Preparation and characterization of bis(guanidinium) and bis(aminotetrazolium)dodecahydroborate salts: Green high energy nitrogen and boron rich compounds

Article abstract of DOI:10.1016/j.poly.2018.09.055

This paper describes the syntheses, crystallization, characterization and energetic properties (calorimetry) of closo-dodecahydroborate salts with guanidinium and aminotetrazolium based cations. The salts were readily produced in good yields by metathesis (ion exchange) reactions depending on the water solubility of the dodecahydroborate salts. Water insoluble salts can be synthesized from the potassium or sodium salts by a simple metathesis reaction with the corresponding halide of the desired organic cation. Water soluble salts can be prepared via two consecutive metathesis reactions: the halide is first converted to the corresponding sulfate, which is reacted in turn with barium dodecahydroborate, yielding the water soluble organic closo(B₁₂H₁₂)^2? salt and the insoluble BaSO₄. The product salt is conveniently isolated by water evaporation. The aminotetrazolium salt 12d gives nice crystals when recrystallized from DMF and NMP but incorporate solvent. Guanidinium salt 15a recrystallizes from nicely from water as a monohydrate. Thermogravimetric analyses established the thermal stabilities of these compounds. The enthalpies of combustion of representative salts were determined using a constant volume bomb calorimetry. The data shows that these salts possess relatively high heats of combustion (ΔU_c, ca. 35 kJ·g^?1), and have the potential to serve as green high-energy materials.

Full text of DOI:10.1016/j.poly.2018.09.055

Accepted Manuscript
Preparation and characterization of bis(guanidinium) and bis(aminotetrazoli-
um)dodecahydroborate salts: Green high energy nitrogen and boron rich com-
pounds
Pessia Sharon, Michal Afri, Sergey Mitlin, Levi Gottlieb, Bruria Schmerling,
Dan Grinstein, Shmuel Welner, Aryeh A. Frimer
PII:
S0277-5387(18)30608-9
https://doi.org/10.1016/j.poly.2018.09.055
POLY 13454
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
24 July 2018
16 September 2018
19 September 2018
Please cite this article as: P. Sharon, M. Afri, S. Mitlin, L. Gottlieb, B. Schmerling, D. Grinstein, S. Welner, A.A.
Frimer, Preparation and characterization of bis(guanidinium) and bis(aminotetrazolium)dodecahydroborate salts:
Green high energy nitrogen and boron rich compounds, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.
2018.09.055
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REVISED Ms. Ref. No.: POLY-D-18-00694
Preparation and characterization of bis(guanidinium) and
bis(aminotetrazolium)dodecahydroborate salts:
Green high energy nitrogen and boron rich compounds
Pessia Sharon,[a] Michal Afri,[a] Sergey Mitlin,[b] Levi Gottlieb[c],
Bruria Schmerling,[a] Dan Grinstein,[a] Shmuel Welner[a] and Aryeh A. Frimer[a]*
E-mail addresses of Authors
Dr. Pessia Sharon
Dr. Michal Afri
Pessia.Sharon@biu.ac.il
Michal.Afri@biu.ac.il
Dr. Sergey Mitlin
Dr. Levi Gottlieb
Ms. Bruria Schmerling
Dr. Dan Grinstein
Dr. Shmuel Welner
Dr. Aryeh A. Frimer
Sergey.Mitlin@mail.huji.ac.il
Gottlieb969@gmail.com
Bruria.Schmerling@mail.biu.ac.il
Dan_Grinstein@mod.gov.il
Shmuel_Welner@mod.gov.il
Aryeh.Frimer@biu.ac.il
Affiliation of Authors:
[a] The Department of Chemistry, Bar-Ilan University, Ramat Gan 5290002, Israel
[b] The Institute of Chemistry, Hebrew University of Jerusalem, Jerusalem 9190401, Israel
[c] RAFAEL, Department M4, POB 2250 Haifa 31021 Israel
Corresponding Author: Prof. Aryeh A. Frimer
The Ethel and David Resnick Chair in Active Oxygen Chemistry,
The Department of Chemistry, Bar-Ilan University, Ramat Gan 5290002, Israel
E-mail: Aryeh.Frimer@biu.ac.il
Tel.: 972-3-5318610 FAX: 972-3-7384053
1

Graphical Abstract
This paper describes the convenient syntheses, crystallization, characterization and energetic
properties (calorimetry) of various guanidinium and aminotetrazolium closo-
dodecahydroborate salts – green, high energy, nitrogen and boron rich compounds.
2

Abstract
This paper describes the syntheses, crystallization, characterization and energetic properties
(calorimetry) of closo-dodecahydroborate salts with guanidinium and aminotetrazolium based
cations. The salts were readily produced in good yields by metathesis (ion exchange)
reactions depending on the water solubility of the dodecahydroborate salts. Water insoluble
salts can be synthesized from the potassium or sodium salts by a simple metathesis reaction
with the corresponding halide of the desired organic cation. Water soluble salts can be
prepared via two consecutive metathesis reactions: the halide is first converted to the
corresponding sulfate, which is reacted in turn with barium dodecahydroborate, yielding the
water soluble organic closo(B12H12)2- salt and the insoluble BaSO4. The product salt is
conveniently isolated by water evaporation. The aminotetrazolium salt 12d gives nice crystals
when recrystallized from DMF and NMP but incorporate solvent. Guanidinium salt 15a
recrystallizes from nicely from water as a monohydrate. Thermogravimetric analyses
established the thermal stabilities of these compounds. The enthalpies of combustion of
representative salts were determined using a constant volume bomb calorimetry. The data
shows that these salts possess relatively high heats of combustion (ΔUc, ca. 35 kJ∙g−1), and
have the potential to serve as green high-energy materials.
Keywords:
dodecahydroborate salts; high energy materials; propellants; green materials; nitrogen rich
cations
3

