Base-Promoted Ammonia Borane Hydrogen-Release

Syntheses, Structures, and Reactivities of N-Heterocyclic Carbene-Coordinated Aminoborane Complexes

Recent research has attracted considerable attention toward N-heterocyclic carbene-coordinated boranes (NHC-borane) and their B-substituted derivatives because of their unique characteristics. In the present work, we focused on the syntheses, structures, and reactivities of such types of amine complexes, [NHC·BH₂NH₃]X ((NHC = IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and IMe (1,3-dimethylimidazol-2-ylidene); X = Cl, I, OTf). We have developed a synthetic method to access NHC·BH₂NH₂ through the reaction of NaH with [IPr·BH₂NH₃]I, which was synthesized by the reaction of IPr·BH₂I with NH₃. As a Lewis base, NHC·BH₂NH₂ could further react with HCl or HOTf to produce the corresponding salts of [IPr·BH₂NH₃]⁺. IPr·BH₂NH₂BH₂X (X = Cl, I) were synthesized by the reaction of HCl/I₂ with IPr·BH₂NH₂BH₃ and then converted to [IPr·BH₂NH₂BH₂·IPr]X (X = Cl, I) by reacting with IPr. The IMe-coordinated boranes reacted quite similarly. The preliminary results revealed that the introduction of the NHC molecule has a considerable impact on the solubility and reactivities of aminoboranes.

Unraveling the Mechanistic Details of Ru-Bis(pyridyl)borate Complex Catalyst for the Dehydrogenation of Ammonia Borane

Ru-Bis(pyridyl)borate complex (CAT) is an efficient catalyst for ammonia borane (AB) dehydrogenation. Although the mechanistic pathway of this catalyst has been theoretically investigated previously, the gap between the experimental findings and the computational results could not be bridged thus far. In our study, using density functional theory calculations, we elucidate the mechanism of AB dehydrogenation of CAT at a variable degree of ligand hydrogenation. Our results confirm that the acetonitrile ligands get reduced in the presence of AB and remain hydrogenated. Moreover, in line with experiments, we find that AB dehydrogenation on CAT proceeds via a concerted mechanism (with the free energy energetic span between 25.4 and 32.5 kcal/mol). We find that the ligand reduction alters the electronic structure and activity of CAT and the highest activity of the catalyst is expected at the fifth degree of hydrogenation of ligands with an energetic span of 25.4 kcal/mol. Additionally, the mechanism for the removal of molecular H₂ from the catalysts also alters with the degree of ligand hydrogenation. Furthermore, our results show that optimal H₂ binding free energy calculations can be used as a descriptor to identify the most active sites. Finally, this work demonstrates that ligand reduction improves the activity of the catalyst. These results highlight the importance of ligand hydrogenation in probing the activity and operating mechanism of the Ru-bis(pyridyl)borate complexes for AB dehydrogenation. Further, we identify a plausible dimer structure and rationalized experimental observation that the deactivation chemistry of this catalyst is different from the Shvo's catalyst.

B(C6F5)3-Catalyzed Transfer Hydrogenation of Esters and Organic Carbonates Towards Alcohols with Ammonia Borane

Herein, we report an efficient metal-free system for the transfer hydrogenation of esters and carbonates by-passing the otherwise ubiquitous formation of transesterification side-products. The Lewis acid B(C6F5)3 is used as...

Cobalt sandwich-stabilized rhodium nanocatalysts for ammonia borane and tetrahydroxydiboron hydrolysis

A bulky organocobalt sandwich-supported Rh nanoparticle is an efficient, stable and recyclable nanocatalyst for hydrolysis of both ammonia borane and tetrahydroxydiboron to H2.

Facile Synthetic Method of Na[BH3(NH2BH2)2H] Based on the Reactions of Sodium Amidoborane (NaNH2BH3) with NiBr2 or CoCl2

The reactions of sodium amidoborane (NaNH₂BH₃) with NiBr₂ have been investigated, and the results showed that black precipitate 1 including the NiBNH_x composites could be obtained. From the aqueous solution of the precipitate 1, the hydrolysis product Ni-B (2) was isolated and characterized. Both the in situ formed precipitate 1 and the hydrolysis product 2 can catalyze the formation of Na[BH₃(NH₂BH₂)₂H]. CoCl₂ showed comparable performance with NiBr₂. Based on these results, a facile method for the synthesis of Na[BH₃(NH₂BH₂)₂H] has been developed. This work provides insights into studying experimental methods for the synthesis of long B/N chain complexes and developing boron and nitrogen chemistry.

