Tunable Hydrogen Release from Amine–Boranes via their Insertion into Functional Polystyrenes

Polystyrene‐g‐boramine random copolymers are dihydrogen reservoirs with tunable dehydrogenation temperatures, which can be adjusted by selecting the boramine content in the copolymers. They display a unique dihydrogen thermal release profile, which is a direct consequence of the insertion of the amine–boranes in a polymeric scaffold, and not from a direct modification of the electronics or sterics of the amine–borane function. Finally, the mixture of polystyrene‐g‐boramines with conventional NH₃‐BH₃ (borazane) allows for a direct access to organic/inorganic hybrid dihydrogen reservoirs with a maximal H₂ loading of 8 wt %. These exhibit a dehydrogenation temperature lower than that of either the borazane or the polystyrene‐g‐boramines taken separately.     

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.     

Dihydrogen generation from amine/boranes: synthesis, FT-ICR, and computational studies

A Fourier transform ion cyclotron resonance spectrometry (FT-ICR) study of the gas-phase protonation of ammonia-borane and sixteen amine/boranes R(1)R(2)R(3)N-BH(3) (including six compounds synthesized for the first time) has shown that, without exception, the protonation of amine/boranes leads to the formation of dihydrogen. The structural effects on the experimental energetic thresholds of this reaction were determined experimentally. The most likely intermediate and the observed final species (besides H(2)) are R(1)R(2)R(3)N-BH(4)(+) and R(1)R(2)R(3)N-BH(2)(+), respectively. Isotopic substitution allowed the reaction mechanism to be ascertained. Computational analyses ([MP2/6-311+G(d,p)] level) of the thermodynamic stabilities of the R(1)R(2)R(3)N-BH(3) adducts, the acidities of the proton sources required for dihydrogen formation, and the structural effects on these processes were performed. It was further found that the family of R(1)R(2)R(3)N-BH(4)(+) ions is characterized by a three-center, two-electron bond between B and a loosely bound H(2) molecule. Unexpected features of some R(1)R(2)R(3)N-BH(4)(+) ions were found. This information allowed the properties of amine/boranes most suitable for dihydrogen generation and storage to be determined.     

Addition of boranes to N-aryl-salicylaldimines: intramolecular hydrogenation of imines

Addition of boranes to N-aryl-salicylaldimines takes place initially at the reactive phenolic O-H bond to give an activated boron-containing imine and dihydrogen. In some cases a subsequent intramolecular hydrogenation step is observed and the C=N imine bond is reduced to the corresponding amine. Reactions with dimesitylborane in THF are unique in that the reduced amine product is the major product observed in solution.     

Evidence of Dihydrogen Bonding of a Chiral Amine–Borane Complex in Solution by VCD Spectroscopy

IR and vibrational circular dichroism (VCD) spectra of a chiral amine–borane in solution are investigated. By comparison of experimental and calculated spectra, unique VCD spectral signatures, which can be attributed to the formation of dihydrogen‐bonded dimers in solution, are identified for the first time. These VCD features are highly sensitive to the specific dihydrogen‐bonding topologies utilized by the chiral amine–borane subunits and thus provide direct structural information of these dihydrogen‐bonded species in solution. Differences in the dihydrogen binding arrangements in solution and in solid state are also revealed.     

Dihydrogen Generation from Amine/Boranes: Synthesis,FT‐ICR,and Computational Studies

A Fourier transform ion cyclotron resonance spectrometry (FT‐ICR) study of the gas‐phase protonation of ammonia‐borane and sixteen amine/boranes R¹R²R³N? BH₃ (including six compounds synthesized for the first time) has shown that, without exception, the protonation of amine/boranes leads to the formation of dihydrogen. The structural effects on the experimental energetic thresholds of this reaction were determined experimentally. The most likely intermediate and the observed final species (besides H₂) are R¹R²R³N? BH₄⁺ and R¹R²R³N? BH₂⁺, respectively. Isotopic substitution allowed the reaction mechanism to be ascertained. Computational analyses ([MP2/6‐311+G(d,p)] level) of the thermodynamic stabilities of the R¹R²R³N? BH₃ adducts, the acidities of the proton sources required for dihydrogen formation, and the structural effects on these processes were performed. It was further found that the family of R¹R²R³N? BH₄⁺ ions is characterized by a three‐center, two‐electron bond between B and a loosely bound H₂ molecule. Unexpected features of some R¹R²R³N? BH₄⁺ ions were found. This information allowed the properties of amine/boranes most suitable for dihydrogen generation and storage to be determined.     

