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Mechanistic insights into the thermal decomposition of ammonia borane, a material Cite this: Inorg. Chem. Front., 2021, 8, 1900 studied for chemical hydrogen storage

Umit B. Demirci

Though ammonia borane NH3BH3 (AB) was discovered in the 1950s, it is fair to state that AB as a potential chemical hydrogen storage material was discovered more recently, in the 2000s. Unlike the isoelectronic

ethane CH3CH3, AB is polar; three of its hydrogens are protic (NH3 group) and the other three are hydridic δ+ δ− (BH3 group); the material is solid at ambient conditions owing to dihydrogen N–H ⋯H –B interactions; and AB decomposes from 90 °C under thermogravimetric conditions. With such properties, AB has attracted much attention, even though AB in neat form is not suitable for the application mentioned above because it decomposes more than it dehydrogenates. Hence, strategies (based on solubilization,

catalysis, chemical doping and nanosizing) aiming at destabilizing AB to make it release pure H2 at <100 °C have been developed. Beyond the performance targeted for hydrogen storage, this provided us with better understanding of the mechanisms of decomposition. Indeed, studies on thermal decomposition of neat AB have revealed just how complex the mechanisms are (due to the involvement of two possible key intermediates initiating the decomposition, the formation of various volatile products, the existence of counterintuitive homopolar reactions, and the formation of polymeric residues of complex composition, for example). Studies on destabilized AB have provided insights into several mechanistic aspects including the reaction intermediates, the decomposition pathways, and the nature of the residue

forming upon the release of 1 and ≥2 equiv. H2. We presently have a fairly good understanding of the mechanisms of decomposition of AB, which is discussed in more detail below. In that respect, this review Received 17th November 2020, focuses ﬁrstly on the complexity of thermal decomposition of neat AB, secondly on what we know with Accepted 1st February 2021 Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. regard to thermal decomposition of destabilized AB, and thirdly on all outstanding questions. It is very DOI: 10.1039/d0qi01366h important to have an excellent knowledge of the reaction mechanisms if technological progress is to be rsc.li/frontiers-inorganic made with AB as a chemical hydrogen storage material.

AB is a much studied chemical hydrogen storage 1. Introduction 5–16 material, despite the fact that it does not store H2 reversi- Ammonia borane NH3BH3 (AB, Chart 1) and ethane CH3CH3 bly. AB stores the H atoms through its N–H and B–H bonds. It are isoelectronic. They both carry six H atoms and, due to the follows that, under heating, AB dehydrogenates by a reaction δ δ− lightness of the B, N and C atoms, they have high gravimetric between H + and H through an exothermic process. hydrogen densities with 19.6 and 20.1 wt% H respectively. Consequently, the dehydrogenated form of AB, a polymeric

Similarities between these two compounds end here. In AB, solid also called residue and denoted BNHx with x < 6, cannot – ﬀ the N B bond is polarized, the H atoms of the NH3 group are be hydrogenated at a ordable conditions in terms of pressure δ+ 17–19 protic H , and the H atoms of the BH3 group are hydridic of H2 and temperature. In contrast, porous materials and δ− δ δ− H .1 This allows heteropolar dihydrogen N–H +⋯H –B inter- metal hydrides for example can be dehydrogenated and hydro- 2 actions to occur. As a result of these electrostatic interactions, genated in a reversible way by tuning the pressure of H2 and – AB is a solid.3 It dehydrogenates very slowly at ambient con- the temperature.20 24 As it is, the only sustainable solution to δ+ δ− 4 ditions by reaction between H and H . Also, it possesses recover the starting AB is to chemically recycle BNHx. Hausdorf

attractive intrinsic properties for chemical hydrogen storage. et al. developed a stepwise process where BNHx is digested in hydrochloric acid HCl; the as-obtained ammonium chloride

NH4Cl produces ammonia NH3, and the other product boron Institut Européen des Membranes, IEM – UMR 5635, ENSCM, CNRS, Univ trichloride BCl3 is hydrodechlorinated into diborane B2H6 Montpellier, Montpellier, France. E-mail: [email protected] (DB); in a last step AB is synthesized by a reaction between

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25 NH3 and DB. Sutton et al. developed a simpler process that of decomposition should help us be more eﬀective in de- applies to polyborazylene (PB) only.26 The process just requires stabilizing AB. This article aims at answering the above ques-

suspending PB in liquid NH3 (−77 °C) and reducing it with tion, first by focusing on what we know with regard to thermal

hydrazine N2H4 at 40 °C. Despite these achievements, the decomposition of neat AB, and second by surveying the ﬃ ﬀ e cient and cost-e ective regeneration of AB from BNHx is open literature dedicated to thermal decomposition of destabi- still challenging. lized AB. Neat AB is not really suitable for chemical hydrogen storage because, under heating, it decomposes more than it dehydro-

genates and it transforms into BNHx that is of complex composition. Decomposition takes place at temperatures higher than 2. Neat AB 100 °C, which is too high from a viewpoint of technological 2.1. A stepwise thermal decomposition implementation. This has paved the way for the development of strategies aiming at destabilizing AB and promoting its A common technique to study the thermal decomposition of dehydrogenation at <100 °C. neat AB is thermogravimetric analysis. A sample is heated at a There are four strategies for destabilizing AB. The first strat- constant heating rate, under an inert gas flow transporting any egy is based on the solubilization of AB in aprotic solvent volatile product away from the furnace. At a heating rate of − (organic or ionic liquid). The second strategy is an evolution of >1 °C min 1, neat AB undergoes a stepwise decomposition the first one since a catalyst is added into the solution. The that features two successive mass losses occurring within the third strategy is chemical doping: a solid-state dopant is added temperature range 90–200 °C. For the first decomposition step to solid-state AB. The fourth strategy aims at nanosizing AB (Fig. 1, at 90–130 °C), Hu et al. reported the release of 1 equiv. thanks to the use of a scaﬀold. It is worth mentioning another H2 and the formation of polyaminoborane (NH2BH2)n (PAB) by strategy (the fifth one). AB can be chemically modified in dehydrocoupling.32 The reaction is exothermic. For instance, − −1 33 order to produce metal amidoboranes M(NH2BH3)α (with M an Wolf et al. determined a mean enthalpy of 21.7 kJ mol . In alkaline, alkaline-earth, transition or group-13 metal, and α = the second decomposition step (Fig. 1, at 130–200 °C), PAB 27–31 1, 2, 3 and 4). Amidoboranes are ionic salts and are releases n equiv. H2 and transforms into polyiminoborane beyond the scope of this review. (NHBH)n (PIB). This is the AB-to-PAB-to-PIB pathway. The The four destabilization strategies introduced above allow mass loss measured at 200 °C is generally high (for example AB to improve its dehydrogenation properties. Indeed, AB ∼35 wt%) because of formation of volatile impurities. dehydrogenates much more than it decomposes, thereby There is a certain consistency in the nature of the volatile

releasing purer H2; dehydrogenation takes place at lower temp- impurities. Baitalow et al. detected small quantities of DB, bor- erature; and PB favorably forms upon the release of ≥2 equiv. azine B3N3H6 (BZ) and aminoborane NH2BH2 during the first 5–16 H2. This raises the question as to what are the reaction decomposition step, and a large amount of BZ in the second 34 mechanisms behind the improvements. This is all the more decomposition step. Wolf et al. detected NH2BH2 prior to Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. important given that a good understanding of the mechanisms BZ.33 They concluded that BZ forms upon the formation of NH2BH2:

3NH2BH2 ! B3N3H6 þ 3H2 ð1Þ Umit B. Demirci (UBD; https:// Frueh et al. reported similar observations.35 Shimoda et al.

sites.google.com/view/umitbde- detected DB, NH3 and NH2BH2 as the main volatile impurities, mirci/home) is full professor and as well as BZ but to a much lesser extent.36 A plausible expla- materials chemist at the nation of the low amount of BZ may be the rapid flow rate − University of Montpellier. UBD (300 mL min 1) that the authors set for the thermogravimetric works on boron- and/or nitrogen- analysis. Such a rate is likely to favor rapid transportation of based materials for solid-/liquid- volatile products away from the furnace, thereby mitigating the

state hydrogen storage (such as formation of BZ from NH2BH2. sodium borohydride, ammonia It is worth noting that thermogravimetric results are greatly borane, metal hydrazinidobor- dependent on operation conditions. Petit et al. showed that anes, and boron nitride-based two AB samples synthesized in diﬀerent conditions and having derivatives). Recently, he similar purity are able to display dissimilar thermogravimetric Umit B. Demirci initiated new projects to explore results.37 They also showed that a given AB sample is able to novel B–N-based materials for overcome a mass loss of <20 wt% with one thermogravimetric 38 reversible storage of H2 and other applications. With respect to his analyzer and a mass loss of >50 wt% with another analyzer. curriculum vitae, he got his PhD in Physical Chemistry at the Comparison of mass losses thus needs to be done with University of Strasbourg in 2002; then, he had experience in the caution. automotive industry and academic institutions (e.g. the University The mechanism proposed by Hu et al. (Fig. 1) is straight- of Lyon 1). forward. It has been used often to describe the thermal dehy-

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Chart 1

34–40 þ ! ð Þ drogenation of AB in a clear way. However, the real mecha- NH3BH3 NH2BH2 NH3NH2BH2BH3 3 nism is considerably more complex. Roy et al. suggested that the dehydrocoupling initiates fol-

lowing an autocatalytic mechanism where NH2BH2 is respon- sible for the autocatalytic eﬀect (Fig. 2).46 NH BH reacts with 2.2. Intermediates initiating the thermal decomposition 2 2 AB to produce various intermediates such as B-(cyclodiboraza-

