Sintesi, caratterizzazione e reattività di oxoclusters a base di zirconio

Oxoclusters of early transition metals are a class of compounds built up of a small number of 4 – 5 groups metal atoms linked by oxygen bridges. The coordination number of the metal ions can usually vary between 6 and 8 and the connectivity modes are different (corner, edge or face sharing). These compounds are globally neutral and discrete species that are conveniently used as building blocks in inorganic-organic hybrid materials. During the synthesis (a controlled hydrolysis of metal alkoxide), carboxylic acids bearing a double C=C bond, typically methacrylic or vinylacetic acid, are introduced as ligands in order to stabilize the oxocluster structure and to provide the organic moiety needed to start the copolymerization with the suitable monomers in the synthesis of hybrid materials. One new bimetallic oxocluster [ZrMg(O)(OMc)2(OnPr)(OMeOiPr)]4, displaying a totally new connectivity between metallic centers, was synthesized starting from a solution of a mixed Mg/Zr alkoxyde and methacrylic acid in molar ratio 1:1.7 (with respect to the number of moles of each metal) and was characterized with single-crystal-XRD, melting point and FT-IR spectroscopy. A further new monometallic zirconium-based oxocluster, [Zr5(O)3(OMeOiPr)2(OMc)12], having remarkable similarities with known pentanuclear zirconium-based structures4, was instead obtained from a solution of a mixed Sr/Zr alkoxide and methacrylic acid (molar ratio 1:4) and was characterized in the same way as the other one. Moreover the well known monometallic zirconium oxoclusters Zr4, Zr6 and Zr121-3, having a general formula ZryOx(OH)w(OR)z were synthesized and their stability toward hydrolysis was checked by leaving them in air and in acid, basic, neutral and hydrogen peroxide solutions for 24 days and by characterizing them with FT-IR/ATR and EXAFS spectroscopy. An appreciable degradation was noticed only after treatment at extreme pHs and in oxidant conditions. In the last case, the formation of peroxides species was supposed on the basis of evidence from Raman spectroscopy and qualitative iodometry. The known catalytic activity exhibited by Zr6 and Zr12 in the oxidation of substrate containing sulfide functional groups as methyl-p-tolylsulfide with hydrogen peroxide was studied in acetonitrile at room temperature by means of GC/FID. A kinetic treatment of the data led to the conclusion that Zr12 has a lower electrophilic character than Zr6, since methyl-p-tolylsulfone can be easily obtained in the presence of excess hydrogen peroxide. This property makes Zr12 a potentially suitable catalyst in the desulfurization of oil and gases. Some clues on the lack of stability of the catalysts were obtained by acquiring FT-IR and EXAFS spectra on the species precipitated from the reaction mixture. Further attempts were made to obtain novel adducts between lacunary polyoxotungstates of the Keggin family (POMs) and Zr6 or Zr12. Indeed, POMs like Na8HPW9O34 or Na7SbW9O33 also exhibit oxidative catalytic properties that could be enhanced through a cooperative action with the zirconium based oxoclusters5. The syntheses were run under reflux and under both idro- or solvothermal conditions. However, no formation of new adduct between the POM and the clusters could be assumed by FT-IR and 31P-NMR spectroscopy and by single-crystal-XRD.