Co-ligand-directed structural and magnetic diversities in an anisotropic CoII-triazolate system

Article abstract of DOI:10.1039/c1dt10394f

Three novel 3,5-diamino-1,2,4-triazole (Hdatrz)-based Co^II coordination complexes, [Co(Hdatrz)_0.5(H₂O) ₂(btec)_0.5]_n (1), {[Co(Hdatrz)(Hbtc)] ·H₂O}_n (2) and [Co₂(datrz) ₂(nb)₂]_n (3) (H₄btec = 1,2,4,5-benzenetetracarboxylic acid, H₃btc = 1,3,5- benzenetricarboxylic acid and Hnb = 4-nitrobenzoic acid), were synthesized by incorporating different carboxylate-containing co-ligands and then were structurally and magnetically characterized. Complex 1 is a 3D pillared-layer framework with corrugated Co^II-btec^4- layers supported by neutral μ₂-N1, N4-Hdatrz ligands. In contrast, the other two complexes are chiral (4, 4)- and racemic (4, 8²)-topological layers with asymmetric μ₂-N1, N4-Hdatrz-bridged helical chains extended by bis-monodentate Hbtc^2- ligands for 2 and with a μ₃-N1, N2, N4-datrz^- aggregated Shastry-Sutherland magnetic layer for 3. More interestingly, different magnetic phenomena with a field-induced metamagnetic transition from antiferromagnetic ordering to a ferromagnetic state for 1, spin-canted antiferromagnetism with a T_N lower than 2.0 K for 2, as well as the coexistence of spin frustration and spin-flop transitions for 3 were observed, which, significantly, are governed by the local low-dimensional magnetic motifs mediated by the carboxylate and/or triazolate heterobridges in the anisotropic Co^II-triazolate system. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10394f

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PAPER
Co-ligand-directed structural and magnetic diversities in an anisotropic
CoII–triazolate system†
En-Cui Yang,* Zhong-Yi Liu, Tian-Yu Liu, Li-Ly Li and Xiao-Jun Zhao*
Received 9th March 2011, Accepted 20th May 2011
DOI: 10.1039/c1dt10394f
Three novel 3,5-diamino-1,2,4-triazole (Hdatrz)-based CoII coordination complexes,
[Co(Hdatrz)0.5(H2O)2(btec)0.5]n (1), {[Co(Hdatrz)(Hbtc)]·H2O}n (2) and [Co2(datrz)2(nb)2]n (3) (H4btec =
1,2,4,5-benzenetetracarboxylic acid, H3btc = 1,3,5-benzenetricarboxylic acid and Hnb = 4-nitrobenzoic
acid), were synthesized by incorporating different carboxylate-containing co-ligands and then were
structurally and magnetically characterized. Complex 1 is a 3D pillared-layer framework with
corrugated CoII-btec4- layers supported by neutral m2-N1, N4–Hdatrz ligands. In contrast, the other
two complexes are chiral (4, 4)- and racemic (4, 82)-topological layers with asymmetric m2-N1,
N4–Hdatrz-bridged helical chains extended by bis-monodentate Hbtc2- ligands for 2 and with a m3-N1,
N2, N4-datrz- aggregated Shastry–Sutherland magnetic layer for 3. More interestingly, different
magnetic phenomena with a field-induced metamagnetic transition from antiferromagnetic ordering to
a ferromagnetic state for 1, spin-canted antiferromagnetism with a TN lower than 2.0 K for 2, as well as
the coexistence of spin frustration and spin–flop transitions for 3 were observed, which, significantly,
are governed by the local low-dimensional magnetic motifs mediated by the carboxylate and/or
triazolate heterobridges in the anisotropic CoII–triazolate system.
