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Ammonia borane and amine boranes are main group analogues of alkanes, which are characterised by their large gravimetric hydrogen content. This hydrogen can be released in dehydrocoupling and dehydropolymerisation reactions to obtain B−N oligomers and polymers that are of importance as precursors for functional B−N materials. Furthermore, amine boranes are potent reagents for application in transfer hydrogenation reactions, representing a versatile, easy‐to‐handle alternative to the use of gaseous hydrogen for the reduction of organic compounds. Compared to late transition metals, complexes of readily available and comparatively inexpensive electropositive group 4 metals have been used to a much lesser extent. This review summarises the developments in the field of dehydrocoupling of amine boranes and transfer hydrogenation with these reagents, catalysed by complexes of group 4 metals. We analyse the background for these developments using examples and reaction mechanisms and provide an outlook for future developments in this field of research. [ABSTRACT FROM AUTHOR]

This study focused on investigating the potential of allylamine borane (AABH3) as a hydrogen‐releasing compound and its viability as an alternative energy source. The investigation involved a comprehensive analysis of its structure and thermochemistry during the dehydrogenation process using density functional theory (DFT) methods. The structural parameters computed with the M06‐2X method were observed to be closer to the experimental values than those of B3LYP and wB97XD3, particularly, the bond lengths. The C−N−B bond angle was observed to increase with dehydrogenation, and higher with doubly dehydrogenated adduct. Also, the dihedral angle was observed to change from negative to positive and back to negative in AABH3, AABH2 and AABH respectively. The reaction enthalpy (▵Hr) for the first dehydrogenation step obtained at M06‐2X/cc‐pVTZ level is exothermic (−19.6 kJ/mol), while the second step is endothermic (132.6 kJ/mol). However, the values of the entropy in the first and second dehydrogenation steps are 122.0 and 124.0 kJ/mol respectively, indicating the feasibility of the reaction. The values of the reaction free energy (▵Gr), in the first and second steps were calculated to be −56.0 kJ/mol and 95.7 kJ/mol respectively, indicating that the second step of the reaction is rate limiting. The relative free energies of the reaction also corroborated this observation. The mechanism of the reaction was observed to be initiated by the abstraction of AABH3 proton by the BH3, over a distance of 1.37 Å. Importantly, the calculated dehydrogenation reaction energies demonstrated minimal energy requirements for the hydrogen release reactions, indicating the suitability of allylamine borane for chemical hydrogen storage onboard various applications. [ABSTRACT FROM AUTHOR]

Amine boranes are solid hydrogen storage materials and are difficult to transport within systems unless dissolved in solvents. The solubility and free energy of solution were experimentally determined for ammonia borane, methylamine borane, dimethylamine borane, trimethylamine borane, and tert-butylamine borane in methanol, ethanol, 1-propanol, 2-propanol, acetonitrile, dimethyl sulfoxide, and tetrahydrofuran. The solubilities of ammonia borane/methylamine borane mixtures in methanol were also measured. The enthalpy of solution was experimentally determined for ammonia borane, methylamine borane, dimethylamine borane, and 30/70 wt% ammonia borane/methylamine borane eutectic in four different solvents: methanol, ethanol, acetonitrile, and dimethyl sulfoxide. For all solutes, dimethyl sulfoxide solutions were the most exothermic or least endothermic. Increased methyl-substitution on the amine led to more endothermic processes. For methanol, ethanol, and acetonitrile, the entropy of solution increased as solubility increased. The entropy of solution for ammonia borane dissolving in dimethyl sulfoxide is negative. COSMO-RS solubility calculations using density functional theory optimized geometries agree relatively well with experimental values for amine borane materials dissolved in polar solvents. Image 1 • 30/70 wt% ammonia borane/methylamine borane eutectic is highly soluble in methanol. • 30/70 wt% eutectic weakens intermolecular forces compared to ideal mixture. • Amine borane/DMSO solution enthalpy was the most exothermic or least endothermic. • Entropy of solution is smallest for dissolving in DMSO. • Predicted solubilities from COSMO-RS agree with relative experimental solubilities. [ABSTRACT FROM AUTHOR]

