Preparation of Mixed Bis-N-Heterocyclic Carbene Rhodium(I) Complexes.

A series of mixed bis-NHC rhodium(I) complexes of type RhCl(η-olefin)(NHC)(NHC') have been synthesized by a stepwise reaction of [Rh(-Cl)(η-olefin)] with two different NHCs (NHC = N-heterocyclic carbene), in which the steric hindrance of both NHC ligands and the η-olefin is critical. Similarly, new mixed coumarin-functionalized bis-NHC rhodium complexes have been prepared by a reaction of mono NHC complexes of type RhCl(NHC-coumarin)(η,η-cod) with the corresponding azolium salt in the presence of an external base. Both synthetic procedures proceed selectively and allow the preparation of mixed bis-NHC rhodium complexes in good yields.