Crown-6 rotator in spin-ladder compound of m-aminoanilinium([18]crown-6) [Ni(dmit)2]-

Article abstract of DOI:10.1039/c0dt00308e

Supramolecular cations of HOPD⁺([18]crown-6) and HMPD ⁺([18]crown-6) were introduced into [Ni(dmit)₂] ^- salts (HOPD⁺: o-aminoanilinium, HMPD⁺: m-aminoanilinium, and dmit^2-: 2-thioxo-1,3-dithiole-4,5-dithiolate). Alternate layers of cations and anions were observed in the two new salts of (HOPD⁺)([18]crown-6)[Ni(dmit)₂]^- (1) and (HMPD⁺)([18]crown-6)[Ni(dmit)₂]^- (2). From X-ray crystal structural analyses, solid state ¹H nuclear magnetic resonance (NMR) spectra, and dielectric constants, the thermal rotations of [18]crown-6 in both of the (HOPD⁺)([18]crown-6) and (HMPD ⁺)([18]crown-6) supramolecules occurred at temperatures above ～200 K based on the orientational disorder in the crystal structures. The two-fold flip-flop motions of HOPD⁺ and HMPD⁺ cations in salts 1 and 2 were suppressed, due to the relatively large potential energy barriers. The [Ni(dmit)₂]^- anion formed π-dimer arrangements in both salts, with a layer structure by lateral sulfur-sulfur interatomic contacts. Although the simple dimer model reproduced the magnetic properties of salt 1, the ladder arrangement of the π-dimer in salt 2 yielded the magnetic behavior of a spin-ladder with a spin-gap of 92.5 K. The temperature dependent magnetic susceptibility of salt 2 was well reproduced by the magnetic anisotropy of J₁/J₂≈ 8 between the ladder-leg (J₁: intra-dimer interaction) and ladder-rung (J ₂: inter-dimer interaction). The [18]crown-6 supramolecular rotator of (HMPD⁺)([18]crown-6) was coexistent with the spin-ladder chain of [Ni(dmit)₂]^- anions in salt 2.

Full text of DOI:10.1039/c0dt00308e

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PAPER
www.rsc.org/dalton | Dalton Transactions
[18]Crown-6 rotator in spin-ladder compound of
m-aminoanilinium([18]crown-6)[Ni(dmit)2]-†
Tomoyuki Akutagawa,*a Daisuke Sato,b Qiong Ye,c Toru Endo,b Shin-ichiro Noro,b,c Sadamu Takedad and
Takayoshi Nakamura*b,c
Received 14th April 2010, Accepted 10th June 2010
First published as an Advance Article on the web 5th August 2010
DOI: 10.1039/c0dt00308e
Supramolecular cations of HOPD+([18]crown-6) and HMPD+([18]crown-6) were introduced into
[Ni(dmit)2]- salts (HOPD+: o-aminoanilinium, HMPD+: m-aminoanilinium, and dmit2-:
2-thioxo-1,3-dithiole-4,5-dithiolate). Alternate layers of cations and anions were observed in the two
new salts of (HOPD+)([18]crown-6)[Ni(dmit)2]- (1) and (HMPD+)([18]crown-6)[Ni(dmit)2]- (2). From
X-ray crystal structural analyses, solid state 1H nuclear magnetic resonance (NMR) spectra, and
dielectric constants, the thermal rotations of [18]crown-6 in both of the (HOPD+)([18]crown-6) and
(HMPD+)([18]crown-6) supramolecules occurred at temperatures above ~200 K based on the
orientational disorder in the crystal structures. The two-fold flip-flop motions of HOPD+ and HMPD+
cations in salts 1 and 2 were suppressed, due to the relatively large potential energy barriers. The
[Ni(dmit)2]- anion formed p-dimer arrangements in both salts, with a layer structure by lateral
sulfur–sulfur interatomic contacts. Although the simple dimer model reproduced the magnetic
properties of salt 1, the ladder arrangement of the p-dimer in salt 2 yielded the magnetic behavior of a
spin-ladder with a spin-gap of 92.5 K. The temperature dependent magnetic susceptibility of salt 2 was
well reproduced by the magnetic anisotropy of J1/J2 ª 8 between the ladder-leg (J1: intra-dimer
interaction) and ladder-rung (J2: inter-dimer interaction). The [18]crown-6 supramolecular rotator of
(HMPD+)([18]crown-6) was coexistent with the spin-ladder chain of [Ni(dmit)2]- anions in salt 2.
by the intermolecular transfer integral (t) and intermolecular
1. Introduction
magnetic exchange energy (J).
Precise control of electronic structures and molecular arrange-
Although the molecular arrangement in the crystals mainly
ments in molecular crystals is essential to realize the desired
determines the physical properties, the dynamic disorder with
electrical conducting and magnetic properties.1,2 For instance,
respect to the molecule arrangement sometimes plays an im-
metallic or superconducting cation radical salts have been suc-
portant role in realizing novel physical properties of molecular
crystals.4–9 For example, the order–disorder transition of counter
anion orientation in the superconducting cation radical salts of
the TTF derivative affect the electrical conducting properties,5
and the order–disorder transition of molecular orientation yields
the ferroelectric–paraelectric transition.6–8 Control of the dynamic
properties of molecules in the crystals has the potential to achieve
unique conducting, magnetic, and dielectric functions.
A large number of dynamic molecular rotators based on rotax-
anes and catenanes have been developed to realize uni-directional
molecular rotators in the solution phase,9 where the intramolecular
motional freedom can be controlled by external stimuli such
as thermal energy, photo-irradiation, and chemical reactions.
Stepwise molecular conformational changes using isomerization
processes are important design strategies to obtain rotaxane- and
catenane-based uni-directional molecular rotators. On the other
hand, attempts to construct solid state molecular rotators have
been achieved by Garcia-Garibay for the molecular gyroscope
compounds,10 where the rotator and stator units are connected
through covalent bonds to maintain the crystalline rotation space.
