Mixed-valence biferroceniums: Pronounced effects of cation-anion interactions on the intramolecular electron-transfer rate

Article abstract of DOI:10.1021/om00004a034

Relatively minor perturbations caused by the cation-anion interactions in 1′,2′,3′,1?,2?,3?-hexaethylbiferrocenium triiodide (1) and 1′,2′,4′,1?,2?,4?-hexaethylbiferrocenium triiodide (2) have pronounced effects on the electronic structure and the rate of intramolecular electron transfer. The X-ray structure of 1 has been determined at 298 K: P2₁/c, a = 9.433(3) ?, b = 18.597(3) ?, c = 10.425(3) ?. β = 110.24(2)°, Z = 2, D_calcd = 1.779 g cm^-3, R_F = 0.037, and R_wF = 0.035. The isomeric 2 crystallizes in the monoclinic space group P2₁/n with two molecules in a unit cell with dimensions a = 12.214(9) ?, b = 11.516(10) ?, c = 12.222(7) ?. β = 103.36(5)°; R_F = 0.055, and R_wF = 0.055. The neutral compound 1′,2′,3′,1?,2?,3?-hexaethylbiferrocene crystallizes in the triclinic space group P1 with one molecule in a unit cell with dimensions a = 7.999(1) ?, b = 8.981(3) ?, c = 11.222(2) ?. α = 112.08(2)°, β = 76.47(1)°, γ = 113.80(2)°; R_F = 0.027, and R_wF = 0.032. The variable-temperature ⁵⁷Fe M?ssbauer data indicate that 1 is delocalized on the M?ssbauer time scale in the solid state above 170 K. However, the electron-transfer rate in 2 is localized on the M?ssbauer time scale at 300 K (electron-transfer rate <～10⁷ s^-1). We suggest that the difference in rates of electron transfer in 1 and 2 is a result of difference in the cation-anion interactions. A comparison of electron-transfer rates in solid state and solution state is also presented.