Substituent Influence on Charge Transfer Interactions in α,α′-Diferrocenylthiophenes

Within
a series of α,α′-diferrocenylthiophenes,
the influence of electron-withdrawing substituents at the ferrocenyl
moieties and electron-donating functionalities at the thiophene bridge
on the electronic behavior was studied. The synthesis of the appropriate
2,5-fc2-cC4H2S and 2,5-fc2-3,4-(OCH2CH2O)-cC4S compounds (fc = Fe­(η5-C5H4)­(η5-C5H4R); R = H (1, 5), 3,5-(CF3)2-cC6H3 (2, 6), CHO (3, 7), CN (4, 8)) was carried
out using the Negishi C,C cross-coupling protocol to effect the coupling
of the thiophene and the ferrocenyl moieties. The corresponding diferrocenylthiophenes
were characterized spectroscopically. Within this context, the structural
properties of 2–5, [5]­[B­(C6F5)4], 6, and 8 in the solid state were investigated by single-crystal X-ray
diffraction studies. Electrochemical investigations of thiophenes 2–8 demonstrated an increasing redox separation
and hence thermodynamic stability of the corresponding mixed-valent
species in the series 1 < 3 < 5 < 4 ≈ 2 < 6 < 7 < 8. Moreover, UV/vis/near-IR
and infrared spectroelectrochemical studies verify these observations.
These studies further reveal a valence trapped situation in corresponding
mixed-valent species and allow a class II classification according
to Robin and Day.