The gaseous structures, thermochemical properties and dehydrogenation reaction energy profiles of the borane complexes of pyrrolidine and piperidine have been investigated using gas electron diffraction (GED) and state-of-the-art computational methods. These complexes are of interest because of their potential as hydrogen storage materials for future onboard transport applications. A comparative structural and thermochemical analysis revealed structures with a slight difference in the essential B–N bond length, with the piperidine borane having a longer bond even though it has a stronger B–N bond according to predicted bond dissociation energies, a trend common with amine boranes. To identify the most favourable dehydrogenation pathway, BH3-catalysed and BH3-uncatalysed dehydrogenation channels have been explored, where the former has been shown to be the favourable process for both complexes. The energy requirements for the hydrogen release reactions are expected to be minimal as evidenced from the calculated dehydrogenation reaction energies, implying their suitability for onboard chemical hydrogen storage. [ABSTRACT FROM AUTHOR]

The Lewis acidities of a series of [n]magnesocenophanes (1 a–d) have been investigated computationally and found to be a function of the tilt of the cyclopentadienyl moieties. Their catalytic abilities in amine borane dehydrogenation/dehydrocoupling reactions have been probed, and C[1]magnesocenophane (1 a) has been shown to effectively catalyze the dehydrogenation/dehydrocoupling of dimethylamine borane (2 a) and diisopropylamine borane (2 b) under ambient conditions. Furthermore, the mechanism of the reaction with 2 a has been investigated experimentally and computationally, and the results imply a ligand‐assisted mechanism involving stepwise proton and hydride transfer, with dimethylaminoborane as the key intermediate. [ABSTRACT FROM AUTHOR]

The reactions of NH3BH2R (R = Me, Ph and Cl) with THFBH3 have been investigated and it was found that different substituents on the B atom help to proceed the reactions in different ways. The expected doubly‐bridged B‐substituted aminodiborane products, similar to aminodiborane (ADB, BH2(µ‐H)(µ‐NH2)BH2) via the reaction of ammonia borane (AB, NH3BH3) and tetrahydrofuran borane (THFBH3), are not obtained. Two competitive reactions occurred with the change of R = Me or Ph. When R is a Me group, an "open" version of B‐substituted µ‐aminodiborane, THFBH(Me)(µ‐NH2)BH3, is formed as a major product; when R is a Ph group, AB and THFBH2Ph are formed as main products via the intermolecular NH3–THF exchange reaction. However, if R is Cl, then NH3BH2Cl reacts with THFBH3 through reversible intermolecular Cl–H exchange mechanism. Furthermore, DFT calculations are performed to elucidate the formation mechanism of THFBH(Me)(µ‐NH2)BH3 via the reaction of NH3BH2Me and THFBH3 as well as the exchange mechanism of Cl–H in the reaction of NH3BH2Cl and THFBH3. [ABSTRACT FROM AUTHOR]

The reactions of NH3BH2R (R = Me, Ph and Cl) with THFBH3 have been investigated and it was found that different substituents on the B atom help to proceed the reactions in different ways. The expected doubly‐bridged B‐substituted aminodiborane products, similar to aminodiborane (ADB, BH2(µ‐H)(µ‐NH2)BH2) via the reaction of ammonia borane (AB, NH3BH3) and tetrahydrofuran borane (THFBH3), are not obtained. Two competitive reactions occurred with the change of R = Me or Ph. When R is a Me group, an "open" version of B‐substituted µ‐aminodiborane, THFBH(Me)(µ‐NH2)BH3, is formed as a major product; when R is a Ph group, AB and THFBH2Ph are formed as main products via the intermolecular NH3–THF exchange reaction. However, if R is Cl, then NH3BH2Cl reacts with THFBH3 through reversible intermolecular Cl–H exchange mechanism. Furthermore, DFT calculations are performed to elucidate the formation mechanism of THFBH(Me)(µ‐NH2)BH3 via the reaction of NH3BH2Me and THFBH3 as well as the exchange mechanism of Cl–H in the reaction of NH3BH2Cl and THFBH3. [ABSTRACT FROM AUTHOR]

Abstract: The first spontaneous, metal‐free, room‐temperature hydrogen transfer from ammonia borane to an iminoborane is reported. Mechanistic studies of the reaction indicate a concerted transfer of H+ and H− from donor to acceptor with an activation energy far below those of comparable concerted transfer hydrogenations. This reaction was employed in the syntheses and isolation of new B,N‐disubstituted aminoboranes, a comparatively rare subset within the aminoborane family. This successful transfer hydrogenation to a highly dehydrogenated BN system may serve as a starting point for the design of new systems capable of reversible dehydrogenation/rehydrogenation. [ABSTRACT FROM AUTHOR]

