Observation of magnetic bistability in polymorphs of the [Ni(dmit) 2]- complexes

Article abstract of DOI:10.1021/ic900090e

Four ion-pair complexes of [Ni(dmit)₂]^- with [NO ₂bzql]⁺ have been obtained, which belong to two kinds of polymorph forms, [NO₂bzql][Ni(dmit)₂] (1α and 1β) and [NO₂bzql][Ni(dmit)

Full text of DOI:10.1021/ic900090e

Inorg. Chem. 2009, 48, 9623–9630 9623
DOI: 10.1021/ic900090e
Observation of Magnetic Bistability in Polymorphs of the [Ni(dmit)2]- Complexes
Shuangquan Zang,†, Xiaoming Ren,† Yang Su,† You Song,† Wenjun Tong,† Zhaoping Ni,† Hanhua Zhao,‡
Song Gao,§ and Qingjin Meng*,†
†State Key Laboratory of Coordination Chemistry, Department of Chemistry, Nanjing University, Nanjing
210093, P. R. China, ‡Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station,
Texas 77842-3012, and §State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of
Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China. Current address:
Department of Chemistry, Zhengzhou University, Zhengzhou 450000, Henan, P. R. China.
Received January 16, 2009
Four ion-pair complexes of [Ni(dmit)2]- with [NO2bzql]þ have been obtained, which belong to two kinds of polymorph
forms, [NO2bzql][Ni(dmit)2] (1r and 1β) and [NO2bzql][Ni(dmit)2] CH3COCH3 (2r and 2β) (where dmit = 2-thioxo-1,3-
3
dithiole-4,5-dithiolate and [NO2bzql]þ = 1-(4-nitrobenzyl)quinolinium). Though 1r, 2r, and 2β all show anionic
dimerization structures at room temperature, they have different anionic and cationic arrangement fashions, which give
rise to different magnetic behaviors for these polymorphs or pseudo-polymorphs. Compounds 1r, 1β, and 2r exhibit
magnetic bistabilities. In particular, 1r has a hysteretic loop at ∼55 K, while 2β does not display a spin transition in the
2-300 K range. On the basis of the crystal structure data of 2rin high- and low-temperature phases, the magnetic coupling
feature within the [Ni(dmit)2]- spin dimer was explored with the broken-symmetry approach at the UBPW91/LANL2DZ
level; combined with the experimental data and theoretical analyses, the relationship between the magnetic coupling nature
and the stacking pattern of [Ni(dmit)2]- anions as well as the origin of the phase interconversion are discussed.
Introduction
information storage, and other molecular electronic devices.1
The most well-known example of these materials is transition
metal spin-crossover (SC) complexes.1-4 Additionally, new
bistability systems, such as charge transfer complexes,5,6
valence ordering compounds,7 and pure organic radical
compounds,8-10 have been developed.
Molecular spin bistability materials have aroused a great
deal of attention owing to their potential applications in
many technical fields, such as magneto-thermal switching,
*To whom correspondence should be addressed. E-mail: mengqj@
nju.edu.cn.
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2009 American Chemical Society
Published on Web 09/15/2009
pubs.acs.org/IC

9624 Inorganic Chemistry, Vol. 48, No. 20, 2009
Zang et al.
