Synthesis, crystal structure and phase transitions of a series of imidazolium iodides

Article abstract of DOI:10.1039/c3ce40559a

The reaction of imidazole with hydroiodic acid leads to three products crystallizing as ionic salts; [C₃N₂H₅ ⁺][I^-], [C₃N₂H₅ ⁺]₂[I₄^2-] and [C₃N ₂H₃I₂⁺][I^-]. All the analogs were characterized by single-crystal X-ray diffraction, while the first two were additionally studied by calorimetric, dilatometric, dielectric and proton magnetic resonance methods. At room temperature (RT), [C ₃N₂H₅⁺][I^-] adopts the centrosymmetric, trigonal space group (R3). The crystal structure consists of disordered imidazolium cations and discrete I^- ions. [C ₃N₂H₅⁺][I^-] undergoes two discontinuous phase transitions (PTs) at 180/185 K and 113/123 K (cooling-heating), both of them governed by the imidazolium cation dynamics. [C₃N₂H₅⁺]₂[I ₄^2-] consists of disordered imidazolium cations and quite rare and exotic [I₄]^2- tetraiodide counterion. It undergoes continuous PT at 204 K of the ferroelastic type with a symmetry change from orthorhombic Fddd to monoclinic C2/c. The mechanism of PT is complex and consists of 'order-disorder' and 'displacive' contributions that are assigned to the dynamics of cations and to the distortion of the [I₄ ^2-] rods, respectively. [C₃N₂H ₃I₂⁺][I^-] is built up of discrete 4,5-diiodoimidazolium cations and isolated I^- ions. A characteristic feature of this compound is the presence of a layered structure in which moieties are held together by strong I...I halogen interactions and N-H...I hydrogen bonds.

Full text of DOI:10.1039/c3ce40559a

CrystEngComm
View Article Online
View Journal | View Issue
PAPER
Synthesis, crystal structure and phase transitions of a
series of imidazolium iodides3
Cite this: CrystEngComm, 2013, 15,
5633
Magdalena We˛cławik,a Anna Ga˛gor,b Anna Piecha,*a Ryszard Jakubasa
and Wojciech Medyckic
The reaction of imidazole with hydroiodic acid leads to three products crystallizing as ionic salts;
[C3N2H5+][I2], [C3N2H5+]2[I422] and [C3N2H3I2+][I2]. All the analogs were characterized by single-crystal
X-ray diffraction, while the first two were additionally studied by calorimetric, dilatometric, dielectric and
proton magnetic resonance methods. At room temperature (RT), [C3N2H5+][I2] adopts the centrosym-
¯
metric, trigonal space group (R3). The crystal structure consists of disordered imidazolium cations and
discrete I2 ions. [C3N2H5+][I2] undergoes two discontinuous phase transitions (PTs) at 180/185 K and 113/
22
123 K (cooling–heating), both of them governed by the imidazolium cation dynamics. [C3N2H5+]2[I4
]
consists of disordered imidazolium cations and quite rare and exotic [I4]22 tetraiodide counterion. It
undergoes continuous PT at 204 K of the ferroelastic type with a symmetry change from orthorhombic
Fddd to monoclinic C2/c. The mechanism of PT is complex and consists of ‘order–disorder’ and ‘displacive’
contributions that are assigned to the dynamics of cations and to the distortion of the [I422] rods,
respectively. [C3N2H3I2+][I2] is built up of discrete 4,5-diiodoimidazolium cations and isolated I2 ions. A
characteristic feature of this compound is the presence of a layered structure in which moieties are held
Received 29th March 2013,
Accepted 10th May 2013
DOI: 10.1039/c3ce40559a
…
…
www.rsc.org/crystengcomm
together by strong I I halogen interactions and N–H I hydrogen bonds.
Cl, Br). Three imidazolium ferroelectrics with this composition
Introduction
have already been synthesized and characterized.6–8 The
‘order–disorder’ mechanism of ferroelectric transition in
R5M2X11 type compounds was assigned to the dynamics of
imidazolium cations. Very recently, bis(imidazolium) L-tar-
trate, a hydrogen-bonded ‘displacive’ type ferroelectric mate-
rial was discovered.9 The simple (1 : 1) ionic complexes of
imidazolium and tetrahedral anions were found to also exhibit
ferroelectric properties at the room temperature phases;
Ionic liquids based on various substituted imidazolium salts
are characterized by low melting points, many of them are
liquid at room temperature. Extensive studies of ionic liquids
have revealed a great number of useful properties such as
extremely low volatility, high thermal stability and high ionic
conductivity.1–4 Therefore, the above-mentioned ionic liquids
have found application in solar cells, electrochemical devices
and ion transport systems.5
2 10
+
[C3N2H5+][ClO4
]
and [C3N2H5 ][BF42].11
Bulky plane organic cations like non-substituted imidazo-
lium are widely used in numerous organic–inorganic hybrids.
