Isomorphous Co(ii) and Ni(ii) antiferromagnets based on mixed azide- and carboxylate-bridged chains: Metamagnetism and single-chain dynamics

Article abstract of DOI:10.1039/c1dt11068c

Two isomorphous Co(ii) and Ni(ii) coordination polymers with azide and the 4-(4-pyridyl)benzoic acid N-oxide ligand (4,4-Hopybz) were synthesized, and structurally and magnetically characterized. They are formulated as [M(4,4-opybz)(N₃)(H₂O)]_n (M = Co, 1 and Ni, 2). The compounds consist of 2D coordination networks, in which 1D coordination chains with (μ-N₃)(μ-COO) bridges are interlinked by the 4,4-opybz spacers, and the structure also features intra- and interchain O-H...O hydrogen-bonding bridges between metal ions. Both compounds exhibit ferromagnetic interactions through the intrachain (μ-N₃)(μ- COO)(O-H...O) bridges and antiferromagnetic interactions through the interchain O-H...O bridges. The ferromagnetic chains are antiferromagnetically ordered, and the antiferromagnetic phases exhibit field-induced metamagnetic transition. It is found that 1 displays slow relaxation of magnetization, typical of single-chain magnets, while 2 does not. The difference emphasizes the great importance of large magnetic anisotropy for single-chain-magnet dynamics.

Full text of DOI:10.1039/c1dt11068c

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PAPER
Isomorphous Co(II) and Ni(II) antiferromagnets based on mixed azide- and
carboxylate-bridged chains: metamagnetism and single-chain dynamics†
Xiu-Mei Zhang,a,b Kun Wang,a Yan-Qin Wanga and En-Qing Gao*a
Received 7th June 2011, Accepted 2nd September 2011
DOI: 10.1039/c1dt11068c
Two isomorphous Co(II) and Ni(II) coordination polymers with azide and the 4-(4-pyridyl)benzoic acid
N-oxide ligand (4,4-Hopybz) were synthesized, and structurally and magnetically characterized. They
are formulated as [M(4,4-opybz)(N3)(H2O)]n (M = Co, 1 and Ni, 2). The compounds consist of 2D
coordination networks, in which 1D coordination chains with (m-N3)(m-COO) bridges are interlinked
by the 4,4-opybz spacers, and the structure also features intra- and interchain O–H ◊ ◊ ◊ O
hydrogen-bonding bridges between metal ions. Both compounds exhibit ferromagnetic interactions
through the intrachain (m-N3)(m-COO)(O–H ◊ ◊ ◊ O) bridges and antiferromagnetic interactions through
the interchain O–H ◊ ◊ ◊ O bridges. The ferromagnetic chains are antiferromagnetically ordered, and the
antiferromagnetic phases exhibit field-induced metamagnetic transition. It is found that 1 displays slow
relaxation of magnetization, typical of single-chain magnets, while 2 does not. The difference
emphasizes the great importance of large magnetic anisotropy for single-chain-magnet dynamics.
Recently we have been investigating magnetic systems with
simultaneous carboxylate and azide bridges. We have established
Molecular magnetic materials have been one of the most active
an efficient synthetic approach to such systems: making use
Introduction
topics for decades.1,2 The huge diversity and versatility of co-
of zwitterionic mono- or dicarboxylate ligands to facilitate the
simultaneous coordination of carboxylate and azide.7a,9–11 Bifunc-
tional ligands bearing carboxylate and pyridyl or pyridyl-N-oxide
coordinative groups have also been used to construct simulta-
neous bridging systems, but the study using the pyridyl-N-oxide
derivatives has only been limited to the simple pyridylcarboxylate-
N-oxide.7d,12 Noticeably, the simultaneous bridges usually induce
FM interactions in Co(II) and Ni(II) systems, some of which
are composed of FM chains and display metamagnetic and/or
SCM properties.7,9,13 To extend the study, here we report the
structures and magnetic properties of two isomorphous com-
pounds of formula [M(4,4-opybz)(N3)(H2O)]n (4,4-opybz = 4-(4-
pyridyl)benzoate N-oxide, M = Co, 1 and Ni, 2). They consist of 2D
coordination layers in which 1D chains with [(N3)(COO)] double
bridges are interlinked by 4,4-opybz ligands, and the structure
is reinforced by intra- and interchain O–H ◊ ◊ ◊ O hydrogen bond
bridges between the metal ions. Magnetically, both compounds
exhibit intrachain FM interactions, AF order, and field-induced
metamagnetism. However, the different metal ions lead to dra-
matic differences in dynamic properties: 1 shows complex relax-
ation dynamics related to the SCM components, while 2 does not.
