Stack pattern of the countercation-modulating magnetic property of low-dimensional [Pt(mnt)2]- monoanion spin systems

Article abstract of DOI:10.1021/ic102406u

Three [1-N-(4′-R-benzyl)-4-aminopyridinium][Pt(mnt)₂] compounds were structurally and magnetically characterized, where the substituent was attached to the para-position of the phenyl ring (R = CN (1), Cl (2), and H (3); mnt^2- = male

Full text of DOI:10.1021/ic102406u

ARTICLE
pubs.acs.org/IC
Stack Pattern of the Countercation-Modulating Magnetic Property
of Low-Dimensional [Pt(mnt)2]ꢀ Monoanion Spin Systems
Wen-Bo Pei,† Jian-Sheng Wu,† Zheng-Fang Tian,† Xiao-Ming Ren,*,†,‡ and You Song‡
†State Key Laboratory of Materials-Oriented Chemical Engineering and College of Science, Nanjing University of Technology,
Nanjing 210009, P.R. China
‡State Key Lab & Coordination Chemistry Institute, Nanjing University, Nanjing 210093, P.R. China
S Supporting Information
b
ABSTRACT: Three [1-N-(40-R-benzyl)-4-aminopyridinium][Pt(mnt)2] com-
pounds were structurally and magnetically characterized, where the substituent
was attached to the para-position of the phenyl ring (R = CN (1), Cl (2), and H
(3); mnt2ꢀ = maleonitriledithiolate). 1 and 2 crystallized in the monoclinic
space group P2(1)/c, with the cations and anions forming segregated columnar
stacks. Their structural differences involved two aspects: (1) both anion and
cation stacks were regular in 1 and irregular in 2; (2) the neighboring cations
were arranged in the boat-type pattern in 1, whereas these cations were in the
chair-type pattern in 2 within the cation stack. 3 belonged to the triclinic space
group P1, where the anions were assembled into the stack with a tetrameric
[Pt(mnt)2]ꢀ subunit, but the cations did not form the columnar stack. Magnetic
measurements disclosed that a spin-Peierls-type transition occurred around 240
K for 1, whereas a long-range, antiferromagnetic ordering took place at about 5.8
K, and a metamagnetic phenomenon was observed with HC ≈ 1000 Oe for 2; 3 showed very strong antiferromagnetic interactions
with diamagnetism in the temperature range 5ꢀ300 K. Combined with our previous studies, the correlation between the stacking
pattern of benzylpyridinium derivatives in a cation stack and the spin-Peierls-type transition is discussed for the series of quasi-1-D
[M(mnt)2]ꢀ (M = Ni, Pd and Pt) compounds.
’ INTRODUCTION
determined not only by the Madelung energy but also by the
Lennard-Jones potential (which depended on molecular proper-
ties, such as the shape, size, polarity, local charge distribution,
types and location of the functional groups of the molecules,
which were generally less important than the Madelung energy).
The behavior of the packing structure for planar paramagnetic
[M(mnt)2]ꢀ (M = Ni, Pd, or Pt) monoanions was strongly
affected by the type of counterions;8 for instance, a mixed stack
was favorably formed when the [M(mnt)2]ꢀ (M = Ni, Pd, or Pt)
monoanions were combined with the planar monovalent counter-
cations.9 In earlier studies, we designed Λ-shape 1-N-benzylpyr-
idinium derivatives as the counterions of [M(mnt)2]ꢀ (M = Ni,
Pd, or Pt) monoanions, which could prevent the anion (A) and
cation (C) from forming a mixed stack arranged in the alternating
ACAC manner, and we obtained a series of compounds with
segregated anion and cation stacks; each [M(mnt)2]ꢀ (M = Ni,
One-dimensional (1-D) molecular solids have recently at-
tracted widespread attention because of their novel physical
properties, such as the Peierls transition,1 spin-Peierls transition,2
charge-density-wave (CDW)/spin-density-wave (SDW),3 spin-
charge separation,4 and valence-ordering states5 in addition to
the many advantages (e.g., a straightforward synthetic approach,
easily tailored molecular structure and functional properties) of
the organic component.
Bis-1,2-dithiolene complexes of transition metals have been
widely studied because of their novel properties in the areas of
magnetic6 and conducting7 materials. In our present studies,
we focused on the assembly of quasi-1-D S = 1/2 spin systems,
which have been based on the molecular architectures of bis-
(maleonitriledithiolato)metalate monoanions ([M(mnt)2]ꢀ
and M = Ni, Pd, or Pt). To achieve such types of quasi-1-D
magnetic molecular solids, a critical issue has been the control of
the [M(mnt)2]ꢀ anions in the columnar arrangement. The
crystal packing mode of an inorganic salt, where the simple
inorganic ions could be considered as point electric charges, and
the electrostatic interactions between the pairs of point electric
charges obey Coulomb’s law, dependent mostly upon the Madelung
energy (ionic lattice energy), while the arrangements of organic
ions in an organic or inorganicꢀorganic hybrid crystal were
1
Pd, or Pt) monoanion stack behaved as an S = /2 spin chain.
