3D chiral and 2D achiral cobalt(II) compounds constructed from a 4-(benzimidazole-1-yl)benzoic ligand exhibiting field-induced single-ion-magnet-type slow magnetic relaxation

Article abstract of DOI:10.1039/c6dt00676k

Organizing magnetically isolated 3d transition metal ions, which behave as single-ion magnet (SIM) units, in a coordination network is a promising approach to design novel single-molecule magnets (SMMs). Herein 3D chiral and 2D achiral cobalt(ii) coordination compounds based on single metal nodes with a 4-(benzimidazole-1-yl)benzoic acid (Hbmzbc) ligand, namely, [Co(bmzbc)₂(1,2-etdio)]_n (1) (1,2-etdio = 1,2-ethanediol) and [Co(bmzbc)₂(Hbmzbc)]_n (2), have been synthesized and structurally characterized. The 3D chiral structure 1 with 2-fold interpenetrating qtz topological nets consisting of totally achiral components was obtained via spontaneous resolution, while the achiral structure 2 is a 2D (4,4) net. In both structures, individual cobalt(ii) ions are spatially well separated by the long organic ligands in the well-defined networks. Magnetic measurements on 1 and 2 showed field-induced slow magnetic relaxation resulting from single-ion anisotropy of the individual Co(ii) ions. Analysis of the dynamic ac susceptibilities with the Arrhenius law afforded an anisotropy energy barrier of 16.8(3) and 31.3(2) K under a 2 kOe static magnetic field for 1 and 2, respectively. The distinct coordination environments of the Co(ii) ions in 1 and 2 lead to the different anisotropic energy barriers.

Full text of DOI:10.1039/c6dt00676k

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3D Chiral and 2D achiral cobalt(II) compounds constructed of 4-
(benzimidazole-1-yl)benzoic Ligand exhibiting field-induced single-
ion-magnet-type slow magnetic relaxation
Received 00th January 20xx,
Accepted 00th January 20xx
a
Yu-Ling Wang,a Lin Chen,a Cai-Ming Liu,b Zi-Yi Du,c Li-Li Chen a and Qing-Yan Liu*
DOI: 10.1039/x0xx00000x
Organizing magnetically isolated 3d transition metal ions, which behave as single-ion magnets (SIMs) units, in a
coordination network is a promising approach to design novel single-molecule magnets (SMMs). Herein a 3D chiral and a
2D achiral cobalt(II) coordination compounds based on the single metal nodes with the 4-(benzimidazole-1-yl)benzoic acid
(Hbmzbc) ligand, namely, [Co(bmzbc)2(1, 2-etdio)]n (1) (1, 2-etdio =1, 2-ethanediol) and [Co(bmzbc)2(Hbmzbc)]n (2), have
been synthesized and structurally characterized. The 3D chiral structure 1 with 2-fold interpenetrating qtz topological nets
consisting of totally achiral components was obtained via spontaneous resolution. While achiral structure 2 is a 2D (4, 4)
net. In both structures, the individual cobalt(II) ions are spatially well separated by the long organic ligands in the well-
defined networks. Magnetic measurements on 1 and 2 showed field-induced slow magnetic relaxation resulting from
single-ion anisotropy of the individual Co(II) ions. Analysis of the dynamic ac susceptibilities with Arrhenius law afforded
the anisotropy energy barrier of 16.8(3) and 31.3(2) K under a 2 kOe static magnetic field for 1 and 2, respectively. The
distinct coordination environments of the Co(II) ions in 1 and 2 lead to the different anisotropic energy barriers.
