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

Abstract

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.