Hydrogen Release from Dialkylamine–Boranes Promoted by Mg and Ca Complexes: A DFT Analysis of the Reaction Mechanism

Mg and Ca β‐diketiminato silylamides [HC{(Me)CN(2,6‐iPr₂C₆H₃)}₂M(THF)_n{N(SiMe₃)₂}] (M=Mg, n=0; M=Ca, n=1) were studied as precatalysts for the dehydrogenation/dehydrocoupling of secondary amine–boranes R₂HNBH₃. By reaction with equimolar quantities of amine–boranes, the corresponding amidoborane derivatives are formed, which further react to yield dehydrogenation products such as the cyclic dimer [BH₂?NMe₂]₂. DFT was used here to explore the mechanistic alternatives proposed on the basis of the experimental findings for both Mg and Ca amidoboranes. The influence of the steric demand of amine–boranes on the course of the reaction was examined by performing calculations on the dehydrogenation of dimethylamine–borane (DMAB), pyrrolidine–borane (PB), and diisopropylamine–borane. In spite of the analogies in the catalytic activity of Mg‐ and Ca‐based complexes in the dehydrocoupling of amine–boranes, our theoretical analysis confirmed the experimentally observed lower reactivity of Ca complexes. Differences in catalytic activity of Mg‐ and Ca‐based complexes were examined and rationalized. As a consequence of the increase in ionic radius on going from Mg²⁺ to Ca²⁺, the dehydrogenation mechanism changes and formation of a key metal hydride intermediate becomes inaccessible. Dimerization is likely to occur off‐metal in solution for DMAB and PB, whereas steric hindrance of iPr₂NHBH₃ hampers formation of the cyclic dimer. The reported results are of particular interest because, although amine–borane dehydrogenation is now well established, mechanistic insight is still lacking for many systems.     

New boryl radicals derived from N-heteroaryl boranes: generation and reactivity

Recently, boryl radicals have been the subject of revived interest. These structures were generated by hydrogen-abstraction reactions from the corresponding boranes (i.e., from amine or phosphine boranes). However, the classical issue remains their high B--H bond-dissociation energy (BDE), thereby preventing a very efficient hydrogen-abstraction process. In the present paper, new N-heteroaryl boranes that exhibiting low B--H BDE are presented; excellent hydrogen-transfer properties have been found. Both the generation and the reactivity of the associated boryl radicals have been investigated through their direct observation in laser flash photolysis. The boryl radical interactions with double bonds, oxygen, oxidizing agent, and alkyl halides have been studied. Some selected applications of N-heteroaryl boryl radicals as new polymerization-initiating structures are proposed to evidence their high intrinsic reactivity.     

Mechanistic analysis of the transition metal-catalyzed hydrogenation of imines and functionalized enamines

The asymmetric hydrogenation of prochiral imines has been mainly studied on cationic rhodium or iridium complexes and involves the coordination of the nitrogen atom on the metal centre. Oxidative addition of dihydrogen followed by one hydride transfer onto the carbon atom produces a σ-bonded nitrogen-metal intermediate from which reductive elimination occurs with the second hydride to produce the amine. From all these reactivity studies we propose that in an early step a hydrogen atom transfer could occur on the nitrogen atom to generate an iminium species which coordinates the metal centre through the CN double bond. The Shvo catalyst, an active neutral ruthenium(II) complex, presents the particularity to transfer almost simultaneously a proton from the hydroxycyclopentadienyl ligand and a hydride bonded to the metal center. In this case too, we suggest the first proton transfer generates an iminium species.Morever, a few titanium complexes have been shown to catalyze the hydrogenation of imines.Concerning enamines, literature is rather scarce on catalytic success. On the contrary, many studies focused on hydrogenation of enamides, which represent functionalized enamines, and enantiomeric excesses very close to 100% have been obtained. Extensive studies of the effect of the (chiraldiphosphine)Rh⁺ framework on the asymmetric induction have shown that several reversible steps can occur related to the oxidative addition of dihydrogen before or after the coordination of the enamide; the irreversible step directly connected to the asymmetric induction is the formation of the chiral alkyl-rhodium species.     

