Redox Asymmetry Research Articles

Primitive Molecular Recognition Effects in Electron Transfer Processes: Modulation of ((Trimethylammonio)methyl)ferrocenium/ferrocene Self-Exchange Kinetics via Hydrophobic Encapsulation.

1H NMR line broadening measurements show that the electron self-exchange rate constant for ((trimethylamino)methyl)ferrocenium/ferrocene (TMAFc(2+/+)) in D(2)O as solvent is decreased by ca. 20-50 fold in the presence of excess beta-cyclodextrin. The rate effect is associated with the selective hydrophobic encapsulation of the ferrocene form of the redox couple (i.e., the ferrocenium form is not significantly encapsulated). Selective encapsulation leads to a coupling of electron transfer to host (cyclodextrin) transfer. Optical intervalence absorption measurements for a closely related mixed-valence system strongly suggest that the coupling decreases the self-exchange rate by increasing the thermal activation barrier-an inference that is corroborated by activation parameter measurements. The barrier increase ultimately can be understood in terms of a redox asymmetry effect upon the isolated electron transfer event, where the overall exchange mechanism likely entails sequential electron and host transfer.

Perturbation of the electronic structure of the Creutz-Taube ion via asymmetric encapsulation with macrocyclic ether species

Ligand-bridged mixed-valence ions can exist in either valence-localized double-minimum ground potential energy surface or valence-delocalized (single-minimum ground PE surface) form. The determining factor is the balance between twice the initial-state/final-state electroniccoupling energy (H[sub if]) and the sum of the zeroth-order reorganization or trapping energy ([chi]) and any redox asymmetry [delta]E. For the Creutz-Taube ion (NH[sub 3])[sub 5[minus]] Ru[sup 11][sup 1/2](NH[sub 3])[sub 5][sup 5+], (1), redox asymmetry is absent, the valencies are completely delocalized, and 2H[sub if] evidently exceeds [epsilon]. We reasoned, however, that [open quotes]electronic isomerization[close quotes] to a valence-localized form might be possible if significant redox asymmetry could be introduced. We report here the introduction of the desired asymmetry in an unusual way: via binding of a macrocyclic ether to just one of the two available ruthenium ammine sites. We also report optical evidence for the onset of valence localization in both 1 and a trans pyridine (py) substituted analog (2), in the presence of either dibenzo-36-crown-12(DB-36-C-12) or dibenzo-30-crown-10 (DB-30-C-10) species. 17 refs., 3 figs.

ChemInform Abstract: Evidence for Specific Solvent‐Solute Interactions as a Major Contributor to the Franck‐Condon Energy in Intervalence‐Transfer Absorptions of Ruthenium Ammine Complexes.

AbstractThe relationship between the electrochemically determined redox asymmetry, ΔE1/2, and the spectroscopic intervalence‐transfer band energy, EIT, over a series of the asymmetric Ru(II)/Ru(III) dimers (I) and (II) is investigated by variation of the ligand L and by variation of the solvent (nitromethane, nitrobenzene, benzonitrile, acetonitrile, acetone, DMF, Me2SO).

Redox asymmetry and metal-metal coupling in pyrazine-bridged ruthenium dimers

Redox asymmetry and metal-metal coupling in pyrazine-bridged ruthenium dimers

Intervalence transfer in mixed-metal dimers of ruthenium and osmium: relationship between redox asymmetry and the intervalence-transfer absorption band

Intervalence transfer in mixed-metal dimers of ruthenium and osmium: relationship between redox asymmetry and the intervalence-transfer absorption band