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Borole systems tend to undergo various reactions driven by the disruption of its destabilizing antiaromatic character. As a consequence, the isolation and characterization of free boroles is challenging, especially when the substituents around the C4B framework are sterically less demanding. In the present paper we report the synthesis of 1-bromo-2,3,4,5-tetraphenylborole. The title compound readily undergoes a dimerization/rearrangement reaction in analogy to the previously reported 1-chloro-2,3,4,5-tetraphenyborole to form an isostructural product identified by X-ray crystallography. Additionally we present the formation of Lewis acid–base adducts of the title compound with 3,5-lutidine, PCy3, N-hetrocyclic carbene, cyclic (amino)(alkyl)carbene, and THF. The latter compounds were analyzed by single-crystal X-ray diffraction and compared. [ABSTRACT FROM AUTHOR]

Because of the ubiquity of fluoride ions and their potential toxicity at high doses, researchers would like to design receptors that selectively detect this anion. Fluoride is found in drinking water, toothpaste, and osteoporosis drugs. In addition, fluoride ions also can be detected as an indicator of uranium enrichment (via hydrolysis of UF6) or of the chemical warfare agent sarin, which releases the ion upon hydrolysis. However, because of its high hydration enthalpy, the fluoride anion is one of the most challenging targets for anion recognition. [ABSTRACT FROM AUTHOR]

While we have previously reported that [1-(Mes2B)-8-(Me3NCH2)-C10H6]+ ([2]+) complexes fluoride ions to form [1-(Mes2FB)-8-(Me3NCH2)-C10H6] (2-F), we now show that this cationic borane also complexes cyanide to form [1-(Mes2(NC)B)-8-(Me3NCH2)-C10H6] (2-CN). This reaction also occurs under biphasic conditions (H2O–CHCl3) and may serve to transport cyanide in organic phases. The zwitterionic cyanoborate 2-CN has been fully characterized and its crystal structure determined. UV-vis titration experiments carried out in THF indicate that [2]+ has a higher affinity for fluoride (K > 108 M−1) than cyanide (K = 8.0 (±0.5) × 105 M−1). Steric effects which impede cyanide binding to the sterically congested boron center of [2]+ are most likely at the origin of this selectivity. Finally, electrochemical studies indicate that [2]+ is significantly more electrophilic than its neutral precursor 1-(Mes2B)-8-(Me2NCH2)-(C10H6) (1). These studies also show that reduction of [2]+ is irreversible, possibly because of elimination of the NMe3 moiety under reductive conditions. In fact, [2]OTf reacts with NaBH4 to afford 1-(Mes2B)-8-(CH3)-(C10H6) (4) which has also been fully characterized. [ABSTRACT FROM AUTHOR]

Electrophilic quenching of a carbene-stabilized π-boryl anion with the Brønsted acid Et3NHCI provides a convenient synthetic route to the parent NHC-stabiized 1-hydroborole, which isomerizes to the corresponding 3-hydroborole via two successive nondegenerate [1,5]-sigmatropic hydrogen migrations. Molecular structures of both isomers have been determined by X-ray crystallography. [ABSTRACT FROM AUTHOR]

Mes 2BF and Ar N2BF (Ar N= 4-(Me 2N)-2,6-Me 2-C 6H 2) react with trimethylsilyltriflate and p-dimethylaminopyridine in chlorobenzene to afford the corresponding borenium salts [Mes 2B-DMAP] +OTf −([ 1] +OTf −) and [Ar N2B-DMAP] +OTf −([ 2] +OTf −), which have been fully characterized. [ABSTRACT FROM AUTHOR]

The article focuses on addressing the question that whether the introduction of multiple cationic groups would have additive effects on the electron deficiency and Lewis acidity of the boron center. A synthesis and study of cationic analogues of trimesitylborane. It concluded that the electron deficiency and Lewis acidity of triarylboranes can be incrementally enhanced through pripheral decoration of the aryl ligands with cationic groups.

The article offers information about a study that describes the synthesis of a cationic Lewis acidic borane which forms a highly stable zwitterionic ammonium/flouroborate complex possesing a C-H...F-B hydrogen bond. Furthermore, this article also demonstrates how this cationic borane can be used for the extraction of fluoride from water.

The reaction of 10-bromo-9-oxa-10-boraanthracene with the tetrakis(tetrahydrofuran)lithium salt of dimesityl-1,8-naphthalenediylborate in diethyl ether affords 1-(dimesitylboryl)-8-(10′-bora-9′-oxaanthryl)naphthalene (2). This diborane reacts with [Me3SiF2][S(NMe2)3)] to afford the anionic complex [2-μ2-F]-, which has been isolated as a [S(NMe2)3]+ salt. The cyclic voltammograms of diborane 2 as well as 1-(dimesitylboryl)-8-(10′-bora-9′-thiaanthryl)naphthalene (1) exhibit two reversible reductions at E1/2 = -2.200 and -2.566 V (vs FcH/FcH+) for 1 and E1/2 = -2.248 and -2.620 V (vs FcH/FcH+) for 2 corresponding to the sequential reduction of the two boron centers. These two waves simultaneously disappear upon fluoride addition, thus indicating the formation of fluoride chelate complexes [1-μ2-F]- and [2-μ2-F]-. To identify the origin of the high fluoride affinity displayed by these diboranes, the structures of 2 and [2-μ2-F]- have been studied experimentally and computationally. The crystallographic studies show that the structure of 2 is distorted, thus indicating the presence of important steric repulsions between the neighboring boryl moieties. By contrast, the structure of the anionic complex [2-μ2-F]- is much more sterically relaxed than that of 2, as indicated by a reduction of the B-B distance from 3.279(4) Å in 2 to 2.922(7) Å in [2-μ2-F]-. The structural results suggest that the high fluoride affinity displayed by 2 results, at least in part, from the relief of steric repulsions induced by fluoride binding. Finally, the nature of the bonding as well as the strength of the interactions involved in the B-F-B bridge of [2-μ2-F]- has been studied using density functional theory calculations and Atoms-In-Molecules analyses. These calculations indicate that the enthalpic gain associated with the formation of two B-F bonds in [2-μ2-F]- only amounts to a fraction of the energy of a terminal B-F bond. These calculations also suggest that the relief of steric repulsions induced by fluoride binding in 2 may contribute to the high fluoride affinity of these types of molecules. [ABSTRACT FROM AUTHOR]

