Thermodynamics of boroxine formation from the aliphatic boronic acid monomers R-B(OH)2 (R = H, H3C, H2N, HO, and F): a computational investigation

Enhancing Cancer Therapy: Boron-Rich Polyboronate Ester Micelles for Synergistic Boron Neutron Capture Therapy and PD-1/PD-L1 Checkpoint Blockade

Malignant cancers, known for their pronounced heterogeneity, pose substantial challenges to monotherapeutic strategies and contribute to the risk of metastasis. Addressing this, our study explores the synergistic potential of combining boron neutron capture therapy (BNCT) with immune checkpoint blockade to enhance cancer treatment efficacy. We synthesized boron-rich block copolymer micelles as a novel boron drug for BNCT. Characterization was conducted using nuclear magnetic resonance, gel-permeation chromatography, transmission electron microscopy, and dynamic light scattering. These micelles, with an optimal size of 91.3 nm and a polydispersity index of 0.18, are suitable for drug delivery applications. In vitro assessments on B16-F10 melanoma cells showed a 13-fold increase in boron uptake with the micelles compared to borophenyl alanine (BPA), the conventional boron drug for BNCT. This resulted in a substantial increase in BNCT efficacy, reducing cell viability to 77% post-irradiation in micelle-treated cells, in contrast to 90% in BPA-treated cells. In vivo, melanoma-bearing mice treated with these micelles exhibited an 8-fold increase in boron accumulation in tumor tissues versus those treated with BPA, leading to prolonged tumor growth delay (5.4 days with micelles versus 3.3 days with BPA). Moreover, combining BNCT with anti-PD-L1 immunotherapy further extended the tumor growth delay to 6.6 days, and enhanced T-cell infiltration and activation at tumor sites, thereby indicating a boosted immune response. This combination demonstrates a promising approach by enhancing cytotoxic T-cell priming and mitigating the immunosuppressive effects of melanoma tumors.

Solid-State Synthesis of B←N Adducts by the Amine-Facilitated Trimerization of the Phenylboronic Acid

Stable boroxine-amine adducts comprising dative B←N bond(s) were prepared by mechanochemically-induced reactions of phenylboronic acid (PBA) and amines (pyridine, DMAP, 1H-pyrazole, piperidine, DABCO, hexamethylenetetramine, or 4,4'-bipyridine). In-situ Raman monitoring, ex-situ PXRD and DFT calculations were used for product identification. Stoichiometry of the product (3 : 1, 3 : 2 or 6 : 1 adduct) was controlled by the amine structure and the molar ratio of the reactants. The 1 : 2 H-bonded assembly of PBA and 4,4'-bipyridine (bpy) was confirmed as an intermediate in the adduct formation for bpy. Competitive binding experiments indicated that the exchange of the amines in the 3 : 1 adducts follows the computed adduct stabilities that increase with the amine basicity. Following the DFT prediction, the first adduct with two different amines, DMAP and pip, bound to one boroxine moiety was isolated and structurally characterized. Results show that calculations can be used to predict possible and preferred product(s) and their spectral characteristics.

Isolation, bonding and reactivity of a monomeric stibine oxide

In contrast to phosphine oxides and arsine oxides, which are common and exist as stable monomeric species featuring the corresponding pnictoryl functional group (Pn=O/Pn⁺-O^-; Pn = P, As), stibine oxides are generally polymeric, and the properties of the unperturbed stiboryl group (Sb=O/Sb⁺-O^-) remain unexplored. We now report the isolation of the monomeric stibine oxide, Dipp₃SbO (where Dipp = 2,6-diisopropylphenyl). Spectroscopic, crystallographic and computational studies provide insight into the nature of the Sb=O/Sb⁺-O^- bond. Moreover, isolation of Dipp₃SbO allows the chemistry of the stiboryl group to be explored. Here we show that Dipp₃SbO can act as a Brønsted base, a hydrogen-bond acceptor and a transition-metal ligand, in addition engaging in 1,2-addition, O-for-F₂ exchange and O-atom transfer. In all cases, the reactivity of Dipp₃SbO differed from that of the lighter congeners Dipp₃AsO and Dipp₃PO.

Boronic-acid-derived covalent organic frameworks: from synthesis to applications

Modular, well-defined, and robust hierarchical functional materials are targets of numerous synthesis endeavors.