In contrast with molecular rotators in the solution phase, solid
state rotators should be a key target to realize device functions
and also to clarify the rotation mechanism,10–12 although their
construction is quite difficult, due to their closest-packed crystal
structures. However, the rotation frequency and symmetry have
cessfully obtained by the chemical design of the well-known
tetrathiafulvalene (TTF) derivative.1 The formation of a partially
occupied p-band is important to obtain highly electrical conduct-
ing molecular crystals. On the other hand, the monovalent cation
radical salts of the TTF+ derivative can generate one S = 1/2
spin on the molecule, forming a variety of magnetic crystals.2,3
The conducting and magnetic properties in these crystals are
dominated by the molecular arrangement, which is represented
aPolymer Hybrid Materials Research Center, Institute of Multidisciplinary
Research for Advanced Materials (IMRAS), Tohoku University, 1-1
Katahira, Aobaku, Sendai, 980-8577, Japan. E-mail: akuta@tagen.tohoku.
ac.jp; Fax: +81 22 217 5655; Tel: +81 22 217 5653
bResearch Institute for Electronic Science, Hokkaido University, Sapporo,
001-0020, Japan. E-mail: tnaka@es.hokudai.ac.jp; Fax: +81 11 706 9420;
Tel: +81 11 706 9417
cGraduate School of Environmental Science, Hokkaido University, Sapporo,
001-0020, Japan
dFaculty of Science and Graduate School of Science, Hokkaido University,
Sapporo, 060-0810, Japan
† Electronic supplementary information (ESI) available: The atomic
numbering scheme, structural analysis of salts 1 and 2, calculated
1
structural units, H NMR spectra of salt 2, vibrational spectra, nearest-
neighboring packing of the rotators in salts 1 and 2, ESR spectra at
room temperature, and DSC diagram. CCDC reference numbers 773234–
773237. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0dt00308e
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been recently designed in the modification of rotator or stator
structures, even in the solid state.
We have been constructing supramolecular rotators of
the cation-crown ether molecular assemblies in the magnetic
[Ni(dmit)2]- salts (dmit2-: 2-thioxo-1,3-dithiole-4,5-dithiolate).13
pletely suppressed in the solid state, thermally induced rotation
of [18]crown-6 was realized in salts 1 and 2. Furthermore, salt
2 yielded a spin-ladder magnetic arrangement of the [Ni(dmit)2]-
anion, forming a new hybrid system between the molecular rotator
and the spin-ladder magnetic property.
Recently,
a ferroelectric–paraelectric transition was found
using the molecular rotator of the (m-fluoroanilinium)-
(dibenzo[18]crown-6) supramolecule in [Ni(dmit)2]- salt, which
realized a two-fold flip-flop motion of m-fluoroanilinium (m-
FAni+) and a dipole inversion environment in the crystal.14 In
this salt, the molecular motion and dipole orientation of the
m-FAni+ cation were controllable by an external electric field.
When the molecular rotations can be combined with the electrical
conducting, magnetic, and dielectric properties, functional solid
state molecular rotator devices are expected.
Since supramolecular cation structures have structural diversity
through combinations between cations (alkali metal, alkali
earth metals, arylammonium etc.) and crown ethers, these
cationic supramolecular assemblies are effective for the control
of the [Ni(dmit)2]- anion arrangements and magnetic properties
in the crystals. For instance, K+([18]crown-6), (meso-1,2-
diphenylethylenediammonium)([18]crown-6)2, (anilinium)([18]-
2. Results and discussion
2.1 Cation conformations
Cation exchange reactions from (n-Bu4N+)[Ni(dmit)2]- to HOPD+
and HMPD+ in the presence of [18]crown-6 yielded monovalent
[Ni(dmit)2]- salts of (HOPD+)([18]crown-6)[Ni(dmit)2]- (1) and
(HMPD+)([18]crown-6)[Ni(dmit)2]- (2). In both salts, the 1 : 1
adducts of supramolecular cations between the organic cation and
[18]crown-6 were constructed by six N–H+ ◊ ◊ ◊ O hydrogen-bonding
interactions, forming a stand-up configuration of the C–N bond
of the anilinium unit with respect to the mean oxygen plane of
[18]crown-6 (Fig. 1). The angles between the C–N bond and the
mean oxygen plane of [18]crown-6 in salts 1 and 2 were 84.5 and
88.0◦, respectively. The steric repulsion between the o-amino group
of HOPD+ and [18]crown-6 induced slight deviation from the right
angle of the C–N bond with respect to the mean oxygen plane of
[18]crown-6.
crown-6) resulted in
a one-dimensional antiferromagnetic
Heisenberg chain, two-dimensional antiferromagnetic Heisenberg
chain, and spin-ladder chain, respectively.13 Among this
structural diversity, specific cationic assemblies between organic
ammonium-crown ethers have yielded molecular rotators such as
(anilinium)([18]crown-6), (adamantylammonium)(crown ethers),
and (m-fluoroanilinium)(dibenzo[18]crown-6) in [Ni(dmit)2]-
salts.15 The crystallization of these functional units occurred
in a one-pot reaction by a self-assembly process through
selective cation recognition by the crown ethers, and because
the magnetically active [Ni(dmit)2]- anions can coexist with
the supramolecular cations. The design of the anilinium cation
and crown ether was essential to yield the ferroelectric crystals
and rotational freedom in [Ni(dmit)2]- salts. In the ferroelectric
(m-FAni+)(dibenzo[18]crown-6)[Ni(dmit)2]- salt,14 the m-FAni+
and dibenzo[18]crown-6 act as rotator and stator, respectively.
