Tuning the structure and magnetism of azido-mediated CuII systems by coligand modifications

Article abstract of DOI:10.1021/ic802066a

Through the use of a series of structurally related benzoates bearing different substituents as coligands, three new azido-copper compounds, [Cu(benzoate)(N₃)]_n (1), [Cu(2-methyl-benzoate)(N ₃)]_n (2), and [Cu(1-

Full text of DOI:10.1021/ic802066a

Inorg. Chem. 2009, 48, 2482-2489
Tuning the Structure and Magnetism of Azido-Mediated CuII Systems by
Coligand Modifications
Jiong-Peng Zhao,† Bo-Wen Hu,† E. C. San˜udo,‡ Qian Yang,† Yong-Fei Zeng,† and Xian-He Bu*,†
Department of Chemistry, Nankai UniVersity, Tianjin 300071, P. R. China, and Departament de
Qu´ımica Inorga`nica, UniVersitat de Barcelona, Diagonal, 647, 08028-Barcelona, Spain
Received October 27, 2008
Through the use of a series of structurally related benzoates bearing different substituents as coligands, three new
azido-copper compounds, [Cu(benzoate)(N3)]n (1), [Cu(2-methyl-benzoate)(N3)]n (2), and [Cu(1-naphthoate)(N3)]n
(3), have been successfully obtained and structurally and magnetically characterized. Single-crystal structure analyses
indicated that the uncoordinating substituents in the benzoates greatly affect the structure of the complexes. Complex
1 displays isolated ferromagnetic chains with the largest Cu-N-Cu angle in known carboxylate/end-on-azido
mixed-bridged copper systems, while complexes 2 and 3 were 2D coordination polymers, containing µ-(1,1,3,3)
and µ-(1,1,3) bridging azides and exhibiting new azido-copper networks with (44) and (4 · 82) topologies, respectively.
Furthermore, 2 was a chiral complex obtained through spontaneous resolution. In the low-temperature range, both
2 and 3 showed spontaneous magnetization with characteristics of soft ferromagnetic magnetism with phase transition
temperatures of 13 and 10 K, respectively.
Chart 1
Introduction
The azido ligand is able to link metal ions in different
coordination modes, which induces rich structural diversity
as well as a range of different magnetic properties in the
azido-metal complexes.1-6 The azido-based CuII coordina-
azido-metal complexes.7,8 End-on (EO) and end-to-end (EE)
tion polymers are among the most important kinds of
are two typical bridging modes for the azido linker in
azido-metal systems, but the azido ligand can bridge more
* To whom correspondence should be addressed. Fax: +86-22-23502458.
than two metals in a combination of EE and EO modes
(Chart 1). All of the bridging modes can be symmetric or
asymmetric and with or without the presence of coli-
Tel: +86-22-23502809. E-mail: buxh@nankai.edu.cn.
†
Nankai University.
Universitat de Barcelona.
‡
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2482 Inorganic Chemistry, Vol. 48, No. 6, 2009
10.1021/ic802066a CCC: $40.75  2009 American Chemical Society
Published on Web 02/09/2009

Azido-Mediated CuII Systems
Chart 2
Herein, we report the synthesis, structural characterization,
and magnetic properties of three new azido-Cu coordination
polymers, [Cu(L1)(N3)]n (1), [Cu(L2)(N3)]n (2), and [Cu(L3)-
(N3)]n (3). Interestingly, our results indicate that altering the
substitutional groups of coligands greatly influences the
structure of the resulting complexes, which in turn results
in different magnetic behaviors. Complex 1 consists of
isolated one-dimensional (1D) ferromagnetic chains with a
large Cu-N-Cu angle of 126.8°, which to the best of our
knowledge is the largest in known carboxylate/EO-azido
mixed-bridged copper compounds,17a while 2 and 3 are two-
dimensional (2D) ferromagnetic networks exhibiting new
azido-copper structures with (44) and (4 · 82) topology,
respectively. Additionally, complex 2 is a chiral complex
obtained through spontaneous resolution.
gands.9-11 It is well-established that the EE modes usually
propagate antiferromagnetic while the EO modes are usually
ferromagnetic, although the coupling between metal ions
bridged by EO azido ligands can be antiferromagnetic in the
presence of other bridging ligands or for very large
metal-N-metal angles.12 Several structural parameters,
especially the Cu-N-Cu angle, affect the superexchange
mechanism.13,14 Modifying the coligand would also influence
the geometry of the azido-metal moiety and would thus have
an effect on the magnetic properties of the complexes.2
Although azido can mediate strong coupling between CuII
ions, there are still few examples that exhibit long-range
ordering phenomena.15 The common strategy for the en-
hancement of bulk magnetic properties is introducing a
second neutral organic linker such as amine, bipyridine, or
bisimidazole.16 Recently, three-dimensional (3D) azido-metal
complexes were reported by employing a negatively charged
pyridyl carboxylate as a coligand, which resulted in a novel
topology of the materials and enhancement of the bulk
magnetic properties of the azido-metal systems, with some
of them exhibiting ferromagnetic ordering.15a,17 In this work,
we investigate the use of benzoate and substituted benzoates
(Chart 2) as coligands to azido-Cu complex systems. With
this modification in the coligand sustituents, we expect to
be able to tune the structure and magnetic properties of the
resulting azido-Cu systems.
