Electronic modification of redox active ferrocenyl termini and their influence on the electrontransfer properties of 2,5-diferrocenyl-N-phenyl-1H-pyrroles

Abstract

A series of 2,5-diferrocenyl-N-phenyl-1H-pyrroles of type 2,5-Fc2-cC4H2NPh (Fc = Fe(η5-C5H4)(η5-C5H4R); 3a, R = cC4H7O2; 3b, H; 3c, 3,5-(CF3)2-C6H3) was synthesized by Negishi C,C cross-coupling reactions to connect the heterocyclic unit with the redox active ferrocenyl moieties. The reaction of 3a with p-toluene sulfonic acid gave 3d (R = CHO). A subsequent cyanation reaction of 3d produced 3e (R = CN). Characterization of these molecules was performed by spectroscopic means and single crystal X-ray diffraction analysis (3c–e). It could be shown that the electronic properties of the 2,5-diferrocenyl-N-phenyl-1H-pyrroles is influenced by the substitution of electron withdrawing/donating functional groups in 1′-position of the ferrocenyls. The influence of the electronic nature of the substituents on the charge transfer character of the heterocyclic bridging system was investigated using electrochemical (cyclic and square-wave voltammetry) and spectroelectrochemical methodologies (in situ UV–Vis/NIR spectroscopy). The observed redox splittings ΔE°′ between the redox active ferrocenyls are 390 mV for 3a, 485 mV for 3c, 555 mV for 3d and 600 mV for 3e using [NnBu4][B(C6F5)4] as electrolyte. Furthermore, the electrochemistry studies were performed in [NnBu4][PF6] to determine changes of the electrostatic stabilization. The redox separation increases with increasing electron withdrawing ability of the ferrocenyl substituents in the order R = cC4H7O2 < H < 3,5-(CF3)2-C6H3 < CHO < CN, while the spectroelectrochemical studies reveal decreasing strength of the intramolecular electronic coupling between the iron atoms. All compounds show intense and broad absorptions in the NIR region and can be classified as strongly coupled class II systems according to Robin and Day.