Lithium Hydrazinidoborane Ammoniate LiN₂H₃BH₃·0.25NH₃, a Derivative of Hydrazine Borane

Abstract

Boron- and nitrogen-based materials have shown to be attractive for solid-state chemical hydrogen storage owing to gravimetric hydrogen densities higher than 10 wt% H. Herein, we report a new derivative of hydrazine borane N₂H₄BH₃, namely lithium hydrazinidoborane ammoniate LiN₂H₃BH₃·0.25NH₃. It is easily obtained in ambient conditions by ball-milling N₂H₄BH₃ and lithium amide LiNH₂ taken in equimolar amounts. Both compounds react without loss of any H atoms. The molecular and crystallographic structures of our new compound have been confirmed by NMR/FTIR spectroscopy and powder X-ray diffraction. The complexation of the entity LiN₂H₃BH₃ by some NH₃ has been also established by thermogravimetric and calorimetric analyses. In our conditions, LiN₂H₃BH₃·0.25NH₃ has been shown to be able to release H₂ at temperatures lower than the parent N₂H₄BH₃ or the counterpart LiN₂H₃BH₃. It also liberates non-negligible amounts of NH₃ at temperatures lower than 100 °C. This is actually quite detrimental for chemical H storage, but alternatively LiN₂H₃BH₃·0.25NH₃ might be seen as a potential NH₃ carrier.

1. Introduction

Hydrogen storage is one of the major obstacles restricting the development of an “economy of hydrogen energy”. Within the past decade, many studies have been done to find new solutions, and particular attention has been paid to chemical H storage [1], and interestingly there has been new interest in old molecules and materials [2]. An example of this is hydrazine borane N₂H₄BH₃. It was discovered in the 1960s and the first applied study presented it as an energetic material [3]. It was then barely investigated until recent years. In 2009, N₂H₄BH₃ was re-discovered owing to its high gravimetric hydrogen density of 15.4 wt%. However, it was considered as being unsuitable for solid-state chemical H storage because of hazardous dehydrogenation properties [4]. Indeed, when heated it releases large amounts of hydrazine N₂H₄ together with H₂, and generates a shock-sensitive solid residue above 300 °C [5]. It is worth mentioning that hydrogen storage via hydrazine borane and any other B–N–H compounds (e.g., ammonia borane NH₃BH₃) is irreversible. Upon thermolytic dehydrogenation, they transform into a polymeric residue of complex composition and they cannot be hydrogenated directly under H₂ pressure. The only way to regenerate the starting materials is chemical recycling [1,2].

Yet, N₂H₄BH₃ is still of interest in the field because it can be modified by reaction with an alkali hydride MH (M⁺ = Li⁺, Na⁺, K⁺) resulting in the formation of an alkali hydrazinidoborane derivative, MN₂H₃BH₃ [6]:

N₂H₄BH₃ (s) + MH (s) → MN₂H₃BH₃ (s) + H₂ (g)

(1)