Bis-dithiolate metal ion-pair complexes have been actively
studied for a long time as a wide range of conducting and
magnetic materials as well as nonlinear optical materials.11,12
Experimental and theoretical investigations indicated that
the unusual physical properties in such kinds of ion-pair
complexes originate from the intermolecular interactions of
bis-dithiolate metal anions; as a result, the anionic stacking
pattern strongly influences the physical properties of the
materials. It is notable that the structural features of the
countercation play an important role in tuning the stacking
pattern of bis-dithiolate metal anions.13-15 We have recently
obtained a series of quasi-one-dimensional molecular mate-
rials that are [Ni(mnt)2]--based (where mnt2- = maleonitri-
ledithiolate) with interesting magnetic behaviors, such as
spin-Peierls-like transition, ferromagnetic ordering, and me-
tamagnetic transformation, employing benzylpyridinium
derivatives with λ-shaped and flexible molecular conforma-
tions to adjust the packing pattern of [Ni(mnt)2]- anions in
crystals.13
Scheme 1. Molecular Structure of [NO2bzql][Ni(dmit)2]
block employed for the construction of molecular magnetic
materials apart from its well-known electric conductivity.14,15
In comparison with the [Ni(mnt)2]- complex, the existence of
versatile intermolecular S S contacts in the crystal of the
3 3 3
[Ni(dmit)2]- complex (for example, head-to-head and lateral-
to-lateral S S contacts between [Ni(dmit)2]- anions besides
3 3 3
face-to-face S S contacts often observed in the crystal of
3 3 3
[Ni(mnt)2]- complex could serve as valid cooperative effects in
molecular magnetic or conducting materials) probably give
hysteretic loops; on the other hand, the [Ni(dmit)2]- com-
plexes exhibit a rich polymorphism14,15 (which can be defined
as a chemical substance possessing at least two different
arrangements in the crystalline solid state,17,18 giving rise to
materials with different physical properties).
Most recently, we extended our previous studies of
the series of [Ni(mnt)2]- complexes and focused our
research attention on [Ni(dmit)2]- complexes,16 because the
[Ni(dmit)2]- anion is the analog of [Ni(mnt)2]- (both anions
possess planar molecular geometry and delocalized electronic
structure with S = 1/2 spin) and also is an excellent building
Herein, we report four ion-pair complexes of [Ni(dmit)2]-
with[NO2bzql]þ (NO2bzqlþ = 1-(4-nitrobenzyl)quinolinium,
and for its molecular structure, see Scheme 1). [NO2bzql]-
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Starting Materials and Chemicals. 4,5-Bis(thiobenzoyl)-1,3-
dithiol-2-thione and [NO2bzql]Cl were prepared as described in
the literature.19,20 Other chemicals were purchased as analytical-
grade reagents.
Powder Sample. To the suspended solution of 4,5-bis-
(thiobenzoyl)-1,3-dithiol-2-thione (812 mg, 2.0 mmol) in dry
methanol (20 mL) was added sodium (92 mg, 4.0 mmol) under a
nitrogen atmosphere at room temperature. After all sodium
balls disappeared, NiCl2 6H2O (238 mg, 1 mmol) was added to
3
the above-mentioned mixture with strong stirring for 20 min. I2
(127 mg, 0.5 mmol) and [NO2bzql]Cl (1 mmol, 301 mg) in
methanol were added in turn; the resulting dark blue powder
was collected by filtration.
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Article
The addition of 20 mL of acetone to the saturated acetone
Inorganic Chemistry, Vol. 48, No. 20, 2009 9625
The interaction between two spins SB1 and SB2 can be described
with the Heisenberg-Dirac-van Vleck Hamiltonian:23-25
solution (10 mL) of the above powdered sample gave a dilute
solution. Evaporation of such a dilute solution of the above
powdered sample in acetone at room temperature over 1-2
weeks gave rise to needle-shaped 1r as a single phase in the
bottom of the beaker. Evaporation of the acetone/isopropyl
alcohol, acetonitrile/isopropyl alcohol, or acetonitrile dilute
solutions of the powdered sample all gave 1r as a single
phase. Elem anal. calcd for C22H13N2O2S10Ni (1r): C, 36.87;
H, 1.83; N, 3.91%. Found: C, 36.87; H, 1.65; N, 4.10%. IR:
absent νCdO (acetone) vibration band.
ꢂ
H ¼ -2JSB1SB2
ð1Þ
where J is the magnetic exchange constant between SB1 and SB2.
