and
catalysis.
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 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.
Nucleophilic dissociation 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.
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[Al[OC(CF3)3]4] or Li[Al[OC(CF3)3]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. 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 (DFT) calculations of isolable borenium ions show that the strongly
Lewis acidic boron can be stabilized by pi-donation from
aromatic substituents such as pyridine. The left structure shows loss of electron density; the right structure shows gain of electron density.
N-heterocyclic carbenes (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 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.
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 N-heterocyclic carbene (NHC)-stabilized borenium ions have been demonstrated to be potent metal-free H2 activation and
hydrogenation catalysts. Unlike the neutral boranes typically used in
frustrated Lewis pair (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 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. 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 hydrogenation. 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. 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 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. .
Hydroboration The electrophilicity of borenium ions can drive the
trans-
hydroboration of
alkynes. 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. File:Enantioselective hydroboration.png|alt=|center|thumb|596x596px|Mechanism of hydroboration with a borenium ion electrophile. ==Borinium cations==