Role of Halide Ions in the Nature of the Magnetic Anisotropy in Tetrahedral CoII Complexes

Article abstract of DOI:10.1002/chem.201606031

A series of mononuclear tetrahedral Co^II complexes with a general molecular formula [CoL₂X₂] [L=thiourea and X=Cl (1), Br (2) and I (3)] were synthesized and their structures were characterized by single-crystal X-ray diffraction. Direct-current (dc) magnetic susceptibility [χ_MT(T) and M(H)] and its slow relaxation of magnetization were measured for all three complexes. The experimental dc magnetic data are excellently reproduced by fitting both χ_MT(T) and M(H) simultaneously with the parameters D=+10.8 cm^?1, g₁=2.2, g₂=2.2, and g₃=2.4 for 1; D=?18.7 cm^?1, g_iso=2.21 for 2; and D=?19.3 cm^?1, g_iso=2.3 for 3. The replacement of chloride in 1 by bromide or iodide (in 2 and 3, respectively) was accompanied by a change in both sign and magnitude of the magnetic anisotropy D. Field-induced out-of-phase susceptibility signals observed in 10 % diluted samples of 1–3 imply slow relaxation of magnetization of molecular origin. To better understand the magnetization relaxation dynamics of complexes 1–3, detailed ab initio CASSCF/NEVPT2 calculations were performed. The computed spin Hamiltonian parameters are in good agreement with experimental data. In particular, the calculations unveil the role of halide ions in switching the sign of D on moving from Cl^? to I^?. The large spin–orbit coupling constant associated with the heavier halide ion and weaker π donation reduces the ground state–excited state gap, which leads to a larger contribution to negative D for complex 3 compared to complex 1. Further magnetostructural D correlations were developed to understand the role of structural distortion in the sign and magnitude of D values in this family of complexes.

Full text of DOI:10.1002/chem.201606031

A Journal of
Accepted Article
Title: Role of Halide Ions on the Nature of Magnetic Anisotropy in
Tetrahedral Co(II) Complexes
Authors: Maheswaran Shanmugam, Shefali Vaidya, Saurabh Kumar
Singh, Pragya Shukla, Kamaluddin Ansari, and Gopalan
Rajaraman
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201606031
Link to VoR: http://dx.doi.org/10.1002/chem.201606031
Supported by

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
Role of Halide Ions on the Nature of Magnetic Anisotropy in
Tetrahedral Co(II) Complexes
Shefali Vaidya,[a] Saurabh Kumar Singh,[a],[b] Pragya Shukla,[a] Kamaluddin Ansari,[a] Gopalan
Rajaraman*[a] and Maheswaran Shanmugam*[a]
Abstract: A series of mononuclear tetrahedral Co(II) complexes with
a general molecular formula of [CoL2X2] (where L= thiourea and X =
Cl (1), Br (2) and I (3)) were synthesized and their structures were
characterized by single crystal x-ray diffraction. Detailed direct
current (dc) magnetic susceptibility (χMT(T) and M(H)) and its slow
relaxation of magnetization measurements were performed on all the
three complexes. The experimental dc magnetic data is excellently
reproduced by fitting both χMT(T) and M(H) simultaneously using the
parameters D = +10.8 cm-1, g1 = 2.2, g2 = 2.2, and g3 = 2.4 for 1; D
= -18.7 cm-1, giso = 2.21 for 2; D = -19.3 cm-1, giso = 2.3 for 3. The
replacement of chloride anion in 1 by bromide or iodide (in 2 and 3
respectively) accompanied by not only change in sign of magnetic
anisotropy (D), but also in magnitude. Field induced out-of-phase
susceptibility signals are observed in 10% diluted sample of 1-3
implies the slow relaxation of magnetization behavior of molecular
origin. To better understand the magnetization relaxation dynamics
of complexes 1-3 detailed ab initio CASSCF/NEVPT2 calculations
were performed. The computed spin Hamiltonian parameters are in
good agreement with experimental data. Particularly calculations
unveil the role of halogen atom in switching the sign of D as we
move from –Cl- to –I-. Large spin-orbit coupling constant associated
with the heavier halide ion and weaker π donation reduces the
ground state-excited state gap leads to larger contribution to
negative D for the complex 3 compared to complex 1. Further
magneto-structural D correlations are developed to understand the
role of structural distortion on the sign and magnitude of D values in
this family of complexes.
can be envisaged for many potential applications such as high
density storage, spin valves, spintronic, Quantum computing
etc.[5] Over the period of three decades, it has been realized
that due to the random easy axis orientations in larger cluster,
overall magnetic anisotropy (D) becomes small, although
complexes were stabilized with relatively large ground state.
Ideally, there are no parameters available to enhance the D and
S simultaneously in oligomeric complexes which is also evident
that D is approximately equals to 1/S2 of the complex.[6] Due to
this persistent problem researchers focused their attention to
modulate the D value of the mononuclear transition metal
complexes. The majority of the transition metal complexes suffer
with low spin-orbit coupling as the orbital angular momentum is
reduced by the ligand field. Restricting the coordination number
around the transition metal is found to be a fruitful way to gain
orbital angular momentum[7] and such synthetic strategy was
proven successful in a two coordinate Co(II) and Fe(I) complex
with the largest anisotropic barrier 578.2
K and 225 K
respectively reported to date for any transition metal SMMs by
Gao and Co-workers and Long and co-workers independently.[8]
In addition, several other approaches have been reported in the
literature to gain orbital angular momentum, to maximize the
magnetic anisotropy in mononuclear complexes. For examples,
the substituent on the ligand modulate Ueff (by means of D),[2e, 9]
in-plane and out-of plane shift of metal ion in a five coordinate
Co(II) complexes[10], by changing the halides in an octahedral
and certain tetrahedral complexes,[11] structural distortion around
metal centers and even more intricate factors such as secondary
coordination sphere has a significant effect on modulating D-
value which was elegantly explained by Neese and co-workers
theoretically and experimentally proven by us recently.[12] To
modulate the sign of magnetic anisotropy, we have proposed a
novel synthetic strategy recently i.e. employing a soft donor
ligand (such as sulfur) stabilizes easy axis anisotropy, while the
hard donor favours easy plane magnetization orientation.[13]
Introduction
Slow relaxation of magnetization arises in certain oligomeric
complexes due to the presence of easy or Ising type magnetic
anisotropy associated with the overall ground state of
a
molecule. Such phenomenon is termed as Single-molecule-
magnets (SMM), if the phenomenon originates due to a single
metal ion or monomeric coordination complexes it is known as
single-ion magnets (SIM). Such SMM property was first
discovered in {Mn12OAc} complex.[1] After the discovery,
numerous transition metal complexes[2] were flooded in the
literature with a record breaking ground state (83/2 for a Mn19
cluster) reported by Powell and co-workers[3] and an effective
energy barrier (Ueff, 86 K for Mn6 cluster) for the magnetization
relaxation by Brechin and co-workers[4] independently, as SMM
In this line of interest, we intend to probe the influence of other
commonly known ligand such as halides in controlling the spin
Hamiltonian parameters of the complexes, apart from the soft
donor “L” ligand. For this purpose, we have synthesized a series
of monomeric Co(II) tetrahedral thiourea complexes and their
magnetic properties were investigated in details. The observed
change in Spin Hamiltonian (SH) parameters and its slow
magnetization relaxation behavior of these complexes were
rationalized by detailed theoretical calculations.
[a] S. Vaidya, Dr. S.K. Singh, P. Shukla, K. Ansari, Prof. G. Rajaraman and
Prof. M. Shanmugam
Department of Chemistry, Indian Institute of Technology Bombay
Powai, Mumbai-400076, Maharashtra, India.
E-mail: rajaraman@chem.iitb.ac.in, eswar@chem.iitb.ac.in
[b] Dr. S. K. Singh
Department of Molecular Theory and Spectroscopy, Max-Planck Institute
for Chemical Energy Conversion, Mülheim an der Ruhr, D-45470,
Germany
Results and Discussion
Recently we and others have pointed out that by employing soft
donor ligand such as sulfur in a tetrahedral Co(II) environment,
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
stabilize easy axis magnetic anisotropy.[13-14] This proposed idea
has been proven by us recently by employing a totally different
soft donor (thiourea and its derivatives) ligands.[12c] In order to
probe the influence of the other ligated atoms such as halides
apart from the soft donor ligands, we have synthesized a series
of monomeric Co(II) complexes. When one equivalent of
alcoholic solution of CoX2.xH2O (where X = Cl or Br or I) is
treated with two equivalent of thiourea, yields blue color single
crystal which are suitable for single crystal x-ray diffraction. The
structure solution and refinement reveals the molecular formulae
of all the three complexes as [CoX2L2] where L = thiourea
((NH2)2CS); X = Cl (1) or Br (2) or I (3) (Figure 1). The
complexes 1-3 were crystallized in a monoclinic Cc (for 1) and
complexes is larger than the Co-S bond length. The bond angle
of ∠X11-Co1-X12 in all the three complexes are close to the
tetrahedral angle ie. 107.82°(3), 108.40°(3) and 107.25°(2) for 1-
3 respectively. However, drastic change is noticed with the bond
angle of ∠S11-Co-S12 96.63°(3) (for 1), 101.43°(3) (for 2), and
100.32°(4) (for 3). Selected bond lengths and bond angles of
complexes 1-3 are given in Table 2. Detailed structural analysis
of all the three complexes reveal that there is hydrogen bonding
network across all the directions. In complexes 1-3, the protons
on the amino group of thiourea ligand are involved in hydrogen
bonding with halide ions as well as the sulfur atom. Apart from
the intermolecular hydrogen bonding, intramolecular hydrogen
bonding also exists in the crystal structure. The atoms involved
in both intra and intermolecular hydrogen bonding are listed in
Tables S1-S3 for all the three complexes.
