Krebs-type polyoxometalate-based crystalline materials: synthesis, characterization and catalytic performance

Abstract

Three new Krebs-type polyoxometalate-based crystalline materials (H2en)4{[M(Hen)(H2O)]2(WO2)2(B-β-SbW9O33)2}·12H2O (M = Ni2+ in 1, M = Co2+ in 2, M = Mn2+ in 3) (en = ethylenediamine) have been synthesized via hydrothermal method and characterized by single crystal X-ray diffraction, IR spectroscopy, X-ray powder diffraction and thermogravimetric analysis. Compounds 1-3 display isomorphic structures and contain similar disubstituted Krebs-type {[M(Hen)(H2O)]2(WO2)2(B-β-SbW9O33)2}8- polyoxoanions, in which sandwich-type {(WO2)2(B-β-SbW9O33)2} subunits are linked by [M(Hen)(H2O)]3+ fragments to form two-dimensional layer structures. By employing K3[Fe(CN)6]-Na2S2O3 reaction as the redox model, the influence of metal linkers on the catalytic electron transfer properties of POMs have also been investigated. A series of control experiments demonstrated that the Co-disubstituted compound 2 had better catalytic performance for reducing K3[Fe(CN)6] into K4[Fe(CN)6] among 1-3. While Ni- or Mn-disubstituted compounds 1 and 3 only presented weak effect. Such differences in catalytic properties may arise from their different φѳ of linked heterometal atoms. The current work may provide guidance for the design of Krebs-type polyoxometalate-based heterogeneous catalysts and the exploration of catalytic redox reaction mechanism at the molecular level.