Unusual magnetic property associated with dimerization within a nickel tetramer

Article abstract of DOI:10.1021/ic0255860

A new ion-pair complex, 1-benzyl-4-aminopyridinium bis(maleonitriledithiolato)nickelate(III)(1), has been synthesized. The variable magnetic susceptibility results of 1 show a discontinuity around 184 K, which is phenomenologically similar to that observed for first-row transition metal complexes undergoing a spin crossover transition. The crystal structure analyses of 1 at high and low temperatures indicate that this unusual magnetic property is associated with a packing structure that changes from tetrameric spin clusters to dimers between neighboring spin carriers.

Full text of DOI:10.1021/ic0255860

Inorg. Chem. 2002, 41, 5931−5933
Unusual Magnetic Property Associated with Dimerization within a Nickel
Tetramer
,†
Xiaoming Ren,† Qingjin Meng,* You Song,† Chuanjiang Hu,† Changsheng Lu,† Xiaoyuan Chen,‡ and
Ziling Xue§
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry,
Nanjing UniVersity, Nanjing, 210093, P. R. China, Department of Chemistry,
Syracuse UniVersity, Syracuse, New York 13244, and Department of Chemistry,
Tennessee UniVersity, KnoxVille, Tennessee 37996
Received March 12, 2002
A new ion-pair complex, 1-benzyl-4-aminopyridinium bis(maleo-
nitriledithiolato)nickelate(III) (1), has been synthesized. The variable
magnetic susceptibility results of 1 show a discontinuity around
184 K, which is phenomenologically similar to that observed for
first-row transition metal complexes undergoing a spin crossover
transition. The crystal structure analyses of 1 at high and low
temperatures indicate that this unusual magnetic property is
associated with a packing structure that changes from tetrameric
spin clusters to dimers between neighboring spin carriers.
countercation in [M(mnt)2]- complexes play an important
role in controlling the stacks of anions and cations. We have
recently developed a new class of salts [RbzPy]+[Ni(mnt)2]-
([RbzPy]+ ) derivative of benzylpyridinium). Some signifi-
cant and interesting results are described as follows:6 (1) The
structural feature of this class of complexes is that the well-
separated anions and cations form regular stacked columns
in which [Ni(mnt)2]- anions form a 1-D magnetic chain of
S ) 1/2. (2) The topology and size of the [RbzPy]+ ion, which
is related to the molecular conformation of the [RbzPy]+ ion,
can be modified by systematic variation of the substituent
groups in aromatic rings. Therefore, the stacking structure
of those complexes can be finely tuned by controlling the
molecular conformation of the [RbzPy]+ ion. (3) These
classes of 1-D chain complexes are strongly correlated
electron systems, and magnetic interactions in these systems
are very sensitive to intermolecular separations.
In this contribution, an ion-pair complex (1) with an
unusual magnetic property, which consists of [Ni(mnt)2]-
and 1-benzyl-4-aminopyridinium (Scheme 1), is described.
Upon decreasing the temperature, the packing structure of 1
in the solid state converts from tetrameric spin clusters to
the dimers. As a consequence, the temperature dependence
of the magnetic susceptibility exhibits a discontinuity
phenomenologically similar to that observed for first-row
transition metal complexes undergoing a spin crossover
transition. The transition feature has been characterized by
magnetic susceptibility measurement, crystal structure de-
One-dimensional (1-D) molecular solids have attracted
widespread attention because they show novel physical
properties such as Peierls transition, spin-Peierls transition,
charge density wave (CDW) states, spin density wave (SDV)
states, molecular magnetic bistability, molecular magnetic
nanowire, etc.1 In addition, 1-D compounds have also
stimulated theoretical investigations.
Among the most studied 1-D transition metal complexes
are complexes containing [M(mnt)2]- (M ) Ni(III), Pd(III),
or Pt(III)) ions. In these compounds, the constituent planar
molecules [M(mnt)2]- form columnar stacking, in which
2
intermolecular dz or π orbital interactions result in a 1-D
electronic nature.2-5 Generally, the topology and size of the
* Corresponding author. E-mail: Xmren@netra.nju.edu.cn.
