Isolation of Elusive Tetranuclear and Pentanuclear M(II)–Hydroximate Intermediates in the Assembly of Lanthanide [15-Metallacrown-5] Complexes

Two intermediates in the assembly
of lanthanide metallacrowns (MCs) of divalent transition metals and
ligands in the picoline hydroxamic acid (picHA)/α-amino hydroxamic
acid family were synthesized and crystallographically characterized.
Structures of the elusive MII[12-MCMII,L-4]2+ were obtained with M = Ni, Zn and L = picHA, quinaldic
hydroxamic acid. Consistent with previous calculations, the complex
is highly concave, particularly with Zn­(II). ESI-MS and 1H NMR reveal that the complexes retain their structure in solution.
The Zn­(II) analogue reacts with Ln­(III) ions to form LnIII[15-MCZn(II),picHA-5]3+ in pyridine. The greater
stability of ZnII[12-MCZn(II),picHA-4]2+ relative to the Cu­(II) and Ni­(II) analogues is inferred and attributed
to the square-pyramidal Zn­(II) ions being complementary with the concave
MC topology. A Zn4(picHA)2(OAc)4(DMF)2 species bearing a tetranuclear [6-MCZnII,picHA-2] motif was also isolated. A mechanism for MII[12-MCMII,L-4]2+ formation is proposed on the
basis of structural analysis of tetranuclear [6-MCMII,L-2] complexes. These results contribute to the goal of controlling
the reactivity of intermediates in the assembly of lanthanide MCs,
and coordination driven macrocycles in general, to prepare complexes
with greater stability or enhanced physical properties.

本研究合成并经晶体学表征了二价过渡金属与吡啶羟肟酸（picoline hydroxamic acid, picHA）/α-氨基羟肟酸类配体构建的镧系金属冠醚（lanthanide metallacrowns, MCs）组装过程中的两类中间体。成功获得了M为Ni、Zn且配体L为picHA、喹啉羟肟酸的罕见MII[12-MCMII,L-4]2+型配合物的晶体结构。与此前的计算结果一致，该配合物具有高度凹陷的拓扑结构，以Zn(II)配合物尤为显著。电喷雾电离质谱（ESI-MS）与1H NMR表征结果显示，该类配合物在溶液中可保持其结构。Zn(II)对应配合物可与Ln(III)离子在吡啶溶剂中反应，生成LnIII[15-MCZn(II),picHA-5]3+型配合物。研究推断，ZnII[12-MCZn(II),picHA-4]2+的稳定性相较于Cu(II)与Ni(II)对应配合物更高，这一现象可归因于四方锥配位构型的Zn(II)离子与凹陷的金属冠醚拓扑结构高度互补。本研究还分离得到了一种含有四核[6-MCZnII,picHA-2]结构基元的Zn4(picHA)2(OAc)4(DMF)2物种。基于四核[6-MCMII,L-2]型配合物的结构分析，本研究提出了MII[12-MCMII,L-4]2+型配合物的形成机理。上述研究成果有助于实现调控镧系金属冠醚乃至配位驱动大环组装过程中间体反应活性的目标，以制备具有更高稳定性或更优异物理性能的配合物。