chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, a boranylium ion is an

inorganic An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemistry''. Inor ...

cation with the

chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...

, where R represents a non-specific substituent. Being electron-deficient, boranylium ions form adducts with

Lewis base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

s. Boranylium ions have historical names that depend on the number of coordinated ligands: *: borinium *: borenium *: boronium

Borenium ions

A borenium ion is an

cation with the

. In this class of molecules, the electron-deficient boron center has two valence electrons involved in sigma bonding with two ligands, while the third ligand is a two-electron donor such that the overall charge of the complex is +1. Depending on the nature of the ligands around the central boron, this positive charge can be localized on the boron center or delocalized across the entire molecule. Borenium ions can be made in a number of different ways and are of interest for applications in

organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

and

catalysis Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

Synthesis

Synthetic methods for preparing borenium ions include halide abstraction, nucleophilic dissociation, and protic addition to aminoboranes.

Halide or hydride abstraction

Borenium ions can be made from tetracoordinate Lewis acid-base adducts of boron halides. In this method, halide abstraction by a

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...

such as AlCl₃ results in a borenium cation and AlCl₄⁻ anion. The first borenium ion to be isolated and characterized was made by Ryschkewitsch and Wiggins in 1970 using this method. They found that

aluminium chloride Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms a hexahydrate with the formula , containing six water molecules of hydration. Both the anhydrous form and the hexahydrate are col ...

dissolved in

dichloromethane Dichloromethane (DCM, methylene chloride, or methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odor is widely used as a solvent. Although it is not miscible with ...

in the presence of the adduct of

4-methylpyridine 4-Methylpyridine is the organic compound with the formula CH3C5H4N. It is one of the three isomers of methylpyridine. This pungent liquid is a building block for the synthesis of other heterocyclic compounds. Its conjugate acid, the 4-methylpyri ...

and BCl₃. A positive charge on boron was then inferred from

proton NMR Proton nuclear magnetic resonance (proton NMR, hydrogen-1 NMR, or 1H NMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the stru ...

spectroscopy.

Similar to the halide abstraction method, borenium ions can be made through abstraction of a hydride from a tetracoordinate boron complex.

Nucleophilic dissociation

Displacement of a ligand from a neutral tricoordinate boron halide by a neutral donor such as pyridine results in the generation of a borenium cation. For this reaction to yield the desired borenium cation, the ligand must be a good leaving group and the neutral donor must have enough steric bulk that nucleophilic dissociation is favored over Lewis acid-base adduct formation with the neutral BR₃ starting material, as demonstrated by competition experiments.

Protic addition to aminoboranes

Aminoboranes can be protonated by various acids to make borenium ions. This synthetic method was developed in 1983 by Narula and Noth who used triflic acid to protonate 1,3-dimethyl-2-(dimethylamino)-1,3,2-diazaborolidine; however, they were unable to crystallize and structurally characterize this particular cation. Protonation of non-Lewis acidic oxazaborolidines results in the generation of borenium ions that can be used as enantioselective Diels–Alder catalysts. These N-protonated borenium species have been characterized by

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which atomic nucleus, nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing ...

Other methods

Borenium ions can also be made through other methods such as the addition of base to a dicoordinate borinium ion or by metathesis with salts with weakly coordinating anions such as Ag l[OC(CF₃)₃sub>4">C(CF<sub>3<_sub>)<sub>3<_sub>.html" ;"title="l[OC(CF₃)₃">l[OC(CF₃)₃sub>4or Li l[OC(CF₃)₃sub>4].

Structure and electronics

A number of borenium ions have been structurally characterized through x-ray crystallography. The structures of borenium ions generally have two short bonds and one longer bond which is characteristic of a

dative bond In coordination chemistry, a coordinate covalent bond, also known as a dative bond, dipolar bond, or coordinate bond is a kind of two-center, two-electron covalent bond in which the two electrons derive from the same atom. The bonding of metal io ...

. The electron-deficient nature of the boron center of many borenium ions has been confirmed by computational and experimental studies. A Natural Population Analysis treatment of many borenium ions show that the boron center does indeed carry a significant positive charge. For example, the BH₂NH₃⁺ cation has a natural charge of +0.687 on boron. Depending on the nature of the ligands around the central boron, this positive charge can be localized on the boron center or delocalized across the entire molecule. In some cases, pi-donating ligands arranged in the plane of the boron's empty p orbital can act to stabilize the electron deficiency of the boron.

Density functional theory Density functional theory (DFT) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (principally the ground state) of many-body ...

