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. 2024 Mar 11;9(12):14241–14248. doi: 10.1021/acsomega.3c09934

Synthesis and Energetic Characterization of Borane-Amines on High-Nitrogen Heterocycles

Randy Lin †, Nicholas F Scherschel ‡,∥, Matthias Zeller †, Stephen G Hamlin §,∥, Madison Snyder †, Steven Son §,∥, P Veeraraghavan Ramachandran †,∥, Davin G Piercey ‡,§,∥,*
PMCID: PMC10976356  PMID: 38559918

Abstract

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Borane-amines have garnered attention over the last several decades in a variety of applications, ranging from hydrogen storage materials to hypergolic fuel systems. An investigation into the synthesis of borane-amines with high-nitrogen content heterocycles was undertaken in this work. Borane-amines were formed by the reaction of BH3·Me2S in tetrahydrofuran (THF) with the requisite nitrogen-containing heterocycle and isolated by placing the crude reaction mixture in hexanes to precipitate the product. X-ray crystallography, thermogravimetric analysis (TGA), high resolution mass spectroscopy (HRMS), 1H NMR, 13C NMR, and 11B NMR were utilized for product characterization, while impact and friction sensitivity testing were conducted to identify sensitivity in the synthesized compounds. Most isolated borane-amines, except one, were found to decompose in the atmosphere and were more sensitive to mechanical stimuli than their starting materials; however, all synthesized compounds were found to be hypergolic in the presence of white fuming nitric acid (WFNA).

Introduction

Few materials have demonstrated versatility in their use as boron. In its various chemical forms, it may be utilized as a dopant to yield thermal shock-resistant borosilicate glass,1 a hydrogen source,2−4 detergents like borax, hypergolic fuels,5,6 and even act as essential nutrients for certain plant species.7 In chemistry, organoboron compounds, such as diborane and its derivatives, attract much attention for their versatility in synthetic applications.8 Additionally, boronic esters have found widespread use in Suzuki–Miyaura cross-coupling reactions for the C–C bond formation.9 Furthermore, boron finds use in the field of energetic materials, specifically in hypergolic fuels.

Boron-based hypergolic fuels have been a topic of interest over the last several decades due to their reduced toxicity relative to hydrazine-based binary propulsion systems.10−12 Hypergolic propulsion systems are an attractive option where reliability, simplicity, and reignitability are important as they only require valves to bring the hypergolic components together to achieve ignition.13,14 Accordingly, work has been conducted on boron-containing ionic liquids by Shreeve15−17 and hypergolicity studies of borane-amines with nonenergetic backbones by Ramachandran.5,6 However, comparatively little research has been conducted on borane-amines on high-nitrogen content heterocycles as hypergolic fuels.

In the field of energetic materials, there is a continued push for improved densities, detonation pressures, and combustion temperatures as these are the predominant indicators against which the energetic performance is measured. New heteroatoms other than carbon are desirable for investigation, as they may allow for higher-performing systems than those presently available. Boron is a highly sought-after heteroatom as the bond dissociation energy (BDE) of B–O is 787 ± 42 kJ/mol, which is significantly greater than the BDE of the Al–O bond at 485 ± 8 kJ/mol.18 High BDEs translate to higher combustion temperatures, which are important in the performance of rocket motors. Accordingly, boron may be useful not only in high-explosive formulations but also in rocket motors, as it may have desirable characteristics in both areas.

Although borane-amines seem to intersect the field of energetic materials quite well due to their hypergolic characteristics with white fuming nitric acid (WFNA) as well as the high-energy storage capability inherent in boron, there is comparatively little research on borane-amines with high-nitrogen content heterocycles. An article by Bélanger-Chabot et al.19 examined borane-trinitromethyltetrazole and related compounds, yet it remains one of the few papers that has investigated borane-amines on high-nitrogen content energetic materials. In contrast, zwitterionic N-oxides of high-nitrogen content heterocycles have been comparatively more widely investigated and shown to improve the properties of the parent energetic systems, with examples such as dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50),20 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105),21 nitrotetrazole-2N-oxide,22 and 3,6-diamino-1,2,4,5-tetrazine 1,4-dioxide (LAX-112).23 In principle, N–BH3 bonds may behave as zwitterions, which may yield a good comparison to N-oxides in energetic systems and possible related stabilization of such systems.24−26 Accordingly, we have attempted the synthesis and characterization of various borane-amines of high-nitrogen content heterocycles and evaluated their hypergolicity with WFNA.

