Some reactions and thermochemistry of NbO3-:: oxidation and reduction, hydrogen bond strength, and catalytic activation of primary alcohols

Some reactions of the Nb(V) anion NbO3- have been studied in the gas phase using Fourier transform ion cyclotron resonance mass spectrometry. In most instances, this ion behaves as a closed shell species, however, reactions where both the ion and neutral products were radicals were also observed. In these cases, the niobium was invariably reduced to a lower oxidation state. A value for the NbO3--H bond strength is proposed, based on the observed reactivity: D(NbO3--H) = 103 +/- 9 kcal mol(-1). The reactions of NbO3- with methanol and ethanol were also studied due to their relevance in catalytic processes. For the NbO3-/CH3OH couple, the favoured pathway involves reduction of the parent ion to NbO3H2-, dihydroxyniobium(III) oxide, and concomitant liberation of formaldehyde in a single step dehydrogenation reaction. The dehydration pathway, which liberates the neutral H2O, competes less efficiently with the oxidation/reduction pathway. The primary product NbO3H2- does not appear to react with methanol. In contrast, the dehydration pathway is kinetically favoured for ethanol, with liberation of neutral ethene and formation of NbO4H2- observed. This reaction is extremely inefficient (k(exp)/k(ADO) = 0.02). The primary hydration product reacts even less efficiently with ethanol, in a two step process, ultimately resulting in formation of NbO4C2H6-. Radio-frequency acceleration of NbO4H2- results in regeneration of the parent ion. Overall, the results are in agreement with the hypothesis that higher order Nb-O bonds are the catalytically active centres on Nb-O surfaces. (C) 2000 Elsevier Science B.V.