Field-induced ferrimagnetic state in a molecule-based magnet consisting of a CoII ion and a chiral triplet bis(nitroxide) radical

Article abstract of DOI:10.1021/ja064828i

We present the synthesis, crystal structure, and temperature and field dependence of the magnetic properties of a new molecule-based magnet, [Co(hfac)₂]7bull;BNO* (1), where hfac = 1,1,1,5,5,5- hexafluoroacetylacetonato and BNO* is the chiral t

Full text of DOI:10.1021/ja064828i

Published on Web 07/21/2007
Field-Induced Ferrimagnetic State in a Molecule-Based
Magnet Consisting of a CoII Ion and a Chiral Triplet
Bis(nitroxide) Radical
Youhei Numata,†,‡ Katsuya Inoue,*,‡,§ Nikolai Baranov,| Mohamedally Kurmoo, and
Koichi Kikuchi#
Contribution from the Department of Structural Molecular Science, The Graduate UniVersity for
AdVanced Studies, Okazaki, 444-8585, Japan, Department of Chemistry, Faculty of Science,
Hiroshima UniVersity, Kagamiyama, Higashi-Hiroshima 739-8526, Japan, Institute for AdVanced
Materials Research, Faculty of Science, Hiroshima UniVersity, Higashi-Hiroshima 739-8530,
Japan, Institute of Metal Physics RAS and Ural State UniVersity, Ekaterinburg 620083, Russia,
Laboratoire de Chimie de Coordination Organique, CNRS-UMR 7140, UniVersite´ Louis Pasteur,
4 rue Blaise Pascal, 67000 Strasbourg Cedex, France, and Department of Chemistry, Tokyo
Metropolitan UniVersity, Hachioji, Tokyo 192-0367, Japan
Received July 7, 2006; E-mail: kxi@hiroshima-u.ac.jp
Abstract: We present the synthesis, crystal structure, and temperature and field dependence of the magnetic
properties of a new molecule-based magnet, [Co(hfac)2]‚BNO* (1), where hfac ) 1,1,1,5,5,5-hexafluoro-
acetylacetonato and BNO* is the chiral triplet bis(nitroxide), 1,3-bis(N-tert-butyl-N-oxylamino)-5-{1′-methyl-
1′-[2′′-(S)-methylbutoxy]ethyl}benzene. The presence of enantiomer-pure BNO induces the formation of
chiral one-dimensional chains that are packed parallel to each other in the noncentrosymmetric P1 space
group. 1 exhibits four magnetic ground states: paramagnetic; antiferromagnetic; forced ferrimagnetic; field-
induced metastable ferrimagnetic. In the paramagnetic state (T > 20 K), it presents short-range
antiferromagnetic interaction between Co ion and nitroxide radical and has a minimum of ømT value at 220
K. The Weiss temperature estimated in the temperature range 220-300 K is found to be -89.9 K. At 20
K (TN), an antiferromagnetic long-range ordering is established. In the temperature range 4 K < T < 20 K,
the isothermal magnetization curve show a spin-flip transition to the forced ferrimagnetic state at around
850 Oe. Below 4 K, this compound enters into a field-induced ferrimagnetic state, which is metastable and
stabilized by the Ising character of the Co ion. In the low-temperature phase, the material becomes a very
hard magnet with wide hysteresis loop whose coercive field reaches 25 kOe at 2 K. The magnetic phase
diagram based on these magnetic data is presented.
Introduction
various structures such as zero-dimensional clusters, one-
dimensional chains, two-dimensional sheets, and three-dimen-
Molecule-based magnets have been investigated for several
decades, not only for their molecular structures but also for the
fact that their molecular arrangements and structural dimen-
sionalities can be controlled. For example, pure organic radical
magnets,1 organic radical metal complexes,2 and metal com-
plexes bridged by nonmagnetic ligands such as oxalato,3 azide,4
polycarboxylic acids,5 cyanide,6 etc.,7 are reported. They have
sional networks. Among the large number of compounds,
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† The Graduate University for Advanced Studies.
‡ Faculty of Science, Hiroshima University.
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§ Institute for Advanced Materials Research, Hiroshima University.
| Institute of Metal Physics RAS and Ural State University.
Universite´ Louis Pasteur.
# Tokyo Metropolitan University.
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9902
J. AM. CHEM. SOC. 2007, 129, 9902-9909
10.1021/ja064828i CCC: $37.00 © 2007 American Chemical Society

Ferrimagnetic State in a Molecule-Based Magnet
A R T I C L E S
Chart 1. [Co(hfac)2]‚BNO*
discrete magnetic clusters and quasi one-dimensional magnetic
chains are widely studied as candidates for single molecule
magnets (SMMs)8 and single chain magnets (SCMs),9 respec-
tively. These examples present very characteristic magnetic
properties among the molecule-based magnets. They show
unique magnetic hysteresis and relaxation properties on the basis
of their molecular structures and magnetic anisotropy. One of
their remarkable magnetic properties is magnetic quantum
tunneling. This is the current focus of interest because these
compounds are good examples where quantum phenomenon can
be observed on a macroscale.
phase diagram of one of our designed chiral structured molecule-
based magnets, [Co(hfac)2]·BNO*, where the chirality is
incorporated by the use of a chiral organic biradical. This
approach has facilitated the production of several chiral
structured magnets.13 It is a novel antiferromagnetic compound
[Co(hfac)2]‚BNO* (Chart 1) consisting of a large magnetic
anisotropic CoII ion and a chiral triplet biradical ligand. This
compound has one-dimensional chain structure and displays
unusual magnetic behavior under the influence of a magnetic
field.
