Boron Center Research Articles

Design of a Boron-Containing PTHF-Based Solid Polymer Electrolyte for Sodium-Ion Conduction with High Na+ Mobility and Salt Dissociation

Solid polymer electrolytes based on semi-interpenetrating polymer networks (semi-IPNs) were developed for sodium-ion conduction using a boron-centered moiety with polytetrahydrofuran (PTHF) chains. The resulting solid polymer electrolytes (SPEs) combined the anion-trapping properties of the boron with the weakly coordinating PTHF to achieve enhanced salt dissociation and transference number. The structure of the polymer was confirmed by 1H NMR, 13C NMR, and 11B NMR, and the cross-linking reaction with hexamethylene diisocyanate (HMDI) was confirmed by Fourier transform infrared (FTIR) spectroscopy. The mechanical and thermal stabilities of the samples were evaluated, and FTIR spectroscopic analysis showed the efficiency of the boron centers in complexing with ClO4–. Dielectric studies also indicated the predominance of free Na+ in samples. The highest room-temperature ionic conductivity was delivered by boron-containing electrolytes with O/Na: 5, 7.54 × 10–5 S cm–1, and O/Na: 10, 1.13 × 10–5 S cm–1. Linear sweep voltammetry (LSV) revealed suitable electrochemical stability against Na metal, and the measured transference number was 0.88, confirming that electrolytes benefit from the looser coordination of Na+ ions with PTHF chains combined with the anion-trapping performance of boron moieties, indicating a potential direction in polymer electrolyte design.

The Conversion of Superoxide to Hydroperoxide on Cobalt(III) Depends on the Structural and Electronic Properties of Azole-Based Chelating Ligands.

Conversion from superoxide (O2−) to hydroperoxide (OOH−) on the metal center of oxygenases and oxidases is recognized to be a key step to generating an active species for substrate oxidation. In this study, reactivity of cobalt(III)-superoxido complexes supported by facially-capping tridentate tris(3,5-dimethyl-4-X-pyrazolyl)hydroborate ([HB(pzMe2,X)3]−; TpMe2,X) and bidentate bis(1-methyl-imidazolyl)methylborate ([B(ImN-Me)2Me(Y)]−; LY) ligands toward H-atom donating reagent (2-hydroxy-2-azaadamantane; AZADOL) has been explored. The oxygenation of the cobalt(II) precursors give the corresponding cobalt(III)-superoxido complexes, and the following reaction with AZADOL yield the hydroperoxido species as has been characterized by spectroscopy (UV-vis, resonance Raman, EPR). The reaction of the cobalt(III)-superoxido species and a reducing reagent ([CoII(C5H5)2]; cobaltocene) with proton (trifluoroacetic acid; TFA) also yields the corresponding cobalt(III)-hydroperoxido species. Kinetic analyses of the formation rates of the cobalt(III)-hydroperoxido complexes reveal that second-order rate constants depend on the structural and electronic properties of the cobalt-supporting chelating ligands. An electron-withdrawing ligand opposite to the superoxide accelerates the hydrogen atom transfer (HAT) reaction from AZADOL due to an increase in the electrophilicity of the superoxide ligand. Shielding the cobalt center by the alkyl group on the boron center of bis(imidazolyl)borate ligands hinders the approaching of AZADOL to the superoxide, although the steric effect is insignificant.

Boosting nitrogen reduction reaction with boron sites supported by defective Mo2B2O2 MBene

Developing a novel strategy based on boron centers as active sites for high-efficient catalytic nitrogen reduction reaction (NRR) to produce ammonia is highly desired. Herein, we have systematically investigated the properties of single and double B atoms supported on defective Mo2B2O2 MBene (B@Mo2B2O2 and B2@Mo2B2O2) for efficient N2 capture and reduction. Our results suggest that B@Mo2B2O2 and B2 @Mo2B2O2 show high activity for N2 reduction through the enzymatic pathway with quite low limiting potentials of − 0.42 V and − 0.34 V, respectively. We highlight that B2 @Mo2B2O2 exhibit much higher selectivity to suppress the competitive HER reaction than B@Mo2B2O2, which is illuminated to be originated from the synergistic effect and covalent bonding between the double B atoms. Further analysis on electronic structures suggests that the limiting potentials of NRR can be well described by the p-band center of B atom as well as its overlap position with N2-p orbitals. Finally, we reveal that compared with B@Mo2B2O2, the double B active sites (B2 @Mo2B2O2) can selectively modulate the adsorption free energy of *NH3, further leading to a lower limiting potential. We believe that the present findings provide depth insight into the superior catalytic performance of B center catalysts for NRR and further guide the design of single/double-atom catalysts.

