Pairing heterocyclic cations with closo-lcosahedral borane and carborane anions. i. benchtop aqueous synthesis of binary triazolium and imidazolium salts with limited water solubility

Article abstract of DOI:10.1021/ol900550v

Ten new salts that pair triazolium and imidazolium cations with c/oso-icosahedral anions [B₁₂H₁₂]² and [B ₁₁H₁₂]^-were synthesized in water solvent using an open-air, benchtop method. These unreported [Heterocyclium] ₂[B₁₂H₁₂] and [Heterocyclium][CB ₁₂H₁₂] salts extend reports of [Imidazolium][CB ₁₂H₁₂] and [Pyridinium][CB₁₂H₁₂] salts that were synthesized in anhydrous organic solvents under an inert atmosphere with glovebox or Schlenk techniques. Spectroscopic data, melting points, and densities are reported for each salt. Single-crystal X-ray structures are provided for the five new [B₁₂H₁₂] ^2- salts.

Full text of DOI:10.1021/ol900550v

ORGANIC
LETTERS
2009
Vol. 11, No. 12
2623-2626
Pairing Heterocyclic Cations with
closo-Icosahedral Borane and Carborane
Anions. I. Benchtop Aqueous Synthesis
of Binary Triazolium and Imidazolium
Salts with Limited Water Solubility
Scott A. Shackelford,* John L. Belletire,† Jerry A. Boatz, Stefan Schneider,
Amanda K. Wheaton,† Brett A. Wight,† Leslie M. Hudgens,† Herman L. Ammon,‡
and Steven H. Strauss§
Air Force Research Laboratory, Propellants Branch (AFRL/RZSP), 10 East Saturn
BouleVard, Edwards AFB, California 93524-7680, Department of Chemistry and
Biochemistry, UniVersity of Maryland, College Park, Maryland 20742, and Department
of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523
scott.shackelford@edwards.af.mil
Received March 16, 2009
ABSTRACT
Ten new salts that pair triazolium and imidazolium cations with closo-icosahedral anions [B12H12]2- and [CB11H12]- were synthesized in water
solvent using an open-air, benchtop method. These unreported [Heterocyclium]2[B12H12] and [Heterocyclium][CB11H12] salts extend reports of
[Imidazolium][CB11H12] and [Pyridinium][CB11H12] salts that were synthesized in anhydrous organic solvents under an inert atmosphere with
glovebox or Schlenk techniques. Spectroscopic data, melting points, and densities are reported for each salt. Single-crystal X-ray structures
are provided for the five new [B12H12]2- salts.
Tailoring the chemical and physical properties of molecular
materials by rational design to increase density, reduce
toxicity, or to improve overall stability characteristics
continues to be an important research endeavor. Achieving
this by the structural modification of neutral organic com-
pounds offers a limited scope of opportunities. Organic-like
solid and liquid salts significantly enhance this flexibity for
tailoring properties by pairing different classical heterocyclic
cations with a plethora of different counteranions. Over the
past 25 years, this is re-emphasized with salts based on
neutral heterocyclic compounds. Rekindled interest in this
approach appears to initiate with the unique properties
exhibited by the hygroscopic binary 1-Ethyl-3-methyl-1,3-
imidazolium (EMIM) chloroaluminate salt1 followed by a
subsequent report of nonhygroscopic, air stable, binary
† ERC, Inc. contractor at AFRL/RZSP, Edwards AFB.
‡ University of Maryland, College Park, MD.
§ Colorado State University, Ft. Collins, CO.