ligands with paramagnetic ions under controllable experimental
conditions, which have exhibited unusual spin canting,11–12,16–17
Introduction
spin frustration,13 spin-flop transitions,16 metamagnetism18 and
the coexistence of two or more different magnetic behaviors.18
On the other hand, the magnetic couplings between spin carriers
can also be significantly modified by the co-ligand,19 counter-
ion20 and template molecule.3 In particular, the collaboration
or competition between different superexchange pathways by
mixed magnetic bridges can probably lead to unexpected mag-
netic couplings.19 For our continuing efforts on the design and
synthesis of metal triazolate systems by incorporating different co-
ligands,21 in this context, three structurally comparable auxiliaries
attached to different numbers of carboxylate groups (1,2,4,5-
benzenetetracarboxylic acid (H4btec), 1,3,5-benzenetricarboxylic
acid (H3btc) and 4-nitrobenzoic acid (Hnb)) were selected as co-
ligands for assembly with 3,5-diamino-1,2,4-triazole (Hdatrz) and
an anisotropic CoII ion. Obviously, the special interest in the
introduction of secondary ligands was focused on the modulating
effects on the structure and magnetic behavior of the target com-
plexes. It is well-known that a polycarboxylate-attached aromatic
ligand can act as a poly-topic linker to extend discrete magnetic
cores into high-dimensional robust frameworks by the variety of
bridging modes and flexible conformation-dependent magnetic
transmission of the carboxylate group.22 As a consequence, three
new Hdatrz-based CoII complexes with both structural and
magnetic diversity were generated. Complex 1 is a 3D pillared-
layer framework with corrugated CoII-btec4- layers supported
by neutral m2-N1, N4–Hdatrz ligands. In contrast, the chiral
Molecule-based magnets have always attracted considerable atten-
tion due to their impressive structural diversity, intriguing physical
properties and complicated magneto–structural correlations.1–2
In contrast to the traditional three-atom short bridges (azido,
formato, thiocyanate and so on),3–4 five-membered heterocyclic
polyazoles with two or more N donors separated by ring carbon
atoms (midazole,5 triazole6–9 and tetrazole10) and their various
derivatives have recently become excellent candidates for active
magnetic bridges.11–16 Undoubtedly, in addition to exhibiting
diverse bridging modes that aggregate different spin carriers
into a magnetic motif, as well as mediating interesting magnetic
couplings, polyazole-based magnetic bridges can also create anti-
symmetric binding modes to lead to a systematic alteration in the
relative orientations of the neighboring chromophores,6,11 which
can produce significant spin canting and long-range magnetic
ordering. Thus, some intriguing magnetic systems with diverse
zero-dimensional (0D) to three-dimensional (3D) structural mo-
tifs have been successfully obtained by combining polyazole-based
College of Chemistry, Tianjin Key Laboratory of Structure and Performance
for Functional Molecules, Tianjin Normal University, Tianjin, 300387, P. R.
China. E-mail: xiaojun_zhao15@yahoo.com.cn; Fax: +86-22-23766556
† Electronic supplementary information (ESI) available. CCDC reference
numbers 809885–809887. These data can be obtained free of charge
upon application to CCDC, 12 Union Road, Cambridge, CB2 1EZ,
U.K., (fax (+44) 1223-336-033, e-mail deposit@ccdc.cm.ac.uk). For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10394f
8132 | Dalton Trans., 2011, 40, 8132–8139
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(4, 4)- and racemic (4, 82)-topological layer was constructed from
helical CoII-Hdatrz chains and bridging Hbtc2- ligands for 2 and
from a m3-N1, N2, N4-datrz- anion and terminal nb- anions for
3. As expected, the structural differences between 1–3 have caused
interesting changes in their magnetic behavior. Due to the presence
of anti, syn-COO- and the m2-N1,N4–Hdatrz heterobridges in the
2D magnetic structure, a metamagnetic transition of 1 from an-
tiferromagnetic ordering to a ferromagnetic state occurs. Instead
3417(w), 3234(br), 1665(s), 1620(s), 1558(s), 1509(m), 1436(m),
1385(s), 1139(m), 983(w), 840(w), 758(w), 581(w), 543(w).
{[Co(Hdatrz)(Hbtc)]·H2O}n (2).
A
mixture of Hdatrz
(19.8 mg, 0.2 mmol), H3btc (21.0 mg, 0.1 mmol) and
Co(NO3)2·6H2O (43.6 mg, 0.15 mmol) was dissolved in a mixed
water–methanol solution (v : v = 3 : 2, 10.0 mL) and the initial
pH of the mixture was adjusted to 7 by the slow addition
of triethylamine with constant stirring. The mixture was then
transferred into a p◦arr Teflon-lined stainless steel vessel (23.0 mL)
and heated at 160 C for 72 h under autogenous pressure. After
the mixture was cooled to room temperature at a rate of 5.4 ◦C h-1,
blue block-shaped crystals suitable for X-ray analysis were directly
obtained, washed with ethanol and dried in air. Yield: 23% based
on H3btc. Anal. calcd for C11H11N5O7Co (%): C 34.39, H 2.89, N
18.23. Found (%): C 34.41, H 2.88, N 18.22. IR (KBr, cm-1):
3431(w), 3340(br), 1719(s), 1614(s), 1558(s), 1542(s), 1427(m),
1365(s), 1261(w), 1124(w), 1062(w), 748(w), 723(m), 699(w).
of this, 2 exhibits a spin-canted antiferromagnetism with TN
<
2.0 K by the antisymmetric exchange of the m2-N1, N4–Hdatrz
linkages. Complex 3, with a strong antiferromagnetically coupled
Shastry–Sutherland magnetic layer, exhibits the coexistence of
spin frustration and a spin-flop transition. These interesting results
encourage us to continue to investigate the magneto–structural
correlations of polyazole–carboxylate-based magnetic complexes.
Experimental section
[Co2(datrz)2(nb)2]n (3).