Therefore, it was reasonable to consider such an ion-pair com-
pound as a quasi-1-D spin system. We noted that (1) the anions
and cations showed a regular arrangement in some [M(mnt)2]ꢀ
compounds and irregular alignment in others within a stack, and
Received: December 2, 2010
Published: April 04, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Figure 1. Correlation between spin-Peierls-type transition temperature TC and cell volume V for the series of [40-R-BzPy][Ni(mnt)2] compounds
(where 40-R-BzPyþ = the monosubstituted benzylpyridinium derivatives and the substituent located in the para-position of the phenyl ring).8
most compounds with regular anion and cation stacks as well as
only a few compounds with the irregular anion and cation stacks
exhibited a novel spin-Peierls-type transition below a critical
temperature; (2) a correlation seemed to exist between the
stacking pattern of the neighboring cations in a stack and the
spin-Peierls-type transition; most of the compounds, which
possessed a boat-type arrangement between the neighboring
cations, showed the regular columnar stacks for both anions and
cations and a spin-Peierls-type transition; (3) the local structural
fluctuation, which is a common phenomenon in 1-D conductors
and magnets and arises from electronꢀphonon or magnetoelas-
tic coupling interactions, was observed with the spin-Peierls-type
transition; (4) most of the spin-Peierls-type transitions were
associated with a structural transition and the nonzero transition
enthalpy; (5) the correlation between the characteristics of the
substituent groups attached to the phenyl ring of the cation and
the spin-Peierls-type transition temperature TC was observed in
some instances. The variation of TC was independent of the
electronic effect of the substituent in the series of [1-N-(40-R-
benzyl)pyridinium][Ni(mnt)2] (R = F, Cl, Br, I, NO2, CN, CH3,
and CHdCH2). For example, the electron affinity of the sub-
stituent should follow the order: NO2, CN > F > Cl > Br > I >
CHdCH2 > CH3; whereas the variation of TC in this family did
not show this simple order. NO2 and CH3 possessed different
electron donating and withdrawing effects, but the correspond-
ing compounds had a similar transition temperature. As demon-
strated in Figure 1, the variation of TC appeared to be dependent
on the size of the substituent group because the spin-Peierls-type
transition temperature increased with the cell volume of the
crystal, which increased with the atomic sizes and atomic
numbers of the substituent located in the para-position of the
phenyl ring.8,10ꢀ12
Scheme 1. Molecular Structures of Compounds 1ꢀ3
’ EXPERIMENTAL SECTION
Chemicals and Materials. All reagents and chemicals were
purchased from commercial sources and used without further purifica-
tion. The starting materials, disodium maleonitriledithiolate (Na2mnt),13
1-N-(40-cyanobenzyl)-4-aminopyridinium bromide ([40-CN-Bz-NH2Py]-
Br), 1-N-(40-chlorobenzyl)-4-aminopyridinium bromide ([40-Cl-Bz-
NH2Py]Br), and 1-N-benzyl-4-aminopyridinium bromide ([Bz-
NH2Py]Br),14 were prepared following the published procedures. [40-
CN-Bz-NH2Py]2[Pt(mnt)2], [40-Cl-Bz-NH2Py]2- [Pt(mnt)2], and [Bz-
NH2Py]2[Pt(mnt)2] were synthesized utilizing a procedure similar to
the preparation of [TBA]2[Pt(mnt)2].13
Syntheses of Compounds. [40-CN-Bz-NH2Py][Pt(mnt)2] MeCN
3
(1 MeCN). A MeCN solution (10 mL) of I2 (150 mg, 0.59 mmol) was
3
slowly added to a MeCN solution (10 mL) of [40-CN-Bz-NH2Py]2[Pt-
(mnt)2] (896 mg, 1.0 mmol); the mixture was stirred for 20 min, and
MeOH (80 mL) was then added. The mixture was allowed to stand
overnight; the 548 mg of microcrystals that formed were filtered off, washed
with MeOH, and dried in vacuum (∼80% yield). Single crystals of
To understand the above-mentioned issues and establish the
general validity of the correlation between the structural features
of the countercation and the arranging pattern of the cationic
stack column as well as the relationship between the arranging
pattern of cationic stack column and the feature of the spin-
Peierls-type transition, we sought to systematically investigate
the crystal structures and magnetic behaviors for the series of
[M(mnt)2]ꢀ (M = Ni, Pd, and Pt) with benzylpyridinium
derivatives. In this paper, the crystal structures and magnetic
properties were studied for three ion-pair compounds of
[Pt(mnt)2]ꢀ wþith disubstituted benzylpyridinium derivatives,
40-R-Bz-NH2Py (ref Scheme 1).
1 MeCN that were suitable for X-ray structure analysis were obtained by
3
slowly evaporating the solution of 1 in MeCN at room temperature for
6 days. The analytical calculations for C21H12N7PtS4 C2H3N included the
3
following values: C, 38.01; H, 2.08; N, 15.42%. The experimental values
found for this compound were the following: C, 37.37; H, 2.58; N, 14.61%
(these values were close to the calculations based on the formula of
C21H12N7PtS4 0.5C2H3N: C, 37.40; H, 1.91; N, 14.88%). The peaks in
3
the infrared (IR) spectrum (KBr disk, cmꢀ1) were found at the following
values: 3442(s), 3342(s), 3238(s) for the νNꢀH of -NH2; 3097(w),
3066(m) for the νCꢀH of the phenyl and pyridyl rings; 2208(vs) for the
νCtN of the mnt2ꢀ ligands and the νCtN of 40-CN-Bz-NH2Pyþ; 1658(vs)
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Inorganic Chemistry
ARTICLE
Table 1. Crystal and Structural Refinement Data for
Scheme 2. Illustration for the Characteristic Intermoleþcular
Distances in a Tetrameric Subunit of 40-CN-Bz-NH2Py
Cationsa
1 MeCN, 2, and 3
3
1 MeCN
2
3
3
chemical formula
formula weight
CCDC number
temp./K
wavelength/Å
space group
a/Å
C23H15N8PtS4 C20H12ClN6PtS4 C20H13N6PtS4
726.79 695.17 660.72
CCDC 781810 CCDC 776573 CCDC 777239
293(2)
296(2)
296(2)
0.71073
P2(1)/c
12.404(3)
31.060(7)
7.2419(17)
90.00
0.71073
P2(1)/c
7.3903(8)
26.318(3)
12.7696(15)
90.00
0.71073
P1
11.533(4)
14.191(5)
15.786(5)
73.318(7)
74.034(7)
68.030(6)
2253.9(13), 4
1.947
b/Å
c/Å
R/deg
β/deg
105.571(4)
90.00
105.945(2)
90.00
γ/deg
V/Å3, Z
a d1 is the distance of the N N of the CN groups; c1 and c2 represent
2687.7(11), 4 2388.1(5), 4
3 3 3
Density(calc)/g cmꢀ3 1.796
1.931
the centroid-to-centroid distances of the phenyl rings.
3
Abs. Coeff. (mmꢀ1
)
5.560
6.358
6.600
peaks at the following values: 3435(s), 3348(s), 3247(s) for the νNꢀH of
-NH2; 3099(w), 3066(s), 2976(w) for the νCꢀH of the phenyl and
pyridyl rings; 2206(s) for the νCtN of mnt2ꢀ ligands; 1652(vs) for the
νCdN of the pyridyl ring and the νCdC of the phenyl ring; 1446(s) for
the νCdC of mnt2ꢀ ligands.
