Imido complexes with metal carbonyls

In general high pressure and temperature are required for these reactions to occur. However there are significant examples of reactions catalysed at atmospheric pressure, in part icular for the synthesis of isocyanates (4.2.5.). In the majority of cases the most important steps of these reactions are supposed to be the deoxygenation of the nitro function by carbon monoxide iving a nitrene residue bound to the metal centre, followed by insertion of carbon monoxide into the metal-nitrene bond. This is a likely hyphotesis since nitrene complexes can be obtained by stoichiometric reactions of nitro compounds with metal carbonyls. Conversion of the imido metal complex to the observed... 

In a model study [34], nitrobenzene has been observed to react with an heterogeneous rhodium catalyst to afford surface oxo and imido species. Reduction with CO gives a surface carbonyl complex, with the metal in a low oxidation state and this is supposed to favour the CO insertion into the Rh=NR bond, which is considered to be the rate determining step. However, rhodium metals requires pyridine [34] (a base) or FeCb [146] (an acid) in order to be active (an induction period is also observed at least with the pyridine-promoted system) and these characteristics are not explained by the mechanism above reported. 

For M = Ru, formamides and amines were the principal products. Substitution of Fe3(CO)i2 for Ru3(CO)i2 results in the formation of carbamate esters (ArNHCOOMe) as the major products, with ureas as the main by-products. Without the metal carbonyl, nitroarenes are recovered imchanged from the reaction mixture. An imido-alcoxycarbonyl complex was suggested as an intermediate in the reaction and the difference between iron and ruthenium was proposed to be due to the different facility of this intermediate to undergo protonation and reductive elimination to give carbamate, or insertion of CO into the M-N bond, followed by hydrogenation, affording formamide (Scheme 13) ... 

The CP3UCHPR3 complexes also undergo insertion into the CsN bonds of nitriles and the C s O bonds of metal carbonyls. Thus, Cramer et al. [39] reported that insertion reactions with acetonitrile yield uranium imido complexes ... 

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