Efficient Synthesis of an Aluminum Amidoborane Ammoniate

Abstract

A novel species of metal amidoborane ammoniate, [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] has been successfully synthesized in up to 95% via the one-step reaction of AlH₃·OEt₂ with liquid NH₃BH₃·nNH₃ (n = 1~6) at 0 °C. This solution based reaction method provides an alternative pathway to the traditional mechano-chemical ball milling methods, avoiding possible decomposition. MAS ²⁷Al NMR spectroscopy confirms the formulation of the compound as an Al(NH₂BH₃)₆³⁻ complex anion and an Al(NH₃)₆³⁺ cation. Initial dehydrogenation studies of this aluminum based M-N-B-H compound demonstrate that hydrogen is released at temperatures as low as 65 °C, totaling ~8.6 equivalents of H₂ (10.3 wt %) upon heating to 105 °C. This method of synthesis offers a promising route towards the large scale production of metal amidoborane ammoniate moieties.

1. Introduction

A critical challenge facing the advancement of hydrogen fuel cells for automotive applications is the development of safe and energy efficient hydrogen storage materials. Metal amidoboranes (MNH₂BH₃, MAB) and metal borohydride ammoniates (MBH₄·nNH₃, MBA) are currently among the most promising candidate materials [1,2,3,4,5,6]. Recent demonstration of the regeneration of ammonia borane derivatives using hydrazine in liquid ammonia point to the feasibility of off-board reversibility [7,8,9]. Substitution of one protic H atom in the [NH₃] of NH₃BH₃ by a metal atom leads to the formation of MAB complexes.

Aluminum amidoborane (Al(NH₂BH₃)₃, AlAB), first synthesized by Hawthorne et al. [10], possesses one of the highest theoretical hydrogen capacities among MABs (12.9 wt % H), capable of releasing 6 wt % H₂ at 190 °C and approximately 8 wt % H₂ in the presence of an ionic liquid at lower temperatures [10]. As such, this material has already experienced intensive explorations, although up to now only a few reports have identified its existence owing to its poor stability and spontaneous H₂ loss caused by the chemically vulnerable Lewis-acidic Al³⁺ center [10,11,12]. The improved dehydrogenation properties of AlAB (Al χ_p = 1.5), relative to ammonia borane [10], makes the Al-N-B-H systems attractive, albeit difficult to synthesize. Recently, Guo et al. reported on the stability of [Al(NH₃)₆](BH₄)₃ in air, which differs quite significantly from the analogous volatile liquid Al(BH₄)₃ [13,14]. Strong N-H^δ+∙∙∙^−δH-B dihydrogen bonds contribute to the stability of this compound resulting in its long term stability in air. Another recently reported Al based amidoborane complex includes Li₃AlH₆·n(NH₂BH₃) which releases 9 wt % H₂ at a temperature of 130 °C [15].

A variety of B-N amidoborane ammoniates, have previously been synthesized by reacting MAB and NH₃, including LiNH₂BH₃·NH₃ [16], Mg(NH₂BH₃)₂·3NH₃ [17], and Ca(NH₂BH₃)₂·NH₃ [18]. However, to the best of our knowledge, there has been no prior report of the synthesis of an aluminum analog. Herein we report the first synthesis of aluminum amidoborane ammoniate, [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺].

2. Results and Discussion

The synthesis of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] (according to Equations (1) and (2)) was achieved using a specially-designed polytetrafluoroethylene reactor, which allowed the reactants to be stirred at low temperature under ammonia pressure. Under these conditions, ammonia borane reversibly absorbs up to 6 equivalents of NH₃, forming liquid NH₃BH₃·nNH₃ (n = 1–6) complexes [19]. AlH₃·OEt₂, which is insoluble in NH₃BH₃·nNH₃, was utilized as a highly reactive Al source [20,21]. Immersing the Al source in ammonia borane ammoniate complex permits the selective uptake of ammonia in a one-step synthesis of Al(NH₂BH₃)₃·3NH₃ as a solid precipitate that can be isolated in up to 95% purity (based on AlH₃·OEt₂) by filtration (Equations (1) and (2), details in Section 3, Experimental Section). It should be emphasized that this method avoids the high-energy impact generally encountered in traditional ball milling methods and further prevents possible decomposition of components. This synthesis strategy may also be effective for other amidoborane ammoniates.