Assuming the so-called “weak bonding” regime, Noodleman
et al.26-28 evaluated J values within a broken symmetry frame-
work using
EBS -ET
Sm2 ax
Jð1Þ
¼
ð2Þ
The 1β phase was obtained through the heating of 2r (see
below) at 120 ꢀC for 10-30 min. Elem anal. (%) calcd for
C22H13N2O2S10Ni (1β): C, 36.87; H, 1.83; N, 3.91. Found: C,
37.02; H, 1.47; N, 4.08. IR: absent νCdO (acetone) vibration band.
The 2r-phase was obtained through the evaporation of the
saturated solution (10 mL) of the above powdered sample in
acetone over 1-2 weeks, gaving rhombus-shaped 2r as a single
phase in the bottom of the beaker. Elem anal. (%) calcd for
C25H19N2O3S10Ni (2r): C, 38.76; H, 2.47; N, 3.62. Found: C,
where EBS and ET denote the total energies in the broken
symmetry (BS) singlet state and the triplet state, respectively,
and Smax corresponds to the total spin of the high-spin state. It
has been suggested that the following expression might give
more reasonable solutions in the strong overlap region:29,30
38.88; H, 2.13; N, 3.70. IR: νCdO(acetone) = 1702 cm-1
.
EBS -ET
SmaxðSmax þ 1Þ
Jð2Þ
¼
ð3Þ
Block 2β could be obtained as a single phase from the wall of
the beaker by quick evaporation of the saturated solution
(10 mL) of the powder sample in acetone. Elem anal. (%) calcd
for C25H19N2O3S10Ni (2β): C, 38.76; H, 2.47; N, 3.62. Found: C,
However, Yamaguchi et al. claimed that J obtained by the
approximate spin projection procedure reproduces the charac-
teristic feature of J in the whole region:31-33
38.76; H, 2.23; N, 3.71. IR: νCdO(acetone) = 1690 cm-1
.
Detailed IR spectra, thermogravimetric analysis, and XRD
patterns for the four phases can be found in Figures S1, S2, and
S3 (Supporting Information).
EBS -ET
ÆS2æT - ÆS2æBS
Jð3Þ
¼
ð4Þ
Physical Measurements. Elemental analyses for C, H, and N
were performed on a Perkin-Elmer 240C elemental analyzer. IR
spectra were collected on a VECTOR 22 spectrometer (KBr
disk). Thermal analyses were carried out in a TGA V5.1A
Dupont 2100 instrument from room temperature to 500 ꢀC with
a heating rate of 10 ꢀC/min in the air. Powder X-ray diffraction
patterns were recorded on a RigakuD/max-RA rotating anode
X-ray diffractometer with graphite monochromatic Cu KR (λ =
where ÆS2æT and ÆS2æBS denote the total spin angular momentum
of the triplet state and broken-symmetry singlet state, respec-
tively. Thus, the J(3) values obtained from eq 4 are employed in
the following discussion.
X-Ray Crystallography. Single-crystal X-ray diffraction data
were collected on a Bruker SMART APEX CCD diffractometer
using graphite-monochromatized Mo KR radiation (λ =
˚
˚
0.71073 A) at 293 K for 1r, at 293 K and 50 K for 2r; and at
1.542 A) radiation at room temperature. Variable-temperature
293 K for 2β. Lorentz polarization and absorption corrections
were applied. The structures were solved by direct methods and
refined with the full-matrix least-squares technique using the
SHELXS-97 and SHELXL-97 programs.34 Anisotropic ther-
mal parameters were assigned to all non-hydrogen atoms. The
crystallographic data and selected bond lengths for 1r, 2r, and
2β are listed in Table 1 and Table S1 (Supporting Information).