The behavior of NaBH 4 (THB) and the amine boranes, NH 3 BH 3 (AB), tert butylNH 2 BH 3 (TBAB), Me 2 NHBH 3 (DMAB) was investigated in continuous flow chemical vapor generation of H 2 Se from aqueous Se IV coupled with atomic absorption spectrometry. Unexpected higher efficiency of H 2 Se generation was obtained with amine boranes compared to THB (TBAB > AB > THB) using millimolar concentration of reductant (0.001–0.1 mol L −1 ) under strongly acidic conditions (HCl, HClO 4 , H 2 SO 4 , HNO 3 , 0.5–5 mol L −1 H + ). Analytical applicability of the CVG system was tested by the determination of Se IV in natural water samples certified reference materials, using 0.01 mol L −1 TBAB in 0.5 M H 2 SO 4 . In order to explain this unexpected higher efficiency of amine boranes with respect of THB, the kinetic of hydrolysis of AB, TBAB and DMAB was investigated in acid media typically employed in chemical vapor generation for trace element determination. The kinetic was investigated by monitoring the rate the hydrogen gas evolved during hydrolysis, using a laboratory made thermostated reaction cell. Kinetics were measured for AB, TBAB and DMAB in 0.1, 0.5, 5 mol L −1 HCl or HClO 4 reaction media and in 0.1 mol L −1 cysteine +0.1 mol L −1 HCl or HClO 4 buffer, for reaction times from 0 to 30 min. Under strongly acidic conditions, the rates of hydrogen evolution produced by amine boranes hydrolysis appear to be much slower than those predicted by a pseudo-first order reaction and using the rate constants reported in the literature. This suggests that, at elevated acidities (5 mol L −1 HCl or HClO 4 ), the hydrolysis of amine boranes takes place in two steps, generating a first amount of H 2 (0.67–1.15 mol) much faster than the remaining about 2 mol. This evidence indicates a different mechanism of hydrolysis to the one accepted in the literature for amine boranes. The relatively high efficiencies of H 2 Se observed with amine borane reduction of inorganic Se IV at elevated acidities can be addressed to the action of borane intermediates, most probably amine borane cations, formed during amine borane hydrolysis in the same reaction conditions. [ABSTRACT FROM AUTHOR]

A novel strategy for post-synthesis amine borane functionalization of MOFs under gas-solid phase transformation, utilizing gaseous diborane, is reported. The covalently confined amine borane derivative decorated on the framework backbone is stable when preserved at low temperature, but spontaneously liberates soft chemical hydrogen at room temperature, leading to the development of an unusual borenium type species (-NH=BH2+) ion-paired with a hydroborate anion. Furthermore, the unsaturated amino borane (-NH=BH2) and the μ-iminodiborane (-μ-NHB2H5) were detected as final products. A combination of DFT based molecular dynamics simulations and solid state NMR spectroscopy, utilizing isotopically enriched materials, were undertaken to unequivocally elucidate the mechanistic pathways for H2 liberation. [ABSTRACT FROM AUTHOR]

An efficient synthetic method of ionic hydrogenation of azine to the corresponding 1,2-dialkylhydrazines was accomplished. Reaction time was fast and isolation and purification of the 1,2-dialkylhydrazines were operationally simple. Yields were almost quantitative for most of the products with good functional group tolerance. Moreover, the byproduct of reduction gave the opportunity for the selective synthesis, in situ , of the mono hydrazide extending the potentiality of the synthetic method described. [Display omitted] [ABSTRACT FROM AUTHOR]

Cover Feature: Magnesocenophane-Catalyzed Amine Borane Dehydrocoupling (Chem. Eur. J. 28/2020) Keywords: amine boranes; catalysis; dehydrocoupling; magnesium; metallocenophanes EN amine boranes catalysis dehydrocoupling magnesium metallocenophanes 6088 6088 1 05/18/20 20200515 NES 200515 B A C[1]magnesocenophane b catalyzes the dehydrocoupling reaction of dimethylamine borane to the cyclic tetramethyldiborazane under ambient conditions. While there are many transition-metal-based catalysts known to facilitate this reaction at room temperature, it is quite remarkable for a magnesium compound, as previous magnesium-based systems required elevated temperatures. [Extracted from the article]