An interesting magnetic behavior in molecular solid containing one-dimensional Ni(III) chain

Article abstract of DOI:10.1016/j.cplett.2004.08.060

The preparation, crystal structure and magnetic properties of a new ion-pair complex, [BrFBzNH₂Py][Ni(mnt)₂] (1) [BrFBzNH ₂Py⁺ = 1-(4′-bromo-2′-flurobenzyl)-4- aminopyridinium, mnt^2- = maleonitriledithiolate] are reported. The Ni(mnt)2- anions and [BrFBzNH₂Py]⁺ cations of 1 form completely segregated uniform stacking columns. The intrachain Ni?Ni separation is 4.045 ? in the Ni(mnt)2- stacking column. Magnetic susceptibility measurements for 1 in the temperature range 2.0-300 K show the occurrence of significant ferromagnetic interaction in the high-temperature phase (HT), spin gap in the low-temperature phase (LT) and weak ferromagnetism due to spin canting below 5 K.

Full text of DOI:10.1016/j.cplett.2004.08.060

Chemical Physics Letters 396 (2004) 353–358
www.elsevier.com/locate/cplett
An interesting magnetic behavior in molecular solid
containing one-dimensional Ni(III) chain
a
Chunlin Ni , Dongbin Dang , You Song , Song Gao , Yizhi Li , Zhaoping Ni ,
a
a
b
a
a
a
Zhengfang Tian , Lili Wen , Qingjin Meng
a
a,
*
a
State Key Laboratory of Coordination Chemistry, Department of Chemistry, Coordination Chemistry Institute, Nanjing University,
Hankou Road 22, Nanjing, Jiangsu 210093, PR China
b
State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, PR China
Received 26 June 2004; in final form 16 August 2004
Abstract
The preparation, crystal structure and magnetic properties of a new ion-pair complex, [BrFBzNH2Py][Ni(mnt)2] (1)
[BrFBzNH2Py+ = 1-(40-bromo-20-flurobenzyl)-4-aminopyridinium, mnt2ꢀ = maleonitriledithiolate] are reported. The NiðmntÞꢀ ani-
2
ons and [BrFBzNH2Py]+ cations of 1 form completely segregated uniform stacking columns. The intrachain Niꢁ ꢁ ꢁNi separation is
ꢀ
˚
4.045 A in the NiðmntÞ stacking column. Magnetic susceptibility measurements for 1 in the temperature range 2.0–300 K show the
2
occurrence of significant ferromagnetic interaction in the high-temperature phase (HT), spin gap in the low-temperature phase (LT)
and weak ferromagnetism due to spin canting below 5 K.
ꢀ 2004 Elsevier B.V. All rights reserved.
1. Introduction
molecular magnets [8]. Recently, much of our attention
has been directed at a new class of ion-pair complexes
[RbzPy]+[Ni(mnt)2]ꢀ ([RbzPy]+ = derivatives of ben-
zylpyridinium). These efforts are motivated by the de-
The design of new molecule-based magnets with
higher critical temperatures Tc has been one of the ma-
jor challenges in magneto-chemistry [1,2]. Recently,
bis-1,2-dithiolene transition metal complexes have at-
tracted intensive interest because of their attractive
properties and application in many fields [3,4]. The
ion-pair complexes which are comprised of organic or
inorganic cations and anions have a unique position
in this field. One class of these complexes is exemplified
by [Ni(mnt)2] complexes [5–7]. Especially, the discovery
in 1996 of the ferromagnetic complex containing
sire to tune the stacking pattern of NiðmntÞꢀ
2
complexes, and modulate the magnetic properties by
the molecular conformation of the attractive RbzPy+
cation. Versatile magnetic properties, such as magnetic
transition from ferromagnetic coupling to diamagnet-
ism, meta-magnetism, ferromagnetic ordering at low
temperature, and Spin-Peierls-like transitions, had been
founded in our pervious Letters [9,10]. With the view
to further determine the structure-function relationship
for this new class of magnetic materials, in this contri-
bution we wish to report the synthesis, crystal struc-
ture, and magnetic properties of a novel ion-pair
complex [BrFBzNH2Py][Ni(mnt)2] (1) which exhibits a
complicated magnetic transition from ferromagnetic
interaction to spin gap, then to spin canting, as the
temperature is lowered.
