Synthesis and characterization of a novel chiral molecular-based ferrimagnet prepared from a chiral nitronyl nitroxide radical and manganese(II) ion

Article abstract of DOI:10.1246/bcsj.80.204

A new chiral organic radical, 4,4,5,5-tetramethyl-2-{4-[(S)-2-methylbutoxy] phenyl}-3-oxido-4,5-dihydroimidazol-1-oxyl (1) and its Mn^II complex ([1·Mn^II(hfac)₂]_n (hfac = hexafluoroacetylacetonato)) were synthesi

Full text of DOI:10.1246/bcsj.80.204

204 Bull. Chem. Soc. Jpn. Vol. 80, No. 1, 204–207 (2007)
Ó 2007 The Chemical Society of Japan
Synthesis and Characterization of a Novel Chiral
Molecular-Based Ferrimagnet Prepared from a Chiral
Nitronyl Nitroxide Radical and Manganese(II) Ion
y
Motoko Akita-Tanaka,y Hitoshi Kumagai,yy Ashot Markosyan,yyy and Katsuya Inoue
ꢀ;
Institute for Molecular Science, 38 Nishigounaka, Myoudaiji, Okazaki 444-8585
Received February 28, 2006; E-mail: kxi@hiroshima-u.ac.jp
A new chiral organic radical, 4,4,5,5-tetramethyl-2-{4-[(S)-2-methylbutoxy]phenyl}-3-oxido-4,5-dihydroimidazol-
1-oxyl (1) and its MnII complex ([1 MnII(hfac)2]n (hfac = hexafluoroacetylacetonato)) were synthesized and character-
ꢁ
ized. From single crystal X-ray structure analysis, 1 and [1 MnII(hfac)2]n have a chiral space group (P212121), and
ꢁ
[1 MnII(hfac)2]n has a one-dimensional helical structure. Magnetic susceptibility measurements of [1 MnII(hfac)2]n
ꢁ
ꢁ
indicate that this heterospin system behaves as a ferrimagnet with very weak interchain ferromagnetic exchange inter-
action.
on a Perkin-Elmer Spectrum BX FT-IR system with the samples
as KBr pellets. The temperature and field dependence of the mag-
netization of the complex were performed on a Quantum Design
MPMS-5S SQUID operating in the temperature range of 2–
300 K and fields up to 5 Tesla. AC measurements were performed
in frequency range of 1–10 Hz with AC field amplitude of 3 Oe.
Preparation. The synthetic route to prepare radical 1 is illus-
trated in Scheme 1. Compound 2: N,N0-2,3-dimethylbutane-2,3-
diylbishydroxylamine was condensed with p-[(S)-2-methylbut-
oxy]benzaldehyde (3) in methanol. Mp 147.2–150.9 ꢂC, MS
(m=z ¼ 322), HRMS m=z found 322.2247 calcd for C18H30N2O3
322.2256, 1H NMR (270 MHz, CDCl3) ꢁ 7.40 (d, 2H), 6.88 (d,
2H), 4.90 (brs, 2H), 4.70 (s, 1H), 3.9–3.6 (m, 2H), 1.95–1.75
(m, 1H), 1.4–1.2 (m, 2H), 1.16 (s, 12H), 1.02 (s, 3H), 0.94 (s, 3H).
Compound 1: Compound 2 was oxidized with NaIO4/water in
dichloromethane at 0 ꢂC and purified by column chromatography
on silica gel eluted with ether. Mp 84.2 ꢂC, MS (m=z ¼ 319),
HRMS m=z found 319.2006 calcd for C18H27N2O3 319.2022,
EPR (9.4522 GHz, hexane) g ¼ 2:0058, aN ¼ 7:63 G; UV–vis
(hexane) ꢂmaxð"Þ 297 (8180) nm, 366 (9900) nm, 634 (550) nm,
There has been considerable interest in synthesizing organ-
ic–inorganic hybrid materials due to their intriguing structural
diversity and potential dual or multiple functions, such as su-
perconductivity, non-linear optical activity, and magnetism.1–5
Rational synthesis of 1D-, 2D-, and 3D-coordination network
with appropriate bridging ligands is fundamental in developing
these materials. A strategy using ꢀ-conjugated polyaminoxyl
radicals with high-spin ground states as bridging ligands for
magnetic metal ions to assemble and align the electron spins
on a macroscopic scale has been reported.6–8 The crystal struc-
tures and magnetic structures of these complexes can be tuned
using this strategy.9 The majority of these materials are based
on achiral organic radicals and the resulting complexes form
achiral crystals. We have reported the crystal structure and
magnetic properties of MnII complex made up of a chiral di-
radical.10 The synthesis and study of chiral molecule-based
magnets, which are transparent to light, are of great inter-
est.3,11–13 Although novel properties are expected for such
compounds, few examples are still known.14,15 To expand our
chemistry, we designed and synthesized a new chiral organic
radical, 4,4,5,5-tetramethyl-2-{4-[(S)-2-methylbutoxy]phenyl}-
3-oxido-4,5-dihydroimidazol-1-oxyl (1), which can be used to
construct chiral molecule-based magnets. This work concerns
with synthesis and characterization of a novel 1D chiral com-
plex made up of new chiral organic radical 1 and MnII ion.
