Department of Chemistry, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan.
Dalton Trans. 2018 Jul 24;47(29):9657-9664. doi: 10.1039/c8dt01226a.
Four kinds of cobalt-polyoxomolybdenum clusters were synthesized by a combination of polyoxomolybdates and a Co-tacn (tacn = 1,4,7-triazacyclononane) complex. The heterometallic polynuclear clusters were obtained by the reaction of Na2MoO4 and Co(tacn)(H2O)33·H2O in water. The peripherals of polyoxomolybdates are capped by thermodynamically stable [Co(tacn)]3+ protecting groups to prevent further hydrolysis and condensation reactions. Intra-molecular hydrogen bonds between N-H groups on tacn and oxygen atoms on the molybdate core contribute to the stability of the complexes in water. Under the optimized synthetic conditions, the molar ratios of Na2MoO4 and Co(tacn)(H2O)33·H2O were adjusted to 1 : 0.3, 1 : 0.9, or 1 : 1.5, producing [{Co(tacn)}2Mo3O12]·2NaCF3SO3·7H2O (1), {Co(tacn)}4H2Mo7O272·13H2O (2), and {Co(tacn)}4H3Mo4O175·6H2O (3), respectively. As one of the most important factors, the adjustments of the pH values by changing the molar ratio enabled us to isolate these clusters. The presence of pyridine produces a neutral complex, [Co2(tacn)2(pyridine)Mo5O18]·4.5H2O (4) containing a lacunary Lindqvist-type polyoxomolybdate. The synthesis of complexes 2-4 is also possible starting from cluster 1, implying that 1 is a precursor for the formation of 2-4 in aqueous solution. Complexes 2-4 have a common structural building block, [{Co(tacn)}2Mo3O13], in each of the cluster units. The 1H NMR spectra of 2 and 3 in aqueous solution show multiple splitting patterns of methylene signals on tacn ligands according to the chemical environments indicated by the crystallographic structure. The 59Co NMR spectrum of cluster 3 in CH3NO2 has two signals from two different Co units with a 3 : 1 integration ratio ensuring the structural integrity in solution. The broadening of one of the signals for the 59Co NMR spectrum of cluster 3 in water as a protic solvent reveals the presence of the deprotonation equilibrium of the acidic protons on the cluster framework in water.
四种钴-多钼氧簇合物通过聚钼酸盐和 Co-tacn（tacn = 1,4,7-三氮杂环壬烷）配合物的组合合成。杂多核簇合物通过在水中反应 Na2MoO4 和 Co(tacn)(H2O)33·H2O 得到。聚钼酸盐的外围由热力学稳定的 [Co(tacn)]3+ 保护基团封闭，以防止进一步的水解和缩合反应。tacn 上的 N-H 基团与钼酸盐核上的氧原子之间的分子内氢键有助于复合物在水中的稳定性。在优化的合成条件下，调整 Na2MoO4 和 Co(tacn)(H2O)33·H2O 的摩尔比为 1：0.3、1：0.9 或 1：1.5，分别得到 [{Co(tacn)}2Mo3O12]·2NaCF3SO3·7H2O（1）、{Co(tacn)}4H2Mo7O272·13H2O（2）和 {Co(tacn)}4H3Mo4O175·6H2O（3）。作为最重要的因素之一，通过改变摩尔比来调节 pH 值使我们能够分离这些簇合物。吡啶的存在产生了一种中性配合物，[Co2(tacn)2(pyridine)Mo5O18]·4.5H2O（4），其中含有一个缺角 Lindqvist 型多钼酸盐。从簇合物 1 开始，也可以合成配合物 2-4，这意味着在水溶液中 1 是形成 2-4 的前体。配合物 2-4 在每个簇单元中都具有共同的结构构建块，[{Co(tacn)}2Mo3O13]。在水溶液中，2 和 3 的 1H NMR 光谱根据晶体结构所示的化学环境显示出 tacn 配体上亚甲基信号的多重分裂模式。在 CH3NO2 中 3 的 59Co NMR 谱有两个来自两个不同 Co 单元的信号，它们的积分比为 3：1，确保了溶液中的结构完整性。作为质子溶剂的水的 3 的 59Co NMR 谱中一个信号的展宽表明簇合物骨架上的酸性质子存在去质子化平衡。
