Synthesis and Application of a Chiral Diborate

"Homoleptic" Tetracoordinate Boron Compounds

"Homoleptic" tetracoordinate boron compounds, in which the central boron atom links to four identical atoms, are a special and important family of boron compounds. During the past decades, they have been extensively employed in inorganic, organic, macromolecular, and materials chemistry. Many of them exhibit a diverse range of outstanding properties, and therefore, the synthesis and application of those compounds have emerged as a hot research topic in modern boron chemistry. This review summarizes and discusses the "homoleptic" tetracoordinate boron compounds, which are organized according to the kinds of atoms coordinated to the central boron.

Probing Catalyst Function - Electronic Modulation of Chiral Polyborate Anionic Catalysts

Boroxinate complexes of VAPOL and VANOL are a chiral anionic platform that can serve as a versatile staging arena for asymmetric catalysis. The structural underpinning of the platform is a chiral polyborate core that covalently links together alcohols (or phenols) and vaulted biaryl ligands. The polyborate platform is assembled in situ by the substrate of the reaction, and thus a multiplex of chiral catalysts can be rapidly assembled from various alcohols (or phenols) and bis-phenol ligands for screening of catalyst activity. In the present study, variations in the steric and electronic properties of the phenol/alcohol component of the boroxinate catalyst are probed to reveal their effects on the asymmetric induction in the catalytic asymmetric aziridination reaction. A Hammett study is consistent with a mechanism in which the two substrates are hydrogen-bonded to the boroxinate core in the enantiogenic step. The results of the Hammett study are supported by a computational study in which it is found that the H-O distance of the protonated imine hydrogen bonded to the anionic boroxinate core decreases with an increase in the electron releasing ability of the phenol unit incorporated into the boroxinate. The results are not consistent with a mechanism in which the boroxinate catalyst functions as a Lewis acid and activates the imine by a Lewis acid/Lewis base interaction.

Ionic Covalent Organic Frameworks for Energy Devices

Covalent organic frameworks (COFs) are a class of porous crystalline materials whose facile preparation, functionality, and modularity have led to their becoming powerful platforms for the development of molecular devices in many fields of (bio)engineering, such as energy storage, environmental remediation, drug delivery, and catalysis. In particular, ionic COFs (iCOFs) are highly useful for constructing energy devices, as their ionic functional groups can transport ions efficiently, and the nonlabile and highly ordered all-covalent pore structures of their backbones provide ideal pathways for long-term ionic transport under harsh electrochemical conditions. Here, current research progress on the use of iCOFs for energy devices, specifically lithium-based batteries and fuel cells, is reviewed in terms of iCOF backbone-design strategies, synthetic approaches, properties, engineering techniques, and applications. iCOFs are categorized as anionic COFs or cationic COFs, and how each of these types of iCOFs transport lithium ions, protons, or hydroxides is illustrated. Finally, the current challenges to and future opportunities for the utilization of iCOFs in energy devices are described. This review will therefore serve as a useful reference on state-of-the-art iCOF design and application strategies focusing on energy devices.

Chiral anionic layers in tartramide spiroborate salts and variable solvation for [NR4][B(TarNH2)2] (R = Et, Pr or Bu)

The spiroborate anion, namely, 2,3,7,8-tetracarboxamido-1,4,6,9-tetraoxa-5λ⁴-boraspiro[4.4]nonane, [B(TarNH₂)₂]^-, derived from the diol L-tartramide TarNH₂, [CH(O)(CONH₂)]₂, shows a novel self-assembly into two-dimensional (2D) layer structures in its salts with alkylammonium cations, [NR₄]⁺ (R = Et, Pr and Bu), and sparteinium, [HSpa]⁺, in which the cations and anions are segregated. The structures of four such salts are reported, namely, the tetrapropylazanium salt, C₁₂H₂₈N⁺·C₈H₁₂BN₄O₈^-, the tetraethylazanium salt hydrate, C₈H₂₀N⁺·C₈H₁₂BN₄O₈^-·6.375H₂O, the tetrabutylazanium salt as the ethanol monosolvate hemihydrate, C₁₆H₃₆N⁺·C₈H₁₂BN₄O₈^-·C₂H₅OH·0.5H₂O, and the sparteinium (7-aza-15-azoniatetracyclo[7.7.1.0^2,7.0^10,15]heptadecane) salt as the ethanol monosolvate, C₁₅H₂₇N₂⁺·C₈H₁₂BN₄O₈^-·C₂H₅OH. The 2D anion layers have preserved intermolecular hydrogen bonding between the amide groups and a typical metric repeat of around 10 × 15 Å. The constraint of matching the interfacial area organizes the cations into quite different solvated arrangements, i.e. the [NEt₄] salt is highly hydrated with around 6.5H₂O per cation, the [NPr₄] salt apparently has a good metric match to the anion layer and is unsolvated, whilst the [NBu₄] salt is intermediate and has EtOH and H₂O in its cation layer, which is similar to the arrangement for the chiral [HSpa]⁺ cation. This family of salts shows highly organized chiral space and offers potential for the resolution of both chiral cations and neutral chiral solvent molecules.

