Effect of phenylacetic acid coligands on the structures and magnetic properties of azido-bridged copper(ii)-chain compounds

Article abstract of DOI:10.1039/c3ce42532k

To explore and alter the structures of azido-copper(ii) chain compounds, as well as investigate how their magnetic properties are influenced by substituents on the phenylacetic acid coligands, three new azido-bridged Cu(ii) compounds, [Cu(o-npa)(N₃)(H₂O)]_n (1), [Cu(p-npa)(N₃)]_n (2) and [Cu(p-mpa)(N₃)] _n (3) (o-Hnpa = o-nitrophenylacetic acid, p-Hnpa = p-nitrophenylacetic acid, and p-Hmpa = p-methylphenylacetic acid), have been successfully obtained, and structurally and magnetically characterized. Single-crystal structure analyses indicate that the azido ligands adopt the single end-on (EO) mode to connect adjacent Cu(ii) centers in all three compounds. In compounds 1 and 2, the carboxylate groups exhibit the same μ₂-bridging bidentate mode. Although compound 2 exhibits a 2D layer-structure formed through connections between the p-NO₂ groups on the aromatic rings and 1D Cu(ii)-chains, 2 shows similar intrachain ferromagnetic coupling to 1. When p-methylphenylacetic acid is employed as a coligand, compound 3 exhibits a 2D layer-structure, in which the carboxylate adopts a μ₃-bridging tridentate mode to shorten the distance between Cu(ii)-chains. Due to the counter-complementarity of the superexchange pathways in this system, compound 3 behaves as a magnet with a spontaneous magnetization temperature of 7 K. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3ce42532k

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Effect of phenylacetic acid coligands on the
structures and magnetic properties of
Cite this: CrystEngComm, 2014, 16,
azido-bridged copper(II)-chain compounds†
4194
Lan Yang,a Sheng Zhang,a Xiangyu Liu,ab Qi Yang,a Qing Wei,a Gang Xiea
a
*
and Sanping Chen
To explore and alter the structures of azido–copper(II) chain compounds, as well as investigate how their
magnetic properties are influenced by substituents on the phenylacetic acid coligands, three new
azido-bridged Cu(II) compounds, [Cu(o-npa)(N3)(H2O)]n (1), [Cu(p-npa)(N3)]n (2) and [Cu(p-mpa)(N3)]n (3)
(o-Hnpa
=
o-nitrophenylacetic acid, p-Hnpa
=
p-nitrophenylacetic acid, and p-Hmpa
=
p-methylphenylacetic acid), have been successfully obtained, and structurally and magnetically charac-
terized. Single-crystal structure analyses indicate that the azido ligands adopt the single end-on (EO)
mode to connect adjacent Cu(II) centers in all three compounds. In compounds 1 and 2, the carboxylate
groups exhibit the same μ2-bridging bidentate mode. Although compound 2 exhibits a 2D layer-structure
formed through connections between the p-NO2 groups on the aromatic rings and 1D Cu(II)-chains, 2
shows similar intrachain ferromagnetic coupling to 1. When p-methylphenylacetic acid is employed as a
coligand, compound 3 exhibits a 2D layer-structure, in which the carboxylate adopts a μ3-bridging
tridentate mode to shorten the distance between Cu(II)-chains. Due to the counter-complementarity of
the superexchange pathways in this system, compound 3 behaves as a magnet with a spontaneous
magnetization temperature of 7 K.