Experimental Section
Materials. All of the chemicals used for synthesis are of
analytical grade and are commercially available. Cu(NO3)3 ·3H2O,
sodium hydroxide, sodium benzoate, 2-methyl-benzoic acid, R-naph-
thalic acid, and sodium azide were purchased from commercial
sources and used as received.
Caution! Azide-metal complexes are potentially explosiVe; only
a small amount of material should be prepared and with care.
Physical Measurements. Elemental analyses (C, H, N) were
performed on a Heraeus CHN-Rapid elemental analyzer (at Institute
of Chemistry, CAS). IR spectra were measured on a Tensor 27
OPUS (Bruker) FT-IR spectrometer with KBr pellets. The X-ray
powder diffraction (XRPD) was recorded on a Rigaku D/Max-2500
diffractometer at 40 kV and 100 mA using a Cu-target tube and a
graphite monochromator. Simulation of the XRPD spectra was
carried out using the single-crystal data and diffraction-crystal
module of the Mercury (Hg) program, available free of charge via
the Internet at http://www.iucr.org.
Magnetic data were collected using crushed crystals of the sample
on a Quantum Design MPMS-XL SQUID magnetometer equipped
with a 5T magnet. The data were corrected using Pascal’s constants
to calculate the diamagnetic susceptibility, and an experimental
correction for the sample holder was applied.
(10) (a) Tangoulis, V.; Panagoulis, D.; Raptopoulou, C. P.; Samara, C. D.
Dalton Trans. 2008, 1752. (b) Han, Y.-F.; Wang, T.-W.; Song, Y.;
Shen, Z.; You, X.-Z. Inorg. Chem. Commun. 2008, 11, 207. (c) Feng,
P. L.; Beedle, C. C.; Wernsdorfer, W.; Koo, C.; Nakano, M.; Hill, S.;
Hendrickson, D. N. Inorg. Chem. 2007, 46, 8126.
Synthesis. Single crystals of complexes 1-3 suitable for X-ray
analysis were obtained using a method similar to that described
below for 1, except 2-methyl-benzoic acid and R-naphthalic acid
were used instead of sodium benzoate for 2 and 3, respectively,
while NaOH in this procedure was in a slight excess (1 mmol).
[Cu(benzoate)(N3)]n (1). Complex 1 was hydrothermally syn-
thesized under autogenous pressure. A mixture of Cu(NO3)·3H2O
(1 mmol), sodium benzoate (0.4 mmol), NaOH (0.5 mmol), NaN3
(1.5 mmol), and H2O (10 mL) was sealed in a Teflon-lined
autoclave and heated to 140 °C. After being maintained for 48 h,
the reaction vessel was cooled to room temperature over 12 h. Pure
green-black crystals were collected. Yield: ∼20% based on
Cu(NO3)·3H2O. FT-IR (KBr pellets, cm-1): 3132, 2096, 1589,
1534, 1401, 1266, 723, 684. Anal. calcd for C7H5CuN3O2: C, 37.09;
H, 2.22; N, 18.54%. Found: C, 37.35; H, 2.31; N, 18.01%.
Synthesis of [Cu(2-methyl-benzoate)(N3)]n (2). Yield: ∼30%.
FT-IR (KBr pellets, cm-1): 3415, 3134, 2080, 1617, 1527, 1401,
1125, 738, 618. Anal. calcd for C8H7CuN3O2: C, 39.92; H, 2.93;
N, 17.46%. Found: C, 39.50; H, 2.90; N, 16.74%.
(11) (a) Chen, H.-J.; Mao, Z.-W.; Gao, S; Chen, X.-M. Chem. Commun.
2001, 2320. (b) Woodard, B.; Willett, R. D.; Haddad, S.; Twamley,
B.; Garcia, C. J.; Coronado, E. Inorg. Chem. 2004, 43, 8126.
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Chem. 1983, 22, 2877. (b) Cortes, R.; Urtiaga, M. K.; Lezama, L.;
Larramendi, J. I. R.; Arriortua, M. I.; Rojo, T. J. Chem. Soc., Dalton
Trans. 1993, 3685. (c) Thompson, L. K.; Tandon, S.S. Comments
Inorg. Chem. 1996, 18, 125. (d) Zhang, L.; Zuo, J.-L.; Gao, S.; Song,
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1998, 120, 11122. (b) Cabrero, J.; Graaf, C.; Bordas, Caballol, E.; R.;
Malrieu, J.-P. Chem.sEur. J. 2003, 9, 2307. (c) Triki, S.; García,
C. J. G.; Ruiz, E.; Pala, J. S. Inorg. Chem. 2005, 44, 5501.
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D. Chem. Commun. 2006, 3181.