It was not possible to collect the single-crystal X-ray diffraction
data for 1r in the low-temperature phase owing to the single
crystal being severely cracked, even though the crystal was
cooled very slowly.
magnetic susceptibility data were collected on microcrystalline
samples from 2 to 300 K under a magnetic field of 2000 Oe, using
a Quantum Design MPMS-XL SQUID magnetometer, and
diamagnetic correction for each compound was estimated from
Pascal’s constants.21
Calculation Details. All density functional theory (DFT)
calculations were carried out utilizing the Gaussian 98 pro-
gram.22 The whole nonmodelized molecular structures of the
real complex 2r in high-temperature (HT) and low-temperature
(LT) phases were taken directly from X-ray crystallography
complete structures, and calculations for magnetic exchange
constants J were performed at the BPW91 level with the basis
sets of LANL2DZ. The convergence criterion of SCF is 10-8
.
Results and Discussion
Complex 1r at 293 K. Complex 1r crystallizes in
monoclinic space group P21/c, and its asymmetric unit
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20, 1391–1400.
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Koga, N.; Iwamura, H. J. Am. Chem. Soc. 2002, 124, 450–461.
(23) Heisenberg, W. Z. Phys. 1928, 49, 619–636.
::
::
(24) Dirac, P. A. M. The Principles of Quantum Mechanics; Clarendon
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(34) Sheldrick, G. M. SHELXL-97; University of Gottingen: Gottingen,
Germany, 1997.

9626 Inorganic Chemistry, Vol. 48, No. 20, 2009
Zang et al.
contains one anion together with one cation, as depicted
in Figure 1. The anion shows an approximated planar
geometry, and its bond lengths and angles are comparable
to the reported [Ni(dmit)2]- complexes.14,15 The cation
[NO2bzql]þ exhibits a λ-shaped conformation with an
88.50ꢀ dihedral angle between benzene and quinoline
rings, which is similar to the reported benzylpyridinium
derivatives. Neighboring cations form a molecular layer
via weak intermolecular hydrogen bonds as well as van
der Waals forces (please see Figure 2 and Figure S4,
Supporting Information). The π-type dimers of
[Ni(dmit)2]- anions (the atom-to-atom distances: 3.643
˚
˚
˚
A for S5 C13, 3.492 A for S6 N1, 3.570 A for
3 3 3 3 3 3
˚
C2 C10, 3.342 A for C4 C15, and 3.659 A for
˚
3 3 3
3 3 3
3 3 3
C3 C9), which are filled in the channels formed by
the cations, aligned through lateral S S interactions to
give a 2D supramolecular structure (Figure 2).
3 3 3
Complex 1β. Complex 1β was only obtained as a
powder through heating of 2r. Its composition was
verified by element analyses, IR spectra, and TG analyses
(see Supporting Information). The DSC measurements of
2r showed the existence of a phase transition following
the loss of the acetone molecules (Figure S5). The XRD
Figure 1. Molecular structure of 1r.
Table 1. Crystallographic Data and Refinement Parameters for 1r, 2r, and 2β
1r
2r
2β
temp (K)
293
C22H13N2NiO2S10
716.65
monoclinic
P21/c
17.496(4)
10.956(3)
14.690(4)
293 (HT)
C25H19N2NiO3S10
774.73
triclinic
P1
8.026(2)
12.499(4)
16.496(5)
106.482(5)
102.477(5)
92.807(5)
1538.5(8)
2
50 (LT)
C25H19N2NiO3S10
774.73
triclinic
P1
7.337(3)
13.616(5)
15.137(5)
106.398(6)
95.967(6)
91.973(7)
1439.6(8)
2
293
C25H19N2NiO3S10
774.73
monoclinic
P21/n
12.219(3)
8.134(2)
31.928(8)
empirical formula
fw
cryst syst
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
99.611(5)
98.855(4)
3
˚
V (A )
Z
2776.3(12)
4
1.715
1452
1.052
3135.6(13)
4
1.641
1580
1.081
F (g cm-1
F(000)
)
1.672
790
1.098
0.0566, 0.1328
0.0791, 0.1392
1.787
790
1.048
0.0411, 0.1127
0.0554, 0.1162
goodness of fit on F2
R1, wR2a [I > 2σ(I)]
R1, wR2a [all data]
0.0528, 0.1379
0.0651, 0.1414
0.0565, 0.1295
0.0802, 0.1367
P
P
P
P
a R1 = ||Fo| - |Fc||/|Fo|, wR2 = [ w( Fo2 - Fc2)2/ w(Fo2)2]1/2
.
