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Pronounced effects of crystal structures on intramolecular electron transfer in mixed-valence biferrocenium cations: Structural, EPR, and 57Fe Moessbauer characteristics
Crystallographic properties play an important role in controlling the rate of electron transfer in mixed-valence 1?,1?-disubstituted-biferrocenium triiodide salts. The X-ray structure of neutral 1?,1?-dinaphthylmethylbiferrocene has been determined at 298 K. The corresponding mixed-valence 1?,1?-dinaphthylmethylbiferrocenium triiodide exhibits two crystalline morphologies at 298 K. Dark crystals, formed when a CH2Cl2 solution of triiodide salt was allowed to evaporate slowly, crystallize in space group P1. Dark crystals, obtained when a layer of hexane was allowed to slowly diffuse into a CH2Cl2 solution of triiodide salt, crystallize in monoclinic space group P21/n. The observations of the structural characteristics of 1?,1?-dinaphthylmethylbiferrocenium triiodide are also consistent with our Moessbauer studies. The cation with space group P1 shows two doublets in the variable-temperature Moessbauer spectra at temperatures below 100 K. An increase of temperature causes the two doublets to move together, resulting in an average-valence doublet at 130 K. At 300 K, the spectrum of this sample shows a single doublet which is characteristic of a valence-detrapped cation in which the electron-transfer rate exceeds ?108 s-1. On the other hand, the cation with P21n phase exhibits a Moessbauer spectrum characteric of a valence-trapped cation at 300 K. Obviously, the intramolecular electron-transfer rate is quite sensitive to environment perturbations caused by different crystal packing arrangements. At 77 K, the EPR spectrum of cation with P1 phase is clearly a typical axial-type spectrum with g? = 3.16 and g? = 1.91. Surprisingly, the EPR spectrum of the cation with P21/n phase consists of two g? signals (3.67 and 2.85) and two g? signals (2.01 and 1.79). We suggest that the origin arises from the interaction of spin-spin exchange resulting from a dipole-dipole interaction that develops between cations. The syntheses, characterizations, and physical properties of mixed-valence 1?,1?-di(4-biphenylmethyl)biferrocenium and 1?,1?-di(9-anthracenylmethyl)biferrocenium triiodide salts are also described.
Pronounced effects of crystal structures on intramolecular electron transfer in mixed-valence biferrocenium cations: Structural, EPR, and 57Fe Moessbauer characteristics
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