When the bulky dibenzo[18]crown-6 is replaced with [18]crown-6,
the rotation of [18]crown-6 could occur with the two-fold flip-flop
motion of the anilinium cation, forming a dual rotation of
anilinium and [18]crown-6 with different rotation symmetry and
frequency.
Here, we report new supramolecular cations of o-
aminoanilinium (HOPD+)([18]crown-6) and m-aminoanilinium
(HMPD+)([18]crown-6) in [Ni(dmit)2]- salts. Two single crystals of
(HOPD+)([18]crown-6)[Ni(dmit)2]- (1) and (HMPD+)([18]crown-
6)[Ni(dmit)2]- (2) were prepared (Scheme 1), where the [18]crown-6
and HOPD+ (and HMPD+) cation acted as the rotator and stator,
respectively. Although the rotation of both the cations was com-
Fig. 1 Supramolecular cation structures of a) (HOPD+)([18]crown-6) in
salt 1 and b) (HMPD+)([18]crown-6) in salt 2, viewed normal to the p-plane
of the aminophenyl ring. c) Orientational disorder of [18]crown-6 in salt 1
at temperatures above 300 K. Two [18]crown-6 orientations (I and II) were
overlapped with each other (right). d) Structure of [18]crown-6 in salts 1
and 2 at 150 K.
Fig. 1a and
b
show the (HOPD+)([18]crown-6) and
(HMPD+)([18]crown-6) structures viewed along the mean oxy-
gen plane of [18]crown-6. Each (HOPD+)([18]crown6) and
Scheme
1
Molecular structures of o-aminoanilinium (HOPD+),
m-aminoanilinium (HMPD+), [18]crown-6, and [Ni(dmit)2]-.
8220 | Dalton Trans., 2010, 39, 8219–8227
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(HMPD+)([18]crown-6) structural unit was crystallographically
independent in salts 1 and 2. Average hydrogen-bonding N ◊ ◊ ◊ O
distances between the ammonium nitrogen and oxygen atoms
Table 1 Transfer integrals (t) and magnetic parameters of salts 1 and 2
1
2
˚
of salts 1 and 2 were 2.97 and 2.94 A, respectively, which were
T/K
300
150
300
-52.6
18.3
5.17
3.62
Two-leg ladder
2.0402
12.24
150
comparable to the standard N–H+ ◊ ◊ ◊ O hydrogen-bond length.16
In salt 1, a weak N–H ◊ ◊ ◊ O hydrogen-bonding interaction between
the o-amino group of the HOPD+ cation and the oxygen atom
of [18]crown-6 was observed in the N–O distance of 3.013(3) A,
whereas interatomic contacts within the (HMPD+)([18]crown-
t1/meV a
t2/meV a
t3/meV a
89.7
-18.9
-8.87
3.69
92.7
-20.0
-0.23
3.63
-64.3
20.0
5.38
3.59
˚
d
dimer/A
˚
[Ni(dmit)2]- arrangement Dimer
g (298 K) b
2.0522
15.9
0.398
-82.6
Dn (298 K)/mT b
C/emu K mol-1
q or J/kB/K
6) unit were not observed in salt 2.
0.371
Orientational disorder of the o- and m-amino groups in the
HOPD+ and HMPD+ cations was not observed by structural
analyses at 300 K, which suggests that the two-fold flip-flop
motions of o-aminophenyl and m-aminophenyl groups were
suppressed in the crystals, which was consistent with the potential
energy calculations. On the other hand, orientational disorder
of [18]crown-6 was observed in the crystal structural analysis
of salt 1 at temperatures above 300 K. The second [18]crown-
6 orientation-II began to appear at 300 K (Fig. 1d), in which
the twelve carbon atoms were shared by the first orientation-I
and the second orientation-II. The oxygen atoms of orientation-II
appeared at the 30◦ rotated phase of the first orientation-I. The
occupancy factor of orientation-II at 300 K was approximately
0.1, which suggests that orientation-I was a major component
at 300 K with respect to orientation-II. The occupancy factor
of orientation-II increased with increase in the temperature from
0.90 : 0.10 at 300 K, 0.80 : 0.20 at 330 K, to 0.75 : 0.25 at 360 K.
Thermally activated rotation of [18]crown-6 in salt 1 resem-
J1 = -102
J2 = -10.2
S-L
Magnetismc
S-T
a The transfer integrals (t) were obtained from the LUMO of [Ni(dmit)2]-
based on the extended Hu¨ckel calculation (t = -10S eV, where S is
the overlap integral) at 150 K. b The g-value (g) and line-width (Dn) of
the [Ni(dmit)2]- anion were determined from the electron spin resonance
spectra for single crystals at 298 K. c S-T and S-L are the singlet–triplet
thermal excitation and spin-ladder models, respectively.
bled that in Cs+ ([18]crown-6)3[Ni(dmit)2]2 salt (3),15a which also
2
exhibited two [18]crown-6 orientations at 300 K. Although the
orientational disorder of [18]crown-6 was not observed in salt
2, the [18]crown-6 rotation was confirmed from temperature
dependent solid state 1H NMR measurements (see Solid state 1H
NMR). The average thermal parameters of the six oxygen atoms
of [18]crown-6 at 300 K (Beq = 6.16) in salt 2 were two times
larger than those at 150 K (Beq = 3.31), which suggests the
thermal motions of [18]crown-6 around 300 K. The differences in
disorder from the X-ray crystal structural analyses corresponded
to the different shapes of the hindering potential for rotation of
[18]crown-6 in salts 1 and 2.
Fig. 2 Crystal structures of salt 1. a) Unit cell viewed along the a-axis.
b) (HOPD+)([18]crown-6) cation arrangement within the ab-plane viewed
along the c-axis. c) [Ni(dmit)2]- anion arrangement within the ab-plane.