Such organic molecules play a role of simple dielectric rotator
units which have been used to obtain new ferroelectric
materials. The known representatives of this group of ferroic
compounds are based on haloantimonates(III) and
halobismuthates(III) with the R5M2X11 chemical stoichiometry
Imidazolium analogs in which the anionic network is
formed by the halogen atoms appeared to be the simplest as
regards the crystal structure; however, they have been studied
only fragmentarily. Recently, we have characterized the
+
molecular-ionic compound, [C3N2H5 ][Br2] (ImBr), which
shows very interesting structural and dynamic properties.12
The crystal structure of the RT phase is described by the polar,
R3 space group and consists of a highly disordered imidazo-
lium cations and discrete Br2 anions. ImBr reveals a first order
phase transition at 199/201 K (cooling–heating cycle) which
leads to the monoclinic centrosymmetric space group P21/m.
The mechanism of the phase transition is mixed; related to the
freezing of cation motions (‘order–disorder’ (‘o–d’) contribu-
tion) and to a significant rebuilding of the anionic substruc-
ture (‘displacive’ (‘d’)-type mechanism).
+
(where R = imidazolium cation (C3N2H5 ), M = Sb, Bi and X =
aFaculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław,
Poland. E-mail: anna.piecha@chem.uni.wroc.pl; Fax: +48713757270;
Tel: +48713757288
bW. Trzebiatowski Institute of Low Temperature and Structure Research PAS, P. O.
Box 1410, 50-950 Wrocław, Poland
c
´
Institute of Molecular Physics, PAS, M. Smoluchowskiego 17, 60-179 Poznan, Poland
Electronic supplementary information (ESI) available: Details of the thermal
3
(TGA), X-ray diffraction and dielectric and 1H NMR measurements. CCDC
929449–929453. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3ce40559a
The aim of this paper is to present details of physicochem-
+
ical properties of a series of imidazolium salts; [C3N2H5 ][I2]
This journal is ß The Royal Society of Chemistry 2013
CrystEngComm, 2013, 15, 5633–5640 | 5633

View Article Online
Paper
CrystEngComm
+
(1) and [C3N2H5 ]2[I422] (3) including preparation, crystal
instrument in the temperature range 300–700 K with a ramp
rate 2 K min21 the scans was performed in flowing nitrogen
(flow rate: 1 dm3 h21).
The complex electric permittivity, e* = e9 2 ie99 was measured
between 100 and 290 K by the Agilent E4980A Precision LCR
Meter in the frequency range between 135 Hz and 2 MHz. The
overall error was less than 5%. The single crystal samples had
dimensions ca. 2 6 2 6 0.5 mm3. Silver electrodes were
printed on opposite faces. The dielectric measurements were
carried in a controlled atmosphere (N2).
structures in various phases, thermal and dielectric properties.
Structural characterization is presented also for
a new
substituted imidazolium compound; [C3N2H3I2 ][I2] (2). The
molecular motions in (1) and (3) are studied by means of
proton magnetic resonance measurements (1H NMR) (for
+
more information, see ESI ). The molecular mechanism of the
phase transitions in the crystals under investigation is also
proposed.
3
Single crystal X-ray diffraction crystallographic data were
collected on a KM4 diffractometer with CCD detector and
graphite-monochromated Mo Ka radiation (l = 0.71073 Å).
Low temperature was maintained using an open flow unit
(Oxford Cryosystem). The CrysAlis software version 1.171.32
was used for data processing.13 An empirical absorption
correction was applied using spherical harmonics implemen-
ted in SCALE3 ABSPACK scaling algorithm. The structure was
solved by direct methods and refined against F2 by means of
SHELX-97 program package.14 All non-hydrogen atoms were
refined anisotropically. H atoms were included in geometric
positions and treated as riding atoms. The Uiso(H) were
constrained to be xUeq (carrier atom), where x = 1.2.
Structural phase transitions observed with temperature
lowering led to twinning of the (1) and (3) crystals. In (3)
symmetry changed from orthorhombic to monoclinic which
resulted in crystal twinning with two dominating domains,
with partially overlapped diffraction peaks. Refinement of the
structure was performed using the data from both domains
and HKLF 5 instruction.
In the case of (1) two structural transformations were
encountered. During the cooling cycle the crystal symmetry
changed from trigonal to triclinic (at 180 K) and with further
cooling to a monoclinic one (113/123 K cooling–heating).
Lowering of the crystal class led to non-merohedral twinning
of the sample. Six triclinic crystal domains were encountered
at 150 K from the splitting of diffraction peaks. Partial
overlapping of the peaks together with weak intensities did
not allow satisfactory indexing, thus the crystal structure of
this intermediate phase has not been determined. Below 113 K
the symmetry of (1) increased to monoclinic and the number
of domains was reduced to three. The structure refinement
was based on the diffraction intensities from one dominating
crystal domain, indexing 60% of the measured diffraction
peaks. All peaks were completely separated. Table 1 contains
crystal data and structure refinement results of measured
crystals.