I. Introduction:
Over the past decades substantial interest has developed in insensitive munitions (IM)
- high energy compounds which are relatively stable to unplanned stimuli such as heat, shock,
fragment or bullet impact, or electromagnetic pulse (EMP) [1]. Ideally, the IM should be an
environmentally friendly or “green” explosives as well, releasing neither smoke nor toxic
gases when they detonate. Other properties desired are high crystal density and heat of
formation, with a low oxygen balance. A class of compounds which would seem to answer
all these criteria, are those which are rich in nitrogen and/or boron and produce primarily N2
and boron oxides on oxidative decomposition. These materials are thermally stable and
insensitive to electrical discharge, friction and impact [2,3]. Since these compounds contain
fewer hydrogen and carbon atoms, oxygen balance is less of a problem.
Turning first to polynitrogen compounds, many are known to be thermallly stable
energetic systems. Examples of stable nitrogen rich compounds are pyrazoles (1), imidazoles
(2), 1,2,3- and 1,2,4-triazoles (3a and 3b, respectively) and tetrazoles (4), all aromatic five-
membered ringed compounds containing two, three and four nitrogen atoms respectively (see
Scheme 1) [2]. Among the six-membered aromatic systems are the 1,2,3,4 and isomeric
1,2,4,5,-tetrazines (5a and 5b, respectively) with four nitrogens [4].
N
N
N
NH
N
HN
HN
HN
N
N
N
HN
N
N
1,2,3-triazole
1,2,4-triazole
tetrazole
imidazole
pyrazole
3a
3b
4
2
1
N
N
N
N
N
N
N
N
1,2,3,4-tetrazine
1,2,4,5-tetrazine
5a
5b
Scheme 1: Assorted polynitrogen heterocyclic aromatics.
These heterocyclic systems have been prepared with nitro, azido, nitrate and ester
substituents to raise the thermal energy of these molecules.[2,5] In addition, if there is an
amino group α to these groups, the thermal energy increases, due to intermolecular hydrogen
bonding [5].
Regarding column IIIA elements boron and aluminium, they release large amounts of
4

energy upon combustion, because of the stability of their oxidation products B2O3 or Al2O3.
As a results, these elements are often added to various formulations of high-energy materials
(HEM) in order to enhance their energy output [2,3,5]. Alkylammonium dodecahydroborate
salts have been known since 1963 [6], and are starting materials for various metal
dodecahydroborate salts [7,8,9[. The latter can be converted to organic dodecahydroborates
by a metathesis (or cation exchange) reaction [10,11,12,13]. Lower hydroborate anions, such
-
as closo-(B10H10(2- [14], closo-(B6H6(2- [7] or B3H3 [15] have also been investigated,[16 ] but
are less stable to oxidation and hydrolysis. 11B NMR spectra of various hydroborate
compounds have also been reported [17].
Boron salts containing a closo-(B12H12(2- system (6 in Scheme 2) are among the most
stable boron compounds. This dianionic system is a symmetrical icosahedron that contains
clusters of 12 boron atoms. Each boron atom within the dodecahedryl closo system 6 is
surrounded by four other delocalized boron valence electrons, which creates a δ-bonded
framework on the cluster surface. The number of electrons involved in the cluster is 4n+2,
where n=6, similar to that of an aromatic system. The double negative charge in closo 6 is
dispersed equally over the surface [18]. Common counter ions for 6 are alkali metal [Li+,
+
+
Na+, K+, and Cs+], ammonium [NEt3H+ and NBu4 ], trityl [CPh3 ] and nitosonium [NO+]
cations [3]. The physical and chemical properties of inorganic and alkylammonium closo-
(B12H12(2- compounds have been investigated [9,14]. Organic salts combining closo(B12H12)2-
anions to imidazolium, triazolium or tetrazolium cations [13] (Scheme 2) are expected to be
very stable, because the heterocyclic cations maintain an aromatic structure.
5

Counter Cation
a:
b:
c:
Alkali (Li+, Na+, K+ and Cs+)
2M
Closo-(B12H12)2
2 NHR3
R
Ammonium
Imidazolium
2
HB
HB
CH3
CH3
N
N
cation
2
HB
HB
BH
BH
HB
HB
BH
BH
H2N
N
Aminotriazolium
d:
e:
f:
2
N
N
BH
BH
6
NH2
R
Aminotetrazolium
(R=H, CH3)
N
N
2
CH3
N
N
NHR
C
Guanidinium (R=H) or
2
RHN
Triaminoguanidinium (R=NH2)
NHR
Scheme 2: Various dodecahydroborate salts
Our goal in this project was to synthesize novel polyboron salts containing polynitrogen
organic cations. Bis(imidazolium) and bis(triazolium)dodecahydroborate salts (6c and 6d,
respectively) are well-known in the literature [10,11,12]. We, therefore, focused our efforts
on the preparation of aminotetrazolium and aminoguanidinium analogs 6e and 6f. The latter
are good candidates for green energetic materials because of the high percentage of nitrogen.
[19,20] Tetrazole rings have also been shown to be applicable in a variety of high energy
salts, with the tetrazole ring serving as either part of the nitrogen rich cation, or the anion or
both. [21] Aminoguanidines [22] and tetrazoles [23] also have a well-documented role in
high energy compounds. Indeed, several papers have even appeared describing the synthesis
of aminoguanidinium [24,25,26] and aminotetrazolium [11,27] dodecahydroborate salts.
However, the procedures are not convenient and the identity of the prepared compounds was
for the most part determined only by IR spectroscopy and elemental analysis. Below we
describe general and relatively simple procedures for the synthesis of the desired
closo(B12H12)2- salts and report their complete spectral properties.
II. Results and Discussion
1. General Synthetic Strategies for the Preparation of Organic Dodecahydroborate Salts
Based on previous efforts [10,11,12], we have developed two general and convenient
synthetic pathways for the preparation the dodecahydroborate organic salts, which depend on
6