ordination and supramolecular chemistry have provided great
opportunities to obtain new magnetic materials and to better
understand fundamental magnetic phenomena. Field-induced
metamagnets are a special type of antiferromagnets in which the
weak antiferromagnetic (AF) order between magnetic chains or
layers can be broken up by an applied field to induce a first-order
transition,3 while single-chain magnets (SCMs) are magnetizable
1D systems exhibiting slow dynamics of magnetization.4–6 They
both require strong magnetic correlations to generate ferromag-
netic (FM), ferrimagnetic or spin-canted AF chains (and layers for
metamagnets). Large uniaxial magnetic anisotropy is required for
SCMs, and interchain interactions may impede SCM dynamics. By
contrast, weak interchain/interlayer AF interactions are required
for metamagnets.3a Generally speaking, the requirements of SCMs
are more difficult to meet, and sometimes the attempts to
obtain SCMs led to metamagnets.7 Interestingly, it has been
recently demonstrated that metamagnetism and SCM dynamics
can coexist, but the examples are still very limited.8,9
aShanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal University, Shanghai, 200062,
China. E-mail: eqgao@chem.ecnu.edu.cn
Experimental
Materials and physical measurements
bSchool of Chemistry and Materials Science, Huaibei Normal University,
Huaibei, Anhui, 235000, China
† Electronic supplementary information (ESI) available: Supplemen-
tary graphics. CCDC reference number 826223 (1). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt11068c
The reagents were obtained from commercial sources and used
without purification. The ligand (4,4-Hopybz) was prepared
according to a procedure for similar compounds.14
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Elemental analyses were determined on an Elementar Vario
ELIII analyzer. The FT-IR spectra were recorded in the range
500–4000 cm-1 using KBr pellets on a Nicolet NEXUS 670
spectrophotometer. Temperature- and field-dependent magnetic
measurements were performed on a Quantum Design MPMSXL5
SQUID magnetometer. The experimental susceptibilities were
corrected for the diamagnetism of the constituent atoms (Pascal’s
tables).15
Powder X-ray diffraction
Powder X-ray diffraction data for 1 and 2 were collected on
a Bruker D8-ADVANCE diffractometer equipped with Cu-Ka
◦
radiation (l = 1.5406 A) at a scan speed of 2 min-1 in the 2q range
of 5–35◦. Since the patterns of 1 and 2 are very similar, the unit cell
parameters of 2 were refined in the P21/c space group using the
CELREF program,18 with the parameters of 1 as initial values. A
total of 40 reflections were used in the refinements, and the final
˚
˚
refinement gave a = 7.299(9), b = 25.87(2), c = 14.02(2) A, b =
Synthesis
ꢀ
◦
105.7(1) and V = 2548(5) A with R = ( (2qobs - 2qcald) /Nref)1/2
=
3
2
˚
[Co(4,4-opybz)(N3)(H2O)]n (1). A mixture of CoCl2·6H2O
(0.1 mmol, 0.024 g), NaN3 (0.5 mmol, 0.032 g), and 4,4-Hopybz
(0.025 mmol, 0.0062 g) was dissolved in H2O (3 ml) in a Teflon-
lined stainless steel vessel (25 ml), heated at 70 ◦C for 3 days under
autogenous pressure, and then cooled to room temperature. Purple
plate crystals of 1 were obtained in a yield of 83% based on 4,4-
Hopybz. Anal. (%): Found: C, 43.41; H, 3.39; N, 16.55. Calc. for
C12H10CoN4O4: C, 43.26; H, 3.03; N, 16.82. Main IR bands (KBr,
cm-1): 2062s [n(N3)], 1593m [nas(COO)], 1560m, 1521m, 1477w,
1398s [ns(COO)], 1186m, 851m, 777w.
