Heterobimetallic Complexes of Bi- or Polydentate N-Heterocyclic Carbene Ligands and Their Catalytic Properties

This review summarizes developments in the synthesis and catalytic applications of those heterobimetallic carbene complexes in which at least two different metals are bound to the same ligand by at least one M-C(carbene) bond each. Several new synthetic methods for such complexes yielding well-defined and thoroughly characterized compounds are presented. The new complexes were found to be catalytically active in several (most often tandem) reactions. In certain cases, the incorporation of two different metals into the same imidazole- or triazol-based NHC-carbene complex resulted in the substantially higher catalytic activity of the heterobimetallic complex compared either to its homobimetallic analogs or to mixtures of comparable mononuclear complex fragments containing the two metals independently. This is a clear demonstration of advantageous metal–metal cooperation within the catalyst. Opposite examples are also discussed, where the heterobimetallic carbene complex proved inferior in relation to its homobimetallic analogs or to mixtures of homonuclear fragments.