And as another remarkable feature of molecule-based com-
pounds, the new materials, which have hybridized physical
properties, can be constructed by the combination of two (or
more) components which individually may have different
properties. For example, some compounds designed from such
ideas of combining magnetism and conductivity10 or supercon-
ductivity11 and magnetism and photoreactivity12 and optical
properties13 are reported. In this respect, we have been interested
in designing chiral molecule-based magnets with unique and
novel magnetic properties. Here, we report the preparation,
crystal structure, dc and ac magnetic properties, and the magnetic
Experimental Section
General Procedures and Materials. 1-3 and 6 were synthesized
under N2 atmosphere and using freshly distilled solvents. [Co(hfac)2-
(H2O)2] was prepared according to a literature method.14 All starting
reagents are available commercially. Organic compounds were char-
acterized by NMR measurements.
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Preparation of 1,3-Dibromo-5-{(1′-hydroxy-1′-methyl)ethyl}-
benzene (6). To a solution of 1,3,5-tribromobenzene (2.00 g, 6.35
mmol) in 30 mL of distilled diethyl ether was slowly added a 1.46 M
tert-butyllithium (8.86 mL, 13.02 mmol) n-pentane solution at -78
°C, and the mixture was warmed to -30 °C and stirred for 2 h. Distilled
acetone (0.5 mL, 6.67 mmol) was added to the solution, and the new
solution was stirred for 4 h. After addition of an aqueous solution of
NH4Cl and diethyl ether, the organic layer was separated, washed with
water, dried over anhydrous MgSO4, and concentrated under reduced
pressure. The orange oil was chromatographed on silica gel and
n-hexane as eluent at first and subsequently flashed with diethyl ether.
The diethyl ether fraction was concentrated and distilled under reduced
pressure (5 mmHg, 105 °C) to give 1.54 g (82.5%) of 6 as colorless
1
crystals. H NMR (400 MHz, CDCl3): δ 7.55 (d, 2H), 7.53 (d, 1H),
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Hashimoto, K. J. Am. Chem. Soc. 2006, 128, 5320.
1.54 (s, 6H).
Preparation of (S)-((2-Methylbutyl)oxyl)toluenesulfonyl (5). To
a solution of p-toluenesulfonyl chloride (8.87 g, 46.5 mmol) in 20 mL
of pyridine was added (S)-3-methyl-1-butanol (5.0 mL, 46.5 mmol),
which was then stirred at 0 °C for 1 day. To this suspension, water
and n-hexane were added and separated. The organic layer was
neutralized with dilute hydrochloric acid and separated again. The
organic layer was dried over with MgSO4 and NaHCO3, filtered, and
concentrated under reduced pressure to give 9.50 g (84.7%) of 5 as a
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Mart´ınez-Agudo, J. M.; Neuz, A.; Romero, F. M.; Stoeckli-Evans, H.
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Murcia-Mart´ınez, A.; Canadell, E. Inorg. Chem. 2004, 43, 8072. (n) Inoue,
K.; Imai, H.; Ohkoshi, S. A Chiral Molecule-Based Magnet. In Magne-
tism: Magnetic Materials Vol. 5; Miller, J. S., Drillon, M., Eds.; Wiley-
VCH: Weinheim, Germany, 2005; p 41. (o) Cinti, F.; Affronte, M.;
Lascialfari, A.; Barucci, M.; Olivieri, E.; Pasca, E.; Rettori, A.; Risegari,
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Gatteschi, D.; Rovai, D. Polyhedron 2005, 24, 2568. (p) Imai, H.; Inoue,
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Go´mez-Garc´ıa, C. J.; Nuez, A.; Romer, F. M.; Waerenborgh, J. C. Chem.
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1
colorless oil. H NMR (400 MHz, CDCl3): δ 7.78 (d, 2H), 7.34 (d,
2H), 3.84 (m, 2H), 2.45 (s, 3H), 1.71 (m, 1H), 1.39 (m, 1H), 1.14 (m,
1H), 0.87 (d, 3H), 0.82 (t, 3H).