10.1021/ol900550v CCC: $40.75
Published on Web 05/14/2009
 2009 American Chemical Society

EMIM salts containing other anions.2 Later, triazolium
cations are used to form salts with common anions.3
In contrast, this first paper describes heterocyclium borane/
carborane salts that are relatively water insoluble at rt where
a high water solubility of the potassium halide byproduct is
advantageous. Our aqueous-based metatheses require reaction
times of only several minutes and are conducted in a
benchtop fumehood with standard glassware open reaction
vessels and deionized (DI) water solvent. With the relatively
low water solubilities exhibited by the heterocyclium salts
of [B12H12]2- 1-5 and [CB11H12]- 6-11, anhydrous solvents
can be avoided and air-stable products obtained. Because
all the heterocylium halides used in this work, as well as
the K2[B12H12] and K[CB11H12] reactant salts,11 are water-
soluble, reactions are rapid. The two reagents are mixed in
stoichiometric amounts, and precipitation or slow recrystal-
lization yields the desired salt product 1-1112 which are
readily separated from the potassium halide-containing
aqueous supernatant by filtration and washing with cold DI
water. Crude product yields are in the 71% to 96% range.
Recrystallization from hot DI water, in some cases containing
a small proportion of methanol, removes any small amounts
of potassium and halide ions to give overall yields ranging
from 44% to 84%. A trace of the residual potassium cation
should not afford the catalysis problems sometimes caused
by trace silver cations following a silver salt metathesis.
We now report the synthesis and characterizaton of salts
containing heterocyclic cations that are paired with two
different three-dimensional polyhedral borane/carborane an-
ions,4 [B12H12]2- and [CB11H12]-, as well as a new aqueous
preparation method that does not require air-free techniques.
These unique salts are comprised of aromatic planar π-de-
localized cations and aromatic polyhedral σ-delocalized
anions. Triazolium and imidazolium cations paired with the
closo-icosahedral, weakly nucleophilic [B12H12]2- dianion
represent the first reported examples of these borane salts
1-5.
The [B12H12]2- dianion, first synthesized in 1960 as the
triethylammonium salt,5 is known to form salts of low
toxicity and high thermal stability6 and is purported to be
the most stable covalently bonded chemical entity in all of
chemistry.7a It is a “super-aromatic” polyhedral-shaped
“counterpart of the planar benzene molecule with 26 delo-
calized valence electrons in its σ-bonded framework”7 and
possesses an overall negative two charge. This unique
[B12H12]2- dianion is paired with either two planar aromatic
triazolium or two imidazolium cations, which each possess
six delocalized valence electrons in its π-bonded cyclic
structure and an overall positive one charge. These new
[Heterocyclium]2[B12H12] salts 1-5 previously have not been
reported. The icosahedral monoanion, closo-[CB11H12]-, first
reported in 1967,8 also forms thermally stable salts. Analo-
gous [Triazolium][CB11H12] 6-9 and one new [Imidazo-
lium][CB11H12] 10 salts also are described which extend
properties characterization beyond several [Imidazo-
lium][CB11H12]9 and one [Pyridinium] [CB11H12]10 salts that
previously were reported.
Figure 1 displays the five heterocyclinium borane salts
syntheszied by our benchtop, open-air, aqueous procedure.
The former [Imidazolium][CB11H12] salts were prepared
by stirring the Cs+ or Ag+ salt of [CB11H12]- with the
appropriate imidazolium halide in an anhydrous organic
solVent, or mixture of solVents, inside an inert-atmosphere
gloVebox for 10 h. The solid phase CsCl or AgCl byproduct
was separated from the supernatant by filtration through
Celite. Removal of solvent afforded white crystals.9 The
[N-Pentylpyridinium][CB11H12] salt was prepared similarly
oVer 20 h using Schlenk air-free techniques and column
chromatography purification.10
Figure 1. [Heterocyclium]2[B12H12] borane salts prepared.
The synthesis and spectroscopic data for [4-Amino-1-methyl-
1,2,4-triazolium]2[B12H12] 1 are typical.13 More detailed
syntheses for 1 and for the four other [B12H12]2- salts,
1-Amino-3-methyl-1,2,3-triazolium borane 2, 1-Amino-3-H-
1,2,3-triazolium borane 3, 1-Methylimidazolium borane 4,
and 1-Ethyl-3-methylimidiazolium borane 5, appear in the
Supporting Information.
(1) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg.
Chem. 1982, 21, 1263.
(2) Wilkes, J. S.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1992,
965.
(3) Drake, G.; Hawkins, T.; Brand, A; Hall, L; Mckay, M. Propellants,
Explos., Pyrotech. 2003, 28, 174.
