Effect of Coordinating Impurities on the Electrochemical Stability of Polymeric Nickel(II) Schiff-Base Complexes

Polymer films of nickel Schiff-base complexes were investigated to clarify degradation mechanisms induced by coordinating impurities—specifically, the protic solvents methanol and isopropanol. Films of poly[Ni(Salen)] and its sterically protected derivatives were electropolymerized in situ and subjected to cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) measurements in dry acetonitrile electrolyte with 1% vol. alcohol added. In situ monitoring of redox activity and mass changes revealed something. It was revealed that traces of alcohols act as axial ligands to the Ni center. This disrupts the conjugated π-system and conductivity of the polymer. The rate of electrochemical stability strongly depends on the complex structure. The unsubstituted poly[Ni(Salen)] film showed the fastest loss of capacity in both methanol and isopropanol, whereas complexes with methyl substituents in the diimine bridge (poly[Ni(Salpn-1,2)] and poly[Ni(Saltmen)]) exhibited significantly improved stability. EQCM measurements revealed irreversible changes in the mass of all polymer films upon exposure to alcohol-containing electrolytes. These observations are consistent with the axial coordination of alcohol molecules to the Ni centers and the concomitant ingress of solvent species into the polymer matrix. The results demonstrate that molecular design—specifically, introducing steric hindrance around the metal center—markedly enhances resistance to coordinating impurities.