Single crystals of 2 and 3 that were suitable for X-ray structure analysis
were obtained by slowly evaporating the solution with the correspond-
ing compounds in MeCN at room temperature for 6ꢀ8 days.
Physical Measurements. Elemental analyses (C, H, and N) were
performed with an Elementar Vario EL III analytical instrument. IR
spectra were recorded on a Bruker Vector 22 Fourier Transform Infrared
Spectrometer (170SX) (KBr disk). Magnetic susceptibility data for
polycrystalline samples were measured over a temperature range of
1.8ꢀ300 K using a Quantum Design MPMS-5S superconducting
quantum interference device (SQUID) magnetometer.
F(000)
1404.0
1328
1268
θ ranges of data
collection
1.83ꢀ24.99
1.83ꢀ27.0
1.58ꢀ26.0
index ranges
ꢀ8 e h e 14 ꢀ7 e h e 9
ꢀ14 e h e 12
ꢀ35 e k e 36 ꢀ33 e k e 32 ꢀ16 e k e 17
ꢀ8 e l e 8
13023
ꢀ16 e l e 14
15094
ꢀ19 e l e 11
13172
reflections collected
Rint
0.0379
0.0399
0.1213
independent
reflections
4672
5160
8622
refinement method
on F2
goodness-of-fit on F2 1.026
full-matrix least-squares
0.999
0.884
final R indices
[I > 2σ(I)]
R1 = 0.0983
R1 = 0.0341
R1 = 0.0396
X-ray Single Crystallography. The single-crystal X-ray diffrac-
tion data for 1 MeCN, 2 and 3 were collected at 296 K with graphite
3
wR2 = 0.2493 wR2 = 0.0999
wR2 = 0.0825
R1 = 0.0754
wR2 = 0.0926
monochromated Mo KR (λ = 0.71073 Å) on a CCD area detector
(Bruker-SMART). Data reductions and absorption corrections were
performed with the SAINT and SADABS software packages,
respectively.15 Structures were solved by a direct method using the
SHELXL-97 software package.16 The non-hydrogen atoms were aniso-
tropically refined using the full-matrix least-squares method on F2. All
hydrogen atoms were placed at the calculated positions and refined
riding on the parent atoms. The details about data collection, structure
refinement and crystallography are summarized in Table 1.
R indices (all data)
R1 = 0.1219
R1 = 0.0519
wR2 = 0.2618 wR2 = 0.1383
residual (e Åꢀ3
)
2.849/ ꢀ3.275 1.014/ ꢀ1.642 1.308/ ꢀ1.803
R1 = ∑||Fo| ꢀ |Fc||/∑|Fo|, wR2 = [∑w(Fo ꢀ Fc2)2/∑w-
2
2 2 1/2
(Fo ) ]
.
for the νCdN of the pyridyl ring and the νCdC of the phenyl ring; 1442(m)
for the νCdC of mnt2ꢀ ligands.
DFT Calculations. All density functional theory (DFT) calcula-
tions were carried out using the Gaussan98 program17 on an SGI 380þ0
workstation. The dipole moments of the cations, 40-CN-Bz-NH2Py
and 40-Cl-Bz-NH2Pyþ, were calculated at the b3lyp/6-31gþ(d,p)
level18 on the non-modelized molecular geometries, which were directly
taken from the single crystal X-ray analyses. The single-point energies of
the 40-CN-Bz-NH2Pyþ tetramers were calculated at the b3lyp/6-31gþ-
(d,p) level for the regular and dimerized stacks. For these calculations,
the molecular geometry for each 40-CN-Bz-NH2Pyþ cation in the
tetrameric subunit was directly taken from the singlþe crystal X-ray
analysis, and the positions of four 40-CN-Bz-NH2Py cations were
not changed for the case of the regular cation stack, while the positions of
two terminal cations were kept fixed and two central cations were
For the preparation of 2 and 3 similar procedures were used, but [40-
CN-Bz-NH2Py]2[Pt(mnt)2] was replaced with the corresponding salts
of the benzylpyridinium derivative with the [Pt(mnt)2]2ꢀ dianion.
[40-Cl-Bz-NH2Py][Pt(mnt)2] (2). The product yield was ∼78%. The
analytical calculations for C20H12ClN6PtS4 were the following: C, 34.56;
H, 1.74; N, 12.09%. The experimental values included the following: C,
34.30; H, 1.88; N, 12.31%. The IR spectrum (KBr disk, cmꢀ1) showed
peaks at the following values: 3477(s), 3373(vs), 3238(w) for the νNꢀH
of -NH2; 3099(w), 3072(w) for the νCꢀH of the phenyl and pyridyl
rings; 2206(s) for the νCtN of mnt2ꢀ ligands; 1649(vs) for the νCdN of
the pyridyl ring and the νCdC of the phenyl ring; 1452(s) for the νCdC of
mnt2ꢀ ligands.
[Bz-NH2Py][Pt(mnt)2] (3). The product yield was ∼82%. The analy-
tical calculations for C20H13N6PtS4 were the following: C, 36.36; H,
1.98; N, 12.72%. The experimental values included the following: C,
36.22; H, 2.067; N, 12.72%. The IR spectrum (KBr disk, cmꢀ1) showed
translated to two opposite terminal cations along the N(7) N(7)
3 3 3
direction for the case of the dimerized stack; each case was identified by
the characteristic intermolecular separations (ref Scheme 2). All SCF
convergence criteria for DFT calculations were 10ꢀ8
.
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ARTICLE
Table 2. Characteristic Bond Lengths and Angles in the
[Pt(mnt)2]ꢀ Moiety of 1 MeCN, 2, and 3
3
1 MeCN
2
3*
3
a/Å
2.272(6)
2.259(5)
2.235(5)
2.262(6)
89.8(2)
2.2717(18)
2.2608(18)
2.2740(18)
2.2589(19)
89.90(6)
2.290(2)
2.282(2)
2.277(2)
2.280(2)
90.00(7)
90.43(7)
89.67(7)
89.86(7)
2.2730(19)
2.2803(19)
2.2864(19)
2.2675(19)
90.13(7)
b/Å
c/Å
d/Å
—1/deg
—2/deg
—3/deg
—4/deg
90.0(2)
91.90(7)
90.09(7)
90.10(18)
90.20(17)
88.47(7)
89.99(7)
89.74(7)
89.81(7)
* There were two crystallographically different [Pt(mnt)2]ꢀ anions
in 3.
