Sizeable Effect of Lattice Solvent on Field Induced Slow Magnetic Relaxation in Seven Coordinated CoII Complexes

Article abstract of DOI:10.1021/acs.inorgchem.9b00615

We have demonstrated the effect of a solvent at the second coordination sphere on slow relaxation of magnetization for hepta-coordinated cobalt(II) complexes with the formulas [Co(H₄L)(DMF)(H₂O)](NO₃)₂·(DMF) (1)

Full text of DOI:10.1021/acs.inorgchem.9b00615

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Sizeable Effect of Lattice Solvent on Field Induced Slow Magnetic
Relaxation in Seven Coordinated CoII Complexes
Arpan Mondal, Ajit Kumar Kharwar, and Sanjit Konar*
Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066,
MP, India
S
* Supporting Information
ABSTRACT: We have demonstrated the effect of a solvent at the second
coordination sphere on slow relaxation of magnetization for hepta-coordinated
cobalt(II) complexes with the formulas [Co(H4L)(DMF)(H2O)](NO3)2·(DMF) (1),
[Co(H4L)(MeOH)(H2O)](NO3)2·(MeOH) (2), and [Co(H4L)(DEF)(H2O)]-
(NO3)2 (3) (H4L = 2,2′-(pyridine-2,6-diylbis(ethan-1-yl-1-ylidene))bis(N-phenyl-
hydrazinecarboxamide). Structural analysis reveals that the presence of lattice solvent
molecule in 1 and 2 dramatically changes the crystal packing and noncovalent
interactions as compared to 3 where no solvent molecule is present in the crystal
lattice. The dc and ac magnetic susceptibility measurements reveal the presence of
easy-plane magnetic anisotropy for all the complexes, and field induced slow relaxation
behavior has been observed above 2 K for 1 and 2 in contrast to 3 due to the
availability of the solvent molecules in the crystal lattice. The ab initio calculations
further support the sign of D and the negligible effect of the first co-ordination sphere,
as almost similar D values were obtained for all the complexes. The field and
temperature dependence of relaxation time confirm that quantum tunnelling of magnetization (QTM) plays a major role in
slow magnetic relaxation, and thermal dependence like an optical or acoustic Raman pathway is also important. To further
analyze the effect of dipole−dipole interaction on slow magnetic relaxation, a dilution experiment has been performed.
more effective for 3d metal ions.6 Considering all the reported
3d-based SIMs, the most demanding feature is a low
INTRODUCTION
■
In the field of molecular magnetism, single-molecule magnets
coordination number of the metal center as the anisotropy is
(SMMs)1 have attracted considerable attention, as their
enhanced due to unrestricted orbital angular momentum. In
magnetic bistability can be used as a prospective component
this regard, CoII complexes are the most desirable candidates
for molecular spintronics,2 high information storage, and
for SIMs7 because of their large magnetic anisotropy and
quantum computing.3 After the discovery of SMM behavior of
noninteger spin ground state, which reduce the possibility of
Mn12, more efforts were dedicated to prepare the multinuclear
complexes with a large spin ground state to get a high energy
quantum tunneling of magnetization (QTM).6h−k,8 Recently,
barrier for slow magnetic relaxation.4 However, this strategy
Bunting et al. reported the highest spin reversal barrier of 450
cm−1 among transition metal complexes for a linear two
was not very successful because in a multinuclear system the
coordinated cobalt complex with maximal orbital angular
different orientation of the resultant magnetization vector
momentum.8c As low coordinated cobalt complexes are not
reduces the anisotropy, which must be large to observe better
SMM behavior.4c,d On the other hand, the complexes with one
stable, more efforts have been made to tune their geometry to
enhance their magnetic anisotropy.8,9 However, hepta-
paramagnetic metal center, referred to as single ion magnets
coordinated CoII SIMs with pentagonal bipyramidal (PBP)
(SIMs), are relatively easier in terms of controlling the
geometry are still limited where only the effect of coordination
anisotropy, having a high energy barrier (Ueff) and blocking
temperature (TB).4,5 In addition, substantial effort has been
spent in optimization of uniaxial anisotropy of these molecules
with a minimum rhombic term by applying a proper ligand
field and suitable geometry around the metal centers. Recently,
environment on easy-plane magnetic anisotropy has been
studied.10 The spin−orbit coupling between ground electronic
states with two excited electronic states results in the easy-
plane magnetic anisotropy for those systems. Thus, the
anisotropy can be tuned by changing the extent of mixing
the highest hysteresis temperature found was 80 K for a two
between ground and excited states with proper ligand field. In
coordinated dysprosium metallocene complex reported by Gao
our previous report on CoII−PBP complexes, we showed that
et al.5f The observed high hysteresis temperature is close to
liquid nitrogen temperature, bringing hope for real application
prospects in the future. Besides the lanthanides, transition
metal complexes also display interesting SMM behavior in a
proper ligand field and geometry as the ligand field is much
weak sigma donor ligand can increase the magnitude of the
Received: March 2, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00615
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Figure 1. View of molecular structure for complexes 1−3 (a−c). Solvent molecules and H atoms are omitted for clarity.
