Syntheses, spectra, crystal structures and magnetic properties of molecular solids based on the M(mnt)2- ion (M = Ni, Pt)

Article abstract of DOI:10.1016/j.poly.2005.04.032

Two new ion-pair complexes [1-(4′-nitrobenzyl)-4-aminopyridinium]- bis(maleonitriledithiolato)nickel ([NO₂BzPyNH₂][Ni(mnt) ₂]) (1) and [1-(4′-nitrobenzyl)-4-aminopyridinium]- bis(maleonitriledithiolato)platinum ([NO₂BzPyNH₂][Pt(mnt) ₂]) (2) have been prepared and characterized by elemental analyses, IR, electronic and MS spectra, single crystal X-ray diffraction and magnetic susceptibility. 1 and 2 crystallize in the monoclinic space group P2 ₁/c, a = 12.331(3) ?, b = 26.937(6) ?, c = 7.322(2) ?, V = 2349.0(10) ?³, Z = 4 for 1, and a = 10.600(3) ?, b = 8.376(2) ?, c = 27.089(7) ?, V = 2362.2(11) ?³, Z = 4 for 2. The Ni(III) ions of 1 form a 1D zigzag chain within a Ni(mnt)2- column, while 2 is a dimer. Magnetic susceptibility and DSC measurements for 1 in the temperature range 2.0-300 K show the occurrence of an unusual magnetic phase transition around 160 K, and an antiferromagnetic interaction in the high-temperature phase (HT) and a spin gap in the low-temperature phase (LT), while 2 is diamagnetic.

Full text of DOI:10.1016/j.poly.2005.04.032

Polyhedron 24 (2005) 1669–1676
www.elsevier.com/locate/poly
Syntheses, spectra, crystal structures and magnetic properties
ꢀ
of molecular solids based on the MðmntÞ2 ion (M = Ni, Pt)
a
a
a
b
a
Chun-Lin Ni , Yi-Zhi Li , Dong-Bin Dang , Song Gao , Zhao-Ping Ni ,
a
a
a,
*
Zheng-Fang Tian , Li-Li Wen , Qing-Jin Meng
a
State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing University, 22 Hankou Road, 210093 Nanjing, PR China
b
State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, 100871 Beijing, PR China
Received 14 January 2005; accepted 15 April 2005
Available online 14 July 2005
Abstract
Two new ion-pair complexes [1-(40-nitrobenzyl)-4-aminopyridinium]-bis(maleonitriledithiolato)nickel ([NO2BzPyNH2][Ni(mnt)2])
(1) and [1-(40-nitrobenzyl)-4-aminopyridinium]-bis(maleonitriledithiolato)platinum ([NO2BzPyNH2][Pt(mnt)2]) (2) have been pre-
pared and characterized by elemental analyses, IR, electronic and MS spectra, single crystal X-ray diffraction and magnetic
˚
˚
˚
susceptibility. 1 and 2 crystallize in the monoclinic space group P21/c, a = 12.331(3) A, b = 26.937(6) A, c = 7.322(2) A, V =
3
3
˚
˚
˚
˚
˚
2349.0(10) A , Z = 4 for 1, and a = 10.600(3) A, b = 8.376(2) A, c = 27.089(7) A, V = 2362.2(11) A , Z = 4 for 2. The Ni(III) ions
ꢀ
of 1 form a 1D zigzag chain within a NiðmntÞ2 column, while 2 is a dimer. Magnetic susceptibility and DSC measurements for 1 in
the temperature range 2.0–300 K show the occurrence of an unusual magnetic phase transition around 160 K, and an antiferromag-
netic interaction in the high-temperature phase (HT) and a spin gap in the low-temperature phase (LT), while 2 is diamagnetic.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Maleonitriledithiolate; 1-(40-Nitrobenzyl)-4-aminopyridinium; Nickel (III) complex; Platinum (III) complex; Crystal structures; Magnetic
properties
1. Introduction
[12]. One of the interesting works in our laboratory is
to develop a new class of ion-pair complexes [RbzPy]+-
[M(mnt)2]ꢀ ([RbzPy]+ = benzylpyridinium derivative,
M = Ni, Pt). We have found that the prominenꢀt feature
of these ion-pair complexes is that the NiðmntÞ2 anions
and [RbzPy]+ cations stack into well-segregated columns
in the solid state, and they exhibit versatile magnetic
properties such as ferromagnetic ordering at low temper-
ature [13], magnetic transitions from ferromagnetic cou-
pling to diamagnetism [14], meta-magnetism [15] and
spin-Peierls-like transitions [16–18]. In the continuing re-
search with [M(mnt)2]ꢀ complexes, we have focused on
finding more suitable multifunctional organic cations
to tune the crystal stacking structure of the [M(mnt)2]ꢀ
anion with a view to obtaining ideal molecular magnets.
