Isolation and structural characterization of the elusive 1:1 adduct of hydrazine and carbon dioxide

Article abstract of DOI:10.1039/c1cc14542h

A solid hydrazine was isolated as a crystalline powder by reacting aqueous hydrazine with supercritical CO₂. Its structure determined by single crystal X-ray diffraction shows a zwitterionic form of NH₃ ⁺NHCO₂^-. The solid hydrazine is remarkably stable but is as reactive as liquid hydrazine even in the absence of solvents.

Full text of DOI:10.1039/c1cc14542h

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COMMUNICATION
Isolation and structural characterization of the elusive 1 : 1 adduct of
hydrazine and carbon dioxidew
Byeongno Lee,a Seung Hee Kang,a Donghyeon Kang,a Kyu Hyung Lee,a Jaeheung Cho,b
Wonwoo Nam,b Oc Hee Hancd and Nam Hwi Hur*a
Received 26th July 2011, Accepted 22nd August 2011
DOI: 10.1039/c1cc14542h
A solid hydrazine was isolated as a crystalline powder by reacting
aqueous hydrazine with supercritical CO2. Its structure determined
by single crystal X-ray diffraction shows a zwitterionic form of
NH3+NHCO2À. The solid hydrazine is remarkably stable but is as
Carbazic acid (NH2NHCO2H) has been known for a century,11
but it does not exist in the free-state and is considered an
intermediate compound. Carbazic acid has been presumed to
form when a stream of CO2 gas is passed through cold hydrazine
in water. The sticky product thus formed probably comprisÀes
reactive as liquid hydrazine even in the absence of solvents.
Hydrazine is a volatile liquid that is extensively used as a rocket
fuel,1 blowing agent,2 fuel cell,3 and a precursor to make various
high-value chemicals.4,5 However, it is extremely hazardous
and the risks of transportation and handling in its liquid form are
a high deterrent to its extensive usage. This drawback can be over-
come by preparing solid hydrazine whose usage can be considered
chemically equivalent to liquid hydrazine. In this communication,
we report the preparation of stable solid hydrazine by reacting
aqueous hydrazine with supercritical CO2 (sc CO2). The product,
formulated as NH3+NHCO2À, is a white crystalline powder that
is quite stable but is as reactive as liquid hydrazine. Under mild
conditions, the solid hydrazine readily produces anhydrous
hydrazine via the decarboxylation process. This transportable
solid hydrazine will lead to novel applications not only for the
green synthesis of fine chemicals but also in energy-related
fields such as hydrogen storage materials and fuels.6
carbazic acid and hydrazinium carbazate (N2H5+CO2N2H3
)
that is ascribed to water present in liquid hydrazine. Pure carbazic
acid can be considered as a simple 1 : 1 adduct of hydrazine
and CO2. However, its isolation and characterization have
remained elusive1,11 despite its great importance in various
disciplines. This prompted us to employ sc CO2 for the isolation
of pure solid hydrazine. Our synthetic strategy was to exploit sc
CO2 as both a reactant and a solvent and to react sc CO2 with
hydrazine while sustaining its solvating capability under sc CO2
conditions.
The reaction was conducted in a stainless steel reactor
(eqn (1)). An aqueous solution of hydrazine was introduced into
a reactor charged with dry ice (solid CO2). The mixture was
heated at 80 1C and 100 bar for 10 h (see ESIw). A colourless
crystalline solid—zwitteÀrionic hydrazinium carboxylate with the
formula NH3+NHCO2 (1)—was formed on the reactor wall.
The solid hydrazine 1 is a remarkably stable solid and is almost
insoluble in most solvents when stored in a closed bottle; its
stability was carefully verified by X-ray powder diffraction (see
Fig. S1, ESIw). Moreover, 1 remains intact even in the presence
of water. Unlike amino acids, 1 slowly sublimes in air (see Fig. S2,
ESIw) and reverts to NH2NH2 and CO2 by various methods,
which can thus be used as a source of anhydrous hydrazine.
Single crystals of 1 were grown from the reaction mixture of
NH2NH2ÁH2O with sc CO2 and the reactor was cooled very
slowly down to 0 1C after the reaction was completed. As
illustrated in Fig. 1, the solid-state structure of 1 determined
by single crystal X-ray diffraction clearly reveals a zwitterionic
form of NH3+NHCO2À, which is structurally analogous to
the glycine molecule (NH3+CH2CO2À) known as the smallest
amino acid.12 However, the N–C bond (1.398 A) of 1 is con-
siderably shorter than that of glycine (1.475 A).12a In addition,
a quasi-planar NCO2 geometry is formed by the N(2), C(1), O(1),
and O(2) atoms, which implies that the N–C bond is inter-
mediate between single and double bonds. The N–N bond length
Supercritical carbon dioxide (sc CO2) has been widely used as
an industrial solvent because of its environmentally benign nature
and easy separation from products.7,8 In recent years, sc CO2 has
also been considered to be an ideal synthetic feedstock.9 More-
over, it is well documented that sc CO2 can be an effective reagent
for a variety of synthetic reactions. In particular, sc CO2 reacts
readily with amines, leading to the formation of carbamic acids.10
(1)
a Department of Chemistry, Sogang University, Seoul 121-742, Korea.
E-mail: nhhur@sogang.ac.kr
b Department of Bioinspired Science, Department of Chemistry and
Nano Science, Ewha Womans University, Seoul 120-750, Korea
c Daegu Center, Korea Basic Science Institute, Daegu, 702-701, Korea
d GRAST, Chungnam National University, Daejeon, 305-764, Korea
w Electronic supplementary information (ESI) available: Experi-
mental details, data for the solid hydrazine (1), characterization of
azines (2), XRD and crystallographic data. CCDC 822062. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc14542h
c
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Chem. Commun., 2011, 47, 11219–11221 11219