0.0309.
Results and discussion
Crystal structures
Single-crystal X-ray analyses revealed that compound 1 consists
of 2D layers in which 1D [Co(N3)(COO)]n chains are connected
by the 4,4-opybz spacers. Selected bond lengths and angles
are summarized in Table 2. There are three crystallographically
independent CoII ions. The Co1 ion assumes the octahedral [N2O4]
geometry defined by two transoid carboxylate oxygen atoms (O1,
O4), two cisoid coordinated water molecules (O7, O8) and two
cisoid azide nitrogen atoms (N3, N6). The Co1–N (O) distances
[Ni(4,4-opybz)(N3)(H2O)]n (2). A procedure similar to that
for 1 was followed to prepare 2 using NiCl2·6H2O instead of
CoCl2·6H2O. Green polycrystals of 2 were obtained in a yield of
78% based on 4,4-Hopybz. Our attempts to get single crystals of 2
by different methods were fruitless. Anal. (%): Found: C, 43.50; H,
3.49; N, 17.26. Calc. for C12H10NiN4O4: C, 43.29; H, 3.03; N, 16.83.
Main IR bands (KBr, cm-1): 2060s [n(N3)], 1596m [nas(COO)],
1564m, 1516m, 1483w, 1386s [ns(COO)], 1199m, 850m, 646w.
˚
range from 2.064(2) to 2.162(2) A. The Co2 and Co3 are located
at different inversion centers in similar trans-octahedral [N2O4]
environments, with only minor differences in bond parameters.
Four oxygen atoms (two from carboxylate groups and two from
N-oxide groups) define an equatorial plane, with the axial sites
occupied by two azide nitrogen atoms. The equatorial Co–O
distances are somewhat shorter than the Co–N ones. The metal
ions alternate in the –Co1–Co2–Co1–Co3– repeating sequence,
and the adjacent metal ions are doubly bridged by a carboxylate
group in the syn,syn mode and an azide ligand in the end-on
(EO) mode, generating a 1D chain along the [101] direction
(Fig. 1). The two double bridges between Co1 and Co2 and
between Co1 and Co3 are somewhat different in terms of their
structural parameters. The [Co1(COO)(N3)Co2] moiety has a Co–
N–Co bridging angle of 123.3(1)◦, a Co–O ◊ ◊ ◊ O–Co torsion angle
Crystal structure analysis
Diffraction data for 1 was collected at 293 K on a Bruker Apex II
CCD area detector equipped with graphite-monochromated Mo-
Ka radiation (l = 0.71073 A). Empirical absorption corrections
˚
were applied using the SADABS program.16 The structures were
solved by direct methods and refined by the full-matrix least-
squares method on F2, with all non-hydrogen atoms refined with
anisotropic thermal parameters.17 All the hydrogen atoms attached
to carbon atoms were placed in calculated positions and refined
using the riding model, and the water hydrogen atoms were located
from the difference maps. Selected crystal data are listed in Table 1.
◦
˚
of 27.6(1) , and a Co ◊ ◊ ◊ Co distance of 3.778(4) A, while the
corresponding parameters for the [Co1(COO)(N3)Co3] moiety
are 120.4(1)◦, 35.8(1)◦ and 3.704(5) A, respectively. It is noted
˚
Table 1 Crystal data and structure refinement for compound 1
Compound
1
Formula
Mr
C12H10CoN4O4
333.17
Crystal system
Space group
Monoclinic
P21/c
˚
a/A
7.2227(7)
26.181(2)
14.2067(12)
106.151(4)
2580.4(4)
8
1.715
1.353
5025
0.0365
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
Dc/g cm-3
m/mm-1
Unique reflections
Rint
R1 [I > 2s(I)]
wR2 (all data)
0.0348
0.0856
Fig. 1 A chain structure with azide, carboxylate and hydrogen bonding
bridges in 1.
