Variable coordination of amine functionalised N-heterocyclic carbeneligands to Ru, Rh and Ir: C–H and N–H activation and catalytic transfer hydrogenation

Abstract

Chelating amine and amido complexes of late transition metals are highly valuable bifunctional catalysts in organic synthesis, but complexes of bidentate amine–NHC and amido–NHC ligands are scarce. Hence, we report the reactions of a secondary-amine functionalised imidazolium salt 2a and a primary-amine functionalised imidazolium salt 2b with [(p-cymene)RuCl₂]₂ and [Cp*MCl₂]₂ (M = Rh, Ir). Treating 2a with [Cp*MCl₂]₂ and NaOAc gave the cyclometallated compounds Cp*M(C,C)I (M = Rh, 3; M = Ir, 4), resulting from aromatic C–H activation. In contrast, treating 2b with [(p-cymene)RuCl₂]₂, Ag₂O and KI gave the amine–NHC complex [(p-cymene)Ru(C,NH₂)I]I (5). The reaction of 2b with [Cp*MCl₂]₂ (M = Rh, Ir), NaO^tBu and KI gave the amine–NHC complex [Cp*Rh(NH₂)I]I (6) or the amido–NHC complex Cp*Ir(C,NH)I (7); both protonation states of the Ir complex could be accessed: treating 7 with trifluoroacetic acid gave the amine–NHC complex [Cp*Ir(C,NH₂)I][CF₃CO₂] (8). These are the first primary amine– or amido–NHC complexes of Rh and Ir. Solid-state structures of the complexes 3–8 have been determined by single crystal X-ray diffraction. Complexes 5, 6 and 7 are pre-catalysts for the catalytic transfer hydrogenation of acetophenone to 1-phenylethanol, with ruthenium complex 5 demonstrating especially high reactivity.