Heterocyclic carbenes. One-pot synthesis of rhodium and iridium carbene complexes

Rhodium carbene complexes of the type Rh(X)(η⁴-1,5-cod)(L_carbene) (X = halide anion, L_carbene = imidazole, triazole, pyrazole and benzimidazole carbene) are directly accessible from [Rh(η⁴-1,5-cod)Cl]₂ following a simple new one-pot synthesis. A slurry of this complex in ethanol is successively treated with an excess of both sodium ethoxide and heteroaromatic azolium salt. An in situ formed rhodium ethoxide complex acts as deprotonating agent. Cyclooctadiene may be displaced by two molecules of carbon monoxide yielding complexes of the type Rh(CO)₂X(L_carbene), and one carbon monoxide ligand may again be replaced by phosphines or phosphites to yield trans-Rh(CO)X(L_carbene)(L_p). The described method also works for analogous iridium complexes. The σ-donor/π-acceptor characteristics of imidazole carbenes are compared with those of phosphines.