Substituent effects on the stabilities of polymeric and small molecule bis-terpyridine complexes

Metallo-supramolecular assemblies of polymeric, particulate, or small-molecule building blocks based on 2,2′:6′,2′′-terpyridine most commonly rely on chemical substitution of this ligand at the 4′ position. Here, we investigate the effects that this modification to the ligand have on the kinetic stabilities of bis transition metal complexes, and therefore the lifetime of supramolecular bonds that it forms. The decay of aqueous cobalt(II) and iron(II) complexes in the presence of excesses of competing metal ions is studied by ultraviolet visible (UV-vis) absorption spectroscopy, and rate constants for the decay of amino-, ether-, and non-substituted ligands are determined. Remarkably, for cobalt(II), amino substituents lead to rapid oxidation in air to kinetically inert cobalt(III) complexes, while ether substituents yield decay constants three- to six-fold larger than unsubstituted complexes. Ether linkages apparently weaken cobalt complexes by inductive withdrawal of electrons, while amino substituents donate electron density, thus lowering the oxidation potential of the complexes. For iron(II), both ether- and amino-linked complexes decay at least an order of magnitude faster than unsubstituted complexes. Our results highlight the importance of considering linking chemistry when designing dynamic supramolecular assemblies based on terpyridine ligands, as it yields significant, and occasionally dramatic, changes in the kinetic stabilities of these complexes.