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the Fe(II),7 Fe(III),8 Mn(II),9 Co(II)10 ions have been reported
due to their single-ion anisotropy. On the other hand, the
association of polynuclear SMM unit into coordination
Introduction
In the field of molecule-based magnetic materials much effort
has so far been concentrated on the area of single-molecule
magnets (SMMs),1 which demonstrate slow relaxation of
magnetic moments after removing an external magnetic field,
with potential applications in magnetic information storage,
quantum computation and spintronics.2 The large ground-
state spin S and zero-field splitting parameter D, which give
the high relaxation energy barriers (Ueff), are the prerequisites
for a molecule to be a SMM.3 Thus the polynuclear compounds
based on the high-spin 3d metal ions have afforded the most
diversity of the SMMs.4 However, a single lanthanide ion-
based SMM, which is referred to as a single-ion magnet (SIM),
with a high energy barrier has been documented in 2003.5 The
4f and 5f metal ions-based SIMs display slow magnetic
relaxation resulting from a large ground-state spin and a
strong easy-axis magnetic anisotropy for these metal ions.6
Very recently, the exploration of new SIMs has been extended
to 3d transition-metal ions. A few examples of SIMs based on
networks has provided a unique approach to create new
SMMs.11 Could slow magnetic relaxation occur in coordination
networks containing magnetically isolated 3d transition metal
ions? Actually, very recently two cobalt(II) coordination
polymers exhibiting slow magnetic relaxation have been
documented.12 This type of slow magnetic relaxation arises
from an individual metal ion in a certain ligand field, which is a
typical SIM. Thus the design of SIMs can be achieved by
organizing magnetic ions with suitable local magnetic
anisotropy into polymeric architectures, which can be
considered as a promising approach to a new generation of
molecular magnetic materials. For metal ions behaving as SIM
units in a well-defined network, it requires long connections
between the magnetic centers to avoid the magnetic
interaction. Following this approach, we synthesized a large
spatial ligand of 4-(benzimidazole-1-yl)benzoic acid (Hbmzbc).
It should be noted that the synthetic method for this organic
ligand (Scheme S1) is totally different from the reported one.13
The Hbmzbc ligand has a large skeleton with the coordination
donors of N and O atoms are spatially separated, which can
effectively separate the metal centers from each other. In
additional, the Hbmzbc ligand is suitable as a bridging ligand
that can exhibit an adaptable conformation in a certain
structure as a result of the free rotation of the C–N single bond
between the benzene moiety and benzimidazole moiety.
Reaction of CoCl2∙6H2O and Hbmzbc in different solvents
a.College of Chemistry and Chemical Engineering, Jiangxi Normal University,
Nanchang, Jiangxi 330022, P. R. China. E-mail: qyliuchem@hotmail.com.
b.Beijing National Laboratory for Molecular Sciences, Institution of Chemistry,
Chinese Academy of Sciences, Center for Molecular Sciences, Beijing 100190, P. R.
China.
c. College of Chemistry and Chemical Engineering, Gannan Normal University,
Ganzhou 341000, P.R. China.
Electronic Supplementary Information (ESI) available: X-ray crystallographic data in
CIF format, materials and methods, figures of structure, XRPD, and TGA curve and
magnetic data. CCDC-1443010 and 1422328. For ESI and crystallographic data in
CIF or other electronic format See DOI: 10.1039/x0xx00000x
afforded two compounds, [Co(bmzbc)2(1, 2-etdio)]n (1) and
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[Co(bmzbc)2(Hbmzbc)]n
(2). Compounds 1 and 2 exhibit
different structural dimensionalities with chiral 3D and achiral
2D structures, respectively, demonstrating the adaptable
character of the Hbmzbc ligand. Detailed magnetic studies
show that
1 and 2 exhibit field-induced slow magnetic
relaxation wherein the single Co(II) ion behave as a SIM unit in
the polymeric structures.
Results and discussion
Syntheses
It is well-known that the reaction solvent plays an important
role in the formation of the final crystalline products. In the
present case, reactions of CoCl2∙6H2O and Hbmzbc in different
solvents lead to different structures. Using the 1, 2-ethanediol
(1,2-etdio) solvent with high viscosity, 3D structure of
obtained. Single-crystal X-ray diffraction analyses revealed that
crystallizes in the chiral trigonal space group P3121 (Table 1).
The Flack parameter is – 0.005(6), which is nearly zero,
indicating that each individual crystal of consists of a single
1 was
Fig. 1 The Solid-state CD spectra for two synthetic batches of bulk
samples 1.
1
Table 1 Crystal data and structural refinements for 1 and 2.
