THE QUEST FOR STABLE BOROZENE CORE IN MAIN-GROUP CAPPED INVERSE SANDWICH COMPLEXES, [(HE)2B6H6]2- (E=B, Al, Ga, In, and Tl).

The ubiquitous chemistry of benzene led us to explore ways to stabilise analogous borozene, by capping them with appropriate groups. The mismatch in overlap of ring-cap fragment molecular orbitals in [(HB)2B6H6]2- is overcome by replacing the two BH caps with  higher congeners of boron. We calculated the relative energies of all the polyhedral structural candidates for [(HE)2B6H6]2- (E = Al-Tl) and found hexagonal bipyramid (HBP) to be more stable with Al-H caps. A global minimum search also gives HBP as the most stable structure for [Al2B6H8]2-. The capped B6H6 ring in [(HAl)2B6H6]2- has aromaticity comparable to that of benzene.

1,2,3-Triazole-Containing Azamacrocycles from Chiral Triazolopeptoids: Synthesis and Solid-State Studies.

Two new chiral 1,2,3-triazole-containing macrocyclic oligoamides (i. e.: triazolopeptoid 4 and 5) were obtained through solid-phase synthesis of linear precursors followed by high dilution macrocyclization reaction. Theoretical (DFT) and spectroscopic (NMR) studies revealed the intricate interplay between the Nα-chiral side chains and their conformational attitudes. BH3-mediated reduction of the tertiary amide groups of known 1-3 and newly synthesized 4 gave novel azamacrocycles 6-9. Detection of borane complexes of azamacrocycles 6 and 9 (i. e.: 10 and 11), corroborated by X-ray diffraction studies, demonstrated the peculiar properties of 1,2,3-triazole-containing macrorings.

Regulation of Electronic Structures of the Urchin-Like NiCoP/CoP Nanocatalysts for Fast Hydrogen Evolution.

The exploration of stable, efficient, and low-cost catalysts toward ammonia borane hydrolysis is of vital significance for the practical implementation of this hydrogen production technology. Integrating interface engineering and nano-architecture engineering is a favorable strategy to elevate catalytic performance, as it can modify the electronic structure and provide sufficient active sites simultaneously. In this work, urchin-like NiCoP/CoP heterostructures are prepared via a three-step hydrothermal-oxidation-phosphorization synthesis route. It is demonstrated that the original Ni/Co molar ratio and the amount of phosphorus are crucial for adjusting the morphology, enhancing the exposed surface area, facilitating charge transfer, and modulating the adsorption and activation of H2O molecules. Consequently, the optimal Ni1Co2P heterostructure displays remarkable catalytic properties in the hydrolysis of ammonia borane with a turnover frequency (TOF) value of 30.3 molH2 ⋅ min-1 ⋅ molmetal -1, a low apparent activation energy of 25.89 kJ ⋅ mol-1, and good stability. Furthermore, by combining infrared spectroscopy and isotope kinetics experiments, a possible mechanism for the hydrolysis of ammonia borane was proposed.

Fluorination/Dearomatization of C6F5 Groups: An FLP Route to an Electrophilic Borane and Non-Coordinating Anions.

B(C6F5)3 and the corresponding anion [B(C6F5)4]- are ubiquitous in main group and transition metal chemistry. Known derivatives are generally limited to the incorporation of electron donating substituents. Herein we describe electrophilic fluorination and dearomatization of such species using XeF2 in the presence of BF3 or Lewis acidic cations. In this fashion the anions [HB(C6F5)3]-, [B(C6F5)4]- and [(C6F5)3BC≡NB(C6F5)3]-, are converted to [FB(C6F7)3]-, [B(C6F7)4]-, and [(C6F7)3BC≡NB(C6F7)3]-, respectively. Similarly, the borane adducts (L)B(C6F7)3 (L=MeCN, OPEt3) are produced. These rare examples of electrophilic attack of electron deficient rings proceed as [XeF][BF4] acts as a frustrated Lewis pair effecting fluorination and dearomatization of C6F5 rings.

Novel simultaneous analysis of 18 types of glycosaminoglycan-derived disaccharides using 4-aminobenzoic acid ethyl ester derivatization by HPLC with fluorescence detection.