Herein, we report the syntheses, spectra, crystal struc-
tures and magnetic properties of two novel 1D molecular
Metal complexes of bis-dithiolenes have been widely
investigated due to their application as building blocks
in molecular-based materials showing magnetic, super-
conducting and optical properties [1–3]. The complexes
containing [M(mnt)2]ꢀ(mnt2ꢀ = maleonitriledithiolate,
M = Ni(III) or Pt(III) ions) have attracted much interest
because they show novel magnetic properties [4–11].
Especially the discovery in 1996 of the ferromagnetic
ꢀ
complex containing the NiðmntÞ2 ion, NH4 Æ Ni(mnt)2 Æ
H2O, strongly stimulated the study on Ni(mnt)2 com-
plexes as building blocks for new molecular magnets
*
Corresponding author. Tel: +86 25 83597266; fax: +86 25
83314502.
E-mail address: njuchem1024@163.com (Q.-J. Meng).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.04.032

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C.-L. Ni et al. / Polyhedron 24 (2005) 1669–1676
solids [1-(40-nitrobenzyl)-4-aminopyridinium]-bis(male-
onitriledithiolato)nickel([NO2BzPyNH2][Ni(mnt)2]) (1)
Single crystals suitable for X-ray structure analysis
were obtained by evaporating a solution of 1 in a mixed
solution of MeCN and i-PrOH (1:2 v/v).
and
[4-aminopyridinium]-bis(maleonitriledithiolato)-
platinum ([NO2BzPyNH2][Pt(mnt)2])(2). To the best of
our knowledge, a uniform chain exhibiting an unusual
magnetic transition is very rare in the Ni(mnt)2
complexes.
2.2.2. [NO2BzPyNH2][Pt(mnt)2] (2)
The procedure for preparing 2 was similar to that for
1. Yield: 87.5%. Anal. Calc. for C20H12PtN7S4O2: C,
34.04; H, 1.71; N, 13.89. Found: C, 34.12; H, 1.84; N,
13.76%. IR (cmꢀ1): 3371.9s, 3070.4w, 2205.9s, 1648.2s,
1604.8m, 1564.4m, 1535.2s, 1508.5m, 1449.8s, 1332.8m.
Single crystals suitable for X-ray structure analysis
were obtained by evaporating a solution of 2 in a mixed
solution of MeCN and i-BuOH (1:2 v/v).
2. Experimental
2.1. General materials and techniques
The starting materials, 1-(40-nitrobenzyl)-4-aminopy-
ridinium bromide ([NO2BzPyNH2]Br), and disodium
maleonitriledithiolate (Na2mnt) were synthesized fol-
lowing the literature procedures [19,20]. A similar method
for preparing [Bu4N]2[Ni(mnt)2] was utilized to pre-
pare [NO2BzPyNH2]2[Ni(mnt)2] and [NO2BzPy-
NH2]2[Pt(mnt)2] [20].
2.3. Single-crystal X-ray structure determination
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 the x scan mode within the angular range
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) spectropho-
tometer in KBr pellets. Electronic spectra were recorded
on a Shimadzu UV-3100 spectrophotometer. All solu-
tion concentrations were ca. 10ꢀ5 mol dmꢀ3 in CH3CN.