1
Empirical formula
Formula weight
Crystal system
Space group
a (Ǻ)
C30H24N4O6Co
595.46
Trigonal
P3121
C42H28N6O6Co
771.63
enantiomer. However, there is no chiral source in the reaction
mixture. Thus, the overall ensemble of the crystals in a batch
Triclinic
P-1
of
1 can be expected to be racemic, which indicates that
spontaneous resolution occurred during crystal growth. The
solid-state circular dichroism (CD) spectra for samples
obtained from two separately synthetic batches further
12.9013(4)
12.9013(4)
15.3095(9)
90
11.7983(7)
11.9044(7)
14.4291(8)
72.807(3)
81.143(3)
61.863(2)
1706.98(18)
2
b (Ǻ)
c (Ǻ)
confirmed the racemic of the bulk material of
aqueous solvent in the similar reaction a 2D achiral structure
of was obtained, which indicates that the final outcomes are
1 (Fig. 1). With
α/deg
β/deg
90
2
significantly affected by the reaction medium. Furthermore, a
2D compound {[Co(bmzbc)2]∙2DMF}n exhibiting interesting
adsorption properties and slow magnetic relaxation has been
reported by our group very recently, which was obtained using
the DMF as the solvent.14
γ/deg
V/Å3
120
2206.78(19)
3
Z
Dcalcd/g cm-3
μ/mm-1
1.344
1.501
0.631
0.565
F(000)
921
794
Independent
reflections
3324
7849
Structural description
The asymmetric unit of
1 consists of half a Co(II) ion, one
bmzbc– anion, and half a 1, 2-etdio molecule with a 2-fold
rotation axis symmetry. As depicted in Fig. 2, the Co(II) ion
located on a 2-fold rotation axis is six-coordinated by two
benzimidazole N atoms from two bmzbc– ligands and two
hydroxyl O atoms of a 1, 2-etdio in a square-planar geometry,
with the Co–N and Co–O bond lengths of 2.095(2) to 2.167(2)
Å, respectively (Table 2). The two axial positions of the
[CoN2O4] octahedron are occupied by two carboxylic O (O1A
and O1B) atoms from another two bmzbc– ligands (Co–O =
2.0887(18) Å). The bmzbc– ligand bridges two Co(II) ions
through the monodentate carboxylic and benzimidazole
groups with the dihedral angle between benzene ring and
benzimidazole ring of 69.69o (Scheme S2). The 1, 2-etdio
coordinates to one Co(II) ion in a chelating fashion with its two
hydroxyl groups. The octahedral Co(II) ions are linked by the
bmzbc– ligands to generate a 3D framework as shown in Fig.
3a. It is interesting that in the 3D structure the Co(II) ions are
bridged by bmzbc– ligands to form an infinite left-handed
Observed reflections.[ 2861
7173
I >2σ(I)]
R[int]
GOF on F2
0.0209
1.009
0.0201
1.003
2θ
range[o]
1.823 to 27.455 1.477 to 27.647
R1, wR2 [I>2σ(I)]
R1, wR2 (all data)
Largest diff. Peak and
hole[e∙ Å–3]
0.0286, 0.0771
0.0396, 0.0919
0.354 and
0.233
0.0305, 0.0836
0.0340, 0.0866
0.287 and
0.293
–
–
helical chain running along the c-axis (Fig. 3b). The helix is
generated around the crystallographic 31 screw axis.
Additionally, the 3D structure contains the double-stranded
right-handed helical chains running along the c axis to give 1D
helical channels (Fig. 3c), which indicates that the structure
1
has the hexagonal pseudo-61 symmetry (Fig. 3a). All indicate
structure
1
has a chiral α-quartz (qtz) topology.15 Indeed, each
Co(II) ion is bridged by four bmzbc– ligands in a bidentate
2 | J. Name., 2012, 00, 1-3
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bridging fashion, which bridge the Co(II) ions to form the 3D
structure. Thus structure can be reduced into a chiral qtz
1
topological net based on the 4-connected cobalt nodes with
the Schläfli symbol of 64∙82 (Fig. 4). With Co∙∙∙Co separation of
12.34(3) Å, a large void is generated within a single qtz net (Fig.