Glycosaminoglycans (GAGs), including hyaluronic acid (HA), chondroitin sulfate (CS)/dermatan sulfate (DS), heparan sulfate (HS)/heparin (HP), and keratan sulfate (KS), play pivotal roles in living organisms. Generally, GAGs are analyzed after enzymatic digestion into unsaturated or saturated disaccharides. Due to high structural similarity between disaccharides, however, separation during analysis is challenging. Additionally, little is known about the structures of GAGs and their functional relationships. Elucidating the function of GAGs requires highly sensitive quantitative analytical methods. We developed a method for the simultaneous analysis of 18 types of disaccharides derived from HA (1 type), CS/DS (7 types), HS/HP (8 types), and KS (2 types) potentially detectable in analyses of human urine. The simple method involves HPLC separation with fluorescence detection following derivatization of GAG-derived disaccharides using 4-aminobenzoic acid ethyl ester (ABEE) as a pre-labeling agent and 2-picoline borane as a reductant. The ABEE derivatization reaction can be performed under aqueous conditions, and excess derivatization reagents can be easily, rapidly, and safely removed. This method enables highly sensitive simultaneous analysis of the 18 abovementioned types of GAG-derived disaccharides using HPLC with fluorescence detection in small amounts of urine (1 mL) in a single run. The versatile method described here could be applied to the analysis of GAGs in other biological samples.

Efficient Reductive N-11C-Methylation Using Arylamines or Alkylamines and In Situ-Generated [11C]Formaldehyde From [11C]Methyl Iodide.

Reductive N-11C-methylation using [11C]formaldehyde and amines has been used to prepare N-11C-methylated compounds. However, the yields of the N-11C-methylated compounds are often insufficient. In this study, we developed an efficient method for base-free reductive N-11C-methylation that is applicable to a wide variety of substrates, including arylamines bearing electron-withdrawing and electron-donating substituents. A 2-picoline borane complex, which is a stable and mild reductant, was used. Dimethyl sulfoxide was used as the primary reaction solvent, and glacial acetic acid or aqueous acetic acid was used as a cosolvent. While reductive N-11C-methylation efficiently proceeded under anhydrous conditions in most cases, the addition of water to the reductive N-11C-methylation generally increased the yield of the N-11C-methylated compounds. Substrates with hydroxy, carboxyl, nitrile, nitro, ester, amide, and phenone moieties and amine salts were applicable to the reaction. This proposed method for reductive N-11C-methylation should be applicable to a wide variety of substrates, including thermo-labile and base-sensitive compounds because the reaction was performed under relatively mild conditions (70°C) without the need for a base.

Transfer Hydrogenation of Vinyl Arenes and Aryl Acetylenes with Ammonia Borane Catalyzed by Schiff Base Cobalt(II) Complexes.

A series of bench-stable Co(II) complexes containing hydrazone Schiff base ligands were evaluated in terms of their activity and selectivity in carbon-carbon multiple bond transfer hydrogenation. These cobalt complexes, especially a Co(II) precatalyst bearing pyridine-2-yl-N(Me)N=C-(1-methyl)imidazole-2-yl ligand, activated by LiHBEt3, were successfully used in the transfer hydrogenation of substituted styrenes and phenylacetylenes with ammonia borane as a hydrogen source. Key advantages of the reported catalytic system include mild reaction conditions, high selectivity and tolerance to functional groups of substrates.

Borane-Trimethylamine Complex: A Versatile Reagent in Organic Synthesis.

Borane-trimethylamine complex (Me3N·BH3; BTM) is the most stable of the amine-borane complexes that are commercially available, and it is cost-effective. It is a valuable reagent in organic chemistry with applications in the reduction of carbonyl groups and carbon-nitrogen double bond reduction, with considerable examples in the reduction of oximes, hydrazones and azines. The transfer hydrogenation of aromatic N-heterocycles and the selective N-monomethylation of primary anilines are further examples of recent applications, whereas the reduction of nitrobenzenes to anilines and the reductive deprotection of N-tritylamines are useful tools in the organic synthesis. Moreover, BTM is the main reagent in the regioselective cleavage of cyclic acetals, a reaction of great importance for carbohydrate chemistry. Recent innovative applications of BTM, such as CO2 utilization as feedstock and radical chemistry by photocatalysis, have extended their usefulness in new reactions. The present review is focused on the applications of borane-trimethylamine complex as a reagent in organic synthesis and has not been covered in previous reviews regarding amine-borane complexes.

Borane-Pyridine: An Efficient Catalyst for Direct Amidation.

Borane-pyridine acts as an efficient (5 mol%) liquid catalyst, providing improved solubility for the direct amidation of a wide range of aromatic and aliphatic carboxylic acids and amines to form secondary and tertiary carboxamides. Tolerance of potentially incompatible halo, nitro, and alkene functionalities has been demonstrated.

Design, Synthesis, and Evaluation of B-(Trifluoromethyl)phenyl Phosphine-Borane Derivatives as Novel Progesterone Receptor Antagonists.

