Ir-III/Au-I and Rh-III/Au-I Heterobimetallic Complexes as Catalysts for the Coupling of Nitrobenzene and Benzylic Alcohol

Two simple bisimidazolium salts derived from m- or p-bisimidazolebenzenes have been used as ligand precursors for the preparation of a series of heterobimetallic Rh-III/Au-I and Ir-III/Au-I complexes. All complexes have been fully characterized, and the molecular structures of selected derivatives have been determined by X-ray diffraction studies. These analyses showed that, regardless of the ligand used, the direct coordination environment of the metals is composed of identical donors (M-III: C-NHC, C-benzene, eta(5)-Cp*, and I- ; Au-I: C-NHC and I-) and is rather similar in the heterobimetallic complexes. The catalytic activity of the heterobimetallic complexes in the coupling of nitrobenzene with benzylic alcohol has been investigated. In all cases, the performance observed for the heterobimetallic complexes was superior to that shown by mixtures of homometallic M-III/Au-I complexes, suggesting a cooperative effect between the two metal centers. In addition and in spite of the similarity of the donor groups of the two bis-NHC ligands used, the slightly different relative orientation of the two metals in the heterobimetallic complexes also plays a role in the catalytic conversion observed.