Multielectron redox processes in polymeric cobalt complexes with N2O2 Schiff base ligands

A series of polymeric cobalt (II) complexes with tetradentate N2O2 Schiff base ligands (poly-[Co(Schiff)]) are prepared via oxidative electrochemical polymerization and characterized by scanning electron microscopy, cyclic voltammetry, quartz crystal microgravimetry, and in situ UV/Vis spectroscopy in inert atmosphere. Integration of data from all techniques revealed that (a) all studied poly-[Co(Schiff)] films demonstrate multi-electron redox switching between 0 V and +1.4 V (vs. Ag/AgCl); (b) the Co(II) to Co(III) conversion takes place in the early stage of polymer oxidation, which is followed by both one- and two-electron ligand-based redox processes in the later stages; (c) the number of electrons exchanged by each monomer unit during oxidation/reduction of the polymer depends on the substituents in the imine bridge and aldehyde moieties of the ligand, as well as on the donor number of the electrolyte solvent. The poly-[Co(CH3O-SaltmEn)] film demonstrates the ability to be reversibly oxidized by three electrons per a monomer unit in an acetonitrile-based electrolyte, which makes it a viable candidate for energy storage applications.