As mentioned above, NH2BH2 is one of the volatile products nyl)amine-borane (NH2BH2)2NH2BH3 (BCDB) and cyclotribora- 41–43 forming first. Its formation implies an intramolecular zane (NH2BH2)3 (CTB). Zhong et al. reached contradictory pre- δ+ δ− reaction between one H and one H of AB: dictions. Their first-principles calculations suggested the preferential occurrence of an intramolecular reaction between two NH3BH3 ! NH2BH2 þ H2 ð2Þ AB molecules:47 By first-principles molecular dynamics calculations, Liang ! þ ð Þ and Tse predicted the predominance of this reaction (eqn 2NH3BH3 NH3NH2BH2BH3 H2 4 (2)).44 Zimmerman et al. went a step further and, by using Another way to study the thermal decomposition of neat AB

ab initio CCSD(T) simulations, they predicted that NH2BH2 is is to be in isothermal conditions: the temperature is fixed the intermediate initiating the dehydrocoupling of AB:45 below the melting point of AB (∼100 °C). For instance,

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high-level electronic calculations (correlation-consistent aug- cc-pVTZ basis set at the second-order perturbation MP2 level), Nguyen et al. confirmed the predominant role of DADB.54 – Other studies arrived at the same conclusion.55 61 On the basis of calculations using ab initio molecular dynamics and metadynamics, Rizzi et al. predicted the formation of two successive intermediates, ammonia diborane + 62 NH3BH2(µ-H)BH3 (ADB) and then NH4 :

2NH3BH3 ! NH3BH2ðμ-HÞBH3 þ NH3 ð5Þ

þ NH3BH2ðμ-HÞBH3 þ NH3 ! NH2BH2 þ NH4 þ BH4 ð6Þ The initiation intermediate that plays the key role is sup- + posed to be NH4 . It triggers an autocatalytic mechanism

resulting in the formation of NH2BH2 and H2:

þ þ NH4 þ NH3BH3 ! NH3 þ NH3½BH4 ð7Þ

þ þ NH3 þ NH3½BH4 ! NH4 þ NH2BH2 þ H2 ð8Þ However, as stated by Gao et al.,63 the simulation conditions do not represent the realistic experimental conditions of the thermal decomposition of neat AB.

2.3. BNHx, a residue of complex composition

The residue forming upon the release of 1 equiv. H2 is PAB (Fig. 1), more exactly linear and branched PABs.36,53,64 The

residue forming upon the release of 2 equiv. H2 has often been reported to be PIB. In reality, PB also forms, as evidenced by Shimoda et al.36 Likewise, Kobayashi et al. concluded with the dehydrocyclization of branched PAB into PB.64 Roy et al. Fig. 1 Stepwise thermal decomposition of neat AB, according to Hu modeled the second decomposition step of AB with a nucleus et al.32 In the ﬁrst decomposition step at 90–130 °C, PAB forms by the growth mechanism and concluded with the formation of PB dehydrocoupling of AB. In the second step of decomposition, at by dehydrocoupling of BZ.46 This model was also supported by – 130 200 °C, PAB dehydrogenates and PIB forms. This is the AB-to-PAB- 65 Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. to-PIB pathway. The volatile impurities that can form at each step (DB, DFT calculations.

NH3,NH2BH2 and BZ) are also shown. With the formation of PAB and then the formation of PI and/or PIB, it is understood that all of the AB molecules dehydrocouple simultaneously and similarly.66 However, Petit et al. reported nuclear magnetic resonance results that are in contradiction.67 They analyzed the residues forming upon the Heldebrant et al. measured the release of ∼1 equiv. H2 at 75 or release of 0.5 and then 1 equiv. H in isothermal conditions 90 °C, and observed that the release of H2 starts after an induc- 2 tion period that shortens with the increase of the temperature (60 °C). The spectra indicated a complex composition with the – (Fig. 3).48 Comparable results were reported elsewhere.49 52 coexistence of AB, DADB, PAB (linear and branched), PIB and Stowe et al. studied the mechanism leading to the release PB. It is concluded that all of the AB molecules do not de- 53 hydrocouple simultaneously (Fig. 5), resulting in a hetero- of 1 equiv. H2 at 88 °C. Two initiation intermediates were identified by means of 11B magic angle spinning nuclear mag- geneous composition involving the species mentioned above. netic resonance spectroscopy (Fig. 4). The first one is a mobile This is in line with the predictions reported by Miranda and 68 phase of AB (denoted AB*). It forms by disruption of the dihy- Ceder. By using DFT, they predicted that the AB-to-PAB-to- δ δ− drogen N–H +⋯H –B bonding during the induction period. PIB pathway and the AB-to-BZ-to-PB pathway are both thermo- The second initiation intermediate is diammoniate of dibor- dynamically favored and are likely to be concomitant (Fig. 6). + − ane [(NH3)2BH2] [BH4] (DABD). This is the ionic dimer of AB that forms by the combination of two AB* during the nuclea- 2.4. Counterintuitive homopolar dihydrogen interactions tion period. DADB is reactive and initiates the formation of One of the key properties of AB (more broadly of amine borane PAB by reaction with AB during the growth period. There are adducts) is the existence of heteropolar dihydrogen N– δ δ− parallel reactions, for instance, one leading to the dimerization H +⋯H –B interactions between molecules. In view of this, of AB (eqn (4)) and another one leading to the formation of we have long seen the dehydrogenation of AB as the result of δ+ δ− the cyclic dimer, cyclodiborazane (NH2BH2)2 (CDB). By using heteropolar N–H ⋯H –B reactions (Fig. 4).

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Fig. 2 Mechanism of decomposition of neat AB, according to Roy et al.46 The reaction initiates with the dehydrogenation of AB and the formation

of NH2BH2. Then, NH2BH2 autocatalyzes the dehydrocoupling of AB, which leads to the stepwise formation of BCDB and CTB.

δ δ δ δ N–D +⋯D +–N(N–H +⋯H +–N) interactions. They also showed δ− δ− that the contribution of the homopolar B–H ⋯H –B

Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. δ− δ− (B–D ⋯D –B) interactions was comparable to that of the het- δ δ− δ δ− eropolar N–D +⋯H –B(N–H +⋯D –B) interactions. These observations were confirmed by Roy et al.70 Elsewhere, the con- δ δ− δ δ− tribution of heteropolar N–H +⋯D –B(N–H +⋯D –B) interactions was found to be greater than that of homopolar δ δ N–H +⋯H +–N interactions, whereas the contribution of homo- δ− δ− polar B–D ⋯D –B interactions was negligible.71

Al-Kukhun et al. studied ND3BH3. They confirmed the contribution of homopolar interactions throughout the dehydro- δ δ− genation of AB.72 They noticed that heteropolar N–D +⋯H –B interactions predominated in the first decomposition step:

Fig. 3 Release of H2 from neat AB at isothermal conditions (75, 80, 85 ND3BH3 ! ND2:3BH1:9 þ 0:28 H2 þ 0:55 HD þ 0:05 D2 ð9Þ and 90 °C). The release of H2 is preceded by an induction period that is temperature-dependent, and the induction period shortens with the – δ+⋯ δ+– – δ−⋯ δ−– increase of the temperature. Reprinted with permission from ref. 48. Homopolar N D D N and B H H B interactions Copyright 2008 American Chemical Society. made a greater contribution in the second decomposition step:

ND2:3BH1:9 ! ND1:8BH0:8 þ 0:68 H2 þ 0:85 HD þ 0:19 D2 69 Wolstenholme et al. called this into question. Exploring ð10Þ the thermal decomposition of deuterated derivatives of AB

such as ND3BH3 and NH3BD3, they evidenced the formation of There was thus a disproportion in the degree of dehydro-

HD as well as that of both H2 and D2, which was possible by genation of the groups ND3 and BH3 of ND3BH3 and, by exten- – δ−⋯ δ−– – δ−⋯ δ−– the existence of homopolar B H H B(B D D B) and sion, the groups NH3 and BH3 of AB.

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Fig. 4 Mechanism of decomposition of neat AB, according to Stowe et al.53 The mechanism is understood as the formation of AB* during the induction period, the formation of DADB during the nucleation period, and the formation of PAB during the growth mechanism. Parallel reactions also take place: the AB dimer forms by dehydrocoupling of 2 AB molecules; and CDB forms from DADB. Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM.

number of volatile products (NH3, DB, NH2BH2, BZ) that form. Second, there are two possible key intermediates that initiate

the decomposition of AB. The first one is NH2BH2. The second one is DADB. It is worth mentioning that NH2BH2 is more reactive than DADB, thereby having a shorter lifetime, and it has not been proposed as a possible precursor of DADB.