Preparation of 1,3-Dibromo-5-{1′-methyl-1′-[2′′-(S)-methylbutoxy]-
ethyl}benzene (4). To a solution of 6 (1.0 g, 3.40 mmol) in 20 mL of
diethylene glycol dimethyl diethyl ether (diglyme) was added ground
KOH (85%) powder (422 mg, 6.80 mmol) in small fractions, and the
solution was stirred for 2 h at room temperature. After the color of the
solution changed to red, it was warmed to 110 °C. A solution of 5
(0.82 g, 3.40 mmol) in 20 mL of diglyme was slowly added to the
mixture during 3 h, and the new solution was stirred until the color
(14) (a) Petrukhina, M. A.; Henck, C.; Li, B.; Block, E.; Jin, J.; Zhang, S.;
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9
J. AM. CHEM. SOC. VOL. 129, NO. 32, 2007 9903

A R T I C L E S
Numata et al.
Table 1. Crystallographic Data for 1
changed to creamy yellow. To the mixture, 10 mL of water was added
and separated, dried over with anhydrous MgSO4, filtered, and
concentrated under reduced pressure to give 773 mg (62.4%) of 4 as a
colorless oil. H NMR (400 MHz, CDCl3): δ 7.53 (d, 1H), 7.47 (d,
2H), 2.95 (m, 2H), 1.59 (m, 2H), 1.47 (m, 6H), 1.12 (m, 1H), 0.88 (q,
formula
mol wt
cryst system
space group
a, Å
C32H40CoF12N2O7
851.59
triclinic
1
P1h (No. 1)
10.704(4)
11.573(5)
17.429(7)
81.647(7)
84.643(7)
63.195(6)
1905.6(14)
2
9717
7856
1.000
0.0714 (0.1960)
-0.03(2)
6H).
b, Å
Preparation of 1,3-Bis[N-tert-butyl-N-(hydroxyamino)]-5-{1′-
methyl-1′-[2′′-(S)-methylbutoxy]ethyl}benzene (3). A solution of 4
(1.0 g, 2.75 mmol) in 20 mL of diethyl ether was cooled to -78 °C,
and a 1.46 M tert-butyllithium (7.6 mL, 11.14 mmol) n-pentane solution
was slowly added to the solution. The mixture was warmed gradually
to -30 °C and stirred for 2 h. A solution of 2-methyl-2-nitrosopropane
dimer (0.49 g, 5.64 mmol) in 10 mL of diethyl ether was added in
small fractions to the solution, and the new solution was stirred for 3
h at -30 °C. After addition of an aqueous solution of NH4Cl and diethyl
ether, the organic layer was separated, washed with water and dried
over anhydrous MgSO4, filtered, and concentrated under reduced
pressure. To the mixture, a small amount of n-hexane was added and
cooled for 1 day at -20 °C, to give 567 mg (54.2%) of a white powder
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
no. of unique reflcns
no. of reflcns used
GOF
R1 (wR2)
Flack factor
Results and Discussion
1
of 3. H NMR (400 MHz, CDCl3): δ 7.05 (s, 1H), 6.89 (s, 2H), 2.91
Crystal Structure. X-ray structural analysis of 1 revealed
the absence of all symmetry, and thus, the triclinic P1 (No. 1)
space group was chosen. Details of crystallographic data are
presented in Table 1. In the unit cell, there are two crystallo-
graphically independent units of [Co(hfac)2]‚BNO*. [Co(hfac)2]
and the BNO* radical form a uniform one-dimensional R-type
helical chain structure along the crystallographic c-axis. Figure
1 presents a view of the 1-D chain structure. In the unit cell,
there is only one helical chain consisting of the two different
building blocks of [Co(hfac)2]‚BNO*. The aminoxyl groups of
BNO* are rotated out of the phenylene ring plane. The chiral
substitute of all BNO* molecules have the S-configuration.
These alkyl groups are situated between the 1-D chains;
therefore, the chains are well isolated from each other (Figures
2 and 1S). All cobalt ions exhibit octahedral coordination with
the four oxygen atoms of two hfac ligands in the equatorial
plane and the two oxygen atoms of the aminoxyl groups of two
different BNO* molecules in trans- configurations. The Co-
(m, 2H), 1.58 (m, 4H), 1.37 (m, 6H), 1.15 (m, 18H), 1.04 (q, 1H),
0.84 (m, 6H).
Preparation of 1,3-Bis[N-tert-butyl-N-(oxylamino)]-5-{1′-methyl-
1′-[2′′-(S)-methylbutoxy]ethyl}benzene (2). A solution of 3 (100 mg,
0.26 mmol) in 15 mL of dichloromethane was cooled in an ice bath to
0 °C, and to this solution was added Ag2O (500 mg, 2.16 mmol), which
was then stirred for 2 h. The mixture was purified by silica gel column
chromatography with dichloromethane as eluent to give 98 mg (98%)
of 2 as an orange colored oil. It was subsequently used for the next
step.
Preparation of [Co(hfac)2]‚BNO* (1). [Co(hfac)2(H2O)2] (132 mg,
0.26 mmol) was refluxed in n-hexane to remove water of hydration by
azeotropic distillation. To this solution, a solution of BNO* (98 mg,
0.26mmol) in dichloromethane was added and stirred for 30 min at 80
°C. The mixture was cooled at -20 °C for 1 week to give 106 mg
(47.8%) of the dark red crystals of 1. These crystals were suitable for
X-ray structure determination. Anal. Calcd for C32H40CoF12N2O7 (1):
C, 45.13; H, 4.73; N, 3.29. Found: C, 45.75; H, 4.05; N, 3.29. IR
(KBr): ν ) 1644, 1554, 1504, 1257, 1196, 1150, 795, 669, 587 cm-1
.
