OXYGEN-EXCHANGE PATHWAYS IN ALUMINUM POLYOXOCATIONS

Abstract

Using molecular dynamics simulations and electronic structure methods, we postulate a mechanism to explain the complicated reactivity trends that are observed for oxygen isotope exchange reactions between sites in aluminum polyoxocations of the ɛ-Keggin type and bulk solution. Experimentally, the molecules have four nonequivalent oxygens that differ considerably in reactivity both within a molecule, and between molecules in the series: Al 13, GaAl 12, and GeAl 12 [ MO 4Al 12(OH) 24(H 2O) 12n+ (aq); with M = Al(III) for Al 13, n = 7; M = Ga(III) for GaAl 12, n = 7; M = Ge(IV) for GeAl 12, n = 8]. We find that a partly dissociated, metastable intermediate molecule of expanded volume is necessary for exchange of both sets of μ 2-OH and that the steady-state concentration of this intermediate reflects the bond strengths between the central metal and the μ 4-O. Thus the central metal exerts extraordinary control over reactions at hydroxyl bridges, although these are three bonds away. This mechanism not only explains the reactivity trends for oxygen isotope exchange in μ 2-OH and η-OH 2 sites in the ɛ-Keggin aluminum molecules, but also explains the observation that the reactivities of minerals tend to reflect the presence of highly coordinated oxygens, such as the μ 4-O in boehmite, α-, and γ-Al 2O 3 and their Fe(III) analogs. The partial dissociation of these highly coordinated oxygens, coupled with simultaneous activation and displacement of neighboring metal centers, may be a fundamental process by which metals atoms undergo ligand exchanges at mineral surfaces.