$${\text{NH}}_3{\text{BH}}_{3\left(s\right)}+n{\text{NH}}_{3\left(g\right)}\underset{n=1~6}{\overset{0 °\text{C}}{\to }}{\text{NH}}_3{\text{BH}}_3·n{\text{NH}}_{3\left(l\right)}$$

(1)

$${\text{AlH}}_3·{\text{OEt}}_{2\left(l\right)}+3{\text{NH}}_3{\text{BH}}_3·n{\text{NH}}_{3\left(l\right)} \underset{-3H_2,-E{t}_{2}O,-{NH}_3}{\overset{0 °\text{C}}{\to }}\text{Al}{\left({\text{NH}}_2{\text{BH}}_3\right)}_3·3{\text{NH}}_{3\left(s\right)}$$

(2)

Figure 1a illustrates the XRD pattern obtained for a sample of Al(NH₂BH₃)₃·3NH₃ prepared via the method described above. The pattern does not index to any previously reported Al-N-B-H quaternary compound and contains at most, only very minor contributions from unreacted starting material. FTIR analysis of Al(NH₂BH₃)₃·3NH₃ featured a N-B stretch at 875 cm⁻¹ and peaks at 426 and 461 cm⁻¹ which were assigned to Al-N lattice vibrations, (Figure S1, Table S1). Attempts were also made to prepare Al(NH₂BH₃)₃·3NH₃ using ball milling techniques. As shown in Figure 1c, no new species evolved from a mixture of AlH₃·OEt₂ + 3NH₃BH₃, which was ball milled under ammonia atmosphere at 0 °C, at a speed of 150 rpm for at least 2 hours. Moreover, increasing the ball milling energy, such as higher rotational speed or temperature (>40 °C) during the synthesis causes dissociation of the ether adduct, which often leads to the production of γ-AlH₃ (Figure 1b). This alane polymorph incidentally shows much lower reactivity in liquid NH₃BH₃·nNH₃ than pure AlH₃·OEt₂, and inhibits the formation of an Al-N bond [22].

Figure 1. XRD patterns of (a) as-prepared Al(NH₂BH₃)₃·3NH₃; (b) mixture of γ-AlH₃ + 3NH₃BH₃; (c) ball milled AlH₃·OEt₂ + 3NH₃BH₃. λ = 1.5406 Å.

Figure 1. XRD patterns of (a) as-prepared Al(NH₂BH₃)₃·3NH₃; (b) mixture of γ-AlH₃ + 3NH₃BH₃; (c) ball milled AlH₃·OEt₂ + 3NH₃BH₃. λ = 1.5406 Å.

The ²⁷Al MAS NMR spectrum (Figure 2b) verifies the formation of Al(NH₂BH₃)₃·3NH₃ and provides key information about its molecular structure. After reaction, only traces of the characteristic resonance for AlH₃·OEt₂ at 109.9 ppm are observed [23]. Two major resonances at 65.5 ppm and 33.6 ppm dominate the spectrum, clearly indicating that Al(NH₂BH₃)₃·3NH₃ contains equal amounts of aluminum in two very different coordination environments. The MAS ¹¹B NMR spectrum of the product contains a major resonance for Al(NH₂BH₃)₃·3NH₃ at 19.6 ppm and a and a minor resonance at −38 ppm which is due the presence of [(NH₃)₂BH₂⁺][BH₄⁻] DADB or a related decomposition product that was also observed in the starting material [24]. As seen in Figure 2a, the ¹¹B chemical shift of Al(NH₂BH₃)₃·3NH₃ is 2.7 ppm upfield from the 22.3 ppm shift observed for NH₃BH₃·nNH₃. Similar upfield shifts have been observed for other metal amidoboranes and hence this observation confirms the substitution of an H atom by an Al atom in the ammonia borane molecule [25,26].

Figure 2. (a) ¹¹B MAS NMR spectra of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] and NH₃BH₃·nNH₃; (b) ²⁷Al MAS NMR spectra of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] and AlH₃·OEt₂. The molecular structure of the octahedral Al complexes are also depicted (yellow balls represent Al, red for N, blue for B, and grey for H).

Figure 2. (a) ¹¹B MAS NMR spectra of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] and NH₃BH₃·nNH₃; (b) ²⁷Al MAS NMR spectra of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] and AlH₃·OEt₂. The molecular structure of the octahedral Al complexes are also depicted (yellow balls represent Al, red for N, blue for B, and grey for H).