Three types of intermolecular interactions (t1–t3) were observed.
dimers of [18]crown-6 molecules. Around the o-aminophenyl
ring, two nearest-neighboring [18]crown-6 molecules presented
steric hindrance, which restricted the two-fold flip-flop motion
of the o-aminophenyl group within HOPD+ cation. Fig. 2c
shows the [Ni(dmit)2]- anion arrangements within the ab-plane.
2.2 Packing structures of cations and [Ni(dmit)2]- anions
Each [Ni(dmit)2]- anion had one S = 1/2 spin, and the arrange-
ment of [Ni(dmit)2]- anions directly determined the magnetism.
Transfer integrals based on extended Hu¨ckel molecular orbital
calculations were used to evaluate the magnitudeof the intermolec-
ular interactions between [Ni(dmit)2]- anions within the crystals.17
The magnetic exchange energy (J) is proportional to the square of
the transfer integral, J ª 4t2/Ueff, where Ueff is the effective on-site
Coulomb repulsive energy in the solid.18 Table 1 summarizes the
transfer integrals (T = 300 and 150 K) and magnetic parameters
of salts 1 and 2.
Alternate layer structures of cations and anions were observed
in salts 1 and 2. Fig. 2a shows the unit cell of salt 1 viewed along
the a-axis. The [Ni(dmit)2]- anionic and (HOPD+)([18]crown-6)
cationic layers within the ab-plane were elongated along the c-axis.
Each (HOPD+)([18]crown-6) cation was arranged at antiparallel
configurations along the b-axis (Fig. 2b), forming face-to-face
Three types of intermolecular interactions of t1 = 92.7, t2
=
-20.0, and t3 = -0.23 meV (T = 150 K) were observed with
magnitudes of t1 > t2 ꢀ t3 (Table 1). Among these, the p-stacking
interaction between the [Ni(dmit)2]- anions (t1) was the most
effective interaction to determine the magnetic exchange energy
(see Magnetic susceptibility).
The cation and anion packing in salt 2 was different from that in
salt 1. Fig. 3a shows the unit cell of salt 2 viewed along the a-axis,
where the alternate cationic and anionic layers were elongated
along the b-axis. The (HMPD+)([18]crown-6) supramolecular
cations are stacked along the a-axis, which is the same cation ar-
rangement of the spin-ladder compound of (anilinium)([18]crown-
6)[Ni(dmit)2]- (4).13i Although the two-fold rotation of anilinium in
salt 4 has been confirmed, the orientation of the HMPD+ cation in
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Fig. 4 Temperature-dependent solid state wide-line 1H-NMR spectra
of salts 1 and 2. a) Spectral changes of salt 1 by decreasing the
temperature from i) 350, ii) 295, iii) 250, iv) 230, v) 190, to vi) 130 K.
b) Temperature-dependent maximum slope width (Dn) of salts 1 (᭺) and
2 (᭹).
Fig. 3 Crystal structures of salt 2. a) Unit cell viewed along the a-axis.
b) (HMPD+)([18]crown-6) cation arrangement within the ac-plane viewed
along the b-axis. c) [Ni(dmit)2]- anion arrangement within the ac-plane.
Three types of intermolecular interactions (t1–t3) were observed.
lowering of the temperatures from 350 K (spectrum i) to 130 K
(spectrum vi). The (HOPD+)([18]crown-6) cation in salt 1 has
thirty-three protons in total (24 aliphatic protons of [18]crown-
6, 4 aromatic protons of HOPD+, and 5 amino and ammonium
protons of HOPD+). Since the number of [18]crown-6 protons is
approximately three times greater than those of the others, the
changes in the Dn values are mainly governed by the motion
of the [18]crown-6 molecule of salt 1. The Dn narrowing at
high temperatures corresponds to the thermally induced motional
(rotational) narrowing arising from [18]crown-6. The Dn of 16
kHz at 350 K was about three times smaller than that of 53 kHz at
130 K. Although the orientational disorder of [18]crown-6 was not
observed in the X-ray crystal structural analysis of salt 2, almost
the same temperature dependent Dn changes were observed in the
1H NMR spectra (see Fig. S4†).
Fig. 4b shows the temperature dependent Dn of salts 1 and 2.
A rapid decrease in the Dn values from ca. 55 to ca. 15 kHz at
around 230 K was observed in salt 1, whereas that from ca. 55 to
ca. 12 kHz occurred at almost the same temperature in salt 2. The
Dn–T plots of salts 1 and 2 indicate the rotational frequency of
the [18]crown-6 molecules increased up to a few tens of kHz at ca.
200 K, although the orientational disorder of [18]crown-6 was not
observed in salt 2. The difference in the X-ray crystal structures of
salts 1 and 2 at 300 K indicates the difference in shape hindering
potential for rotation.
salt 2 was fixed without the orientational disorder of the m-amino
group (Fig. 3b). In addition, the dual rotation of anilinium and
[18]crown-6 in salt 4 was confirmed in the (anilinium)([18]crown-
6) cation arrangement in the ac-plane. However, the large steric
hindrance between the m-amino group of the HMPD+ cation
and two nearest-neighboring [18]crown-6 molecules in the ac-
plane completely suppressed the two-fold flip-flop motion of the
HMPD+ cation. Therefore, the pinwheel rotation of [18]crown-6
in the fixed HMPD+ cation and [Ni(dmit)2]- anion configuration
occurred in the crystal.
The [Ni(dmit)2]- anion arrangement in salt 2 is the same as
that of salt 4, resulting in a two-leg spin ladder arrangement of
[Ni(dmit)2]- anions. The [Ni(dmit)2]- p-dimer with the intermolec-
ular interaction of t1 = -52.6 meV (T = 300 K) is connected
through the lateral sulfur–sulfur interaction of t2 = 18.3 meV
along the long-axis of the anions. Two equivalent t2-interactions
connect each [Ni(dmit)2]- p-dimer along the a+c-axis, forming the
two-leg ladder [Ni(dmit)2]- anion arrangement. The directions of
the t1- and t2-interactions correspond to the ladder-leg and ladder-
rung interactions, respectively. The small magnitude of the inter-
ladder t3-interaction (5.17 meV) along the a-axis can be ignored.