Received 12th December 2013,
Accepted 14th February 2014
DOI: 10.1039/c3ce42532k
www.rsc.org/crystengcomm
Introduction
relationship between the coordination mode of azido and the
way in which coupling is transmitted: the μ-1,1 (end-on, EO)
In recent years, the field of molecular magnetism has been a
hot research topic, in which great progress has been
achieved.1,2 The design and synthesis of molecule-based
magnets, especially one-dimensional (1D) magnets (so-called
single-chain magnets (SCMs)), with organic bridging ligands
and transition metals continues to be a productive area due
to their intriguing structures, unique physical characteristics,
and promising novel applications, such as magnetic sensors,
magnetic switches and multifunctional magnetic devices.3,4
Coordination chemistry is a powerful tool for accessing such
systems, and a common strategy is to connect paramagnetic
metal centers with short bridging groups (C2O42−, CN−, SCN−,
and μ-1,3 (end-to-end, EE) coordination modes generally
transmit ferromagnetic (FM) and antiferromagnetic (AFM)
exchange interactions respectively, which is helpful for
predicting the magnetism of the compounds,5 although there
are several exceptions.6 In addition, plenty of compounds fea-
ture mixed azido bridging modes and also show interesting
magnetic properties.7 Among metal–azido-bridged systems,
transition metal–azido-bridged compounds have attracted
considerable attention, especially Cu(II)–azido-bridged coordi-
nation polymers.8 Recently, combining azido and coligands
in one system has been an interesting approach for constructing
new structures and modulating magnetic behaviour.9
Considering that the carboxylate group can also efficiently
transmit magnetic exchange, introducing carboxylate into
metal–azido systems would increase the structural diversity
and modulate the magnetic behaviour of the compounds,
giving a large number of coordination polymers with diverse
topologies and magnetic properties,9 for example, pyridyl-
carboxylates,10a pyridylacrylates,10b pyridine carboxylate N-oxide,10c
dicarboxylates,10d and aromatic carboxylate derivatives.11a
Similarly, non-coordinating substituents in the o, m, or p
position of the aromatic ring of the benzoic acid coligand
could influence the structural properties of the compounds,
such as Cu⋯Cu distances, Cu–N–Cu angles and intrachain
−
N3 , etc.) to transmit magnetic coupling.5 There is a clear
a Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of
Ministry of Education, College of Chemistry and Materials Science, Northwest
University, Xi'an, 710069, China. E-mail: sanpingchen@126.com,
sanpingchen312@gmail.com; Fax: +8602988302604; Tel: +8602988302604
b School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan
750021, China
† Electronic supplementary information (ESI) available: Selected bond lengths
and angles are listed in Tables S1–S3. Fig. S3 shows XRPD graphs of
compounds 1–3. CCDC reference numbers are 972618 (1), 972616 (2) and
972617 (3). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3ce42532k
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separations of the Cu(II) ions, and further affect the magnetic
behaviour of the compounds.11 Extending the idea above, if
phenylacetic acid is selected as coligand instead of benzoic
acid, do the substituents on the aromatic ring affect the coor-
dination mode of the carboxylate groups? Furthermore, how
do the intrachain structure and the magnetic behaviour of the
Cu(II)–azido system change?
In order to answer the questions above, three phenylacetate
coligands with o-nitro, p-nitro and p-methyl substituents were
introduced into the Cu(II)–azido system. Herein we report the
synthesis and characterization of three new azido–Cu(II)
compounds which are formulated as [Cu(o-npa)(N3)(H2O)]n (1),
[Cu(p-npa)(N3)]n (2) and [Cu(p-mpa)(N3)]n (3). Compounds
1 and 2, in which nitro groups are located at the o and p posi-
tions of the phenylacetic acids, respectively, display similar
intrachain structures with mixed EO-azido and syn–syn
carboxylate bridges. 1 is an infinite 1D chain, while 2 has a
2D layer-structure. Magnetic data indicate that both 1 and 2
exhibit intrachain ferromagnetic interactions. Interestingly,
compound 3 behaves as a 2D ordered magnet with a phase
transition temperature of about 7 K because the carboxylate
groups adopt a μ3-bridging tridentate mode due to the methyl
group in the phenylacetic acid.
Synthesis
Preparation of [Cu(o-npa)(N3)(H2O)]n (1). Compound 1 was
hydrothermally synthesized under autogenous pressure. A
mixture of CuCl2·6H2O (0.0517 g, 0.3 mmol), NaN3 (0.0196 g,
0.3 mmol), and o-Hnpa (0.0908 g, 0.5 mmol) in 6 mL H2O
was sealed in a Teflon-lined stainless steel autoclave and
heated to 140 °C. After 72 h, the reaction vessel was cooled to
room temperature at 10 °C h−1. Large block-like dark green
crystals were collected. Yield: 68% (based on Cu). Anal. calcd
for C8H8CuN4O5 (%): C, 31.61; H, 2.63; N, 18.44. Found (%):
C, 31.71; H, 2.59; N, 18.49. IR (KBr, cm−1): 3497 (m), 2968 (s),
2085 (vs), 1684 (vs), 1568 (s), 1523 (s), 1394 (s), 1292 (w),
791 (w), 719 (m), 690 (w), 494 (w).