NiðmntÞꢀ ion, NH4 Æ Ni(mnt)2 Æ H2O, revived the inter-
2
est in Ni(mnt)2 complexes as building blocks for new
*
Corresponding author. Fax: +86 25 83314502.
E-mail address: njuchem1024@163.com (Q. Meng).
0009-2614/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.08.060

354
C. Ni et al. / Chemical Physics Letters 396 (2004) 353–358
2. Experimental
3. Results and discussion
[1-(40-bromo-20-flurobenzyl)]-4-aminopyridinium bro-
mide was prepared by literature method [11]. A similar
method for preparing [Bu4N][Ni(mnt)2] was used to pre-
pare [BrFBzNH2Py]2[Ni(mnt)2] [12]. A acetone solution
(10 cm3) of I2 (160 mg, 0.62 mmol) was slowly added to
3.1. Description of structure of 1
Complex 1 crystallizes in the monoclinic space group
P21/c. The structure and an ORTEP drawing with non-
hydrogen atomic labeling of the asymmetric are shown
in Fig. 1. Selected bond lengths and bond angles are
listed in Table 2. The molecular structure of the
a
acetone solution (50 cm3) of [BrFBzNH2Py]2
[Ni(mnt)2] (903 mg,1 mmol) and the mixture was stirred
for 1 h. MeOH (90 cm3) was then added, and the mix-
ture allowed to stand overnight; The resulting black mi-
cro-crystals of 1 formed were filtered off, washed with
i-PrOH and dried in vacuo. Yield 480 mg, 77.2%. Anal.
Calc: for C20H11 N6NiFBrS4: C, 38.67; H, 1.78; N,
13.53. Found: C, 38.62; H, 1.85; N, 13.37%. Infrared
spectrum (cmꢀ1): m(N–H), 3378.8 s, 1652.0 s, m(CN)
2205.8 s, m(C@C) of mnt2ꢀ, 1452.6 s. The black single
crystals suitable for the X-ray structure analysis were
obtained by evaporating the MeCN and i-PrOH (v/
v = 1:1) mixed solution of 1 about two weeks at room
temperature. Elemental analyses were run on a Model
240 Perkin–Elmer CHN instrument. IR spectra were re-
corded on an IF66V FT-IR (400–4000 cmꢀ1 region)
spectrophotometer in KBr pellets. Magnetic susceptibil-
ity data on crushed polycrystalline sample of 1 were col-
lected over the temperature range of 2–300 K using a
Quantum Design MPMS-5S super-conducting quantum
interference device (SQUID) magnetometer, and dia-
magnetic corrections were made using Pascalꢀs con-
stants. Crystallographic details of 1 are listed in Table 1.
NiðmntÞꢀ anion consists of two mnt2ꢀ ligands symmet-
2
rically chelated to the nickel cation which adopts the ex-
pected square–planar coordination geometry. The Ni–S
bond distances and the S–Ni–S bond angles within the
five-membered rings compare well with those found in
NiðmntÞꢀ complexes [13]. The CN groups of NiðmntÞꢀ
2
2
anion are slightly tipped out of the plane; the deviations
˚
˚
from the plane are ꢀ0.4606 A for N(1), ꢀ0.5102 A for
˚
˚
N(2), ꢀ0.1654 A for N(3) and 0.2850 A for N(4). The
[BrFBzNH2Py]+ cation adopts a conformation, where
(a)
(b)
+
NC
NC
S
S
CN
CN
S
S
Br
CH2
N
NH2
Ni
F
Table 1
Crystal data and structure refinement for 1
Temperature (K)
293(2)
Empirical formula
Formula weight
Wavelength
C
621.22
20H11N6NiBrFS4
˚
0.71073 A
Monoclinic
P21/c
Crystal system
Space group
Unit cell dimensions
Fig. 1. (a) The structure of 1. (b) ORTEP plot (30% probability
ellipsoids) showing the molecule structure of 1.