Whether the initiation intermediate is NH2BH2 or DADB, they both produce linear and cyclic intermediates such as Fig. 5 Schematized thermal decomposition of one microsized grain of NH BH NH BH , CDB, CTB and/or BCDB. Third, a mixture of neat AB, at isothermal conditions, according to Petit et al.67 It is 3 2 2 3 assumed that all of the AB molecules in the grain do not decompose in linear and branched polymers forms upon the release of 1 and a simultaneous way. The inner part of the grain consists of a mixture of then 2 equiv. H2. These polymers are PAB, PIB and PB, when δ+ δ− AB* and DADB upon the evolution of 0.5 and then 1 equiv. H2. With heteropolar N–H ⋯H –B reactions drive dehydrocoupling. respect to the molecules in the outer part (as highlighted by the light Polymers containing B–B and N–N bonds form when homopo- green box), they release the H and they transform into PAB, branched 2 lar reactions contribute to the decomposition of AB. Fourth, PAB (denoted b-PAB), PIB and PB. the AB molecules in a microsized grain are likely to decompose unevenly, resulting in a mixture of AB*, DADB, linear and

branched PAB, PIB and PB upon the release of 1 equiv. H2. 2.5. Conclusion about thermal decomposition of neat AB Studying neat AB from a mechanistic point of view is tricky. Thermal decomposition of neat AB has proven more complex Furthermore, the tendency of neat AB to decompose into vola- than the AB-to-PAB-to-PIB pathway originally reported. It is tile impurities and a residue of complex composition makes it

complex for at least four reasons. First, there are, besides H2,a unsuitable for chemical hydrogen storage. AB has thus been

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In diglyme, AB (0.15 M) generated H2 and CTB as well as BZ as a product of CTB: ð Þ ! þ ð Þ NH2BH2 3 B3N3H6 3H2 12 In glyme, AB (0.15 M) generated a small amount of μ μ -aminodiborane ( -NH2)B2H5 (µADB). Following the seminal study mentioned above, some eﬀorts were devoted to understanding the destabilizing eﬀect of glymes on AB. Shaw et al. performed in situ 11B and 15N nuclear magnetic resonance analyses to get an insight into the initiation mechanism of the thermal decomposition of AB in glyme (Fig. 7).74 They suggested that, in such conditions, two AB molecules dimerize to form a DADB intermediate. Being unstable in the solvent, DADB quickly dehydrocyclizes to form

CDB. As a next step, CDB reacts with AB to produce H2 and BCDB. CTB was also detected as a product. Similar observations were reported for AB solubilized in diglyme,72 or – tetraglyme.75 77 Aside from CDB, Kostka et al. detected µADB as a product of DADB via the formation of CTB (Fig. 8).78 They also observed that high concentrations of DADB are prone to promote the formation of BCDB. All of these molecules are reaction intermediates leading up to BZ and ultimately to PB. None of the experimental studies discussed above mentions

NH2BH2 as an initiation intermediate. In contrast, compu-

tational simulations emphasize the key role of NH2BH2 in the – initiation mechanism of dehydrogenation.79 81 They however

emphasize the high instability of NH2BH2, which may explain why it has not been detected and identified by nuclear magnetic resonance. Fig. 6 Mechanism of decomposition of neat AB, according to Miranda Bluhm et al. studied the thermal decomposition of AB in and Ceder.68 The mechanism was predicted on the basis of DFT calcu- 1-butyl-3-methylimidazolium chloride (bmimCl).82,83 At 85 °C, lations. The full-line black arrows indicate the thermodynamically AB in bmimCl immediately released H2. The ionic liquid pro-

Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. favored pathways (at 0 K). The dashed-line grey arrows show the thermodynamically disfavored pathways (at 0 K). It is suggested that the moted the rapid formation of DADB. This was supported by AB-to-PAB-to-PIB and the AB-to-BZ-to-PB pathways are both thermo- the second-order Møller–Plesset perturbation calculation dynamically favored. results reported by Mahato et al.84 AB in bmimCl was able to

release ∼1 equiv. H2 in 3 h. The decomposition until the

release of 2 equiv. H2 was described as a stepwise process where PAB forms by dehydrocoupling of DADB and/or AB, PAB ﬀ destabilized by using di erent strategies, allowing for a much dehydrogenates into branched PAB and/or PIB, and branched better understanding of its mechanisms of decomposition. PAB and/or PIB dehydrocyclizes into PB. Nakagawa et al. evidenced the formation of PB.85 Studies focusing on bmimCl – and other ionic liquids reached the same conclusions.86 88 3. AB destabilized in an aprotic solvent

As mentioned in the introduction, there are four strategies for 4. AB in solution destabilized by a destabilizing AB, and the first of them is based on the solubil- catalyst ization of AB in aprotic solvent (organic or ionic liquid). 4.1. Homogeneous catalysis using metal complexes Acetonitrile and pyridine are unsuitable solvents because they react with AB. Ethereal solvents are more appropriate. For In the presence of a catalyst, the kinetics and degree of dehy- 89,90 instance, Wang and Geanangel observed that, heated at 85 °C, drogenation of AB are expected to be further improved. AB (0.15 M) in tetrahydrofuran decomposed into CTB and Denney et al. reported the first homogeneous transition metal 73 complex able to catalyze the thermal dehydrogenation of AB in H2: 91 tBu tetrahydrofuran. The complex is (POCOP )Ir(H)2, with NH BH ! 1=3 ðNH BH Þ þ H ð11Þ tBu η3 3 3 2 2 3 2 POCOP as [ -1,3-(OPtBu2)2C6H3] (denoted [Ir](H)2 hereafter,

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diﬀerent initiation mechanisms were reported to explain its formation. Keaton et al., who studied the thermal dehydrogenation of AB in diglyme in the presence of a N-heterocyclic carbene

nickel complex Ni(NHC)2, proposed a mechanism where Ni 108 activates one B–H bond to form H–Ni–BH2NH3. This step is δ− followed by a β-elimination (reaction between H of H–Ni and δ+ H of NH3) that produces H2 and NH2BH2. For the same catalyst, Yang and Hall predicted a diﬀerent, more complex initiation mechanism, by using DFT with the ab initio TPSS functional and the all-electron cc-pVDZ basis set.109,110 The activation of AB implies the coordination of Ni with AB via the δ+ BH3 group and the transfer of H of NH3 to a Ni-bound C (Fig. 10). The ligand is cooperative and acts as a co-catalyst. δ Subsequently, Ni transfers the H + of the activated C–H bond δ− and the H of the coordinating B–H bond to form ⋯ ⋯ – H2 Ni H2B NH2. As a final step, H2 and NH2BH2 are released. This initiation mechanism was also suggested as – being valid for Ni, Fe and Ru complexes.111 115 Another initiation mechanism where the ligand plays an active role was reported by Marziale et al. for the complex 116 [Ru(H)2(PMe3)(HPNP)], with PNP as N(CH2CH2PiPr2)2. The mechanism is ligand-centered (Fig. 11). The ligand activates AB (in tetrahydrofuran). This is shown by the formation of Ru– δ+ H2 via the transfer of H of NH3 and coordination of NH2– – – ⋯ – BH3 with the N H of the ligand such as N H NH2 BH3. Then, Ru–H2 releases H2 and the unsaturated Ru coordinates δ− with H of BH3 to form N–H⋯NH2–BH2–H⋯Ru. Finally,

NH2BH2 is released. An analogous ligand-centered initiation mechanism was suggested for the complex [Fe(H)CO(PNP)] 117 with PNP as N-(CH2CH2PiPr2)2, and a heterocyclic phosphenium complex of Mn.118 Kim et al. studied the thermal dehydrogenation of AB in tet- Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. raglyme in the presence of the Pd complex [Pd 119 (MeCN)4][BF4]2. They conducted experiments using the four Fig. 7 Mechanism of decomposition of AB in glyme, according to Shaw isotopes of AB (NH3BH3,ND3BD3,NH3BD3 and ND3BH3) and et al.74 AB in solution dimerizes to form a DADB intermediate that, being DFT calculations. They found an initiation mechanism unstable in the solvent, quickly dehydrocyclizes. CDB forms and reacts (Fig. 12) that diﬀers from the mechanisms reported above. It is with AB to produce H2 and BCDB. Another reaction product is CTB but proposed that Pd activates AB via an α-agostic B–H⋯Pd inter- the mechanism of its formation is not well understood yet. action, and AB replaces one MeCN ligand. The ligand remains in the close vicinity of the complex since it interacts with AB

via a hydrogen bond with NH3. This is the BH3 group of AB for clarity). AB catalyzed by 0.5 mol% of [Ir](H)2 released 1 that predominantly dehydrogenates, such as:

equiv. H2 in 14 min at room temperature, confirming the posi- NH BH ! 1=n ðH NBÞ þ 2H ð13Þ tive eﬀect of the catalyst. In these conditions, PAB formed.92 3 3 2 n 2 Following this seminal study, particular emphasis was put Such a mechanism suggests that each AB molecule is action the initiation mechanism of dehydrogenation. Paul and vated by Pd. The same initiation mechanism was reported else- Musgrave predicted, on the basis of DFT calculations, that where for other Pd and Ru complexes.120,121

[Ir](H)2 releases H2 in a preliminary step so that H-free Ir is The initiation mechanisms discussed above diﬀer, but they – able to bind AB to form an intermediate where one B H bond all agree that NH2BH2, which is highly reactive, is the initiation and one N–H bond have been simultaneously activated intermediate of the dehydrogenation of AB in solution. 93 (Fig. 9). Then, the intermediate releases NH2BH2, while NH2BH2 reacts with AB to form, by dehydrocyclization, oligo- 122–124 freeing up [Ir](H)2. A similar initiation mechanism was mers such as BCDB, CTB and BZ. Upon the release of 1 94–107 125 reported for Ir, Ru, Rh, Os, Fe and Ni complexes. equiv. H2, PAB forms. Using DFT calculations, Ghatak and There is full agreement on the formation of NH2BH2 as a Vanka predicted that the oligomerization of AB in the presence

key intermediate initiating the decomposition of AB. However, of [Ir](H)2 follows a chain growth mechanism involving the

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Fig. 8 Mechanism of decomposition of AB in triglyme, according to Kostka et al.78 Two pathways are proposed. The ﬁrst one takes place for high concentrations of DADB (>1.5 M): DADB reacts with AB to form BCDB; BCDB dehydrogenates and BZ forms; BZ dehydrocouples to produce PB. The second pathway takes place for low concentrations (of AB): two AB molecules dehydrocouple to produce CDB; CDB reacts with another AB to form

CTB that then produces µADB (together with an intermediate proposed to be NH2BHNH2) and BZ; BZ dehydrocouples to produce PB.