Amine-directed hydroboration: scope and limitations

Iodine activation induces intramolecular hydroboration of homoallylic and bis-homoallylic amine boranes with good to excellent control of regiochemistry compared to control experiments using excess THF*BH 3. Deuterium labeling and other evidence confirm that the iodine-induced hydroboration reaction of homoallylic amine boranes occurs via an intramolecular mechanism equivalent to the classical 4-center process and without competing retro-hydroboration. Longer carbon chain tethers result in lower regioselectivity, whereas the shorter tether in allylic amines results in a switch to dominant intermolecular hydroboration. Regioselectivity in THF*BH 3 control experiments is higher for the allylic amine boranes compared to the iodine activation experiments, whereas the reverse is true for homoallylic amine borane activation.     

The synthetic utility of amine borane reagents in the reduction of aldehydes and ketones

Primary and secondary amine boranes have been shown to be mild, efficient, and stereoselective reducing agents for aldehydes and ketones in protic or non-protic solvents with reactivity patterns different than diborane or sodium borohydride.     

Magnesium-Catalyzed Tandem Dehydrogenation-Dehydrocoupling: An Atom Economical Access to Alkynylboranes

Owing to the unusual reactivity of sterically hindered amine–borane complexes, a catalytic system based on magnesium salts was designed to perform a tandem dehydrogenation–dehydrocoupling between terminal alkynes and boranes. The reaction is providing pure alkynylboranes within few minutes at room temperature, with only two molecules of hydrogen as a byproduct.     

On the Use of Amine–Borane Complexes To Synthesize Iron Nanoparticles

The effectiveness of amine–borane as reducing agent for the synthesis of iron nanoparticles has been investigated. Large (2–4 nm) Fe nanoparticles were obtained from [Fe{N(SiMe₃)₂}₂]. Inclusion of boron in the nanoparticles is clearly evidenced by extended X‐ray absorption fine structure spectroscopy and Mössbauer spectrometry. Furthermore, the reactivity of amine–borane and amino–borane complexes in the presence of pure Fe nanoparticles has been investigated. Dihydrogen evolution was observed in both cases, which suggests the potential of Fe nanoparticles to promote the release of dihydrogen from amine–borane and amino–borane moieties.     

Are Boryl Radicals from Amine–Boranes and Phosphine–Boranes the Most Stable Radicals?

The relative stability of the radicals that can be produced from amine–boranes and phosphine–boranes is investigated at the G3‐RAD level of theory. Aminyl ([RNH]^.:BH₃) and phosphinyl ([RPH]^.:BH₃) radicals are systematically more stable than the boryl analogues, [RNH₂]:BH₂^. and [RPH₂]:BH₂^.. Despite similar stability trends for [RNH]^.:BH₃ and [RPH]^.:BH₃ radicals with respect to boryl radicals, there are significant dissimilarities between amine– and phosphine–boranes. The homolytic bond dissociation energy of the N?H bond decreases upon association of the amines with BH₃, whereas that of the P?H bond for phosphines increases. The stabilization of the free amine is much smaller than that of the corresponding aminyl radical, whereas for phosphines this is the other way around. The homolytic bond dissociation energy of the B?H bond of borane decreases upon complexation with both amines and phosphines.     

Photoinduced Single-Electron Transfer as an Enabling Principle in the Radical Borylation of Alkenes with NHC–Borane

A photoinduced SET process enables the direct B−H bond activation of NHC–boranes. In contrast to common hydrogen atom transfer (HAT) strategies, this photoinduced reaction simply takes advantage of the beneficial redox potentials of NHC–boranes, thus obviating the need for extra radical initiators. The resulting NHC–boryl radical was used for the borylation of a wide range of α-trifluoromethylalkenes and alkenes with diverse electronic and structural features, providing facile access to highly functionalized borylated molecules. Labeling and photoquenching experiments provide insight into the mechanism of this photoinduced SET pathway.     

