Design and Synthesis of Kekulé and Non-Kekulé Diradicaloids via Radical Peri-Annulation Strategy

This work describes a simple and general approach to the design of new polyaromatic diradicaloids. The systematic approach starts with privileged core structures with the maximum number of Clar's sextets, and explores the electron pairing topology by addition of CH2 radicals as spin probes at different positions. Computational results clearly illustrate how, for the same core, gain of a larger number of Clar's sextets in the open-shell form increases the diradical character parameter y0 from 0 to 0.98 (where 1.0 indicates a perfect diradical). To test this design, 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 ΔES-T values, in both of these systems approach zero: -0.2 kcal/mol for the Kekulé and +0.3 kcal/mol for the non-Kekulé diradicaloids. The isomeric Kekulé (K) and non-Kekulé (NK) systems were assembled using a sequence of radical peri-annulations, cross-coupling and C-H activation. Both molecules are NMR-inactive but EPR-active at room temperature. The magnetic properties of molecule K was further studied by superconducting quantum interference device (SQUID) and experimentally measured singlet-triplet energy gap, ΔES-T (K) = -0.8 kcal/mol which is close to calculated values. The diradicals are kinetically stabilized by six tert-butyl substituents and (triisopropylsilyl)acetylene groups. The Kekulé diradicaloid has a half-life of 42h under ambient conditions (i.e., exposure to air at the room temperature) while the non-Kekulé diradicaloid has a half-life of 2h.