Quantitative Estimation of Ising-Type Magnetic Anisotropy in a Family of C3-Symmetric CoII Complexes

Article abstract of DOI:10.1002/chem.201702108

In this paper, the influence of the structural and chemical effects on the Ising-type magnetic anisotropy of pentacoordinate Co^II complexes has been investigated by using a combined experimental and theoretical approach. For this, a deliberate design and synthesis of four pentacoordinate Co^II complexes [Co(tpa)Cl]?ClO₄ (1), [Co(tpa)Br]?ClO₄ (2), [Co(tbta)Cl]?(ClO₄)?(MeCN)₂?(H₂O) (3) and [Co(tbta)Br]?ClO₄ (4) by using the tripodal ligands tris(2-methylpyridyl)amine (tpa) and tris[(1-benzyl-1 H-1,2,3-triazole-4-yl)methyl]amine) (tbta) have been carried out. Detailed dc and ac measurements show the existence of field-induced slow magnetic relaxation behavior of Co^II centers with Ising-type magnetic anisotropy. A quantitative estimation of the zero-field splitting (ZFS) parameters has been effectively achieved by using detailed ab initio theory calculations. Computational studies reveal that the wavefunction of all the studied complexes has a very strong multiconfigurational character that stabilizes the largest m_s=±3/2 components of the quartet state and hence produce a large negative contribution to the ZFS parameters. The difference in the magnitudes of the Ising-type anisotropy can be explained through ligand field theory considerations, that is, D is larger and negative in the case of weak equatorial σ-donating and strong apical π-donating ligands. To elucidate the role of intermolecular interactions in the magnetic relaxation behavior between adjacent Co^II centers, a diamagnetic isostructural Zn^II analog (5) was synthesized and the magnetic dilution experiment was performed.

Full text of DOI:10.1002/chem.201702108

A Journal of
Accepted Article
Title: Quantitative Estimation of Ising-type Magnetic Anisotropy in a
Family of C3 Symmetric CoII Complexes
Authors: Sanjit Konar, Amit Kumar Mondal, Jesús Jover, and Eliseo
Ruiz
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Quantitative Estimation of Ising-type Magnetic Anisotropy in a
Family of C3 Symmetric CoII Complexes
Amit Kumar Mondal,a Jesús Jover,b Eliseo Ruiz*b and Sanjit Konar*a
Abstract: In this paper, the influence of the structural and chemical effects on the Ising-type magnetic anisotropy of penta-coordinate CoII
complexes has been investigated using a combined experimental and theoretical approach. For this, a deliberate design and synthesis of four
penta-coordinate CoII complexes [Co(tpa)Cl]·ClO4 (1), [Co(tpa)Br]·ClO4 (2), [Co(tbta)Cl]·(ClO4)·(MeCN)2·(H2O) (3) and [Co(tbta)Br]·ClO4 (4)
using the tripodal ligands tris(2-methylpyridyl)amine (tpa) and tris[(1-benzyl-1H-1,2,3-triazole-4-yl)methyl]amine) (tbta) have been carried out.
Detailed dc and ac measurements show the existence of field induced slow magnetic relaxation behaviour of CoII centres with Ising-type
magnetic anisotropy. A quantitative estimation of ZFS parameters has been effectively achieved using detailed ab initio theory calculations.
Computational studies reveal that the wavefunction of all the studied complexes has a very strong multiconfigurational character that
stabilizes the largest Ms = ±3/2 components of the quartet state and hence produce a large negative contribution to ZFS parameters. The
difference in the magnitudes of the Ising-type anisotropy can be explained via ligand field theory considerations, i.e. D is larger and negative
in the case of weak equatorial σ-donating and strong apical π-donating ligands. In order to elucidate the role of intermolecular interactions in
the magnetic relaxation behavior between adjacent CoII centres, a diamagnetic isostructural ZnII analogue (5) was synthesized and the
magnetic dilution experiment was performed.
Introduction
where the single ion anisotropy is enhanced because of the
partially unquenched orbital angular momentum.[6] Among
transition metals, CoII based complexes are mostly attractive
candidates for designing SIMs, since they have a non-integer
spin ground state,[7] which decreases the possibility of quantum
tunnelling of magnetization (QTM).[8]
Single-molecule magnets (SMMs), a kind of molecular nano-
magnets that show slow relaxation of the magnetization and
magnetic hysteresis below the blocking temperature (TB), have
attracted a lot of research interest over the last two decades.[1]
The magnetism of SMMs originates from an energy barrier (U)
that precludes reversal of the magnetization at low temperatures,
that is a result of the combined effect from the ground-state spin
and magnetic anisotropy of the molecule [U = |D| S2 and U = |D|
(S2 – 1/4) for integer and non-integer S, respectively].[2] The first
generation of SMMs were ferro-magnetically coupled transition
metal clusters which have a large spin ground state along with a
large Ising-type anisotropy.[1b,d] Nevertheless, the control of the
anisotropy in cluster systems was very difficult, and the increase
in total spin of these systems led to a decrease in the total
anisotropy in most of the cases. In the last decade, extensive
efforts have been dedicated towards the synthesis of a new
class of mononuclear SMMs that are commonly referred to as
single-ion magnets (SIMs), where the SMM-like behavior comes
from only one paramagnetic center. While most of the reported
SIMs are based on the lanthanide complexes,[3] the search for
new SIMs has been extended to transition-metal based
complexes.[4-5] Low-coordinate metal centres are commonly
observed among the reported transition-metal based SIMs,
Mononuclear penta-coordinate CoII SMMs have been reported to
typically display trigonal bipyramidal (TBY) or square pyramidal
(SPY)[4k-m] coordination geometries and many attempts have
aimed to rationally control the single ion anisotropy in recent
years.[4l] However, the elements regulating single ion anisotropy
are not well comprehended and intricate control over magnetic
anisotropy remains still a challenging task. In the last few years
significant contributions have been made in order to understand
the nature of the magnetic anisotropy in trigonal bipyramidal CoII
complexes;[5i-k,6g-h] although many things are still to be
understood. For transition metal complexes, an axial symmetry
imposed by the ligands is usually an essential requirement to
obtain large magnetic anisotropies. Herein, we report the
synthesis of four different CoII complexes bearing the tripodal
ligands tpa and tbta (tpa = tris(2-methylpyridyl)amine and tbta =
tris[(1-benzyl-1H-1,2,3-triazole-4-yl)methyl]amine) that were
anticipated to impose C3v axial symmetry in the resulting
complexes. The dynamic magnetization behaviour of penta-
coordinate CoII complexes [Co(tpa)Cl]·ClO4 (1), [Co(tpa)Br]·ClO4
(2), [Co(tbta)Cl]·(ClO4)·(MeCN)2·(H2O) (3) and [Co(tbta)Br]·ClO4
(4) have been studied and the influence of structural and
chemical effects on the magnitude of the magnetic anisotropy
have been investigated using a combined experimental and
theoretical approach. The detailed ab initio theory calculations
disclose that the wavefunction of complexes 1-4 has a very
strong multiconfigurational character that stabilizes the largest
Ms = ±3/2 components of the quartet state and hence produce a
large negative contribution to D parameters.
