Evans-Showell-type polyoxometalate constructing novel 3D inorganic architectures with alkaline earth metal linkers: syntheses, structures and catalytic properties

Four new architectures containing [Co₂Mo₁₀H₄O₃₈]^6- polyoxoanions, (C₂N₂H₁₀)₂[Sr(H₂O)₅][Co₂Mo₁₀H₄O₃₈]·2H₂O 1, (C₂N₂H₁₀)₂[Ba(H₂O)₃][Co₂Mo₁₀H₄O₃₈]·3H₂O 2, (C₃N₂H₁₂)₂[Sr(H₂O)₅][Co₂Mo₁₀H₄O₃₈]·3H₂O 3 and (C₃N₂H₁₂)[Ba(H₂O)₄][Ba(H₂O)₄][Co₂Mo₁₀H₄O₃₈]·2H₂O 4 (C₂N₂H₁₀ = ethylenediamine; C₃N₂H₁₂ = 1,3-propanediamine) have been synthesized and characterized by elemental analysis, IR spectroscopy, solid diffuse reflective spectroscopy, TG analysis, powder X-ray diffraction and single crystal X-ray diffraction. Compounds 1 and 2 obtained in the presence of ethylenediamine, are made of Evans-Showell-type anions [Co₂Mo₁₀H₄O₃₈]^6-, linked by Sr²⁺ or Ba²⁺ cations to form 3D frameworks. To our knowledge, compound 1(2) represents the first example of 3D architecture in which the Evans-Showell anions [Co₂Mo₁₀H₄O₃₈]^6- were linked by pure alkaline earth cations. When propanediamine was used instead of ethylenediamine, compounds 3 and 4 with 2D networks were obtained. This phenomenon indicates that the organic cations, which adjust the reaction pH values, can induce different dimensional inorganic frameworks. As heterogeneous catalysts, compounds 1-4 show excellent catalytic performance in the cyanosilylation of carbonyl compounds. Furthermore, these catalytic reactions were performed under solvent-free conditions using only a low amount of the catalysts, and these catalysts can be recovered and reused without displaying any significant loss of activity. As far as we know, compounds 1-4 represent the first examples of cyanosilylation catalyzed by POM-based species containing alkaline earth metal cations.