the aqueous solubility of the desired product salt. Water insoluble dodecahydroborate salts
can be synthesized from the potassium or sodium salts of dodecahydroborate (7 and 8,
respectively) by a simple metathesis (ion exchange) reaction with the corresponding chloride
of the desired organic cation (Scheme 3, Path 1). The organic closo(B12H12)2- salt precipitates
and the water soluble starting materials and resulting inorganic KCl or NaCl are washed
away.
Water soluble salts can be prepared via two consecutive metathesis reactions (Scheme 3,
Path 2): in the first, the organic cation chloride is converted with silver sulfate to the
corresponding water soluble organic sulfate salt and insoluble silver chloride; following
filtration, the cation sulfate is reacted with barium dodecahydroborate (9), yielding the water
soluble organic closo(B12H12)2- salt and the insoluble BaSO4. The product salt is conveniently
isolated by water evaporation and purified by recrystallization, if necessary.
2 Cat Cl
K2B12H12 (7) or
Na2B12H12 (8)
Ag2SO4
Path 2
Path 1
(Cat)2 SO24 + 2 AgCl (s)
(Cat)2 B12H122 + 2KCl (aq)
or
2NaCl (aq)
BaB12H
12 (9)
(Cat)2 B12H122 + BaSO4 (s)
Scheme 3: General synthesis of closo(B12H12) salts.
Turning now to the inorganic sodium closo(B12H12)2- salt (Na2B12H12, 8) starting material,
synthetic procedures for its preparation have been known [7,15,28,29,30] for close to 50
years. They were fraught, however, with two major drawbacks. Firstly, the starting materials
used, B2H6 and B4H10, are toxic and costly; secondly, product isolation is often a daunting
task. We did, however, succeed in preparing 8 according to the more recent and convenient
procedure of Geis et al. [8], outlined in eq. 1-5.
7

diglyme
3NaBH4 + I2
NaB H
NaB3H8 + 2NaI + 2H2
(1)
(2)
100 C
, 48hrs.
diglyme
5
+ 3NaBH4 + 8H2
3
8
Na2B12H12
170 C, 24hrs.
(3)
(4)
NaBH +
4H
HCl + 3H2O
2NEt3 HCl
B(OH)3 + NaCl +
2
4
2
+ 2NaCl
Na2B12H12
+
B12H12
)
( HNEt3
2
2
H2O
(
B12H12
HNEt3)
+ 2NaOH
Na2B12H12 + 2NEt3 +
2H2O
2
(5)
8
When we compared the chemical reactivity of the sodium salt 8 with the commercially
available (Strem Chemicals) potassium salt 7, we found no substantial difference in the yield
of the resulting organic closo(B12H12)2- salts. Commercial K2B12H12 (7) does not contain
complexed water and is relatively non-hygroscopic. By contrast, Na2B12H12 (8) crystallizes
from water as a solid leaf-shaped brilliant white crystal. According to elemental analysis via
inductively coupled plasma (ICP) and NMR, 8 is a pentahydrate, with a molecular formula of
Na2B12H12●5H2O.
The barium salt BaB12H12 (9), used above in Path B of Scheme 3, was synthesized in a
fashion analogous to the Geis procedure [8], by refluxing a dilute aqueous solution of
bis(triethylammonium)dodecahydroborate (10) and barium hydroxide for two hours, as
outlined in Equation 6. High dilution and heat are required for this metathesis because of the
relative insolubility of the barium hydroxide. In earlier reports [31,32] the barium salt (9) was
prepared from the ammonium salt (10) in two steps, by converting it to the acid
(H2O)2B12H12 and reacting with BaCO3.
H2O
2
B12H12 + Ba(OH)2
)
HNEt
3 2
BaB12H12
(6)
(
+ 2NEt3
+
2H2O
reflux, 2h
10
9
2. Bis(guanidinium)dodecahydroborates 12a-e
2.1 Preparation
The starting materials for various derivatives of the guanidinium ions were the
corresponding chlorides 11. Guanidinium-, aminoguanidinium- and diaminoguanidinium
chloride (11a, b and c, respectively) are commercially available. Triaminoguanidinium
8

chloride 11d was prepared according to the procedure of Coburn et al. [33,] while guanylurea
hydrochloride (or guanyluronium chloride) 11e was prepared according to the approach of
Klapotke and coworkers [34]. Since the corresponding bis(guanidinium) closo(B12H12) salts
(12b-e) are water insoluble, they can be prepared directly by a single metathesis reaction (as
outlined in Scheme 3, path 1). Thus chlorides 11b-e reacted with dipotassium or disodium
dodecaborohydrates (7 or 8, respectively), yielding the desired bis(guanidinium)
closo(B12H12) salts (12b-e) in 70-80% yield (Scheme 4).
2
HB
HB
R1 NH
Path 1
H2O
Cl
R1 NH
K2B12H12 (7)
or
Na2B12H12 (8)
HB
HB
BH
BH
2
HB
HB
BH
BH
C
C
HN
NH
R3
- 2 KCl
or
-2 NaCl
HN
NH
R3
BH
BH
R2
R2
2
11b-e
12b-e
,
,
e: R1=R2 =H R3=CONH2
b: R1=R2 =H R3=NH2; c: R1=H, R2=R3=NH2 d: R1=R2=R3=NH2
Scheme 4: Synthesis of bis(guanidinium) closo(B12H12) salts (12b-e)
The procedure of Path 1 failed for analog 12a, because of the good aqueous solubility of
this closo salt. For that reason, we used the double metathesis approach (Path 2 in Scheme
3). As shown in Scheme 5, the preparation of 12a involves two ion exchange reactions. In the
first step the halogen ion is replaced by sulfate ion, using Ag2SO4. Following filtration of the
AgCl, BaB12H12 was added, the resulting BaSO4 precipitate was filtered off, and the water
evaporated to give 12a. Organic closo salts 12a-e were characterized by NMR, MS and ICP
spectroscopy, as will be described later.
NH2
NH2
C
H2O
2
Cl
C
2AgCl
SO4
NH2
Ag2SO4
H2N
2 H2N
NH2
2
11a
H2O
NH2
C
2
NH2
C
2
BaB12H12
B12H12
BaSO4
SO4
H2N
NH2
H2N
NH2
2
2
12a
Scheme 5: Synthesis of bis(guanidinium) closo(B12H12) salt 12a
9