Figure 2. (a) Packing of 1r viewed along c axis (b) Intra- and interdimer S S and S Ni contacts of the [Ni(dmit)2]- anion: 4.013 A for Ni1 Ni1a,
˚
3 3 3
3 3 3
3 3 3
˚
˚
˚
˚
˚
˚
3.985 A for Ni1 S1a, 3.663 A for S1 S9b, 3.514 A for S4 S9b, 3.554 A for S4 S2b, 3.438 A for S5 S2b, and 3.608 A for S5 S3b (c) 2D
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
supramolecular structure of [Ni(dmit)2]- anions through S S contacts in 1r.
3 3 3

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9627
pattern (Figure S3, Supporting Information) further
confirmed that it is a new phase different from the 1r
and 2r phases. Further work is in progress to establish the
cell parameters and space group for 1β, with the aim of
solving its crystal and molecular structure.
Complex 2r at 293 K and 50 K. The crystal of 2r
belongs to the triclinic system with space group P1 in both
the HT phase (293 K) and the LT phase (50 K), and an
asymmetric unit is comprised of one [NO2bzql]þ cation
and one [Ni(dmit)2]- ion together with one acetone
molecule (Figure 3). The bond parameters for the planar
anion and the cation are similar to those in 1r. The
cations build staircase-like columns along [011] via weak
molecular planes, defined via Ni and four coordination
˚
S atoms, only reduces by 0.113 A), while obvious slippage
is found along the long molecular axis of the [Ni(dmit)2]-
anion (which is reflected in the Ni Ni distance reducing
3 3 3
˚
˚
from 3.966 A in the HT phase to 3.535 A in the LT phase),
and such a slippage results in the two anions being nearly
completely eclipsed in the LT phase (Figure 4b). (3) The
interdimer interactions (including both terminal and
lateral along [100] and [011] as mentioned above in the
HT phase) are also strengthened at LT (please see
Table 2).
Complex 2β at 293 K. Although 2β and 2r take the
same stoichiometry of [NO2bzql][Ni(dmit)2] CH3COCH3
3
π
π interactions between two quinoline rings as well as
(see Figure S7, Supporting Information), the crystal
symmetry of 2β (P21/n) was higher than that of 2r (P1),
and the arrangement of cations and anions in the two
3 3 3
hydrogen bond interactions between cations and acetone
molecules (Figure 4a). The [Ni(dmit)2]- dimers link into
one-dimensional (1D) zigzag chains along [100] through
intermolecular lateral S S interactions (Figure 4), and
˚
3 3 3
Table 2. Ni S and S S Contacts (A) in 2r at HT and LT phases
3 3 3
3 3 3
such anionic chains fill in the cavity formed by the cation
columns. In comparison with the HT phase, the structural
distinctions for 2r in the LT phase involve the following
aspects: (1) The dihedral angle between benzene and the
quinoline rings is 77.8ꢀ at 293 K versus 70.4ꢀ at 50 K. (2)
Within the [Ni(dmit)2]- dimer, less shrinking is observed
along the stack direction (the distance between mean
lengths
atoms
HT
LT
Intradimer (face to face)
Ni1
Ni1a
S4
S3
S5
Ni1a
S3a
S2a
S1a
S9a
S8a
3.966
3.575
3.535
3.536
3.554
3.601
3.657
S6
Interdimer (lateral to lateral)
S6
S3
S3
S2
S9b
S9b
S2b
S2b
3.731
3.700
3.615
3.601
3.685
3.537
3.498
3.379
Interdimer (head to head)
S10’
Figure 3. An asymmetric unit of 2r.
