Iridium(III) Complexes Bearing Chelating Bis-NHC Ligands and Their Application in the Catalytic Reduction of Imines

Aznarez F., Iglesias M., Hepp A., Veit B., Sanz Miguel P., Oro L., Jin G., Hahn F.

Abstract

The IrIIIcomplexes 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)}2] (Cp*=η5-C5Me5) 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 IrIIIcenter. Subsequently, NaOAc promoted C–H bond activation at the pendant imidazole moiety of the intermediate IrIIImono-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 IrIIIcomplexes 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 H2pressure of 3 bar in MeOH.