2.2 Characterization
Bis(guanidinium)) closo salts 12a-e (Schemes 4 and 5) were characterized by 1H, 13C and
11B NMR, MS and ICP spectroscopy. The literature records that the counter anion has little if
any affect on the 1H or 13C NMR chemical shifts of the cation within an organic salt [20,35].
Indeed, the spectra of the various cations in 12 are almost identical to the corresponding
guanidinium hydrochloride starting materials 11.
1
The H NMR chemical shift of the (B12H12)2- anion is also identical in its various salts
[10,11,12]. Thus, there is a broad high field (0.7-1 ppm) multiplet, due to coupling of the H-B
11
12
10
groups, which includes the B and B isotopes of boron (11B - 80.42% and B - 19.58%)
[17]. The integration ratio between the boron hydrogens to the guanidinium hydrogens
11
confirms the stoichiometric ratio between the anion and the cation. In the B NMR spectra,
there is a doublet at -15.5 ppm, with JB-H = 388 Hz. To exemplify this observation, Figure 1
13
11
shows the 1H, C and B NMR spectra of 12d. The proton ratio of internal amines:terminal
amines: borohydrides is 6:12:12.
10

1H- NMR
TMS
H2O
H2N
NH
NH
C
DMSO
H2N
NH2
NH
-2
B12H12
13C- NMR
DMSO
H2N
NH
H2N
NH2
C
NH
NH
TMS
11B- NMR
-2
B12H12
Figure 1: 1H, 13C and 11B NMR spectra of bis(triaminoguanidinium)dodecahydroborate 12d.
1
13
11
The ICP elemental analysis of boron percentage was consistent with H, C and B NMR
spectra. The ICP analysis also indicated that compound 12d is a monohydrate. All these
compounds have characteristic IR stretching appearing around 2500 cm-1 corresponding to
stretching vibrations of the B-H bounds [10,11,24].
We
attempted
to
determine
the
molecular
structure
of
bis(guanidinium)dodecahydroborates analogs 12a-e by X-ray crystallography; however, we
were only able to obtain amorphous crystals from water which are unsuitable for crystal
11

structure analysis. The literature has already indicated that most of alkylimidazolium
closo(B12H12) salts don't yield good [10] quality crystals. Indeed, only methyl and methyl-
ethyl imidazolium derivatives have been analyzed by X-ray crystallography [12].
Aminoguanidinium
closo(B12H12)
12b
has
been
crystalized
[25].
as
an
(aminoguanidine)copper(II):complex
closo(B12H12)
Although
bis(triaminoguanidinium)dodecahydroborate 12d has been previously prepared [26], no X-
ray crystal structure has been reported.
After some trial and error, we succeeded in obtaining good quality crystals when
recrystallizing the amorphous monohydrate 12d from DMF (dimethylfomamide).
Unfortunately, the crystals obtained were far from simple and incorporated solvent. Thus,
when pure DMF was used, the crystal analysis (Figure 2) showed stoichiometric unit cell
with five elements: a dodecahydroborate dianion complexed with two DMF molecules and
two triaminoguanidinium cations; water is absent from the crystal. Figure 3 is a fuller
picture of the cell structure and gives us a better understanding of the complex situation.
From the latter we see that the charge separated DMF molecules function to bridge between
the dodecahydroborate dianion and the two guanidinium cations. In particular, the positive
nitrogen end of the DMF molecules helps solvate the dianion, while the negative DMF
carbonyl oxygen solvates the guanidinium cations (Equation 7).
H3C
H3C
O
H
H3C
H3C
O
H
(7)
N C
N C
Figure 2: Representation (hydrogens not shown) of stoichiometric unit cell of
bis(triaminoguanidinium)dodecahydroborate (12d) recrystallized from pure DMF.
12

Figure 3: Larger representation of crystal structure obtained from the
bis(triaminoguanidinium)dodecahydroborate (12d) recrystallized from pure DMF (hydrogens
not shown). The bridging/solvating role of the DMF can be clearly seen.
13

Table 1: Crystallographic data and structure refinement for
bis(triaminoguanidinium)dodecahydroborate (12d) recrystallized from pure DMF
Empirical formula
C8 H44 B12 N14 O2
498.29
Formula weight
Temperature
294(1) K
Wavelength
0.71073 Å
Monoclinic
C2/c
Crystal system
Space group
Unit cell dimensions
a = 9.046(1) Å
B = 17.402(2) Å
C = 17.822(2) Å
2755.6(6) Å3
4
α= 90°.
β= 100.807(2)°.
γ = 90°.
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
1.201 Mg/m3
0.078 mm-1
1064
Crystal size
0.36 x 0.33 x 0.33 mm3
2.34 to 28.01°.
Theta range for data collection
Index ranges
-11<=h<=11, -22<=k<=22, -23<=l<=23
15032
Reflections collected
Independent reflections
Completeness to theta = 28.01°
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I>2sigma(I)]
R indices (all data)
Largest diff. peak and hole
3265 [R(int) = 0.0276]
98.2 %
Semi-empirical from equivalents
0.9747 and 0.9724
Full-matrix least-squares on F2
3265 / 0 / 225
1.067
R1 = 0.0446, wR2 = 0.1302
R1 = 0.0521, wR2 = 0.1369
0.185 and -0.285 e.Å-3
When, the crystallization solvent was DMF containing a very small amount of NMP
(N-methyl-2-pyrrolidone) as antisolvent, the stoichiometric unit cell (Figure 4) contained
seven elements: the dodecahydroborate dianion, two triaminoguanidinium cations, and two
14

NMP molecules each complexed with a water molecule. Figure 5 is a fuller picture of the cell
structure, from which we see that the NMP molecules bridge between the dodecahydroborate
dianion and the two guanidinium cations. In particular, the positive end of the included NMP
molecules helps stabilize and solvate the dianion, while the negative NMP carbonyl oxygen
does the same for the guanidinium cations (Eq. 8). What is fascinating is that from Figure 5
we see that each water molecules lies in between the NMP carbonyls and the guanidinium
moiety. Presumably, the water proton hydrogen bonds with the pyrrolidone carbonyl oxygen,
while the relatively negative oxygen end of the water interacts with the guanidinium moiety.
O
O
H3C
H3C
N
N
(8)
Figure 4: Representation (hydrogens not shown) of stoichiometric unit cell of
bis(triaminoguanidinium)dodecahydroborate (12d) recrystallized from DMF/NMP.
15