3a). Therefore structural interpenetration is difficult to avoid
for such an empty framework. Actually, the void space in
1 is
filled by the formation of two independent qtz nets. The
closest Co∙∙∙Co separation between the two interpenetrating
nets is 8.61(5) Å, indicating a spatially separated the Co(II)
centers. Careful investigation showed that the two mutually
interpenetrated chiral qtz networks have the same
handedness (Fig. 4b), which is different from that of the
recently reported Zn(II) compound with the same ligand.16 In
that Zn(II) compound, two enantiomeric qtz nets are mutually
interpenetrated. The formation of these qtz nets indicates the
tetrahedral metal node and the adaptive linear 4-
Fig. 2 The coordination environment of the Co(II) center (Symmetric
codes: A x – y + 1, – y + 1, – z + 2/3; B – y + 1, x – y + 1, z – 2/3; C y,
x, – z).
(benzimidazol-1-yl)benzoic
conformations facilitate the construction of the chiral qtz net.
Compound crystallizes in a triclinic P-1 space group (Table
rod
with
the
feasible
Table 2 Selected bond distances (
Å
) and bond angles(o).
Compound
2
2
Compound
1
1). One Co(II) ion and three crystallographically independent 4-
(benzimidazole-1-yl)benzoic ligands are in the asymmetric unit
Co1–O1A
Co1–O1B
Co1–N2C
Co1–N2
Co1–N6
2.0875(11)
2.1040(10)
2.1045(10)
2.1562(12)
2.1571(10)
2.2384(10)
91.46(5)
2.0887(18)
of 2. There should be an organic ligand with protonated
Co1–O6A
2.0887(18)
2.095(2)
2.095(2)
2.167(2)
2.167(2)
179.51(14)
86.71(9)
93.62(10)
93.62(9)
86.71(9)
95.96(15)
90.43(8)
89.19(9)
92.58(10)
89.1799)
90.43(8)
92.58(10)
78.98(14)
Co1–O1
Co1–N4B
Co1–O3
Co1–O3
Co1–O3C
Co1–O5A
O1A–Co1–O1B
O1A–Co1–N2C
O1B–Co1–N2C
O1A–Co1–N2
O1B–Co1–N2
N2C–Co1–N2
O1A–Co1–O3
O1B–Co1–O3
N2–Co1–O3
N6–Co1–O1
O6A–Co1–O1
N6–Co1–N4B
O6A–Co1–N4B
O1–Co1–N4B
N6–Co1–O3
O6A–Co1–O3
O1–Co1–O3
N4B–Co1–O3
N6–Co1–O5A
O6A–Co1–O5A
N4B–Co1–O5A
O3–Co1–O5A
104.35(4)
93.93(5)
95.08(5)
87.43(4)
Fig. 3 (a) 3D framework of
1 (The 1, 2-etdio molecules are
omitted for clarity). (b) the left-handed helix in
channel formed by double right-handed helix in
1. (c) the chiral
85.84(4)
1
.
83.34(4)
99.59(4)
172.98(4)
103.62(4)
60.69(4)
O1A–Co1–O3C
O1B–Co1–O3C
N2C–Co1–O3C
O3–Co1–O3C
Symmetric codes:
91.79(4)
81.46(4)
Fig. 4 (a) A single qtz net of
1 and (b) 2-fold interpenetrating
1
, A x – y + 1, – y + 1, – z + 2/3; B – y + 1, x –
qtz network with same handedness in
1 discriminated by
y + 1, z – 2/3; C y, x, – z. 2,
A x, y – 1, z; B x – 1, y + 1, z.
colors.
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carboxylic group for charge balance, as suggested by the
strong absorption at 1700 cm–1 in the IR spectrum. The
carboxylic hydrogen atom was added at the O4 atom due to
the much different C21–O4 (1.2824(19) Å) and C21–O3
(1.2302(17) Å) bond distances and strong H-assisted hydrogen
bond between the carboxylic groups (O4∙∙∙O2 = 2.4389(16) Å).
