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
Ty. Dong et al., 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, Journal of the American Chemical Society, 115(14), 1993, pp. 6357-6368
Relatively minor perturbations caused by Cp ring substituents in a ser
ies 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) angst
rom, 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 compoun
d 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 = 83.4
76(6)degrees; R(f) = 0.046, and R(wf) = 0.055. The mixed-valence compo
und 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 = 10.91
82(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 t
hree-dimensional hydrogen bonding network is clearly found between the
Cp ring hydrogen atoms and iodine counterion atoms. The two crystallo
graphically equivalent metallocene moieties of the cation have dimensi
ons intermediate between those of Fe(II) and Fe(III) metallocenes. The
variable-temperature Fe-57 Mossbauer data indicate that there is a pr
onounced dependency on sample history for 1',1'''-diethyl-, 1',2',1'''
,2'''-tetraethyl-, and 1',3',1''',3'''-tetraethylbiferrocenium triiodi
des. Crystalline samples of 1',2',1''',2'''-tetraethyl-and 1',3',1''',
3'''-tetraethylbiferrocenium triiodide are delocalized on the Mossbaue
r 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 s
eries of mixed-valence biferrocenium salts is a result of difference i
n the degree of tilting of the Cp rings from the parallel geometry. De
viations of the Cp rings from the parallel position were found to corr
elate quite well with the critical temperature for electronic delocali
zation-localization in mixed-valence biferrocenium salts. Based on ext
ended Huckel MO calculations, a theoretical explanation of the influen
ce of bending back the Cp rings on the electron-transfer rates is also
presented.