Design of crystalline spaces for molecular rotations in crystals

Article abstract of DOI:10.1021/cg4013262

4-Methylanilinium derivatives were used to introduce spaces for molecular rotation in crystals. The [Ni(dmit)₂]^- (dmit^2- = 2-thioxo-1,3-dithiole-4,5-dithiolate) salts with supramolecular cations of dibenzo[18]crown-6 (DB[18]crown-6) and 4-methylanilinium derivatives, (4-methylanilinium⁺)(DB[18]crown-6)[Ni(dmit)₂] ^- (1), (2-fluoro-4-methylanilinium⁺)(DB[18]crown-6) [Ni(dmit)₂]^- (2), and (3-fluoro-4-methylanilinium ⁺)(DB[18]crown-6)[Ni(dmit)₂]^- (3) were synthesized. The potential energy curves for the molecular rotations of the cations in the crystals had double minimum shapes with maxima of 100, 210, and 230 kJ mol^-1 for crystals 1, 2, and 3, respectively. Introduction of a methyl substituent at the p-position was effective in reducing the potential energy maxima. For crystals 2 and 3, large dielectric responses originating from the flip-flop motions of the cationic molecules were observed upon applying an AC voltage. The temperature-dependent magnetic susceptibilities of complexes 1, 2, and 3 followed the Curie-Weiss law, showing weak antiferromagnetic interactions.

Full text of DOI:10.1021/cg4013262

Article
pubs.acs.org/crystal
Design of Crystalline Spaces for Molecular Rotations in Crystals
Zun-qi Liu,† Kazuya Kubo,*,†,‡ Shin-ichiro Noro,†,‡ Tomoyuki Akutagawa,§ and Takayoshi Nakamura*,†,‡
†Graduate School of Environmental Science, Hokkaido University, N10W5, Kita-ku Sapporo, Hokkaido 060-0810, Japan
‡Research Institute for Electronic Science, Hokkaido University, N20W10, Kita-ku Sapporo 001-0020, Japan
§Institute of Multidisciplinaly Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
S
* Supporting Information
ABSTRACT: 4-Methylanilinium derivatives were used to introduce
spaces for molecular rotation in crystals. The [Ni(dmit)2]− (dmit2−
=
2-thioxo-1,3-dithiole-4,5-dithiolate) salts with supramolecular cations
of dibenzo[18]crown-6 (DB[18]crown-6) and 4-methylanilinium
derivatives, (4-methylanilinium+)(DB[18]crown-6)[Ni(dmit)2]− (1),
(2-fluoro-4-methylanilinium+)(DB[18]crown-6)[Ni(dmit)2]− (2), and
(3-fluoro-4-methylanilinium+)(DB[18]crown-6)[Ni(dmit)2]− (3)
were synthesized. The potential energy curves for the molecular
rotations of the cations in the crystals had double minimum shapes
with maxima of 100, 210, and 230 kJ mol−1 for crystals 1, 2, and 3,
respectively. Introduction of a methyl substituent at the p-position was effective in reducing the potential energy maxima. For
crystals 2 and 3, large dielectric responses originating from the flip-flop motions of the cationic molecules were observed upon
applying an AC voltage. The temperature-dependent magnetic susceptibilities of complexes 1, 2, and 3 followed the Curie−
Weiss law, showing weak antiferromagnetic interactions.