Lithium hydrazinidoborane LiN₂H₃BH₃ is synthesized by ball-milling and depending on the milling conditions, two polymorphs form. The α phase (monoclinic, s.g. (space group) P2₁/c), already reported [7], is the high-temperature phase. It can be synthesized directly [7]. Otherwise it can be formed from the β phase (orthorhombic, s.g. Pbca) at about 95 °C [8]. The latter phase is the low-temperature phase. Lithium hydrazinidoborane is thermally less stable than the parent N₂H₄BH₃ as a result of the substitution of one H^δ+ of the N₂H₄ moiety by Li⁺ and of the subsequent lengthening and concomitant weakening of the H^δ+···H^δ− interactions (which refer to the so-called dihydrogen bonding). For example, the β phase is able to dehydrogenate from 40 °C and release 7.8 wt% of almost pure H₂ up to 144 °C. Lithium hydrazinidoborane is thus more suitable for solid-state chemical H storage than the parent N₂H₄BH₃. This is also the case for the sodium derivative NaN₂H₃BH₃ [9]. It starts to liberate H₂ below 60 °C, and over the range 60–100 °C it loses 6 wt% of almost pure H₂. The interesting feature with NaN₂H₃BH₃ is that synthesis by ball-milling has to be performed below −30 °C because of the high reactivity of sodium hydride NaH with N₂H₄BH₃. A reaction enthalpy of −27,756.7 J mol⁻¹ was determined by Calvet calorimetry at 25 °C [10]. The reaction is highly exothermic, especially when compared to the enthalpy of the reaction between LiH and N₂H₄BH₃ at the same temperature (−62.8 J mol⁻¹). In fact, the bigger the alkali cation is, the more exothermic the reactivity of the alkali hydride. The enthalpy of reaction of KH with N₂H₄BH₃ was measured as being −70,247.2 J mol⁻¹. Accordingly, the safest way to get potassium hydrazinidoborane KN₂H₃BH₃ is by wet synthesis (i.e., suspension in tetrahydrofuran) in an autoclave reactor [11]. All of these alkali hydrazinidoboranes have shown better and safer dehydrogenation properties than the parent hydrazine borane. To our knowledge, there is no other derivative of N₂H₄BH₃. Attempts in synthesizing the hydrazinidoboranes of magnesium, calcium, and aluminum M(N₂H₃BH₃)_n (Mⁿ⁺ = Mg²⁺, Ca²⁺, Al³⁺) by ball-milling have failed. Composites MH_n-N₂H₄BH₃ have been found to form where the hydrides act as destabilizers of the borane [10].

The substitution of one H^δ+ of the N₂H₄ moiety of N₂H₄BH₃ by M⁺ negatively affects the gravimetric hydrogen density of the as-formed material because of the loss of 1 equivalent of H₂. For instance, the gravimetric hydrogen density drops from 15.4 wt% for N₂H₄BH₃ to 11.7, 8.9, and 7.2 wt% for LiN₂H₃BH₃, NaN₂H₃BH₃, and KN₂H₃BH₃, respectively. The decrease might be compensated by using an amide MNH₂ instead of a hydride MH, only if the NH₂ group of the amide interacts by trapping one H^δ+ of the N₂H₄ moiety of hydrazine borane. Consequently, the formation of a new compound may be expected with a formula resembling MN₂H₃BH₃·xNH₃. Such an approach has shown to be successful for synthesizing metal amidoborane ammoniates like Mg(NH₂BH₃)·NH₃ and Ca(NH₂BH₃)·NH₃ where all of the hydrogen atoms of the precursors were kept in the final material [12,13]. We therefore explored the possibility of synthesizing novel derivatives of N₂H₄BH₃. Lithium amide LiNH₂ was selected owing to the lightness of Li⁺. Another reason for this choice is that LiH is less reactive than NaH or KH, and thus is safer. Similar behavior was expected for LiNH₂. Then, by ball-milling N₂H₄BH₃ and LiNH₂, the formation of lithium hydrazinidoborane ammoniate LiN₂H₃BH₃·xNH₃ was expected; it has a higher gravimetric hydrogen density (13.2 wt% for x = 1; 12.1 for x = 0.25) than LiN₂H₃BH₃ (11.6 wt%). This is discussed hereafter.

2. Results

2.1. Molecular Structure

Ball-milling of LiNH₂ and N₂H₄BH₃ in our conditions resulted in the formation of a pasty solid. More details are given in the section dedicated to the experimental conditions. The pasty aspect is otherwise discussed in the discussion section. Hereafter, the precursors N₂H₄BH₃ and LiNH₂ will be denoted 1 and 2 respectively, while the ball-milling product will be identified as 3.