† Nanjing University.
‡ Syracuse University.
§ Tennessee University.
(1) (a) Coomber, A. T.; Beljonne, D.; Friend, R. H.; Bre´das, J. L.; Charlton,
A.; Robertson, N.; Underhill, A. E.; Kurmoo, M.; Day, P. Nature 1996,
380, 144. (b) Manabe, T.; Kawashima, T.; Ishii, T.; Matsuzaka, H.;
Yamashita, M.; Mitani, T.; Okamoto, H. Synth. Met. 2001, 116, 415.
(c) Wei, J. H.; Zhao, J. Q.; Liu, D. S.; Xie, S. J.; Mei, L. M.; Hong,
J. Synth. Met. 2001, 122, 305. (d) Caneschi, A.; Gatteschi, D.; Lalioti,
N.; Sangregorio, C.; Sessoli, R.; Venturi, G.; Vingingni, A.; Rettori,
A.; Pini, M. G.; Novak, M. A. Angew. Chem., Int. Ed.2001, 40, 1760.
(e) Fujita, W.; Awaga, K. Science 1999, 286, 261. (f) Sugano, T.
Polyhedron 2001, 20, 1285 and references therein. (g) Kommandeur,
J.; Vegter, J. G. Mol. Cryst. Liq. Cryst. 1975, 30, 11. (h) Kosaki, A.;
Sorai, M.; Suga, H.; Seki, S. Bull. Chem. Soc. Jpn 1977, 50, 810. (i)
Hobi, M.; Zu¨rcher, S.; Gramlich, V.; Burckhardt, U.; Mensing, C.;
Spahr, M.; Togni, A. Organometallics 1996, 15, 5342.
(2) Nishijo, J.; Ogura, E.; Yamaura, J.; Miyazaki, A.; Enoki, T.; Takano,
T.; Kuwatani, Y.; Iyoda, M. Solid State Commun. 2000, 116, 661.
(3) Pullen, A. E.; Faulmann, C.; Pokhodnya, K. I.; Cassoux, P.; Tokumoto,
M. Inorg. Chem. 1998, 37, 6714.
(4) Allan, M. L.; Coomber, A. T.; Marsden, I. R.; Martens, J. H. F.; Friend,
R. H.; Charlton, A.; Underhill, A. E. Synth. Met. 1993, 55-57, 3317.
(5) Kobayashi, A.; Sasaki, Y.; Kobayashi, H.; Underhill, A. E.; Ahmad,
M. M. J. Chem. Soc., Dalton Trans. 1982, 390.
(6) (a) Ren, X. M.; Lu, C. S.; Liu, Y. J.; Zhu, H. Z.; Li, H. F.; Hu, C. J.;
Meng, Q. J. Transition Met. Chem. 2001, 26 (1/2), 136. (b) Ren, X.
M.; Li, H. F.; Wu, P. H.; Meng, Q. J. Acta Crystallogr., Sect. C 2001,
C57, 1022. (c) Ren, X. M.; Meng, Q. J.; et al. Submitted to Inorg.
Chem. (d) Unpublished materials.
10.1021/ic0255860 CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/25/2002
Inorganic Chemistry, Vol. 41, No. 23, 2002 5931

COMMUNICATION
Figure 1. The plot of ømT versus T for 1. Inset: d(ømT)/dT versus T.
Scheme 1. Molecule Structure of 1
Figure 2. The subunit of BAAB consisted of [Ni(mnt)2]- anions and the
infinite chains formed by S‚‚‚S interactions.