[a] A. K. Mondal, Prof. Dr. S. Konar
Department of Chemistry, Indian Institute of Science Education
and Research Bhopal, Bhopal Bypass road, Bhauri, Bhopal
462066, MP (India). Tel: (+91) 755-6691313. Fax: (+91) 755-
6692392.
E-mail: skonar@iiserb.ac.in
[b] Dr. J. Jover, Prof. Dr. E. Ruiz
Departament de Química Inorgànica I Orgànica, Secció de
Química Inorgànica and Institut de Recerca de Química Teòrica i
Computacional, Universitat de Barcelona, Diagonal 645, E-08028
Barcelona, Spain.
E-mail: eliseo.ruiz@qi.ub.es
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Results and Discussion
distorted trigonal bipyramidal, with minimum CShM values of
1.880 and 2.298 for 1 and 2 respectively (Table S4). In both the
complexes, significant π-interaction and intermolecular
hydrogen-bonding network are present, which supports the
formation of a supramolecular arrangement (Figures S2-S3 and
Table S6-S7). In complex 1, hydrogen atoms of pyridyl rings are
involved in intermolecular hydrogen bonding (Table S4) with
chloride atoms and perchlorate molecules and resulted in the
formation of a supramolecular two dimensional arrangement
(Figure S2). In addition to the H-bonding interactions, strong
CH···π interactions are also noticed with CH to centroid
distances of 3.598(3) Å and 3.346(5) Å for 1 and 2, respectively.
Single-crystal X-ray analysis show that complexes 3 and 4
crystallize in monoclinic P21/n and cubic Pa-3 space groups
respectively (Table S1). The CoII center is coordinated by the
three nitrogen atoms of triazole rings of tbta ligand. The bond
distance between the cobalt center and the central amine
nitrogen of tbta is 2.344(1) Å, which is longer than the terminal
ones (2.037(2) - 2.049(1) Å). In complex 3, the Co–Cl distance is
equal to 2.260(1) Å whereas in 4 the Co–Br distance is equal to
2.402(1) Å (Table S3). The CoII center lies 0.531 Å above the
plane containing the three triazole nitrogen-donor atoms in
complex 3 (0.514 Å in complex 4). Both the complexes are
involved in intermolecular hydrogen bonding (Table S8-S9) with
the lattice solvent molecules and perchlorate anions and these
interactions support the formation of supramolecular two
dimensional arrangement (Figures S4-S5).
Structural description of complexes 1 - 4:
Single-crystal X-ray analysis of 1 and 2 reveals that the complex
1 crystallizes in the monoclinic P21/c space group and complex 2
crystallizes in the triclinic P-1 space group (Table S1). The
molecular structures of the complexes are shown in Figure 1.
Figure 1. View of the molecular structures for complexes 1 and 2 (left), where
X = Cl (1) and X = Br (2). The bond angles (α, β, γ and δ) around CoII centres
have been emphasized in this figure (left); View of the molecular structures for
complexes 3 and 4, where X = Cl (3) and X = Br (4) (right); hydrogen atoms
are omitted for clarity.
The tripodal ligand coordinates in a tetradentate fashion and the
geometry at the CoII centre is distorted trigonal bipyramidal. The
ligands (tpa) are dispersed at the apices of a trigonal bipyramid
with crystallographic C3 symmetry; the three equatorial sites are
employed by the terminal amines and the axial sites are
occupied by the central amine and the chloride ion. In complex 1,
the largest angles within the four atoms N6, N7, N8 and Cl2 are
β = 178.64(2)° for N7–Co3–Cl2, and α = 118.67(1)° for N8–
Co3–N6 (Table S2). Thus, τ is (178.64–118.67)/60 = 0.999.[9]
This indicates that the geometry around CoII is highly distorted
trigonal bipyramidal. Additionally, a modified index parameter, χ,
has been proposed to describe the trigonality of five-coordinate
complexes, where χ = (β + γ + δ - 2α)/180 (γ and δ represent the
remaining bond angles around the metal ion without the donor
atoms defining β). The χ value for 1 is 0.937, which suggests
that the geometry around CoII is better described as a distorted
trigonal bipyramidal (Figure S1) geometry. The CoII ion lies
0.449 Å below the equatorial plane of the three nitrogen atoms
in complex 1 (0.455 Å in complex 2). The Co-N axial bond length
(2.201(1) Å) is slightly longer than the equatorial ones (2.056(2)
- 2.069(1) Å). In complex 1, the Co–Cl distance is equal to
2.283(1) Å whereas in 2 the Co–Br distance is equal to 2.431(1)
Å. Additional SHAPE analysis confirms that the five-coordinate
CoII centres adopt geometries which can be better described as
Some important structural features for complexes 1-4 have been
summarized in table S5. For comparison, the Co-N distances to
the pyridine rings of tpa ligands in complex 1 and 2 are slightly
longer than the Co-N(triazole) distances observed in the case of 3
and 4. The main difference, however, is the Co-N(amine) distance,
which is 2.344(1) Å for complex 3, whereas for 1 it is 2.201(1) Å.