In-depth analysis of the photophysics of BOPAHY dyes in solution, glass and film

BOPAHY dyes are a recently synthesized class of asymmetric highly fluorescent double boron complexed dyes. The photophysics of 5-substituted BOPAHY dyes was investigated by steady-state and time-resolved spectroscopy in liquid solution, dissolved in an organic glass and in neat spin-coated films. In the latter samples some derivatives show efficient fluorescence. The experimental results were compared with the predictions from DFT and time dependent DFT. The influence of the nature of the hetero-aromatic substituent in the 5-position on the features of the absorption and fluorescence spectra and the decay of the S1-state is linked to the molecular morphology. The experiments show the choice of the 5-substituent is critical to avoid loss of a boron center in solution.

Bis-BODIPY-based fluoride and cyanide sensor mediated by unconventional deprotonation of C−H proton

A novel bis-BODIPY-based colorimetric and fluorescent sensor (BODIPY-NN) for F− and CN− recognition with high selectivity and high sensitivity is reported. The designed sensor, BODIPY-NN, was synthesized by a one-step click reaction of azido-BODIPY with N,N′-dipropargylbenzene-1,2-diamine. The BODIPY-NN sensor displays a marked color change from pink to colorless and a remarkable turn-off fluorescence behavior exclusively toward F− and CN− ions. Analysis of 1H, 13C, 19F, 11B NMR and mass spectra, together with theoretical calculations, provides insight into the mechanism of anion binding to BODIPY-NN. The sensing mechanism was found to be unconventional deprotonation at pseudo-benzylic carbons attached to BODIPY cores in the presence of F− and CN− ions, to form the less highly conjugated BODIPY moieties, thereby displaying fluorescence quenching. Addition of water in the BODIPY-NN + F− system regenerates the sensor with restoration of its spectroscopic properties. In contrast, the CN− ions permanently disrupt the BODIPY moieties by nucleophilic displacement at the central boron atoms, which is irreversible upon addition of water. The detection limits for F− and CN− in THF are 83 and 72 nM, respectively. In particular, BODIPY-NN embedded in cetyltrimethylammonium micelles can be used as colorimetric detection of CN− ions in aqueous media.

On Neutral Unsaturated Ouroboric Borylenes.

The search is on for stable isolated borylenes. Potential roles in modern synthetic chemistry for boron analogues of carbenes continue to motivate interest in locating them. Using density functional and ab initio methods, we posit and examine the thermochemistry, and chemical bonding, including aromaticity, of several classes of 5- and 6-membered borylenic rings. In these systems, cyclization relies on dative bonding (ouroboric coordination) and π donation to a monovalent boron center from an adjacent O center. Certain neutral five-membered rings (heterocyclic cyclopentadienyl analogues) in particular are found to exhibit exceptionally strong preferences for the singlet multiplicity, each with singlet-triplet (S-T) gaps in excess of 40 kcal·mol-1. The singlet five-membered rings with the largest S-T gaps and some of the six-membered rings show evidence of weak aromaticity. Relationships of the form N = A·r-b, in line with Gordy's and other functions linking bond order, N, and covalent bond length, are identified for dative B←O contacts, r, reinforced in rings by π-delocalization.