We previously revealed that phosphine-boranes can function as molecular frameworks for biofunctional molecules. In the present study, we exploited the diversity of available phosphines to design and synthesize a series of B-(trifluoromethyl)phenyl phosphine-borane derivatives as novel progesterone receptor (PR) antagonists. We revealed that the synthesized phosphine-borane derivatives exhibited LogP values in a predictable manner and that the P-H group in the phosphine-borane was almost nonpolar. Among the synthesized phosphine-boranes, which exhibited PR antagonistic activity, B-(4-trifluoromethyl)phenyl tricyclopropylphosphine-borane was the most potent with an IC50 value of 0.54 µM. A docking simulation indicated that the tricyclopropylphosphine moiety plays an important role in ligand-receptor interactions. These results support the idea that phosphine-boranes are versatile structural options in drug discovery, and the developed compounds are promising lead compounds for further structural development of next-generation PR antagonists.

Exploring Enhanced Hydrolytic Dehydrogenation of Ammonia Borane with Porous Graphene-Supported Platinum Catalysts.

Graphene is a good support for immobilizing catalysts, due to its large theoretical specific surface area and high electric conductivity. Solid chemical converted graphene, in a form with multiple layers, decreases the practical specific surface area. Building pores in graphene can increase specific surface area and provide anchor sites for catalysts. In this study, we have prepared porous graphene (PG) via the process of equilibrium precipitation followed by carbothermal reduction of ZnO. During the equilibrium precipitation process, hydrolyzed N,N-dimethylformamide sluggishly generates hydroxyl groups which transform Zn2+ into amorphous ZnO nanodots anchored on reduced graphene oxide. After carbothermal reduction of zinc oxide, micropores are formed in PG. When the Zn2+ feeding amount is 0.12 mmol, the average size of the Pt nanoparticles on PG in the catalyst is 7.25 nm. The resulting Pt/PG exhibited the highest turnover frequency of 511.6 min-1 for ammonia borane hydrolysis, which is 2.43 times that for Pt on graphene without the addition of Zn2+. Therefore, PG treated via equilibrium precipitation and subsequent carbothermal reduction can serve as an effective support for the catalytic hydrolysis of ammonia borane.

Metal-Free B-B Dehydrocoupling Reaction of a Simple Borane Adduct: Convenient Access to a Nucleophilic Diborane(4).

The selective formation of homonuclear bonds is of key importance in synthetic chemistry. Especially, dehydrocoupling reactions are attractive as ecologically and economically friendly alternatives to established reductive bond forming reactions, since they do not require the use of stoichiometric amounts of a reducing reagent and produce only valuable dihydrogen as by-product. Here, we report on a metal-free B-B dehydrocoupling reaction that starts directly from a simple, easily accessible BH3 adduct, providing convenient access to a new nucleophilic dihydridodiborane in excellent yield. The dihydridodiborane in turn activates dihydrogen, allowing to obtain quantitatively the dideuteridodiborane from the dihydridodiborane by D2 activation. On the basis of detailed quantum-chemical calculations, the mechanism of this unprecedented reaction is elucidated. Some key points that are essential for metal-free dehydrocoupling are disclosed, paving the way for their systematic evaluation and application.

Modulating Hydrogen Shuttling in Ammonia by Neutral and Cationic Boron-Containing Frustrated Lewis Pairs (FLPs).

Xanthene-backbone FLPs featuring secondary borane functions -B(ArX)H (where ArX=C6F5 (ArF) or C6Cl5 (ArCl)) have been targeted through reactions of the dihydroboranes Me2S ⋅ BArXH2 with [4,5-xanth(PR2)Li]2 (R=Ph, iPr), and investigated in the synthesis of related cationic systems via hydride abstraction. The reactivity of these systems (both cationic and charge neutral) with ammonia have been probed, with a view to probing the potential for proton shuttling via N-H bond 'activation.' We find that in the case of four-coordinate boron systems (cationic or change neutral), the N-H linkage remains intact, supported by a NH⋅⋅⋅P hydrogen bond which is worth up to 17 kcal mol-1 thermodynamically, and enabled by planarization of the flexible xanthene scaffold. For cationic three coordinate systems, N-to-P proton transfer is viable, driven by the ability of the boron centre to stabilise the [NH2]- conjugate base through N-to-B π bonding. This proton transfer can be shown to be reversible in the presence of excess ammonia, depending on the nature of the B-bound ArX group. It is viable in the case of C6F5 substituents, but is prevented by the more sterically encumbering and secondary donor-stabilising capabilities of the C6Cl5 substituent.