S7
3.477
Figure 4. (a) Top: the packing fashion of 2r along [011] at 293 K. Cations form stair-step columns, and anions lie between these cation columns. Bottom:
the packing fashion of 2r along [100] at 293 K. Anions form zigzag chains along [100]. NO2bzqlþ cations, [Ni(dmit)2]- anions, and acetone molecules are
marked green, orange, and pink, respectively; hydrogen bonds are represented as blue dotted lines. (b) Intra- and interdimer S S, S Ni, and Ni Ni
3 3 3
3 3 3
3 3 3
contacts in 2r along [100] at (top) 293 K and (bottom) 50 K.

9628 Inorganic Chemistry, Vol. 48, No. 20, 2009
Zang et al.
Figure 5. (a) Cations are linked into both right- and left-handed helices via hydrogen bonds (cations and acetone molecules marked as green and pink for
˚
clarity). (b) Side view of the [Ni(dmit)2]- anion stack in 2β with dNi1 Ni1a = 3.979 A, dNi1 S4a = 3.732 A, dS2 S6b = 3.669 A, dS2 S3b = 3.661 A,
˚
˚
˚
3 3 3
3 3 3
3 3 3
3 3 3
˚
˚
˚
dS3 S3b = 3.530 A, dS3 S2b = 3.661 A, and dS6 S2b = 3.669 A (c). Anion arrangement with two orientations in 2β.
3 3 3
3 3 3
3 3 3
Figure 6. Plots of χmT versus T for (a) 1r, (b) 1β, (c) 2r, and (d) 2β.
crystals was also quite different. In 2β, benzene and
quinoline rings form a dihedral angle of 84.43ꢀ (versus
77.8ꢀ in 2r), and the cations form supramolecular helical
chains with alternative right- and left-handed helices via
other, while in 2β, there are two [Ni(dmit)2]- plane
orientations with a dihedral angel of 72.5ꢀ(Figure 5d).
Magnetic Properties. Figure 6a-d show the tempera-
ture dependence of χMT of 1r, 1β, 2r, and 2β. At room
temperature, the χMT value is 0.380 cm3 K mol-1 for 1r,
which is close to the spin-only value for the S = 1/2 spin
system. Upon cooling, the χMT value remains nearly
constant to 130 K, drops quickly around 112 K, and
reaches 0.015 cm3 K mol-1 at 55 K. The warming mode
reveals the occurrence of thermal hysteresis with an
approximated 55 K loop. Complex 1β exhibits similar
magnetic bistability behavior with a ca. 30 K loop and
lower critical temperatures (cooling Tdown ≈ 70 K and
warming Tup ≈ 100 K). Weakly antiferromagnetic
(AFM) coupling behavior is observed for 2r in the
weak hydrogen bonds with a C15#1 O2 distance of
˚
3 3 3
3.237(7) A (symmetry code: #1 = 1 - x, 1/2 þ y, -1/2 - z;
Figure 5). Acetone molecules are suspended alongside the
cation helical chains through hydrogen bonds with a
˚
C16 O3 distance of 3.395(5) A. Anion dimers are
located in the pores of cations and acetone through
3 3 3
π
π interactions (Figure 5c; the atom-to-atom dis-
tances: S1 C14, 3.654; S4 C13, 3.497; S4 C12,
3 3 3
3 3 3 3 3 3 3 3 3
˚
3.597;C9 C16, 3.696 A) and form a 1D chain along [010]
through lateral-to-lateral S S interactions (Figure 5b).
3 3 3
3 3 3
In 2r, the [Ni(dmit)2]- anions are all parallel to each

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9629
300-120 K range, and a magnetic transition occurs
around 100 K in the cooling process with a ca. 8 K loop
(Figure 6c), while 2β exhibits the AFM coupling feature
without magnetic transition in the temperature range
2-300 K (Figure 6d). Bistable materials with a wide
hysteretic loop are rarely observed in the dithiolate Ni
complexes; as far as we known, only smaller hysteretic
loops, ∼9 K for [Me4N][Ni(tfadt)2] and ∼1 K and
∼10 K have been reported for r and β phases of
[H2DABCO][Ni(mnt)2]2.35 Additionally, bis-dithiolate
Ni complexes with the countercation Fe(Cp*)2 show
thermal hysteresis.36 Bistable materials with a hysteresis
loop derived from the spin crossover of Fe3þ using Ni
dithiolate as counterions were also reported.37 As far as
we know, the thermal hysteresis loop with a width of
∼55 K is a notably large for transition-metal-containing
molecular materials, except for SC and cyano-bridged
metal-to-metal CTIST complexes.
