Magnetic behavior of a 3:2 mixture of bis(hexafluoroacetylacetonato)copper(II) and 1,3,5-benzenetriyltris(4-pyridyldiazomethane) in a frozen solution after irradiation: Photochemical formation of a solid solution magnet [7]

Full text of DOI:10.1021/ja010936n

J. Am. Chem. Soc. 2001, 123, 9685-9686
Magnetic Behavior of a 3:2 Mixture of
9685
Bis(hexafluoroacetylacetonato)copper(II) and
1,3,5-Benzenetriyltris(4-pyridyldiazomethane) in a
Frozen Solution after Irradiation: Photochemical
Formation of a Solid Solution Magnet
Satoru Karasawa, Harumi Kumada, Noboru Koga,* and
Hiizu Iwamura†
Graduate School of Pharmaceutical Sciences
Kyushu UniVersity, 3-1-1 Maidashi, Higashi-ku
Fukuoka 812-8582, Japan
UniVersity of the Air, 2-11 Wakaba
Mihama-ku, 261-8586 Chiba, Japan
ReceiVed April 12, 2001
Figure 1. M/Ms vs H plots at 2 K after irradiation of 3:2 mixtures of
Cu(hfac)2 and 2 in 5(0), 10 (O), and 20 (4) mM concentrations in frozen
solutions together with the data (3) before irradiation of a 20 mM sample.
The low-field region (<0.5 kOe) is enlarged in Inset.
For the construction of a molecular magnet, it is required to
assemble as many spins as possible in the two and three-
dimensional mesoscopic network and couple them in a ferro- or
ferrimagnetic fashion.1 We have employed a strategy of construct-
ing heterospin systems consisting of 3d metal and free radical 2p
spins.2,3 The 1:1 mixed ligand complexes, [Mn(hfac)2‚1]n and [Cu-
(hfac)2‚1]n, of bis(hexafluoroacetylacetonato)manganese(II) and
copper(II), Mn(hfac)2, and Cu(hfac)2, coordinated with a photo-
responsive magnetic coupler, diazodi(4-pyridyl)methane 1, formed
one-dimensional chains and gave ferri- and ferromagnetic super
high-spin molecules with S ) 317 (at 1.9 K)2b and 34 (at 3.0
K),3a respectively, after generation of the carbene centers by
photolysis of the diazo moieties in the complexes. To increase
the dimension of the spin-networks from 1D to 2D and 3D
structures, two approaches can be considered; one is the use of
coordinatively highly unsaturated metal ion without hfac ligand,
and the other is the design of highly branched base ligand. On
the basis of the latter strategy, 1,3,5-benzentriyltris(4-pyridyldiazo-
methane) 2 was designed and prepared as a novel photoresponsive
magnetic coupler. In this paper,4 we report the magnetic behavior
after photolysis of the 3:2 molar mixture of Cu(hfac)2 and 2 in
frozen solutions.5
4-formylpyridine and then the oxidation with activated manga-
nese(IV) dioxide. The carbonyl groups were converted to the diazo
groups by a standard procedure reported previously.2,3 The
tridiazo-tripyridyl derivative 2 was recrystallized from CHCl3 to
afford red plates. The crystal and molecular structure of 2 was
revealed by X-ray structure analysis.4 Carbene 2c generated by
photolysis of 2 was confirmed to be a ground-state septet by EPR
spectrum and magnetization measurement under the conditions
similar to the ones for the following experiments on the complex.
Each carbene center was also expected to interact ferromagneti-
cally with copper(II) ions through the pyridine rings in the
complex with Cu(hfac)2.3a,c
The magnetic properties before and after photolysis of self-
assemblies consisting of Cu(hfac)2 and 2 were studied by means
of SQUID magneto/susceptometer. In these experiments, 50 µL
of solutions of Cu(hfac)2 in MTHF and 2 in MTHF-CH2Cl2 (4:
1) by 3:2 molar ratio were employed as the sample for SQUID
measurements.6 The light from an argon ion laser (λ ) 514 nm)
was used, and the irradiation of the sample was performed in a
sample room inside SQUID apparatus. Photolysis of the samples
was confirmed to take place quantitatively by taking the difference
of absorptions at 492 nm due to the diazo moieties before and
after SQUID measurements. Saturation magnetization values (Msat)
obtained from the following experiments ranged 92.4-95.7% of
the theoretical ones.
7
Magnetization data, Mmol before and after irradiation of the
3:2 mixtures of Cu(hfac)2 with 2 in 5, 10, and 20 mM
concentrations were measured at 2 and 5 K in the field range
0-10 kOe. Before irradiation, the magnetization data exhibited
a large contribution due to diamagnetic solvents.8 When the
irradiation started, the Mmol values at 5 K gradually developed
and leveled off after 2 h. The Mmol versus H plots at 2 K before
and after irradiation of the 3:2 mixtures in various concentrations
are shown in Figure 1.