Experimental section
Materials and instrument
Synthesis of the complexes. All compounds were obtained in
the reaction between imidazole amine and iodic acid, however,
both synthesis-reactions were carried out in different condi-
tions (temperature and molar ratio) applied for each com-
pound.
(1) crystals were prepared at ambient temperature by
dissolving imidazole in the 20% HI solution. Single crystals
were grown by slow evaporation from the solution. The
crystalline product was twice recrystallized. Single transparent
crystals were grown by slow evaporation from an aqueous
solution and characterized by an elemental analysis: C: 18.37%
(theor. 18.39%), N: 14.25% (theor. 14.29%), H 2.49% (theor.
2.57%).
(2) crystals were prepared by dissolving imidazole in the
20% HI solution. Subsequently, the mixture was stirred (at 320
K for about 30 min). After a few days, tiny minuscule
transparent solids were formed by a slow evaporation from
the colorless solution. Nevertheless, due to small amounts of
obtained material (only X-ray diffraction measurements were
undertaken) we were not able to make the physicochemical
characterization of this material. We obtained successfully the
4,5-diiodoimidazolium iodide crystals only during the first
crystallization. The following crystallization produced exclu-
sively the (1) crystals.
(3) crystals were prepared at ambient temperature by
dissolving imidazole in concentrated (57%) HI. Single crystals
were grown by slow evaporation from the solution. The dark,
navy blue, needle like crystals were characterized by an
elemental analysis: C: 11.18% (theor. 11.16%), N: 8.55%
(theor. 8.68%), H 1.59% (theor. 1.56%).
General details
All the solvents and starting materials for the synthesis were
purchased commercially and were used as received.
DSC traces were obtained using a Perkin Elmer model 8500
differential scanning calorimeter calibrated using n-heptane
and indium. Hermetically sealed Al pans with the polycrystal-
line material were prepared in a controlled-atmosphere N2
glovebox. The measurements were performed between 100 and
470 K. The thermal hysteresis was estimated from the scans
performed at various rates (20, 10 and 5 K min21) extrapolated
Results and discussion
Thermal properties
The DSC runs of (1) (Fig. 1(a)) revealed two PTs at 180/185 K
and 113/123 K (cooling–heating) of the first order type. The
PTs are reversible, however, the enthalpy effects differ on
cooling and heating significantly. The entropy values were
estimated to be ca. 7 J mol21 K21 for the I « II PT and 2 J
to
a
scanning rate of
0
K
min21
.
Simultaneous
Thermogravimetric Analysis (TGA) and Differential Thermal
Analysis (DTA) were performed on a Setaram SETSYS 16/18
5634 | CrystEngComm, 2013, 15, 5633–5640
This journal is ß The Royal Society of Chemistry 2013

View Article Online
CrystEngComm
Paper
Table 1 Crystal data and structure refinement results for [C3N2H5+][I2] (1), [C3N2H3I2+][I2] (2) and [C3N2H5+]2[I422] (3)
Crystal data
(1)
(2)
(3)
Chemical formula
Temperature (K)
Mr
Crystal system, space group
Lattice parameters (Å)
C3H5IN2
295, Phase I
195.99
C3H5IN2
110, Phase III
195.99
Monoclinic, P21/n
a = 7.8732(5),
b = 15.4592(7),
c = 9.6771(6),
b = 106.64(1)u
1128.5(1)
C3H3I3N2
295
447.77
C6H10I4N4
295, Phase I
645.98
C6H10I4N4
115, Phase II
645.98
¯
Trigonal, R3
Orthorhombic, Pmmn Orthorhombic, Fddd Monoclinic, C2/c
a = 7.8956(9),
c = 8.077(2),
c = 120u
a = 7.2366(3),
b = 8.4314(3),
c = 7.2292(3)
a = 14.154(1),
b = 15.736(1),
c = 27.108(2)
a = 13.963(2),
b = 15.660(1),
c = 14.940(1),
b = 116.778(7)
2916.4(5)
V (Å3)
436.1(1)
3(1)
441.09(3)
2
6037.7(7)
16(4)
Z (Z for primitive cell)
Data collection
Radiation type
8
8(4)
Mo Ka
5.37
m (mm21
)
5.53
10.55
8.24
8.52
Crystal size (mm)
Tmin, Tmax
0.25 6 0.20 6 0.20
0.777, 1.000
0.25 6 0.22 6 0.19
0.552, 1.000
0.25 6 0.21 6 0.01
0.860, 1.000
No. of measured, independent 1677, 118, 118
and observed [I . 2s(I)]
11 112, 2126, 1881 1414, 489, 432
17 210, 2088, 894
17 335, 17 352, 14 614
0.061
reflections
Rint
0.022
0.092
0.021
0.051
Refinement
R[F2 . 2s(F2)], wR(F2), S
No. of reflections
No. of parameters
No. of restraints
D . max, D . min (e Å23
H-atom treatment
0.025, 0.074, 1.20 0.063, 0.138, 1.31
0.023, 0.051, 1.06
489
29
1
0.034, 0.106, 0.83
2088
49
13
0.046, 0.129, 1.20
17 352
115
0
118
5
2126
109
0
0
)
0.35, 20.22
4.82, 21.60
0.47, 20.99
0.97, 20.94
1.29, 22.26
H atoms treated by a mixture of independent and constrained refinement
mol21 K21 for the II « III one (the heating cycle) and are
characteristic of the ‘o–d’ mechanism of phase transition.