Figure 2. Crystal packing diagram of complexes 1 (left) and 3 (right) along the crystalographic b axis.
anisotropy parameters (D-axial and E-rhombic anisotropy) as
well as energy barrier.9e However, all methods can be summed
up in two aspects which have been used to increase the energy
barrier and blocking temperature: first, regulating the
coordination and geometry around metal center; second,
accepting suitable counterions or solvents. Usually, the
magnetization reversal is highly sensitive toward the first
coordination sphere, local symmetry, and single ion anisotropy
of the metal center.6,7,11 At the same time, the solvent
molecule present in the crystal lattice may lead to different
dipole−dipole interactions, which controls the relaxation rate
of incoherent quantum tunneling and results in a drastically
different energy barrier of magnetization reversal.12 Thus, the
lattice solvent may also play an important role in relaxation
dynamics.
In this work, we have reported the first example of Co(II)
SIMs with PBP geometry with the formulas [Co(H4L)(DMF)-
(H2O)](NO3)2·(DMF) (1), [Co(H4L)(MeOH)(H2O)]-
(NO3)2·(MeOH) (2), and [Co(H4L)(DEF)(H2O)](NO3)2
(3) (H4L = 2,2′-(pyridine-2,6-diylbis(ethan-1-yl-1-ylidene))-
bis(N-phenylhydrazinecarboxamide), where lattice solvent
plays an important role in the slow relaxation of magnetization.
neigboring molecules in all the complexes. The asymmetric
unit contains one CoII ion and one molecule of water in the
axial position for all complexes. The other axial position is
occupied by solvent molecules DMF, MeOH, and DEF in
complexes 1, 2, and 3, respectively. Even under a similar ligand
field environment, the equatorial coordination shows little
difference [ligand bite angles from 69.72° (for Npyridyl−Co−
N) to 77.97° (for O−Co−O) in complex 1, compared to
70.96−74.51° and 70.39−76.99° for the equivalent angles in 2
and 3, respectively] (Table S3). The free −NH group of the
ligand is strongly hydrogen bonded to the free nitrate anions
present in the crystal lattice (Figure S2 and Table S2) of all the
complexes. In addition, one free solvent molecule (DMF in
complex 1 and MeOH in complex 2) is present in the
asymmetric unit, which is strongly hydrogen bonded with the
axial water molecules, whereas in 3, the same hydrogen bond is
observed only by the nitrate anion (Figure S1). In complex 3,
no free solvent molecule is present in the crystal lattice. The
presence of the lattice solvent in 1 and 2 dramatically changes
the crystal packing (Figures 2, S3, and S4) as compared to 3,
which shows alternative π−π interaction (3.71 Å) in the crystal
packing (Figures 2 and S5). Interestingly, the π−π interactions
are totally absent in the former complexes (1 and 2). Further,
the analysis of the crystal packing reveals that the nearest Co−
Co distance is in the range of 8.13−9.34 Å for all complexes.
The continuous shape measurements (CShM),13 used to
calibrate the deviation of the structures from the reference
polygons, were calculated to be 0.293, 0.067, 0.253 for 1−3,
respectively, which are very close to zero of the ideal D5h
geometry. Complete results of the geometric analyses are
shown in Tables S3, S4, and S5.