Biomimetic Boroxine-Based Multifunctional Thermosets via One-Pot Synthesis

Boroxine-based thermosets with remarkable mechanical tunability, self-healing ability, recyclability, and adhesive strength are of significant importance in various applications. However, complex multistep reactions are often required to prepare such thermosets. Herein, a facile one-pot approach to synthesize boroxine-based malleable thermosets is proposed. Random copolymers with pendant boronic acid groups were synthesized from alkenyl monomers containing boronic acids [4-vinylphenylboronic acid (4-VPBA), 3-vinylphenylboronic acid, or 3-acrylamidophenylboronic acid] and octadecanoxy polyethylene glycol methacrylate. Then, the as-prepared copolymers were cured to form thermosets with boroxine bonds. The tensile strengths of the thermosets were tailored to range from 9.3 to 27.5 MPa by increasing the concentration of 4-VPBA. Moreover, because of the reversible nature of dynamic boroxine bonds (transformation between boroxines and boronic acids) induced by water, the thermosets exhibit remarkable self-healing efficiency (up to 99%), tunable mechanical properties, and excellent recyclability. Additionally, the thermosets also demonstrate superior adhesive strength (as high as 73.9 MPa) on different substrates.

Boron-based inorganic heterocyclic clusters: electronic structure, chemical bonding, aromaticity, and analogy to hydrocarbons

This Perspective article deals with recent computational and experimental findings in boron-based heterocyclic clusters, which focuses on binary B-O and B-S clusters, as well as relevant ternary B-X-H (X = O, S, N) species. Boron is electron-deficient and boron clusters do not form monocyclic rings or linear chains. Boron-based heterocyclic clusters are intuitively even more electron-deficient and feature exotic chemical bonding, which make use of O 2p, S 3p, or N 2p lone-pairs for π delocalization over heterocyclic rings, facilitating new cluster structures and new types of bonding. Rhombic, pentagonal, hexagonal, and polycyclic clusters are discussed herein. Rhombic species are stabilized by four-center four-electron (4c-4e) π bonding, that is, the o-bond. An o-bond cluster differs from a typical 4π antiaromatic system, because it has 4π electrons in an unusual bonding/nonbonding combination, which takes advantage of the empty 2p_z atomic orbitals from electron-deficient boron centers. A variety of examples (notably including boronyl boroxine) possess a hexagonal ring, as well as magic 6π electron-counting, making them new members of the inorganic benzene family. Pentagonal clusters bridge rhombic o-bond systems and inorganic benzenes, but they do not necessarily favor 6π electron-counting as in cyclopentadienide anion. In contrast, pentagonal 4π clusters are stable, leading to the concept of pentagonal o-bond. One electron can overturn the potential energy landscape of a system, enabling rhombic-to-hexagonal structural transition, which further reinforces the idea that 4π electron-counting is favorable for rhombic systems and 6π is magic for hexagonal rings. The bonding analogy between heterocyclic clusters and hydrocarbons goes beyond monocyclic species, which allows rational design of boron-based inorganic analogs of polycyclic aromatic hydrocarbons, including s-indacene as a puzzling aromatic/antiaromatic system. Selected linear B-O clusters are also briefly discussed, featuring dual 3c-4e π bonds, that is, ω-hyperbonds. Dual ω-hyperbonds, rhombic or pentagonal o-bond, and inorganic benzenes share a common chemical origin. The field of boron-based heterocyclic clusters is still in its infant stage, and much new chemistry remains to be discovered in forthcoming experimental and theoretical studies.

Constructing new metal-organic frameworks with complicated ligands from "One-Pot" in situ reactions

Metal-organic frameworks (MOFs) have emerged as one of the most fascinating libraries of porous materials. In spite of their myriad merits, practical application of most MOFs is restricted due to their high preparation cost because of the complicated organic ligands involved. To address this limitation, we propose to use simple and cheap organic precursors to synthesize MOFs with complicated ligands via "one-pot" in situ reactions of these precursors along with the formation of new MOFs. In this work, we have carefully screened several organic reactions, through which target ligands were generated in situ from easily available reactants during the MOF construction. With this "one-pot" approach, the fabrication of a series of novel MOFs by integrating the organic covalent bond and the coordinate bond has thus been realized through the judicious selection of organic reactions, which effectively simplifies the MOF synthesis process and thus reduces the cost.

Monosubstituted Phenylboronic Acids, R-B(OH)2 (R = C6H5, C6H4CH3, C6H4NH2, C6H4OH, and C6H4F): A Computational Investigation

