Organic salt of hydrogen L-tartaric acid: A novel wide-temperature-range ferroelectrics with a reversible phase transition

Article abstract of DOI:10.1039/c0ce00109k

4-Ethylanilinium hydrogen (2R,3R)-tartrate, a novel wide-temperature-range ferroelectric was synthesized. DSC measurement discloses that the homochiral organic salt undergoes an isosymmetric reversible phase transition at about 186 K with a sharply narrow

Full text of DOI:10.1039/c0ce00109k

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PAPER
Organic salt of hydrogen L-tartaric acid: a novel wide-temperature-range
ferroelectrics with a reversible phase transition†
*
De-Hong Wu, Jia-Zhen Ge, Hong-Lin Cai, Wen Zhang and Ren-Gen Xiong
*
Received 18th April 2010, Accepted 14th July 2010
DOI: 10.1039/c0ce00109k
4-Ethylanilinium hydrogen (2R,3R)-tartrate, a novel wide-temperature-range ferroelectric was
synthesized. DSC measurement discloses that the homochiral organic salt undergoes an isosymmetric
reversible phase transition at about 186 K with a sharply narrow heat hysteresis of 0.7 K. The heat
capacity Cp obtained from the calorimetric measurement exhibits a sharp peak at 185.8 K,
characteristic of a first-order phase transition. However temperature-dependence dielectric constant
measurements reveal no dielectric anomaly near the phase transition point. The measurement of the
unit cell parameters except for c axis versus temperature suggests that the values change abruptly and
remarkably between 180 and 190 K with the cell volume doubled. The crystal structures determined at
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
123(2) K (a ¼ 7.461 A, b ¼ 11.930 A, c ¼ 14.873 A, a ¼ 95.34 , b ¼ 91.95 , g ¼ 107.92 ) and 298(2) K
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
(a ¼ 6.078 A, b ¼ 7.478 A, c ¼ 14.951 A, a ¼ 87.66 , b ¼ 82.69 , g ¼ 71.80 ) also show that the phase
transition could be a type of isosymmetric change with the same triclinic space group P1 (No. 1).
Structural analysis shows that the different modes of hydrogen bonds probably affect the
configurations of the phenyl rings from the cations, consequently leading to a reversible structural
phase transition.
crystallize in the chiral group which meets with the requirements
Introduction
of ferroelectric space groups or ten polar point groups (C1, Cs,
C2, C2v, C3, C3v, C4, C4v, C6, C6v or 1, 2, 3, 4, 6, m, mm2, 3m,
4mm, 6mm) while the ferroelectric materials have found wide-
spread applications in the modern high-technology field.10,11
Along this line, herein we report a novel salt composed of
[EtPhNH3]+$[HOCOCH(OH)CH(OH)COO]ꢁ (1), of which is
interesting to note that the homochiral organic salt undergoes
a reversible isosymmetric phase transition at about 186 K. To the
best of our knowledge, 1 represents the first example of
a homochiral organic salt undergoing a reversible phase transi-
tion in the known homochiral compounds.12
The influence of hydrogen-bonded interactions on the crystal