RESULTS AND DISCUSSIONS
■
Structural Description. From the single crystal X-ray
analysis, it has been observed that complexes 1 and 2 (Figure
1) crystallize in monoclinic P21/c and P21 (Table S1) space
groups, whereas complex 3 (Figure 1) crystallizes in the
̅
triclinic P1 space group (Table S1). All the complexes have a
N3O4 coordination environment where the H4L ligand binds
with the CoII center equatorially by five donor sites, leaving the
axial position free for the solvent and water molecules. The
charge of the metal center is balanced by the nitrate
counterions, which are strongly hydrogen bonded with the
coordinated water (Figure S1 and Table S2) and the
Magnetic Property Studies. The magnetic susceptibility
measurements were performed under an applied field of 0.1T
in the temperature range of 2−300 K. For all the complexes,
the purity of the bulk sample was confirmed by the powder X-
ray diffraction data compared to the simulated ones (Figure
B
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Figure 3. χMT vs T plots measured at 0.1 T for complex 1 (a) and 3 (c). The 1/χM vs T plots are shown in the inset. M/NμB vs H plots for 1 (b)
and 3 (d) at the indicated temperatures. The red lines are the best fit.
S6). At room temperature, the χmT values (χm = molar
magnetic susceptibility) were obtained as 2.52, 2.47, and 2.44
cm3 mol−1 K for complexes 1−3 (Figures 3 and S7),
respectively. These values are slightly higher than the spin
only value (1.87 cm3 mol−1 K, g = 2.0) for the noninteracting
CoII high spin ion, but they fall within the usual range of 2.1−
3.4 cm3 mol−1 observed for highly anisotropic CoII ions (where
g > 2.0). Upon cooling the temperature from 300 K, the χmT
value slowly decreases for all the complexes down to 110 K,
below which it decreases rapidly and reaches a minimum value
of 1.52(1), 1.5(2), and 1.42(3) cm3 mol−1 K at 2 K. The rapid
decrease at low temperature is due to the presence of inherent
anisotropy of the CoII centers. In addition, the field
dependence magnetization data have been collected, which
reached the highest values of 2.26 (1), 2.15 (2), and 1.97 (3)
at 7 T magnetic fields (Figures 3 and S7). The sharp increase
of the magnetization at a low temperature (2 K) specifies the
degenerate ground states, and at high field, (7 T) they become
almost saturated, indicating the higher energy of the first
excited states for all the complexes. Furthermore, the reduced
magnetization curves (Figure S8) do not fall on the same
master curve for all the complexes, showing the anisotropy of
the system. Considering the large intermolecular distance
(8.13−9.34Å) between CoII centers, the interaction between
local spin quartets, if any, is expected to be very weak. Thus,
the following spin Hamiltonian has been used to extract the
ZFS parameter (D) as given in eq 1.10a−e
are in excellent agreement with those obtained from the fitting
of the experimental data (Tables S6 and S7). It is also noticed
that in all complexes the electronic transition occurs between
the different ml values of the d orbital (Figures 4 and S9),
which results in the positive signs of the D parameters.
Figure 4. AILFT computed d-orbital splitting for complex 1 (a), 2
(b).
Additionally, the CASSCF/RASSI-SO/SINGLE_ANISO
type calculations were performed by MOLCAS 8.2 where
the orientation and easy plane type anisotropy are also
observed in the ground Kramers doublet state (KDs; Figure
S10 and Table S8) as the quartet S = 3/2 states splits further in
two KDs 1/2 and 3/2. For all the complexes, the g tensor
of the ground KDs is pretty close to the ideal values of gX = 6,
gY = 4, and gZ = 2 for the 1/2 type of the Kramers doublet
(where D > 0, E/D = 0), which further aligns with the positive
D value (Table S8). Also, the anisotropy of the first excited
KDs is closer to the ideal values of gX = 0, gY = 0, and gZ = 6 for
3/2 type KDs (Table S9). Thus, theoretical calculations also
nicely produce the ZFS parameters (Tables S6 and S7) close
to the experimental results, which further confirms the sign of
ZFS parameters for all the complexes. The theoretically
calculated D values are very close to each other as the
coordination environments are almost similar in all complexes.
To check the relaxation dynamics in complexes 1−3,
alternating current (ac) susceptibility measurements have
been performed under a 3.5 Oe ac field. None of the
complexes show any frequency as well as temperature
dependence in the absence of an external magnetic field.