Al³⁺ generally has either tetrahedral or octahedral coordination. Thus a priori there are four possible coordination geometries for Al(NH₂BH₃)₃·3NH₃: 1) Al coordinates three (NH₂BH₃)⁻ anions and three ammonia molecules to give a neutral Al(NH₃)₃(NH₂BH₃)₃ complex; 2) Al coordinates octahedrally with only ammonia giving a hexamminealuminum cation [27], Al(NH₃)₆³⁺ and leaving three free (NH₂BH₃)⁻ anions; 3) Al coordinates tetrahedrally with (NH₂BH₃)⁻ anions, forming an Al(NH₂BH₃)₄⁻ anion and three of these anions pair with one Al(NH₃)₆³⁺; and 4) Al octahedrally coordinates with (NH₂BH₃)⁻ anions giving a Al(NH₂BH₃)₆³⁻ complex anion and ion pairs with the Al(NH₃)₆³⁺ cation. The observation of two peaks with equal intensity in the ²⁷Al MAS NMR spectrum is consistent with only the [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] formulation and as such the ²⁷Al NMR resonances are assigned as follows: Al(NH₂BH₃)₆³⁻ at 65.5 ppm and Al(NH₃)₆³⁺ at 33.6 ppm (Figure 2b). This is quite similar to the reported structure of Mg(NH₂BH₃)₂·3NH₃ where Mg²⁺ exhibits both tetrahedral and octahedral coordination [17]. Elemental analysis (Table S2) also shows the ratio of Al:N:B is 1:6:3, and supports the [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] formulation.

No apparent reaction was observed after exposure of a sample of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] to dry air for 3 days (Figure S2). The time-programmed-desorption/mass spectroscopy (TPD/MS) results reveal that the thermal decomposition of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] occurs in the temperature range of 65–180 °C, with the release corresponding to 7.5 wt % (Figure 3a,c). The desorbed gaseous product comprises of both H₂ and NH₃.

Figure 3. (a) TPD (∆) and gas release (□) profiles of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] at a heating rate of 5 °C min⁻¹ under argon flow; (b) Isothermal desorption of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] in a closed vessel; the temperature ramping shown by dash dot line; (c) MS signals in (a): □ H₂, ○ NH₃, ∆ B₂H₆, ▽ B₃N₃H₆; (d) MS signals in (b) measured using Calibration Injection Mode: □ H₂, ○ NH₃, ∆ B₂H₆, ▽ B₃N₃H₆; (e) H₂ purity comparison between different systems.

Figure 3. (a) TPD (∆) and gas release (□) profiles of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] at a heating rate of 5 °C min⁻¹ under argon flow; (b) Isothermal desorption of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] in a closed vessel; the temperature ramping shown by dash dot line; (c) MS signals in (a): □ H₂, ○ NH₃, ∆ B₂H₆, ▽ B₃N₃H₆; (d) MS signals in (b) measured using Calibration Injection Mode: □ H₂, ○ NH₃, ∆ B₂H₆, ▽ B₃N₃H₆; (e) H₂ purity comparison between different systems.

Figure 4 shows the N 1s XPS results of AlAB·3NH₃ before and after thermal decomposition (experimental details described in Supplemental Information). The peaks at ~396.6, ~398.0 and ~399.6 eV are attributed to N-Al, N-B and NH₃, respectively. After decomposition, the evolution of NH₃ and the corresponding peak at ~189.8 eV in B 1s XPS (Figure S3) suggests the formation of an Al-N-B matrix. Combined with the remaining B-H vibrations in micro-FTIR (Figure S4, Table S3), the reaction under dynamic inert gas flow can be described by Equation (3). The anticipated borazine-derivated structure is illustrated in Figure S5 representing AlN₃B₃H₆.

[Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] → 2AlN₃B₃H₃ + 6NH₃ + 12H₂

(3)

Figure 4. The N 1s XPS results of Al(NH₂BH₃)₃·3NH₃ (1) before (a) and after thermal decomposition in an open system (b) and in a closed system (c). The experiment data are in black, while the fitted ones are in red.

Figure 4. The N 1s XPS results of Al(NH₂BH₃)₃·3NH₃ (1) before (a) and after thermal decomposition in an open system (b) and in a closed system (c). The experiment data are in black, while the fitted ones are in red.

