EFFECTS OF RING TILTING ON RATES OF INTRAMOLECULAR ELECTRON-TRANSFER IN MIXED-VALENCE 1',2',1''',2'''-TETRAETHYLBIFERROCENIUM, 1',3',1''',3'''-TETRAETHYLBIFERROCENIUM, AND 1',2',4',1''',2''',4'''-HEXAETHYLBIFERROCENIUM TRIIODIDES

Relatively minor perturbations caused by Cp ring substituents in a series of mixed-valence biferrocenium cations have pronounced effects on the electronic structure and rate of intramolecular electron transfer. The X-ray structure of 1',2',1''',2'''-tetraethylbiferrocene has been determined at 298 K: C2/c, a = 18.760(3) angstrom, b = 9.568(3) angstrom, c = 16.441(3) angstrom, beta = 130.494(13)degrees, Z = 4, D(calcd) = 1.42 g cm-1, R(f) = 0.038, and R(wf) = 0.041. The isomeric compound 1',3',1''',3'''-tetraethylbiferrocene crystallizes in the triclinic space group P1BAR with one molecule in a unit cell with dimensions a = 5.8649(6) angstrom, b = 8.8462(6) angstrom, c = 12.4822(11) angstrom, alpha = 109.814(6)degrees, beta = 103.063(8)degrees, and gamma = 83.476(6)degrees; R(f) = 0.046, and R(wf) = 0.055. The mixed-valence compound 1',3',1''',3'''-tetraethylbiferrocenium triiodide crystallizes in the triclinic space group P1BAR with one molecule in a unit cell with dimensions a = 8.4567(20) angstrom, b = 9.0268(15) angstrom, c = 10.9182(12) angstrom, alpha = 105.777(12)degrees, beta = 101.606(13)degrees, and gamma = 106.301(15)degrees; R(f) = 0.042, and R(wf) = 0.042. A three-dimensional hydrogen bonding network is clearly found between the Cp ring hydrogen atoms and iodine counterion atoms. The two crystallographically equivalent metallocene moieties of the cation have dimensions intermediate between those of Fe(II) and Fe(III) metallocenes. The variable-temperature Fe-57 Mossbauer data indicate that there is a pronounced dependency on sample history for 1',1'''-diethyl-, 1',2',1''',2'''-tetraethyl-, and 1',3',1''',3'''-tetraethylbiferrocenium triiodides. Crystalline samples of 1',2',1''',2'''-tetraethyl-and 1',3',1''',3'''-tetraethylbiferrocenium triiodide are delocalized on the Mossbauer time scale in the solid state above 195 and 125 K, respectively. We suggest that the difference in the rates of electron transfer in the series of mixed-valence biferrocenium salts is a result of difference in the degree of tilting of the Cp rings from the parallel geometry. Deviations of the Cp rings from the parallel position were found to correlate quite well with the critical temperature for electronic delocalization-localization in mixed-valence biferrocenium salts. Based on extended Huckel MO calculations, a theoretical explanation of the influence of bending back the Cp rings on the electron-transfer rates is also presented.