Theoretical Study on the Mechanism of the Hydrogenation of Azo (N═N) Bonds to Amines Catalyzed by Manganese.

The mechanism of the transition metal manganese complex Mn(PNN)(CO)Br () that catalyzed the hydrogenation of the azo (N═N) bond to amines has been investigated using the PBE0 function. The results show that the whole reaction involves three basic processes: (1) the addition of H to gives , which can hydrogenate the azo (N═N) bond at the later stage; (2) hydrogenation of azobenzene by , which gives 1,2-diphenylhydrazine (PhNHNHPh); and (3) hydrogenation of 1,2-diphenylhydrazine by , which affords aniline (PhNH). The results suggest that the hydrogenation of and hydrogenation of azobenzene by to afford PhNHNHPh are easy to occur due to the low barriers, and the overall rate-determining step is the formation of and PhNH by breaking the N-N bond in the stage of hydrogenation of 1,2-diphenylhydrazine by , with an energy barrier of 39.1 kcal/mol. The computed results are in good agreement with the experimental results. The mechanism of the azobenzene reaction catalyzed by manganese was analyzed by charge and orbital analysis in detail. The theoretical results provide a deeper understanding of the mechanism and fully explain the experimental facts.