A homometallic ferrimagnet based on mixed antiferromagnetic and ferromagnetic interactions through oxamato and carboxylato bridges

Article abstract of DOI:10.1246/cl.2008.64

Two novel coordination polymers (CPs), {Cu[Cu(L)(H₂O)-(py)] ₂}_n (1) and {Cu[Cu(L)(MeOH)]₂·2H ₂O}_n (2), were synthesized using 2-(oxaloamino)benzoic acid (H₃L) providing oxamate and carboxylate groups, and magnetically characterized. Different 2-D layer structures were formed based on oxamato-bridged tricopper units bridging via the carboxylate group. In compound 1, a simple antiferromagnetic interaction operates through the oxamato bridge in the tricopper core, whereas, 2 showed a tendency for ferrimagnetic ordering at 2 K, resulting from mixed antiferromagnetic and ferromagnetic interactions through oxamato and syn-anti-type carboxylato bridges. Copyright

Full text of DOI:10.1246/cl.2008.64

64
Chemistry Letters Vol.37, No.1 (2008)
A Homometallic Ferrimagnet Based on Mixed Antiferromagnetic and Ferromagnetic
Interactions through Oxamato and Carboxylato Bridges
Ko Yoneda, Yukari Hori, Masaaki Ohba,ꢀ and Susumu Kitagawaꢀ
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510
(Received October 29, 2007; CL-071193; E-mail: ohba@sbchem.kyoto-u.ac.jp)
Two novel coordination polymers (CPs), {Cu[Cu(L)(H2O)-
copper (Cu3) core with L3ꢁ
.
.
(py)]2}n (1) and {Cu[Cu(L)(MeOH)]2 2H2O}n (2), were synthe-
sized using 2-(oxaloamino)benzoic acid (H3L) providing oxa-
mate and carboxylate groups, and magnetically characterized.
Different 2-D layer structures were formed based on oxamato-
bridged tricopper units bridging via the carboxylate group. In
compound 1, a simple antiferromagnetic interaction operates
through the oxamato bridge in the tricopper core, whereas, 2
showed a tendency for ferrimagnetic ordering at 2 K, resulting
from mixed antiferromagnetic and ferromagnetic interactions
through oxamato and syn–anti-type carboxylato bridges.
H3L was prepared as the ethyl ester by the reaction of an-
thranilic acid and excess diethyloxalate upon heating in EtOH.
{Cu[Cu(L)(H2O)(py)]2}n (1) was prepared by the reaction of
.
Cu(CH3COO)2 H2O, H3L, and pyridine in 3:2:20 mole ratio
in H2O/MeOH (1:1 v/v). Greenish blue crystals of 1 appeared
after allowing the reaction mixture to stand at room temperature
.
(rt) for several days. {Cu[Cu(L)(MeOH)]2 2H2O}n (2) was ob-
tained as green crystals by slow reaction of Cu(CH3COO)2 H2O
.
and H3L (3:2) in H2O/MeOH (1:1 v/v). ORTEP drawings
of 1 and 2 are shown in Figure 1.5 In both cases, the ligand
L3ꢁ coordinates to Cu1 with the tridentate NO2 site to afford a
[Cu1(L)]ꢁ unit, and two [Cu1(L)]ꢁ units coordinate to Cu2 af-
fording an oxamato-bridged Cu3 core. In the case of 1, pyridine
coordinates to the equatorial position of Cu1 as a terminal li-
gand; on the other hand, the carboxylate oxygen atom of the
nearest Cu3 unit (O2#6: symmetry operations are listed in
Ref. 5) occupies the same position in 2. The geometry about
Cu1 is square pyramidal with the NO2 site of L3ꢁ and nitrogen
of pyridine for 1 or carboxylate oxygen for 2 in the equatorial
plane, and an axial water for 1 or MeOH for 2. Cu2 of 1 is locat-
ed on the inversion center and has an elongated octahedral
geometry consisting of two oxamato groups of L3ꢁ (O3, O5,
O3#1, O5#1) and two axial carboxylate oxygen atoms of the
adjacent Cu3 cores (O2#2 and O2#3) with bond distance of
Coordination polymers have attracted much attention in the
past decade because of their unique structures and properties.