˚
a (A)
11.848(4)
26.611(8)
˚
b (A)
˚
c (A)
7.744(10)
100.64(1)
b (ꢁ)
Volume (A ), Z
Table 2
Selected bond lengths, bond angles and intermolecular contacts for 1
3
˚
2400(3), 4
1.720
Density (calculated) (mg/m3)
Absorption coefficient (mmꢀ1
F(000)
ꢀ
Bond lengths (A)
Ni(1)–S(1)
Ni(1)–S(3)
)
2.851
1236
2.142(3)
2.149(3)
Ni(1)–S(2)
Ni(1)–S(4)
2.153(3)
2.133(3)
Crystal size (mmꢀ1
)
0.4 · 0.2 · 0.2
1.91–26.00
h range for data collection
Limiting indices
Bond angles (ꢁ)
S(1)–Ni(1)–S(2)
S(1)–Ni(1)–S(4)
ꢀ14 6 h 6 12, ꢀ32 6 k 6 29,
ꢀ9 6 l 6 9
92.47(4)
85.23(4)
S(3)–Ni(1)–S(4)
S(2)–Ni(1)–S(3)
92.38(4)
89.89(4)
Reflections collected
Independent reflections
Refinement method
12619
4683 (Rint = 0.085)
Full-matrix least-squares on F2
4683/0/299
ꢀ
Intermolecular contacts (A)
Ni(1)ꢁ ꢁ ꢁNi(1)a
Ni(1)ꢁ ꢁ ꢁNi(1)b
Ni(1)ꢁ ꢁ ꢁS(4)a
Ni(1)ꢁ ꢁ ꢁS(4)b
4.045
4.045
3.753
3.753
S(1)ꢁ ꢁ ꢁS(4)a
3.791
3.746
3.853
3.766
Data/restraints/parameters
Goodness of fit on F2
Final R indices (I > 2r(I))
R indices (all data)
Br(1)ꢁ ꢁ ꢁC(9)a
Br(1)ꢁ ꢁ ꢁC(13)b
Br(1)ꢁ ꢁ ꢁC(14)b
0.99
R1 = 0.0421, wR2 = 0.0838
R1 = 0.0613, wR2 = 0.0855
0.48 and ꢀ0.86
ꢀ3
˚
Largest diff. peak and hole e A
Symmetry codes: a = x, 0.5 ꢀ y, 0.5 + z; b = ꢀ1 + x, y, z.

C. Ni et al. / Chemical Physics Letters 396 (2004) 353–358
355
both the phenyl ring and pyridine ring are twisted to the
C(14)–C(15)–N(5) reference plane. The dihedral angles
that pyridine ring and phenyl ring make with the refer-
ence plane are 80.6ꢁ and 106.8ꢁ, respectively. An inter-
esting structural features of 1 is the presence of the
completely segregated uniform stack columns of
The slipped zig–zag configuration of NiðmntÞꢀ anions
2
are displayed in Fig. 2b. The [BrFBzNH2Pyl]+ cations
orient themselves in the solid state and also form a 1D
chain through p–p interactions between Br atom and
the neighboring phenyl rings duo to the shorter intermo-
lecular separations occurring between Br(1) and C(9)a,
Br(1) and C(13)b, Br(10) and C(14)b of adjacent benzene
rings. This p–p interaction between the halogen atoms
and the benzene rings has also been found in some hal-
ogeno-benzene derivatives in the solid state [10,14–16].