The electrospray-mass spectra [ESI-MS] were deter-
mined on a Finnigan LCQ mass spectrograph, sample
concentration ca. 1.0 mmol dmꢀ3. DSC was carried
out with a Perkin–Elmer calorimeter. Thermal analyses
of crushed polycrystalline samples for 1 placed in an
aluminum crucible were performed on warming (rate
of 20 K molꢀ1) from 98 to 293 K. Magnetic susceptibil-
ity data on crushed polycrystalline samples of 1 and 2
were collected over the temperature range of 2–300 K
using a Quantum Design MPMS-5S super-conducting
quantum interference device (SQUID) magnetometer,
and the experimental data were corrected for diamagne-
tism of the constituent atoms estimated from Pascalꢀs
constants.
1.87ꢁ < h < 26.0ꢁ for 1 and 1.96ꢁ < h < 25.0ꢁ 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 Brukerꢀs SHELXTL [21]. All non-hydrogen
atoms were refined with anisotropic thermal parameters.
All H atoms were placed in calculated positions, as-
signed 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.
3. Results and discussion
3.1. Descriptions of the structures
3.1.1. X-ray structure of 1
Complex 1 crystallizes in the monoclinic space group
P21/c at room temperature. Selected bond distances,
bond angles and intermolecular Niꢁ ꢁ ꢁNi, Niꢁ ꢁ ꢁS and
Sꢁ ꢁ ꢁS distances are summarized in Table 2. An ORTEP
drawing of 1 with non-hydrogen atomic labelling in an
asymmetric unit is shown in Fig. 1. 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
2.2. Syntheses of complexes
2.2.1. [NO2BzPyNH2]2[Ni(mnt)2] (1)
[NO2BzPyNH2]2[Ni(mnt)2] (0.800 g, 1.0 mmol) was
dissolved in 30 cm3 MeCN, then a MeCN solution
(10 cm3) of I2 (0.150 g, 0.59 mmol) was slowly added,
the mixture was stirred for 1 h, and then 100 cm3
CH3OH was added. After the mixture was allowed to
stand overnight, 0.508 g dark microcrystals produced
were filtered off, washed with CH3OH and Et2O, and
dried in a vacuum. Yield: 89.3%. Anal. Calc. for
C20H12NiN7S4O2: C, 42.19; H, 2.12; N, 17.22. Found:
C, 42.12; H, 2.23; N, 17.16%. IR (cmꢀ1): 3380.4s,
3073.1w, 2207.1s, 1651.3s, 1606.6m, 1565.4m, 1536.6s,
1518.6m, 1454.2s, 1346.1s.
˚
˚
plane are ꢀ0.4477 A for N(1), ꢀ0.4875 A for N(2),
˚
˚
ꢀ0.1285 A for N(3) and ꢀ0.2619 A for N(4). The aver-
age S–Ni–S bond angle within the five-membered ring is
90.0(5)ꢁ, and the average Ni–S bond distance is
˚
2.139(1) A, these values are in agreement with those
found in other [Ni(mnt)2]ꢀ complexes [22]. The cation
[NO2BzPyNH2]+, adopts a conformation in which the
benzene and pyridine rings are almost perpendicular to
the reference plane C(14)–C(15)–N(5) (dihedral angles

C.-L. Ni et al. / Polyhedron 24 (2005) 1669–1676
1671
Table 1
Crystal data and structure refinements for 1 and 2
Compound
1
2
Temperature
293 (2)
293 (2)
Empirical formula