Two aryl amino borinium cations derived from Cl(Mes)B-NR2 (NR2 = TMP, HMDS) faced divergent outcomes. As the HMDS-substituted one underwent methyl migration from silicon to boron transforming the putative borinium ion to a silylium ion, [Mes-B-TMP]+ can initiate cyanosilylation and catalyse hydrosilylation of ketones and aldehydes. [ABSTRACT FROM AUTHOR]

In an attempt to prepare the heavier analogue of NHC-stabilized borenium cation, we have prepared a silylene-coordinated borenium equivalent (1a), which can be viewed as a contact ion pair of borenium and triflate. The triflate anion coordinates to the oxophilic silicon center, leaving the boron center existing in a trigonal-planar geometry in the solid state and in solution. Nucleophilic substitution reactions of 1a all result in the release of free silylene ligand, demonstrating the borenium-ion-like reactivity of 1a. Chemical reduction of 1a leads to a neutral radical possessing a partial Si-B double bond. [ABSTRACT FROM AUTHOR]

The reaction mechanism of the [Et3BH]--induced conversion of a dicationic boron compound ([Cp*B-IMes]2+, [1]2+) to a planar neutral carbene-coordinated borabenzene (2) is investigated experimentally and theoretically. Owing to the steric congestion around the boron center, bulky [Et3BH]- attacks the less encumbered carbon atom of Cp*, leading to a metastable 5-borabicyclo[2.1.1]hex-2-ene borenium cation ([3-CH]+) at low temperature. Upon raising the temperature, the bicyclic-borenium ion undergoes sequential pericyclic reactions, including suprafacial [1,3]-sigmatropic shift, two-electron electrocyclic ring opening, and then [1,2]-hydrogen shift to a planar cyclic borenium ion, [4-CH]+, which can then be deprotonated to yield the neutral aromatic borabenzene. The attack of hydride at the boron center of [1]2+ was excluded through the preparation and isolation of 6-borabicyclo[3.1.0]hexenylium, [3-BH]+, which could not be transformed into [3-CH]+ or [4-CH]+. The borenium ion rearrangement follows first-order kinetics with activation parameters of ΔH- = 19.6 (±0.31) kcal/mol and ΔS- = -2.03 (±1.15) cal/mol·K. The observed kinetic isotope effect of 0.87 at 265 K is consistent with the DFT-calculated reaction mechanism, which predicts an overall KIE value of 0.90. These results show that [1]2+ reacts with [Et3BH]- as a carbon-based electrophile and the formation of an electron-deficient borenium center is responsible for the skeletal rearrangement of the nido-cluster. [ABSTRACT FROM AUTHOR]

A twisted mono-cationic bis-benzimidazolium salt was serendipitously isolated from the dehydrative condensation of 5,6-dihydroxyl-1,3- dimesityl-benzimidazolium and 1,4-benzenediboronic acid. Subsequent deprotonation of the benzimidazolium salt led to the formation of a spiroborate-linked free bis-NHC, which was further transformed into the corresponding diborane adduct and the di-Rh complex. [ABSTRACT FROM AUTHOR]

The reaction of [Li(THF)4][1,8-μ-(Mes2B)C10H6] with HgCl2 affords [1,1′-(Hg)-[8-(Mes2B)C10H6]2] (3) or [1-(ClHg)-8-(Mes2B)C10H6] (4), depending on the stoichiometry of the reagents. These two new compounds have been characterized by 1H, 13C, 11B and 199Hg NMR, elemental analysis and X-ray crystallography. The cyclic voltammogram of 3 in THF shows two distinct waves observed at E1/2 −2.31 V and −2.61 V, corresponding to the sequential reductions of the two boron centers. Fluoride titration experiments monitored by electrochemistry suggest that 3 binds tightly to one fluoride anion and more loosely to a second one. Theses conclusions have been confirmed by a UV-vis titration experiment which indicates that the first fluoride binding constant (K1) is greater than 108 M−1 while the second (K2) equals 5.2 (± 0.4) × 103 M−1. The fluoride binding properties of 3 have been compared to those of [1-(Me2B)-8-(Mes2B)C10H6] (1) and [1-((2,6-Me2-4-Me2NC6H2)Hg)-8-(Mes2B)C10H6] (2). Both experimental and computational results indicate that its affinity for fluoride anions is comparable to that of 2 but significantly lower than that of the diborane 1. In particular, the fluoride binding constants of 1, 2 and 3 in chloroform are respectively equal to 5.0 (± 0.2) × 105 M−1, 1.0 (± 0.2) × 103 M−1 and 1.7 (± 0.1) × 103 M−1. Determination of the crystal structures of the fluoride adducts [S(NMe2)3][1–μ2-F] and [S(NMe2)3][3–μ2-F] along with computational results indicate that the higher fluoride binding constant of 1 arises from a strong chelate effect involving two fluorophilic boron centers. [ABSTRACT FROM AUTHOR]