Efficient nickel based catalyst for the homogeneous reduction of aromatic nitro compounds

The reduction of aromatic nitro compounds under CO under relatively mild conditions in the presence of NiX₂L₂ (XCl, Br, I; LPR₃, RMe, Et, Ph) and NiI₂(PhNH₂)₄ as catalyst precursors is described. The nitrobenzene is quantitatively converted into N,N′ diphenylurea in aniline solution and into alkylcarbanylate with a good yield, in aniline-alcohol mixtures. Nitrobenzene derivatives (p-RC₆H₄NO₂; RCH₃, Cl) and 2,4-dinitrotoluene, are also reduced in aniline solution; diphenylurea and amine, corresponding to the nitroderivatives, are the reaction products in these cases. This different behaviour is explained in terms of aminolysis of the expected N-aryl-N′-phenylurea. Sufficient data are produced to say that the observed increase of the catalytic activity, increasing in the sequence Cl, Br, I by changing the NiX₂L₂ complex, is due to an additional co-catalytic effect of the halide. On the basis of the chemical behaviour and IR evidence, the true catalytic species and some intermediates of reaction are suggested and a possible catalytic cycle is discussed.