When the t1-interaction is much larger than the t2-interaction,
the magnetic behavior should be governed by the simple dimer
model. In contrast, the comparable magnitudes of the t1- and t2-
interactions manifest in the spin-ladder magnetic behavior. The
t1 : t2 ratio of approximately 3 : 1 at 300 K formed the magnetic
interactions between the ladder-leg direction (J1) and the ladder-
rung (J2) of the J1 : J2ª9:1 at 300 K. Spin-ladder behavior has been
reported in the J1 : J2 ª 8 : 1 of salt 4;13i therefore, similar magnetic
behavior was expected in salt 2 (see Magnetic susceptibility).
2.4 Potential energy of phenyl rotation within a cation
The rotation modes of supramolecular cations in the solid state
were determined from the potential energy curve and the magni-
tude of the potential energy barrier (DE). The potential energy
curves for the molecular rotation of aminophenyl groups within
cations in salts 1 and 2 were calculated using the RHF/6-31(d)
basis set.19 Since the energy changes for the rotation of NH3+ group
with the fixed configuration of o-aminophenyl and m-aminophenyl
groups was quite significantly small in magnitude (<5 kJ mol-1),
the rigid rotations of o-aminophenyl and m-aminophenyl groups
around the C–N bond with the fixed configuration of –NH3
group were evaluated. Besides the supramolecular cation, the
nearest-neighboring molecules of [18]crown-6 or [Ni(dmit)2]- were
included in the calculations (Fig. 5a and b).
2.3 Solid state 1H NMR
On the basis of the temperature dependent crystal structures, the
orientational disorder of [18]crown-6 in salt 1 occurred at 300 K
due to the appearance of the second [18]crown-6 orientation. The
1
temperature-dependent solid state wide-line H NMR spectra of
salts 1 and 2 were evaluated to provide evidence of the thermally
induced [18]crown-6 rotations.
Fig. 4a shows the temperature-dependent spectral changes
of salt 1. The line-width (Dn) shows a clear broadening by
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the rotators will decrease the potential energy barrier in contrast
with the rigid rotations, the flip-flop motions of o-aminophenyl
and m-aminophenyl gorups should be restricted due to the quite
large potential energy barriers.
The 1H NMR measurements of salts 1 and 2 suggest [18]crown-6
rotations at temperatures above 200 K. In the temperature depen-
dent crystal structural analysis of salt 1, the second [18]crown-
6 orientation-II appeared at temperatures above 300 K, where
the occupancy factors of orientation-I and -II were 0.9 and 0.1,
respectively. When the rotation environments of orientation-I and
-II are same as each other, almost equivalent occupancy factors
are expected in the crystal structural analysis at 300 K. An
idealized C3v molecular symmetry of [18]crown-6 would result
in alternate oxygen atoms at the upper and lower positions to
the mean oxygen plane of [18]crown-6, so that equivalent oxygen
sites in the C3v symmetry would appear at every 120◦ rotation
of [18]crown-6.20 The idealized C3v symmetry of the [18]crown-6
in salt 2 corresponds to a three-fold rotation symmetry, whereas
the appearance of orientation-II in salt 1 suggested a twelve-fold
rotation symmetry (Fig. 1).
Fig. 5 Potential energy curves for the two-fold flip-flop motion of
o-aminophenyl and m-aminophenyl group in salts 1 and 2, respectively.
Calculated structures of a) (HOPD+)([18]crown-6)3 in salt 1 and b)
(HMPD+)([18]crown-6)3[Ni(dmit)2]2 in salt 2. c) Potential energy (DE) vs.
rotation angle f1 plots for salts 1 (red) and 2 (blue) around the C–N bond.
The blue and red lines show line fitting to the data.
The structures of (HOPD+)([18]crown-6)3 and (HMPD+)-
([18]crown-6)3[Ni(dmit)2]2 were firstly evaluated for calculations
of the two-fold flip-flop motions (Fig. 5a and b). In the rotational
angle (f1) dependence of the relative energy (DE) in salts 1 and
2, the relative energy at f1 = 0◦ corresponds to the atomic
coordinate from the X-ray crystal structural analysis and is defined
as zero (Fig. 5c). In both salts, two nearest-neighboring [18]crown-
6 molecules largely affected and restricted the two-fold flip-flop
motion. The steric hindrance of the o-aminophenyl ring from the
two nearest-neighboring [18]crown-6 molecules in salt 1 was larger
than that in salt 2.
The intermolecular interactions between the cation and
[18]crown-6 in salts 1 and 2 were different. The hydrogen-bonding
interaction between the o-amino group of the HOPD+ cation and
oxygen atoms of [18]crown-6 was observed by a N–O distance
+
˚
of 3.013(3) A in the (HOPD )([18]crown-6) of salt 1 (Fig. S7†),
whereas the hydrogen-bonding interaction between the m-amino
group of the HMPD+ cation and [18]crown-6 was observed at an
˚
N–O distance of 3.0162(16) A along the stacking direction of the
(HMPD+)([18]crown-6) cations (Fig. S7†). From this perspective,
the rotation environments of [18]crown-6 in salts 1 and 2 were
different.
Aside from the first potential energy minimum at f1 = 0◦, the
second potential energy minimum was observed around f1 = 180◦.
