Electronically Intercommunicating Iron Centers in Di- and Tetraferrocenyl Pyrroles

Abstract

Novel 2,5-diferrocenyl-1-phenyl-1H-pyrrole (4) and 2,3,4,5-tetraferrocenyl-1-phenyl-1H-pyrrole (6) have been prepared by a 2- or 4-fold Negishi cross-coupling reaction of 2,5-dibromo-1-phenyl-1H-pyrrole (3) and 2,3,4,5-tetrabromo-1-phenyl-1H-pyrrole (5), respectively, with FcZnCl (2) (Fc = Fe(η5-C5H4)(η5-C5H5)) in the presence of [(Ph3P)4Pd] as catalyst. The electronic and structural properties of 4 and 6 were investigated with UV−vis spectroscopy and single-crystal X-ray diffraction (6). Comparison of the appropriate bond distances in the pyrrole core system of 6 demonstrates considerable electron delocalization. Cyclic, square wave, and linear sweep voltammetry as well as in situ NIR spectro-electrochemistry highlight the electrochemical properties of both compounds. Molecules 4 and 6 display two (4) or four (6) electrochemically reversible one-electron transfer processes with remarkably high ΔE1/2 values and reduction potentials of E0′ = −238 and E0′ = 212 mV for 4 (ΔE1/2 = 450 mV) and E0′ = −280, E0′ = 51, E0′ = 323, and E0′ = 550 mV for 6 (ΔE1/2 = 322, 264, and 233 mV) using [NBu4][B(C6F5)4] as the supporting electrolyte. The pyrroles could be classified as class II systems according to Robin and Day. Additionally, 4[PF6]n (n = 1, 2) were synthesized and studied, giving CV responses and NIR spectra identical to those obtained for 4 from electrochemical oxidations.