PRONOUNCED EFFECT OF SUBSTITUENTS ON THE INTRAMOLECULAR ELECTRON-TRANSFER RATES IN MIXED-VALENCE BIFERROCENIUM TRIIODIDE COMPLEXES

The factors controlling the rate of intramolecular electron transfer in the solid state have been studied for a series of binuclear mixed-valence 1',1'''-bis(substituted benzyl)biferrocenium triiodide salts, where the substituents in the benzyl unit are p-iodo (1), p-bromo (2), p-chloro (3), o-iodo (4), and o-bromo (5). The X-ray structure of neutral 1',1'''-bis(p-bromobenzyl)biferrocene has been determined at 298 K: P1BAR, a = 5.853 (4) angstrom, b = 8.380 (3) angstrom, c = 14.2295 (18) angstrom, alpha = 74.622 (15)-degrees, beta = 83.940 (23)-degrees, and gamma = 86.69 (4)-degrees; Z = 1, D(calcd) = 1.758 g cm-8, R(F) = 0.036, and R(wF) = 0.038. The Fe-Fe distance is 5.119 (3) angstrom. Compound 2 crystallizes in the triclinic space group P1BAR with one molecule in a unit cell with dimensions a = 9.5505 (24) angstrom, b = 10.984 (3) angstrom, c = 11.248 (5) angstrom, alpha = 119.71 (3)-degrees, beta = 116.04 (3)-degrees, and gamma = 76.946 (22)-degrees. The final discrepancy factors are R(F) = 0.052 and R(wF) = 0.039. A decrease in Fe-Fe distance (5.058 (7) angstrom) has been observed when 1',1'''-bis(p-bromobenzyl)biferrocene is oxidized. The solid-state structure of 2 is composed of parallel sheets of cations and polyiodide ions which contain zigzag chains of alternate I3- and I2 units. The distance of 3.487 (2) angstrom between neighboring iodine atoms in I2 and I3- units within a chain indicates a nonnegligible interaction. Compound 4 crystallizes in the triclinic space group P1BAR with one molecule in a unit cell with dimensions a = 8.149 (4) angstrom, b = 9.567 (3) angstrom, c = 12.491 (4) angstrom, alpha = 111.78 (3)-degrees, beta = 85.97 (4)-degrees, and gamma = 108.39 (3)-degrees. The final discrepancy factors for 4 and R(F) = 0.056 and R(wF) = 0.04. The Fe-Fe distance in 4 is 5.082 (8) angstrom. As expected, the triiodide anion in 4 is situated differently. In contrast to compound 2, the I3- anion in 4 is perpendicular to the fulvalenide ligand. The features in the variable-temperature (77-300 K) Fe-57 Mossbauer spectra of 1 include two doublets, which are expected for a mixed-valence cation localized on the time scale of the Mossbauer technique (electron-transfer rates less than approximately 10(7) s-1). The valence-trapped electronic structure is also seen for compound 3. In the case of 2, at temperatures below 150 K it shows two doublets in the Fe-57 Mossbauer spectra and increasing the temperature causes two doublets to become a single ''average-valence'' doublet at a temperature of approximately 200 K. In comparison with compounds 1-3, a single ''average-valencec doublet (DELTA-E(Q) almost-equal-to 1.1 mm s-1) is seen even at 77 K for 4 and 5. Thus, there is a dramatic change in electron-transfer rate as the position of halide substituent in the benzyl unit is changed from the para position to the ortho position. EPR and IR data are also presented for 1-5.