approach because of the relatively small hydrogen-bonding
energy compared with the crystalline energy. The latter usually
becomes a maximum in the close-packed structure without void
spaces. Thus, careful design of constituent molecules is
necessary. One of the strategies was to use spherical molecules
instead of planar arylammonium ions. The adamantylammo-
nium cation has a spherical shape with C3 symmetry. Indeed,
the energy barrier for the molecular rotation in the supra-
molecular cation salt of (adamantylammonium+)(DB[18]-
crown-6)[Ni(dmit)2]− was considerably smaller than that of
(anilinium+)(DB[18]crown-6)[Ni(dmit)2]−.8 Another ap-
proach was the use of heteroaromatics without any substituent
at the peri-positions of the ring. The pyridazinium+ cation was
successfully introduced in (pyridazinium+)(DB[18]crown-6)-
[Ni(dmit)2]−,9 in which the energy barrier for the molecular
motion was estimated to be smaller than that for (m-
fluoroanilinium+)(DB[18]crown-6)[Ni(dmit)2]−.10 In the
present study, we applied a new strategy for the reduction of
the energy barrier for the flip-flop motion of the arylanilinium
ion in the solid state. We used 4-methylanilinium+ derivatives as
the molecular rotator. The methyl group introduced at the
para-position of the anilinium+ cation parallel to the rotation
axis expanded the space for the flip-flop motion of the rotator
in the solid state. We synthesized three crystals, (4-
methylanilinium+)(DB[18]crown-6)[Ni(dmit)2]− (1), (2-fluo-
ro-4-methylanilinium+)(DB[18]crown-6)[Ni(dmit)2]− (2), and
INTRODUCTION
■
Solid-state rotators1 have been used to construct functional
molecular materials such as ferroelectrics. A crystal design that
provides sufficient space for molecular rotations in the solid
state is one of the most important points. Khuong et al. have
reported molecular gyroscopes in the solid state based on the p-
diethynylbenzene group as the rotator and triphenylmethyl
(trityl) and triptycyl groups as the stators connected through
covalent bonds.2 The molecular gyroscopes ensured sufficient
spaces around the diethynylbenzene group through molecular
design of the bulky stator moiety. Although the molecular
gyroscope approach guarantees molecular rotation in the solid
state, advanced organic synthesis of the molecules was required.
As an alternative approach, we focused on the supramolecular
approach to developing solid-state rotators.3−6 We have already
reported that a ferroelectric crystal of (m-fluoroanilinium+)-
(DB[18]crown-6)[Ni(dmit)2]− (DB[18]crown-6 =
dibenzo[18]crown-6) exhibited a ferroelectric transition at
348 K, in which the molecular rotator was constructed from an
m-fluoroanilinium rotator and a DB[18]crown-6 stator
connected through hydrogen bonds. The crystal was an
order−disorder-type ferroelectric, whose flip-flop motion of
the m-fluoroanilinium rotator caused an inversion of the dipole
moment in the solid state.7 Since the m-fluoroanilinium rotator
has a C2 rotation axis parallel to the C−N bond, the successive
flip-flop in the same direction (i.e., molecular rotation) may be
possible in the paraelectric phase at higher temperature. The
supramolecular approach simplified the synthesis of the solid-
state rotators. It is difficult to form adequate void spaces for
molecular rotation in the crystal through the supramolecular
Received: September 4, 2013
Revised: November 14, 2013
Published: December 16, 2013
© 2013 American Chemical Society
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Article
Table 1. Crystallographic Data for Salts 1, 2, and 3 at 173 K
1
2
3
chemical formula
formula weight
crystal size (mm3)
crystal system
space group
a (Å)
C
16.5H17N0.5Ni0.5O3S5
C16.5H16F0.5N0.5Ni0.5O3S5
468.45
C16.5H16F0.5N0.5Ni0.5O3S5
468.45
459.96
0.40 × 0.20 × 0.10
monoclinic
C2/c
0.40 × 0.22 × 0.10
monoclinic
C2/c
0.38 × 0.20 × 0.08
monoclinic
C2/c
29.1206(15)
9.0225(4)
20.0291(11)
131.7874(11)
3923.8(3)
8
28.969(2)
9.0811(6)
20.0477(13)
131.4924(14)
3950.5(5)
8
29.2740(11)
9.0709(3)
20.0025(9)
131.9151(9)
3952.5(3)
8
b (Å)
c (Å)
β (°)
V (Å3)
Z
Dcalc (g cm−3
)
1.557
1.575
1.574
F(000)
μ (mm−1
1900
1928
1928
)
1.071
1.069
1.068
measured 2θ range (°)
no. of reflections collected
independent reflections
observed reflections with I > 2.00σ(I)
Rint
6.18−54.88
18457
6.14−54.88
18514
6.16−54.84
18325
4473
4500
4488
4158
2767
3591
0.0251
0.0846
0.0357
a
R [I > 2σ(I)]
0.0329
0.0650
0.0446
b
wR (all data)
0.0790
0.1732
0.1124
GOF
1.159
1.062
1.201
a
b
2
2
2
R = Σ(|Fo| − |Fc|)/Σ|Fo|. Rw2 = Σw(Fo2 − Fc2)2/Σw(Fo )2; w−1 = σ2(Fo ) − (0.0466P)2 − 1.6414P for 1, w−1 = σ2(Fo ) − (0.1398P)2 for 2, w−1
=
σ2(Fo ) − (0.0715P)2 − 17.4945P for 3, where P = (Fo − 2Fc2)/3.