A mixture of Hdatrz (19.8 mg,
Materials and instruments
0.2 mmol), Hnb (8.35 mg, 0.05 mmol) and Co(NO3)2·6H2O
(43.6 mg, 0.15 mmol) was dissolved in a mixed water–methanol
solution (v : v = 3 : 2, 10.0 mL) and the pH of the mixture was
adjusted to 5–6 by the slow addition of triethylamine with constant
stirring. The resulting mixture was then transferred into a Teflon-
lined stainless steel vessel (23.0 mL) and heated at 140 ◦C for
72 h. After the mixture was cooled to room temperature at
All of the starting materials employed were commercially pur-
chased (Hdatrz, H4btec, H3btc and Hnb were from Acros and
other analytical grade reagents were from the Tianjin Chemical
Reagent Factory) and used as received without further purifi-
cation. Doubly deionized water was used for the conventional
synthesis. Elemental analyses for C, H and N were carried out
with a CE-440 (Leeman Labs) analyzer. Fourier transform (FT)
IR spectra (KBr pellets) were taken on an Avatar-370 (Nicolet)
spectrometer in the range 4000–400 cm-1. Thermogravimetric
analysis (TGA) experiments were carried out on a Shimadzu
simultaneous DTG-60A compositional analysis instrument from
room temperature to 800 ◦C in a N2 atmosphere at a heating rate
◦
a rate of 5 C h-1, brown block-shaped crystals suitable for
X-ray analysis were directly obtained, washed with methanol
and dried in air. Yield: 44% based on Hnb. Anal. calcd for
C18H16Co2N12O8 (%): C 33.45, H 2.50, N 26.01. Found (%):
C 33.40, H 2.40, N 25.94. IR (KBr, cm-1): 3447(w), 3411(w),
3366(w), 3329(w), 3164(w), 1628(s), 1598(s), 1572(s), 1530(s),
1407(s), 1345(s), 1131(w), 1100(w), 1061(w), 1011(w), 871(w),
832(m), 793(m), 757(m), 717(w), 669(w), 580(w), 529(m), 494(m),
439(m).
◦
of 5 C min-1. Powder X-ray diffraction (PXRD) patterns were
obtained using a Bruker D8 ADVANCE diffractometer at 40 kV
˚
and 40 mA for Cu-Ka radiation (l = 1.5406 A), with a scan speed
of 0.1 s per step and a step size of 0.01◦ in 2q. The simulated PXRD
patterns were calculated using single-crystal X-ray diffraction data
and processed by the free Mercury v1.4 program provided by the
Cambridge Crystallographic Data Center. Variable-temperature
susceptibility measurements of the complexes were carried out on
a Quantum Design (SQUID) magnetometer MPMS-XL-7 with
phase-pure crystalline samples. The diamagnetic corrections were
calculated using Pascal’s constants. The experimental correction
for a sample holder was also applied.
X-ray crystallography
Diffraction intensities of the complexes were collected on Bruker
APEX-II CCD for 1 and 2, as well as a Bruker SMART
diffractometer (for 3) equipped with graphite-monochromatic
˚
Mo-Ka radiation with a radiation wavelength of 0.71073 A by
using the j–w scan technique at 173(2) (for 1 and 2) or 296(2) K
(for 3). There was no evidence of crystal decay during data
collection. Semiempirical absorption corrections were applied
(SADABS)23 and the program SAINT was used for the integration
of the diffraction profiles.24 The structures were solved by direct
methods and refined with the full-matrix least-squares technique
using the SHELXS-97 and SHELXL-97 programs.25 Anisotropic
thermal parameters were assigned to all non-hydrogen atoms.
The organic hydrogen atoms were geometrically generated; the H
atoms of the water molecules were located from difference maps
and refined with isotropic temperature factors. One coordinated
water molecule (O6) and two carboxylate O atoms (O2 and O3) in
1 were varied between two positions with occupancies of 0.539 for
O6 and 0.461 for O6¢, 0.57 for O2 and 0.43 for O2¢, as well as 0.6 for
O3 and 0.4 for O3¢. Additionally, the Hdatrz ligand in 1 was varied
between two centrosymmetric positions. The crystallographic data
and the selected bond lengths and angles are given in Tables 1–3.
Preparation of complexes 1–3
[Co(Hdatrz)0.5(H2O)2(btec)0.5]n (1).