Efficient Hydrogen Generation from Ammonia Borane Hydrolysis on a Tandem Ruthenium-Platinum-Titanium Catalyst.

Hydrolysis of ammonia borane (NH3BH3, AB) involves multiple undefined steps and complex adsorption and activation, so single or dual sites are not enough to rapidly achieve the multi-step catalytic processes. Designing multi-site catalysts is necessary to enhance the catalytic performance of AB hydrolysis reactions but revealing the matching reaction mechanisms of AB hydrolysis is a great challenge. In this work, we propose to construct RuPt-Ti multi-site catalysts to clarify the multi-site tandem activation mechanism of AB hydrolysis. Experimental and theoretical studies reveal that the multi-site tandem mode can respectively promote the activation of NH3BH3 and H2O molecules on the Ru and Pt sites as well as facilitate the fast transfer of *H and the desorption of H2 on Ti sites at the same time. RuPt-Ti multi-site catalysts exhibit the highest turnover frequency (TOF) of 1293 min-1 for AB hydrolysis reaction, outperforming the single-site Ru, dual-site RuPt and Ru-Ti catalysts. This study proposes a multi-site tandem concept for accelerating the dehydrogenation of hydrogen storage material, aiming to contribute to the development of cleaner, low-carbon, and high-performance hydrogen production systems.

Creating Remarkably Moisture- and Air-Stable Macromolecular Lewis Acid by Integrating Borane within the Polymer Chain: A Highly Active Catalyst for Homo(co)polymerization of Epoxides.

Borane-based Lewis acids (LA) play an indispensable role in the Lewis pair (LP) mediated polymerization. However, most borane-based LPs are moisture- and air-sensitive. Therefore, development of moisture and air-stable borane-based LP is highly desirable. To achieve this goal, the concept of "aggregation induced enlargement effects" by chemically linking multiple borane within a nanoscopic confinement was conceived to create macromolecular LA. Accordingly, an extremely moisture and air stable macromolecular borane, namely, PVP-1B featuring poly(4-vinylphenol) backbone, was constructed. The concentration of borane active site is greatly higher than average concentration due to local confinement. Therefore, an enhanced activity was observed. Moreover, the local LA aggregation effects allow its tolerance to air and large amount of chain transfer agent. Consequently, PVP-1B showed remarkable efficiency for propylene oxide (PO) polymerization at 25 °C (TOF=27900 h-1 ). Furthermore, it enables generation of well-defined telechelic poly (CHO-alt-CO2 ) diol (0.6-15.3 kg/mol) with narrow Ds via copolymerizing cyclohexene oxide and CO2 at 80 °C. This work indicates unifying multiple borane within a polymer in a macromolecular level shows superior catalytic performance than constructing binary, bi(multi)functional systems in a molecular level. This paves a new way to make functional polyethers.

Silica Confinement for Stable and Magnetic Co-Cu Alloy Nanoparticles in Nitrogen-Doped Carbon for Enhanced Hydrogen Evolution.

Ammonia borane (AB) with 19.6 wt % H2 content is widely considered a safe and efficient medium for H2 storage and release. Co-based nanocatalysts present strong contenders for replacing precious metal-based catalysts in AB hydrolysis due to their high activity and cost-effectiveness. However, precisely adjusting the active centers and surface properties of Co-based nanomaterials to enhance their activity, as well as suppressing the migration and loss of metal atoms to improve their stability, presents many challenges. In this study, mesoporous-silica-confined bimetallic Co-Cu nanoparticles embedded in nitrogen-doped carbon (CoxCu1-x@NC@mSiO2) were synthesized using a facile mSiO2-confined thermal pyrolysis strategy. The obtained product, an optimized Co0.8Cu0.2@NC@mSiO2 catalyst, exhibits enhanced performance with a turnover frequency of 240.9 molH2 ⋅ molmetal ⋅ min-1 for AB hydrolysis at 298 K, surpassing most noble-metal-free catalysts. Moreover, Co0.8Cu0.2@NC@mSiO2 demonstrates magnetic recyclability and extraordinary stability, with a negligible decline of only 0.8 % over 30 cycles of use. This enhanced performance was attributed to the synergistic effect between Co and Cu, as well as silica confinement. This work proposes a promising method for constructing noble-metal-free catalysts for AB hydrolysis.

Double-Heterostructured Nickel Phosphate-Phosphides as High-Activity Electrocatalyst for Ammonia Borane Electrooxidation.