2
2
techniques and refined on F2 by the full-matrix least-squares method
(SHELXL97) compiled into Yadokari-XG.11 The parameters were
refined using anisotropic temperature factors, except for the hydrogen
atoms, which were refined using the riding model with a fixed C−H
bond distance of 0.95 Å. The crystallographic data for salts 1, 2, and 3
at 173 K are summarized in Table 1. The data for 300 K are given in
Table S1 of the Supporting Information. CCDC numbers of the
crystals are 969609−969614.
Magnetic Susceptibility. The temperature-dependent magnetic
susceptibilities of salts 1, 2, and 3 were measured using a Quantum
Design MPMS-XL SQUID magnetometer for the polycrystalline
samples. The DC magnetic susceptibility was measured at temper-
atures from 2 to 300 K in an applied field of 1 T.
Calculations. The relative energies of the structures were
calculated using a semiempirical method with the RHF/6-31(d)
basis set.12 The nearest-neighboring molecules around the 4-
methylanilinium+ derivatives were included in the calculation. The
structural units of the salts 1, 2, and 3 used in the calculations were (4-
methylanilinium+)(DB[18]crown-6)2, (2-fluoro-4-methylanilinium+)-
(DB[18]crown-6)2, and (3-fluoro-4-methylanilinium+)(DB[18]-
crown-6)2, respectively, which are different from the real stoichiome-
tries of the salts (Figure S1 of the Supporting Information). The
atomic coordinates of the salts based on the X-ray crystal structural
analyses at 173 K were used in the calculations. The relative energies
of the structures were obtained for rigid rotations around the C(1)−
N(1) axes for the flip-flop motion (Figure S2 of the Supporting
Information). Rotations were performed in 30° steps.
(3-fluoro-4-methylanilinium+)(DB[18]crown-6)[Ni(dmit)2]−
(3). The effect of the p-methyl substituent is discussed.
EXPERIMENTAL SECTION
■
General. Elemental analyses were carried out using a Yanaco CHN
Corder MT-6a at the Instrumental Analysis Division, Equipped
Management Center, Creative Research Institution, Hokkaido
University. IR (400−7800 cm−1) spectra were measured using a
Thermo Scientific Nicolet 6700 FT-IR. Thermogravimetric analysis
was carried out using a Rigaku Thermoplus TG8120 thermal analysis
station employing an Al2O3 reference, in the temperature range from
298 to 773 K, with a heating rate of 10 K min−1 under flowing nitrogen
gas.
Preparation of (4-Methylanilinium+)(DB[18]-crown-6)[Ni-
(dmit)2]− (1), (2-Fluoro-4-methylanilinium+)(DB[18]crown-
6)2[Ni(dmit)2]− (2), and (3-Fluoro-4-methylanilinium+)(DB[18]-
crown-6)2[Ni(dmit)2]− (3). An acetone solution (20 mL) of (n-
Bu4N+)[Ni(dmit)2]− (20 mg, 0.028 mmol)10 was added to an acetone
7
−
solution (20 mL) of (4-methylanilinium+)(BF4 ) (50 mg, 0.25
mmol) and DB[18]-crown-6 (200 mg, 0.54 mmol). The solvent was
evaporated for one week at room temperature to give crystal 1 as black
blocks. Crystals 2 and 3 were obtained as black blocks using similar
methods by using (n-Bu4N+)[Ni(dmit)2]− (20 mg, 0.028 mmol), (2-
−
fluoro-4-methylanilinium+)(BF4 ) (50 mg, 0.25 mmol) DB[18]-
crown-6 (200 mg, 0.54 mmol), and (n-Bu4N+)[Ni(dmit)2]− (20 mg,
−
0.028 mmol), (2-fluoro-4-methylanilinium+)(BF4 ) (50 mg, 0.25
mmol) DB[18]-crown-6 (200 mg, 0.54 mmol), respectively. The
chemical formulas of the salts were determined by elemental and X-ray
crystallographic analyses. Anal. Calcd C33H34NO6S10Ni for salt 1: C,
43.13; H, 3.62; N, 1.52%. Found: C, 43.17; H, 3.76; N, 1.57%. Anal.
