Effect of Alkyl Group on MxOy- + ROH (M = Mo, W; R = Me, Et) Reaction Rates

A systematic comparison of MxOy- + ROH (M = Mo vs W; R = Me vs Et) reaction rate coefficients and product distributions combined with results of calculations on weakly bound MxOy-center dot ROH complexes suggest that the overall reaction mechanism has three distinct steps, consistent with recently reported results on analogous MxOy- + H2O reactivity studies. MxOy- + ROH -> MxOy+1- + RH oxidation reactions are observed for the least oxidized clusters, and MxOy- + ROH -> MxOyROH- addition reactions are observed for clusters in intermediate oxidation states, as observed previously in MxOy- + H2O reactions. The first step is weakly bound complex formation, the rate of which is governed by the relative stability of the MxOy-center dot ROH charge-dipole complexes and the Lewis acid-base complexes. Calculations predict that MxOy- clusters form more stable Lewis acid-base complexes than WxOy-, and the stability of EtOH complexes is enhanced relative to MeOH. Consistent with this result, MxOy- + ROH rate coefficients are higher than analogous WxOy- clusters. Rate coefficients range from 2.7 X 10(-13) cm(3) s(-1) for W3O8 + MeOH to 3.4 X 10(-11) cm(3) s(-1) for Mo2O4 + EtOH. Second, a covalently bound complex is formed, and anion photoelectron spectra of the several MxOyROH- addition products observed are consistent with hydroxyl-alkoxy structures that are formed readily from the Lewis acid-base complexes. Calculations indicate that addition products are trapped intermediates in the MxOy- + ROH -> MxOy+1- + RH reaction, and the third step is rearrangement of the hydroxyl group to a metal hydride group to facilitate RH release. Trapped intermediates are more prevalent in MxOy- reaction product distributions, indicating that the rate of this step is higher for WxOy+1RH- than for MoxOy+1RH-. This result is consistent with previous computational studies on analogous MxOy- + H2O reactions predicting that barriers along the pathway in the rearrangement step are higher for MxOy- reactions than for WxOy-.