Primary Amine Functionalized N-Heterocyclic Carbene Complexes of Iridium: Synthesis, Structure, and Catalysis

Abstract

The square-planar iridium(I) complex [Ir(cod)(C–NH2)]PF6 (3·PF6) containing a primary amine based N-heterocyclic carbene ligand (C–NH2) ligand was synthesized by a transmetalation reaction of the nickel(II) complex [Ni(C–NH2)2](PF6)2 (1) with [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene). An addition reaction of HCl with 3·PF6 afforded a mixture of two isomers of the octahedral iridium(III) hydrido chloride complex [IrHCl(cod)(C–NH2)]PF6 (5). One of the isomers, 5-cis-HCl, was found to have a rare geometry containing trans-H–Ir–NH2 and cis-H–Ir–Cl moieties. All of these iridium complexes were tested in the hydrogenation of molecules with unsaturated bonds. They are active catalysts in the H2-hydrogenation of acetophenone, with activity superior to that of the phosphine–amine (P–NH2) complex [Ir(cod)(P-NH2)]PF6 (4) and half-sandwich complexes [Ir(C5Me5)(C–NH2)Cl]PF6 (2a) and [Ir(C5H5)(C-NH2)Cl]PF6 (2b). Complex 3·PF6, however, was ineffective as a catalyst for the hydrogenation of trans-α-methylstilbene and N-(1-phenylethylidene)aniline.