Synthetic methods for preparing ionic liquids containing hypophosphite and carbon-extended dicyanamide anions

Article abstract of DOI:10.1002/chem.201203740

Chemical methodology: New synthetic methods to prepare ionic liquids containing the hypophosphite (HP) and carbon-extended dicyanamide (DCA) anions (vinylogous DCA and N,N′-dicyanoformamide (DCF)) have been developed (see scheme). The performance of these materials as hypergolic ionic liquids has also been evaluated. Copyright

Full text of DOI:10.1002/chem.201203740

COMMUNICATION
DOI: 10.1002/chem.201203740
Synthetic Methods for Preparing Ionic Liquids Containing Hypophosphite
and Carbon-Extended Dicyanamide Anions
John P. Maciejewski,[a] Haixiang Gao,[b] and Jeanꢀne M. Shreeve*[a]
The ability to design small molecules to perform specific
tasks is critical to advance fields that range from pharma-
ceuticals and therapeutics[1,2] to materials science.[3] In addi-
tion to molecular compounds, ionic liquids (ILs, salts that
melt below 1008C) represent a specific class of compounds
that often demonstrate low volatility and broad applicability
to the aforementioned fields.[4,5] The emergence of ionic liq-
uids specifically designed for particular roles[6] has demon-
strated that these molten salts can be employed over a wide
range of applications, which include solvents,[7] electrolytes,[8]
pharmaceuticals and therapeutics,[9] and hydrogen-storage
materials.[10]
uids that serve as benchmark fuels often contain the dicy
AHCTUNGTREGaNNNU mide (DCA, 1), nitrocyanamide (NCA, 2), or the dicyano-
ACHTUNGTRENNUNGan-
borohydride (DCB, 3) anions, in which the latter has pro-
duced ignition-delay (ID) times below 10 ms when paired
with various organic cations (Figure 1).[14] The synthesis and
characterization of ionic liquids containing anions 4, 5, and
6 have been scarce. In addition, the hypergolic properties of
ionic liquids containing these anions have not been report-
ed. Our choice for studying anion 4 was due to its ability to
act as a strong reducing reagent. Anions 5 and 6 were stud-
ied to prepare carbon-extended derivatives of the DCA
anion.
A hypergolic chemical reaction is defined as the sponta-
neous combustion reaction between a fuel and an oxidiz-
er.[11,12] During our efforts to prepare new classes of ionic
liquids for hypergolic fuel studies, we have developed new
synthetic methods for the preparation of ionic liquids. We
became interested in studying the physical properties and
hypergolic reactivity of ionic liquids containing anions 4–6
(Figure 1). Common hypergolic fuels are hydrazine based
and are volatile, toxic substances.[12] Hypergolic ionic liquids
are considered to be potential surrogates for hydrazine-
based fuels in aerospace propulsion applications.[13] Ionic liq-
Initial attempts to use a Brønsted acid/base metathesis re-
action between an imidazolium hydroxide and phosphinic
acid were unsuccessful. This required the development of an
alternative metathesis protocol. The conventional method
used to prepare ionic liquids involving the formation of a
silver salt of the anion was also attempted. However, these
conditions are not applicable to the hypophosphite anion,
because the sodium salt readily reduces silver nitrate to
silver metal.[15] It was necessary to develop a new and effi-
cient methodology to prepare hypophosphite-containing
ionic liquids.[16] To utilize the efficiency of silver-salt meta-
thesis, an intermediate of chemical orthogonality, a bisam-
monium sulfate, was incorporated into our general synthetic
approach (Scheme 1). These intermediates were prepared
Figure 1. Anions studied in hypergolic ionic-liquid applications.
[a] Dr. J. P. Maciejewski, Prof. Dr. J. M. Shreeve
Department of Chemistry, University of Idaho
Moscow, ID 83844-2343 (USA)
Fax : (+1)208-885-9146
Scheme 1. Preparation of hypophosphite ionic liquids.
