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

Abstract

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(-)·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(-)·ROH charge-dipole complexes and the Lewis acid-base complexes. Calculations predict that MoxOy(-) 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, MoxOy(-) + ROH rate coefficients are higher than analogous WxOy(-) clusters. Rate coefficients range from 2.7 × 10(-13) cm(3) s(-1) for W3O8(-) + MeOH to 3.4 × 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 MoxOy(-) 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 MoxOy(-) reactions than for WxOy(-).

Reactions of metal cluster anions with inorganic and organic molecules in the gas phase

“…84,174,239 Furthermore, the unique electronegativity of noble metal atoms that sits well between the non-noble metal atom and the O atom enable the not fully oxidized noble metal atom to coexist with the highly oxidative O •− radical in a cluster, which is a key bonding character to drive catalytic reactions such as CO oxidation by O 2 . [246][247][248] Recently, the mass spectrometry, photoelectron imaging spectroscopy, and quantum chemistry calculations have been integrated to study the ion-molecule reactions 157,175,176,233 and the structure-reactivity relationship of ligated metal clusters has been clarified clearly. According to the proposed influencing factors on cluster reactivity, one may preliminarily predict whether or not a newly generated metal cluster anion is reactive toward reductive or oxidative molecules.…”