Formula weight
Wavelength
C20H12NiN7S4O2
569.32
0.71073
C20H12PtN7S4O2
705.70
0.71073
Space group
P21/c
P21/c
Crystal system
Unit cell dimensions
monoclinic
monoclinic
˚
a (A)
12.331(3)
26.937(6)
7.322(2)
105.02(1)
2349.0(10), 4
1.610
1.216
10.600(3)
8.376(2)
27.089(7)
100.84(1)
2362.2(11), 4
1.984
˚
b (A)
˚
c (A)
b (ꢁ)
Volume (A ), Z
Dcalc (g/cm3)
3
˚
Absorption coefficient (mmꢀ1
F(000)
)
6.328
1356
1156
Crystal size (mm3)
Absorption correction
0.35 · 0.30 · 0.20
empirical (SHELXA)
0.78 and 0.66
1.87–26.00
0.40 · 0.35 · 0.25
empirical (SHELXA)
0.21 and 0.09
1.96–25.00
ꢀ9 6 h 6 12,
ꢀ 6 k 6 9,
ꢀ32 6 l 6 21
11200
Maximum and minimum transmission
h Range for data collection (ꢁ)
Limiting indices
ꢀ15 6 h 6 15,
ꢀ33 6 k 6 21,
ꢀ8 6 l 6 8
Reflections collected
Independent reflections
Rint
12474
4604
0.084
4142
0.047
Refinement method
Goodness-of-fit on F2
Final R indices [I > 2r (I)]
Final R indices (all data)
full-matrix least-squares on F2
1.016
full-matrix least-squares on F2
1.014
R1 = 0.0545, wR2 = 0.1104
R1 = 0.0973, wR2 = 0.1195
0.34 and ꢀ0.47
R1 = 0.0335, wR2 = 0.0678
R1 = 0.0431, wR2 = 0.0699
1.07 and ꢀ0.81
ꢀ3
˚
Largest difference in peak and hole (e A
)
Table 2
Selected bond parameters and intermolecular contacts for 1 and 2
Compound
1
2
˚
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)
2.134(1)
2.146(1)
2.141(1)
2.136(1)
1.716(4)
1.715(4)
1.706(4)
1.702(4)
Pt(1)–S(1)
Pt(1)–S(2)
Pt(1)–S(3)
Pt(1)–S(4)
S(1)–C(2)
S(2)–C(3)
S(3)–C(6)
S(4)–C(7)
2.264(2)
2.253(2)
2.257(2)
2.255(2)
1.716(6)
1.699(6)
1.716(6)
1.714(5)
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)
92.50(5)
86.63(5)
92.23(5)
88.60(5)
S(1)–Pt(1)–S(2)
S(1)–Pt(1)–S(4)
S(3)–Pt(1)–S(4)
S(2)–Pt(1)–S(3)
89.88(6)
90.71(5)
89.95(5)
89.46(6)
˚
Intrachain distances (A)
Niꢁ ꢁ ꢁNi (nearest separation)
Niꢁ ꢁ ꢁS
3.855
3.644
3.725
Ptꢁ ꢁ ꢁPt (nearest separation)
3.511
4.066
3.488
Ptꢁ ꢁ ꢁS
Sꢁ ꢁ ꢁS
Sꢁ ꢁ ꢁS
of 90.4ꢁ and 91.0ꢁ, respectively). The phenyl ring and the
pyridine ring make a dihedral angle of 112.5ꢁ. The plane
defined by S(1)S(2)Ni(1)S(3)S(4) with the phenyl and
pyridine rings make dihedral angles of 17.2ꢁ and
113.8ꢁ, respectively. The most notable structural feature
of 1 is that the anions and cations possess a stacking pat-
tern with well-separated columns along the direction of
the c-axis (Fig. 2). Within an anionic column, the near-
˚
est Niꢁ ꢁ ꢁNi distance is 3.855 A, and the nearest Sꢁ ꢁ ꢁS
˚
and Niꢁ ꢁ ꢁS contacts are 3.725 and 3.664 A, respectively.

1672
C.-L. Ni et al. / Polyhedron 24 (2005) 1669–1676
a
Fig. 1. ORTEP plot (30% probability ellipsoids) showing the molecule
structure of 1.
b
Fig. 3. (a) Side view of the anions stack of 1 showing the uniform space
linear-chain of [Ni(mnt)2]ꢀ. (b) Side view of the cations stack of 1.
