Dianionic and Neutral Diboron-Centered Classical Diradicaloids

Abstract

Herein, we report the syntheses and electronic structures of crystalline dianionic as well as neutral diboron-centered classical diradicaloids as boron analogues of classical Thiele, Chichibabin, and Muller (this only for dianionic diradicaloids!) hydrocarbons. These are based on borane radical anion and NHC-stabilized boryl radical spin carriers, respectively. All these dianionic diboron-centered diradicaloids exhibit triplet population at room temperature regardless of the π-conjugated spacer: p-phenylene, p,p′-biphenylene, or p,p″-terphenylene. In the case of neutral diboron-centered diradicaloids, the employed π-conjugated spacer plays a crucial role for the triplet population at room temperature: EPR inactive for p-phenylene vs EPR active for p,p′-biphenylene. The findings emphasize the importance of the spin carriers for the resulting ground-state: borane radical anion vs NHC-stabilized boryl radical along with the pivotal role of the πconjugated spacer as spin-coupler between two spins. Notably, 100 years (a century) after the first report by Krause of the triphenyl borane radical-anion, being isoelectronic to the triphenylmethyl radical, we convey borane radical anion-based diradicaloids. Furthermore, while donor-stabilized boryl radicals were introduced in the 1980s by Giles and Roberts, said concept is herewith being extended to NHC-stabilized boryl radical-based diradicaloids.

Crystalline Radical Anion of a Diboratriazole and Its Conversion to a Neutral Radical Driven by a Carbene

“…In particular, diradicals without significant electron coupling, namely, dis-biradicals, are extremely limited. To date, only a small number of boron-containing diradical compounds have been reported, , among which dis-biradicals are nearly unprecedented. , Given the successful synthesis of neutral radical 3 by the simple addition of a carbene to 1 •– •K + , we anticipated that the employment of a bis-carbene could allow constructing the dis-biradical system. Accordingly, we next examined the reaction with the bis-carbene, in which two N -heterocyclic carbene rings are separated by a long −(CH 2 ) 4 – linker (Scheme ).…”