A complex sequence of PTs is also encountered in (3)
(Fig. 1(b)). For the cooling cycle only one heat anomaly
characteristic of the second order transition is revealed at ca.
204 K, whereas on heating two thermal anomalies are seen; at
136 K typical of the first order type and another one at higher
temperature corresponding to the PT detected on cooling at
204 K. The presence of a lower temperature heat anomaly may
suggest the possibility of existence of a metastable phase
below 204 K.
with the linear thermal expansion measurements. Both
compounds are stable up to ca. 500 K, however, above this
temperature a continuous decomposition takes place (see Fig.
S1(a) and S1(b) in ESI ).
3
Structure discussion
The crystal structure of (1) (Phase I). (1) crystallizes in the
2
¯
trigonal system in the centrosymmetric R3 space group. I
ions occupy all corners of the unit cell as well as two positions
on the [2111] diagonal whereas imidazolium cations are
located in the middle of the line connecting two adjacent
anions in the c direction. The packing is similar to those
reported for ImBr (ref. 12) although these two simple
structures are not isomorphic. (1) adopts higher symmetry
The sequence of the PTs observed in the calorimetric
measurements (DSC) for (1) and (3) is in a good agreement
compared to Br2 analog (R3 vs. R3) and reveals a much more
¯
complicated disorder of the counterions.
+
[C3N2H5 ] are disordered over a three-fold axis adopting 6
possible orientations. High values of displacement parameters
of anions together with an unstable position of imidazolium
cations indicate a low stability of the trigonal structure.
Thermal motions dominate over possible directional bonding
down to 180 K. Below this point, a drastic symmetry lowering
occurs and the crystal structure transforms to triclinic in the
first order phase transition. All cell parameters show discrete
changes that are much pronounced for the angles and the
bmono direction (Fig. S2, ESI ). The transition from Phase I to II
3
is accompanied by a crystal expansion in the bmono direction
by about 2.3% and a slight contraction of amono and cmono
axes; the most pronounced changes are observed for all unit
cell angles that deviate greatly from 90u indicating a triclinic
symmetry of Phase II.
Fig. 1 DSC curves for (a) (1) (b) (3) upon cooling and heating runs (2 K min21
(a) m = 20.8 mg, (b) m = 21.2 mg).
,
The lack of any anomalies of pyroelectric current in the
vicinity of the 180 K temperature points to the centrosym-
This journal is ß The Royal Society of Chemistry 2013
CrystEngComm, 2013, 15, 5633–5640 | 5635

View Article Online
Paper
CrystEngComm
Fig. 3 Hydrogen bonded chains of I2–[C3N2H5+]–I2 in Phase III, T = 110 K.
Table 2 Selected hydrogen-bond parameters in (1) at 110 K, Phase IIIa
…
…
…
…
D–H
A
D–H/Å
H
A/Å
D
A/Å
D–H A (u)
…
N1–H1 I2
0.86
0.86
0.86
0.86
2.91
2.75
2.83
2.83
3.591(8)
3.533(8)
3.551(8)
3.525(8)
136.9
151.5
143.1
139.6
i
…
N3–H3 I2
Fig. 2 Crystal structure of (1) (a) and (b) trigonal Phase I with disordered
…
N6–H6 I1
imidazolium counterions, six possible orientations of imidazolium are possible in
ii
…
N8–H8 I1
¯
R3 symmetry, selected positions with the site occupation factor equal to 0.17
a
are presented as well as the averaged picture, T = 295 K, (c) Phase III with
ordered, pseudo-hexagonal arrangement of ions, T = 110 K.
Symmetry code(s): (i) 2x + 1/2, y 2 1/2, 2z + 3/2; (ii) x 2 1/2, 2y +
1/2, z 2 1/2.
¯
metric space group, P1, for Phase II. The intermediate, triclinic
direction from 8.08(1) Å at 295 K to 7.87(2) Å at 110 K that
results in contraction of the amono lattice parameter whereas
shifts of anions in hexagonal (001) plane are at the origin of a
pronounced increase of the bmono parameter.
phase is preserved down to 113 K. Then, the crystal symmetry
increases to the monoclinic one, P21/n (Phase III). Both phase
transitions have a clear first-order character, there is no
group–subgroup relation between the phases and translation
symmetry also changes distinctively.
The crystal structure of (1) (Phase III). The unit cell
periodicity is not preserved during the transformation from I
to III Phase. The volume of the unit cell changes from 436 Å3 at
295 K to 1128 Å3 at 110 K. The monoclinic lattice parameters
The crystal structure of (2). The crystal structure of (2) is
…
built of planes in which moieties are held together by I
halogen and N–H I hydrogen bonds (see Fig. 4).