It is worth noting that the intermolecular hydrogen
bonds between anions and cations were observed in
crystal structure and molecular structure. The intermo-
lecular contact is N(6) atom with the N(2)(1 ꢀ x, 1/
2 + y, ꢀ1/2 ꢀ z ) and N(4)(1 ꢀ x, 1 ꢀ y, ꢀz ) atom being
NiðmntÞꢀ anions and [BrFBzNH2Py]+ cations, each
2
flanked by two columns of cations, as revealed by the
projection along the crystallographic c-axis inꢀFig. 2a.
˚
The Niꢁ ꢁ ꢁNi distance is 4.045 A in the NiðmntÞ anions
2
stacking column; the nearest Niꢁ ꢁ ꢁS and Sꢁ ꢁ ꢁS distances
˚
are 3.679 and 3.791 A. While the closest Niꢁ ꢁ ꢁNi separa-
˚
tion between anion columns is 13.459 A, which is signif-
icantly longer than the distance of Niꢁ ꢁ ꢁNi separation
within a column. Obviously, there exist intermolecular
interactions between neighboring NiðmntÞꢀ anions with-
3.028(7) and 3.151(6) A, respectively. The hydrogen
˚
2
in an anion column. Therefore, each NiðmntÞꢀ anion
atom associated with these atoms has the following con-
˚
2
can be considered as a quasi-one-dimensional uniform
magnetic chains through the shorter intermolecular
interactions from the viewpoint of the crystal structure.
tacts with N(6)ꢁ ꢁ ꢁH(6) 2.590 and 2.490 A. These con-
tacts play important roles in crystal packing and
stability of 1.
3.2. Magnetic properties
(a)
Variable-temperature (2–300 K) magnetic susceptibil-
ity data is collected on crushed polycrystals of 1. The
temperature dependent of the vm, measured under a field
of 1000 Oe, are shown in Fig. 3a. As the temperature is
lowered, the vm varies in a complex way: it first increases
smoothly a maximum at 20 K, then decreases abruptly
to a rounded minimum at 10 K, and finally increases
again upon further cooling to 2 K. It is worthy to note
that the vm vs. T curve undergoes an inflection around
10 K. The room temperature vmT, value of 0.372
emu K molꢀ1 is in good agreement with the expected va-
lue for NiIII (S = 1/2) that are magnetically isolated (ex-
pected value is 0.375 emu K molꢀ1). On cooling down,
the vmT increases gradually and reaches a maximum
of 0.742 emu K molꢀ1 at 20 K. This behavior indicates
the ferromagnetic interactions between the metal spin
centers in this temperature range. This origin of the fer-
romagnetic interaction can be illustrated by means of
McConnellꢀs theory because the structure of 1 meets
two conditions of magnetic coupling [8,17]. The mag-
netic susceptibility data at 20–300 K for 1 are best fit
(b)
by the Baker equation for a 1D magnetic chain with
S = 1/2 spin [18,19]. Eq. (1) for H = ꢀ2 SiSj, with
P
y = J/2kT
ꢀ ꢁ
2=3
Ng2b2
4kT
C
D
vm ¼
;
ð1Þ
where
C = 1.0 + 5.79799y + 16.902653y2 + 29.376885y3 +
29.832959y4 + 14.036918y5 and D = 1.0 + 2.797991y +
7.0086780y2 + 8.653844y3 + 4.5743114y4.
Fig. 2. (a) The packing diagram of a unit cell for 1 as viewed along c-
axis. (b) Side view of the anions stack of 1 showing the uniform space
linear-chain of [Ni(mnt)2]ꢀ.