Experimental Section

General Procedure for Addition of Borane

To an oven-dried three-neck round-bottom flask with a stir bar, 1 equiv of the relevant nitrogen-containing precursor was added. Next, the round-bottom flask was connected to a Schlenk line and purged with an inert gas atmosphere, typically neat N2 or argon gas, using conventional Schlenk techniques. Tetrahydrofuran was then added to the reaction mixture, followed by 2.0 equiv of 9 M BH3·Me2S (BMS) to make a 0.5 M solution. The reaction mixture was then stirred for 3 h; however, precursors with low solubility in tetrahydrofuran (THF) were instead stirred for a total of 16 h. The reaction progress was monitored by 11B NMR spectroscopy of the crude material. After the reaction was complete, the crude reaction mixture was then placed in hexanes, which precipitated the borane-amine product. The liquid organics were then removed by pipette, and the remaining solids were dried under vacuum, yielding the required borane-amine.

Procedure for Attempted Transamination of Borane-Ammonia with 19a

To an oven-dried three-neck round-bottom flask, 1 equiv of the precursor of 19a and 1 equiv of borane-ammonia were added and placed along with a stir bar. Next, the round-bottom flask was connected to a Schlenk line and purged with an atmosphere of neat N2 using conventional Schlenk techniques. Then, 1 M tetrahydrofuran was added to the reaction mixture, and the mixture was stirred under reflux for 10 h. The reaction progress was monitored by 11B NMR of the crude material. After the reaction was complete, the mixture was dried under vacuum, yielding a mixture of 19b and the parent compound, 19a. Isolation of 19b from 19a was not attempted, as the general procedure for borane-amines discussed previously yielded better results.

Results and Discussion

The general procedure for the addition of borane via adduct exchange of BMS was followed for all compounds shown in Figures 1, 2, and 3, but did not demonstrate any detectable addition of borane by 11B NMR for 1 – 14 (Figure 1). We also attempted an alternate method to synthesize the product compounds via the transamination of borane-ammonia27 in a trial run with the parent compound 19a. While the expected N–BH3 peak was detected in the 11B NMR spectrum, noticeable amounts of the parent amine were also detected in the 1H NMR spectrum. In the case of utilizing BMS as the borane source for 19a, primarily the product was observed. As such, despite the toxic and malodorous hazards associated with the dimethyl sulfide byproduct, the adduct exchange of BMS and the presented heterocycles was primarily used here as the relatively mild reaction conditions and straightforward isolation (compared to other methods like transamination) were found to be critical in handling these moisture-sensitive products.

Figure 1.

Figure 1

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Nitrogen-rich compounds did not exhibit any detectable interactions with BH3.

Figure 2.

Figure 2

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Transiently stable borane-amines.

Figure 3.

Figure 3

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Examples of isolatable borane-amines, in which the Lewis-acid–Lewis-base pairs are presented as zwitterionic pairs.

Figure 1 shows the nitrogen-rich heterocycles that failed to form borane-amines. This also shows the difficulty of finding an acceptable backbone for forming a borane-amine with a high-nitrogen heterocyclic backbone, yet it is also a useful guide for observing the failure patterns of borane addition. For example, no borane-amine was formed with any compounds that had an acidic proton or anion (3, 4, 5, 10, 12, 13, and 14). Of this group, it should be noted that the borane of 12 was not successfully synthesized, despite there being literature precedent for trinitro analogues successfully forming borane-amines.19 Additionally, it seems that tetrazoles with substitutions at one position do not form stable boraneamines, while substitutions at the two positions tend to allow for a transiently stable product, an unusual substitution pattern. Furthermore, heterocycles with nitro-substituents have never demonstrated successful borane-amine addition, which may be attributed to the deactivation of the ring’s electrons by the electron-withdrawing nature of the nitro-substituents. Lastly, the N-oxide zwitterions of 1,214,226,23 and 8(28) are all well-known and exhibit good performance as explosives; however, the corresponding N–BH3 zwitterion was not detectable for any of these compounds, even in situ. Accordingly, it seems that a compound’s proclivity for the addition of zwitterionic N-oxides is not to be considered a good predictor of how borane may add to the same high-nitrogen content backbone.