P21/c (for
2 and 3) space group. The crystallographic
parameters for all the three complexes are given in Table 1.
Table 1. Crystallographic data for complexes 1-3.
1
CoC2H8Cl2N4S2
0.20 x 0.16 x 0.11
Monoclinic
Cc
2
CoC2H8Br2N4S2
0.35 x 0.17 x 0.08
Monoclinic
P21/c
3
CoC2H8I2N4S2
0.58 x 0.22 x 0.12
Monoclinic
P21/c
Formula
Size (mm)
System
Space grp.
a [Å]
b [Å]
c [Å]
α [°]
β [°]
8.1970(16)
11.528(2)
10.794(2)
90.0000
103.56(3)
90.0000
991.5(3)
4
10.163(4)
6.977(2)
14.562(5)
90.0000
93.207(5)
90.0000
1030.9(6)
4
10.483(3)
7.3377(17)
14.813(4)
90.0000
91.344(3)
90.0000
1139.1(5)
4
γ [°]
V [Å3]
Z
ρcalcd[g/cm-3]
2ϴmax
1.890
58.28
2.390
58.3
2.711
58.34
radiation
λ [Å]
Mo Kα
0.71073
Mo Kα
0.71073
Mo Kα
0.71073
T [K]
100
100
100
reflns
5985
19068
14607
Ind. reflns
reflns with
I>2σ(I)
R1
2585
2310
2770
2065
3046
2251
0.0242
0.0683
0.0304
0.0478
0.0275
0.0597
wR2
The packing diagram of 1 is distinctly different from complex 2 or
3 (Figure 2A). Since complexes 2 and 3 possess a similar
packing arrangement, a representative packing diagram is
shown in Figure 2B. In complex 1, hydrogen bonding exists
between N-H….S11 (2.624(3) Å) is relatively stronger than N-
H….Cl (3.093(2) Å).
Table 2. Selected bond lengths and bond angles parameter for complexes 1-
3.
Label
1 (Å)
2 (Å)
3 (Å)
Figure 1. Thermal ellipsoid (50% probability) representation of complexes A) 1
B) 2 C) 3.
Co1-S11
Co1-S21
Co1-X11
Co1-X12
2.297(10)
2.307(10)
2.249(9)
2.268(9)
2.303(9)
2.311(9)
2.397(6)
2.405(5)
2.308(10)
2.330(11)
2.595(6)
2.608(5)
In all the three complexes the divalent cobalt ion exists in a
distorted tetrahedral geometry. Two out of four coordination sites
are occupied by thiourea ligand while the other two sites are
completed by two halide ions in respective complexes. The
average Co-S bond distance found to be 2.302(10) Å, 2.307(9)
Å, 2.319(10) Å for complexes 1-3 respectively. While the
average bond length for Co-X (where X = Cl or Br or I; 2.258(9)
Å for 1; 2.401(5) Å for 2; 2.602(5) Å for 3) increases upon
increasing the atomic radii of the halide ion in complex. But the
extent of increase in bond lengths of Co-X in all the three
Bond angle (°)
X11-Co1-X12
X11-Co1-S11
X12-Co1-S11
X11-Co1-S21
X12-Co1-S21
S11-Co1-S21
107.82(3)
116.02(4)
106.96(4)
113.46(4)
115.72(4)
96.63(3)
108.40(3)
111.65(3)
109.87(3)
110.05(3)
115.36(3)
101.43(3)
107.25(2)
111.86(3)
116.73(3)
110.45(3)
110.07(3)
100.32(4)
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
Variation in the H-bonding strength in 1 is likely due to the short
Co(II)….Co(II) distance found between two molecules (see
Figure 2 for details). Such supramolecular interaction is likely to
play a significant role in magnetization relaxation dynamics.
From the packing diagram of complex 1, it is clearly witnessed
that a layer of molecules along the c-axis are orienting in the
same direction, while the adjacent layer is generated by c-glide.
Although the interatomic distance of Co(II)….Co(II) found in 2
(5.853(3) ) and 3 (5.932(2) Å) is shorter than distance found in 1,
the H-bonding strength observed in complex 2 and 3 is weaker
than in 1 (see Tables S1-S3 and figure S1 for details).
Figure 2. Packing diagram of complex 1 (panel A) and complex 2 (panel B). Sky blue dotted bonds represent intermolecular H-bonding between sulfur and proton
(-NH2) and wine red dotted bond denotes H-bonding between halide and proton (-NH2) in the crystal lattice. Colour code: Magenta = Co(II), green = X (X = Cl (for
1) or Br (for 2)), blue = N, yellow = S.
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
Figure 3. Panels A-C) Direct current magnetic susceptibility measurement performed on polycrystalline sample of complexes 1-3 respectively in the presence of
an external magnetic field of 1 kOe. The open circle in those panel represents the simulation of the experimental magnetic data using the SH parameters
computed from CASSCF / NEVPT2 calculations described in main text. Panels D-F) Field dependent magnetization measurements were performed at the
indicated temperatures. The solid red line in all the panels represents the simultaneous magnetic data (χMT(T) and M(H)) fitting using the parameters described in
Table 3 (vide infra).
Direct current (dc) magnetic susceptibility data of
complexes 1-3.
complexes 1-3 respectively (Figure 3). The observed magnetic
moment at this temperature is significantly lower than the
expected value indicates the presence of magnetic anisotropy
associated with the ground state in all the complexes. The non-
superimposable nature of the reduced magnetization curve of all
the three complexes further supports the presence of magnetic
anisotropy (Figure S2). In order to extract the spin Hamiltonian
(SH) parameters of all the three complexes, we have fitted the
magnetic data of both χMT(T) and M(H) simultaneously using
PHI software.[15] The Hamiltonian used for fitting the data is
given below.
Variable temperature direct current magnetic susceptibility
measurements were performed on polycrystalline samples of all
the three complexes in the temperature range of 2.0 - 300 K in
the presence of an external magnetic field of 1 kOe (Figure 3).
The room temperature (RT) χMT value for all the three
complexes 1 – 3 are 2.40, 2.52, 2.63 cm3 K mol-1 respectively
which are significantly higher than the expected value for a
mononuclear Co(II) (S = 3/2) ion with no first order orbital
angular momentum (1.875 cm3Kmol-1, g = 2). The temperature
dependent χMT(T) behavior of all the three complexes is almost
similar, i.e. a gradual decrease in χMT value is noticed upon
lowering the temperature from room temperature (RT) to 50 K.
The observed temperature dependency in this temperature
!!!
ꢀ = ꢀ ꢀ!! − !
+ ꢀ ꢀ!! − ꢀ!! + ꢀµ!ꢀ. ꢀ
(1)
!
In order to reduce the over parameterization, we have fitted the
magnetic data using isotropic g-value for all the complexes
except for complex 1 and the obtained parameters are listed in
Table 3. An excellent agreement between the fit and the
experimental magnetic data obtained using the parameters D =
+10.8 cm-1 (gxx = 2.2, gyy = 2.2, gzz = 2.4; |E/D| = 0.11, for 1.
While the simultaneous χMT(T) and M(H) data fit of 2 and 3
yields the D-value of -18.7 cm-1 (giso = 2.21) and -19.3 cm-1 (giso
= 2.3) respectively (Table 3). Here we would like to point out that,
the experimental χMT(T) fit alone is insensitive to the sign of D-
value i.e. either with positive D or negative D, experimental
χMT(T) data could be modeled. However, the parameters
region (RT to 50 K) for
a mononuclear Co(II) complex
designates the depopulation of the Kramers-state. There is a
sharp decrease in χMT value below 50 K and reaches a value of
0.861, 0.900, 1.240 cm3 K mol-1 at 2.0 K for complexes 1-3
respectively. Several factors such as I) magnetic anisotropy
associated with the complex II) intermolecular antiferromagnetic
interaction III) dipolar interactions are likely to contribute to the
sharp decrease in χMT value at low temperature.
Field dependent magnetization measurements were
performed at various temperatures (2-10 K) for complexes 1-3
up to 70 kOe. The magnetic moment gradually increases upon
increasing the external magnetic field and the magnetic moment
tend to saturate around 1.95, 2.01, and 2.04 NµB at 2.0 K for
extracted while incorporating negative
D for 1 results in
unreliable SH parameters (data not shown) while positive D for 2
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
and 3 respectively did yield poor fit for M(H) data for these
complexes (see Table S4 and Figure S3 of ESI). This evidently
suggest that simultaneous fit of magnetic data (χMT(T) and
M(H)) facilitate to reliably extract the sign and magnitude of D
value for all the complexes (Table 3). Such an approach is
fruitful for unambiguous determination of sign of D using dc
magnetic data for majority of the complexes reported in the
literature.[13a, 16]
Table 3. Spin Hamiltonian parameters extracted from CASSCF
/ NEVPT2[a] calculations and PHI[b] fitting[15] for the complexes
1-3.
[a]
[b]
[a]
[b]
Dcal
Dfit
|E/D|
Cal
0.25 0.11 2.16,2.30,2.41
gxx, gyy, gzz
gxx, gyy, gzz
(cm-1)
(cm-1)
fit
17.4
±14.9
-18.3
+10.8
-18.7
-19.3
2.2, 2.2, 2.4
2.21 (giso
2.3 (giso
1.
2.
3.
0.32
0.25
-
-
2.18,2.29,2.42
2.19,2.29,2.46
)
)
The observation of positive D-value for 1 is quite different from
our earlier prediction[13a] and we also notice that there is a drastic
change in magnitude of magnetic anisotropy in all the
complexes compared to the series of [CoS4]2- complexes
reported by us recently.[12c] The notable changes observed in the
Spin Hamiltonian parameters of complexes 1-3 are rationalized
by detailed computational calculations (vide infra).
Electron paramagnetic resonance (EPR) measurements of
complexes 1-3.