The C–N bond was inclined approximately 5◦ with respect to the
mean oxygen plane of [18]crown-6, so that an asymmetric double-
minimum potential energy curve with energy barriers of 800 and
1700 kJ mol-1 was observed for salt 1. Assuming the relaxation
process of the HOPD+ cationic arrangement for each f1 angle,
the average potential energy barrier of ca. 1200 kJ mol-1 would
represent the energy barrier for the two-fold flip-flop motion of
the o-aminophenyl group in salt 1. The large energy barrier for
salt 1 completely suppressed the two-fold flip-flop motion, which
is consistent with the X-ray crystal structural analysis.
2.5 Dielectric properties
Temperature-dependent dielectric constants (e1) of salts 1 and 2
were examined to evaluate the molecular motions of supramolecu-
lar cations.21 Fig. 6a and b show plots of the frequency dependence
e1–T for single crystals 1 and 2 measured along the c- and b+c-
axes, the directions of which are normal to the molecular axis
of the HOPD+ and HMPD+ cations. From the crystal structural
analyses and 1H NMR measurements, the thermally activated
rotations of [18]crown-6 or the thermal fluctuations of the HOPD+
and HMPD+ cations along the C–N axis could contribute to
The potential energy curve for the two-fold flip-flop motion
of the m-aminophenyl group in salt 2 is affected by the two
nearest-neighboring [18]crown-6 and two [Ni(dmit)2]- anions
(Fig. 5b). The p-plane of HMPD+ cations in salt 2 are arranged
at an angle of 45◦ with respect to the c-axis, so that the two
[Ni(dmit)2]- anions affect the potential energy profile. The first
and second energy minima were observed at f1 = 0◦ and 180◦
with a potential energy barrier of ca. 500 kJ mol-1, whereas
the third energy minimum was observed at f1 = 270◦ with an
energy barrier of 400 kJ mol-1. The interaction between the
HMPD+ cation and the two nearest-neighboring [Ni(dmit)2]-
anions yielded an asymmetric triple-minimum potential energy
curve. However, the two-fold flip-flop motion of the m-aminphenyl
group was suppressed due to a relatively large energy barrier of ca.
500 kJ mol-1, which was one order of magnitude larger than that
of salt 4 (40 kJ mol-1). Although the structural relaxation around
Fig. 6 Temperature- and frequency-dependent dielectric constants (e1)
of salts a) 1 and b) 2. The dielectric constants of salts 1 and 2 were
measured along the c- and b+c-axes, respectively, the directions of which
were perpendicular to the rotation axis of the HOPD+ and HMPD+ cations,
respectively.
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the temperature- and frequency-dependent dielectric behavior.
Because the rotation of [18]crown-6 resulted in a very small
magnitude dipole moment change, the dielectric response should
also be of small magnitude.
In both salts, the frequency- and temperature-dependent e1
were observed at temperatures above ca. 200 K at low-frequency
(ca. 1 kHz). Relatively slow molecular motion in the crystals
can be assigned to the thermal rotation of [18]crown-6 and/or
thermal fluctuations of the HOPD+ or HMPD+ cations. 1H NMR
measurements indicate that the frequency of rotation of the
[18]crown-6 in salts 1 and 2 increased up to a few tens of kHz
at ca. 200 K, which is almost consistent with the appearance
of the frequency-dependent e1 responses at temperatures above
200 K. Broad frequency-dependent e1 maxima of salts 1 and 2 were
observed at around 300 and 270 K, respectively, with increasing
temperature.
Fig. 7 Temperature dependent magnetic susceptibility of a) salt 1 and
b) salt 2. The blue and red lines represent the line fits obtained using
the singlet–triplet thermal excitation model and the spin-ladder model,
respectively.
with the large intra-dimer t1 interaction (ca. 90 meV) in the transfer
integral.
When the frequency of the molecular motions coincides with
the measurement frequency, a dielectric peak should appear in
the e1–T plots. The typical rotation frequency of [18]crown-6 in
Although the overall cmol–T behavior of salt 2 almost resembles
that of salt 1, the magnetic interaction between [Ni(dmit)2]- anions
in salt 2 does not follow the simple dimer model (blue line
in Fig. 7b). In the crystal structure, the formation of a spin-
ladder arrangement of [Ni(dmit)2]- anions was confirmed by the
magnitude of the ladder-leg (t1 = -52.6 meV) and ladder-rung
interactions (t2 = 18.3 meV). The almost zero magnitude cmol
value at 2 K was rapidly increased with increasing temperature,
and exhibited a broad maximum at ca. 50 K. The spin-ladder
model reproduces the magnetic behavior of the isomorphous
(anilinium)([18]crown-6)[Ni(dmit)2]- salt (4) well; therefore, we
applied the same spin-ladder model to the cmol–T behavior of
salt 2 (red line in Fig. 7b). Firstly, a low temperature limit eqn (1)
was applied to the spin-ladder model,23
the Cs+ ([18]crown-6)3[Ni(dmit)2]- 15a and (anilinium+)([18]crown-
2
6)[Ni(dmit)2]- 15d salts around 230 K was estimated to be ~10 kHz
from the solid state 1H NMR spectra. Since the weak e1 maxima
around ca. 300 K in salts 1 and 2 were inconsistent with the rotation
frequency of ~10 kHz at 230 K, the thermal fluctuation of HOPD+
and HMPD+ cations contributed to the dielectric responses. On
the other hand, relatively large e1 responses at the temperatures
above 300 K were observed at the low-frequency measurements.
Such a large e1 response was not observed in the molecular motion
of the nonpolar molecule. Therefore, the thermal motion of the
polar molecule contributes to the large dielectric responses at high
temperature.