E-mail: jshreeve@uidaho.edu
[b] Prof. Dr. H. Gao
by the addition of ammonium halide salts (7–12) to an aque-
ous suspension of silver sulfate. Filtration of the reaction
Department of Applied Chemistry
China Agricultural University
Beijing, 100193 (P.R. China)
mixture into
a flask containing barium hypophosphite
(Ba(OP(O)H2)2) provided water-soluble ammonium hypo-
phosphite ionic liquids, as well as barium sulfate as a precip-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203740.
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2947

itate. During this work, an improved synthesis of barium hy-
pophosphite was developed, which involves the reaction of
BaCO3 and in situ generated phosphinic acid. This scalable
reaction was used to prepare up to 47 g (74% yield) of
Ba(OP(O)H2)2, and is less cumbersome than known meth-
ods of preparation involving the reaction between white
phosphorus and barium hydroxide.[17]
As a result of our intended applications, our substrate
scope was limited to the production of room-temperature
ionic liquids. The instability of aliphatic ammonium (e.g.,
pyrrolidinium) hypophosphites limited our substrate scope
to study the chemistry of hydrazinium- and imidazolium-
based cations. According to the method described in
Scheme 1, a series of six HP ionic liquids have been pre-
pared in 63–80% yield. The 1-butyl-1,1-dimethylhydrazini-
um HP salt 18 was isolated as a waxy, hygroscopic solid at
room temperature, and the other products in the HP series
exist as liquids at room temperature. The physical properties
of the prepared hypophosphite ionic liquids are shown in
Table 1.
vDCA) was accomplished in two steps. The condensation of
malononitrile with triethylorthoformate provided the vinyl
ethyl ether 19, which was subsequently treated with sodium
cyanamide to give 20 (Scheme 2a). The preparation of the
N,N’-dicyanoformamidine sodium salt (Na-DCF) was ac-
complished through the condensation of cyanamide with
triethylorthoformate to give ethyl N-cyanoformimidate 21,
which underwent an analogous addition/elimination reaction
with sodium cyanamide to provide 22 (Scheme 2b).
The preparation of ionic liquids 23–26 containing anions 5
and 6 was accomplished by adding a freshly prepared silver
salt of the respective anion to an aqueous solution of the re-
spective cation (7 or 9) as shown in Scheme 3. These materi-
Table 1. Properties of hypophosphite (HP) ionic liquids.[a]
[b]
Entry
ILs
Tm [8C]
Td [8C][b]
h [cP][b]
1 [gcmÀ3
]
ID [ms]
1
2
3
4
5
6
13
14
15
16
17
18
<À80
<À80
<À80
<À80
<À80
<À80
219
200
214
171
217
202
56.2
177.8
154.1[c]
308.9[d]
251.6
–
1.18
1.19
203
125
230
NH
241
–
Scheme 3. Preparation of vDCA and DCF ionic liquids.
1.11[c]
1.12[d]
1.25
1.11[c]
als were isolated in 86–90% yield (see Experimental Section
in the Supporting Information).[18]
[a] IL=ionic liquids prepared; Tm =melting temperature; Td =decompo-
sition temperature; h=viscosity; 1=density; ID=ignition delay (average
of three measurements by using WFNA as the oxidizer); NH=not hyper-
golic. [b] Measurements at 258C. [c] Measurements at 268C. [d] Measure-
ments at 278C.