The Co(II) atom is bonded to four carboxylic O atoms from two
bmzbc− ligands and one Hbmzbc ligand, and two
benzimidazole N atoms from the second Hbmzbc ligand and
the third bmzbc− ligand (Fig. 5). The four equatorial positions
of Co(II) are occupied by three carboxylic O (O1, O5A and O6A)
and a N6 atoms; O3 and N4B take up the apical positions to
complete the octahedral coordination geometry of each Co
center. The Co–O bond distances vary from 2.1040(10) to
2.2383(10) Å with the longest bond distance associated with
the chelating carboxylic group (Table 2). The Co–N bond
distances are 2.0875(11) and 2.1561(12) Å. The axial O–Co–N
bond angle of 172.98(4)o and the equatorial O/N–Co–O/N
bond angles ranging from 60.69(4) to 104.36(4)o all deviate
from the corresponding angles of 180 and 90o, respectively, for
an ideal octahedron.
Fig. 6 The 2D layered structure of 2.
11.9 and 12.1 Å, affording nanosized square cavities. It is
interesting that the terminal bmzbc− ligands as suspensions
hang at one side of the layer (Fig. S1). In the crystal packing,
two neighboring layers are interdigitated with each other
through the hanging bmzbc− ligands penetrating into the
naosized cavities of the metal-organic squares (Fig. S2). Each
nanosized cavity is enough to encapsulate one bmzbc− ligand
from neighboring layer. The two interdigitated layers are
packed along the c direction to generate a 3D packing (Fig. S3).
Examination of the 3D packing indicates the closest
interlayered Co∙∙∙Co separation is 7.73 Å, which suggests an
isolated magnetic environment for the Co(II) ion.
The three independent organic ligands exhibit three types of
coordination modes (Scheme S2). One of the bmzbc− ligand as
a terminal ligand monodentately coordinated to a Co(II) ion
through a carboxylic oxygen and the other bmzbc− ligand
bridges two Co(II) ions through its chelating carboxylic group
and the monondentate benzimidazole N atom. The Hbmzbc
bridges two Co(II) ions through its one monodentate carboxylic
O and the benzimidazole N atoms. As depicted in Fig. 5, each
Co(II) ion is surrounded by five organic ligands, four of which
link the Co(II) ions to give a 2D (4,4) net with metal-organic
squares (Fig. 6). The 2D layer is extended along the ab plane.
Within the metal-organic squares, the Co∙∙∙Co separations are
In structure
2, the three crystallgraphically independent
organic ligands exhibit three kinds of conformations with the
dihedral angles between benzene ring and benzimidazole ring
are 47.7, 50.8, 53.2o, respectively, demonstrating the diverse
conformations for this organic ligand. It should be noted that
the compound
2 is not a common (4,4) net constructed from
the single metal ion surrounded by four bridging organic
ligands, as observed in its reported cobalt(II) and zinc(II)
compounds with the bmzbc− ligand.14,17 In
2, the single metal
ion is surrounded by four bridging organic ligands to form the
(4,4) network and an additional terminal bmzbc− ligand to fill
the cavity of the neighboring (4,4) network. Thus a novel
structure is generated.
IR spectra, thermogravimetric analyses and powder X-ray
diffraction patterns
In the IR spectra of 1 and 2, the strong bands between 1651
and 1459 cm–1 can be attributed to the stretching bands of
carboxylic groups of bmzbc− ligand. The broad band centered
at 3474 cm–1 in the IR spectrum of
1 can be assigned to the O–
H stretching band of the 1, 2-ethanediol. The C–O stretching
band of the 1, 2-ethanediol is appeared at 1238 cm–1 in the IR
spectrum of
spectrum of
1
. The strong peak at 1700 cm–1 in the IR
Fig.
5 The coordination environment of the Co(II) ion in 2
2
indicates the presence of the protonated
. The phase purity of the
were checked by the powder X-ray
(Symmetric codes: A x, y – 1, z; B x – 1, y + 1, z).
carboxylic group of the Hbmzbc in
bulky samples of and
2
1
2
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diffraction (PXRD). As shown in Fig. S4, the experimental PXRD
patterns match well with the simulated ones calculated from
single-crystal diffraction data, indicative of the single phase of
both materials. The thermal behavior of
investigated by the thermogravimetric analyses (TGA) (Fig. S5).
