Oxidation Products of Doubly Trimethylene-Bridged Tetrabenzyl <i>p</i>‑Phenylenediamine Paracyclophane

Abstract

We report synthesis and investigation of doubly trimethylene-bridged tetrabenzyl-<i>p</i>-phenylenediamine <b>1­(Bz)</b> in its singly and doubly charged redox states. The singly oxidized monoradical cation, which is a mixed-valence (MV) system with directly interacting charge-bearing units, shows broad and solvent-sensitive intervalence bands consistent with class II compounds according to the Robin–Day classification. The doubly oxidized diradical dication of <b>1­(Bz)</b> exists in the spin-paired singlet state with thermally accessible triplet state. It has similar conformations as the other dimeric <i>p</i>-phenylenediamines, such as derivatives <b>1­(Me)</b> and <b>1­(Et)</b>, in both the solid-state and solution phases. The successful isolation of the single-crystalline <b>1­(Bz)</b>^<b>2+</b> diradical dications with two different in nature counteranions, relatively highly coordinating SbF₆^– and weakly coordinating carborane [undecamethylcarborane HCB₁₁Me₁₁^– (CB^–)], reveals the distinct effect of the nature of counterions on the structural features of diradical dication. Cyclic voltammetry measurements of <b>1­(Bz)</b> in dichloromethane reveal separation of the first and second oxidation potential by 0.12 V (2.8 kcal/mol), indicating relatively stable mixed-valence state in the dichloromethane, whereas in the acetonitrile both the first and second oxidation potentials overlap into one unresolved redox peak with minimal separation