Tug‐of‐War between Two Distinct Catalytic Sites Enables Fast and Selective Ring‐Opening Copolymerizations

AbstractRing‐opening copolymerizations have emerged as a powerful approach towards the creation of sustainable polymers. Typical H‐bonding catalysts for ring‐opening are subject to a single catalytic site. Here we describe a H‐bond‐donor/Lewis‐acidic‐boron organocatalyst featuring two distinct catalytic sites in one molecule. The ring‐opening copolymerization of epoxides with anhydride mediated by these modular, and tunable catalysts achieves high selectivity (>99 % polyester selectivity) and markedly higher activity compared to either of the di‐thiourea analogues or any combinations of them. Calculations and experimental studies reveal that the superior catalytic performance arises from tug‐of‐war between two differentiated catalytic sites: thiourea pulls off the propagating chain‐end from boron center, simultaneously enhancing the role of monomer activation and also nucleophilicity of the propagation intermediates.

Tug‐of‐War between Two Distinct Catalytic Sites Enables Fast and Selective Ring‐Opening Copolymerizations

AbstractRing‐opening copolymerizations have emerged as a powerful approach towards the creation of sustainable polymers. Typical H‐bonding catalysts for ring‐opening are subject to a single catalytic site. Here we describe a H‐bond‐donor/Lewis‐acidic‐boron organocatalyst featuring two distinct catalytic sites in one molecule. The ring‐opening copolymerization of epoxides with anhydride mediated by these modular, and tunable catalysts achieves high selectivity (>99 % polyester selectivity) and markedly higher activity compared to either of the di‐thiourea analogues or any combinations of them. Calculations and experimental studies reveal that the superior catalytic performance arises from tug‐of‐war between two differentiated catalytic sites: thiourea pulls off the propagating chain‐end from boron center, simultaneously enhancing the role of monomer activation and also nucleophilicity of the propagation intermediates.

Photonics of Halogenated Zinc(II) and Cadmium(II) Dipyrromethene Complexes

This article compares spectroscopic properties of the series of dipyrromethene dyes, namely their complexes of boron (III), zinc(II) and cadmium(II) with the halogenated ligands of the same structure. Absorption and emission spectra, lifetimes of long-lived emission and quantum yields of luminescence were studied as the functions of molecular structure of dipyrromethene complexes. The role of the position and nature of a substituent in a ligand, polarity of a solvent and temperature of media were also investigated. The studies demonstrate that replacing the central atom boron(III) by zinc(II) decreases the fluorescence quantum yield, indicating the increased role of non-radiative processes in excitation energy deactivations such as intersystem crossings. In addition, according to the heavy atom effect, the efficiency of intersystem crossings in halogen-substituted zinc(II) and cadmium(II) dipyrromethene complexes is higher than in the corresponding boron fluoride dipyrromethenes (BODIPY), which leads to increase in phosphorescence at low temperatures (frozen solutions). The obtained results make it possible to carry out further investigations of potential sensory properties that are required for systematic use of halogenated dipyrromethene complexes for the creation of modern optical oxygen sensors and singlet oxygen photosensitizers for photodynamic therapy or photocatalytic oxidative reactions.

Nucleophilic Substitution at a Coordinatively Saturated Five-Membered NHC∙Haloborane Centre

In this paper, we have used a saturated five-membered N-Heterocyclic carbene (5SIDipp = 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-ylidine) for the synthesis of SNHC-haloboranes adducts and their further nucleophilic substitutions to put unusual functional groups at the central boron atom. The reaction of 5-SIDipp with RBCl2 yields Lewis-base adducts, 5-SIDipp·RBCl2 [R = H (1), Ph (2)]. The hydrolysis of 1 gives the NHC stabilized boric acid, 5-SIDipp·B(OH)3 (3), selectively. Replacement of chlorine atoms from 1 and 2 with one equivalent of AgOTf led to the formation of 5-SIDipp·HBCl(OTf) (4) and 5-SIDipp·PhBCl(OTf) (5a), where all the substituents on the boron atoms are different. The addition of two equivalents of AgNO3 to 2 leads to the formation of rare di-nitro substituted 5-SIDipp·BPh(NO3)2 (6). Further, the reaction of 5-SIDipp with B(C6F5)3 in tetrahydrofuran and diethyl ether shows a frustrated Lewis pair type small molecule activated products, 7 and 8.