Preparation of [Cu(p-npa)(N3)]n (2).
A
mixture of
CuCl2·6H2O (0.0342 g, 0.2 mmol), p-Hnpa (0.0362 g, 0.2 mmol),
and NaN3 (0.0131 g, 0.2 mmol) in 15 mL H2O was stirred for
2 h at room temperature. A deep green solution was formed
and filtered, and then the filtrate was left to slowly evaporate at
room temperature. Dark green crystals of 2 were obtained
within two weeks. Yield: 52% (based on Cu). Anal. calcd for
C8H6CuN4O4 (%): C, 33.60; H, 2.10; N, 19.60. Found (%): C,
33.56; H, 2.02; N, 19.69. IR (KBr, cm−1): 3458 (m), 2916 (w),
2080 (vs), 1649 (s), 1551 (s), 1409 (s), 1229 (w), 732 (m), 598 (w),
534 (w).
Preparation of [Cu(p-mpa)(N3)]n (3). The synthetic
procedure for 3 was similar to that for 2, but p-Hnpa was
replaced with p-Hmpa. Dark green crystals were collected
after a week. Yield: 42% (based on Cu). Anal. calcd for
C9H9CuN3O2 (%): C, 42.40; H, 3.53; N, 16.49. Found (%): C,
42.50; H, 3.46; N, 16.58. IR (KBr, cm−1): 3447 (m), 2926 (w), 2093
(vs), 1634 (s), 1531 (s), 1400 (s), 1269 (w), 723 (m), 588 (w), 513 (w).
Experimental section
Physical measurements
Elemental analyses (C, H, N) were performed on a Perkin-
Elmer 2400 CHN elemental analyzer. The FT-IR spectra were
recorded in the range 400–4000 cm−1 using KBr pellets on an
EQUINOX55 FT/IR spectrophotometer. The phase purity of
the bulk or polycrystalline samples was verified by X-ray
powder diffraction (XRPD) measurements performed on a
Rigaku RU200 diffractometer at 60 kV and 300 mA, with Cu
Kα radiation (λ = 1.5406 Å), a scan speed of 5° min−1 and a
2θ step size of 0.02°. Temperature-dependent magnetic
measurements were performed on a Quantum Design MPMS
XL-7 SQUID magnetometer. Magnetic susceptibility measure-
ments were collected in the range of 2–300 K, under an
applied field of 1 kOe, and ac magnetization measurements
were taken at 1.8 K from 0 to 50 kOe. All data were corrected
for diamagnetism estimated from Pascal's constants, and an
experimental correction for the sample holder was applied.
Crystallographic data collection and refinement
Suitable single crystals of the three compounds were
mounted on glass fibers for X-ray measurements. Reflection
data were collected at room temperature on a Bruker SMART
APEX-CCD-based diffractometer using graphite monochromated
Mo Kα radiation (λ = 0.71073 Å). An empirical absorption
correction was applied using the SADABS program.12 Data
processing was accomplished with the SAINT processing
program. The structures were solved by direct methods and
refined on F2 with full-matrix least-squares using the
SHELXTL 97 program.13 All non-hydrogen atoms were refined
with anisotropic displacement parameters. Selected crystallo-
graphic data and structural refinement details for 1, 2 and 3
are summarized in Table 1. Selected bond lengths and bond
angles are listed in Tables S1–S3.†
Materials and methods
All of the solvents and reagents for synthesis are of analytical
grade and are commercially available. CuCl2·6H2O,
o-nitrophenylacetic acid (o-Hnpa), p-nitrophenylacetic acid
(p-Hnpa), p-methylphenylacetic acid (p-Hmpa) and NaN3 were
purchased from commercial sources and used without
further purification.
