Triferrocenes Built on a <i>C</i>₃-Symmetric Ligand Platform: Entry to Redox-Active <i>pseudo</i>-Triphenylenes via Chelation-Driven Stereoselection of Triple Schiff Bases

An expedient tandem deprotonation−trapping protocol was employed to prepare a tris(difluoroboronyl) complex of a triferrocenyl ligand that is geometrically analogous to substituted triphenylenes. A triple Schiff base condensation reaction between 1,3,5-triformylphloroglucinol and aminoferrocene afforded the tris(N-salicylideneamine) adducts 5a + 5b in ca. 1:1 ratio. The keto−enamine tautomeric core of this isomeric mixture could be converted to a common enolate−imine intermediate. Subsequent trapping with BF3·Et2O cleanly afforded the tris(difluoroboronyl) adduct 6 in essentially quantitative yield. The electronic and structural properties of this new class of ferrocene compounds were investigated using various methods including UV−vis, cyclic voltammetry (CV), differential pulse voltammetry (DPV), and X-ray crystallography. In CH2Cl2−CH3CN, 6 displayed a reversible three-electron oxidation process at E1/2ox = +210 mV (vs Fc/Fc+). Despite the sharing of a common [π,π]/[n,π]-conjugated core, no significant electronic communication was observed among the three ferrocenyl units in 6 under either CV or DPV conditions. On the other hand, the broad oxidation wave of 5a + 5b at E1/2ox = +60 mV in CH2Cl2−CH3CN was comprised of at least two major components at +20 and +90 mV, which collapsed to become a single peak in DMF electrolyte, despite that the ratios between the two isomers 5a,b remained essentially invariant to the change in solvent.