Hydrogenation of phenyl-substituted C≡N, C=N, C=C, C=C and C=O functional groups by Cr, Mo and W PNP pincer complexes - a DFT study

The hydrogenation of phenyl-substituted C=N, C=N, C=C, C=C and C=O functional groups catalyzed by PNP pincer amido M(NO)(CO)(PNP) and amino M-H(NO)(CO)((PNP)-P-H) complexes [M = Cr, Mo and W; PNP = N(CH2CH2P(isopropyl)(2))(2)] has been computed at the B3PW91 level of density functional theory. The computed structure and stability of the Mo and W complexes are in agreement with the experimental results. The hydrogenation of Ph-C=N, Ph-CH=CH2 and Ph-CHO undergoes a stepwise mechanism, while that of Ph-C=CH, Ph-CH=NH, Ph-CH=NH-Ph, Ph-CH=N-CH2-Ph and Ph-CO-CH3 follows a one-step mechanism. The computed barrier in the increasing order of Ph-C=N < Ph-CH=NH < Ph-CH=N-Ph < Ph-CH=N-CH2-Ph is in agreement with the experimentally observed hydrogenation activity for the Mo and W complexes. In addition, the hydrogenation of Ph-C=N has a lower barrier than that of Ph-CH=CH2, and this is also found for the hydrogenation of 4-vinylbenzonitrile catalyzed not only by the Mo complexes but also by the corresponding PNP Fe pincer complexes, in disagreement with the experiment. The finding that the hydrogenation of Ph-CHO has a lower barrier than those of Ph-CO-CH3 and Ph-CH=NH-Ph is just opposite to the experimental results for the Mo complexes. It is found that the Mo complexes have higher catalytic activity than the W and Cr complexes. In contrast to the Mo and W complexes, the Cr complexes have not yet been reported experimentally.