Solvent-induced and polyether-ligand-induced redox isomerization within an asymmetrically coordinated mixed-valence ion: trans-(py)(NH[sub 3])[sub 4]Ru(4-NCpy)Ru(2,2[prime]-bpy)[sub 2]Cl[sup 4+]

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)(NH[sub 3])[sub 4]Ru[sup II](NCpy)Ru[sup III](bpy)[sub 2]Cl[sup 4+] (py is pyridine; NCpy is 4-cyanopyridine; bpy is 2,2[prime]-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)(NH[sub 3])[sub 4]Ru[sup III](NCpy)Ru[sup II](bpy)Cl[sup 4+]. 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 released 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 stoichiometic) are required, in comparison to basic solvent (several-fold 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 opticalmore » studies of competitive hole- and electron hole- and electron-transfer pathways is mentioned.« less