Coligand effects on the architectures and magnetic properties of octahedral cobalt(ii) complexes with easy-axis magnetic anisotropy

Article abstract of DOI:10.1039/c9ce01871a

Two mononuclear azido-cobalt(ii) complexes, with the formulas [Co(3,3-Hbpt)₂(N₃)₂(H₂O)₂] (1) and [Co(abpt)₂(N₃)₂]·H₂O (2) (3,3-Hbpt = 1H-3,5-bis(3-pyridyl)-1,2,4-triazole; abpt = 4-amino-3,5-bis(2-pyridyl)-1,2,4-triazole), have been prepared by altering the pyridyl-triazole coligands. In both complexes, the Co(ii) centers feature hexa-coordinated environments with distorted octahedra in which the axial sites are identical, whereas the equatorial environments are finely modulated by the distinct chemical nature of the different coligands. It is worth noting that the distinct intermetallic distances in the two complexes (10.302 ? for 1 and 6.576 ? for 2) unambiguously cause a disparity in intermolecular interactions, implying their dissimilar magnetic behaviours. As a result, alternating current dynamic susceptibility measurements show that only 2 exhibits field-induced slow relaxation of magnetization with an effective energy barrier of 11.29 K, though large easy-axis magnetic anisotropies for both complexes are unveiled by combined analyses of the magnetic data and ab initio calculations.

Full text of DOI:10.1039/c9ce01871a

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DOI: 10.1039/C9CE01871A
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ARTICLE
Coligand effects on the architectures and magnetic properties of
octahedral cobalt(II) complexes with easy-axis magnetic
anisotropy
Received 00th January 20xx,
Accepted 00th January 20xx
a
Yuewei Wu,a# Jing Xi,a# Jinhui Yang,a Weiming Song,a Shuchang Luo,*b Zheng Wang and Xiangyu
DOI: 10.1039/x0xx00000x
Liu*a c
www.rsc.org/
Two mononuclear azido-cobalt(II) complexes, with formulas [Co(3,3-Hbpt)2(N3)2(H2O)2] (1) and [Co(abpt)2(N3)2]·H2O (2)
(3,3-Hbpt = 1H-3,5-bis(3-pyridyl)-1,2,4-triazole, abpt = 4-amino-3,5-bis(2-pyridyl)-1,2,4-triazole), have been prepared by
alternating the pyridyl-triazole coligands. In both complexes, Co(II) centers feature hexa-coordinated environments with
distorted octahedrons in which the axial sites are identical, whereas the equatorial environments are finely modulated by
the varying chemical natures of the different coligands. It is worth noting that the distinct intermetallic distances in two
complexes (10.302 Å for 1 and 6.576 Å for 2) unambiguously cause the disparity of intermolecular interactions, implying
the dissimilar magnetic behaviours. As a result, alternating current dynamic susceptibility measurements show that only 2
exhibits field-induced slow relaxation of the magnetization with an effective energy barrier of 11.29 K, though large easy-
axis magnetic anisotropies for both complexes are unveiled by the combined analyses of the magnetic data and the ab
initio calculations.
containing complexes are explored the most because they are
air-stable in various cases. More importantly, such cases
govern strong magnetic anisotropy attributed to significant
Introduction
Single-molecule magnets (SMMs) have boundless prospects
and could be considered as admirable models for
comprehending quantum phenomena in the mesoscopic world
and have promising potential for high-density magnetic
information storage and quantum computation.1 Such
materials containing a unique slow-relaxing metal ion (the so
called single-ion magnets (SIMs)) are an emerging class of
SMMs that possess a crucial preponderance for tuning zero-
field splitting (ZFS) through the regulation of the coordination
number and ligand field of the coordination centers.2 After the
initial work on lanthanoid-based SIMs,3 the focus for the
experimental and theoretical analysis of SIMs shifted to also
include transition metals since Long and coworkers reported a
mononuclear Fe(II) complex in a trigonal pyramidal geometry
in 2010,4 and the SIMs of 3d transition metal ions have then
endlessly emerged.5-8 Among them, monometallic cobalt(II)-
spin-orbit coupling in diverse geometric symmetries.9 For a
series of mononuclear Co(II) complexes, strong magnetic
anisotropies with both a negative ZFS parameter D (D < 0) and
an easy-plane anisotropy (D > 0) were clarified, exactly as Ruiz,
Luis, and co-workers proposed that, for a Kramers ion of
strong magnetic anisotropy, the field-induced slow magnetic
relaxation seemingly has no inevitable relation to the sign of
the D values.10 In light of this aforementioned situation, the
slow magnetic relaxation mechanism for Co(II) SIMs is still
indefinable.