Figure 5: Larger representation of crystal structure obtained from the
bis(triaminoguanidinium)dodecahydroborate (12d) recrystallized from DMF/NMP
(hydrogens not shown) . The bridging/solvating role of the NMP can be clearly seen.
16

Table 2: Crystallographic data and structure refinement for
bis(triaminoguanidinium)dodecahydroborate (12d) recrystallized from DMF/NMP
Empirical formula
C12 H52 B12 N14 O4
586.40
Formula weight
Temperature
293(1) K
Wavelength
0.71073 Å
Crystal system
Space group
Monoclinic
C2/c
Unit cell dimensions
a = 13.970(3) Å
b = 12.978(2) Å
c = 17.563(3) Å
α= 90°.
β= 97.633(3)°.
γ = 90°.
3
Volume
3156.0(10) Å
Z
4
3
Density (calculated)
Absorption coefficient
F(000)
1.234 Mg/m
-1
0.083 mm
1256
3
Crystal size
0.42 x 0.21 x 0.18 mm
2.15 to 27.00°.
Theta range for data collection
Index ranges
-17<=h<=17, -16<=k<=16, -22<=l<=22
16612
Reflections collected
Independent reflections
Completeness to theta = 27.00°
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
3423 [R(int) = 0.0328]
99.2 %
Semi-empirical from equivalents
0.9852 and 0.9660
2
Full-matrix least-squares on F
3423 / 0 / 294
2
Goodness-of-fit on F
1.076
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0471, wR2 = 0.1254
R1 = 0.0564, wR2 = 0.1312
-3
0.290 and -0.252 e.Å
Largest diff. peak and hole
This incorporation of solvent into the crystal structure is characteristic of
dodecahydroborate salts. For example, in the case of metal dodecahydroborate [31] crystalized
from water, the salts form polyhydrates - often including as many as 15 water molecules.
Similarly, tetramethyl ammonium closo(B12H12) derivatives crystalize as CH3CN
complexes.[30]
17

2.3 Thermal studies
Thermogravimetric analysis (TGA) of bis(guanidinium) closo(B12H12) salts 12a-e were
carried out under N2. The results are shown in Figure 6.
Figure 6: TGA curves of bis(guanidinium) closo(B12H12) salts 12a-e
Table 3: Energetic and chemical properties of guanidinium closo(B12H12) salts 12a-e
Calculated
% Wt loss of one
guanidinium unit
Measured
% weight
loss
MW
Anhyd.
% Wt
B12H12
Td(°C)
onset
Compd.
2-
243
340
323
323
340
262
343
233
253
242
54
32
32
32
23
23
18
25
30
38
40
12a
12b
12c
12d
12e
44
44
43
44
Figure 6 indicates that increasing amination of the guanidinium cation decreases the
stability of the salt. Thus, bis(di- and triaminoguanidinium)dodecahydroborate salts 12c and
18

12d decompose at ca. 220 °C, while the corresponding unsubstituted and monoamino analogs
12a and 12b and guanylurea analog 12e are somewhat more stable up to about 350 °C,
presumably due to the increased number and strength of the hydrogen bonds (see Table 3).
This stability decrease with increasing amination seems to be well correlated to the weight
percentage of the anion in the dodecahydroborate salt. Thus, in the unsubstituted guanidine
analog 12a, the closo(B12H12)2- anion comprises 54% of the molecular weight, while in the
triaminoguanidinium analog 12d and in the guanylurea analog 12e the dodecahydroborate
anion weight percentage drops to 40%. Presumably, the organic part of the molecule
undergoes decomposition first leaving the closo(B12H12) cluster essentially untouched. This
correlation is preecedented and first observed in the TGA traces of
dialkylimidazoliumdodecahydroborate salts.[10] The maximum weight loss of 40% for the
compounds matches the percentage weight loss of only one guanidinium group, which
consists of approximately 30% of the molecular weight in the bis(guanidinium) closo(B12H12)
salts 12c-d and in the guanylurea analog 12e (Table 3). In the unsubstituted and monoamino
guanidine dodecaborate salts 12a and 12b, respectively, the weight loss is only ca. 25% due
to the corresponding lower molecular weight of the cation.
After the initial weight loss, the residual weight of the dodecahydroborate salts 12a-e
remains constant weight (80-60%, Table 3) - even at temperatures as high as 1000 °C. This is
presumably because of the thermal stability of the closo(B12H12) anion.
The 223 °C melting point of 12d was the lowest of the dodecahydroborate salts, while
those of the other derivatives are substantially higher (>300 °C). Interestingly, this low value
may correspond to the fact that the onset of decomposition for this salt occurs at
approximately the same temperature as its melt (Figure 6 and Table 3), a common
phenomenon in high energy density materials..
We should note in closing this section that the aminoguanidine salts 12a-d have been
previously studied by Rao and Muralidharan in 2016 [26], and were prepared by reacting
Ag2B12H12 with the corresponding organic cation halides. These salts were reported to show a
complicated TGA decomposition pattern with substantially lower melting points, compound
stability and residual weight percentages (only around 30%) than recorded in the present
paper. By contrast, our aminoguanidinium closo(B12H12) salts were obtained via metathesis
reaction with Na2B12H12 or K2B12H12 (7 or 8, respectively, for water insoluble salts) or
through double metathesis reactions using BaB12H12 (9, for water soluble salts). They showed
residual weight percentages of 60-85%, and melting points as much as 200 °C higher than
19

