Crystal design of monometallic single-molecule magnets consisting of cobalt-aminoxyl heterospins

Article abstract of DOI:10.1021/ja0767579

Five N-aryl-N-pyridylaminoxyls, which have no substituent (PhNOpy), one substituent (MeOPh-NOpy and tert-BuPhNOpy) at the 4-position, and three substituents (TPPNOpy and TBPNOpy) at the 2, 4, and 6-positions of the phenyl ring, were prepared as new ligands for cobalt-aminoxyl heterospin systems. The 1:4 complexes, [Co(NCS)₂(PhNOpy)₄] (1), [Co(NCS) ₂(MeOPhNOpy)₄] (2), [Co(NCS)₂(tertBuPhNOpy) ₄] (3), [Co(NCS)₂(TPPNOpy)₄] (4), [Co(NCS) ₂(TBPNOpy)₄] (5a), and [Co(NCO)₂(TBPNOpy) ₄] (5b), were obtained as single crystals. The molecular geometry revealed by X-ray crystallography for all complexes except 4 is a compressed octahedron. In the crystal structure of 1, 2, and 3, the organic spin centers have various short contacts within 4 A with the neighboring molecules to form 3D and 2D spin networks. On the other hand, complexes 5a and 5b have no significant short intermolecular contacts, indicating that they are magnetically isolated. 1 and 2 behaved as a 3D antiferromagnet with a Neel temperature, T_N, of 22 K and as a weak 3D antiferromagnet with a T_N of 2.9 K and a spin-flop field at 1.9 K, H_sp(1.9), of 0.7 kOe, respectively. 3 was a canted 2D antiferromagnet (a weak ferromagnet) with T _N = 4.8 K and showed a hysteresis loop with a coercive force, H _c, of 1.3 kOe at 1.9 K. On the other hand, the trisubstituted complexes 4, 5a, and 5b functioned as single-molecule magnets (SMMs). 5b had an effective activation barrier, U_eff, value of 28 K in a microcrystalline state and 48 K in a frozen solution.

Full text of DOI:10.1021/ja0767579

Published on Web 02/15/2008
Crystal Design of Monometallic Single-Molecule Magnets
Consisting of Cobalt-Aminoxyl Heterospins
Shinji Kanegawa,† Satoru Karasawa,† Masataka Maeyama,‡ Motohiro Nakano,§ and
Noboru Koga*,†
Graduate School of Pharmaceutical Sciences, Kyushu UniVersity, 3-1-1 Maidashi, Higashi-ku,
Fukuoka, 812-8582 Japan, Rigaku Corporation, 3-9-12 Matsubara, Akishima, Tokyo, 196-8666
Japan, and Department of Applied Chemistry, Graduate School of Engineering, Osaka
UniVersity, 2-1 Yamadaoka, Suita, Osaka, 565-0871 Japan
Received September 18, 2007; E-mail: koga@fc.phar.phyushu-u.ac.jp
Abstract: Five N-aryl-N-pyridylaminoxyls, which have no substituent (PhNOpy), one substituent (MeOPh-
NOpy and tert-BuPhNOpy) at the 4-position, and three substituents (TPPNOpy and TBPNOpy) at the 2,
4, and 6-positions of the phenyl ring, were prepared as new ligands for cobalt-aminoxyl heterospin systems.
The 1:4 complexes, [Co(NCS)2(PhNOpy)4] (1), [Co(NCS)2(MeOPhNOpy)4] (2), [Co(NCS)2(tertBuPhNOpy)4]
(3), [Co(NCS)2(TPPNOpy)4] (4), [Co(NCS)2(TBPNOpy)4] (5a), and [Co(NCO)2(TBPNOpy)4] (5b), were
obtained as single crystals. The molecular geometry revealed by X-ray crystallography for all complexes
except 4 is a compressed octahedron. In the crystal structure of 1, 2, and 3, the organic spin centers have
various short contacts within 4 Å with the neighboring molecules to form 3D and 2D spin networks. On the
other hand, complexes 5a and 5b have no significant short intermolecular contacts, indicating that they
are magnetically isolated. 1 and 2 behaved as a 3D antiferromagnet with a Neel temperature, TN, of 22 K
and as a weak 3D antiferromagnet with a TN of 2.9 K and a spin-flop field at 1.9 K, Hsp(1.9), of 0.7 kOe,
respectively. 3 was a canted 2D antiferromagnet (a weak ferromagnet) with TN ) 4.8 K and showed a
hysteresis loop with a coercive force, Hc, of 1.3 kOe at 1.9 K. On the other hand, the trisubstituted complexes
4, 5a, and 5b functioned as single-molecule magnets (SMMs). 5b had an effective activation barrier, Ueff,
value of 28 K in a microcrystalline state and 48 K in a frozen solution.
Single-molecule magnets (SMM)s1-3 exhibiting slow mag-
netic relaxation are of interest as new functional materials,
because they are a few nanometers in size and have unique
quantum tunneling effects. Therefore, many metal complexes
containing anisotropic metal ions4 have been reported as SMMs
over the past decade. The slow magnetic relaxation for the
characteristic SMM behavior is derived from a thermal activa-
tion barrier, U, between the up spin and the down spin
corresponding to |D|S2; D (negative) is a zero-field splitting
parameter and S is a spin quantum number. Most of the
complexes reported to date except the Mn12 family5 and Mn6
complex6a have an effective activation barrier, Ueff, for the
reversal of magnetism of less than 30 K (the Mn12 and Mn6
complexes are Ueff ) 60 and 86 K, respectively). Therefore,
the construction of SMMs with large Ueff values become one
of the challenging targets in the field of the molecule-based
magnet.6 For the construction of SMMs, we proposed the use
of a heterospin system7 consisting of the 3d spins of the metal
† Kyushu University.
‡ Rigaku Corporation.
§ Osaka University.
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J. AM. CHEM. SOC. 2008, 130, 3079-3094
3079

A R T I C L E S
Kanegawa et al.
Table 1. Crystallographic Data Collection and Structural Refinement Information for 1, 2, and 3
1
2
3
empirical formula
formula weight
crystal class
space group
a /Å
C46H36N10O4S2Co
915.91
C54H44N10O9S2Co
1100.06
C62H68N10O4S2Co
1140.34
monoclinic
P21/c (no. 14)
8.9163(3)
20.8603(7)
11.1551(5)
90
91.3879(15)
90
2074.20(14)
5.740
monoclinic
P21/n (no. 14)
11.2616(5)
19.5979(7)
23.6461(9)
90
90.5730(12)
90
5218.5(4)
4.761
monoclinic
P21/c (no. 14)
17.0307(11)
21.0325(13)
10.8981(8)
90
129.7325(18)
90
3002.1(3)
4.101
b /Å
c /Å
R /deg
â /deg
γ /deg
V /Å3
µ /cm-1
Z
2
4
2
crystal size /mm
D
0.6 × 0.4 × 0.1
0.5 × 0.5 × 0.2
0.6 × 0.2 × 0.1
calc /gcm-3
1.466
1.400
1.261
F(000)
946.00
2276.00
1202.00
radiation
T /K
Mo-KR
123
Mo-KR
123
Mo-KR
123
no. reflections measured
no. unique reflections
no. reflections observed
no. parameters
2930
632
3610(I > 2.00σ(I))
304
0.0394(I > 2.00σ(I))
0.1091 (I > 2.00σ(I))
1.000
7370
1601
6150(I > 2.00σ(I))
704
0.0631 (I > 2.00σ(I))
0.1603(I > 2.00σ(I))
1.005
24397
5586
5586(all)
360
0.0580 (I>2.00σ(I))
0.1720 (all reflections)
1.077
a
R1
wR2
a
GOF
2
2
2
a R1 ) Σ ||F0| - |Fc||/ Σ |F0|; wR2 ) {Σ w(F0 - Fc )2/ Σ w(F0 )2}1/2
.
Table 2. Crystallographic Data Collection and Structural Refinement Information for 5a, 5b, TPPNOpy, and TBPNOpy
5a
5b
TPPNOpy
TBPNOpy
empirical formula
formula weight
crystal class
space group
a /Å
C
102H152N10O6S2Co
C102H152N10O8Co
1896.80
triclinic
C29H21N2O
413.50
C23H33N2O
353.53
1737.44
triclinic
monoclinic
P21/c (no. 14)
18.2162(6)
10.8471(5)
22.0773(8)
90
98.4008(10)
90
4315.3(3)
0.775
monoclinic
P21/c (no. 14)
9.9900(4)
19.4317(8)
11.8283(5)
90
116.3259(15)
90
2058.00(15)
0.693
P1h (no. 2)
10.1796(8)
12.5417(9)
20.2773(15)
74.475(2)
83.843(2)
83.886(2)
2483.1(3)
2.712
P1h (no. 2)
10.3678(5)
12.4883(7)
20.1384(8)
75.1828(14)
83.0490(11)
84.7460(16)
2497.2(2)
2.296
b /Å
c /Å
R /deg
â /deg
γ /deg
V /Å3
µ /cm-1
Z
1
1
8
4
crystal size /mm
D
0.6 × 0.2 × 0.1
0.7 × 0.6 × 0.6
0.5 × 0.3 × 0.3
0.7 × 0.6 × 0.4
calc /gcm-3
1.162
1.134
1.273
1.141
F(000)
941.00
925.00
1736.00
772.00
radiation
T /K
Mo-KR
123
Mo-KR
123
Mo-KR
123
Mo-KR
123
no. reflections measured
no. unique reflections
no. reflections observed
no. parameters
3644
1535
4080(I > 3.00σ(I))
623
0.0863(I > 3.00σ(I))
0.2415(I > 3.00σ(I))
0.955
33675
9147
9147(I > 3.00σ(I))
549
0.0627(I > 2.00σ(I))
0.2011 (all refrections)
1.140
9705
1321
3803(I > 2.00σ(I))
619
0.0525(I > 2.00σ(I))
0.0971(I > 2.00σ(I))
1.004
2908
631
2728(I > 3.00σ(I))
268
0.0924(I > 3.00σ(I))
0.2685(I > 3.00σ(I))
1.001
a
R1
wR2
a
GOF
2
2
2
a R1 ) Σ ||F0| - |Fc||/ Σ |F0|; wR2 ) {Σ w(F0 - Fc )2/ Σ w(F0 )2}1/2
.
ion and the 2p spins of the unpaired electrons of the organic
radicals coupled magnetically through a linker. According to
our strategy, the 1:4 complexes of Co(NCS)2 with (4-pyridyl)-
phenylcarbene, C1py, and Co(NCO)2 with N-tert-butyl-N-4-
pyridylaminoxyl, 4NOpy, were prepared and showed SMM
behavior with Ueff ) 89 and 50 K, respectively, in frozen
solution.8,9 In a series of 1:4 complexes, [Co(X)2(4NOpy)4] (X
) Br-, NCS-, and NCO-), furthermore, SMM behavior was
found to be controlled by the axial ligand X; the Ueff values
increased in the order of Br-, NCS-, and NCO-.10 However,
SMM magnetic behavior was observed in frozen solution but
not in a crystalline state. The absence of a SMM magnetic
behavior in the crystalline state is mainly due to the strong
intermolecular interaction. In the 1:4 complexes of [Co(X)2-
(7) (a) Karasawa, S.; Kumada, H.; Koga, N.; Iwamura, H. J. Am. Chem. Soc.
2001, 123, 9685-9686. (b) Karasawa, S.; Koga, N. Polyhedron 2003, 22,
1877-1882. (c) Morikawa, H.; Karasawa, S.; Koga, N. Appl. Magn. Reson.
2003, 23, 507-515. (d) Karasawa, S.; Koga, N. Polyhedron 2001, 20,
1387-1389. (e) Morikawa, H.; Imamura, F.; Tsurukami, Y.; Itoh, T.;
Kumada, H.; Karasawa, S.; Koga, N.; Iwamura, H. J. Mater. Chem. 2001,
11, 493-502.
(8) (a) Karasawa, S.; Zhou, G.; Morikawa, H.; Koga, N. J. Am. Chem. Soc.
2003, 125, 13676-13677. (b) Tobinaga, H.; Suehiro, M.; Ito, T.; Zhou,
G.; Karasawa, S.; Koga, N. Polyhedron, 2007, 26, 1905-1911.
(9) Kanegawa, S.; Karasawa, S.; Nakano, M.; Koga, N. Chem. Commun. 2004,
1750-1751.
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2006, 79, 1372-1382.
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3080 J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008