Mechanistic insights into the thermal decomposition of ammonia borane, a material studied for chemical hydrogen storage

We have now a better understanding of the mechanisms of thermal decomposition of ammonia borane, a widely studied hydrogen storage material.

Surface-Controlled Conversion of Ammonia Borane from Boron Nitride

“One-pot regeneration”, which is simple regneneration method of ammonia borane (AB) using hydrazine and liquid ammonia, enables conversion of AB from hexagonal boron nitride (h-BN) after milling hydrogenation. Solution 11B-NMR revealed the presence of AB after NH3/N2H4 treatment of milled h-BN (BNHx) although the yield of AB was less than 5%. The conversion mechanism was clarified as B-H bonds on the h-BN surface created by ball-milling under hydrogen pressure have an ability to form AB, which was confirmed by Thermogravimetry-Residual Gas Analysis (TG-RGA) and Infrared (IR) analysis. The reaction routes are also the same as regeneration route of polyborazylene because intermediates of AB such as (B(NH2)3 and hydrazine borane were found by solution 11B-NMR after soaking BNHx in liquid NH3 and hydrazine, respectively. Because of the fact that all reactions proceed on the h-BN surface and no reaction proceeds when neat h-BN is treated, breaking of B3N3 ring structure and then creation of B-H bond is the key issue to increase conversion yield of AB.

Use of silica-based homogeneously distributed gold nickel nanohybrid as a stable nanocatalyst for the hydrogen production from the dimethylamine borane

In this study, the effects of silica-based gold-nickel (AuNi@SiO2) nanohybrid to the production of hydrogen from dimethylamine borane (DMAB) were investigated. AuNi@SiO2 nanohybrid constructs were prepared as nanocatalysts for the dimethylamine borane dehydrogenation. The prepared nanohybrid structures were exhibited high catalytic activity and a stable form. The resulting nanohybrid, AuNi@SiO2 as a nanocatalyst, was tested in the hydrogen evolution from DMAB at room temperature. The synthesized nanohybrids were characterized using some analytical techniques. According to the results of the characterization, it was observed that the catalyst was in nanoscale and the gold-nickel alloys showed a homogenous distribution on the SiO2 surface. After characterization, the turn over frequency (TOF) of nanohybrid prepared for the production of hydrogen from dimethylamine was calculated (546.9 h-1). Also, the prepared nanohybrid can be used non-observed a significant decrease in activity even after the fifth use, in the same reaction. In addition, the activation energy (Ea) of the reaction of DMAB catalyzed AuNi@SiO2 nanohybrid was found to be 16.653 ± 1 kJmol-1 that facilitated the catalytic reaction. Furthermore, DFT-B3LYP calculations were used on the AuNi@SiO2 cluster to investigate catalyst activity. Computational results based on DFT obtained in the theoretical part of the study support the experimental data.

All-around coating of CoNi nanoalloy using a hierarchically porous carbon derived from bimetallic MOFs for highly efficient hydrolytic dehydrogenation of ammonia-borane

In situ synthesis of core–shell carbon enclosed CoNi alloys achieves efficient heterogeneous catalysis.

Direct Experimental Observation of in situ Dehydrogenation of an Amine-Borane System Using Gas Electron Diffraction

In situ dehydrogenation of azetidine-BH₃, which is a candidate for hydrogen storage, was observed with the parent and dehydrogenated analogue subjected to rigorous structural and thermochemical investigations. The structural analyses utilized gas electron diffraction supported by high-level quantum calculations, while the pathway for the unimolecular hydrogen release reaction in the absence and presence of BH₃ as a bifunctional catalyst was predicted at the CBS-QB3 level. The catalyzed dehydrogenation pathway has a barrier lower than the predicted B-N bond dissociation energy, hence favoring the dehydrogenation process over the dissociation of the complex. The predicted enthalpy of dehydrogenation at the CCSD(T)/CBS level indicates that mild reaction conditions would be required for hydrogen release and that the compound is closer to thermoneutral than linear amine boranes. The entropy and free energy change for the dehydrogenation process show that the reaction is exergonic, energetically feasible, and will proceed spontaneously toward hydrogen release, all of which are important factors for hydrogen storage.