In principle, the magnetic behaviour of SIMs primarily arises
from the inherent magnetic anisotropy of the metal center,
which is strongly impacted upon by the ligand field and
coordination geometry as well as the coordination
environment.11 So far, reported examples have proven that
fine-tuning of the coordination microenvironment through the
modification of some structural factors, including the
replacement of a substituent in a ligand, the ligand-field
strength, counter anion nature and structural isomerism, can
effectively contribute to a large magnetic anisotropy, and
consequently enhancement of the anisotropy energy barrier
(Ueff) of SIMs.12 Moreover, the single Co(II) ion is very
appealing since they offer the possibility to conserve the highly
anisotropic nature of the individual spins while potentially
reducing QTM through appropriate magnetic coupling and
suitable applied magnetic field. Therefore, fine-tuning the
a. State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical
Engineering, National Demonstration Center for Experimental Chemistry
Education, College of Chemistry and Chemical Engineering, Ningxia University,
Yinchuan 750021, China. E-mail: xiangyuliu432@126.com
b. School of Chemical Engineering, Guizhou University of Engineering Science, Bijie,
551700, China E-mail: luosc@gues.edu.cn
c. State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210023,
China.
# These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: Figures S1-S7, Tables S1-S9,
crystal structures of 1 and 2 in CIF format. CCDC numbers are 1963359 (1) and
1963361 (2). These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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structure of ligands is one of the possible influencing factors in
Caution! Although we have not encountered any problems
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DOI: 10.1039/C9CE01871A
modulating the ligand fields and intermolecular interactions as in our experiments, azido and its complexes are potentially
well as improving the anisotropy barriers for magnetic explosive; only a small amount of the materials should be
properties of Co2+ complexes. In the context, we focus here on prepared and disposed with care.
the bent ligands which introduce 1H-1,2,4-triazole and 4- Synthetic procedures of 1 and 2
amino-1,2,4-triazole moieties between the two pyridyl groups
Synthesis
of
[Co(3,3-Hbpt)2(N3)2(H2O)2]
(1).
The
and change the position of the pyridyl N atoms. The different hydrothermal method was adopted to synthesize the
degrees of bending which are caused by the diverse N atom coordination polymer. A mixture of Co(NO3)2·6H2O (0.0291 g,
positions could be conducive to the formation of different 0.1 mmol), 3,3-Hbpt (0.0446 g, 0.2 mmol), NaN3 (0.0130 g, 0.2
structures and, moreover, may give an antisymmetric mmol) and water (6 mL) was sealed in a 15 mL Teflon-lined
interaction between the spin carries.13
stainless-steel vessel and heated at 140 oC for 3 days and then
Along with our investigation in this field, we have reported cooled to room temperature at a rate of 5 C h-1. Pink block
two new mononuclear Co(II) complexes, [Co(3,3- crystals of 1 were collected in a yield of 69% (based on Co).
Hbpt)2(N3)2(H2O)2] (1) (3,3-Hbpt = 1H-3,5-bis(3-pyridyl)-1,2,4- Anal. calcd for C24H22CoN16O2 (M = 625.51): C, 46.09; H, 3.55;
triazole) and [Co(abpt)2(N3)2]·H2O (2) (abpt = 4-amino-3,5- N, 35.83. Found: C, 46.02; H, 3.52; N, 35.80. IR (KBr, cm-1):
o
bis(2-pyridyl)-1,2,4-triazole),
performing
different 3423 (s), 2976 (w), 2922 (w), 2083 (m), 1627 (m), 1472 (w),
architectures. The Co(II) centers of both complexes are 1417 (w), 1381 (m), 1043 (m), 989 (w), 706 (w).
indicative of distorted octahedral geometries, in which the
Synthesis of [Co(abpt)2(N3)2]·H2O (2). 2 was prepared in the
axial sites occupied by azido moieties remain very similar to same way as 1, except that 3,3-Hbpt was substituted with abpt
each other, whereas the equatorial environments are fine- (0.0476 g, 0.2 mmol). Brown crystals were obtained (yield:
tuned by different pyridyl-triazole coligands. Magnetic 65%, based on Co). Anal. calcd for C24H22CoN18O (M = 637.53).
measurements reveal that easy-axis magnetic anisotropies (D 2: C 45.22, H 3.48, N 39.55. Found: C 45.19, H 3.44, N 39.52. IR
< 0) are observed in both cases, which are further confirmed data (KBr, cm-1): 3441 (s), 3058 (w), 2046 (s), 1636 (m), 1600
by ab initio calculations. However, only complex 2 presents (m), 1490 (w), 1454 (m), 1345 (w), 1253 (w), 788 (m), 697 (m),
field-induced slow magnetic relaxation.
