Synthesis and (Spectro)electrochemical Behavior of 2,5-Diferrocenyl-1- phenyl-1 H-phosphole

2,5-Diferrocenyl-1-phenyl-1H-phosphole (3) has successfully been prepared by a cyclization reaction of phenylphosphine with 1,4-diferrocenyl butadiyne. Subsequent reaction with elemental sulfur and selenium, respectively, leads to the formation of the appropriate phosphole sulfide (4) or selenide (5). Molecules 4 and 5 have structurally been characterized by single-crystal X-ray diffraction. Despite the tetrahedral environment at the phosphorus atom, the cC4P ring itself is planar and coplanar with the cyclopentadienyl rings of the ferrocenyl termini. Electrochemical measurements revealed that the two ferrocenyl groups could be oxidized at discrete potentials, with separation of the individual redox waves of 280 (3), 240 (4), and 235 mV (5), respectively. These values agree with other examples of heterocyclic-bridged diferrocenyl compounds such as diferrocenylthiophene (260 mV) and diferrocenylfuran (290 mV). Compounds [3]+–[5]+ exhibit IVCT absorptions of weak to moderate strength, which conforms well to the predictions of the Hush two-state model for weakly coupled mixed-valence systems. These conclusions are supported by DFT and TD-DFT results, which satisfactorily model the observed structural and spectroscopic parameters. The computational work assists in assigning the various low-energy (LF, IVCT) electronic transitions and also highlights the key role of the unsaturated cis-diene-like C4H2 building block of the heterocycle in promoting the Fc → Fc+ electron-transfer transition.