Assembly of Lanthanide-Containing Tungstotellurates(VI): Syntheses, Structures, and Catalytic Properties

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 = Dy(3+), Ho(3+), Er(3+), Tm(3+), Yb(3+), and Lu(3+)), dimeric (DMAH)(n)[H(22−n){Ln(H(2)O)(3)[TeW(17)O(61)]}(2)](·)mH(2)O (abbreviated as {Ln(2)Te(2)W(34)}; DMAH(+) = dimethylammonium), mono-substituted (DMAH)(7)Na(2){H(2)Ln(H(2)O)(4)[TeW(17)O(61)]}(·)mH(2)O (abbreviated as {LnTeW(17)}), and three-dimensional (3D) inorganic frameworks (DMAH)(n){H(3−n)Ln(H(2)O)(4)[TeW(6)O(24)]}(·)mH(2)O (abbreviated as {LnTeW(6)}), 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, {Ln(2)Te(2)W(34)} were obtained at pH 1.7, where two Dawson-like monovacant [TeW(17)O(61)](14−) are linked by two Ln(3+) ions; mono-substituted Dawson-like {LnTeW(17)} were isolated at pH 1.9, and 3D inorganic framework {LnTeW(6)} based on Anderson-type [TeW(6)O(24)](6−) were formed at pH 2.3. It was also found that the assembly of Ln-containing POMs depends on the type of Ln(3+) ions. The three types of POMs can be prepared by using Ln(3+) ions with a relatively smaller ionic radius, such as Tb(3+)-Lu(3+), while the use of Ln(3+) ions (La(3+)-Eu(3+)) results in the formation of precipitation or {TeW(18)O(62)} clusters. Furthermore, three {LnTeW(6)} (Ln = Tb(3+), Er(3+), Lu(3+)) were used as Lewis acid catalysts for the cyanosilylation of benzaldehydes, and their catalytic activity decreases with the decrease of Ln(3+) ionic radius, giving the order: {TbTeW(6)} > {ErTeW(6)} > {LuTeW(6)}. Notably, {TbTeW(6)} is stable to leaching and can be reused for five cycles without a significant loss of its activity.