Influence of extrinsic factors on electron transfer in a mixed-valence Fe2+/Fe3+ complex:: Experimental results and theoretical considerations

The Crystal structure of the mixed-Valence complex (NEt4)[Fe-2(salmp)(2)]. xMeCN(crystal) (x = 2,3) [1]. xMeCN(crystal) (crystal) was determined at temperatures between 153 and 293 K. The complex shows distinct Fe2+ and Fe3+ sites over this temperature interval. Variable temperature Mossbauer spectra confirm the valence-localized character of the complex. In contrast, spectroscopic,investigation of powder samples generated from [1]. xMeCN(crystal) indicate the presence of a valence-averaged component at temperatures above 150 K. To elucidate this apparent contradiction we have conducted a variable-temperature Mossbauer investigation of different forms of 1, including [1]. xMeCN(crystal), [1]. 2DMF(crystal), [1]. yMeCN(powder), and solution samples of 1 in acetonitrile. The low-temperature Mossbauer spectra of all forms are virtually identical-and confirm the valence-localized nature. of-the S = 9/2 ground state, The high-temperature spectra reveal a subtle control of electron hopping by the environment of the complexes. Thus, [1]. xMeCN(crystal) has valence-localized spectra at all explored temperatures, [1]. 2DMF- (crystal) exhibits a complete collapse into a valence-averaged spectrum over a narrow, temperature range, the powder exhibit's partial valence averaging over a broad temperature interval, and the solution sample shows at 210 K the presence of a valence-saveraged component in a minor proportion. The spectral transformations are characterized by a coexistence of valence-localized and valence-averaged spectral components. This. phenomenon cannot be explained by intramolecular electron hopping between the valence-localized states FeA2+FeB3+ and FeA3+FeB2+ in a homogeneous ensemble of complexes, but requires relaxation processes involving at least three distinguishable states of the molecular anion. Hopping rates for [1]. 2DMF(crystal) arid [1]. xMeCN(pow der) have been determined from spectral simulations, based on stochastic line shape. theory. Analysis of the temperature dependences of the transfer rates reveals the:existence of thermally activated processes between (quasi) degenerate excited states in both forms. The preexponential factors in the rate law for the hopping processes in the [1]. yMeCN(powder) and [1]. 2DMF(crystal) differ dramatically and suggest an important-influence of the asymmetry of the complex environment (crystal) on Intramolecular electron hopping. The differences between the spectra for the crystalline sample [1]. xMeCN and those for powders generated under vacuum from these crystals indicate that solvate depletion has a profound effect on the dynamic behavior. Finally, two interpretations for,the three state's involved in the relaxation processes in 1 are given and critically discussed (salmp bis(salicyledeneamino)-2-methylphenolate(3-)).