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 FcZnCl ( 2 ) (Fc = (η 5 -C 5 H 4 )(η 5 -C 5 H 5 )Fe) in the presence of [(Ph 3 P) 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 °′ = −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 Fc/Fc + for the 4 3+ / 4 4+ couple. Δ E °′ 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+ .