Hydrosilylation with Bis(alkynyl)(1,5-cyclooctadiene)platinum Catalysts: A Density Functional Study of the Initial Activation

At elevated temperatures bis(alkynyl)(1,5-cyclooctadiene)platinum complexes catalyze the cross-linking of polyorganosiloxanes containing Si−H and vinyl groups. Density functional calculations with medium-size basis sets and effective core potentials are reported for reactions that may activate these precatalysts for hydrosilylation. For a model system consisting of the bis(ethynyl) complex, trimethylsilane, and ethylene, the computations provide two plausible pathways for gaining access to the Chalk−Harrod cycle. The first one involves a sequence of four oxidative additions and reductive eliminations, while the second one requires a reductive coupling that is induced by olefin coordination. In both cases, the initial step is rate-determining, with a computed barrier of 27 kcal/mol. Experiments for polysiloxane systems of industrial interest favor the first pathway and yield barriers of 25−30 kcal/mol. Substituents in the alkynyl groups affect the measured barriers and the barriers computed for the rate-determining initial step of the first pathway in a qualitatively similar manner. We propose that the activation of the precatalysts is initiated by oxidative addition of Si−H.