Progress on Boron Subnaphthalocyanines (BsubNcs) and Associated Hybrids Towards Organic Electronic Applications and Their Electrochemical Properties

For some time, our group has been focused on the molecular design, synthesis and application of boron subphthalocyanines (BsubPcs) and subnaphthalocyanines (BsubNcs), which are macrocycles with a chelated central boron atom via nitrogen and a p-conjugated ligand. Our focal point balances between the basic and applied chemistry of the BsubPcs and BsubNcs, their physical properties (electrochemistry included) and their application as light emitting, light absorbing and electronic conducting materials for application in organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs)/organic solar cells (OSCs), the basic electrochemical and photophysical properties being critical to these applications.For this presentation, I will focus on our progress on the development of BsubNcs. In the past we have shown that BsubNcs end up being a mixed alloyed composition based on bay-position halogenation that was formed randomly during the reaction of BCl3 with 2,3-dicyanonaphthalene at temperature to form the BsubNcs. The random bay-position halogenation has been shown to be impactful in a positive way within OPV devices, negative within OLED devices and also has electrochemical variations. However given it is random halogenation, it is desirable to truly understand its impact systematically.We have recently been able to develop a separation method and therefore separate the mixed alloyed BsubNc compositions and acquire data to show the impact of the percentage/number of bay-position halogens, chlorine and bromine included, on the electrochemical potentials and the photoluminescence. I will also present a new synthetic methodology to avoid the random bay-position halogenation of the associated BsubNcs.We have also applied a computational model to look at the relative impact of the random bay-position halogenation on the electronics. We have found that the frequency of halogenation has a larger impact on the predicted HOMO/LUMO energy levels than does the random halogen positioning around the bay-positions of the BsubNcs. As this was in parallel with the separations method that was developed, this computational data is therefore comparable to the acquired electrochemical data.We have also developed BsubNc + BsubPc hybrid materials. For the hybrids, there is a way to avoid bay-position halogenation and once this was avoided, we have the first example of electrochemical and photoluminescence data for the associated BsubNc + BsubPc hybrids. I will also outline our approach to accelerated development of BsubNcs, BsubPcs and the hybrids whereby their molecular design and synthesis was first been justified through a re-adopted computational model. Figure 1

Borylation Directed Borylation of Indoles Using Pyrazabole Electrophiles: A One‐Pot Route to C7‐Borylated‐Indolines

AbstractPyrazabole (1) is a readily accessible diboron compound that can be transformed into ditopic electrophiles. In1(and derivatives), the B⋅⋅⋅B separation is ca. 3 Å, appropriate for one boron centre bonding to N and one to the C7 of indoles and indolines. This suitable B⋅⋅⋅B separation enables double E−H (E=N/C) functionalisation of indoles and indolines. Specifically, the activation of1with HNTf2generates an electrophile that transforms N−H indoles and indolines into N/C7‐diborylated indolines, with N−H borylation directing subsequent C7−H borylation. Indole reduction to indoline occurs before C−H borylation and our studies indicate this proceeds via hydroboration—C3‐protodeboronation to produce an intermediate that then undergoes C7 borylation. The borylated products can be converted in situ into C7‐BPin‐N‐H‐indolines. Overall, this represents a transient directed C−H borylation to form useful C7‐BPin‐indolines.

Borylation Directed Borylation of Indoles Using Pyrazabole Electrophiles: A One-Pot Route to C7-Borylated-Indolines.

Pyrazabole (1) is a readily accessible diboron compound that can be transformed into ditopic electrophiles. In 1 (and derivatives), the B⋅⋅⋅B separation is ca. 3 Å, appropriate for one boron centre bonding to N and one to the C7 of indoles and indolines. This suitable B⋅⋅⋅B separation enables double E−H (E=N/C) functionalisation of indoles and indolines. Specifically, the activation of 1 with HNTf2 generates an electrophile that transforms N−H indoles and indolines into N/C7‐diborylated indolines, with N−H borylation directing subsequent C7−H borylation. Indole reduction to indoline occurs before C−H borylation and our studies indicate this proceeds via hydroboration—C3‐protodeboronation to produce an intermediate that then undergoes C7 borylation. The borylated products can be converted in situ into C7‐BPin‐N‐H‐indolines. Overall, this represents a transient directed C−H borylation to form useful C7‐BPin‐indolines.