A mixture of Hdatrz
(19.8 mg, 0.2 mmol), H4btec (25.4 mg, 0.1 mmol) and CoSO4
(46.4 mg, 0.3 mmol) was dissolved in a mixed water–methanol
solution (v : v = 3 : 2, 10.0 mL). The resulting mixture was then
transferred into a p◦arr Teflon-lined stainless steel vessel (23.0 mL)
and heated at 160 C for 72 h under autogenous pressure. After
the mixture was cooled to room temperature at a rate of 5.4 ◦C h-1,
pink block-shaped crystals suitable for X-ray analysis were directly
obtained, washed with ethanol and dried in air. Yield: 30% based
on H4btec. Anal. calcd for C12H15N5O12Co2 (%): C 26.73, H 2.80,
N 12.99. Found (%): C 26.77, H 2.62, N 13.00. IR (KBr, cm-1):
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Table 1 Crystal data and structure refinement for 1–3
1
2
3
Empirical formula
Fw
C12H15N5O12Co2
539.15
monoclinic
P21/c
7.4406(5)
15.9099(11)
9.3763(5)
90
128.827(3)
90
864.70(10)
2
2.071
C11H11N5O7Co
384.18
tetragonal
C18H16N12O8Co2
646.29
Crystal system
triclinic
¯
Space group
P43212
P1
˚
a/A
9.8163(4)
9.8163(4)
28.0099(15)
90
9.7813(8)
9.9448(9)
12.5211(11)
84.787(2)
80.6880(10)
88.7970(10)
1196.92(18)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90
90
2699.0(2)
8
1.891
1.325
13810
2386
1560
3
˚
V/A
Z
Dc/g cm-3
m/mm-1
1.793
1.459
6183
4204
652
1.037
2.002
4310
1516
544
Reflections collected
Unique reflections
F(000)
GOF
1.084
1.005
Rint
0.0166
0.0254
0.0145
b
R1a, wR2 [I > 2s(I)]
0.0987, 0.2387
0.0993, 0.2389
1.672, -1.490
0.0186, 0.0477
0.0192, 0.0479
0.288, -0.208
0.0330, 0.0760
0.0423, 0.0813
0.569, -0.562
R1a, wR2 [all data]
Residuals/e A
b
-3
˚
2
a R1 = RꢀFo|-|Fcꢀ/|Fo|. b wR2 = [Rw(Fo - Fc2)2/Rw(Fo2)2]1/2
.
◦
◦
˚
˚
Table 2 Selected bond lengths (A) and angles ( ) for 1
Table 3 Selected bond lengths (A) and angles ( ) for 2–3
Co(1)–O(4)
Co(1)–O(5)
Co(1)–O(1)
2.061(8)
2.090(7)
2.118(8)
104.2(5)
96.8(4)
87.7(5)
88.2(3)
168.5(3)
85.3(6)
88.6(4)
83.7(4)
Co(1)–N(3)
2.062(10)
2
Co(1)–O(6)
2.115(11)
2.174(14)
82.7(3)
166.8(5)
90.4(4)
92.1(4)
97.1(4)
Co(1)–O(2)a
Co(1)–O(6)a
1.9810(15)
2.0106(16)
97.37(6)
106.51(6)
99.65(6)
Co(1)–O(1)
1.9993(14)
2.0346(17)
110.40(6)
129.38(7)
109.66(7)
Co(1)–N(1)b
Co(1)–N(3)
O(4)–Co(1)–N(3)
N(3)–Co(1)–O(5)
N(3)–Co(1)–O(6)
O(4)–Co(1)–O(1)
O(5)–Co(1)–O(1)
O(4)–Co(1)–O(2)a
O(5)–Co(1)–O(2)a
O(1)–Co(1)–O(2)a
O(4)–Co(1)–O(5)
O(4)–Co(1)–O(6)
O(5)–Co(1)–O(6)
N(3)–Co(1)–O(1)
O(6)–Co(1)–O(1)
N(3)–Co(1)–O(2)a
O(6)–Co(1)–O(2)a
O(6)a–Co(1)–O(1)
O(1)–Co(1)–N(1)b
O(1)–Co(1)–N(3)
3
O(6)a–Co(1)–N(1)b
O(6)a–Co(1)–N(3)
N(1)b–Co(1)–N(3)
169.6(7)
83.3(6)
Co(1)–O(1)
1.953(2)
2.004(2)
Co(1)–N(1)
Co(1)–N(8)
1.987(2)
2.005(2)
Co(1)–N(2)c
Co(2)–O(5)
1.967(2)
1.985(2)
Co(2)–N(6)
1.983(2)
2.028(2)
98.30(9)
108.03(10)
108.85(9)
119.17(10)
100.78(9)
104.90(9)
Co(2)–N(7)d
Co(2)–N(3)e
a Symmetry codes: x, 1/2 - y, z - 1/2.
O(1)–Co(1)–N(1)
N(1)–Co(1)–N(2)c
N(1)–Co(1)–N(8)
O(5)–Co(2)–N(6)
N(6)–Co(2)–N(7)d
N(6)–Co(2)–N(3)e
121.16(10)
105.90(9)
113.05(9)
110.26(10)
111.93(9)
108.58(9)
O(1)–Co(1)–N(2)c
O(1)–Co(1)–N(8)
N(2)c–Co(1)–N(8)
O(5)–Co(2)–N(7)d
O(5)–Co(2)–N(3)e
N(7)d–Co(2)–N(3)e
See CCDC 809887 for 1, 809886 for 2 and 809885 for 3, found in
the supplementary crystallographic data.†
Symmetry codes:a x + 1/2, 1/2 - y, 1/4 - z; b x + 1/2, -1/2 - y, 1/4 - z
c
-x, 2 - y, 1 - z; d 1 - x , 1 - y, 1 - z e x + 1, y, z.