To support the parameters extracted from the magnetic
data fit, we have recorded X-band EPR spectra of complexes 1 -
3 at 5 K both in both solid (100% and diluted sample) and frozen
solutions. Complex 1 was EPR silent above 100 K both in solid
and frozen solution spectra recorded, while complexes 2 and 3
show a broad EPR signals at 100 K (see Figures S4-S6). This is
probably due to the intricate electronic structure associated with
59Co(II) ion possessing an hyperfine of I=7/2 and its associated
fast relaxation phenomenon. Further, we would like to point out
that accurate determination of zero field splitting (ZFS) is
extremely difficult using low frequency EPR spectroscopy as its
microwave quanta is considerably smaller than the ZFS
observed in complexes 1-3. At 5.0 K, EPR spectral features of
100% polycrystalline sample of 1 are distinctly different from
Figure 4. X-band EPR spectra of frozen ethanol-toluene solution of complexes
1-3 recorded at 5 K. Condition: microwave power = 10 dB (1), 18 dB(2), 20
dB(3) (20 mW (1), 3.17 mW (2), 2mW (3)), modulation amplitude = 0.4 mT (1),
1.45 mT (2), 0.4 mT, microwave frequency = 9.37 GHz, Temp = 5K,
In contrast to complexes 2 and 3, both frozen solution and
2% diluted sample of 1 shows well resolved peaks indicative of
the signal arising from ±1/2 ground Kramers state (ΔMs = ±1)
(see Figure S4). Variable temperature EPR spectra recorded on
frozen solution spectra evidently shows that the intensity of the
all the signal decreases upon increasing the temperature. Since
the entire EPR spectral features of complex 1 are not feasible
with low frequency EPR instrument, we have not attempted to
do EPR simulation for 1, which might lead to estimation of
unreliable spin Hamiltonian parameters.
complexes
2 and 3 suggest that the electronic structure
Although EPR experiments performed does not facilitate to
associated with these complexes must be different, while
complexes 2 and 3 show similar EPR spectral features (see
Figure 4). It is very well witnessed in the literature that a high
spin Co(II) complex stabilized with easy axis anisotropy is
expected to be EPR silent at 5.0 K under strictly axial condition
quantify the magnitude of
D accurately, it gives strong
experimental evidence for the sign of magnetic anisotropy
extracted from the magnetic data fit of complexes 1-3, ie.
complex 1 stabilized with easy plane anisotropy while 2 and 3
possess easy axes anisotropy. This is further strongly
corroborated by ab initio calculations (vide infra).
17]
due to the forbidden ΔMs = ±3 intra Kramers transition.[11h,
However, for 2 and 3, broad EPR transitions arising from ground
Kramers doublet observed around 3500 G and 9000 G is due to
non-zero rhombicity (E/D = 0.32 (for 2) and 0.25 (for 3) )
associated with these complexes, which mixes the pure wave
functions of two Kramers doublet. Similar EPR spectral features
(broad EPR signals around 3500 G and 9000 G) were observed
in frozen solution and magnetically diluted samples of 2 and 3
(see Figures S5-S6). Such scenario have been witnessed in
many high spin Co(II) complexes stabilized with easy axis
Alternating current (ac) magnetic susceptibility data of
complexes 1-3.
To probe the magnetization relaxation dynamics of all the three
complexes, ac susceptibility measurements were performed on
polycrystalline sample of 1-3 with 3.5 Oe ac oscillating field with
and without the external bias field between 1.8 and 8.0 K. For
complex 1, no frequency dependent out-of-phase susceptibility
signals (χM”) were observed under a zero applied field. This is
not surprising for a complex (1) that possesses easy plane
[11h, 17]
anisotropy.
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
magnetic anisotropy. However, ac data were collected in the
presence of 2 kOe dc bias field, we do observe χM” signals
indicative of field induced slow relaxation of magnetization
shown in Figure 5.
Figure 5. Frequency dependent out-of-phase (χM”) susceptibility signal
observed for 100% sample of complex 1 in the presence of external magnetic
field of 2 KOe at the indicated frequency.
Similar behavior has been observed for the majority of
tetrahedral complexes stabilized with positive anisotropy in the
literature.[18] Ruiz and co-workers elegantly explain the rationale
for the slow relaxation of magnetization in a molecule with
positive D value.[19] Although complex 1 shows χM” signals, the
maxima are observed well below the instrument limit which
hampers to extract the barrier for the magnetization relaxation.
To our surprise, although complexes 2 and 3 possess negative
anisotropy, there are no frequency dependent out-of-phase
susceptibility signals observed in ac measurement, both in the
presence and absence of an external magnetic field. The
absence of χM” even in the presence of dc bias field suggests
that quantum tunneling of magnetization is extremely fast over
the thermal relaxation mechanism (Orbach Process). Further,
the magnetization relaxation can be triggered by the nuclear
hyperfine interaction of Co(II) and the coordinated halide ions, in
addition to the supramolecular interaction mediated through the
hydrogen bonding.
As pointed out earlier, in all the complexes (1-3) both intra
and intermolecular hydrogen bonding spread across all direction.
Often such supramolecular interaction leads to faster
magnetization relaxation or the observed slow relaxation
phenomenon could be due to magnetic ordering. In order to
understand the role of dipolar interaction and to identify the slow
relaxation magnetization behavior is single molecular origin or
not, we have performed ac relaxation dynamics studies on the
magnetically diluted sample of all the complexes (see
experimental section for details). In all the diluted samples, we
do observe frequency dependent out-of-phase susceptibility
signals in the presence of optimum external magnetic field
suggests that the slow relaxation of magnetization behavior in
the complexes 1-3 is of single-molecule origin rather magnetic
ordering phenomenon.
Figure 6. Ac measurement for 10% diluted sample of complex 1. A)
Frequency dependent dependent out-of-phase susceptibility signals at the
indicated optimum dc magnetic field B) Cole-Cole plot at the indicated
temperatures C) Arrhenius plot of complex 1. The solid red line represents the
fit of the data.
From figure 6A (also see Figure S8A) it is evident that there are
two kinds of relaxation i.e. majority of the fraction undergo faster
relaxation and a non-negligible fraction follows a slow relaxation
process. The fast relaxation (major relaxation) and slow
relaxation phenomenon observed in 1 is very well witnessed in
Cole-Cole plot of the complex, particularly at lower temperature
(see Figure 6C). The Cole-Cole plot of 1 was fitted considering
two relaxation processes using generalized Debye model
(equation 2) given below.
To gain more insight into the magnetization relaxation
phenomenon, the ac data of all the complexes were analyzed in
details. The frequency dependent out-of-phase susceptibility
data of diluted complex 1 is shown in Figure 6 carried out at an
optimum field of 5.5 kOe (see Figure S7).
ꢀꢁ
ꢀꢁ
ꢀ
ꢀ
ꢀꢀꢁ ꢀ = ꢀꢀ,ꢀꢁꢀ
+
+
… (2)
)
ꢀ
(ꢀ!ꢀ
)
(ꢀ!ꢀ
ꢀ
ꢀ! (ꢀꢁꢀ )
ꢀ! (ꢀꢁꢀ )
ꢀ
ꢀ
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
The α1 values ranges from 0.21 to 0.06 and α2 ranges from 0.07
The α values are ranging between 0.09 and 0.15 in the
to 0.6 between 1.8 K – 2.4 K temperature ranges (see Table S5). temperature range of 3.6 K to 1.8 K. This suggests the presence
The relaxation time extracted (τ1 and τ2) from Cole-Cole fitting is
employed to construct the Arrhenius plot (Figure 6C). The linear
fit of this data considering only the Orbach process results in the
effective energy barrier of 13.5 K (τ0 = 1.37 x 10-7 s) and 8.15 K
(τ0 = 2.2 x 10-4 s).
of narrow distribution of relaxation times. Using the τ values
obtained from the Cole-Cole fit, we have constructed the
Arrhenius plot (Figure 7C). Below 3.5 K, apart from Orbach
process, other relaxation processes (such as direct, QTM and
Raman processes) appears to be operative. The data was fitted
according to equation 4, which takes into account of various
relaxation processes.
On the contrary to 1 (diluted sample), complex 2 (10%
diluted sample) indeed shows
a well resolved frequency
dependent χM” signals in the presence of optimum external
magnetic field of 2 kOe (Figure 7 and Figure S7). The presence
one single major relaxation is firmly corroborated by the Cole-
Cole plot of 2 (Figure 7B and Figure S8B). The Cole-Cole plot
was fitted by considering a single relaxation process using the
generalized Debye equation (equation 3) and the extracted
parameters are shown in Table S6.
+ ꢀꢀ!ꢀ + ꢀꢀ! + exp(!!
)
(4)
!
!
!
!""
=
!
!
!
!
!
!"#
!
!
Where ꢀ ꢀꢀꢁꢂ represents relaxation via QTM, ꢀꢀꢀꢀ
represents direct process, ꢀꢀꢀ is for Raman process and the
last terms describes the relaxation via Orbach process.
! ! !
!
!
It is not necessary to use all the relaxation process to fit the
Arrhenius plot of 2. By considering only Orbach (28.6 K (τ0 =
1.66 x 10-7 s)), QTM (0.0088 s) and Raman (C = 0.065 s-1K-3 and
n = 6), excellent fit to the experimental data obtained.
ꢀ!" ꢀ = ꢀ! +
(3)
(!!!)
!! (!"#)
Figure 8. Ac measurement for 10% diluted sample of complex 3. A)
Frequency dependent dependent out-of-phase susceptibility signals at the
indicated optimum dc magnetic field B) Cole-Cole plot at the indicated
temperatures C) Arrhenius plot of complex 3. The solid red line represents the
fit of the data.
