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 te mperatures between 153 and 293 K. The complex shows distinct Fe2+ and Fe3sites over this temperature interval. Variable temperature Mossbauer spectr a confirm the valence-localized character of the complex. In contrast, spec troscopic,investigation of powder samples generated from [1]. xMeCN(crystal ) indicate the presence of a valence-averaged component at temperatures abo ve 150 K. To elucidate this apparent contradiction we have conducted a vari able-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 cont rol of electron hopping by the environment of the complexes. Thus, [1]. xMe CN(crystal) has valence-localized spectra at all explored temperatures, [1] . 2DMF- (crystal) exhibits a complete collapse into a valence-averaged spec trum 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 proporti on. The spectral transformations are characterized by a coexistence of vale nce-localized and valence-averaged spectral components. This. phenomenon ca nnot be explained by intramolecular electron hopping between the valence-lo calized states FeA2+FeB3+ and FeA3+FeB2+ in a homogeneous ensemble of compl exes, but requires relaxation processes involving at least three distinguis hable states of the molecular anion. Hopping rates for [1]. 2DMF(crystal) a rid [1]. xMeCN(pow der) have been determined from spectral simulations, bas ed on stochastic line shape. theory. Analysis of the temperature dependence s of the transfer rates reveals the:existence of thermally activated proces ses between (quasi) degenerate excited states in both forms. The preexponen tial factors in the rate law for the hopping processes in the [1]. yMeCN(po wder) and [1]. 2DMF(crystal) differ dramatically and suggest an important-i nfluence of the asymmetry of the complex environment (crystal) on Intramole cular electron hopping. The differences between the spectra for the crystal line sample [1]. xMeCN and those for powders generated under vacuum from th ese crystals indicate that solvate depletion has a profound effect on the d ynamic behavior. Finally, two interpretations for,the three state's involve d in the relaxation processes in 1 are given and critically discussed (salm p bis(salicyledeneamino)-2-methylphenolate(3-)).