30
½ꢃꢃD ¼ 323ꢂ (c 0.0124, hexane). ꢄB value (300 K) 1.74 ꢄB.
1)n-BuLi
OH
Br
O
CH3
*
2)DMF
3)H+
TsO
CH3
*
KOH
THF
-78 °C
THF
CHO
Experimental
3
All chemicals were obtained from Wako and Tokyo Kasei Co.,
Ltd. and used without purification. Solvents were distilled under
nitrogen atmosphere before use. Infrared spectra were recorded
O
CH3
O
CH3
NHOH
NHOH
*
*
NaIO4
CH2Cl2/H2O
y Present address: Graduate School of Science, Hiroshima
University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526
MeOH
HO
OH
O
O
N
N
N
N
yy Present address: Toyota Central R&D Labs. Inc., 41-1
Yokomichi, Nagakute, Aichi-gun, Aichi 480-1131
2
1
yyy Present address: M.V. Lomonosov Moscow State University,
119899 Moscow, Russia
Scheme 1.
Published on the web January 12, 2007; doi:10.1246/bcsj.80.204

M. Akita-Tanaka et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 1 (2007)
205
Preparation of [1 MnII(hfac) ] . Chiral radical 1 was mixed
Á
2
n
with an equimolar amount of dehydrated Mn(hfac)2 in diethyl
ether/heptane. The mixture was evaporated to ca. 10 mL and
green block crystals were obtained at ꢄ30 ꢂC within 1 week.
Mp 141.6–143.6 ꢂC (decomp.), Anal. Found: C, 42.19; H, 3.37;
N, 3.47%, Calcd for C28H29F12MnN2O7: C, 42.65; H, 3.71; N,
3.55%, EPR (9.4522 GHz, polycrystalline sample) g ¼ 2:0100,
UV–vis (hexane) ꢂmaxð"Þ 300 (13600) nm, 366 (5200) nm, 635
30
(1100) nm, ½ꢃꢃD ¼ 1890ꢂ (c 0.0200, hexane).
X-ray Crystallography and Structure Solution. Selected
single crystals were glued on the tip of glass fibers. The dif-
fractometer is equipped with a graphite monochromated Mo Kꢃ
˚
(0.7107 A) radiation. The data were corrected for Lorentz and po-
larization effects. The structure was solved by direct methods and
expanded using Fourier techniques. The non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were placed at the calcu-
lated ideal positions. The final cycle of full-matrix least-squares
refinement was based on the number of observed reflections and
n variable parameters. They converged (large parameter shift
was ꢅ times its e.s.d.) with agreement factors of R ¼ ꢀjjFoj ꢄ
Fig. 1. A view of the helical chain formed by the chiral
nitronyl nitroxide 1 and MnII(hfac)2. Hydrogen atoms
are omitted for clarity.
interchain contact between the MnII ion and the oxygen atom
2
2
jFcjj=ꢀjFoj, Rw ¼ ½ꢀwðjFoj ꢄ jFcjÞ =ꢀwjFoj ꢃ1=2. No extinction
II
II
˚
of 1 is 9.2 A, while the shortest Mn –Mn interchain distance
corrections were applied.