Cobalt-Aminoxyl Heterospins
A R T I C L E S
(4NOpy)4], the spin centers of aminoxyl are located at the
periphery of the complex and easily come close to those in the
neighboring complex molecules. Actually, short contacts (<4
Å) between the aminoxyl centers and between the aminoxyl
center and the pyridine ligands belonging to the neighboring
molecules are often observed in the crystal structure of [Co-
(X)2(4NOpy)4] and many other heterospin complexes. For the
observation of SMM behavior, therefore, we are compelled to
use frozen solutions to avoid intermolecular interactions.
Although intermolecular magnetic interaction in the crystal-
line state is undesirable for SMMs, the interaction can be
regarded as the formation of a new spin network. This means
that a discrete cobalt(II) complex in the crystalline state is
expected to form a low-dimensional spin network and show
unique magnetic behavior affected by the magnetic anisotropy
of the cobalt(II) ion.11 Indeed, the crystalline samples of [Co-
(X)2(4NOpy)4] (X ) NCS- and NCO-,10) and [CoCl2(4NOPy-
OMe)2],12 showed anisotropic magnetic behavior. The former
were anitiferromagnet with TN ) 15 and 4.5 K, respectively,
and the latter showed anisotropic 2D magnetic behavior with a
magnetic phase transition of Tc ) 2.14 K and a glass-like
magnetic behavior below Tc.
in the crystalline state and confirmed that they exhibit similar
SMM behavior in frozen solution.
Experimental Section
General Methods. Infrared spectra were recorded on a JASCO 420
FT-IR spectrometer. Vis-Nir spectra were recorded on a JASCO V570
spectrometer. 1H NMR spectra were measured on a JEOL 270 Fourier
transform spectrometer and a Varian UNITY INOVA (400 MHz) using
CDCl3 (or DMSO-d6) as solvent and referenced to TMS. EPR spectra
were recorded on a Bruker Biospin EMX EPR X-band (9.4 GHz)
spectrometer. FAB mass spectra (FAB MS) were recorded on a JEOL
JMS-SX102 spectrometer. Melting points were obtained with a MEL-
TEMP heating block and are uncorrected. Elemental analyses were
performed in the Analytical Center of the Faculty of Science in Kyushu
University.
X-Ray Molecular and Crystal Structure Analyses. Crystal-
lographic data and experimental details are summarized in Table 1 for
1, 2, and 3 and in Table 2 for 5a and 5b together with free aminoxyls,
TPPNOpy and TBPNOpy. Suitable single crystals were glued onto a
glass fiber using epoxy resin. All X-ray data were collected on a Rigaku
Raxis-Rapid diffractometer with graphite monochromated Mo-KR
radiation (λ ) 0.71069 Å). Reflections were collected at 123 ( 1 K.
The molecular structures were solved by direct methods (SIR program)
and expanded using Fourier techniques (DIRDIF94). The refinements
were converged using the full-matrix least-squares method from the
Crystal Structure software package13 to give the P21/c (no. 14) for 1,
3, TPPNOpy, and TBPNOpy, P21/n (no. 14) for 2, and P1h (no. 2) for
5a and 5b. All non-hydrogen atoms were refined anisotropically,
hydrogen atoms were included at standard positions (C-H ) 0.96 Å,
C-C-H ) 120 °) and refined isotropically using a rigid model.
Crystallographic data for the structure reported in this paper have been
deposited with the Cambridge Crystallographic Data Center as supple-
mentary publications no. CCDC-626043, -626044, -652128, -626045,
-652127, -626041, and -626042 for 1, 2, 3, 5a, 5b, TPPNOpy, and
TBPNOpy, respectively.
SQUID Measurements. Direct current (dc) and alternating current
(ac) magnetic susceptibility data were obtained on Quantum Design
MPMS5S and MPMS2 SQUID magneto/susceptometer, respectively,
and corrected for the magnetization of the sample holder and capsule
and for diamagnetic contributions to the samples, which were estimated
from Pascal’s constants.14 A pulverized sample was prepared by
grinding the complex crystal to powder and restrained by eicosane in
a sample capsule to avoid partial reorientation of the crystal during
the measurement. For the magnetic measurement of a direction-arranged
sample of 5b, a single crystal (1.0 × 0.8 × 0.4 mm3) was selected and
used. The orientation of the crystal’s axis was determined by X-ray
analysis, and then the crystal was arranged on a sample holder. Solution
samples (5 or 10 mM) were prepared as follows: the crystal of the
complex was dissolved in toluene, and 150 µL of solution sample was
placed in a sample capsule. Dc magnetic susceptibility was measured
by a sequence of ZFCM (zero field cooled magnetization), FCM (field
cooled magnetization), and RM (remnant magnetization) at a dc field
To understand the correlation between the magnetism and
the crystal structure in the heterospin cobalt complex, this time
we considered controlling the crystal lattice of the 1:4 complex
by changing systematically the size of the substituents on the
aminoxyl-pyridine ligands and aimed at the observation of the
monometallic SMM in the crystalline state. Until now, 4NOpy
has been used as a metal ligand carrying an organic spin source
in our heterospin SMM. However, it is inadequate for this study.
Based on previous studies of the heterospin complexes, an
N-phenyl-N-pyridylaminoxyl, PhNOpy, in which the tert-butyl
group of 4NOpy was replaced with a phenyl ring, was selected
as a framework of the new ligand for introducing different-
sized substituent(s). In this study, N-aryl-N-pyridylaminoxyls
(MeOPhNOpy, tert-BuPhNOpy, TPPNOpy, and TBPNOpy)
having one or three substituents (-OMe, -tert-Bu, and -Ph)
on the phenyl ring were prepared in addition to PhNOpy and
the 1:4 complexes of Co(X)2 (X ) NCS- and NCO-) with
N-aryl-N-pyridylaminoxyls, [Co(NCS)2(PhNOpy)4] (1), [Co-
(NCS)2(MeOPhNOpy)4] (2), [Co(NCS)2(tert-BuPhNOpy)4]
(3), [Co(NCS)2(TPPNOpy)4] (4), [Co(NCS)2(TBPNOpy)4]
(5a), and [Co(NCO)2(TBPNOpy)4] (5b), were obtained as
crystals. The crystal structures of the 1:4 cobalt complexes were
characterized by X-ray crystallography and their magnetic
properties in the crystalline state were investigated. Finally, we
found the 1:4 complexes, 4, 5a, and 5b, showing SMM behavior
(13) SIR 92: Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Cryst. 1994, 27, 435.
(b) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de
Gelder, R.; Israel, R.; Smits, J. M. M. DIRDIF99; The DIRDIF-99 program
system, Technical Report of the Crystallography Laboratory; University
of Nijmegen: The Netherlands, 1994. (c) Crystal Structure 3.5.1; Crystal
Structure Analysis Package; Rigaku and Rigaku/MSC: The Woodlands,
TX, 2000-2003.
(11) (a) Murrie, M.; Teat, S. J.; Stoeckli-Evans, H.; Gu¨del, H.-U. Angew. Chem.,
Int. Ed. 2003, 42, 4653-4656. (b) Galan-Mascaros, J. R.; Dunbar, K. R.
Angew. Chem., Int. Ed. 2003, 42, 2289-2293. (c) Yang, E. C.; Hendrickson,
D. N.; Wernsdorfer, W.; Nakano, M.; Zakharov, L. N.; Sommer, R. D.;
Rheingold, A. L.; Ledezma-Gaeraud, M.; Christou, G. J. Appl. Phys. 2002,
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A R T I C L E S
Kanegawa et al.
of 4.7 Oe. Ac magnetic susceptibility measurements were carried out
with a 3.9 Oe oscillating field and a zero dc field at a frequency of
1-997 Hz.
4-{N-(2,4,6-Triphenylphenyl)-N-hydroxylamino}pyridine, TPP-
NOHpy. This was prepared in a manner similar to PhNOHpy using
2,4,6-triphenylnitrosobenzene in place of nitrosobenzene. The dark-
colored oil was chromatographed on silica gel with chloroform/methanol
(8/1) as eluent to give the hydroxylamine (0.81 g, 6.5% yield) as a
Materials. Diethyl ether and toluene were distilled from sodium
benzophenone ketyl. Dichloromethane was distilled under high-purity
N2 after drying with calcium hydride. The nitroso derivatives,
nitrosobenzene,15a 4-methoxynitrosobenzene, 4-tert-butylnitroso-
benzene, 2,4,6-triphenylnitrosobenzene,15b and 2,4,6-tri-tert-butylnitro-
sobenzene15c were prepared by procedures reported in the literature.
Co(NO3)2‚6H2O, KNCO, and Co(NCS)2 were purchased and used
without purification.
1
white solid: mp (dec) 243-244 °C, H NMR (400 MHz, DMSO-d6)
δ 9.80 (br, 1H), 7.88 (d, J ) 5.80 Hz, 2H), 7.83 (d, J ) 8.61 Hz, 2H),
7.68 (s, 2H), 7.51-7.41 (m, 7H), 7.32-7.26 (m, 6H) 6.29 (br, 2H),
FAB MS (in m-NBA matrix), 415.3 (M+1); Anal. Calcd for
C29H22N2O: C 84.03, H 5.35, N 6.76%; found: C 83.25, H 5.46, N
6.69%.
4-(N-Phenyl-N-oxylamino)pyridine, PhNOpy. Ag2O (187 mg, 0.81
mmol) was added to a solution of PhNOHpy (100 mg, 0.54 mmol) in
diethyl ether (20 mL) at room temperature. The suspension was stirred
for 20 min and filtered. n-Hexane (30 mL) was added to the filtrate.
The solution was slowly evaporated, and a resulting precipitate was
collected by filtration. PhNOpy (71 mg, 71.4% yield) was obtained as
a red powder: mp (dec) 68 °C; ESR (CH2Cl2) aN ) 9.3 G. FAB MS
(in m-NBA matrix), 186.2 (M+1), Anal. Calcd for C11H9N2O: C 71.34,
H 4.90, N 15.13%; found: C 71.35, H 4.95, N 15.02%.
4-{N-Phenyl-N-hydroxylamino}pyridine, PhNOHpy. A 1.58 M
solution of n-butyllithium (7.6 mL) in n-hexane was added at -78 °C
to a solution of 4-bromopyridine (1.57 g, 10.0 mmol) in 100 mL of
anhydrous diethyl ether. After 30 min of stirring, a solution of
nitrosobenzene (1.51 g, 14.0 mmol) in dry diethyl ether (50 mL) was
added dropwise. The reaction mixture was stirred for 15 min at -78 °C
and allowed to warm slowly to room temperature overnight. The
reaction was quenched with saturated aqueous ammonium chloride.
The resulting light-brown precipitate was collected by filtration and
washed with diethyl ether/n-hexane to give the hydroxylamine PhNO-
Hpy (0.85 g, 48.8% yield) as a white solid: mp 158-160 °C, 1H NMR
(270 MHz, DMSO-d6) δ 8.24 (d, J ) 6.01 Hz, 2H), 7.45 (t, J ) 7.73
Hz, 2H), 7.36 (d, J ) 6.71 Hz, 2H), 7.26 (t, J ) 7.38 Hz, 1H), 6.87 (d,
J ) 6.71 Hz, 2H), FAB MS (in m-NBA matrix), 187.2 (M+1), Anal.
Calcd for C11H10N2O: C 70.95, H 5.41, N 15.04%; found: C 70.88, H
5.41, N 15.03%.
4-{N-(4-Methoxyphenyl)-N-oxylamino}pyridine, MeOPhNOpy.
This was prepared in a manner similar to PhNOpy using MeOPh-
NOHpy (100 mg, 0.46 mmol). MeOPhNOpy (63 mg, 63.2% yield)
was obtained as a red powder: mp (dec) 112 °C; ESR (CH2Cl2) aN
)
9.4 G. FAB MS (in m-NBA matrix), 216.2 (M+1), Anal. Calcd for
C12H11N2O2: C 66.97, H 5.15, N 13.02%; found: C 66.95, H 5.19, N
13.01%.
4-{N-(4-tert-Butylphenyl)-N-oxylamino}pyridine, tert-BuPhNOpy.
This was prepared in a manner similar to PhNOpy using tert-
BuPhNOHpy (100 mg, 0.41 mmol). tert-BuPhNOpy (74 mg, 74.3%
yield) was obtained as a red powder: mp (dec) 80 °C; ESR (CH2Cl2)
aN ) 9.4 G. FAB MS (in m-NBA matrix), 242.2 (M+1), Anal. Calcd
for C15H17N2O: C 74.66, H 7.10, N 11.61%; found: C 74.54, H 7.25,
N 11.59%.
4-{N-(2,4,6-tert-Butylphenyl)-N-oxylamino}pyridine, TBPNOpy.
This was prepared in a manner similar to PhNOpy using TBPNOHpy.
TBPNOpy (76 mg, 76.2% yield) was obtained as a yellow crystal:
mp (dec) 144 °C; ESR (CH2Cl2) aN ) 9.2 G. FAB MS (in m-NBA
matrix), 354.4 (M+1), Anal. Calcd for C23H33N2O: C 78.14, H 9.41,
N 7.92%; found: C 78.03, H 9.49, N 7.83%.
4-{N-(2,4,6-Triphenylphenyl)-N-oxylamino}pyridine, TPPNOpy.
This was prepared in a manner similar to PhNOpy using TPPNOHpy.
TPPNOpy (84 mg, 84.2% yield) was obtained as a red crystal: mp
(dec) 202 °C; ESR (CH2Cl2) aN ) 9.2 G; FAB MS (in m-NBA matrix),
414.3 (M+1); Anal. Calcd for C29H21N2O‚3/7CH2Cl2: C 78.57, H 4.90,
N 6.23%; found: C 78.52, H 4.91, N 6.29%.
4-{N-(4-Methoxyphenyl)-N-hydroxylamino}pyridine, MeOPh-
NOHpy. This was prepared in a manner similar to PhNOHpy using
4-nitrosoanisole in place of nitrosobenzene. The hydroxylamine
MeOPhNOHpy (1.25 g, 57.8% yield) was obtained as a white solid:
mp (dec) 143-145 °C, 1H NMR (270 MHz, DMSO-d6) δ 8.19 (d, J )
6.05 Hz, 2H), 7.27 (d, J ) 8.73 Hz, 2H), 7.01 (d, J ) 8.73 Hz, 2H),
6.73 (d, J ) 6.04 Hz, 2H), 3.79 (s, 3H), FAB MAS (in m-NBA matrix)
217.4 (M+1), Anal. Calcd. for C12H12N2O2: C 66.65, H 5.59, N
12.96%; found: C 66.93, H 5.61, N 12.99%.
4-{N-(4-tert-Butylphenyl)-N-hydroxylamino}pyridine, tert-BuPh-
NOHpy. This was prepared in a manner similar to PhNOHpy using
4-tert-butylnitrosobenzene in place of nitrosobenzene. The hydroxy-
lamine (0.98 g, 40.6% yield) was obtained as a white solid: mp (dec)
170-172 °C, 1H NMR (270 MHz, DMSO-d6) δ 8.22 (d, J ) 6.04 Hz,
2H), 7.47 (d, J ) 8.06 Hz, 2H), 7.27 (d, J ) 8.72 Hz, 2H), 6.83 (d, J
) 6.04 Hz, 2H), 1.30 (s, 9H), FAB MS (in m-NBA matrix), 243.3
(M+1), Anal. Calcd for C15H18N2O: C 74.35, H 7.49, N 11.56%;
found: C 74.14, H 7.50, N 11.57%.
4-{N-(2,4,6-Tri-tert-butylphenyl)-N-hydroxylamino}pyridine, TB-
PNOHpy. A mixture of a 1.58 M solution of n-butyllithium (38 mL)
in n-hexane and TMEDA (6.96 g, 60 mmol) was stirred for 5 min in
a nitrogen atmosphere. The solution was cooled to -78 °C and diethyl
ether (300 mL) was added. A solution of 4-bromopyridine (7.9 g, 50
mmol) in diethyl ether (200 mL) was added dropwise. Next a solution
of 2, 4, 6-tri-tert-butylnitrosobenzene (13.8 g, 50 mmol) in diethyl ether
(100 mL) was added immediately. The reaction mixture was stirred
for 1 h at -78 °C and allowed to warm slowly to room temperature
overnight. The reaction was quenched with saturated aqueous am-
monium chloride. The resulting precipitate was collected by filtration
and washed with diethyl ether/n-hexane to give hydroxylamine (1.12
g, 6.3% yield) as a white solid: mp (dec) 233-235 °C, 1H NMR (270
MHz, DMSO-d6) δ 9.39 (br, 1H), 8.16 (br, 1H), 7.89 (br, 1H), 7.52 (s,
2H), 6.76 (br, 1H), 5.50 (br, 1H), 1.32 (s, 9H), 1.27 (s, 18H); FAB MS
(in m-NBA matrix), 355.4 (M+1); Anal. Calcd for C23H34N2O: C 77.92,
H 9.67 N 7.90%; found: C 78.01, H 9.62, N 7.98%.,
Tetrakis{4-(N-Phenyl-N-oxylamino)pyridine}di(thiocyanato-N)-
cobalt(II), [Co(NCS)2(PhNOpy)4] (1). A solution of Co(NCS)2 (35.5
mg, 0.20 mmol) in EtOH (0.5 mL) and a solution of PhNOpy (150
mg, 0.81 mmol) in dichloromethane (30 mL) were mixed. n-Hexane
(15 mL) was added to the mixture, and then the volume of the solution
was reduced to ca. 30 mL on a rotary evaporator. The resulting
precipitates were removed by filtration, and the solution was kept at
-30 °C. The complex 1 was obtained as dark-red platelet crystals (123
mg): mp (dec) 124-126 °C,IR (KBr pellet) ν ) 2074 cm-1 (NCS);
Anal. Calcd for C46H36N10O4S2Co‚1/6CH2Cl2: C 59.79, H 3.94, N
15.12%; found: C 59.80, H 4.15, N 14.90%.
Tetrakis[4-{N-(4-Methoxyphenyl)-N-oxylamino}pyridine]di-
(thiocyanato-N)cobalt(II), [Co(NCS)2(MeOPhNOpy)4] (2). This was
prepared in a manner similar to the procedure for 1 by using
MeOPhNOpy. The complex 2 was obtained as dark-red brick-like
crystals (142 mg): mp (dec) 182-184 °C,IR (KBr pellet) ν ) 2059
cm-1 (NCS); Anal. Calcd for C50H44N10O8S2Co‚3/4Et2O: C 58.32, H
4.76, N 12.83%; found: C 58.57, H 4.76, N 12.99%.
(15) (a) Porta, F.; Prati, L. J. Mol. Catal. 2000, 157, 123-129. (b) Kanaya, S.;
Kozaki, M.; Shiomi, D.; Sato, K.; Takui, T.; Okada, K. Synth. Met. 2001,
121(1-3), 1808-1809. (c) Hedayatullah, M.; Thevenet, F.; Denivelle, L.
Tetrahedron Lett. 1977, 18, 1595-1596.
Tetrakis[4-{N-(4-tert-Butylphenyl)-N-oxylamino}pyridine]di-
(thiocyanato-N)cobalt(II), [Co(NCS)2(tert-BuPhNOpy)4] (3). This
9
3082 J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008