Figure 2. (a) ORTEP view with non-hydrogen atomic numbering at
30% thermal ellipsoid probability level; (b) packing diagram showing the
segregated and regular stacks of anions and cations along the crystal-
lographic c-axis direction as well as the hydrogen bond between the
cation and the solvent acetonitrile for 1 MeCN.
3
’ RESULTS AND DISCUSSION
Description of Crystal Structures. Compound 1 MeCN
3
crystallized in a monoclinic space group P2(1)/c. As shown in
Figure 2a, an asymmetric unit comprised one [Pt(mnt)2]ꢀ
monoanion, explicitly one 40-CN-Bz-NH2Pyþ cation together
with a MeCN molecule. In the [Pt(mnt)2]ꢀ moiety, the Pt-atom
was coordinated to four sulfur atoms from two mnt2ꢀ ligands,
exhibiting a square planar coordination geometry. The average
distance of PtꢀS bonds was 2.26 Å, and the average SꢀPtꢀS bite
angle was 90.45ꢀ; the other bond lengths and bond angles, listed
in Table 2, were comparable to those of the reported
[Pt(mnt)2]ꢀ compounds.11a The 40-CN-Bz-NH2Pyþ cation
adopted a Λ-shape conformation where the bond lengths and
bond angles were normal, and both pyridyl and phenyl rings were
twisted to the CBzꢀCꢀNPy reference plane with the correspond-
ing dihedral angles 91.91ꢀ and 83.30ꢀ.
The anions and cations in 1 MeCN formed regular stacks
3
along the crystallographic c-axis direction (Figure 2b). Within an
anion stack, the neighboring [Pt(mnt)2]ꢀ anions were arranged
in a rotational manner with an angle of 167.97ꢀ between their
Figure 3. (a) ORTEP view with non-hydrogen atomic numbering at
30% thermal ellipsoid probability level; (b) packing diagram that shows
the segregated and irregular stacks of anions and cations along the
crystallographic c-axis direction for 2.
long molecular axes, the Pt Pt distance of 3.986 Å, and the
3 3 3
nearest S S and Pt S contacts of 3.794 and 3.785 Å. These
3 3 3
3 3 3
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Inorganic Chemistry
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rings formed a dihedral angle of 108.45ꢀ and 72.39ꢀ, respectively,
with the CBzꢀCꢀNPy reference plane in the Λ-shape 40-Cl-Bz-
NH2Pyþ cation. These dihedral angles were larger than the
corresponding values in 40-CN-Bz-NH2Pyþ of 1 MeCN. The
3
[Pt(mnt)2]ꢀ and 40-Cl-Bz-NH2Pyþ stacks ran parallel to the
crystallographic a-axis direction, with each stack flanked by four
stacks of opposite charge (ref Figure 3b). Such an arrangement
was obviously different from that in 1 MeCN.
3
In an anion stack, two neighboring [Pt(mnt)2]ꢀ anions
2ꢀ
formed the eclipsed π-type [Pt(mnt)2]2
dimer with
an intradimer separation of dplane-to-plane = 3.520 Å (the
mean-molecular plane of [Pt(mnt)2]ꢀ anion was defined by
four S atoms), dPt1
= 3.500 Å, dS1
= 3.564 Å,
S3#1
Pt#1
3 3 3
dS2 S4#1 = 3.486 Å3(3s3ymmetric code: #1 = 2 ꢀ x, 1 ꢀ y, ꢀz).
3 3 3
As shown in Figure 3b, the slippages occurred between the
adjacent [Pt(mnt)2]2 dimers along both the long and the
2ꢀ
short molecular axes; the interdimer plane-to-plane separa-
tion was 3.669 Å, the interdimer Pt
Å, and the nearest interdimer S
Pt distance was 4.147
3 3 3
S and Pt
S contacts
3 3 3
3 3 3
were 3.962 and 3.825 Å, respectively. In a 40-Cl-Bz-NH2Pyþ
stack, two adjacent cations arranged in a chair-type fashion to
form a cationic dimer (Figure 3b). The phenyl rings in the
benzyl moieties were parallel to each other, owing to the
geometric constraint by an inversion center, and the inter-
plane distance between the parallel phenyl rings was 3.502 Å
within a cationic dimer and 3.512 Å between the cationic
dimers.
The crystal of 3 belonged to a triclinic system with space group
P1 and was isomorphic with its analogue [Bz-NH2Py][Ni-
(mnt)2].14 The asymmetric unit depicted in Figure 4a contained
two pairs of [Pt(mnt)2]ꢀ and Bz-NH2Pyþ ions. Two crystal-
lographically different planar anions were almost parallel to each
other (two mean molecular planes defined by four S atoms
formed a dihedral angle of 1.62ꢀ and built an angle of 17.82ꢀ for
the Pt(1)-anion versus 16.75ꢀ for the Pt(2)-anion with the
crystallographic bc-plane). The geometric parameters in two
crystallographically different [Pt(mnt)2]ꢀ anions were normal
Figure 4. (a) ORTEP view with non-hydrogen atomic numbering at
30% thermal ellipsoid probability level; (b) packing diagram that shows a
1-D arrangement of anions with a tetramer subunit along the crystal-
lographic a-axis direction as well as the shorter S S, S Pt and
3 3 3
3 3 3
and in agreement with those in 1 MeCN and 2 (ref Table 3).