Results and discussion
Synthesis and IR spectra
Complexes 1–3 were successfully obtained by controlling the
temperature of the solvothermal reaction. In the IR spectra, a
weak peak, together with a broad band, for both 1 (3417 and
3234 cm-1) and 2 (3431 and 3340 cm-1) should be ascribed to
the stretching vibrations of nO–H and nN–H, respectively, suggesting
the presence of an amino group and water molecule in the
two complexes. In contrast, two pairs of weak bands located at
3447, 3411, 3366, and 3329 cm-1 are indicative of the exocyclic
amino group of the Hdatrz ligand in 3. In contrast to the full
deprotonated co-ligands in 1 and 3, the characteristic strong
band at 1719 cm-1 in 2 confirms the incomplete deprotonation
of H3btc. Correspondingly, the multiple bands for the asymmetric
and symmetric stretch vibrations of the carboxylate group are
observed between 1600–1400 cm-1.26
Crystal structures
[Co(Hdatrz)0.5(H2O)2(btec)0.5]n (1). Complex 1 is a 3D pillared-
layer framework with wave-like CoII–btec4- layers supported by
bidentate bridging Hdatrz ligands. As shown in Fig. 1a, the sole
crystallographically independent CoII ion in 1 is in a distorted NO5
octahedron surrounded by one heterocyclic N donor belonging to
one neutral Hdatrz ligand, three carboxylate O atoms from two
btec4- anions and two coordinated water molecules. The Co–O and
˚
Co–N distances fall in the normal range of 2.061(8)–2.174(14) A
(Table 2), which is comparable to previously reported values.12,15
As shown in Fig. 1b, each fully deprotonated btec4- lig-
and in 1 is centrosymmetric and adopts a hexadentate m6-
2
1
1
2
k O1,O4:k O2:k O2¢:k O1¢,O4¢ mode to periodically connect
8134 | Dalton Trans., 2011, 40, 8132–8139
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Fig. 1 (a) The local coordination environment of the CoII ion in 1 (hydrogen atoms are omitted for clarity). Symmetric codes: A = x, 1/2 - y, z - 1/2;
B = 1 - x, - y, 2 - z). (b) The 2D CoII–btec4- layer of 1. (c) The 3D pillared-layer framework of 1 with the 2D magnetic layer formed by m2-N1, N4–Hdatrz
and anti,syn-COO-.
the four crystallographically equivalent CoII ions into a wave-
like CoII–btec4- layer in the bc plane. Furthermore, the neutral
Hdatrz ligand supports the adjacent corrugated CoII–btecII layer
through a m2-N1,N4-bridging mode, leading to the 3D pillared-
layer framework of 1 (Fig. 1c). Magnetically, the separate CoII
ions in 1 are aggregated by two mixed heterobridges (anti,syn-
COO- and m2-N1, N4–Hdatrz) to generate an important low-
dimensional magnetic layer (Fig. 1d). The nearest CoII ◊ ◊ ◊ CoII
distances bridged by the anti,syn-COO- and m2-N1, N4–Hdatrz
which can produce an antisymmetric exchange in the CoII entities
for canted magnetic behavior. The separate chiral layers of 2 are
stacked into a 3D supramolecular network through O–H ◊ ◊ ◊ O
hydrogen bonding interactions between the lattice water molecule
and the protonated carboxylic group of Hbtc2- ligand (Table S1
and Fig. S1 in the ESI†).
[Co2(datrz)2(nb)2]n (3). Complex 3 is a bimetallic [Co2(datrz)2]
core-based (4, 82) topological layer with terminally coordinated
nb- anions located on both sides. The asymmetric unit of 3
contains two crystallographically distinct CoII atoms and pairs
of deprotonated nb- and datrz- ligands. As shown in Fig. 3a, both
of the crystallographically unique CoII ions (Co1 and Co2) in 3 are
in distorted tetrahedrons (Table 3) completed by one monodentate
carboxylate O from one nb- anion and three heterocyclic N donors
belonging to three individual datrz- ligands.
˚
ligands are 5.37912 and 6.3295(3) A, respectively.
{[Co(Hdatrz)(Hbtc]·H2O}n (2). Complex 2 crystallizes in a
chiral P43212 space group, exhibiting a 2D (4, 4) sheet with helical
CoII–Hdatrz chains connected by incomplete deprotonated bis-
monodentate-Hbtc2- co-ligands. In contrast to the octahedral
coordination environment in 1, the unique CoII ion in 2 is in
a distorted tetrahedron completed by pairs of heterocyclic N
and carboxylate O atoms from two symmetry-related Hdatrz and
Hbtc2- ligands (Fig. 2a). The bond lengths of Co–N and Co–
O (Table 3) are comparable to 1 and other CoII complexes with
mixed tetrazolyl and carboxylate ligands.12,15
As shown in Fig. 2b, the adjacent CoII ions in 2 are infinitely
connected by asymmetric m2-N1,N4–Hdatrz ligands to generate
a helical –CoII-m2-N1,N4–Hdatrz–CoII– chain. Parallel helices
are further connected by bis-monodentate Hbtc2- ligands to
generate a grid-based (4, 4) chiral layer, in which the CoII ◊ ◊ ◊ CoII
separations across the Hbtc2- and Hdatrz bridges are 7.9261(2) and
In the crystal, pairs of symmetry-related CoII ions (Co1 and
Co1A, as well as Co2 and Co2B) are aggregated together by
two m2-N1,N2–datrz- ligands to generate bimetallic [Co2(datrz)2]2+
subunits. The neighboring unique [Co2(datrz)2]2+ subunits are
perpendicular to each other and are further linked together
by the N3 donors of the datrz- ligands coordinating to the
CoII atom of the adjacent units, generating a wavy 2D layer
with (4, 82) topology in the crystallographic ab plane (Fig.