Caution! Although we have not experienced any problems
in our experiments, azido and its compounds are potentially
explosive; only a small amount of material should be
prepared, and should be handled with care.
Results and discussion
Description of structures
Crystal structure of [Cu(o-npa)(N3)(H2O)]n (1). Single-crystal
X-ray diffraction analysis reveals that compound 1 crystallizes
in the monoclinic space group P21/n. The asymmetric unit of 1
consists of one Cu(II) ion, one azido ligand, one o-npa ligand
and one coordinated water molecule. As shown in Fig. S1(a),†
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Table 1 Selected crystallographic data for the three compounds 1, 2 and 3
Compound
1
2
3
Empirical formula
Formula weight
C8H8CuN4O5
303.72
C8H6CuN4O4
285.71
C9H9CuN3O2
254.73
Crystal system
Space group
a (Å)
Monoclinic
P21/n
13.5498(16)
Triclinic
P1
6.785(5)
Monoclinic
P21/c
17.787(3)
¯
b (Å)
6.5985(8)
7.594(6)
6.6764(13)
c (Å)
13.6226(16)
10.591(8)
8.4841(16)
α (°)
90
70.714(15)
90
β (°)
116.118(2)
77.11(14)
102.003(3)
γ (°)
90
1093.6(2)
4
1.845
2.018
88.869(14)
501.3(7)
2
1.893
2.188
90
985.5(3)
4
1.717
2.197
V (Å3)
Z
D (g cm−3
)
Mu (mm−1
)
F(0 0 0)
612
286
516
Unique reflections
Observed reflections
Rint
1937
5269
0.0358
1736
1736
0.0000
1816
4869
0.0696
Final R indices [I > 2σ(I)]
R indices (all data)
Goodness-of-fit on F2
R1 = 0.0331, wR2 = 0.0795
R1 = 0.0439, wR2 = 0.0855
1.075
R1 = 0.0858, wR2 = 0.1803
R1 = 0.1716, wR2 = 0.2360
1.049
R1 = 0.0508, wR2 = 0.1210
R1 = 0.0907, wR2 = 0.1628
1.003
the Cu(II) ion has a slightly distorted octahedral geometry.
The equatorial square is composed of two nitrogen atoms
(Cu1–N1 = 2.004 Å, Cu1–N1A = 2.021 Å) from two azido ligands
and two oxygen atoms (Cu1–O1 = 1.922 Å, Cu1–O1A = 1.939 Å)
from two carboxylate groups of the o-npa ligands. The apical
positions are occupied by two oxygen atoms (Cu1–O2 = 2.807 Å,
Cu1–O2A = 2.427 Å) from two coordinated water molecules.
Adjacent Cu(II) ions are bridged by EO-azido, μ2-bridging
bidentate carboxylate bridges and water oxygen atoms, with a
Cu–N–Cu angle of 110.3°, a Cu–O–Cu angle of 77.9° and a
Cu⋯Cu distance of 3.303 Å to form an isolated 1D mixed
carboxylate/EO-azido/H2O bridged copper chain along the b
direction (Fig. 1).
and two oxygen atoms (Cu1–O1 = 1.917 Å, Cu1–O1A = 1.917 Å)
from two equivalent carboxylate groups. Two nitro oxygen
atoms (Cu1–O2 = 2.514 Å, Cu1–O2A = 2.514 Å) from another
two p-npa ligands occupy the axial sites. Cu2 is equatorially
coordinated by two nitrogen atoms (Cu2–N1A = 1.993 Å,
Cu2–N1B = 1.993 Å) from two azido ligands and two oxygen
atoms (Cu2–O3 = 1.934 Å, Cu2–O3A = 1.934 Å) from two
carboxylate groups. In 2, one carboxylate group bridges two
Cu(II) ions via a μ2-bridging bidentate mode. Unlike in com-
pound 1, one nitro group of the p-npa ligand binds another
Cu(II) ion via a monodentate mode. This may be attributed to a
steric hindrance effect resulting from the different position of
the nitro group. The metal ions alternate in the Cu1–Cu2–Cu1
Crystal structure of [Cu(p-npa)(N3)]n (2). Compound 2
sequence to generate a 1D mixed carboxylate/EO-azido
¯
crystallizes in the triclinic space group P1 and contains two
bridged chain along the a direction (Fig. 2) in 2. The intra-
chain Cu⋯Cu separation is 3.393 Å, and the Cu1–N1–Cu2
angle is 120.4°, which is much larger than the 108° Cu–N–Cu
angles in other Cu(II)–EO-azido systems.5c,8b,11a,14,15 Further-
more, neighbouring chains are cross-linked by coordinated
nitro groups from the p-npa ligands to afford a 2D layer along
the ac plane (Fig. S2†), with the nearest interchain Cu⋯Cu
separation of 10.375 Å spanned by the p-npa ligands.