Figure 7. Ac measurement for 10% diluted sample of complex 2. A)
Frequency dependent out-of-phase susceptibility signals at the indicated
optimum dc magnetic field B) Cole-Cole plot at the indicated temperatures C)
Arrhenius plot of complex 2. The solid red line represents the fit of the data.
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
In line with the magnetization relaxation behavior of 2, complex
3 (10% diluted sample) also exhibit a similar relaxation behavior
(Figure 8), however, slightly higher dc-bias field (Hdc = 5.5 kOe,
Figure S7) is required to observe the frequency dependent out-
of-phase susceptibility signals in 3 compared to 2 (Hdc = 2 kOe).
The τ value extracted from Cole-Cole fit data employed to
construct the Arrhenius plot. The experimental data were fitted
considering multiple relaxations (equation 4). Reasonably good
fit were obtained by taking into account of only Orbach (9 K, τ0 =
4.06 x 10-4 s), Raman (C = 0.034 s-1K-3 and n = 6 value) and
QTM (0.007 s) processes (see Figure 8C).
better results. The calculated absorption spectrum for all
complexes 1-3 along with experimental observations are
provided in the ESI (see Figure S9). Despite the high-level
theory employed to reproduce the UV-vis spectra, apparent
deviations from experiments are still noted. This is attributed to
the fact that the calculations performed includes only selected
electrons in the reference space, while this is generally sufficient
for magnetic anisotropy, very large reference space including
that of ligands are required to reproduce the exact positions of
the absorption spectra. This has been witnessed earlier in
20]
several mononuclear complexes.[12a,
Both experimental and
computed d-d transitions show a red shift as we move from –Cl
to –I in complexes 1-3 and this is essentially due to the fact that,
iodide ligand offers smaller crystal field splitting and small inter-
electronic repulsion which results into low-lying excited states.
For all the complexes 1-3, the ground state wavefunction heavily
mixed with other excited states. For complex 1, the ground state
To fully understand the magnetization relaxation dynamics
observed in all the three complexes, trend found in the
estimated magnetic anisotropy and to unequivocally establish
the role of halide ions in modulating the D and E values, ab initio
calculations were performed on complexes 1-3.
2
2
2
2
has 48% of (dx
)
(dz )2 (dxy)1 (dxz)1 (dyz)1 composition while
-y
2
2
26% of (dx 2)1 (dz )2 (dxy)2 (dxz)1 (dyz)1 composition.
-y
Computational studies of complexes 1-3.
CASSCF and NEVPT2 calculations have been performed on
complexes 1-3 to understand the reason behind the origin of
positive (for 1) and negative (for 2 and 3) zero field splitting (zfs)
parameters
in
these
complexes
(
see computational details). We have also attempted to shed light
on the role of structural distortions on the magnetic anisotropy of
these complexes. The computed Spin-Hamiltonian (SH)
parameters (D, E and g values) for all the three complexes are
listed in Table 3. The simulation of experimental magnetic
susceptibility data using the computed SH parameters is in good
agreement (see Figure 3), which reflects the reliability of the
computed parameters. It is evident from table 3 that for all the
three complexes, ab initio calculated |E/D| values are quite large.
Particularly for complex 2, the |E/D| value is found to be
significantly large and is close to the rhombic limit (|E/D|~0.3),
therefore the sign of D in complex 2 cannot be predicted
unambiguously by calculation. Although single crystal X-ray
diffraction studies reveal that all the three complexes possess
distorted Td geometry, in reality they have only C2v symmetry.
Thus, lowering of symmetry allows rigorous mixing between the
ground and the excited states, leading to a large zfs in these
Figure 9. UV-Vis absorption spectroscopy for complexes 1-3 were performed
in ethylacetate solution in the range of 450-900 nm. Colored sticks represent
the computed absorption bands for 4A2→4T1(P) transitions with CAS(13,8)
level of theory.
The mixing is even more rigorous for –Br and –I analogues (see
ESI for details). Not only the single-excitations, but also the
states arising from the double excitations are found to be
strongly mixed with the ground state wavefunction. This
highlights the need for a genuine multi-reference method to
compute the spectroscopic properties of these systems.[21] To
understand the origin of magnetic anisotropy in these class of
complexes, we have first analyzed the state-by-state
contributions and then correlated these transitions with the
orbital ordering. The state-by-state contribution to the D value for
all the complexes 1-3 is detailed in table 4. From table 4, it is
evident that the largest contribution to the D value arises from
the 4T2(F) excited state (assuming Td environment), while other
quartet states marginally contribute to D value. Among these
three transitions, two transitions offer positive contribution to the
D value, while third transition always contributes to the negative
D value.
4
complexes. The tetrahedral Co(II) ligand field terms are T2(F),
4T1(F) and 4T1(P) which further splits in to [(4A2+4B1+4B2),
(4A1+4B1+4B2) and (4A2+4B1+4B2) states respectively in C2v
symmetry, thus total nine spin-allowed transitions are possible.
The computed spin allowed d-d transitions are listed in Tables
S8-S11 (also see Figure S9). The d-d transitions belong to the
single excitation 4A2→4T2(F) and 4A2→4T1(F) are found below
<10000 cm-1 (see table 4 for details) and this explains why the
UV-VIS absorption spectrum recorded for all the three
complexes did not capture these transitions. On the other hand,
the transition which arises from the double excitations 4A2→
4T1(P) are captured in the UV-vis absorption spectrum. Three
distinct peaks are observed for all the three complexes (features
of low-symmetry) in the range of 525 to 825 nm (see Figure 9)
The d-d transitions computed at CAS (7,5 and 13,8) level of
theory are overestimated compared to experiment and this is
likely due to the lack of dynamic correlation. However, inclusion
of the dynamic correlations using NEVPT2 method yield slightly
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
Table 4. CASSCF (7,5)+NEVPT2 computed Spin-Hamiltonian parameter (g, D, |E/D|) along with listed state-by-state
contribution to the D values.
States
1
2
3
CASSCF
NEVPT2
CASSCF
NEVPT2
CASSCF
NEVPT2
Contribution to D (cm-1)
Contribution to D (cm-1)
Contribution to D (cm-1)
4T2(F)
26.46
18.99
10.94
-13.48
-2.949
27.46
19.03
11.05
-16.67
-2.760
12.22
7.74
8.09
16.14
16.64
- 25.04
-2.747
12.75
-59.81
-2.289
-40.01
-2.277
-19.52
-2.909
2G
5.322
1.089
5.084
1.054
5.409
5.454
4.829
4.853
-1.375
-1.391
-1.848
-1.813
21.69
17.38
18.84
14.89
-24.68
-18.31
Dtol
|E/D|
0.26
0.25
0.33
0.31
0.23
0.25
2.2172
2.1596
2.3001
2.4101
2.2663
2.4211
2.5774
2.1878
2.2983
2.4154
2.2856
2.4238
2.6644
2.1941
2.2936
2.4675
gxx
gyy
gzz
2.4125
2.5539
This is due to the different nature of transition dipole moments
transitions within t2-subshell. Inclusion of the dynamic
correlations slightly decreases the magnitude of the D value;
however, the sign of the D value remains unchanged. This is
due to the fact that the dynamic correlation strongly stabilizes
the ground state compared to the excited quartet state, and this
led to the increase in the energy gap between the ground and
excited quartet state, which eventually decreases the magnitude
of the D value. On the other hand, doublet states are strongly
affected by the dynamic correlation, however, transitions of
similar magnitude and opposite sign cancels out each other
leading to a negligible contribution to the total D value. The
orientation of the D-tensor for complexes 1-3 are provided in the
figure 12.
(vide infra). Apart from the quartet 4T2 state, we have also
2
noticed some contribution from the low-lying G state; however,
it has little effect on the overall zfs parameter. Here, we have
provided the splitting pattern of the low-lying excited states for
the complex 3 (see figure 10). The observed splitting pattern are
in line with the previous studies on the low-symmetric Co(II)
complexes. Here, we have analyzed CAS (7,5) orbitals to
rationalize these different contributions to the D value. The
energy ordering of the CAS (7,5) orbitals are in provided in
figure 11, which is accordance with ligand field paradigm of the
C2v symmetry. The three different excitations corresponding to
the 4T2 can be assigned as excitation from the (dx2-y2)2→t2
subshell ((dxy)1 (dxz)1 (dyz)1). The negative contribution to the D
value is due to spin-conserved excitation between (dx2-
The negative contribution to the D value increases as we move
down from –Cl to –I, and this might be due to the fact that large
spin-orbit coupling constant associated with the heavier halide,
brings the excited states closer to the ground state, and hence
2)2→(dxy)1 orbitals as both orbital belong to same |ml| level. The
y
other two excitations (dx -y2)2→(dyz)1/(dxz)1 leading to a positive
2
zfs contribution (see table 4 for details).[19]
the increase in the magnitude of the D value (see table 4 for
[22]
details).
To cross-check, whether it is an effect of heavier
halide or local structural distortion (∠S-Co-S angle 96°(1) and
100°(3)), we have performed additional calculations on a model
complex of 3 where –I ligand is substituted by –Cl, without
altering the Co-Cl bond distance (named as 3a). Calculations on
model complex 3a yields a D value of +17.4 cm-1 and |E/D| value
of 0.19, very similar to the complex 1 (see Figure S10 and
Figure S11). This clearly highlights that the zfs of Co(II) ion in
complexes 1-3 are influenced strongly by the presence of
heavier ligand like iodide in the first coordination sphere than the
local structural distortions.
This is in line with previous observations on pseudo-tetrahedral
Co(II) complexes, where the sign of the D values is found to be
sensitive to the nature of metal-ligand interaction (negative D for
soft ligands).[12-14] The rhombic zero-field splitting E arises due
to difference between the DXX and DYY components; the larger
the difference, large is the E value. The difference in the DXX and
DYY components can be rooted back to the different strength of
the single-electron excitation from the e-subshell to dxz and dyz
orbitals of t2-subshell. We have observed a significant splitting
Figure 10. The splitting pattern of the of few low-lying quartet and doublet
states (4T2(F), 4T1(F), 4T1(P) and 2G) for complex 3.