Pt Pt distances (d1 = 3.586, d2 = 4.066, d3 = 3.717, d4 = 3.513, d5 =
3
3 3 3
The dihedral angles between pyridyl and phenyl rings and the
CBzꢀCꢀNPy reference plane were distinct in two crystallogra-
phically independent Λ-shape Bz-NH2Pyþ cations; these angles
were 103.5ꢀ and 89.6ꢀ in one cation and 92.9ꢀ and 80.2ꢀ in
another cation. As illustrated in Figure 4b, the [Pt(mnt)2]ꢀ
anions formed a tetrameric subunit in which two types of
crystallographic anions were arranged in a manner of Pt(1)Pt-
(2)Pt(2)Pt(1), where a terminal Pt(1) anion and a central Pt(2)
anion stacked into an eclipsed π-type [Pt(mnt)2]22ꢀ dimer with
an intradimer plane-to-plane separation of 3.514 Å (the mean-
molecular plane of the [Pt(mnt)2]ꢀ anion was defined by four S
atoms), Pt Pt separation of d1 = 3.586 Å, S S contacts of
3.535, d6 = 3.587, d7 = 3.612, d8 = 3.844, d9 = 3.587, and d10 = 3.802 Å)
within the 1-D tetrameric arrangement for 3.
interatomic separations were slightly longer than the sums of the
van der Waals radii of the corresponding atoms.19 Within a cation
stack, the adjacent Λ-shape cations adopted the boat-type
arrangement, the phenyl rings of neighboring benzyl moieties
were almost parallel to each other with a dihedral angle of 4.79ꢀ
and the CN groups were superimposed on the phenyl rings with
3.462 and 3.413 Å as the shortest distances of the C and N atoms,
respectively, to the mean molecular plane of the phenyl ring. The
anion and cation stacks were arrayed into two-dimensional (2-D)
molecular sheets that were parallel to the crystallographic
3 3 3
3 3 3
d4 = 3.513, d5 = 3.535, d6 = 3.587, and d7 = 3.612 Å; two dimers in
a tetrameric subunit slipped along a PtꢀS bond direction, leading
bc-plane. In an anion sheet, the closest Pt Pt separation
3 3 3
between the anion stacks was 12.068 Å along the crystallographic
to an increase of Pt Pt separation (d2 = 4.066 Å) and a
decrease of Pt S distance (d3 = 3.717 Å). The adjacent
tetrameric subunits were connected into a zigzag-type stack via
3 3 3
b-axis direction; this distance was significantly longer than that of
3 3 3
the Pt Pt separation within a stack. Therefore, from the
3 3 3
structural viewpoint, 1 MeCN could be considered a uniform
weakly lateral S S contacts (with the shorter S S separa-
3
3 3 3
3 3 3
chain spin system.
tions of d8 = 3.844, d9 = 3.587, and d10 = 3.802 Å) along the
crystallographic a-axis direction. It was noted that the structure of
Compound 2 crystallized in a monoclinic space group P2(1)/c,
and its asymmetric unit consisted of one pair of [Pt(mnt)2]ꢀ and
40-Cl-Bz-NH2Pyþ ions, as shown in Figure 3a. The bond
parameters in the planar [Pt(mnt)2]ꢀ entry were comparable
3 was remarkably different from that of 1 MeCN and 2. First, the
3
cations filled the space between the tetrameric anion stacks but
did not form a cofacial stack. Second, strong intermolecular
H-bond interactions were formed between the CN groups of the
to those in 1 MeCN, as listed in Table 2. The pyridyl and phenyl
3
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Table 3. Stacking Manner of Cations and Spin-Peierls-Type Transition in Reported Compounds
substituents
R1
R2
M
stacking manner of cations
stacking manner of anions
SP transition
ref.
F
H
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Pd
Pd
Pd
Pd
Pd
Pt
boat-type
boat-type
boat-type
boat-type
boat-type
boat-type
boat-type
boat-type
chair-type
chair-type
chair-type
chair-type
chair-type
boat-type
boat-type
chair-type
chair-type
boat-type
boat-type
boat-type
boat-type
boat-type
boat-type
boat-type
boat-type
chair-type
boat-type
regular stack
regular stack
regular stack
regular stack
regular stack
regular stack
regular stack
regular stack
irregular stack
irregular stack
irregular stack
irregular stack
irregular stack
regular stack
regular stack
irregular stack
irregular stack
regular stack
regular stack
regular stack
regular stack
regular stack
regular stack
regular stack
regular stack
irregular stack
regular stack
TC ≈ 90 K
TC ≈ 102 K
TC ≈ 110 K
TC ≈ 120 K
TC ≈ 181 K
TC ≈ 182 K
TC ≈ 190 K
TC ≈ 208 K
no
10b
10a
Cl
Br
I
H
H
10a
H
10e
NO2
CH3
CN
CHCH2
F
H
10a
H
10f
H
10c
H
10d
11c
H
Cl
H
no
11b
11b
11b
11b
11a
Br
H
no
I
H
no
NO2
Cl
H
no
H
TC ≈ 275 K
TC ≈ 269 K
TC ≈ 80 K
TC ≈ 184 K
no
Br
H
Pt
11a
I
H
Pt
11d
11a
NO2
CNa
Fb
H
Pt
H
Pt
12a
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Pt
no
12b
12c
Cl
TC ≈ 87 K
TC ≈ 105 K
no
Br
12d
12e
I
NO2
CN
CH3
Cl
TC ≈ 160 K
TC ≈ 210 K
TC ≈ 155 K
no
12f
12g
10f
this work
CN
Pt
TC ≈ 240 K
this work
a There was a larger slippage between the neighboring anions along the shorter molecular axis in an anion stack, and the neighboring phenyl rings did not
form a face-to-face overlap in the cation stack. b The neighboring pyridyl rings in this compound and the phenyl rings in others were superimposed within
a cation stack.
Pt(2)-anions and the NH2 groups in the N(12)-cation moieties,
which gave rise to a 2 þ 2 H-bond supramolecular ring (two
anions with two cations, as shown in Figure 5).
