Two quasi-one-dimensional molecular solids based on Ni(mnt)2 anion: Crystal structures, spectra, and magnetic properties

Article abstract of DOI:10.1016/j.ica.2005.12.050

Two new molecular solids, [DiClBzPyNH₂][Ni(mnt)₂] (1) and [DiClBzPyN(CH₃)₂][Ni(mnt)₂] (2) ([DiClBzPyNH₂]⁺ = 1-(2′,4′-dichlorobenzyl)-4- aminopyridinium, [DiClBzPyN(CH₃)₂]⁺ = 1-(2′,4′-dichlorobenzyl)-4-dimethylaminopyridinium, and mnt ^2- = maleonitriledithiolate) have been prepared and characterized by elemental analyses, IR, MS spectra, single-crystal X-ray diffraction and magnetic susceptibility. The Ni(III) ions of both 1 and 2 form a 1D zigzag magnetic chain within a Ni(mnt)2- column through Ni?S, S?S, Ni?Ni, or π?π interactions. The chain is alternating in 1 with Ni?Ni distances of 3.869 and 4.074 A?, while it is uniform in 2 with Ni?Ni distances of 4.159 A?. Magnetic susceptibility measurements in the temperature range 1.8-300 K show that 1 exhibits antiferromagnetic behavior, while 2 shows an unusual magnetic phase transition around 140 K, and antiferromagnetic interaction in the high-temperature (HT) phase and spin gap in the low-temperature (LT) phase. The phase transition for 2 is second order by determination of DSC analyses.

Full text of DOI:10.1016/j.ica.2005.12.050

Inorganica Chimica Acta 359 (2006) 1383–1389
www.elsevier.com/locate/ica
Two quasi-one-dimensional molecular solids based on Ni(mnt)2
anion: Crystal structures, spectra, and magnetic properties
a,b,
a
a,b
Chunlin Ni
*, Linliang Yu , Lemin Yang
a
Department of Applied Chemistry, College of Science, South China Agricultural University, Wushan, Tianhe District, Guangzhou 510642, PR China
b
Centre of Inorganic Functional Materials, South China Agricultural University, Guangzhou 510642, PR China
Received 17 August 2005; received in revised form 6 December 2005; accepted 29 December 2005
Available online 20 February 2006
Abstract
Two new molecular solids, [DiClBzPyNH2][Ni(mnt)2] (1) and [DiClBzPyN(CH3)2][Ni(mnt)2] (2) ([DiClBzPyNH2]+ = 1-(20,40-dichlo-
robenzyl)-4-aminopyridinium, [DiClBzPyN(CH3)2]+ = 1-(20,40-dichlorobenzyl)-4-dimethylaminopyridinium, and mnt2ꢀ = maleonitril-
edithiolate) have been prepared and characterized by elemental analyses, IR, MS spectra, single-crystꢀal X-ray diffraction and
magnetic susceptibility. The Ni(III) ions of both 1 and 2 form a 1D zigzag magnetic chain within a NiðmntÞ2 column through Niꢁ ꢁ ꢁS,
˚
Sꢁ ꢁ ꢁS, Niꢁ ꢁ ꢁNi, or pꢁ ꢁ ꢁp interactions. The chain is alternating in 1 with Niꢁ ꢁ ꢁNi distances of 3.869 and 4.074 A, while it is uniform in 2
˚
with Niꢁ ꢁ ꢁNi distances of 4.159 A. Magnetic susceptibility measurements in the temperature range 1.8–300 K show that 1 exhibits anti-
ferromagnetic behavior, while 2 shows an unusual magnetic phase transition around 140 K, and antiferromagnetic interaction in the
high-temperature (HT) phase and spin gap in the low-temperature (LT) phase. The phase transition for 2 is second order by determi-
nation of DSC analyses.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Maleonitriledithiolate; 1-(20,40-Dichlorobenzyl)-4-aminopyridinium; 1-(20,40-Dichlorobenzyl)-4-dimethylaminopyridinium; Crystal structures;
Antiferromagnetic coupling; Spin gap
1. Introduction
diamagnetism, meta-magnetism and spin-Peierls-like tran-
sitions [9–19]. The quasi-one-dimensional magnetic prop-
erty of these complexes was attributed to the segregated
columnar stacks of anions and cations, and intermolecular
Niꢁ ꢁ ꢁS, Sꢁ ꢁ ꢁS, Niꢁ ꢁ ꢁNi or pꢁ ꢁ ꢁp interactions within the anio-
nic columns. Following the approach of using multifunc-
tional organic cations containing substituted pyridinium
that promote short Niꢁ ꢁ ꢁS, Sꢁ ꢁ ꢁS, Niꢁ ꢁ ꢁNi or pꢁ ꢁ ꢁp interac-
tions in a [M(mnt)2]ꢀ column, two novel molecular solids
1-(20,40-dichlorobenzyl)-4-aminopyridinium-bis(maleonitri-
ledithiolato)nickel ([DiClBzPy][Ni(mnt)2]) (1) and 1-(20,40-
dichlorobenzyl)-4-dimethylaminopyridinium-bis(maleoni-
triledithiolato)nickel ([DiClBzPy(CH3)2][Ni(mnt)2]) (2)
have been prepared. Their spectra, crystal structures and
magnetic properties have been investigated. To the best of
our knowledge, 2 is the first 1D magnetic chain that exhibits
a spin gap with substituted 4-dimethylaminopyridinium in
the Ni(mnt)2 complexes.