Therefore, the central amine nitrogen binds much more strongly
to the CoII center in 1 as compared to 3 and hence the ligand tpa
is coordinated to the CoII center in a more compact manner.[10]
Magnetic Property Studies:
The purity of the as-synthesized products is demonstrated by
the good agreement of the bulk phase powder X-ray diffraction
patterns with the simulated ones (Figure S6-S7). Magnetic
susceptibility measurements have been carried out under direct
current (DC) and an applied field of 0.1 T. At room temperature,
χΜΤ values (χΜ = molar magnetic susceptibility) of 2.29 (1), 2.17
(2), 2.53 (3) and 2.47 (4) cm3 K mol−1 have been obtained, which
are larger than the spin-only value of 1.875 cm3 mol−1K for a
high-spin CoII ion. These values fall well in the range of 2.1−3.4
cm3 mol−1 K for the highly anisotropic CoII ions with a significant
(d)
(b)
(c)
(a)
Figure 2. χMT vs. T plots measured at 0.1 T for complex 2 (a) and 4 (b). 1/χM vs. T plots shown in the inset; M/NµB vs. H plots for complexes 2 (c) and 4 (d) at the
indicated temperatures. The red lines correspond to those obatined with the best fit.
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(a)
(b)
(c)
(d)
Figure 3. Frequency dependency of the in-phase (χM′) (a) and out-of-phase (χM″) (b) AC magnetic susceptibility plots for complex 2; Frequency dependency of the
in-phase (χM′) (c) and out-of-phase (χM″) (d) AC magnetic susceptibility plots for complex 4 at 1000 Oe.
orbital contribution.[11] Upon cooling from room temperature, the
χΜΤ values remain constant down to 70 K, below which they
collapse, reaching values of 1.48 (1), 1.39 (2), 1.65 (3) and 1.40
corresponding to the ±Ms levels owing to parity effects;[8] and
thus, the quantum tunnelling is possibly due to hyperfine and
dipolar arbitrated relaxation processes.[14] The application of a
1000 Oe dc field splits the energy of the ±Ms doublets,
supresses the QTM relaxation pathway and all complexes show
frequency-dependent ac signals typically observed for field-
induced 3d-SIM species (Figure 3 and S10). The Cole-Cole
plots (Figure 4 and S11) have been generated from the
frequency-dependent ac susceptibility data. The fit of the χM" vs
χM′ data at each temperature using the generalized Debye
model[15] produces the corresponding α values. This parameter
determines the width of the distribution of relaxation times, so
that α = 1 corresponds to an infinitely wide distribution of
relaxation times. In contrast, α = 0 indicates a relaxation with a
single time constant. Values within the ranges 0.02-0.21 (1),
0.06-0.24 (2), 0.07-0.26 (3) and 0.04-0.30 (4), have been
obtained for our complexes, suggesting in all cases a narrow
distribution of the relaxation time. The effective energy barrier
(Ueff) and relaxation times (τ0) have been determined using the
Arrhenius eqn (2):[16]
1
(4) cm3 mol− K at 2 K (Figure 2 and S8). The decline of
χΜΤ curves at low temperature can be attributed to intrinsic
magnetic anisotropy of the CoII ions. Magnetization data (M/NµB
vs. H) have been collected and reach the highest values of 2.67
(1), 2.71 (2), 2.61 (3) and 2.52 (4) NµB at 2 K and 7 T (Figure 2
and S8). These values are well below the theoretical saturation
for an S = 3/2 system (Msat = 3.3 with g = 2.2). The
magnetization values do not saturate even at the highest
available fields and the M/NµB vs. H/T plots display that all
isotherm magnetization plots do not fall on the same master
curve signifying the highly magnetic anisotropic systems (Figure
S9). The spin Hamiltonian shown eqn (1) is used to qualitatively
define the magnetic anisotropy
2
H = gµBS·B + D[Sz2 − S(S + 1)/3] + E(Sx2 – Sy )
(1)
where µB, S and B represent the Bohr magneton, spin (S = 3/2
for 1−4), and magnetic field vectors, respectively; D and E terms
represent the single-ion axial and rhombic ZFS parameters. The
PHI code[12] has been employed to quantify the anisotropy
parameters of the CoII centres by concomitant fitting of the χΜΤ
vs. Τ and M/NµB vs. H plots; during the fitting procedure the g
tensor has been assumed to be isotropic. The best fits of the
reduced magnetization data gave D = -10.1(2) cm−1, |E| = 1.8(4)
cm−1, and g = 2.28 for 1; D = -7.8(7) cm−1, |E| = 2.1(3) cm−1, and
g = 2.23 for 2; D = -7.5(4) cm−1, |E| = 0.4(6) cm−1, and g = 2.24
for 3; D = -4.3(8) cm−1, |E| = 0.03(2) cm−1, and g = 2.21 for 4.