The ¹¹B MAS NMR spectra of 1 and 3 are shown in Figure 1. With the former (i.e., N₂H₄BH₃), the signal is typical of a quadrupolar coupling due to anisotropy around the boron atom because of strong intermolecular H^δ+···H^δ− interactions. In contrast, the new compound 3 shows a resonance of high intensity at δ −19.6 ppm; the signal is otherwise broad. Such features are indicative of isotropy around the boron atom of the NBH₃ environment, change of the environment of the N–H bonds, and lengthening of the intermolecular H^δ+···H^δ− interactions. Similar observations were reported for LiN₂H₃BH₃ as well as for NaN₂H₃BH₃ [6,7,8,9]. There are also two other signals. The first one is centered at δ −41.3 ppm. It is typical of a BH₄ environment [14]. The second one is centered at δ −6.7 ppm. It may be ascribed to N₂BH₂ and N₃BH environments [15]. The presence of such N₂BH₂ and N₃BH environments might indicate some decomposition of the starting borane, but the concomitant apparition of the BH₄ environment does not support such an assumption. In the open literature dedicated to B–N–H compounds, the presence of both signals is generally ascribed to the formation of an ionic dimer [16]: for example, with ammonia borane NH₃BH₃, a similar ¹¹B MAS NMR spectrum can be collected when part of the borane transforms to the ionic dimer diammoniate of diborane of formula [NH₃BH₂NH₃⁺][BH₄⁻]. We therefore believe that in our conditions a little part of N₂H₄BH₃ dimerized into an ionic intermediate of formula [N₂H₄BH₂N₂H₄⁺][BH₄⁻] (dihydrazinoboronium borohydride or dihydrazinate of diborane):

2N₂H₄BH₃ (s) → [N₂H₄BH₂N₂H₄⁺][BH₄⁻] (s)

(2)

Such a compound shows both of the N₂BH₂ and BH₄ environments, while the BH₃ environment comes from the new solid 3.

The FTIR spectrum of 3 (Figure 2) is different from that of 1. It displays a less complex fingerprint in the N–H stretching region (3450–2950 cm⁻¹) and broadened N–H bending bands (1700–1300 cm⁻¹), thereby suggesting that the intermolecular H^δ+···H^δ− interactions are less important and that the H^δ+···H^δ− network is weaker. There are more bands in the B–H stretching region (2600–1800 cm⁻¹) owing to interactions of Li⁺ and N of 2 with H^δ− of BH₃ groups. The two small bands at 1913 and 2017 cm⁻¹ observed on the spectrum of N₂H₄BH₃ are indicative of strong H^δ+···H^δ− interactions [5]. They cannot be seen for 3, confirming a weakened H^δ+···H^δ− network. It may be reasonably concluded that 2 has induced electronic modification in 1. In fact, the FTIR spectrum of 3 resembles that of LiN₂H₄BH₃ reported elsewhere and similar conclusions were made for this derivative [8]. The N–H stretching vibration at 3375 cm⁻¹ is in the range of the degenerate stretching N–H mode of ammonia, thereby suggesting weakly bound NH₃ molecules [17].

The aforementioned spectroscopy results are indicative of the formation of a compound with the speculated molecular formula LiN₂H₃BH₃·xNH₃ (lithium hydrazinidoborane ammoniate).

2.2. Crystallography

The powder XRD pattern recorded for 3 is shown in Figure 3. It was compared to the patterns of the reactants 1 [3,7,18] and 2 [19]. The presence of a new crystalline phase was confirmed, though some residual diffraction peaks coming from 2 have been also detected.

The diffraction peaks of 3 have been successfully indexed as a single phase using DICVOL06 [20] after removing the peaks supposed to be overlapped with the LiNH₂ diffraction lines. The new phase 3 crystallizes in the monoclinic system, with a space group P2₁/n (No. 14) and Z = 4; the unit cell parameters a = 7.649(1) Å, b = 7.502(1) Å, c = 5.973(1) Å, and β = 97.81(1)° were found. The unit cell volume (~340 ų) is larger than the one reported for α–LiN₂H₃BH₃ (328 ų) [7]. The structure factor of 3 and that of the α [7] and β [8] phases of LiN₂H₃BH₃ were compared; they present strong differences. This is indicative of the formation of a new phase. The position of the Li element in the unit cell was located by Direct Methods using EXPO2014 [21]. The crystal structure was then successfully refined by the Rietveld method (Figure 3) using the Jana 2006 program [22]. The structure parameters that were obtained are presented in

Table 1. Space group (s.g.), unit cell parameters, goodness of fit, and R-values for the refined structures for 3 (LiN₂H₃BH₃·xNH₃) and 2 (LiNH₂) at room temperature.