Since the discovery in 1996 of the ferromagnetic ion-pair
complex, NH4 Æ Ni(mnt)2 Æ H2O (mnt2ꢀ = maleonitriledi-
thiolate) [1], a large variety of molecular solids based on
[M(mnt)2]ꢀ (M = Ni, Pd, or Pt ions) have been synthesized
and investigated [2–8]. More recently, substantial efforts
have been devoted to finding more suitable cations to tune
the crystal stacking structure of [M(mnt)2]ꢀ anion with a
view to obtaining ideal molecular magnets. By using ben-
zylpyridinium derivatives ([RbzPy]+) as the counter-cation
of [M(mnt)2]ꢀ (M = Ni, Pd, or Pt), we have obtained a ser-
ies of new ion-pair complexes that exhibit versatile magnetic
properties such as ferromagnetic ordering at low tempera-
ture, magnetic transition from ferromagnetic coupling to
*
Corresponding author. Tel.: +86 20 85280318; fax: +86 20 85282366.
E-mail address: scauchemnicl@163.com (C. Ni).
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.12.050

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C. Ni et al. / Inorganica Chimica Acta 359 (2006) 1383–1389
2. Experimental
and the experimental data were corrected for diamagnetism
of the constituent atoms estimated from Pascal’s constants.
2.1. General materials and techniques
2.2. Syntheses of complexes
The starting materials, 1-(20,-40-dichlorobenzyl)-4-amin-
opyridinium bromide ([DiClBzPyNH2]Br), 1-(20,40-dichlo-
robenzyl)-4-dimethylaminopyridinium bromide ([DiClBz-
PyN(CH3)2]Br) and disodium maleonitriledithiolate
(Na2mnt), were synthesized following the literature
procedures [20,21]. A similar method for preparing
[Bu4N]2[Ni(mnt)2] was utilized to prepare [DiClBz-
PyNH2]2[Ni(mnt)2] and [DiClBzPyN(CH3)2]2[Ni(mnt)2]
[21].
Elemental analyses were run on a Model 240 Perkin–
Elmer C H N instrument. IR spectra were recorded on
an IF66V FT-IR (400–4000 cmꢀ1 region) spectrophotome-
ter in KBr pellets. The electrospray mass spectra (ESI-MS)
were determined on a Finnigan LCQ mass spectrograph,
with a sample concentration of ca. 1.0 mmol dmꢀ3. Ther-
mal analyses of crushed polycrystalline samples for 2
placed in an aluminum crucible were performed on warm-
ing (rate of 20 K molꢀ1) from 96 to 293 K. Magnetic sus-
ceptibility data on crushed polycrystalline samples of 1
and 2 were collected over the temperature range of 1.8–
300 K using a Quantum Design MPMS-5S super-conduct-
ing quantum interference device (SQUID) magnetometer,
2.2.1. [DiClBzPyNH2][Ni(mnt)2] (1)
[DiClBzPyNH2]2[Ni(mnt)2] (0.848 g, 1.0 mmol) was dis-
solved in 20 cm3 of MeCN. A solution of I2 (0.150 g,
0.59 mmol) in MeCN (10 cm3) was slowly added. The mix-
ture was stirred and a deep red solution was formed almost
immediately. The mixture was continuously stirred for 1 h,
and then 50 cm3 i-PrOH was added. After the mixture was
allowed to stand overnight, the produced 0.498 g dark
microcrystals were filtered off, washed with CH3OH and
Et2O and dried in a vacuum. Yield: 84.0%. Anal. Calc.