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◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for compound 1
Co1–O4
Co1–O7
Co1–O8
2.064(2)
2.103(2)
2.154(2)
Co1–O1
Co1–N6
Co1–N3
2.097(2)
2.119(2)
2.162(2)
Co2–O2
Co2–N3
Co3–O3E
2.070(2)
2.132(2)
2.128(2)
Co2–O6B
Co3–O5
Co3–N6
2.115(2)
2.079(2)
2.148(2)
O4–Co1–O1
O1–Co1–O7
O1–Co1–N6
O4–Co1–O8
O7–Co1–O8
O4–Co1–N3
O7–Co1–N3
O8–Co1–N3
O2–Co2–O6C
177.62(8)
90.92(8)
90.41(8)
89.91(7)
78.94(7)
88.28(8)
88.79(8)
167.68(8)
95.25(7)
O4–Co1–O7
O4–Co1–N6
O7–Co1–N6
O1–Co1–O8
N6–Co1–O8
O1–Co1–N3
N6–Co1–N3
O2A–Co2–O2
O2–Co2–O6B
86.70(1)
91.95(8)
171.46(8)
89.62(7)
92.63(8)
91.68(8)
99.61(9)
180.000(1)
84.75(7)
O6B–Co2–O6C
O2–Co2–N3A
O6B–Co2–N3
O5–Co3–O5D
O5–Co3–O3E
O5–Co3–N6
O3F–Co3–N6
N6–Co3–N6D
Co1–N6–Co3
180.00(9)
89.89(8)
91.43(8)
180.00(5)
92.74(7)
90.81(8)
88.84(8)
180.0
O2–Co2–N3
90.11(8)
88.57(8)
180.000(1)
87.26(7)
180.00(8)
89.19(8)
91.16(8)
123.3(1)
O6C–Co2–N3
N3A–Co2–N3
O5–Co3–O3F
O3E–Co3–O3F
O5–Co3–N6D
O3E–Co3–N6
Co2–N3–Co1
120.4(1)
Symmetry codes: (A) -x + 1, -y, -z + 2; (B) x, -y + 1/2, z + 1/2; (C) -x + 1, y - 1/2, -z + 3/2; (D) -x + 1, -y, -z + 1; (E) -x + 1, y + 1/2, -z + 3/2; (F)
x, -y - 1/2, z - 1/2.
that the chain structure is reinforced by two sets of intrachain
hydrogen bonds between the coordinated water molecules (O7 and
O8) and the coordinated N-oxide oxygen◦atoms (O6B and O3E)
˚
(Fig. 1), with O7–H7C ◊ ◊ ◊ O6B = 168.5(1) , H ◊ ◊ ◊ O = 1.772(3) A,
◦
˚
O7 ◊ ◊ ◊ O6 = 2.678(4) A; and O8–H8C ◊ ◊ ◊ O3E = 169.9(1) , H ◊ ◊ ◊ O =
˚
˚
1.778(3) A, O8 ◊ ◊ ◊ O3 = 2.625(5) A. The hydrogen bonds provide
triatomic O–H ◊ ◊ ◊ O bridges between adjacent Co(II) ions, and
therefore, it can be stated that the Co(II) ions are linked by the
triple [(m-COO)(m-EO-N3)(O–H ◊ ◊ ◊ O)] bridges.
The coordinated water molecules (O7 and O8) also form
interchain hydrogen bonds. The O8 water molecule as donor is
hydrogen-bonded to an azide nitrogen atom (N5) from another
chain with O8–H8B ◊ ◊ ◊ N5 (1 + x, y, z) = 165.3(2)◦, H ◊ ◊ ◊ N =
˚
˚
2.165(3) A and O8 ◊ ◊ ◊ N5 = 2.992(4) A, while the O7 water
molecule forms a hydrogen bond with an N-oxide oxygen atom
(O6) from anothe◦r chain, with O7–H7B ◊ ◊ ◊ O6 (2 - x, -0.5 + y,
˚
1.5 - z) = 161.3(2) , H ◊ ◊ ◊ O = 2.033(3) A and O7 ◊ ◊ ◊ O6 = 2.850(4)
A. Noticeably, the latter hydrogen bond builds up a short triatomic
˚
O–H ◊ ◊ ◊ O bridge between Co(II) ions from different chains, with
˚
a relatively short Co ◊ ◊ ◊ Co distance of 5.98(1) A. This feature
may be relevant to magnetic behaviors. The interchain hydrogen
bonds serve to interlink the [Co(COO)(N3)]n chain into a 2D
sheet along the ac plane (Fig. 2a). The sheet features an eight-
membered hydrogen-bonded ring with graph set R4 (8),19 which is
2
formed from two pairs of triatomic O–H ◊ ◊ ◊ O bridges involving
two water molecules and two N-oxide oxygen atoms from different
coordination chains.