This may be due to the presence of fast resonant zero field
quantum tunnelling of magnetization (QTM) between the
ground KDs for all complexes. Thus, to reduce the QTM, we
have applied 0.2 T as an optimal dc field (Figure S11). It was
observed that out-of-phase (χM″) susceptibility shows a
maximum value at 0.2 T almost for all complexes, and ac
H = gμ S·B + D[Sz2 − S(S + 1)/3] + E(Sx2 − Sy2)
(1)
B
In the above equation, axial and rhombic anisotropy
parameters are represented by D and E respectively whereas
other parameters have their usual meaning. The simultaneous
fitting of χmT vs T and M/NμB vs H plots using the PHI
program14 provides D = 35.92 cm−1, |E| = 1.42 cm−1, and g =
2.3 for 1; D = 37.23 cm−1, |E| = 0.93 cm−1, and g = 2.29 for 2;
and D = 43.76 cm−1, |E| = 0.84 cm−1, and g = 2.28 for 3. In
addition, the inclusion of the intermolecular interaction during
the fitting shows very weak dipole−dipole interaction (zJ =
−0.025 cm−1 for 1, zJ = −0.013 cm−1 for 2, and zJ = −0.009
cm−1 for 3) for all complexes. The obtained positive D values
are in good agreement with previously reported CoII PBP
complexes.10 The fitting using a negative D value gives a
poorer fit for all the complexes. The spin−orbit coupling
between ground state and excited states results in a positive D
value due to the electronic excitation between different ml
values of the d orbital as previously reported for hepta-
coordinated CoII complexes. To get further insight into the
electronic structures and relaxation dynamics, we have studied
ab initio calculations with the help of ORCA 4.015 and
MOLCAS 8.2 packages.16 In both calculations, the parameters
C
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Figure 5. Temperature dependency of the in-phase (χM′) for complex 1 (a), 2 (b), and 3 (c) and out-of-phase (χM″) 1 (d), 2 (e), and 3 (f) under a
0.2 T dc field.
measurements were performed under this external field. The
complexes 1 and 2 show nice temperature (Figure 5) and
frequency dependency (Figures S12 and S13). However, no
such dependency is observed above 2 K for complex 3 (Figure
5). Additionally, the temperature dependence of in-phase
(χM′) and out-of-phase susceptibility (χM″) shows peak
maxima below 2 K (Figure 5) for 3, which gives the indication
of slow relaxation of magnetization. A generalized Debye
model17 has been used to fit Cole−Cole plots (Figure 6) for 1
time (τ0) as Ueff = 25 K and τ0 = 6.5 × 10−6 s for 1 and Ueff
=
15 K and τ0 = 3.5 × 10−6 s for 2. Since complex 3 does not
show any peak maxima above 2 K, the Debye equation ln(χM″/
χM′) = ln(ωτ0) + Ueff/kBT has been used to estimate the
energy barrier (Figure S14). The linear fit of ln(χM″/χM′) vs 1/
T gives the energy barrier and pre-exponential relaxation time
(τ0) as Ueff = 4 K and τ0 = 1.9 × 10−6 s. For all complexes, the
pre-exponential relaxation time is in the order of observed
SIMs behavior (10−6−10−12) as reported previously.6,7,10
Although, in 3d-SIMs with positive D, it is suggested that
relaxation dynamics is either controlled by the transverse
anisotropy present in the easy (XY) plane or field-induced
phonon bottleneck effect of the direct relaxation of the ground
KDs. Also, the direct process is represented with a high
anisotropic system setting aside the sign of the D parameters.
In addition, the experimental and theoretical results for the
said complex deduce that the relaxation dynamics are mainly
controlled by the transverse anisotropy. To gain more
understanding about the magnetization reversal process, we
have studied the average relaxation time at 4 K in a different
external field. For this, eq 2 was used, where the first term
represents the field dependence process (QTM process) and
the second term, the weak field dependence (Raman and
Orbach process). which is kept as constant. Since the average
relaxation time increases with field (Figure S15), the direct
term contribution was neglected for the studied complexes.11a
τ−1 = B1/(1 + B2H2) + C
(2)
Figure 6. Cole−Cole plots for 1 (a) and 2 (c) and Arrhenius plots for
1 (b) and 2 (d). The solid lines represent the best fitting.