The CPs’ frameworks show a rich diversity due to the combina-
tion of various components (metal ion, bridging ligand, coli-
gand, etc.), which makes it possible to tune the CPs’ frameworks
and properties.1–3 Consequently, CPs are expected to be a plat-
form to deliver multiple functions. From this aspect, it is one
of the rational strategies to incorporate physical properties,
e.g., magnetic,2 dielectric properties,3 etc., in the CPs’ frame-
works. Here, the design and construction of sensible frameworks
providing target properties are the principal significant challeng-
es. To construct a new magnetic framework, we selected a bridg-
ing ligand, 2-(oxaloamino)benzoic acid (H3L; Scheme 1), hav-
ing oxamate and carboxylate groups. H3L can form an oxama-
to-bridged trinuclear complex, MII[MII(L)]2, which contains
mononuclear complexes, [MII(L)]ꢁ. Furthermore, the trinuclear
complex provides coordinatable sites and can form a self-assem-
bled structure because of the terminal carboxylate groups. Be-
cause the carboxylate groups bridge the terminal MII ions in a
syn–anti style, which mediates a ferromagnetic interaction4
due to steric demand, the coexistence of oxamato and carboxy-
lato bridges is expected to exhibit a unique magnetic behavior
resulting from the mixed antiferromagnetic and ferromagnetic
interactions. Here, we report the synthesis, crystal structures,
and magnetic properties of new CPs, {Cu[Cu(L)(H2O)(py)]2}n
˚
2.491(4) A. Cu2 of 2 is in a square planar environment with
two oxamate groups (O3, O5, O3#5, O5#5).
In the lattice, 1 forms a 2-D sheet structure on the bc plane
through the carboxylato bridges between the equatorial position
of the terminal Cu1 and the adjacent axial position of the central
Cu2. The 2D sheets are linked through complementary hydrogen
bonds between the carboxylate oxygen O2 and the axial water on
(a)
(b)
.
(1) and {Cu[Cu(L)(MeOH)]2 2H2O}n (2), which consist of a tri-
(c)
(d)
H3L
{Cu[Cu(L)]2} core
O
O
O
N
O
O
O
O
N
O
O
Cu1
Cu2
Cu1'
O
NH
O
OH
b
b
O
O
c
c
HO
Figure 1. Tricopper core structures of 1 (a) and 2 (b) and projections of
the 2D sheet structures of 1 (c) and 2 (d) onto the bc plane.
Scheme 1.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.1 (2008)
65
Cu3 core. The observation that saturation occurs faster than
the theoretical curve based on the Brillouin function for
S ¼ 1=2 confirms the magnetic ordering. In the light of these
magnetic data of 2, its magnetic behavior is classified to ‘‘ferri-
magnetic’’ based on antiferromagnetic coupling between the
syn–anti-type carboxylato-bridged ferromagnetic chain and the
central Cu2 through the oxamato bridge. Because cusps were
not observed in the ac magnetic response down to 2 K, a
complete ferrimagnetic ordering is expected below 2 K.
9
1.2
0.9
β
6
0.6
0.3
3
We have successfully prepared two new CPs using 2-(oxa-
loamino)benzoic acid as a bridging unit. Compounds 1 and 2
formed different types of 2-D layer structures consisting of oxa-
mato-bridged Cu3 cores. These structures were derived by the
use of the terminal pyridine ligand. In the case of 1, the central
CuII in the Cu3 core was linked with the terminal CuII in the
nearest Cu3 core through carboxylato bridges, whereas, the 2-
D layer structure of 2 was constructed by linking the terminal
CuII through syn–anti-type carboxylato bridges. Compound 1
showed a relatively strong antiferromagnetic interaction (J ¼
ꢁ142 cmꢁ1) through the oxamato bridge as a magnetically iso-
lated Cu3 core. On the other hand, 2 demonstrated a tendency to-
wards ferrimagnetic ordering around 2 K, in which the residual
spin in the antiferromagnetically coupled Cu3 core was aligned
by a weak ferromagnetic interaction through the syn–anti-type
carboxylato bridge. This is a rare homometallic ferrimagnetic
system9 based on an odd-numbered metal unit with mixed anti-
ferromagnetic and ferromagnetic interactions.
χ
0.0
0
10
20
30
40
50
H / kOe
0
0
100
200
300
T / K
Figure 2. ꢁMT vs. T plots of 1 ( ) and 2 ( ). Insert: Field dependence of
magnetization of 1 ( ) and 2 ( ) at 2 K. The solid lines are the Brillouin
function for S ¼ 1=2 with g ¼ 2:00 (1) and g ¼ 2:11 (2).
#4
˚
Cu1 (O6 ) with a distance of 2.805 A, the nearest intersheet
CuꢂꢂꢂCu distance being Cu1ꢂꢂꢂCu1#4 with a value of 5.707 A. In
˚
contrast, compound 2 forms a different 2-D sheet structure from
1 extended on the (101) plane, where the Cu3 cores are connect-
ed to each other by the syn–anti-type carboxylato bridge be-
tween terminal Cu1 atoms, complementary hydrogen bonds be-
tween oxamate oxygen O5 and the axial MeOH on Cu1 (O6#8),
and ꢀꢂꢂꢂꢀ contacts (e.g., C8ꢂꢂꢂC6#9 = 3.440 A, C9ꢂꢂꢂC1
=
#9
˚
˚
3.473 A), and forms a 1-D 21 helical arrangement of Cu1 atoms
along the b axis. The helical chains are linked by oxamato
bridges forming a 2-D sheet structure. The nearest intersheet
This work was supported by a Grant-in-Aid for Young
Scientists (B) (No. 18750046) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan and CREST/
JST, Japan. K. Y. is grateful to JSPS Research Fellowships for
Young Scientists.