Phenylboronic acids (PBAs) are an important class of compounds with diverse applications in synthetic, biological, medicinal, and materials chemistry. In this investigation we report structural and thermochemical parameters for several monosubstituted ortho, meta, and para PBAs, R-B(OH)2 (R = C6H5, C6H4CH3, C6H4NH2, C6H4OH, and C6H4F). Equilibrium geometries of all the PBAs discussed in this article were obtained using second-order Møller-Plesset perturbation theory (MP2) with the Dunning-Woon aug-cc-pVDZ basis set; heats of formation (HOF) were calculated at the Gaussian-3 (G3) level of theory. The endo-exo conformers of all the positional isomers of these PBAs were lowest in energy. Using HOF for the monosubstituted PBAs calculated at the G3 level of theory, in conjunction with the experimental HOF for benzene, toluene, aniline, phenol, and fluorobenzene, the values of [Formula: see text] for the transfer processes C6H6 + C6H4X-B(OH)2 → C6H5X + C6H5-B(OH)2 (X = CH3, NH2, OH, and F) are found to be in good agreement with values of [Formula: see text] calculated at the MP2(FC)/aug-cc-pVTZ//MP2(FC)/aug-cc-pVTZ computational level; the bonding in the reactants and products for these transfer reactions are well-matched and thermochemical calculations at this level are expected to be very accurate, providing checks on the G3 HOF calculations.

Pentagonal five-center four-electron π bond in ternary B3N2H5 cluster: an extension of the concept of three-center four-electron ω bond

Ternary B₃N₂H₅ (C_2v, ¹A₁) cluster has a heteroatomic B₃N₂ ring, with 4π electrons in a robust bonding/nonbonding combination, which is proposed as a five-center four-electron o-bond.

A Comparison of the Structure and Bonding in the Aliphatic Boronic R-B(OH)2 and Borinic R-BH(OH) acids (R=H; NH2, OH, and F): A Computational Investigation

Boronic acids, R-B(OH)2, play an important role in synthetic, biological, medicinal, and materials chemistry. This investigation compares the structure and bonding surrounding the boron atoms in the simple aliphatic boronic acids, R-B(OH)2 (R = H; NH2, OH, and F) and the analogous borinic acids, R-BH(OH). Geometry optimizations were performed using second-order Møller-Plesset perturbation theory (MP2) with the Dunning-Woon aug-cc-pVTZ, aug-cc-pVQZ and aug-cc-pV5Z basis sets; single-point CCSD(FC)/aug-cc-pVTZ//MP2(FC)/aug-cc-pVTZ level calculations were used to generate a QCI density for Natural Bond Orbital analyses of the bonding. The optimized boron-oxygen bond lengths for the X-B-Ot-H trans-branch of the endo-exo form of the boronic acids and for the X-B-O-H cis-branch of the boronic and borinic acids (X = N, O, and F respectively) decrease as the electronegativity of X increases. The boron-oxygen bond lengths are generally longer in the endo-exo or anti forms of the boronic acids than in the corresponding borinic acids. NBO analyses suggest the boron-oxygen bond in H2BOH is a double bond; the boron-oxygen bonding in the remaining boronic and borinic acids in this study have a significant contribution from dative pπ-pπ bonding. Values for [Formula: see text] for the highly balanced reaction, R-B(OH)2 + R-BH2 → 2 R-BH(OH), suggest that the bonding surrounding the boron atom is stronger in the borinic acid than in the corresponding boronic acid.

Synthesis of a boron/nitrogen-containing compound based on triazine and boronic acid and its flame retardant effect on epoxy resin

With the aim of developing a novel organic flame retardant, an organic boronic acid derivative containing a triazine ring (2,4,6-tris(4-boronic-2-thiophene)-1,3,5-triazine (3TT-3BA)) was synthesized. The thermal properties of 3TT-3BA and its corresponding intermediate products were investigated by thermogravimetric analysis. The results show that 3TT-3BA has a high char yield (56.9%). The flame retardant properties of epoxy resin (EP) with 3TT-3BA were investigated by cone calorimeter, limiting oxygen index (LOI) test, and vertical burning test (UL 94). The LOI of EP with 20% 3TT-3BA is 31.2% and the UL 94 V-0 rating is achieved for EP with 20% 3TT-3BA. The flame retardant mechanism of 3TT-3BA in EP was investigated using TGA–Fourier transform infrared spectroscopy and scanning electron microscopy.

Spectroscopic and Computational Investigations of The Thermodynamics of Boronate Ester and Diazaborole Self-Assembly

The solution phase self-assembly of boronate esters, diazaboroles, oxathiaboroles, and dithiaboroles from the condensation of arylboronic acids with aromatic diol, diamine, hydroxythiol, and dithiol compounds in chloroform has been investigated by (1)H NMR spectroscopy and computational methods. Six arylboronic acids were included in the investigations with each boronic acid varying in the substituent at its 4-position. Both computational and experimental results show that the para-substituent of the arylboronic acid does not significantly influence the favorability of forming a condensation product with a given organic donor. The type of donor, however, greatly influences the favorability of self-assembly. (1)H NMR spectroscopy indicates that condensation reactions between arylboronic acids and catechol to give boronate esters are the most favored thermodynamically, followed by diazaborole formation. Computational investigations support this conclusion. Neither oxathiaboroles nor dithiaboroles form spontaneously at equilibrium in chloroform at room temperature. Computational results suggest that the effect of borylation on the frontier orbitals of each donor helps to explain differences in the favorability of their condensation reactions with arylboronic acids. The results can inform the use of boronic acids as they are increasingly utilized in the dynamic self-assembly of organic materials and as components in dynamic combinatorial libraries.

