Ferromagnetic coupling of [Ni(dmit)2]- anions in (m-fluoroanilinium)(dicyclohexano[18]crown-6)[Ni(dmit)2]

Article abstract of DOI:10.1039/b920600k

Ferromagnetic coupling of the [Ni(dmit)₂]^- anion layer through lateral sulfur-sulfur interactions along the short-axis of the anion was achieved using the flexible supramolecular cation of (m-fluoroanilinium⁺)(meso-dicyclo

Full text of DOI:10.1039/b920600k

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Ferromagnetic coupling of [Ni(dmit)2]- anions in
(m-fluoroanilinium)(dicyclohexano[18]crown-6)[Ni(dmit)2]†
Tomoyuki Akutagawa,*a,b Daisuke Sato,b Qiong Ye,a Shin-ichiro Noroa,b and Takayoshi Nakamura*a,b
Received 2nd October 2009, Accepted 14th December 2009
First published as an Advance Article on the web 15th January 2010
DOI: 10.1039/b920600k
Ferromagnetic coupling of the [Ni(dmit)2]- anion layer
through lateral sulfur–sulfur interactions along the
short-axis of the anion was achieved using the flexible
supramolecular cation of (m-fluoroanilinium+)(meso-
dicyclohexano[18]crown-6).
in which molecular motions can couple with J values. Herein,
we report a new [Ni(dmit)2]- salt with ferromagnetic coupling
(J > 0) induced by the (m-FAni+)(DCH[18]crown-6) supramolec-
ular cation (Scheme 1).
Ferromagnetic ordering of metal–organic coordination com-
pounds has been reported in [Fe(C5Me5)2][tetracyanoethylene],
MnCu[2-hydroxy-1,3-propylenebis(oxamato)]·H2O, and [Ni(1,1-
dimethylethylenediamine)2]2[Fe(CN)6]CF3SO3·3.5H2O,1
where
Scheme
1
Molecular structures of m-fluoroanilinium (m-FAni+),
the magnetic exchange energy (J) between the metal centers plays
an important role in achieving a positive J value. The p-orbitals
of the organic ligands consist of the frontier orbitals of several
metal–organic compounds such as [Ni(mnt)2]- and [Ni(dmit)2]-
anions, where mnt2- and dmit2- are maleonitriledithiolate
and 2-thioxo-1,3-dithiole-4,5-dithiolate, respectively.2 The
effective contribution of the Ni orbital to the LUMO orbital
of the [Ni(mnt)2]- anion results in ferromagnetic ordering of
meso-dicyclohexano[18]crown-6 (DCH[18]crown-6), and [Ni(dmit)2].
Slow diffusion between (n-Bu4N+)[Ni(dmit)2]- and (m-
-
FAni+)(BF4 ) in the presence of DCH[18]crown-6 yielded single
crystals of (m-FAni+)(DCH[18]crown-6)[Ni(dmit)2]- (1).‡ The
1 : 1 adduct of the supramolecular cation between m-FAni+
and DCH[18]crown-6 was observed through the six N–H+
~ O hydrogen bonds, forming a stand-up configuration of the
NH4 [Ni(mnt)2]·H2O at 4.5 K.3 On the other hand, since the
+
+
C-NH3 unit of m-fluorophenyl groups with respect to the mean
oxygen plane of the DCH[18]crown-6 (Fig. 1a). The average
N–O hydrogen-bonding distances between the ammonium nitro-
LUMO orbital of the [Ni(dmit)2]- anion was constructed from the
p-orbital of dmit2- ligands,2 the J value between the [Ni(dmit)2]-
anions was smaller than those of the [Ni(mnt)2]- anion. However,
the LUMO orbital is delocalized onto the entire [Ni(dmit)2]-
anion, and can participate in diverse intermolecular interactions
such p-stacking, p-dimers, lateral dimers, and linear-chains. These
conformations are thought to be responsible for the diverse
magnetic properties observed.4
˚
gen and six oxygen atoms of DCH[18]crown-6 (dN–O = 2.914 A)
was comparable to the standard N–H+ ~ O hydrogen bond
length.7 The fixed orientation of m-FAni+ suggested restriction
of the molecular motions of m-FAni+ due to the steric hindrance
We previously reported a useful technique to control the
[Ni(dmit)2]- anion arrangements using supramolecular cation
structures of organic ammonium–crown ether assemblies. For
example, (anilinium+)([18]crown-6), (p-xylylenediammonium)-
([18]crown-6)2, and meso-1,2-diphenylethylenediammonium)-
([18]crown-6)2 supramolecules yielded the spin-ladder, p-dimer,
and two-dimensional square lattice of S = 1/2 spin on the
[Ni(dmit)2]- anions.5 One aspect of this approach is the possible
structural diversity in terms of size, valence, and flexibility of the
cation structures. In addition, dynamic cation structures such
as molecular rotators can be designed in [Ni(dmit)2]- salts,4,6
aResearch Institute for Electronic Science, Hokkaido University,
Sapporo, 001-0020, Japan. E-mail: takuta@es.hokudai.ac.jp, tnaka@
es.hokudai.ac.jp; Fax: 81-11-7069420; Tel: 81-11-7069418
bGraduate School of Environmental Earth Science, Hokkaido University,
Sapporo, 060-0810, Japan
† Electronic supplementary information (ESI) available: Crystal growth,
atomic numbering scheme, structural analysis of salt 1, calculated model
structures, IR spectra, Raman spectra, dielectric measurements, potential
energy curve for molecular rotation, and DFT calculation. CCDC
reference numbers 749640 & 749641. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b920600k
Fig. 1 Crystal structure of salt 1. Unit cell viewed along (a) the c-axis
and (b) the a-axis.
