Synthesis And Catalytic Performance Of Hybrid Dimers Based On Metal-substituted Keggin Polyoxometalates

The assembly of lacunary POM clusters with transition metal（TM）or lanthanide（Ln）cations produces a family of metal-substituted POMs,which not only possess diverse structures,but also have significant applications in catalysis,magnetism,luminescence or medicine.In particular,catalysis materials of metal-substituted POMs have attracted increasing interest.In this paper,six novel organic-inorganic hybrid polyoxometalate derivatives have been synthesized under the routine water bath process and characterized by elemental analysis,IR,solid state UV-vis spectroscopy,thermal gravimetric analysis,powder X-ray diffraction and single-crystal X-ray diffraction.Furthermore,all these compounds as heterogeneous Lewis acid-base catalysts for the cyanosilylation of different carbonyl compounds have been investigated.1.Three novel organic–inorganic hybrid polyoxometalate derivatives have been successfully synthesized by[PW₁₁Ln O₃₉]^4-and 4-picolinic acid（4-Hpic）.H[（H₂O）₇（4-Hpic）La][（4-Hpic）₂LaPW₁₁O₃₉]·14.5H₂O（1）,H[（H₂O）₇（4-Hpic）Ce][（4-Hpic）₂CePW₁₁O₃₉]·7.5H₂O（2）,K（4-Hpic）_0.5[（H₂O）₆（4-Hpic）Nd][（4-Hpic）₂NdPW₁₁O₃₉]·10H₂O（3）.Compounds 1–3 are also dimers constructed from two lanthanide-substituted Keggin-type polyoxoanions[PW₁₁LnO₃₉]^4-and four 4-Hpic ligands.Then these subunits are joined together by strong hydrogen-bonding interactions between polyoxoanions and coordinated water molecules to produce a 2D supramolecular network.2.Three hybrid compounds based on[PW₁₀Ti₂O₄₀]^7-,lanthanide cations(Ce³⁺,Nd³⁺and Sm³⁺)and 3-picolinic acid（3-Hpic）,have been synthesized.K[（H₂O）₄（3-Hpic）₂Ce][（H₂O）₅（3-Hpic）₂Ce][PW₁₀Ti₂O₄₀]·11H₂O（4）,K[（H₂O）₄（3-Hpic）₂Nd][（H₂O）₅（3-Hpic）₂Nd][PW₁₀Ti₂O₄₀]·12H₂O（5）,K[（H₂O）₄（3-Hpic）₂Sm][（H₂O）₅（3-Hpic）₂Sm][PW₁₀Ti₂O₄₀]·8H₂O（6）.Compounds 4–6 are isostructural,and display a dimer composed of two[PW₁₀Ti₂O₄₀]^7-anions and four Ln-3-pic units.Theses dimers are joined up together to yield a 1D supramolecular chain by the strong hydrogen-bonding interactions between oxygen atoms from3-Hpic molecules and coordinated water molecules.All these compounds as heterogeneous Lewis acid-Lewis base catalysts show high activity and high selectivity for the cyanosilylation of different carbonyl compounds in solvent-free condition,and can be recycled without any obvious inactivation.Furthermore,we can find a remarkable difference in activity between Ti-substituted polyoxometalate derivatives4–6 and Ln-substituted polyoxometalate derivatives 1–3.Compounds 4–6 show better catalytic performance.