CuꢂꢂꢂCu distance is Cu1ꢂꢂꢂCu1#7 with a value of 7.316 A.
˚
The temperature dependences of ꢁMT per Cu3 unit for 1 and
2 are shown in Figure 2. In the case of complex 1, the value of
ꢁMT at 300 K was 0.719 emuꢂKꢂmolꢁ1 (2.40 ꢂB), decreased
with decreasing temperature and reached a plateau (0.333
emuꢂKꢂmolꢁ1; 1.63 ꢂB), which is a characteristic behavior of a
References and Notes
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discrete antiferromagnetically coupled tricopper complex.6,7
A
2
W. Kaneko, M. Ohba, S. Kitagawa, J. Am. Chem. Soc. 2007, 129, 13706; N.
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˜
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least-squares fitting of the magnetic behavior based on the equa-
tion for S ¼ 1=2 þ 1=2 þ 1=2 system with Heisenberg Hamilto-
Cu2Þ ꢁ 2J0ðSCu1
S
Cu1 Þ) was
0
0
nian (H ¼ ꢁ2JðSCu1
SCu2 þ SCu1
ꢂ
S
ꢂ
ꢂ
performed, where the magnetic interaction between Cu2 and
Cu1#2 in the next Cu3 unit through the carboxylato bridge was
treated as an overall intermolecular interaction ꢃ. The best-fit pa-
rameters obtained for 1 were J ¼ ꢁ142 cmꢁ1, J0 ¼ 0:00, g ¼
2:00, and ꢃ ¼ ꢁ0:56 K, which suggests that an antiferromagnet-
ic interaction through the oxamato bridge dominantly operated,
and the interaction between Cu2 and Cu1#2 was negligible be-
cause of a low overlap integral of the magnetic orbitals between
the equatorial and axial planes of CuII.6 The magnetic behavior
of 2 was notably different from that of 1. The value of ꢁMT grad-
ually decreased with decreasing temperature from 0.839
emuꢂKꢂmolꢁ1 (2.59 ꢂB) at rt to 0.490 emuꢂKꢂmolꢁ1 (1.98 ꢂB)
at 44 K. Upon further cooling, the ꢁMT sharply increased to a
maximum value of 8.87 emuꢂKꢂmolꢁ1 (8.42 ꢂB) at 2 K, which
suggests a tendency to magnetic ordering. The temperature
dependences of the ac magnetic susceptibility and weak-field
magnetization also support an onset of magnetic ordering.8
The M vs. H curve of 2 showed a sharp increase with a
value for the saturation magnetization (Ms) of 1.10 Nꢄ at
50 kOe. The value of Ms means that one spin remains in the
¯
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4
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´
Crystal data for 1: C28H22N4Cu3O10, T ¼ 223 K, fw 765.14, monoclinic, C2=c,
ꢃ
˚
Z ¼ 4, a ¼ 19:294ð2Þ, b ¼ 10:2228ð9Þ, c ¼ 15:446ð1Þ A, ꢄ ¼ 112:785ð1Þ ,
V ¼ 2808:9ð5Þ A , ꢂ(Mo Kꢅ) = 23.28 cmꢁ1, R1 ¼ 0:0584, wR2 ¼ 0:133 (all
3
˚
data, F2), G.O.F. ¼ 1:145. CCDC 664979; for 2: C18H8N2Cu3O10, T ¼
183 K, fw 602.9, monoclinic, P21=n, Z ¼ 2, a ¼ 10:8891ð7Þ, b ¼ 6:1482ð8Þ,
c ¼ 17:437ð2Þ A, ꢄ ¼ 98:640ð2Þꢃ, V ¼ 1154:1ð2Þ A , ꢂ(Mo Kꢅ) = 27.93
3
˚
˚
cmꢁ1
.
R1 ¼ 0:0461, wR2 ¼ 0:143 (all data, F2), G.O.F. ¼ 1:001. CCDC
664980. Symmetry operations: #1: ꢁx þ 3=2, ꢁy þ 1=2, ꢁz þ 1; #2: x, ꢁy,
z ꢁ 1=2; #3: ꢁx þ 3=2, y þ 1=2, ꢁz þ 1=2; #4: ꢁx þ 2, ꢁy, ꢁz þ 2; #5: ꢁx,
ꢁy, ꢁz; #6: ꢁx ꢁ 1=2, y ꢁ 1=2, ꢁz þ 1=2; #7: ꢁx ꢁ 1, ꢁy þ 1, ꢁz; #8: x,
y ꢁ 1, z; #9: ꢁx, 1 ꢁ y, ꢁz.
6
7
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¯
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8
9
Published on the web (Advance View) December 15, 2007; doi:10.1246/cl.2008.64