Multiscale Insight into the pH-Dependent Connectivity of Metallo-Supramolecular Polymer Networks with a Bidentate Protic Ligand

Metal–ligand complexes play an important role in metallo-supramolecular materials, but how the protonation state of protic ligands controls coordination geometry and material properties is still not clearly addressed. In this work, we study the pH-dependent coordination behavior of a protic bidentate ligand, 4-nitro-catechol (nCAT), in polymer networks formed from tetra-arm poly(ethylene glycol) functionalized with nCAT (tetra-nCAT) and iron ions. Hydrogels were prepared with Fe 3+ and Fe 2+ ions and studied using rheology and UV–Vis spectroscopy over a wide pH range. The results show that increasing pH gradually shifts the coordination structure from mono- to bis- and finally tris-complexes. In Fe 3+ systems, this shift leads to higher network connectivity, which appears as a moderate increase in plateau modulus and a strong increase in relaxation time and flow activation energy. Time–temperature superposition shows a single relaxation mode, suggesting that bis and tris complexes interconvert dynamically. In contrast, Fe 2+ networks are significantly weaker and show lower connectivity, consistent with a lower coordination strength, while they are surprisingly more stable and more temperature dependent. Density functional theory (DFT) calculations unveil that the partially deprotonated ligand can contribute as a bidentate chelating ligand, including specifically the coordination of sulfate counterions, which stabilizes bis and mono complexes, particularly in the presence of Fe 2+ . Together, these results provide a multiscale picture of how ligand deprotonation controls coordination geometry and how this molecular change is reflected in the macroscopic properties of metallo-supramolecular polymer networks.