Pronounced effect of substituents on the intramolecular electron-transfer rates in mixed-valence biferrocenium triiodide complexes

Article abstract of DOI:10.1021/om00038a012

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: P1, a = 5.853 (4) ?, b = 8.380 (3) ?, c = 14.2295 (18) ?, α = 74.622 (15)°, β = 83.940 (23)°, and γ = 86.69 (4)°; 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) ?. Compound 2 crystallizes in the triclinic space group P1 with one molecule in a unit cell with dimensions a = 9.5505 (24) ?, b = 10.984 (3) ?, c = 11.248 (5) ?, α = 119.71 (3)°, β = 116.04 (3)°, and γ = 76.946 (22)°. The final discrepancy factors are R_F = 0.052 and R_wF = 0.039. A decrease in Fe-Fe distance (5.058 (7) ?) 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) ? between neighboring iodine atoms in I₂ and I₃^- units within a chain indicates a nonnegligible interaction. Compound 4 crystallizes in the triclinic space group P1 with one molecule in a unit cell with dimensions a = 8.149 (4) ?, b = 9.567 (3) ?, c = 12.491 (4) ?, α = 111.78 (3)°, β = 85.97 (4)°, and γ = 108.39 (3)°. 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) ?. 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) ⁵⁷Fe M?ssbauer spectra of 1 include two doublets, which are expected for a mixed-valence cation localized on the time scale of the M?ssbauer technique (electron-transfer rates less than ～10⁷ 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 ⁵⁷Fe M?ssbauer spectra and increasing the temperature causes two doublets to become a single average-valence doublet at a temperature of ～200 K. In comparison with compounds 1-3, a single average-valence doublet (ΔE_Q ≈ 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.