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from the two nearest-neighboring cyclohexane rings. However,
increasing the temperature changed the thermal parameter of the
fluorine atom from Beq = 2.58 at 100 K to Beq = 8.66 at 300 K,
suggesting that thermal fluctuation of m-FAni+ occurred around
room temperature.
the calculations taking the steric repulsions into account (see
Fig. S5†). An asymmetrical double-minimum type potential
energy curve with large DE values of 3200 and 1600 kJ mol-1 was
obtained. Although the same cation–anion arrangements were
observed in salt 2 with DE ~ 330 kJ mol-1, the modification of the
cation structure from Ani+ to m-FAni+ significantly enhanced DE
for the flip-flop motion of the cation. Such a dipole inversion was
not allowed in salt 1 due to quite a high potential energy barrier.
The temperature- and frequency-dependent dielectric constant
(e1) of salt 1 showed a slight anomaly around 240 K (Fig. 3). The
crystal structure and large DE values confirmed that the small
amplitude thermal fluctuation of m-FAni+ can affect the dielectric
responses. A small hump in the e1–T plots was observed along
the a-axis, parallel to the p-plane of m-FAni+. From the potential
energy calculations, small amplitude thermal fluctuations within
the limit of 20◦ were assigned to a pendulum motion of the m-
FAni+ cation. Since the dielectric anomaly along the c-axis was not
observed, the pendulum motion of m-FAni+ occurred along the a-
axis at temperatures above 240 K. The dielectric response at 1 kHz
was larger than that at 1 MHz, therefore, a slow molecular motion
around room temperature influenced the dielectric properties of
the crystal. The constant e1 value (~275) at temperatures below
150 K was due to the polarization of p-electrons along the short-
axis of the [Ni(dmit)2]- anion.
Fig. 1a and 1b show the unit cell of salt 1 viewed along the
c- and a-axis, respectively. The cation–anion packing in salt 1
is the same as that of (anilinium+)(DCH[18]crown-6)[Ni(dmit)2]-
(2).4 Alternating layers of cations and anions were elongated
along the b-axis. The transfer integral was based on extended
Hu¨ckel molecular orbital calculations and was used to evaluate
the magnitude of the intermolecular interactions between the
[Ni(dmit)2]- anions within the crystal.21 The two-dimensional
[Ni(dmit)2]- layer was observed in the ac-plane, where lateral
sulfur–sulfur contacts (t1 = -1.21 meV and t2 = 2.27 meV)
were observed along the a-axis. Two other interactions of t3 =
-10.0 meV and t4 = -2.76 meV connected the lateral dimer
along the c-axis. Our previous findings showed that a uniform
[Ni(dmit)2]- interaction along the long axis of the anion resulted in
a one-dimensional antiferromagnetic Heisenberg chain. The same
ferromagnetic coupling has been confirmed in the isomorphous
salt 2, where the lateral J1- and J2-interactions were essential for
the ferromagnetic coupling.
Fig. 2b shows the (m-FAni+)(DCH[18]crown-6) cation arrange-
ment in the ac-plane. The orientation of the m-FAni+ cation
was fixed in the layer. The molecular motion of m-FAni+ in
the solid state was evaluated by the potential energy curve and
the magnitude of the potential energy barrier (DE), which was
calculated using the RHF/6-31(d) basis set with the atomic
coordinates determined from the X-ray crystal structural analysis.8
The two nearest-neighboring DCH[18]crown-6 were included in
Fig. 3 Temperature- and frequency- (1, 10, 100, and 1000 kHz) dependent
dielectric constants (e1) of salt 1 along the a-axis. The a-direction was
parallel to the p-plane of m-FAni+.