A fit of the data to Eq. (1) gives J = 0.42 cmꢀ1
and g = 2.26, TIP = 1.4 · 10ꢀ4 emu molꢀ1 with a final

356
C. Ni et al. / Chemical Physics Letters 396 (2004) 353–358
0.08
(a) 0.07
0.06
(a)
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0.006
0.005
0.004
0.003
0.002
0.001
)
l
-1
0.05
m
K
u
)
-1
(me
0.04
)
m
-1
X
l
o
mol
0.03
0.02
0.01
0.00
m
mu
(e
(emuG
0
2
6
8
10
12
14
16
18
20
4
x'
m
X
T(K)
-0.01
30
40
50
0
10
20
T(K)
0
50
100
150
200
250
300
T(K)
0.010
(b)
(b)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
0.005
0.000
-0.005
-0.010
)
-1
)
mol
-1
eum
(
mol
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
X'
)
-1
l
o
m
T(emuG
K
u
m
X
m
(
T
d
T/
m
dX
0
10
20
30
40
50
0
50
100
150
200
250
300
T(K)
T(K)
Fig. 4. In-phase (a) and out-phase (b) components of the a.c.
susceptibilities for 1 at 1633 Hz.
0
50
100
150
200
250
300
T(K)
Fig. 3. (a) Plot of vm vs. T for 1. The solid line are reproduced from the
theoretic calculations and detailed fitting procedure described in the
text. (b) The plot of vmT vs. T for 1 (inset: d(vmT)/dT vs. T).
for 1 is shown in Fig. 3a, and the corresponding param-
eters are given as follows: a = 7.71, D/kb = 92.0 K,
v0 = 2.3 · 10ꢀ4 emu molꢀ1
,
C = 1.06 · 10ꢀ2 emu K
P
agreement factor R = 2.1 · 10ꢀ4 (R is defined as
molꢀ1, and R = 4.5 · 10ꢀ8 (R is defined as
ðvcmalcd
ꢀ
P
2
2
2
2
ðvmcalcꢀ vombsdÞ =ðvombsdÞ Þ, which is in agreement with
the ferromagnetic coupling between adjacent S = 1/2
Ni spin carriers within the anion chain of 1 mentioned
above.
vombsdÞ = ðvmobsdÞ ). According to the results, the values
of the parameter 2D/kbTc (Tc is the transition tempera-
ture) are estimated to be 9.20 for 1, which is higher than
the ideal value of 3.53 derived from the BCS formula in
a weak coupling regime. This result thus means that the
short-range magnetic correlations within a chain are not
fully developed and intrinsic magneto elastic instability
of a 1D system cannot be considered as a driving force
for this transition; namely, the transition is not a pure
Spin-Peierls transition [10]. The origins of the phase
transition for 1 are attributed to cooperative interac-
tions of Niꢁ ꢁ ꢁS bonding, inter-plane repulsion of the
[Ni(mnt)2]ꢀ anions [20], pꢁ ꢁ ꢁp stacking interactions be-
tween of adjacent cations, spin-lattice interaction [21]
and spin–spin coupled interaction between nearest-
neighbor anions [22].
In the low-temperature phase (2–20 K), the vmT de-
creases exponentially and drops from 0.742
emu K molꢀ1 at 20 K to a minimum of 0.00837
emu K molꢀ1 at about 5 K as the sample temperature
decreases, and exhibits the characteristics of a spin gap
system. The transition temperature, 20 K, may be esti-
mated from the d(vmT)/dT vs. T plot (the inset of Fig.
3b). The magnetic susceptibility may be estimated by
the formula vm = [aexp(ꢀD/kBT)]/T + C/T + v0, where
a is a constant value corresponding to the dispersion
of excitation energy, D is the magnitude of the spin
gap, v0 contributes from the core diamagnetism and
the possible Van Vleck paramagnetism, and the other
symbols have their usual meaning. The best fit curve
To further determine the precise temperature of mag-
netic transition, ac magnetic susceptibility measure-

C. Ni et al. / Chemical Physics Letters 396 (2004) 353–358
357
ments are performed. In 1, v0 shows a maximum and the
peak (Fig. 4a) is very sharp at 20.75 K (1633 Hz), indi-
cating there exists effective magnetic transition but not
be suggests as the ferromagnetic ordering temperature.
It can be confirmed by the linear relationship of M vs.