	LiN2H3BH3·xNH3	LiNH2
s.g.	P21/n (N° 14)	$I\overline{4}$ (N° 82)
Z	4	8
a (Å)	7.6498(18)	5.1158(11)
b (Å)	7.482(3)	5.1158(11)
c (Å)	5.968(17)	10.103(3)
β (°)	97.803(12)	-
V (Å3)	338.91(17)	264.41(12)
R.P.A. (wt%) 1	95.4(5)%	4.6(6)%
GoF	2.94	2.94
Rp	3.66	3.66
wRp	486	4.86
R(obs)/R(all)	13.85/15.47	11.17/12.49
wR(obs)/wR(all)	11.66/11.77	11.92/12.00

¹ Relative phase amounts in weight.

and

Table 2. Experimental structural parameters of 3 (LiN₂H₃BH₃·xNH₃) and 2 (LiNH₂) at room temperature. The atomic positions for LiNH₂ were kept fixed during the refinement [19].

Sample	Atom	Site	Occupancy	x	y	z	Uiso (Å2)
3	Li1_1	4e	1	0.4025(11)	0.4401(9)	0.767(2)	0.0229(1)
	B2_1	4e	1	0.7210(11)	0.3147(17)	0.5489(13)	0.0213(1)
	N3_1	4e	1	0.6451(11)	0.2980(11)	0.7827(11)	0.0202(1)
	N4_1	4e	1	0.6373(18)	0.1096(14)	0.867(3)	0.0202(1)
2	Li1_2	2a	1	0.00000	0.500000	0.25000	0.0177(1)
	Li2_2	2d	1	0.00000	0.00000	0.00000	0.0177(1)
	Li3_2	4e	1	0,00000	0.00000	0.25300	0.0177(1)
	N4_2	8g	1	0.23400	0.25400	0.13700	0.0065(1)

. For the refinement the lengths of the N–N and N–B bonds and the N–N–B angle were fixed to 1.50 Å, 1.58 Å, and 113.8° respectively. The refinement was carried out while taking into account the presence of 2. As a result, the relative weight amounts of 3 (LiN₂H₃BH₃·xNH₃) and 2 (LiNH₂) were found to be close to be 95.4 and 4.6 wt%, respectively. Nevertheless, the refinement of the position of the x molecule of NH₃ into the crystalline network has not been possible. Hence, considering the difference of volume (14 ų) between the cell volume of 3 and that of α–LiN₂H₃BH₃, it can be assumed the presence of one non-H atom per unit cell. This atom is proposed to be N of NH₃, suggesting then the molecular formula LiN₂H₃BH₃·0.25NH₃ for 3.

2.3. Thermal Analyses and Evolving Gas Analyses

Under heating at 5 °C min⁻¹, 3 is stable up to 75 °C (Figure 4). Then, it decomposes according to a complex pathway. The DSC curve is characterized by two major (maximum at 119.4 and 180.2 °C) and two minor exothermic events (107.4 and 145.5 °C). This is consistent with the occurrence of several weight losses observed by TGA. Over 75 °C, 3 starts to liberate NH₃, which is characterized by the first exothermic event (0.9 kJ mol⁻¹) peaking at 107.4 °C. This is consistent with the molecular structure of an adduct like LiN₂H₃BH₃·xNH₃. The first major weight loss (8.3 wt%) that occurs over the temperature range 75–145 °C is associated with the second exothermic event (11.5 kJ mol⁻¹) peaking at 119.4 °C. It is mainly due to the dehydrogenation of 3. There is then an exothermic signal of low heat (0.15 kJ mol⁻¹). It is followed by the second major decomposition (12.9 wt% and 50.5 kJ mol⁻¹) that takes place over the temperature range 145–300 °C. At 300 °C, the overall weight loss is 21.2 wt% and it cannot be rationalized with the loss of H₂ only (maximum of 13.2 wt% H in 3 if x = 1; 12.1 wt% H if x = 0.25). The weight proportion of NH₃ in the adduct LiN₂H₃BH₃·xNH₃ is theoretically of 24.7 wt% NH₃ if x = 1, and 7.6 wt% NH₃ if x = 0.25. Hence, 3 releases H₂ and significant amounts of NH₃.

In our conditions, 3 releases most of the NH₃ during the first decomposition step. Similar trends were reported for lithium amidoborane ammoniate Li(NH₂BH₃)·NH₃, for which the evolution of NH₃ peaks at 52 °C while that of H₂ shows the maximum at around 100 °C [23]. Similar observations were reported for calcium amidoborane ammoniate Ca(NH₂BH₃)·NH₃, making the authors suggest that the adducted NH₃ is weakly bound to the cation Ca²⁺ [13].