Development of a fluorescent strategy for quantification of fluoride ions in foods and toothpaste

Fluoride is one of the key chemical elements in human life and chemical industry, which also shows the high toxicity. A fluorescent probe (ZYP-1) based 1,8-naphthalimide has been prepared for fluoride ions detection, and exhibited high selectivity and sensitivity towards real samples. 19F NMR and 1H NMR titration was further determined the interaction between analyte and ZYP-1, and show the strong Lewis acidic boron atom of probe can complex basic fluoride. Theoretical calculations demonstrate that the coordination process will inhibit the photoinduced electron transfer (PET) from fluorophore to boron center that results in the increased fluorescence intensity at 540 nm. The limit of detection (LOD) of ZYP-1 for F− was found as low as 33 nM in acetonitrile. The probe can successfully applied in various liquid and solid samples of foods (fish fillets, tea leaves, water, and milk) and toothpaste to detect fluoride, which render broad application prospect.

Modulation of Properties by Ion Changing Based on Luminescent Ionic Salts Consisting of Spirobi(boron ketoiminate).

We report development of luminescent ionic salts consisting of the boron ketoiminate structure, which is one of the robust skeletons for expressing aggregation-induced emission (AIE) properties. From the formation of the boron-centered spiro structure with the ketoiminate ligands, we obtained stable ionic salts with variable anions. Since the ionic salts show Tms below 100 °C, it was shown that these salts can be classified as an ionic liquid. By using PF6 anion, the single crystal—which is applicable for X-ray crystallography—was obtained. According to the optical measurements, it was proposed that electronic interaction should occur through the boron center. Moreover, intense emission was observed both in solution and solid. Finally, we demonstrated that the emission color of the PF6 salt was altered from crystal to amorphous by adding mechanical forces. Based on boron complexation and intrinsic solid-state luminescent characters, we achieved obtainment of emissive ionic materials with environmental responsivity.

Planarized Phenyldithienylboranes: Effects of the Bridging Moieties and π-Extension on the Photophysical Properties and Lewis Acidity.

Two kinds of planarized phenyldithienylboranes, which contain (CH3 )2 C- or CH2 -bridging moieties, were synthesized. The difference of the bridging moieties affects their packing structures and photophysical properties. In particular, the (CH3 )2 C-bridged derivative exhibits a large Stokes shift, unusual for such planarized compounds, that results from a large structural relaxation in the excited state. A series of π-extended derivatives was synthesized, among which a p-(diphenylamino)phenyl-substituted derivative shows large solvatochromism in the fluorescence spectra, while maintaining high quantum yields even in polar solvents. The Lewis acidity of the phenyldithienylborane derivatives was also assessed by titration with pyridine. The Lewis acidity of the boron center is affected not only by the difference in the steric bulk of the bridging moieties, but also by the electronic effect of the substituents introduced at remote positions relative to the boron atom. These results demonstrate the characteristic features of planarized phenyldithienylboranes as building blocks for boron-based π-electron materials.

Cationic Axial Ligand Effects on Sulfur-Substituted Subphthalocyanines.

Herein, we report the synthesis of sulfur-substituted boron(III) subphthalocyanines (SubPcs) with cationic axial ligands. Subphthalocyanines were synthesized by a condensation reaction using the corresponding phthalonitriles and boron trichloride as a template. An aminoalkyl group was introduced on the central boron atom; this process was followed by N-methylation to introduce a cationic axial ligand. The peripheral sulfur groups shifted the Q band of SubPcs to a longer wavelength. The cationic axial ligands increased the polarity and enhanced the hydrophilicity of SubPcs. The effect of axial ligands on absorption and fluorescence properties is generally small. However, a further red shift was observed by introducing cationic axial ligands into the sulfur-substituted SubPcs. This change is similar to that in sulfur-substituted silicon(IV) phthalocyanines. The unique effect of the cationic axial ligand was extensively investigated by theoretical calculations and electrochemistry. In particular, the precise oxidation potential was determined using ionization potential measurements. Thus, the results of the present study provide a novel strategy for developing functional dyes and pigments based on SubPcs.