The relaxation times were found to be in conjecture (Figure
S15) with eq 2 (τQTM was calculated as 1.9 × 10−4 s for 1 and
2.2 × 10−4 s for 2), which further signifies that QTM has a
sizable effect on the slow relaxation process. Additionally, by
considering the thermally activated process (either Orbach or
Raman), the temperature dependence of τ at 0.2 T can be
modeled well (Figure S15) by the power law as eq 3 (where
τQTM was fixed as 1.9 × 10−4 s (1) and 2.2 × 10−4 s (2), n = 4.9
(1), 4.6 (2)), which indicates QTM has been well suppressed.
The values of n in the range of n = 1−6 also suggest the
possibility of spin−lattice relaxation through the admixture of
and 2 where the semicircle nature indicates a single relaxation
process. The α (indicating the width of the distribution of
relaxation time) values were obtained as 0.13−0.27 (1) and
0.09−0.26 (2), indicating the narrow distribution of the
relaxation time. Further, to extract the energy of the
magnetization reversal, the classical Arrhenius equation ln(τ)
= ln(τ0) + Ueff/KT has been used (Figure 6) for 1 and 2 where
clear peak maxima were observed above 2 K.6,8 The best fit
gives the energy barrier (Ueff) and pre-exponential relaxation
D
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single phonon direct and optical acoustic Raman processes as
reported by Colacio et al.18
ically obtained D values as calculated without solvent
molecules, which are almost same for all the complexes. At
the same time, the lattice solvent molecules are also involved in
several noncovalent interactions which lead to the formation of
different assemblies at low temperatures. These assemblies also
may help for slower spin relaxation than the pure mononuclear
system where such interactions are absent. On the other hand,
the dilution study reveals that dipolar interaction has an
important role in the slow relaxation process in 1 and 2,
whereas no such behavior is observed in 3. The presence of
solvent in the crystal lattice results in different crystal packing
as well as supramolecular reorganization which further affect
the orientation of the anisotropic axis. In addition, the
experimentally observed different transverse anisotropy
parameters (E) caused by the dipole−dipole interactions,
which also tune the relaxation rate, arise from the incoherent
QTM.12f Probably, the different crystal packing/supramolec-
ular arrangements and the dipole−dipole interactions are
responsible for different relaxation dynamics in all the
complexes.12b,g Additionally, we have done thermal analysis
(Figure S21) for complex 1 and 2, which indicates the loss of
lattice and coordinated solvent molecules. Thus, we are unable
to measure the magnetic data for the desolvated samples.
τ−1 = τQTM−1 + bTn
(3)
Moreover, from both field and temperature dependence of
the relaxation time, it is obvious that QTM plays a major role
in the slow relaxation process. Also, thermal dependence like
an optical or acoustic Raman pathway is also important.
Additionally, to examine the effect of dipolar interaction on the
slow magnetic relaxation process, we have prepared diluted
compounds for complexes 1, 2, and 3 to obtain 1a, 2a, and 3a,
respectively, by using the mixture of Co(ClO4)2·6H2O and
Zn(ClO4)2·6H2O in a 5:95 percentage ratio. The presence of
Zn and Co elements and a doped level in the diluted samples
were verified by energy dispersive X-ray spectroscopy (EDS;
Figures S16, S17, and S18). The compounds 1a and 2a show
the temperature dependence of out-of-phase susceptibility
(Figure S19) at little higher temperature range (2−5.9 K for 1a
and 2−4.8 K for 2a) as compared to 1 (2.0−5.2 K) and 2
(2.0−4.3 K) under the same external field. But no such
difference was found in ac susceptibilities for 3a (Figure S19).
Thus, it can be specified that in 1 and 2 the dipolar interaction
plays a role in the slow relaxation process, which does not arise
from the single ion anisotropy.
CONCLUSION
We have computed energy levels of the lowest two KDs
using the SINGLE_ANISO module by MOLCAS (Figures 7
■
In summary, we have shown three hepta-coordinated CoII
complexes with different relaxation dynamics because of the
lattice solvent molecules. The noncovalent interactions caused
by the lattice solvent molecules lead to different supra-
molecular reorganization, which further has a dramatic effect to
alter the magnetic behavior of the complexes. Solvent
molecules are present in both 1 and 2, which show field
induced SIM behavior above 2 K with Ueff = 25 and 15 K,
whereas due to the absence of the solvent molecule in 3 (Ueff
=
4 K), no peak maxima are observed above 2 K. In all the
complexes, it has also been observed that QTM through the
ground states is the major relaxation process, and thermal
dependence also makes it clear that an optical or acoustic
Raman pathway is important too.