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À
Fig. 2 Solid state NMR spectra of NH3+NHCO2 (1) at 295 K.
(a) 1H magic angle spinning (MAS) NMR spectrum recorded at 14.1 T
and 23 kHz spinning rate. The blue and red dotted lines correspond to
two distinctive resonances at 7.2 and 10.2 ppm, respectively; they were
obtained from the deconvolution of the peaks using a Voigt function.
(b) 13C CP MAS NMR spectrum recorded at 9.4 T and 3 kHz spinning
rate. Asterisks (*) denote spinning side bands.
Fig. 1 X-Ray crystal structure of NH3+NHCO2À(1). (a) Molecular
structure of 1; here, thermal ellipsoids have 30% probability. The
selected bond lengths and angles are as follows. O1–C1: 1.264(3) A;
O2–C1: 1.256(3) A; N1–N2: 1.438(3) A; N2–C1: 1.402(3) A;
C1–N2–N1: 114.1(2)1; O2–C1–O1: 126.8(2)1; O2–C1–N2: 117.8(2)1;
O1–C1–N2: 115.3(2)1. (b) Illustration of hydrogen bonds in
NH3+NHCO2À. Covalent bonds are linked by solid lines. Green
dotted lines represent the hydrogen bonds in which the HÁ Á ÁO
distances are shorter than 2.0 A. For clarity, other HÁ Á ÁO distances
longer than 2.0 A are omitted. Red, blue, grey, and pale pink spheres
represent O, N, C, and H atoms, respectively.
(2.467 and 2.573 A) are considerably longer than those of the
three short OÁÁÁH bonds (Table S2, see ESIw). The considerable
stability of 1 in the solid state is presumably associated with
the intermolecular hydrogen bonding; this is well documented
in the case of amino acids.12
(1.438 A) of 1 is similar to that of free hydrazine (1.450 A).
An important feature is that both O atoms of the CO2À group
are involved in the formation of intermolecular hydrogen
Fig. 2 shows the 1H magic angle spinning (MAS) NMR
spectrum of 1 recorded at 295 K with a spinning rate of 23 kHz.
Two broad signals with different intensities have been detected
at 10.2 and 7.2 ppm. The integration ratio of the two peaks,
which is estimated by the deconvolution of the peaks using a
Voigt function of multi-peaks fitting, is very close to 3 : 1. The
proton signals originating from the COOH group can typically
be observed in the range of 11.0–18.1 ppm, while the signals
of the N–H protons are observed at 6.3–10.3 ppm.13 Hence,
the assignment of the peaks in the 1H MAS spectrum is
straightforward. One peak at 7.2 ppm can be assigned to the
NH proton signal while the other peak at 10.2 ppm can be
+
bonds with the H atoms in the NH3 group. Each molecule
(NH3+NHCO2À) is hydrogen-bonded to three neighbouring
molecules, thereby forming a three-dimensional network
(Fig. 1). The O(1) atom having a long C–O bond (1.258 A)
+
closely interacts with two different H atoms in the NH3
groups; here, the two intermolecular OÁ Á ÁH bond distances are
1.889 and 1.893 A. The O(2) atom with a short C–O bond
+
(1.250 A) contacts an H atom in the NH3 group, and the
distance is 1.920 A. In addition to the three short OÁ Á ÁH
bonds, the H atom attached to the NH group also weakly
interacts with the two O atoms. However, their bond distances
+
ascribed to the signal of the NH3 protons. The large values
a w
Table 1 Reactions of solid hydrazine (1) with carbonyl compounds
Entry
Reactant
Product
Reaction time/h
Yieldb (%)
2a, 497
2b, 497
Remarkc
1
2
3
3
Yellow crystal
Yellow crystal
3
3
2c, 497
Yellow crystal
4
5
a
3
2d, 498
2e, 497
b
Yellow crystal
Yellow crystal
24
Reaction conditions: 1 (5.0 mmol, 0.38 g), carbonyl reactant (10.0 mmol), no solvent, at 45 1C, in 10 mL vial. Isolated yield based on 1.