While the aforementioned compounds were unsuccessful coupling partners, other backbones yielded borane-amines that were transiently stable using the general procedure for borane addition (Figure 2). Oftentimes, a borane-amine was detected in the crude solution via 11B NMR spectroscopy; however, upon attempted isolation of the borane-amine, the product would decompose even under more gentle drying techniques such as vacuum drying. Of the transiently stable borane-amines, 15b is the most stable. Nevertheless, 15b shows visible signs of decomposition (the dry powdery solid turns into an oily solid) even under argon gas. In contrast, 16b–18b are unstable in situ and decompose readily on their own.

Isolable borane-amines whose products are relatively stable are shown in Figures 3 and 4, along with their crystallographic unit cells in Figures 5, 6 and 7. Borane-amines 19b, 20b, and 21b were the most stable and were successfully isolated and characterized by utilizing single-crystal X-ray diffraction (SCXRD). Of these, 21b is by far the most stable and shows no signs of decomposition, even after two months of exposure to the atmosphere, unlike 19b and 20b. The B–N bond lengths of 19, 20b, and 21b were found to be 1.58, 1.57, and 1.61, respectively, in single-crystal X-ray analysis at 150 K. The B–N bond lengths of 19b and 20b are slightly shorter than the B–N bond length of tetragonal borane-ammonia (1.599 Å), while that of 21 is slightly longer (1.61 Å).29 The B–H distances are all ≤1.11, which is the usual B–H bond length of borane-amines (Table 1). The identified B–H bond lengths are significant as they were elucidated by HFIX/AFIX 138 constraints to allow for refinement of the B–H bond lengths. Notably, no intermolecular H-bonding was observed for 19b, 20b, and 21b in their crystal structures. Furthermore, 21b was the only long-term air-stable borane-amine, yet it possessed the longest B–N bond length of all synthesized compounds and was even longer than borane-amine’s B–N bond.

Figure 4.

Figure 4

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ORTEP representation of the molecular structures of 19b, 20b, and 21b in the crystalline state. Non-H atomic displacement ellipsoids are shown at a 50% probability level.

Figure 5.

Figure 5

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Unit cell packing of 19b: (A) viewed along the c-axis and (b) viewed along the a-axis. Bond distances of 1.58 are observed for B–N bonds at 150 K.

Figure 6.

Figure 6

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Unit cell packing of 20b: (a) viewed along the c-axis and (b) viewed along the a-axis. Bond distances of 1.57 are observed for the B–N bonds at 150 K.

Figure 7.

Figure 7

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Unit cell packing of 21b: (a) viewed along the c-axis and (b) viewed along the a-axis. Bond distances of 1.61 are observed for B–N bonds at 150 K.

Table 1. Comparison of the Crystallographic B–N and B–H Distances of 19b, 20b, 21b, and Borane-Ammoniaa.

Property 19b 20b 21b Borane-Ammonia29,30b
B–N distance [Å] 1.58 1.57 1.61 1.60
B–H distance [Å] 1.10 1.11 1.09 1.11
ρ (g/cm3) 1.232 1.270 1.224 0.78
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a

Densities of 19b, 20b, and 21b Originate from Single-Crystal X-ray Diffraction at 150 K while those of Borane-Ammonia Is Taken from 200 K.30

b

Bond lengths for borane-ammonia are essentially unchanged from 90 to 298 K.

Comparisons of the densities of the borane-amine products to the respective precursor molecules allow for the analysis of the energetic properties of borane-amines. Although the starting compounds range in density from 1.175 to 1.75 g/cm3, the crystalline density of all borane adducts is ∼1.25 g/cm3. 21b marginally increases in density when compared to its starting material, which is the only example of an increase in density during borane-amine formation in this work. In high explosives, density is the most important measure of performance, as a compound’s density directly influences its detonation velocity and detonation pressure.31−33 Since the density of all the borane adducts is ∼1.25 g/cm3, it is unlikely that the addition of borane groups is desirable for high-explosive formulations as acceptable densities are at least comparable to TNT at 1.65 g/cm3.

TGA analysis was used to determine the decomposition temperatures of borane-amines. For 19b and 20b, the decomposition temperatures were recorded as 36.7 and 50.5 °C, respectively (Table 2). In contrast, 19a and 20a had decomposition temperatures of 84 and 159 °C, which are substantially higher than those of their borane-amine counterparts. As will be seen in future data, 21b is an anomaly in the data set as its precursor, 21a, sublimates under standard conditions, while its borane-amine, 21b, decomposes at 108 °C. It should be noted that the decomposition temperature of 21b is higher than that of borane-ammonias, which stands at 90 °C and is of interest for hypergolic fuel use.34 While it seems that the stability of the formed borane-amine adduct increases with the basicity of the parent amine, the trend would need other examples to ensure the veracity of the observation.

