Boronium Cation Research Articles

Borenium Ion Equivalents Stabilized by BICAAC and Their Implementation as Catalysts in Hydrosilylation of Carbonyls.

Hydride abstraction from the borane adduct, (BICAAC)‧BH3 afforded the hydride bridged dinuclear borenium ion equivalent complexes 1 and 2 that have been characterized by various spectroscopic and spectrometric techniques followed by the assessment of Lewis acidity using the Gutmann-Beckett method. The single crystal X-ray structure of complex 2 revealed the presence of discrete ions in the solid state. The complex (BICAAC)‧BH2(OTf) (3), obtained from the reaction of (BICAAC)‧BH3 with MeOTf, forms the corresponding boronium cations [(BICAAC)‧BH2(L)]+(OTf)- on reaction with Lewis bases (L = pyridine (4) and DMAP (5)). Complexes 1 and 2 demonstrated notable catalytic activity in the hydrosilylation of a diverse array of carbonyls using 1.0 mol% catalyst loading (achieving the highest turnover frequency (TOFmax) of up to 1200 h-1 with benzaldehyde. A broad substrate scope has been presented for aldehydes and ketones decorated with various electron-donating and withdrawing substituents along with this the hydrosilylation of a few para-quinone methides has also been presented.

Synthesis and Characterization of Neoteric Boronium Ionic Liquids

Ionic liquids (ILs) are highly tailorable materials with unique physical and chemical properties that set them apart from conventional organic solvents. As the library of readily accessible ILs continues to grow, so too does their relevance in applications ranging from material processing to electrochemical energy storage as solvents capable of accessing new chemistries disallowed by traditional chemicals. While a great deal of interest has been directed towards imidazolium and quaternary ammonium based ionic liquids, there are other understudied classes of cations which have potentially favorable properties for energy related applications. One such class is that with boronium cations. These cations have a unique structure with a formally negative boron flanked by positive nitrogens. This inherently zwitterionic structure presents interesting possibilities for electrochemical applications. To date only a handful of boronium cation-based ionic liquids have been thoroughly characterized despite exhibiting impressive electrochemical stabilities (> 5.0 V). In the present study we synthesized a series of ILs with novel boronium cations coupled with the bis(trifluoro-methanesulfonyl)imide anion. We then characterized the electrochemical and physical properties of these boronium ionic liquids by techniques such as cyclic voltammetry, broadband dielectric spectroscopy, oscillatory shear rheology, and thermogravimetric analysis. We will discuss how systematic variations in boronium cation structure impacted electrochemical and physical properties.

Electrochemical Approach for Studying Local Dynamic Heterogeneity of Ionic Liquids Applied to Neoteric Boroniums

Ionic liquids (ILs) are liquid salts (below ca. 100 °C) whose ions can be designed to impart unique physicochemical properties. An emergent class of ILs based on the boronium cation (BILs) can have unique zwitterionic structure within the cation that presents potential for electrochemical applications. In the present study we first show how broadband dielectric spectroscopy (BDS) can be used to evaluate local dynamic heterogeneity in imidazolium-based ionic liquid systems. We then apply this approach to a series of BILs (coupled with the bis(trifluoro-methanesulfonyl)imide ([Tf2N]–) anion), whose cationic structures show similar features, suggesting may have potential to promote more favorable ion transport for application in supercapacitors. Preliminary cyclic voltammetry and electrochemical impedance spectroscopy data will also be shown that evaluates the chemical stability of each BILs when combined with common electrode materials.

Physical and Electrochemical Analysis of N-Alkylpyrrolidinium-Substituted Boronium Ionic Liquids.