packing and geometry of organic molecular solids is well
known.1,2 Ever since Pasteur carried out the first reported sepa-
ration of crystalline enantiomers,3 tartaric acid (TA) has held
a firm place in the archives of ground-breaking chemical
discoveries. TA has two kinds of different proton-donor centers,
two carboxylic and two hydroxyl groups (pKa1 ¼ 3.0 and pKa2
¼
4.3 in H2O)4 and, therefore, is potentially capable of forming
monovalent (semi-tartrate) or divalent (tartrate) anions to afford
1 : 1 or 1 : 2 proton-transfer salts with most Lewis bases. However,
with stoichiometric control it is possible to selectively form 1 : 1
hydrogen tartrates, and the crystal structures of a large number of
these 1 : 1 salts have been reported. More recently, its utility as an
agent for the introduction of chirality in achiral organic compounds
for the generation of crystalline materials with potentially useful
non-linear optical properties has been also explored.5–9
Experimental
L-(+)-TA (Aldrich) and 4-ethylbenzenamine (Aldrich) were used
as commercial products without further purification. The
4-ethylanilinium hydrogen L-(+)-tartrate was prepared by dis-
solving equimolar amouts of L-(+)-TA and 4-ethylbenzenamine
in a methanol solution at room temperature, the mixtures were
set aside to crystallize to obtain the transparently colorless single
crystals within a few days. Yield: 97%. m.p. 159–161 ꢀC with
decomp. 1H NMR (DMSO-d6): d 1.09 (t, J ¼ 7.5 Hz, 3H, CH3),
2.42 (q, J ¼ 7.5 Hz, 2H, CH2), 4.35 (s, 2H, CHOH), 6.54 (d, J ¼
8.0 Hz, 2H, C6H4), 6.86 (d, J ¼ 8.0 Hz, 2H, C6H4), 7.57 (s, 6H,
OH and NH). 13C NMR (DMSO-d6): d 16.51 (CH3), 27.84
(CH2), 72.62 (CHOH), 115.10, 128.53 (CH in phenyl ring),
(2R,3R)-tartaric acid (L-(+)-TA) was chosen for this study
because a great number of potential hydrogen-bond donors and
acceptors are present, and its chirality guarantees its salts
Ordered Matter Science Research Center, Southeast University, Nanjing,
211189, P. R. China. E-mail: zhangwen@seu.edu.cn; xiongrg@seu.edu.
cn; Fax: (+86)-25-52090626; Tel: (+86)-25-52090626
† Electronic supplementary information (ESI) available: IR and XRD
for 1. CCDC reference numbers 773838 (LT (123 K) phase) and
773839 (RT (298 K) phase). For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c0ce00109k
This journal is ª The Royal Society of Chemistry 2011
CrystEngComm, 2011, 13, 319–324 | 319

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Fig. 1 A view of asymmetric unit of 1 with atomic numbering scheme at 298(2) K (left) and 123(2) K (right). Displacement ellipsoids were dawn at 30%
probability level. Hydrogen atoms in 123(2) K were omitted for clarity.
Table 2 Hydrogen-bond parameters (A, ꢀ) in LT (123 K), RT (298 K)
ꢀ
Table 1 Crystallographic data and structural refinement details of LT
(123 K) and RT (298 K) phases of 1
phases of 1a
D–H/A
H/A
D/A
:DHA
LT (123 K) phase
RT(298 K) phase
(A) Intra-anion hydrogen bonds
LT phase
CCDC number
chemical formula
Crystal size/mm
Formula weight
T/K
773838
773839
O3–H3/O2
