Frontiers in Chemistry (Nov 2020)
Assembly of Lanthanide-Containing Tungstotellurates(VI): Syntheses, Structures, and Catalytic Properties
Abstract
Lanthanide (Ln)-containing polyoxometalates (POMs) have attracted particular attention owing to their structural diversity and potential applications in luminescence, magnetism, and catalysis. Herein three types of Ln-containing tungstotellurates(VI) (Ln = Dy3+, Ho3+, Er3+, Tm3+, Yb3+, and Lu3+), dimeric (DMAH)n[H22−n{Ln(H2O)3[TeW17O61]}2]·mH2O (abbreviated as {Ln2Te2W34}; DMAH+ = dimethylammonium), mono-substituted (DMAH)7Na2{H2Ln(H2O)4[TeW17O61]}·mH2O (abbreviated as {LnTeW17}), and three-dimensional (3D) inorganic frameworks (DMAH)n{H3−nLn(H2O)4[TeW6O24]}·mH2O (abbreviated as {LnTeW6}), have been synthesized by using simple metal salts and characterized by single-crystal X-ray diffraction and other routine techniques. Interestingly, the assembly of these POMs is pH dependent. Using the same starting materials, {Ln2Te2W34} were obtained at pH 1.7, where two Dawson-like monovacant [TeW17O61]14− are linked by two Ln3+ ions; mono-substituted Dawson-like {LnTeW17} were isolated at pH 1.9, and 3D inorganic framework {LnTeW6} based on Anderson-type [TeW6O24]6− were formed at pH 2.3. It was also found that the assembly of Ln-containing POMs depends on the type of Ln3+ ions. The three types of POMs can be prepared by using Ln3+ ions with a relatively smaller ionic radius, such as Tb3+-Lu3+, while the use of Ln3+ ions (La3+-Eu3+) results in the formation of precipitation or {TeW18O62} clusters. Furthermore, three {LnTeW6} (Ln = Tb3+, Er3+, Lu3+) were used as Lewis acid catalysts for the cyanosilylation of benzaldehydes, and their catalytic activity decreases with the decrease of Ln3+ ionic radius, giving the order: {TbTeW6} > {ErTeW6} > {LuTeW6}. Notably, {TbTeW6} is stable to leaching and can be reused for five cycles without a significant loss of its activity.
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