Planar B3S2H3−and B3S2H3clusters with a five-membered B3S2ring: boron–sulfur hydride analogues of cyclopentadiene

Boron–sulfur hydride clusters,C_2vB₃S₂H₃⁻and B₃S₂H₃, possess a five-membered B₃S₂ring as the core, which is analogous to cyclopentadiene in terms of π bonding.

Ternary B2X2H2(X = O and S) rhombic clusters and their potential use as inorganic ligands in sandwich-type (B2X2H2)2Ni complexes

Boron-based ternary B₂O₂H₂and B₂S₂H₂clusters possess a rhombic, heteroatomic ring with 4π electrons in a nonbonding/bonding combination, differing from cyclobutadiene.

Growth of boronic acid based two-dimensional covalent networks on a metal surface under ultrahigh vacuum

An extensive analysis of the complex mechanisms governing the on-surface polymerisation of boronic acid on a metal surface under vacuum.

[(B3O3H3)(n)M]+ (n = 1, 2;M = Cu, Ag, Au): a new class of metal-cation complexes

A density functional theory (DFT) investigation into the structures and bonding characteristics of [(B3O3H3)nM](+)(n = 1, 2;M = Cu, Ag, Au) complexes was performed. DFT calculations and natural bond orbital (NBO) analyses indicate that the ΙB metal complexes of boroxine exhibit intriguing bonding characteristics, different from the typical cation-π interactions between ΙB metal-cations and benzene. The complexes of [B3O3H3M](+) and [(B3O3H3)2 M](+) (M = Cu, Ag, and Au) favor the conformation of perfectly planar structures with the C2v and D2h symmetry along one of the threefold molecular axes of boroxine, respectively. Detailed natural resonance theory (NRT) and canonical molecular orbitals (CMOs) analyses show that interaction between the metal cation and the boroxine in [B3O3H3M](+) (M = Cu, Ag, and Au) is mainly ionic, while the ΙB metal-cations←π donation effect is responsible for the binding site. In these complexes, boroxine serves as terminals η(1)-B3O3H3 with one O atom of the B3O3 ring. The infra-red (IR) spectra of [B3O3H3M](+) were simulated to facilitate their future experimental characterization. The complexes all give two IR active modes at about 1,300 and 2,700 cm(-1), which are inactive in pure boroxine. Simultaneously, the B-H stretching modes of the complexes are red-shifted due to the interaction between the metal-cation and boroxine. To explore the possibility of the structural pattern developed in this work forming mesoporous materials, complexes [(B3O3H3M)6](6+) (M = Cu, Ag, and Au) were also studied, which appear to be unique and particular interesting: they are all true minima with D6h symmetries and pore sizes ranging from 12.04 Å to 13.65 Å.

Heats of Formation for the Boronic Acids R-B(OH)2 and Boroxines R3B3O3 (R=H, Li, HBe, H2B, H3C, H2N, HO, F, and Cl) Calculated at the G2, G3, and G4 Levels of Theory

Boronic acids (R-B(OH)₂) and their boroxine (R₃B₃O₃) dehydration products have emerged as important classes of compounds with a multitude of diverse applications. However, the available heats of formation for these compounds are not always as accurate as would be required for further use. In this study the heats of formation at 298.15 K of R-B(OH)₂ and R₃B₃O₃ (R = H, Li, HBe, H₂B, H₃C, H₂N, HO, F, and Cl) have been calculated at the G2, G3[G3B3], and G4 levels of theory and used to determine the enthalpy changes for the dehydration reactions: 3 R-B(OH)₂ → R₃B₃O₃ + 3 H₂O; comparisons are made with other rigorous levels of theory, e.g. CBS-Q[CBS-QB3] and W1U, as well as with experimental values wherever possible. Enthalpy changes for the dehydration reactions have also been calculated using second-order Møller-Plesset perturbation theory (MP2) with the Dunning-Woon correlation-consistent aug-cc-pVDZ and aug-cc-pVTZ basis sets, and B3LYP density functional theory with the 6-311++G(2df,2pd) basis set. With the exception of H₂N-B(OH)₂, the dehydration reactions are consistently predicted to be exothermic. Our results provide a cautionary note for the use of the B3LYP functional in the calculation of structures and energies of boronic acids and boroxines. Where comparisons could be made, the G4 and W1U predictions for the heats of formation of these boron compounds differ significantly.