(DFT) calculations of isolable borenium ions show that the strongly Lewis acidic boron can be stabilized by pi-donation from

aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...

substituents such as pyridine. Bh2nhc nocv

N-heterocyclic carbene A persistent carbene (also known as stable carbene) is an organic molecule whose natural resonance structure has a carbon atom with octet rule, incomplete octet (a carbene), but does not exhibit the tremendous instability typically associated with ...

s (NHCs) can also be used to stabilize borenium ions through pi-conjugation, albeit acting as weaker pi-donors than neutral N-donors. The interaction energy between a BH₂⁺ fragment and various NHCs has been calculated using the extended

transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked w ...

method for energy decomposition analysis combined with the natural orbitals for chemical valence (NOCV) theory. This analysis showed a net pi-donating effect of the NHC ligand – in this case, the positive charge is delocalized over the entire pi system rather than localized on the boron. In other cases the dative ligand has been observed to be twisted out of the BR₃ plane due to steric crowding. This nonplanar geometry leads to a reduction in pi-donation to the boron center, making it even more electron-deficient. It has been found that increased localization of charge on the boron increases the Lewis acidity of the borocation. The

Gutmann–Beckett method In chemistry, the Gutmann–Beckett method is an experimental procedure used by chemists to assess the Lewis acidity of molecular species. Triethylphosphine oxide (, TEPO) is used as a probe molecule and systems are evaluated by 31P-NMR spectr ...

has been used by many researchers in this field to benchmark the Lewis acidities of these cations. Early crystal structures of borenium cations indicate that the corresponding anion is non-coordinating. Further studies have shown that the reactivity of borocations is highly tied to the identity of its counter ion. In catalytic applications, weakly coordinating anions have allowed for the most active borenium catalysts. A commonly used counter ion for borenium cations is tetrakis(pentafluorophenyl)borate, B(C₆F₅)₄⁻; however, other counterions such as AlCl₄⁻, halides, and triflate are also possible. The synthetic viability of a borenium ion is often determined by its reactivity relative to its counterion. Halides are often unable to stabilize borenium ions, preferring instead to coordinate to the boron center to make a tetracoordinate species. A systematic evaluation of counterion effects on the synthetic viability of NHC-dicholoroborenium ions was conducted by Muthaiah and coworkers in 2013.

Reactivity and applications

Borenium ions are highly Lewis acidic. Their Lewis acidity is of the boron atom is determined by the electronic and steric effects of its ligands.

Hydrogen activation and FLP chemistry

(NHC)-stabilized borenium ions have been demonstrated to be potent metal-free H₂ activation and

hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to redox, reduce or Saturated ...

catalysts. Unlike the neutral boranes typically used in

frustrated Lewis pair A frustrated Lewis pair (FLP) is a compound or mixture containing a Lewis acid and a Lewis base that, because of steric hindrance, cannot combine to form a classical adduct. Many kinds of FLPs have been devised, and many simple substrates exhibit ...

(FLP) chemistry of this type, borenium ions are inherently electrophilic and do not require electron-withdrawing ligands to perform these small-molecule activations. Because electron-withdrawing substituents can hamper

hydride In chemistry, a hydride is formally the anion of hydrogen (H−), a hydrogen ion with two electrons. In modern usage, this is typically only used for ionic bonds, but it is sometimes (and has been more frequently in the past) applied to all che ...

delivery during hydrogenation catalysis, borenium ions can be more potent catalysts than neutral boron species because they are effective hydride donors. Indeed, in 2012, Stephan and coworkers were able to develop a borenium-based FLP system capable of activating H₂ stoichiometrically in the presence of

phosphine Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...

. In 2015, Devillard ''et al.'' synthesized a naphthyl-bridged intramolecular borenium-containing FLP capable of activating H₂ with concomitant hydrogenolysis of a mesityl ligand. A second-order perturbation theory analysis of the

natural bond orbital In quantum chemistry, a natural bond orbital or NBO is a calculated ''bonding orbital'' with maximum electron density. The NBOs are one of a sequence of natural localized orbital sets that include "natural atomic orbitals" (NAO), "natural hybrid o ...

s (NBOs) of the intermediate in this reaction involved with H₂ activation showed a 281.8 kcal/mol interaction between the sigma bond of H₂ and the 2p orbital of the cationic boron. Borenium ions have also been used catalytically for various hydrogenations. Stephan and coworkers were able to use a borenium ion catalyst to activate H₂ catalytically to be used for

imine In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bon ...

hydrogenation. A similar NHC-stabilized borenium ion was used to catalyze the

enantioselective In chemistry, stereoselectivity is the property of a chemical reaction in which a single reactant forms an unequal mixture of stereoisomers during a non- stereospecific creation of a new stereocenter or during a non-stereospecific transformation o ...

reduction of ketimines. In this example, enantioselectivity was afforded through the use of a

chiral Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek language, Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is dist ...