‘ ’ 129–132 transient species Ir(H)2(NH2BH2) and free NH2BH2 Upon the release of >2 equiv. H2, PB forms. PB forms molecules:126 from BZ that is a product of various cyclic oligomers as men- – tioned above.133 138 For instance, Bhunya et al., using DFT cal- ½ ð Þ þ ! ð Þ ð Þþ ð Þ Ir H 2 NH3BH3 Ir H 2 NH2BH2 H2 14 culations, stressed the involvement of CTB and BCDB as intermediates of BZ and PB.133 Kalviri et al. introduced a cyclic ami- ð Þ ð Þþ ! ð Þ ð Þ ð Þ Ir H 2 NH2BH2 4NH2BH2 Ir H 2 NH2BH2 5 15 noborane tetramer, B-(cyclotriborazanyl)amine-borane (BCTB), as the initiation intermediate of the second step of dehydro- According to Bhunya et al., the oligomerization is termi- genation of AB.134 nated by reaction with AB:127

IrðHÞ ðNH BH Þ þ NH BH ! IrðHÞ þ BH ðNH BH Þ NH 2 2 2 5 3 3 2 3 2 2 5 3 4.2. Heterogeneous catalysis using metals ð16Þ Shrestha et al. studied Pt (0.5%) supported on alumina for the Kumar et al. discussed the oligomerization of AB in a thermal dehydrogenation of AB in 2-methoxyethyl ether at similar way while considering the occurrence of doubly metal- 70 °C.139 Improved dehydrogenation properties were observed 128 ∼ bound oligomers such as Ir(NH2BH2)n−1(NH2BH2)n′−1. in terms of kinetics and dehydrogenation extent (with 2

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3 nm Pd nanoparticles (2.5 mM) for catalyzing the thermal dehydrogenation of AB (200 mM) in tetrahydrofuran. AB 141 released 2 equiv. H2 in 100 min at 25 °C. Cyclic oligomers and PAB, and then PB, were suggested to form. By using the DFT/UB3LYP method, Tong et al. predicted that two Pd sites

are required for the dehydrogenation of AB into H2 and 142 NH2BH2. In summary, heterogeneous metal nanoparticles – eﬃciently catalyze the thermal decomposition of AB.143 147

4.3. Metal-free catalysis Frustrated Lewis pairs are potential bifunctional metal-free cat- – alysts for thermal dehydrogenation of AB.148 150 For instance, Fig. 9 Initiation mechanism of dehydrogenation of AB in tetrahydro- Guo et al. explored, by DFT, the catalytic capacity of 93 ﬁ 151 furan catalyzed by [Ir](H2), according to Paul and Musgrave In a rst N-PMTN-CH2C6H4B(C6F5)2 (denoted as N/B pair for clarity). step, [Ir](H)2 releases its H2 and H-free Ir binds to AB. In a second step, The calculations predicted an activation of AB by the N/B pair as-activated AB is dehydrogenated and NH BH is released. This way, 2 2 through N–H⋯N and B–H⋯B interactions, the formation of [Ir](H2) is regenerated. NH2BH2 at low temperature, and the release of H2 at 110 °C t (Fig. 14). Miller and Bercaw studied P Bu3/B(C6F5)3, a P/B pair.152 It eﬀectively catalyzed the dehydrogenation of AB in

equiv. H2). Reaction intermediates such as CDB, CTB and BZ chlorobenzene at 25 °C. AB released 1 equiv. H2 and branched form thanks to NH2BH2 that is supposed to play the key role in PAB formed. Boom et al. focused on another P/B pair, namely 153 the initiation mechanism (Fig. 13). Finally, PB forms. Ayvali tBu2PCH2BMes2. AB in tetrahydrofuran dehydrogenated at et al. synthesized Rh nanoparticles of 2 nm (15 mM) to cata- room temperature and released 2 equiv. H2. Various reaction lyze the thermal dehydrogenation of AB (1 M) in tetrahydro- intermediates were identified by 11B{1H} and 31P{1H} nuclear 140 furan at 25 °C. AB dehydrogenated immediately and magnetic resonance (Fig. 15). It is suggested that two P/B pairs released 1.4 equiv. H2 in 7 h. PAB and PB were identified as are required to activate one AB molecule. One of the pairs the main components of the residue. Metin et al. developed binds to two H of AB through H–P and H–B bonds, and the Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM.

109,110 Fig. 10 Initiation mechanism of dehydrogenation of AB catalyzed by Ni(NHC)2, according to Yang and Hall. There are four steps. In the ﬁrst δ+ step, AB loses one H by transfer from N to unsaturated C in the carbene of Ni(NHC)2; concomitantly, NH2BH3 binds to Ni. In the second step, the as-formed C–H is activated, which implies that the H is transferred from C to Ni. In the third step, one Hδ− of AB is transferred from B to Ni, resulting

in the formation of H2 bound to Ni. In the fourth step, NH2BH2 and H2 are released, and Ni(NHC)2 is regenerated. In this mechanism, the ligand is cooperative and acts as co-catalyst.

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Fig. 11 Initiation mechanism of dehydrogenation of AB in tetrahydrofuran catalyzed by the complex [Ru(H)2(PMe3)(HPNP)], with PNP as 116 δ+ N(CH2CH2PiPr2)2, according to Marziale et al. AB is activated by the catalyst by transfer of H from the N of AB to Ru–H. The as-formed H2 binds to Ru, and NH2BH3 binds to the complex via N⋯H⋯N interaction. Upon the release of the H2, Ru binds to NH2BH3 via aB–H–N bridge. Finally,

NH2BH2 is released and the complex is regenerated.

borane B(C6F5)3, and a Brønsted acid that is trifluoromethane 154 sulfonic acid HOSO2CF3. With both acids, the mechanism δ− was initiated by the abstraction of H from AB (in glymes) + resulting in the formation of the cation [NH3BH2] :

ð Þ þ !½ ð Þ þ½ þ ð Þ B C6F5 3 NH3BH3 HB C6F5 3 NH3BH2 17

þ HOSO2CF3 þ NH3BH3 !½OSO2CF3 þ½NH3BH2 þ H2 ð18Þ

+ [NH3BH2] , an unstable cation, readily reacts with AB (Fig. 16). Various cationic intermediates form, before their dehydrocyclization and the formation of BZ. Similar cations Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. were reported for crown ethers used as Lewis acids.155 Lu et al. studied bis(borane) Lewis acids; 9,10-dichlorodiboraanthra- cene emerged as the most eﬃcient catalyst.156 It catalyzed the

release of ∼2.5 equiv. H2 in 7 h at 60 °C. BZ and PB formed. The dehydrogenation of AB initiates with the formation of

NH2BH2. Hasenbeck et al. screened a series of pyridines and isolated the most eﬃcient one, namely 6-tert-butyl-2-thiopyri- 157 ∼ done. It catalyzed the release of 1.8 equiv. H2 at 60 °C. The Fig. 12 Initiation mechanism of dehydrogenation of AB in tetraglyme authors stressed the key role of NH2BH2. 119 catalyzed by the complex [Pd(MeCN)4][BF4]2, according to Kim et al. Bases also act as catalysts. Himmelberger et al. studied bis Pd activates AB via an α-agostic B–H⋯Pd interaction. This way, AB (dimethylamino)naphthalene, also called Proton Sponge.158 replaces one MeCN ligand, and this ligand binds to AB via a hydrogen AB in the ionic liquid bmimCl showed improved dehydrogena- δ− δ− bond with NH3. One H of AB is transferred to Pd. This H reacts with δ− tion properties. The mechanism is initiated as follows. The another H of AB to release H2 and produce the species NH3BH. For δ+ − base extracts one H of AB and the anion [NH2BH3] forms. this mechanism, the dehydrogenation of AB is mainly driven by the BH3 group, leading to a residue such as (H NB)n. Dehydrocoupling then takes place by a reaction between 2 − [NH2BH3] and AB molecules. Linear and branched anionic PABs form, and then dehydrogenate to produce anionic PIB and finally PB. Ewing et al. reported a similar mechanism for

other one binds to NH2BH2 through N–P and B–B bonds. In the thermal dehydrogenation of AB catalyzed by a Verkade’s further steps that have not been detailed, as-activated AB pro- base, i.e. 2,8,9-triisobutyl-2,5,8,9-tetraaza-1-phosphabicyclo duces CTB, BCDB and BZ. [3.3.3]undecane.159 They evidenced the formation of anionic − Acids are other potential catalysts. Stephens et al. studied aminoborane oligomers such as [HB(NH2BH3)3] and − two strong acids: a Lewis acid that is tris(pentafluorophenyl) [H3BNH2BH2NH2BH2NH2BH3] .

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Fig. 13 Mechanism of decomposition of AB in 2-methoxyethyl ether catalyzed by Pt (0.5%) supported on alumina, according to Shrestha et al.139

The key intermediate is NH2BH2. It forms in a ﬁrst step by catalytic dehydrogenation of AB. It then reacts with AB molecules to produce CDB, BCDB and CTB.