Cobalt-Aminoxyl Heterospins
A R T I C L E S
Scheme 1. Preparations of N-Aryl-N-pyridylaminoxyls and Their Co Complexes
was prepared in a manner similar to 1 by using tert-BuPhNOpy.
Complex 3 was obtained as dark-red chip crystals (114 mg): mp (dec)
157-159 °C, IR (KBr pellet) ν ) 2067 cm-1 (NCS); Anal. Calcd for
C62H68N10O4S2Co: C 65.30, H 6.01, N 12.28%; found: C 65.14, H
6.11, N 12.16%.
Tetrakis[4-{N-(2,4,6-Triphenylphenyl)-N-oxylamino}pyridine]di-
(thiocyanato-N)cobalt(II), [Co(NCS)2(TPPNOpy)4] (4). This was
prepared in a manner similar to 1 by using TPPNOpy. Immediately
after mixing, complex 4 was obtained as a dark-red powder (162 mg):
mp (dec) 198-200 °C, IR (KBr pellet) ν ) 2058 cm-1 (NCS); Anal.
Calcd for C118H84N10O4S2Co‚5/2Et2O: C 75.98 H 5.13, N 7.20%;
found: C 75.89, H 5.05, N 7.11%.
Tetrakis[4-{N-(2,4,6-Tri-tert-butylphenyl)-N-oxylamino}pyridine]-
di(thiocyanato-N)cobalt(II), [Co(NCS)2(TBPNOpy)4] (5a). This was
prepared in a manner similar to 1 by using TBPNOpy. Complex 5a
was obtained as dark-red brick-like crystals (42 mg): mp (dec) 156-
158 °C, IR (KBr pellet) ν ) 2051 cm-1 (NCS); Anal. Calcd for
C94H132N10O4S2Co: C 71.04, H 8.37, N 8.81%; found: C 71.19, H
8.52, N 8.54%.
Tetrakis[4-{N-(2,4,6-tert-Butylphenyl)-N-oxylamino}pyridine]di-
(isocyanato-N)cobalt(II), [Co(NCO)2(TBPNOpy)4] (5b). A solution
of TBPNOpy (200 mg, 0.56 mmol) in diethyl ether (20 mL) and a
solution of Co(NO3)2‚6H2O (31 mg, 0.10 mmol) and KNCO (19 mg,
0.23 mmol) in methyl alcohol (3 mL) were mixed and then stirred for
10 min. The solvents were removed to dryness under reduced pressure.
The dark-green solid obtained was dissolved in a small amount of
dichloromethane (ca. 1 mL). Diethyl ether (10 mL) and n-hexane (15
mL) were added to the mixture and the volume of the solution was
reduced to 15 mL on a rotary evaporator. The resulting precipitates
were removed by filtration, and the filtrate was kept at -30 °C. The
complex 5b was obtained as dark-red brick-like crystals (115 mg): mp
(dec) 88-90 °C; IR (KBr pellet) ν ) 2198 cm-1 (NCO); Anal. Calcd
for C94H132N10O6Co‚2/3CH2Cl2: C 70.46, H 8.33, N 8.68%; found: C
70.42, H 8.50, N 8.69%.
uents were expected to play two important roles as follows. (1)
The substituents prevent the access of neighboring molecules
to the aminoxyl center. (2) They prevent the electron spin of
aminoxyl from delocalizing onto the phenyl ring by sterically
twisting the phenyl ring from the aminoxyl plane.
The intermolecular magnetic interactions in a crystalline state
take place not only between the radical spin centers but also
between the atoms bearing the spin density. To understand the
magnetic coupling in a crystal lattice, therefore, it is necessary
to know the spin distribution of the aminoxyl ligand. For the
estimation of the spin density distributions in N-aryl-N-
pyridylaminoxyls, PhNOpy, MeOPhNOpy, tert-BuPhNOpy,
TPPNOpy, and TBPNOpy, a molecular orbital calculation
based on the density function theory (DFT) was performed. The
popular unrestricted B3LYP hybrid density functional16,17 with
the standard split valence 6-31G(d) basis set was used for the
computation using the GAUSSIAN 98 program package. The
molecular geometries of PhNOpy, MeOPhNOpy, and tert-
BuPhNOpy were determined by MM2 calculation, and those
of TPPNOpy and TBPNOpy were used from the data of X-ray
analyses described in the following section. The spin density
populations revealed by the calculation for five N-aryl-N-
pyridylaminoxyls are shown in Figure S1 (Supporting Informa-
tion). In monosubstituted aminoxyls, PhNOpy, MeOPhNOpy,
and tert-BuPhNOpy, the spin density distributed onto both the
benzene and pyridine rings, whereas in trisubstituted aminoxyls,
TPPNOpy and TBPNOpy, it distributed mainly onto the
pyridine ring. The insignificant spin distribution onto the phenyl
ring is due to the large dihedral angle between the phenyl ring
Results and Discussion
Molecular Design and Preparations. In this study, we
attempted to modulate the intermolecular distance in the crystal
lattice by varying the bulkiness and number of diamagnetic
substituents. To this end, a N-aryl-N-pyridilaminoxyl was used
as a frame for the ligand in the heterospin system. Taking the
bulkiness of the substituent into account, a methoxy group and
a tert-butyl group were selected and introduced at the 4-position
of the phenyl ring. Furthermore, bulky substituents, a tert-butyl
group and a phenyl ring, were introduced at the 2, 4, and
6-positions of the phenyl ring, in which the substituents at the
2 and 6-positions lie near the aminoxyl center. Those substit-
Figure 1. ORTEP drawing of the molecular structure of 1. Hydrogen atoms
are omitted for the sake of clarity.
(16) Becke, A. D. Phys. ReV. 1988, A38, 3098.
(17) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785.
9
J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008 3083

