Mixed-valence dialkylbiferrocenium salts: an explanation for the observed temperature dependence of electron-transfer rates

Abstract

Data are presented to indicate that the rate of inrramolecular electron transfer in mixed-valence biferrocenium triiodide, 1, and four mixed-valence dialkyl-substituted biferrocenium triiodide salts is controlled by dynamics in the solid state. The X-ray structure of 1 at 296 K has been determined: Pi, a = 7.5779 (20) A, b = 8.4742 (14) A, c = 9.5577 (21) A, LY = 112.619 (14)', p = 104.646 (20)O, and y = 94.610 (19)'; Z = 1, Dcalcd = 2.32 g ~m-~, R,, = 0.042 for 2185 observed (F, 2 2.5u(F0)) reflections. The mixed-valence biferroceqium cation assumes a trans conformation where the Fe(q5-C5Hs) moieties are on opposite sides of a planar fulvenide ligand. The two rings bonded to each atom are eclipsed. Both the cation and the anion sit on centers of symmetry in 1. The 300 K Mossbauer spectrum of 1, however, shows equal-area Fe and Fe' doublets. It is probable that a well-formed single crystal has Mossbauer-delocalized molecules, whereas a microcrystalline sample tends to have localized molecules. Several different samples of 1 were examined with the Mossbauer technique to find that at 300 K the ratio of delocalized to localized molecules can vary from -0 to - 1 depending on the history of the sample. At temperatures above 300 K the Mossbauer spectrum of compound 1 does change to eventually become a single average-valence doublet at -357 K. Differential scanning calorimetry curves in the range of -303 to -363 K were run for several samples of 1. For samples that show 300 K Mossbauer spectra comprised essentially of a localized signal, a relatively sharp endothermic peak is seen at -335 K. Samples of 1 which show appreciable amounts of a signal for the delocalized species at 300 K give a broad DSC peak shifted to lower temperature than -335 K. In one case the peak was apparently even so broad as to be not detectable. The X-ray structure of 1',6'-di-n-butylbiferrocenium triiodide, 7, has been determined at 150, 298, and 363 K. At all three temperatures 7 crystallizes in the P2,/c space group. At 150 K the structure of 7 is characterized as a = 10.0201 (8) A, b = 13.5203 (9) A, c = 21.6080 (23) A, and p = 106.015 (8)'; Z = 4, RF = 0.042, and RwF = 0.052 for 5145 observed (IqZ > 2.0~14~) reflections. The mixed-valence di-n-butylbiferrocenium cation assumes a trans conformation with a planar fulvenide ligand. The two halves of the cation are inequivalent at 150 K by virtue of the positioning of the 13- anion and differences in the configuration of the two butyl groups. Analysis of the X-ray diffraction results for 7 at 298 and 363 K shows that at these higher temperatures positional disorder sets in for both the cation and the anion. One of the butyl groups develops pronounced positional disorder at the higher temperatures. It is suggested that this butyl group is dynamically disordered at the higher temperatures. This is supported by the observation that the c axis of a crystal of 7 expands appreciably from 150 to 363 K. Both 7 and 1',6'-dibenzylbiferrocenium triiodide, 8, exhibit an interesting phenomenon. Microcrystalline samples of 7 and 8 give 300 K Mossbauer spectra which show appreciable intensity signals from valence-localized species, whereas recrystallized samples give 300 K Mossbauer spectra exhibiting only a signal for a delocalized species. At temperatures below -200 K there are two doublets for crystalline 7 and 8. The two doublets became a single doublet at -275 and -260 K, respectively, for 7 and 8 as the temperature is increased. The above results indicate that these mixed-valence compounds are undergoing phase transitions. It is suggested that the observed temperature dependence of the Mossbauer spectra for the dialkyl-substituted complexes 7 and 8 and the analogous ethyl- and propyl-substituted complexes results from I< anions undergoing a transformation from a static situation which tends to localize the charge in the mixed-valence cation to a dynamic situation where on the average the environment about each half of a mixed-valence cation is identical. ~~~~~~~ ~