Iridium(III) complexes bearing chelating Bis-NHC ligands and their application in the catalytic reduction of imines

Abstract

The Ir complexes 4 and 5 bearing bis-NHC ligands (NHC = N-heterocyclic carbene) composed of one classical NR,NR NHC and one N,NR NHC donor were prepared by the reaction of the azolium/azole compounds 2I and 3Br, respectively, with [{Cp*IrCl(µ-Cl)}] (Cp*=η-CMe) in the presence of NaOAc as base. Most likely, the salts 2I and 3Br were first selectively deprotonated at the C2 position of the disubstituted (NR,NR) diazaheterocycle to generate an NHC donor, which then coordinated to the Ir center. Subsequently, NaOAc promoted C–H bond activation at the pendant imidazole moiety of the intermediate Ir mono-NHC complexes led to the formation of the six-membered iridacycles 4 and 5, which bear a chelating, doubly C-metalated C(NHC)C(NHC′) bis-NHC ligand. The Ir complexes 4 and 5 were tested as precatalysts for the reduction of imines with molecular hydrogen. Moderate to good activity was observed at a catalyst loading of 5 mol-% and an H pressure of 3 bar in MeOH.