for C20H11NiN6S4Cl2: C, 40.50; H, 1.87; N, 14.17. Found:
C, 40.35; H, 2.00; N, 14.06%. IR (cmꢀ1): 3418s, 3095w,
3071w, 2207s, 1656s, 1590s, 1561s, 1537m, 1474s, 1450s,
1375s, 1349s.
2.2.2. [DiClBzPyN(CH3)2][Ni(mnt)2] (2)
The procedure for preparing 2 was similar to that for 1.
Yield: 90.2%. Anal. Calc. for C22H15NiN6S4Cl2: C, 42.53;
H, 2.43; N, 13.53. Found: C, 42.36; H, 2.51; N, 13.42%.
IR (cmꢀ1): 3096w, 3071w, 2209s, 1657s, 1579s, 1558s,
1535m, 1475s, 1435s, 1387s, 1353s.
Table 1
Crystal data and structure refinements for 1 and 2
Compound
1
2
Temperature
293 (2)
293 (2)
Empirical formula
Formula weight
Wavelength
C
593.20
0.71073
20H11NiN6S4Cl2
C22H15NiN6S4Cl2
621.25
0.71073
Pnma
ꢀ
Space group
P1
Crystal system
Unit cell dimensions
triclinic
orthorhombic
˚
a (A)
7.211(2)
20.634(5)
˚
b (A)
12.476(3)
14.664(4)
70.57(1)
76.04(1)
76.37(1)
1189.9(5)
2
1.656
1.413
598
7.058(2)
18.035(5)
90
90
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
90
3
˚
V (A )
2626.5(12)
4
1.571
1.284
1260
0.40 · 0.20 · 0.10
empirical (SHELXA)
0.74 and 0.88
2.46–27.38
Z
Dcalc (g/cm3)
Absorption coefficient (mmꢀ1
F(000)
)
Crystal size (mm3)
Absorption correction
0.26 · 0.20 · 0.15
empirical (SHELXA)
0.69 and 0.81
2.99–27.39
Maximum and minimum transmission
h Range for data collection (ꢁ)
Limiting indices
ꢀ8 6 h 6 8, ꢀ14 6 k 6 14, ꢀ17 6 l 6 16
ꢀ16 6 h 6 25, ꢀ8 6 k 6 8, ꢀ22 6 l 6 22
Reflections collected
5912
13677
Independent reflections
Rint
4114
0.040
2801
0.046
Refinement method
full-matrix least-squares on F2
1.068
full-matrix least-squares on F2
1.005
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
Final R indices (all data)
R1 = 0.0620, wR2 = 0.1695
R1 = 0.0703, wR2 = 0.1768
0.88 and ꢀ0.88
R1 = 0.0381, wR2 = 0.0531
R1 = 0.0826, wR2 = 0.0846
0.54 and ꢀ0.98
ꢀ3
˚
Largest difference in peak and hole (e A
)

C. Ni et al. / Inorganica Chimica Acta 359 (2006) 1383–1389
1385
Single crystals suitable for X-ray structure analyses were
obtained by evaporating the solution of 1 and 2 in a mixed
solution of MeCN and i-PrOH (1:2 v/v).