Similar quality fits have been obtained starting from both positive
and negative E parameters, demonstrating that the fits are not
sensitive to the sign of E parameter. The results of D, E and g
values of 1-4 agree well with the values obtained from EPR
studies of similar trigonal bipyamidal CoII complexes reported by
Gatteschi et al.[13]
The negative zero-field splitting parameters (D) for complexes 1-
4 indicate the possibility of exhibiting SMM behaviour. To probe
the relaxation dynamics of 1-4, AC magnetic susceptibility
measurements were carried out in the temperature range of 1.8-
10 K at a 3.5 Oe ac field. No out-of-phase ac susceptibility (χM")
signal was observed for them under a zero dc field. This can be
ascribed to the presence of quantum tunnelling of the
magnetization (QTM) through the thermal relaxation barrier
between the degenerate ground ±3/2 levels. For a non-integer
spin system with D < 0, transverse anisotropy cannot stimulate
the QTM process through mixing of the wave functions
ln(1/τ) = ln(1/τ0) - Ueff/kT
(2)
where k is the Boltzmann constant and 1/τ0 is the pre-
exponential factor. A linear fit according to Arrhenius equation
produces Ueff = 12 cm−1 and τ0 = 7.2 × 10−6 s for 1; Ueff = 8.7
cm−1 and τ0 = 5.8 × 10−6 s for 2; Ueff = 8.1 cm−1 and τ0 = 3.5 ×
10−6 s for 3; Ueff = 5 cm−1 and τ0 = 2.1 × 10−6 s for 4 (Figure 4
and S11). The τ0 values are at the higher end of the
experimental range found for SMMs[17a,b] and are similar to those
found for other CoII SIMs.[17c] This strongly suggests that the
quantum pathway of relaxation at very low temperatures is not
fully suppressed by the effects of the applied dc field. The
thermally activated relaxation process at the high-temperature
regime specifies that an Orbach pathway[18] through the excited
Ms = ±1/2 level should be followed. In this regime, the system is
excited to the Ms = ±1/2 level by absorption of phonons followed
by an emission of phonons to reach the Ms = ±3/2 ground
state.[19] The energy gap between the Ms = ±1/2 and Ms = ±3/2
levels is given by 2(D2+3E2)1/2. Considering the D values
resulting from the fitting of the magnetization data, the effective
energy barriers (Ueff) are obtained: 20.2 (1), 15.6 (2), 15.0 (3)
and 8.6 (4) cm-1. Normally, these values are larger than the Ueff
values, obtained from ac susceptibility measurements, this is
probably due to the existence of
relaxation pathway which is not fully supressed by the applied dc
field. The single-phonon direct process and the Raman process
a substantial quantum
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(a)
(b)
(d)
(c)
Figure 4. Cole-Cole plots for complex 2 (a) and 4 (b). Solid lines represent the best fit; ln (τ) vs. T-1 plots for complex 2 (c) and 4 (d). The red lines are the best fit
of the Arrhenius equation.
may have also a considerable contribution to the spin relaxation
behaviour.
Ab initio calculations:
The magnetic properties of the low-lying states of complexes 1-4
were analyzed by means of an ab initio multireference
On a comparative point of view, the difference in the magnitude
of D between 1 and 2 can be explained via ligand field theory.
The Co–Cl distance in 1 (2.283(1) Å) is shorter than the Co–Br
distance in 2 (2.431(1) Å), as a consequence Br- is found to
have a weaker π-donating effect than Cl-. That results in a higher
stabilization of the dxy/dx2-y2 pair of orbitals for complex 1, which
at the end produces a smaller energy gap between occupied
and semioccupied orbitals and therefore a higher |D| value. This
behaviour should be also expected for complexes with the tbta
ligand (3 and 4). Furthermore, the average Co–N bond distance
in the equatorial plane is larger for 1 (2.061 Å) than for 3 (2.042
Å). Therefore, the σ-donor effect of the equatorial amine nitrogen
atoms is larger for 3 than for 1 and the dxy/dx2-y2 pair of orbitals is
higher in energy in the former than in the latter. This produces a
larger energy gap between occupied and semioccupied orbitals
for complex 3 (the computed energy gaps are 2796.3 and
3601.8 cm-1 for complexes 1 and 3, respectively) and hence |D|
is likely to be smaller in 3. As a whole, it should be expected that
weak equatorial σ-donating and strong apical π-donating ligands
would produce larger Ising-type anisotropy; providing an
implement to forecast the single-ion magnetic anisotropy in
trigonal bipyramidal CoII complexes. The idea of explaining
Ising-type anisotropy of trigonal bipyramidal CoII complexes was
first suggested by Guihéry and Mallah coworkers.[5i,5k,6h]
methodology;
the
computed
second-order
anisotropy
parameters and excitation energies for all the compounds are
collected in Table 1. These values have been obtained from two
different electronic structure calculations that have been carried
out with the ORCA[20] and MOLCAS 8.0[21-23] software packages.
ORCA produces two sets of results: CASSCF and CASSCF +
NEVPT2 (which is the method of choice to introduce the
dynamic correlation effects), both including spin-orbit
contributions. On the other hand MOLCAS has been only able to
provide CASSCF results, including spin-orbit effects that have
been introduced with the SO-RASSI method. Additional
CASPT2 calculations for all the complexes were also carried out
but they showed serious convergence problems and were thus
finally discarded.
The different computational methods employed produce similar
values for most of the anisotropy parameters of the studied CoII
complexes, although the inclusion of the dynamic correlation
effects with NEVPT2 tends to produce slightly lower values. The
computed D, |E| and giso values are similar to those obtained in
the experimental fit (Table 1). In all cases, a 3/2 ground state is
found for complexes 1-4 before including the spin-orbit effects.