2.07
2.13
2.06
2.15
2.08
2.10
2.58
2.08
2.59
2.15
2.07
2.08
2.54
2.580(3)
2.625(3)
2.573(3)
2.638(3)
2.578(2)
2.727(3)
2.927(2)
2.586(2)
2.922(3)
2.640(3)
2.578(3)
2.583(2)
2.898(3)
120.3
118.9
120.0
118.1
119.3
133.7
107.4
119.3
105.8
118.1
119.4
119.0
108.0
C8H12N C4H5O6
0.36 ꢂ 0.32 ꢂ 0.30
271.27
123(2)
Mo-Ka (0.71073 A)
C8H12N C4H5O6
0.36 ꢂ 0.32 ꢂ 0.30
271.27
298(2)
Mo-Ka (0.71073 A)
O4–H4D/O5
O9–H9/O8
O10–H10/O12
O15–H15A/O14
O15–H15A/O23
O16–H16A/O15
O21–H21A/O20
O22–H22/O21
O3–H3/O1
ꢀ
ꢀ
Radiation
Crystal system
Space group
Triclinic
P1
Triclinic
P1
ꢀ
a/A
7.4606(12)
11.9298(18)
14.8726(9)
95.338(2)
91.950(1)
107.916(1)
1251.3(3)
4
6.0782(12)
7.4777(15)
14.9506(10)
87.655(10)
82.692(10)
71.802(1)
640.29(19)
2
1.407
0.113
288
3.18–27.48
6591 (Rint ¼ 0.0357)
2930
2745
2930/3/353
1.057
0.0411/0.1009
0.0440/0.1029
0.235/ꢁ0.194
ꢀ
b/A
ꢀ
RT phase
c/A
a (ꢀ)
b (ꢀ)
g (ꢀ)
O4–H4A/O6
O10–H10/O12
O9–H9/O10
3
ꢀ
(B) Inter-anion hydrogen bonds
V/A
Z
LT phase
O6–H6/O1ii
1.78
1.78
2.05
1.74
2.09
2.06
1.73
1.80
1.75
2.07
2.14
2.587(3)
2.588(3)
2.728(3)
2.551(2)
2.713(3)
2.739(3)
2.538(3)
2.601(3)
2.553(3)
2.755(3)
2.757(3)
167.5
170.0
140.2
170.8
132.3
139.2
169.8
165.9
167.5
140.8
132.1
O11–H11/O7ii
O16–H16A/O7iv
O18–H18/O13ii
O21–H21A/O17v
O22–H22/O1vi
O24–H24A/O19ii
O2–H2/O5x
rc/g cmꢁ3
m/mmꢁ1
F(000)
1.440
0.116
576
q range (deg)
Reflns collected
Indep. reflns
Reflns obs.[I > 2s(I)]
Data/restr./paras
GOF
R1, wR2[I > 2s(I)]
R1, wR2 (all data)
ꢀ
larg. peak/hole/e A
3.11–27.49
13753 (Rint ¼ 0.0298)
5681
4661
5681/3/705
1.001
0.0386/0.0867
0.0495/0.0929
0.245/ꢁ0.219
RT phase
O7–H7A/O11x
O9–H9/O5xi
O10–H10/O8xi
(C) Cation-anion hydrogen bond
ꢁ3
LT phase
N1–H1D/O14
N1–H1E/O1
N1–H1F/O15i
N2–H2C/O3
N2–H2D/O7ii
N2–H2D/O10ii
N2–H2E/O2ii
N3–H3A/O9
N3–H3B/O1iii
N3–H3C/O8ii
N4–H4A/O20ii
N4–H4B/O13iii
N4–H4C/O21
N1–H1D/O4vii
N1–H1E/O5
N1–H1E/O3
N1–H1F/O6vii
N2–H2C/O12ix
N2–H2D/O11x
N2–H2E/O10vii
1.84
1.89
1.92
1.98
2.41
2.48
1.89
2.00
2.38
1.87
1.81
1.90
1.94
2.00
2.49
2.57
1.88
1.83
1.92
1.95
2.712(3)
2.775(3)
2.753(3)
2.862(3)
3.260(3)
2.939(3)
2.762(3)
2.883(3)
3.225(3)
2.736(3)
2.679(3)
2.783(3)
2.784(3)
2.880(3)
3.350(4)
3.048(3)
2.749(3)
2.704(3)
2.798(3)
2.792(3)
165.3
170.7
154.6
174.0
159.5
112.6
165.7
169.2
159.3
165.0
163.6
171.6
157.6
169.6
161.6
114.6
164.9
165.8
170.8
156.9
132.21, 145.62 (quaternary C in phenyl ring), 173.68 (C]O). IR
(cmꢁ1, KBr pellet): 3325, 2907, 2597, 1714, 1575, 1513.
Results and discussion
Structure discussion
RT phase
Complex 1 crystallizes in triclinic space group P1 (No. 1). Fig. 1
shows its molecular structure with labeling scheme of LT phase
(low temperature, 123 K) and RT phase (room temperature,
298 K). Crystallographic data and structural refinement details
of the LT phase and RT phase are listed in Table 1.
a
Symmetry codes: (i) x + 1, y, z; (ii) x ꢁ 1, y, z; (iii) x, y + 1, z; (iv) x ꢁ 1, y,
z + 1; (v) x + 1, y + 1, z; (vi) x, y + 1, z + 1; (vii) x ꢁ 1, y + 1, z; (viii) x ꢁ 1,
y, z; (ix) x ꢁ 1, y ꢁ 1, z; (x) x, y ꢁ 1, z; (xi) x + 1, y, z.
