Halide anions as co-catalysts

Some years ago, research in our group showed that Ru3(CO)i2, when activated by a tetraalkylammonium halide, is a very active catalyst for the carbonylation ol nitroarenes in the presence of alcohols to afford the corresponding carbamates. Because it was already known that Ru3(CO)i2 reacts with nitrobenzene to afford a 

It was also tentatively proposed the chloride attacks the imido cluster generating the equivalent of an acyl chloride, which might then react more easily with methanol. Protonation of the imido nitrogen and reductive elimination would yield the final product, regenerating Ru3(CO)i2. 

Other studies, again conducted by Geoffrey s group, also led to the discovery of interesting promotion effects of halides. It was found that the addition of [PPN][X] (PPN = (PPh3)2N+, X = Cl, Br, I) strongly accelerates the reaction between Ru3(CO)i2 and nitrosobenzene, to afford an imido cluster in which the 

The promoting role of halides in the isocyanate-producing reaction was ascribed to their ability to assume a bridging position in the cluster, which is likely to weaken one of the N-Ru bonds, so rendering the imido fragment more likely to attack a 

Some model reactions on related trinuclear ruthenium clusters, especially related to the reduction reactions of nitrobenzene to aniline, have also been reported by Bhaduri and coworkers and several papers have also been published by different groups on related reactions of ruthenium and osmium clusters containing imido. amido or isocyanate fragments, although the latter were not intended as models for catalytic reactions. 

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