A Review of Electrochemistry of osmium(II)-polypyridines and supporting DFT studies

Abstract

This work presents a critical review of the electrochemistry of osmium(II)-polypyridines, supported by Density Functional Theory (DFT) studies, focusing on phenanthroline, bipyridine, terpyridine, and their substituted derivatives. The results demonstrate that osmium(II)-bipyridines, osmium(II)-phenanthrolines, and osmium(II)-terpyridines exhibit similar redox behavior. The reversible one-electron oxidation of these d⁶ osmium(II)-polypyridines leads to the formation of d⁵ osmium(III)-polypyridines, which maintain a similar molecular structure. The one-electron reduction of these osmium(II)-polypyridines produces a reduced molecule with a d⁶ osmium(II) center, accompanied by one ligand radical (containing one unpaired electron) and one neutral ligand (for terpyridine) or two neutral ligands (for phenanthroline or bipyridine). The electronic structures of DFT-optimized osmium(III)-polypyridines, osmium(II)-polypyridines, and their reduced forms support the experimental assignment of the observed redox processes. Additionally, the oxidation potentials of osmium(II)-polypyridines are determined to be directly related to the highest occupied molecular orbital (HOMO) energies of the DFT-optimized osmium(II)-polypyridines making it possible to predict redox potentials of related systems. In conclusion, the limitations of the published work are critically assessed, and potential future perspectives are thoughtfully explored.