Axially chiral dimeric Ir and Rh complexes bridged by flexible NHC ligands

A family of Ag complexes of the type [{BnN(CH2CH2RIm)2}Ag]·[AgX2] (X = I, RIm = 1-methylimidazole (3a); X = Cl, RIm = 1-tert-butylimidazole (3b), 1-benzylimidazole (3c) or 1-methylbenzimidazole (3d)) or [{BnN(CH2CH2CH2RIm)2}Ag]·[AgCl2] (RIm = 1-methylimidazole (4a), 1-tert-butylimidazole (4b), 1-benzylimidazole (4c) or 1-methylbenzimidazole (4d)) with a flexible unsaturated linker connecting the two NHC ligands has been prepared. These silver complexes undergo facile transmetalliation with [Ir(COD)Cl]2 to generate the bimetallic species (COD)ClIr{BnN(CH2CH2RIm)2}IrCl(COD) (RIm = 1-methylimidazole (5a), 1-tert-butylimidazole (5b), 1-benzylimidazole (5c) or 1-methylbenzimidazole (5d)) or (COD)ClIr{BnN(CH2CH2CH2RIm)2}IrCl(COD) (RIm = 1-methylimidazole (6a), 1-tert-butylimidazole (6b), 1-benzylimidazole (6c) or 1-methylbenzimidazole (6d)) with a bridging bidentate NHC ligand. A similar reaction can be performed using [Rh(COD)Cl]2 to generate the analogous Rh species (COD)ClRh{BnN(CH2CH2CH2RIm)2}RhCl(COD) (RIm = 1-methylimidazole (7a), 1-benzylimidazole (7c) or 1-methylbenzimidazole (7d)). Complexes 5d, 6d and 7d were characterized by X-ray crystallography. All of our new Rh and Ir species possess axial chirality and are prepared as a mixture of isomers. However, crystals of 5d, 6d and 7d contain only one diastereomer. Dissolution of the diastereometrically pure crystals results in epimerization and the formation of an equilibrium mixture. Our new Ir complexes are active catalysts for both olefin and transfer hydrogenation and we present a comparison of their catalytic activity based on the linker length of the ligand.