Ohfac bond lengths range from 2.037(5) to 2.056(6) Å, and Co-
Physical Measurements. Dc and ac magnetic susceptibilities were
measured by use of a Quantum Design MPMS-XL SQUID magne-
tometer. The magnetic susceptibilities were corrected for diamagnetism
using Pascal’s constants. The IR spectrum was measured on KBr disk
with a Jasco FT/IR-660 plus spectrometer. The X-band ESR spectra
of polycrystalline samples were measured by a Bruker ESR 500
equipped Oxford ESR 900 cryostat for the measurements at 300-15
K and ESR 910 cryostat at 15-2 K, respectively.
Orad ranges from 2.075(6) to 2.096(6) Å. The bond angles made
Crystallography. A selected single crystal of size 0.50 × 0.25 ×
0.18 mm3 was mounted on a glass fiber. The X-ray data were collected
at 100 K using a Bruker SMART-APEX diffractmeter, equipped with
a CCD area detector and graphite-monochromated Mo KR radiation, λ
) 0.710 73 Å, employing a ω-scan mode (0.3° step) and semiempirical
absorption correction on Laue equivalents. The structure was solved
by direct methods and refined by full-matrix least squares against F2
of all data, using SIR-97 and SHELXTL softwares.15,16 All non-
hydrogen atoms were refined anisotropically. All hydrogen atoms were
placed in calculated positions and not refined. The refinement converges
with R1 ) 7.14% for 7856 data (I > 4σ(I)), wR2 ) 19.60% for 9717
unique data (1.18 e θ e 28.39), Flack parameter ) -0.03(2), and
maximim/minimum residual electron density 1.34/-1.03 e Å-3
.
(15) Altamore, A.; Burla, M. C.; Camalli, M.; Gascarano, G. L.; Giacovazzo,
C.; Guaglirdi, A.; Molitemi, G. G.; Polidori, G.; Spagna, R. SIR-97. J.
Appl. Crystallogr. 1999, 32, 115.
(16) Sheldrick, G. M. SHEXL-97; University of Go¨ttingen: Go¨ttingen, Germany,
1997.
Figure 1. ORTEP drawing of the crystallographically independent unit of
1. Thermal ellipsoids are at the 40% level. H atoms are omitted for clarity.
Two Co and some selected atoms are labeled. Key: black, carbon; red,
oxygen; blue, nitrogen; green, fluorine.
9
9904 J. AM. CHEM. SOC. VOL. 129, NO. 32, 2007

Ferrimagnetic State in a Molecule-Based Magnet
A R T I C L E S
All magnetic measurements were performed for the polycrys-
talline samples. The ømT vs T and 1/øm vs T plots are presented
in Figure 3. The temperature dependence of ømT has a minimum
at 220 K, which suggests ferrimagnetic behavior due the
antiparallel alignment of the noncompensated moments within
each chain in 1. Weiss temperature, Θ, which was estimated to
be -89.9 K from the 1/øm vs T plot in the temperature range
220-300 K, indicates J2 , 0. The peak temperature in the M-T
curve is around 20 K and shifts to lower temperatures by
increasing the dc field (Figure 3S).
The magnetic susceptibilities for the zero-field-cooled (øZFC
)
sample and field-cooled sample (øFC) were measured in applied
fields of 500 and 5000 Oe (Figure 4). In 500 Oe, øZFC and øFC
exhibit similar values in the temperature region 4-100 K. At
temperatures 4 < T < 16 K, a shoulder is evident for both øZFC
and øFC curves. At 4 K, the øZFC value increases abruptly;
subsequently, it saturates and decreases gradually with further
increasing temperature. Below 4 K, øZFC(T) and øFC(T) are split
and demonstrate constant values below 3.2 K. In 5000 Oe, the
øZFC value is also a constant in the temperature range 2-3.2 K,
but the peak at 20 K is not present, suggesting reversal of all
moments in this high field. The sharp drop in the susceptibility
in 500 Oe suggests long-range magnetic ordering of the resultant
moments within each chain. However, as the CoII ion becomes
more anisotropic as the temperature is lowered due to the
depopulation of the upper Kramers doublets, a small canting is
generated resulting in the weak ferromagnetism below 4 K with
consequent magnetic hardness.