Crystals were obtained from the re-crystallization of powders in ether/pentane at ambient temperature.
c
c
11220 Chem. Commun., 2011, 47, 11219–11221
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+
of the chemical shifts of the NH3 protons indicate that all
Ae Chae and Ms Sun Ha Kim at KBSI are acknowledged for
their technical assistance in acquiring the NMR spectra.
the three protons are involvÀed in strong hydrogen bonding
with the O atoms in the CO2 group. As illustrated in Fig. 2,
the 13C cross polarization (CP) MAS NMR spectrum of 1
exhibits a sharp peak at 164.0 ppm along with side bands, and
this corresponds to the carbon signal emanating from the
Notes and references
1 E. A. Schmidt, Hydrazine and Its Derivatives: Preparation,
Properties, Applications, Wiley-Interscience, New York, 2001.
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À
CO2 group. The carboxylate carbon in amino acids is
typically visible at the region of 167–173 ppm.14 Further, the
1H and 13C solid-state NMR data are in good agreement with
the solid state structure of 1 determined by a single crystal
X-ray diffraction.
The reactivity of solid hydrazine was examined by the
reaction of the liquid carbonyl compound with 1, which was
conducted in the absence of solvents. The addition of 1 to the
carbonyl compounds yielded solid azines as the sole products
with over 97% yields at 25 1C (Table 1 and ESIw). The
reactions are essentially the condensation reactions of carbonyl
compounds with 1, and they appear to proceed via the
dissociation of 1 into NH2NH2 and CO2. A notable feature
is that the reaction is highly selective without the production of
any side products; this is probably a result of the generation of
pure anhydrous hydrazine from 1. As given in Table 1 (entry
4), the reaction of 1 with cinnamaldehyde produces exclusively
the azine, 2d. However, the same reaction performed with
hydrazine hydrate gives a significant amount of unknown
compounds (over 15%). It is noteworthy that hydrazine
hydrate or hydrazine sulfate used as a hydrazine source often
leads to the formation of byproducts15,16 due presumably to
the presence of water or acid in the reactant. However, all the
reactions using 1 as a hydrazine source are environmentally
benign ‘‘waste-free’’ processes in which no side products are
generated except CO2 and water. The solid hydrazine (1) will
thus be beneficial for the extensive application of hydrazine to
many organic syntheses, and this will not only enhance the
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and CO2 and is very stable. Moreover, its reactivity is similar
to that of liquid hydrazine and exhibits better selectivity due
probably to the absence of water. A remarkable benefit of
solid hydrazine is that it can not only be transported safely but
it also permits the production of anhydrous hydrazine at local
areas because it readily produces anhydrous hydrazine via the
decarboxylation process. This transportable solid hydrazine
can be expected to significantly extend the current applications
of liquid hydrazine and also lay the groundwork for major
technological challenges in energy-related fields.
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We thank the Converging Research Center Program
(2010K001050) and the NRL program (2010-0018937) funded
by the Ministry of Education, Science, and Technology
through the National Research Foundation of Korea. Dr Seen
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11219–11221 11221