606 (w).
Crystallographic data collection and refinement
Suitable single crystals of complexes 1 and 2 were selected for
indexing, and the intensity data were recorded on a Bruker
Experimental
Physical Measurements
Smart APEX II CCD diffractometer equipped with
a
Elemental analysis (C, H, N) was implemented on
a
graphitemonochromated Mo-Kα radiation (λ = 0.71073 Å)
source. Using Olex2,15 the structures of 1 and 2 were solved
with the ShelXT16 structure solution program using intrinsic
phasing, and refined with the ShelXL17 refinement package
using least squares minimisation. All of the non-hydrogen
atoms were refined anisotropically. All of the hydrogen atoms
of complexes 1 and 2 were located from difference maps using
the program Olex2. Basic information pertaining to the crystal
parameters and structure refinement is summarized in Table
S1, and selected bond lengths and angles are listed in Tables
S2 and S3.
PerkinElmer 2400 CHN elemental analyzer. The FT-IR spectra
were conducted in the range 400-4000 cm-1 using KBr pellets
on an EQUINOX55 FT/IR spectrophotometer. The phase purity
of the polycrystalline samples was recorded by powder X-ray
diffraction (PXRD) measurements performed on a Rigaku
RU200 diffractometer at 60 kV, 300 mA and Cu Kα radiation (λ
= 1.5406 Å), with scan speeds of 2° min-1 for 1 and 5° min-1 for
2 and step size of 0.02° in 2θ. Magnetic measurements on
polycrystalline samples of 1 and 2 were accomplished using a
Quantum Design MPMS-XL7 superconducting quantum
interference device (SQUID) magnetometer (restrained in
eicosane to prevent torquing under high fields). The measured
magnetic data were corrected for the diamagnetism of the
constituent atoms using Pascal’s tables.
Results and discussion
Materials and general procedures
Crystal structure of 1 and 2
All reagents were obtained from commercially available
sources and used as received unless otherwise noted. 1H-3,5-
bis(3-pyridyl)-1,2,4-triazole and 4-amino-3,5-bis(2-pyridyl)-
1,2,4-triazole were synthesized following the previously
reported method (Scheme 1).14
X-ray analyses suggest that complexes 1 and 2 crystallize in the
monoclinic system with P21/n space group (Table S1). Both
complexes are mononuclear motifs where the Co(II) ions
display hexa-coordinated environments with distorted
octahedrons. As depicted in Fig. 1a, the Co(II) ion in 1 is
surrounded by two N atoms from two 3,3-Hbpt ligands and
two O atoms from two water molecules that lie on the
equatorial plane, and two N atoms from two azido anions are
located at the axial positions. The axial Co-N bond distances
(Co1-N6 = 2.131 Å) are longer than the equatorial Co-O bond
distances (Co1-O1 = 2.043 Å) and a little shorter than
Scheme 1 syntheses of 3,3-Hbpt (a) and abpt (b).
2 | J. Name., 2012, 00, 1-3
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equatorial Co-N bond distances (Co1-N1 = 2.175 Å). It is worth monotonously to 2.0 K, reaching 1.18 cm3 K moVl-ie1wfAorrtic1le.OTnlhinee
noting that the metal centers are well-separated with a χMT decrease of values is concurrentlDyOId: u10e.10t3o9/Cs9pCinE–01o8r7b1iAt
shortest Co···Co distance of 10.302 Å, thus excluding potential coupling of Co(II) ion and/or intradimeric antiferromagnetic
intermolecular dipole-dipole interactions. In contrast, the coupling between Co(II) ions. Interestingly, for complex 2, as
metal center in 2 is chelated by two bidentate abpt ligands and the temperature is lowered, χMT first decreases smoothly to
two azido ligands, thus completing the CoN6 coordination 2.65 cm3·K·mol-1 at 20.0 K, then increases sharply to 2.81
group. In 2, the axial average Co-N bond distances (Co-N = cm3·K·mol-1 at 14.0 K, and finally rapidly drops to 1.72
2.132 Å) are longer than the equatorial Co-N bond distances cm3·K·mol-1 at 2.0 K. Although such curve rarely appeared in
(Co-N = 2.117 Å). The smallest intermetallic distance in 2 is mononuclear Co(II)-SIMs, it could also be observed for a small
6.576 Å, which illustrates nonnegligible intermetallic number of known Co(II)-containing complexes with the similar
interaction. Additionally, π-π interactions between bpy rings situation.19 The high-temperature magnetic behaviours
exist in 1 and 2, yielding three-dimensional supramolecular indicate that the magnetic interactions between Co(II) ions are
structures (Fig. S2 and S3).