II
˚
is 10.7 A. The radicals are bound to a Mn ion in a cis-fashion
X-ray structure analysis for 1: Reflection intensities were mea-
sured at ꢄ100 ꢂC on a Rigaku RAXIS-IV imaging plate area
detector. Crystal data for 1; C18H27N2O3, Mw ¼ 319:43, crystal
to each other. A detailed description of the coordination sphere
of MnII must take into account the possible configurations re-
sulting from the cis-coordination arrangement, which can lead
to ꢁ or ꢂ configurations. In this complex, the metal centers all
exhibit a ꢁ configuration. The absolute configuration of a met-
al center is often affected by chirality of the organic ligand,
and our results show a similar effect from the chiral carbon
atom of 1.10 Since no inversion centers are present in this space
group, the chains are completely isotactic, and the crystal
3
˚
size, 0:20 ꢅ 0:10 ꢅ 0:05 mm ; Orthorhombic, a ¼ 11:494ð3Þ A,
3
˚
˚
˚
b ¼ 25:328ð3Þ A, c ¼ 6:1281ð5Þ A, V ¼ 1748:0ð4Þ A , Space
group P212121 (No. 19), Z ¼ 4, Dcalcd ¼ 1:189 g cmꢄ3, absorption
coefficient, 0.81 cmꢄ1
(Rw) = 0.067 (0.083), parameters, 208; maximum and minimum
;
Reflections, 1389 (I > 2:00ꢅðIÞ);
R
ꢄ
3
ꢄ
3
˚
˚
residual electron densities, 0.30 e /A and ꢄ0:41 e /A ; respec-
tively. The crystal data have been deposited at CCDC, Cambridge,
UK and given the reference numbers CCDC 284812.
lattice as a whole is chiral. The UV–vis spectrum of [1 MnII-
ꢁ
X-ray structure analysis for [1 MnII(hfac)2]n: Reflection
ꢁ
(hfac)2]n were measured in hexane solution and the crystalline
state in KBr disk. Both spectra exhibit absorption around 300,
365, and 635 nm. They are very similar to each other and dif-
ferent from that of the free radical 1. This results indicate that
the interaction between 1 and MnII(hfac)2 in the complex is re-
intensities were measured at 25 ꢂC with ! scan mode on a
Bruker SMART CCD diffractometer. Crystal data for complex
[1 MnII(hfac)2]n; C28H29F12MnN2O7, Mw ¼ 788:47, crystal size,
ꢁ
3
˚
0:46 ꢅ 0:23 ꢅ 0:22 mm ; Orthorhombic, a ¼ 14:081ð1Þ A, b ¼
3
˚
˚
˚
15:940ð1Þ A, c ¼ 16:075ð1Þ A, V ¼ 3608:1ð4Þ A , Space group
P212121 (No. 19), Z ¼ 4, Dcalcd ¼ 1:452 g cmꢄ3, absorption coef-
ficient, 4.71 cmꢄ1; Reflections, 8695 (I > 2:00ꢅðIÞ); R1 (wR2) =
0.0690 (0.1692). Absolute structure parameter, ꢄ0:03ð3Þ; param-
eters, 4ꢄ51; maximum and minimum residual electron densities,
tained in hexane. The solution of [1 MnII(hfac)2]n in hexane
ꢁ
exhibits optical rotation, which indicates that [1 MnII(hfac)2]n
is chiral in solution.
ꢁ
The temperature dependence of ꢆT of a microcrystalline
sample of [1 MnII(hfac)2]n is shown in Fig. 2. At 300 K, ꢆT ¼
3
ꢄ
3
˚
˚
0.477 e /A and ꢄ0:393 e /A ; respectively. The crystal data
have been deposited at CCDC, Cambridge, UK and given the
reference numbers CCDC 284813. Copies of the data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; Fax: +44
1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
ꢁ
5:11 emu K molꢄ1, which is larger than the theoretical value
(4.75 emu K molꢄ1) for isolated spins of organic radicals and
MnII ions, and it increases monotonically with decreasing tem-
perature. In the paramagnetic range, the complex was treated
as a heterospin 1D (ꢁ ꢁ ꢁ–1/2–5/2–ꢁ ꢁ ꢁ)-chain compound, and
a fit to the experimental data was made using the Heisenberg
quantum-classical approximation: H ¼ JꢀðSi þ Siþ1ÞSi. For
a quantum spin 1/2 and a classical spin 5/2, the expression
for ꢆT.
Results and Discussion
From the X-ray crystal structure analysis, both 1 and
[1 MnII(hfac)2]n crystallize in the same chiral space group
ꢀ
ꢁ
Nꢄ2B
2
ꢁ
ðꢆTÞCh
¼
38 þ
ð25P ꢄ 10Q þ Q2Þ
ð1Þ
P212121 (No. 19). The molecular structure of [1 MnII(hfac)2]n
ꢁ
3k
1 ꢄ P
is depicted in Fig. 1. The asymmetric unit of [1 MnII(hfac)2]n
ꢁ
where
consists of one MnII ion, two hfac anions, and one chiral radi-
cal 1. The MnII ion has an octahedral coordination environ-
ment with four oxygen atoms of two hfac anions and two oxy-
gen atoms of different nitronyl nitroxide molecules. As the re-
sult, MnII and 1 form a 1D chain along the a-axis. The shortest
1 þ 2 cosh ꢇ
1 ꢄ cosh ꢇ þ ꢇ sinh ꢇ
P ¼ 1 þ 12ꢇꢄ2 ꢄ 2
;
cosh ꢇ
ꢇꢄ1 ꢄ ꢇꢄ1 cosh ꢇ þ sinh ꢇ
J
16
Q ¼
and ꢇ ¼ 2:5
:
kT

206 Bull. Chem. Soc. Jpn. Vol. 80, No. 1 (2007)
Synthesis of a Chiral Molecular-Based Magnet
Fig. 3. The temperature dependence of the effective mag-
netic moment of [1 MnII(hfac)2]n.