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Chem. Commun. 2006, 2227. (b) Gu, Z.-G.; Song, Y.; Zuo, J.-L.; You,
X.-Z. Inorg. Chem. 2007, 46, 9522. (c) Gu, Z.-G.; Zuo, J.-L.; You,
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Commun. 2008, 281.
Synthesis of [Cu(1-naphthoate)(N3)]n (3). Yield: ∼20%. FT-
IR (KBr pellets, cm-1): 3130, 2086, 1595, 1515, 1460, 1402, 1288,
798, 788, 776, 659, 595, 542, 507, 481, 417. Anal. calcd for
(17) (a) He, Z.; Wang, Z.-M.; Gao, S.; Yan, C.-H. Inorg. Chem. 2006, 45,
6694. (b) Zeng, Y.-F.; Zhao, J.-P.; Hu, B.-W.; Hu, X.; Liu, F.-C.;
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Inorganic Chemistry, Vol. 48, No. 6, 2009 2483

Zhao et al.
Table 1. Crystal Data and Structure Refinement Parameters for
Complexes 1-3
1
2
3
chemical formula C7H5CuN3O2
C8H5CuN3O2
240.71
C11H5CuN3O2
276.74
fw
226.68
Pnma
6.8436(14)
7.0732(14)
6.600(3)
90
803.6(3)
4
1.874
space group
a (Å)
b (Å)
c (Å)
ꢀ/deg
V/Å3
P2(1)2(1)2(1)
4.9590(10)
6.4582(13)
27.249(5)
90
872.7(3)
4
1.832
P2(1)/c
15.912(12)
6.799(5)
9.948(7)
101.981(11)
1052.7(13)
4
1.746
2.065
293
Z
D/g cm-3
µ/mm-1
T
2.682
293
2.475
293
Ra/wRb
0.1049/ 0.2586 0.0481/ 0.0951 0.1026/0.2717
Figure 1. The 1D carboxylato/EO azido mixed-bridged copper chain of
1.
a R ) ∑|Fo| - |Fc|/∑|Fo|. b Rw ) [∑[w(Fo - Fc2)2]/∑w(Fo )2]1/2
.
2
2
mixed-bridged copper compounds.10a,15a,17a,21 The nearest
separation of CuII ions between two neighbor chains is 6.844
Å. The key bond distances and angles are listed in Table 2.
In the structure of complex 2, the 2-methyl-benzoate
replaces the benzoate ligand, and different from 1, the CuII
ion is hexacoordinated with tetragonal axial elongation. Two
nitrogen atoms and two oxygen atoms coordinated to the
CuII ion in the equatorial plane [Cu1-N1 ) 1.986 Å,
Cu1-N1A ) 2.003 Å, Cu1-O1 ) 1.932 Å, and Cu1-O2A
) 1.918 Å], while the axial is taken by two azido anions
[Cu1-N3A ) 2.604 Å and Cu1-N3B ) 2.695 Å]. The
carboxylate group bridges two CuII ions adopting the syn,syn
mode. There is a 32.0° torsion of the carboxylate group from
the benzene ring. The azido ions are in the µ-(1,1,3,3)
bridging mode, using one end to coordinate to two CuII ions
on the equatorial plane while the other end links to one CuII
ion using its apical position of the square-based pyramid
(Figure 2a). The distances of CuII ions linked by azido in
the EO mode are 3.230 Å, while the EE modes are 4.959 Å
and 5.956 Å in the cis and trans forms, respectively. The
Cu-N-Cu angles are 108.16° in the equatorial plane and
75.09° in the axial positions. The Cu1-N1-N2 and
Cu1A-N1-N2 angles are 118.3° and 117.2°, while the
angles Cu1G-N3-N2 and Cu1G-N3-N2 are 109.4° and
110.5°, respectively. The structure of 2 can be easily
described as well-isolated layers of hexacoordinated CuII ions
along the ab plane of the unit cell; each CuII center exhibits
a tetragonal axial elongation, in which the unpaired electron
C11H7CuN3O2: C, 47.74; H, 2.55; N, 15.18%. Found: C, 47.77; H,
2.62; N, 15.20%.
X-Ray Data Collection and Structure Determinations. X-ray
single-crystal diffraction data for complexes 1-3 were collected
on a Bruker Smart 1000 CCD diffractometer at 293(2) K with Mo
KR radiation (λ ) 0.71073 Å) in the ω scan mode. The program
SAINT18 was used for integration of the diffraction profiles. All
of the structures were solved using direct methods using the
SHELXS program of the SHELXTL package and refined using full-
matrix least-squares methods with SHELXL (semiempirical absorp-
tion corrections were applied using the SADABS program).19 Metal
atoms in each complex were located from the E-maps, and other
non-hydrogen atoms were located in successive difference Fourier
syntheses and refined with anisotropic thermal parameters on F2.
