Metal cluster catalysis. 2. Selective reduction of nitrobenzene catalyzed by rhodium carbonyl cluster anions. Evidence for water gas shift reaction

The rhodium cluster complex, Rh₆(CO)₁₆, has been found to catalyze the homogeneous reduction of nitrobenzene to aniline at temperatures above 80 °C in the presence of N,N-dimenthylbenzylamine, using any one of the following reducing gases: (1) H₂/CO, (2) H₂, (3)CO/H₂O. The reductions are highly selective and aniline was the only product detected. The same result was obtained using Amberlyst A-21 resin beads which contained polymerbound N,N-dimethylbenzylamine moieties which, in turn, immobilize the rhodium clusters. It was shown, by using D₂O, and following deuterium incorporation into the resulting aniline, that the water is the source of hydrogen when CO/H₂O was used. When nitrobenzene is absent, this catalyst system promotes the water—gas shift reaction. Unlike aromatic nitro groups, aliphatic nitro groups were not reduced. However, the water—gas shift reaction was catalyzed. The kinetics of aniline formation were first order in nitrobenzene and the effect of pressure on the reaction is described. Use of the resin catalyst led to lower rates and a remarkably air sensitive catalyst system relative to the homogeneous reactions.