I
…
The distance between the planes is equal to 3.618(1) Å. In
the plane, each diiodoimidazolium is involved in two hydro-
gen and two halogen bonds. I2 acts as an acceptor in these
four interactions.
are equal to amono = 7.8732(5) Å, bmono = 15.4592(7) Å, cmono
=
9.6771(6) Å, b = 106.64(1)u. The asymmetric unit contains two
independent tetraiodide anions and two imidazolium counter-
ions. Fig. 2 presents the crystal packing in Phase I (a, b) and III
(c) showing an hexagonal arrangement of ions in Phase I and
pseudo-hexagonal arrangement of imidazolium counterions
and I2 anions in Phase III.
Phase transitions affect both the imidazolium cations that
are stabilized in the low temperature structure and positions
of the anions that shift from ‘trigonal’ sites distinctively. The
phase transitions are thus of a mixed character possessing
both ‘o–d’ and ‘d’ contributions. The local symmetry of the
¯
cation stays the same in all phases whereas the high 3m
symmetry site of I2 in Phase I lowers to 1 in Phase III. The
reorientation motions of imidazolium cations are frozen in
Phase III, the arrangement of the ions is established due to the
…
weak N–H I interactions. The low temperature phase is
composed of two non-equivalent chains of I2–[C3N2H5+]–I2
propagating perpendicular to each other within the adjacent
planes.
Fig. 3 presents the hydrogen-bonded chains propagating in
the b and c directions. The geometry of the hydrogen bonds is
given in Table 2. The I2 anions exhibit large shifts from their
high temperature positions. The I–I distance shrinks in chex
Fig. 4 Crystal structure of (2) (a) view along a direction, the crystal is composed
of layers with halogen and hydrogen bonds as motif-defining interactions,
green bonds – top layer, black bonds – down layer, dense dashed lines stand for
…
halogen I–I bonds, thicker ones represent N–H I interactions. (b) View along
the b direction, the interlayer spacing is equal to 3.62 Å (c) model of interactions
between 4,5-diiodoimidazolium and iodide, the atoms from the asymmetric
unit are labeled.
5636 | CrystEngComm, 2013, 15, 5633–5640
This journal is ß The Royal Society of Chemistry 2013

View Article Online
CrystEngComm
Paper
Table 3 I I and I–I distances in [I422] complexesa
…
…
…
I/[Å]
Compound
I
I/[Å]
I–I/[Å]
I
Ref.
Phenancetin?H2I4?2H2O
(C10H14N4Se2)(I3)(I4)0.5
[Ni(OSMe2)6]I4
[Co(OSMe2)6]I4
[V(MeCN)6]I4
3.403
3.405
3.345
3.338
3.166
3.201
3.294
3.357
3.362
3.363
3.403
3.360
3.346
3.440
3.415
3.207
2.773
2.819
2.848
2.845
2.846
2.792
2.814
2.814
2.792
2.762
2.826
2.825
2.761
2.829
2.811
2.805
3.403
3.405
3.452
3.338
3.516
3.547
3.429
3.357
3.418
3.363
3.403
3.360
3.346
3.440
3.415
3.511
19
20
21
22
23
24
a-(1)?3HI?I2
b-(1)?3HI?I2
24
(Pentafluorobenzyldibenzyl-ammonium)3?I32?I4
17
17
25
25
22
1,3-Bis(diphenylpentafluoro-benzylphosphonium)propane I2?I4
22
[2{tHPMT)22+}(2I2)?(I422)]b
[2{PMTH)2+}?(I422)]c
BTMAH2+?I4
26
22d
(3) Phase I, orthorhombic
(3) Phase III, monoclinic
This work
This work
a
d
b
c
1-Macrocyclic azocyclophane. tHPMT – 2-mercapto-3,4,5,6-tetrahydropyrimidine analog. PMTH – 2-mercaptopyrimidine analog.
BTMAH2+ – (CH3)3N+(CH2)6N+(CH3)3.
Both bonds are considerably strong although the geome-
1,3-bis(diphenylpentafluorobenzylphosphonium)propane
salts.17
trical parameters indicate that halogen bonds predominate
hydrogen interactions. The I–I distance of 3.5410(1) Å is
significantly shorter then the sum of the van der Waals radii
(4.12 Å), and the angle h1 that is equal to 177.6u gives almost a
[I422] tetraiodide dianions and imidazolium counterions
stack along one direction giving pseudo-hexagonal crystal
packing. Fig. 5(c) shows the arrangement of ions at 295 K.
Imidazolium cations, distributed in large channels, are able to
perform thermally activated librations and rotations in and
out of the plane of the rings.
