Hydrogenation of phenyl-substituted CN, CN,CC, CC and CO functional groups by Cr, Mo and W PNP pincer complexes – a DFT study

Abstract

The hydrogenation of phenyl-substituted CN, CN, CC, CC and CO functional groups catalyzed by PNP pincer amido M(NO)(CO)(PNP) and amino ^HM(NO)(CO)(PN^HP) 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–CN, Ph–CHCH₂ and Ph–CHO undergoes a stepwise mechanism, while that of Ph–CCH, Ph–CHNH, Ph–CHNH–Ph, Ph–CHN–CH₂–Ph and Ph–CO–CH₃ follows a one-step mechanism. The computed barrier in the increasing order of Ph–CN < Ph–CHNH < Ph–CHN–Ph < Ph–CHN–CH₂–Ph is in agreement with the experimentally observed hydrogenation activity for the Mo and W complexes. In addition, the hydrogenation of Ph–CN has a lower barrier than that of Ph–CHCH₂, 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–CHNH–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.