A R T I C L E S
Kanegawa et al.
Table 3. Selected Bond Lengths (Å) and Dihedral Angles (deg) for 1, 2, and 3
1
2
3
Bond lengths (Å)
Co-N(1)
Co-N(3)
Co-N(5)
2.205
2.200
2.075
Co-N(1)
Co-N(3)
Co-N(5)
Co-N(7)
Co-N(9)
Co-N(10)
2.214
2.199
2.201
2.190
2.075
2.076
Co-N(1)
Co-N(3)
Co-N(5)
2.198
2.209
2.063
Dihedral angles (deg) between pyridine ring and XY plane
CoN(1)N(3)-N(1)C(2)C(4)
CoN(1)N(3)-N(3)C(13)C(15)
67.75
69.53
N(1)N(3)N(5)N(7)-N(1)C(2)C(4)
N(1)N(3)N(5)N(7)-N(3)C(14)C(16)
N(1)N(3)N(5)N(7)-N(5)C(26)C(28)
N(1)N(3)N(5)N(7)-N(7)C(38)C(40)
76.41
61.85
71.12
63.82
Co(1)N(1)N(3)-N(1)C(2)C(4)
Co(1)N(1)N(3)-N(3)C(17)C(19)
71.46
61.76
Dihedral angles (deg) between pyridine ring and aminoxyl plane
N(1)C(2)C(4)-O(1)N(2)C(3)
N(3)C(13)C(15)-O(2)N(4)C(14)
27.78
24.72
N(1)C(2)C(4)-O(1)N(2)C(3)
N(3)C(14)C(16)-O(3)N(4)C(15)
N(5)C(26)C(28)-O(5)N(6)C(27)
N(7)C(38)C(40)-O(7)N(8)C(39)
15.13
27.54
9.00
N(1)C(2)C(4)-O(1)N(2)C(3)
N(3)C(17)C(19)-O(2)N(4)C(18)
24.79
8.30
28.13
Dihedral angles (deg) between phenyl ring and aminoxyl plane
C(7) C(9) C(11)-O(1)N(2)C(6)
O(2)N(4)C(17)-C(18)C(20)C(22)
29.08
O(1)N(2)C(6)-C(7)C(9)C(11)
O(3)N(4)C(18)-C(19)C(21)C(23)
O(5)N(6)C(30)-C(31)C(33)C(35)
O(7)N(8)C(42)-C(43)C(45)C(47)
32.02
26.52
41.30
25.74
O(1)N(2)C(6)-C(7)C(9)C(11)
34.15
40.88
31.58
O(2)N(4)C(21)-C(22)C(24)C(26)
and the aminoxyl plane caused by the bulky substituents at the
2- and 6-positions of the phenyl ring, as expected.
symmetry centered at the cobalt ion for 1 and 3 and with no
symmetric center for 2. The ORTEP drawing of the molecular
structure of 1 is demonstrated in Figure 1. The drawings for 2
and 3 are shown in Figure S2 (Supporting Information).
As shown in Scheme 1, all the N-aryl-N-pyridylaminoxyls
were prepared using similar procedures: metalation of 4-bro-
mopyridine, a reaction with corresponding nitroso derivatives
to give hydroxyamine derivatives, and oxidation with freshly
prepared Ag2O. All nitroso derivatives were prepared by
procedures (or modified versions thereof) reported in the
literature.15 Although analogous 4NOpy is unstable in a solid
state, the N-aryl-N-pyridylaminoxyls were relatively stable in
a solid state. Indeed, trisubstituted N-aryl-N-pyridylaminoxyls,
TPPNOpy and TBPNOpy, were obtained as single crystals and
their molecular structures were revealed by X-ray crystal-
lography.
The 1:4 complexes of high-spin cobalt(II) with N-aryl-N-
pyridylaminoxyls, [Co(NCS)2(PhNOpy)4] (1), [Co(NCS)2-
(MeOPhNOpy)4] (2), [Co(NCS)2(tert-BuPhNOpy)4] (3), [Co-
(NCS)2(TBPNOpy)4] (5a), and [Co(NCO)2(TBPNOpy)4] (5b),
were prepared by mixing the solutions of Co(NCS)2 (or a
mixture of Co(NO3)2 and KNCO for 5b) and the corresponding
N-aryl-N-pyridylaminoxyls in 1:4 ratios at room temperature.
The complexes were crystallized from suitable solvents at
-30 °C and were obtained as dark-red crystals. Complex 4 was
obtained only as a red microcrystal under similar conditions.
Crystal Structural Analysis. The single crystals of two
N-aryl-N-pyridylaminoxyls, TPPNOpy and TBPNOpy, and five
cobalt(II) complexes, 1, 2, 3, 5a, and 5b, were analyzed by X-ray
crystallography. A single crystal of 4 with a size amenable to
the X-ray analysis could not be obtained.
In the octahedral structure of three complexes, the bond
lengths (Co-NNCS) between the axial ligand and the cobalt ion
are short by 0.13-0.14 Å compared with those (Co-NPy)
between the nitrogen of pyridine and the cobalt ion. The bond
lengths of Co-NNCS are 2.075, 2.075 and 2.076, and 2.063 Å
for 1, 2, and 3, respectively. The dihedral angles between the
pyridine ring and the XY plane defined by the nitrogen atoms
of four pyridines in three complexes are in the range of 62-
76°, which may affect the zero-field splitting parameters of the
complexes. The dihedral angles between the pyridine ring and
the aminoxyl plane and between the phenyl ring and the
aminoxyl plane are 9-28 and 26-41°, respectively. The former
angles suggest that the magnetic coupling between the aminoxyl
and the cobalt ion takes place effectively. Both dihedral angles
are relatively small (the latter angles are slightly larger than
the former), indicating that the aminoxyl spins delocalize not
only on the pyridine ring but also on the phenyl ring, as expected
by the calculation. In the crystalline state for three complexes,
therefore, the magnetic interaction resulting from the intermo-
lecular short contacts between the phenyl ring and the neighbor-
ing molecules must be taken into account. Observed bond
lengths and dihedral angles are similar to those for [Co(NCS)2-
(4NOpy)4] reported previously.10 The selected bond lengths and
the dihedral angles for 1-3 are summarized in Table 3.
The three complexes have a similar crystal structure, shown
in Figure 2 for 1 and Figure S2 (Supporting Information) for 2
and 3, respectively. In the crystal lattice, there are two complex
molecules tilting in the opposite direction of each other, which
are shown by the green and tan colors in Figure 2. Each
molecule aligns in one direction to form a chain structure (chain
A and chain B in Figure 2a), and the resulting chains alternately
align in parallel. One complex molecule (for example, the green
molecule in Figure 2b) is surrounded by six complex molecules
(two green and four tan colored molecules) at a significantly
The crystals contain one molecule for complex 2 and two
molecules for complex 5a and 5b of diethyl ether per unit cell.
Five cobalt(II) complexes have a similar hexacoordinated
structure, in which each cobalt ion is surrounded by four nitrogen
atoms of pyridine rings (NPy) at the basal plane and two
counterions at the apical positions.
(A) [Co(NCS)2(PhNOpy)4] (1), [Co(NCS)2(MeOPhNOpy)4]
(2), and [Co(NCS)2(tert-BuPhNOpy)4] (3). In molecular
geometry, the three complexes are compressed octahedrons with
9
3084 J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008