A crystal structure analysis9 of 1 at 293 K [high-
temperature phase (HT phase)] shows that an asymmetric
unit in a cell comprises two [Ni(mnt)2]- anions and two
[1-benzyl-4-aminopyridinium]+ cations. Most of the bond
lengths and bond angles are in agreement with those in other
[Ni(mnt)2]- complexes,6 except that the N atoms of the CN
group in [Ni(mnt)2]- anions have unusual deviations from
S4 planes. The average deviation containing Ni(1) anion is
around 0.1 Å, and the deviation containing Ni(2) anion is
around 0.32 Å. The anions are stacked face-to-face along
the a-axis (Figure 2) to form a 4-fold anion subunit. In each
subunit, the stacking is regular, and the overlap of the
neighboring anion is slightly shifted. The two independent
Ni‚‚‚Ni distances in the 4-fold subunit are 3.946 and 3.950
Å, respectively. The conformations of two [1-benzyl-4-
aminopyridinium]+ cations are slightly different, and the
pyridine ring and benzene ring with the CBz-C-NPy refer-
ence plane in two [1-benzyl-4-aminopyridinium]+ cations
make dihedral angles of 105.5°, 89.8° and 96.5°, 84.3°,
respectively.
termination in high- and low-temperature phases, and DSC
analysis.
The starting materials, Na2mnt and [1-benzyl-4-aminopy-
ridinium]Cl, were synthesized by the literature procedures.7,8
A similar method for preparing [Bu4N]2[Ni(mnt)2] was
utilized to prepare [1-benzyl-4-aminopyridinium]2[Ni(mnt)2].7
1 was prepared by the reaction of 1.0 equiv of [1-benzyl-
4-aminopyridinium]2[Ni(mnt)2] with 0.6 equiv of I2 in
MeCN-MeOH mixed solution. Single crystals suitable for
the X-ray structure analysis were obtained by diffusing Et2O
into 1 solution in MeCN.
The molecular structure of 1 at 89 K [the low-temperature
phase (LT phase)] is similar to that at 293 K, except that
the molecular packing in the LT phase is significantly
different from that in the HT phase (Figure 3).
Magnetic susceptibilities were measured with a Quantum
Design MPMS-5S superconducting quantum interference
device (SQUID) magnetometer in the range 2-350 K. The
diamagnetic correction was evaluated by using Pascal’s
constants.
The nearest-neighbor intermolecular arrangements [Ni(1)
anion‚‚‚Ni(2A) anion, and Ni(1A) anion‚‚‚Ni(2B) anion]
exhibit a farther slipped overlap in the HT phase compared
A plot of ømT versus T for 1 is shown in Figure 1. The
ømT value per mole of 1 at 350 K is 0.340 emu‚K‚mol-1,
and slightly lower than that expected for independent S )
1/2. As a sample 1 was cooled from 350 K, the ømT value
first decreases smoothly, then abruptly at 188 K, before
dropping to zero around 145 K. Below this temperature, this
complex is diamagnetic. When the temperature is increased
from 2 to 350 K, an identical curve is observed with no
hysteresis effect. Around 184 K, which is the temperature
at the sharp peak of d(ømT)/dT shown in the inset of Figure
1, the value of ømT abruptly drops from 0.295 to 0.018
emu‚K‚mol-1 within 10 K. The material thus exhibits a
switching from paramagnetic to diamagnetic around 184 K.
(9) Crystal data (C20H13N6NiS4): T ) 293(2) K, triclinic, space group
P1h, a ) 11.220(2) Å, b ) 14.295(3) Å, c ) 15.609(3) Å, R ) 74.00-
(3)°, â ) 77.30(3)°, γ ) 68.04(3)°, V ) 2212.7(8) Å3, Z ) 4, Fcalcd
) 1.574 g‚cm-3; FR590 CAD4 diffractometer, Mo KR radiation λ )
0.71073 Å; 8027 measured (1.37° e θ e 24.97°) and 7766
independent reflections used in the refinement; Lorentz polarization
but no absorption corrections (µ ) 1.275 mm-1). The structure was
solved by direct methods by using SHELXS 97 and refined with
SHELXL 97, 559 parameters. R1(on F2) ) 0.0322, wR2(on F2) )
0.0996, residual electron density 0.324 e‚Å-3. T ) 89(3) K, triclinic,
space group P1h, a ) 11.3376(15) Å, b ) 13.9757(18) Å, c ) 15.481-
(2) Å, R ) 73.070(2)°, â ) 74.367(2)°, γ ) 67.640(2)°, V ) 2135.1-
(5) Å3, Z ) 4, Fcalcd ) 1.631 g‚cm-3; CCD area detector, Mo KR
radiation λ ) 0.71073 Å; 9653 measured (1.9° e θ e 28.29°) and
7442 independent reflections used in the refinement; Lorentz polariza-
tion but no absorption corrections (µ ) 1.321 mm-1). The structure
was solved by direct methods by using SHELXS 97 and refined with
SHELXL 97, 575 parameters. R1(on F2) ) 0.0316, wR2(on F2) )
(7) Davison, A.; Holm, H. R. Inorg. Synth. 1967, 10, 8.
