Selective Anion Binding by a Cofacial Binuclear Zinc Complex of a Schiff-Base Pyrrole Macrocycle

The synthesis of the new cofacial binuclear zinc complex [Zn2(L)] of a Schiff-base pyrrole macrocycle is reported. It was discovered that the binuclear microenvironment between the two metals of [Zn2(L)] is suited for the encapsulation of anions, leading to the formation of [K(THF)6][Zn2(μ-Cl)(L)]·2THF and [Bun4N][Zn2(μ-OH)(L)] which were characterized by X-ray crystallography. Unusually obtuse Zn−X−Zn angles (X = Cl: 150.54(9)° and OH: 157.4(3)°) illustrate the weak character of these interactions and the importance of the cleft preorganization to stabilize the host. In the absence of added anion, aggregation of [Zn2(L)] was inferred and investigated by successive dilutions and by the addition of coordinating solvents to [Zn2(L)] solutions using NMR spectroscopy as well as isothermal microcalorimetry (ITC). On anion addition, evidence for deaggregation of [Zn2(L)], combined with the formation of the 1:1 host−guest complex, was observed by NMR spectroscopy and ITC titrations. Furthermore, [Zn2(L)] binds to Cl− selectively in THF as deduced from the ITC analyses, while other halides induce only deaggregation. These conclusions were reinforced by density functional theory (DFT) calculations, which indicated that the binding energies of OH− and Cl− were significantly greater than for the other halides.

本文报道了一种新型共面双核席夫碱吡咯大环锌配合物[Zn₂(L)]的合成。研究发现，[Zn₂(L)]中两金属离子间的双核微环境适于阴离子包封，由此得到了经X射线晶体学表征的两种配合物：[K(THF)₆][Zn₂(μ-Cl)(L)]·2THF与[Bun₄N][Zn₂(μ-OH)(L)]。异常的钝角Zn−X−Zn键角（X=Cl：150.54(9)°；X=OH：157.4(3)°）表明这些金属-阴离子相互作用较弱，同时也印证了主体裂隙预组织结构对稳定主客体配合物的重要性。在未添加外源阴离子的条件下，我们通过逐级稀释法以及向[Zn₂(L)]溶液中加入配位溶剂，并结合核磁共振（NMR）光谱与等温滴定量热法（ITC），对[Zn₂(L)]的聚集行为进行了推断与研究。向体系中添加阴离子后，通过NMR光谱与ITC滴定实验观察到了[Zn₂(L)]解聚集的现象，同时生成了1:1型主客体配合物。此外，ITC分析结果表明，在四氢呋喃（THF）溶剂中[Zn₂(L)]对Cl⁻具有选择性结合作用，而其他卤化物仅能引发[Zn₂(L)]解聚集。密度泛函理论（DFT）计算进一步验证了上述结论：OH⁻与Cl⁻的结合能显著高于其他卤化物。