A crystalline radical cation derived from Thiele’s hydrocarbon with redox range beyond 1 V

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.

Redox‐Active Hydrocarbons: Isolation and Structural Determination of Cationic States toward Advanced Response Systems

“…Indeed, aromatic hydrocarbons are known to exhibit unique electrochemical and spectroscopic properties based on their π-conjugated systems, and several systems capable of redox interconversion between neutral and cationic (anionic) states have been reported by effectively stabilizing the charged state by embedding heteroatoms or introducing heteroatom-substituents into the π-skeleton. [21][22][23][24][25][26][27][28][29][30][31][32] On the other hand, since it is difficult to stabilize such charged species without heteroatoms, the development of pure hydrocarbonbased redox systems is challenging from the viewpoint of the stability of cationic (anionic) species. In general, even though the incorporation of heteroatoms is an effective method for stabilizing charged species and modulating the electronic properties of organic molecules, the potential instability of bonds between carbon and heteroatoms in the neutral state may reduce fatigue resistance in organic electronic applications.…”

“…In addition, delocalization of a charge over an extended π‐conjugation is effective for the synthesis and isolation of charged species as stable entities with carbon‐centered ions. Indeed, aromatic hydrocarbons are known to exhibit unique electrochemical and spectroscopic properties based on their π‐conjugated systems, and several systems capable of redox interconversion between neutral and cationic (anionic) states have been reported by effectively stabilizing the charged state by embedding heteroatoms or introducing heteroatom‐substituents into the π‐skeleton [21–32] . On the other hand, since it is difficult to stabilize such charged species without heteroatoms, the development of pure hydrocarbon‐based redox systems is challenging from the viewpoint of the stability of cationic (anionic) species.…”

Geometrical and Electronic Structure of Cation Radical Species of Tetraarylanthraquinodimethane: An Intermediate for Unique Electrochromic Behavior

“…Organic redox systems undergo reversible electron transfer when the resulting charged species are stable enough [1–5] . Charge delocalization and/or formation of additional aromatic ring upon electron transfer are often adopted strategies to stabilize the organic ions [6–15] . Quinodimethanes are the cross‐conjugated π‐systems, [16,17] which are suitable scaffolds to design the reversible redox systems, especially because of the formation of planar π‐skeleton with an additional aromatic ring in the corresponding ion radicals and doubly‐charged ions [18–20] …”

“…[1][2][3][4][5] Charge delocalization and/or formation of additional aromatic ring upon electron transfer are often adopted strategies to stabilize the organic ions. [6][7][8][9][10][11][12][13][14][15] Quinodimethanes are the crossconjugated π-systems, [16,17] which are suitable scaffolds to design the reversible redox systems, especially because of the formation of planar π-skeleton with an additional aromatic ring in the corresponding ion radicals and doubly-charged ions. [18][19][20] Based on the general consideration shown above, the pentacenequinodimethane-type dication (I 2 + ) has a quite peculiar structure, in which the two positive charges are located on the one side of the molecular skeleton (Scheme 1a).…”

Crystalline radical cations of bis-BN-based analogues of Thiele's hydrocarbon

“…4 Therefore, numerous efforts have been devoted to preparing its derivatives and analogues with enhanced stability and stable redox states. Recently, neutral carbon, 5 silicon, 6 and BN-based, 7 mono-/dianionic boron, 8 and dicationic nitrogen 9 analogues of Thiele's hydrocarbon have been successfully isolated. Despite these advances, structurally characterized radical species derived from analogues of Thiele's hydrocabons are limited to I–IV (Fig.…”