SOLVENT-INDUCED AND POLYETHER-LIGAND-INDUCED REDOX ISOMERIZATION WITHIN AN ASYMMETRICALLY COORDINATED MIXED-VALENCE ION - TRANS-(PY)(NH3)4RU(4-NCPY)RU(2,2'-BPY)2CL4+

Advantage is taken of oxidation-state-dependent ligand (ammine)/solvent interactions to shift redox potentials and effect redox isomerization in the title complex. In poorly basic solvents, the stable isomeric form is trans-(py)(NH3)4Ru(II)(NCpy)Ru(III)(bpy)Cl4+ (py is pyridine; NCpy is 4-cyanopyridine; bpy is 2,2'-bipyridine). In contrast, in stronger Lewis bases or in a mixture of strong and weak bases (dimethyl sulfoxide + nitromethane), the preferred isomer is trans-(py)(NH3)4Ru(III)(NCpy)Ru(II)(bpy)Cl4+. Evidence for redox isomerization was obtained, in part, from plots of formal potentials versus solvent Lewis basicity. Confirmatory evidence was obtained from a combination of electrochemical reaction entropy and resonance Raman spectroscopic experiments. UV-vis-near-IR absorption experiments, however, were not found to be useful in demonstrating isomerization. In a related series of experiments, redox isomerization was also demonstrated based on ammine binding by either a low molecular weight poly(ethylene glycol) species or by a macrocyclic ligand, dibenzo-36-crown-12. Much smaller molar amounts of either the polymer (substoichiometric) or crown (approximately stoichiometric) are required, in comparison to basic solvent (severalfold excess), in order to induce isomerization in nitromethane as the initial solvent. The possible general utility of the redox isomerization concept in time-resolved intramolecular charge-transfer studies and in optical studies of competitive hole- and electron-transfer pathways is mentioned.