Calcd C33H34FNO6S10Ni for salt 2: C, 42.26; H, 3.55; N, 1.49%.
Found: C, 42.21; H, 3.54; N, 1.30%. Anal. Calcd C33H34FNO6S10Ni
for salt 3: C, 42.26; H, 3.55; N, 1.49%. Found: C, 42.33; H, 3.61; N,
1.32%.
The overlap integrals (s) between the lowest unoccupied molecular
orbitals (LUMOs) of the [Ni(dmit)2]− anions were calculated with the
tight-binding approximation using the extended Huckel method.13
̈
Semiempirical parameters for the Slater-type atomic orbitals were
obtained from the literature.14 Transfer integrals (t) were obtained
from the equation t = −10s.
Dielectric Measurement. The temperature-dependent dielectric
constants were measured using the two-probe AC impedance method
at frequencies of 1, 10, 100, and 1000 kHz using an Agilent 4294A. A
single crystal was placed into a cryogenic refrigerating system
(Cryocooler model SRDK-101, Sumitomo Heavy Industries Ltd.).
Crystal Structure Determination. Crystallographic data for
single crystals of 1, 2, and 3 were collected using a Rigaku R-AXIS
RAPID diffractometer with Mo Kα radiation (λ = 0.71075 Å) from a
graphite monochromator at 173 and 300 K. The structures were
solved by the direct method (SIR 2004) and expanded using Fourier
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Figure 1. Packing diagrams of crystal 1 along the b axis at 173 K. Thermal ellipsoids of the atoms indicate 50% of electron densities. Hydrogen atoms
are omitted for clarity. Gray, carbon; red, oxygen; blue, nitrogen; yellow, sulfur.
The electrical contacts were prepared using gold paste (Tokuriki
8560) to attach the 10 μm ϕ gold wires to the single crystal.
RESULTS AND DISCUSSION
■
Crystal Structure. Crystals 1, 2, and 3 contained half the
anilinium derivatives, half DB[18]crown-6, and half [Ni-
(dmit)2]− molecules in the asymmetric unit, which were
isostructural with each other and with (m-fluoroanilinium+)-
(DB[18]crown-6)[Ni(dmit)2]−.7,8 Here, we will describe the
structure of crystal 1. Figure 1 shows a packing diagram for
crystal 1 along the b axis at 173 K. Anionic and cationic layers
are arranged alternately along the c axis. In the cationic layers,
the 4-methylanilinium+ derivatives and DB[18]crown-6 formed
1:1 adducts of supramolecular cations through the N−H+···O
hydrogen bonds with a stand-up configuration of the C−N
bond of the phenyl groups with respect to the mean plane of
the six oxygen atoms in DB[18]crown-6 (Figure 2). The
distances between the nitrogen atoms of the methylanilinium+
derivatives and oxygen atoms of DB[18]crown-6 of complexes
1, 2, and 3 are summarized in Table 2. The average distances
between the ammonium nitrogen of the cation and the oxygen
atoms of the crown ether were 2.7778, 2.9033, and 2.8647 Å for
crystals 1, 2, and 3, respectively, indicating formation of typical
hydrogen bonding. In the supramolecular cations, the DB[18]-
crown-6 had a V-shaped conformation. The most distinct
difference between crystals 1−3 and the (m-fluoroanilinium+)-
(DB[18]crown-6)[Ni(dmit)2]− crystal was that the ammonium
moiety of the methylanilinium cation was inserted from outside
the V-shaped DB[18]crown-6, whereas the m-fluoroanilinium
cation was included from inside the V-shaped DB[18]crown-6
(Figure 2).7 As a result, the angles between the two benzene
rings were 83.54°, 83.71°, and 86.49° in crystals 1, 2, and 3,
Figure 2. Supramolecular cation structures of (a) (4-
methylanilinium+)(DB[18]crown-6) in 1, (b) (2-fluoro-4-
methylanilinium+)(DB[18]crown-6) in 2, (c) (3-fluoro-4-
methylanilinium+)(DB[18]crown-6) in 3, and (d) (m-
fluoroanilinium+)(DB[18]crown-6) in (m-fluoroanilinium+)(DB[18]-
crown-6)[Ni(dmit)2]− viewed parallel to the mean planes of the
oxygen atoms of the DB[18]crown-6 molecules. Hydrogen atoms are
omitted for clarity.