(4) Shackelford, S. A. U.S. Patent 7 521 564, 2009.
(5) Pitochelli, A. R.; Hawthorne, M. F. J. Am. Chem. Soc. 1960, 82,
3228.
(6) Kaszynski, P. Collect. Czech. Chem. Commun. 1999, 64, 895, and
references therein.
Figure 2 shows the six new [Heterocylium][CB11H12] salts
synthesized, 4-Amino-1-H-1,2,4-triazolium carborane 6,
4-Amino-1-methyl-1,2,4-triazolium carborane 7, 1-Amino-
3-methyl-1,2,3-triazolium carborane 8, 1-Amino-3-H-1,2,3-
triazolium carborane 9, 1-Methylimidazolium carborane 10,
and 1-Ethyl-3-methylimidazolium carborane 11. The syn-
thesis of [1-Amino-3-H-1,2,3-triazolium][CB11H12] 9 is typi-
(7) (a) Grimes, R. N. Angew. Chem., Int. Ed. 2003, 42, 1198, and
references therein. (b) Muetterties, E. L.; Knoth, W. H. Polyhedral Boranes;
M. Dekker: New York, 1969.
(8) Knoth, W. H. J. Am. Chem. Soc. 1967, 89, 1274.
(9) Larsen, A. S.; Holbrey, J. D.; Tham, F. S.; Reed, C. A. J. Am. Chem.
Soc. 2000, 122, 7264.
(10) Zhu, Y.; Ching, C.; Carpenter, K.; Xu, R.; Selvaratnam, S.;
Hosmane, N. S.; Maguire, J. A. Appl. Organometal. Chem 2003, 17, 346.
2624
Org. Lett., Vol. 11, No. 12, 2009

The density, melting point, and reaction yields for salts 1
to 11 are listed in Tables 1-3. Experimental densities are
Table 1. 4-Amino-1,2,4-Triazolium Borane and Carborane Salts
1: X ) CH3
6: X ) H
Borane
-
181-182
1.22/1.24
(1.20)
260-262
1.19/-
(1.14)
161-162
1.12/-
91.0
(79.7)
Figure 2. [Heterocyclium][CB11H12] carborane salts prepared.
-
-
Carborane
Carborane
71.0
(43.5)
cal.14 More detailed [Heterocyclium][CB11H12] salt syntheses
for salts 6-11 appear in the Supporting Information.
Initial syntheses were conducted on a small 150-200 mg
scale based on K2[B12H12] and K[CB11H12]. Once optimum
conditions were determined for each salt, the metathesis
reactions were conducted on a 1-4 g scale. The [B12H12]2-
salt of 3 was directly scaled from 2 to 5.5 g for K2[B11H12]
with a comparable yield and final purity.
7: X ) CH3
92.0
(65.5)
(1.12)
Table 2. 1-Amino-1,2,3-Triazolium Borane and Carborane Salts
(11) The K2[B12H12]2- and K[HCB11H11]- reactants were purchased from
Katchem Ltd., E. Krasnohorske 6, 110 00 Prague 1, Czech Republic and
were used as received.
(12) Depending upon the salt product solubility, some heterocyclium
salts immediately precipitate; in other cases, the desired heterocyclium salt
crystallizes overnight at +3.5 °C.
2: X ) CH3
3: X ) H
Borane
Borane
-
-
199-218a
1.19/1.1316
(1.18)
78.2
(66.7)
(13) [4-Amino-1-methyl-1,2,4-triazolium]2[closo-B12H12],1:Solid[4-Amino-
1-methyl-1,2,4-triazolium] iodide (4.52 g; 20.00 mmol), CH3I methylated
plus recrystallization from methanol) and commercial K2[closo-B12H12] (2.20
g; 10.00 mmol) were placed into a 50 mL beaker. To this was added 18.0
mL DI water. The contents were stirred and heated at gentle reflux for 4
min. The resulting clear tan solution was cooled overnight in a refrigerator
and off-white crystals deposited. Filtration and rinsing the filter-cake with
2 × 1.0 mL portions of pre-chilled DI water followed. The crystals were
dried under high vacuum to give anhyd spectroscopically pure salt product
by 1H NMR (3.096 g; 91.05%). Recrystallization of a portion (1.04 g) was
conducted in 40.0 mL boiling DI water containing 5 mL anhydrous
methanol. Most of the methanol boiled off. The resulting clear solution
was cooled overnight in a refrigerator (3.5 °C) to give nicely formed crystals.