The two-fold flip-flop motions of the o-aminophenyl and m-
aminophenyl groups were suppressed from the large potential
energy barriers. The thermal fluctuation of the polar cations,
i.e., the pendulum motion of the HOPD+ and HMPD+ cations,
resulted in the large e1 responses. The thermal molecular motion
of supramolecular cations should be frozen at temperatures below
200 K, which results in the temperature independent e1 behavior.
The e1 values of salts 1 and 2 at temperatures below 200 K
(e1 ca. 1500) are dominated by the polarized p-electron on the
[Ni(dmit)2]- anion. The anisotropic p-electron distribution on the
[Ni(dmit)2]- anions causes the anisotropic dielectric behavior (Fig.
S8–S12†), where large e1 values are observed in the measurement
along the long-axis of the [Ni(dmit)2]- anion.
cmol = aT-1/2exp(-D/T)
(1)
where a is the constant used to determine the spin-gap (D). The
low temperature cmol–T behavior of salt 2 was well reproduced by
eqn (1) using D = 92.5 K (low temperature red line in Fig. 7b). The
magnetic exchange energies along the ladder-leg (J1) and ladder-
rung (J2) directions could be estimated from the D value, assuming
a ratio of the squares of the transfer integrals (J ª t2). A ratio of
t1 /t2 ª 10 (T = 150 K) was applied for the J1/J2 ratio, which
resulted in J1 = 102 K and J2 = 10.2 K based on D = 92.5 K using
eqn (2),
2
2
D = |J1| - |J2| + 1/2(|J2|2/|J1|)
(2)
2.6 Magnetic properties
On the other hand, the high temperature cmol–T behavior was
reproduced by the high-temperature limit eqn (3),
The temperature dependent molar magnetic susceptibility (cmol
)
per [Ni(dmit)2]- anion was directly affected by the anion arrange-
ments in the crystals. Fig. 7a shows a plot of cmol vs. T for salt 1.
The strong p-dimerization of [Ni(dmit)2]- anions in salt 1 results
in the singlet–triplet thermal excitation behavior of the two S =
cmol = C[T-1 - 1/2(|J2| + 1/2|J2|)T-2 + 3/16|J1J2|T-3] (3)
The cmol–T behavior at high temperature was well fitted using
J1 = 92.5 and J2 = 13.6 K (Fig. 7b) with C = 0.371 emu K
mol-1. The J1/J2 ª 7 from the high temperature limit eqn (3) was
almost consistent with J1/J2 ª 8.3 from the squares of the transfer
integrals at 300 K. The magnetic behavior of salt 2 followed
the spin-ladder model, where the three-fold [18]crown-6 rotation
coexists with the spin-ladder arrangements of [Ni(dmit)2]- anions.
1/2 spins.22
cmol has a broad maximum at around 100 K that tends
toward zero at lower temperatures. The behavior corresponds well
to the singlet–triplet dimer model (blue line in Fig. 7a) with a Curie
constant (C) of 0.398 emu K mol-1 and a magnetic exchange energy
(J) of -82.6 K. The singlet spin state between the p-dimer is the
essential intermolecular interaction in salt 1, which is consistent
8224 | Dalton Trans., 2010, 39, 8219–8227
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3. Conclusions
4. Experimental
Hydrogen-bonding assemblies between HOPD+ and/or
HMPD+ and [18]crown-6 formed supramolecular cations
of (HOPD+)([18]crown-6) and (HMPD+)([18]crown-6) in
[Ni(dmit)2]- salts. Alternate layers of cations and anions were
observed in the (HOPD+)([18]crown-6)[Ni(dmit)2]- (1) and
(HMPD+)([18]crown-6)[Ni(dmit)2]- (2) salts. The large potential
energy barriers for the two-fold flip-flop motion along the C–
4.1 Synthesis of (o-aminoanilinium+)(BF4 ) and
-
(m-aminoanilinium+)(BF4 )
-
42% HBF4 aq. solution (9.8 mmol) was slowly added dropwise
into a solution of o-aminoaniline or m-aminoaniline (10 mmol) in
CH3CN (30 mL) over a period of 10 min. The solvent was removed
under reduced pressure and a white powder was recrystallized from
CH2Cl2–hexane. Elemental analyses. (HOPD+)(BF4 ); Calcd. for
C6H9N2BF4; C: 36.78, H: 4.63, N: 14.30. Found. C: 36.03, H: 4.87,
N: 13.93. (HMPD+)(BF4 ); Calcd. for C6H9N2BF4; C: 36.78, H:
-
NH3 bond of the HOPD+ and HMPD+ cations fixed the cation
+
orientation in the crystals. On the other hand, thermally induced
[18]crown-6 rotations were confirmed in both salts at temperatures
above 200 K, as evidenced from temperature dependent solid
state 1H NMR and dielectric constant measurements. The
thermally induced [18]crown-6 rotation was observed in the
1H NMR measurements at ca. 200 K with the frequency
dependent dielectric response (T > 200 K), whereas the thermal
fluctuations of the HOPD+ and HMPD+ cations contributed to
the large dielectric responses at temperatures above 300 K. The
magnetic properties of salt 1 were dominated by the strong
p-dimerization of [Ni(dmit)2]- anions, which followed the
singlet–triplet dimer model with the magnetic exchange energy
of J = -82.6 K. The [Ni(dmit)2]- p-dimers were connected as
two-leg ladder arrangements, where the ratio of the ladder-leg
and ladder-rung interactions was approximately 8 : 1 at 150 K,
based on the squares of the transfer integrals. The temperature
dependent magnetic susceptibility of salt 2 was well explained
by the spin-ladder model in both the low- and high-temperature
regions. The magnetic exchange energies for the ladder-leg and
ladder-rung directions were 92.5 and 13.6 K, respectively, using
the low temperature limit equation. A spin-ladder chain of
[Ni(dmit)2]- anions coexists in the supramolecular rotator of
[18]crown-6 in salt 2. The coupling between the magnetism and
molecular rotators should be an important target to realize novel
phase transition systems.