Dehydrogenation of Amine-Boranes Using p-Block Compounds

Amine-boranes have gained a lot of attention due to their potential as hydrogen storage materials and their capacity to act as precursors for transfer hydrogenation. Therefore, a lot of effort has gone into the development of suitable transition- and main-group metal catalysts for the dehydrogenation of amine-boranes. During the past decade, new systems started to emerge solely based on p-block elements that promote the dehydrogenation of amine-boranes through hydrogen-transfer reactions, polymerization initiation, and main-group catalysis. In this review, we highlight the development of these p-block based systems for stoichiometric and catalytic amine-borane dehydrogenation and discuss the underlying mechanisms.

A Highly Active Bidentate Magnesium Catalyst for Amine-Borane Dehydrocoupling: Kinetic and Mechanistic Studies

A magnesium complex (1) featuring a bidentate aminopyridinato ligand is a remarkably selective catalyst for the dehydrocoupling of amine-boranes. This reaction proceeds to completion with low catalyst loadings (1 mol %) under mild conditions (60 °C), exceeding previously reported s-block systems in terms of selectivity, rate, and turnover number (TON). Mechanistic studies by in situ NMR analysis reveals the reaction to be first order in both catalyst and substrate. A reaction mechanism is proposed to account for these findings, with the high TON of the catalyst attributed to the bidentate nature of the ligand, which allows for reversible deprotonation of the substrate and regeneration of 1 as a stable resting state.

Large Scale Solid-state Synthesis of Catalytically Active Fe3O4@M (M = Au, Ag and Au-Ag alloy) Core-shell Nanostructures

Solvent-less synthesis of nanostructures is highly significant due to its economical, eco-friendly and industrially viable nature. Here we report a solid state synthetic approach for the fabrication of Fe3O4@M (where M = Au, Ag and Au-Ag alloy) core-shell nanostructures in nearly quantitative yields that involves a simple physical grinding of a metal precursor over Fe3O4 core, followed by calcination. The process involves smooth coating of low melting hybrid organic-inorganic precursor over the Fe3O4 core, which in turn facilitates a continuous shell layer post thermolysis. The obtained core-shell nanostructures are characterized using, XRD, XPS, ED-XRF, FE-SEM and HR-TEM for their phase, chemical state, elemental composition, surface morphology, and shell thickness, respectively. Homogeneous and continuous coating of the metal shell layer over a large area of the sample is ascertained by SAXS and STEM analyses. The synthesized catalysts have been studied for their applicability towards a model catalytic hydrogen generation from NH3BH3 and NaBH4 as hydrogen sources. The catalytic efficacy of the Fe3O4@Ag and Ag rich alloy shell materials are found to be superior to the corresponding Au counterparts. The saturation magnetization studies reveal the potential of the core-shell nanostructured catalysts to be magnetically recoverable and recyclable.

Designing Metal-Free Frustrated Lewis Pairs Catalyst for the Efficient Dehydrogenation of Ammonia Borane

Ammonia borane (AB) has been in the spotlight for the chemical storage of hydrogen over the past decade. However, the development of methods for efficient and controlled hydrogen release from AB under mild conditions is still underway. Herein, using density functional theory (DFT) computations, we designed a metal-free frustrated Lewis pair (FLP) catalyst o-(BPh₂ )C₆ H₄ (NiPr₂ ) (M1) that can efficiently dehydrogenate AB to release more than two equivalents of H₂ under mild conditions. Catalyst M1 can dehydrogenate not only AB to H₂ N=BH₂ (AOB) and H₂ , but also oligomers of AOB with rather low free-energy barriers. The high dehydrogenation activity of M1 is the key of new oligomerization routes to the efficient dehydrogenation of AB to borazine (BZ) or H₂ B-(NH=BH)_n -NH₂ (PIB) and finally to polyborazylene (PBZ) so that more than two equivalents of H₂ can be released. A first-principle kinetic Monte Carlo (KMC) study reveals that the activity of our catalytic system can be tuned by varying the initial concentration of M1 and AB. This work can guide the design of catalyst for the highly efficient utilization of AB as a hydrogen storage material.

