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

Abstract

Supercrowded 2,3,4,5-tetraferrocenylthiophene (4) has been synthesized by a fourfold Negishi ferrocenylation of tetrabromothiophene (3) with FcZnCl (2) (Fc = (η5-C5H4)(η5-C5H5)Fe) in the presence of [(Ph3P)4Pd] 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+/42+ couple, 418 mV for the 42+/43+ couple, and 604 mV vs Fc/Fc+ for the 43+/44+ couple. ΔE°′ 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 44+.