…
linear configuration. The I N distance is equal to 3.505(5) Å in
…
N(1)–H(1) I(1) bond and donor-to-acceptor angle equals
170(7)u. The basic hydrogen and halogen bonded pattern is a
ten membered ring. The asymmetric unit contains half of the
diiodoimidazolium counter ion, laying on a m lattice plane
and being intersected by a m mirror plane and 2 axis, and one
I2 anion adopting mm2 symmetry site. The exceptional feature
of this simple salt is the layered crystal structure that has not
been observed in other dihalogenoimidazolium salts and its
derivatives. It is formed probably as a consequence of
halogen–halogen interactions. These interactions are impor-
tant packing forces in brominated hydroxyimidazoles.15
The crystal structure of (3). The anionic substructure of (3)
comprises a rare and exotic [I422] tetraiodide dianion that may
be treated as a charge-transfer complex of two donating iodide
anions and an accepting iodine molecule.16 Electrostatic
repulsion between the two iodide anions bound to a central
diiodine moiety implies low stability of the anion. Although
[I422] species has been observed in some complexes (see
Table 3) the (3) is a rare example of a crystal in which very weak
hydrogen bond interactions stabilize the anionic substructure
and lead to the phase transformation that change the
symmetry of [I422] dianion.
The rings are almost perpendicular to the iodide rods.
Despite the fact that two NH groups are in a bonding distance
to [I422] the H-bonds are not strong enough to overcome
thermally activated motions. Thus the possible hydrogen
bonds have a dynamical character. Below 204 K directional
The room temperature structure is disordered. The asym-
metric unit comprises one-half of [I422] dianion, one imida-
zolium with occupancy 0.5 and two one-half imidazolium
cations with occupancy 0.5, see Fig. 5(a) and (b).
The bonds I–Iterminal are equivalent and equal to 3.415 Å, the
internal I–I bond is equal to 2.811 Å. The terminal iodides are
weakly bonded to iodine molecule. Similar distances to
terminal iodides in other crystals are shorter, e.g. 3.363 Å in
Fig. 5 Crystal structure of (3) in Phase I, T = 295 K (a) pseudo-hexagonal
arrangement of ions along the a direction (b) model of disorder of imidazolium
counterions together with the structure of [I422] dianion, site occupation factor
for each imidazolium position equals 0.5; atoms from asymmetric unit are
labeled, double symmetry axis (.2.) are marked (c) arrangement of counterions
in respect to [I422] rods, hydrogen atoms are omitted for the clarity.
This journal is ß The Royal Society of Chemistry 2013
CrystEngComm, 2013, 15, 5633–5640 | 5637

View Article Online
Paper
CrystEngComm
bonding starts dominating over thermal motions that stands
at the origin of a second-order phase transition. Due to this
transformation the symmetry changes from orthorhombic
Fddd to monoclinic C2/c (C2/c is a maximal non-isomorphic
subgroup of Fddd). The phase transition is group–subgroup
related, the crystal class changes from mmm to 2/m. According
to the Aizu classification (3) belongs to the ferroelastic species
mmmF2/m.18 In both phases of (3) the size of the primitive cells
coincides and the number of C6H10I4N4 formal molecules in
primitive cells (Z) is equal to 4. Fig. S3, ESI, presents the
3
thermal evolution of the lattice parameters. Deformation of
the crystal structure is much pronounced in the direction of
the iodide rods and stacking imidazolium counter ions.
Observed temperature changes have continuous character.
Phase transition is associated with ordering of the cationic
substructure and deformation of the [I422] dianions that lose
their symmetrical arrangement. The internal distance between
iodides remains almost identical as in the high temperature
phase whereas distances between I–Iterminal change signifi-
cantly from 3.43 Å in Phase I to 3.51 Å and 3.21 Å in Phase II.
Independent content of the unit cell consists now of three,
fully occupied imidazolium positions A, B and C and four
iodides forming a highly asymmetric [I422] dianion, see Fig. 6.
In Phase II imidazolium rings are not perpendicular to the
iodide rods, they bend aside approx. 12u from the initial
positions. H-bond interactions stabilize three different spatial
orientations of imidazolium ions in the crystal structure.
A basic hydrogen bonded pattern consists of two iodide rods
interconnected via a counterion by two symmetrical N(6)–
Fig. 7 Temperature dependence of the real part of the complex electric
¯
permittivity along the [1 1 1] direction for (1) during the cooling cycle.
Despite the fact that the linear [I422] ion has been found to
be unstable, nine of twelve compounds possess symmetrical
configuration at room temperature. The present work is a first
report on the temperature stability of the [I422] rods.
Electrical properties of (1) and (3). Fig. 7 shows temperature
dependence of the real part of the complex electric permittivity
(e*) measured between 170 kHz and 2 MHz for a single-crystal
¯
sample of (1) along the [1 1 1] direction, in the temperature
region covering the two low temperature PTs. Both PTs are
dielectrically active. The low frequency dielectric response
possesses interesting features which are encountered both in
antiferroelectric compounds and crystals possessing high
temperature plastic-like phases. When the temperature
approaches 180 K e9c increases linearly up to 16 units and
below 180 K it suddenly drops to 8 units. A similar behavior of
e9(T) is observed in the vicinity of the II A III PT, however, the
reduction of e9 is smaller, ca. 25%.