Filtration, washing the crystals with 0.5 mL of pre-chilled DI water, and
drying at 65 °C under high vacuum (50 mTorr) in a Chem-Dry apparatus
over 72 h gave 0.91 g white solid; (recrysallized portion ) 87.6% recovery);
overall purified yield ) 79.7%): 1H NMR 400 MHz, std. DMSO-d6 (2.51):
δ 10.04 (s, 2H), 9.15 (s, 2H), 6.92 (s, 4H), 4.04 (s, 6H), 1.70-0.10 (complex
m, 12H); 13C NMR (100 MHz, std. DMSO-d6 (39.51): δ 145.05, 142.92,
38.97; FTIR (HATR) 3350, 3309, 3252, 3220, 3134, 3093, 2978, 2472,
2447, 1585, 1479, 1462, 1442, 1254, 1201, 1164, 1123, 1062, 988, 882,
850, 792, 711, 657, 621, 600, 568 cm-1; X-ray in Figure 3 and Supporting
Information.
-
148-152a
1.26/1.2716
(1.27)
76.4
(61.1)
8: X ) CH3
9: X ) H
Carborane
Carborane
259-262
1.10/-
95.8
(76.5)
(1.14)
-
182-183a
1.16/-
70.8
(54.9)
(1.16)
a Melts with gas evolution.
obtained by pycnometry at rt and are compared to a newly
developed predictive density additivity code15 that initially
was updated for heterocyclium closo-icosahedral borane and
carborane salts. This updated code predicts an average 2.0%
variance from the 11 pycnometry-based density values.
Density differences range from 4.2% with salt 6 to 0% with
salts 7 and 9. Pycnometry densities for salts 2 and 3 do not
contain occluded solvent molecules which are present in the
single crystal X-ray analyses.16
(14) [1-Amino-3-H-1,2,3-triazolium][closo-CB11H12], 9: Solid [1-Amino-
3-H-1,2,3-triazolium] chloride (2.41 g; 20.00 mmol) and commercial
K[closo-CB11H12] (3.64 g; 20.01 mmol) are placed into a 50 mL Pyrex
beaker. Addition of 10.0 mL DI water was followed by heating to gentle
reflux and then pouring the nearly clear solution into a fresh 50 mL Pyrex
beaker leaving behind a small amount of a water insoluble impurity in the
K[CB11H12] reactant. The reaction mixture was cooled overnight in the
refrigerator (3.5 °C) to effect crystallization. Filtration, rinsing the filter-
cake with 2.0 mL of pre-chilled DI water, and pumping at high vacuum
(60 mTorr) gave a 4.59 g of solid with some entrained water. Two
recrystallizations from 7.0 mL refluxing DI water gave 2.51 g of pure salt;
overall purified yield (two recrystallizations) ) 54.9%): 1H NMR 400 MHz,
std. DMSO-d6 (2.50): δ 8.71 (brs, 3H), 7.96 (d, J ) 1.1 Hz, 1H), 7.74 (d,
J ) 1.1 Hz, 1H), 2.39 (brs, 1H), 2.25-0.75 (complex m, 11H); 13C NMR
(100 MHz, std. DMSO-d6 (39.51): δ 132.09, 124.28, 50.79; FTIR (HATR)
3714, 3694, 3653, 3636, 3346, 3264, 3154, 3134, 2521, 1659, 1605, 1540,
1462, 1442, 1299, 1168, 1111, 1078, 1017, 956, 890, 788, 711, 694, 657,
625, 600, 564, 543 cm-1; HRMS calcd for [M - 3H]- m/z ) 225.2315,
found 225.2325; calcd for [CB11H12]- m/z ) 143.2035, found 143.2045.