Table
1.
ORCA/CASSCF,
ORCA/CASSCF
+
NEVPT2,
and
In order to investigate the effect of inter- and intramolecular
exchange on the magnetic behavior, we have studied the effect
of magnetic dilution on relaxation of the magnetization. In order
to do this we have synthesized the diamagnetic isostructural ZnII
analogue (5) (see ESI for experimental and X-ray details, Figure
S12 and Table S10). After that, we have prepared the doped
sample in which the CoII complex (3) has been magnetically
diluted with the ZnII analogue in a 5:95 percentage ratio. AC
MOLCAS/CASSCF + RASSI computed D, |E|, and giso values for the ground
state of complexes 1-4. δ and Δ are the computed first excitation energies
before and after including the spin-orbit effects, respectively. The Δ value
corresponds to the energy difference between the ground and the first excited
Kramers’ doublets.
Dexpt
Dcalc
(cm-1)
-9.0
|E|
(cm-1)
2.0
1.6
2.0
1.7
1.3
1.8
0.5
0.4
0.5
0.0
0.0
0.0
δ
Δ
Complex
giso
(cm-1)
(cm-1)
(cm-1)
19.2
13.3
20.2
15.3
10.0
16.1
16.7
12.6
17.7
12.0
8.9
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
3219.7
4458.1
3281.6
3215.2
4471.2
3268.7
3692.7
5006.2
3728.0
3619.6
4904.5
2.27
2.20
2.27
2.27
2.20
2.27
2.26
2.20
2.27
2.27
2.20
2.28
-10.1(2)d
-7.8(7)d
-7.5(4)d
-6.0
susceptibility measurements have been carried out on
a
polycrystalline sample of the diluted complex (Figure S13 (left)).
In this case, the effective energy barrier (Ueff) and relaxation time
(τ0) have been determined using the Arrhenius equation; the
best fit provides Ueff = 7.5 cm−1 and τ0 = 3.2 × 10−6 s (Figure S13
(right)). Therefore, the relaxation energy barriers show no
significant difference between the diluted sample and the
undiluted one. In view of this, we may conclude that
intermolecular forces and dipolar interactions are negligible; and
therefore, the relaxation processes in the case of the studied
mononuclear CoII complexes are of single ion origin.
-9.5
-7.0
-4.4
-7.5
-8.3
-6.3
-8.8
-6.0
-4.3(8)d
-4.4
-6.3
3654.5
12.7
a
b
c
ORCA/CASSCF.
ORCA/CASSCF
+
NEVPT2.
MOLCAS/CASSCF +
RASSI. d Experimental values.
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In these conditions, the lowest-lying spin-orbit free excited states
are found at relatively high energies ca. 3200-5000 cm-1 above
the ground state (δ in Table 1). Once the spin-orbit effects are
included a set of Kramers’ doublets (KDs, Δ) for each complex is
obtained. In all cases the first KD is found at a very low energy,
typically below 20 cm-1, and may actively participate in the spin
relaxation processes (see below). The remaining KDs are found
at much higher values, usually above 3500 cm-1, and are
probably not able to participate in the relaxation mechanism. A
complete list of g and D values, their tensors, the excited states
energies without (δ) and with (Δ) spin-orbit effects for complexes
1-4 can be found in Tables S11-S22. The orientation of the g-
and D-tensors, which is similar for each complex regardless the
calculation method, is shown in Figure S14.
(AILFT)[25,26] approach as implemented in ORCA, have also
been included. As may be observed the electronic configuration
of the quartets shown in Figure 5 indicate that the D value
should be positive because any excitation leading to an excited
quartet state involves the transfer of an electron between
orbitals possessing different |ml| values i.e. from dxz (or dyz) to dxy
(or dx2-y2). This situation has already been reported for other
penta-coordinated trigonal bipyramidal CoII complexes and the
negative D is usually attributed to the highly multiconfigurational
character of the ground and low-lying excited states.[6i,27] Indeed,
this can be confirmed by analyzing the d orbital symmetry in the
point group C3v; in this point group, two pairs of orbitals: dxz/dyz,
and dxy/dx2-y2 belong to the same irreducible representation E,
which enables their mixing and produces ground and excited
states with significant contributions of several determinants
(Table S23). It should be expected that the small negative D
values, found both by the experimental fit and the calculations,
are obtained because the large negative contribution of the first
excited state to the overall D cannot be compensated by the
smaller positive contributions of the higher energy excited states
(Table 2 shows the energies and the contributions to D and E of
the low-lying excited states found with ORCA/CASSCF
methodology).
This fact can be explained by using complex 1 as an example.
The first four excited states for this complex, denoted as 4Φn, are
close enough in energy to the ground state to have a significant
impact on the magnitude of D (Table 2).[5i-j,6g] As stated above
the ground state and the low-lying excited states are highly
multideterminant (Table S23); Figure 6 shows the wave function
compositions for the ground state (4Φ0) and the first excited state
(4Φ1) of complex 1. As may be observed the negative
contribution to D arises from the coupling between the b and e
determinants of 4Φ1 with the c and d determinants of 4Φ0,
because they involve excitations between orbitals having the
same ml values. The interaction between the determinants b
(4Φ1) and e (4Φ0), which should imply an excitation with “double”
negative contribution, may play a role as well.[5i-j,6g]
Figure 5. Schematic d orbital splitting for a mononuclear CoII complex in a
trigonal bipyramidal geometry (left) and AILF computed relative orbital
energies and occupancies for complexes 1-4 (right).
