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 = (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+.