Photoinduced Single‐Electron Transfer as an Enabling Principle in the Radical Borylation of Alkenes with NHC–Borane

A photoinduced SET process enables the direct B?H bond activation of NHC–boranes. In contrast to common hydrogen atom transfer (HAT) strategies, this photoinduced reaction simply takes advantage of the beneficial redox potentials of NHC–boranes, thus obviating the need for extra radical initiators. The resulting NHC–boryl radical was used for the borylation of a wide range of α‐trifluoromethylalkenes and alkenes with diverse electronic and structural features, providing facile access to highly functionalized borylated molecules. Labeling and photoquenching experiments provide insight into the mechanism of this photoinduced SET pathway.     

Amine–Boranes: Green Hypergolic Fuels with Consistently Low Ignition Delays

Complexation of amines with borane converts them to hypergols or decreases their ignition delays (IDs) multifold (with white fuming nitric acid as the oxidant). With consistently low IDs, amine–boranes represent a class of compounds that can be promising alternatives to toxic hydrazine and its derivatives as propellants. A structure–hypergolicity relationship study reveals the necessary features for the low ID.     

Production of Dihydrogen Using Ammonia Borane as Reagent and Pyrazole as Catalyst

Theoretical chemistry (DLPNO-CCSD(T)/def2-TZVP//M06-2x/aug-cc-pVDZ) was used to design a system based on ammonia boranes catalyzed by pyrazoles with the aim of producing dihydrogen, nowadays of high interest as clean fuel. The reactivity of ammonia borane and cyclotriborazane were investigated, including catalytic activation through 1H-pyrazole, 4-methoxy-1H-pyrazole, and 4-nitro-1H-pyrazole. The results point toward a catalytic cycle by which, at the same time, ammonia borane can initially store and then, through catalysis, produce dihydrogen and amino borane. Subsequently, amino borane can trimerize to form cyclotriborazane that, in presence of the same catalyst, can also produce dihydrogen. This study proposes therefore a consistent progress in using environmentally sustainable (metal free) catalysts to efficiently extract dihydrogen from small B−N bonded molecules.     

Dihydrogen Activation with a Neutral,Intermolecular Silicon(IV)-Amine Frustrated Lewis Pair

The heterolytic cleavage of dihydrogen constitutes the hallmark reaction of frustrated Lewis pairs (FLP). While being well-established for planar Lewis acids, such as boranes or silylium ions, the observation of the primary H₂ splitting products with non-planar Lewis acid FLPs remained elusive. In the present work, we report bis(perfluoro-N-phenyl-ortho-amidophenolato)silane and its application in dihydrogen activation to a fully characterized hydridosilicate. The strict design of the Lewis acid, the limited selection of the Lewis base, and the distinct reaction conditions emphasize the narrow tolerance to achieve this fascinating process with a tetrahedral Lewis acid.     

Do Rhodium Bis(σ‐amine‐borane) Complexes Play a Role as Intermediates in Dehydrocoupling Reactions of Amine‐boranes?

The recently synthesized rhodium complex [Rh{P(C₅H₉)₂(η²‐C₅H₇)}(Me₂HNBH₃)₂]BAr^F₄ ( 2 ), which incorporates two amine‐boranes coordinated to the rhodium center with two different binding modes, namely η¹ and η², has been used to probe whether bis(σ‐amine‐borane) motifs are important in determining the general course of amine‐boranes dehydrocoupling reactions. DFT calculations have been carried out to explore mechanistic alternatives that ultimately lead to the formation of the amine‐borane cyclic dimer [BH₂NMe₂]₂ ( A ) by hydrogen elimination. Sequential concerted, on‐ or off‐metal, intramolecular dehydrogenations provide two coordinated amine‐borane molecules. Subsequent dimerization is likely to occur off the metal in solution. In spite of the computationally confirmed presence of a BH???NH hydrogen bond between amine‐borane ligands, neither a simple intermolecular route for dehydrocoupling of complex 2 is operating, nor seems [Rh{P(C₅H₉)₂(η²‐C₅H₇)} B ]⁺ to be important for the whole dehydrocoupling process.