Passadis Stamatis S, Hadjithoma Sofia, Papanikolaou Michael G, Keramidas Anastasios D, Miras Haralampos N, Kabanos Themistoklis A
Section of Inorganic and Analytical Chemistry, Department of Chemistry, University of Ioannina, Ioannina 45110, Greece.
Department of Chemistry, University of Cyprus, Nicosia 2109, Cyprus.
Dalton Trans. 2022 Feb 1;51(5):1806-1818. doi: 10.1039/d1dt03641f.
The hydrolytically stable dioxime ligand (2-6)-piperidine-2,6-dione (Hpidiox) acts as a strong chelator mainly with hard metals in high oxidation states, a pre-requisite for potential applications in metal sequestering processes from aqueous solutions. Reaction of ZrCl with Hpidiox in methanol gives the mononuclear compound [Zr(η,η,η-Hpidiox-,,')(OH)]Cl·HO·CHOH (1), while the same reaction mixture in the presence of KOH gave the pentanuclear ZrOC [ZrIV5(μ-OH)(OH)(μ-η,η,η-Hpidiox-,,')(η,η,η-Hpidiox,,')]·5KCl·3CHOH·8HO (2). Compound 1 is formed at very acidic pH = 0, and the pentanuclear ZrOC 2 at higher pH values (pH = 2). Compounds 1 and 2 were characterized by single crystal X-ray structure analysis, multi-nuclear NMR spectroscopy and ESI-MS spectrometry. The single crystal X-ray structure analysis of 1 revealed a mononuclear zirconium(IV) compound containing an eight-coordinate zirconium atom bound to two singly deprotonated Hpidiox ligands and two water molecules in a severely distorted bicapped octahedral geometry. The pentanuclear ZrOC 2 constitutes the second example of a Zr cluster to be reported and the first one in which the four zirconium atoms are arranged in a tetrahedral arrangement with the fifth occupying the center of the tetrahedron. 1D and 2D NMR spectroscopies of the acidic CDOD solutions of complex 1 reveal a fast equilibrium between 1 and 2. Addition of KOH into a CHOH solution of 2 results in the controlled fast transformation of 2 to an asymmetric hexanuclear ZrOC 3 as evidenced by the NMR and real-time ESI-MS solution studies. Further addition of KOH to the solution of 3 leads to the ZrOC 4, and on the basis of NMR and ESI-MS data and in comparison with the known hexanuclear titanium(IV)/Hpidiox cluster, it is concluded that the cluster 4 should have a hexanuclear structure. Electrospray ionization mass spectrometry (ESI-MS) demonstrated not only the structural stability 1 and 2 in solution, but also revealed the reversible pH driven dis-assembly/re-assembly process between the monomeric 1 and the pentanuclear ZrOC 2.
水解稳定的二肟配体（2 - 6）-哌啶 - 2,6 -二酮（Hpidiox）主要作为一种强螯合剂，与高氧化态的硬金属结合，这是其在从水溶液中进行金属螯合过程中潜在应用的一个先决条件。ZrCl与Hpidiox在甲醇中反应生成单核化合物[Zr(η,η,η - Hpidiox -,,')(OH)]Cl·HO·CHOH（1），而在KOH存在下的相同反应混合物则生成五核ZrOC [ZrIV5(μ - OH)(OH)(μ - η,η,η - Hpidiox -,,')(η,η,η - Hpidiox,,')]·5KCl·3CHOH·8HO（2）。化合物1在非常酸性的pH = 0时形成，五核ZrOC 2在较高pH值（pH = 2）时形成。化合物1和2通过单晶X射线结构分析、多核NMR光谱和ESI - MS光谱进行了表征。1的单晶X射线结构分析表明，它是一种单核锆（IV）化合物，含有一个八配位的锆原子，与两个单去质子化的Hpidiox配体和两个水分子以严重扭曲的双帽八面体几何构型结合。五核ZrOC 2是报道的第二个Zr簇实例，也是第一个其中四个锆原子呈四面体排列且第五个占据四面体中心的实例。配合物1的酸性CDOD溶液的1D和2D NMR光谱显示1和2之间存在快速平衡。向2的CHOH溶液中加入KOH导致2可控地快速转化为不对称六核ZrOC 3，NMR和实时ESI - MS溶液研究证明了这一点。向3的溶液中进一步加入KOH会生成ZrOC 4，基于NMR和ESI - MS数据并与已知的六核钛（IV）/Hpidiox簇进行比较，得出簇4应具有六核结构的结论。电喷雾电离质谱（ESI - MS）不仅证明了1和2在溶液中的结构稳定性，还揭示了单体1和五核ZrOC 2之间由pH驱动的可逆解聚/再组装过程。