types of crystallographically independent Cu(II) ions, Cu1 and
Cu2, each lying at an inversion center but possessing
different geometries. As illustrated in Fig. S1(b),† Cu1 adopts
a slightly distorted octahedral geometry, while Cu2 is in a
square-planar environment. The equatorial plane of Cu1 con-
sists of two nitrogen atoms (Cu1–N1 = 1.917 Å, Cu1–N1A =
1.917 Å) from two equivalent single EO mode azido ligands
Fig. 1 The Cu(II)-chain in compound 1 with mixed water oxygen,
carboxylate and azido bridges. Benzene rings and H atoms have been
omitted for clarity.
Fig. 2 The uniform chain with mixed double bridges (syn–syn
carboxylate and EO-azido) in compound 2. Benzene rings and H atoms
have been omitted for clarity.
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Crystal structure of [Cu(p-mpa)(N3)]n (3). Although
obtained by the same synthetic method, compound
3
displays an entirely different structure compared to 2. The
single-crystal X-ray diffraction analysis suggests that
compound 3 crystallizes in the monoclinic space group P21/c
and adopts
a 2D coordination network structure. The
asymmetric unit is built up from one Cu(II) ion, one azido
ligand and one p-mpa ligand. All Cu(II) ions are five-
coordinate with a slightly distorted square-based pyramidal
geometry (Fig. S1(c)†). The apical position of Cu1 is occupied
by one oxygen atom (Cu1–O1 = 2.594 Å) from one carboxylate
group, and the equatorial plane is formed by two oxygen
atoms (Cu1–O2 = 1.926 Å, Cu1–O3 = 1.960 Å) from another
two carboxylate groups and two nitrogen atoms (Cu1–N1 =
1.979 Å, Cu1–N1A = 1.973 Å) from two azido ligands.
Single EO-azido and carboxylate bridges link two Cu(II) ions to
form a 1D Cu(II) chain (Fig. 3), in which the Cu1–N–Cu1A
angle is 122.4° and the Cu1⋯Cu1A distance is 3.463 Å.
Furthermore, the carboxylate groups link the Cu1B ions in
these 1D Cu(II) chains to form a 2D coordination network
(Fig. 4). The carboxylate groups adopt the μ3-bridging
tridentate coordination mode, which significantly lessens
the distance between two neighbouring chains. The shortest
Cu(II) ion separation between two neighbouring chains is
3.662 Å. For 3, the methyl groups on the p-mpa ligands
affect the coordination mode of the carboxylate groups
and the conformation of the 2D layers, which may
stem from the electronic effect of the substituent on
the coligand.
Fig.
4
The μ3-bridging tridentate carboxylate groups connect
adjacent chains, forming a 2D sheet in compound 3. Benzene rings
and H atoms have been omitted for clarity.