In our laboratory, TGA has been used as an efficient screening tool to evaluate the potential of any new B–N–H compound as well as to compare several of them. Accordingly, the thermal behavior of the sample 3 was first compared to that of the reactants 1 and 2. The TGA curves are shown in Figure 5. In comparison, 2 is quite stable under heating at 5 °C min⁻¹. The weight loss of about 0.6 wt% is negligible before 200 °C and increases to 2.5 wt% up to 300 °C. With respect to 1, it has a much different TGA profile. Therefore, 3 is a compound that is different from the precursors. As mentioned in the previous paragraphs, 3 is somehow comparable to the ammonia-free derivative LiN₂H₃BH₃ (denoted 4 in Figure 5). So, the thermal behavior of 3 was also compared to that of 4. It is worth mentioning that 4 is known to dehydrogenate according to a complex mechanism not releasing unwanted by-products like NH₃ [7,8]. The thermal behaviors of these two compounds are different. 3 starts its decomposition at a lower temperature than 4 and shows a higher weight loss due to the release of NH₃ molecules from both the complexation and the decomposition phenomena. In other words, the presence of NH₃ in 3 leads to a thermolytic behavior which is different from that observed with 4.

3. Discussion and Concluding Remarks

In good agreement with our primary objective, a new hydrogen-rich B–N–H compound has been successfully synthesized by solid-state reaction (mechanosynthesis/ball-milling) of N₂H₄BH₃ and LiNH₂. The molecular analyses suggest a compound with the formula LiN₂H₃BH₃·xNH₃. This is a new type of coordination compound consisting of a metal cation Li⁺, a [N₂H₃BH₃]⁻ anionic unit, and an xNH₃ ligand [24]. The crystallographic analysis allowed quantifying the x value thanks to the difference in the cell volumes of LiN₂H₃BH₃·xNH₃ and another well-described derivative—LiN₂H₃BH₃. Indeed, x was found to be equal to 0.25 suggesting then the following reaction:

LiNH₂ (s) + N₂H₄BH₃ (s) → LiN₂H₃BH₃·0.25NH₃ (s) + 0.75NH₃ (g)

(3)

Note that we also worked on a mixture of LiNH₂ and N₂H₄BH₃ (results not reported) where the mole ratio was lower than 1. We indeed considered 0.75 mole of LiNH₂ and 1 mole of N₂H₄BH₃. Like for LiN₂H₃BH₃·0.25NH₃, a paste-like solid was obtained. The as-obtained product showed NMR and FTIR spectra that were comparable to those of LiN₂H₃BH₃·0.25NH₃. With respect to the powder XRD pattern, the diffraction peaks were comparable to those observed for LiN₂H₃BH₃·0.25NH₃, but with additional peaks belonging to the excess of N₂H₄BH₃. The TGA results were also comparable. In other words, the results and the observations were consistent with the occurrence of the reaction shown by Equation (3).

The reaction of LiNH₂ and N₂H₄BH₃ in our conditions can be interpreted as follows. The Lewis base NH₂ of LiNH₂ would favorably react with the acidic hydrogen H^δ+ of the N₂H₄ moiety of the borane resulting in NH₃. In parallel, the Li⁺ cation would combine with the H^δ+-deficient N₂H₃⁻ entity towards the formation of the entity LiN₂H₃BH₃. It seems that a quarter of the as-formed NH₃ complexes LiN₂H₃BH₃ leading to the formation of a LiN₂H₃BH₃·0.25NH₃ like-compound. The rest of the NH₃ (the 0.75 equivalent) can be evacuated under vacuum at ambient conditions.

