Direct Evidence of Heteroleptic Complexation in the Macroscopic Dynamics of Metallo-supramolecular Polymer Networks

Abstract

Heteroleptic complexes are widely employed in small-molecule supramolecular arrays for the construction of complex architectures or for inducing nanoscopic rearrangements upon application of external stimuli that change the coordination-geometry preference. Despite this potential, they are rarely employed in the development of metallo-supramolecular polymer networks; this is unfortunate as that strategy might actually provide a tool to build highly homogeneous model-type networks that could form a basis for both a myriad of elementary investigations on transient networks and for their use in rational soft-functional materials design. To close that gap, we mix aqueous solutions of terpyridine- and phenanthroline-functionalized tetra-arm poly(ethylene glycol) (tetraPEG) precursors right at the overlap concentration and form physical hydrogels by introducing various types of divalent transition metal ions. The formation of heteroleptic complexes is comprehended by the persistence of the network percolation at intermediate network compositions, as revealed by rheological measurements. Specifically, shear stress curves and the resulting relaxation time spectra demonstrate the emergence of a third relaxation mode, on top of those associated with homoleptic complexes, which is indicative of heteroleptic complexation. Spectroscopic analyses and DFT calculations suggest the possible formation of heteroleptic complexes with all studied metal ions, whereby their fraction and lifetime are traceable using rheological measurements in the case of networks formed by Co2+ ions. Moreover, employing over-stoichiometric Co2+-to-ligand ratios eventually results in the selective formation of heteroleptic complexes. These results not only suggest new paradigms for devising smart soft materials, but they also propose new dimensions for characterizing heteroleptic complexes.

Topics & Concepts