Pyridinedicarboxamide (PDCA) ligands have been widely integrated into different polymer chemistries and provided promising self-healing properties in bulk upon adding specific metal ions. However, their application in the development of synthetic hydrogels is limited due to the lack of clear kinetics and thermodynamic data. To fill this gap, we study the dynamics of polymer hydrogels formed by the condensation of linear poly(ethylene glycol) (PEG) precursors and the PDCA ligand, followed by the complexation with different metal ions at various pH values. Rheological studies reveal an unprecedented ability of PDCA in complexation with Co^2+/3+ and Ni²⁺ at elevated pH values, but in contrast with former reports, not with Fe³⁺ and Zn²⁺. Moreover, Co^2+/3+ gels demonstrate lower equilibrium constants resulting in faster and more efficient self-healing compared to those made by Ni²⁺. Spectroscopic UV and Fourier transform infrared (FTIR) studies also suggest structural modifications in the presence of the gel-forming metal ions but not the others. To explain these results, we employ static and periodic density functional theory (DFT) simulations. Static DFT indeed reveals that the binding interaction is clearly stronger for Co and Ni, and Fe shows the poorest values by far. The most stable complex geometry is octahedral, in which the central pyridyl, one lateral deprotonated amide, and one carbonyl participate in the metal coordination. Moreover, the periodic calculations unveil an overstabilization effect due to the noncovalent interactions between the PEG segments, which results in their torsion and formation of a helicoidal-like structure.

中文翻译：

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基于质子吡啶二甲酰胺配体的金属超分子聚合物中的 pH 响应凝胶化

吡啶二甲酰胺 (PDCA) 配体已广泛整合到不同的聚合物化学中，并在添加特定金属离子后提供有希望的自愈特性。然而，由于缺乏明确的动力学和热力学数据，它们在合成水凝胶开发中的应用受到限制。为了填补这一空白，我们研究了由线性聚（乙二醇）（PEG）前体和 PDCA 配体缩合形成的聚合物水凝胶的动力学，然后在不同的 pH 值下与不同的金属离子络合。^{流变学研究揭示了 PDCA 在升高的 pH 值下与 Co 2+/3+}和 Ni ²⁺络合的前所未有的能力，但与以前的报道相反，与 Fe ³⁺和 Zn ^{2+没有络合}. 此外，与由 Ni ^{2+制成的凝胶相比，Co 2+/3+}凝胶表现出较低的平衡常数，从而导致更快和更有效的自愈. 光谱紫外和傅里叶变换红外 (FTIR) 研究也表明在存在形成凝胶的金属离子而不是其他金属离子的情况下进行结构修饰。为了解释这些结果，我们采用静态和周期性密度泛函理论 (DFT) 模拟。静态 DFT 确实表明，Co 和 Ni 的结合相互作用明显更强，而 Fe 显示出迄今为止最差的值。最稳定的复杂几何结构是八面体，其中中心吡啶基、一个侧向去质子化酰胺和一个羰基参与金属配位。此外，周期性计算揭示了由于 PEG 片段之间的非共价相互作用导致的过度稳定效应，这导致它们的扭转和螺旋状结构的形成。