It has been stated that the sign and value of the D-parameter
can be rationalized using the spin-orbit operator, which couples
the ground and excited states.[24] When the excited state results
from the excitation between orbitals with the same |ml| values,
the MS = ±3/2 components are stabilized and a negative
contribution to D is obtained. On the other hand, an excitation
between orbitals involving a |Δml| = 1 change, i.e. stabilizing the
MS = ±1/2 components, leads to a positive contribution to the D
value.[5b] The expected d-orbital splitting for the trigonal
bipyramidal CoII mononuclear complexes 1-4 is shown in Figure
5; the computed energies of the d orbitals for each complex,
which were obtained from the ab initio ligand field theory
Table 2. ORCA/CASSCF calculated energies (in cm−1) and contributions to D
and E for the first four excited states (4Φn) of complexes 1-4.
Complex
Excited state
4Φ1
Energy (cm-1)
3219.7
4773.1
5212.6
5651.8
3215.2
4620.0
5047.0
5360.1
3692.7
4959.8
5075.4
6410.0
3619.6
4808.9
4808.9
5956.5
Cont. D
-32.0
10.8
6.4
Cont. E
0.0
4Φ2
4Φ3
4Φ4
4Φ1
4Φ2
4Φ3
4Φ4
4Φ1
4Φ2
4Φ3
4Φ4
4Φ1
4Φ2
4Φ3
4Φ4
-10.4
4.3
1
4.6
4.3
-30.9
10.5
7.3
-0.2
-10.6
5.2
2
3
4
4.7
4.0
Figure 6. Composition of the wavefunctions corresponding to the ground state
4Φ0 (top) and the first excited state 4Φ1 (bottom) for complex 1, including only
the determinants that have the contributions larger than 5%.
-30.9
11.3
10.9
0.0
0.0
-10.3
9.9
0.01
0.0
This approach can be also employed to confirm that the main
interaction between the ground and the higher excited states
leads to positive D contributions. However, the large negative
contribution of 4Φ1 cannot be compensated by the smaller
positive contributions of 4Φ2, 4Φ3 and 4Φ4 because those are
-30.5
11.7
11.7
0.0
-9.8
9.8
0.0
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10.1002/chem.201702108
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located at higher energies. The same approach can be used to
rationalize the negative sign of D obtained for complexes 2-4,
although in the case of the latter two complexes only the first
three excited states show a significant contribution to the overall
D value.
The d orbital splitting obtained with the AILF method explains
the differences in the magnitude of D between both pairs of
complexes with tpa (1-2) and tbta (3-4) ligands. As mentioned
above, the presence of a chloride ligand should favor the
their low-lying first KDs, which are found at 20.2 and 16.1 cm-1
for 1 and 2, respectively. The second excited KDs are much
higher in energy (ca. 3200 cm-1) and therefore they are not
expected to participate in the relaxation mechanism. The
complexes containing the tbta ligand (3-4) show a slightly
different behavior; in complex 3 the direct QTM and Orbach
mechanisms are very close to their operational limit although
they may be still active. On the other hand the thermally
assisted-QTM relaxation pathway seems very likely since the
matrix elements for vertical excitation and tunneling at the first
KD, found 17.7 cm-1 above the ground state KD, are quite large.
A similar situation is found in complex 4 although in this case the
direct QTM and Orbach pathways seem to be completely shut
down. The thermally assisted-QTM process is, however,
plausible through the first excited KD, which is just 12.7 cm-1
higher in energy than the ground state. As above, the second
excited KDs for 3 and 4 are much higher in energy (ca. 3600 cm-
1) and are probably not involved in the spin relaxation process.
As a whole, the computed barriers for the spin relaxation
mechanism are close to those obtained in the fitting of the
magnetization data, with experimental and computed values of:
12 and 20.2 cm-1 for 1, 8.7 and 16.1 cm-1 for 2, 8.1 and 17.1 cm-1
for 3, and 5 and 12.7 cm-1 for 4.
stabilization of the equatorial (dxy/dx2-y2
) orbital pair, thus
producing a smaller energy gap between the occupied and the
semi-occupied d-orbitals and hence delivering a larger D value
for complexes 1 and 3. The computed energy gaps are 2796.3,
2929.5, 3601.8 and 3636.6 cm-1 for complexes 1-4, respectively,
in a perfect agreement with what should be expected for both
pairs of complexes (Table S24 contains a complete summary of
the d-orbital splitting for each complex).
The computed relative energies of the lowest-lying KDs and the
spin relaxation pathways of 1-4, computed with SINGLE_ANISO
code implemented in MOLCAS,[28,29] are shown in Figure 7. In
the case of complexes with tpa ligand (1-2) the spin relaxation
mechanisms show a plausible pathway via a direct quantum
tunneling (QTM) in the ground state. The matrix elements of the
transition magnetic moments between states 1- and 1+ are 0.45
and 0.49 µB for 1 and 2, respectively, much higher than the 0.1
µB required value associated to an efficient relaxation
mechanism.[4l,29] In addition, thermally assisted-QTM or Orbach
processes seem very plausible for both compounds through
Conclusions
The study shows that slow magnetic relaxation can be observed
under an applied dc field in a family of C3 symmetric CoII
complexes with two different tripodal ligands tpa and tbta. The
presence of different relaxation processes in all complexes have
been investigated by detailed ab initio calculations.
Computational studies reveal that all the complexes have a very
strong multiconfigurational character of the wave function that
stabilizes the largest Ms = ±3/2 components of the quartet state
and hence produce a large negative contribution to ZFS
parameters. The σ-donating effect of the equatorial amine
nitrogen atoms is larger in the case of tbta ligand than tpa and
results a higher |D| value for complexes with tpa ligand (1-2);
and therefore providing an implement to estimate the single-ion
anisotropy in C3 symmetric CoII complexes. Besides reporting
the SIMs with trigonal bipyramidal penta-coordinated CoII
geometries, the present results also demonstrate the correlation
between Ising-type magnetic anisotropy and structure; therefore
attaining a magneto-structural correlations.