Dielectric dispersion is not observed both in the high and
low temperature phase. It means that the possible motion of
the dipolar groups should be relatively fast in the disordered
Phase I and II.
…
H(6) I(1) bonds with a donor-to-acceptor distance of 3.52 Å
and two imidazolium cations type B interacting via N(1)–
…
H(1) I(2) with terminal iodides with the distance of 3.61 Å.
Imidazolium C does not interact by hydrogen bonds with the
rods. Fig. 6 shows the structure of hydrogen bonds in Phase II.
To the best of our knowledge there are only twelve crystal
structures containing discrete polyiodide [I422] moieties. The
[I422] species may be described as an I2 molecule bound to two
I2 ions through linear halogen bonding that is the inter-
molecular interaction involving the halogen atoms as electro-
philic species with the central I–I bond length ranging from
2
I
…
2.76 and 2.85 Å and the external I–I
distances ranging
It is interesting to compare dynamic properties of closely
related halogen analogs: ImBr (ref. 12) and (1), taking into
account their sequence of PTs presented in Fig. 8.
from ca. 3.17 to 3.55 Å.
The common feature of the RT Phases I of both compounds
is a high dynamic disorder of the imidazolium cations. The
low temperature PTs are governed by a change in the motions
of dipoles of imidazole rings. The completely freezing of these
motions in the case of ImBr takes place exactly at 199 K,
whereas in (1) a two-stage freezing was observed at 180 and
Fig. 6 Arrangement of counterions in respect to [I422] rods in Phase II. Dashed
lines represent hydrogen bond interactions. Hydrogen bonded motif is marked
on red. Independent imidazolium ions are labeled with A, B and C.
Fig. 8 Sequence of PTs for ImBr and (1) (C-cooling, H-heating).
This journal is ß The Royal Society of Chemistry 2013
5638 | CrystEngComm, 2013, 15, 5633–5640

View Article Online
CrystEngComm
Paper
+
three molecular-ionic compounds: [C3N2H5 ][I2] (1),
[C3N2H3I2 ][I2] (2) and [C3N2H5+]2[I4
] (3). The crystal
+
22
structure of all the compounds consisted of isolated discrete
imidazolium cations and simple iodide anions, however, in
the case of (3) we have a rarely encountered polyiodide form
[I422]. The characteristic feature of compounds under inves-
…
…
tigation is the presence of N H–I hydrogen bonds and I
I
halogen interactions. As a result a two-dimensional (2D)
framework structure of (2) and (3) complexes is generated,
+
whereas in (1) a 1D structure, as I2–[C3N2H5 ]–I2 chains, is
observed. The structure of (2) appeared to be very rigid since
the I2 anion interacts with four atoms (via N–H I and I I). It
is an exceptional feature because such a layered crystal
structure is not observed in other dihalogenoimidazolium
derivatives.
…
…
Fig. 9 Temperature dependence of the (a) real and (b) imaginary part of the
complex electric permittivity along the [231] direction for (3) during the heating
cycle.
Two complexes were found to undergo reversible PTs; (1) at
ca. 180/185 K and 113/123 K (cooling–heating) and (3) at ca.
204 K and 136 K.
1
The X-ray, thermal, H NMR and dielectric studies used to
113 K. The total entropy transition effect (DStr) for (1)
corresponds to a three-site model describing the cations
disorder in Phase I.
Compared compounds adopt different symmetries at the RT
phase: ImBr has polar space group (R3) while (1) a centrosym-
study the molecular mechanism of the PTs in (1) and (3)
indicated a mixed molecular mechanism (‘o–d’ and ‘d’
contributions). It should be emphasized that (3) is the first
example of an ionic salt among all of the known compounds
(12) containing polyiodide anions, [I422], which undergo
structural PTs. The most unexpected in this case is the fact
¯
metric one (R3). In general, the electric response of both
analogs in the vicinity of I A II PT reveals some similarities
(e.g. the shape of e vs. T), however, in the case of ImBr the
emax(Tc) is nearly three times larger than that in (1). As for ImBr
the polar character of the Phase I results from two contribu-
tions; permanent electric dipole of organic cations (DP9) and
dipoles arising from mutual positions of the centre of gravity
of the C3N2H5+ cations and Br2 anions along the c-axis (DP). In
the case of (1) we deal only with the (DP9) contribution of the
cations which justifies the lower e9 value over the Phase I.
The temperature dependence of real (e9) and imaginary (e99)
parts of the complex electric permittivity for (3) along the [2 3 1]
direction (on heating) is presented in Fig. 9(a) and (b),
respectively. The expected PTs at 136 and 204 K are visible as a
rapid increase in the e9 value. In the low temperature region over
the Phases II and III one can observe a relaxation process which
is characteristic of the glass-like phases, where the value of e99max
increases with the frequency of the measuring electric field.
The activation energy for the temperature region 110–140 K
that the instability of the crystal structure of (3) at 204 K is
22
governed by the distortion of an unstable, symmetric [I4
species.
