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-fc 2 - c C 4 H 2 S and 2,5-fc 2 -3,4-(OCH 2 CH 2 O)- c C 4 S compounds (fc = Fe(η 5 -C 5 H 4 )(η 5 -C 5 H 4 R); R = H ( 1, 5 ), 3,5-(CF 3 ) 2 - c C 6 H 3 ( 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(C 6 F 5 ) 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.