The small contributions from the low-lying doublet 2T2 (2G) states
arises due to the intra singly occupied molecular orbital
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
between the dxz and dyz orbitals, this eventually led to large
difference between the DXX and DYY components; hence the
large |E/D| values. The large splitting between the dxz and dyz
orbitals are due to the presence of two different donor atoms
possessing varying σ/π-donor abilities. The dxz orbital is found to
interact with the sulphur, while the dyz orbital interacts with -X
ion leading to a large splitting between these two orbitals in all
three complexes. On the other hand, the presence of four
sulphur atoms would results in a nearly degenerate dxz/dyz
orbitals, thus a lower |E/D| values as observed earlier by our
group on [CoS4]2+ SMMs.[12c]
with the minimal active space of CAS(7,5). Interestingly, the
increment in the D value is found to be largest for the complex 3
and smallest for the complex 1. To correlate these changes in
the D values with the metal-ligand covalency, we have analyzed
the eigenvalue plots of complex 1-3. If we look at the figure 13,
we notice two important features, (i) splitting of the d-manifold
falls in the narrow region as we move towards the heavier
halides, (ii) the σ bonding orbitals are much closer in energy to
the metal-based d-orbitals for complex 3 followed by complex 2
and 1 (Figure 13).
The first feature is essentially due to the weak ligand field
offered by the heavier halides, which results into a smaller
crystal field splitting. The second feature highlights the increase
in the metal-ligand covalency, where gap between the metal and
ligand based orbitals decreases as we move towards the
heavier halide. The decrease in the gap between these orbitals
led to strong overlap which is directly proportional to the degree
of covalency (see the coefficients in figure 13) [23]. Thus, the
near degeneracy between the ligand and metal-based orbitals
explains the important contribution of the ligand to metal charge
transfer excitations in the ground state wavefunction. Not only
the D value, incorporation of such non-dynamic correlations by
extending the active space shows pronounced effect on the
absorption spectrum (see Table S11). Furthermore,
incorporation of double shell which increases the radial electron
correlation may further improvise the obtained spectroscopic
properties.[24] Finally, our method suggests that the extended
active space calculations offer a clear picture of metal-ligand
covalency and its effect on the magnetic anisotropy. Our
calculations demonstrate that, presence of large spin-orbit
coupling associated with heavier halide is not only the sole
factor for increase in the zfs of transition metal complexes, as
metal-ligand covalency drastically affects the nature of
[25]
excitations and thus the zfs.
Figure 11. A) CASSCF computed energies of metal based d-orbitals of
complex 3. B) Colour Code. Color Code Co (green); S (light yellow); I
(turquoise); N (blue); C(grey) and H (white).
Magneto-Structural D-correlation
To further understand the influence of structural parameters on
the sign and magnitude of D value in Co(II) tetrahedral
complexes, we have developed a magneto-structural correlation
on complex 3. Here, we have systematically varied the ∠S-Co-S
and ∠I-Co-I bond angles to generate the tetragonally
compressed and elongated structures.[24] In complex 3, the ∠S-
Co-S and ∠I-Co-I bond angles are found to be 100° and 107°
To understand the impact of metal-ligand covalency on the zfs
parameter, it is of prime importance to extend the active space
by incorporating some ligand orbitals which strongly mix with
metal based orbitals, i.e. ligand orbitals with sizable tails on the
metal center.
For the tetrahedral Co(II) complexes, here, we have considered
the three σ bonding orbitals (dxz+L, dyz+L and dxy+L (where L =
ligand orbitals)) in the active space to describe the metal-ligand
covalency. Incorporation of these three σ bonding orbitals offers
a way to analyze the effect of ligand to metal charge transfer on
magnetic anisotropy. One can also incorporate the π-orbitals in
active space, however, σ-bonds offers a better picture of metal-
ligand covalency as the σ-overlaps are more pronounced that π-
overlaps. With an extended active space of CAS(13,8), we have
computed all the ten quartets and forty doublets in the
configuration interaction module for all the three complexes. The
computed D (|E/D|) values are found to be +19.2 (0.27) for 1,
±16.9(0.33) for 2 and -22.4 (0.21) for complex 3 The computed
|E/D| values are in line with previous calculations with minimal
active space of CAS(7,5) i.e. seven Co(II) based electrons in five
Co(II) based orbitals. However, the computed D values with
CAS(13,8) are marginally (~2-4 cm-1) higher than that computed
respectively, which suggests that complex
3 possess an
elongated tetrahedral geometry. Here, we have defined a
parameter called δ (δ = 2Td-(α+β)) (where Td represent the angle
of 109.5°, while α represents the ∠S-Co-S bond angle while β
represents the ∠I-Co-I bond angle) and developed a correlation
based on the variation in the δ value. The negative value of δ
represents the flattening of tetrahedral geometry while positive
value of δ represents tetragonal elongation geometry (see figure
14 for details). It is evident from the figure 14, that with the
increase in the δ value, the D value increases linearly and for
the largest δ value of 50°, the D value raised as high as -86 cm-1.
Thus, we observed the fourfold increase in the D value
compared to the parent complex 3 (δ = 11.4).
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
Figure 12. NEVPT2 calculated orientation of the main magnetic axes (D-tensor) for complexes A) 1, B) 2 and C) 3. Color Code Co (green); S (yellow); Cl (light
green);Br (brown); I (turquoise); N (blue); C (grey) and H (white).
correlations, decrease in the δ value slowly decreases the D
value at δ value of +3.45 (close to tetrahedron), however, we
have not noticed a change in the sign of the D value.
Figure 13. CASSCF computed orbital energies for complex 1-3. The thick
black lines represent the σ-bonding orbital while the red lines represent the d-
orbital splitting
Besides, the |E/D| value also decreases as the δ increases.
Here, we have plotted the variation of the D and |E/D| values
with change δ value along with variation of the first three excited
4
state E1, E2 and E3 (which belong to the T2(F) in Td symmetry,)
and ground state with δ value. The gradual increase in the D
value with the increase in δ value is due to the lowering of the
first excited state (E1) close to the ground state. However, the
other two excited states E2 and E3 show an antagonizing
behavior, moving away from the ground state, as the δ value
increases. The splitting pattern of the first three excited state for
the structures corresponding to the large δ values show a typical
splitting pattern associated with D2d symmetry (where 4A2
4
ground state transforms into B2 state and the first excited state
4T2) splits into 4B1 and 4E state).
The close proximity of the 4B1 state to the 4B2 ground state is the
Figure 14. A) Variation of the D and |E/D| values with change in the δ value
along with the corresponding parameters observed for complexes 1-3; The D-
value observed for complexes 1-3 is mapped (open circle symbol) in the
key behind the giant zfs associated with Co(II) complexes in D2d
point group.[12, 26] On the other hand, the non-degenerate nature
correlation developed based on the δ-value in respective complexes. B)
Energy variation of the ground and first three excited states originating from 4
T
of second and third excited states (4E for D2d symmetry) is due
to the asymmetry in the bond angle and difference in the donor
strength of ligands. In the other half of the magneto-structural
with respect to the change in the δ value. The graphs constructed here, are
based on the values computed at NEVPT2 level of theory. The SH parameters
of the complexes 1-3 is mapped in left panel.
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
The sign of D cannot be determined unambiguously for this
structure as the |E/D| reaches as high as 0.3 at this δ value
(+3.45). The decrease in the magnitude of the D value can be
directly correlated with energy of the first excited state, which
becomes higher in the energy for complexes with small δ values.
sample of all the complexes indicates the presence of magnetic
anisotropy associated with the complexes. The parameters
extracted from magnetic data fitting (simultaneous fitting of
χMT(T) and M(H)) of all the complexes are in excellent
agreement with the experimental magnetic data, suggest the
Unlike the scenario observed in tetragonal elongation correlation, reliability of the parameters extracted. The magnetic data fitting
the D-value does not change abruptly in tetragonal compressed
Co(II) geometry. The developed correlations highlight the
importance of tetragonal compression and elongation, which
directly correlates to the sign, and magnitude of D-value of
{CoL2X2} type of complexes.
evidently shows that complex 1 stabilizes easy plane magnetic
anisotropy while 2 and 3 found to possess easy axis magnetic
anisotropy. This is qualitatively supported by EPR
measurements on these complexes. Alternating current
magnetic susceptibly measurements performed on all
complexes, but none of the complexes show frequency
dependent out-of-phase susceptibility (χM”) in the absence of
Finally, to shed the light on SMM characteristic of these
complexes, we have first analyzed the wavefunction of the
ground state KD for all the complexes. In all the three complexes, external magnetic field. On the other hand, ac measurements on
the ground state KD are not well isolated, they found to be
strongly mixed with other excited state KD. For complex 3, the
ground state KD (represented as |S,±Ms〉) is comprised of 80%
|3/2,±3/2〉 and 18% |3/2,±1/2〉 components, which is in principle
restricted for a Kramers ion. As for the Kramers ion, these two
states must be orthogonal in zero field, and therefore QTM
effects are expected. However, the large E term allows the
mixing between the ground and excited KDs. This mixing of the
state and the apparent splitting of these states by, the large
hyperfine spin of the Co(II) triggers the QTM, even in zero field.
This could be one of the possible reasons behind the absence of
zero field SMM characteristic in the complex 3. This has been
witnessed earlier in other mononuclear Co(II) complexes (see
Table S12).[19] Interestingly, for complex 1, we have observed
the out-of-phase signal in the presence of applied magnetic
field.[18, 27] To understand this scenario, we have first analyzed
decomposition of ground state wavefunction and for complex 1,
the ground state KD is comprised of 70% |3/2,±1/2〉 and 22%
|3/2,±3/2〉 Such strong mixing between the ground and excited
KD occurs due to the presence of large E/D term (E/D ~ 0.24).
