N-Benzyl Substituted N-Heterocyclic Carbene Complexes of Iridium(I): Assessment in Transfer Hydrogenation Catalyst

Iridium()) complexes of N-heterocyclic carbenes (NHCs) (1a-3a) were obtained by transmetalation reactions from the corresponding Ag(I)-NHC complexes. These complexes have been fully characterized by H-1, C-13, heteronuclear multiple-bond correlation NMR spectroscopies, and elemental analysis. X-ray diffraction studies on single crystals of la and 2a confirm the square planar geometry at the metal center. [IrCl(CO)(2)(NHC)] complexes 1b-3b were also synthesized to compare sigma-donor/pi-acceptor strength of NHC ligands. Transfer hydrogenation (TH) reactions of various ketones and imines have been studied using complexes 1a-3a as precatalysts. N-Benzyl substituted NHC complexes of Ir(I) proved to be highly efficient precatalysts in the reduction of aromatic and aliphatic ketones to afford the corresponding alcohol products with turnover frequencies values up to 24 000 h(-1).