Triketone derivative was prepared by trilithiation of 1,3,5-
tribromobenzene with n-butyllithium followed by the reaction with
† University of the Air.
(1) Kahn, O. Molecular Magnetism; VCH Publishers: Weinheim, 1993.
(b) Inoue, K.; Hayamizu, T.; Iwamura, H.; Hashizume, D.; Ohashi, Y. J. Am.
Chem. Soc. 1996, 118, 1803.
The magnetization curves were analyzed to consist at least of
two components, fast and slow saturating fractions (FF and SF,
respectively). The amounts of FF observed in low-field region
(2) (a) Koga, N.; Iwamura, H. Magnetic Properties of Organic Materials;
Lahti, P., Ed.; Marcel Dekker: New York, 1999; Vol. 30, p 629. (b) Karasawa,
S.; Sano, Y.; Akita, T.; Koga, N.; Itoh, T.; Iwamura, H.; Rabu, P.; Drillon,
M. J. Am. Chem. Soc. 1998, 120, 10080 (c) Koga, N.; Ishimaru, Y.; Iwamura,
H. Angew. Chem., Int. Ed. Engl. 1996, 35, 755.
(3) (a) Sano, Y.; Tanaka, M.; Koga, N.; Matsuda, K.; Iwamura, H.; Rabu,
P.; Drillon, M. J. Am. Chem. Soc. 1997, 119, 8246. (b) Koga, N.; Iwamura,
H. Mol. Cryst. Liq. Cryst. 1997, 305, 415. (c) Karasawa, S.; Tanaka, M.;
Koga, N.; Iwamura, H. J. Chem. Soc., Chem. Commun. 1997, 1359.
(4) Partial results in this work have been briefly presented in the 7th ICMM,
San Antonio, TX. Karasawa, S.; Koga, N. Polyhedron 2001. In press.
(5) (a) Morikawa, H.; Imamura, F.; Tsurukami, Y.; Itoh, T.; Kumada H.;
Karasawa, S.; Koga, N.; Iwamura, H., J. Mater. Chem. 2001, 11, 493 (b)
Matsuda, K.; Iwamura, H. J. Am. Chem. Soc. 1997, 119, 7412.
(6) In the UV-vis spectrum of a 3:2 mixture (5 mM) at room temperature,
the shift of the absorption from 492 to ∼480 (shoulder) nm due to n-π*
transition of diazo moiety was observed.
(7) The formula weight of 3:2 complex, [(Cu(hfac)2)3‚22], was used.
(8) The value of ødia was estimated to be -4.76 × 10-4 emu‚Oe from M
vs H plot before irradiation and it was negligibly small (<0.7%) in the
experimental data after irradiation.
10.1021/ja010936n CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/08/2001

9686 J. Am. Chem. Soc., Vol. 123, No. 39, 2001
Communications to the Editor
Figure 2. Temperature dependence of ømolT values after irradiation of a
3:2 mixture (20 mM) of Cu(hfac)2 and 2 in a frozen solution. The inset
shows the enlarged view in the temperature range of 15-30 K.
Figure 3. Temperature dependence of øM′ and øM′′ after irradiation of a
3:2 mixture (20 mM) of Cu(hfac)2 and 2 in a frozen solution with a 1 Oe
ac field oscillating (zero applied dc field) at the various frequencies.
Arrows indicate the øM′ and øM′′ axes.
(<40 Oe) depended on the concentration of the complex; FF
occupied ca. 10, 25, and 70% of the Msat values in 5, 10, and 20
mM concentrations, respectively. Weak hysteresis of M with
respect to H was observed for FF; the coercive force and the
remnant magnetization were ca. 2 Oe and 6.3 × 103 emu‚Oe‚mol-1,
repectively.4,9 Furthermore, it is worth noting that the curvature
of SF seems to be constant (S ) ∼26-30), suggesting that an
assembly having single structure might be mainly produced in
frozen solution.
frequency decreased from 750 to1 Hz. The observed frequency
dependence of øM′ and øM′′ corresponds better to a spin-glasslike
response.13
The magnetic behavior observed in the dc and ac magnetic
susceptibility measurements was reproducible below 24 K.
However, a decrease of the amount of FF in dc and the shift of
peak-top to lower temperature in ac measurements were observed
after annealing the sample in the temperature range 30-40 K.
Both FF and, øM′ and øM′′, completely disappeared at temperatures
greater than 50 K at which temperature the carbene centers start
to decompose chemically. (The changes in the range 30-40 K
are not clear at the present stage).