Step-growth titanium-catalysed dehydropolymerisation of amine-boranes

Precatalysts active for the dehydropolymerisation of primary amine-boranes are generally based on mid or late transition metal. We have found that the activity of the precatalyst system formed from Cp^R₂TiCl₂ and 2nBuLi towards the dehydrogenation of the secondary amine-borane Me₂NH·BH₃, to yield the cyclic diborazane [Me₂N-BH₂]₂, increases dramatically with increasing electron-donating character of the cyclopentadienyl rings (Cp^R). Application of the most active precatalyst system (Cp^R = η-C₅Me₅) to the primary amine-borane MeNH₂·BH₃ enabled the first synthesis of high molar mass poly(N-methylaminoborane), [MeNH-BH₂] _n , the BN analogue of polypropylene, by an early transition metal such as catalyst. Significantly, unlike other dehydropolymerization precatalysts for MeNH₂·BH₃ such as [Ir(POCOP)H₂], skeletal nickel, and [Rh(COD)Cl]₂, the Ti precatalyst system was also active towards a range of substrates including BzNH₂·BH₃ (Bz = benzyl) yielding high molar mass polymer. Moreover, in contrast to the late transition metal catalysed dehydropolymerisation of MeNH₂·BH₃ and also the Ziegler-Natta polymerisation of olefins, studies indicate that the Ti-catalyzed dehydropolymerization reactions proceed by a step-growth rather than a chain-growth mechanism.

Depolymerization of Poly(phosphinoboranes): From Polymers to Lewis Base Stabilized Monomers

We report on depolymerization reactions of poly(phosphinoboranes). The cleavage of the polymers [H₂ PBH₂ ]_n (2 a), [tBuHPBH₂ ]_n (2 c), [PhHPBH₂ ]_n (2 e) and the oligomer [Ph₂ PBH₂ ]_n (2 b), with strong Lewis bases (LBs), in particular with NHCs, leads to the corresponding monomeric phosphanylboranes R¹ R² PBH₂ LB. It is observed that the depolymerization depends on the strength and stability of the LBs as well as on the substitution pattern of the poly(phosphinoboranes). The solid state structures of the monomeric phosphinoboranes H₂ PBH₂ NHC^Me (NHC=N-heterocyclic carbene) (4 a), H₂ PBH₂ NHC^dipp (5 a) and tBuHPBH₂ NHC^Me (4 c) were determined. DFT calculations support the experimentally observed reaction behavior.

Highly efficient hydrolysis of ammonia borane by anion (−OH, F−, Cl−)-tuned interactions between reactant molecules and CoP nanoparticles

An anion-assisted hydrolytic H₂-releasing ultra-high performance system involving ammonia borane and a CoP nanoparticle catalyst has been established under ambient conditions.

The superior desorption properties of MgCl2-added ammonia borane compared to MgF2-added systems—the unexpected role of MgCl2 interacting with [NH3] units

MgCl₂ unexpectedly interacts with [NH₃] units in NH₃BH₃, analogous to the amine complex of Mg(NH₃)_xCl₂, resulting in superior desorption.

Frustrated Lewis Pair Catalyzed Dehydrogenative Oxidation of Indolines and Other Heterocycles

An acceptorless dehydrogenation of heterocycles catalyzed by frustrated Lewis pairs (FLPs) was developed. Oxidation with concomitant liberation of molecular hydrogen proceeded in high to excellent yields for N-protected indolines as well as four other substrate classes. The mechanism of this unprecedented FLP-catalyzed reaction was investigated by mechanistic studies, characterization of reaction intermediates by NMR spectroscopy and X-ray crystal analysis, and by quantum-mechanical calculations. Hydrogen liberation from the ammonium hydridoborate intermediate is the rate-determining step of the oxidation. The addition of a weaker Lewis acid as a hydride shuttle increased the reaction rate by a factor of 2.28 through a second catalytic cycle.