paramagnetic susceptibility should become zero as the tempera-
ture decreases below the spin-Peierls-type transition; however, a
Curie-type paramagnetic behavior was observed in the low-
temperature region, which was attributed to magnetic impurity
(due to, for example, a lattice defect). As a consequence, the
variable-temperature magnetic susceptibility in the low-tempera-
ture phase could be fitted using eq 1,
Magnetic Properties. The temperature-dependent magnetic
susceptibility of 1 MeCN, with both χm and 1/χm, formed at
3
1.8ꢀ300 K is displayed in Figure 6, where χm represents the
molar magnetic susceptibility corresponding to one [Pt(mnt)2]ꢀ
anion per formula unit, and the diamagnetic correction was not
made. From the plots, it could be determined that a spin-Peierls-
type transition occurred at around 240 K, and no sizable
hysteresis loop was detected in the cooling and heating proce-
dures. Below 240 K, a spin gap opened in the magnetic excitation
spectrum, which separated the nonmagnetic ground state from a
continuum of magnetically excited states. Theoretically, the
C
χm
¼
þ χ0
ð1Þ
T
where χ0 is contributed by the core diamagnetism and the
possible van Vleck paramagnetism, and the C/T term represents
the paramagnetism from the magnetic impurity. The best fit was
obtained in the temperature region of 1.8ꢀ120 K, which gave rise
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Figure 5. Bifurcated hydrogen bond interactions between NH2 groups
in N(12)-cation moieties and CN groups of Pt(2)-anions with geo-
metric parameters of dN(12)
= 3.018 Å, —N(12)ꢀH-
N(5)#1
137.51ꢀ3 3 3and dN(12)
(12B) N(5)#1
—N(12)ꢀH(12A) N(5)#1 = 150.82ꢀ (symmetric codes: #1 = x, y,
=
= 3.108 Å,
N(5)#2
3 3 3
3 3 3
3 3 3
1 þ z and #2 = 2 ꢀ x, 1 ꢀ y, 1 ꢀ z).
Figure 7. (a) plots of χm vs T (black, blue, red, and green solid squares
represent the magnetic susceptibility data under the applied magnetic
field of 2000 Oe, 1000 Oe, 500 and 10 Oe, respectively; (b) the plots of
χm vs T and χmT vs T (under an applied field of 2000 Oe) and the
theoretically produced plots in the range 9ꢀ300 K (red line) for 2.
The variable-temperature magnetic susceptibilities were mea-
sured under different magnetic fields (10, 500, 1000, and 2000
Oe) for 2; their plots of χm versus T are shown in Figure 7a, and
the curve of χmT versus T under the applied magnetic field of
2000 Oe is illustrated in Figure 7b (where χm represents the
corrected molar magnetic susceptibility, corresponding to one
[Pt(mnt)2]ꢀ anion per formula unit, and the molar diamagnetic
susceptibility was approximately equal to that of the
[Au(mnt)2]ꢀ analogue, χd = ꢀ2.9 ꢁ 10ꢀ4 emu molꢀ1).20
3
The magnetic properties of 2 showed two characteristics:
(1) the dominant magnetic coupling interactions in 2 were
weakly ferromagnetic over the temperature range of 6ꢀ300 K
and (2) the molar magnetic susceptibility was independent of the
applied magnetic field above ∼6 K but dependent on the applied
magnetic field below ∼6 K; a magnetic susceptibility maximum
was observed at ∼6 K under lower applied magnetic fields, and
this maximum disappeared when the applied magnetic field was
greater than 1000 Oe. The variable-temperature alternating
current (ac) magnetic susceptibility measurements were carried
out, and the corresponding χ0 versus T and χ00 versus T curves are
illustrated in Figure 8a. A broad peak in the temperature range of
2.5ꢀ15 K, with a maximum around 5.8 K, appeared in the χ0
versus T plot, indicating that the magnetic ordering may have
occurred near this temperature, whereas no out-of-phase signal
(χ00) was observed in the same temperature interval (i.e., there
was no net magnetic moment in this magnetic ordering state).
The magnetic field dependence of the magnetization was further
Figure 6. Plots of (a) χm vs T in both heating and cooling procedures
(blue line represents the fit using eq 1 in the temperature range of 2ꢀ120
K) (b) 1/χm vs T (red line is the fit using the CurieꢀWeiss law in the
temperature range of 240ꢀ300 K) for 1 MeCN.
3
to C = 0.04 emu K molꢀ1, χ0 = ꢀ3.0 ꢁ 10ꢀ4 emu molꢀ1, and
3
3
3
this diamagnetic susceptibility agreed well with the experimental
value for the diamagnetic and isostructural [Au(mnt)2]ꢀ com-
pound (ꢀ2.9 ꢁ 10ꢀ4 emu molꢀ1).20 To analyze the magnetic
3
behavior in the high-temperature phase, the temperature-depen-
dent magnetic susceptibility (in the temperature range of
240ꢀ300 K) was further fitted utilizing the CurieꢀWeiss law,
and the corresponding parameters were estimated to be C ≈
0.218 emu K molꢀ1 and θ ≈ ꢀ161 K, indicating that the
3
3
antiferromagnetic coupling interactions dominated between
the neighboring spins in the high-temperature phase.
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Figure 9. Plot of χm versus T for 3 (open circles: experimental data; red
line: fitting in the temperature range of 1.8ꢀ200 K).
where A=1.0þ 5.7980xþ 16.9027x2 þ 29.3769x3 þ 29.8329x4 þ
14.0369x5, B = 1.0 þ 2.7980x þ 7.0087x2 þ 8.6538x3 þ
4.5743x4, and x = J/2kBT, which was proposed by Baker and
Rushbrooke22 and derived from a high-temperature series ex-
pansion combined with a 2-D approach involving a “chain of
chains”23 was used in an attempt to fit the magnetic susceptibility
data from room temperature to 9 K. However, the fit failed to
offer reasonable parameters and to reproduce the experimental
susceptibility curve (Supporting Information, Figure S3). This
result suggested that the two magnetic exchange constants that
corresponded to two alternating Pt Pt distances within a
3 3 3
[Pt(mnt)2]ꢀ stack were significantly different. It was also note-
worthy that the modified BlaneyꢀBowers equations, eq 3 and
eq 4, were not able to provide reasonable results (where J and zJ0
represent the magnetic exchange constants of intradimer and
interdimer, respectively):
Figure 8. (a) ac magnetic susceptibility with f = 10 Hz and (b) M vs H
plot for 2.
measured at 1.8 K, as shown in Figure 8b. The M versus H plot
exhibited an S-shape, a typical feature of metamagnetic behavior,
with a small hysteresis loop. The critical magnetic field was
estimated to be 1000 Oe from the peak in the dM/dH versus H
curve (Supporting Information, Figure S2). These results suggest
that 2 was an antiferromagnet with TN ≈ 5.8 K and metamag-
netic behavior.
