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Kuriakose, Febin; Commodore, Michael; Hu, Chaowei; Fabiano, Catherine J.; Sen, Debashis; Li, Run R.; Bisht, Shubham; Üngör, Ökten; Lin, Xinsong; Strouse, Geoffrey F.; DePrince, A. Eugene; Lazenby, Robert A.; Mentink-Vigier, Frederic; Shatruk, Michael; Alabugin, Igor V.
Journal of the American Chemical Society, 12/2022, Volume: 144, Issue: 51Journal Article
This work introduces an approach to uncoupling electrons via maximum utilization of localized aromatic units, i.e., the Clar’s π-sextets. To illustrate the utility of this concept to the design of Kekulé diradicaloids, we have synthesized a tridecacyclic polyaromatic system where a gain of five Clar’s sextets in the open-shell form overcomes electron pairing and leads to the emergence of a high degree of diradical character. According to unrestricted symmetry-broken UCAM-B3LYP calculations, the singlet diradical character in this core system is characterized by the y 0 value of 0.98 (y 0 = 0 for a closed-shell molecule, y 0 = 1 for pure diradical). The efficiency of the new design strategy was evaluated by comparing the Kekulé system with an isomeric non-Kekulé diradical of identical size, i.e., a system where the radical centers cannot couple via resonance. The calculated singlet–triplet gap, i.e., the ΔE ST values, in both of these systems approaches zero: −0.3 kcal/mol for the Kekulé and +0.2 kcal/mol for the non-Kekulé diradicaloids. The target isomeric Kekulé and non-Kekulé systems were assembled using a sequence of radical periannulations, cross-coupling, and C–H activation. The diradicals are kinetically stabilized by six tert-butyl substituents and (triisopropylsilyl)acetylene groups. Both molecules are NMR-inactive but electron paramagnetic resonance (EPR)-active at room temperature. Cyclic voltammetry revealed quasi-reversible oxidation and reduction processes, consistent with the presence of two nearly degenerate partially occupied molecular orbitals. The experimentally measured ΔE ST value of −0.14 kcal/mol confirms that K is, indeed, a nearly perfect singlet diradical.
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