Effect of Coordination Environment and Electronic Coupling on Redox Entropy in a Family of Dinuclear Complexes.

Abstract

The elucidation of factors that govern the temperature sensitivity of the electrochemical potential is essential to the development of electrochemical systems with target properties. Toward this end, we report a series of isostructural homo- and heterometallic M₂ (M = Fe^II, Fe^III, Zn^II) complexes supported by a phenoxo-centered tetrapyridyl ligand and ancillary carboxylate ligands that enables independent change in (i) charge, (ii) coordination environment of the redox-active center(s), and (iii) electronic coupling strength between redox centers. Variable-temperature electrochemical analysis of the series reveals the temperature coefficient for Fe-based redox couples to be highly dependent on the coordination environment of the redox-active center(s), with Fe centers in a pseudo-octahedral [FeN₃O₃] coordination environment affording a 2-fold greater temperature coefficient for the Fe^III/Fe^II redox couple than those in ancillary ferrocenyl groups. In contrast, identical temperature coefficients for the Fe^III/Fe^II redox event in Fe₂ and FeZn complexes establish electronic coupling strength to have a minimal impact on the temperature dependence of the Fe-based redox couple. Taken together, these results provide important insights for the design of molecular compounds with target redox properties, and they provide the first examination of how electronic coupling influences the temperature dependence of the redox potential and the associated redox entropy in molecular compounds.