SOLVENT-DEPENDENT REDOX THERMODYNAMICS OF METAL AMINE COMPLEXES - DELINEATION OF SPECIFIC SOLVATION EFFECTS

Solvent-dependent formal potentials, Ef, and reaction entropies, ΔSrc°, for four reversible Co(III/II) couples featuring macrobicyclic “cage” ligands have been obtained. These are utilized, together with corresponding data for other Co(III/II) and Ru(III/II) amine couples and for M(III/II) polypyridine couples, to unravel effects on the redox thermodynamics arising from specific ligand-solvent and other interactions. The solvent dependencies of Ef(vs ferrocenium-ferrocene) for each redox couple are fitted by using a multiparameter approach, incorporating various well-known empirical quantities describing solvent basicity, acidity, polarity, and internal ordering. For most amine couples, solvent basicity appears to play a prevailing role, associated with interactions involving the amine hydrogens, although solvent polarity also exerts a substantial, albeit less solvent-dependent, influence upon Ef. The presence of a deprotonated amide group in the Co(III/II) amine couple is signaled by the additional presence of a significant solvent acidity component in the multiparametric fit. The solvent basicity and other specific contributions to the Ef-solvent behavior for the polypyridine couples are markedly smaller than for the amines. While the inclusion of a “solvent internal ordering” term does not exert a statistically significant influence on the Ef-solvent dependence for any of the redox couples, the ΔSrc°-solvent dependencies are correlated most successfully with this parameter. Besides identifying the major specific solvation factors upon the redox thermodynamics for such inorganic redox couples, the present multiparametric solvent-dependent analysis also provides a novel, although approximate, means by which the extent of such obfuscating influences upon Efcan be assessed. © 1990, American Chemical Society. All rights reserved.