Κβαντοχημική μελέτη των καταλυτικών κύκλων της υδροσιλυλίωσης, υδροκυανίωσης και υδροαμίνωσης αλκενίων και αλκινίων με καταλύτες διπυρηνικά σύμπλοκα του λευκόχρυσου
Authors: Κεφαλίδης Χρήστος 
Published: January 2014

A detailed investigation of the mechanism of hydrosilylation, hydrocyanation and hydroamination of olefins and alkynes catalyzed by hydrido-bridged diplatinum complexes has been carried out in the framework of electronic structure calculations at the B3LYP and CCSD(T) levels of theory, using the LANL2DZ+BSII(Pt)?6- 31G**(L) basis set. The present investigation examines, in terms of located structures, energies and activation barriers the participation of postulated intermediates. Prior qualitative mechanistic assumptions are substituted by the presented theoretically well-founded and detailed analysis of both the thermodynamic and kinetic aspects that substantially improve the insight into the reaction course and enlarge them with novel mechanistic proposals. Our calculations show that the catalytically "active species" are the 16e coordinatively unsaturated mononuclear species resulted upon dissociation of the hydrido-bridged dinuclear precursors promoted by the incoming olefin substrate. The theoretical results allowed us first to locate the bridging hydride ligands, which was not able to be located experimentally by X-ray diffraction methods, and understand the apparently abnormally low chemical shift for the Pt(μ-Η)Pt resonances and the relatively high-frequency ν(Pt-H) band in the IR spectra of the hydrido-bridged dimers. Overall, the reactions can be partitioned into three main sections: (i) the hydride migration to the acceptor C atom of the coordinated ethene substrate, (ii) the reductive elimination of the hydrosilylated, hydrocyanated or hydroaminated product and (iii) the oxidative addition process that regenerates the catalytic species. Analysis of the computed potential energy surfaces allowed us to identify effective barriers, consisting of combined elementary steps, for these three stages of the hydrosilylation, hydrocyanation and hydroamination reactions. Firstly, the complete geometric and energetic profile of the catalytic cycles has exhaustively been explored and the role of the hydrosilane, the “spectator” phosphane ligand and the olefin and alkyne substrate on the catalytic activity and regioselectivity of the hydrosilylation process was assessed. Next, the catalytic cycles of hydrocyanation and hydroamination of ethene and ethyne catalyzed by bis(hydrido-bridged)diplatinum complexes have been explored. The efficiency of the catalytic cycles determined by the energy span quantity, δE, follows the trend: hydrocyanation ? hydrosilylation > hydroamination. Moreover, the hydrosilylation, hydrocyanation and hydroamination of ethene catalyzed by bis(hydrido-bridged)diplatinum complexes are predicted to be exothermic by -13.5 (-8.0), -18.9 (-10.9) and -38.8 (-46.7) kcal/mol, respectively at the B3LYP(CCSD(T)) levels of theory, in line with existing experimental observations in the case of hydrosilylation. In addition, the hydrosilylation, hydrocyanation and hydroamination of ethyne catalyzed by the hydrido-bridged diplatinum catalysts are predicted to be exothermic by -32.1 (-25.5), -37.8 (-28.2) and -53.7 (-45.1) kcal/mol, respectively at the B3LYP (CCSD(T)) levels of theory, in line with the experimental observations in the case of hydrosilylation. Some theoretical treatment should represent a potent tool for further development of platinum-based catalysts in particular for the hydrocyanation and hydroamination of unsaturated substrates, processes that are of potential synthetic interest.