Disparate product distributions observed in Mo_(3-x)W_xO_y~-(x=0-3; y=3-9)reactions with D_2Oand CO_2

摘要

Results of gas phase reactivity studies on group six transition metal suboxide clusters, Mo_3O_y~-,Mo_2WO_y~-, MoW_2O_and W_3O_y~-(Mo_((3-x))xO_y~-, x=0-3; y =ca. 9) with both D_2Oand CO_2are reported. Sequential oxidation for the more reduced species, Mo(3_x)Wx0y-+D20/CO2→Mo_((3-x))W_xO_(y+1)~-+D_2/ CO, and dissociative addition for certain species,Mo_((3-x))W_xO_y + D_2O /CO_2→Mo_((3-x))W_xO_(y+1)D_2~-/CO~-, is evident in the product distributionsobserved in mass spectrometric measurements. Reactions with D_2Oproceed at a rate that is on theorder of 10~2higher than for CO_2. The pattern of reaction products reveals composition-dependentchemical properties of these group six unary and binary clusters. At the core of this variation is thedifference in Mo-O and W-O bond energies, the latter of which is significantly higher. This resultsin 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 areobserved 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 havehigher activation barriers for W-O bonds, since the vibrational potentials will be narrower. Thebinary (x=1, 2) clusters generally show sequential oxidation to higher values of y. This again isattributed to higher W-O bond energy, the result being that stable binary structures have W atomsin 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 readilyexplained is the chemically inert behavior of Mo_3O_6~-.