Hybrid dimers based on metal-substituted Keggin polyoxometalates (metal = Ti, Ln) for cyanosilylation catalysis

Six novel organic-inorganic hybrid polyoxometalate derivatives, K[(H2O)4(3-Hpic)2Ce][(H2O)5(3-Hpic)2Ce][PW10Ti2O40]·11H2O 1, K[(H2O)4(3-Hpic)2Nd][(H2O)5(3-Hpic)2Nd][PW10Ti2O40]·12H2O 2, K[(H2O)4(3-Hpic)2Sm][(H2O)5(3-Hpic)2Sm][PW10Ti2O40]·8H2O 3, H[(H2O)7(4-Hpic)La][(4-Hpic)2LaPW11O39]·14.5H2O 4, H[(H2O)7(4-Hpic)Ce][(4-Hpic)2CePW11O39]·7.5H2O 5, and K(4-Hpic)0.5[(H2O)6(4-Hpic)Nd][(4-Hpic)2NdPW11O39]·10H2O 6, (3-Hpic = 3-picolinic acid; 4-Hpic = 4-picolinic acid) have been synthesized and characterized by elemental analysis, IR, solid state UV-vis spectroscopy, thermal gravimetric analysis, powder X-ray diffraction and single-crystal X-ray diffraction. Isostructural compounds 1-3 are hybrid dimers that consist of two Ti-substituted Keggin-type polyoxoanions [PW10Ti2O40]7- and four Ln-3-Hpic coordination groups. The strong hydrogen bonds in 1-3 can make these dimers yield a 1D supramolecular chain. Compounds 4-6 are also dimers constructed from two lanthanide-substituted Keggin-type polyoxoanions [PW11LnO39]4- and four 4-Hpic ligands, which are further linked by two Ln-4-Hpic groups to form a bi-supporting structure. Then these subunits are joined together by strong hydrogen-bonding interactions between polyoxoanions and coordinated water molecules to produce a 2D supramolecular framework. All these compounds as heterogeneous Lewis acid-base catalysts show high activity and high selectivity for the cyanosilylation of different carbonyl compounds under solvent-free conditions, and can be recycled without any obvious inactivation.