Mechanistic insights into hydrogen generation for catalytic hydrolysis and alcoholysis of silanes with high-valent oxorhenium(V) complexes

The high-valent oxorhenium(V) complex[Re(O)(hoz)(2)](+) (1)-catalyzed hydrolytic oxidation of silanes to produce dihydrogen was studied computationally to determine the underlying mechanism. Our results suggested that the oxorhenium.V) complex 1-catalyzed hydrolysis/alcoholysis of silanes proceeds via the ionic out-sphere mechanistic pathway. The turnover-limiting step was found to be the heterolytic cleavage of the Si-H bond and featured a S(N)2-Si transition state, which corresponds to the nucleophilic anti attack of water or alcohol on the silicon atom in a cis eta(1)-silane rhenium(V) adduct. Dihydrogen was generated upon transferring the hydride from the neutral rhenium hydride[Re(O)(hoz)(2)H] to the solvated[Me3SiOHR](+) ion. The activation free energy of the turnover-limiting step along the ionic outer-sphere pathway was calculated to be 15.7 kcal mol(-1) with water, 15.4 kcal mol(-1) with methanol, and 15.9 kcal mol(-1) with ethanol. These values are energetically more favorable than the [ 2 + 2] addition pathway by similar to 15.0 kcal mol(-1). Furthermore, the previously proposed catalytic pathways involving transient rhenium(VII) complexes or via the silicon attack on a rhenium hydroxo/alkoxo complex are shown to possess higher barriers.