The direct electrooxidation reaction of ammonia borane (ABOR) as the anodic reaction of direct ammonia borane fuel cells (DABFCs) is greatly dependent on the properties of electrocatalysts. Both the active sites and charge/mass transfer characteristics are the key to promoting the processes of kinetics and thermodynamics, which can further improve the electrocatalytic activity. Hence, the catalyst double-heterostructured Ni2 P/Ni2 P2 O7 /Ni12 P5 (d-NPO/NP) with the optimistic redistribution of electrons and active sites is prepared for the first time. The d-NPO/NP-750 catalyst obtained after pyrolysis at 750 °C shows the outstanding electrocatalytic activity toward ABOR with an onset potential of -0.329 V vs RHE which is better than all the published catalysts. The density functional theory (DFT) computations illustrate that the Ni2 P2 O7 /Ni2 P acts as the activity enhancement heterostructure with a high d-band center (-1.60 eV) and the low activation energy barrier, while the Ni2 P2 O7 /Ni12 P5 acts as the conductivity enhancement heterostructure with the highest density of valence electrons.

Complexation-Triggered Fluctuation of π-Conjugation on an Antiaromatic Dicyanoanthracene Dianion.

The formation of Lewis pairs is an important chemical concept. Recently, the complexation of Lewis acidic tris(pentafluorophenyl)borane with Lewis basic moieties and subsequent reduction has emerged as a fascinating strategy for designing novel reactions and structures. The impact of the complexation and subsequent reduction of antiaromatic systems bearing Lewis base moieties has been investigated. We found how Lewis adduct formation stabilizes an antiaromatic system consisting of 9,10-dicyanoanthracene and tris(pentafluorophenyl)borane by using synthesis, X-ray crystallography, spectroscopic analysis, and quantum chemical calculations.

Dehydropolymerization of Amine-Boranes using Bis(imino)pyridine Rhodium Pre-Catalysis: σ-Amine-Borane Complexes, Nanoparticles, and Low Residual-Metal BN-Polymers that can be Chemically Repurposed.

The sigma amine-borane complexes [Rh(L1)(η2 :η2 -H3 B⋅NRH2 )][OTf] (L1=2,6-bis-[1-(2,6-diisopropylphenylimino)ethyl]pyridine, R=Me, Et, n Pr) are described, alongside [Rh(L1)(NMeH2 )][OTf]. Using R=Me as a pre-catalyst (1 mol %) the dehydropolymerization of H3 B ⋅ NMeH2 gives [H2 BNMeH]n selectively. Added NMeH2 , or the direct use of [Rh(L1)(NMeH2 )][OTf], is required for initiation of catalysis, which is suggested to operate through the formation of a neutral hydride complex, Rh(L1)H. The formation of small (1-5 nm) nanoparticles is observed at the end of catalysis, but studies are ambiguous as to whether the catalysis is solely nanoparticle promoted or if there is a molecular homogeneous component. [Rh(L1)(NMeH2 )][OTf] is shown to operate at 0.025 mol % loadings on a 2 g scale of H3 B ⋅ NMeH2 to give polyaminoborane [H2 BNMeH]n [Mn =30,900 g/mol, Ð=1.8] that can be purified to a low residual [Rh] (6 µg/g). Addition of Na[N(SiMe3 )2 ] to [H2 BNMeH]n results in selective depolymerization to form the eee-isomer of N,N,N-trimethylcyclotriborazane [H2 BNMeH]3 : the chemical repurposing of a main-group polymer.

Molecules that Turn Themselves Inside-Out: Tuning in/out Equilibria and Homeomorphic Isomerization in Macrobicyclic Dibridgehead Diphosphines P((CH2 )n )3 P Newly Accessible by Earth-Abundant-Metal Templates.

Photolyses of trans-Fe(CO)3 (P((CH2 )n )3 P) (n=10 (a), 12 (b), 14 (c), 16 (d), 18 (e)) in the presence of PMe3 provide the first economical and scalable route to macrobicyclic dibridgehead diphosphines P((CH2 )n )3 P (1). These are isolated as mixtures of in,in/out,out isomers that equilibrate with degenerate in,out/out,in isomers at 150 °C via pyramidal inversion at phosphorus. For the entire series, VT 31 P NMR data establish or bound Keq , rates, and activation parameters for a variety of phenomena, many of which involve homeomorphic isomerizations, topological processes by which certain molecules can turn themselves inside out (e. g., in,in⇌out,out). This provides the first detailed mapping of such trends in homologous series of aliphatic bicyclic compounds XE((CH2 )n )3 EX with any type of bridgehead. Isomeric diborane adducts 1 a,d ⋅ 2BH3 are also characterized. Crystal structures of out,out-1 a and in,in-1 a ⋅ 2BH3 aid isomer assignments and reveal unusual cage conformations.