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the steric hindrance for the flip-flop motion in the solid state.
Noticeable changes for the cell parameter a between crystal 3
[29.2740(11) Å] and the m-fluoroanilinium+ salt [22.9465(10)
Å] were observed.7 In the (m-fluoroanilinium+)(DB[18]crown-
6)[Ni(dmit)2]− crystal, the phenyl groups of nearest-neighbor-
ing DB[18]crown-6 molecules were slightly overlapped
through weak C−H−π interactions along the a axis because
of the V-shaped conformation with the obtuse folding angle
between the phenyl rings of the DB[18]crown-6, causing
restriction of the flip-flop motion of the m-fluoroanilinium+
molecules.7 However, no intermolecular interactions between
two nearest-neighboring DB[18]crown-6 molecules through
the C−H−π interaction of the phenyl rings were observed in
the cationic layer of 3 because of the sharp dihedral angle,
indicating that the effective intermolecular interactions between
the 3-fluoro-4-methylanilinium+ cation and DB[18]crown-6
were reduced. Crystals 1 and 2 showed similar cell parameters
to crystal 3. Orientational disorder of the fluorine atoms with a
large magnitude for the thermal ellipsoids was observed in salts
2 and 3, suggesting flip-flop motions of the arylanilinium+
molecules, as observed in (m-fluoroanilinum+)(DB[18]crown-
6)[Ni(dmit)2]−,7 which further supported the formation of
adequate crystalline void space around the rotator molecules.
Potential Energy of Cation Rotations. The potential
energy curves for the molecular rotations of the methylanili-
nium+ derivatives in salts 1, 2, and 3 were calculated with a
semiempirical method using the RHF/6-31(d) basis set.12 The
model structures for the calculations of 1, 2, and 3 included the
nearest-neighboring molecules of [Ni(dmit)2]− and DB[18]-
crown-6 molecules (Figure S1 of the Supporting Information).
The relative energies as a function of the rotation angle ϕ
around the C−N bonds of the 4-methylanilium+ derivatives
were calculated at every 30° rotation. Figure 4 shows the
rotation angle dependence of relative energies (ΔE) in salts 1,
Table 2. Distances between the Nitrogen and Oxygen Atoms
in Crystals 1, 2, and 3
a
1
2
3
N(1)−O(1)
N(1)−O(2)
N(1)−O(3)
2.7792(17)
2.9124(10)
2.9460(14)
2.791(7)
2.941(4)
2.978(6)
2.775(5)
2.893(3)
2.926(4)
a
Atom numbering schemes of the atoms are summarized in Figure S2
of the Supporting Information.
respectively, which is much smaller than that of the (m-
fluoroanilinium+)(DB[18]crown-6)[Ni(dmit)2]− salt with an
angle of 125.75°.5,7,8 The “anti-V-shape” conformations with
sharp dihedral angles in crystals 1−3 played important roles in
achieving sufficient crystalline void spaces for the flip-flop
motions. The one-dimensional columnar arrangements of the
supramolecular cation were constructed in both of the cationic
layers of crystals 1−3 as well as (m-fluoroanilinium+)(DB[18]-
crown-6)[Ni(dmit)2]− (Figure 3).7,8 The sharp dihedral angles
of 1, 2, and 3 directly affected their cell parameters. For
example, the parameter c of crystal 3, which was the stacking
direction of the cationic and anionic layers, indicated similar
magnitudes for the intermolecular interactions between the
phenyl rings of the methylanilinium+ cation and [Ni(dmit)2]−
anions in (m-fluoroanilinium+)(DB[18]crown-6)[Ni(dmit)2]−
with a similar cell parameter c.7 However, the cell parameters a
and b were clearly different from those of (m-fluoroanilinium+)-
(DB[18]crown-6)[Ni(dmit)2]−. The cell parameter b of crystal
3 [9.0709(3) Å], which was the stacking direction of the
methylanilinium+ cation and DB[18]crown-6, was larger than
that of (m-fluoroanilinium)(DB[18]crown-6)[Ni(dmit)2]−
[8.1007(4) Å]. The methyl group expanded the cell parameter
b and provided sufficient crystalline void space for the phenyl
rings along the b axis, which reduced the energy barriers from
Figure 3. Molecular arrangements of the supramolecular cations in crystals (a) 1, (b) 2, (c) 3, and (d) (m-fluoroanilinium+)(DB[18]crown-
6)[Ni(dmit)2]− viewed along the c axis. Hydrogen atoms are omitted for clarity.
