A two-coordinate boron cation featuring C–B+–C bonding

Abstract

Two-coordinate boron cations (R2B(+)), referred to as borinium ions, are chemical species in which the boron bears only four valence electrons, and that are isoelectronic with hypothetical carbon dications (R2C(2+)). Although lone-pair-donating substituents such as amino groups have enabled the isolation of several borinium ions, diarylated and dialkylated borinium derivatives remain entirely unexplored. Here, we present the synthesis, structure and reactivity of the dimesitylborinium ion, which displays unexpectedly high thermal stability. X-ray crystallography and (11)B NMR spectroscopy, supported by density functional theory calculations, reveal that the borinium ion adopts a linear two-coordinate structure in both the solid state and in solution. The boron centre is stabilized by pπ bonding from the mesityl groups and is free from coordination by the counterion or solvent molecules. This diarylborinium ion possesses exceptional Lewis acidity, accepting a pair of electrons from CO2 to cause an unusual deoxygenation reaction.

Fundamental and Applied Properties of Borocations

“…Simply defining the reactivity of borocations towards π nucleophiles other than arenes and alkynes is in itself a vast research area, whilst the ability to activate sigma bonds opens up a multitude of possibilities, both stoichiometric and catalytic in borocation. Furthermore, the recent development of a simple route to the highly electrophilic borinium cation, [(Mes) 2 B] + , and the subsequent unusual reactivity with CO 2 suggests that even the most reactive member of the borocation family, the borinium, is poised for wider exploitation [89]. In comparison to synthetic applications, the incorporation of borenium and boronium ions into functional materials is less developed and opportunities abound in using the unique properties of borocations to produce desirable photophysical properties.…”

“…31 is a linear (at boron) cation stabilised by π donation from the two orthogonally orientated mesityl groups. Cation 31 reacts with CO 2 by 1,2-carboboration, ultimately forming aroyl cations, [MesCO] + and MesBO, with the latter undergoing oligomerisation and further reactions with 31 to give a complex mixture of boron-containing products [89,90].…”

The molecular and electronic structures of a thioaroyl cation formed by borinium ion-mediated CS double bond cleavage of CS2

“…5,6 Since Mes 2 B + is devoid of lone-pair donation from heteroatoms, the boron center is extremely electron deficient, causing unusual CQO double bond cleavage of CO 2 to yield the aroyl cation [MesCO] + [(C 6 F 5 ) 4 B] À . 4 We extended this finding to the activation of CS 2 and found that the CQS double bond cleavage likewise occurs to give [MesCS] + [(C 6 F 5 ) 4 B] À (2). Typically, Mes 2 B + [(C 6 F 5 ) 4 B] À (1, Scheme 1) was added at 25 1C to an o-dichlorobenzene (ODCB) solution of CS 2 (one equivalent to 1), and the mixture was stirred for 5 minutes.…”

“…As part of our study on the chemistry of a two-coordinate boron cation, i.e., diarylborinium ion [Mes 2 B + ; Mes (mesityl) = 2,4,6-trimethylphenyl], 4 we investigated the reaction of Mes 2 B + [(C 6 F 5 ) 4 B] À (1, Scheme 1) and carbon disulfide (CS 2 ), which affords the thioaroyl cation salt [MesCS] + [(C 6 F 5 ) 4 B] À (2, Scheme 1) in moderate yield. In the course of this study, we successfully obtained high-quality single crystals of 2 suitable for X-ray crystallography, allowing us to unveil the true structural perspective of [ArCS] + .…”

Using Ylide Functionalization to Stabilize Boron Cations

“…[12] These ligands facilitated their stabilization and thus their isolation. [13,14] Nevertheless, almost all isolated boron cations rely on the use of amido or aryl groups as anionic substituents at boron. They are capable to efficiently stabilize the cation by minimizing the electron deficiencyatboron through additional p-donation.…”