In this work, a series of novel boronium-bis(trifluoromethylsulfonyl)imide [TFSI-] ionic liquids (IL) are introduced and investigated. The boronium cations were designed with specific structural motifs that delivered improved electrochemical and physical properties, as evaluated through cyclic voltammetry, broadband dielectric spectroscopy, densitometry, thermogravimetric analysis, and differential scanning calorimetry. Boronium cations, which were appended with N-alkylpyrrolidinium substituents, exhibited superior physicochemical properties, including high conductivity, low viscosity, and electrochemical windows surpassing 6 V. Remarkably, the boronium ionic liquid functionalized with both an ethyl-substituted pyrrolidinium and trimethylamine, [(1-e-pyrr)N111BH2][TFSI], exhibited a 6.3 V window, surpassing previously published boronium-, pyrrolidinium-, and imidazolium-based IL electrolytes. Favorable physical properties and straightforward tunability make boronium ionic liquids promising candidates to replace conventional organic electrolytes for electrochemical applications requiring high voltages.

Physical and Electrochemical Analysis of Novel Boronium Cation-Based Ionic Liquids

The designer nature of ionic liquids makes them a promising material for applications spanning disciplines from material processing to energy generation. While a great deal of interest has been directed towards dialkylimidazolium-based ionic liquids, there are many understudied classes of cations with favorable properties for energy related applications, such as disubstituted boroniums. To date only a handful of boronium cation-based ionic liquids have been thoroughly characterized despite exhibiting electrochemical stabilities (> 5.0 V) far surpassing what is seen with their alkylimidazolium and alkylpyrrolidinium counterparts. This work investigates a series of novel boronium-bistriflimide [TFSI-] ionic liquids using electrochemical and physical characterization techniques such as cyclic voltammetry, broadband dielectric spectroscopy, oscillatory shear rheology, and thermogravimetric analysis. Systematic variations in boronium cation structure were employed to help identify the structural motifs that lead to optimal electrochemical and physical properties. The best performing boronium-based ionic liquids were then tested for initial lithium stability to further probe their potential for use in battery applications.

Physical and Electrochemical Analysis of Novel Boronium Cation-Based Ionic Liquids

This work investigates a series of novel boronium-bistriflimide [TFSI-] ionic liquids using electrochemical and physical characterization techniques such as cyclic voltammetry, broadband dielectric spectroscopy, shear rheology, and thermogravimetric analysis. Systematic variations in boronium cation structure were employed to help identify the structural motifs that lead to optimal electrochemical and physical properties. The best performing boronium-based ionic liquids were then tested for initial lithium stability to further probe their potential for use in battery applications.

NHC induced radical formation via homolytic cleavage of B-B bonds and its role in organic reactions.

New borylation methodologies have been reported recently, wherein diboron(4) compounds apparently participate in free radical couplings via the homolytic cleavage of the B–B bond. We report herein that bis-NHC adducts of the type (NHC)2·B2(OR)4, which are thermally unstable and undergo intramolecular ring expansion reactions (RER), are sources of boryl radicals of the type NHC–BR2˙, exemplified by Me2ImMe·Bneop˙ 1a (Me2ImMe = 1,3,4,5-tetramethyl-imidazolin-2-ylidene, neop = neopentylglycolato), which are formed by homolytic B–B bond cleavage. Attempts to apply the boryl moiety 1a in a metal-free borylation reaction by suppressing the RER failed. However, based on these findings, a protocol was developed using Me2ImMe·B2pin23 for the transition metal- and additive-free boryl transfer to substituted aryl iodides and bromides giving aryl boronate esters in good yields. Analysis of the side products and further studies concerning the reaction mechanism revealed that radicals are likely involved. An aryl radical was trapped by TEMPO, an EPR resonance, which was suggestive of a boron-based radical, was detected in situ, and running the reaction in styrene led to the formation of polystyrene. The isolation of a boronium cation side product, [(Me2ImMe)2·Bpin]+I−7, demonstrated the fate of the second boryl moiety of B2pin2. Interestingly, Me2ImMe NHC reacts with aryl iodides and bromides generating radicals. A mechanism for the boryl radical transfer from Me2ImMe·B2pin23 to aryl iodides and bromides is proposed based on these experimental observations.

Spectroscopic and Computational Study of Boronium Ionic Liquids and Electrolytes.