Figure 7. Single_Aniso computed relaxation mechanism for
complexes 1 (left) and 2 (right). Black, green, blue, and red line
represent KDs, QTM, TA-QTM, and Orbach processes, respectively.
EXPERIMENTAL SECTION
■
and S20). The computed energies of first excited states are
relatively higher (Table S10) as compared to the observed
energy barrier, which further suggests that the Orbach process
is not playing a major role for all complexes. In addition, the
matrix elements of the transition magnetic moment between
the states +1 and −1 are quite high (Figures 7 and S20)
suggesting a strong QTM present in the ground KDs, which
quenched the magnetization and makes it impossible to show
zero field SIMs behavior for all the complexes.
Although the solvent molecules coordinate to all the
complexes, in 1 and 2 they are also present in the crystal
lattice, which makes a dramatic change in the crystal packing.
The large difference in relaxation dynamics between complexes
1 and 2 and complex 3 may not be possible only because of the
coordinating solvent. As in the first coordination sphere, the
metal-solvent bond distances are almost similar in all
complexes. On the other hand, distortion and the axial O1−
Co−O2 bond angle between 1 and 3 both are in distorted PBP
geometry, whereas 2 is in almost perfect PBP geometry. Thus,
bond distances and local symmetry of all the complexes
suggest that crystal field effects may not be responsible for
different relaxation dynamics. It further consisted of theoret-
Materials and General Procedure. All the chemicals and
reagents were analytical grade and used without further purification.
The magnetic susceptibilities were measured by a Quantum Design
SQUID-VSM magnetometer. The sample holder correction was done
for the experimental measure values, and the magnetic susceptibilities
were calculated after diamagnetic correction estimated from Pascal’s
table.19 Elemental analysis was performed on an Elementar Micro-
vario Cube Elemental Analyzer. The IR spectrum was recorded on
KBr pellets using a PerkinElmer spectrometer. Powder X-ray
diffraction (PXRD) data were collected on a PANalytical EMPYR-
EAN instrument using Cu Kα radiation.
Crystal Data Collection and Structure Determination.
̈
Intensity data were collected on a Bruker APEX-II CCD
diffractometer using graphite monochromated Mo Kα radiation (α
= 0.71073 Å) at 140 K. Data collections were performed using a φ
and ω scan. All the structures were solved using the ShelXT20,21
structure solution program using intrinsic phrasing, and olex222 was
used as the graphical interface. The models were refined with
ShelXL23 with full matrix least-squares minimization on F2. All non-
hydrogen atoms were refined anisotropically. Crystallographic data of
complexes 1, 2, and 3 have been summerized in Table S1
Computational Details (ORCA and MOLCAS 8.2). All the
calculations have been performed with the ORCA 4.0 and MOLCAS
8.2 software packages. The coordinates obtained from single crystal X-
E
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ray structure were used without optimization. In ORCA, we have used
ZORA (zeroth-order regular approximation) throughout the calcu-
lation. The def2-TZVPP basis set was used for Co and def-SV(P) for
other atoms in combination with auxiliary basis set def2/JK. In the
active space, we have considered seven electrons in 5 d orbital CAS
(5,7), and in the CI configuration (configuration interaction)
procedure, 10 quartets and 40 doublets were computed. Further, to
consider the dynamic correlation effects, N-electron valence
perturbation theory (NEVPT2) was included on top of the CASSCF
wave function. Also we employed the quasi-degenerate perturbation
theory (QDPT)24,25 approach for spin−orbit coupling effects. Both
the second order perturbation theory and an effective Hamiltonian
approach (EHA)26 have been used to estimate the ZFS parameters D
and E. In MOLCAS, the calculations of the CASSCF/RASSI-SO/
SINGLE_ANISO type and the ANO-RCC basis set of the function
have been used for all the atoms ([ANO-RCC-VTZP] for Co, [ANO-
RCC-VTZ] for O and N, and [ANO-RCC-VDZ] for C and H) with
relativistic effects using the Douglas−Kroll−Hess Hamiltonian.27 In
active space, we consider that, similar to the previous calculation,
[CAS(5,7)] and the RASSI program28 have been used to include the
spin−orbit coupling effects with the mixed optimized states in
previous calculations (10 spin quartet and 40 spin doublet states).