The structural determination of LT phase or RT phase reveals
distinctive hydrogen bonded anion bilayer-sheets with cations
being pendant from both faces of the sheet, via three N–H/O
320 | CrystEngComm, 2011, 13, 319–324
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hydrogen bonds. Another feature of interest in the tartrate
structures was the very wide range of anion substructures
observed, including a variety of chains of fused rings and
a variety of sheet substructures, as well as three-dimensional
frameworks of anions encapsulating large voids which enclose
the cations.
carboxylate O5i and O8i atoms in an inter-anion hydrogen bond,
respectively. Furthermore the two layer sheets are interlocked by
inter-anion O–H/O hydrogen bonds. The N atom of the cation is
linked to the O atoms of the anions, acting as a threefold donor in
N2–H2C/O12ix, N2–H2D/O11x, N2–H2E/O10vii hydrogen
bonds or a fourfold donor in N1–H1D/O4vii, N1–H1E/O5, N1–
H1E/O3, N1–H1F/O6vii ones (Table 2). The majority of these
hydrogen bonds are of a two-centre type and the minority a three-
centre type of N1–H1E/(O)2, O9–H/(O)2 and O10–H/(O)2.
In the structure of the LT phase, the hydrogen tartrate ions are
connected in infinite two-dimensional bilayer-sheets via relatively
short O–H/O hydrogen bonds between the hydroxyl, carboxyl,
and carboxylate groups. The hydroxyl O3, O4, O9, O10, O21,
O16 and O22 atoms act as donors to the carboxylate O2, O5, O8,
O12, O20, and the hydroxyl O15 and O21 atoms in an intra-
anion hydrogen bond, respectively. The hydroxyl O15 atom acts
as a donor to the carboxylate O14 and O23 atoms in an intra-
anion with a three-centre type of hydrogen bond O15–H/(O)2.
The carboxyl O6, O11, O18 and O24 atoms act as donors to the
carboxylate O1ii, O7ii, O13ii and O19ii atoms in an inter-anion
hydrogen bond, respectively. The hydroxyl O16, O21 and O22
atoms act as donors to the carboxylate O7iv, O17v and O1vi atoms
in an inter-anion hydrogen bond, respectively. Furthermore the
two layer sheets are interlocked by inter-anion O–H/O
hydrogen bonds. The N atom of the cation is linked to the
O atoms of the anions, acting as a threefold donor in N1–H/O,
N3–H/O, N4–H/O hydrogen bonds or as a fourfold donor in
N2–H/O (Table 2). The majority of these hydrogen bonds are
of a two-centre type and the minority a three-centre type of
N2–H2D/(O)2, O15–H/(O)2, O16–H/(O)2, O21–H/(O)2
and O22–H/(O)2.
Furthermore, the crystal structure of 4-ethylanilinium
hydrogen L-(+)-tartrate consists of two L-(+)-tartrate and two
4-ethylanilinium cations in the asymmetric unit at RT phase. In
comparison, four L-(+)-tartrate and four 4-ethylanilinium ions in
the asymmetric unit were found at LT phase. Both the structure
of the RT phase and that of the LT phase are characterized by an
extensive hydrogen bonding network. There are infinite chains of
hydrogen bonds of the type COOꢁ/HO2C, involving the usually
ꢀ
found short O–H/O (ꢃ2.6 A) contacts between the monoanionic
L-(+)-tartrate in a ‘‘head-to-tail’’ fashion. 4-Ethylanilinium cations
are antiparallel to each other. At the same time, the two adjacent
infinite chains of anions are interlinked by N–H/O types of
hydrogen bonds provided by the cations. When 4-ethylbenzen-
amine was co-crystallized with L-(+)-TA, the salt, 1 was produced
showing the presence of complete single-proton transfer from
L-(+)-TA to the N atom of 4-ethylbenzenamine. The hydrogen
tartrate anions then form a three-dimensional hydrogen-bonded
substructure through carboxylate interactions with other tartrate
carboxylic acid and hydroxyl groups (Table 2).