increases smoothly between room temperature and 50 K, and
then sharply reaches a maximum value of 6.86 cm3 K mol−1 at
2.58 K. Eventually, it decreases rapidly to 6.40 cm3 K mol−1 at
2 K. This type of curve is typical of strong ferromagnetic
coupling between Cu(II) ions, noticeable at very low
temperatures. The final decrease in χMT may be attributed to
the saturation effect or the presence of interchain AFM
interactions. The data above 70 K follow the Curie–Weiss law with
a Curie constant C of 0.55 cm3 K mol−1 and a Weiss constant Θ
of 37.68 K (Fig. 5, inset). The positive Θ value confirms the
ferromagnetic coupling between the Cu(II) centers, and the C
value falls in the usual range expected for Cu(II) ions. The
magnetic data of 1 were fitted using the following equations:7
2
2
3
4
5
Ng2 2 1 5.7979916x 16.902653x  29.376885x  29.832959x 14.036918x
4kT
3


(1)
 



1 2.7979916x  7.008678x2  8.6538644x3  4.5743114x4

T
MT 
(2)
(3)
2
2
Magnetic studies

1 (zJ /Ng  )
The crystalline samples of 1–3 were all phase-pure, as con-
firmed by XRPD (see Fig. S3 in the ESI†). According to the
obtained data, a dominant ferromagnetic coupling between
the Cu(II) ions in compounds 1–3 can be suggested.
Magnetic properties of [Cu(o-npa)(N3)(H2O)]n (1). The magnetic
properties of 1 are shown in Fig. 5 in the form of a χMT versus T
plot ( χM is the molar magnetic susceptibility per Cu(II) ion). The
χMT value at 300 K is equal to 0.55 cm3 K mol−1, which is higher
than the expected value (0.375 cm3 K mol−1) for magnetically
isolated Cu(II) ions (S = 1/2, g = 2.0). Upon cooling, the χMT value
x = J/2kT
The best fit of the magnetic susceptibility data resulted in
g = 2.26, J = 35.44 cm−1, and zJ′ = −0.13 cm−1 (zJ′ describes the
interchain antiferromagnetic interaction). The large J value
confirms the strong ferromagnetic coupling between the
Cu(II) centers, while the small negative zJ′ value indicates the
presence of weak interchain AFM interactions over the long
interchain distance, consistent with the drop in the χMT
product at low temperature. Magnetization versus field
data were collected for 1 at 2 K (Fig. S4†). The M value
increases very rapidly in the low field region and rises
gradually as the field increases, reaching a saturation value
of 1.18Nβ at higher fields, as expected for one Cu(II) ion with
g = 2.0, demonstrating ferromagnetic coupling along the
Cu(II) chain. To corroborate the existence of ferromagnetic
long-range ordering, measurements of the ac susceptibility
and hysteresis loop were carried out, but showed negative
results.
Fig. 3 The mixed carboxylate/EO-azido bridges form a 1D chain in
compound 3. Benzene rings and H atoms have been omitted for clarity.
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Fig. 5 χMT vs. T plot for compound 1. The solid line is the fit to the
experimental data. Inset: 1/χM vs. T plot; the red solid line is the best fit
to the Curie–Weiss law.
Fig. 6 χMT vs. T plot for compound 2. The solid line is the fit to the
experimental data. Inset: 1/χM vs. T plot; the red solid line is the best fit
to the Curie–Weiss law.
According to the structural data, the system can be treated
as an infinite uniform chain in which magnetic coupling is
mediated through the triple bridges (one EO-azido, one
syn–syn carboxylate and one coordinated water molecule
bridge). It is well known that a syn–syn carboxylate bridge
can transmit AFM interactions between neighbouring metal
ions.7 EO-azido is certainly one of the most interesting
magnetic couplers in molecular magnetism, and the magni-
tude of the J parameter depends on several factors, but
mainly the Cu–N–Cu angle (θ). The single EO azido bridge
should also mediate FM coupling, according to a number of
studies on Cu(II) systems with such bridges. According to pre-
vious reports,8 EO-azido bridging of Cu(II) ions with low θ
results in ferromagnetic coupling, but the coupling is anti-
ferromagnetic if the angle is above a critical θ value, which
has been evaluated to be about 108°.5c,8b,11a,14,15 However,
compound 1 displays ferromagnetic behaviour and has a
Cu–N–Cu angle of 110.3°, which is similar to some refuted
reports.11a For two metal ions linked by multiple bridges, the
overall coupling cannot be simply described as an algebraic
sum of the contributions arising from each bridge, which
may exhibit counter-complementarity of the superexchange
pathways in such systems.16–18 Thus the AFM contributions
of each bridge in 1 cancel each other out, resulting in strong
intrachain FM exchange between the Cu(II) ions.
