Electron Transfer in Mixed-Valence Complexes in the Solid State

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 [M3O(O2CR)6L3]-S, where M is either Fe or Mn, L is a substituted pyridine, and S is a solvate molecule such as C6H6, CHCl3, 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 Fe3O acetate complexes the positioning of the solvate molecule S can lead to valence trapping, even though only van der Waals interactions between the S and mixed-valence Fe3O 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 cooperative 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.