Ammonia is known to have good affinity with the B–N–H compounds. A first example is ammonia borane. Exposed to an atmosphere of NH₃, NH₃BH₃ is able to complex up to six molecules of NH₃, resulting in the formation of a pasty solid [25]. Furthermore, NH₃ is an excellent solvent of NH₃BH₃; the solubility is 259.7 g of NH₃BH₃ in 100 g of NH₃ and the solvated borane shows good stability [26]. A second example is an alkali derivative of NH₃BH₃. It was observed that exposure of LiNH₂BH₃ to NH₃ produces a sticky liquid containing a 1:1 molar ratio of LiNH₂BH₃ to NH₃ [23]. Keeping in mind these reported observations, we did a simple experiment. We synthesized LiN₂H₄BH₃ as reported in our previous report [8] and exposed it to a stream of NH₃. Like for the aforementioned LiNH₂BH₃ [23], our sample changed to a sticky liquid/pasty material. Hence, we may conclude that the “excess” of 0.75 mole of NH₃ (Equation (3)) explains the paste-like state of 3 right after ball-milling.

The new compound LiN₂H₃BH₃·0.25NH₃ was primarily synthesized for assessing its potential as chemical H storage material. Ammoniates of B–N–H compounds are of interest as, compared to their ammonia-free counterparts, they should have better dehydrogenation properties owing to the active participation of NH₃ in the dehydrogenation process [13]. In our conditions, LiN₂H₃BH₃·xNH₃ has been shown to release H₂ at temperatures lower than LiN₂H₃BH₃, which is consistent with the previous remark. However, LiN₂H₃BH₃·0.25NH₃ liberates non-negligible amounts of NH₃. According to Chua et al. [13], contamination of H₂ by small amounts of NH₃ is unavoidable, but in our conditions the amount of NH₃ represents 7.6 wt% of the starting material. Under heating, the x molecules of NH₃ are first released at temperatures lower than 100°C, and then the remaining solid, supposed to be mainly LiN₂H₃BH₃, decomposes into H₂ and NH₃. Taking into account the overall weight loss of 21.2 wt% between 75 and 300 °C and the contents of H (12.1 wt%) and NH₃ (7.6 wt%) in LiN₂H₃BH₃·0.25NH₃, there is a difference of 1.5 wt% (assuming the loss of all H atoms). This is indicative of the decomposition of LiN₂H₃BH₃·0.25NH₃ to some extent. For example, if one assumes the formation of 2 equivalents of H₂ up to 300 °C (i.e., 7.7 wt%, which means the loss of 4 H atoms over a maximum of 6 in LiN₂H₃BH₃), the proportion of NH₃ stemming from the decomposition of LiN₂H₃BH₃ would be 5.9 wt% at 300 °C. This is illustrated in Figure 6 where a mechanism of decomposition is suggested on the basis of both the present results and the results reported for LiN₂H₃BH₃ elsewhere [8]. A polymeric product of unknown composition forms, similar to the residues recovered upon the dehydrogenation of most of the B–N–H compounds [5,6,7,8,9,10,11,12,27]. The nature of such residues is unknown yet because of their difficult characterization (amorphous to X-ray and too complex composition for FTIR and NMR spectroscopy techniques).

With respect to the aforementioned release of NH₃, such a feature is unfortunately quite detrimental for the targeted application, namely chemical H storage. Indeed, the stored hydrogen is intended to be generated on demand for fueling a low-temperature fuel cell, but traces of ammonia (as low as 1 ppm) are able to severely degrade the fuel cell performance to impractical levels [28]. Given that the release of the 0.25 NH₃ cannot be avoided with LiN₂H₃BH₃·0.25NH₃, its ammonia-free counterpart LiN₂H₃BH₃ [8] seems to be more suitable for chemical H storage.

Yet, our new compound LiN₂H₃BH₃·0.25NH₃ might have potential for another application. It might be seen as a potential NH₃ carrier as it is theoretically able to release 7.6 wt% of NH₃ from 75 °C.

4. Materials and Methods

Hydrogen-storage grade LiNH₂ was purchased from Sigma-Aldrich and used as received. Hydrazine borane N₂H₄BH₃ was synthesized by salt metathesis according to an optimized procedure reported in details elsewhere [5]. Typically, an equimolar mixture of sodium borohydride NaBH₄ (Acros Organics, Geel, Belgium) and hydrazine hemisulfate N₂H₄·1/2H₂SO₄ (Sigma-Aldrich, St Quentin Fallavier, France) in dioxane were prepared in a 250-mL three-necked round-bottom flask kept under argon flow. The mixture was allowed to react at 40 °C for 48 h. Then, the solution of N₂H₄BH₃ was separated from insoluble Na₂SO₄ by filtration, the solvent was removed by extraction under vacuum at room temperature for 4 h, and the as-obtained borane was dried under dynamic vacuum at room temperature for 24 h. Both LiNH₂ and N₂H₄BH₃ were stored and handled in an argon-filled glove box (MBraun M200B, O₂ < 0.1 ppm, H₂O < 0.1 ppm).