Fig. 2 (a) 2D sheet through hydrogen bonds. The dashed lines represent
the intrachain (grey) and interchain (purple) hydrogen bonds. (b) 2D
network formed by the 4,4-opybz ligands connecting the chains with mixed
bridges.
The 4,4-opybz ligand serves as a m3 bridge, with the syn,syn
carboxylate group binding two metal ions from one chain and
the N-oxide group binding a metal ion from another chain,
Thus the chains are interlinked into a 2D layer along the
(Fig. 3), indicating that 2 is isomorphic with 1. The peaks observed
for 2 can be indexed in the same space group with similar cell
parameters. The cell volume of 2 is slightly smaller than that of 1,
as expected from the smaller radius of Ni(II) compared to Co(II).
¯
(101) plane (Fig. 2b). The nearest interchain Co ◊ ◊ ◊ Co distance
˚
spanned by the 4,4-opybz ligand is 13.090(1) A. The coordination
layers are stacked into a 3D structure through the above-
mentioned O–H ◊ ◊ ◊ N and O–H ◊ ◊ ◊ O hydrogen bonds (Fig. S1,
ESI†). Alternatively, the 3D structure may be described as a
layer-pillared structure with hydrogen-bonded layers (Fig. 2a) and
organic pillars (the backbones of the 4,4-opybz ligands) (Fig. S2,
ESI†).
We failed in obtaining single crystals of 2 for X-ray crystal-
lographic analysis. However, the PXRD pattern of 2 is in good
agreement with that calculated from the single-crystal data of 1
Magnetic properties
Compound 1. The magnetic susceptibility (c) of compound 1
was measured using powdered crystals under 1 kOe in the range
of 2–300 K (Fig. 4). The cT value per Co(II) at 300 K is about 3.64
emu K mol-1, falling within the usual range for octahedral Co(II)
with an unquenched orbital momentum. As the temperature is
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below), the FM chains are long-range ordered by interchain AF
interactions.
The isothermal magnetization behaviors from 0 to 5 T were
measured at 2 K (Fig. 5a). The overall rise of magnetization
upon increasing the field to 4 kOe is much more rapid than
expected for isolated Co(II) systems, confirming the presence of
FM coupling between Co(II) ions. The magnetization increases
slowly and quasi-linearly in the high-field range and shows no
indication of saturation even at the high field of 5 T, attributable
to the large magnetic anisotropy of the octahedral Co(II) system.
Moreover, the low-field region of the magnetization curve shows a
sigmoid shape, indicative of field-induced metamagnetism: the FM
chains are long-range ordered in an AF phase at zero field, and the
AF order can be broken up by an applied field. The critical field,
corresponding to the maximum position in the (dM/dH)–H plot,
is estimated to be 950 Oe at 2 K (Fig. 5a inset). The magnetization
loop measured by cycling the field between 5 and -5 T at 2 K
shows the double sigmoid shape typical of metamagnets. Narrow
hysteresis was observed in the AF region, with very small coercive
field and remnant magnetization at zero field (Fig. 5b).
Fig. 3 Comparison of the XRD pattern measured for 2 (black) with that
calculated from the single-crystal data of 1 (red).
Fig. 4 Temperature dependence of cT for 1. Inset: ln(cT) vs. 1/T plots,
the solid line representing the linear fit.
lowered, the cT value increases continuously to a maximum at
8.0 K and then drops rapidly. The data above 30 K follow the
Curie–Weiss law with C = 3.50 emu K mol-1 and q = 11.8 K.
The behaviors evidently indicate the presence of intrachain FM
interactions.
