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，通过pH调控及亚硫酸根阴离子（sulfite anion）导向组装，合成了5种含三价铈（cerium(III)）的多钨酸盐（polyoxotungstate）纳米团簇（nanoclusters），具体产物及合成条件如下：
在pH 5.0条件下得到[C₂H₈N]₃Na₇[Ce₂(H₂O)₆W₂₂O₇₂(OH)₄]·20H₂O（化合物1）；pH 4.5时得到[C₂H₈N]₈Na₁₆[Ce₄(H₂O)₁₂W₄₄O₁₄₄(OH)₁₂]·23H₂O（化合物2）；pH 2.8~3.3时得到[C₂H₈N]₂Na₄Ce₂[Ce₂(H₂O)₁₀W₂₈O₉₂(OH)₂]·27H₂O（化合物3）；pH 2.5时得到[C₂H₈N]₂Na₇[{α-SW₇O₂₈}·{Ce₂(H₂O)₆}·(W₃O₆)·{α-SW₉O₃₂}·{α-SW₉O₃₁(OH)}]·18H₂O（化合物4）；pH 1.5时得到[C₂H₈N]₂Na₁₈[Ce₂(H₂O)₉W₃₆O₁₁₀(OH)₁₂]₂·30H₂O（化合物5）。

上述化合物均通过单晶X射线结构分析（single-crystal X-ray structure analysis）、红外光谱（IR spectroscopy）、热重（TG）分析、X射线光电子能谱（X-ray photoelectron spectroscopy, XPS）及电喷雾电离质谱（electrospray ionization mass spectrometry, ESI-MS）完成表征，并对其电化学性质（electrochemical properties）开展了研究。

单晶X射线结构分析结果显示，化合物1和2分别为二核、四核三价铈桥联的多钨酸盐，由两种不同的缺位型{W₁₁}结构单元构筑而成。化合物3由经典的三价铈稳定的{W₂₈}结构单元与有机胺-钠-铈阳离子构成，可在pH 2.8~3.3的反应范围内分离得到。在该反应体系中，当pH降至2.5时，亚硫酸根阴离子（SO₃²⁻）发挥了杂阴离子模板（heteroanion template）的作用。化合物4由三价铈中心与亚硫酸根杂阴离子模板共同组装得到，是首例含硫杂原子的含稀土多钨酸盐（lanthanide-containing polyoxotungstates）；其中化合物4中的4f金属三价铈中心均采用八配位模式，而亚硫酸根杂阴离子模板则分别呈现μ₇和μ₉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，其由两个三角形{W₃₆}亚单元通过三价铈离子桥联而成，是目前已知的最大尺寸含稀土同多钨酸盐（iso-polyoxotungstates）。