Cobalt-Aminoxyl Heterospins
A R T I C L E S
Figure 2. (a) Packing diagram along the bc plane showing 1D chains (chain
A and B) and 2D plane and (b) short contacts with the surrounding molecules
for 1. The intermolecular short distances between the aminoxyl center and
the R-, â-carbon of pyridine in the neighboring molecules are indicated by
red and blue arrows, which show the inter- and intrachain short contacts,
respectively.
Table 4. Intermolecular Distances between the Aminoxyl Center
and the R-, â-Carbon of Pyridine Groups in the Neighboring
Molecule in the Crystals of 1, 2, and 3
Figure 3. Packing diagrams of (a) 1, (b) 2, and (c) 3 showing two layers
of the 2D-plane units. Plane-S and plane-S′ are those containing the sulfur
atoms of the axial ligands.
1, Å
2, Å
3, Å
O1‚‚‚C1′
3.293
3.196
3.144
3.084
O1‚‚‚C17′
3.181
3.159
3.110
2.973
3.089
3.206
3.135
3.002
O1‚‚‚C4′
3.138
3.161
2.981
3.057
O1‚‚‚C25′
O3‚‚‚C40′′
O3‚‚‚C41′′
O5‚‚‚C1′′′
O5‚‚‚C37′′′
O7‚‚‚C13′′′′
O7‚‚‚C14′′′′
O1‚‚‚C12′
O2‚‚‚C15′′
O2‚‚‚C16′′
O1‚‚‚C5′
O2‚‚‚C1′′
O2‚‚‚C2′′
balt ions, and the shortest distance for Co-Co′ is 8.92-10.77
Å. All cobalt ions in the plane reside in the resulting same
one.
The resulting planes are layered parallel to the bc plane for
1 and 3 and the ab plane for 2 to form the 3D structure. As
observed in Figure 3, the substituents at the 4-position of the
phenyl ring in these three complexes project the space between
the two planes. Therefore, the distances between the planes
depend on the bulkiness of the substituents. When the distances
between the 2D planes were compared by using the plane
(plane-S and -S′ in Figure 3) containing the sulfur atoms of the
axial ligands, the distance between plane-S and plane-S′ is ∼0,
2.66, and 4.11 Å for 1, 2, and 3, respectively. In addition, the
distance between the cobalt planes, which is defined by all cobalt
ions in the plane, is 8.91, 11.52, and 13.10 Å for 1, 2, and 3,
respectively. Observed distances are consistent with the order
of the bulkiness of the substituents (-H < -OMe <-tert-Bu),
short distance (<4.0 Å) resulting in intermolecular magnetic
coupling. In addition to the distance between the aminoxyl
centers (rNO-NO′ ) 3.99-4.38 Å), the four aminoxyl centers
in a molecule locate in the distance range of 3.0-3.3 Å
from the R-, â-carbons of pyridines in the surrounding
six molecules (Figure 2b) to form a 2D spin network. The
short contacts are indicated by red and blue arrows in Figure 2
and are listed in Table 4. The red and blue arrows also
show the intra- and interchain short contacts, respectively. On
the other hand, there are no short contacts between the co-
9
J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008 3085

A R T I C L E S
Kanegawa et al.
dihedral angles are in the range of 2.3-6.9° and 58-62°,
respectively. On the other hand, the dihedral angles between
the aminoxyl plane and the phenyl ring become large (80-
86°), as expected. No large differences in the molecular structure
of TBPNOpy between the bound and free pyridine ligands were
observed. The large dihedral angles arising from the bulky
substituents at the 2 and 6-position of the phenyl ring mean
that the distributions of the electron spin on the phenyl ring
become insignificant. The selected bond lengths and the dihedral
angles for 5a and 5b together with TBPNOpy are summarized
in Table 5. The complexes have a similar crystal structure in
which short distances, within 4 Å, are observed neither between
the aminoxyl radical centers nor between the radical center and
the R and â carbons of the pyridine ring of the neighboring
molecules. The distance between the cobalt ions is in the range
of 10.2-20.3 Å. Therefore, 5a and 5b having 2,4,6,-trisubsti-
tuted benzene units are expected to suppress effectively the
intermolecular interaction in the crystal state. The crystal
packings for 5a and 5b are shown in Figure S4 (Supporting
Information) and Figure 5, respectively.
In these five complexes, the substituents on the phenyl ring
effectively tune the intermolecular distance in the crystal lattices.
The substituent at the 4-position of the phenyl ring in mono-
substituted complexes modulates the distance between the
planes, and the distance increases according to the bulkiness of
the substituent. The distance between the planes is large in the
order of 1, 2, and 3. On the other hand, the bulky substituents
at the 2, 4, and 6-positions of the phenyl ring in the complex
control the intermolecular distance. In the crystal structures of
5a and 5b having tert-butyl groups at the 2, 4, and 6-positions
of the phenyl ring, significant short contacts within 4 Å among
the molecules were not observed, indicating that the complex
molecules are magnetically well separated.
Figure 4. ORTEP drawing of the molecular structure of 5b. Hydrogen
atoms are omitted for the sake of clarity.
indicating that the substituent at the 4-position of the phenyl
ring controls the distance between the 2D planes in the
crystal.
The existence of the two complex molecules with different
tilting directions in the 2D planes and the distance between the
2D planes strongly affected the magnetic properties of the
complexes, as discussed in the section on the magnetic properties
for the complexes. A 3D structure similar to 1 was also observed
in an analogous complex, [Co(NCS)2(4NOpy)4].10
(B) TPPNOpy, TBPNOpy, [Co(NCS)2(TBPNOpy)4] (5a)
and [Co(NCO)2(TBPNOpy)4] (5b). (B)-1 TPPNOpy and
TBPNOpy. The bond angle of C(3)N(2)C(6) in aminoxyl units
is 123.4° for TPPNOpy and 124.2° for TBPNOpy, both close
to that for PhNOpy (125.1°). On the other hand, the dihedral
angles between the phenyl and pyridine rings are largely twisted
by the 2,6-substituents compared with the angle (51.09°) for
PhNOpy, being 76.63 and 88.51° for TPPNOpy and TBP-
NOpy, respectively. The dihedral angles between the phenyl
ring and the aminoxyl plane and between the pyridine ring and
the aminoxyl plane for TBPNOpy are 87 and 3.6°, respectively,
indicating that the aminoxyl spin mainly delocalized on the
pyridine ring. Similarly, TPPNOpy has dihedral angles of 71
and 68° for the former and 8.4 and 3.3° for the latter. The
ORTEP drawings of the molecular structure of TPPNOpy and
TBPNOpy are shown in Figure S3 (Supporting Information).
(B)-2 [Co(NCS)2(TBPNOpy)4] (5a) and [Co(NCO)2-
(TBPNOpy)4] (5b). The molecular geometry for both 5a and
5b is also a compressed octahedron with symmetry centered at
the cobalt ion. The ORTEP drawing of the molecular structure
of 5b is shown in Figure 4.
Magnetic Properties. The magnetic properties of the 1:4
complexes 1, 2, 3, 4, 5a, and 5b were investigated using SQUID
magneto/susceptometry.
(A) [Co(NCS)2(PhNOpy)4] (1), [Co(NCS)2(MeOPhNOpy)4]
(2), and [Co(NCS)2(tert-BuPhNOpy)4] (3). The dc magnetic
susceptibilities (ømol) for the pulverized samples of three
complexes were measured at a constant field of 5 kOe in the
temperature range of 2-300 K. Temperature dependence of the
-1
ømol of 1, 2, and 3 is shown as ømol vs T and ømol vs T plots
in the entire temperature region in Figure 6a-c, respectively,
and as a ømolT vs T plot in the range of 2-50 K in Figure 6d.
-1
The ømol vs T plots of three complexes were analyzed by
the Curie-Weiss equation, ømol ) C/(T - θ) where C and θ
are Curie and Weiss constants, respectively. Best fits of the ømol
data above 100 K for 1, 2, and 3, to the Curie-Weiss equation
gave C ) 4.78, 4.92, and 5.23 cm3mol-1K and θ ) -6.74,
-32.4, and -41.7 K, respectively. These Curie constants are
much higher than the spin-only value of 3.38 cm3mol-1
K
3
expected for an isolated one high-spin Co(II) ion (S ) /2, gCo
) 2.00) and four aminoxyls (S ) 1/2 and gNO ) 2.00) owing to
the orbital contribution of the Co(II) ion.18 The negative Weiss
constants suggest the presence of antiferromagnetic coupling.
The bond lengths (Co-NNCX) between the axial ligand and
the cobalt ion are short by 0.10-0.18 Å compared with those
(Co-NPy) between the nitrogen of pyridine and the cobalt ion.
Both dihedral angles between the aminoxyl plane and the
pyridine ring and between the aminoxyl radicals and the XY
plane containing cobalt(II) ion become small compared with
those for monosubstituted complexes. The former and latter
In the ømol vs T plot of 1 (Figure 6a), a sharp maximum at
22 K was observed, indicating that the antiferromagnetic
(18) Hossain, M. J.; Yamasaki, M.; Mikuriya, M.; Kuribayashi, A.; Sakiyama,
H. Inorg. Chem. 2002, 41, 4058-4062.
9
3086 J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008

