Design and Synthesis of Kekulè and Non-Kekulè Diradicaloids via the Radical Periannulation 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 π-sextets. To illustrate the utility of this concept to the design of Kekulédiradicaloids, we have synthesized a tridecacyclic polyaromatic system where a gain of five Clar's sextets in the openshell 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 Kekulésystem with an isomeric non-Kekulédiradical 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 Kekuléand +0.2 kcal/mol for the non-Kekulédiradicaloids. The target isomeric Kekuléand non-Kekulésystems were assembled using a sequence of radical periannulations, cross-coupling, and C−H activation. The diradicals are kinetically stabilized by six tertbutyl 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.

Dihydrophenanthrene Open-Shell Singlet Diradicals and Their Roles in the Mallory Photocyclization Reaction

“…Many studies of fully unsaturated OS singlet diradicals have emphasized the need to generate aromatic rings during the formation of the diradical to pay the price for the loss of a double bond during the process. 43 Fortunately, CS calculations, ωB97X-D/6-31G(d), using all 8 open shell DHPs in Scheme 3 successfully located their higher energy CS* DHP isomer. Consequently, we can provide evidence for the importance of Clar sextets in the formation of partially saturated polyaromatic hydrocarbon OS diradicals by using the HOMA 44 (Harmonic Oscillator Model of Aromaticity) index to compare optimized CS*-and OSisomeric structures.…”

“…Many studies of fully unsaturated OS singlet diradicals have emphasized the need to generate aromatic rings during the formation of the diradical to pay the price for the loss of a double bond during the process . Fortunately, CS calculations, ωB97X-D/6-31G­(d), using all 8 open shell DHPs in Scheme successfully located their higher energy CS* DHP isomer.…”

Bi-Blatter diradicals: convenient access to regioisomers with tunable electronic and magnetic properties

“…1) and, consequently, recovery of the aromaticity and stabilization of the diradical form with a significant open-shell singlet (OSS) character (diradical index 41,42 y = 0.32; where y = 0 for pure CS and y = 1 for pure OSS) 43 and a much smaller separation between the S and T states (DFT energy gap, DE S-T = À5.1 kcal mol À1 ). 44 For this reason II constitutes an attractive fundamental structural element for open-shell systems, such as type III (Fig. 2), in which the diradicaloid character and the S-T gap can be tuned by a judicious choice of substituents at the terminal positions and the degree of spin delocalization.…”

Stable Diradical on the Dimethyldihydropyrene Scaffold

“…The electron pair in diradicaloids can be used as a two-qubit system, provided that the resultant states arise from the superposition of the electrons entangled with each other . However, the presence of two unpaired electrons in single-occupied molecular orbitals makes these species short-lived and highly reactive under standard laboratory conditions, posing a major obstacle to their detailed studies and potential applications. − Aromatic resonance energy plays a crucial role in tuning the diradical character. − The diradical character can be tailored in the molecular architecture mainly in two ways. , First of all, a quinoidal pro-aromatic system with net energy gained upon its aromatization can compensate for the energy required to generate the diradical species. , The Kekule-type diradicals, such as the Chichibabin’s hydrocarbons, are typical examples of such species . A second source of the diradical species are the annulenes with 4 n π electrons (where n = 1, 2, 3, ...). − The contribution from their first singlet state (S 1 ) or triplet excited state (T 1 ) can introduce an inherent diradical character, popularly known as Baird aromaticity. − Pentalene, heptalene, and their derivatives could serve as prime examples of such diradicals. − On the basis of these two principles, a number of diradical systems have been reported. − A remarkable example is bis-periazulene, an isomer of pyrene having 14π peripheral electrons, which was recently reported by Yasuda’s group.…”

“…18,19 First of all, a quinoidal pro-aromatic system with net energy gained upon its aromatization can compensate for the energy required to generate the diradical species. 20,21 The Kekule-type diradicals, such as the Chichibabin's hydrocarbons, are typical examples of such species. 22 A second source of the diradical species are the annulenes with 4nπ electrons (where n = 1, 2, 3, ...).…”