The hydrogen atoms of the ligands were generated theoretically
onto the specific atoms and refined isotropically with fixed thermal
factors. Detailed crystallographic data are summarized in Table 1.
Results and Discussion
Crystal Structure of the Complexes. The single-crystal
X-ray determination reveal that 1 consists of an isolated 1D
azido-copper chain. The asymmetric unit of this complex
consists of half a CuII ion, half an azide anion, and half a
benzoate ligand. The CuII ion located at the inverse center
takes a square-planar environment coordinated by two
nitrogen atoms from two azide anions and two oxygen atoms
from two carboxylates [Cu1-N1 ) 1.977 Å, Cu1-O1 )
1.954 Å, Cu1-N1A ) 1.977 Å, and Cu1-O1A ) 1.954
Å]. The azido takes the EO mode linking two CuII ions, while
the carboxylate bridges two CuII ions together with the azido
anions forming a carboxylate/EO-azido mixed-bridged cop-
per chain (Figure 1). In the chain, CuII ions and the benzoate
ligands are almost in one plane. The chains take the ABAB
packing mode in the lattice. The Cu1 · · · Cu1A distance is
3.537 Å, while the Cu1-N1-Cu1A angle is 126.8°. It is
noticeable that both the copper-copper separation and the
µ-(1,1)-N3 bridge angle are greater than the previously
reported examples containing µ-(1,3)-carboxylate/µ-(1,1)-N3
2
2
is on the dx -y orbital (the two elongated Cu-N distances
are 2.695 and 2.604 Å, respectively). Each layer is formed
by the chains of CuII ions bridged by an EO azido and a
syn,syn-carboxylate group, and these chains are in turn linked
to form the 2D layer by end-to-end azido ligands. The layers
are magnetically isolated from each other, with Cu-Cu
distances of 14 Å. It is interesting that both the azido groups
and CuII ions in the 2D layer can be considered to be
4-connected nodes; thus, the 2D network can be described
as having (44) topology constructed by azido and copper
(Figure 3).
For 2, the methyl substitutional groups of the coligand
affect not only the linkage of azido anions but also the
(18) SAINT Software Reference Manual; Bruker AXS: Madison, WI, 1998.
(19) Sheldrick, G. M. SHELXTL NT, version 5.1; University of Go¨ttingen:
Go¨ttingen, Germany, 1997.
(20) Thompson, L. K.; Tandon, S. S.; Lloret, F.; Cano, J.; Julve, M. Inorg.
Chem. 1997, 36, 3301.
(21) Escuer, A.; Vicente, R.; Mautner, F. A.; Goher, M. A. S. Inorg. Chem.
1997, 36, 1233.
2484 Inorganic Chemistry, Vol. 48, No. 6, 2009

Azido-Mediated CuII Systems
Table 2. Selected Bond Lengths [Å] and Angles for 1-3 [deg]
1
2
Cu(1)-O(1)#1
Cu(1)-O(1)
Cu(1)-N(1)#1
Cu(1)-N(1)
#1-x + 1, -y, -z + 1
1.954(6)
1.954(6)
1.978(5)
1.978(5)
Cu(1)#2-N(1)-Cu(1)
O(1)-Cu(1)-N(1)
N(1)#1-Cu(1)-N(1)
O(1)#1-Cu(1)-O(1)
126.8(6)
91.2(4)
180.0(7)
180.0(4)
#2-x + 1, y + 1/2, -z + 1
Cu(1)-O(2)#1
Cu(1)-O(1)
Cu(1)-N(1)#1
Cu(1)-N(1)
Cu(1)-N(3)#2
Cu(1)-N(3)#3
#1-x + 2, y-1/2, -z + 1/2
#5x + 1, y, z
1.919(4)
1.932(3)
1.986(4)
2.003(4)
2.604(5)
2.695(5)
#2x-1, y, z
#6-x + 3, y + 1/2, -z + 1/2
Cu(1)#4-N(1)-Cu(1)
Cu(1)#5-N(3)-Cu(1)#6
N(1)-Cu(1)-N(3)#2
108.16(19)
75.09(13)
85.69(16)
94.28(16)
90.20(16)
89.99(16)
#4-x + 2, y + 1/2, -z + 1/2
N(1)#1-Cu(1)-N(3)#2
O(2)#1-Cu(1)-N(1)#1
O(1)-Cu(1)-N(1)#1
#3-x + 3, y-1/2, -z + 1/2
3
Cu(1)-O(2)#1
Cu(1)-O(1)
1.932(9)
1.932(9)
1.987(10)
2.014(10)
2.726(13)
#2-x + 2, y + 1/2, -z + /2
Cu(1)-N(1)-Cu(1)#1
N(2)-N(3)-Cu(1)#3
O(2)#1-Cu(1)-N(1)
O(1)-Cu(1)-N(1)
N(1)-Cu(1)-N(3)#3
#3-x + 2, -y, -z + 1
116.8(5)
111.3(10)
91.0(4)
91.8(4)
88.8(4)
Cu(1)-N(1)
Cu(1)-N(1)#2
Cu(1)-N(3)#3
#1-x + 2, y-1/2, -z + 3/2
conformation of the layers compared to 1, which results in
the chiral structure of the complex obtained through spon-
taneous resolution. When coordinated to the metal ions, the
methyl groups of the 2-methyl-benzoates all point along the
b axis deviated from the phenyl, and the phenyl groups at
the topside and the underside of the copper-azido layer have
a dihedral angle of 64.05°. Although the azido and the
coligand 2-methyl-benzoate are all achiral, in the process of
crystallization, a chiral layer was formed. When the layers
pack to high dimension, the layers are related by a 2-fold
screw axis. Hence, such sheets stack along the c axis, forming
a chiral crystal of 2, and spontaneous resolution occurs with
a generated conglomerate. The Flack parameter of 2 is
0.14(5); thus, the absolute structure cannot be determined.