χdimer
χm
¼
ð3Þ
1 ꢀ ðzJ0=Ng2β2Þχdimer
ꢂ
ꢀ
ꢁꢃꢀ1
Ng2β2
ꢀ2J
χdimer
¼
3 þ exp
ð4Þ
3
kBT
kBT
Antiferromagnets with a large anisotropy do not show a spin-
flop phase but, in the presence of competing interactions, they
may undergo a first-order transition to a phase in which there
exists a net magnetic moment.21 The choice of an appropriate
magnetic model and the investigation of the magnetostructure
correlations were helpful for understanding the metamagnetic
behavior in 2. Thus, we tried to fit the magnetic susceptibility
data in the paramagnetic phase by utilizing different magnetic
exchange models. From the viewpoint of crystal structure, an
irregular [Pt(mnt)2]ꢀ stack in 2 could be considered an alter-
Finally, we fitted the molar magnetic susceptibilities in the
temperature region of 9ꢀ300 K for 2, employing the simple
CurieꢀWeiss law to produce the parameters C ≈ 0.387 emu
3
K molꢀ1 and θ ≈ 8.1 K (for the reproduced magnetic suscept-
3
ibility curve, refer to Figure 7b). The fitted Curie constant was
close to 0.375 emu K molꢀ1 (which was deduced from the spin-
3
3
1
only magnetic moment with an S = /2 spin system), and the
small positive Weiss constant indicated the existence of domi-
nant ferromagnetic coupling interactions in 2. The peculiar
metamagnetic behavior of 2 was caused by the competition
between the relatively stronger ferromagnetic (FM) and the
relatively weaker antiferromagnetic (AFM) coupling interac-
tions. Two possibilities existed: (1) the existence of FM interac-
tions in an intrastack and AFM interactions between the
interstacks of [Pt(mnt)2]ꢀ anions and (2) the occurrence of
FM interactions within a [Pt(mnt)2]ꢀ dimer of the anion stack
and AFM interactions between [Pt(mnt)2]ꢀ dimers. At present,
these two types of magnetic exchange schemes can not be
distinguished, and further theoretical investigation is needed.
For 3, the molar magnetic susceptibility as a function of
temperature, as illustrated in Figure 9, showed three striking
1
nating 1-D S = /2 chain or a dimer spin system. However, no
analytical expression has yet been established for a ferromagnetic
alternating chain of an S = 1/2 system. Given the assumption that
two magnetic exchange constants within an alternating chain are
not so different from each other and the consideration of the
[Pt(mnt)2]ꢀ stack as a ferromagnetic uniform chain, the expres-
sion given in eq 2,
ꢀ ꢁ
2=3
Ng2β2
4kBT
A
B
χm
¼
ð2Þ
3
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Figure 10. Cartesian coordinate syþstem chosen for dipole moment
calculations of (a) 40-CN-Bz-NH2Py and (b) 40-Cl-Bz-NH2Pyþ (the
magenta and olive arrows represent the calculated dipole vectors; the
total dipole and its three components along the X, Y and Z-directions,
respectively, were 14.5771, 14.5327, 0.7979, and 0.8101 D in 40-CN-Bz-
NH2Pyþ versus 12.0502, 12.0419, 0.4243, and 0.1355 D in 40-Cl-Bz-
NH2Pyþ).
Figure 11. Schematic illustrations of dipole arrangements in a cationic
stack for (a) 1 MeCN (with the boat-type alignment between the
3
neighboring cations) and (b) 2 (with the chair-type alignment between
the neighboring cations).
characteristics: the diamagnetism in the temperature range of
5ꢀ300 K, the weak paramagnetism below 5 K, and the magnetic
susceptibility that slightly increased as the temperature rose in
the higher-temperature region. The magnetic susceptibility data
over the 1.8ꢀ200 K temperature range were further fitted to eq 1,
giving rise to C ≈ 4.48 ꢁ 10ꢀ3 emu K molꢀ1 and χ0 ≈ ꢀ8.9 ꢁ
Table 4. Relative Energy and the Characteristically
Geometric Parameters (ref. Scheme 2) for a Tetrameric
Subunit of 1-N-(40-CN-benzyl)-4-aminopyridinium in
1 MeCN
3
3
3
10ꢀ4 emu molꢀ1. These observations indicate the existence of a
d1 (Å)
c1 (Å)
c2 (Å)
ΔE (kJ molꢀ1
)
3
3
nonmagnetic ground state and a larger energy gap between the
nonmagnetic ground state and the magnetic excited state. The
crystal structure analysis for 3 disclosed that two crystallographi-
cally different [Pt(mnt)2]ꢀ anions were arranged in the tetra-
meric stack, where a terminal anion and a central anion within a
regular stack
7.242
7.300
7.350
7.400
4.303
4.255
4.214
4.172
4.303
4.401
4.486
4.572
0.0
dimerized stack-1
dimerized stack-2
dimerized stack-3
ꢀ2.3103
ꢀ2.9979
þ0.1930a
a Notes: the repulsive energy between two face-to-face phenyl rings
increased when the terminal and the central cations were close to each
other, which led to an increase in the energy of the dimerized stack.
2ꢀ
tetrameric unit stacked into an eclipsed π-type [Pt(mnt)2]2
dimer with shorter intradimer separation and shorter interatomic
distances. The strong AFM interaction that led to 3, which
showed almost diamagnetism in the temperature range of 5ꢀ300
K, was due to the large overlap of magnetic orbitals between the
two eclipsed [Pt(mnt)2]ꢀ monomers.24
Peierls-type magnetic transition for the [M(mnt)2]ꢀ (M = Ni or
Pt) compounds.