Figure 7. SOMOs of (a) 2r-HT and (b) 2r-LT calculated by UBPW91/
LANL2DZ in the triplet state.
In the LT-phase, temperature-dependent magnetic sus-
ceptibilities show a thermally activated character for 1r,
1β, and 2r, indicating the existence of a spin gap (see
Figure S8, Supporting Information). The magnetic sus-
ceptibilities of 1r (in 2-96 K range), 1β (in 2-55 K
range), and 2r (in 2-100 K range) are better described by
eq 1:38,39
Figure 8. HOMO of 2r-LT obtained by UBPW91/LANA2DZ in the
BS state.
R
C
T
χm
¼
expð -Δ=kBTÞ þ
þ χ0
ð1Þ
the temperature dependences of magnetic susceptibility
for 1r, 1β, and 2r in HT phases as well as for 2β over the
entire temperature range (2-300 K). The fits revealed
that both the dimer and Heisenberg alternating chain
models with S = 1/2 cannot yield reasonable parameters
for 1β and 2β, while the isolated dimer model with S =
1/2 is described quite well and explains the magnetic
behavior of 1r and 2r in HT phases, which gives rise to
the energy gap between the singlet and triplet states at 46
K with a fixed g factor = 2 in the temperature range of
120-300 K for 1r versus 168 K with a fixed g factor = 2
in the temperature range of 154-300 K for 2r (see
Figure S8, Supporting Information). Obviously, the
AFM interaction within an intradimer is greatly en-
hanced from the HT phase to the LT phase owing to
the improved π-π overlap.
Tγ
0
where R is a constant corresponding to the dispersion of
the excitation energy, Δ/kB is the magnitude of the spin
gap, γ0 is a constant (γ0 = 0.5), and the C/T term
represents the contribution from magnetic impurities.
Simulations gave the following results: R = 1.52, Δ/kB =
476 K, C = 4.38 ꢀ 10-3 cm3 K mol-1, and χ0 = 1.8 ꢀ
10-4 cm3 mol-1 for 1r; R = 1.68, Δ/kB = 432 K, C =7.81ꢀ
3
10-3 cm3 K mol-1, and χ0 = 6.3 ꢀ 10-4 cm3 mol-1 for 1β;
R = 1.49, Δ/kB = 816 K, C = 1.62 ꢀ 10-3 cm3 K mol-1, and
χ0 = 2.0 ꢀ 10-5 cm3 mol-1 for 2r.
The crystal structure data of 1r, 1β, and 2r in HT
phases disclosed that the manners of anionic arrangement
are similar to each other; namely, two face-to-face
[Ni(dmit)2]- anions form a π-type dimer, and the adjacent
dimers are linked into a 1D zigzag chain through inter-
Theoretical Calculations. To get a deeper understand-
ing of the relationship between the anionic stacking
pattern and magnetic coupling nature, the magnetic
coupling constant J in the spin dimer has been calculated
with the broken-symmetry approach using a DFT theo-
retical framework for 2r-HT and 2r-LT, and the
Kohn-Sham type of SOMOs in the triplet states are
displayed in Figure 7, which show two unpaired electrons
almost localized in each monomer of the dimer in 2r-HT
but delocalized over the dimeric pair owing to two
monomers getting closer in 2r-LT. Such a delocalization
picture is also clearly shown in the HOMO of 2r-LT
(Figure 8). Further, the calculated parameters in both
the paramagnetic and nonmagnetic states are respecti-
vely given below: ÆS2æBS = 0.9277, ÆS2æT = 2.0030, and
J = -48.31 cm-1; ÆS2æBS = 0.0, ÆS2æT = 2.0031, and J =
-1518.18 cm-1, and these results are in qualitative agree-
ment with the analyses of the temperature-dependent
magnetic susceptibility in both HT and LT phases.