It is well known that the correlation length (x) of a 1D FM chain
with uniaxial anisotropy diverges exponentially with temperature:
x μ cT ª Cexp(Dx/T) (where Dx is the exchange energy cost to
create a domain wall along the chain), which suggests a linear
variation of ln(cT) with 1/T.1c,20 The ln(cT) vs. 1/T plot for
1 (Fig. 4 inset) displays a linear region between 12 and 35 K
with a slope of Dx = 22.3 K, confirming the uniaxially anisotropic
character of the chain. According to the Ising-chain model with
ꢀ
uniform magnetic exchange (H = -JS2 riri +1, where si = 1),
Dx is related to magnetic exchange by Dx = 2JS2.1c,21 In our case,
handling Co(II) at low temperature as an S = 1/2 effective spin, we
obtained J = 44.6 K (31.0 cm-1). Since the structural data reveal
alternating bridging motifs along the chain, we have attempted to
evaluate the alternating magnetic interactions using a two-J Ising-
chain model,22 but the best fits in appropriate temperature ranges
(from 35 to 12 K or lower) led to two widely different parameters
(e.g., 20 and 64 cm-1), which seem to be unrealistic because the two
bridging motifs have similar structural parameters.
Fig. 5 (a) Plots of M vs. H and dM/dH vs. H (inset) for 1 at 2 K. (b)
Magnetic hysteresis at 2 K.
To confirm the metamagnetic behaviours, field-cooled (FC)
magnetization measurements were performed under different
fields. The FC curves under low field (20 Oe in Fig. 6a) show
a maximum at about 7.9 K, supporting the occurrence of
AF ordering. When the external field is increased (Fig. 6b),
the maximum value shifts toward low temperature and finally
disappears, confirming the field-induced phase transition from AF
to FM.
The ln(cT) vs. 1/T plot deviates significantly from linearity
at lower temperature and shows a maximum at 8.0 K. This may
be due to saturation effects and interchain AF interactions. As
will be shown by low-field dc and zero-field ac measurements (see
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Fig. 7 Ac susceptibilities of 1 measured at frequencies of 1, 3, 10, 33, 100,
330 and 997 Hz under zero dc fields with a driving ac field of 3.5 Oe. Inset:
temperature dependent relaxation time fitted to the Arrhenius equation.
at low frequencies. These may suggest the operation of multiple
relaxation processes.25 Therefore, the above-derived parameters for
a single thermal activation process are rough approximations.
Considering the structural feature of 1, it is most likely that
the slow relaxation of magnetization in the AF phase arises from
the SCM component (the FM and anisotropic Co(II) chains). The
retention of SCM behaviors in AF phases has been experimentally
and theoretically demonstrated recently in a few compounds.9 The
Glauber dynamics for Ising chains26 predicts that Dt = 2Dx for
“infinite” chains and Dt = Dx for “finite-size” chains.1c,4b,27 For
1, Dt < 2Dx, suggesting that the slow relaxation occurs in the
“finite-size” regime, where the increase of x is limited by naturally
occurring defects. The difference between Dt and Dx can be justified
by assuming that the spin flip in absence of magnetic exchange
is also thermally activated with an anisotropy barrier, DA. The
activation barrier Dt for relaxation depends not only upon the
correlation energy Dx but also upon the single-ion anisotropy
barrier DA. For “finite-size” chains, Dt = Dx + DA. Thus, we get
DA = Dt - Dx = 9.7 K. The comparison that DA ꢀ Dx indicates that
broad domain walls are relevant in this system.1c,20
Fig. 6 (a) ZFC and FC magnetization curves of 1 at 20 Oe. (b) FC
magnetization curves of 1 at different fields.
The zero-field-cooled (ZFC) magnetization at 20 Oe is also
measured. As shown in Fig. 6a, the ZFC and FC magnetization
curves diverge below about 5 K. The magnetization irreversibility
is unusual for an AF ordered phase. To gain insight into the
origin of the magnet-like irreversibility, thermal ac susceptibility
measurements (Fig. 7) were performed on 1 under a zero dc field
with a driving ac field 3.5 Oe oscillating at different frequencies.
