[Rh(CO)4]-, [Rh5(CO)15]-, and bimetallic clusters as catalysts for the carbonylation of nitrobenzene to methyl phenylcarbamate

Bimetallic clusters of the general formula [PPN]2[MRh4(CO)15] (M Fe, Ru, or Os; PPN+ = (PPh3)2N+) are active catalysts in the presence of methanol for the carbonylation of nitrobenzene to methyl phenylcarbamate. The clusters are active even without co-catalyst, but the addition of bipy (2,2′-bipyridine) greatly enhances both rates and selectivities. In the absence of methanol the conversion was much lower, with a low selectivity in phenyl isocyanate. Interchanging Fe, Ru, and Os has a negligible effect on the activity and a moderate effect on the selectivity, with ruthenium being best. However, the monometallic rhodium cluster [Rh5(CO)15]- proved to be both more active and selective, and the monomer [Rh(CO)4]- was found to be even better. This last catalytic system has been optimized with respect to temperature, CO pressure and the bipy/Rh ratio. The influence of solvent was also examined.