those previously reported. Even the proton NMR data reported in the 2016 paper are replete
with errors. For example, as shown in Fig. 1 above, the proton values for the internal
amines:terminal amines: borohydrides in 12d is: 8.59 (s, 6H), 4.48 (s, 12H), 1.60-0.30 (br m,
12H) ppm. By contrast, Rao and Muralidharan report the following values: 8.59 (s, 4H), 6.53
(s, 2H), 4.49 (s, 8H), 1.25-0.39 (br m, 12H). Because of resonance and rapid exchange, all six
of the internal amine hydrogens must be identical, while the integration for the terminal
hydrogens should be 12, not 8. All this clearly indicates that our aminoguanidine salts 12a-d
are by far more pure.
3. Bis(aminotetrazolium)dodecahydroborates 15 and 16
3.1 Preparation
Dodecahydroborate salts containing imidazolium or triazolium cations are known
[10,11,12] as is the aminotetrazolium analog 15c [11,13,27]. We synthesized derivatives of 5-
aminotetrazolium cations, 15a-d and 16a-b (see
Scheme 6), starting from 1,5-
diaminotetrazole (13) 36,37 or the commercially available 5-aminotetrazolium monohydrate
(17) [38,39,40], as outlined in Scheme 7.
closo-(B12H12)2
aminotetrazolium
cation
closo-(B12H12)2
aminotetrazolium
cation
2
2
HB
HB
HB
R1
HB
BH
R1
HB
HB
BH
BH
HB
HB
N
N
N
N
HB
HB
BH
BH
HB
HB
BH
BH
NH2
NH2
N
N
N
N
R3
R2
BH
BH
BH
BH
BH
2
2
(a-d)
15
R =R =CH
3
(a,b)
16
a:
1
2
a:
R1=R2=CH3
b:R1=H, R2 = CH3
R =H, R =CH
b:
c:
d:
R1=R =2H
3
2
R1=CH , R = NH
1
3
2
2
Scheme 6: Structure of various closo(B12H12) tetrazolium salts 15a-d and 16a-b
20

H
N
NH2
Path A:
CH3I
CH3CN
reflux
Path B:
HCl conc.
N
.
H2O
N
N
17
1. NaOH, CH3I
2. Acetone/H2O
reflux
Path C:
K2B12H12, H2O
or
CH3
N
Na2B12H12, H2O
NH2
N
NH2
N
N
NH2
N
+
NH2
Path D:
1. Ag2SO4, H2O
2. BaB12H12, H2O
N
N
N
N
N
Path B
18
CH3
Path
N
N
19
13
Path
B
Path
A
Path
B
A
Path A
CH3
H
CH3
H
N
NH2
N
H
Cl
5
I
1
Cl
1
N
1
N
N
N
N
5 NH2
5
NH2
I
5
NH2
NH2
NH2
NH2
N
N
N
N
N
5
1
N
5
1
I
CH3
N
N
Cl
N N
N
N
N
2
N
3
N
N
N
4
H
CH3
CH3
CH3
CH3
20a
14
20c
Path D
21a
20b
21b
Path D
Path C
Path C
Path D
Path D
2
2
2
2
2
2
B12H12
B12H12
B12H12
B12H12
B12H12
B12H12
CH3
H
N
CH3
H
N
H
N
H2N
N
NH2
N
NH2
NH2
NH2
N
NH2
NH2
N
N
N
N
N
N
N
N
N
N
N N
N
N
N
N
N N
CH3
CH3
H
CH3
CH3
CH3
2
2
2
2
2
2
15a
16a
15d
15b
16b
15c
Scheme 7: Synthesis of various tetrazolium salts 15a-c and 16a-b.
21

Thus, methylation of 17 with methyl iodide under basic conditions yields monomethyl
aminotetrazoles 18 and 19 [39]. The isomeric compounds were separated by crystallization of the
product mixture from acetone-water solution. 1-Methyl-5-aminotetrazole (18) is the main product
and crystallized from the solution in a 50% yield. After evaporating the mother liquid, 2-methyl-5-
aminotetrazole (19) was recrystallized from water in a 20% yield. Methylation of 18 and 19 with
methyl iodide in refluxing acetonitrile (Path A) yields 1,4- and 1,3-dimethyl 5-aminotetrazolium
iodide (20a or 21a), respectively. Under the same “Path A” conditions, 1,5-diaminotetrazole 13 is
converted to methyldiaminotetrazolium iodide 14
Aminotetrazolium chloride (20c) [41] or hydrochloride salts 20b and 21b were prepared by
refluxing aminotetrazoles 18, 19 and 17 respectively with conc. HCl (in methanol) (Path B). [We
note that in early studies, we also prepared the corresponding sulphates [42] with conc. H2SO4 (in
water), but found that their isolation pure was more difficult, and the overall yields leading to
dodecahydroborates 15b, 15c and 16b were not improved.]
The last step was to prepare the dodecahydroborate salts via metathesis. The unmethylated
and monomethyl aminotetrazolium dodecahydroborates are water soluble due to the polar character
of the cation. Hence, compounds 15b-d and 16b were synthesized via Path D from the
aminotetrazolium chloride via in situ dual ion exchange of chloride with sulfate and the latter with
dodecahydroborate (see Scheme 7). The dimethylated analogs 15a and 16a salts are only slightly
soluble in water and could, therefore, be prepared via Path C (a one step ion exchange) which
concludes with precipitation from water.
3.2 Characterization
1
13
11
Characterization of these molecules was conveniently accomplished by H, C and B NMR,
ICP and MS (see Fig. 7k). As before, the counter anion has little if any affect on the 1H or 13C NMR
chemical shifts of the cation within an organic salt [20,35]. Thus the chemical shifts of the 5-
aminotetrazolium dodecahydroborates are nearly identical to those of the 5-aminotetrazolium
halides.
1
11
The H and B NMR chemical shift of the (B12H12)2- anion is also identical in its various salts
[10,11,12]. The ratio between the boron's hydrogens to the aminotetrazolate's methyl hydrogens
1
13
11
confirms the stoichiometric ratio (1:2) between the anion and the cation. H, C and B NMR
spectra of 15a are presented in Figure 7.
1
The ICP elemental analysis of boron percentage was consistent with H, 13C and 11B NMR
spectra. It also indicated that compound 15a is a monohydrate. All these compounds have
characteristic IR-stretches appearing at ca. 2500 cm-1 corresponding to stretching vibrations of the
22