A chiral spiroborate anion from diphenyl-l-tartramide [B{l-Tar(NHPh)2}2]−applied to some challenging resolutions

A chiral spiroborate anion [B{l-Tar(NHPh)₂}₂]⁻is effective in challenging high yield, 1-pot resolutions, as for the S-2-phenylpropylammonium salt shown.

Isolation and enantiostability of the B-chiral bis(salicylato)borate anions [BR(Sal)2] and [BS(Sal)2]

Both chiral hands of bis(salicylato)borate anion [B_S(Sal)₂] and [B_R(Sal)₂] are isolated and CD spectroscopy indicates they can be configurationally stable.

Chiral bis(mandelato)borate salts for resolution via metathesis crystallization

Spiroborate anions have potential for crystallization or resolution and chiral bis(mandelato)borate anions can be used for the efficient resolution of a diverse range of racemic cations via diastereomeric salt formation. The syntheses, X-ray crystal structures and solubilities of three chiral bis(mandelato)borate salts, namely poly[[aqua-μ3-bis[(R)-mandelato]borato-lithium(I)] monohydrate], [Li(C16H12BO6)(H2O)] n or Li[B(R-Man)2]·H2O, (1), ammonium bis[(R)-mandelato]borate, NH4 +·C16H12BO6 − or NH4[B(R-Man)2], (2), and tetra-n-butylammonium bis[(R)-mandelato]borate, C16H36N+·C16H12BO6 − or NBu4[B(R-Man)2], (3), are reported. They all have a B S configuration and show a reasonably well-conserved anion geometry. The main conformational variation is the orientation of the two phenyl groups, supporting the idea that [B(Man)2]− is a semi-rigid anion. The salts are differentially soluble in a range of solvents, meaning they could be useful as reagents for resolution via a metathesis crystallization approach.

Ionic Covalent Organic Frameworks with Spiroborate Linkage

A novel type of ionic covalent organic framework (ICOF), which contains sp(3)  hybridized boron anionic centers and tunable countercations, was constructed by formation of spiroborate linkages. These ICOFs exhibit high BET surface areas up to 1259 m(2)  g(-1) and adsorb a significant amount of H2 (up to 3.11 wt %, 77 K, 1 bar) and CH4 (up to 4.62 wt %, 273 K, 1 bar). Importantly, the materials show good thermal stabilities and excellent resistance to hydrolysis, remaining nearly intact when immersed in water or basic solution for two days. The presence of permanently immobilized ion centers in ICOFs enables the transportation of lithium ions with room-temperature lithium-ion conductivity of 3.05×10(-5)  S cm(-1) and an average Li(+) transference number value of 0.80±0.02. Our approach thus provides a convenient route to highly stable COFs with ionic linkages, which can potentially serve as absorbents for alternative energy sources such as H2, CH4, and also as solid lithium electrolytes/separators for the next-generation lithium batteries.

Bis(mandelato)borate: an effective, inexpensive spiroborate anion for chiral resolution

Bis(mandelato)borate [B(Man)2](-) (R- or S-) anions are simply prepared and appear widely effective for resolution of racemic cations. Three examples demonstrate their scope; the alkaloid tetrahydropalmatine (THP), 1,2-diaminopropane (1,2-dap) and the metal-organic complex [Co(phen)3](3+) are readily resolved, either by a facile one-pot procedure, or via counter-ion metathesis.