2.3. Single-crystal X-ray structure determination
The measurements of the studied complexes 1 and 2
were performed on a Smart APEX CCD area detector
using graphite-monochromated Mo Ka radiation
˚
(k = 0.71073 A) by x scan mode within the angular range
2.99ꢁ < h < 27.39ꢁ for 1 and 2.42ꢁ < h < 27.38ꢁ for 2. Space
group, lattice parameters, and other relevant information
are listed in Table 1. The structures were solved by direct
methods and refined on F2 by full-matrix least-squares,
employing Bruker’s SHELXTL [22]. All non-hydrogen atoms
were refined with anisotropic thermal parameters. All H
atoms were placed in calculated positions, assigned fixed
isotropic displacement parameters 1.2 times the equivalent
isotropic U value of the attached atom, and allowed to ride
on their respective parent atoms.
a
3. Results and discussion
3.1. Descriptions of structures
Complex 1 crystallizes in the monoclinic space group
P21/n with one couple of [Ni(mnt)2]ꢀ and [DiC-
lBzPyNH2]+ in an asymmetric unit. An ORTEP drawing
of 1 with atomic labeling is shown in Fig. 1a. Selected
bond distances, bond angles, and intermolecular Niꢁ ꢁ ꢁNi,
Niꢁ ꢁ ꢁS and Sꢁ ꢁ ꢁS distances are summarized in Table 2.
The Ni(III) ion in the [Ni(mnt)2]ꢀ anion is coordinated
by four sulfur atoms of two mnt2ꢀ ligands, and exhibits
square-planar coordination geometry. The CN groups
are slightly tipped out of the plane, and the deviations
˚
˚
from the plane are ꢀ0.1517 A for N(1), ꢀ0.0723 A for
˚
˚
N(2), 0.1103 A for N(3) and 0.0729 A for N(4). The aver-
age S–Ni–S bond angle within the five-membered ring is
90.00(6)ꢁ, and the average Ni–S bond distance is
b
˚
Fig. 1. (a) ORTEP plot (30% probability ellipsoids) showing the molec-
ular structure of 1. (b) The packing diagram of a unit cell for 1 as viewed
along a-axis.
2.141(2) A; these values are in agreement with those of
the previously reported [Ni(mnt)2]-complexes [23]. The
cation, [DiClBzPyNH2]+, adopts a conformation in which
the phenyl and pyridine rings are twisted to the reference
plane C(14)–C(15)–N(5) (dihedral angles of 31.6ꢁ and
79.8ꢁ, respectively). The phenyl ring and the pyridine ring
make a dihedral angle of 90.4ꢁ. The two Cl atoms are
slightly away from the phenyl ring plane; deviations from
˚
to the center of the neighboring benzene ring is 3.464 A)
(Fig. 2b), and the cation–cation interactions give rise to
a cation column with a rectangular channel extending
along the crystallographic a-axis. It is worth noting that
three intermolecular hydrogen bonds between the anions
and cations were observed in the crystal structure of 1
as listed in Table 3. The anion–anion, anion–cation and
cation–cation contacts may play important roles in the
packing and stabilizing of 1. The closest Niꢁ ꢁ ꢁNi separa-
˚
˚
the plane are ꢀ0.0134 A for Cl(1) and 0.0091 A for Cl(2).
As shown in Fig. 1b, the well-segregated columns of the
anions and cations are stacked along the direction of a-
axis. Within an anion column, the Ni(III) ions form a
1D alternating chain with Niꢁ ꢁ ꢁNi distances being
˚
˚
˚
tion between anion columns is 12.633 A, which is signifi-
3.869 A, and 4.074 A through intermolecular Niꢁ ꢁ ꢁS,
Sꢁ ꢁ ꢁS, Niꢁ ꢁ ꢁNi or pꢁ ꢁ ꢁp interactions (Fig. 2a). The signif-
icant interaction in adjacent cations is p–p interaction
between chlorine atom of one phenyl ring and the plane
of another phenyl ring (the contact distance of Cl(2) atom
cantly longer than that of Niꢁ ꢁ ꢁNi separations within a
˚
chain (3.869 and 4.074 A). Therefore, 1 can be considered
a 1D alternating magnetic chain system from the point of
view of the structure.