NHC ligand. It has been shown that the steric and electronic properties of the NHC ligand used in these borenium catalysts is of great importance to catalytic activity: NHCs that were too bulky prevented intermolecular hydride delivery and ligands that were highly electron donating weakened the borenium cation's ability to act as a Lewis acid.

Enantioselective catalysis

Borenium ions have been used as metal-free

catalysts for a number of organic transformations. An early example of such is the Corey–Itsuno reduction. One proposed mechanism for this enantioselective reduction involves the ''in situ'' generation of a borenium-like species using BH₃ as a Lewis acid. Further work on borenium ions generated from neutral oxazaborolidines has expanded the scope of their applications. In 2002, it was reported by E. J. Corey and coworkers that N-protonation of non-Lewis acidic oxazaborolidines results in the generation of borenium ions which can catalyze the enantioselective

Diels–Alder reaction In organic chemistry, the Diels–Alder reaction is a chemical reaction between a Conjugated system, conjugated diene and a substituted alkene, commonly termed the Diels–Alder reaction#The dienophile, dienophile, to form a substituted cyclohexe ...

of 1,3-dienes with 2-methacrolein or 2-bromoacrolein. This particular borenium ion could be made ''in situ'' by protonating a neutral oxazaborolidine with triflic acid. Corey and coworkers suggest that the stereoselectivity of this reaction is a result of aldehyde-catalyst association in the pre-transition state which governs stereoselectivity. The use of borenium ions as Diels–Alder catalysts has been further extended to the use of borenium

ionic liquid An ionic liquid (IL) is a salt (chemistry), salt in the liquid state at ambient conditions. In some contexts, the term has been restricted to salts whose melting point is below a specific temperature, such as . While ordinary liquids such as wate ...

s as catalysts for the Diels–Alder reaction by Matuszek ''et al.'' in 2017. Borenium catalyzed diels alder

Electrophilic aromatic borylation

Borenium ions have also been implicated as intermediates in electrophilic aromatic

borylation Metal-catalyzed C–H borylation reactions are transition metal catalyzed organic reactions that produce an organoboron compound through functionalization of aliphatic and aromatic C–H bonds and are therefore useful reactions for carbon–hydroge ...

reactions. In many examples of this reaction, a catalyst is used to activate a borane, producing a highly reactive borenium ion. The formation of this highly electrophilic species drives the formation of the

Wheland intermediate An arenium ion in organic chemistry is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution. For historic reasons this complex is also called a Wheland intermediate, after American chemist George ...

, a key step in the electrophilic aromatic addition mechanism. wIn 2013, Stahl ''et al.'' used a ruthenium(II) thiolate catalyst to generate borenium ions capable of effecting direct borylation of nitrogen-containing heterocycles. In 2017, Oestreich and coworkers developed a metal-free method for effecting this transformation. In their work, B(C₆H₅)₃ was used to activate catecholborane, generating a borenium ion capable of borylating various electron-rich heterocycles.

Hydroboration

The electrophilicity of borenium ions can drive the ''trans''-

hydroboration In organic chemistry, hydroboration refers to the addition of a hydrogen-boron bond to certain double and triple bonds involving carbon (, , , and ). This chemical reaction is useful in the organic synthesis of organic compounds. Hydroboration ...

alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...

s. In 2016, McGough ''et al.'' were able to successfully accomplish metal-free ''trans-''hydroboration with a variety of arylacetylene substrates using a borenium ion electrophile and B(C₆F₅)₃ as a catalyst. Enantioselective hydroboration

Polymerization catalysis

Borenium ions have been shown to form

s capable of catalyzing the

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...

of polyalphaolefins (PAOs). While not yet widely adopted by industry, this technology could provide an alternative to the use of BF₃, a toxic and corrosive gas, in the industrial synthesis of PAOs.

Borinium cations

Borinium ions have the formula X₂sup>+, where X⁻ is usual a bulky amide (R₂N⁻). They have linear geometry at boron and are coordinatively unsaturated.

Boronium cations

Boronium ions have the formula ₂BR₂sup>+ (L =

). Boronium ions are tetrahedral and coordinatively saturated. A well-known example is H₃N)₂BH₂sup>+. Reaction of

diborane Diborane(6), commonly known as diborane, is the chemical compound with the formula . It is a highly toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has att ...

with

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

mainly gives (diammoniodihydroboronium tetrahydroborate).

Related boron cations

Other non-classical boron cations are mononuclear boron di- and tri-cations with formula ₃BXsup>2+ and ₄Bsup>3+, respectively. Other reported boron cations are dibora-dications (bis(borenium) dications), some examples are depicted below. Dibora dication

References

{{Reflist Boron compounds