The different kinds of hysteresis loops were observed from
1.8 to 30 K. In the temperature interval 4-18 K, the isothermal
magnetization, M(H) dependence (shown in Figure 5a) seem
to be typical for antiferromagnets revealing a metamagnetic
transition under application of an external field. This field-
induced phase transition occurs when an applied field reaches
a critical value of the order of the AF interchain exchange. The
saturated magnetization value (MS) of 1 is approximately 1.16
µB/formula unit. This value is higher than that expected (1 µB
if the average g-value of Co is 2 and S ) 3/2 or 0.15 µB if the
average g-value of Co is 4.3 and S ) 1/2) for a simple
ferrimagnetic model consists of the unit of CoII and BNO* (S
) 1) which are coupled antiferromagnetically. As shown in
Figure 5a, the phase transition from the AF state to the forced
ferrimagnetic (FoFi) state around 850 Oe is accompanied by
hysteresis. However, the width of the hysteresis, ∆Hc, does not
exceed the critical field and the remnant magnetization remains
zero above 4 K; that is, the magnet remains soft. However, when
the temperature is set below 4 K, the width of the hysteresis
loop increases abruptly leading to the constricted shape of the
loop at temperatures 3.6 K < T < 4 K (see Figure 5b) and to
the “magnet-type” hysteresis loop with further lowering of
temperature (shown in Figure 5c). The coercive field reaches a
value of 25 kOe below 2.6 K. This behavior has some
characteristics as those observed for Co2(OH)2(terephthalate)
and Co2(pyromellitate).19c
Figure 2. Projection of crystal structure of 1 along the a-axis (top) and
c-axis (bottom). H and F atoms are omitted for clarity. Key: red, oxygen;
blue, nitrogen.
by two oxygen atoms located at the diagonal corners of the
coordination octahedra and central Co ion are almost 180°. (The
angles lie in the range 178.4(3)-179.5(3)°.) The intrachain Co-
Co distances of Co(1)-Co(2) and Co(2)-Co(1′) are 8.722 and
8.696 Å, respectively; additionally, the shortest interchain Co-
Co distance is 10.697 Å. Other selected Co-Co distances are
given in the Supporting Information (Figure 2S). Selected bond
lengths, bond angles, torsion angles, and intrachain Co-Co
distances are given in Tables 2 and 1S.
Magnetic Properties. The m-phenylenebis(nitroxide) radical
has previously been shown to have the triplet ground state where
its intramolecular ferromagnetic interaction, J1/kB, is large (430
K) for the nonchiral BNOH (BNOH ) 1,3-bis(N-tert-butyl-N-
(oxylamino))benzene).17 The magnetic interaction between
nitroxide radical and CoII ion (J2) appears to be antiferromag-
netic owing to the overlapping of magnetic orbitals of the
nitroxide radical and CoII. This value is not available in the
literature, and the observed susceptibility suggests J2 , 0.18
Additionally, the hysteresis loops measured in the ranges 1.8-
2.8, 2-3.8, and 4-22 K are shown in Figure 5S. It is considered
that the change of the shape and large increase of the width of
the hysteresis loops with decreasing temperature below 4 K
(17) (a) Calder, A.; Forrester, Alexander, R.; James, P. G.; Luckhurst, G. R. J.
Am. Chem. Soc. 1969, 91, 3724. (b) Hosokoshi, Y.; Katoh, K.; Nakazawa,
Y.; Nakano, H.; Inoue, K. J. Am. Chem. Soc. 2001, 123, 7921.
(18) Field, L. M.; Moro´n, M. C.; Lahti, P. M.; Palacio, F.; Paduan-Filho, A.;
Oliveira, N. F. Inorg. Chem. 2006, 45, 2562.
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A R T I C L E S
Numata et al.
Table 2. Selected Bond Lengths (Å), Bond Angles (deg), and Intrachain Co-Co Distances (Å) of 1
Co1
Co2
Co1-O1
Co1-O9
Co1-O4
Co1-O5
Co1-O6
Co1-O7
2.075(6)
2.081(5)
2.054(6)
2.048(5)
2.056(6)
2.049(6)
N1-O1
N2-O2
Co1-Co2
1.303(9)
1.296(9)
8.722
Co2-O2
Co2-O8
Co2-O11
Co2-O12
Co2-O13
Co2-O14
2.093(5)
2.096(6)
2.037(5)
2.046(5)
2.040(6)
2.056(6)
N3-O8
N4-O9
Co2-Co1
1.283(9)
1.286(9)
8.696
O1-Co1-O9
O4-Co1-O7
O5-Co1-O6
O1-Co1-O4
O1-Co1-O5
O1-Co1-O6
O1-Co1-O7
O4-Co1-O5
179.1(3)
179.5(3)
179.3(3)
86.1(2)
85.7(2)
94.6(2)
93.8(2)
88.3(2)
O4-Co1-O6
O4-Co1-O9
O5-Co1-O7
O5-Co1-O9
O6-Co1-O7
O6-Co1-O9
O7-Co1-O9
91.0(2)
93.7(2)
92.2(2)
93.4(2)
88.5(2)
86.3(2)
86.4(2)
O11-Co2-O14
O12-Co2-O13
O2-Co1-O8
O2-Co2-O11
O2-Co2-O12
O2-Co2-O13
O2-Co2-O14
O8-Co2-O11
179.0(3)
178.4(3)
179.3(3)
95.9(2)
94.4(2)
85.2(2)
84.8(2)
84.2(2)
O8-Co2-O12
O8-Co2-O13
O8-Co2-O14
O11-Co2-O12
O11-Co2-O13
O12-Co2-O14
O13-Co2-O14
86.5(2)
94.0(2)
95.1(2)
88.5(2)
93.1(2)
90.7(2)
87.7(2)
Figure 3. Temperature dependence plots of 1/øm (crosses) and ømT (open
circles) for 1. Inset: Expanded ømT curve around the minimum point.
