Hydrosilylation, hydrocyanation, and hydroamination of ethene catalyzed by bis(hydrido-bridged)diplatinum complexes: Added insight and predictions from theory

Details on the reaction mechanism of the catalytic cycle of hydrosilylation, hydrocyanation and hydroamination of ethene catalyzed by bis(hydrido-bridged)diplatinum complexes were obtained with the aid of DFT by calculating the relevant intermediates and transition state structures. The catalytically “active” species identified are the 16e coordinatively unsaturated mononuclear [Pt(X)(H)(PH3)(η2-C2H4)] (X = SiH3, CN, NH2) species formed upon addition of the ethene molecule on the monomeric [Pt(X)(H)(PH3)] precursors. All crucial reaction steps encapsulated in the entire catalyzed courses have been scrutinized. The following three steps are found to be critical for these catalytic reactions: (i) the migration of the hydride to the acceptor C atom of the coordinated ethene substrate, (ii) the reductive elimination of the final product and (iii) the oxidative addition process that regenerates the catalyst with activation barriers of 13.1, 16.5 and 13.3 kcal/mol for hydrosilylation, 7.1, 31.0 and 2.8 kcal/mol for hydrocyanation and 11.7, 39.7 and 39.0 kcal/mol for hydroamination reactions. In all cases the rate-determining step is that of the reductive elimination of the final product having always the highest activation barrier. The overall catalytic processes are exergonic with the calculated exergonicities being −13.5 (−8.0), −16.1 (−10.4) and −38.8 (−46.7) kcal/mol for the hydrosilylation, hydrocyanation and hydroamination of ethene, respectively, at the B3LYP (CCSD(T)) levels of theory. According to energetic span of the cycle called δE, which determines the frequency of the catalytic cycle, we found that the catalytic efficiency of the hydrido-bridged diplatinum complexes follows the trend: hydrocyanation ≈ hydrosilylation > hydroamination.