In principle, for Ms =1/2, there no preferred easy axis for
magnetization in presence of pure axial symmetry. However,
presence of large E term ((Dxx-Dyy)/2), creates a preferred easy
axis of orientation within the –xy plane. The relaxation enabled
10% diluted sample of 1-3 apparently show χM” signals in the
presence of optimum external magnetic field suggests that the
slow relaxation phenomenon is originates from single molecule,
which also signifies the influence of dipolar interaction on
magnetization relaxation dynamics. The nature of the spin-
Hamiltonian parameters extracted and observed magnetization
relaxation behavior was rationalized by the electronic structure
calculations. Calculations suggests that, the large negative D
value for complex 3, as compared to 1 and 2 is due to the larger
metal-ligand covalency of Co-I bond compared to Co-Cl and Co-
Br bonds in 1 and 2 respectively. The increase in the metal-
ligand covalency has positive impact on the stabilizing easy
axes of anisotropy in Co(II) complexes. Also, we rationalize for
the absence of slow magnetic relaxation behavior in 1 - 3 is due
to the lack of pure ground state in all complexes, further the
large |E/D| value triggers the QTM effectively rather thermally
assisted Orbach process.[2l, 28] The lack of isolated ground state
and the large |E/D| is correlated and routed back to the structural
distortion present in all the complexes. In addition, the hyperfine
interaction is likely to have non-negligible contribution to the
QTM behaviour observed in all the complexes. Overall, the
present study reveals that not only, the soft donors such as
sulfur modulate the sign and magnitude of D (which is the case
for some of the recent reports) but also other ligands such as
via –x(-y) to +x(+y) direction depending on the sign of the E term. halides holds the key to alter the magnitude and sign of D-value
However, for the Ms =1/2 as the ground state, the QTM is strong
leading to faster relaxation. On the other hand, if one applies the
static d.c field, the QTM gets suppressed and a slow relaxation
can be observed In literature, there are several examples, where
Co(II) complexes show field induced SMM characteristics, even
with positive zfs.[19] In such cases, the barrier height can be
assigned as 2|E| (Dxx -Dyy) rather than 2|D|. The computed
energy barrier for complex 1 is found to be (~2|E| = 6.09 K),
which is good agreement with Ueff of complex 1 (Ueff = 13.5 and
of complexes. Further, the study reveals that heavier ligated
atoms with large spin-orbit coupling enhances the metal ligand
covalency which tend to stabilize easy axis magnetic anisotropy
in tetrahedral Co(II) complexes. To generalize further, if a
tetrahedral Co(II) ion surrounded by similar π/σ strength soft
donor ligand ought to stabilize easy axis anisotropy with small
⏐E/D⏐ ratio is which is an useful finding, particularly for synthetic
chemist involved in revealing new generation of single ion
magnets.
8.15
K for major and minor relaxation respectively). The
presence of field induced SIM characteristic in complex 1, is due
to the presence of positive zfs with remarkably large |E/D| value.
Interestingly, the large |E/D| terms offers a key for molecules to
show slow relaxation of magnetization with positive D values.
Experimental Section
All the chemicals were purchased from commercially available sources
(Alfa Aesar and Sigma-Aldrich). All the reactions were performed under
aerobic conditions. A Perkin-Elmer FT-IR spectrometer (in 400 to 4000
cm-1 range) was used to collect the infrared spectra for the polycrystalline
samples using KBr pellets. Magnetic susceptibility measurements were
performed using a MPMS SQUID magnetometer equipped with 7 Tesla
magnet in 300–2.0 K. The single crystal X-ray data were collected in
Rigaku-Saturn CCD diffractometer. Details of data collection and
structure solution methods were reported elsewhere.[12c] Elemental
analysis was carried out on a Thermo Finnigan model. The powder XRD
data was collected on a Panalytical MRD System. The absorption profile
Conclusions
To conclude, we have isolated a series of tetrahedral Co(II)
complexes with the general molecular formula [CoL2X2] ( X = Cl
(1) or Br (2) or I (3)) which are structurally characterized by the
single crystal X-ray diffraction. Direct current magnetic
susceptibility measurements performed on the polycrystalline
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
for all the complexes were collected on
a
Jasco V-530 UV/Vis
The bulk samples phase purity (complexes 1-3) was confirmed by
powder x-ray diffraction. The experimental data is in well agreement with
simulated data (see Figure S12).
spectrophotometer. Electron paramagnetic resonance spectra were
recorded on a broker EMX-plus X-band (9.37 GHz).
Computational Details
Complex [Zn(L)2Cl2] (1-Zn)
All the quantum-chemical calculations were performed on the X-ray
Similar synthetic procedure was followed as for 1, ZnCl2 (0.895g, 6.57
mmol) was used in place of CoCl2.6H2O. Yield 0.9g (42.4%). Calc: C,
8.33%; H, 2.79%; N, 19.42%; S, 22.23% Found: C, 8.5%; H, 2.6%; N,
19.3%; S, 22.4%. IR (KBr):- 3312 and 3386 cm-1 (ν-NH2) and 1591 cm-1
(ν-C=S).
29]
structures using ORCA[21b,
suite of code. To compute the nature of
low-lying excited states and zero-field splitting parameters, we have
opted the multi-reference ab initio calculations. State-average complete
active space self-consistent field (CASSCF) calculations along with N-
electron valence perturbation theory (NEVPT2) were performed on
complexes 1-3. NEVPT2 calculations have been performed on top of the
converged CASSCF wavefunction to recover the dynamic correlation.[30]
Scalar relativistic effects are treated using second-order Douglas-Kroll-
Hess (DKH) method.[31] All these procedures were carried out with all
electron segmented basis set def2-TZVP for all the atoms. The resolution
of identity (RI) approximation has been used with the corresponding
auxiliary basis sets in order to speed up the calculations.[32] Two set of
calculations have been performed, where we have chosen two different
active spaces. The first active space is the minimal active space which
comprised of seven active d-electron in the five Co(II) based d-orbitals
(CAS(7,5)). Here, we have computed all the 10 quartet and 40 doublet
states. To understand the effect of the ligands, especially the nature of
metal-ligand covalency, we have extended an active space by
incorporating three σ bonding orbitals in the active space. Hence, the
new active space comprises CAS(13,8); 6 electrons of the 3 σ bonding
orbitals along with seven d-electrons in the five active Co(II) based d-
orbitals. The 10 quartet and 40 doublets are computed with CAS(13,8)
active space. Apart from computing the Spin-Hamiltonian parameters, we
have also done survey for the fitting of magnetization and susceptibility
data to check the quality of fit.
Complex [Zn(L)2Br2] (2-Zn)
Similar synthetic procedure was followed as for 2, ZnBr2 (1.48g, 6.57
mmol) was used in place of CoBr2. Yield 0.59g (34.7%). Calc: C, 6.36%;
H, 2.14%; N, 14.7%; S, 17.2% Found: C, 6.3%; H, 2.22%; N, 14.7%; S,
17.2%. IR (KBr):- 3342 and 3401 cm-1 (ν-NH2) and 1618 cm-1 (ν-C=S).
Complex [Zn(L)2I2] (3-Zn)
Similar synthetic procedure was followed as for 3, ZnI2 (2.1g, 6.57 mmol)
was used in place of CoI2. Yield 0.62g (41%). Calc: C, 5.10%; H, 1.71%;
N, 11.88%; S, 13.6% Found: C, 5.17%; H, 1.65%; N, 11.96%; S, 13.42%.
IR (KBr):- 3285 and 3362 cm-1 (ν-NH2) and 1584 cm-1 (ν-C=S).
The unit cell was checked for the single crystals obtained for 1-Zn, 2-Zn
and 3-Zn and the unit cell of all the complexes is in excellent agreement
with unit cell of the corresponding parent Co(II) complexes. This implies
that 1-Zn, 2-Zn and 3-Zn possess similar packing diagram of 1, 2 and 3
respectively. To further give concrete evidence, we have recorded
powder x-ray diffraction pattern for 1-Zn, 2-Zn and 3-Zn which is in
excellent agreement with the simulated data derived from their parent
Co(II) single crystal data. (see Figure S13).
Synthetic procedure for the isolation of complexes 1-3
Complex [Co(L)2Cl2] (1)
Preparation of 10% diluted sample of 1.
To warm (35-40°C) ethanol, solid CoCl2.6H2O (1.55g, 6.5 mmol) was
added. Into this solution L (1g, 13.2 mmol) was added and the reaction
mixture was heated under reflux for 12 hours. The ethanol solvent was
removed under reduced pressure (roto vap) after cooling the reaction
mixture. The complex of interest was extracted from acetonitrile. Needle
shaped blue colour single crystals were grown from filtrate by diffusion of
diethylether into acetonitrile solution after one week at room temperature.
Yield: 1.2 g (32.4%) Elemental analysis: Calc: C, 8.5%; H, 2.8%; N,
19.8%; S, 22.7% Found: C, 8.42%; H, 2.3%; N, 19.6%; S, 22.1%. IR
(KBr): 3329 and 3391 cm-1 (ν-NH2) and 1620 cm-1 (ν-C=S).
To warm (35-40°C) ethanol, solid CoCl2.6H2O (0.156g, 0.657 mmol) and
ZnCl2 (0.807g, 5.92 mmol) was added. Into this solution L (1g, 13.2
mmol) was added and the reaction mixture was heated under reflux for
12 hours. The ethanol solvent was removed under reduced pressure
(roto vap) after cooling the reaction mixture. The complex of interest was
extracted from acetonitrile. Needle shaped light blue single crystals were
grown from filtrate by diffusion of diethylether into acetonitrile solution
after one week at room temperature.