(8) Bulgarevich, S. B.; Bren, D. V.; Movshovic, D. Y.; Finocchiaro, P.;
Failla, S. J. Mol. Struct. 1994, 317, 147.
0.0809, residual electron density 0.545 e‚Å-3
.
5932 Inorganic Chemistry, Vol. 41, No. 23, 2002

COMMUNICATION
Figure 3. The side views of the 4-fold anion subunit in the HT and LT
phases. Symmetric codes: 1A ) 2 - x, 1 - y, 1 - z; 2A ) 1 - x, 1 - y,
1 - z; 2B ) 1 + x, y, z.
Figure 4. DSC of 1 showing the phase transition at 191 K (peak
temperature).
with that in the LT phase.1e The distance of Ni(1)‚‚‚Ni(2A)
and Ni(1A)‚‚‚Ni(2B) in the LT phase is reduced by 0.4 Å
(from 3.950 Å in the HT phase to 3.554 Å in the LT phase),
and the distance of Ni(1)‚‚‚Ni(1A) in the LT phase is reduced
only by 0.04 Å. The geometry in the LT phase maximizes
the exchange energy because of the better overlap between
the unlocalized electrons on the [Ni(mnt)2]- anions.1e As a
consequence, there are stronger interactions between Ni(1)
and Ni(2A), Ni(1A) and Ni(2B) to form a dimer structure
in the LT phase. The conformations of two cations in the
LT phase are slightly different from those in the HT phase;
the pyridine ring and benzene ring with the CBz-C-NPy
reference plane in two cations make dihedral angles of
104.5°, 88.7° and 96.3°, 80.7°, respectively. It is noted that
the crystallographic phase transition from the HT phase to
the LT phase is accompanied by a paramagnetic to diamag-
netic switching.
Differential scanning calorimetry (DSC) measurements
were performed with a Perkin-Elmer calorimeter in the
temperature range 93-293 K (-180 to 20 °C). The power-
compensated DSC trace at the warming rate 20 K min-1 is
shown in Figure 4. At 191 K, which is near the phase
transition temperature obtained from magnetic susceptibility
measurements, an endothermic effect of this phase transition
was observed, and the enthalpy change, ∆H, was 1.52
kJ‚mol-1. The result of DSC indicated that the enthalpy and
entropy are discontinuous when this sample changes from
one phase to another phase; the phase transition is thus first
order.
In conclusion, a complex with a phenomenologically spin-
transition-like nature has been characterized by magnetic
susceptibility measurements, its crystal structure determina-
tion in both HT and LT phases, and DSC analyses. The
abrupt change of the temperature dependence of the magnetic
susceptibility is associated with packing structure changing
from tetrameric spin clusters to dimers between neighboring
spin carriers, and shows feature of spin switching. The DSC
analyses further indicate that the phase transition is first order.
These results in current study reveal a magnetic switching
feature from paramagnetic to diamagnetic as the result of a
crystallographic phase transition. Such systems may poten-
tially be used in molecular devices.
Acknowledgment. The authors thank Prof. Q. P. Dai and
Ms. Z. R. Yuan of DSC Lab in the center of analysis and
determining of Nanjing University for determining DSC
data and acknowledge funding from the National Natural
Science Foundation and the State Education Commission of
China.
Supporting Information Available: Characterization data and
detailed descriptions of the synthesis of 1, tables of crystal data,
structure solution and refinement, atomic coordinates, bond lengths
and angles, and anisotropic thermal parameters for 1 in the HT
and LT phases (PDF). X-ray crystallographic files (CIF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
IC0255860
Inorganic Chemistry, Vol. 41, No. 23, 2002 5933