It is interesting that, although the steric bulk of the side
group of coligand carboxylate becomes larger gradually in
1, 2, and 3, the structure of 3 can be considered as the
integration of 1 and 2. Different from 1 and 2, the CuII ions
in 3 were pentacoordinated [Cu1-N1 ) 1.987 Å, Cu1-N1B
) 2.014 Å, Cu1-O1 ) 1.933 Å, Cu1-O2A ) 1.932 Å,
and Cu1-N3A ) 2.726 Å] (Figure 2b). The angle between
the carboxylate group and naphthalene ring is 40°. The
carboxylate group bridges two CuII ions with the syn,syn
mode. The azido anion of 3 takes the µ-(1,1,3) mode
coordinating to CuII ions in the equatorial plane, giving a
carboxylate/EO-azido mixed-bridged copper chain like 1 and
2; then, the chains are linked by µ-(1,1,3)-N3, forming a 2D
net. The Cu-N-Cu angle is 116.8°, and the Cu1D-N3-N2
bond angle is 111.3°. The distances of CuII ions bridged by
the same azido anion are 3.409, 5.493, and 5.118 Å. In 3,
the azide and CuII are only connected to three of each other.
The azido-copper net could be described as (4 ·82) topology
(Figure 3). The key bond distances and the angles are listed
in Table 2. It is worth noting that the uncoordinating
substitutional groups of carboxylate in 2 and 3 take a key
role in determining the bridge mode of the azido anions.
Unlike most other heterocyclic carboxylate or di-carboxylate
groups used as coligands in an azido-copper system forming
multidimensional complexes via the long bridging coligand,
in 2 and 3, the azido anions link the CuII ion, forming high-
dimensional complexes,10a,b,17a,21 which favors interchain
magnetic coupling, especially at low temperatures. The layers
of 3 are also chiral like that in 2, but in stacking to high
dimension, the neighboring layers are related by a glide
plane, making the adjacent layers have opposite handedness.
The chirality is annihilated; complex 3 is therefore a racemate
in the 3D crystal. The difference in structure between 1, 2,
and 3 reflected the steric repulsion effect of the coligands.
In 1, without a substituent group on the aryl ring, the azido
groups are held very close to the aryl ring, with a small
dihedral angle with the plane of the aryl ring. This prevents
the azido coordinating to other metal in an end-to-end
fashion. For 2 and 3, with a substituent group on the ligand,
the steric repulsions should be stronger. In complex 2, the
substituent is not very large, and thus the azido anions are
held apart from the aryl ring, are not very sterically hindered,
and can link four metal ions. However, in 3, the substituent
is too bulky, and the azido ligand is too sterically hindered
by the substituent of the aryl group to bridge four metal ions.
Magnetic Studies. Magnetic measurements have been
carried out on crystalline samples of complexes 1-3, which
were all pure-phase, as confirmed by XRPD (see Figure S1
in the Supporting Information). According to the obtained
data, a dominant ferromagnetic coupling between the CuII
ions in 1-3 can be suggested. However, in the low-
temperature region, they show various magnetic behaviors,
as discussed below.
Magnetic susceptibility data of 1 were collected for the
crushed crystalline sample of the complex in the 2-300 K
temperature range (Figure 4). The ꢁmT value per CuII ion at
300 K is 0.56 cm3 K mol-1, larger than the predicted 0.375
cm3 K mol-1 for an isolated CuII ion (S ) 1/2) with g )
2.0. As the temperature decreases, the ꢁmT product increases,
indicating ferromagnetic coupling between the CuII ions in
1. This behavior is not field-dependent. The susceptibility
of 1 follows the Weiss law at temperatures above 100 K,
Inorganic Chemistry, Vol. 48, No. 6, 2009 2485

Zhao et al.