Now, it is clear that the origin of the magnetic transition
molecular lateral S S interactions. On the other hand,
3 3 3
2r loses solvent molecules from the lattice to give 1β, and
it is reasonable to assume the existence of a similar anionic
arrangement in 2r and 1β. From the above analyses, the
magnetic exchange model for a dimer or a Heisenberg
alternating chain with S = 1/2 could be used to simulate
(35) (a) Ren, X. M.; Akutagawa, T.; Nishihara, S.; Nakamura, T.; Fujita,
W.; Awaga, K. J. Phys. Chem. B 2005, 109, 16610–16615. (b) Ren, X. M.;
Nishihara, S.; Akutagawa, T.; Noro, S.; Nakamura, T. Inorg. Chem. 2006, 45,
2229–2234. (c) Ren, X. M.; Nishihara, S.; Akutagawa, T.; Noro, S.; Nakamura, T.;
Fujita, W.; Awaga, K. Chem. Phys. Lett. 2006, 418, 423–427.
(36) Mochida, T.; Koinuma, T.; Akasaka, T.; Sato, M.; Nishio, Y.;
Kajita, K.; Mori, H. Chem.;Eur. J. 2007, 13, 1872–1881.
(37) (a) Dorbes, S.; Valade, L.; Real, J.; Faulmann, C. Chem. Commun
2005, 69–71. (b) Faulmann, C.; Jacob, K.; Dorbes, S.; Lampert, S.; Malfant, I.;
Doublet, M.-L.; Valade, L.; Real, J. A. Inorg. Chem. 2007, 46, 8548–8559.
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(38) Johnston, D. C.; Kremer, R. K.; Troyer, M.; Wang, X.; Klumper, A.;
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13527.

9630 Inorganic Chemistry, Vol. 48, No. 20, 2009
Zang et al.
occurs in the [Ni(dmit)2]- polymorphs; that is, upon
cooling, the lattices shrink nonuniformly to enhance the
π-orbital overlap of the [Ni(dmit)2]- anions in a dimer,
which induces a magnetic transition from a paramagnetic
state to a nonmagnetic state.
ment where the reorientation of [Ni(dmit)2]- anions is easily
achieved via intermolecular slippage upon cooling that is a
requirement for a paramagnetic state to nonmagnetic state
transition. Additionally, since these kinds of complexes often
act as the charge carrier for molecular conductors, they are
also promising in the investigation of potential electromag-
netic bifunctional switches.
Concluding Remarks
This work presented the crystal structures and magnetic
properties for four [Ni(dmit)2]--based pseudo-polymorphs.
Combined the experimental data and theoretical analysis
results, the following conclusions could be made: (1) The
magnetic coupling nature between [Ni(dmit)2]- anions is
sensitive to the stacking pattern. Also, for a face-to-face
stacked [Ni(dmit)2]- dimer, there exists a weak AFM cou-
pling interaction in a slippage pattern due to less π-orbital
overlapping, while there is a stronger AFM coupling inter-
action in an eclipsed fashion owing to greater π-orbital
overlapping. (2) The countercation with a flexible molecular
conformation is favored to build a tunable crystal environ-
Acknowledgment. This work was financially supported
by the National Basic Research Program of China
(2007CB925102), National Nature Science Foundation
of China (Grant No. 20490218), Jiangsu Science and
Technology Department, and the Center of Analysis
and Determining of Nanjing University.
Supporting Information Available: Additional figures and
tables and crystallographic data for all complexes in CIF format
can be obtained free of charge via the Internet at http://pubs.
acs.org.