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.