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C. Ni et al. / Inorganica Chimica Acta 359 (2006) 1383–1389
Table 2
Table 3
˚
Selected bond parameters and intermolecular contacts for 1 and 2
Hydrogen bonds for 1 and 2 (A and ꢁ)
Compound
1
2
D–Hꢁ ꢁ ꢁA
Complex 1
d(D–H)
d(Hꢁ ꢁ ꢁA)
d(Dꢁ ꢁ ꢁA)
\(DHA)
˚
Bond distances (A)
Ni(1)–S(1)
Ni(1)–S(2)
Ni(1)–S(3)
Ni(1)–S(4)
S(1)–C(2)
S(2)–C(3)
S(3)–C(6)
S(4)–C(7)
2.146(2)
2.146(2)
2.135(2)
2.135(2)
1.709(6)
1.723(6)
1.714(6)
1.715(6)
2.132(2)
2.142(1)
2.138(2)
2.137(1)
1.719(6)
1.710(4)
1.705(6)
1.708(4)
N(6)–H(6A)ꢁ ꢁ ꢁN(1)#1
N(6)–H(6B)ꢁ ꢁ ꢁN(1)#2
C(15)–H(15)ꢁ ꢁ ꢁN(3)#3
0.86
0.86
0.97
2.40
2.35
2.53
3.256(10)
3.098(10)
3.455(10)
179.0
145.0
160.0
Complex 2
C(10)–H(10)ꢁ ꢁ ꢁN(2)#4
0.93
0.93
2.42
2.42
3.346(5)
3.346(5)
174.0
174.0
C(10)–H(10)ꢁ ꢁ ꢁN(2)#5
Symmetry transformations used to generate equivalent atoms: #1: ꢀx,
ꢀy + 1, ꢀz + 1; #2: x, y + 1, z; #3: ꢀx + 2, ꢀy + 1, ꢀz; #4: x + 1/2,
ꢀy + 1/2, ꢀz + 1/2; #5: x + 1/2, y, ꢀz + 1/2.
Bond angles (ꢁ)
S(1)–Ni(1)–S(2)
S(1)–Ni(1)–S(4)
S(3)–Ni(1)–S(4)
S(2)–Ni(1)–S(3)
92.82(6)
87.57(5)
92.15(6)
87.46(6)
92.29(5)
87.75(5)
91.95(5)
88.01(5)
and the geometry and stacking pattern for [DiC-
lBzPyN(CH3)2]+ in 2 are significantly different from that
of 1. The C(15)–C(14)–N(5) reference plane is completely
˚
Intrachain distances (A)
Niꢁ ꢁ ꢁNi (nearest separation)
Niꢁ ꢁ ꢁS
3.869, 4.074
3.734, 3.542
3.957, 3.935
4.159
3.649
3.743
Sꢁ ꢁ ꢁS
a
a
b
Fig. 2. (a) Side view of the anions stack of 1 and 2 showing the uniform
space linear-chain of [Ni(mnt)2]ꢀ. (b) Intermolecular contacts in two
neighboring cations of 1.
The coordination geometry and stacking pattern for
[Ni(mnt)2]ꢀ anion of 2 are essentially identical to those
described above for 1. Although the anions and the cations
also form segregated columns (Fig. 3a), the anions in an
anionic column of 2 form a zigzag uniform chain
(Fig. 2a) through Niꢁ ꢁ ꢁNi, Sꢁ ꢁ ꢁS, and pꢁ ꢁ ꢁp interactions,
b
Fig. 3. (a) The packing diagram of a unit cell for 2 as viewed along b-axis.
(b) Boat-type conformation and a 1D chain through p–p stacking
interactions among the phenyl rings of neighboring cations for 2.

C. Ni et al. / Inorganica Chimica Acta 359 (2006) 1383–1389
1387
perpendicular to the pyridine ring, which is sharing a plane
with the benzene ring. The benzene rings of the neighbor-
ing benzyl moieties are parallel to each other. This confor-
3.0
2.5
2.0
1.5
1.0
0.5
0.0
ꢀ
mation of the cation is very rare in the NiðmntÞ2
complexes. In a cationic column, the adjacent cations are
stacked in boat-type conformation (Fig. 3b), and form a
1D column through weak pꢁ ꢁ ꢁp stacking interactions
among the phenyl rings of the neighboring cations (the
contact distances of C(9)ꢁ ꢁ ꢁC(12), C(10)ꢁ ꢁ ꢁC(13), and
˚
C(11)ꢁ ꢁ ꢁC(14) are 3.813, 3.797, and 3.813 A). The most
interesting fact in 2 is that the four columns of cations cre-
ate a cubic-like large channel (extending along the crystal-
ꢀ
lographic b-direction) within which the NiðmntÞ2 columns
are located(Fig. 3a). It is worth noting that two intermolec-
ular hydrogen bonds between anions and cations were
observed in the crystal structure of 2 as listed in Table 3.
