Electron Transfer in Mixed-Valence Complexes in the Solid State

Abstract

ABSTRACT. The factors influencing the rate of intramolecular electron transfer (ET) in the solid state have been investigated for mixed-valence oxo-bridged trinuclear metal acetate complexes and salts of biferrocenium cations. Mixed-valence complexes which have potential-energy barriers for ET and are trapped on the vibrational timescale are studied. These include mixed-valence [M30(02CR)6L3lS, where M is either Fe or Mn, L is a substituted pyridine, and S is a solvate molecule such as C6H6, CHCI3, etc., and salts of the mixed-valence 1 ',1 '''-disubstituted biferrocenium cation. It is shown that it is not the level of intramolecular electronic coupling between metal centers which controls the rate of intramolecular ET. The environment about the mixed-valence complex controls the rate of ET. For salts of mixed-valence 1 ',1 '''-disubstituted biferrocenium cations the positioning of the counteranion relative to the cation controls the rate of ET. If the anion is positioned so that the two iron ions in a cation are inequivalent, the rate of ET is small and the cation is trapped in one valence description. In the case of Fe30 acetate complexes the positioning of the solvate molecule S can lead to valence trapping, even though only van der Waals interactions between the Sand mixed-valence Fe30 complexes exist. Valence detrapping, where the rate of ET increases appreciably, occurs frequently in a phase transition. In the solid-state intermolecular interactions valence trap complexes. As the temperature is increased, the coopenitive onset in a phase transition of motion associated with ligands, counterions, or even solvate molecules leads to valence detrapping. 57Fe Mossbauer, EPR, IR, solid-state 2H NMR and heat capacity data are used to probe these catastrophic events.

Observation and Interpretation of Temperature-Dependent Valence Delocalization in the [2Fe−2S]+ Cluster of a Ferredoxin from Clostridium pasteurianum

“…Studies of synthetic and protein-bound Fe−S clusters have indicated that the fine details of cluster structure and concomitant ground spin state are properties sensitive to the cluster environment. − Hendrickson et al have argued that the environment of mixed-valence synthetic clusters in crystalline structure has an essential influence on their intramolecular ET rates . Changes in the charge ordering with temperature have been proposed as the mechanism for the localization-to-delocalization transition observed for these mixed-valence solids.…”

Density Functional Study of the Valence-Tautomeric Interconversion Low-Spin [CoIII(SQ)(Cat)(phen)] ⇌ High-Spin [CoII(SQ)2(phen)]

“…The presence of two or more electronic states close in energy leads to significant vibronic interactions and a sensitivity to environment. Other examples of electronic lability are found in mixed-valence and spin-crossover complexes …”

Anion-Ordering Phase Transitions in Biferrocenium and Ferrocenium Salts with the Bis(trifluoromethanesulfonyl)amide Anion

“…Biferrocenyl compounds exhibit intriguing redox properties and valence states. − In particular, the valence tautomerization of biferrocenium cations containing Fe­(II) and Fe­(III) (Figure ) has attracted significant attention in terms of mixed-valency and crystal engineering. − The biferrocenium cation has an inherently unsymmetrical structure. However, rapid electron exchange in the biferrocenium cation leads to the observation of an average-valence structure on the time scales of X-ray diffraction and Mössbauer spectroscopy.…”