TGA curve of exhibits two weight loss steps. The first weight
1 and 2 were
1
loss of 13.81% between 110 to 260 °C can be assigned to the
loss of the loss of the 1, 2-ethanediol molecule (calcd. 10.43%)
and the 1, 2-ethanediol solvent attached at the surface of the
crystals. The decomposition of the framework of
1 occurs at
400 °C. The 2D compound 2 is robust and stable up to 318 °C.
Above this temperature, the collapse of the organic ligand
starts.
Magnetic properties
The magnetic susceptibilities of
1 and 2 were measured in a
direct-current (dc) field of 1 kOe in the temperature range of
2–300 K. The plots of χMT (χM is the molar magnetic
susceptibility per Co(II)) versus T are depicted in Fig. 7. At 300
K, the χMT values are 3.10 and 2.94 cm3 K mol–1, which are
significantly larger than the expected value of 1.875 cm3
K
mol–1 for an isotropic Co(II) ion (S = 3/2 and g0 = 2.0). However,
the values are within the range of those observed for
compounds containing the high-spin Co(II) with an
unquenched angular momentum.18 The effective geff = 2.53
and 2.50 for
1 and 2, respectively, can be deduced from the
room temperature χMT value, which are larger than the spin-
only value of g0 = 2.0, suggesting the considerable orbital
contributions to the spin-only value. As the temperature is
lowered, the χMT products decrease slowly between 300–100
K and fall sharply to a minimum of 2.01 and 1.84 cm3 K mol–1
Fig. 7 The χMT versus T plots for
1 (a) and 2 (b). Inset:
Experimental M versus H/T plots at different temperatures.
traditional SQUID magnetometer (Fig. S7), verifying the QTM
at 2 K for
1 and 2, respectively, revealing the occurrence of
effect for 1 and 2.
significant spin-orbit coupling. The isothermal magnetization
The application of a dc field of 2 kOe suppresses the QTM
and the temperature- and frequency- dependence of χ′ and χ″
measurements of
temperatures. For
1
and
, M vs H/T plots are not superimposed
, the bifurcation of the M vs H/T plots
2 were measured at different
1
are observed for
is the typical for a field-induced SIM. As shown in Fig. 8a, the
peak of the χ″ can be detected in
when the frequency larger
than 250 Hz. While for
, full peaks of the χ″ can be observed
1 and 2 in the test frequencies (Fig. 8), which
(Figure 7a insert). For
2
appears to be more significant (Fig. 7b insert), especially at
lower temperatures and higher field range, confirming the
presence of significant magnetic anisotropy, which is
attributed to the strong spin−orbital coupling of the Co(II) ion.
1
2
in the frequency larger than 10 Hz. Analysis of the frequency
dependence of the χ″ peaks through Arrhenius law (τ =
τ0exp(Ueff/kT), where T is the temperature of the maximum χ″
at different frequencies, τ = 1/2πν, and Ueff is the anisotropic
energy barrier) afforded an anisotropic energy barrier of
16.8(3) K (11.6(8) cm–1) with a pre-exponential factor τ0 of
As described above, the closest Co∙∙∙Co separations in
are 8.61 and 7.73 Å, respectively. Thus, the magnetic
interaction between the local spin quartets can be negligible.
So the individual Co(II) ions in and maybe behave as a SIM
unit that will exhibit slow magnetic relaxation. In order to
investigate the dynamic magnetic behaviour of and , the
temperature dependencies of the alternating-current (ac)
magnetic susceptibilities of and were collected at zero dc
1 and
2
1
2
1.2(2) × 10–7 s for
1
and 31.3(2) K (21.7(6) cm–1) with a pre-
, respectively (Fig. 8
exponential factor τ0 of 1.7(3) × 10–7 s for
2
1
2
insert). The Ueff values are slightly lower than that of 1D
cobalt(II)-triazole compound under 1.5 kOe (31.6 cm–1),12a but
higher than that of dicyanamide-bridged 2D coblat(II)
compound under 1 kOe (5.1 cm–1).12b The value of the pre-
exponential factor τ0 is within the typical range for SIMs (1 ×
10−7 to 1 × 10−11 s).