Table 2. Comparison of Select Physical and Chemical Properties of Successful Borane-Amine Products with a Comparison to Their Precursors, Where Applicable.

  19b 20b 21b TNT35 RDX20
density (g/cm3)a 1.232 1.270 1.224 1.65 1.82
% N 61.9 57.1 18.2 18.5 37.8
Tdec (°C)b 36.7 50.5 108.0 295 210
precursor Tdec (°C) 84 159 N/Ag N/A N/A
Hf, 298 K (kJ/mol)c 186.3 232.7 –293.1 –80.5 86.3
detonation velocity (m/s)d 7138 7379 5566 6880 8983
detonation pressure (GPa)d 14.5 15.9 9.66 19.5 38.0
impact sensitivity (J)e 30 1 >40 40 7.5
friction sensitivity (N)f 30 40 >360 360 120
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a

Densities of 19b, 20b, and 21b were identified from single-crystal X-ray crystallography values at 150 K.

b

Decomposition temperatures from TGA analysis are defined as the onset of mass loss.

c

Solid-state enthalpies of formation were calculated with Gaussian 16W.

d

Calculated using Explo5 V6.05.02.

e

Impact sensitivities performed according to STANAG 448936 with modifications to an OZM drop hammer by the BAM method according to instructions37

f

Friction sensitivity characterization was carried out as outlined in STANAG 448738 with modifications to instructions39 on a BAM friction tester.

g

Sublimates at room temperature.

The enthalpy of formation calculations were conducted with the Gaussian 16W package40 using the previously described CBS-4 M methodology,41 which was initially developed by Petersson and co-workers.42 The method of the complete basis set (CBS) of Petersson and co-workers obtains accurate energies by using the known asymptotic convergence of pair natural orbital expressions to extrapolate via calculations of a finite basis set to the estimated complete basis limit.43 First, CBS-4 uses an HF/3-21G(d) structure optimization, at which the zero-point energy is also computed. Next, a large basis set SCF calculation as a base energy and an MP2/6-31+G calculation are used with CBS extrapolation to correct the energy through the second order. Higher-order contributions are approximated with an MP4(SDQ)/6-31+(d,p) calculation. Presently, the modified CBS-4 M methodology (M is the use of minimal population localization) was used, which includes additional empirical corrections in addition to the reparameterization of the original CBS-4 M method. Optimized Gaussian structures are available in the Supporting Information. Such methods are consistent with previous reports of the enthalpy of formation in the energetics literature.44−48

Occasionally, computational calculations may result in overestimations; TKX-50 is a pertinent example.44,49,50 An alternative approach to identifying the enthalpy of formation for these materials is by group contribution calculations, which may be accomplished by subtracting the enthalpy of formation of a borane-amine from its free amine. For example, the enthalpies of borane-hydrazine (BH3·N2H4) and hydrazine (N2H4) are −42.6851 and 42.7 kJ/mol,52 respectively. Subtracting the enthalpy of formation of borane-hydrazine from that of hydrazine affords the group contribution of borane to the enthalpy of formation, which is −85.4 kJ/mol. Using 5-amino-2-methyl-tetrazole as an example, we subtract the enthalpy of formation of the borane group contribution (−85.4 kJ/mol) from the parent amine (206.8 kJ/mol),53 and find the enthalpy of formation for 19b to be 121.4 kJ/mol by the group contribution calculation. Such an estimation reveals that the enthalpy of formation for 19b may be overestimated by the CBS-4 M methodology. Despite possibly overestimating the enthalpy of formation, CBS-4 M remains the standard method for reporting the enthalpy of formation in the literature on energetic materials.45−48,54

The detonation properties were calculated with Explo5 V6.05.0255 using the density from single-crystal X-ray diffraction and the previously calculated CBS-4 M solid-state enthalpies of formation. The calculations revealed that 21b possessed the lowest performance of all three borane-amines with a detonation velocity of 5566 m/s and a detonation pressure of 9.66 GPa (Table 2). 20b was found to be the best-performing compound in this series with a detonation velocity of 7379 m/s and a detonation pressure of 15.9 GPa, 19b was just behind 20b in terms of its performance with a detonation velocity of 7138 m/s and a detonation pressure of 14.5 GPa. Energetic performance was generally greatest for compounds 19b and 20b, which boasted heats of formation of 186.3 and 23,278 kJ/mol, respectively, and worst for 21b with a heat of formation of −293.1 kJ/mol. For a summary of the energetic properties, see Table 2.