Boronium cation-based ionic liquids (ILs) have demonstrated high thermal stability and a >5.8 V electrochemical stability window. Additionally, IL-based electrolytes containing the salt LiTFSI have shown stable cycling against the Li metal anode, the "Holy grail" of rechargeable lithium batteries. However, the basic spectroscopic characterisation needed for further development and effective application is missing for these promising ILs and electrolytes. In this work, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and density functional theory (DFT) calculations are used in combination to characterise four ILs and electrolytes based on the [NNBH2 ]+ and [(TMEDA)BH2 ]+ boronium cations and the [FSI]- and [TFSI]- anions. By using this combined experimental and computational approach, proper understanding of the role of different ion-ion interactions for the Li cation coordination environment in the electrolytes was achieved. Furthermore, the calculated vibrational frequencies assisted in the proper mode assignments for the ILs and in providing insights into the spectroscopic features expected at the interface created when they are adsorbed on a Li(001) surface. A reproducible synthesis procedure for [(TMEDA)BH2 ]+ is also reported. The fundamental findings presented in this work are beneficial for any future studies that utilise IL based electrolytes in next generation Li metal batteries.

The (In‐)Stability of the Ionic Liquids [(TMEDA)BH2][TFSI] and −[FSI] on the Li(001) Surface

AbstractElectrolytes that can enable the use of a Li metal anode at a vast 3860 mAh/g, in place of currently used graphite anodes (372 mAh/g), are required for the advancement of next‐generation rechargeable Li batteries. Both quaternary ammonium and boronium (trimethylamine)(dimethylethylamine)dihydroborate [NNBH2]+ cation‐based ionic liquids (ILs) show high electrochemical stability windows and thermal stability for use in Li batteries. Cyclization of the former cation shows improved electrolyte stability compared to the open‐chain counterpart. However, it is not known whether this is the case for the cyclic derivative of [NNBH2]+, N,N,N’,N’‐tetramethylethylenediamine)dihydroborate [(TMEDA)BH2]+. Here, the details of the initial stages of solid−electrolyte interphase (SEI) layer formation on a lithium metal surface, Li(001), for the [(TMEDA)BH2]+ based ILs are revealed using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. These indicate that [(TMEDA)BH2]+ remains intact, displaying a similarly weak interaction with the Li metal surface as the open‐chain analogue. The chemical stability shown by the boronium cation indicates spontaneous and unwanted side reactions with the Li anode are unlikely to occur, which could help to facilitate long‐term cycling stability of the battery. Altogether, the findings suggest the [(TMEDA)BH2]+ ILs, like their [NNBH2]+ IL counterparts, are viable candidates for rechargeable Li metal batteries.

Comparing the Physicochemical, Electrochemical, and Structural Properties of Boronium versus Pyrrolidinium Cation-Based Ionic Liquids and Their Performance as Li-Ion Battery Electrolytes

Ionic liquid-type electrolytes (ILELs) based on boronium cations, (trimethylamine)(dimethylethylamine)dihydroborate [N111N112BH2]+, are revisited as they have barely been studied for Li battery app...

Development of an Unsymmetrical Cyclopropenimine-Guanidine Platform for Accessing Strongly Basic Proton Sponges and Boron-Difluoride Diaminonaphthalene Fluorophores.

An unsymmetrical guanidine-cyclopropenimine proton sponge DAGUN and the related BF2 -chelate DAGBO are reported. Insight into the structural, electronic, bonding and photophysical properties of these two molecules are presented. Joint experimental and theoretical studies reveal the protonated form of DAGUN possesses an intramolecular N⋅⋅⋅H-N hydrogen bond which affords a high experimental pKBH+ of 26.6 (computed=26.3). Photophysical studies show that in solution DAGUN displays a green emission at 534 nm, with a large Stokes shift of 235 nm (14,718 cm-1 ). In contrast, the conjugate acid DAGUN-H+ is only weakly emissive due to attenuated intramolecular charge transfer. X-ray diffraction studies reveal that DAGBO contains a stable tetracoordinate boronium cation, reminiscent of the well-established BODIPY family of dyes. In solution, DAGBO exhibits a strong blue emission at 450 nm coupled with a large Stokes shift (Δλ=158 nm, Δν=11,957 cm-1 ) and quantum yield of 62 %, upon excitation at 293 nm. DAGBO sets the stage as the first entry into a new class of boron-difluoride diaminonaphthalenes (BOFDANs) that represent highly fluorescent and tunable next-generation dyes with future promise for biosensing and bioimaging applications.

