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Synthesis and catalytic properties of manganese (II) and oxovanadium (IV) complexes anchored to mesoporous MCM-41

Department of Chemistry and Biochemistry, Faculty of Sciences, CQB, University of Lisbon, 1749-016, Campo Grande, Lisboa, Portugal
Microporous and Mesoporous Materials (Impact Factor: 3.21). 10/2007; 112(1). DOI: 10.1016/j.micromeso.2007.09.009

ABSTRACT Reaction of [Mn III (acac) 3 ] and [V IV O(acac) 2 ] with the 1,4-diazabutadiene (DAB) ligands Ph–DAB–(CH 2) 3 R [R = Si(OEt) 3 (1a); H (1b)] (L) leads to complexes of the type [Mn II (acac) 2 L] and [V IV O(acac)L]Cl in good yields. These complexes were characterised by spec-troscopic techniques, and magnetic measurements showed that in the case of the manganese the metal was reduced during the reaction with the nitrogen ligand. The oxovanadium and manganese complexes bearing the ligand with triethoxysilyl groups Ph–DAB–(CH 2) 3 Si(OEt) 3 (1a) were immobilised in ordered MCM-41 by carrying out a grafting reaction. Tethered complexes of the same type were prepared by treating MCM-41 first with a toluene solution of 1a under reflux; manganese(III) and oxovanadium(IV) precursors were then introduced into the MCM-41-ligand by pore volume impregnation of complex solutions. The modified materials were characterised by powder X-ray diffraction, solid-state NMR (13 C, 29 Si), FTIR, thermogravimetric studies and low temperature nitrogen adsorption isotherms. The grafted materials contained 0.7 wt% Mn and 0.8 wt% V, while higher metal loadings were achieved when the materials were prepared by tethering (2.9 wt% Mn and 2.4 wt% V). The modified materials are active catalysts for the oxidation of cis-cyclooctene using tert-butylhydroperoxide as oxygen donor, at 328 K, yielding 1,2-epoxycyclooctane as the main reaction product and 1,2-cyclooctanediol as a by-product. The vanadium-containing materials are more efficient epoxidation catalysts than the manganese ones. The catalytic behaviour of the heterogenised catalysts was also compared with that observed in homogeneous phase for the complexes [Mn(acac) 2 L] and [VO(acac)L]Cl. The vanadium catalysts exhibited the highest catalytic activity, whereas the manganese catalysts were the least active and selective to epoxide formation.

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