Topological Effects on Intramolecular Electron Transfer via Quantum Interference

The three isomers of diferrocenylbenzenes (ortho, 1o;
meta, 1m; para, 1p) as well as 5-substituted
derivatives of
m-diferrocenylbenzene with R = NH2
(2), Cl (3), CH3 (4), CN
(5), NO2 (6), and
N(CH3)33+ (7) have
been prepared.
Crystal structures of 1o, 3, and
5 have been solved. In 3 and 5,
the cyclopentadienyl rings are nearly parallel to
the benzene mean planes with angles ranging from 9.99(5)° to
14.74(5)°. One ferrocene group is above and
the
other below the mean molecular plane. For 1o, there is
an important twist between the benzene and cyclopentadiene
rings (68.6(8)° and 32.5(8)°) for steric reasons.
Controlled potential electrolysis yields the mixed-valence
ferrocene/ferrocenium species in comproportionation equilibrium with homovalent
species. Intervalence transitions have
been observed and corrected from comproportionation. From the
intervalence band parameters, metal−metal
couplings (Vab) are calculated using Hush's
equation. The values are much higher for 1o (0.025 eV)
and 1p
(0.043 eV) than for 1m (0.012 eV) and exhibit little or
no variation for the substituted
m-diferrocenylbenzenes
2−6. These results are rationalized by
extended Hückel molecular orbital calculations. The weakness
of the
interaction in 1m can be ultimately traced to a quantum
Interference effect, i.e., a cancellation of the
contributions
of two electron transfer paths. This cancellation occurs because
each path implies a mixing of metal orbitals
with a different ligand orbital, and the resulting molecular orbitals
exhibit different symmetries.