Design and Synthesis of Kekulè and Non-Kekulè Diradicaloids via the Radical Periannulation Strategy: The Power of Seven Clar’s Sextets

• Published in: Journal of the American Chemical Society Vol. 144; no. 51; pp. 23448 - 23464
• Main Authors: 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.
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 28.12.2022; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Multidisciplinary; Physical Sciences; Science & Technology; EDGE STATE; GRAPHENE NANORIBBONS; POLYCYCLIC AROMATIC-HYDROCARBON; GROUND-STATE; RYLENE RIBBONS; 2ND HYPERPOLARIZABILITIES; SINGLET FISSION; OPEN-SHELL; INDEPENDENT CHEMICAL-SHIFTS; BOTTOM-UP SYNTHESIS
• DOI: 10.1021/jacs.2c09637
• ISSN: 0002-7863; 1520-5126

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.