SINGLE_ANISO29 was used to compute the anisotropy of the low
lying states using ab initio wave function and spin−orbit eigenstates.
Synthesis of the Ligand (H4L). The ligand was synthesized using
a simple Schiff base condensation between 2,6-diacetylepyridine and
4-phenylesemicarbazide. To a solution of 2,6-diacetylepyridine (1
mmol, 163 mg) dissolved in 10 mL of ethanol under hot conditions,
ethanolic solution (10 mL) of 4-phenylesemicarbazide (2 mmol, 310
mg) was added. Further, four to six drops of glacial acetic acid were
added to the solution and the stirring continued under refluxed
conditions for 4 h. After cooling to room temperature, a pure white
microcrystalline product was obtained and the solution filtered. The
precipitate was washed with cold ethanol three to five times and dried
under a vacuum, which results in white powder as a pure product. The
ligand was used without further purification.
Synthesis of [Co(H4L)(DMF)(H2O)](NO3)2·(DMF) (1). H4L (0.1
mmol, 42 mg) was added to the 5 mL of acetone, and after few
minutes of stirring, Co(NO3)2·6H2O (0.1 mmol, 29 mg) was added at
room temperature. The stirring was continued for the next 3 h at 50
°C. A brown yellowish precipitate (ppt) was formed and the reaction
mixture cooled to room temperature. After the filtration, the ppt was
washed with acetone and dried under a vacuum. Then, the ppt was
dissolved in hot DMF solvent and cooled to room temperature. The
resultant solution was then kept for vapor diffusion by using diethyl
ether (DEE). After 3 days, orange color blocked shaped crystals
suitable for X-ray were obtained (yield 70%). Anal. Calcd for
C29H39CoN11O11: C, 44.85; H, 5.06; N, 19.84%. Found: C, 45.01; H,
5.02; N, 19.95%. IR (KBr pellet, 4000−400 cm−1), ν /cm−1: 3233,
3090, 3047, 2982, 2962, 1537, 1383, 1333, 1261, 1107, 1040, 756.
Synthesis of [Co(H4L)(MeOH)(H2O)](NO3)2·(MeOH) (2). The
ligand (0.1 mmol, 42 mg) was added in 3 mL of methanol solvent and
the solution stirred for 15 min. The methanolic solution of
Co(NO3)2·6H2O (0.1 mmol, 29 mg in 3 mL of MeOH) was then
slowly added to the ligand solution and the solution stirred for 4 h at
60 °C. The solution was cooled at room temperature and filtered, and
the filtrate was kept for slow evaporation. Brown colored block shaped
crystals were obtained after 4 days and washed with Et2O (yield 90%).
Anal. Calcd for C25H33CoN9O11: C, 43.23; H, 4.79; N, 18.15%.
Found: C, 43.37; H, 4.84; N, 18.19%. IR (KBr pellet, 4000−400
cm−1), ν /cm−1: 3198, 3098, 2944, 2826, 1569, 1383, 1329, 1261,
1095, 1034, 759.
Preparation of Diluted Sample. The diluted sample has been
prepared using a method similar to that for complexes 1, 2, and 3;
however, a mixture of Zn(ClO4)2·6H2O and Co(ClO4)2·6H2O in a
95:5 percentage ratio was used.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00615.
It includes PXRD, plots, and additional data of magnetic
characterizations (PDF)
Accession Codes
CCDC 1895210−1895212 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*Tel.: +91-755-6691313. E-mail: skonar@iiserb.ac.in.
■
ORCID
Sanjit Konar: 0000-0002-1584-6258
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Subhadip Roy for his valuable and scientific
support. A.M. thanks IISER Bhopal for the SRF fellowship.
A.K.H. thanks CSIR for the SRF fellowship, and S.K. thanks
CSIR (Project No. 01(2974)/19/EMR-II) Government of
India and IISER Bhopal for generous financial support.
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F
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