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Figure 4. Potential energy curves for the 4-methylanilinium+
derivatives in the supramolecular cationic structures of (a) 1, (b) 2,
and (c) 3. Solid lines are guides for the eye.
2, and 3, where the relative energies at ϕ = 0° were defined as
zero. For crystals 1−3, two potential minima were observed at
ϕ = 0° and 180°. The double minimum potentials with two
stable molecular orientations indicated 180° flip-flop motion of
the aryl rings in salts 1, 2, and 3, which were the same as those
in (m-fluoroanilinium+)(DB[18]crown-6)[Ni(dmit)2]−.7,8 The
magnitude of maximum ΔE for salt 1 (≈100 kJ mol−1) was
about half that for (anilinium+)(DB[18]crown-6)[Ni(dmit)2]−
(≈180 kJ mol−1),8 indicating that rotation of the 4-
methylanilinium+ cation in 1 was much easier than that in
(anilinium+)(DB[18]crown-6)[Ni(dmit)2]−. A sufficient crys-
talline void space was introduced in the crystal by the
substitution of the methyl group at the p-position of anilinium+.
The maxima of ΔE for salts 2 and 3 (≈200 and ≈230 kJ mol−1,
respectively) became larger than that of 1 by the introduction
of a fluorine atom. However, the values were still smaller than
that of (m-fluoroanilinium+)(DB[18]crown-6)[Ni(dmit)2]−
(≈270 kJ mol−1).7 These results suggested that the
introduction of a methyl group at the p-position was effective
in providing crystalline spaces for the flip-flop motion in the
solid state.
Figure 5. Temperature and frequency dependence of the dielectric
constant ε1 of salts (a) 2 and (b) 3 measured along the a axis. Black,
red, blue, and green points indicate measurement frequencies of 1, 10,
100, and 1000 kHz of the electric field, respectively.
Dielectric Properties. Molecular motions such as the flip-
flop motion in the solid state can be evaluated by measure-
ments of temperature and frequency dependence of the
dielectric constant (ε1), where the change in the dipole
moment responds to the external electric field resulting in large
dielectric constants. In the case of 1, the flip-flop motion of 4-
methylanilinium+ should not cause a change in the dipole
moment. Consequently, no change in dielectric response was
observed in the temperature- and frequency-dependent
dielectric measurement, although adequate crystalline void
space for the flip-flop motion of the phenyl group was achieved
(Figure S5 of the Supporting Information). The rotator
molecules, 2- and 3-fluoro-4-methylanilinium+ cations, have
dipole moments perpendicular to the C−N rotation axis
(Figure S6 of the Supporting Information). Figure 5 shows ε1
versus T plots for crystals 2 and 3 at measurement frequencies
(f) of 1, 10, 100, and 1000 kHz from 10 to 300 K with the
electric field applied parallel to the a axis, which was
perpendicular to the rotation axis of the rotator. Enhanced
dielectric responses (ε1 = 84 and 37 for crystals 2 and 3,
respectively, at 300 K) at 1 kHz were observed, indicating that
the response originated from the molecular rotation.15 Almost
no temperature dependence of the dielectric response along the
b axis was observed because no dipole inversion occurred by
the flip-flop motion.
Figure 6. Molecular arrangement of the [Ni(dmit)2]− anion in crystal
1 viewed along the c axis. Green and red dashed lines indicate
distances d1 and d2 between the sulfur atoms shorter than the sum of
the van der Waals radii. The arrowed line indicates the intermolecular
interaction t1.
the van der Waals radii (<3.7 Å) at d1 and d2 with distances of
3.3646(7) and 3.4911(6) Å, respectively. A relatively strong
intermolecular interaction between the [Ni(dmit)2]− anions, t1,
was observed along a + b and −a + b. The calculated transfer
integral, t , based on the extended Huckel molecular orbital
̈
1
Magnetic Properties. Figure 6 shows the molecular
arrangement of the [Ni(dmit)2]− anions in crystal 1. The
[Ni(dmit)2]− anion formed a two-dimensional layer in the ab
plane through sulfur−sulfur contacts shorter than the sum of
method was 2.62 meV. In crystals 2 and 3, similar two-
dimensional molecular arrangements in the anionic layers were
observed with the sulfur−sulfur contacts in the ranges of
3.366(2)−3.475(2) Å and 3.382(2)−3.485(1) Å, respectively.
