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

A series of mixed bis-NHC rhodium(I) complexes of type RhCl(eta(2)-olefin)(NHC)(NHC') have been synthesized by a stepwise reaction of [Rh(mu-Cl)(eta(2)-olefin)(2)](2) with two different NHCs (NHC = N-heterocyclic carbene), in which the steric hindrance of both NHC ligands and the eta(2)-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)(eta(2),eta(2)-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.