Fig. 2. The packing diagram of a unit cell for 1 as viewed along the
c-axis.
3.1.2. X-ray structure of 2
The coordination geometry of the [Pt(mnt)2] anion is
essentially identical to that described above for 1, while
the stacking pattern of the [Pt(mnt)2] anion and [NO2Bz-
PyNH2] cation in 2 is significantly different from that of
1. Selected bond distances, bond angles and intermolec-
ular Ptꢁ ꢁ ꢁPt, Ptꢁ ꢁ ꢁS and Sꢁ ꢁ ꢁS distances are summarized
in Table 2. The cation is situated above the plane of
the [Pt(mnt)2] anion (Fig. 4(a)) in which the dihedral an-
gles between the S(1)S(2)Pt(1)S(3)S(4) plane and phenyl
and pyridine rings are 84.8ꢁ and 135.6ꢁ, respectively.
The reference plane defined by C(14)–C(15)–N(5) and
the aromatic rings make dihedral angles of 37.4ꢁ
for the phenyl ring and 115.8 for pyridine ring. The
Therefore, the Ni(III) ions within a [Ni(mnt)2]ꢀ anionic
column form a 1D uniformly spaced chain through
intermolecular Niꢁ ꢁ ꢁS, Sꢁ ꢁ ꢁS, Niꢁ ꢁ ꢁNi or pꢁ ꢁ ꢁp interac-
tions (Fig. 3(a)). The adjacent cations stack into a col-
umn via a boat-type conformation (Fig. 3(b)). The
contact distances between the nitrogen atom of the
NO2 group and neighboring phenyl rings are 3.562
˚
and 3.799 A, respectively. So the cations form a 1D
chain via p–p interactions between the NO2 group and
benzene rings. The closest Niꢁ ꢁ ꢁNi separation between
˚
anionic chains is 12.604 A, which is significantly longer
than that of the Niꢁ ꢁ ꢁNi separation within a chain
ꢀ
˚
PtðmntÞ2 anions of 2 form a dimer with a Ptꢁ ꢁ ꢁPt dis-
(3.855 A). Therefore, 1 is an ideal 1D uniform magnetic
˚
˚
tance 3.511 A and a Sꢁ ꢁ ꢁS distance 3.488 A, which is sig-
nificantly different from that of 1. Obviously, the short
contacts of Mꢁ ꢁ ꢁM, Sꢁ ꢁ ꢁS and p–p interactions in 2 are
smaller than those of 1, therefore it is possible that the
chain system from the point of view of the structure.
There are three intermolecular hydrogen bonds between
the anions and cations in the crystal structure of 1
(Table 3).

C.-L. Ni et al. / Polyhedron 24 (2005) 1669–1676
1673
Table 3
˚
Hydrogen bonds for 1 and 2 (A and ꢁ)
D–Hꢁ ꢁ ꢁA
Complex 1
d(D–H)
d(Hꢁ ꢁ ꢁA)
d(Dꢁ ꢁ ꢁA)
\(DHA)
N(6)–H(6A)ꢁ ꢁ ꢁN(4)#1
N(6)–H(6A)ꢁ ꢁ ꢁN(1)#2
N(6)–H(6A)ꢁ ꢁ ꢁN(2)#3
0.86
0.86
0.86
2.56
2.62
2.43
3.132(6)
3.323(6)
3.059(6)
125.0
140.0
131.0
Complex 2
N(6)–H(6A)ꢁ ꢁ ꢁN(1)#4
N(6)–H(6A)ꢁ ꢁ ꢁN(3)#5
C(15)–H(15A)ꢁ ꢁ ꢁN(4)#6
C(16)–H(16A)ꢁ ꢁ ꢁO(1)#7
C(19)–H(19A)ꢁ ꢁ ꢁN(2)#8
0.86
0.86
0.97
0.93
0.93
2.49
2.55
2.55
2.52
2.56
3.273(8)
3.207(8)
3.397(8)
3.275(8)
3.310(8)
153.0
134.0
146.0
138.0
138.0
Symmetry transformations used to generate equivalent atoms: #1 = ꢀx + 1, ꢀy, ꢀz + 1; #2 = x ꢀ 1, ꢀy + 1/2, z ꢀ 1/2; #3 = ꢀx + 1, y ꢀ 1/2,
ꢀz + 1/2; #4 = ꢀx + 1, ꢀy, ꢀz + 2; #5 = ꢀx, y ꢀ 1/2, ꢀz + 3/2; #6 = x, y + 1, z; #7 = ꢀx + 1, ꢀy + 1, ꢀz + 2; #8 = x, ꢀy + 1/2, zꢀ1/2.
