Results of gas phase reactivity studies on group six transition metal suboxide clusters, Mo3Oy-, Mo 2WOy-, MoW2Oy -, and W3Oy- (Mo (3-x)WxOy-, x=0-3; y=ca. 3-9) with both D2O and CO2 are reported. Sequential oxidation for the more reduced species, Mo(3-x)WxOy - + D2O/ CO2 → Mo(3-x)W xOy+1- + D2/CO, and dissociative addition for certain species, Mo(3-x)WxOy - + D2O/ CO2 → Mo(3-x)W xOy+1D2-/Mo(3-x)W xOy+1CO-, is evident in the product distributions observed in mass spectrometric measurements. Reactions with D2O proceed at a rate that is on the order of 102 higher than for CO2. The pattern of reaction products reveals composition-dependent chemical properties of these group six unary and binary clusters. At the core of this variation is the difference in Mo-O and W-O bond energies, the latter of which is significantly higher. This results in a larger thermodynamic drive to higher oxidation states in clusters with more tungsten atoms. However, addition products for more oxidized W-rich clusters are not observed, while they are observed for the more Mo-rich clusters. This is attributed to the following: In the higher oxides (e.g., y=8), addition reactions require distortion of local metal-oxygen bonding, and will necessarily have higher activation barriers for W-O bonds, since the vibrational potentials will be narrower. The binary (x=1,2) clusters generally show sequential oxidation to higher values of y. This again is attributed to higher W-O bond energy, the result being that stable binary structures have W atoms in higher oxidation states, and Mo centers both in more reduced states and sterically unhindered. The reduced Mo center provides a locus of higher reactivity. An unusual result that is not readily explained is the chemically inert behavior of Mo3O6-.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry