Probing the Effects of Ligand Field and Coordination Geometry on Magnetic Anisotropy of Pentacoordinate Cobalt(II) Single-Ion Magnets

Article abstract of DOI:10.1021/acs.inorgchem.7b00233

In this work, the effects of ligand field strength as well as the metal coordination geometry on magnetic anisotropy of pentacoordinated Co^II complexes have been investigated using a combined experimental and theoretical approach. For that, a strategic design and synthesis of three pentacoordinate Co^II complexes [Co(bbp)Cl₂]·(MeOH) (1), [Co(bbp)Br₂]·(MeOH) (2), and [Co(bbp)(NCS)₂] (3) has been achieved by using the tridentate coordination environment of the ligand in conjunction with the accommodating terminal ligands (i.e., chloride, bromide, and thiocyanate). Detailed magnetic studies disclose the occurrence of slow magnetic relaxation behavior of Co^II centers with an easy-plane magnetic anisotropy. A quantitative estimation of ZFS parameters has been successfully performed by density functional theory (DFT) calculations. Both the sign and magnitude of ZFS parameters are prophesied well by this DFT method. The theoretical results also reveal that the α → β (SOMO-SOMO) excitation contributes almost entirely to the total ZFS values for all complexes. It is worth noting that the excitation pertaining to the most positive contribution to the ZFS parameter is the d_xy → d_x²-y² excitation for complexes 1 and 2, whereas for complex 3 it is the d_z² → d_x²-y² excitation.

Full text of DOI:10.1021/acs.inorgchem.7b00233

Article
pubs.acs.org/IC
Probing the Effects of Ligand Field and Coordination Geometry on
Magnetic Anisotropy of Pentacoordinate Cobalt(II) Single-Ion
Magnets
Amit Kumar Mondal,† Tamal Goswami,‡ Anirban Misra,‡ and Sanjit Konar*,†
†Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066,
Madhya Pradesh, India
‡Department of Chemistry, University of North Bengal, Siliguri, Darjeeling 734013, West Bengal, India
S
* Supporting Information
ABSTRACT: In this work, the effects of ligand field strength
as well as the metal coordination geometry on magnetic
anisotropy of pentacoordinated CoII complexes have been
investigated using a combined experimental and theoretical
approach. For that, a strategic design and synthesis of three
pentacoordinate CoII complexes [Co(bbp)Cl2]·(MeOH) (1),
[Co(bbp)Br2]·(MeOH) (2), and [Co(bbp)(NCS)2] (3) has
been achieved by using the tridentate coordination environ-
ment of the ligand in conjunction with the accommodating
terminal ligands (i.e., chloride, bromide, and thiocyanate).
Detailed magnetic studies disclose the occurrence of slow
magnetic relaxation behavior of CoII centers with an easy-plane
magnetic anisotropy. A quantitative estimation of ZFS
parameters has been successfully performed by density functional theory (DFT) calculations. Both the sign and magnitude of
ZFS parameters are prophesied well by this DFT method. The theoretical results also reveal that the α → β (SOMO−SOMO)
excitation contributes almost entirely to the total ZFS values for all complexes. It is worth noting that the excitation pertaining to
2
2
the most positive contribution to the ZFS parameter is the dxy → dx −y excitation for complexes 1 and 2, whereas for complex 3 it
2
2
2
is the dz → dx −y excitation.
In this regard, for the control of the magnetic anisotropy of
single-ion magnets, efforts have recently been increased.4m
Nevertheless, factors governing single-ion anisotropy are not
well-studied, and control over magnetic anisotropy remains a
challenging task. Ideally, the magnetic anisotropy and relaxation
dynamics of mononuclear complexes can be tuned by
controlling the ligand around the metal ion and its geometry,
but a rational approach using combined experimental and
theoretical studies is yet to be achieved. In this work, we have
prepared a V-shaped ligand bbp [bbp = 2,6-bis(2-
benzimidazolyl)pyridine] with the anticipation that the rigid
base of the ligand could favor a square pyramidal rather than
trigonal bipyramidal geometry. To modulate the magnetic
property, we chose thiocyanate ligands which may influence the
geometry of the coordinated metal center. Herein, the dynamic
magnetization study of three pentacoordinate CoII complexes
[Co(bbp)Cl2]·(MeOH) (1), [Co(bbp)Br2]·(MeOH) (2), and
[Co(bbp)(NCS)2] (3) has been reported, and the combined
effects of both terminal ligands and metal coordination
INTRODUCTION
■
Single-molecule magnets (SMMs) can show slow magnetic
relaxation upon removal of an applied magnetic field.1 The
importance of these molecules lies in their potential
applications in molecular spintronics and data storage.2
Recently, significant attention has been paid to the molecular
systems having only one metal center, considerable anisotropy,
no intermetallic interactions, and magnetic properties related to
SMMs, and these molecules are termed as single-ion magnets
(SIMs). The interesting feature of single-ion magnets lies in the
probable forecast of anisotropy established from ligand field
theory. In parallel with the extensive study on lanthanide-based
SIMs,3 substantial efforts have also been made to develop
transition-metal-based SIMs.4 For all the reported 3d-SIMs,
low-coordinated metal centers are commonly observed, where
the anisotropy is enhanced because of the unrestricted orbital
angular momentum.5 Among 3d-metal-based SIMs, CoII
compounds are generally of interest because of the noninteger
spin ground state,6a which decreases the possibility of quantum
tunneling of magnetization (QTM).6b Different from tradi-
tional SMMs where D is negative (D = axial ZFS parameter), a
few mononuclear 3d-complexes have been reported as showing
field-induced SMM behavior with a positive D value.4h,5b,c
Received: January 26, 2017
© XXXX American Chemical Society
A
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geometry on the magnetic anisotropy have been investigated
using a collective experimental and theoretical approach.
RESULTS AND DISCUSSION
■
Structural Description of 1−3. A single-crystal X-ray
study revealed that 1−3 crystallize in the triclinic P1 space
̅
groups (Table S1 in the Supporting Information). All
complexes contain a mononuclear five-coordinate CoII center
(Figure 1). The X-ray crystal structure of complex 1 was
reported previously,7 and only the lattice solvent molecule is
different for the present case.
Figure 2. Shape map for trigonal bipyramid (TBPY) and square
pyramid (SPY) in complexes 1 and 3. The circles designate the
locations of three ideal shapes labeled in boldface.
curvedness (front and back side) of complex 3 shows a
significant overlap between two benzimidazole groups (Figure
S4a−d). The 2D fingerprint plots were generated for complex 3
to explore the different types of contacts. In the 2D fingerprint
plot, the wing region indicates the presence of the C···C
contact (11.5%) involved in π···π stacking present in the crystal
packing for complex 3 (Figure S4e,f). Therefore, this study
emphasizes the usefulness of Hirshfeld surface analysis and the
quick observation of rare structural features9e as well as
supramolecular interactions.9f
Magnetic Property Studies. The phase purities of the as-
synthesized complexes were confirmed by the good agreements
of powder X-ray diffraction patterns with the simulated ones
(Figure S5). Magnetic susceptibility studies were carried out for
1−3 under a 0.1 T field. At 300 K, the χMT values (χM = molar
magnetic susceptibility) of 2.57, 2.28, and 2.39 cm3 K mol−1
were observed for 1−3, respectively, which are higher than the
obtained spin-only value of 1.875 cm3 K mol−1 for the high-spin
CoII center. These values are in the range 2.1−3.4 cm3 K mol−1
for anisotropic CoII centers.10 Upon the cooling of the
complexes from room temperature, the χMT values of 1−3
do not change down to 60 K, below which they suddenly fall,
attaining values of 1.69, 1.46, and 1.52 cm3 K mol−1,
respectively, at 2 K (Figure 3 and Figure S6). The decline of
χMT values could be because of the inherent anisotropy of the
CoII centers. Reduced magnetization data attained the values of
2.3, 2.0, and 2.1 NμB (NμB = nuclear magneton) for 1−3,
respectively, at 2 K and 7 T (Figure 3 and Figure S6). These
values are lower than the theoretical saturation for an S = 3/2
system (Msat = 3.3 for g = 2.2) and do not saturate at a 7 T field,
and it can be observed from Figures S6−S7 that all M/NμB
versus H/T plots do not merge with each other signifying
anisotropic systems. A spin Hamiltonian has been used to
describe the magnetic anisotropy qualitatively (eq 1):
Figure 1. View of the molecular structures of complexes 1 (left) and 3
(right) emphasizing the square pyramidal and trigonal bipyramidal
coordination geometries around CoII centers, respectively. Color
codes: Co (magenta), Cl (green), N (blue), S (yellow), C (gray).
Both complexes 1 and 2 are isostructural with each other,
and the structure of 2 is presented in Figure S1. In complex 2,
the CoII center is coordinated by three nitrogen atoms from the
bbp ligand, and two Br− ions are located in the equatorial and
axial positions. In complex 3, the CoII center has trigonal
bipyramidal coordination geometry where the CoII center
resides on the plane of the bbp ligand. There are three nitrogen
atoms [one pyridyl (N3) and two thiocyanate groups (N6 and
N7)] located in the equatorial plane, and N1 and N4 are placed
at apical sites. The angles in the trigonal plane are close to the
ideal value of 120° [N3−Co1−N7 = 134.07(6)°, N3−Co1−N6
= 116.37(6)°, and N6−Co1−N7 = 109.04(3)°]. However, the
axial angle is considerably deviated from the ideal value [180°;
N1−Co1−N4 = 151.33(8)°; Tables S2 and S3]. The five-
coordinate CoII centers assume geometries which are best
defined as distorted square pyramidal and trigonal bipyramidal
using SHAPE 2.1,8 with minimum continuous shape measure
(CShM) values of 1.735, 2.163, and 2.146 for 1−3, respectively
(Table S4). To analyze the distortion pathways, we have
plotted the shape map for TBPY (trigonal bipyramid)
compared to that for SPY (square pyramid). It can be observed
from the shape map that complex 3 follows the interconversion
path between the SPY and TPBY (Berry pseudorotation), but
complex 1 (or 2) seems to be off this pathway and following
another distortion pathway between a square pyramid and a
vacant octahedron (VOC; umbrella distortion; Figure 2).
In all complexes, the mononuclear units are involved in
intermolecular H-bonding with each other and also with
interstitial solvent molecules (Tables S5 and S6). H-bonding
interactions favor the construction of a supramolecular 2D-
arrangement (Figures S2 and S3). Additionally, the π···π
interactions are also observed between the phenyl rings of the
benzimidazole groups with centroid-to-centroid distances of
3.754(2), 3.791(3), and 3.606(2) Å for 1−3, respectively. For
an understanding of the packing arrangement, the Hirshfeld
surfaces9a and fingerprint plots9b,c were constructed using
CrystalExplorer 3.1.9d A large, flat region appearing in the
2
2
H = gμ S × B + D[Sz − S(S + 1)/3] + E(Sx − Sy2)
B
(1)
where D and E represent the axial and rhombic terms of ZFS
parameters. The PHI code11 was used to estimate the ZFS
values of CoII by concurrent fitting of the χMT versus T and M/
NμB versus H plots. The best fit gave D = 14.5(2) cm−1, E = 0,
gx = 2.41, gy = 2.25, and gz = 2.01 for 1; D = 8.4(7) cm−1, E = 0,
gx = 2.30, gy = 2.20, and gz = 1.99 for 2; D = 10.7(4) cm−1, E =
1.1(2) × 10−4 cm−1, gx = 2.32, gy = 2.23, and gz = 1.99 for 3.
B
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Figure 3. χMT vs T plots measured at 0.1 T for complexes 1 (a) and 3 (b). 1/χM vs T plots shown in the inset; M/NμB vs H plots for complexes 1
(c) and 3 (d) at the indicated temperatures. The red  are the best fit.
Figure 4. Frequency dependency of the in-phase (χM′, a and b) and out-of-phase (χM″, c and d) ac magnetic susceptibility plots for complexes 1 and
3 under a 1000 Oe dc field.
Figure 5. Cole−Cole plots for complexes 1 (a) and 3 (b). The  represent the best fit. ln(1/τ) vs 1/T plots for complexes 1 (c) and 3 (d). The red
 are the best fit of the Arrhenius relationship.
The positive sign of the anisotropic parameter reveals the
presence of a significant interaction between the excited and
ground states by spin−orbit coupling (SOC). The D
parameters of previously reported pentacoordinated CoII
complexes have been shown in Table S7. It is clear that
pentacoordinated high-spin CoII complexes with square
pyramidal and trigonal bipyramidal geometries are systems of
great interest for inducing strong magnetic anisotropy and can
have different types of values of D parameters that varied from
easy-axis- to easy-plane-type magnetic anisotropy. Both the sign
and the magnitude of the ZFS parameters D’s obtained for 1−3
are found to be similar to those of the previously reported
pentacoordinated CoII complex [Co(TPMA)(CH3CN)]-
(BF4)2·CH3CN (Table S7).5e However, the studied complexes
offer the opportunity to probe the influence of the ligand field
strength as well as metal coordination geometry on their easy-
plane magnetic anisotropy.
For investigation of the relaxation dynamics of 1−3, ac
susceptibility measurements were carried out in the temper-
ature range 1.8−10 K, and the out-of-phase (χM″) signal was
not detected under a zero dc field. However, all complexes
exhibit temperature- and frequency-dependent ac signals under
a 1000 Oe dc field (Figure 4 and Figures S8−S10). Also, the
Cole−Cole plots (Figure 5 and Figure S10) were constructed
from the frequency-dependent ac data. The fit of the χM″ versus
χM′ data by the generalized Debye model12 produced the α
values (α parameter defines the width of the distribution of
relaxation times; α = 1 relates to an infinitely wide distribution,
while α = 0 signifies a relaxation with a single time constant)
within the ranges 0.04−0.26 (1), 0.02−0.29 (2), and 0.08−0.31
(3). The effective energy barrier (Ueff) and relaxation times
(τ0’s) were calculated using the Arrhenius equation13 ln(1/τ) =
ln(1/τ0) − Ueff/kT. The best fit gave the following: Ueff = 19.6
K and τ0 = 5.8 × 10−5 s for 1; Ueff = 8.2 K and τ0 = 3.1 × 10−5
s
for 2; Ueff = 9.7 K and τ0 = 4.6 × 10−5 s for 3 (Figure 5 and
Figure S10). Recently, two types of mechanisms are proposed
for the SMM behavior in 3d-SIMs with D > 0. The first type
comes from the transverse anisotropy barrier proposed by
Pardo et al.,4h and the effective barrier is controlled by a
substantial E value. This type is not relevant for these
complexes, because experimental E values were found to be
almost zero. The other type was suggested by Luis et al., and it
was governed by the direct relaxation of the MS = 1/2 states
by the phonon bottleneck effect.14 Actually, these processes are
repressed in a Kramers system with a substantial anisotropy
regardless of the sign of the D parameter. For complexes 1−3,
as the energy barriers obtained from ac susceptibility
measurements are lower than the gap between the MS = 1/
C
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Table 1. Experimental (Dexptl) and Calculated (Dcalc) ZFS Parameters for Complexes 1−3 and Individual Excitation
Contribution to the Total ZFS Parameter D
in cm−1
calc
SOC
D
total Dcalc in cm−1
Dexptl in cm−1
calc
complex
α → α
α → β
β → α
β → β
D
SS
1
2
3
−0.414
0.469
11.862
7.108
9.002
0.033
0.902
−0.665
−1.059
−0.299
0.272
0.361
11.09
7.79
8.44
14.5(2)
8.4(7)
−0.141
−0.053
−0.061
10.7(4)
2 and MS = 3/2 states, the Orbach relaxation process is not
applicable for the present case. Therefore, this supports the
suggestion that quantum tunneling and Raman processes have
contributed exclusively to the slow relaxation behavior of 1−3.
To gain more understanding of the origin of magnetization
relaxation, the relaxation times were re-evaluated considering
different relaxation processes. The experimental relaxation time
has been examined starting from its field dependence at 2 K
[Figure S11 (left)]. The first term in eq 2 exemplifies the QTM
which is intensely affected by magnetic field; however, the
second term (C) includes the weakly field-dependent processes
(Raman, Orbach, etc.) and is therefore kept as constant, C, in
eq 2.15 The relaxation time is well-described by this method
(τQTM was calculated to be 1.82 × 10−4 s from eq 2), which
confirms the significant role of the QTM of the ground state in
the relaxation mechanism.
τ−1 = B1/(1 + B2H2) + C
(2)
Figure 6. Electronic configuration and d-orbital energy level diagram
for complexes 1 (left) and 3 (right) from DFT calculations. CoX
bonds are placed along the z axis in 1 and 3, separately (X = Cl, NCS).
At higher temperatures, Raman or Orbach should be the
predominant process. The temperature dependence was
examined at 0.1 T [Figure S11 (right)], including thermally
active processes, which are either an Orbach process or follow a
Raman mechanism.15 The experimental relaxation time is well-
described by the method with a single power law, and values of
n = 5.5, 4.6, and 4.2 are obtained for complexes 1−3,
respectively (τQTM was fixed at 1.82 × 10−4 s), which are close
to the reported value by Colacio et al.16
molecular orbital (MO) diagram along with the electron
occupations for complex 1 are presented in Figure 6 (left)
which shows that the three unpaired electrons reside in the dxy,
2
2
2
dz , and dx −y orbitals. As is evident from the excitation
contributions to SOC, the α → β excitation corresponds to the
2
2
dxy → dx −y excitation which renders the highest contribution
to the ZFS parameter D. This is further established from the
TDDFT (time-dependent DFT) difference density plot
portrayed in Figure 8 (left), where the cyan and the light
purple surfaces represent electron-rich and electron-deficient
areas (electron and hole densities), respectively. The apparent
movement of electrons from the singly occupied dxy orbital to
τ−1 = τQTM−1 + bTn
(3)
Therefore, from the collective field and temperature
dependence of the relaxation time, it can be concluded that
QTM is the foremost pathway for the relaxation of the
magnetization. However, the relaxation process is steered by an
optical or acoustic Raman pathway that explains the thermal
dependence.
2
2
the singly occupied dx −y orbital (α → β) resulting in the
excited doublet state unambiguously dictates that the mixing of
the excited doublet state with the ground quartet state brings
about the SOC-led ZFS parameter of the CoII center. The MO
diagram and corresponding difference density plot for complex
2 are presented in the Supporting Information (Figures S13
and S15). As expected, the electronic excitation picture is much
the same as that for complex 1. A similar pictorial
representation of the MOs for complex 3 is given in Figure 6
(right). It is worth noting from Figure 8 (right) that the
excitation pertaining to the most positive contribution to the
Theoretical Calculations. To gain insight into the
magnetic interaction at the molecular level, we have carried
out density functional theory (DFT) studies of the magnetic
anisotropy of complexes 1−3. The results for the DFT
calculation of the D parameter are reported in Table 1. The
comparison of the DFT results with the experimental data
shows very good agreement. The four types of excitations
which can be observed to contribute to the D tensor are α → β,
β → α, β → β, and α → α. The different excitation contribution
to the total ZFS reveals that it is the α → β (SOMO−SOMO)
excitation that contributes almost entirely to the total ZFS
value. This information is in compliance with the fact that the
SOC arises out of the mixing of the ground quartet and the
excited doublet state. A close inspection of Table 1 also exposes
a negligible spin−spin coupling (SSC) contribution toward the
total ZFS parameter. A graphical depiction of 3d-orbitals and a
molecular orbital (MO) diagram for all the complexes have
been shown in Figures 6 and 7 and Figures S12−S14. The
2
2
2
ZFS parameter is the dz → dx −y excitation. As the α → β
excitations take a major role in defining the ZFS parameter of
complexes 1−3, the orbital splitting represented here confirms
the positive ZFS parameter of these complexes. This fact
conforms to the previous observation that such an excitation
gives rise to the positive D value.4l,m,5d
It can be observed from the structural parameters of
complexes 1 and 3 that changing the terminal ligand led to a
change in the coordination geometry around the CoII ion.
D
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Figure 7. Graphical representation of the five 3d-orbitals of complex 1. Co−Cl bond is placed along the z axis in 1.
3′ (Table 2). For the pair 2 and 2′, also, the major contribution
arises from the α → β excitation akin to those of complexes 1
and 3. Hence, the above numerical experiment clarifies the
effect of both geometry and ligand field toward the ZFS in the
synthesized complexes. A more precise analysis of the results
from the point-charge computations exposes that the lack of π-
bonding is the foremost reason behind the change in the D
value of complexes 1 and 2. From the orbital diagrams, it is
clear that the dxy orbital has a large π-type contribution from
the Cl ligands, and the change in D seems to arise from the lack
of π-character of the Co−Cl bond in complex 1. The effect of
π-donation in the ZFS parameter is further fortified from the
example of complex 2. Thus, a good π-donor should generally
be capable of introducing a more positive contribution toward
the ZFS parameter of the molecule. This is similar for complex
3, for which the same analysis suggests that a good π-acceptor
will have a negative contribution toward the total D of the
molecule.
We have also performed ab initio calculations with CASPT2
methodology. This calculation was done with the ANO-RCC
basis set in the MOLCAS7.8 suite of software,18a and the D
values were found to be −13.06, −3.65, and −44.31 cm−1 for
complexes 1, 2, and 3, respectively. Thus, compared to the
experimental results, the CASPT2 method is found to give a
poor prediction of the sign and magnitude of the ZFS
parameters. We have also calculated the ZFS parameter using
the NEVPT2 formalism implemented in ORCA. The D values
computed using this method are found to be 41.13, 42.73, and
35.97 cm−1 for complexes 1, 2, and 3, respectively (Table S8).
Thus, the results are also in disagreement with the experimental
results. Therefore, the DFT calculations on these complexes are
in better agreement with the experimental results. Previous
reports have also shown that DFT can calculate ZFS
parameters for CoII complexes.17,18b−e
Figure 8. Charge transfer difference density plots for complexes 1
(left) and 3 (right), where light purple and cyan depict electron-
deficient and electron-rich surfaces, respectively.
Thus, for understanding the effect of altering the ligand, a
deeper insight from DFT is necessary. DFT calculations on all
the geometries of complexes 1, 2, and 3 are performed with the
respective Cl, Br, and NCS ligands replaced by point charges of
magnitude −1.0.17 This calculation, on one hand, shows the
effect of altering geometries on the ligand substitution as is
performed experimentally in a very straightforward and
convincing way, while, on the other hand, it also shows the
effect of individual ligands, i.e., Cl, Br, and NCS, on the value of
the ZFS parameter. We further denoted the point-charge-
replaced geometries of complexes 1, 2, and 3 as complexes 1′,
2′, and 3′, respectively. The D value for 1′ is found to be 7.24
cm−1 while for complex 1 it is 11.09 cm−1 (Table 2). Thus, it is
Table 2. Values of the Total ZFS Parameter (D) after
Replacement of the Terminal Ligands with Point Charges of
the Same Magnitude for Complexes 1−3
complex
α → β excitation (cm−1
)
total D (cm−1
)
1
11.86
7.11
11.09
7.79
2
It was reported that structural changes around the
coordination environment can modulate the magnetic aniso-
tropy and relaxation behaviors of the resulting complexes.19
However, the present series of mononuclear CoII complexes 1−
3 reveals only a minor effect of the geometry and the terminal
ligand field on the magnitude of the magnetic anisotropy
parameter D. We assume that this is because of the stronger
ligand field effect of the benzimidazolylpyridine. Thus, the little
variances in the magnetic properties of complexes 1−3 come
from two combined influences; the first one is the difference in
the ligand field strength imposed by the terminal ligands, and
the second one is the slight changes of the metal coordination
geometry.
3
9.00
8.44
1′
2′
3′
8.78
7.24
−8.54
6.22
−6.95
9.23
apparent from the comparison of D values, for the same
geometry of complex 1 in the presence and absence of Cl as the
ligand, that Cl induces a positive contribution to the total D. It
is also noted that the major contribution (8.78 cm−1) in 1′
comes from the α → β excitation which is in parity with
complex 1. The similar calculation for the geometry of complex
3 manifests a negative contribution of the NCS ligand toward
the total D value. Complex 3′ has a ZFS parameter value of
9.23 cm−1, while the same geometry with the ligand NCS
instead of the point charge, i.e., complex 3, revealed a D value
of 8.44 cm−1 (vide supra). A major contribution of the
individual α → β excitation is 6.22 cm−1 in the case of complex
CONCLUSION
■
In conclusion, it is conceivable to stipulate the coordination
geometry around the CoII center by using the 2,6-bis(2-
E
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data (KBr pellet, 4000−400 cm−1) ν/cm−1: 3187 (νNH), 1610
(νCC), 1566 (νCN), 1460 (νCN), 1272 (νCN).
benzimidazolyl)pyridine ligand in conjunction with the
terminal ligands. The present results demonstrate the slight
change in the magnetic anisotropy of pentacoordinate CoII
SIMs by structural modification. Hence, propagation of the
present study is anticipated to explore interesting routes for
magnetochemists to determine ZFS parameters in low-
coordinate complexes.
Synthesis of Complex 2. Both the ligand bbp (31 mg, 0.1 mmol)
and CoBr2 (22 mg, 0.1 mmol) were dissolved in 10 mL of MeOH, and
the mixture was stirred at room temperature for 3 h. The resulting
mixture forms a deep green solution, and that was filtered and kept for
slow evaporation that yields green crystals of [Co(bbp)Br2]·(MeOH)
(2) after 4 days. The crystals were washed with Et2O and air-dried:
yield (74%). Anal. Calcd for C20H17CoN5OBr2: C, 42.71; H, 3.05; N,
12.46%. Found: C, 42.81; H, 3.13; N, 12.40%. Selected IR data (KBr
pellet, 4000−400 cm−1) ν/cm−1: 3186 (νNH), 1612 (νCC), 1565
(νCN), 1458 (νCN), 1270 (νCN).
EXPERIMENTAL SECTION
■
Materials and General Procedure. Magnetic studies were done
using a Quantum Design SQUID-VSM magnetometer. The obtained
values were amended for the experimentally measured contribution of
the sample holder, whereas the derived susceptibilities were corrected
for the diamagnetic contribution of the sample, assessed from Pascal’s
tables.20 Elemental analysis was performed on an Elementar
Microvario Cube elemental analyzer. The IR spectrum was measured
on KBr pellets with a PerkinElmer spectrometer. Powder X-ray
diffraction (PXRD) data was measured on a PANalytical EMPYREAN
instrument using Cu Kα radiation.
Synthesis of Complex 3. Both the ligand bbp (31 mg, 0.1 mmol)
and Co(ClO4)2·6H2O (36 mg, 0.1 mmol) were dissolved in 10 mL of
MeOH, and KSCN (20 mg, 0.2 mmol) dissolved in H2O (10 mL) was
added to the previous solution. The resulting mixture forms a deep
brown solution and was further stirred at room temperature for 6 h.
The green precipitate was filtered and recrystallized from a MeOH/
MeCN mixture (1:1), which yields green crystals of [Co(bbp)-
(NCS)2] (3) after 4 days. The crystals were washed with Et2O and air-
dried: yield (61%). Anal. Calcd for C21H13CoN7S2: C, 51.87; H, 2.69;
N, 20.16%. Found: C, 51.98; H, 2.57; N, 20.23%. Selected IR data
(KBr pellet, 4000−400 cm−1) ν/cm−1: 3188 (νNH), 2049 (νNCS),
1607 (νCC), 1569 (νCN), 1458 (νCN), 1275 (νCN).
Crystal Data Collection and Structure Determination.
Intensity data were measured on a Bruker APEX-II CCD
̈
diffractometer by graphite monochromated Mo Kα radiation (α =
0.710 73 Å) at 140, 120, and 296 K for complexes 1−3, respectively.
Data collection was done using φ and ω scan. The structure was
solved by direct methods followed by full matrix least-squares
refinements using SHELXTL.21 After that, difference Fourier synthesis
and least-squares refinement exposed the positions of the remaining
non-hydrogen atoms. Determinations of the crystal structure,
orientation matrix, and cell dimensions were made following reported
methods. Lorentz polarization and a multiscan absorption correction
were incorporated. Non-hydrogen atoms were refined with anisotropic
displacement, and H atoms were located geometrically and polished by
riding model. All calculations were performed by SHELXL 97,22
PLATON 99,23 and WinGX systemVer-1.64.24 Crystallographic data
for complexes 1−3 have been given in Table S1.
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.7b00233.
Theoretical background, analysis parameters, figures of
1−3 structures, bond distances and angles, and PXRD
and magnetic characterizations (PDF)
Accession Codes
CCDC 1437903−1437904 and 1538781 contain the supple-
mentary crystallographic 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.
Computational Details. The DFT computations were done with
the help of the ORCA package.25 The D parameter was calculated
using the TPSSH functional and TZV basis set in the unrestricted
Kohn−Sham framework. An auxiliary TZV/J Coulomb fitting basis set
was used during the calculation. The estimation of the SOC effects was
performed using the mean-field approximation (SOMF) incorporating
the spin−own-orbit as well as spin−other-orbit interactions in the
exchange term. The computation of the ZFS parameter was performed
through the popular coupled perturbed (CP-SOC) technique
developed by Frank Neese which is known to produce results that
are more realistic compared to those of any other method.26 We have
also used the zero-order regular approximation (ZORA) for relativistic
corrections in all the computations.
Synthesis of Ligand bbp. A 3.34 g (19.8 mmol) portion of
pyridine-2,6-dicarboxylic acid and 4.68 g (43.7 mmol) of 1,2-
phenylenediamine were mixed with 155 g of polyphosphoric acid
under N2 atmosphere. This was heated at 220 °C for 5 h, and was then
cooled to 140 °C and added into 1 L of H2O. The resulting blue
precipitate was collected by filtration, dispersed in 500 cm3 of 10%
Na2CO3(aq), and further stirred for 30 min. Then, the mixture was
filtered and dispersed in cold H2O (800 cm3). The pH of the solution
was adjusted to 4 by 10% HCl(aq) and recrystallized from MeOH.
The ligand was found as a pure white crystal of 72% yield (4.5 g).
Selected IR data (KBr pellet, 4000−400 cm−1) ν/cm−1: 3185 (νNH),
1600 (νCC), 1573 (νCN), 1460 (νCN), 1278 (νCN).
AUTHOR INFORMATION
Corresponding Author
*E-mail: skonar@iiserb.ac.in. Phone: +91-755-6691313.
■
ORCID
Anirban Misra: 0000-0002-9525-1886
Sanjit Konar: 0000-0002-1584-6258
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.K.M. thanks UGC, India, for the SRF fellowship. T.G. and
A.M. thank DST, India, for financial support. A.K.M. thanks
Mr. Dhananjay Dey and Dr. Jordi Cirera for their helpful
scientific discussions. S.K. thanks BRNS DAE (Project 37(2)/
14/09/2015-BRNS) and IISER Bhopal for generous financial
and infrastructural support.
Synthesis of Complex 1. Both ligands bbp (31 mg, 0.1 mmol)
and CoCl2·6H2O (23 mg, 0.1 mmol) were dissolved in 10 mL of
MeOH, and the mixture was stirred at room temperature for 4 h. The
resulting mixture forms a deep green solution which was filtered and
kept for slow evaporation that yields green crystals of [Co(bbp)Cl2]·
(MeOH) (1) after 3 days. The crystals were washed with Et2O and air-
dried: yield (80%). Anal. Calcd for C20H17CoN5OCl2: C, 50.76; H,
3.62; N, 14.80%. Found: C, 50.89; H, 3.71; N, 14.86%. Selected IR
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