Effect of Alkyl Group on M x O y – + ROH (M = Mo, W; R = Me, Et) Reaction Rates

A systematic comparison of M x O y – + ROH (M = Mo vs W; R = Me vs Et) reaction rate coefficients and product distributions combined with results of calculations on weakly bound M x O y –·ROH complexes suggest that the overall reaction mechanism has three distinct steps, consistent with recently reported results on analogous M x O y – + H2O reactivity studies. M x O y – + ROH → M x O y+1 – + RH oxidation reactions are observed for the least oxidized clusters, and M x O y – + ROH → M x O y ROH– addition reactions are observed for clusters in intermediate oxidation states, as observed previously in M x O y – + H2O reactions. The first step is weakly bound complex formation, the rate of which is governed by the relative stability of the M x O y –·ROH charge–dipole complexes and the Lewis acid–base complexes. Calculations predict that Mo x O y – clusters form more stable Lewis acid–base complexes than W x O y –, and the stability of EtOH complexes is enhanced relative to MeOH. Consistent with this result, Mo x O y – + ROH rate coefficients are higher than analogous W x O y – clusters. Rate coefficients range from 2.7 × 10–13 cm3 s–1 for W3O8 – + MeOH to 3.4 × 10–11 cm3 s–1 for Mo2O4 – + EtOH. Second, a covalently bound complex is formed, and anion photoelectron spectra of the several M x O y ROH– 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 M x O y – + ROH → M x O y+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 Mo x O y – reaction product distributions, indicating that the rate of this step is higher for W x O y+1RH– than for Mo x O y+1RH–. This result is consistent with previous computational studies on analogous M x O y – + H2O reactions predicting that barriers along the pathway in the rearrangement step are higher for Mo x O y – reactions than for W x O y –.