B-H bonds [10,11,24].
1H- NMR
H3C
H3C
N
N
N
N
H2N
-2
B12H12
13C- NMR
H3C
H3C
N
N
N
H2N
N
11B- NMR
-2
B12H12
Figure 7: 1H, 13C and 11B NMR spectra of
bis(1,4-dimethyl-5-aminotetrazolium)dodecahydroborate monohydrate15a
3.3 Molecular structure
The crystal structure of the dodecahydroborate 15a monohydrate was determined by low
temperature X-ray diffraction on suitable single crystals, which were selected from a distilled water
recrystallization mixture, as previously described [43]. The unit cell is shown in Figure 7 and the
23

corresponding data is given Table 4. As seen from Figure 8, the unit cell of 15a includes a water
molecule which is hydrogen bonded to an amino group of one of the aminotetrazolium cations.
Figure 8: Representation of stoichiometric unit cell of
bis(1,4-dimethyl-5-aminotetrazolium)dodecahydroborate monohydrate (15a)
24

Table 4: Crystallographic data and structure refinement for
bis(1,4-dimethyl-5-aminotetrazolium)dodecahydroborate monohydrate (15a)
Empirical formula
C6H30B12N10O
388.12
Formula weight
Temperature
295(2) K
Wavelength
0.71073 Å
Triclinic
Crystal system
Space group
P-1
Unit cell dimensions
a = 8.410(2) Å
B = 10.357(2) Å
C = 13.226(3) Å
α= 80.185(3)°.
β= 80.907(3)°.
γ = 71.338(3)°.
3
Volume
1068.8(4) Å
2
Z
3
Density (calculated)
Absorption coefficient
F(000)
1.206 Mg/m
-1
0.074 mm
408
3
Crystal size
0.30 x 0.24 x 0.18 mm
2.45 to 28.11°.
Theta range for data collection
Index ranges
-11<=h<=11, -13<=k<=13, -17<=l<=17
Reflections collected
Independent reflections
Completeness to theta = 28.11°
Absorption correction
Refinement method
Data / restraints / parameters
12410
4981 [R(int) = 0.0259]
95.6 %
None
2
Full-matrix least-squares on F
4981 / 0 / 330
2
Goodness-of-fit on F
1.026
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0536, wR2 = 0.1492
R1 = 0.0666, wR2 = 0.1582
-3
0.217 and -0.230 e.Å
Largest diff. peak and hole
Crystal structures of closo(B12H12) salts with metal cations have been widely investigated
[30,31,32,44,45]. Some triazolium and imidazolium dodecahydroborate salts crystal structures are
also known in literature [12,27] , with the closo(B12H12) dianion essentially unchanged in all its
compounds. Similarly, the structure of the dimethylaminotetrazolium cation remains the same in
various aminotetrazolate salts [38].
25

3.4 Thermal Studies
The thermogravimetric analysis (TGA) under N2 of aminotetrazolium dodecahydroborate salts
15a-d and 16a-b appear in Figure 9. The data indicates that 16b is relatively speaking the most
unstable of the dodecahydroborate salts, with an onset of decomposition at 150 °C and a weight loss
of 60% by 228 °C. By contrast, the onset of decomposition of the other closo salts is ca. 300 °C.
Figure 9: TGA curves of aminotetrazolium dodecahydroborates 15a-d and 16a-b.
Table 5: Thermal properties of aminotetrazolium closo(B12H12) 15a-d and 16a-b
Calculated
% wt loss of one
tetrazolium unit
MW
Anhyd.
% Wt
B12H12
Measured
% wt loss
Td(°C)
onset
Compd.
2-
320
250, 350a
100, 280a
200
370
342
314
372
273
243
22
31
29
27
24
24
34
33
30
40
90
25
53
15a
15b
15c
15d
16a
16b
43
45
22
280
22
150
43
a:Two decomposition stages.
26

Figure 9 indicates that dimethyl substituted aminotetrazolium dodecahydroborate salts 15a
and 16a undergo gradual decomposition at ca. 300 °C, losing 25-30% of weight (Table 3). The TGA
decomposition curve of the monomethylaminotetrazolium dodecahydroborate 15b is similar. These
three compounds each have at least one methyl group substituted α to the amine moiety which seems
to render it stability. The unmethylated aminotetrazole salt 15c is still less stable, losing 40% of its
weight gradually between 100-300 °C. This is followed by the β-substituted monomethyl 16b, which
begins decomposition at 150 °C, losing 50% of its weight by 200 °C. The diamino analog 15d is the
most unstable, decomposing precipitously at 200 °C losing 90% of its weight – and may prove
promising for propellant use.
This stability decrease seems to be well correlated with the weight percentage of the
dodecahydroborate anion in each of the salts. Presumably, the aminotetrazolium organic portion of
the molecule breaks down first releasing the energy needed to decompose the closo(B12H12) cluster.
A similar phenomenon has been previously observed for the TGA measurements of
dialkylimidazolium dodecahydroborate salts.[10] In our aminotetrazolium closo(B12H12) salts 15a-d
and 16a-b, the dodecahydroborate anion weight percentage is ca. 40% in each case (see Table 5).
Thus, the difference in decomposition temperatures can be attributed to the relative thermal stability
of the aminotetrazolium cations.
Interestingly, by ca 300 °C, most of the decomposition in the tetrazolium dodecaborohydrate
salts 15a-c and 16a-b is complete, and the residue weight remains essentially constant (50-75%,
Table 5) even up to very high temperatures (1000 °C). As noted above, however, bis(1,5-diamino-4-
methyltetrazolium)dodecahydroborate salt 15d loses most of its weight (90%) at its decomposition
temperature of 200 °C. The profound instability of this compound would seem to stem from the
cation, since a similar rapid TGA decomposition at 192 °C is reported 46 for the
hydrazinetetrazolate salt of the same bis(1,5-diamino-4-methyl-tetrazolium) cation.
4. Combustion studies and calculated heats of formation of compounds 12d and 15a
To explore the possibility of using closo(B12H12) salts as high energy fuels, we studied the
energetic properties of two representative compounds: triaminoguanidinium closo(B12H12) 12d and
dimethylaminotetrazolium dodecahydroborate 15a. Both were monohydates, though the former was
amorphous, while the latter was crystalline. Since the oxidation of boron is an exothermic reaction,
it is expected to increase the overall energy release from a propellant or explosive mixed with these
compounds. The standard enthalpy of formation, enthalpy of combustion and heat of explosion are
the essential characteristics of a high energy material. In our bomb calorimetric experiments (carried
27