the interchain magnetic interactions through the long p-npa
ligands should be much weaker than the intrachain inter-
actions through the short EO-azido and syn–syn carboxylate
bridges, the 2D structural framework can be regarded as
quasi-1D chains from a magnetic point of view.19 Thus, using
the same simulation equations as for 1, the fit yields g = 2.28,
J = 52.25 cm−1, and zJ′ = −0.36 cm−1. The positive J value can
be interpreted as a consequence of the EO-azido bridges,
which result in ferromagnetic coupling. The small negative
zJ′ value indicates the presence of interchain antiferro-
magnetic interactions, consistent with the drop in the χMT
value at 1.8 K. The reduced magnetization is shown in Fig. S5.†
The magnetization rises very quickly as the field increases
and tends towards saturation at higher fields, as expected
for one Cu(II) ion. Indeed, the rapid and abrupt increase in
M/Nβ at low field indicates a ferromagnetically ordered state.
No hysteresis was observed, and ac magnetic measurements
revealed no out-of-phase ac susceptibility signals (Fig. S6 in
ESI†), suggesting no long-range ordering or SCM behaviour.
In compound 2, the 2D layers actually consist of 1D chains
cross-linked by the ligands, where the 1D chains are created
by double syn–syn carboxylate and symmetric EO-azido
bridges, which link two consecutive Cu(II) ions. Magnetically,
the system can be treated as a uniform chain in which
magnetic exchange is mediated through the EO-azido and
syn–syn carboxylate double bridges, in spite of the fact that
adjacent Cu(II)-chains are also connected by nitro groups.
The nearest interchain Cu⋯Cu separation spanned by the
p-npa ligands is 10.375 Å. In general, the EO-azido bridge will
enable ferromagnetic exchange, while the syn–syn carboxylate
Magnetic properties of [Cu(p-npa)(N3)]n (2). The magnetic
behaviour of
2 is similar to that of 1, suggesting
ferromagnetic coupling. As shown in Fig. 6, the χMT value per
Cu(II) at room temperature is about 0.40 cm3 K mol−1, close
to the value expected for a magnetically isolated Cu(II) ion. As
the temperature is lowered, the χMT value increases smoothly
to a maximum at about 5.095 cm3 K mol−1 at 3 K, and then
decreases rapidly to 4.266 cm3 K mol−1 at 1.8 K. This
indicates that the magnetic interactions among the Cu(II)
centers are ferromagnetic, which is further confirmed by a
positive Weiss constant Θ of 19.66 K and a Curie constant C
of 0.365 cm3 K mol−1 obtained by fitting the data to the
Curie–Weiss law above 25 K (Fig. 6, inset). Considering that
will provide
a pathway for antiferromagnetic coupling
between the Cu(II) centers. However, compound 2 seems to
be in disagreement with the expectations mentioned above,
exhibiting intrachain ferromagnetic exchange between the
Cu(II) ions ( J = 52.25 cm−1). With expansion of the distance
between two chains, the interchain antiferromagnetic coupling
is weak. In 2, the Cu–N–Cu angle of the EO-azido bridge is
120.4°, which is much larger than the above-mentioned
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range of bond angles,5c,8b,11a,14,15 but overall ferromagnetic
behaviour is observed. In addition, the J parameter confirms
that the ferromagnetic interactions are enabled by the single
EO-azido bridges in compound 2.