As shown in Fig. 7, the real component (c¢) of the ac sus-
ceptibility exhibits a frequency-independent maximum at 7.7 K,
while the imaginary component (c¢¢) is zero down to 6 K. The
phenomenon is typical of antiferromagnets. However, at lower
temperature, the c¢ plots display shoulders and meanwhile the
c¢¢ data become nonzero and show peaks. Both components are
strongly dependent upon frequency, with the c¢¢ maximum shifting
from 3.6 K at 997 Hz to <2 K at 1 Hz, suggesting the occurrence
of slow magnetization relaxation in the AF phase. The frequency
dependence is measured by f = (DTp/Tp)/D(logf ) ª 0.15 (Tp is
the temperature at which c¢¢ reaches a maximum). This value
is out of the range for spin glass (f £ 0.08)23 but consistent
with superparamagnets including SCMs (0.1 £ f £ 0.3).23,24 The
relaxation time t(T) data derived from the c¢¢(T) peaks can be
fitted to the Arrhenius law t = toexp(Dt/T) (Fig. 7 inset), with
to = 2.44 ¥ 10-8 s and Dt = 32.0 K (Dt is the energy barrier
to reverse the magnetization), suggesting a thermally activated
mechanism of the magnetic relaxation. The to and Dt values lie
in the typical range for superparamagnets including SCMs.4a,24
Notably, the c¢¢(T) peaks are dissymmetric, and small shoulders
are present at the high-temperature sides of the peaks measured
Isothermal ac measurements in the frequency range 0.1–
1500 Hz were performed at 2.6 K. The c¢¢–c¢ plot (Cole–Cole
diagram) is highly dissymmetric, and seems to be an envelope
of two semicircles corresponding to relaxation processes (Fig. 8).
Thus we made attempts to fit the frequency-dependent data to the
sum of two modified Debye functions:28
⎡
⎢
⎢
⎣
⎤
⎥
⎥
⎦
b
1−b
c(w) = cS +(cT − cS )
+
1−a
1
2
1+(iwt1)
1+(iwt2 )1−a
where b is the weight of the first process, and the other symbols
have their usual meanings. The best-fit led to cT = 1.06 emu mol-1,
cS = 0.19 emu mol-1, a1 = 0.47, t1 = 1.5 ¥ 10-3 s, a2 = 0.50, t1 =
0.16 s and b = 0.58. The relatively large a values indicate a wide
12746 | Dalton Trans., 2011, 40, 12742–12749
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Fig. 8 Cole–Cole diagram of 1 at 2.6 K. The solid line is the best fit to
the two-Debye-relaxation model.
Fig. 9 Temperature dependence of cT and c for 2 under 1 kOe. The solid
lines represent the best fit to the appropriate model (see text).
distribution of relaxation time for both processes, but still lie in
the range for reported SCMs.4a The occurrence of two separate
relaxation processes may be related to the presence of two distinct
types of anisotropic Co(II) sites along the chain (Co1 in the cis-
octahedral [N2O4] environment while Co2 and Co3 are in similar
trans-octahedral [N2O4] environments).25a,c Further investigations
are needed to confirm the hypothesis, although it is well known
that the relaxation rate is sensitive to the single-ion anisotropy and
to variations of the coordination environment.1c,29
ꢀ
H = - (J1S2iS2i+1 + J2S2iS2i-1
)
AX 3 + BX 2 +CX +1
DX 2 + EX +1
c = (2N b2g2/3kT )
where X = J1/kT, and the coefficients A–E are polynomial
functions of a = J2/J1, The best fit of the data above 30 K produced
J1 = 34.4 cm-1, J2 = 29.9 K (a = 0.87) with g = 2.28. The FM
interactions, confirmed by the positive J values, are consistent with
previous studies on Ni(II) compounds with similar (COO)(N3)
bridges (J = 24–36 cm-1).7d,31
Compound 1 is similar to another Co(II) compound (3) reported
recently by us,9a which is composed of 1D chains with the (m-
COO)2(m-N3) triple bridge. They both exhibit intrachain FM
coupling, interchain AF ordering, field-induced metamagnetic
transition, and SCM-based slow dynamics. The Co–N–Co angles
The presence of the c maximum at 5.9 K suggests the occurrence
of long-range AF ordering of the FM chains. Further magneti-
zation measurements performed under different fields (Fig. 10a)
revealed that the maximum shifts to lower temperature as the
field is increased and disappears at the high field of 3.0 kOe. The
behaviors are characteristic of field-induced metamagnetism, and
the disappearance of the maximum indicates that the interchain
AF ordering is broken up by the field. The metamagnetic
transition is confirmed by the sigmoidal shape of the isothermal
magnetization curve measured at 2 K (Fig. 10b), from which the
critical field (Hc) is estimated to be about 2.3 kOe. No evident
hysteresis was observed upon cycling the field between -5 and 5
T at 2 K (Fig. S3, ESI†). Finally, the ac magnetic measurements
were performed under zero dc field at different frequencies (Fig.
