pH-Controlled and Sulfite Anion-Directed Assembly of a Family of Cerium(III)-Containing Polyoxotungstates Clusters

A versatile one-pot strategy was employed to synthesize five cerium­(III)-containing polyoxotungstate nanoclusters through pH-controlled and sulfite anion-directed assembly: [C2H8N]3Na7[Ce2(H2O)6W22O72(OH)4]·20H2O (1) at pH 5.0; [C2H8N]8Na16[Ce4(H2O)12W44O144(OH)12]·23H2O (2) at pH 4.5; [C2H8N]2Na4Ce2[Ce2(H2O)10W28O92(OH)2]·27H2O (3) at pH 2.8–3.3; [C2H8N]2Na7[{α-SW7O28}­{Ce2(H2O)6}­(W3O6)­{α-SW9O32}­{α-SW9O31(OH)}]·18H2O (4) at pH 2.5; [C2H8N]2Na18[Ce2(H2O)9W36O110(OH)12]2·30H2O (5) at pH 1.5. These compounds were characterized by single-crystal X-ray structure analysis, IR spectroscopy, thermogravimetric (TG) analysis, X-ray photoelectron spectroscopy (XPS), and electrospray ionization mass spectrometry (ESI-MS). Moreover, their electrochemical properties were investigated. Single-crystal X-ray structure analysis revealed that 1 and 2 were di- and tetra-cerium­(III)-bridged polyoxotungstates, constructed from two different types of lacunary {W11} units. 3 composed of the well-known cerium­(III)-stabilized {W28} unit and organic amine–sodium–cerium cations, was isolated in the pH range 2.8–3.3. In this reaction system, the SO32– anion acted as a heteroanion template at a lower pH 2.5. 4 was isolated by the combination of cerium­(III) centers and SO32– heteroanion template, which is the first lanthanide-containing polyoxotungstates with sulfur heteroatoms, and the 4f metal cerium­(III) centers in 4 both have eight-coordinated modes and the SO32– heteroanion templates display μ7 and μ9 coordination modes. At a much lower pH 1.5, the polyanion of 5 was obtained, two triangular-shaped {W36} subunits were bridged by the cerium­(III) ions, resulting in the largest lanthanide-containing iso-polyoxotungstates known to date.

本研究采用通用一锅法(versatile one-pot strategy)，通过pH调控且亚硫酸根导向的组装过程，合成了5种含铈(III)的多钨酸盐纳米团簇(cerium(III)-containing polyoxotungstate nanoclusters)：在pH 5.0条件下得到[C2H8N]3Na7[Ce2(H2O)6W22O72(OH)4]·20H2O（化合物1）；在pH 4.5条件下得到[C2H8N]8Na16[Ce4(H2O)12W44O144(OH)12]·23H2O（化合物2）；在pH 2.8–3.3条件下得到[C2H8N]2Na4Ce2[Ce2(H2O)10W28O92(OH)2]·27H2O（化合物3）；在pH 2.5条件下得到[C2H8N]2Na7[{α-SW7O28}{Ce2(H2O)6}(W3O6){α-SW9O32}{α-SW9O31(OH)}]·18H2O（化合物4）；在pH 1.5条件下得到[C2H8N]2Na18[Ce2(H2O)9W36O110(OH)12]2·30H2O（化合物5）。

采用单晶X射线结构分析(single-crystal X-ray structure analysis)、红外光谱(IR spectroscopy)、热重(TG)分析(thermogravimetric (TG) analysis)、X射线光电子能谱(X-ray photoelectron spectroscopy, XPS)与电喷雾电离质谱(electrospray ionization mass spectrometry, ESI-MS)对上述化合物进行了表征，并对其电化学性能展开了研究。

单晶X射线结构分析结果显示，化合物1与2分别为二核铈(III)桥联与四核铈(III)桥联多钨酸盐，均由两类不同的缺位型{W11}结构单元构筑得到。化合物3由经典的铈(III)稳定化{W28}结构单元与有机胺-钠-铈阳离子构成，可在pH 2.8–3.3的区间内分离获得。在pH 2.5的较低酸度条件下，反应体系中的SO32–阴离子充当杂阴离子模板(heteroanion template)。化合物4由铈(III)中心与SO32–杂阴离子模板协同构建而成，是首例含硫杂原子的含镧系元素多钨酸盐；其中4中的4f金属铈(III)中心均采用八配位模式，而SO32–杂阴离子模板则展现出μ7与μ9两种配位模式。在更低的pH 1.5条件下，得到了化合物5的多阴离子结构，其由两个三角形{W36}亚单元通过铈(III)离子桥联得到，是迄今为止已知的最大含镧系元素同多钨酸盐(iso-polyoxotungstates)。