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

• Published in: Inorganica Chimica Acta Vol. 380; pp. 399 - 410
• Main Authors: Ashley, John M., Farnaby, Joy H., Hazari, Nilay, Kim, Kelly E., Luzik, Eddie D., Meehan, Robert E., Meyer, Eric B., Schley, Nathan D., Schmeier, Timothy J., Tailor, Amit N.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.01.2012
• Subjects: Axial chirality; NHC ligands; Olefin hydrogenation; Rhodium and iridium compounds; Transfer hydrogenation; Rhodium and iridium compounds; Transfer hydrogenation; NHC ligands; Olefin hydrogenation; Axial chirality
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2011.11.034

A family of dimeric Rh and Ir complexes supported by bidentate NHC ligands with a flexible amine linker have been prepared. The new complexes are axially chiral and are active catalysts for both transfer and olefin hydrogenation. [Display omitted] ► Bidentate NHC ligands support Ir and Rh complexes. ► X-ray shows that complexes are axially chiral. ► Complexes catalyze transfer hydrogenation and olefin hydrogenation. 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.