above. The bands at 3380.4(s) and 1651.3(s) cmꢀ1 for 1,
a
and 3371.9 and 1648.2 cmꢀ1 for 2 are attributable to
N–H stretching and bending of the amino group. The
bands at 3073.1 and 3070.4 cmꢀ1 are due to the stretch-
ing vibration frequencies of C–H in the aromatic ring.
The complexes 1 and 2 show one strong band at
2207.1 and 2205.9 cmꢀ1, respectively, which are CN
stretching bands. The m(C@N) and m(C@C) bands for
the pyridine ring and phenyl ring are located at
1606.6, 1565.4 and 1518.6 cmꢀ1 for 1, and 1604.8,
1564.4, and 1508.5 cmꢀ1 for 2. The m(C@C) band of
mnt2ꢀ is at 1452.2 cmꢀ1 for 1 and 1449.8 cmꢀ1 for 2.
The m(NO2) bands of complexes 1 and 2 are presented
at 1536.6, 1346.1 cmꢀ1 and 1535.2, 1332.8 cmꢀ1
.
b
3.3. Electronic absorption spectra and electrospray mass
spectra
The UV–Vis absorption spectra of 1 and 2 were mea-
sured in CH3CN solvent in the region of 230–1200 nm
(Table 4). All bands are attributed to the anionic por-
tions of these compounds. The characteristic bands of
1 at 859.6, 473.0, 374.0 and 271.0 nm, are assigned as
L(p) ! M, L(r) ! M, L* ! L and L(r) ! M, respec-
tively, which is basically similar to those of
[(n-C4H9)4N][Ni(mnt)2] [23]. The bands of 2 are similar
to those of 1. 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 230.2 peak,
which is due to [NO2BzPyNH2]+, the peaks at
340.3 and 476.3 are assigned to [Ni(mnt)2 + H]ꢀ and
[Pt(mnt)2 + H]ꢀ, respectively.
Fig. 4. (a) ORTEP plot (30% probability ellipsoids) showing the
molecule structure of 2. (b) The packing diagram of a unit cell for 2 as
viewed along the b-axis.
interaction between MðmntÞ2ꢀ anions in 2 may be stron-
ger than that in 1, and 2 forms a dimer structure (Fig.
4(b)). It is worth noting that five intermolecular hydro-
gen bonds between anions and cations were observed
in the crystal structure of 2, as listed in Table 3. The an-
ion–anion, anion–cation and cation–cation contacts may
play important roles in crystal packing and the stabiliza-
tion of 2.