An X-ray crystal packing structure of [4-Amino-1-methyl-
1,2,4-triazolium]2[B12H12] 1 appears in Figure 3. Single
(15) Ammon, H. L. Propellants, Explos., Pyrotech. 2008, 33, 92.
(16) Preparing acceptable single crystals for X-ray analysis results in
two occluded CH3CN molecules for salt 2 and two occluded H2O molecules
for salt 3. Both salts become amorphorus powders when the solvent
molecules are removed for pycnometry density analyses.
Org. Lett., Vol. 11, No. 12, 2009
2625

Table 3. Imidazolium Borane and Carborane Salts
4: X ) CH3
Y ) H
Borane
Borane
-
-
260-262a
1.08/1.11
(1.12)
82.3
(60.0)
5: X ) CH3
Y ) CH2CH3
-
290-292a
1.06/1.10
(1.08)
81.6
(74.2)
10: X ) CH3
Y ) H
Carborane
Carborane
368-369a
1.11/-
93.2
(83.5)
(1.07)
175-176a 17
1.03/-
93.6
(69.5)
,
11: X ) H
Y ) CH2CH3
-
(1.05)
a Melts with gas evolution.
Figure 3
a-axis.
. Crystal packing of salt 1 viewed along the crystallographic
crystal X-ray data for [Heterocyclium][B12H12] salts 1-5 are
found in the Supporting Information.16 Melting points are
obtained in open capillary tubes and are uncorrected. Both
the crude and overall product yield are given for each salt
in Tables 1-3. This overall yield is determined by multiply-
ing the crude yield percent with the percentage of salt product
recovered after recrystallization.
The previouly reported [1-Ethyl-3-methylimidazolium]-
[CB11H12] salt 11 was synthesized by a silver salt-based
metathesis procedure.9 The earlier measured X-ray single
crystal density of 1.07 g/cm3 compares well with our
pycnometry density of 1.03 g/cm3, taken at rt, and the
predicted density15 of 1.05 g/cm3 (Table 3). Our visually
determined melting point (1 °C/min) is 175-176 °C com-
pared to the reported DSC-based melting point of 122 °C.9
Apparently, an endothermic DSC peak at 122 °C was taken
to be a melting point.17 Our yields of 94% (crude), 74%
(recrystallized), and 70% (overall) compare favorably with
the 90% yield previously reported for salt 11.9
challenges surfaced in its synthesis. Treating it like water-
soluble binary triazolium and tetrazolium borane/carborane
salts encountered in this research effort was the answer.
These will be reported in a future publication as well as an
option for pairing equimolar amounts of two dissimilar
heterocyclic cations with the [B12H12]2- dianion to give
mixed heterocyclium borane salts. Initial results have given
three such mixed cation borane salts: e.g. [4-Amino-1-methyl-
1,2,4-triazolium][1-Ethyl-3-methylimidazolium][B12H12] us-
ing the heterocyclic cations seen in salts 1 and 11.
Acknowledgment. Mr. Milton B. McKay verified the ion
chromatography halide ion content procedure prior to it being
used for our Cl- analyses on salts 1-11. AFRL/RZS,
Edwards AFB, CA, provided administrative, financial, and
technical support.
Supporting Information Available: Experimental pro-
cedures, textual spectroscopy data, detailed melting point
notes, and ion chromatography Cl- analyses for salts 1-11,
DSC scans for salt 11, single crystal X-ray for salts 1-5,
and HRMS for salts 6-11. This material is available free of
charge via the Internet at http://pubs.acs.org.
The [4-Amino-1H-1,2,4-triazolium]2[B12H12] salt, analo-
gous to [CB11H12]- salt 6, is absent from Table 1. Unique
(17) Our DSC analyses of salt 11 (1 °C/min) show a small endotherm
at 128 °C, which we attribute to a solid phase transition, and a larger
endotherm at 175-176 °C. This latter temperature matches the visually-
determined melting point where a white solid changes to a colorless liquid.
Visually-determined solid softening occurs from 106-132 °C.
OL900550V
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Org. Lett., Vol. 11, No. 12, 2009