The isothermal desorption in a closed vessel was examined using a Sieverts method at 105 °C (experimental details described in Supplemental Information). The gas evolved is calculated to be 10.3 wt % (Figure 3b), while only 0.05% NH₃ is detectable (Figure 3d). Obviously, the mass difference (Figure 3e) indicates that a significant partial pressure of NH₃ in a closed system suppresses further NH₃ desorption. This phenomenon is in accordance with the decomposition pathways of other metal amidoborane ammoniates [17,28,29]. Element analysis shows that the composition of Al, N, B and H are 15.83%, 48.18%, 18.36% and 3.83%, respectively, indicating an empirical formula of AlN₆B₃H_6.5^*. Similarly, N-H or B-H vibrations are not observed in the micro-FTIR spectrum, while Al-N stretching vibrations and weak H wagging vibrations are apparent (Figure S4 and Table S3). Meanwhile, the peaks at 1367 and 1627 cm⁻¹ are typical of N-B stretching in h-BN [30]. The N 1s XPS peak ~396.4 eV (Figure 4c) and is attributed to the formation of an Al-N bond, while the two overlapped B 1s XPS peaks in Figure S3 suggests that the decomposed product comprises of not only [AlNBH] but also another B moiety. On the other hand, the ¹¹B MAS NMR spectrum presents at least two overlapping resonances at 6.3 ppm and 18.3 ppm (Figure S6), which is due to the second-order quadrupolar interaction. Thus, the B atoms are likely in a BN₃ or BN₂H environment [31,32]. The N 1s peak at ~399.1 eV is possibly a N-N bond. Clearly, the decomposition mechanism of [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] in a closed system is much more complicated than that of an open system. On the basis of 8.6 equivalents of H₂, the dehydrogenation process can be briefly described by Equation (4).

[Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] → AlN₆B₃H_6.5^* + (8.5~9.0)H₂

(4)

3. Experimental Section

All starting materials, LiAlH₄ 99% (Sigma-Aldrich, Shanghai, China), AlCl₃ 99.99%, NH₃BH₃ 99% (Sigma-Aldrich), and NH₃ (Alfa Aesar, Shanghai, China), were obtained commercially and used without further purification. All manipulations were carried out under inert atmosphere conditions, either in an argon-filled glovebox or using standard Schlenk line techniques under a nitrogen atmosphere. The organometallic synthesis of AlH₃·Et₂O is a chemically simple process, but a brief summary is presented. Generally, AlCl₃ was reacted with LiAlH₄ in diethyl ether with the LiCl precipitate being removed by filtration [21,33]. The excess diethyl ether was then removed under dynamic vacuum. AlH₃·Et₂O was ground in a mortar with excess NH₃BH₃ and then sealed in a self-designed polytetrafluoroethylene (PTFE) reactor. The reactor was attached to the gas/vacuum manifold and rapidly evacuated/backfilled with 0.3–0.5 MPa NH₃. The system was cooled to −70 °C using acetone and dry ice, and gradually warmed to 0 °C in an ice bath. At this temperature and under the NH₃ atmosphere, ammonia borane reversibly absorbed up to at least 6 equivalents of NH₃, forming liquid NH₃BH₃·nNH₃ (n = 1–6) complexes. AlH₃·OEt₂ was dissolved in liquid NH₃BH₃·nNH₃, and the solution stirred for 2 h until the reaction was complete. The internal temperature and pressure of the reactor and manifold were recorded for the duration of the experiment. The ammonia and reaction produced hydrogen were then removed in vacuo at room temperature. Anhydrous diethyl ether was then added to the remaining products, thereby dissolving the excess NH₃BH₃ of which was removed by filtration. The residual solid [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺] was then heated to 45 °C for 12h to remove residual solvent to yield a solid white powder.

4. Conclusions

In summary, a novel aluminum amidoborane ammoniate, [Al(NH₂BH₃)₆³⁻][Al(NH₃)₆³⁺], has been successfully synthesized. A reaction vessel has been designed that allows a one-step synthesis from the reaction of AlH₃·OEt₂ with liquid NH₃BH₃·nNH₃ (n = 1~6) at 0 °C. MAS ²⁷Al NMR spectroscopy confirms the formulation of the compound as an Al(NH₂BH₃)₆³⁻ complex anion and a Al(NH₃)₆³⁺ cation. This aluminum based M-N-B-H compound begins to release hydrogen at 65 °C, amounting to ~8.6 equivalents of H₂ (10.3 wt %) upon heating to 105 °C. This method of synthesis offers a promising route towards the large scale production of metal amidoborane ammoniate moieties.