3.4. Magnetic properties
The temperature dependence of the magnetic suscep-
tibility for 1 and 2 is measured under an applied field
of 1000 Oe in the temperature range 2–300 K. The plot
of vm versus T for 1 is shown in Fig. 5, where vm is the
magnetic susceptibility per nickel atom corrected by
the diamagnetic contribution. For the complex 1, the
3.2. Infrared spectra
The infrared spectra of the two complexes 1 and 2 are
very much consistent with the structural data presented

1674
C.-L. Ni et al. / Polyhedron 24 (2005) 1669–1676
ꢀ
ꢁꢀ ꢁ
Table 4
D
1
C
T
vm ¼ a exp
ꢀ
þ
þ v0;
ð1Þ
The main UV–Vis spectra (CH3CN) of 1 and 2
kbT
T
Complex
k (nm)
e (molꢀ1 dm3 cmꢀ1 103)
where a is a constant value corresponding to the disper-
sion of the 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 [29]. The best
fit curves are shown in Fig. 5, and the corresponding
parameters are given as follows: a = 1.28, D/kb =
494.97 K, v0 = ꢀ5.3 · 10ꢀ4 emu molꢀ1, C = 3.7 · 10ꢀ3
emu K molꢀ1 and R = 2.2 · 10ꢀ5 (R is defined as
1
1178.0
859.6
629.0
473.0
374.0
271.0
0.33
0.76
0.18
0.34
7.68
133.47
2
1180.0
850.0
629.0
455.0
333.6
273.0
0.27
4.91
0.35
3.27
10.70
147.66
2
2
Rðvmcalcd ꢀ vombsdÞ =ðvombsdÞ ). The value of the parameter
2D/kbTc (Tc is the transition temperature) is estimated
to be 7.84 on the basis of the above results, which is
higher than the ideal value of 3.53 derived from the
BCS formula in a weak coupling regime. These results
suggest 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 transi-
tion is not a pure spin-Peierls transition [18]. The origins
of the phase transition for 1 is attributed to cooperative
interactions of Niꢁ ꢁ ꢁS bonding, interplane repulsion of
the [Ni(mnt)2]ꢀ anions, pꢁ ꢁ ꢁp stacking interactions be-
tween of adjacent cations, spin–lattice interactions and
spin–spin coupled interaction between nearest-neighbor
anions [30–32].
0.012
0.0024
Peak Temperature = 160 K
0.008
0.004
0.000
T
0.0020
0.0016
0.0012
0.0008
0.0004
T
m
0
50 100 150 200 250 300
T (K)
Complex 2 is expected to be diamagnetic on account
of the dimerized structure.
0
50
100
150
T (K)
200
250
300
3.5. Thermodynamic properties
Fig. 5. Plots of vm vs. T for 1 (inset: plots of d(vmT)/dT vs. T). The
solid line are reproduced from the theoretical calculations and detailed
fitting procedure described in the text.
To obtain additional information about the thermo-
dynamic properties of the observed phase transition of
1, DSC measurements on 1 were performed. The
power-compensated DSC traces for 1 from 98 to
293 K at a warming rate of 20 K minꢀ1 are displayed
in Fig. 6. For 1, an abruptly endothermic peak in the
overall magnetic behaviors correspond to a paramag-
netic system with an antiferromagnetic coupling inter-
action. As the temperature decreases, the value of
vmT slightly decreases from 0.391 emu K molꢀ1 at
300 K (the value is somewhat higher than that expected
18
16
for
magnetically
isolated
Ni(III)
ions)
to
0.187 emu K molꢀ1 at 160 K for 1. In the LT phase,
as the samples are continuously cooled, the magnetic
susceptibilities decrease exponentially (Fig. 5), indicat-
ing that 1 exhibits the characteristics of a spin gap sys-
tem [24]. On increasing of temperature back to the
original one, the same vm ꢀ T curves are obtained
without hysteresis effect. These findings indicate that
complex 1 undergoes a reversible phase transition.
The phase transition temperature is evaluated as the
temperature at the maximum of the d(vmT)/dT deriva-
tive, that is ꢂ160 K for 1 (inset of Fig. 5).
14
12
10
8
Tpeak = 166.4 K
Delta H = 385.43 J mol-1
6
The spin-gapped system has attracted extensive inter-
est recently [25–28], but it is very rare for Ni(III) com-
plexes [18]. The magnetic susceptibilities of 1 may be
estimated by the formula:
100 125 150 175 200 225 250 275 300
T (K)
Fig. 6. DSC plot for 1.

C.-L. Ni et al. / Polyhedron 24 (2005) 1669–1676
1675
DSC trace is observed, and the phase transition temper-
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Acknowledgments
The authors thank for financial support from the Na-
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20490210). The Center of Analysis and Determination
of Nanjing University are acknowledged.
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