Regioselective 1,4-hydroboration of pyridines catalyzed by an acid-initiated boronium cation.

The reaction of the commercially available ammonium salt NH4BPh4 with a pyridine-activated pinacolborane species generates a boronium cation that facilitates the 1,4-selective hydroboration of pyridines in polar solvents. This catalytic reaction is amenable to a host of reactive functional groups and provides access to sterically bulky hydroboration products, previously inaccessible by metal-free routes. Further, the regioselectivity of this reaction can be altered by reducing the polarity of the reaction solvent, resulting in greater proportions of the 1,2-hydroboration product.

Mechanistic Details of Ru–Bis(pyridyl)borate Complex Catalyzed Dehydrogenation of Ammonia–Borane: Role of the Pendant Boron Ligand in Catalysis

The role of pendant boron ligands in ammonia–borane (AB) dehydrogenation has been investigated using hybrid density functional theory for two very efficient ruthenium-based catalysts developed by Williams and co-workers. Our findings reveal that the catalytic action initiates through opening of the labile metal–ligand bridging group associated with the boron-based pendant ligand arm for both catalysts. In case of the hydroxyl-bridged catalyst, the ligand (B–OH moiety) backbone plays an active role along with the metal center to perform concerted dehydrogenation of ammonia–borane by overcoming a free energy activation barrier of 24.3 kcal/mol, and this dehydrogenation step is the rate-determining step of the catalytic cycle. However, for the trifluoroacetate-bridged complex, H2 is released in a stepwise fashion with active participation of the solvent. It involves formation of a boronium cation with a rate-determining free energy activation barrier of 23.7 kcal/mol for the solvent-assisted B–H bond-breakin...

Extremely Facile Transformations of Tris(3,5‐dimethylpyrazolyl)borate: a Bidentate Nitrogen Ligand and a C2‐Chiral Cation

AbstractTwo novel transformations are reported for the well‐known hydridotris(3,5‐dimethylpyrazolyl)borate (Tp*–). The uncommon Lewis base abstractor aryldiazonium cation (ArN2+; Ar = p‐Br‐C6H4, p‐MeO‐C6H4) if used in equimolar amounts, removes a hydride from Tp*– in acetone to produce the formal acetone adduct of tris(3,5‐dimethylpyrazolyl)borane (pz*3B·C3H6O, compound 1). Frustrated Lewis pair (FLP) like behavior is thus found for a tris(pyrazolyl)borane. If 2 equiv. of ArN2+ are used, 3,5‐dimethylpyrazolate is now being abstracted from 1, and the resulting intermediate bifunctional borane adds another equivalent of acetone to form cation 2+ {[pz*2B·(C3H6O)2]+, obtained as 2‐BF4}, a boronium cation that is the formal addition product of 2 equiv. of acetone to the bis(3,5‐dimethylpyrazolyl)borinium cation. Compounds 1 and 2+ are characterized by 1H, 13C, and 11B NMR spectroscopy, mass spectrometry, and single‐crystal X‐ray diffraction. Compound 1 is potentially useful as a charge‐neutral bidentate chelate ligand with controlled sterics, and 2+ is a C2‐chiral cation.

