Di- and Triferrocenyl (Hetero)Aromatics: Synthesis, Characterization, (Spectro-)Electrochemistry, and Calculations

A series of di- and triferrocenyl (hetero)­aromatics including 2,5-diferrocenylpyridine (4), 2,6-diferrocenylpyridine (5), 1,3,5-triferrocenylbenzene (9), 2,4,6-triferrocenylpyridine (10), and 2,4,6-triferrocenyl-1,3,5-triazine (11) have been prepared using the Negishi C,C cross-coupling protocol. Characterization of the molecules was performed by spectroscopic means and single-crystal X-ray diffraction study (10). The electrochemical properties of these compounds were determined by cyclic voltammetry, square wave voltammetry, and in situ NIR spectroelectrochemistry. The cyclic voltammograms show well-defined, separated, and reversible one-electron processes for each ferrocenyl moiety with ΔE°′ values (= separation between two formal potentials) ranging from 140 to 185 mV. The UV–vis/NIR spectra of the partially oxidized mixed-valent complexes of 4, 5, 9, and 10 show weak intervalence charge transfer absorptions, while only electrostatic interactions exist between the different oxidized iron centers of 11. Supporting DFT calculations were carried out to establish a basic understanding of the electronic structure.