Gas-phase reactivity of metavanadate [VO3]- towards methanol and ethanol: experiment and theory

The gas-phase reactivity of the metavanadate anion VO3]- towards methanol and ethanol was examined by a combination of ion-molecule reaction and isotope labelling experiments in a quadrupole ion-trap mass spectrometer. The experimental data were interpreted with the aid of density functional theory calculations. VO3]- dehydrated methanol to eliminate water and form VO2(eta2-OCH2)]-, which features an eta2-C,O-OCH2]2- ligand formed by formal removal of two protons from methanol and which is isoelectronic with peroxide. VO3]- reacted with ethanol in an analogous manner to form VO2(eta2-OCHCH3)]-, as well as by loss of ethene to form VO2(OH)2]-. The calculations predicted that important intermediates in these reactions were the hydroxo alkoxo anions VO2(OH)(OCH2R)]- (R: H, CH3). These were predicted to undergo intramolecular hydrogen-atom transfer to form VO(OH)2(eta1-OCHR)]- followed by eta1-O-->eta2-C,O rearrangements to form VO(OH)2(eta2-OCHR)]-. The latter reacted further to eliminate water and generate the product VO2(eta2-OCHR)]-. This major product observed for VO3]- is markedly different from that observed previously for NbO3]- containing the heavier Group 5 congener niobium. In that case, the major product of the reaction was an ion of stoichiometry Nb, O3, H2]- arising from the formal dehydrogenation of methanol to formaldehyde. The origin of this difference was examined theoretically and attributed to the intermediate alkoxo anion NbO2(OH)(OCH3)]- preferring hydride transfer to form HNbO2(OH)]- with loss of formaldehyde. This contrasts with the hydrogen-atom-transfer pathway observed for VO2(OH)(OCH3)]-.