Fig. 15 Mechanism of decomposition of AB in tetrahydrofuran cata- 153 lyzed by the P/B pair tBu2PCH2BMes2, according to Boom et al. Two Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. P/B pairs are required to dehydrogenate one AB molecule. Unknown Fig. 14 Initiation mechanism of dehydrogenation of AB catalyzed by species are also suggested to form in a ﬁrst step. As-activated AB pro- the N/B pair N-PMTN-CH C H B(C F ) , according to Guo et al.151 The 2 6 4 6 5 2 duces CTB, BCDB and BZ in subsequent steps. N/B pair activates AB via N⋯H–B and B⋯H–N interactions. As a result,

NH2BH2 forms and it is released at <110 °C, whereas the release of H2 requires a temperature of 110 °C. The N/B pair is finally regenerated. − apparent activation energy (∼117 kJ mol 1) than neat AB − (129 kJ mol 1) and did not release BZ. In a further study, He 3+ et al. found that AB reduced the Fe of iron(III) chloride FeCl3 5. AB destabilized by chemical into 2–5 nm nanoparticles of iron boride FeB.162,163 As-doped doping AB transformed into crystalline PAB upon the release of 1 equiv. H2, indicating a FeB catalyst-controlled dehydrogena- 5.1. Metal halides as solid-state dopants tion. Similar catalytic activity was reported for Ni-based – The first evidence of the destabilizing effect of a metal halide dopants.164 167 was reported by De Benedetto et al.160 They mechanically Benzouaa et al. confirmed the positive destabilizing effect 168 mixed 1 mol% hexachloroplatinate(IV) hydrate H2PtCl6·xH2O of both CoCl2 and FeCl3. They understood it in terms of a – 2+ 3+ α+ with AB. A mechanochemical reaction (reduction of H2PtCl6 by Lewis acid base reaction between Co or Fe (denoted M ) AB) took place, which produced Pt nanoparticles. As-doped AB and the NH3 of AB (Fig. 17). The reaction induces the release + showed better decomposition properties (shortened induction of H2 and the formation of N H2BH2. The cation is in inter- period, faster kinetics, and slightly higher dehydrogenation action with the Lewis acid that has partially reduced, such as (α−1)+ + extent at 82–83 °C) than neat AB. Similar observations were M ⋯N H2BH2. The dehydrocoupling of AB then occurs by 161 (α−1)+⋯ + reported by He et al. for cobalt chloride CoCl2-doped AB. a reaction of M N H2BH2 with AB. Pai and Han pre- + Indeed, AB reduced Co2 (2 mol%) to produce finely-dispersed dicted a comparable mechanism for the metal chlorides FeCl2, + 169 α+ nanoparticles (<3 nm) of Co . As-doped AB showed a lower CoCl2, NiCl2, CuCl2 and ZnCl2. M was considered as a

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Fig. 16 Mechanism of decomposition of AB in glymes catalyzed by a Lewis acid B(C6F5)3 or a Brønsted acid HOSO2CF3, according to Stephens 154 δ− + et al. The acid catalyzes the abstraction of one H of AB, resulting in the formation of the cation [NH3BH2] . Then, the cation reacts with another AB molecule to generate cationic transient species such as the examples shown here.

Fig. 18 Initiation mechanism of dehydrogenation of solid-state AB 176 catalyzed by MgCl2, according to Ding et al.

that unidentified reactive Mg-containing species (denoted by Mg-CS in eqn (19)) form:

NH3BH3 þ MgCl2 ! NH4BCl4 þ H2 þ Mg-CS ð19Þ

According to Ding et al., MgCl2 activates AB without decom- δ posing (Fig. 18).176 The occurrence of two Cl⋯H + and one

Mg⋯N interactions between MgCl2 and AB hinders the release

of NH3, changes the bonding state of the N–H and B–H bonds, Fig. 17 Mechanism of decomposition of solid-state AB catalyzed by and catalyzes the release of H . This way, MgCl coordinates Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. 2 2 α+ M (of a metal chloride MClα), a Lewis acid, according to Benzouaa ⋯ ⋯ 168 α+ with NH2BH2 via Mg N and Cl N interactions. et al. The interaction between AB and M induces the release of H2 + Biliskov et al. considered 1 mol KBr as the dopant of 1 mol and the formation of N H2BH2. The cation is in interaction with the par- (α−1)+ (α−1)+ + ﬀ 177 tially reduced Lewis acid M such as M ⋯N H2BH2. The dehy- AB. KBr was found to have two e ects. First, it promotes the (α−1)+⋯ + drocoupling of AB occurs by reaction of M N H2BH2 with AB. formations of DADB and NH2BH2, both reacting then together to form PAB via µADB and BCDB. Second, KBr allows the for- + – + mation of NH4 at the KBr AB interface and the NH4 catalyzes the dehydrogenation of AB while suppressing the formation of

(α−1)+ + volatile impurities. polymerization catalyst and the BH2 of M ⋯(N H2BH2)n−1 α as a cationic polymerization radical. To be active, M + must be reducible.170,171 Toche et al. screened the metal chlorides 5.2. Metal hydrides as solid-state dopants 2+ FeCl2, CoCl2, CuCl2, ZnCl2 and PtCl2, and found that Cu The present section is about metal hydrides used as solid-state oﬀers the optimal electronic properties (electronegativity and dopants of AB. Studies dealing with metal hydrides that preli- electron aﬃnity) to destabilize AB and catalyze its decompo- minarily react with AB to generate ionic salts like metal amido- sition.172 Under heating up to 100 °C, AB and/or decomposing boranes are out of the scope. This is for instance the case AB reduced Cu2+ in its metallic state.173 It is worth noting that when 1 mol LiH and 1 mol AB are mixed.178 The reader is the presence of a metal chloride (10 mol% of LiCl, NaCl, KCl, invited to refer to relevant reviews for information on 27–31 AgCl, MgCl2, CaCl2, CrCl2, MnCl2, FeCl2, CoCl2, NiCl2, CuCl2, amidoboranes. Metal hydrides such as MgH2 and CaH2 ZnCl2, SrCl2, AlCl3, TiCl3 or VCl3) depresses the formation of that reputedly do not readily react with AB are discussed 174 DB, BZ and NH3. hereafter. 175 Li et al. studied MgCl2. AB doped with it (mol ratio AB/ Kang et al. mechanically milled 1 mol MgH2 with 2 mol AB 179 MgCl2 of 2) released H2 from ∼40 °C. Unlike the studies men- and studied the thermal dehydrogenation of the blending.

tioned above, no volatile impurities formed. It was assumed As-doped AB quickly released up to 8 wt% pure H2 at 100 °C

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after a short induction period. It was assumed that MgH2 atoms at the edges of vacancies of a BN sheet catalyze the dis- aﬀects the chemical bonding state of AB via solid-phase inter- sociation of one B–H and one N–H bonds towards the for- actions. In further studies from the same group, it was mation of two surface-adsorbed H atoms and the release of

suggested that, at an early stage, a fraction of MgH2 and AB NH2BH2. The same initiation mechanism was predicted for react together and unidentified amorphous Mg–B–N–H inter- nanotubes of SiC.193 180,181 194 mediates form. In another of their studies, where Mg Zhang et al. studied graphitic carbon nitride C3N4. It was

was used instead of MgH2, the Mg–B–N–H intermediate was mixed with an equal amount of AB. As-doped AB released H2 identified as being metastable (even unstable) magnesium at slightly lower temperature than neat AB. No BZ was 182 amidoborane Mg(NH2BH3)2. The destabilizing eﬀect of detected, but C3N4 promoted the formation and release of

MgH2 (CaH2,Mg2NiH4 and AlH3) was, elsewhere, explained in NH3. This is explained by the contribution of surface NH2 and – – terms of changes in the electronic states of the B N, N H and NH groups that replace the NH3 of AB. Another nitride to 183–186 195 B–H bonds. mention is Mg3N2. Unlike BN and C3N4, it reacts with AB to 187 Choi et al. screened MgH2, CaH2, TiH2 and ZrH2. The generate magnesium amidoborane ammoniate Mg

best dehydrogenation result was found for 2AB/MgH2/0.1TiH2. (NH2BH3)·2NH3. The binary MgH2/0.1TiH2 catalyzes the dehydrogenation of AB There are a few other solid-state dopants to mention.

by destabilizing DADB and favoring the formation of CDB. The Heldebrant et al. found that AB, in the presence of NH4Cl consequences are that the induction period is reduced and the (5 wt%) and at 80 °C, had slightly improved dehydrogenation 48 formation of volatile impurities is suppressed. According to properties (kinetics and amount of H2 released). Komova

Nakagawa et al., the metal’s electronegativity is an indicator to et al. successfully used calcined titania TiO2 as solid-state predict the decomposition properties of the metal hydride- dopant.196 As-doped AB dehydrogenated and B–O bond-con- 188 doped AB. taining BNHx formed. Gangal et al. destabilized AB with silicon nanoparticles (10 wt%),197 zeolite-X and Cs exchanged 198 ﬃ 5.3. Other solid-state dopants K-chabazite. Roy et al. proposed bentonite as an e cient dopant to avoid the formation and release of BZ.199 Zhong Neiner et al. showed that nanoparticles (∼10 nm) of boron et al. focused on the metal–organic framework (MOF) ZIF-8, nitride (BN) promote the release of H2 from AB owing to the 200 that is, [Zn(2-methylimidazolate)2]. It acted as a catalyst and formation of DADB.189,190 However, as-doped AB produced destabilized AB (no induction period, faster kinetics of H2 more BZ than neat AB because of BN surface-enhanced cycliza- release, and much less volatile impurities). tion of AB trimers occurring via B⋯N and N⋯B interactions between BN and AB (Fig. 19). This was further supported by DFT calculations.191 Another initiation mechanism was pro- 5.4. Solid-state protic dopants jected for a BN sheet. Kuang et al. who used the VASP code The B–O bonds of borates are almost as stable as the CvO 192 201 based on DFT predicted the formation of NH2BH2. NorB bonds of carbon dioxide. The formation of borates and poly- Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM.

Fig. 19 Formation of BZ from solid-state AB doped by BN nanoparticles, according to Neiner et al.189,190 The surface of BN enhances the cyclization of AB trimers, which is favored by the occurrence of B⋯N and N⋯B interactions between BN and AB.