3b). Thus, each anionic datrz- ligand in 3 adopts a tridentate
m3-N1,N2,N4 polytopic binding mode, which is very different
from the neutral m2-N1,N4–Hdatrz ligand in both 1 and 2. No-
tably, the CoII ◊ ◊ ◊ CoII separations by two unique m3-N1,N2,N4–
II
˚
6.1216(2) A, respectively. The individual –Co -m2-N1,N4–Hdatrz-
CoII– chain in the 2D covalent layer of 2 should be essentially
responsible for the interesting magnetic couplings of 2, because
the CoII ◊ ◊ ◊ CoII distance by Hbtc2- ligand is too far to efficiently
transmit the magnetic couplings. Moreover, the two CoII ions
adjacent to m2-N1,N4–Hdatrz have a 2-fold screw axis symmetry,
datrz- ligands are 3.7017(6)/3.5679(6), 6.0281(7)/6.0573(7),
˚
and 6.0206(7)/5.9682(7)
A for Co1 ◊ ◊ ◊ Co1A/Co2 ◊ ◊ ◊ Co2B,
Co1A ◊ ◊ ◊ Co2C/Co1 ◊ ◊ ◊ Co2 and Co1 ◊ ◊ ◊ Co2C/Co1 ◊ ◊ ◊ Co2B, re-
spectively. Magnetically, an approximate isosceles triangle ar-
rangement of the spin carriers within the 2D layer of 3 becomes
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Fig. 2 (a) The local coordination environment of the CoII ion in 2
(hydrogen atoms are omitted for clarity. Symmetric codes: A = x + 1/2,
1/2 - y, 1/4 - z; B = x + 1/2, - y - 1/2, 1/4 - z). (b) The chiral 2D (4, 4)
grid layer of 2.
a distorted Shastry–Sutherland magnetic lattice for geometric
spin-frustration.27 Each periodically arranged 2D sheet gives an
–ABAB– stacking sequence along the crystallographic c direction
and these are further connected by N–H ◊ ◊ ◊ O hydrogen-bonding
interactions between the amino group of datrz- and the nitro
group of the nb- anions, which leads to the formation of a 3D
supramolecular network of 3 (Fig. 3c).
Fig. 3 (a) The local coordination environment of the CoII ion in 3
(hydrogen atoms are omitted for clarity). Symmetric codes: A = -x, 2
- y, 1 - z; B = 1 - x, 1 - y, 1 - z; C = x + 1, y, z). (b) The 2D CoII–datrz-
layer of 3 with a distorted Shastry–Sutherland magnetic lattice. (c) The 3D
supramolecular network of 3 by hydrogen bonding interactions.
Thermal stability and PXRD of complexes 1–3
TG experiments of complexes 1–3 were carried out in order to
explore their thermal stability (Fig. S2 in the ESI†). As a result, two
continuous weight loss steps were observed for both 1 and 2, but
in different temperature ranges. The first weight loss process for 1
occurs in the range 166–256 ◦C, due to the removal of coordinated
water molecules (expt. 13.0%, calcd 13.4%). The second step in
the range 371–588 ◦C is attributed to the decomposition of the
mixed organic ligands and the collapse of the 3D framework of 1.
Two weight loss stages for the removal of lattice water molecules
and decomposition of the 2D covalent layer was observed for 2 in
the range 206–533 ◦C. Instead, complex 3 exhibits only a one-step
weight loss process for the decomposition of the covalent layer in
the range 360–685 ◦C. The amorphous power, which is calculated
to be CoO, was left as the final product for 1–3 (expt 28.6%,
calcd 27.8% for 1, calcd 19.5%, observed 20.6% for 2 and calcd
23.2%, observed 23.9% for 3). A comparison of the powder pattern
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calculated from the single-crystal data and the experimental X-ray
powder diffraction (XRPD) pattern obtained at room temperature
confirms the structural consistency and phase purity of the bulk
products for 1–3 (Fig. S3 in the ESI†).