Sensitivity testing of the borane-amines revealed that the addition of borane may act as a sensitizer in some of these systems. Free N-donor compounds 19a, 20a, and 21a are all insensitive materials (>40 J impact, >360 N friction), but 19b and 20b became sensitized toward impact and friction with the addition of borane. 19b had friction and impact sensitivities of 30 N and 30J, respectively, and 20b was found to possess impact and friction sensitivities of 1J and 40N, respectively. in contrast, 21b remained insensitive with respect to both friction and impact stimuli. It is unknown why 19b and 20b became sensitive to impact and friction stimuli while 21b did not.

Lastly, hypergolic testing was conducted on 19b–21b and 15b by dropping WFNA onto borane-amine powder. All 19b-21b demonstrated hypergolicity with white fuming nitric acid, while 15b only produced orange gas. Flames for boron combustion are generally vibrant green and can be seen clearly in Figure 8. 15b decomposed substantially prior to testing, as evidenced by its appearance, which was originally a white powder that became an oily solid after a couple of hours while still under argon gas. 19b and 20b are stable for at least several days, longer if under argon. 21b was found to be air-stable for at least two months at the time of writing.

Figure 8.

Figure 8

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Images captured using a high-speed camera for hypergolic testing on 19b (a) and 20b (b) with white fuming nitric acid.

Conclusions

The synthesis of borane-amines was attempted with a wide variety of high-nitrogen content heterocycles. Although most borane additions failed, the information acquired from successful borane-amine syntheses provides valuable insights into the application of these compounds in energetic materials. N-oxide functionalization was found not to be a predictor of the successful addition of borane-amine. The synthesized borane-amine adducts were all air-sensitive, except 21b, and most demonstrated sensitization to impact and friction stimuli with the addition of borane. The addition of borane to all amines resulted in a final borane-amine density of ∼1.25 g/cm3, regardless of the density of the starting material. Accordingly, the use of borane-amines in highly explosive formulations is likely unsuitable due to their low densities, poor energetic performance, decomposition from atmospheric moisture, and sensitization issues. There may be hypergolic applications, as they ignite upon contact with 100% nitric acid.

However, the use of 21b as an additive in hypergolic propellant formulations may be beneficial for investigation. 21b is more comparable to borane-ammonia than 19b and 20b because 21b is air-stable, insensitive to friction and impact stimuli, possesses a superior decomposition temperature than borane-ammonia, and is hypergolic with WFNA. Perhaps most unusual for 21b is that it has the highest decomposition point of all the synthesized borane-amines, higher than borane-ammonia, despite having a longer B–N bond length than borane-ammonia. Accordingly, 21 may serve as an alternative to borane-amine in hypergolic propellant systems and is likely worthy of further investigation.

Warning

19b and 20b are sensitive energetic materials. While no issues arose during our handling of these materials, appropriate personal protective equipment (face shield, body armor, Kevlar gloves) should be worn during the handling of these materials.

Acknowledgments

Jake Zuckermann is acknowledged for the synthesis of several precursor compounds tested in this work. The National Science Foundation is acknowledged through the Major Research Instrumentation Program under Grant No. CHE1625543 for the single-crystal X-ray diffractometer. This work was supported in part by the Research Instrumentation Center in the Department of Chemistry at Purdue University.

Glossary

Abbreviations

WFNA

white fuming nitric acid

RDX

1,3,5-trinitro-1,3,5-triazinane

BMS

borane dimethyl sulfide

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c09934.

  • Experimental details, characterization data, and CIF files are available for 19b, 20b, and 21b free of charge via the Internet (PDF)

Author Contributions

⊥ R.L. and N.F.S. contributed equally to this work.

Financial support for this work was provided by the Office of Naval Research under grant N00014–19–1–2089. Our lab is also supported by Purdue University and The Army Research Office (ARO).

The authors declare no competing financial interest.

Supplementary Material

ao3c09934_si_001.pdf (556.8KB, pdf)

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