Experimental Section
Materials and Methods
Magnetic measurements were performed using
a Quantum Design
Figure 7. Lowest two Kramers’ doublets and ab initio computed relaxation
mechanism for complexes 1-4. The thick black lines imply KDs as a function of
their magnetic moment along the main anisotropy axis. Red lines indicate the
magnetization reversal mechanism. The blue lines correspond to ground state
QTM and thermally assisted-QTM via the first excited KD, and green lines
show possible Orbach relaxation processes. The values close to the arrows
indicate the matrix elements of the transition magnetic moments (in µB above
0.1 an efficient spin relaxation mechanism is expected).[4l,29]
SQUID-VSM magnetometer. The measured values were corrected for
the experimentally measured contribution of the sample holder, while the
derived susceptibilities were corrected for the diamagnetic contribution of
the sample, estimated from Pascal’s tables.[30] Elemental analysis was
performed on Elementar Microvario Cube Elemental Analyzer. IR
spectrum was recorded on KBr pellets with a Perkin-Elmer spectrometer.
Powder X-ray diffraction (PXRD) data was collected on a PANalytical
EMPYREAN instrument using Cu-Kα radiation.
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10.1002/chem.201702108
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X-ray Crystallography
washed with cold water and Et2O and air-dried yield (80 %). Anal. Calcd
for C25H27CoN5O6: C, 54.36; H, 4.93; N, 12.67 %. Found: C, 54.45; H,
5.03; N, 12.73 %. IR (KBr pellet, 4000 − 400 cm−1) ν /cm-1: 3420, 3064,
2918, 2788, 1528, 1374, 1332, 1291, 1074, 1034.
Intensity data were collected on a Brüker APEX-II CCD diffractometer
using a graphite monochromated Mo-Kα radiation (α = 0.71073 Å). Data
collection was performed using φ and ω scan. The structure was solved
using direct methods followed by full matrix least square refinements
against F2 (all data HKLF 4 format) using SHELXTL.[31] Subsequent
difference Fourier synthesis and least-square refinement revealed the
positions of the remaining non-hydrogen atoms. Determinations of the
crystal system, orientation matrix, and cell dimensions were performed
according to the established procedures. Lorentz polarization and multi–
scan absorption correction was applied. Non-hydrogen atoms were
refined with independent anisotropic displacement parameters and
hydrogen atoms were placed geometrically and refined using the riding
model. All calculations were carried out using SHELXL 97,[32] PLATON
99,[33] and WinGX systemVer-1.64.[34] Crystallographic data for
complexes 1-4 were summarized in Table S1.
Synthesis of [Co(tpa)Br]·ClO4 (2):
TPA (29 mg, 0.1 mmol) was dissolved in MeOH (5 ml) and the solution
was warmed to 40°C. The mixture of CoBr2 (11 mg, 0.05 mmol) and
Co(ClO4)2·6H2O (18 mg, 0.05 mmol) dissolved in MeOH (5 ml) was
added dropwise to the above ligand solution while stirring. The resulting
solution forms an intense red mixture that was stirred further for 30
minutes. The solution was then filtered off and the filtrate was left at open
atmosphere for slow evaporation which yields large X-ray quality red
crystals of [Co(tpa)Br]·ClO4 (2) after 2 days. The crystals were separated,
washed with cold water and Et2O and air-dried yield (55 %). Anal. Calcd
for C27H29CoN7O6S2: C, 48.37; H, 4.36; N, 14.62; S, 9.54 %. Found: C,
48.44; H, 4.44; N, 14.69; S, 9.61 %. IR (KBr pellet, 4000 − 400 cm−1) ν
/cm-1: 3429, 3052, 2921, 2838, 2165, 1525, 1363, 1327, 1295, 1070,
1052.
Synthesis of tpa (tris(2-methylpyridyl)amine):
A solution of 39.0 g (238 mmol) of 2-picolyl chloride hydrochloride in 100
ml of deionized water was cooled to 0°C in an ice bath. To this solution
was added, with stirring, 45 ml of a 5.3 N aqueous solution of NaOH. The
resulting free amine appeared as a bright red emulsion following the
neutralization. To this mixture was then added a solution of 12.8 g (1 19
mmol) of 2-(aminomethy1)pyridine in 150 ml of dichloromethane. The
mixture was then allowed to warm to room temperature and, for 2 days
and an additional 45 ml aliquot of 5.3 N aqueous NaOH solution was
added. During addition of the NaOH solution, the pH of the aqueous
portion of the reaction mixture was not allowed to exceed 9.5. The crude
mixture was then washed with 100 ml of 15% NaOH, and the organic
phase was dried with MgSO4 and filtered. Removal of the
dichloromethane yielded a brown solid mass which was extracted with
boiling diethyl ether (3 × 50 ml). Evaporation of the ether extracts yielded
yellow crystals of ligand. The ligand was purified by recrystallization from
diethyl ether to give 27.2 g (80%) of white crystalline solid. 1H NMR
(CDCl3): δ 8.48 (d), 7.69 (t), 7.60 (d), 7.17 (t), 3.81 (s). 13C NMR (CDCl3):
δ 160.51, 149.85, 137.25, 123.74, 122.94, 60.83.