originates from freezing of domain wall displacement due to
the crossover from a Heisenberg-type antiferromagnet to an
Ising-type antiferromagnet.19,20 Figure 8S shows ESR spectra
measured at selected temperatures and the plot of the positions
of g-values of the half-width of left end peaks. At 300 K, it
shows only one signal at g ) 2.00. Upon lowering of the
temperature, the peaks were split and the g-value increased. In
Figure 8Sc, the g-value shifts following a decrease temperature
show a two times increase at around 70 and 7 K. It is considered
that they are originated from the magnetic interactions and single
ion anisotropy of Co(II).
In the temperature dependence of the ac magnetic susceptibil-
ity measurements in zero magnetic field, the øac′ values show a
frequency dependence in the temperature range 4-8 K. These
data support our suggestion about the crossover from Heisen-
berg-type to Ising-type regime occurred in this temperature
Figure 4. Temperature dependence of molar magnetic susceptibility
measured at 500 Oe (a) and 5000 Oe (b) employing the ZFC (open circles)
and FC (closed circles) mode. The inset shows a magnified view at the
temperature region 2-6 K.
(19) (a) Kurmoo, M.; Kumagai, H.; Green, M. A.; Lovett, B. W.; Blundell, S.
J.; Ardavan, A.; Singleton, J. J. Solid State Chem. 2001, 159, 343. (b)
Lovett, B. W.; Blundell, S. J.; Kumagai, H.; Kurmoo, M. Synth. Met. 2001,
121, 1814. (c) Kumagai, H.; Kepert, C. J.; Kurmoo, M. Inorg. Chem. 2002,
41, 3410. (d) Kurmoo, M.; Kumagai, H.; Hughes, S. M.; Kepert, C. J.
Inorg. Chem. 2003, 42, 6709.
(20) (a) Miller, J. S.; Calabrese, J.; McLean, R. S.; Epstein, A. J. AdV. Mater.
1992, 4, 498. (b) Miller, J. S.; Vazquez, C.; Jones, N. L.; McLean, R. S.;
Epstein, A. J. J. Mater Chem. 1995, 5, 707. (c) Wynn, C. M.; Gˆırt¸u, M.
A.; Sugiura, K.; Brandon, E. J.; Manson, J. L.; Miller, J. S. Epstein A. J.
Synth. Met. 1997, 85, 1695. (d) Wynn, C. M.; Gˆırt¸u, M. A.; Miller, J. S.;
Epstein, A. J. Phys. ReV. B 1997, 56, 14050. (e) Dawe, L. N.; Miglioi, J.;
Turnbow, L.; Taliafferro, M. L.; Shum, W. W.; Bagnato, J. D.; Zakharov,
L. N.; Rheingold, A. L.; Arif, A. M.; Fourmigue´, M.; Miller, J. S. Inorg.
Chem. 2005, 44, 7530.
range.19,20 The position of the øac′ peak at around 20 K is
observed to be frequency independent (Figure 6S). The øac′′(T)
values dependence shows an additional peak which is frequency
dependent (1-1500 Hz) (Figure 6S). In a bias magnetic field
of 500 Oe which is lower than the metamagnetic critical field,
øac′ and øac′′ show behavior similar to that in 0 Oe (Figure 6S).
The frequency-dependent region of øac′ broadens with increasing
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9906 J. AM. CHEM. SOC. VOL. 129, NO. 32, 2007

Ferrimagnetic State in a Molecule-Based Magnet
A R T I C L E S
Figure 6. Magnetic phase diagram of 1. Inset: Diagram magnified from
0 to 2000 Oe.
The change of the magnetization with magnetic field reveals
a different dynamics below and above this crossover temper-
ature. It is clearly seen from dc field dependence of the ac
susceptibility presented in Figure 6Sa,b at selected temperatures.
At T g 20 K, the behavior of the ac susceptibility is quite typical
for paramagnets (Figure 7Sg). There is no frequency dependence
of the øac′ and øac′′ curves within the frequency range used (1-
1000 Hz). The imaginary part of the ac susceptibility is zero,
since the relaxation time of magnetic moments above the
ordering temperature is significantly smaller than the period of
ac magnetic field. Below the ordering temperature, the imaginary
part of the ac susceptibility reflects mainly the energy losses,
which occur during the magnetization rotation within the domain
and domain wall movement. At 7.5 K, i.e., above the crossover
temperature, the øac′(Hdc) dependence shows frequency-depend-
ent peaks at around 850 Oe and øac′′ curves also show field-
dependent peak (Figure 7Sd). This peak indicates the spin-flip
transition, which is associated with the nucleation of the FoFi
phase within AF phase and displacement of the domain walls
separating AF and FoFi phases. The frequency dependence of
both øac′ and øac′′ in the vicinity of the critical field becomes
more pronounced with decreasing temperature especially in the
low-frequency range. This means that the rate of the magnetiza-
tion change decreases with lowering temperature. Finally, below
4 K, we could not observe any peak of the øac′ and øac′′ in the
vicinity of the field-induced AF-FiFi transition in M-H
measurements below 4 K (Figure 7Sa,b). These data are
indicative of the domain wall freezing below the crossover
temperature. øac-H plots at 12, 10, and 5 K are presented in
Figure 7Sf,e,c. To understand its magnetic properties more
clearly, the magnetic phase diagram was constructed.