external magnetic field strength (0–70 KOe, Fig. 4b) and this
was further evidenced by the metamagnetic behaviour of 1. The
magnetization value at the highest field (2.6 Nb) is the same as
that of an octahedral CoII ion with S = 1/2 and g = 4.1–5.0,
indicating a ferromagnetic state of 1 at higher fields. Furthermore,
the weak ferromagnetic state at higher fields can be confirmed by
field-dependent ac magnetic susceptibilities measurements under
a 3.5 Oe alternative field at various frequencies, in which both
the in-phase (c¢) and out-of-phase (c¢¢) peaks are observed at ca
22 KOe. Furthermore, no frequency dependence was observed,
which excludes the possibility of the spin glass of 1 (Fig. S6 in
the ESI†). Additionally, the magnetic phase transition from the
antiferromagnetic to ferromagnetic state can also be realized by
comparing the cM at low temperature with different external fields
(Fig. 4a insert). The critical field of the metamagnetic transition
is ca 22 KOe, determined by both the dM/dH curve (Fig. 4b
insert) and ac measurements (Fig. S6 in the ESI†). Two different
superexchange pathways produced by m2-N1,N4–Hdatrz and
anti,syn-COO- of the btec4- ligand (Fig. 1c) could be responsible
for the interesting magnetic behavior of 1. The adjacent CoII ions
are ferromagnetically coupled through anti,syn-COO- in the –
CoII-anti,syn-OCO-–CoII– chains in the 2D magnetic layer of 1.
Weak antiferromagnetic exchange interactions mediated by m2-
N1,N4–Hdatrz bridges can coexist between the –CoII-anti,syn-
OCO--CoII– chains, although no appropriate model has been
used to quantitatively fit the magnetic data so far. Thus, the
metamagnetism of 1 can be rationalized to be a result of the
competition between the two different superexchange pathways.
At low field, anti-parallel alignment of the magnetic moments
from the adjacent –CoII-anti,syn-OCO-–CoII– chains can lead to
an antiferromagnetic ordering. Once the field exceeds 22 KOe
below 2.5 K, the weak interchain antiferromagnetic interactions
of the m2-N1,N4–Hdatrz ligand can be overcome by the high field
to lead to a weak ferromagnetic state.
Magnetic properties
Variable temperature (2–300 K) magnetic susceptibilities of the
crystalline samples of 1–3 were measured under an applied direct
current (dc) magnetic field of 2000 Oe. As shown in Fig. 4a, the
cMT value per CoII of 1 is 3.24 cm3 K mol-1 at 300 K, which is much
higher than the spin-only value (1.875 cm3 K mol-1) expected for
a high-spin octahedral CoII ion (S = 3/2, g = 2) due to the large
orbital contribution. As the temperature is lowered, the cMT value
continuously decreases and reaches a value of 0.22 cm3 K mol-1
at 2.0 K. The appearance of a round peak at 4.0 K in the cM vs T
curve indicates an antiferromagnetic ordering (Fig. 4 inset). The
Ne´el temperature, TN = 2.5 K, was determined from the sharp
-1
peak in d(cMT)/dT (Fig. S4 in the ESI†). Furthermore, cM vs
T above 40 K obeys the Curie–Weiss law, with C = 3.41 cm3 K
mol-1 and q = -18.2 K (Fig. S5 in the ESI†). The small negative
Weiss constant (q > -20 K) may indicate the coexistence of the
weak antiferro- and ferromagnetic interactions in 1, which could
be covered by the strong spin–orbit coupling. Interestingly, the
peak at 4.0 K in the cM vs T disappears at higher field (> 22 kOe,
inset of Fig. 4a), suggesting an antiferromagnetic ordering at low
fields and a metamagnetic transition at high fields.
The cMT value per CoII ion of 2 is 2.56 cm3 K mol-1 at 300 K (Fig.
5a), which is much higher than one tetrahedral CoII ion due to the
large orbital contribution. The cMT continuously decreases with
the lowering temperature and reaches a minimum of 0.44 cm3 K
mol-1 at 3.0 K. The cMT shows a little rise upon further cooling
to 2.0 K, which is field-dependent and disappears at higher field.
This behavior suggests a spin-canted antiferromagnetism at low
temperatures. The abrupt increase below 3.0 K indicates the onset
of magnetic ordering with a spontaneous magnetization. The plot
-1
of cM vs T above 10 K obeys the Curie–Weiss law with C = 2.62
cm3 K mol-1 and q = -15.1 K (Fig. S5 in the ESI†). Considering
that the Weiss constant for a magnetically uncoupled tetrahedral
CoII ion is close to zero,2 the negative Weiss constant indicates the
presence of antiferromagnetic coupling between the CoII ions in
2. Notably, although 2 exhibits a spin-canted antiferromagnetism,
no obvious TN is observed due to the identical ZFC-FC plot, even
at temperatures as low as 2.0 K (Fig. 5b). Meanwhile, no typical
peaks in the alternating-current (ac) susceptibility signals were
detected, although both the c¢ and c¢¢ components obviously rise
and reach a maximum at 2.0 K (Fig. 5c). All of these magnetic
phenomena suggest that the TN of 2 is lower than 2.0 K. The
lower TN of 1 compared to the previously reported spin-canted
antiferromagnet [Co(bIM)(acetate)] (bIM = benzimidazole) with
TN = 13.0 K,11 is attributed to the long Hbtc2- ligand, which
essentially lowers the magnetic ordering temperature.