]
The proton spin–lattice relaxation time (T1) measurements
confirmed that a free in-plane reorientation of the imidazo-
lium ring takes place over the high temperature phase of (1)
and (3) whereas large-amplitude librations or small-angle
reorientations of cations are permitted at low temperatures
(Phases II and III).
Notes and references
1 P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis,
Wiley-VCH, Weinheim, 2003.
2 J. Dupont, R. F. de Souza and P. A. Suarez, Chem. Rev.,
2002, 102, 3667.
3 C. E. Song, Chem. Commun., 2004, 1033.
4 H. Ohno, Electrochemical Aspect of Ionic Liquids, Wiley-
Interscience, 2005.
5 H. Ohno, Electrochemical Aspect of Ionic Liquids, Wiley-
Interscience, in particular Part V and VI, ch. 24–27, 2005.
6 R. Jakubas, A. Piecha, A. Pietraszko and G. Bator, Phys. Rev.
B, 2005, B72, 104107.
was found to be ca. 17 kJ mol21 (see Fig. S4 and S5, ESI ).
3
Concluding, the dielectric relaxation process taking place
below room temperature in (3) is due to a freezing and slowing
down motion of dipolar units assigned to the imidazolium
cations.
The molecular mechanism of PTs in (3) seems also to be
complex and associated with an ordering of the cationic
moieties and a significant deformation of the [I422] dianions
´
7 A. Piecha, A. Białonska and R. Jakubas, J. Phys.: Condens.
Matter, 2008, 20, 325224.
8 A. Piecha, A. Pietraszko, G. Bator and R. Jakubas, J. Solid
State Chem., 2008, 181, 1155.
9 Z. Sun, T. Chen, J. Luo and M. Hong, Angew. Chem., Int. Ed.,
2012, 51, 3871.
Conclusion
´
´
10 Z. Paj˛ak, P. Czarnecki, B. Szafranska, H. Małuszynska and
The reaction between imidazole amine and hydroiodic acid in
an aqueous solution results in simultaneous crystallization of
Z. Fojud, J. Chem. Phys., 2006, 124, 144502.
This journal is ß The Royal Society of Chemistry 2013
CrystEngComm, 2013, 15, 5633–5640 | 5639

View Article Online
Paper
CrystEngComm
´
´
11 Z. Paj˛ak, P. Czarnecki, B. Szafranska, H. Małuszynska and
20 F. Bigoli, F. Demartin, P. Deplano, F. A. Devillanova,
F. Isaia, V. Lippolis, M. L. Mercuri, M. A. Pellinghelli and E.
F. Trogu, Inorg. Chem., 1996, 3683.
21 D. L. Long, H. M. Hu, J. T. Chen and J. S. Huang, Acta
Crystallogr., Sect. C: Cryst. Struct. Commun., 1999, 55, 339.
22 V. V. Tkachev, E. A. Lavrenteva, O. S. Roshchupkina, I.
P. Lavrentev and L. Y. Atovmyan, Koord. Khim., 1994, 20,
674.
Z. Fojud, Phys. Rev. B, 2004, B69, 132102.
12 A. G˛agor, A. Piecha, R. Jakubas and A. Miniewicz, Chem.
Phys. Lett., 2011, 503, 134.
13 Oxford Diffraction, CrysAlis CCD, Data Collection software,
Oxford Diffraction Ltd., Wrocław, Poland, 2005.
14 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found.
Crystallogr., 2007, A64, 112–122.
23 P. B. Hitchcock, D. L. Hughes, G. J. Leigh, J. R. Sanders,
J. Desouza, C. J. McGarry and L. F. Larkworthy, J. Chem.
Soc., Dalton Trans., 1994, 3683.
24 C. A. Ilioudis and J. W. Steed, CrystEngComm, 2004, 6,
239–242.
25 A. M. Owczarzak, M. Kubicki, N. Kourkoumelis and S.
K. Hadjikakou, RSC Adv., 2012, 2, 2856.
26 A. Abate, M. Brischetto, G. Cavallo, M. Lahtinen,
P. Metrangolo, T. Pilati, S. Radice, G. Resnati,
K. Rissanen and G. Terraneo, Chem. Commun., 2010, 46,
2724.
15 G. Laus, K. Wurst, V. Kahlenberg and H. Schottenberger, Z.
Naturforsch., B: J. Chem. Sci., 2012, 67, 354.
16 O. Hassel, Mol. Phys., 1958, 1, 241.
17 M. Muller, M. Albrecht, V. Gossen, T. Peters, A. Hoffmann,
G. Raabe, A. Valkonen and K. Rissanen, Chem.–Eur. J.,
2010, 16, 12446.
18 K. Aizu, J. Phys. Soc. Jpn., 1969, 27, 387.
19 F. H. Herbstein, M. Kapon and W. Schwotzer, Helv. Chim.
Acta, 1983, 66, 35.
5640 | CrystEngComm, 2013, 15, 5633–5640
This journal is ß The Royal Society of Chemistry 2013