Isolation of a Three‐Coordinate Boron Cation with a Boron–Sulfur Double Bond

The reaction of the bulky bis(imidazolin-2-iminato) ligand precursor (1,2-(L(Mes)NH)2-C2H4)[OTs]2 (1(2+) 2[OTs](-); L(Mes) = 1,3-dimesityl imidazolin-2-ylidene, OTs = p-toluenesulfonate) with lithium borohydride yields the boronium dihydride cation (1,2-(L(Mes)N)2-C2H4)BH2[OTs] (2(+) [OTs](-)). The boronium cation 2(+) [OTs](-) reacts with elemental sulfur to give the thioxoborane salt (1,2-(L(Mes)N)2-C2H4)BS[OTs] (3(+) [OTs](-)). The hitherto unknown compounds 1(2+) 2[OTs](-), 2(+) [OTs](-), and 3(+) [OTs](-) were fully characterized by spectroscopic methods and single-crystal X-ray diffraction. Moreover, DFT calculations were carried out to elucidate the bonding situation in 2(+) and 3(+). The theoretical, as well as crystallographic studies reveal that 3(+) is the first example for a stable cationic complex of three-coordinate boron that bears a B=S double bond.

Generation of aminoborane monomers RR'N=BH2 from amine-boronium cations [RR'NH-BH2L](+): metal catalyst-free formation of polyaminoboranes at ambient temperature.

Protonation of MeRNH·BH3 (R = Me or H) with HX (X = B(C6F5)4, OTf, or Cl), followed by immediate, spontaneous H2 elimination, yielded the amine-boronium cation salt [MeRNH·BH2(OEt2)][B(C6F5)4] and related polar covalent analogs, MeRNH·BH2X (X = OTf or Cl). These species can be deprotonated to conveniently generate reactive aminoborane monomers MeRN=BH2 which oligomerize or polymerize; in the case of MeNH2·BH3, the two step process gave poly(N-methylaminoborane), [MeNH-BH2]n.

Formation, Stability, and Structures of Borenium and Boronium Cations Derived from Pentamethylazaferrocene–Boranes by Hydride or Chloride Abstraction Reactions

Hydride abstraction reactions from 1′,2′,3′,4′,5′-pentamethylazaferrocene–boranes (PMAF-boranes, where borane = BH3, 9-bora[3.3.1]bicyclononane) using trityl cation in CH2Cl2 yielded the corresponding borenium cations, whose stability depends on the counteranion of the trityl salt. If the [BF4]− salt was employed, then fluoride abstraction from the anion occurred immediately, yielding PMAF-BF3 and other identifiable byproducts. If the [B(C6F5)4]− salt was employed, then the borenium ions were stable in solution at room temperature and could be characterized by 1H, 13C, and 11B NMR spectroscopy. Double hydroboration of [PMAF-BH2]+ with 1,5-cycloooctadiene yields the borenium ion of PMAF-9-bora[3.3.1]-bicyclononane, [PMAF-9BBN]+. Reaction of [PMAF-BH2]+ with excess PMAF yields the boronium cation [(PMAF)2BH2]+, which was isolated and crystallographically characterized as its [B(C6F5)4]− salt. Reaction of [PMAF-9BBN]+ with 2 equiv of N-methylimidazole leads to the boronium cation [(N-methylimidazole)2(BBN)]+, which was also isolated and crystallographically characterized as its [B(C6F5)4]− salt. Chloride abstraction from PMAF-BCl3 with AlCl3 in CH2Cl2 yielded the borenium cation [PMAF-BCl2]+, which was characterized in solution by 1H, 13C, and 11B NMR spectroscopy. A single-crystal X-ray structure was also obtained of [PMAF-BCl2]+[AlCl4]−, revealing structural changes upon borenium ion formation, including rehybridization of the B center and a bending of the BCl2 moiety toward the Fe center. The Lewis acidity of all three borenium cations was assessed by the Gutmann–Beckett method, and [PMAF-BH2]+ was found to be more acidic than B(C6F5)3. DFT calculations were performed on [PMAF-BH2]+ using the BP86 method with the LANL2DZ basis set for Fe and the TZVP basis set for C, H, N, and B. The optimized structure revealed substantial bending of the BH2 moiety toward Fe with a dip angle of −23.7°. Inspection of the frontier MOs reveal that the electrophilic B center is stabilized through delocalized interactions that involve orbitals on Fe, the pyrrolyl ring, and the pentamethylcyclopentadienyl ring.

ChemInform Abstract: Haloboration of Internal Alkynes with Boronium and Borenium Cations as a Route to Tetrasubstituted Alkenes.

