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

Abstract

Spectra of the mixed-valence complexes [(bpy)2ClOs(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 OsIII−RuII in solvents of donor number (DN) < 14. In solvents of high donor number (DN > 15) they are OsII−RuIII. 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 OsIII−RuII and OsII−RuIII display broad, solvent-dependent intervalence transfer (IT) bands in the near infrared (NIR). There are two isomers of [(bpy)2ClOs(pz)Ru(NH3)5]4+ as well. In one, which is dominant in solvents of DN < 22, the oxidation states are [(bpy)2ClOsIII(pz)RuII(NH3)5]4+. This is shown by the appearance of dπ → dπ marker bands for OsIII (at 4200 and 5800 cm-1 in CD3CN), ν(bpy) OsIII 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)2ClOsII(pz)RuIII(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 OsIII and RuII in [(bpy)2ClOsIII(pz)RuII(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 dπ(RuII) orbital largely orthogonal to the bridge. In [(bpy)2ClOsII(pz)RuIII(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)2ClOsIII(pz)RuII(NH3)5]4+ ⇌ [(bpy)2ClOsII(pz)RuIII(NH3)5]4+, K(297K) = 2.8, ΔH° = −8.2 ± 2 kcal/mol, and ΔS° = −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.

Solvent as Electron Donor: Donor/Acceptor Electronic Coupling Is a Dynamical Variable

“…We note that Neyhart et al speculated that solvent librations could induce large changes in H DA for electronic coupling across the bridging ligand for intramolecular electron-transfer in binuclear Ru/Os systems. 103 Intramolecular motions that one might expect to correlate with H DA are the torsions and inversions of the dimethylamino group on either DMA or C152. In results from our third trajectory we have found a DMA molecule that is in close proximity to C152 and is undergoing inversion.…”

Intervalence Charge Transfer (IVCT) in Ruthenium Dinuclear and Trinuclear Assemblies Containing the Bridging Ligand HAT {1,4,5,8,9,12‐hexaazatriphenylene}

“…[11,54,55] Alternatively, the asymmetry of the IVCT bands may be attributed to a manifold of discrete, underlying transitions which are identified as spinorbit components. [10,[56][57][58] Spectral deconvolution of the reduced (AEe(n)/n dn) NIR bands reveals three underlying gaussian-shaped components ( Table 2). The differences observed between the energies, intensities and bandwidths for the diastereoisomers lie well outside the limits of experimental error.…”

Diastereoisomers as probes for solvent reorganizational effects on IVCT in dinuclear ruthenium complexes

“…IVCT solvatochromism studies in solvent mixtures have demonstrated that the solvent reorganizational process occurs predominantly within the first solvation layer, and may be profoundly influenced by the systematic replacement of individual solvent molecules in the immediate vicinity of the mixed-valence chromophore [2,5,7,13,14,19,45].…”