The anion–anion, anion–cation and cation–cation contacts
may play important roles in the crystal packing and stabil-
ization of 2. Selected bond distances, bond angles, and
intermolecular Niꢁ ꢁ ꢁNi, Niꢁ ꢁ ꢁS and Sꢁ ꢁ ꢁS distances of 2
are summarized in Table 2.
0
50
100
150
200
250
300
a
T (K)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
3.2. Infrared spectra and electrospray mass spectra
25
20
15
10
5
The infrared spectra of the two complexes 1 and 2 are very
much consistent with the structural data presented above.
The band at 3418 (s) cmꢀ1 for 1 is attributable to N–H
stretching vibration of amino group. The bands at 3095,
3071 cmꢀ1 for 1 and 3096, 3071 cmꢀ1 for 2 are due to the
stretching vibration frequencies of C–H in the aromatic ring.
Complexes 1 and 2 show one strong band at 2207 and
2209 cmꢀ1, respectively, which are CN stretching bands.
The bands at 1656, 1590, and 1561 cmꢀ1 for 1, and 1657,
1579, and 1558 cmꢀ1 for 2 are attributable to m(C@N),
m(C@C) stretching bands for the phenyl and pyridine rings.
The m(C@C) band of mnt2ꢀ is at 1450 cmꢀ1 for 1 and
1435 cmꢀ1 for 2. The negative-ion and positive-ion ESI-
MS spectra of 1 and 2 in MeCN solution show that the mass
spectrum is dominated by the 340.2 peak, which is due to
[Ni(mnt)2+H]ꢀ; the peaks at 254.3 and 282.1 are assigned
to [DiClBzPyNH2]+ and [DiClBzN(CH3)2]+, respectively.
0
0
50 100 150 200 250 300
T (K)
0
50
100
150
200
250
300
T (K)
b
Fig. 4. Plots of vm vs. T for 1 (a) and 2 (b) (inset: plot of d(vmT)/dT vs. T).
The solid lines are reproduced from the theoretic calculations and detailed
fitting procedure described in the text.
ously cooled, the magnetic susceptibilities decrease expo-
nentially (Fig. 4b), indicating that
2 exhibits the
characteristics of a spin gap system [24]. On increasing
the temperature back to the original one, the same vm–T
curves are obtained without hysteresis effect. These findings
indicate that complex 2 undergoes a reversible phase tran-
sition. The phase transition temperature is evaluated as the
temperature at the maximum of the d(vmT)/dT derivative,
that is, ꢂ140 K for 2 (inset of Fig. 4b).
3.3. Magnetic properties
The temperature dependence of the magnetic suscepti-
bility for 1 and 2 is measured under an applied field of
1000 Oe in the temperature range 1.8–300 K. The plots of
vm versus T for 1 and 2 are shown in Fig. 4, where vm is
the magnetic susceptibility per nickel atom corrected by
the diamagnetic contribution. The overall magnetic behav-
iors of 1 and 2 correspond to a paramagnetic system with
an antiferromagnetic coupling interaction, but as for 2,
when the temperature decreases, the value of vmT slightly
decreases from 0.187 emu K molꢀ1 at 300 K (the value is
slightly higher than that expected for the magnetically iso-
lated Ni(III) ions) to 0.0697 emu K molꢀ1 at 140 K. In the
low-temperature (LT) phase, as the samples are continu-
˚
As for 1, due to the Ni(1)ꢁ ꢁ ꢁNi(1A) distance (4.248 A)
˚
being larger than the Ni(1)ꢁ ꢁ ꢁNi(1B) distance (3.842 A) in
nickel(III) ion chains from the point of view of structure
analysis (Fig. 2a), therefore the nickel(III) ions magnetic
interactions can be neglected and the temperature-depen-
dent magnetic susceptibility data of 1 can be modeled by
the following equation [9]:
vm ¼ ðNb2g2=kTÞ=ð3 þ expðD=kT ÞÞ þ C=T þ v0;
where the first term represents contributions from the sus-
ð1Þ
ceptibility of a dimer with spin one-half, D is the energy

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C. Ni et al. / Inorganica Chimica Acta 359 (2006) 1383–1389
separation between triplet and singlet, C is the Curie con-
stant for the uncoupled paramagnetic impurity, and v0 is
the sum of both core diamagnetic and Van Vleck paramag-
netic contributions. The best-fit parameters obtained by
least-squares fit are g = 2.01, D/k = 2384 K, C = 6.2 ·
Thermodynamic property of the phase transition for 2
was determined. The power-compensated DSC trace for 2
from 96 to 293 K at a warming rate of 20 K minꢀ1 is dis-
played in Fig. 5. No detectable endothermic peak in the
corresponding temperature region was observed, indicating
that the magnetic transition of 2 is a second-order transi-
tion [16,18].