Preparation of 10% diluted sample of 2.
Complex [Co(L)2Br2] (2)
To warm (35-40°C) ethanol, solid CoBr2 (0.144g, 0.657 mmol) and ZnBr2
(0.666 g, 5.92 mmol) was added. Into this solution L (1g, 13.2 mmol) was
added and the reaction mixture was heated under reflux for 12 hours.
Further, the product of interest was extracted from ethylacetate rather
than acetonitrile. Light blue single crystals were obtained by slow
evaporation of ethylacetate solution.
A similar synthetic procedure was followed as in 1, CoBr2 (1.43g, 0.0065
mol) was used in place of CoCl2.6H2O. Further, the product of interest
was extracted from ethylacetate rather than acetonitrile. Unlike in 1,
single crystals were obtained by slow evaporation of ethylacetate solution.
X-ray quality blue colour single crystals were grown from filtrate within a
week at room temperature. Yield: 1.7g (35.4%) Calc: C, 6.5%; H, 2.17%;
N, 15.1%; S, 17.3% Found: C, 6.38%; H, 2.5%; N, 14.84%; S, 17.5%. IR
(KBr):- 3313 and 3391 cm-1 (ν-NH2) and 1634 cm-1 (ν-C=S).
Preparation of 10% diluted sample of 3.
To warm (35-40°C) ethanol, solid CoI2 (0.206g, 0.657 mmol) and ZnI2
(1.88g, 5.92 mmol) was added. Into this solution L (1g, 13.2 mmol) was
added and the reaction mixture was heated under reflux for 12 hours.
Further, the product of interest was extracted from ethylacetate rather
than acetonitrile. Light blue single crystals were obtained by slow
evaporation of ethylacetate solution.
Complex [Co(L)2I2] (3)
A similar synthetic procedure was followed as in 2, CoI2 (2.05g, 0.0065
mol) was used in place of CoBr2. Yield 0.7g (11.4%). Calc: C, 5.2%; H,
1.7%; N, 12.1%; S, 13.8% Found: C, 5.16%; H, 2.1%; N, 11.75%; S,
13.1%. IR (KBr):- 3300 and 3417 cm-1 (ν-NH2) and 1606 cm-1 (ν-C=S).
CCDC numbers: 1504900-1504902.
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
Korobkov, M. Murugesu, D. S. Richeson, J. Am. Chem. Soc. 2011, 133,
15814-15817; c) D. Schweinfurth, M. G. Sommer, M. Atanasov, S. Demeshko,
S. Hohloch, F. Meyer, F. Neese, B. Sarkar, J. Am. Chem. Soc. 2015, 137,
1993-2005; d) R. Ruamps, L. J. Batchelor, R. Guillot, G. Zakhia, A.-L. Barra,
W. Wernsdorfer, N. Guihery, T. Mallah, Chem. Sci. 2014, 5, 3418-3424; e) T. J.
Woods, M. F. Ballesteros-Rivas, S. Gomez-Coca, E. Ruiz, K. R. Dunbar, J.
Am. Chem. Soc. 2016, 138, 16407-16416.
Acknowledgements
M.S. thanks the funding agencies DST, DST Nanomission, INSA,
and IIT Bombay for financial support. S.V. and P.S. thanks CSIR
for financial support. K.A. thanks UGC for financial support.
[11] a) F. Yang, Q. Zhou, Y. Zhang, G. Zeng, G. Li, Z. Shi, B. Wang, S. Feng,
Chem. Commun. 2013, 49, 5289-5291; b) M. Idesicova, J. Titis, J. Krzystek, R.
Boca, Inorg. Chem. 2013, 52, 9409-9417; c) L. Smolko, J. Cernak, M. Dusek,
J. Titis, R. Boca, New J. Chem. 2016, 40, 6593-6598; d) L. Smolko, J. Cernak,
J. Kuchar, R. Boca, Monatsh. Chem. 2015, 146, 243-248; e) F. Habib, I.
Korobkov, M. Murugesu, Dalton Trans. 2015, 44, 6368-6373; f) E. Colacio, J.
Ruiz, E. Ruiz, E. Cremades, J. Krzystek, S. Carretta, J. Cano, T. Guidi, W.
Wernsdorfer, E. K. Brechin, Angew. Chem., Int. Ed. 2013, 52, 9130-9134; g)
Y.-Y. Zhu, C. Cui, Y.-Q. Zhang, J.-H. Jia, X. Guo, C. Gao, K. Qian, S.-D. Jiang,
B.-W. Wang, Z.-M. Wang, S. Gao, Chem. Sci. 2013, 4, 1802-1806; h) S.
Sottini, G. Poneti, S. Ciattini, N. Levesanos, E. Ferentinos, J. Krzystek, L.
Sorace, P. Kyritsis, Inorg. Chem. 2016, 55, 9537-9548; i) S. Ziegenbalg, D.
Hornig, H. Goerls, W. Plass, Inorg. Chem. 2016, 55, 4047-4058.
[12] a) E. A. Suturina, D. Maganas, E. Bill, M. Atanasov, F. Neese, Inorg.
Chem. 2015, 54, 9948-9961; b) D. Maganas, S. Sottini, P. Kyritsis, E. J. J.
Groenen, F. Neese, Inorg. Chem. 2011, 50, 8741-8754; c) S. Vaidya, S.
Tewary, S. K. Singh, S. K. Langley, K. S. Murray, Y. Lan, W. Wernsdorfer, G.
Rajaraman, M. Shanmugam, Inorg. Chem. 2016, 55, 9564-9578.
[13] a) S. Vaidya, A. Upadhyay, S. K. Singh, T. Gupta, S. Tewary, S. K.
Langley, J. P. S. Walsh, K. S. Murray, G. Rajaraman, M. Shanmugam, Chem.
Commun. 2015, 51, 3739-3742; b) J. M. Zadrozny, J. Telser, J. R. Long,
Polyhedron 2013, 64, 209-217.
[14] J. M. Zadrozny, J. R. Long, J. Am. Chem. Soc. 2011, 133, 20732-20734.
[15] N. F. Chilton, R. P. Anderson, L. D. Turner, A. Soncini, K. S. Murray, J.
Comput. Chem. 2013, 34, 1164-1175.
[16] a) M. Dey, S. Dutta, B. Sarma, R. C. Deka, N. Gogoi, Chem. Commun.
2016, 52, 753-756; b) A. Eichhoefer, Y. Lan, V. Mereacre, T. Bodenstein, F.
Weigend, Inorg. Chem. 2014, 53, 1962-1974; c) D. V. Korchagin, G. V. Shilov,
S. M. Aldoshin, R. B. Morgunov, A. D. Talantsev, E. A. Yureva, Polyhedron
2015, 102, 147-151.
[17] a) V. V. Novikov, A. A. Pavlov, A. S. Belov, A. V. Vologzhanina, A.
Savitsky, Y. Z. Voloshin, J. Phys. Chem. Lett. 2014, 5, 3799-3803; b) J.
Pilbrow, Transition Ion Electron Paramagnetic Resonance,Clarendon Press
Clarendon Press: Oxford, U.K. 1990.
[18] a) J. M. Zadrozny, J. Liu, N. A. Piro, C. J. Chang, S. Hill, J. R. Long,
Chem. Commun. 2012, 48, 3927-3929; b) S. Gomez-Coca, E. Cremades, N.
Aliaga-Alcalde, E. Ruiz, J. Am. Chem. Soc. 2013, 135, 7010-7018; c) W.
Huang, T. Liu, D. Wu, J. Cheng, Z. W. Ouyang, C. Duan, Dalton Trans. 2013,
42, 15326-15331.
Keywords: Single ion magnet (SIM) • Cobalt(II) • ab initio
calculation • magneto structural correlation
References
[1] a) R. Sessoli, D. Gatteschi, A. Caneschi, M. A. Novak, 1993, 365, 141-143;
b) A. Caneschi, D. Gatteschi, R. Sessoli, A. L. Barra, L. C. Brunel, M. Guillot, J.
Am. Chem. Soc. 1991, 113, 5873-5874.
[2] a) A. Nava, L. Rigamonti, E. Zangrando, R. Sessoli, W. Wernsdorfer, A.
Cornia, Angew. Chem., Int. Ed. 2015, 54, 8777-8782; b) M. Ako Ayuk, V.
Mereacre, Y. Lan, W. Wernsdorfer, R. Clerac, E. Anson Christopher, K. Powell
Annie, Inorg Chem 2010, 49, 1-3; c) R. Pattacini, P. Teo, J. Zhang, Y. Lan, K.
Powell Annie, J. Nehrkorn, O. Waldmann, T. S. A. Hor, P. Braunstein, Dalton
Trans 2011, 40, 10526-10534; d) M. Ibrahim, Y. Lan, S. Bassil Bassem, Y.
Xiang, A. Suchopar, K. Powell Annie, U. Kortz, Angew Chem Int Ed Engl 2011,
50, 4708-4711; e) Y.-Z. Zhang, A. J. Brown, Y.-S. Meng, H.-L. Sun, S. Gao,
Dalton Trans. 2015, 44, 2865-2870; f) Y.-Y. Zhu, C. Cui, K. Qian, J. Yin, B.-W.
Wang, Z.-M. Wang, S. Gao, Dalton Trans. 2014, 43, 11897-11907; g) C.-L.
Zhou, Z.-M. Wang, B.-W. Wang, S. Gao, Polyhedron 2011, 30, 3279-3283;
h)Y.-Y. Zhu, X. Guo, C. Cui, B.-W. Wang, Z.-M. Wang, S. Gao, Chem.
Commun. 2011, 47, 8049-8051; i)A. Perivolaris, A. M. Fidelli, R. Inglis, V. G.
Kessler, A. M. Z. Slawin, E. K. Brechin, G. S. Papaefstathiou, J. Coord. Chem.
