Computational and Experimental Confirmation of the Diradical Character of para-Quinonedimethide

Abstract

The ground-state structure of the parent para-quinonedimethide (p-QDM) molecule is generally represented in its closed shell form, i.e., as a cyclic, nonaromatic, through-conjugated/cross-conjugated hybrid comprising four CC bonds. Nonetheless, p-QDM has been theorized to contain a contribution from its open-shell aromatic singlet diradical form. VBSCF calculations identify an open-shell contribution of 29% to the structure, while CASPT2­(16,16)/def2-TZVP and ωB97XD/aug-cc-pVTZ calculations predict that dimerization proceeds along an open-shell singlet diradical pathway with a low (77 kJ/mol) barrier toward dimerization, which occurs by way of C–C bond formation between the exocyclic methylene carbons. A similar low (98 kJ/mol) barrier exists toward the reaction between a p-QDM molecule and the radical trap TEMPO. These predictions are verified experimentally through the isolation of bis-TEMPO-trapped p-QDM, its C–C coupled dimer, and by demonstrating that a mixture of p-QDM and TEMPO can initiate the radical polymerization of n-butyl acrylate at ambient temperature. In contrast to p-QDM, tetracyanoquinone (TCNQ) neither dimerizes nor reacts with TEMPO, despite having a similar diradical character to p-QDM. This lack of reactivity is consistent with both a higher kinetic barrier and a thermodynamically unfavorable process, which is ascribed to destabilizing steric clashes and polar effects.

An N‐Heterocyclic Quinodimethane: A Strong Organic Lewis Base Exhibiting Diradical Reactivity

“…However, the dimerization of QDMs such as p-xylylene is known, and recent computational and experimental studies provided conclusive evidence that this process is initiated by a CÀ C coupling between the exocyclic methylene carbons along an open-shell diradical pathway. [8,16,14] In addition, the dimerization of heteroatomcontaining QDMs has been reported. [17] It is therefore reasonable to attribute the formation of 8 to such diradical coupling as well.…”

“…[18] The inset shows the open-shell singlet state transition state structure of the CÀ C bond formation with the spin density. The bathochromic shift of 320 nm and the greater intensity compared to 1 is likely caused by the extension of πconjugation in 8 that lowers the HOMO-LUMO gap from 7.0 eV (1) to 5.0 eV (8). Exposure of 8 to air results in immediate color change from blue to purple.…”

Ein N‐Heterozyklisches Chinodimethan: Eine starke organische Lewis‐Base mit Diradikal‐Charakter

“…Die mittels Einkristall-Röntgenstrukturanalyse (SCXRD) bestimmten CÀ C-Bindungslängen des C 6 H 4 -Linkers von 6 unterstützen diese Annahme (Abbildung 3). [15] Wenn eine THF-Lösung von [8,16,14] Darüber hinaus wurde über die Dimerisierung von Heteroatom-haltigen QDMs berichtet. [17] Es ist daher naheliegend, die Bildung von 8 ebenfalls auf eine diradikalische Kupplung zurückzuführen.…”

“…Jüngste Arbeiten von Coote, Sherburn und Mitarbeitern haben gezeigt, dass Chinodimethane (QDMs) die Reaktivität eines Diradikals aufweisen. [8] Des Weiteren zeigten Ghadwal und Mitarbeiter, dass NHC-Substituenten gut geeignet sind, um C-zentrierte Radikale zu stabilisieren. [9] Es stellt sich somit auch die Frage, inwieweit NHQs radikalische Reaktivität aufweisen könnten.…”

Dianionic and Neutral Diboron-Centered Classical Diradicaloids

“…Classical examples of diradicaloids are Thiele, Chichibabin, and Müller hydrocarbons I , which in the open-shell form bears two triphenylmethyl radical scaffolds holding the carbon-centered spin and are separated by p -phenylene, p , p′ -biphenylene, or p , p″ -terphenylene, respectively (Chart ). The synthesis, isolation, and subsequent unique properties along with several applications of this class of diradicaloids are well-known . This goes back to the well-established chemistry of various stable and isolable carbon-centered radicals II (from one-electron oxidation/reduction of corresponding carbanions/carbocations), which has been known for quite some time .…”