Figure 2. The linkage and coordination modes for complexes (a) 2 and
(b) 3. Blue bonds stand for equatorial coordination and the dark red for
axial.
Figure 3. The 2D azido-copper net and topology for (a) 2 and (b) 3. Blue
bonds stand for equatorial coordination, the dark red for axial coordination,
and the light green line for topology analysis.
with C ) 0.504 emu mol-1 and a Weiss constant of Θ )
34.62 K, indicating ferromagnetic coupling in 1 (Figure S2,
Supporting Information). The experimental data above 10
K can be modeled using Baker’s equation for an infinite chain
CuII ion. It appears that antiferromagnetic coupling would
be expected for 1 for the syn-syn carboxylato bridge and
the EO azido for a Cu-N-Cu angle of 126.8°.13a Actually,
in this situation, as Thompson et al. and Escuer et al.
proposed, the countercomplementarity effect of the two
ligands may explain the strong ferromagnetic coupling, not
simply the sum of the two separated components.20,21
The low-temperature magnetic measurements of 2 and 3
show spontaneous magnetization. At 300 K, the ꢁmT product
per CuII ion has a value of 0.59 cm3 K mol-1 (Figure 5a)
and 0.61 cm3 K mol-1 (Figure 5b) for 2 and 3, respectively,
slightly above the expected 0.375 cm3 K mol-1 for an isolated
CuII ion with S ) 1/2 and g ) 2.0. As the temperature
decreases, the ꢁmT product increases steadily, until below
of S ) 1/2 spins22 based on the Hamiltonian H ) -J∑SiSi.
The best fit, shown in Figure 4 as a solid line, was obtained
for g ) 2.28, J ) 33.90 cm-1, and R ) 3.77 × 10-4 (R )
∑[(ꢁmT)obsd - (ꢁmT)calcd]2/∑[(ꢁmT)obsd]2), confirming the strong
ferromagnetic coupling between the CuII centers. Magnetiza-
tions versus field data at 2 K were collected for 1 (Figure 4,
inset). Magnetization rises very fast as the field increases,
indicating ferromagnetic coupling along the CuII chain, and
tends to saturation at the higher fields, as expected for one
ˆ
ˆ ˆ
(22) Baker, G. A.; Rushbrooke, G. S. Phys. ReV. 1964, 135, 1272.
2486 Inorganic Chemistry, Vol. 48, No. 6, 2009

Azido-Mediated CuII Systems
Figure 4. ꢁmT vs T plot for 1 at 0.3 T applied field. The solid line is the
best fit to the experimental data. Inset: the plot of the field dependence of
the magnetization at 2 K.
Figure 6. (a) Zero-field-cooled/field-cooled susceptibility plot for 2 at an
applied field of 50 Oe (black squares, ZFC; white squares, FC). (b) Zero-
field-cooled/field-cooled susceptibility plot for 3 at an applied field of 20
Oe (black squares, ZFC; white squares, FC). Inset: AC magnetic susceptibil-
ity plot with an oscillating AC field of 3.5 Oe at 1000 and 100 Hz. Black
circles, in-phase susceptibility, ꢁm′; white circles, out-of-phase susceptibility,
ꢁm′′.
22
ˆ
ˆ ˆ
(discussed below), the intrachain coupling J (H ) -J∑SiSi)
together with the mean field approximation23 for interchain
coupling zJ were used to fit the magnetic data of 2, giving
the best fit with parameters g ) 2.08, J ) 93.10 cm-1, zJ )
3.40 cm-1, and R ) 1.56 × 10-4 (R ) ∑[(ꢁmT)obsd
-
(ꢁmT)calcd]2/∑[(ꢁmT)obsd]2) above 35 K (Figure 5a). The
susceptibility of 3 above 25 K was simulated using the same
model, giving a best fit with parameters g ) 2.23, J ) 65.63
cm-1, zJ ) 1.96 cm-1, and R ) 3.18 × 10-4 (R ) ∑[(ꢁmT)obsd
- (ꢁmT)calcd]2/∑[(ꢁmT)obsd]2) (Figure 5b). The interchain
interaction zJ was attributed to the interactions of chains in
one layer and the interlayer coupling. However, the layers
are well-separated, and the interactions between them are
very small. Complexes 2 and 3 behave as 2D ordered
magnets. A zero-field-cooled/field-cooled measurement shows
spontaneous magnetization below 20 K for 2 (Figure 6a) and
below 14 K for 3 (Figure 6b), indicating the onset of a
ferromagnetic state. Furthermore, the magnetization versus
field plot at 2 K shows spontaneous magnetization to a
saturation value of 1.18, which indicates a ferromagnetically
Figure 5. ꢁmT vs T plot at 0.2 T (circles) and 1.0 T (triangles). The solid
line is the best fit to the experimental data. Inset: Magnetization vs field
plot at 2.0 K for (a) 2 and (b) 3.
