铌氧簇的合成机制及其过氧铌键断裂、再聚合机理研究

Polyoxoniobates (PONbs) have attracted increasing interest because of their unique characteristics and potential applications in nuclear-waste treatment and virology. However, they could only exist in strong alkaline aqueous solutions, and the yields are relatively low and the repeatability poor. To inhibit the hydrolysis of [Nb6O19]8– polyanion in acid condition, the only reported route often leads to the formation of peroxoniobium groups ((NbO2), involving aqueous peroxide H2O2, which may further affect the structures and properties. On the other hand, the NbO2 groups in the peroxoniobium-containing polyoxometalates (POMs) could be eliminated to NbO groups and then construct poly(POM) species by the formation of Nb–O–Nb linkages. Unfortunately, we have no idea about the forming conditions. Therefore, there is a need to find the mechanism of synthesizing novel PONbs that can be used to promote repeatability, increase yields, and then controllable cleavage and repolymerization of NbO2 groups. . The aim of this project is to design and synthesis of novel PONbs and niobium-substituted POMs that have excellent catalytic activity by using simple precursors (K7[HNb6O19]13H2O) under conventional synthetic conditions or hydrothermal methods, which would be characterized systemically by IR, UV/Vis, elemental analysis, NMR, and single-crystal X-ray diffraction. The second aim is to track their solution behavior by using ultra high resolution time-of-fight Electrospray Ionization Mass Spectrometry under different conditons, and then to illustrate their forming processes. It is expected to explore the mechanism of PONbs and its cleavage and repolymerization of NbO2 groups by detecting the different groups appeared in the synthesis processes of polyanions, which not only broaden the research fields of POM chemistry, but also could be applied to design and synthesize novel PONbs with high yield, good repeatability and excellent property. Therefore, this research has important academic value as well as far-reaching reality significance.

尽管目前已有不少铌氧簇的报道，但这类化合物的合成产率低且重复性差；而且，为了抑制铌氧簇阴离子水解而在体系中引入的双氧水中的过氧基团经常会与铌结合形成过氧铌键，进而影响铌氧簇的结构和性质；尽管过氧铌键在一定的条件下会断裂并导致阴离子的再聚合，但断裂和再聚合的条件不清楚。如何增强铌氧簇合成的可重复性、提高铌氧簇产率并可控制的促使过氧铌键的断裂及铌氧簇阴离子的再聚合，需要从机理上阐明化合物的形成过程。本项目从铌氧簇阴离子[Nb6O19]8–构成的化合物出发，设计、合成新型铌氧簇和取代型铌氧簇化合物，控制反应条件并采用高分辨质谱跟踪铌氧簇阴离子的形成过程，通过检测铌氧簇阴离子形成过程中出现的各种基团，探明铌氧簇的形成机理及过氧铌键断裂和阴离子再聚合的机理，不仅可以稳定铌氧簇合成的可重复性、提高铌氧簇产率，而且可用以指导新型铌氧簇的设计与合成。

新型多酸的合成是多酸研究的基础，是推动多酸研究发展的前沿领域。过氧铌键在一定条件可以断裂、再聚合，但其断裂和再聚合条件不清楚，本项目通过原位形成的[(NbO2)6P2W12O56]12–阴离子与过渡金属、稀土离子反应，制备合成了3个系列54例具有新颖结构的多金属氧酸盐，并对所得新化合物进行了全面表征和结构测定。建立了该类化合物的一般合成方法，实现了过氧铌键的可控断裂、再聚合。结果表明：①[(NbO2)6P2W12O56]12–阴离子中过氧键可以逐步断裂并键合不同数量的稀土离子或过渡金属离子，从而形成二聚、四聚或六聚衍生物；②通常极位过氧键断裂及键合优先于赤道位；③通过Nb-O-Ln桥可以形成超四面体、平行四边形等不同类型的四聚体，且部分四聚体又可通过稀土离子连接形成一维链状结构；④Tb和Ho稀土离子表现出极强的结构多样性；⑤不同类型的结构单元（单体、二聚体或四聚体）随着温度提高、pH降低均可转化为经典超四面体四聚结构，因此我们推测该四聚体结构是热力学稳定结构；⑥研究了其中部分化合物的溶液行为，发现过渡金属配位水的质子缓释能力。.同时，向[M6O19]8–(M = Nb/Ta)或WO42–双氧水体系中引入简单无机酸根离子，成功合成了4个系列33例新型杂多过氧多酸X4(MO2)6 (X = P/Se/As, M= Nb/Ta)及其衍生物，率先开展了杂多过氧钽簇的制备及性质研究，开辟了新型杂多钽的合成途径，得到了制备该类化合物的一般方法。并采用高分辨质谱跟踪阴离子的形成过程，通过检测形成过程中出现的各种基团，初步探明该系列过氧钽/铌氧簇的形成机理及过氧铌键断裂和阴离子再聚合的机理，利于指导新型铌氧簇的靶向设计和铌氧簇产率的提高。.该项目的实施，将开辟多酸合成的新方法，丰富多酸的结构类型，拓展多酸研究前沿，推动多酸化学发展。