Cobalt-Aminoxyl Heterospins
A R T I C L E S
Table 5. Selected Bond Lengths (Å) and Dihedral Angles (deg) for 5a, 5b, and TBPNOpy
5a
5b
TBPNOpy
Bond lengths (Å)
Co-N(1)
Co-N(3)
Co-N(5)
2.191
2.219
2.090
Co-N(1)
Co-N(3)
Co-N(5)
2.202
2.240
2.060
Dihedral angles (deg) between pyridine ring and XY plane
CoN(1)N(3)-N(1)C(2)C(4)
CoN(1)N(3)-N(3)C(25)C(27)
57.87
60.19
CoN(1)N(3)-N(1)C(2)C(4)
CoN(1)N(3)-N(3)C(25)C(27)
59.58
58.15
Dihedral angles (deg) between pyridine ring and aminoxyl plane
N(1)C(2)C(4)-O(1)N(2)C(3)
2.33
5.80
N(1)C(2)C(4)-O(1)N(2)C(3)
2.53
6.90
N(1)C(2)C(4)-O(1)N(2)C(3)
O(1)N(2)C(6)-C(7)C(9)C(11)
3.57
N(3)C(25)C(27)-O(2)N(4)C(26)
N(3)C(25)C(27)-O(2)N(4)C(26)
Dihedral angles (deg) between phenyl ring and aminoxyl plane
84.26
79.73
O(1)N(2)C(6)-C(7)C(9)C(11)
O(2)N(4)C(29)-C(30)C(32)C(34)
O(1)N(2)C(6)-C(7)C(9)C(11)
O(2)N(4)C(29)-C(30)C(32)C(34)
85.72
79.95
86.84
In the ømolT vs T plot of 2 at a constant field of 5 kOe (Figure
6d), the ømolT values gradually decreased on cooling until 10
K, increased steeply to reach a maximum (1.28 cm3mol-1K) at
ca. 3 K, and then decreased. The observed thermal profile of
ømol values below 10 K strongly depended on the applying field.
The ømol vs T plots at constant fields of 1.0, 0.5, and 0.1 kOe
are shown in Figure 7a. The maximum ømol value (1.08
cm3mol-1) at 2.3 K appeared at 1.0 kOe and the peak-top
temperature slightly shifted to a higher temperature with a
decrease in the external field. This field dependence of ømol
suggests an antiferromagnetic transition.20 To determine the Neel
temperature, TN, ac magnetic susceptibility measurements were
carried out with an ac field of 3.9 Oe at 997, 99.9, and 1 Hz. In
the temperature dependence of ac molar magnetic susceptibili-
ties, in-phase signals (ø′mol) without frequency dependence were
observed and showed the maximum at 2.9 K, whereas out-of
phase signals (ø′′mol) were extremely weak (Figure 7b). From
the observed peak-top temperature of the ø′mol signal, the TN
value of 2 was determined to be 2.9 K.
In the ømol vs T plot of 3 (Figure 6d), the thermal profile of
ømol values below 10 K was similar to that for 2 and also showed
the dependence of applying the external field. At a constant
field less than 1 kOe, the ømolT values steeply increased below
5 K, indicating that a long-range magnetic ordering of magnetic
spin takes place. To characterize a long-range magnetic ordering,
dc magnetic measurements using a sequence of zero field-cooled
magnetization (ZFCM), field-cooled magnetization (FCM), and
remnant magnetization (RM) and the ac magnetic measurements
in a zero dc field with a 3.9 Oe ac field at 997, 99.9, and 1 Hz
were carried out in the temperature range of 10-1.9 K. The dc
and ac magnetic measurements are shown in Figure 8a and b,
respectively.
The molar magnetization, Mmol, in the FCM measurement at
a constant field of 4.7 Oe increased rapidly below ca. 5 K,
reached a value of 977 cm3 mol-1Oe at 3.5 K, and then slightly
increased below it. The derivative curve, dM/dT, of the FCM
measurement gave an extremum at 4.5 K corresponding to the
critical temperature, Tc. On the other hand, the Mmol values in
the ZFCM measurement showed a relatively sharp maximum
at ca. 4.5 K and a large discrepancy in Mmol values between
the ZFCM and FCM measurements was observed below Tc )
4.5 K (Figure 8a). In the RM measurement, the Mmol value,
Figure 5. Crystal packings with the space filling model (C, gray; O, red;
and N, blue) projecting along the (a) a-axis and (b) b-axis for 5b. Aryl
units of TBPNOpy in 5b are indicated by the gray lines.
ordering operated below the temperature.19 From the temperature
of the sharp maximum of ømol, the Neel temperature, TN, was
defined to be 22 K for 1. The observed magnetic behavior was
similar to that for [Co(NCS)2(4NOpy)4], which has a TN of
15 K.
(19) (a) King, P.; Cle´rac, R.; Anson, C. E.; Coulon, C.; Powell, A. Inorg. Chem.
2003, 42, 3492-3500. (b) Aoki, C.; Ishida, T.; Nogami, T. Inorg. Chem.
2003, 42, 7616-7625.
(20) Manson, J. L.; Huang, Q.; Lynn, J. W.; Koo, H. J.; Whangbo, M. H.;
Bateman, R.; Otsuka, T.; Wada, N.; Argyriou, D. N.; Miller, J. S. J. Am.
Chem. Soc. 2001, 123, 162-172.
9
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A R T I C L E S
Kanegawa et al.
Figure 6. Plots of ømol vs T (O) and ømol -1 vs T (0) for the pulverized samples of (a) 1, (b) 2, and (c) 3 and (d) the plots of ømolT vs T in the temperature
range 1.9-50 K for 1(O), 2(0), and 3(4). The dashed lines are the fitting results from a Curie-Weiss equation; see the text for the fitting parameters.
Figure 8. (a) M vs T plot in ZFCM (O), FCM (0), and RM (4)
Figure 7. Plots of (a) ømol vs T at a constant field of 1(O), 0.5(0), and
measurements and (b) ø′mol (open) and ø′′mol (filled) vs T plots at 997 (4),
0.1(4) kOe and (b) ø′mol and ø′′mol vs T at an ac field of 3.9 Oe at 997 (4),
99.9 (0), and 1 Hz (O) for the pulverized sample of 3. Solid lines are visual
99.9 (0), and 1 (O) Hz for the pulverized sample of 2.
guides.
which was 1046 cm3mol-1Oe at 2.0 K, decreased nearly along
the curve for the FCM measurement upon warming and then
disappeared above 5 K. In ac magnetic susceptibility measure-
ments, both ø′mol and ø′′mol signals with frequency dependence
were observed (Figure 8b). In the ø′mol and ø′′mol vs T plots,
the thermal profiles of the ø′mol and ø′′mol signals at each
9
3088 J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008

Cobalt-Aminoxyl Heterospins
A R T I C L E S
temperature of 4.7 K obtained for the ø′mol signal was defined
as the TN for 3.21
To investigate the antiferromagnetic nature of 1, 2, and 3 in
more detail, Mmol was measured at 1.9 K, where the temperature
is below TN, in the field range -50 to +50 kOe. When the dc
field varied in the range of -50 to +50 kOe, three complexes
showed characteristic magnetic behavior. The Mmol/NµB vs H
plots for 1, 2, and 3 at 1.9 K are shown in Figure 9.
The values of Mmol/NµB at 50 kOe are 0.45, 1.82, and 1.46
for 1, 2, and 3, respectively, and do not reach the saturation
magnetization, Ms, even at 50 kOe. They are 2.7-11 times
5
smaller than the value of Ms (5/NµB) calculated by Stotal ) /2
(one Co(II) ion with the effective spin quantum number S′ )
1
1/2 and four aminoxyls with S ) /2 are coupled ferromagneti-
cally) and g ) 2.00, indicating that strong antiferromagnetic
interaction takes place in the spin networks. The three complexes
showed the characteristic magnetic profiles individually in the
field dependence of magnetization. In the Mmol/NµB vs H plot
of 1, the Mmol/NµB values increased gradually and slightly, as
the dc field increased from 0 to 50 kOe. As seen in Figure 8a,
a small discrepancy in the Mmol value on increasing and reducing
the field was observed. In a similar plot for 2, the Mmol/NµB
increased slightly until 0.3 kOe, increased steeply to 0.18 at 1
kOe, and then increased linearly. This field dependence of Mmol
/
NµB values in a low field range might indicate the phase
transition from an antiferromagnet phase to a spin-flop phase.22
From the dM/dH vs H plot (inset in Figure 9b), an anomalous
point corresponding to the spin-flop field at 1.9 K, Hsp, was
determined to be 0.7 kOe. On the other hand, the Mmol/NµB
values for 3 steeply increased to 0.3 NµB in 0-1.0 kOe and
then increased with the curvature having a convex downward
at above 1 kOe. The curvature suggests a possible spin-flop
transition on applying a further field. Furthermore, interestingly,
the complex 3 showed a hysteresis with respect to the field.
The hysteresis loop with a remnant magnetization, Mr, of 0.29
NµB and a coercive force, Hc, of 1.3 kOe at 1.9 K is shown in
Figure 9c. This magnetic behavior is typical of a canted
antiferromagnet (a weak ferromagnet).21
The antiferromagnetic behavior of the three complexes was
closely related with their crystal structure revealed by X-ray
crystallography. The proximity of N(O)-Câ (or CR) between
the molecules, which were often observed in analogous het-
erospin complexes, leads to the antiferromagnetic (or ferro-
magnetic) coupling. In these complexes, the antiferromagnetic
couplings operate predominantly to form the antiferromagnetic
2D planes. The difference in the magnetic behavior among the
three complexes was caused by antiferromagnetic interaction
between the 2D planes. The results of dc magnetic susceptibility
measurements suggested that the antiferromagnetic interaction
between the 2D planes decreases in the order of 1, 2, and 3 and
becomes insignificant in 3. The observed order of the magnitude
of antiferromagnetic coupling in the three complexes is con-
sistent with that for the distance between the 2D planes, 1 < 2
< 3, (Figure 3). Notably, only 3 having a similar local structure
in the 2D plane exhibits the magnetic behavior of a canted
antiferromagnet, indicating that the magnetic interaction between
the 2D planes counteracts the magnetic behavior for a canted
Figure 9. Figure 9. Plots of Mmol/NµB vs H for the pulverized samples of
(a) 1, (b) 2, and (c) 3 at 1.9 K. The insets in (b) and (c) show the detail in
a low field (0-3 kOe) together with the dM/dH vs H plot and the field
range of (6 kOe, respectively.
frequency were similar to each other: the ø′mol and ø′′mol signals
decreased as the frequency increased and had peak-top tem-
peratures of 4.7 and 4.3 K, respectively. The peak-top temper-
ature of 4.7 K for the ø′mol signals was consistent with the Tc
obtained in the dc magnetic susceptibility measurements. The
(21) Kim, J.; Han, S. J.; Pokhodnya, K. I.; Migliori, J. M.; Miller, J. S. Inorg.
Chem. 2005, 44, 6983-6988.
(22) Wang, X. Y.; Wang, L.; Wang, Z. M.; Su, G.; Gao, S. Chem. Mater. 2005,
17, 6369-6380.
9
J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008 3089