dominated by antiferromagnetic coupling and/or a significant
spin-orbit coupling, whereas this upturn at low-temperature of
χMT clearly demonstrates the possibility of weak ferromagnetic
behaviour due to the spin-canting.19
Fig. 1 The asymmetric unit of the Co(II) ions in the complexes 1
(a) and 2 (b), showing the octahedral geometry around the
Co(II) ion. H atoms and free H2O molecules are omitted for
clarity.
Fig. 2 Plots of χMT versus T for complexes 1 (a) and 2 (b). Inset:
The experimental plots of
M versus H/T at different
temperatures. The solid lines show the best-fitting curves to
the experimental data.
To ascertain the precise geometries around the metallic
centers and the degree of the distortion from the ideal model
for two complexes, the geometric spheres of Co(II) cations are
calculated by using the SHAPE 2.1 program18 in the light of the
crystal parameters, and the representative coordination
polyhedrons are illustrated in Fig. 1. As listed in Table S4, the
calculated values suggest that the Co(II) ions in both 1 and 2
represent octahedral (OC) geometry with tiny irregulars from
the optimal geometry. The shape measure values for the
octahedral polyhedrons are 0.219 for 1 and 0.886 for 2,
respectively. The relatively large value for the Co(II) ion in 2
corresponds to more deviation from ideal octahedral
geometry.
The field-dependent magnetizations in the form of the M vs. H
plots for 1 and 2 are represented in Fig. S4. For both complexes,
the M vs. H/T plots at different temperatures are not
superimposed (Fig. 2, inset), clearly indicating the presence of
significant magnetic anisotropy, which is derived from the
strong spin-orbital coupling of the Co(II) ion.20 In order to
analyze the magnetic anisotropy, reliable ZFS parameters were
obtained by simultaneously fitting the experimental χMT vs. T
and M vs. HT-1 curves using the PHI program21 based on the
following spin Hamiltonian:
퐻 = 휇 (푔 푆 퐵 + 푔 푆 퐵 + g 푆 퐵
)
z
z
퐵
푥
푥
y
y
z
푥
y
+퐷(푆 2 ― 푆(푆 +1)/3) + 퐸 푆 2 ― 푆 2
(
)
(1)
In addition, the purity of the crystalline powders of 1 and 2
푧
x
푦
was measured by powder X-ray diffraction (PXRD) (Fig. S1). where µB is the Bohr magneton, D is the axial ZFS parameter, E
The experimental PXRD patterns of 1 and 2 are in accord with is the rhombic or transverse ZFS parameter, S is the spin
the corresponding simulated patterns obtained from the operator, and B is the magnetic field vector, respectively. The
single-crystal data, confirming the high purity of the crystal parameters D, E, and g were selected to correlate the data. For
samples.
the χMT versus T curve of 2, the superposition of the
experimental data and the fitting one is not perfect in
measured temperature range. Therefore, the fitting for 2 is
only performed in the high temperature, reaching a good
effect. Meanwhile, both positive and negative signs of D are
considered in the fitting processes. However, only the negative
sign reduces the experimental data very well. The best fit
values were: D = –54.71 cm-1, |E| = 1.62 cm-1, gx,y = 2.02, gz =
2.35 for 1, and D = –35.38 cm-1, |E| = 0.35 cm-1, gx,y = 2.03, gz =
Magnetic studies
Under a direct current (DC) field of 1000 Oe, the χMT products
at 300 K are 2.68 cm3 K mol-1 for 1 and 2.92 cm3 K mol-1 for 2
(Fig. 2), which are clearly larger than the spin-only value (1.875
cm3 K mol-1) for a magnetically isolated Co(II) cation (S = 3/2
and g = 2.0), indicating a significant orbital contribution to the
magnetic moment. Upon cooling, the χMT values decreases
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