10ꢀ3 emu K molꢀ1
,
v0 = 3.8 · 10ꢀ4 emu molꢀ1
and
P
2
R = 3.3 · 10ꢀ7 (R is defined as
ðvcmalcd ꢀ vombsdÞ =
P
2
ðvombsdÞ ). The model provides an excellent fit (the solid
lines Fig. 4a), as indicated by the low value of R.
4. Conclusion
The spin-gapped system has been attracting extensive
interest recently [25–27], but it this situation is very rare
for the Ni(III) complexes [12,18]. The magnetic susceptibil-
ities of 1 may be estimated by the following formula:
In this paper, the syntheses, spectra, crystal structures
and magnetic propeꢀrties of two new ion-pair complexes
ꢀ
containing NiðmntÞ2 anion are reported. The NiðmntÞ2
anions and cations of complexes 1 and 2 form completely
segregated stacking columns, and the Ni(III) ions form a
1D zigzag chain through Niꢁ ꢁ ꢁS, Sꢁ ꢁ ꢁS, Niꢁ ꢁ ꢁNi, or pꢁ ꢁ ꢁp
vm ¼ a expðꢀD=kTÞ=T þ C=T þ 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 meanings [28]. The best fit curves are
shown in Fig. 4b, and the corresponding parameters are
given as follows: a = 12.43, D/k = 742.32 K, C = 3.6 ·
ꢀ
interactions within a NiðmntÞ2 column. The measurement
of the temperature dependence of the magnetic susceptibil-
ities reveals that 1 shows the magnetic behavior of an iso-
lated antiferromagnetic dimer with a large energy gap,
while 2 exhibits an unusual magnetic phase transition
around 140 K, and also exhibits antiferromagnetic interac-
tion in the high-temperature (HT) phase and spin gap in
the low-temperature (LT) phase. The phase transition for
2 is second order by the determination of DSC analyses.
10ꢀ4 emu K molꢀ1, v0 = 5.0 · 10ꢀ5 emu molꢀ1, and R =
P
2
2
5.3 · 10ꢀ6 (R is defined as ðvmcalcd ꢀ vombsdÞ =ðvmobsdÞ ). The
value of the parameter 2D/kTc (Tc is the transition temper-
ature) is estimated to be 10.60 on the basis of the above re-
sults, which is significantly higher than the ideal value of
3.53 derived from the BCS formula in a weak coupling re-
gime. The result suggests 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; that is,
the transition is not a pure spin-Peierls transition [12,18].
The origins of the phase transition for 2 are attributed to
cooperative interactions of Niꢁ ꢁ ꢁS bonding, interplane
repulsion of the [Ni(mnt)2]ꢀ anions, pꢁ ꢁ ꢁp stacking interac-
tions between the adjacent cations, spin–lattice interac-
tions, and spin–spin coupled interaction between the
nearest-neighbor anions [29–32].
5. Supplementary materials
Details of the crystallographic data of 1 and 2 have been
deposited at the Cambridge Crystallographic Data Center
as supplementary publication Nos. CCDC-281115 and
CCDC-281114. Copies of the data may be obtained free of
charge from The Director, CCDC, 12 Union Road, Cam-
bridge CB2 1EZ, UK (fax: +44 1223 336 033; e-mail: depos-
it@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
The authors thank the president’s science foundation of
South China Agricultural University (No. 2005K092) for
financial support of this work. The Center of Analysis
and Determination of South China Agricultural University
is acknowledged.
30
25
20
15
10
5
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