50 K, a pronounced rise is observed. A maximum value is
reached at 14 K for 2 and 10 K for 3. The magnitude of the
maximum is strongly field-dependent. The susceptibility
follows the Curie-Weiss law with C ) 0.49 emu mol-1 and
Θ ) 53 K down to 100 K for 2 (Figure S3, Supporting
Information) and with C ) 0.53 emu mol-1 and Θ ) 43 K
down to 90 K for 3 (Figure S4, Supporting Information),
suggesting strong ferromagnetic coupling between the CuII
ions in 2 and 3. On the basis of the magnetic structure
(23) (a) Myers, B. E.; Berger, L.; Friedberg, S. A. J. Appl. Phys. 1969, 40,
1149. (b) O’Conner, C. J. Prog. Inorg. Chem. 1982, 29, 203.
Inorganic Chemistry, Vol. 48, No. 6, 2009 2487

Zhao et al.
ordered state at this temperature for 2 (Figure 5a inset). The
magnetization versus field plot at 2 K of 3 also shows
spontaneous magnetization to a saturation value of 0.99 per
CuII ion, indicating a ferromagnetically ordered state (Figure
5b inset). AC magnetic susceptibility data were also collected
for 2. A strong out-of-phase signal appeared in the ꢁm′′ versus
T plot at 13 K, as a decrease of the in-phase (ꢁm′) signal
was observed (Figure 6a inset). The shape of the out-of-
phase signal shows a sharp peak overlapping with a broader
one, at lower temperatures, indicating the presence of more
than one slow-relaxation phenomenon taking place below
13 K, which may be attributed to the moving of the domain
wall that was frequent in the 2D structure.24 For 3, the AC
magnetic susceptibility data show the appearance of a weak
out-of-phase signal at 10 K, similarly to what was observed
for 2; the out-of-phase signal seems to have its origin in two
different processes that are likely due to a small amount of
impurity in the sample (Figure 6, inset).
In 2, two distinct magnetic exchange pathways, in the two
directions of the CuII layers, can be described: (i) Along the
chains, the possible magnetic exchange pathways are pro-
vided by the syn,syn-carboxylato group and two end-on azido
II
2
2
ligands bound to the dx -y orbitals of the Cu centers (with
Cu-N distances of 1.98-2.01 Å and a Cu-N-Cu angle of
2
108.16°) and to the dz orbitals (Cu-N distances of 2.6-2.7
Å and a Cu-N-Cu angle of 75.09°). (ii) Perpendicular to
the chains, end-to-end azido ligands mediate a weak ferro-
2
2
magnetic interaction between the magnetic dx -y orbital of
II
II
2
one Cu and the full dz orbitals of the other Cu ions, which
are mismatched for interaction through the azide. A priori,
one might think that end-on azide ligands displaying large
Cu-N-Cu angles like those in 2 should afford antiferro-
magnetic coupling, as calculated by Ruiz et al. for [Cu2(EO-
Figure 7. (a) View of 2 along the c direction of the unit cell (the CuII
layers lay along the ab plane). The xy plane of the CuII centers are shown.
(b) View of 3 along the a axis of the unit cell (the CuII layers lay along the
bc plane). The xy planes on each CuII ion are shown.
N3)2] planar units.13a However, that is not the case in 2. The
magnetic orbital on each Cu center is dx -y , which is not
parallel but at a 64° canting angle (Figure 7a), resulting in
the nonplanar [Cu2(EO-N3)2] units. Furthermore, the second
II
2
2
Just like in 2, there is ferromagnetic coupling along the chains
of CuII ions bridged by a syn,syn-carboxylato group and an
2
azido ligand is bound to the full axial dz orbitals of both
CuII; thus, there is not an allowed superexchange pathway
through this ligand. All of these result in weak ferromagnetic
exchange, both inter- and intrachain.25 This can explain the
two features of the AC measurement: one corresponds to
the ferromagnetic ordering in the lattice, while the weak
interchain ferromagnetic interaction gives rise to the 2D soft
ferromagnetic behavior of 2 and the second AC feature.
However, there is no hysteresis at 2 K, due to the lack of
three dimensional ordering in 2, since the layers are not
magnetically coupled (Figure S5, Supporting Information).
For 3, the structure is very similar to that already discussed
above for 2, and the same magnetic exchange pathways are
applied. The CuII ions in 3 are pentacoordinated in a square-
pyramidal fashion, with a very elongated apical Cu-N
distance of 2.726 Å, defining the z axis on the metal center.
