Design and synthesis of Kekulè and non-Kekulè diradicaloids via radical peri-annulation strategy: the power of seven Clar’s sextets

Abstract

This work introduces an approach to uncoupling electrons via maximum utilization of localized aromatic units, i.e., the Clar’s 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 high degree of diradical character. According to unrestricted symmetry-broken UCAM-B3LYP DFT calculations, the singlet diradical character in this core system is characterized by the y0 value of 0.98 (y0 = 0 for closed shell molecule, y0 = 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 and the high-spin ground state is unavoidable. The calculated singlet-triplet gap, i.e., the 〖ΔE〗_ST values, in both of these systems approach 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 peri-annulations, cross-coupling and C-H activation. The diradicals are kinetically stabilized by six tert-butyl substituents and (triisopropylsilyl)acetylene groups. The Kekulé diradicaloid (K) has a half-life of 42 h under ambient conditions (i.e., exposure to air at the room temperature) while the non-Kekulé diradicaloid (NK) has a half-life of 2h. Both molecules are NMR-inactive but EPR-active at room temperature. The magnetic properties of the Kekulé diradicaloid was studied by superconducting quantum interference device (SQUID) to provide the experimental singlet-triplet energy gap, 〖ΔE〗_ST (K) = -0.8 kcal/mol, which was close to calculated value. Cyclic voltammetry revealed quasi-reversible two-electron oxidation and reduction processes, consistent with the presence of two degenerate partially occupied molecular orbitals.