Abstract
Thiele’s hydrocarbon occupies a central role as an open-shell platform for new organic materials, however little is known about its redox behaviour. While recent synthetic approaches involving symmetrical carbene substitution of the CPh2 termini yield isolable neutral/dicationic analogues, the intervening radical cations are much more difficult to isolate, due to narrow compatible redox ranges (typically < 0.25 V). Here we show that a hybrid BN/carbene approach allows access to an unsymmetrical analogue of Thiele’s hydrocarbon 1, and that this strategy confers markedly enhanced stability on the radical cation. 1•+ is stable across an exceptionally wide redox range (> 1 V), permitting its isolation in crystalline form. Further single-electron oxidation affords borenium dication 12+, thereby establishing an organoboron redox system fully characterized in all three redox states. We perceive that this strategy can be extended to other transient organic radicals to widen their redox stability window and facilitate their isolation.
Highlights
Thiele’s hydrocarbon occupies a central role as an open-shell platform for new organic materials, little is known about its redox behaviour
Recently reported analogues based on N-heterocyclic carbenes (NHCs) by Ghadwal et al, cyclic(amino)carbenes (CAACs) and acyclic diaminocarbenes (ADCs) by Jana et al are characterized either by two single-electron transfer events at potentials which are very close (ΔE < 0.25 V)[28,29], or by a single two-electron redox wave[30,31], alluding to the instability of the respective mixedvalence radical cations towards disproportionation[13,33]
Cyclic voltammetry reveals irreversible one-electron oxidations, hinting at the generation of extremely reactive radical cations and dications derived from these systems that contain a pair of highly electron-deficient boron atoms
Summary
Thiele’s hydrocarbon occupies a central role as an open-shell platform for new organic materials, little is known about its redox behaviour. In regards to anionic boron analogues, three-state redox systems have been reported very recently, but the redox ranges associated with the intermediate radicals are very narrow (ΔE = 0.20 V), limiting the stability of these radicals.
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