Rational design and syntheses of aniline-based diradical dications: isolable congeners of quinodimethane diradicals

Abstract

Two-electron oxidation of five aniline-based compounds 4,4′′-p/m-terphenyldiamines afforded a class of terphenyldiamine dications 1²⁺–5²⁺. Their electronic structures were systematically investigated by theoretical calculations and various experiments, including single-crystal X-ray diffraction, UV/Vis absorption spectroscopy, EPR spectroscopy and SQUID measurements. p-Terphenyldiamine dications 1²⁺–3²⁺ are open-shell singlets in the ground state with large singlet–triplet energy gaps. The inherent instability of dication 4²⁺ precludes the determination of its ground state, whereas the observed linear Curie plot for m-terphenyldiamine dication 5²⁺ indicates that its ground state is triplet. The calculated singlet–triplet energy gap (ΔE_OS–T) value is 1.27 kcal mol⁻¹ for 4²⁺ and 1.10 kcal mol⁻¹ for 5²⁺. This work provides the first examples of isolable aniline-based diradical dications.