To eþxplore this issue, the dipole moments of a 40-CN-Bz-
Cation Arrangement Pattern in a Columnar Stack and
Spin-Peierls-Type Transition. In recent studies, we have de-
signed, synthesized, and characterized more than 30 compounds
of planar [M(mnt)2]ꢀ with Λ-shape 1-N-(40-R1-benzyl)-4-R2-
pyridinium derivatives (M = Ni, Pd, and Pt; R2 = H or NH2, as
listed in Table 3); most of these compounds showed segregated
stacks of anions and cations in the crystal structures. From the
viewpoint of structure, these compounds could be divided into
two main subgroups according to the manner in which they
aligned with neighboring cations in a columnar stack. The
compounds in which the neighboring cations adopted boat-type
and chair-type configurations were classified as structural class I
and II, respectively. In structural class I, all compounds exhibited
regular stacks for both anions and cations, and most of the
compounds displayed a spin-Peierls-type transition below a
critical temperature. In structural class II, all compounds had
irregular stacks for both anions and cations; only two compounds
showed a spin-Peierls-type transition. Thus, there was probably
an intrinsic relationship between the spin-Peierls-type transition
and the stacking manner of the neighboring cations. This type of
magnetic transition was generally associated with a structural
dimerization of anion and cation stacks and the nonzero transi-
tion enthalpy, which was a typical characteristic of the first-order
phase transition; however, the spin-Peierls transition was theo-
retically the second-order phase transition. As a result, it is
obvious that some structural factors other than the magnetoe-
lastic interactions also played an important role in the spin-
NH2Py cation in 1 MeCN and a 40-Cl-Bz-NH2Pyþ cation in 2
3
were calculated. The chosen Cartesian coordinate system, to-
gether with the calculated dipole vectors, is shown in Figure 10.
The cationic stack showed different polarity in the cases of the
neighboring cations adopted in the boat-type (structural class I)
or chair-type (structural class II) arrangement. As illustrated in
Figure 11, the dipole moments were canceleþd out between the
neighboring cations in the 40-Cl-Bz-NH2Py stack (structural
class II) because they were related via an inversion center. There
was a net dipole moment along the columnar direction in the
40-CN-Bz-NH2Pyþ stack (structural class I), and such a polar
cation stack was highly energetically unfavorable, indicating the
existence of structural strain in a boat-type cation stack. Further-
more, the single point energy for a tetrameric subunit of 40-CN-
Bz-NH2Pyþ cations was calculated for the regular and dimerized
stacks (ref Scheme 2). The results summarized in Table 4
revealed that the structural strains of the polar 40-CN-Bz-
NH2Pyþ cation stack could be released via the dimerization.
In addition, we inspected all crystal structure data for the series
of quasi-1-D [benzylpyridinium derivative][M(mnt)2] com-
pounds (M = Ni, Pd, and Pt) and found that the neighboring
cations related by an inversion center were a common feature
when the cation stack possessed a chair-type arrangement. These
findings were helpful in understanding two issues for the series of
quasi-1-D [benzylpyridinium derivative][M(mnt)2] compounds:
(1) the origin of the spin-Peierls-type transition, and (2) the
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ARTICLE
preference of the spin-Peierls-type transition for compounds with a
boat-type cation arrangement.
Cl π interactions in 1 MeCN and 2, and the reproduced
3 3 3
3
magnetic susceptibility plots for 1 MeCN and 2 in PDF format.
3
This material is available free of charge via the Internet at http://
pubs.acs.org.
’ CONCLUDING AND REMARKS
In summary, three ion-pair compounds of [Pt(mnt)2]ꢀ
monoanions with 1-N-(40-CN-benzyl)-4-aminopyridinium deriv-
atives were structurally and magnetically characterized. For
’ AUTHOR INFORMATION
Corresponding Author
*Phone: þ86 25 83587820. Fax: þ86 25 83587438. E-mail:
1 MeCN, 2 and 3, the substituent in the phenyl ring of the
3
xmren@njut.edu.cn.
cation affected the packing structure of the ion-pair compounds.
The anions and cations formed segregated columnar stacks in
1 MeCN and 2, while only the anions formed the zigzag-type
3
stacks with a tetrameric [Pt(mnt)2]ꢀ subunit and the cations
filled the spaces between the anionic stacks in 3. The cation and
anion stacks were regular, and the neighboring 1-N-(40-CN-
benzyl)-4-aminopyridinium cations were arranged in the boat-
’ ACKNOWLEDGMENT
The authors thank the National Nature Science Foundation of
China for their financial support (Grant 20871068 and
20901039), and X.-M.R. thanks Prof. C. J. Fang for reading the
manuscript.
type pattern within a stack for 1 MeCN; in contrast, both cation
3
and anion stacks were irregular and the neighboring 1-N-(40-Cl-
benzyl)-4-aminopyridinium cations were aligned in the chair-
type manner within a stack for 2. A spin-Peierls-type transition
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ing took place with TN = 5.8 K, and below TN, a peculiar
metamagnetic phenomenon was observed with the critical field
of ∼1000 Oe for 2. The very strong AFM interactions within the
tetramer [Pt(mnt)2]ꢀ stack in 3 was due to the large overlap of
magnetic orbitals between the two eclipsed [Pt(mnt)2]ꢀ mono-
mers. Therefore, compound 3 showed almost diamagnetism in
the temperature range of 5ꢀ300 K.
In combination with our previous studies on the family of
quasi-1-D magnetic compounds of [benzylpyridinium derivative]-
[M(mnt)2] (M = Ni, Pd and Pt), the present findings reveal a
correlation between the spin-Peierls-type transition and the
stacking manner of the neighboring benzylpyridinium derivative
cations; the compounds with the boat-type stacking pattern
within a regular cation stack preferably elicited a spin-Peierls-
type transition. Such a transition was probably driven by two
factors acting together, including the magnetoelastic interaction
between the magnetic [M(mnt)2]ꢀ stacks and the lattice and the
release of structural strains within the diamagnetic stacks of the
benzylpyridinium derivative, as net dipole moments existed and
the structural strains could be released via the dimerization of the
stack in such a polar cation column.
A few open issues related to the stacking pattern of the cations
and the nature of the molecular buildings (the electronic and
geometric properties for both anions and cations) still remained
in the series of quasi-1-D [benzylpyridinium derivative][M-
(mnt)2] compounds (M = Ni, Pd, and Pt). For example, all
[Pd(mnt)2]ꢀ compounds showed irregular stacks for both
anions and cations, and the neighboring cations were aligned
in the chair-type configuration within a stack; the same benzyl-
pyridinium derivative cations were adopted in the boat-type
configuration in [Ni(mnt)2]ꢀ compounds, but in the chair-type
pattern in the corresponding [Pt(mnt)2]ꢀ compounds and vice
versa. The question remains as to which factors played a critical
role in the control of the stacking pattern of benzylpyridinium
derivative cations. The research to establish the correlations
between the factors not explained is in progress.
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’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data in CIF
b
format for 1 MeCN, 2, and 3, and the profiles of CN π,
3
3 3 3
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