Figure 5. Temperature dependence of hysteresis loops measured at selected
temperatures: (a) 2-30 K; (b) 3.4-4.0 K; (c) 1.8-3.2 K.
Magnetic Phase Diagram. The magnetic phase diagram in
the low-temperature region is displayed in Figure 6. Three lines
in the phase diagram, HC (critical field of the initial magne-
strength of dc magnetic field above 500 Oe. The øac′′ peak in
the vicinity of 4-6 K corresponds to the phase transition from
AF to field-induced ferrimagnetic (FiFi) phase (vide infra). The
behaviors of øac′ and øac′′ are completely different in the dc
field of 5000 Oe, i.e., in a field well over the metamagnetic
critical field. The øac′ values show a sharp rise at around 5 K
and a frequency-independent peak at around 30 K (Figure 6S).
The peaks at 30 K correspond to the phase transition from
paramagnetic state to forced ferrimagnetic state.
up
down
tization process), HC
(critical field of the demagnetization
up
down
process), and ∆HC (difference between HC and HC
)
represent the boundary of the FiFi phase,20,21 the FoFi phase,
and AF phase, respectively. The HCup curve corresponds to the
(21) (a) Baranov, N. V.; Pirogov, A. N.; Teplykh, A. E. J. Alloys Compd. 1995,
226, 70. (b) Baranov, N. V.; Mushnikov, N. V.; Goto, T.; Hosokoshi, Y.;
Inoue, K. J. Phys.: Condens. Matter 2003, 15, 8881.
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A R T I C L E S
Numata et al.
spin-flip field. This diagram exhibits four magnetic ground
states. The paramagnetic state exists above 20 K. In the
temperature region 4-20 K, this molecule behaves as a usual
metamagnet. The value of the critical field of the AF-FoFi
transition corresponds to the interchain exchange interaction.
An upturn of ∆HC with decreasing temperature below 4 K is
associated with freezing of the domain wall motion in the Ising-
type magnetic structure. Below 3.2 K, the compound displays
a large coercive field (ca. 25 kOe at 2 K) and hysteresis loop
that seem to be typical for hard magnetic materials. After
application of a strong magnetic field in this temperature range,
further variation of an external field does not lead to the regime
of the initial AF state in the compound. Thus, this FiFi state is
a metastable state. The width of the hysteresis loops in the Ising
state is controlled by the exchange interaction within chains
and thermal activation of the domain wall displacement.
However, the increase of the width of the hysteresis loops with
decreasing temperature is stopped below ∼2.8 K, which can
be ascribed to the change in the mechanism of the domain wall
displacement from the thermal activation to macroscopic
quantum tunneling.
magnetic state (0.3 µB) but smaller than that of a ferromagnetic
state (4.3 µB) with parallel alignment of magnetic moments of
Co ions and radicals. For the polycrystalline sample we have
to obtain at least half (2.15 µB) of the saturation value in the
case of uniaxial magnetocrystalline anisotropy. Therefore, one
can suggest that in our case all the moments are tilted. Assuming
same tilt angle for both Co and radicals, as we did above, the
estimated tilt angle is 74° from the hard axis. This may be the
reason for the very square shape hysteresis loop with a low
saturation value. The tilt may be regarded as a fan, which may
well be chiral. Neutron diffraction, if ever a fully deuterated
sample becomes available, may resolve the problem.
Conclusion
We successfully synthesized a novel organic-inorganic
hybrid type chiral structural antiferromagnet, which is comprised
of [Co(hfac)2] and a chiral triplet bis(nitroxide) radical. This
magnet possesses 1-D helical chain structure; additionally, the
structure is characterized by R-type helicity. Initially, in the high-
temperature region above 20 K, this molecule displays behavior
as a paramagnetic state. Below 20 K, antiferromagnetic interac-
tion dominates among 1-D ferrimagnetic chains, and the ground
state of this compound is 3-D antiferromagnetic. In the
temperature range 3.2-20 K, this compound behaves as a
metamagnet and its MS is ∼1.16 µB. However, below ∼4 K,
the thermal activated motion of the domain wall between the
AF and FoFi phases is frozen owing to the high single ion
anisotropy. It is considered that this behavior is caused by
crossover from a Heisenberg-type to an Ising-type antiferro-
magnet with decreasing temperature. The ESR spectra show
g-value shifts, and the plot of the position of half-width of the
left end peak shows a two time increase at around 70 and 7 K.