2014, 67, 3972-3986; j) S. M. Taylor, J. M. Frost, R. McLellan, R. D. McIntosh,
E. K. Brechin, S. J. Dalgarno, Cryst. Eng. Comm. 2014, 16, 8098-8101; k) J.
Martinez-Lillo, N. Dolan, E. K. Brechin, Dalton Trans. 2013, 42, 12824-12827;
l) Y.-Z. Zhang, S. Gomez-Coca, A. J. Brown, M. R. Saber, X. Zhang, K. R.
Dunbar, Chem. Sci. 2016, 7, 6519-6527; m) S. K. Langley, R. A. Stott, N. F.
Chilton, B. Moubaraki, K. S. Murray, Chem. Commun. 2011, 47, 6281-6283; n)
S.-S. Bao, L.-M. Zheng, Coord. Chem. Rev. 2016, 319, 63-85; o) T. K. Prasad,
G. Poneti, L. Sorace, M. J. Rodriguez-Douton, A.-L. Barra, P. Neugebauer, L.
Costantino, R. Sessoli, A. Cornia, Dalton Trans. 2012, 41, 8368-8378; p) M.
Murrie, Chem. Soc. Rev. 2010, 39, 1986-1995; q) Y.-Y. Zhu, Y.-Q. Zhang, T.-T.
Yin, C. Gao, B.-W. Wang, S. Gao, Inorg. Chem. 2015, 54, 5475-5486.
[3] A. M. Ako, I. J. Hewitt, V. Mereacre, R. Clerac, W. Wernsdorfer, C. E.
Anson, A. K. Powell, Angew. Chem., Int. Ed. 2006, 45, 4926-4929.
[19] S. Gomez-Coca, A. Urtizberea, E. Cremades, P. J. Alonso, A. Camon, E.
Ruiz, F. Luis, Nat. Commun. 2014, 5, 4300.
[4] C. J. Milios, R. Inglis, A. Vinslava, R. Bagai, W. Wernsdorfer, S. Parsons, S.
P. Perlepes, G. Christou, E. K. Brechin, J. Am. Chem. Soc. 2007, 129, 12505-
12511.
[20] E. A. Suturina, J. Nehrkorn, J. M. Zadrozny, J. Liu, M. Atanasov, T.
Weyhermueller, D. Maganas, S. Hill, A. Schnegg, E. Bill, J. R. Long, F. Neese,
Inorg. Chem. 2017, 56, 3102-3118.
[5] a) M. N. Leuenberger, D. Loss, Nature 2001, 410, 789-793; b) J. Lehmann,
A. Gaita-Arino, E. Coronado, D. Loss, Nat. Nanotechnol. 2007, 2, 312-317; c)
L. Bogani, W. Wernsdorfer, Nat. Mater. 2008, 7, 179-186; d) R. Sessoli, M.-E.
Boulon, A. Caneschi, M. Mannini, L. Poggini, F. Wilhelm, A. Rogalev, Nat.
Phys. 2015, 11, 69-74.
[21] a) D. Ganyushin, F. Neese, J. Chem. Phys. 2006, 125, 024103/024101-
024103/024111; b) F. Neese, T. Petrenko, D. Ganyushin, G. Olbrich, Coord.
Chem. Rev. 2007, 251, 288-327.
[22] S. Ye, F. Neese, J. Chem. Theory Comput. 2012, 8, 2344-2351.
[23] T. Cundari, Ed. Computational Organometallic Chemistry; Marcel Dekker,
Inc., New York 2001.
[6] F. Neese, D. A. Pantazis, Faraday Discuss. 2011, 148, 229-238.
[24] R. Maurice, R. Bastardis, C. de Graaf, N. Suaud, T. Mallah, N. Guihery, J.
Chem. Theory Comput. 2009, 5, 2977-2984.
[7] a) D. E. Freedman, W. H. Harman, T. D. Harris, G. J. Long, C. J. Chang, J.
R. Long, J. Am. Chem. Soc. 2010, 132, 1224-1225; b) R. Ruamps, R. Maurice,
L. Batchelor, M. Boggio-Pasqua, R. Guillot, A. L. Barra, J. Liu, E.-E. Bendeif, S.
Pillet, S. Hill, T. Mallah, N. Guihery, J. Am. Chem. Soc. 2013, 135, 3017-3026;
c) S. Gomez-Coca, D. Aravena, R. Morales, E. Ruiz, Coord. Chem. Rev. 2015,
289-290, 379-392; d) A. K. Bar, C. Pichon, J.-P. Sutter, Coord. Chem. Rev.
2016, 308, 346-380; e) G. A. Craig, M. Murrie, Chem. Soc. Rev. 2015, 44,
2135-2147; f) J. M. Zadrozny, M. Atanasov, A. M. Bryan, C.-Y. Lin, B. D.
Rekken, P. P. Power, F. Neese, J. R. Long, Chem. Sci. 2013, 4, 125-138; g) R.
C. Poulten, M. J. Page, A. G. Algarra, J. J. Le Roy, I. Lopez, E. Carter, A.
Llobet, S. A. Macgregor, M. F. Mahon, D. M. Murphy, M. Murugesu, M. K.
Whittlesey, J. Am. Chem. Soc. 2013, 135, 13640-13643; h) P. P. Samuel, K. C.
Mondal, N. Amin Sk, H. W. Roesky, E. Carl, R. Neufeld, D. Stalke, S.
Demeshko, F. Meyer, L. Ungur, L. F. Chibotaru, J. Christian, V.
Ramachandran, J. van Tol, N. S. Dalal, J. Am. Chem. Soc. 2014, 136, 11964-
11971; i) C. G. Werncke, E. Suturina, P. C. Bunting, L. Vendier, J. R. Long, M.
Atanasov, F. Neese, S. Sabo-Etienne, S. Bontemps, Chem. - Eur. J. 2016, 22,
1668-1674; j) Y.-S. Meng, Z. Mo, B.-W. Wang, Y.-Q. Zhang, L. Deng, S. Gao,
Chem. Sci. 2015, 6, 7156-7162.
[25] S. K. Singh, J. Eng, M. Atanasov, F. Neese, Coord. Chem. Rev. 2017,
10.1016/j.ccr.2017.1003.1018.
[26] Y. Rechkemmer, F. D. Breitgoff, M. van der Meer, M. Atanasov, M. Hakl,
M. Orlita, P. Neugebauer, F. Neese, B. Sarkar, J. van Slageren, Nat. Commun.
2016, 7, 10467.
[27] J. Vallejo, I. Castro, R. Ruiz-Garcia, J. Cano, M. Julve, F. Lloret, G. De
Munno, W. Wernsdorfer, E. Pardo, J. Am. Chem. Soc. 2012, 134, 15704-
15707.
[28] a) V. V. Novikov, A. A. Pavlov, Y. V. Nelyubina, M.-E. Boulon, O. A.
Varzatskii, Y. Z. Voloshin, R. E. P. Winpenny, J. Am. Chem. Soc. 2015, 137,
9792-9795; b) L. Smolko, J. Cernak, M. Dusek, J. Miklovic, J. Titis, R. Boca,
Dalton Trans 2015, 44, 17565-17571.
[29] F. Neese, Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 73-78.
[30] a) C. Angeli, R. Cimiraglia, S. Evangelisti, T. Leininger, J. P. Malrieu, J.
Chem. Phys. 2001, 114, 10252-10264; b) C. Angeli, R. Cimiraglia, J.-P.
Malrieu, Chem. Phys. Lett. 2001, 350, 297-305.
[31] B. A. Hess, Phys. Rev. A Gen. Phys. 1986, 33, 3742-3748.
[8] a) J. M. Zadrozny, D. J. Xiao, M. Atanasov, G. J. Long, F. Grandjean, F.
Neese, J. R. Long, Nat. Chem. 2013, 5, 577-581; b) X.-N. Yao, J.-Z. Du, Y.-Q.
Zhang, X.-B. Leng, M.-W. Yang, S.-D. Jiang, Z.-X. Wang, Z.-W. Ouyang, L.
Deng, B.-W. Wang, S. Gao, J. Am. Chem. Soc. 2017, 139, 373-380.
[9] a) C. Rajnak, J. Titis, I. Salitros, R. Boca, O. Fuhr, M. Ruben, Polyhedron
2013, 65, 122-128; b) D. Wu, X. Zhang, P. Huang, W. Huang, M. Ruan, Z. W.
Ouyang, Inorg. Chem. 2013, 52, 10976-10982.
[32] F. Neese, J. Comput. Chem. 2003, 24, 1740-1747.
[10] a) F. Habib, O. R. Luca, V. Vieru, M. Shiddiq, I. Korobkov, S. I. Gorelsky,
M. K. Takase, L. F. Chibotaru, S. Hill, R. H. Crabtree, M. Murugesu, Angew.
Chem., Int. Ed. 2013, 52, 11290-11293; b) T. Jurca, A. Farghal, P.-H. Lin, I.
This article is protected by copyright. All rights reserved.

10.1002/chem.201606031
Chemistry - A European Journal
FULL PAPER
FULL PAPER
Shefali Vaidya,Saurabh Kumar Singh,
Pragya Shukla, Kamaluddin Ansari,
Gopalan Rajaraman* and Maheswaran
Shanmugam*
Page No. – Page No.
Role of Halide Ions on the Nature of
Magnetic Anisotropy in Tetrahedral
Co(II) Complexes
We report a family of tetrahedral Co(II) single-ion magnets with general molecular
formula of [Co(L1)2X2] where L1 = thiourea and X = Cl (1), Br (2) and I (3). Effect of
halide ion in modulating the sign as well as magnitude of D in these complexes
been studied in detail and the observed experimental results firmly supported by
theoretical calculations.
This article is protected by copyright. All rights reserved.