Synthesis, Structure Characterization, and Reversible Transformation of a Cobalt Salt of a Dilacunary γ‑Keggin Silicotungstate and Sandwich-Type Di- and Tetracobalt-Containing Silicotungstate Dimers

A cobalt salt of a γ-Keggin dilacunary silicotungstate, {CoL5}2[γ-SiW10O34L2] [Co-SiW10; L = N,N-dimethylformamide (DMF) or H2O], could be synthesized by the cation-exchange reaction of TBA4[γ-H4SiW10O36] (TBA = tetra-n-butylammonium) with 2 equiv of Co­(NO3)2 with respect to TBA4[γ-H4SiW10O36] in a mixed solvent of DMF and acetone (97% yield). Each Co-SiW10 was linked by water molecules via a hydrogen-bonding network. Besides Co-SiW10, various kinds of isostructural M-SiW10 could be synthesized via the same procedure as that for Co-SiW10 (M = Mn2+, Fe2+, Ni2+, Cu2+, Zn2+, and Cd2+). By the reaction of Co-SiW10 with 1 equiv of TBA6[γ-H2SiW10O36] in acetone, a silicotungstate dimer pillared by two cobalt cations with a significantly slipped dimer configuration, TBA6[Co2(γ-H3SiW10O36)2]·3H2O (Co2), could be synthesized. By the reaction of Co-SiW10 with 3 equiv of TBAOH in acetone, a tetracobalt-containing sandwich-type silicotungstate, TBA6[{Co­(H2O)}2(μ3-OH)2{Co­(H2O)2}2(γ-H2SiW10O36)2]·5H2O (Co4), could be synthesized. Compound Co4 possessed the tetracobalt–oxygen core, [{Co­(H2O)}2(μ3-OH)2{Co­(H2O)2}2]6+, identical with those of previously reported Weakley-type sandwich polyoxometalates, [Co4(H2O)2(XM9O34)2]n− (X = P5+, Si4+, Ge4+, As5+ or V5+; M = Mo6+ or W6+). The reversible transformation between these three compounds (Co-SiW10 ⇆ Co2, Co-SiW10 ⇆ Co4, and Co2 ⇆ Co4) took place by the addition and/or subtraction of required components in appropriate solvents, affording the desired products in high yields (71–93% yields).

一种γ-凯金型双空位硅钨酸盐（γ-Keggin dilacunary silicotungstate）的钴盐{CoL5}2[γ-SiW10O34L2]（记为Co-SiW10；L = N,N-二甲基甲酰胺(N,N-dimethylformamide, DMF)或H2O），可通过将TBA4[γ-H4SiW10O36]（TBA为四正丁基铵(tetra-n-butylammonium)）与相对于TBA4[γ-H4SiW10O36] 2当量的Co(NO3)2在DMF与丙酮的混合溶剂中进行阳离子交换反应合成，产率达97%。每个Co-SiW10单元通过水分子形成的氢键网络相互连接。除Co-SiW10外，采用与Co-SiW10相同的合成流程，还可制备一系列同构的M-SiW10（M = Mn²+、Fe²+、Ni²+、Cu²+、Zn²+及Cd²+）。将Co-SiW10与1当量的TBA6[γ-H2SiW10O36]在丙酮中反应，可得到由两个钴阳离子桥连的硅钨酸盐二聚体，其具有显著滑移的二聚体构型：TBA6[Co2(γ-H3SiW10O36)2]·3H2O（记为Co2）。将Co-SiW10与3当量的TBAOH在丙酮中反应，可合成含四核钴的夹心型硅钨酸盐：TBA6[{Co(H2O)}2(μ3-OH)2{Co(H2O)2}2(γ-H2SiW10O36)2]·5H2O（记为Co4）。化合物Co4的四核钴-氧核心[{Co(H2O)}2(μ3-OH)2{Co(H2O)2}2]6+，与此前报道的韦克利型夹心多金属氧酸盐(Weakley-type sandwich polyoxometalates)[Co4(H2O)2(XM9O34)2]n−（X = P5+、Si4+、Ge4+、As5+或V5+；M = Mo6+或W6+）的核心结构完全一致。上述三种化合物（Co-SiW10 ⇌ Co2、Co-SiW10 ⇌ Co4及Co2 ⇌ Co4）可通过在适宜溶剂中添加或移除所需组分实现可逆转化，目标产物产率可达71%~93%。