A R T I C L E S
Kanegawa et al.
Figure 11. Plot of Mmol/NµB vs H/T at 0.8(]), 20(3), 30(4), 40(0), and
50(O) kOe for the pulverized sample of 5b. The solid lines are the fitting
results from the ligand field theory model; see the text for the fitting
parameters.
Figure 10. Plot of ømolT vs T for the pulverized samples of 4 (4), 5a (0),
and 5b (O). The solid line for 5b is the fitting result from the ligand field
theory model; see the text for the fitting parameters.
indicates that the intermolecular antiferromagnetic interactions
observed in 1, 2, and 3 were effectively eliminated and the
complex molecules would be magnetically isolated.
antiferromagnet in the 2D plane. Similar magnetic behavior
based on local and structural anisotropies in a low dimension
was reported previously.23
The dc field dependence of Mmol for the pulverized samples
of 4, 5a, and 5b was measured at 1.9, 2.0, 2.5, 3.0, 3.5, 4.0,
5.0, 6.0, and 7.0 K in the range of 0.8-50 kOe. In the plots of
M/NµB vs H/T for the three complexes, typically large de-
viations from the Brillouin function14,25 were observed, indicat-
ing that the zero-field splitting parameter D/kB operated at the
ground state. The plot of M/NµB vs H/T for 5b is shown in
Figure 11.
(B) [Co(NCS)2(TPPNOpy)4] (4), [Co(NCS)2(TBPNOpy)4]
(5a), and [Co(NCO)2(TBPNOpy)4] (5b). (B-1) Dc Magnetic
Susceptibility Measurements in Microcrystalline State. The
dc magnetic susceptibility of pulverized samples of the three
complexes was measured at a constant field of 5 kOe in the
temperature range of 2-300 K. The thermal profiles of ømol
for the three complexes are similar, which are shown as ømol vs
-1
T and ømol vs T plots in Figure S5 (Supporting Information)
The field dependence of Mmol for the orientated sample of
5b was measured at 0.43 K in the field range of (50 kOe. In
the Mmol/NµB vs H plot, the Mmol values gradually increased
and reached a saturation magnetization, Ms, of 6.35 over 3 kOe
as the field increased. The Ms value of 6.35 corresponds to that
calculated with Stotal ) 5/2 and g ) 2.54. In the cycle of applying
and reducing the field, a small discrepancy in Mmol values
between the increase and the decrease in the field in the range
of 0.5-2 and -(0.5-2) kOe was observed (Figure S6, Sup-
porting Information).
(B-2) Ac Magnetic Susceptibility Measurements. Ac
magnetic susceptibility measurements for the pulverized samples
of 4, 5a, and 5b were carried out. The measurements were made
in a zero dc field with a 3.9 Oe ac field at 997-1 Hz and in the
temperature range of 1.9-10 K. In all three complexes, ø′mol
and ø′′mol signals with frequency dependence were observed,
indicating that the complexes had a slow magnetic relaxation
for the reversal of the magnetism. The plots of ø′molT vs T and
ø′′mol vs T for 4, 5a, and 5b are shown in Figure 12a-c,
respectively.
In the ø′molT vs T plot of 4, the ø′molT values gradually
decreased on cooling from 10 to 3.5 K and then steeply
decreased with frequency dependence below 3.5 K, whereas in
the same plots of 5a and 5b, they slightly increased and were
nearly constant, respectively, until the temperature that the ø′′mol
signals appeared. Although for 4 there are no data from X-ray
crystallography, the decrease in the ø′molT values in the range
of 10-3.5 K might indicate that the weak antiferromagnetic
interaction operates intermolecularly. In the ø′′mol vs T plots
for three complexes, the maxima of ø′′mol signals shifted to the
lower temperature side as the frequency decreased. Complexes
and a ømolT vs T plot in Figure 10. The ømol data above 100 K
in the ømol-1 vs T plots for 4, 5a, and 5b were analyzed by the
Curie-Weiss equation (Figure S5a-c, Supporting Information)
and gave C ) 4.57, 4.75, and 4.75 cm3mol-1K and θ ) 10.1,
2.04, and 13.4 K, respectively. These Curie constants are similar
to those (4.78-5.23 cm3mol-1K) for the monosubstituted
complexes. The positive Weiss constants clearly indicate the
presence of ferromagnetic couplings. In the ømolT vs T plots
for the three complexes, the ømolT values were nearly constant
until ca. 100 K, gradually increased on cooling, reached maxima
at 15-5.5 K, and then decreased. The maximum values of ømolT
are 6.00, 6.71, and 8.22 cm3mol-1K for 4, 5a, and 5b,
respectively. They are larger than the value of 3.6 cm3mol-1
K
(4 × 0.375 for aminoxyls + 2.1 for [Co(NCS)2(py)4] with S′ )
1
/
24) for the isolated four aminoxyl radicals and a cobalt(II)
2
ion, which also indicate that ferromagnetic interaction takes
place between the cobalt ion and the aminoxyls through pyridine
ligand and the high-spin complexes with Stotal ) 5/2 are produced
below the temperature showing the maximum ømolT value.
Observed thermal profiles in the low-temperature region (100-
20 K) showed a clear contrast to those for 1, 2, and 3, where
the ømolT values for 4, 5a, and 5b gradually increase on cooling,
while those for 1, 2, and 3, gradually decrease under similar
conditions (Figure 6d). The increase in ømolT on cooling
(23) (a) Salah, M. B.; Vilminot, S.; Andre, G.; Richard-Plouet, M.; Mhiri, T.;
Takagi, S.; Kurmoo, M. J. Am. Chem. Soc. 2006, 128, 7972-7981. (b)
Takagami, N.; Ishida, T.; Nogami, T. Bull. Chem. Soc. Jpn. 2004, 77, 1125-
1135. (c) Coronado, E.; Galan-Mascaros, J. R.; Gomez-Garcia, C. J.;
Murcia-Martinez, A. Chem.-Eur. J. 2006, 12, 3484-3492. (d) Bellouard,
F.; Clemente-Leon, M.; Coronado, E.; Galan-Mascaros, J. R.; Gomez-
Garcia, C. J.; Romero, F.; Dunbar, K. R. Eur. J. Inorg. Chem. 2002, 43,
1603-1606.
(24) (a) Jankovics, H.; Daskalakis, M.; Raptopoulou, C. P.; Terzis, A.; Tangoulis,
V.; Giapintzakis, J.; Kiss, T.; Salifouglou, A. Inorg. Chem. 2002, 41, 3366-
3374. (b) Thuery, P.; Zarembowitch, J. Inorg. Chem. 1986, 25, 2001-
2008. (c) Zarembowitch, J.; Kahn, O. Inorg. Chem. 1984, 23, 589-593.
(25) M ) NgSµBB(x) where B(x) is a Brillouin function, where x ) gSµBH/kBT,
and the other symbols have their usual meanings.
9
3090 J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008

Cobalt-Aminoxyl Heterospins
A R T I C L E S
Figure 12. Plots of ø′molT vs T (upper) and ø′′mol vs T (lower) for the pulverized samples of (a) 4, (b) 5a, and (c) 5b in the absence of a dc field and (d)
5b in the presence of a dc field of 1 kOe. The solid lines are visual guides
4 and 5b showed maxima at a frequency higher than 10 and 50
Hz, respectively, whereas 5a had no maximum at any frequency
above 1.9 K. As 5a and 5b were similar in molecular structure
as revealed by X-ray crystallography, the maximum of the ø′′
signals for 5a at each frequency might exist below 1.9 K. A
similar axial-ligand dependence was observed in [Co(X)2-
9
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A R T I C L E S
Kanegawa et al.
ref
Table 6. Values of Ueff and τ0 of 4, 5a, and 5b in the Crystalline State and Frozen Toluene Solution
complexes
sample conditions
Ueff [K]
τ0 [s]
4
5a
crystalline state
crystalline state
30
6.9 × 10-9
this work
this work
NDa
27
NDa
frozen toluene solution
crystalline state
frozen toluene solution
frozen MTHF solution
frozen MTHF solution
2.3 × 10-8
b
5b
28 (35)b
48
31
1.8 × 10-7 (5.1 × 10-8
3.7 × 10-8
)
this work
[Co(NCS)2(4NOpy)4]
[Co(NCO)2(4NOpy)4]
1.7 × 10-7
10
9
50
1.8 × 10-7
a Not determined. b The values in parentheses are the ones in the presence of a dc field of 1 kOe.
(4NOpy)4] (X ) NCS- and NCO-) in frozen solution (Table
6). Because each frequency at the peak-top temperature for ø′′mol
signals is consistent with 1/τ, the kinetic activation energy, Ueff,
for the reversal of the magnetism and the pre-exponential factor,
τ0, were estimated from the Arrhenius plot; τ ) 1/2πνac ) τ0
exp(Ueff/kBT). The values of Ueff ) 30 and 28 K and τ0 ) 6.9
× 10-9 and 1.8 × 10-7 s for 4 and 5b were obtained using the
peak-top temperature data above 10 and 50 Hz, respectively
(Figure S7, Supporting Information).
When the ø′′mol vs T plot of 5b was closely examined, the
ø′′mol signals at each frequency seemed to have a second peak
(shoulder) on the lower temperature side. Then, the dc field
dependence of the ø′mol and ø′′mol signals was investigated. In
the ø′molT vs T and ø′′mol vs T plots in the presence of a dc field
of 1 kOe (Figure 12d), the ø′molT values at the lower temperature
reached nearly zero, whereas the main peaks of ø′′mol signals
shifted to the higher temperature side and the second peaks (or
shoulder) almost disappeared. The ø′′mol vs T plot of 5b in the
presence of a dc field of 1 kOe gave values of Ueff ) 35 K and
τ0 ) 5.1 × 10-8 s. (Figure S7). The change of ø′mol and ø′′mol
signals in the presence of the dc field suggests that the magnetic
relaxation, kd, observed by ac magnetic susceptibility measure-
ments might be affected by the relaxation, kq, due to the spin
quantum tunneling effect.2c,26 Fast relaxation, kq, was often
observed in the case of the Co complex carrying organic spins
with a small S value in the heterospin systems.9,10
SMM, a theoretical study of 5b and the ac measurements of 5a
and 5b in frozen solution were carried out.
(B-3) Theoretical Study for 5b. To analyze quantitatively
the magnetic property of 5b showing SMM behavior,the ligand-
field theoretical model14,29 was employed. In this cobalt het-
erospin system, the effective Hamiltonian is written as
4
1
3
2
Hˆ ) ∆ ‚ Lˆ2z - Lˆ2
-
kλLˆ ‚ Sˆ - 2J
Sˆ ‚ Sˆ +
0 i
∑
0
(
)
3
i)1
4
3
µB - kLˆ + ge
Sˆ ‚ B (1)
∑
i
(
)
2
i)0
4
4
where ∆ and Lˆ are the energy splitting between A2g and Eg
states and an effective angular momentum with a magnitude of
L ) 1, respectively, Sˆ0 and Sˆi (i ) 1-4) are spin operators of
the Co(II) ion (S0 ) 3/2) and four radicals (Si ) 1/2), µB and ge
are the Bohr magneton and the Lande´ g factor for free electron,
respectively, and parameters k, λ, and J stand for the Stevens
orbital reduction factor of Co(II), the spin-orbit coupling of
Co(II), and the superexchange interaction between Co(II) and
each radical, respectively. Equation 1 fitted both sets of
experimental data for the plots of ømolT Vs. T (Figure 10) and
M/NµB vs H/T (Figure 11) simultaneously by nonlinear least-
squares fitting based on a downhill simplex method.30-32
The optimized parameters thus obtained are ∆/kB ) -1360
K, λ/kB ) -276 K, k ) 0.866, and J/kB ) 16 K for 5b and
simulation curves calculated using these parameter sets are
shown in Figures 10 and 11.
The strong dependence on frequency of the ø′mol and ø′′mol
signals and the physically reasonable values of τ0 suggest that
4, 5a, and 5b function as an SMM rather than a spin glass.27,28
To better understand the magnetic properties of heterospin
The energy level scheme corresponding to 5b is shown in
Figure 13, where energy eigen values E of the Hamiltonian (eq
1) are plotted by levels with respective degeneracy figures
against the z-component of the total spin quantum number MS.
The lowest six levels form a parabolic barrier profile with a
height of 49 K, partitioning the MS ) +5/2 and -5/2 states.
Although such a system possessing not-fully quenched orbital
angular momentum cannot be described by using a zero-field
splitting parameter D characteristic of the conventional Abrag-
am-
(26) (a) Costes, J. P.; Dahan, F.; Wernsdorfer, W. Inorg. Chem. 2006, 45, 5-7.
(b) Lu, Z. L.; Yuan, M.; Pan, F.; Gao, S.; Zhang, D. Q.; Zhu, D. B. Inorg.
Chem. 2006, 45, 3538-3548.
(27) Mydosh, J. A. Spin Glasses, An Experimental Introduction; Taylor and
Francis: London, 1993.
(28) (a) Gˆırtu, M. A.; Wynn, C. M.; Fujita, W.; Awaga, K.; Epstein, A. J. Phys.
ReV. B 2000, 61, 4117-4130. (b) Sellers, S. P.; Korte, B. J.; Fitzgerald, J.
P.; Reiff, W. M.; Yee, G. J. Am. Chem. Soc. 1998, 120, 4662-4670. (c)
Greedan, J. E.; Raju, N. P.; Maignan, A.; Simon, C.; Pedersen, J. S.;
Niraimathi, A. M.; Gmelin, E.; Subramanian, M. A. Phys. ReV. B 1996,
54, 7189-7200.
(29) (a) Herrera, J, M.; Bleuzen, A.; Dromze´e, Y.; Julve, M.; Lloret, F.;
Verdaguer, M. Inorg. Chem. 2003, 42, 7052-7059. (b) Sakiyama, H.; Ito,
R.; Kumagai, H.; Inoue, K.; Sakamoto, M.; Nishida, Y.; Yamasaki, M.
Eur. J. Inorg. Chem. 2001, 2027-2032. (c) Palii, A. V.; Tsukerblat, B. S.;
Coronado, E.; Clemente-Juan, J. M.; Borra’s-Almenar, J. J. Inorg. Chem.
2003, 42, 2455-2458. (d) Palii, A. V.; Ostrovsky, S. M.; Klokishner, S.
I.; Reu, O. S.; Sun, Z.-M.; Prosvirin, A. V.; Zhao, H.-H.; Mao, J.-G.;
Dunbar, K. R. J. Phys. Chem. A 2006, 110, 14003-14012. (e) Ostrovsky,
S. M.; Falk, K.; Pelikan, J.; Brown, D. A.; Tomkowicz, Z.; Haase, W.
Inorg. Chem. 2006, 45, 688-694. (f) Maspoch, D.; Domingo, N.; Ruiz-
Molina, D.; Wurst, K.; Hernndez, J. M.; Lloret, F.; Tejada, J.; Rovira, C.;
Veciana, J. Inorg. Chem. 2007, 46, 1627-1633.
(30) Nelder, J. A.; Mead, R. Comput. J. 1965, 7, 308-313.
(31) Parkinson J. M.; Hutchinson, D. In Numerical Methods for Non-linear
Optimization; Lootsma, F. A., Ed.; Academic Press: London, 1972; pp
115-135.
(32) Press, W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. P. Numerical
Recipes; Cambridge University Press: Cambridge, 1986.
Figure 13. E/kB vs Ms diagram for 5b obtained from the optimized
parameters. The broken curve is an eye-guide showing the energy-barrier
5
profile for reversal of the resultant spin S ) /2.
9
3092 J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008