The magnetic behavior after photolysis of self-assemblies
formed in the frozen solution strongly depends on the structure
of photoresponsive magnetic coupler. For example, no FF was
observed in a combination of Cu(hfac)2 and 1-bromo-3,5-
benzendiylbis(4-pyridyldiazomethane), which has a partial struc-
ture of 2, under the condition similar to this work; they showed
an increase of average S values (8-17) in proportion to the
concentration in the range of 1.0-35 mM.15 Although it is very
difficult to reveal the structure of the self-assemblies formed in
frozen solution, we took advantage of our hetero-spin system and
could successfully construct complexes having spin-glasslike
magnetic properties16 by the simple procedure of mixing the two
components in solutions and freezing them below 30 K followed
by irradiation. The construction of the spin-glass reported here
suggests a new approach to the single-molecule magnets by
employing anisotropic metal ions in place of copper(II) ions.17
Temperature dependence of the molar paramagnetic suscepti-
7
bility, ømol for 20 mM sample in the range of 2-70 K was
measured at constant field of 0.002, 1, and 5 kOe below 15, 15-
30, and above 30 K, respectively. ømolT versus T plots are shown
in Figure 2. The ømolT value at 20 K amounted to 143
emu‚Oe‚mol-1 which was slightly greater than 116 emu‚Oe‚mol-1
for a tetramer of [{Cu(hfac)2}3‚22]. As the temperature was
decreased, the ømolT value gradually increased in the range 20-
12 K, then steeply increased, reached a maximum of 2.79 × 104
emu‚Oe‚mol-1 at 9 K,10 and finally decreased. The observed
gradual and steep developments of ømolT value on cooling from
30 K may indicate the ferromagnetic interaction (30-15 K)
between the carbene centers and copper(II) ions within the
assemblies and the magnetic phase transition (<12 K), respec-
tively.11
Single-molecule magnets12 due to Mn(III, IV), Fe(II, III), Cr-
(III), and V(III) clusters are topics of recent interest. To know if
FF showing the hysteretic properties might be a single-molecule
magnet, ac magnetic susceptibility data for the same sample (20
mM) were collected in the temperature range 4.5-15 K with a 1
Oe ac field oscillating at the frequency of 1-750 Hz with a zero
dc field. As shown in Figure 3, the temperature dependence of
the in-phase and out-of-phase components, øM′ and øM′′, respec-
tively, revealed relatively large round maxima and their peak-
top temperature depended on the frequency of applied ac field;
the temperature of the round maxima for øM′ and øM′′ (8.4-7.6
and 6.2-5.2 K, respectively) decreased, as the field alternation
Acknowledgment. This work was supported by a Grant-in-Aid for
COE Research (No. 08CE2005) from Ministry of Education, Science and
Culture, Japan.
JA010936N
(13) The value of ∆Tf/Tf(0)∆(log w), where ∆Tf is the sift of the peak-
temperature in ø′M, log w is the logarithm of the applied frequency, and Tf(0)
is the position of the peak at zero frequency, obtained from the ø′M vs T plot
(Figure 3) is 0.04. According to Mydosh,14 the value, 0.04, locates in a spin-
glass region.
(14) Mydosh, J. A. Spin Glasses: An Experimental Introduction; Taylor
and Francis: London, 1993.
(15) To be published elsewhere.
(9) The value of the coercive force depends on the temperature; the value
decreased on warming until 10 K.
(10) The maximum value corresponds to 995 units of 3:2 complex of Cu-
(hfac)2 and 2.
(11) The magnetization measurements by a sequence of zero-field cooled
magnetization (ZFC), field-cooled magnetization (FCM), and remnant mag-
netization (RM) also suggested the magnetic phase transition below 10 K.
(12) (a) Aubin, S. M. J.; Wemple, M. W.; Adams, D. M.; Tsai, H. L.;
Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1996, 118, 7746. (b)
Oshio, H.; Hoshino, N.; Ito, T. J. Am. Chem. Soc. 2000, 122, 12602. (c) Barra,
A. L.; Gatteschi, D.; Sessoli, R. Chem. Eur. J. 2000, 6, 1608. (d) Ferlay, S.;
Mallah, T.; Ouahes, R.; Veillet, P.; Verdaguer, M. Inorg. Chem 1999, 38,
229. (e) Castro, S. L.; Sun, Z. M.; Grant, C. M.; Bollinger, J. C.; Hendrickson,
D. N.; Christou, G. J. Am. Chem. Soc. 1998, 120, 2365.
(16) (a) Girtu, M. A.; Wynn, C. M.; Fujita, W.; Awaga, K.; Epstein, A. J.
Phys. ReV. B 2000, 61, 4117. (b) Sellers, S. P.; Korte, B. J.; Fitzgerald, J. P.;
Reiff, W. M.; Yee, G. T. J. Am. Chem. Soc. 1998, 120, 4662. (c) Grrdan, J.
E.; Raju, N. P.; Maignam, A.; Simon, Ch.; Pedersen, J. S.; Niraimathi, A.
M.; Gmelin, E.; Subramanian, M. A. Phys. ReV. B 1996, 54, 7189.
(17) We thank a reviewer of this paper for reminding us of this possibility.
Contrary to the suggested large zero-field splitting in the copper complexes
with basic radical centers, the present Cu(II) complex having the doublet metal
ion with the triplet carbene centers separated by a pyridine ring has smaller
D value (unpublished results).