The vDCA and DCF anions provided ionic liquids with
similar density and viscosity to the DCA benchmarks. The
DSC analyses of the DCF- and vDCA-based ionic liquids
showed that the decomposition temperatures (Td) for the
DCF class are below those of the vDCA analogues. Both
classes were less thermally stable than the DCA benchmark
ionic liquids (Table 2). Despite the similarities in physical
properties to the DCA ionic liquids, the hypergolic reactivi-
ty for ionic liquids 23–26 was lower than expected. When
using the DCF anion, a hypergolic reaction (ID=381 ms)
was observed when paired with the 1-ethyl-3-methylimida-
zolium cation 7, but not with the 1-butyl-3-methylimidazoli-
When screened for hypergolic reactivity, the ID times
were recorded in triplicate by using white fuming nitric acid
(WFNA) as the oxidizer, and the average ID values are re-
ported in Table 1. In general, the ID times for the room-
temperature HP ionic liquids are two to six times longer
(125–241 ms) compared with those reported for the DCA
class with the respective cations. In one case, the 1-allyl-1,1-
dimethylhydrazinium hypophosphite ionic liquid 16 was not
hypergolic.
Table 2. Properties of vDCA and DCF ionic liquids.[a]
Two new classes of anions similar to the DCA anion have
also been prepared, as described in Scheme 2. The prepara-
tion of the N-(2,2-dicyanovinyl)cyanamide sodium salt (Na-
[b]
Entry
ILs
Tm [8C]
Td [8C][b]
h [cP][b]
1 [gcmÀ3
]
ID [ms]
1
2
3
4
5
6
7
23
24
25
26
27
28
29
<À80
<À80
3
<À80
À21
n/a
205
193
197
186
275
207
240
69.4
126.9
32.8[c]
80.2
21[d]
42
1.12
1.09
1.13[c]
1.08
1.06
n/a
NH
NH
381
NH
36[e]
43
À90
33
1.06
47
[a] IL=ionic liquids; Tm =melting temperature; Td =decomposition tem-
perature; h=viscosity; 1=density; n/a=data not available; ID=ignition
delay (average of three measurements by using WFNA as the oxidizer);
NH=not hypergolic; [b] measurements at 258C; [c] measurements at
268C; [d] measurement at 208C; [e] ID for ionic liquid not previously re-
ported.
Scheme 2. Preparation of vDCA and DCF sodium salts.
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Chem. Eur. J. 2013, 19, 2947 – 2950

Synthetic Methods for Preparing Ionic Liquids
COMMUNICATION
1-Ethyl-3-methyl-imidazolium N-(2,2-dicyanovinyl)cyanamide, 23: Gener-
um cation 9. Although the physical properties of the ionic
liquids are similar to the analogous DCA ionic liquids, upon
the addition of ionic liquids 23 and 24 to WFNA, no hyper-
golic reaction was observed. For comparison, entries 5–7 in
Table 2 outline the physical properties and ID times of
known 1-ethyl-[19], 1-allyl-[12] and 1-butyl-3-methylimidazoli-
um[12,20] DCA ionic liquids (27–29, respectively).
In summary, we have developed new synthetic methods to
prepare ionic liquids that contain hypophosphite and
carbon-extended dicyanamide anions. Although the pre-
pared ionic liquids containing anions 4–6 did not outperform
the benchmark hypergolic ionic liquids that contain anions
1–3, their physical and thermodynamic properties are simi-
lar.
al protocol B: To
a protected from light solution of 20 (1.320 g,
9.423 mmol) in H2O (25 mL) at RT, a solution of AgNO3 (1.600 g,
9.419 mmol) in H2O (15 mL) was added. The resulting suspension was
stirred for 1 h, filtered, and rinsed with H2O. The silver salt was then sus-
pended in a solution of 7 (1.500 g, 7.851 mmol) in H2O (30 mL). After
protected from ambient light mixture was stirred overnight at RT, the
mixture was filtered, rinsed with absolute EtOH, and concentrated under
reduced pressure. The residue was dissolved in EtOH, stirred with acti-
vated carbon, filtered, concentrated, and dried in vacuo to give 23 (87%
yield; 1.555 g, 6.813 mmol) as an amber oil. Td =2058C (onset); IR: n˜ =
3472, 3153, 3111, 2196, 2164, 1549, 1324, 1169 cmÀ1 1H NMR (500 MHz,
;
[D6]DMSO): d=9.09 (s, 1H, CH), 8.08 (brs, 1H, CH), 7.75 (s, 1H, CH),
7.67 (s, 1H, CH), 4.19 (q, J=7.3 Hz, 2H, CH2), 3.85 (s, 3H, CH3),
1.42 ppm (t, J=7.5 Hz, 3H, CH3); 13C NMR (125 MHz, [D6]DMSO): d=
172.5, 154.5, 136.2, 123.5, 121.9, 120.2, 119.0, 116.2, 44.1, 35.7, 15.0 ppm;
elemental analysis calcd (%) for C11H12N6 (228.25): C 57.88, H 5.30, N
36.82; found: C 57.90, H 5.53, N 36.69.