Blending materials composed of boron, nitrogen and carbon to transform approaches to liquid hydrogen stores

Mixtures of hydrogen storage materials containing the elements of boron, nitrogen, carbon, i.e., isomers of BN cyclopentanes are examined to find a 'fuel blend' that remains a liquid phase throughout hydrogen release, maximizes hydrogen storage density, minimizes impurities and remains thermally stable at ambient temperatures. We find that the mixture of ammonia borane dissolved in 3-methyl-1,2-dihydro-1,2-azaborolidine (compound B) provide a balance of these properties and provides ca. 5.6 wt% hydrogen. The two hydrogen storage materials decompose at a faster rate than either individually and products formed are a mixture of molecular trimers. Digestion of the product mixture formed from the decomposition of the AB + B fuel blend with methanol leads to the two corresponding methanol adducts of the starting material and not a complex mixture of adducts. The work shows the utility of using blends of materials to reduce volatile impurities and preserve liquid phase.

Catalytic B-N Dehydrogenation Using Frustrated Lewis Pairs: Evidence for a Chain-Growth Coupling Mechanism

The catalytic dehydrogenation of ammonia- and amine-boranes by a dimethylxanthene-derived frustrated Lewis pair is described. Turnover is facilitated on a thermodynamic basis by the ready release of H2 from the weakly basic PPh2-containing system. In situ NMR studies and the isolation of intermediates from stoichiometric reactions support a mechanism initiated by B-H activation, followed by end-growth BN coupling involving the terminal NH bond of the bound BN fragment and a BH bond of the incoming borane monomer.

Aluminum borate nanowires from the pyrolysis of polyaminoborane precursors

Polyaminoboranes [N(R)H-BH₂]_n (1: R = H, 2: R = Me) were pyrolyzed on a range of substrates: silicon, metal foils (stainless steel, nickel, and rhodium), and sapphire wafers, as well as on Al₂O₃ and AlN powders.

Metal-Free Addition/Head-to-Tail Polymerization of Transient Phosphinoboranes, RPH-BH2: A Route to Poly(alkylphosphinoboranes)

Mild thermolysis of Lewis base stabilized phosphinoborane monomers R(1)R(2)P-BH2⋅NMe3 (R(1),R(2)=H, Ph, or tBu/H) at room temperature to 100 °C provides a convenient new route to oligo- and polyphosphinoboranes [R(1)R(2)P-BH2]n. The polymerization appears to proceed via the addition/head-to-tail polymerization of short-lived free phosphinoborane monomers, R(1)R(2)P-BH2. This method offers access to high molar mass materials, as exemplified by poly(tert-butylphosphinoborane), that are currently inaccessible using other routes (e.g. catalytic dehydrocoupling).

A rationally designed amino-borane complex in a metal organic framework: a novel reusable hydrogen storage and size-selective reduction material

A novel microporous material UiOAB was synthesized, which can act as both a reusable hydrogen chemical storage material and a size-selective reducing agent.

Probing the second dehydrogenation step in ammonia-borane dehydrocoupling: characterization and reactivity of the key intermediate, B-(cyclotriborazanyl)amine-borane

While thermolysis of ammonia-borane (AB) affords a mixture of aminoborane- and iminoborane oligomers, the most selective metal-based catalysts afford exclusively cyclic iminoborane trimer (borazine) and its B-N cross-linked oligomers (polyborazylene). This catalysed dehydrogenation sequence proceeds through a branched cyclic aminoborane oligomer assigned previously as trimeric B-(cyclodiborazanyl)amine-borane (BCDB). Herein we utilize multinuclear NMR spectroscopy and X-ray crystallography to show instead that this key intermediate is actually tetrameric B-(cyclotriborazanyl)amine-borane (BCTB) and a method is presented for its selective synthesis from AB. The reactivity of BCTB upon thermal treatment as well as catalytic dehydrogenation is studied and discussed with regard to facilitating the second dehydrogenation step in AB dehydrocoupling.

Reaction of a bulky amine borane with lanthanide trialkyls. Formation of alkyl lanthanide imide complexes

Lanthanide imidoborane complexes with terminal alkyl groups are synthesized from the reactions of simple lanthanide trialkyls with a bulky amine borane.

Synthesis of a yolk/shell Fe3O4@poly(ionic liquid)s-derived nitrogen doped graphitic porous carbon materials and its application as support for nickel catalysts

The synthesis of yolk/shell spheres including a movable magnetic core, a poly(ionic liquid)s-derived porous carbon shell, and nickel nanoparticles confined within the porous shell is reported.

Improved thermal dehydrogenation of ammonia borane by MOF-5

TPD-MS spectra (left) and solid-state ¹¹B NMR (right) of pristine AB, AB/MOF-5 and AB/JUC-32-Y.