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

Within a series of alpha,alpha'-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-fc(2)-(C4H2S)-C-c and 2,5-fc(2)-3,4-(OCH2CH2O)-(C4S)-C-c compounds (fc = Fe(eta(5)-C5H4)(eta(5)-C5H4R); R = H (1, 5), 3,5-(CF3)(2)-(C6H3)-C-c (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 approximate to 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.