1
2
field with an ac field of 2.5 Oe with oscillating frequencies. The
results are given in Fig. S6 in the form of χ′ and χ″ versus T plots
(χ′ and χ″ are the in-phase and out-of-phase ac molar magnetic
susceptibilities, respectively). No frequency dependent signal
can be observed under zero dc field in the measurements,
indicative of a fast tunnelling of the magnetization (QTM). In
The Cole−Cole plots at 2 K under the 2 kOe dc field were
generated from the frequency-dependent ac susceptibility
data for
1
. The Cole-Cole diagram in the form of χ″ versus χ′ is
addition, the M vs H data for
1 and 2 do not exhibit a
hysteresis effect from MPMS measurements at 1.9 K used in a
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Fig. 9 Cole-Cole plots for 2 under 2 kOe dc field. The solid lines are
the best fit obtained with a sum of two modified Debye functions
Debye model.
wherein the left and right parts correspond to the fast
relaxation phase (FR) and the slow relaxation phase (SR),
respectively. The Cole–Cole curves could be well fitted by the
sum of two modified Debye functions. The fitting results are
depicted in Fig. 9 and Fig. S9. The resulting α1 values varying
from 0.000005 to 0.0249 are much smaller than the α2 values
ranging 0.285 to 0.156 at all three temperatures (Table S1).
These values suggest that the FR phase has a narrower
distribution of the relaxation time in comparation to the SR
phase. As mentioned above, there is only one kind of Co(II) ion
in 1 and 2, so the field-inducing role is ascribed to the two-step
magnetic relaxation.20 Such a behavior is similar to the two
relaxation peaks observed in a mononuclear Dy-based SIM.21
Experimental
Materials and Instrumentations.
All commercially available chemicals are of reagent grade and
used as received without further purification. IR (KBr pellets)
spectra were recorded in the 400–4000 cm-1 range using a
Perkin-Elmer Spectrum One FT-IR spectrometer. Elemental
analyses were carried out on Elementar Perkin-Elmer 2400CHN
microanalyzer. Thermogravimetric analyses were carried out
on a PE Diamond TG/DTA unit at a heating rate of 10 °C/min
under
a nitrogen atmosphere. Powder X-ray diffraction
patterns were performed on a Rigaku Miniflex II powder
Fig. 8 Temperature dependence of the out-of phase and in-
diffractometer using Cu-Kα radiation (λ = 1.5418 Å). The 2θ
phase ac susceptibility signals under 2 kOe dc field for 1 (a) and
(b). Inset: Magnetization relaxation time, lnτ vs. T–1 plot
range was 5–60° with a continuous scan step width of 0.05°.
2
The recorded PXRD patterns were compared with theoretical
patterns calculated from single-crystal structure data.
Magnetic measurements were carried out on crystalline
under 2 kOe dc field. The solid line is fitted with the Arrhenius
law.
samples with
a
Quantum Design MPMS-XL5 SQUID
shown in Fig. S8, which displays
a large semicircle
magnetometer in the temperature range of 2–300 K.
Diamagnetic corrections were estimated from Pascal’s
constants for all constituent atoms.22
accompanying with a small semicircle. This implies that two
relaxation processes are present in 1. The Cole–Cole curve was
fitted by the sum of two modified Debye functions (equ S1).19
However, the data is not fitted well with the Debye function
(Fig. S8). For 2, the Cole−Cole plots at 2, 3 and 4 K in the 2 kOe
Synthesis.