In the structure of the RT phase, the hydrogen tartrate ions are
connected in infinite two-dimensional bilayer-sheets via relatively
short O–H/O hydrogen bonds between hydroxyl, carboxyl, and
carboxylate groups. The hydroxyl O3, O4, O10 and O9 atoms act
as donors to the carboxylate O1, O6, O12 and the hydroxyl O10
atoms in an intra-anion hydrogen bond, respectively. The
carboxyl O3 and O7 atoms act as donors to the carboxylate O5x
and O11x atoms in an inter-anion hydrogen bond, respectively.
Also the hydroxyl O9 and O10 atoms act as donors to the
The hydrogen tartrate ions in both the RT phase and LT
phase are interlinked via hydrogen bonds into infinite two-
dimensional bilayer-sheets. And some other non-classic
Fig. 2 Packing diagrams of 4-ethylanilinium cation moieties in 1 at 123(2) K (left) and 298(2) K (right). The hydrogen tartrate anions and all hydrogen
atoms were omitted for clarity.
Fig. 3 Packing diagrams of 4-ethylanilinium cation moieties in 1 at 123(2) K (left) and 298(2) K (right) viewed along the b axis and a axis, respectively.
The hydrogen tartrate anions and all hydrogen atoms were omitted for clarity.
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Fig. 4 Temperature dependence of unit-cell parameters of 1: (left) unit-cell lengths and (right) cell volume.
intra- and inter-molecular C–H/O weak hydrogen-bonding
interactions are present in the LT and RT phase.
show very small differences, indicating no phase transition from
183 K to 93 K (Fig. 4).
In the structure of the LT phase, the phenyl rings (C3 to C8)
and (C19 to C24) are nearly antiparallel to the phenyl rings (C11
to C16) and (C27 to C32), respectively, with dihedral angles of
3.6ꢀ and 0.9ꢀ. But the phenyl ring (C3 to C8) is un-parallel to the
phenyl ring (C19 to C24) with a dihedral angle of 24.0ꢀ. Also, the
phenyl ring (C11 to C16) is un-parallel to (C27 to C32) with
a dihedral angle of 20.7ꢀ. As a result, in the LT phase, there are
two sets of parallel phenyl rings with a dihedral angle of about
22.3ꢀ. In the structure of the RT phase, the phenyl ring (C3 to C8)
is nearly antiparallel to the phenyl ring (C11 to C16) with
a dihedral angle of 1.5ꢀ. Thus, in RT phase, all the phenyl rings
are parallel (Fig. 2 and 3).
DSC measurement
The DSC measurement was performed to detect whether
compound 1 undergoes a phase transition triggered by temper-
ature or not. Upon heating and cooling, the crystalline sample
undergoes a single phase transition at ca. 186 K, showing an
exothermic peak at 186.4 K and an endothermic peak at 185.7 K
(Fig. 5). The two observed peaks demonstrate a reversible phase
transition with a heat hysteresis of 0.7 K. Both the narrow
thermal hysteresis and the shape of the peaks testify the character
of a first-order phase transition, in good agreement with that
found in variable-temperature X-ray single crystal diffraction.
X-ray crystal structures of 1 under different temperatures at
373 K, 298 K, 233 K, 223 K, 213 K, 203 K, 193 K, 183 K, 173 K
and 123 K were measured. The cell parameters of 1 measured at
the temperature from 373 K to 193 K show very small differences
with those measured at room temperature when the temperature
influence on the cell parameters, i.e., the thermal expansion and
contraction, is excluded. Thus above 193 K, no phase transition
occurred in 1. Interestingly, the cell parameters of 1 at 183 K
change abruptly and the cell volume is nearly doubled at 183 K vs.
193 K, indicating a first-order phase transition.13 The cell
parameters of 1 measured at the temperature from 183 K to 93 K
Heat capacity
The calorimetric measurement exhibits a sharp peak at 185.8 K,
being characteristic of a first-order phase transition (Fig. 6).
