Hydrosilylation with bis(alkynyl)(1,5-cyclooctadiene) platinum catalysts:: A density functional study of the initial activation

被引:31
作者
Jagadeesh, MN
Thiel, W
Köhler, J
Fehn, A
机构
[1] Max Planck Inst Kohlenforsch, D-45470 Mulheim, Ruhr, Germany
[2] Consortium Elektrochem Ind GmbH, D-81379 Munich, Germany
[3] Wacker Chemitron GmbH, Werk Burghausen, D-84480 Burghausen, Germany
关键词
D O I
10.1021/om0200196
中图分类号
O61 [无机化学];
学科分类号
070301 ; 081704 ;
摘要
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.
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页码:2076 / 2087
页数:12
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