Abstract

The relative effectiveness of a range of heteropolyoxometalate catalysts with Keggin, Dawson and transition metal-substituted lacunary structures has been compared using the homogeneous-phase oxidation of isobutyraldehyde to isobutyric acid with H 2O 2 in acetonitrile at 50 °C. For the Keggin ([XM 12O 40] n− , X=P(V), Co(III), Fe(III), Co(II), Zn(II), M=W(VI); X=P(V), M=Mo(VI)) and Dawson ([X 2Mo 18O 62] m− , X=P(V), S(VI)) structures, catalytic activity increased with decreasing anionic charge. In the case of the prototypical [PW 12O 40] 3− ion, kinetic studies indicated that there was association of the isobutyraldehyde and phosphopolyoxotungstate prior to oxidation of the former by H 2O 2. No fragmentation of the tungsten-containing species, such as [PW 12O 40] 3− and [CoW 12O 40] 6−, occurred under the reaction conditions, as shown by 31 P NMR and UV–VIS spectroscopy, respectively while both [PMo 12O 40] 3− and [P 2Mo 18O 62] 6− generated small amounts of peroxo-species based on 31 P NMR studies. The transition metal-substituted lacunary polyoxomolybdate systems of the type [PZ(II)(Br)Mo 11O 39] 6−, where Z=Mn(II), Co(II), Ni(II), Cu(II) and Zn(II), are likely degraded to (PO 4[MoO(O 2) 2] 4) 3− and/or related species by H 2O 2, based on UV–VIS spectroscopic studies and on 31 P NMR studies of diamagnetic [PZn(Br)Mo 11O 39] 6−. The resulting products are only slightly more active catalysts than the Keggin anions (a maximum increase in activity by a factor of ∼7). The initial rates of reaction followed the sequence Co(II)>Ni(II)>Zn(II)>Mn(III)>Cu(II), with Mn(II) undergoing oxidation by H 2O 2 under the reaction conditions. The order likely corresponds to a combination of the catalytic abilities of the transition metal-substituted lacunary anions in conjunction with their rates of decomposition by H 2O 2, and/or the catalytic abilities of the freed transition metals and peroxo-products themselves.

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