H (Fig. 5a). Usually, in a magnet containing net mag-
netic moments in ordered state, there is a maximum in
the in-phase signal (v0) near Tc, and the out-of-phase sig-
nal (v00) appears at the temperature just below Tc. But in
1, a rounded maximum of nonzero v00 was found at
around 21.6 K (Fig. 4b). The increases of v0 and v00 val-
ues below around 5 K as the temperature is lowered,
confirming the occurrence of another magnetic phase
transition suggested by dc magnetic measurements.
The field-dependent magnetization of 1 was measured
at 19.7 K (Fig. 5a); the magnetization at the highest
measured field (70 kG) is 2123 emu molꢀ1, which is be-
characterization in the low-temperature region, the
field-dependent magnetization of 1 at 2 K measured
under different fields (Fig. 5b). The magnetization in-
creases very rapidly before the field reaches 10 kG,
above which the magnetization saturates rapidly upon
increasing the field. The saturation magnetization is
16.34 emu G molꢀ1, far below the saturation value
(5585 emu molꢀ1) expected for an S = 1/2 system,
consistent with weak ferromagnetism duo to spin cant-
ing (the small canting angle a is about 0.17ꢁ deduced
from sina = Mw/Ms). Spin canting arises through a Dzy-
aloshinsky–Moriya interaction [23–26], which minimizes
the coupling energy when two spins are perpendicular to
one another, and this magnitude is proportional to Dg/g.
The results of the structural analysis in the room tem-
perature of 1 indicate that a symmetric element is pre-
sent in
a P21/c space group; the antisymmetric
low the theoretical saturation value 5585 emu molꢀ1
.
exchange is excluded as a factor accounting for the spin
canting. So only the local anisotropy would account for
the magnetic behavior observed. No hysterisis hoop was
observed at 2 K.
The linearity of the curve is consistent with the presence
of short-range magnetic ordering.
The final increase of the magnetic susceptibility at
very low temperature (5 K) may due to the presence of
weak ferromagnetic interaction of 1. To get a closer
4. Conclusion
(a) 2500
The crystal structure of 1 at room temperature illus-
trate that the NiðmntÞꢀ anions and [BrFBzNH2Py]+ cat-
2
2000
ions of 1 posses well-separated stacking column. The
NiðmntÞꢀ anions form a 1D completely segregated uni-
-1
2
1500
1000
500
0
form magnetic chains through intermolecular Sꢁ ꢁ ꢁS,
Niꢁ ꢁ ꢁNi, Sꢁ ꢁ ꢁNi, or pꢁ ꢁ ꢁp interactions. The measure-
ments of the temperature dependence of magnetic sus-
ceptibility for 1 in the temperature range 2–300 K
reveal the occurrence of significant ferromagnetic inter-
action in the high-temperature phase (HT), spin-gapped
system in the low-temperature phase (LT), and weak fer-
romagnetism due to spin canting below 5 K.
euGmol
M
1
4
7
0
2
3
5
6
8
H (T)
5. Supplementary material
18
16
14
12
10
8
(b)
Supplementary crystallographic data are available
from the Cambridge Crystallographic Data Center,
CCDC No. 242815. Copies of this information may be
obtained free of charge from The Director, CCDC, 12
Union Road, Cambridge, CB2 1EZ, UK (fax: +44
1223 336033; deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
-1
euGmol
6
M
4
2
0
Acknowledgements
-2
0
2
4
1
3
5
The authors are grateful for financial support from
the National Natural Science Foundation of China
(Project No. 20171022). The Center of Analysis and
Determination of Nanjing University are acknowledged.
H (T)
Fig. 5. Magnetization vs. H for 1 at the temperature of 19.7 K (a) and
2 K (b).

358
C. Ni et al. / Chemical Physics Letters 396 (2004) 353–358
[13] K. Brunn, H. Endres, J. Weiss, Z. Naturforsch. 42B (1987)
1222.
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