Coordination Geometry Preference Regulates the Structure and Dynamics of Metallo-Supramolecular Polymer Networks

Metal-ligand interactions are extensively used for the development of biomimetic polymers. Macroscopic properties of such systems are closely tied to the microscopic structure and dynamics of not only the polymer precursor but also metallo-supramolecular bonds. Despite many researchers that have tried to develop a clear understanding of the strength and stability of transient bonds, the role of the coordination geometry is often overlooked. In this work, we utilize a flexible platform that allows us to vary the coordination geometry preference of junctions and study the consequences on the macroscopic scale. We graft tetra-and bi-functional poly(ethylene glycol) precursors with bidentate phenanthroline ligands, which, in combination with different divalent transition metal ions, demonstrate different coordination geometries. Specifically, despite the universality of the dynamics in such ideal transient networks, Fe2+ ions form stable gels at ligand-deficient conditions, in sharp contrast to Co2+ or Ni2+. Modeling of the evolution of the UV-vis absorption spectra of the ligand in the presence of various concentrations of metal ions suggests that while they all contain a remarkable fraction of tris-complexes on top of bis-complexes, the equilibrium constants of these two are inversely correlated in networks formed by Fe2+ or the other ions. Moreover, DFT simulations show subtle differences between the structures and ground-state energies of the mono-, bis-, and tris-complexes made by different ions. Accordingly, the corresponding free energies of formation prove that Fe2+ has a significantly larger affinity toward the tris-complex while other ions rather other geometries. These findings propose a new dimension for regulating the structure and dynamics of metallo-supramolecular polymers.

Automated summary

Metal-ligand interactions play a crucial role in the development of biomimetic polymers, with the coordination geometry significantly impacting the macroscopic properties of materials. By studying different coordination geometry preferences, the structure and dynamics of metallo-supramolecular polymers can be effectively regulated.