Experimental Section
Acknowledgements
Safety precautions: Although we have experienced no difficulties in syn-
theses and characterization of these materials, proper protective precau-
tions should be used. Manipulations must be carried out in a hood
behind a safety shield. Eye protection and gloves must be worn. For the
complete experimental details, see the Supporting Information. In addi-
tion to full characterization, the heat of formation and specific impulse
properties of the ionic liquids have been calculated. All calculated data
are provided in the Supporting Information.
The authors are grateful for the support of the CFD Research Corp. Air
Force SBIR Phase II (contract FA9300–11-C-3044); PA clearance
number 12645.
Keywords: anions · hypergolic fuels · hypophosphite · ionic
liquids · synthetic methods
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peak. Melting and decomposition points for ionic liquids were recorded
on a TA Instruments Co., model Q10 differential scanning calorimeter
(DSC) from À80 to 4008C at a scan rate of 58CminÀ1 in compressed
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ACHTUNGTRENNUNG
sured by DSC from 40 to 4008C at a scan rate of 58CminÀ1. IR spectra
were recorded as thin films by using a BIORAD model 3000 FTS spec-
trometer, unless otherwise noted. Densities were measured by using a
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General synthetic procedures: 1-ethyl-3-methyl-imidazolium hypophos-
phite, 13: General protocol A: To a suspension of Ag2SO4 (2.083 g,
6.681 mmol) in H2O (4 mL) cooled in an ice bath and protected from
light was added a solution of 7 (2.502 g, 13.09 mmol) in H2O (8 mL). The
mixture was stirred while protected from light for 2 h. The solution was
then filtered into a flask containing BaACHTNUTRGNE(UNG H2PO2)2 (3.676 g, 13.75 mmol),
and the solid was rinsed with H2O (6 mL). The suspension was stirred for
3 h at RT, diluted with absolute EtOH (10 mL), and the volatile materials
were removed under reduced pressure. The residue was dissolved in
EtOH, stirred with activated carbon, filtered, concentrated under re-
duced pressure, and dried in vacuo to give 13 (70% yield; 1.621 g,
9.202 mmol) as a light yellow oil. Td =2198C (onset); IR: n˜ =3385, 3083,
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2984, 2295, 2258, 1661, 1572, 1455, 1195, 1082, 1050, 807 cmÀ1 1H NMR
;
(300.1 MHz, [D6]DMSO): d=9.29 (brs, 1H, CH), 7.80 (s, 1H, CH), 7.72
(s, 1H, CH), 7.07 (d, J=451.2 Hz, 2H, H2PO2), 4.20 (q, J=7.3 Hz, 2H,
CH2), 3.85 (s, 3H, CH3), 1.41 ppm (t, J=7.3 Hz, 3H, CH3); 13C NMR
(75.5 MHz, [D6]DMSO): d=136.4, 123.5, 121.9, 44.0, 35.6, 15.0 ppm;
31P NMR (121.5 MHz, [D6]DMSO): d=À5.61 ppm (s, H2PO2); elemental
analysis calcd (%) for C6H13N2O2P+1H2O (194.17): C 37.11, H 7.79, N
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Received: October 19, 2012
Revised: December 30, 2012
Published online: January 30, 2013
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Chem. Eur. J. 2013, 19, 2947 – 2950