Synthesis of [Co(tbta)Cl]·(ClO4)·(MeCN)2·(H2O) (3):
Tbta (53 mg, 0.1 mmol) was dissolved in MeCN (5 ml) and the solution
was warmed to 40°C. The mixture of CoCl2·6H2O (12 mg, 0.05 mmol)
and Co(ClO4)2·6H2O (18 mg, 0.05 mmol) dissolved in MeOH (5 ml) was
added dropwise to the above ligand solution while stirring. The resulting
solution forms an intense red mixture that was stirred further for 30
minutes. The solution was then filtered off and the filtrate was left at open
atmosphere for slow evaporation which yields large X-ray quality red
crystals of [Co(tbta)Cl]·(ClO4)·(MeCN)2·(H2O) (3) after
3 days. The
crystals were separated, washed with cold water and Et2O and air-dried
yield (70 %). Anal. Calcd for C26H29ClCoN6O10S: C, 43.87; H, 4.11; N,
11.80; S, 4.49 %. Found: C, 43.96; H, 4.21; N, 11.87; S, 4.55 %. IR (KBr
pellet, 4000 − 400 cm−1) ν /cm-1: 3422, 3073, 2917, 2818, 2175, 1529,
1384, 1318, 1289, 1076, 1028, 921.
Synthesis of [Co(tbta)Br]·ClO4 (4):
Synthesis of tbta (tris[(1-benzyl-1H-1,2,3-triazole-4-yl)methyl]amine):
Tbta (53 mg, 0.1 mmol) was dissolved in MeCN (5 ml) and the solution
was warmed to 40°C. The mixture of CoBr2 (11 mg, 0.05 mmol) and
Co(ClO4)2·6H2O (18 mg, 0.05 mmol) dissolved in MeOH (5 ml) was
added dropwise to the above ligand solution while stirring. The resulting
solution forms an intense red mixture that was stirred further for 30
minutes. The solution was then filtered off and the filtrate was left at open
atmosphere for slow evaporation which yields large X-ray quality red
crystals of [Co(tbta)Br]·ClO4 (4) after 3 days. The crystals were separated,
washed with cold water and Et2O and air-dried yield (50 %). Anal. Calcd
for C27H25ClFeN11O6: C, 49.49; H, 3.84; N, 23.50 %. Found: C, 49.58; H,
3.95; N, 23.56 %. IR (KBr pellet, 4000 − 400 cm−1) ν /cm-1: 3430, 3064,
2927, 2827, 2195, 1518, 1361, 1329, 1297, 1065, 1004, 875.
Tripropargylamine (13.2 g, 0.1 mol) in acetonitrile (100 ml) was treated
sequentially with benzyl azide (59.9 g, 0.45 mol), 2,6-lutidine (10.7 g, 0.1
mol), and Cu(MeCN)4PF6 (1.3 mol % with respect to total alkyne). Upon
addition of the copper salt, the reaction warmed and was cooled in an ice
bath. After the mixture was stirred at room temperature for 4 days, a
white crystalline solid precipitated from the reaction. Filtration and
washing with cold acetonitrile gave white needle like crystals (43.8 g,
83%). The ligand was further purified by recrystallization from a hot 1:1
tert-butyl alcohol/water solution (50 ml) followed by filtration and washing
with water (2 × 30 ml). The white needle like crystals were dried under
high vacuum overnight. Yield: 0.87 g, 87%. Mp: 138-140°C. 1H NMR
(CDCl3): δ 3.70 (s), 5.49 (s), 7.26 (m), 7.34 (m), 7.67 (s). 13C NMR
(CDCl3): δ 134.7, 129.1, 128.7, 128.0, 123.8, 54.1, 47.0. Anal. Calcd for
C30H30N10: C, 67.90; H, 5.70; N, 26.40. Found: C, 67.75; H, 5.77; N,
26.33.
Acknowledgements
A.K.M. thanks UGC, India for the SRF fellowship. S.K.
acknowledges DAE-BRNS (project no. No. 37(2)/14/09/2015-
BRNS), Government of India and IISER Bhopal for generous
financial support. The research reported here was supported by
the Spanish Ministerio de Economía y Competitividad (grant
CTQ2015-64579-C3-1-P, MINECO/FEDER, UE). E.R. thanks
Generalitat de Catalunya for an ICREA Academia award. J.J.
and E.R. thankfully acknowledge the computer resources in the
Consorci Serveis Universitaris de Catalunya (CSUC).
Synthesis of [Co(tpa)Cl]·ClO4 (1):
TPA (29 mg, 0.1 mmol) was dissolved in MeOH (5 ml) and the solution
was warmed to 40°C. The mixture of CoCl2·6H2O (12 mg, 0.05 mmol)
and Co(ClO4)2·6H2O (18 mg, 0.05 mmol) dissolved in MeOH (5 ml) was
added dropwise to the above ligand solution while stirring. The resulting
solution forms an intense red mixture that was stirred further for 30
minutes. The solution was then filtered off and the filtrate was left at open
atmosphere for slow evaporation which yields large X-ray quality red
crystals of [Co(tpa)Cl]·ClO4 (1) after 2 days. The crystals were separated,
This article is protected by copyright. All rights reserved.

10.1002/chem.201702108
Chemistry - A European Journal
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Keywords: Single Ion Magnets • trigonal bipyramidal geometry •
• Ising-type anisotropy • ab initio calculations
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Entry for the Table of Contents
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A
quantitative
of
█ Single Ion Magnets
estimation
magnetic anisotropy
has been done for C3
Amit Kumar Mondal,
Jesús Jover, Eliseo
Ruiz* and Sanjit
Konar*
symmetric
complexes with Ising-
type magnetic
CoII
■■ – ■■
anisotropy by using a
combined
Quantitative
Estimation of Ising-
type Magnetic
Anisotropy in a
Family of C3
experimental
and
theoretical approach.
Symmetric CoII
Complexes
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