Temperature-induced racemic compounds and chiral conglomerates based on polyoxometalates and lanthanides: syntheses, structures and catalytic properties

Abstract

By controlling the reaction temperatures, four new architectures based on Evans–Showell type polyoxometalate, [Ln(H₂O)₅][Ln(H₂O)₇][Co₂Mo₁₀H₄O₃₈] 6H₂O (Ln = Sm 1; Eu 2), and (NH₄)₃[Ln(H₂O)₆][Co₂Mo₁₀H₄O₃₈] 6H₂O (Ln = Sm 3; Eu 4) have been synthesized and characterized by elemental analysis, IR spectroscopy, TG analysis, solid diffuse reflective spectrum, powder X-ray diffraction and single crystal X-ray diffraction. Compounds 1 and 2 are racemic compounds, obtained at the higher temperature (85 °C). They show a 1D ladderlike structure constructed from left-handed chain and right-handed chain linked by lanthanide cations, which represent the first 1D self assembly based on Evans–Showell type polyoxometalate. When the reaction temperature was decreased to room temperature (25 °C), two chiral species 3 and 4 were obtained, showing a mono-supporting structure composed of one [Co₂Mo₁₀H₄O₃₈]⁶⁻ polyoxoanion and one [Ln(H₂O)₆]³⁺ unit. They crystallize in the chiral space group P2₁, as conglomerates of two enantiomerically pure crystals. Their absolute configuration were determined by the Flack parameters and solid state circular dichroism spectroscopy. The structural differences between 1(2) and 3(4) reveal that the reaction temperature is the key factor controlling the products from racemic compounds to conglomerates. Four compounds 1–4 acted as Lewis acid–base catalysts through a heterogeneous manner to catalyse cyanosilylation with excellent efficiency.