Cobalt-Aminoxyl Heterospins
A R T I C L E S
Figure 14. Plots of ø′molT vs T (upper) and ø′′mol vs T (lower) for the toluene solution samples of (a) 5a and (b) 5b. The solid lines are visual
guides.
Pryce spin Hamiltonian, it was found that there exists another
zero-field splitting in the ground spin manifold, which contrib-
utes to the thermodynamic activation barrier (U) for the reversal
of the molecular spin. It should be noted here that the picture
of the resultant molecular spin of S ) 5/2 has a certain relevance
in the low-energy region of the energy level scheme. This
resultant spin value is obtained if it is assumed that a high spin
cobalt ion has a Kramers doublet ground state29 with effective
octahedral structure at 180 K (Figure S8, Supporting Informa-
tion). The magnetic property of 4 in the frozen solution could
not be investigated, because it was insoluble in toluene and other
appropriate organic solvents. The ac magnetic susceptibility
measurements for the toluene solution samples of 5a and 5b
were carried out under conditions similar to those for the
crystalline samples. The plots of ø′molT vs T and ø′′mol vs T are
shown in Figure 14.
1
S′ ) /2 and ferromagnetically interacts with four surrounding
radicals.
The frozen solution samples of both complexes showed a
frequency dependence of ø′ and ø′′ signals similar to the
crystalline samples of the corresponding ones. In the
Because the thermodynamic activation barrier, U, obtained
by the calculation is the thermal activation over the potential
energy for the reversal of the spin and the effective activation
barrier, Ueff, obtained from ac magnetic susceptibility measure-
ments involves quantum tunneling, the difference in the values,
U ) 49 and Ueff ) 28 K, may arise from a resonance quantum
tunneling.33,34 The zfs parameter D/kB for 5b was estimated to
be -8.2 K from the equation U ) |D|{S2 - (1/2)2} for a
complex with a half-integer spin by U ) 49 K.
(B-4) Ac Magnetic Susceptibilities of 5a and 5b in Frozen
Solution. The complexes 5a and 5b were dissolved in dry
toluene and used as solution samples (10 and 5 mM, respec-
tively). The solutions of both complexes in toluene were
measured by Visible spectra in the range of 300-180 K, and it
was confirmed that, in an equilibrium between the tetrahedral
and the octahedral structure, they are mainly the tetrahedral
structure at room temperature and are predominantly the
ø′molT vs T plots for the frozen solution samples, the ø′mol
T
values in the plateau region are 6.20 and 7.80 cm3mol-1K for
5a
and 5b, respectively, and are consistent with those for the
pulverized samples, indicating that the molecular structure of
the complexes formed in frozen solution are octahedral, the
same as those in the crystalline state. In the ø′′mol vs T
plots, the peak-top temperature at each frequency shifted to a
higher temperature. Especially, complex 5a clearly showed a
maxima of ø′′ signal at above 99.9 Hz, although the pulver-
ized sample of 5a showed no maximum under similar condi-
tions. Their Arrhenius plots (Figure S7, Supporting Infor-
mation) gave Ueff and τ0 values of 27 and 48 K, and 2.3 ×
10-8 and 3.7 × 10-8 s for 5a and 5b, respectively. Both values
became large compared with those obtained in the crystalline
state. The observed difference of Ueff values for 5b in the
crystalline state and in the frozen solution might be caused by
small changes in the local environment around the cobalt ion
such as a change of the dihedral angles between the basal plane
and the pyridine rings. The effect of crystal packing in the
crystalline state is considered to be released in solution. The
(33) (a) Friedman, J.; Sarachik, M.; Tejada, J.; Ziolo, R. Phys. ReV. Lett. 1996,
76, 3830-3833. (b) Thomas, L.; Lionti, F.; Ballou, R.; Gatteschi, D.;
Sessoli, R.; Barbara, B. Nature 1996, 383, 145-147.
(34) (a) Soler, M.; Wernsdorfer, W.; Folting, K.; Pink, M.; Christou, G. J. Am.
Chem. Soc. 2004, 126, 2156-2165. (b) Hendrickson, D. N.; Christou, G.;
Ishimoto, H.; Yoo, J.; Brechin, E. K.; Yamaguchi, A.; Rumberger, E. M.;
Aubin, S. M. J.; Sun, Z.; Aromi, G. Polyhedron 2001, 20, 1479-1488. (c)
Aubin, S. M. J.; Dilley, N. R.; Wemple, M. W.; Maple, M. B.; Christau,
G.; Hendrickson, D. N. J. Am. Chem. Soc. 1998, 120, 839-840.
Ueff and τ0 values for 4, 5a, and 5b in the crystalline state and
9
J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008 3093

A R T I C L E S
Kanegawa et al.
frozen solution are summarized in Table 6 together with those
for [Co(NCS)2(4NOpy)4] and [Co(NCO)2(4NOpy)4] in frozen
solution.
The Ueff and τ0 values for 5a and 5b in the frozen solution
are well consistent with those for the corresponding complexes,
[Co(NCS)2(4NOpy)4] and [Co(NCO)2(4NOpy)4], obtained un-
der similar conditions.
ring. In the crystals of 5a and 5b, on the other hand, there were
no significant short contacts between the aminoxyl centers and
between the aminoxyl center and the R- and â carbons of the
pyridine ring of the neighboring molecules. These differences
in crystal structure for the five complexes appeared as unique
magnetic behavior due to the local and the structural magnetic
anisotropy. The 1:4 complexes behaved as an antiferromagnet
with TN ) 22 K for 1, as a weak antiferromagnet with TN
)
Conclusion
2.9 K and Hsp(1.9) ) 0.7 kOe for 2, and as a canted 2D
antiferromagnet (a weak ferromagnet) showing a hysteresis loop
with Hc ) 1.3 kOe below TN ) 4.7 K for 3. In contrast,
complexes 4, 5a, and 5b, having no intermolecular interaction,
functioned as SMMs. Especially, 5b having bulky tert-butyl
groups at the 2, 4, and 6-position of the phenyl ring, was found
to have a Ueff value for the reversal of the magnetism of 28 K
in the crystalline state and 48 K in frozen solution.
Because the magnetic property for an SMM originates from
one molecule, an SMM molecule is required to be magnetically
isolated in the crystal. However, the 1:4 complexes, [Co(NCO)2-
(4NOpy)4] and [Co(NCS)2(C1py)4], exhibiting SMM behavior
in frozen solution, have strong intermolecular magnetic interac-
tions in the crystalline state. To modulate the crystal structure
of the heterospin systems, this time, 4NOpy was replaced with
the new N-aryl-N-pyridylaminoxyl ligand. With the new ligand,
it is possible to introduce substituents on the phenyl ring and
to modulate the crystal structure after the complexation with
cobalt(II) ion by changing the bulkiness, the number, and the
position of the substituent(s). The spin distributions for new
N-aryl-N-pyridylaminoxyl ligands were confirmed by the DFT
calculation. The 1:4 complexes of cobalt(II) ions with N-aryl-
N-pyridylaminoxyls, [Co(NCS)2(PhNOpy)4] (1), [Co(NCS)2-
(MeOPhNOpy)4] (2), [Co(NCS)2(tert-BuPhNOpy)4] (3), [Co-
(NCS)2(TPPNOpy)4] (4), [Co(NCS)2(TBPNOpy)4] (5a), and
[Co(NCO)2(TBPNOpy)4] (5b) were prepared and the relations
between the crystal structure and the magnetic properties were
investigated. The molecular structure of all five complexes
revealed by X-ray crystallography was a similar compressed
octahedra. However, the crystal structure strongly depended on
the bulkiness, number, and position of the substituent. The spin
centers in complexes 1, 2, and 3 had various short contacts
within 4 Å with the neighboring molecules to form magnetically
the 2D structures. The distance between the 2D planes increased
in the order of 1, 2, and 3, which was consistent with the order
of bulkiness of the substituent at the 4-position of the phenyl
In this study, we could control the magnetic properties of
discrete cobalt complexes in a heterospin system by using crystal
engineering and finally succeeded in demonstrating the magnetic
behavior of monometallic SMM in the crystalline state.
Acknowledgment. S.K. acknowledges a scholarship by the
Japan Society for the Promotion of Science (JSPS). This work
was supported by a Grant-in-Aid for Scientific Research (B)-
(2)(No.17350070) from the Ministry of Education, Science,
Sports, and Culture, Japan, and by the “Nanotechnology Support
Project” of the Ministry of Education, Culture, Sports, Science,
and Technology (MEXT), Japan.
Supporting Information Available: X-ray crystallographic
file for five complexes and TPPNOpy and TBPNOPy, the spin
density distributions for five N-aryl-N-pyridylaminoxyls, and
various structural and magnetism figures as mentioned in the
text. This material is available free of charge via the Internet at
http://pubs.acs.org.
JA0767579
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3094 J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008