Synthesis and Optical Properties of Anthryl-substituted Tetracyclic Borepins

Anthracene-substituted triarylboranes have been widely studied as optoelectronic materials because of their unique photophysical properties. In this work, we synthesized new anthryl-substituted boranes with a seven-membered aromatic ring system, so-called borepin. The anthryl-substituted tetracyclic borepins exhibited limited electronic interaction between the borepin structure and the anthryl group in the ground state, whereas their fluorescence spectra were obviously different from those of mesityl-substituted systems. Notably, the fluorescence was enhanced when fluoride was added to the borepin solution by the coordination of fluoride to the boron center.

Stable, π-conjugated radical anions of boron–nitrogen dihydroindeno[1,2-b]fluorenes

We have recently reported the synthesis and application of boron–nitrogen dihydroindeno[1,2-b]fluorene derivatives as acceptors in organic photovoltaic devices. Their modest observed efficiencies may be related to the properties of their reduced congeners. In this work, we report two new members of this family of compounds prepared via the electrophilic borylation of 2,5-di- p-tolylpyrazine followed by an arylation of the boron center with ZnAr2 reagents. Two derivatives, 1 (Ar = 2,4,6-F3C6H2) and 2 (Ar = C6F5) were synthesized, and their radical anions, 1•− and 2•−, were formed via chemical reductions with CoCp*2 and CoCp2, respectively. Through comparison of structural parameters, as well as spectroscopic and computational data, the unpaired electron in the radical anions is localized in the planar core of the molecule, and dimerization is disfavored as a result. However, unlike the neutral starting materials, 1•− and 2•− are reactive toward ambient atmosphere. These observations suggest that the reduced compounds are stable toward intrinsic degradation pathways but subject to extrinsic degradation in device operation.

Highly active bifunctional dual-arm organoboron catalysts bearing cooperative intramolecular structures for the copolymerization of CO2 and epoxides

A series of bifunctional hexanuclear organoboron catalysts with dual-arm structures were facile prepared via a two-step procedure in ~100% yields featuring six boron centers and two quaternary ammonium bromides for the epoxides/CO2 copolymerization. The boron active sites within the p-HexaB, m-HexaB, and o-HexaB catalysts were like two stretched “arms” presenting the cooperative intramolecular structures as para-, meta-, and ortho-locations, respectively. For propylene oxide (PO)/CO2 copolymerization, the catalysts exhibited > 99% polymer selectivity under 25 – 60 ℃. The p-HexaB catalyst provided the highest carbonate linkages (88%) in the poly(propylene carbonate)s while the o-HexaB catalyst generated much more ether linkages as 45%. The o-HexaB catalyst performed higher activity in cyclohexane oxide (CHO)/CO2 copolymerization, obtaining fully alternating poly(cyclohexane carbonate)s with > 99% polymer selectivity under 25 – 150 ℃. For electron-withdrawing epichlorohydrin (ECH) monomer, the catalysts showed highly selective copolymerization (>99% polymer selectivity) and enhanced activity in comparison with tetraboron catalysts, providing poly(chloropropylene carbonate)s with > 99% carbonate linkages under 25 – 40 ℃. A novel bilaterally sequential copolymerization mechanism for alternating epoxide/CO2 insertion was proposed and verified by density functional theory (DFT) calculations. Bearing dual-arm intramolecular structures, the catalysts withstood the simultaneous growth of two polymer chains, achieving the enhanced activity for the copolymerization. Thus, up to seven effective boron pairs were found in the o-HexaB catalysts within the unique cooperative structure, leading to the PO homopolymerization in the copolymer with high ether linkages. Meanwhile, the o-HexaB catalyst with enough steric space exhibited higher activity than p-HexaB and m-HexaB catalysts during the copolymerization.