S4, ESI†). The ac susceptibility exhibits a maximum in the real
component (c¢) at 6.2 K. No frequency dependency and no evident
imaginary signal (c¢¢) were observed, typical of AF ordering.
◦
˚
(av. 121.9(1) ) and Co ◊ ◊ ◊ Co distances (av. 3.741(2) A) for the
Co(COO)(N3)(O–H ◊ ◊ ◊ O)Co bridging moieties in 1 are compa-
rable to those (122.2(2)◦ and 3.659(1) A) for Co(COO)2(N3)Co
˚
in 3, but the intrachain FM coupling (J = 31.0 cm-1) in 1 is
significantly weaker than that (J = 54.1 cm-1) in 3. This suggests
that the intrachain O–H ◊ ◊ ◊ O hydrogen bond bridge is less efficient
in promoting the FM interactions than the O–C–O carboxylate
bridge. Importantly to the bulk magnetic behaviors, the interchain
O–H ◊ ◊ ◊ O bridge between Co(II) ions in 1 provides a short pathway
˚
(Co ◊ ◊ ◊ Co distance, 5.98 A) for interchain magnetic interactions,
whereas the chains in 3 are much farther separated, the shortest
˚
Co ◊ ◊ ◊ Co distance being 7.75 A (associated with weak C–H ◊ ◊ ◊ O
hydrogen bonds). These structural differences may account for
the higher AF ordering temperature and the higher metamagnetic
critical field in 1 (Tc = 7.7 K and Hc = 950 Oe. For comparison,
the data for 3 are 6.4 K and 100 Oe). Furthermore, the presence
of two distinct types of Co(II) sites may be responsible for the
complex relaxation processes in 1. For comparison, there is only
one independent Co(II) site in 3, which shows a single relaxation
process.
Conclusions
We have described the structures and magnetic properties of two
isomorphous compounds of formula [M(4,4-opybz)(N3)(H2O)]n
(4,4-opybz = 4-(4-pyridyl)benzoate N-oxide, M = Co, 1 and
Ni, 2). In the compounds, metal ions are linked into chains
by simultaneous m-EO-N3, syn,syn-COO and O–H ◊ ◊ ◊ O bridges,
and the chains are interlinked by the backbones of the 4,4-
opybz ligands into 2D coordination layers. The structure also
features interchain O–H ◊ ◊ ◊ O bridges between metal ions. Both
compounds exhibit FM interactions through the intrachain (m-
N3)(m-COO)(O–H ◊ ◊ ◊ O) bridges and AF interactions through the
interchain O–H ◊ ◊ ◊ O bridges. The FM chains are AF ordered, and
Compound 2. The cT value of 2 at 300 K (1.56 emu K mol-1)
is typical of Ni(II) systems with g > 2. Upon cooling, the c and cT
values increase smoothly to maxima at 5.9 and 7 K, respectively,
and then drop rapidly (Fig. 9). The 1/c vs. T plot above 140 K
obeys the Curie–Weiss law, with C = 1.31 emu mol-1 K and q =
46.6 K. The positive q value and the increase of cT with decreased
temperature clearly indicate FM coupling between the Ni(II) ions.
According to structural data, the thermal magnetic data of 2
was analyzed using the polynomial expression for Ni(II) chains
with alternating FM interactions: 30
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