complex 12, which are interconvertible by reactions with hydrogen donors and acceptors. Owing to the metal–ligand cooperation, 10 effectively catalyzed transfer hydrogenation of ketones with 2-propanol.

A series of organometallic oxime complexes featuring a β-protic OH group have been synthesized. The hydrido-bridgedoxime–oximato complex (pictured) dissociates into two mononuclear species, which are interconvertible through reactions with hydrogen donors and acceptors. Owing to the metal–ligand cooperation, catalytic transfer hydrogenation of ketones with 2-propanol has been achieved.