-
4.63, N: 14.30. Found. C: 36.77, H: 4.62, N: 14.34%.
4.2 Preparation of salts 1 and 2
The monovalent precursor (n-Bu4N)[Ni(dmit)2] salt was prepared
according to the literature.24 Single crystals of salts 1 and 2 were
obtained by the standard diffusion method in an H-shaped cell
(ca. 50 mL). A green solution of (n-Bu4N)[Ni(dmit)2] (40 mg) in
CH3CN (ca. 20 mL) was poured into a solution of (HOPD+)(BF4 )
-
or (HMPD+)(BF4 ) (ca. 100 mg) in the presence of [18]crown-6
-
(ca. 400 mg) in CH3CN (ca. 30 mL) (detailed conditions for single
crystal preparation are shown in Table S1†). After two weeks,
single crystals (typical dimensions: 0.4 ¥ 0.2 ¥ 0.1 mm) were
obtained as black plates. The stoichiometry of the crystals was
determined by X-ray structural and elemental analyses. Elemental
analyses. Salt 1: Calcd. for C24H33N2O6S10Ni; C: 34.95, H: 4.03,
N: 3.40. Found. C: 34.95, H: 3.97, N: 3.22. Salt 2: Calcd. for
C24H33N2O6S10Ni; C: 34.95, H: 4.03, N: 3.40. Found. C: 34.45, H:
3.99, N: 3.50%.
4.3 Crystal structure determination
Crystallographic data (Table 2) were collected using a Rigaku
RAXIS-RAPID diffractometer with Mo-Ka (l = 0.71073 A)
˚
Table 2 Crystal data, data collection, and reduction parameter of salts 1 and 2
1 (150 K)
1 (300 K)
2 (150 K)
2 (300 K)
Chemical formula
Formula weight
Space group
C24H33N2O6S10Ni
824.83
P1 (#2)
C24H33N2O6S10Ni
824.83
P1 (#2)
C24H33N2O6S10Ni
824.83
P1 (#2)
C24H33N2O6S10Ni
824.83
P1 (#2)
¯
¯
¯
¯
˚
a/A
10.6094(4)
12.5348(7)
13.6440(7)
82.4953(19)
75.4226(14)
77.7356(18)
1710.27(14)
2
150(1)
1.602
12.18
16663
10.780(5)
12.576(7)
13.666(7)
82.92(2)
75.67(2)
78.02(2)
1750.9(15)
2
300(1)
1.564
11.902
16994
8.2754(6)
14.0310(10)
16.2836(10)
110.176 (2)
95.932(2)
91.287(2)
1761.8(2)
2
150(1)
1.555
11.828
16921
8.334(4)
14.025(6)
16.325(9)
109.876(20)
95.93(2)
90.928(18)
1782.2(15)
2
300(1)
1.537
11.693
17208
˚
b/A
˚
c/A
a/deg
b/deg
g /deg
3
˚
V/A
Z
T/K
D
calc/g cm-1
m/cm-1
Reflections measured
Independent reflections
Reflections used
7707
6858
7873
5805
7960
6809
7994
6196
Rint
0.021
0.028
0.042
0.903
0.038
0.020
0.0361
0.0424
1.111
0.018
R a
0.0510
0.0883
0.400
0.0326
0.0430
1.145
Rw(F2) a
GOF
2
1/2
a R = RꢁFo| - |Fcꢁ/R|Fo| and Rw = (Rw(|Fo| - |Fc|)2/RwFo
) .
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radiation from a graphite monochromator. Structural refinements
were made using the full-matrix least-squares method on F2.
Calculations were performed using the Crystal Structure and
SHELX97 software packages.25 Parameters were refined using
anisotropic temperature factors except for the hydrogen atom and
disorderd oxygen atoms of [18]crown-6 in salt 1 at 300 K. CCDC
numbers: 773234–773237.†
a Quantum Design MPMS-XL5 SQUID magnetometer using
polycrystalline samples. The applied magnetic field was 1 T for
all temperature dependent measurements.
Acknowledgements
This work was partly supported by a Grant-in-Aid for Science
Research from the Ministry of Education, Culture, Sports, Science,
and Technology of Japan.
4.4 Calculations
The calculations for potential energy curves of the two-fold flip-
flop motion of o-aminophenyl and m-aminophenyl groups were
evaluated to determine the possible molecular motions in the
crystals. The relative energies of the structures were calculated
using the RHF/6-31(d) basis set.19 The nearest-neighboring
molecules around the HOPD+ and HMPD+ cations were included
in the calculations of the potential energy curves. The struc-
tural units of salts 1 and 2 for the calculations of two-fold flip-flop
motions were (HOPD+)([18]crown-6)3 and (HMPD+)([18]crown-
Notes and references
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structural analysis were used for the calculations. The rigid
rotations of o-aminophenyl and m-aminophenyl groups around
+
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Temperature-dependent dielectric constants were measured by the
two-probe AC impedance method at a frequency of 100 kHz
(HP4194A) with a single crystal placed in a cryogenic refrigeration
system (Daikin PS24SS). Electrical contacts were prepared using
gold paste (Tokuriki 8560) to attach the 10 mm f gold wires
to the single crystal. The measurement axes of salts 1 and 2
were perpendicular to the molecular axis of [18]crown-6 with the
HOPD+ and HMPD+ cations.
4.6 Solid state 1H NMR measurements
Solid-state wide-line 1H nuclear magnetic resonance (NMR)
spectra under static sample conditions were measured by solid
echo pulse sequence p/2x–t–p/2y (p/2 pulse width and t were 1.3
and 10 ms, respectively) using a Bruker DSX 300 spectrometer with
an operating frequency of 300 MHz for protons.
4.7 Magnetic susceptibility
The temperature-dependent magnetic susceptibility and the
magnetization-magnetic field dependence were measured with
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