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−1 borates, such as B(OH)3,NH4B(OH)4, NaB(OH)4 and/or reaction for the release of H2 (−1 kJ mol ) whereas the reac- 202–204 − −1 (NH4)2B4O5(OH)4·2H2O, has thus been a stumbling tion occurring with neat AB is exothermic ( 21 kJ mol ). The block in the technological development of liquid-state hydro- apparent activation energy is also significantly decreased (with −1 −1 gen storage using aqueous sodium borohydride NaBH4 and ∼67 kJ mol , versus 184 kJ mol for neat AB). Improved dehy- – aqueous AB as hydrogen carriers:205 207 drogenation properties were also reported for AB nanocon- − fined into mesoporous silica MCM-41 (726 m2 g 1) and NaBH þ 4HO ! NaBðOHÞ þ 4H ð20Þ − 4 2 4 2 MCM-48 (926 m2 g 1),215,216 silica hollow nanospheres (233 m2 −1 217 2 −1 218 NH3BH3 þ 3H2O ! NH3 þ BðOHÞ þ 3H2 ð21Þ g ), and silica aerogel (887 m g ), as well as other 3 − oxides like halloysite in the form of nanotubes (48 m2 g 1),219 þ ! ð Þ þ ð Þ 2 NH3BH3 4H2O NH4B OH 4 3H2 22 and manganese oxide as hollow particles (257–298 m −1 220,221 Against this background, the use of a protic dopant for g ). thermal dehydrogenation of AB is puzzling since the formation Gutowska et al. explained the improved dehydrogenation ff 214 of B–O is inescapable by an acid–base reaction between the properties of nanoconfined AB by two e ects. The first δ− δ+ effect is due to nanosizing that generates more defect sites in H of BH3 and the H of OH. 208 the AB nanoparticles. These sites initiate the dehydrogenation Pan et al. studied mannitol C6H8(OH)6. It was mixed δ ff with 2 equiv. AB in order to have an equal number of H + and at a lower temperature. The second e ect is related to the δ− acid–base reaction (catalytic activation) occurring between the H . As-doped AB had an onset temperature of dehydrogena- δ− tion of 80 °C (versus 106 °C for neat AB) and did not release surface SiOH of silica and the H of AB. Based on this, Zhang – et al. suggested a temperature-dependent initiation mecha- BZ. However, NH3 was released because the B N bond broke: nism (Fig. 20).217 Between 70 and 95 °C, two AB molecules 1 1 – δ+⋯ δ−– – ð Þ þ ! ð Þ þ þ anchor onto two surface Si(OH)2 via SiO H H B and Si C6H8 OH 6 NH3BH3 n C6H8O6B2 n 3H2 NH3 δ 2 2 O⋯H +–N interactions. The former interaction results in the ð23Þ – release of H2 and the formation of SiO BH2NH3. Above 95 °C – The residue was (C H O B ) , that is, C H O entities con- and up to 160 °C, two as-formed SiO BH2NH3 react and the 6 8 6 2 n 6 8 6 – nected to each other via O–B–O bridges. Similar observations BH2NH3 entities dehydrocouple. According to Lai et al.,Oof were reported for AB destabilized by maleic acid surface SiOSi acts as a Lewis base and coordinates with AB, 209 thereby loosening the B–N bond and the dihydrogen C2H2(COOH)2. Kim et al. also studied C6H8(OH)6. They – δ+⋯ δ−– 222 observed that, with a decrease of the amount of dopant, AB N H H B bonding, and favoring the formation of AB*. 210 Otherwise, this initiation mechanism explains the formation released less NH3 while dehydrocoupling into PB. Yeo et al. destabilized AB with tetraethylene glycol HO and release of NH3 (Fig. 21). Sullivan et al. confirmed the role 211 of both SiOH and SiOSi but reported that interactions of AB (CH2CH2O)3CH2CH2OH. They discussed the improved dehy- 223 drogenation properties of AB while considering two de- with surface SiOH predominate. ff stabilizing effects. The first effect is related to the acid–base Porous carbon was also studied as sca old. Feaver et al. syn-

Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. − δ δ− 2 1 reaction between the H + of the glycol and the H of AB. The thesized a carbon cryogel (300 m g ) that was infiltrated by 224 second effect, which is concomitant, is related to the charge 24 wt% of AB. In thermogravimetric conditions, AB@C δ− transfer from O to N–H, which results in more hydridic H ; released pure H2 between 75 and 100 °C. In reality, a first this promotes the formation of DADB. release of H2 occurred during the preparation of AB@C 212,213 because of the reaction between B–H and surface COOH. Such Hwang et al. doped AB with boric acid B(OH)3. As- a catalytic activation was also reported for activated carbon as doped AB released H2 from ∼60 °C. Two distinct reactions 225 were supposed to take place. The first one is the main porous host. The dehydrogenation of AB was discussed as pathway: AB dehydrocouples and this is catalyzed by the Lewis follows (Fig. 22). In a first step, surface COOH reacts with AB − to form COO–BH2NH3 and H2. In a second step, surface COO– acid B(OH)4 that has formed by the hydroxylation of B(OH)3. The second one is hydrothermolysis occurring at 95–130 °C BH2NH3 reacts with two AB molecules to form, by dehydrocoupling, a surface-adsorbed branched trimer. Concomitantly, 2 thanks to H2O originating from the dehydration of B(OH)3. equiv. H2 are released. Li et al., who used mesoporous carbon − CMK-3 (1150 m2 g 1), analyzed the solid recovered after 226 6. Nanosizing AB decomposition. AB@CMK-3 released NH3 together with H2. The presence of B–O bonds in the residue was observed, thus 6.1. By confining into a porous host providing evidence for reactions between surface COH/COOH Gutowska et al. successfully explored the use of a porous host and AB (Fig. 23). as a scaffold to decrease the size of AB particles to the nano Apart from the aforementioned effects, there are two other scale.214 They used mesoporous silica SBA-15 (specific surface effects that explain the better dehydrogenation properties of − – area of ∼900 m2 g 1) and filled its channels (7.5 nm of dia- AB nanoconfined into porous carbon.227 232 The first one is meter) with AB (50 wt%). Such nanosizing changes the related to the structure of the host that limits the diffusion of thermodynamics of the dehydrogenation of AB. As-obtained AB. Consequently, the hydrogen diffusion distance is reduced, − AB@SBA-15 (<50 m2 g 1) has an almost neutral enthalpy of the frequency of interactions is increased, and the dehydro-

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Fig. 20 Mechanism of decomposition of nanoconﬁned AB by the surface Si(OH) of silica, according to Zhang et al.217 The mechanism is 2 Fig. 21 Release of NH3 from nanoconﬁned AB because of interactions temperature-dependent. Between 70 and 95 °C, two AB molecules are with the surface O of SiOSi and/or SiOH groups of silica, according to δ δ− δ via – +⋯ – – ⋯ +– 222 activated by two surface Si(OH)2 SiO H H B and Si O H N Lai et al. The surface O acts as a Lewis base, and the B of AB as a interactions; this way, 2 equiv. H2 are released. Between 95 and 160 °C, Lewis acid. Interaction between O and B results in a loosened B–N the as-formed SiO–BH2NH3 react together, and the –BH2NH3 entities

Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. bond, thereby favoring the formation of NH3. dehydrocouple; 2 equiv. H2 are released.

ff genation is accelerated. The second e ect is the destabilization − + δ δ− ing cationic dimer [O] [NH BH NH BH ] reacts with another of the dihydrogen N–H +⋯H –B bonding of AB, which lowers 3 2 2 2 AB, and so forth, until PAB forms. Concomitantly 1 equiv. H the barrier to H release. 2 2 is released. PB is produced upon the release of >2 equiv. H . So et al. focused on the pure nanosizing effect.233 They 2 The DFT calculations performed by Kuang et al. supported the studied a series of porous carbons having uniform pore size key role of surface OH.236 The formation of B–O bonds was and no catalytic surface group. Nanoconfined AB showed confirmed by Champet et al.237 improved dehydrogenation properties in comparison with neat A class of porous materials that has been much studied for AB. They highlighted that there is a linear relationship nanosizing AB is that of MOFs. Li et al. reported the first between the reciprocal of the pore size and the temperature of AB@MOF by using JUC-32-Y (Y3+ as unsaturated coordination H release. In other words, the lowest dehydrogenation temp- − 2 site and 1,3,5-benzenetricarboxylate as ligand; 659 m2 g 1).238 eratures for nanoconfined AB are more likely to be achieved AB@JUC-32-Y showed a one-step dehydrogenation peaking at with the narrowest pores. The pure nanosizing effect was also 84 °C (without the formation of volatile impurity). It released reported for AB nanoconfined into BN hollow spheres.234 − 8 wt% H in 10 min when maintained at 85 °C (versus 0 wt% Tang et al. studied graphene oxide (GO; 391 m2 g 1)asa 2 for neat AB). The better dehydrogenation properties were scaffold.235 AB@GO, consisting of 30 wt% AB, released pure explained by two synergetic effects. The first one is the nano- H from 50 °C. The initiation mechanism is as follows: 2 sizing effect. The second effect is due to the unsaturated Y3+ þ sites. These Lewis acid sites play a catalytic role by interacting OH þ NH3BH3 !½O ½NH3BH2 þ H2 ð24Þ with AB and trapping the NH3 group (Fig. 24). The as-formed δ− δ+ – – H of AB reacts with H of surface OH and H2 forms. As- O BH3 and Y NH3 surface entities are then able to react with − + ff formed surface [O] [NH3BH2] then reacts with AB. The result- AB molecules by dehydrocoupling. Both of these e ects were

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Fig. 22 Mechanism of decomposition of nanoconﬁned AB by the surface COOH of activated carbon, according to Moussa et al.225 AB is activated δ+ δ− by surface COOH via an acid–base reaction between the H of COOH and the H of AB. The as-formed COO–BH2NH3 reacts with AB molecules, and a COO-bound branched trimer forms by dehydrocoupling.

226 Fig. 23 Release of H2 and NH3 from nanoconﬁned AB, by reaction with the surface COH of mesoporous carbon CMK-3, according to Li et al. AB is activated by surface COH via an acid–base reaction between the Hδ+ of COH and the Hδ− of AB. As a result, the B–N bond breaks, surface CO– BH forms, and H together with NH are released.

Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. 2 2 3

− also attributed to AB nanoconfined into Mg-MOF-74 (Mg2+ as benzenedicarboxylate as ligand; 1110 m2 g 1).244 Evolution of 2+ unsaturated coordination site and 2,5-dioxido-1,4-benzenedi- NH3 was also observed for AB infiltrated into Cu-BDC (Cu as − carboxylate as ligand; 1100 m2 g 1),239 Zn-MOF-74 (Zn2+ as coordination site 1,4-benzenedicarboxylate as ligand; 550 m2 − unsaturated coordination site and 2,5-dihydroxybenzene-1,4- g 1).245 Initially, Cu2+ was unsaturated. However, it partially − dicarboxylate; 1100 m2 g 1),240 Fe-MIL-53 (Fe3+ as unsaturated reduced and saturated during the decomposition. As a result, coordination site and 1,4-benzenedicarboxylate as ligand),241 the long-range-ordered structure of the MOF degraded, and its and Tm(BTC) (Tm3+ as unsaturated coordination site and 1,4- catalytic effect was lost. The degraded MOF was no longer 242 benzenedicarboxylate as ligand). capable of hindering the release of NH3. With respect to the aforementioned second effect, the unsa- There is actually another effect that is due to the ligand of turation of the metal site is crucial for an effective catalytic the MOF. The O-functional groups of the ligands contribute to activation of AB. Li et al. showed that with saturated Zn2+ sites destabilizing AB: O interacts with the B of AB.240,241,243,245 This − in MOF-5 (1,4-benzenedicarboxylate as ligand; 1032 m2 g 1), results in perturbations of the atomic charge distribution and 243 δ+ δ− the evolution of NH3 cannot be hindered. A comparable of the dihydrogen N–H ⋯H –B bonding. In this way, a B–O result was reported for IRMOF-1 (Zn2+ as coordination site and bond forms between the ligand and the dehydrogenated AB. − 1,4-benzenedicarboxylate as ligand; 1060 m2 g 1), IRMOF-10 Gao et al. developed MIL-101 (Cr3+ as coordination site and 2+ 2 −1 (Zn as coordination site and biphenyl-4,4′-dicarboxylate as 1,4-benzenedicarboxylate as ligand; 1186 m g ) with NH2 or 2 −1 4+ 246 ligand; 320 m g ), UiO-66 (Zr as coordination site and 1,4- NHCOCH3 functionalization. Both functional groups per- − benzenedicarboxylate as ligand; 1010 m2 g 1), UiO-67 (Zr4+ as turbed the charge distribution within AB and the dihydrogen δ δ− coordination site and biphenyl-4,4′-dicarboxylate as ligand; N–H +⋯H –B bonding, because of the formation of 2 −1 3+ ⋯ ⋯ 1920 m g ), and Al-MIL-53 (Al as coordination site and 1,4- NH2 H3NBH3 or CO BH3NH3 interactions.

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Fig. 24 Mechanism of decomposition of nanoconﬁned AB by the surface of JUC-32-Y represented here by C–O–Y (with C as carbon, and with Y3+ 238 as unsaturated yttrium cation), according to Li et al. AB interacts with C–O–Y via O⋯BH3 and Y⋯NH3 bonds, which lead to O–BH3 and Y–NH3 surface entities. These entities then react with AB molecules by dehydrocoupling.

place according to a one-step process (with an enthalpy of reac- − −1 tion of 4.9 kJ mol ). NH3 was however detected as a volatile impurity. A temperature-dependent mechanism was proposed. At 70–90 °C, AB interacts with surface CO. The bonds of AB are weakened, which hinders the formation of DB and BZ, for ⋯ example. Above 95 °C, as-formed CO BH3NH3 decomposes, and H2 and some NH3 are released. A similar initiation mechanism was proposed for the polymeric scaﬀolds poly(vinyl pyrrolidone),249 polyacrylamide,250 and poly(methyl methacry- 251 late). Ploszajski et al. also noticed the release of NH3 when AB blended with polyethylene oxide (PEO) was heated.252,253 In addition, they detected the release of BZ and DB. The level of BZ was greater for AB@PEO, and the level of DB was comparable for AB@PEO and neat AB. It is assumed that PEO promotes the formation of DADB owing to interactions between δ+ ethereal O and all the H of NH3 (Fig. 25). Otherwise, the thermal decomposition of AB@PEO is seen as being as complex as the thermal decomposition of neat AB. Typically, DADB transforms into various cyclic intermediates like CTB

Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. and BCDB, BZ forms from them, and finally PB is produced by dehydrocoupling of BZ. When an O-free polymeric structure like poly(styrene-co-di- Fig. 25 Enhanced formation of DADB due to hydrogen bonding inter- ﬀ δ+ vinylbenzene) is used as a sca old, AB does not release actions between the ethereal O of PEO and the three H of AB, accord- 254,255 256 ing to Ploszajski et al.252,253 NH3. Zhang et al. studied polypyrrole (PPy). In thermogravimetric conditions, AB@PPy started to release pure

H2 from 48 °C, and in isothermal conditions, at 80 °C, it For the studies mentioned above, PB was found to form released 8 wt% H2 in 2 h (versus <1 wt% for neat AB). The N of PPy, a Lewis base, was suggested to play a crucial catalytic role upon the release of >2 equiv. H2. In contrast, Jeong et al., who 257 studied AB nanoconfined into MOF-5 (Zn2+ as coordination (Fig. 26). It allows enhancing the bonding strength between − site and 1,4-benzenedicarboxylate as ligand; 2169 m2 g 1), the polymer and AB, which leads to the deprotonation of 258 − identified PIB as the final residue.247 This was explained as AB. As-formed [NH2BH3] then reacts with the protonated follows. The very small pores of the MOF (1.3 nm) prevent the polymer to produce H2 and NH2BH2. formation of BZ and thus the AB-to-BZ-to-PB pathway from ff taking place. Hence, the dehydrogenation followed the AB-to- 6.3. By following a sca old-free approach PAB-to-PIB pathway. Recent achievements showed that nanosizing AB can be done without using a scaffold. Lai et al. synthesized, by anti-precipi- ff 6.2. By blending with a polymeric sca old tation, nanospheres of AB with a mean diameter of 50 nm.259 Zhao et al. explored the possibility to use a polymer to nano- Melting of AB under heating was not avoided, thereby suppres- confine AB.248 Poly(methyl acrylate) (PMA) and AB (20 wt%) sing any effect related to nanosizing. Lai et al. then encapsu- were combined through a solution-blending process. AB@PMA lated the AB nanospheres within a matrix made of Ni nano- showed a lower onset temperature of dehydrogenation of 70 °C particles (1–7 nm; 45 wt%). This was done by mixing the AB

(versus >90 °C for neat AB), and the dehydrogenation took nanospheres with NiCl2 at room temperature and letting the

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Fig. 26 Initiation mechanism of dehydrogenation of AB blended with PPy, according to Zhang et al.256–258 The N of PPy plays a crucial role by δ+ δ+ δ− − transferring one H of AB to the polymer. The H of the protonated polymer then reacts with H of the anion [NH2BH3] to form H2 and NH2BH2. This way, PPy is regenerated.

Fig. 27 Synthesis of Ni nanoparticles (1–7 nm) supported on AB nanoparticles (50 nm) and decomposition of as-formed AB@Ni at ∼80 and 259 ∼180 °C, according to Lai et al. The synthesis is done by mixing the AB nanospheres with NiCl2 at room temperature and letting the mixture age + − for 1 week. In doing so, [NH3BH2NH3] [BCl4] forms at the interface between the AB and Ni nanoparticles. Under heating, AB@Ni decomposes into H2,NH3 and BZ. Aminoborane oligomers and then iminoborane oligomers form. Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM.

+ − mixture age for 1 week. In doing so, [NH3BH2NH3] [BCl4] hydrogen storage capacity of 13 wt% H (two thirds of 19.6), formed at the interface between the AB nanoparticles and the which is high and thus attractive. Regrettably, neat AB decom- Ni nanoparticles (Fig. 27). As-obtained AB@Ni was able to poses rather than dehydrogenates. This means that it releases

release H2 from ∼50 °C. The formation and release of DB, BZ some H2 that is polluted by volatile impurities like NH3,DB

and NH3 was however not totally suppressed. This is explained and BZ. This also means that some H, as well as some B and by the thermal decomposition of the AB within the nano- N, is lost, which is detrimental in terms of atom economy (one particles, a fraction that is not in the close vicinity of the Ni of the principles of ‘green chemistry’). In other words, neat AB nanoparticles and that behaves like neat AB. Between 80 and is unsuitable for chemical hydrogen storage. 180 °C, aminoborane oligomers and then iminoborane oligo- The decomposition of neat AB follows a complex mecha-

mers form. Prospects for improvement exist. This should go nism involving at least one initiation intermediate (NH2BH2 through smaller AB nanoparticles and a surface protection and/or DADB), heteropolar and homopolar interactions, (catalytic or inert thin layer for example).260 various identified and unidentified reaction intermediates,

several volatile products of decomposition (H2,NH3, DB, BZ), and a polymeric residue of complex composition (linear and 7. Summary and outlook branched PAB, PIB, PB, and polymers with B–B and N–N bonds). A further complication is that it is very diﬃcult to It is obvious that using AB in neat form would have been quantify each of the intermediates, products, and polymers.

much better, particularly in terms of gravimetric hydrogen For instance, an intermediate like NH2BH2 is very reactive storage capacities. AB carries 19.6 wt% H and, releasing 2 (short lifetime) and, though it may be detected as a volatile

equiv. pure H2 (through the AB-to-PAB-to-PIB pathway for product, the fraction of it that has formed and reacted is example) for example, equates to an eﬀective gravimetric unknown. The cyclic intermediates CDB, BCDB, CTB, and the

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dimer/trimer of AB are diﬃcult to distinguish because of quite conclusion is about the prospects for implementing AB as a similar spectra. With respect to the residue, isolating and chemical hydrogen storage material. The formation of PB as quantifying each oligomer/polymer is challenging. In residue is a good point as AB can be eﬀectively regenerated summary, decomposition of neat AB is much more complex from PB. At this stage, it would be important to focus more that the originally reported AB-to-PAB-to-PIB pathway that sees attention on the residue forming upon the release of ≥2 equiv.

