A DENSITY-FUNCTIONAL STUDY ON SIGMA-BOND METATHESIS REACTIONS OF POSSIBLE IMPORTANCE IN DEHYDROGENATIVE SILANE POLYMERIZATION

Possible elementary reaction steps in dehydrogenative polymerization of silanes have been studied by nonlocal density functional calculations. The reaction steps involve silylation L(n)M-H + SiH4 --> L(n)M-SiH3 + H-2 (1), and the silicon-silicon bond forming step, L(n)M-SiH3 + SiH4 --> L(n)M-H + Si2H6 (2). For the model system L(n)M = Cl2Sc, step (1) has a calculated enthalpy of 3 kJ/mol and a calculated activation energy of 7 kJ/mol whereas step (2) has an enthalpy of 10 kJ/mol and a negative activation energy of - 11 kJ/mol. It is concluded that the mechanism proposed in the literature for dehydrogenative polymerization of silanes in terms of the steps (1) and (2) is feasible for L(n)M = Cp2M with M = Sc, Y, La, Lu and Sm. Studies were also carried out on the corresponding steps involving carbon rather than silicon. It was found that the carbon-carbon bond forming process analogous to (2) has an activation energy of 132 kJ/mol and it is suggested that the reaction between L(n)M-CH3 and CH4 instead leads to the methyl exchange process L(n)M-CH3 + CH4 --> L(n)M-CH3 + CH4 with an activation energy of 33 kJ/mol for L(n)M = Cl2Sc.