From the curve, a heat change of DQ z 0.61 J gꢁ1 and an entropy
change of DS z 3.28 ꢂ 10ꢁ3 J gꢁ1 Kꢁ1 ¼ 0.89 J Kꢁ1 molꢁ1 are
obtained. Given that DS ¼ RlnN, where N represents possible
configurations, it is found that N z 1.17, indicating that the
phase transition of the system is not simply a displacive type but
an order-disorder type.14
Fig. 5 DSC curves obtained on heating-cooling for 1 (scanning rate 10
K minꢁ1, sample mass 16.28 mg).
Fig. 6 The temperature dependence of the heat capacity Cp of 1. The
dotted base line was used for the evaluation of the entropy change (DS).
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Fig. 7 Temperature-dependent dielectric constants of 1 at 10, 90 and 1000 kHz: (left) 86 K to 280 K and (right) 286 K to 446 K.
Dielectric constant measurement
isosymmetric phase transition at ca. 186 K. The phase transition
shows no observable dielectric anomaly. The hydrogen bonding
interactions are very complicated for the two structures at 123 K
and 298 K, maybe resulting from the configurations of the phenyl
rings changing significantly with the dihedral angle of ca. 22ꢀ.
This value indicates that the phase transition of the system is not
simply a displacive type but a configurational order-disorder
type.
It is very useful for us to search for phase transitions through
the variable-temperature dielectric constant response, especially
in the relatively high frequency range. Owing to failure to get
large crystals, the powder-pressed pellet of 1 was used in
dielectric constant measurements. The temperature dependence
of the dielectric constant measured at 10, 90 and 1000 kHz is
shown in Fig. 7. Unexpectedly, no observable dielectric
anomaly was observed. The dielectric constant is nearly equal to
4 under the three different frequencies from 86 K to 280 K, and
Acknowledgements
shows
a temperature-independent feature. The dielectric
This work was supported by the China National Natural
Science Foundations (20931002, 90922005) and Jiangsu prov-
ince NSF (BK2008029). De-Hong Wu thanks China Post-
doctoral Science Foundation funded project (20090451147),
Jiangsu Planned Projects for Postdoctoral Research Funds
(0802003B) and SEU Major Postdoctoral Research Funds
(3212000901).
anomaly may be too weak to be observable in the specific
system with narrow heat hysteresis and minor structural
changes across the Tc. Further super-low frequency dielectric
constant studies are in progress. According to D ¼ 3E + P (D ¼
electric displacement, E ¼ electric field, 3 ¼ dielectric constant,
P ¼ polarization), there is no dielectric anomaly at relative high
frequency and the P ꢃ D diagram should definitely be linear, i.e.
1 should be a dielectric not ferroelectric. However, 1 definitely
displays a ferroelectric space group with phase transition (Tc)
and its properties are a typical feature for molecular ferroelec-
trics. Consequently, 1 is still a ferroelectric at very low
frequency because the super-low frequency should be needed for
the inversion of molecules with large size. The phase transition
feature of 1 looks like BaTiO3 in which a ferroelectric-to-ferro-
electric transition occurs at ca. 0 ꢀC, and is different from
Rochelle salt in which a paraelectric-to-ferroelectric phase tran-
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a ferroelectric-to-paraelectric phase transition occurs with the
decrease of the temperature to ꢁ18 ꢀC.12 So, at relative high
frequency, it is reasonable for one to fail to observe the dielectric
anomaly.
On the other hand, similar to the behavior of KTaO3 and
SrTiO3, since the soft-mode is non-polar whose freezing wouldn’t
cause the order of ferroelectric or antiferroelectric phase, no
dielectric anomaly could be observed around phase transition
temperature. However, detailed measurements such as Raman
and optical birefringence still need to be carried out to better
understand its phase transformation.
Conclusion
In summary, the combined DSC, heat capacity measurement and
12 F. Jona, G. Shirane, Ferroelectric Crystals; Pergamon Press: New
York, 1962.
structural analysis reveal that the 1 undergoes a reversible
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