Vinyl boronates are extremely useful precursors, especially for the formation of new C–C bonds by cross-coupling and conjugate addition reactions.1, 2 Although alkyne hydroboration is a powerful synthetic route,3, 4 it is not applicable to the synthesis of trisubstituted vinyl boronates. Thus, alternative regio- and stereospecific methods are needed, particularly for subsequent use in the formation of tetrasubstituted alkenes,5–7 as the production of these important biologically active compounds as single isomers by classical methods is challenging.8 One simple approach to trisubstituted vinyl boronates is the functionalization of internal alkynes by metal-catalyzed 1,2-carboboration9–13 and 1,1-carboboration.14 The introduction of two selectively transformable moieties onto an internal alkyne should enable ready access to tetrasubstituted alkenes by successive cross-coupling reactions. Significant progress has been made in this area, particularly in the dimetalation of internal alkynes to provide two nucleophilic sites of distinct reactivity.15–19 The haloboration of internal alkynes is an attractive alternative to dimetalation, as it generates ambivalent synthetic intermediates that contain both a nucleophilic and an electrophilic position.20 These synthetic intermediates are ideally suited for the diversity-oriented synthesis of tetrasubstituted alkenes. To date, the application of alkyne haloboration with boron trihalides (BX3) has been limited to terminal alkynes, and has proved an effective route to produce trisubstituted alkenes with excellent regio- and stereoselectivity.21–25 The haloboration of internal alkynes is unsuccessful with BCl3, and it is either slow25 or produces isomeric mixtures susceptible to B–C bond cleavage when BBr3 is used.21, 26 Recent calculations found that the haloboration of internal alkynes with BCl3 is endothermic, but as the Lewis acidity of BX3 increases (Cl<Br<I), haloboration becomes exothermic, and the energy of the key transition state is also reduced.27 This result suggested that an increase in the electrophilicity at boron beyond that of BX3 would facilitate the haloboration of internal alkynes.

Dehydrocoupling Reactions of Dimethylamine-Borane by Pt(II) Complexes: A New Mechanism Involving Deprotonation of Boronium Cations

Coordinatively unsaturated Pt(II) complex [Pt(I(t)Bu')(I(t)Bu)](+) stabilized by N-heterocyclic carbene (NHC) ligands dehydrogenates N,N-dimethylamineborane through a mechanism that involves hydride abstraction, assisted by an amine, to yield a platinum-hydride complex [PtH(I(t)Bu')(I(t)Bu)] with concomitant formation of the boronium cation [(NHMe2)2BH2](+). This latter species is very likely in equilibrium with the THF stabilized borenium cation [(NHMe2)(THF)BH2](+), bearing an acidic NH group that is able to protonate the platinum hydride [PtH(I(t)Bu')(I(t)Bu)] releasing H2, the amino borane H2B-NMe2 and regenerating the catalytic [Pt](+) species.

(acridine)BCl2]+: A Borenium Cation That Is a Strong Boron- and Carbon-Based Lewis Acid

[(acridine)BCl2][AlCl4] was synthesized by halide abstraction from (acridine)BCl3 with AlCl3. The hydride ion affinity of the C9 position in [(acridine)BCl2]+ was calculated to be 14 kcal mol–1 greater than that at boron. [(acridine)BCl2][AlCl4] reacts with 1 equiv of acridine to form the strained boronium cation [(acridine)2BCl2][AlCl4] and with P(mesityl)3 by photoinduced one-electron transfer to form the 9,9′-biacridane moiety by radical coupling. A stable frustrated Lewis pair (FLP) was formed on combining [(acridine)BCl2][AlCl4] and 2,4,6-tri-tert-butylpyridine (TBP), which heterolytically activated H2 at 100 °C. The ultimate location of the hydride from H2 activation was the C9 position of acridine and not boron. Carbon Lewis acid based reactivity also occurred when thieno[3,2-b]thiophene was added to the [(acridine)BCl2][AlCl4]/TBP FLP or to [(acridine)2BCl2][AlCl4], with arylation of acridine at C9 observed for both.