H2 as the single volatile product. H2, very probably PB, to check on its purity and its molecular It has been necessary to develop destabilization strategies structure (linear or cross-linked). To that end, one may explore

to make AB release pure H2. Beyond the performance, studies isotopes of AB. dedicated to the destabilization of AB have enhanced our As mentioned above, one of the pathways leading to PB is understanding of thermal decomposition of (destabilized) AB the AB-to-BZ-to-PB one. It assumes that BZ forms from AB via on the grounds of what we knew about neat AB, even though a stepwise reaction involving some intermediates. There is first

certain points are still unclear. the initiation intermediate (NH2BH2 and/or DADB). There are

As initiation intermediates, there are again NH2BH2 and then intermediates that form by the reaction of the initiation DADB. For AB in an aprotic solvent, formation of DADB is pro- intermediate with one or more AB molecules. The most

moted, but the formation and involvement of NH2BH2 cannot common intermediates are µADB, CDB, CTB and BCDB. They

be discarded though it has not been reported yet. NH2BH2 is are all deemed to be precursors of BZ. The mechanisms of for- highly unstable and reactive, which makes its detection tricky. mation of these intermediates are not yet fully understood

Computational simulations support the role of NH2BH2 in the because they are diﬃcult to discriminate by using the usual initiation mechanism of the thermal decomposition of AB in spectroscopy techniques. This remains a challenge. It would solution. In any case, there is large consensus on the for- be important to explore ways to become more selective while

mation and key role of NH2BH2 for AB in solution and in the favoring a single pathway leading to a given intermediate. presence of a catalyst (metal-based homogeneous and hetero- With destabilized AB, the release of volatile impurities is geneous, frustrated Lewis pair). Otherwise, the involvement of either depressed or, better, suppressed. For instance, some

NH2BH2 has been pointed out for solid-state AB destabilized NH3, DB and BZ were detected for solid-state AB destabilized

by solid-state dopants such as MgCl2 and KBr. Apart from by solid-state metal chloride. This is due to the fact that such a

NH2BH2 and DADB, few other initiation intermediates have destabilization strategy is based on grain-to-grain contacts, been reported in specific conditions. There are, among others, and the AB within a grain (not in contact with the dopant) + [NH3BH2] for AB in solution catalyzed by a Lewis or Brønsted behaves like neat AB. Nanosizing AB is eﬃcient in addressing − – acid, [NH2BH3] for AB in solution catalyzed by a base and for that: AB is destabilized owing to a weakened dihydrogen N δ δ− AB blended with a polymer acting as Lewis base, H +⋯H –B bonding, which favors the formation of a reactive (α−1)+ + M ⋯N H2BH2 for solid-state AB destabilized by a solid- initiation intermediate and the subsequent dehydrocoupling. – state metal chloride, O BH2NH3 for AB nanoconfined into a With AB nanoconfined into a MOF with unsaturated metal silica or carbonaceous scaﬀold, and O–BH3 together with M– sites, no volatile products are released. The unsaturated metal Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. NH3 for AB nanoconfined into a MOF having an unsaturated is able to trap the NH3 of AB, thereby hindering its release. metal site. It is worth mentioning that an intermediate like What is clear from this is that dehydrogenation of AB preferen- − [NH2BH3] for AB blended with a polymer acting as Lewis base tially occurs when AB is destabilized according to one of the

then reacts with the protonated polymer to produce NH2BH2 aforementioned strategies. The overall process is thus more

and H2. In summary, the nature of the initiation intermediates selective, particularly towards the formation of H2. depends on the properties of the destabilizing agents. O-Containing functional groups play an active role in de- Nonetheless, uncertainties remain, as for the formation of stabilizing AB. Firstly, there are SiOH, COOH and COH groups. δ+ NH2BH2 before or in parallel to that of DADB. Dehydrogenation of AB is initiated by reaction of H of OH δ− + It is commonly believed that the initiation intermediate cat- with H of AB, and the resulting [NH3BH2] binds to O via a alyzes the decomposition of AB by dehydrocoupling. As a B–O bond. Secondly, there is ethereal O. It interacts with the B result, linear and branched PABs form upon the release of 1 of AB, which changes the electronic states of the B–N, N–H

equiv. H2, and PB forms upon the release of ≥2 equiv. H2. and B–H bonds and thus weakens the dihydrogen N– δ δ− δ− Another pathway leading to the formation of PB is the AB-to- H +⋯H –B bonding of AB. H is more hydridic, and it more δ BZ-to-PB one. PIB may also form. It has been considered as a readily reacts with the H + of AB. In other words, the barrier to

possible intermediate before the formation of PB, and even as H2 release is lowered. the final dehydrocoupling product for AB nanoconfined into a The O atom discussed above has the double advantage of scaﬀold with very small pores. This leads to two conclusions. being more electronegative than N and B (Pauling electro- The first conclusion is about the mechanisms of decompo- negativity χ of 3.44, 3.04 and 2.04 respectively) and of being a α sition. All the aforementioned pathways imply heteropolar N– Lewis base. The counterpart of O is M +. The 3d metal cations, δ δ− H +⋯H –B reactions. Nor is there any reference to the poss- for example, have the double advantage of being less electrone- δ δ ible occurrence of homopolar reactions (N–H +⋯H +–N and gative than N and B (χ of 1.83 and 1.9 for Fe and Cu respect- δ− δ− B–D ⋯D –B). Unlike for neat AB, no systematic study focus- ively) and of acting as Lewis acids. Because AB is a dipole α ing on homopolar interactions is available yet. The second (moment of 4.9 D in dioxane),261 both O and M + electronically

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destabilize AB by binding to B and N respectively. This induces great extent while transforming into a polymeric residue that changes in the electronic states of the B, N, H atoms, the B–N, can be recycled to regenerate AB. Assuming a dehydrogenation δ+ δ− B–H, N–H bonds, and the dihydrogen N–H ⋯H –B bonding. extent of 83% (i.e. 2.5 equiv. H2) with the formation of PB as a It is therefore reasonable to conclude that MOFs made of unsa- polymeric residue, this would mean that AB is able to release turated metal sites and O-containing ligands (as shown in 16.3 wt% H, or that AB confined into a highly-porous host Fig. 24 for example) oﬀer an optimal catalytic environment for (65%–35% in weight) is able to release 10.6 wt% H. It is worth

destabilizing AB. The same environment could be oﬀered by mentioning that the elimination of 3 equiv. H2 must be other porous hosts provided that they have surface O and that avoided because of the formation of the thermally- and chemi- a metal cation is supported onto the surface of the porous cally-stable BN. Third, and as discussed in the introduction, host. This was in fact successfully done by Li et al. for AB has to be regenerated from the polymeric residue, for CMK-3.226 The carbonaceous host was doped with Li+, and as- example PB, to close the hydrogen cycle. A regeneration rate obtained Li+-CMK-3 infiltrated with AB. AB@Li+-CMK-3 higher than 90% would be a great achievement. In all likeli- showed improved dehydrogenation properties in comparison hood, the nanosizing strategy comes closest to the aforemen- with AB@CMK-3 thanks to the synergetic actions of surface O tioned requirements, and further innovations in nanosizing of CMK-3 and Li+. This shows that there are prospects for could serve to match these requirements. α developing M +-doped and surface O-containing porous hosts for nanosizing and destabilizing AB. We however should not ignore that the formation of B–O Conﬂicts of interest bonds between dehydrogenated AB and an O-containing destabilizing agent may impact negatively the real gravimetric There are no conflicts to declare. hydrogen storage capacity of AB. Energetically, the B–O bond is roughly comparable to the CvO bond of carbon dioxide.201 Converting B–O into B–H is thus as challenging as converting Acknowledgements CvO into C–H. If this cannot be done in suitable conditions, the destabilizing agent should be optimized (mol number of O Just to set a precedent, I would like to warmly thank our com- versus mol number of B, for example) in order to minimize the puter service, represented by Mr Henri Bourrasse, Mr Yoann amount of B that cannot be hydrogenated. Gerbaud, and Mr Gilles Mendy. “Behind the scenes”, they are As a final conclusion, it is clear from the foregoing that our daily contributing to provide us with the right computational understanding of the mechanisms of decomposition of AB is tools so that we can communicate our scientific results. much better than it was 15 years ago. At the outset, we had a quite simplistic view of the mechanism. Then, the studies dedicated to neat AB showed how complex the mechanism is References in fact, and the studies focusing on destabilized AB, mainly Published on 02 February 2021. Downloaded 9/27/2021 6:22:53 AM. developed to make AB implementable for chemical hydrogen 1 J. L. M. Abboud, B. Németh, J. C. Guillemin, P. Burk, storage, have provided insights into the initiation intermedi- A. Adamson and E. R. Nerut, Dihydrogen generation from ates, the decomposition pathways, the oligomeric intermedi- amine/boranes: Synthesis, FT-ICR, and computational ates involved, the nature of the residue, and the role of the de- studies, Chem. – Eur. J., 2012, 18, 3981–3991. stabilizing agents. Shortcomings, however, still remain, and 2 X. Chen, J. C. Zhao and S. G. Shore, The roles of dihydro- there are outstanding questions such as the following ones. Do gen bonds in amine borane chemistry, Acc. Chem. Res.,

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is regenerated by the reduction of PB? What is the proportion ammonia-borane, H3NBH3, J. Am. Chem. Soc., 1955, 77, of B of AB that binds to the O of a destabilizing agent? Is it 6084–6085. possible to partially re-hydrogenate such B in order to form O– 4 S. G. Shore and R. W. Parry, Chemical evidence for the

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pure H2 in the temperature range of a low-temperature fuel materials for hydrogen storage, Dalton Trans., 2008, 5400– cell (e.g. 70–120 °C). Second, AB has to be dehydrogenated to a 5413.

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