out at Hebrew University) in an oxygen atmosphere (30 bar, 20-40 mg sample), the enthalpies of
combustion at constant volume (ΔHc, kJ∙mol−1) were determined.
In a bomb calorimeter, chemical processes are carried out at a constant volume, i.e. ΔV=0.
The first law of thermodynamics states that for isochoric (constant volume) process ΔU= QV, where
U is the internal energy (a state function) and QV is the measured heat. The latter value, QV, is
obtained in the course of calorimeter experiments and is reported as ΔUC (kJ∙mol−1 or kJ∙g−1) i.e. the
change in the internal energy of the system due to the combustion process. For computational
convenience, it is conventional to report the standard combustion enthalpy, i.e. ΔH°C, which is
referenced to the standard state of 25 °C (298.15 K) and 1 bar (100 kPa). Recalling that H = U +
PV, we can write directly that ΔH°C = ΔU°C + (PΔV)°C. Furthermore, resorting to the Mendeleev-
Clapeiron gas law, PΔV = Δn∙RT, we obtain ΔH°C = ΔU°C+ΔnC∙RT°. Taking into the account that
the calorimetric experiments are performed at temperatures relatively close to 25 °C (in actuality, T
in these experiments is ca. 22 °C), we get: ΔH°C = QV +ΔnC∙RT°.
To exemplify our calculations, let us turn now to the combustion of bis(1,4-dimethyl-5-
aminotetrazolium)-closo-dodecahydroborate monohydrate (15a). It is important to note that the
literature10,26 generally assumes that the combustion products in the case of organoboron compounds
are dehydrated completely (briefly designated “complete”) yielding boric anhydride (O=B-O-B=O).
This assumption would lead to equation 7a:
.
(C6N10H16)(B12H12) H2O (Solid) + 22O2 (Gas) → 6CO2 (Gas) + 5N2 (Gas) + 6B2O3 (Solid) +15H2O (Liq) (7a)
From the calorimetry experiment we obtain:
QV = ΔU°C = − 35.97 kJ/g or −13960.6 kJ/mole
ΔnC = 6 + 5 – 22 = − 11
ΔnC∙RT° = (−11)×(8.314)×(298.15) = − 27.27 kJ∙mol−1
ΔH°C =−13960.6 − 27.27 = −13988 kJ∙mol−1
The standard enthalpy of formation is defined as (equation 8):
ΔH°C = ∑ni × ΔH°f (producti) - ni × ΔH°f (reactantj) (8)
which leads to the following result for the standard formation enthalpy of 15a
ΔH°f (15a) = -ΔH°C (15a) +6ΔH°f (CO2) +6ΔH°f (B2O3) + 15ΔH°f (H2O) (9a)
For the ΔH°f values, we use those given on the NIST website,47
ΔHo (CO2) = -342.5 KJ/mol; ΔHo (B2O3) = -1273.5 KJ/mol; ΔHo (H2O) = -285.8 kJ/mol
f
f
f
Thus: ΔHf°(15a)complete = − 301 kJ∙mol−1 (or – 71.9 Kcal mol-1)
28

Surprisingly, however, in the case of closo(B12H12) salts 15a and 12d, a non-explosive
oxidation of the substances is observed leading to the formation of nano-crystalline orthoboric acid
[B(OH)3 or H3BO3] as the main crystalline product presumably accompanied by some oxoborinic
acid (HBO2). We note that in both B(OH)3 and O=B-O-B=O, boron has the some oxidation number
(+3); however, the former is incompletely dehydrated (briefly designated “incomplete.” Again,
carbon dioxide and molecular nitrogen are the main gaseous products – but no water is produced.
The formation of crystalline orthoboric acid was confirmed by powder XRD and Raman
spectroscopy on the residual solids collected from the bomb after each experiment. This surprising
result requires a revision of our combustion reaction to equation 7b:
(C6N10H16)(B12H12) H2O (Solid) + 22O2 (Gas) → 6CO2 (Gas) + 5N2 (Gas) + 9H3BO3 (Solid) +3HBO2 (Solid) (7b)
From the calorimetry experiment we obtain:
QV = ΔU°C = − 35.97 kJ/g or −13960.6 kJ/mole
ΔnC = 6 + 5 – 22 = − 11
ΔnC∙RT° = (−11)×(8.314)×(298.15) = − 27.27 kJ∙mol−1
ΔH°C =−13960.6 − 27.27 = −13988 kJ∙mol−1
The standard enthalpy of formation is defined as:
ΔH°C = ∑ni × ΔH°f (producti) - ni × ΔH°f (reactantj) (8)
which leads to the following result for the standard formation enthalpy of 15a
ΔH°f (15a) = -ΔH°C (15a) +6ΔH°f (CO2) +9ΔH°f (H3BO3) + 3ΔH°f (HBO2) (9b)
For the ΔH°f values, we again use those given on the NIST website [47],
ΔHo (CO2) = -342.5 KJ/mol; ΔHo (H2BO2) = -4344.8 KJ/mol; ΔHo (HBO2) = -802.8
f
f
f
Thus: ΔHf°(15a)incomplete = −635 kJ∙mol−1 (or -151.8 Kcal mol-1).
We note that at 1 bar of oxygen, which is only slightly above the stoichiometric ratio of O2 to the
organic material, the oxidation process does not proceed completely to orthoboric acid [B(OH)3],
and a black solid (unidentified at this point) remains. This incomplete oxidation results in reduced
values of the heat of combustion. Nevertheless, the heat of combustion values of the closo(B12H12)
salts 12d and 15a at 1 bar of O2 are close to those observed at 30 bar of O2 - leading to the
conclusion that these substances are of interest for propellant applications. A non-explosive, self-
sustained exothermic chemical reaction is evident. Overall, the dodecahydroborate salts 12d and 15a
are easily ignitable and release good amounts of energy per gram (see below).
29