Fig. 4 (a) The temperature dependence of cMT for 1 at 2 KOe (inset:
plots of cM vsT under different fields at low temperature). (b) The field
dependence of the magnetization for 1 at 2.0 K (inset: the dM/dH vs H
derivative curve).
The field dependence of the magnetization of 1 at 2 K reveals
a sigmoid-shaped increase in magnetization with the increased
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Fig. 5 (a) A plot of cMT vs T for 2 at 2 KOe (inset: plots of cMT vs. T under several magnetic fields at low temperature). (b) FC and ZFC magnetization
of 2 in the dc field of 100 Oe. (c) Real (c¢) and imaginary (c¢¢) ac magnetic susceptibilities in a zero dc field and in an ac field of 3.5 Oe at 10 and 1000 Hz
frequencies for 2. (d) The field dependence of the magnetization of 2 at 2.0 K.
The field dependence of the magnetization of 2 at 2.0 K is
consistent with a spin-canted antiferromagnetic behavior. There is
an abrupt increase in magnetization with fields below 10 KOe and
the magnetization increases linearly to 1.73 Nb when the external
field is increased to 70 KOe (Fig. 5d). The magnetization value
at 70 KOe is less than the expected saturation value (2.4–3.0 Nb)
for one tetrahedral CoII ion, suggesting a canted antiferromagnet.
Based on the structural analysis above, the observed spin canting
of 2 may be attributed to the presence of the anisotropic CoII ion
and the antisymmetric exchange in the –CoII–m2-N1,N4–Hdatrz–
CoII– chain. 1,2
the field-dependent susceptibility at higher fields is characteristic
of a spin-flop transition. The transition field, determined from
the dM/dH derivative curve, is ca. 20 KOe at 2.0 K (Fig. 6b
inset). It is well-known that a spin-flop transition often occurs
in complexes with a small anisotropic ion, so weak single-ion
anisotropy of the tetrahedral CoII ion may be responsible for
the spin-flop transition of 3 in the antiferromagnetically coupled
Shastry–Sutherland layer.
The cMT value for the CoII unit of 3 is 4.83 cm3 K mol-1 at
2
Conclusion
300 K (Fig. 6a), which is higher than those of two magnetically
isolated spin-only tetrahedral CoII ions with g = 2.0. Upon cooling,
the cMT undergoes a gradual decrease down to 0.36 cm3 K mol-1
at 2.0 K. The cM-1 vs T above 40 K obeys the Curie–Weiss law with
C = 5.11 cm3 K mol-1 and q = -32.0 K (Fig. S5†). Obviously, such
a negative q value for 3 represents the strong antiferromagnetic
couplings between the adjacent CoII ions within the Shastry–
Sutherland magnetic layer. The absence of a maximum value in the
In summary, the synthesis, structures and magnetic phenomena
of three 3,5-diamino-1,2,4-triazole-based CoII complexes with
structurally comparable carboxylate-appended co-ligands were
reported. Due to the synergistic co-coordination and competi-
tive transmission of the magnetic couplings by the carboxylate
and/or triazolate ligands, the 3D pillared-layer framework of 1
exhibits a metamagnetism from antiferromagnetic ordering to a
ferromagnetic state resulting from the anti,syn-m2-carboxylate and
m2-N1,N4–Hdatrz heterobridges within the local 2D magnetic
layer. Instead of this, a (4, 4)-chiral layer by the doubly Hdatrz
and Hbtc2- heterobridges for 2 and a (4, 82)-racemic sheet
by the singly m3-N1,N2,N4–datrz- anions for 3 show canted
antiferromagnetism, as well as spin frustration and a spin-flop
transition, which is strongly dominated by the antisymmetric
magnetic superexchange in the –CoII-anti,syn-OCO-–CoII– helical
chain of 2 and 2D Shastry–Sutherland-arranged [Co2(datrz)2]2-
layer of 3.
c
M vs T curve excludes the antiferromagnetic ordering in 3 (Fig. 6a
inset), which is obviously due to the triangular alignment of spin
carriers within the antiferromagnetic coupled Shastry–Sutherland
2D lattice 3.27 Notably, the comparison of cM vs. T.curves
under 2–30 KOe magnetic fields suggests an obvious occurrence
of a field-dependent magnetic transition (Fig. 6a. inset). The
field dependence of the magnetization at 2.0 K reveals that the
magnetization reaches 2.0 Nb at 70 KOe, which is much lower than
the expected saturation value of 4.8–6.0 Nb for two tetrahedral
CoII ions (Fig. 6b). The sigmoidal shape of the M–H curve and
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We gratefully acknowledge the financial support from the National
Natural Science Foundation of China (Grants 20871092 and
20973125), the Program for New Century Excellent Talents in
University (NCET-08-0914) and the Natural Science Foundation
of Tianjin (10JCZDJC21600).
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