It is expected that they originated from magnetic interactions
and the change of single ion anisotropy of Co(II) from
Heisenberg-type to Ising-type AF state. As a result, a sample
undergoing magnetization by application of a high magnetic
field remains in the field-induced ferrimagnetic FiFi state after
removing the field. Moreover, further variation of the field does
not return the sample to the initial AF state.19-21 The initial AF
alignment of the magnetic moments of the chains may be
reinstated upon heating the sample above 5 K, followed by
cooling in zero field. In the FiFi phase, the sample demonstrates
hard magnetic properties characterized by coercive field of 25
kOe below 3 K and with MS ∼ 1.16 µB. With decreasing
temperature below ∼2.5 K, the width of the hysteresis loop
becomes temperature independent (Figure 4S). Above 2.5 K,
the widths of the hysteresis loops show remarkable temperature
dependence. These results are indicative of crossover of the
thermal activated domain wall motion with quantum tunneling
behavior in Ising-type magnets.9,23 Currently, the magnetic
properties of this molecule are under investigation and its
magnetic phase transitions are being identified systematically.
Subsequently, the magnetic properties of single crystals must
be examined. Furthermore, neutron diffraction measurements
are necessary to establish the relationship between magnetic
properties and chirality.
To get an insight into the plausible orientation of the moments
in the different ground state, we looked for different models to
account for the observed magnetization values. Above 20 K,
the compound is in the paramagnetic state with strong intramo-
lecular ferromagnetically coupled S ) 1/2 radicals and less
strong antiferromagnetically coupling between the cobalt mo-
ment and the radicals. At 20 K, long-range magnetic ordering
is observed with a residual moment of 0.03 µB. This value is
obtained from the øm values of below 3 K (Figure 4). This value
is much smaller than the 0.3 µB expected for a ferrimagnetic
state, that is, all the cobalt moments being antiparallel to those
of the radicals assuming the moment of cobalt to be 2.3 µB (S
) 1/2, g > 4) and that of the radical is 2.0 µB (S ) 1, g ) 2.0).
The presence of small residual moment may result from several
origins: (i) the lack of the compensation of magnetic moments
at the ends of magnetically broken chains due the presence of
nonmagnetic impurities and lattice defects;22 (ii) the pinning of
domain walls separating AF and F phases on nonmagnetic
impurities and lattice defects after zeroing the field; (iii) the
presence of canted antiferromagnetic structure with a weak
ferromagnetic component. If one bears in mind the high
anisotropy of Co, the presence of a small canting of the moment
between two sublattices, where one sublattice comprises one
Co atom and a pair of radicals, seems to be reasonable. The
estimated canting is 5.7° from the easy axis, if we assume that
both the cobalt and the radical moments are canted by the same
angle. The double of this angle can be a measure of the angle
between the directions of the moments of the two crystal-
lographic independent cobalt atoms. The observed angle between
O-Co-O (where the oxygen atoms are those of the radicals)
is 11.7°. This agreement, though coincidental, may suggest that
the moments of the cobalt atoms is quite likely perpendicular
to the Co(hfac)2 plane. The second magnetization value of 1.16
µB is that observed upon application of field larger than the
critical metamagnetic field. This is larger than that of the ferri-
Acknowledgment. We thank Dr. J. Nishijyo, Institute for the
Molecular Science, for his great help with the SQUID measure-
ments and Mr. M. Fujiwara, Institute for the Molecular Science,
for his great help with ESR measurements. In crystal structural
(22) Bacerra, C. C.; Paduan-Filho, A.; Palacio, F. J. Phys.: Condens. Matter
2000, 12, 6207.
(23) (a) Lhotel, E.; Khatsko, E. N.; Paulsen, C. Phys. ReV. B. 2006, 74, 020402.
(b) Yuan, S.; Raedt, H. D.; Miyashita, S. J. Phys. Soc. Jpn. 2006, 75,
084073.
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Ferrimagnetic State in a Molecule-Based Magnet
A R T I C L E S
analysis, we were indebted to Yadokari-XG software. This work
is supported by a Grant-in-Aid for Science Research (A) (No.
18205023) from the Ministry of Education, Science, Sports, and
Culture. A part of this work was supported by a “Nanotech-
nology Support Project” of the Ministry of Education, Culture,
Sports, Science, and Technology (MEXT) of Japan.
structures. This material is available free of charge via Internet
at http://pubs.acs.org. Crystallographic data for 1 (CCDC-
603148) can be also be found free of charge via www.
ccdc.cam.ac.uk/conts/retrieving.html (or from Cambridge Crys-
tallographic Centre, 12 Union road, Cambridge CB2 1EZ, U.K.;
fax (+44) 1223-336-033).
Supporting Information Available: X-ray crystallographic
information files (CIF) for 1, magnetic properties, and crystal
JA064828I
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J. AM. CHEM. SOC. VOL. 129, NO. 32, 2007 9909