Influence of Lewis Bases on the Electrochemical Properties of Polymeric Nickel Complexes with Schiff Bases

Grant Number: 15-33-20379

Project Description

Polymeric square-planar transition metal complexes with N₂O₂-type Schiff bases are versatile and accessible materials for electrode modification. These compounds are notable for reversible electrochemical oxidation in aprotic solvents. For nickel complexes, oxidation is linked with formation of a coordinatively unsaturated Ni(III) species, stable only in the presence of external ligands—either solvent molecules or added Lewis bases (electron pair donors).

Key research questions:

• The mechanism of polymer interaction with Lewis bases remains virtually unexplored.
• Such interactions may explain the catalytic activity of polymer films and changes in the overall voltammetric response.
• Even trace Lewis base impurities in solution may lead to rapid loss of polymer electroactivity—problematic for energy storage applications.
• Sometimes, catalytic oxidation currents for solution additives occur, useful for electroanalysis or directed synthesis.

Thus, it is critical to elucidate the interaction mechanism of polymer complexes with different types of Lewis bases. Deeper understanding will enable targeted synthesis of new complexes and broader practical use of electrodes modified with Schiff base polymer complexes.

Project Report

Interactions were investigated between nickel polymer complexes (with varied ligand structures) and different Lewis bases.

Key findings:

• The nickel center is mainly attacked by Lewis bases, but probability and effectiveness depend on ligand structure.
• Donor substituents on phenyl rings lower the attack probability at the metal center.
• If substituents contain oxygen atoms, additional coordination sites for Lewis bases are introduced, leading to new complex degradation pathways.
• Quantum chemical calculations show that incorporating aliphatic substituents into the ring can solve this issue—they do not introduce new coordination sites for water, but their donor effect lowers attack probability at the metal center. Similar effects arise from aliphatic substituents in the ligand bridge.

Practical outcomes:

• Complexes with these substituents were synthesized and tested in electrolyte solutions with added water. These showed low susceptibility to Lewis base attack and high capacity stability in repeated charging/discharging in aqueous electrolytes, making them promising for energy storage devices.
• The observed nickel complex–Lewis base interaction is valuable for electroanalysis, exploiting changes in voltammetric curve shapes and catalytic currents.
• Optimized substrate/analyte pairs and sensor prototypes—volt- and amperometric—were developed for aliphatic, aromatic, and even biogenic amines.