Hydrogenation of single and multiple N–N or N–O bonds by Ru(II) catalysts in homogeneous phase

The ruthenium(II) complexes RuH2(CO)2(PnBu3)2, RuH2(CO)2(PPh3)2, and RuH2(PPh3)4 are catalytically active in the hydrogenation of organic substrates containing a NN, N(O)N or NO2 group. The reduction of the first two groups leads to hydrazine as intermediate and amine as the final product, while reducing a NO2 group the corresponding amine is selectively formed. A complete conversion was reached, depending on temperature, catalyst and substrate concentration. The catalysts are also active in the hydrogenolysis of an N–N group giving the corresponding amine with a 97.3% conversion using RuH2(PPh3)4 as catalyst. A first-order reaction rate with respect to substrate, catalyst or hydrogen pressure was detected in all cases. Finally, the activation parameters and the kinetic constants of these reactions were calculated. In the hydrogenation of azobenzene, the rate determining step involves an associative or a dissociative step depending on the catalyst employed while in the hydrogenation of all other substrates an associative rate determining step is always involved. A catalytic cycle is suggested for the hydrogenation of azobenzene, taking into account the intermediate complexes identified in the reaction medium.