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The calculated transfer integrals in 2 and 3 were 1.53 and 2.63
meV, respectively (Figure S4 of the Supporting Information).
One S = 1/2 spin was on each [Ni(dmit)2]− anion in crystals
1, 2, and 3, where the arrangement of the [Ni(dmit)2]− anion
directly affected the magnetic properties. The transfer integrals
ACKNOWLEDGMENTS
■
This work was partly supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan.
calculated using the extended Huckel molecular orbital method
̈
REFERENCES
■
based on the crystallographic analyses of 1, 2, and 3 indicated
formation of two-dimensional layers through the weak
intermolecular interactions (ca. 2 meV), which would lead to
a weak antiferromagnetic interaction between [Ni(dmit)2]−
anions. Plots of χmol versus T for polycrystalline samples of 1,
2, and 3 exhibited typical Curie−Weiss behavior with Weiss
temperatures (θ) of −3.17, −3.25, and −3.09 K, respectively,
indicating weak antiferromagnetic interactions between the
[Ni(dmit)2]− anions in the salts. Thermal fluctuations of the
supramolecular rotators did not affect the magnetic properties
of 1, 2, and 3.
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CONCLUSION
■
Hydrogen-bonding molecular assemblies between 4-methyl-
anilinium+ derivatives and dibenzo[18]crown-6 (DB[18]crown-
6) formed supramolecular rotators, which were introduced into
[Ni(dmit)2]− crystals (dmit2− = 2-thioxo-1,3-dithiole-4,5-
dithiolate) to form crystals 1−3. The supramolecular rotator
(4-methylanilinium+ derivatives)(DB[18]crown-6) arranged in
an isostructural manner with (m-fluoroanilinium+)(DB[18]-
crown-6)[Ni(dmit)2]−. The methyl group on the rotator
molecule expanded the crystalline space and effectively reduced
the potential energy barrier for the 180° flip-flop motion in the
solid state compared with that of (m-fluoroanilinium+)(DB-
[18]crown-6)[Ni(dmit)2]−. The anti-V-shaped conformations
of DB[18]crown-6 with a sharp dihedral angle between the
terminal phenyl rings in 1−3 provided adequate crystalline void
space around the rotator molecules. The molecular flip-flop
motions of 2- or 3-fluoro-4-methylanilinium+ in the solid state
were confirmed by dielectric measurements. Crystals 1−3
exhibited antiferromagnetic behavior following the Curie−
Weiss law because of weak intermolecular interactions in the
two-dimensional molecular arrangement of [Ni(dmit)2]−
anions. One of our next targets is the introduction of molecular
rotators with small potential energy barriers, such as
adamantylammonium+ derivatives, to achieve molecular
rotation in the solid state, which are expected to exhibit a
ferroelectric transition at a lower temperature than that of the
(m-fluoroanilinium+)(DB[18]crown-6)[Ni(dmit)2]− crystal.3,4
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ASSOCIATED CONTENT
■
S
* Supporting Information
Crystal structures, thermogravimetric analyses, infrared spectra,
model structures for the RHF calculation and CIF files of
crystals 1, 2, and 3. This material is available free of charge via
the Internet at http://pubs.acs.org.
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J. Appl. Crystallogr. 2005, 38, 381−388. (b) Sheldrick, G. M. SHELXL-
97 Program of the Refinement of Crystal Structures; University of
AUTHOR INFORMATION
■
Gottingen: Germany, 1997. (c) Kabuto, C.; Akine, S.; Nemoto, T.;
̈
Corresponding Authors
Kwon, E. J. Cryst. Soc. Jpn. 2009, 51, 218−224.
(12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A. Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
*E-mail: kkubo@es.hokudai.ac.jp.
*E-mail: tnaka@es.hokudai.ac.jp.
Notes
The authors declare no competing financial interest.
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