ꢁ
Fig. 2. The temperature dependence of the product ꢆT
for the complex [1 MnII(hfac)2]n. The inset shows field-
cooled (FC) and zero-field-cooled (ZFC) magnetization
versus T plots.
ꢁ
H is the external field and Sn is the unspecified spin value. In
the fits of Su, the value of the total spin per f.u. was 2 ꢄB.
Hence Sn can be used to determine the average length the cor-
related spins make up at a given temperature. The inset in
Fig. 3 shows the temperature variation of the number of the
correlated units n ¼ Sn=Su. Down to 15 K, it can be approxi-
A weak mean field correction was introduced in order to take
into account the interchain exchange according to the expres-
ꢄ1
sion ꢆT ¼ ½ðꢆTÞ ꢄ ꢂ=Tꢃꢄ1 where ꢂ refers to the interchain
Ch
mated as n ¼ 1 þ 250Tꢄ4=3
.
exchange. In Fig. 2, the solid line is the least squares fit of this
equation to the experimental data in the range of 15–300 K.
The fit parameters that were obtained are: J=k ¼ ꢄ435 ꢆ 5 K
and ꢂ0 ¼ ðzJ0=kÞ ¼ ꢄ0:0275 ꢆ 0:005 K. J0 is interchain mag-
netic exchange interaction parameter. If the number of nearest
neighbors for a given chain z ¼ 4, one obtains J0=J ¼ 1:5 ꢅ
In summary, a novel chiral nitronyl nitroxide radical and its
MnII complex were prepared, and a chiral one-dimensional hy-
brid chain was realized. The complex has a helical-chain struc-
ture and is characterized by an (S) chiral carbon center along
with a ꢁ configuration around the MnII ion. From magnetic
susceptibility measurements, this chiral heterospin complex
behaves as a ferrimagnetic one-dimensional chain system with
an extremely low ratio between the intrachain and interchain
exchange parameters (J0=J ¼ 1:5 ꢅ 10ꢄ5).
10ꢄ5. Hence, [1 MnII(hfac)2]n is characterized by low dimen-
ꢁ
sionality along with a strong negative (ferrimagnetic) metal–
radical exchange interaction.
The field-cooled (FC) and zero-field-cooled (ZFC) magnet-
izations were traced at a low field of 5 Oe over the temperature
range from 1.8 to 20 K. The magnetization signal of both the
traces decreased sharply with increasing temperature. The in-
set in Fig. 2 shows the difference between the two signals
and points to a possible magnetic ordering at ca. 4.5 K. The
magnetic measurements and X-ray crystal structure analysis
both indicate that the interaction between the chains is expect-
We thank Prof. Dr. Hideaki Kanno in Shizuoka University
for the measurement of optical rotation. We thank Dr. Kenji
Yoza and Mr. Shinetsu Igarashi in Bruker Japan Co., Ltd.
for the measurement of X-ray crystallography. This research
was supported by a Grant-in-Aid for scientific research from
the Ministry of Education, Culture, Sports, Science and Tech-
nology. We thank JSPS for the postdoctoral fellowship given
to H.K.
ed to be ferromagnetic, making [1 MnII(hfac)2]n complex,
ꢁ
which is an assembly of chiral one-dimensional chains, a
molecule based ferrimagnet with J0=k > 0. AC susceptibility
measurements also revealed a maximum of ꢆ at TC ꢇ 4:5 K,
which is characteristic of a ferrimagnetic ordering. It should
be noted that the J0=k value evaluated from the fit to the para-
magnetic susceptibility data (Fig. 2) is negative. The positive
interchain exchange interaction obtained from the low-temper-
ature measurements can be ascribed to factors that were not
included in the model: effects of thermal expansion and the
contribution from the positive dipole–dipole interactions that
are important at low temperatures as well as neglect of the
next-nearest neighbor exchange interactions.
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