Magnetic properties of [Cu( p-mpa)(N3)]n (3). The magnetic
behaviour of compound 3 is remarkably different from that
of
1 and 2. The temperature dependence of χMT for
compound 3 is shown in Fig. 7 as a χMT versus T plot. The
value of χMT at 300 K is 0.534 cm3 K mol−1, larger than the
predicted value of 0.375 cm3 K mol−1 for an isolated Cu(II)
ion (S = 1/2) with g = 2.0. As the temperature is lowered from
room temperature, the χMT value continuously increases to
reach a maximum value of 1.418 cm3 K mol−1 at 18 K, and
then it decreases rapidly to a value of 0.424 cm3 K mol−1 at
1.9 K, which is attributed to zero-field splitting of the Cu(II)
ions or AFM interactions between the chains. As the tempera-
ture decreases, the χMT product increases, indicating ferro-
magnetic coupling between the Cu(II) ions within the chains
Fig. 8 FC and ZFC plots at 10 Oe for compound 3.
temperature. To corroborate the existence of ferromagnetic
long-range ordering, measurements of the ac magnetic sus-
ceptibility and hysteresis loop were also carried out for 3. The
ac susceptibility measurements were performed in the range
1.8–10 K at frequencies of 1, 10, 100 and 1000 Hz. Fig. 9
shows a peak in the in-phase (χ′M) signal and an out-of-phase
(χ′′M) signal that is non-zero below 7 K, defining Tc for this
magnet. No frequency dependence was observed in the ac
measurements, thus excluding the possibility of any glassy
behaviour in compound 3. Furthermore, as shown in Fig. 10,
the hysteresis magnetization was measured at 2 K, 4 K and
10 K within 6 kOe. A characteristic hysteresis loop with a
narrow gap is observed at 4 K, which strongly supports the
bulk ferromagnetic ordering of 3.
For 3, the methyl groups of the coligand affect the carbox-
ylate groups linkages, and thus the magnetic behaviour of
the compound. In contrast to 1 and 2, the chains are linked
via tridentate-bridging carboxylate groups, which strengthen
the global ferromagnetic behaviour. Compared to 1 and 2,
the carboxylate groups in 3 significantly lessen the distance
between two neighbouring chains, with a nearest Cu⋯Cu
separation of 3.662 Å. According to the chain topology, there
are three sets of magnetic exchange pathways: one syn–syn
carboxylate bridge, one EO-azido bridge (intrachain) and two
−1
in 3. The Curie–Weiss fit of χM above 60 K results in a Curie
constant C = 0.48 cm3 K mol−1 and a Weiss constant Θ =
33.45 K (Fig. 7, inset). The positive Θ value suggests the
presence of dominant ferromagnetic coupling between the
Cu(II) ions. Within this compound, the main Cu–bridge–Cu
coupling pathways are listed as follows: Cu1–N1–Cu1A,
Cu1–(COO)–Cu1A, and Cu1A–O2–Cu1B. With no appropriate
magnetic model for a system with various bridges, it is diffi-
cult to simulate the magnetic behaviour and calculate the J
values from experimental data. At 1.8 K, the field-dependent
magnetization increases linearly to a maximum value of
0.75Nβ (Fig. S7†), indicating that the magnetic moment is
still unsaturated (the expected saturated value is Ms = nSg =
1Nβ). This behaviour is probably due to the significant anti-
ferromagnetic interactions between the Cu(II) ions, since the
Cu(II) ions are less anisotropic.20 The field-cooled (FC) and
zero-field-cooled (ZFC) magnetization measurements were
performed at a low applied field of 10 Oe in the temperature
range of 1.8–10 K (Fig. 8). The data diverge below 7 K and the
magnetization increases continuously with decreasing
Fig. 7 χMT vs. T plot for compound 3. The solid line is the fit to the
experimental data. Inset: 1/χM vs. T plot; the red solid line is the best fit
to the Curie–Weiss law.
Fig. 9 χ′M and χ′′M vs. T plots for compound 3.
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in phenylacetic acid compared to benzoic acid. The different
substituents on the aromatic ring may fine-tune the
structures and further affect the magnetic properties of the
compounds.
Acknowledgements
We gratefully acknowledge financial support from the
National Natural Science Foundation of China (grant no.
21373162, 21073142 and 21173168), and the Nature Science
Foundation of Shaanxi Province (grant no. 11JS110, FF10091
and SJ08B09).
References
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Conclusions
Our interests are to probe how flexible aromatic carboxylate
coligands with different substituents influence the structure
and magnetism of Cu(II)–azido compounds. Diverse struc-
tures and magnetic properties have been observed in the
three compounds. Structural analyses show that compound 1
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3
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