Solvent-induced electron transfer and electronic delocalization in mixed-valence complexes. Spectral properties

Spectra of the mixed-valence complexes [(bpy)(2)ClOs(L)Ru(NH3)(5)](4+) (L = 4,4'-bipyridine, pyrazine; bpy = 2,2'-bipyridine) are highly solvent dependent. They provide oxidation state specific spectral markers which show that oxidation states for L = 4,4'-bpy are Os-III-Ru-II in solvents of donor number (DN) < 14. In solvents of high donor number (DN > 15) they are Os-II-Ru-III. The isomers coexist at donor numbers 14-15 and the distribution between them tuned in mixtures of acetonitrile and propylene carbonate demonstrating that intramolecular electron transfer can be induced by varying the solvent. Both Os-III-Ru-II and Os-II-Ru-III display broad, solvent-dependent intervalence transfer (IT) bands in the near infrared (NIR). There are two isomers of [(bpy)(2)ClOs(pz)Ru(NH3)(5)](4+) as well. In one, which is dominant in solvents of DN < 22, the oxidation states are [(bpy)(2)ClOsIII(pz)Ru-II(NH3)(5)](4+) This is shown by the appearance of d pi --> d pi marker bands for Os-III (at 4200 and 5800 cm-l in CD3CN), nu(bpy) Os-III bands in the mid-IR, and the pattern of UV-visible bands. For this isomer a narrow, structured IT band is observed at similar to 8100 cm(-1) which is nearly solvent independent in solvents of DN greater than or equal to 11.9. The second isomer, [(bpy)(2)ClOsII(pz)Ru-III(NH3)(5)](4+), is dominant at DN > 24. It has IT bands in the NIR which are broad and solvent dependent. It is concluded that there is significant through-pyrazine electronic coupling between Os-III and Ru-II in [(bpy)(2)ClOsIII(pz)Ru-II(NH3)(5)](4+), but localized oxidation states, and a residual solvent-vibrational barrier to intramolecular electron transfer continue to exist. The exchanging electron is in a d pi(Ru-II) orbital largely orthogonal to the bridge. In [(bpy)(2)ClOsII(pz)Ru-III(NH3)(5)](4+) electronic coupling is much less. Its properties are consistent with those of Class II in the Robin and Day mixed-valence classification scheme. The two isomers coexist in trimethyl phosphate (DN = 23.0) and formamide (DN = 24.0). From the temperature dependence of the mixed-valence equilibrium in trimethyl phosphate, [(bpy)(2)ClOsIII(pz)Ru-II(NH3)(5)](4+) = [(bpy)(?)2ClOS(II)(pz)RU(III)(NH3)(5)](4+), K(297K) = 2.8, Delta H degrees = -8.2 +/- 2 kcal/mol, and Delta S degrees = -27 +/- 6 cal mol(-1) deg(-1). Because of the difference in the extent of electronic delocalization between the two isomers, intramolecular electronic transfer is a complex event involving coupled electronic and nuclear motions. There is no simple relationship between optical and thermal electron transfer.