An Isolable Radical Anion Based on the Borole Framework

Abstract

The element boron is known to have a variety of ways to relieve its inherent electron deficiency. The acceptance of an electron pair (Lewis acidity) has applications in catalysis [1] and activation of element-element bonds (frustrated Lewis pairs). [2] The combination of boron with p-donating substituents (e.g. BF 3 ) and its incorporation into organic p-conjugated systems allows the empty p z orbital of boron to participate in p bonding and p conjugation, respectively, and the latter enables the use of boron in optoelectronic materials with unique properties. [3] The absence of p-donating substituents at the boron center may result in multiple-center bonding to form nonclassical frameworks (e.g. B 2 H 6 or clusters). In addition, organoboranes and -diboranes(4) are prone to accept a single electron by chemical reduction. [4] Likewise, hydrogen atom abstraction from N-heterocyclic carbene (NHC)-stabilized boranes (NHC-BH 3 ) can lead to neutral, persistent [4f] boryl radicals of the type NHC-BH 2 C, [5] which have been studied by means of cyclic voltammetry, EPR, and UV/Vis spectroscopy as well as trapping reactions. [4][5][6] However, examples of isolated boron radicals are rare owing to the reactive nature of the species, and only little is known about their structural properties. Steric protection of the boron center combined with spin delocalization over the organic substituents, both achieved by substitution with mesityl groups (Mes = 2,4,6-trimethylphenyl), has occasionally enabled isolation and structural characterization of radical anions such as [Li ([12]crown-4) 2 ][BMes 3 ] (1) or [K([18]crown-6)-(thf) 2 ][Mes 2 BB(Ph)Mes] (2). [7]Our group has recently studied a persistent radical anion as an intermediate in the stepwise reduction of 1-ferrocenyl-2,3,4,5-tetraphenylborole (3). [8] Boroles are a class of antiaromatic compounds with interesting chemical and photophysical properties [9,10] that are well-known for their ability to accept two electrons with formation of an aromatic borole dianion. [11,12] Encouraged by these recent results on the radical anion [3]C À , which indicated the presence of a highly unusual C 4 B p system bearing five electrons, [8] we set out to isolate and characterize a stable borol radical anion. As we report here, this was possible by choice of steric protection and an appropriate reducing agent.The synthesis of MesBC 4 Ph 4 (1-mesityl-2,3,4,5-tetraphenylborole, 4) by means of the commonly employed tin-boron exchange reaction [11][12][13][14] was unsuccessful because of the low reactivity of dihalo(mesityl)boranes (MesBX 2 ; X = Cl, Br). However, 4 was obtained in 41 % yield by functionalization of the boron center in 1-chloro-2,3,4,5-tetraphenylborole (5) through nucleophilic displacement of the chlorine ligand with LiMes (Scheme 1). [14b] A more efficient alternative was found to be the salt-elimination reaction of MesBCl 2 with 1,4dilithio-2,3,4,5-tetraphenylbuta-1,3-diene (6) which provided 4 in 66 % yield. Formation of a Lewis acid-base adduct with Et 2 O, as previou...

A theoretical study of the aromaticity in neutral and anionic borole compounds

“…In addition, we have investigated corresponding NICS values for the negatively charged derivatives. As previously reported by the groups of Herberich, [52] Yamaguchi [18] and Braunschweig, [15,17,[53][54][55] the synthesis and isolation of various dianionic and a monoanionic species has been successfully achieved ( Figure 4). The addition of one electron noticeably decreases the antiaromatic character of boroles, as judged by both the NICS and multicenter indices.…”

“…60°) which reduces the overlap of the lone pair on nitrogen with the empty p-orbital on boron, an effect that is better reflected by the multicenter indices. A comparison between boroles with different aryl substituents (1: R = phenyl; [7] 7: R = 2,4,6-trimethylphenyl; [17] 8: R = 2,3,5,6-tetramethylphenyl; [50] 9: R = 2,4,6-triisopropylphenyl) 50 The data also suggests that the antiaromaticity of the boroles correlates with the steric demand of the B-aryl substituent: the higher the steric bulk of the R substituent, the higher the antiaromaticity. This can be understood by an increasingly weaker π-overlap of the aryl group with the borole unit due to the steric crowding around the exocyclic B−C bond.…”

“…[9] Nevertheless, isolable boroles are typically highly reactive species and follow various pathways to reduce their antiaromaticity, namely by activation of H−H and Si−H bonds, [10][11][12] [4+2] cycloaddition reactions, [13,14] adduct formation with Lewis bases [15,16] or one-and two-electron reductions to form the corresponding radical anions and dianions, respectively. [17,18] Besides their rich reactivity profile, boroles display chromophoric properties, i.e. they absorb light strongly in the UV-vis region of the spectrum, and are highly Lewis acidic due to the presence of a vacant p-orbital on boron.…”

Bor als überaus starkes Reduktionsmittel: Synthese eines Bor‐ zentrierten Radikalanionen‐Radikalkationen‐Paares

“…[8,18] Es sollte an dieser Stelle zudem hervorgehoben werden, dass 4 eines der am stärksten negativen Reduktionspotentiale aller bislang untersuchten neutralen organischen Moleküle besitzt. [19] Die [7] Tabelle 1: Bindungslängen [] und -winkel [8] [8] darauf hin, dass die ungepaarte Spindichte hauptsächlich auf der B-B-Bindung konzentriert ist. Hierbei ist die verhält-nismäßig große Hyperfeinkopplungskonstante zu den Isopropylprotonen wahrscheinlich auf eine effiziente Spindelokalisierung aus der (p-p)p-Bindung über einen Hyperkonjugations-Mechanismus zurückzuführen.…”

“…Das EPR-Spektrum von 6 wird am besten durch eine Überlage-rung der beiden Signale für das Borol-Radikalanion und das Diboren-Radikalkation beschrieben, woraus die in Abbildung 3 gezeigte charakteristische Resonanz resultiert. Somit kann das experimentell gemessene EPR-Spektrum problemlos durch Kombination der EPR-Signale von 5 mit dem des Borol-Radikalanions (a( 11 B) = 9.7 MHz) [7,8] simuliert werden. Entfernen eines Elektrons aus dem bindenden B-B-pOrbital ist erwartungsgemäß mit der Abnahme der B-BBindungsordnung von 2 auf 1.5 verbunden, was auch aus den Festkçrperstrukturen von 5 und 6 ersichtlich ist (Abbildung 4; Tabelle 1).…”