Solvent-Induced Electron Transfer and Electronic Delocalization in Mixed-Valence Complexes. Spectral Properties

Spectra of the mixed-valence complexes [(bpy) 2 ClOs(L)Ru(NH 3 ) 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(NH 3 ) 5 ] 4+ as well. In one, which is dominant in solvents of DN < 22, the oxidation states are [(bpy) 2 ClOs III (pz)Ru II (NH 3 ) 5 ] 4+ . This is shown by the appearance of dπ → dπ marker bands for Os III (at 4200 and 5800 cm - 1 in CD 3 CN), ν(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 ∼8100 cm - 1 which is nearly solvent independent in solvents of DN ≥ 11.9. The second isomer, [(bpy) 2 ClOs II (pz)Ru III (NH 3 ) 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 ClOs III (pz)Ru II (NH 3 ) 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 dπ(Ru II ) orbital largely orthogonal to the bridge. In [(bpy) 2 ClOs II (pz)Ru III (NH 3 ) 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 ClOs III (pz)Ru II (NH 3 ) 5 ] 4+ ⇌ [(bpy) 2 ClOs II (pz)Ru III (NH 3 ) 5 ] 4+, K (297K) = 2.8, Δ H ° = −8.2 ± 2 kcal/mol, and Δ S ° = −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.