dc field were plotted in Fig. 9. Two-step thermal magnetic
relaxation character was also observed in the Cole–Cole plots
6 | J. Name., 2012, 00, 1-3
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The Hbmzbc ligand was prepared following a modified known
procedure (Scheme S1).23
Conclusions
In conclusion,
a
3D chiral compound with 2-fold
Synthesis of [Co(bmzbc)2(1, 2-etdio)]n (1). CoCl2∙6H2O (0.0119 g,
0.05 mmol), 4-(benzimidazol-1-yl)benzoic acid ligand (0.0238 g,
0.1 mmol) and triethylamine (0.05 mL) were mixed in 5 mL 1,
2-ethanediol solvent. The resulting mixture was sealed in a
Teflon-lined stainless steel autoclave and heated at 120 °C for
72 h. Then the mixture allowed to cool to room temperature
interpenetrating qtz network and a 2D achiral compound with
(4,4) network based on the cobalt(II) nodes and Hbmzbc ligand
have been synthesized and characterized. In both structures,
the six-coordinate Co(II) ions are well isolated from each other
via the long organic bridging ligands. Both compounds exhibit
field-induced slow relaxation consistent with SIM behavior,
which enrichs the 3d transition-metal SIMs. This work
demonstrates that slow magnetic relaxation can be observed
in the high-dimensional Co(II) coordination networks wherein
the highly anisotropic central Co(II) ions are in an isolated
magnetic environment and behave as SIM units. This approach
will provide a new route to the design molecular magnetic
material based on a mononuclear complex unit.
naturally to obtain the red crystals of
1 (yield: 0.0152 g, 50%
on the basis of Co). Anal. calcd. for C30H24N4O6Co (595.46): C,
60.51; H, 4.06; N, 9.41%. Found: C, 58.68; H, 4.31; N, 9.19 %.
Main IR features (KBr pellet, cm−1): 3474 (m), 3145 (w), 2958
(w), 2521 (w), 1938 (w), 1604 (s), 1546 (m), 1516 (m), 1481
(w), 1459 (s), 1392 (s), 1322 (m), 1299 (m), 1238 (s), 1175 (w),
1140 (w), 1091 (w), 1074 (m), 1045 (w), 1014 (w), 989 (m), 934
(w), 910 (w), 870 (m), 831 (m), 792 (s), 778 (w), 748 (s), 726
(w), 704 (s), 648 (m), 620 (m), 585 (m), 561 (w), 532 (m), 491
(m), 427 (w).
Acknowledgements
Synthesis of [Co(bmzbc)2(Hbmzbc)]n (2). A mixture of CoCl2∙6H2O
(0.0119 g, 0.05 mmol) and 4-(benzimidazole-1-yl)benzoic acid
(0.0357 g, 0.15 mmol) in 5 mL H2O was sealed in a 25 mL Parr
Teflon-lined stainless steel vessel. The vessel was sealed and
heated to 140 °C. This temperature was kept for 5 days and
then the mixture was cooled naturally to form red block
This work was supported by the NNSF of China (Grants
21561015, 21361011 and 21101081), the Provincial NSF of
Jiangxi (Grant 20151BAB203002), and the Young Scientist
Training Project of Jiangxi Province (Grant 20153BCB23017).
crystals of
2 (yield: 0.0166 g, 43% on the basis of Co). Anal.
Notes and references
calcd. for C42H28N6O6Co (771.63): C, 65.37; H, 3.66; N, 10.89%.
Found: C, 65.41; H, 3.69; N, 10.83%. Main IR features (cm−1,
KBr pellet): 3854 (w), 3421 (m), 3121 (w), 3057 (w), 2517 (w),
1934 (w), 1700 (s), 1651 (s), 1605 (s), 1547 (s), 1518 (m), 1481
(s), 1426 (m), 1325 (m), 1297(m), 1236 (s), 1173 (w), 1147 (w),
1135 (w), 1102 (w), 1012 (m), 986 (m), 967 (w), 938 (w), 909
(m), 866 (s), 838 (w), 790 (s), 743 (s), 719 (w), 700(s), 643 (m),
622(m), 586 (s), 554 (w), 528 (m), 490 (m), 430 (w).
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Graphical Abstract
A 3D chiral and a 2D achiral cobalt(II) compounds have been synthesized and characterized.
Detailed magnetic measurements on 1 and 2 show field-induced slow magnetic relaxation
resulting from single-ion anisotropy of the individual Co(II) ions in 1 and 2. The different
magnetic anisotropy energy barriers for 1 and 2 are resulted from the distinct coordination
environments of Co(II) ions.