The molar magnetic susceptibility (cmol) was dominated by the
[Ni(dmit)2]- anion arrangements in the crystal. The Curie–Weiss
behavior with a ferromagnetic coupling (Curie constant C = 0.375
emu K mol-1 and Weiss temperature q = +3.42 K) was observed
in the cmolT–T plots (Fig. 4a). Increasing the cmolT values with a
decrease in temperature below 70 K clearly showed ferromagnetic
coupling between [Ni(dmit)2]- anions. The magnetization (M)–
magnetic field (H) plots at 2 K resulted in an S-shaped response
with a saturation of the M value at 1.73 mB, which corresponded
to one S = 1/2 spin. The hysteresis in the M–H curve was not
detected at 2 K, and the AC magnetic susceptibility showed no evi-
dence for the ferromagnetic ordering at 2 K (Fig. S6†). To evaluate
the J1–J4 interactions, symmetry-broken DFT calculations were
applied for each pair.9,10 The UB3LYP method using the basis-
set of Ni atoms for a triple-zeta type designed for an ECP + f
polarization (LanL2TZ) and those of S and C atoms for an aug-
cc-pVDZ were employed for the calculations. The J value was
defined by (Esinglet - Etriplet)/(<S2>singlet - <S2>triplet), where E and
Fig. 2 Cation–anion arrangements in salt 1. (a) Two-dimensional
[Ni(dmit)2]- anion arrangement within the ac-plane. The intermolecular J
are depicted in the ac-plane. (b) The supramolecular cationic layer in the
ac-plane. The m-FAni+ is shown by van der Waals representations.
2192 | Dalton Trans., 2010, 39, 2191–2193
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and DCH[18]crown-6 (~100 mg) were slowly diffused during a period of
two weeks.
Crystal data for 1: C32H43N9O6NFS10Ni, Mr = 935.99, T = 100 K,
¯
˚
triclinic, space group P1 (no. 2), Z = 2, a = 12.3990(6) A,◦ b =
◦
˚
˚
12.5367(5) A, c = 14.1216(6) A, a = 95.4297(12) , b = 104.7569(14) , g =
◦
3
˚
104.5606(13) , V = 2024.78(14) A . Of the 19625 reflections collected, 9072
are independent. On the basis of all these data and 461 refined parameters,
R(int) = 0.029, R1(observed data) = 0.0363 and wR2 (all data) = 0.1310
¯
were obtained. T = 300 K. triclinic, space group P1 (no. 2), Z = 2, a =
◦
˚
˚
˚
12.4557(10) A, b = 12.7411(9) A, c = 14.2746(10) A, a = 94.509(2) , b =
◦
◦
3
˚
104.813(2) , g = 105.509(2) , V = 2083.9(3) A . Of the 20345 reflections
collected, 9365 are independent. On the basis of all these data and 461
refined parameters, R(int) = 0.033, R1(observed data) = 0.0437 and wR2
(all data) = 0.1568 were obtained. Intensity data were collected on a Rigaku
RAXIS-RAPID diffractometer using Mo-Ka radiation. The structure was
solved by direct methods and refined through the full-matrix least-squares
method on F2 using SHELXS-97.11 CCDC number: 749640–749641.
Temperature-dependent dielectric constants were measured by the two-
probe AC impedance method at frequencies from 1 to 1000 kHz
(HP4194A). The electrical contacts were prepared using gold paste
(Tokuriki 8560) to attach the 10-mm ∅ gold wires to the single crystal. The
temperature-dependent magnetic susceptibility and the magnetization-
magnetic field dependence were measured using a Quantum Design
MPMS-XL5 SQUID magnetometer using polycrystalline samples.
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molT vs. T plots (right-scale) and (b) M–H plots at 2 K.
c
<S2> are the total energy and the total spin angular momentum,
respectively. The magnitude of the J1- and J2-interactions were
significantly smaller than those for the J3- and J4-interactions
(see Table S2†), suggesting that the energy difference between the
singlet and triplet states was small for the lateral [Ni(dmit)2]- anion
arrangements.
In conclusion, an alternate layer of supramolecular cations and
anions was observed in (m-FAni+)(DCH[18]crown-6)[Ni(dmit)2]-.
The two-fold rotation of m-FAni+ was suppressed in the two-
dimensional layer, where a small amplitude thermal fluctuation
was identified by the potential energy calculation and dielectric
responses. The two-dimensional [Ni(dmit)2]- anion layer was
constructed from lateral sulfur–sulfur interatomic contacts along
the long- and short-axes of the [Ni(dmit)2]- anion. The magnetic
behavior showed ferromagnetic coupling with a Weiss temperature
of +3.42 K, where the lateral interactions along the short-axis of
the [Ni(dmit)2]- anion was important to achieve the ferromagnetic
coupling. The introduction of molecular rotator structures into
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Acknowledgements
This work was supported in part by a Grant-in-Aid for Science
Research from the Ministry of Education, Culture, Sports, Science,
and Technology of Japan.
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Notes and references
‡ Single crystals of salt 1 were obtained by the standard diffusion
method in an H-shaped cell (CH3CN ~50 mL). The green solution of
(n-Bu4N)[Ni(dmit)2] (30 mg) and a solution of (m-FAni+)(BF4-) (~60 mg)
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 2191–2193 | 2193
©