The synthesis of our new compound was done as follows. In the argon-filled glove box, LiNH₂ and N₂H₄BH₃ were separately weighted (total of about 350 mg; equimolar ratio) and transferred in a 50-mL stainless steel jar. Several stainless steel balls (Ø 10 mm) of a total weight of 40 g were added and the jar was sealed to be taken out of the glove box. The mixture was ball-milled at ambient conditions by using a RETSCH PM 100 planetary ball mill: 2 min of milling at 300 rpm + 2 min of break, 10 times. In doing so, a paste-like solid was obtained. It was subjected to vacuum overnight. The paste-like state was modified, appearing less pasty. The sample was finally transferred into a vial to be kept in the argon-filled glove box. Note that these conditions were optimized in a preliminary systematic study (not reported herein). We indeed worked with different N₂H₄BH₃/LiNH₂ ratios (1:0.75 and 1:0.5), using different ball-milling conditions and a wet synthesis approach (solubilization/dispersion of the reactants in moisture-free organic solvents like tetrahydrofuran and dioxane). For consistency and clarity, only the optimized conditions are presented.

The molecular structure was analyzed by Fourier transform infrared (FTIR) spectroscopy (Nicolet 710, range 3600–600 cm⁻¹, 32 scans). The attenuated total reflection (ATR) mode (enabling sample to be examined directly) was used at ambient conditions and under air. To minimize the contact of the sample with atmospheric O₂ and H₂O, a vial containing a few milligrams of the borane was prepared in the glove box, taken out, and opened just before the measurement. The number of scans was fixed to 32 to avoid excessive contact with air. In such conditions, reliable spectra are generally collected.

The molecular structure was also analyzed by ¹¹B magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy (Varian VNMR400, 128.37 MHz, 20000 rpm, −10 °C). In the glove box, a few milligrams (or grains) of the sample was transferred into the NMR tube (Ø 10 mm) and dissolved by acetonitrile-d3 (Eurisotop, Gif sur Yvette, France). The sealed tube was then taken out of the glove box to be analyzed by NMR according to the standard procedures for this technique.

The crystal structure was analyzed by powder X-ray diffraction (PXRD) using a PANalytical X’PERT Pro multipurpose diffractometer (Cu-K_α1/α2 radiation, λ = 1.5418 Å, 45 kV, 30 mA). All the patterns were collected using Bragg–Brentano geometry on a spinning sample holder loaded into a glove box (Jacomex PBOX, H₂O < 5 ppm, O₂ < 5 ppm). The powders were protected from air and oxygen contamination using a Kapton foil.

The thermal behavior of the samples was analyzed by thermogravimetric analysis (TGA; Q500 TA Instruments; heating rate 5 °C min⁻¹; N₂ flow rate 50 mL min⁻¹) and differential scanning calorimetry (DSC; 2920 MDSC TA Instruments, manufacturer, New Castle, DE, USA). For both techniques, our standard conditions were as follows: sample weight of 2–3 mg; aluminum crucible of 100 mL with a pinhole (Ø 670 mm); temperature range 25–300 °C; heating rate 5 °C min⁻¹; N₂ flow rate of 50 mL min⁻¹. A micro-gas chromatograph-mass spectrometer detector (µGC-MSD; S.R.A. Instruments, Agilent Technologies, Lyon, France) was used in coupling with a TGA/DSC 2 from Mettler-Toledo for the identification and quantification of the gaseous by-products. The µGC is equipped with micro-thermal conductivity detectors and two columns: one molecular sieve column for the detection and quantification of H₂ (10 m × 0.32 mm; 5 Å; carrier gas Ar; 70 °C; head column pressure fixed at 28 psi), and one poraPLOT U column for NH₃ (8 m × 0.15 mm i.d.; carrier gas He; 130°C; head column pressure fixed at 30 psi).