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-^<i>c</i>C₄H₂S and 2,5-fc₂-3,4-(OCH₂CH₂O)-^<i>c</i>C₄S compounds (fc = Fe­(η⁵-C₅H₄)­(η⁵-C₅H₄R); R = H (<b>1</b>, <b>5</b>), 3,5-(CF₃)₂-^<i>c</i>C₆H₃ (<b>2</b>, <b>6</b>), CHO (<b>3</b>, <b>7</b>), CN (<b>4</b>, <b>8</b>)) 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 <b>2</b>–<b>5</b>, [<b>5</b>]­[B­(C₆F₅)₄], <b>6</b>, and <b>8</b> in the solid state were investigated by single-crystal X-ray diffraction studies. Electrochemical investigations of thiophenes <b>2</b>–<b>8</b> demonstrated an increasing redox separation and hence thermodynamic stability of the corresponding mixed-valent species in the series <b>1</b> < <b>3</b> < <b>5</b> < <b>4</b> ≈ <b>2</b> < <b>6</b> < <b>7</b> < <b>8</b>. 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