2
2
end-on azido ligand bound to the dx -y orbitals of the Cu
centers (with Cu-N distances of 1.98-2.01 Å and a
Cu-N-Cu angle of 116.85°). In 3, the basal planes of the
CuII centers are canted at a 59.84° angle (Figure 7). The
chains are linked together to form layers via end-to-end azido
ligands that provide a weak ferromagnetic exchange pathway
II
2
2
2
by bridging full dz orbitals of one Cu to the half-full dx -y
of the CuII in the neighboring chain. The situation is very
similar to that in 2, but in 3, only half the exchange pathways
exist since the CuII ions of 3 are pentacoordinated (Figure
7b). This fact is probably due to the steric bulk of the
naphthyl group of the carboxylate and is most likely
responsible for the fact that the ferromagnetic phase appears
at lower temperatures for 3 than for 2.
The big dihedral angle of the basal planes of the CuII
centers, in 2 and 3, as well as the countercomplementarity
effect lead to the total ferromagnetic exchange in these
complexes in spite of a big Cu-N-Cu angle. It will be
noticed that the difference in J values of the chains in 1, 2,
and 3 reflects the difference in the Cu-N-Cu angle. This
(24) (a) Coronado, E.; Galo´n-Mascaro´s, J. R.; Go´mez-García, C. J.; Murcia-
Martínex, A. Chem. Eur.sJ. 2006, 12, 3484. (b) The´tiot, F.; Sala-
Pala, J.; Galo´n-Mascaro´s, J. R.; Golhen, S. Chem. Commun. 2002,
1078.
(25) Colacio, E.; Costes, J.-P.; Domı´nguez-Vera, J. M.; Maimounac, I. B.;
Sua´rez-Varela, J. Chem. Commun. 2005, 534.
2488 Inorganic Chemistry, Vol. 48, No. 6, 2009

Azido-Mediated CuII Systems
Table 3. Pertinent Cu-N-Cu Angles (deg) and Interactions for the
complex 1 is formed by isolated carboxylato/EO azido
mixed-bridged chains, while 2 and 3 are 2D azido-copper
networks displaying µ-(1,1,3,3) and µ-(1,1,3) azido coordina-
tion modes, respectively. The change of the structure
depending on the R group of the carboxylate was clearly
observed from X-ray analysis. Introducing steric bulk to the
side of the benzoate adjusts the linkage of the azido anions
and the coordination geometries of the metal center. Spon-
taneous resolution occurs in 2, giving a chiral complex. The
magnetic behavior of 1 is that of an isolated ferromagnetic
chain. Complexes 2 and 3 are 2D magnets with transition
temperatures of 13 and 10 K. These three complexes are
good new examples of the azido-bridged copper complex
family. We have shown here a good example of crystal
engineering, in which the addition of substitutents to a simple
aromatic carboxylate alters the structure of the complexes
obtained as well as allows the fine-tuning of their magnetic
properties.
Carboxylato/EO Azido Mixed-Bridged Copper Complex
Cu-N-Cu
angles (deg)
J/cm-1
ref
126.8
124.3
116.85
116.09
111.9
108.16
107.6
106.7
106.6
106.5
105.52
103
39
48
this work1
ref 17a
65.63
69.7
75
93.10
126
80 ( 5
80
126
63
89
this work3
ref 17b
ref 21
this work2
ref 10a
ref 15a
ref 17a
ref 10a
ref 10b
ref 17b
is consistent with a previously reported copper complex with
carboxylato/EO-azido ligands. It is observed that the ferro-
magnetic coupling in the carboxylato/EO-azido CuII systems
increases with the decrease of the Cu-N-Cu angle, with a
maximum near 108°. The correlation is shown in Table 3sall
of the superexchange parameters J correspond to two Cu
ions bridged by one carboxylato and one EO-azido ligand.
Acknowledgment. This work was financially supported
by the 973 Program of China (2007CB815305), the NNSF
of China (50673043 and 20773068), and NSF of Tianjin,
China (07JCZDJC00500). E.C.S. acknowledges financial
support from the Spanish Government (Grant CTQ2006/
03949BQU and Juan de la Cierva fellowship).
ˆ
ˆ ˆ
They were obtained using the Hamiltonian H ) -J∑SiSi.
Conclusion
Three new azido-copper complexes with aromatic car-
boxylates as coligands have been structurally and magneti-
cally characterized. The three complexes reported here,
although having the same chemical formula, that is,
[Cu(RCOO)(N3)]n, where R is in each case a different
aromatic group, display very different solid-state structures
and magnetic properties. Structural analyses show that
Supporting Information Available: X-ray crystallographic data
for complexes 1-3 in CIF format and Figures S1-S5. These
materials are available free of charge via the Internet at http://pubs.
acs.org.
IC802066A
Inorganic Chemistry, Vol. 48, No. 6, 2009 2489