A Star-Shaped Supercrowded 2,3,4,5-Tetraferrocenylthiophene: Synthesis, Solid-State Structure, and Electrochemistry

Supercrowded 2,3,4,5-tetraferrocenylthiophene (4) has been synthesized by a fourfold Negishi ferrocenylation of tetrabromothiophene (3) with FeZnCl (2) (Fe = (eta(5)-C5H4)(eta(5)-C5H4)Fe) in the presence of [(Ph3P)(4)Pd] as catalyst. The electronic and structural properties of 4 were investigated with UV-vis spectroscopy and single-crystal X-ray diffractometry. Complex 4 adopts a solid-state structure, with not one of the ferrocenyl substituents being coplanar with the thiophene ring. Cyclic. square wave, and linear sweep voltammetry and in situ near-IR spectroelectrochemistry highlight the electron-transfer properties of 4. Compound 4 displays four electrochemically reversible one-electron-transfer processes with formal reduction potentials of E degrees' = -161 mV for the 4/4(+) couple, 58 mV for the 4(+)/4(2+) Couple, 418 mV for the 4(2+)/4(3+) couple, and 604 mV vs Fe/Fe+ for the 4(3+)/4(4+) couple. Delta E degrees' values of 219, 360, and 186 mV indicate significant electrostatic interaction among the four terminal ferrocenyl groups as oxidation progresses. The spectroelectrochemistry results showed several UV vis and near-IR absorption bands appearing or disappearing between 280 and 3000 nm as 4 is oxidized in a slow and stepwise fashion to ultimately generate 4(4+).