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This minireview reports the most recent advances in the use of heterometallic catalysts based on singleframe N-heterocyclic carbene ligands. The article describes the synthetic strategies for the preparation of heterometallic catalysts, and their applications in the design of tandem processes by combining the catalytic properties associated with the two (or more) different metal centers. Several examples are discussed in which the use of heterometallic complexes results in a clear enhancement of the catalytic outcome compared to the results provided by mixtures of related homometallic complexes. The field constitutes a research area that is full of potential and is at its very earliest stage.

Chelating bis(imidazolium) salts having (CH 2 ) n chains of different lengths (n ) 1-4) linking the diazole rings show very large reactivity differences on metalation with [(cod)RhCl] 2 . Long linkers favor a square-planar Rh(I) product, while short linkers favor octahedral Rh(III). We ascribe the origin of the effect to the restricted rotation of the highly sterically anisotropic diazole rings and the different preferred orientations of these rings as n changes. Defining the x and y axes as the Rh-carbene bond directions, we find that with short linkers the diazole rings tend to be oriented close to the xy plane. This tends to favor Rh(III) because these complexes, [Rh(bis-carbene)I 2 (OAc)], have the lowest steric hindrance in the xy plane. With long linkers, the diazole rings tend to be aligned face to face along the (z axis. This tends to favor Rh(I) because these complexes, [(cod)Rh(bis-carbene)]PF 6 , have the lowest steric hindrance along the (z axis. Crystallographic studies are reported. Electrospray MS data provide evidence for strong metal-carbene binding.