Single-Ion Magnetic Behavior in CoII–CoIII Mixed-Valence Dinuclear and Pseudodinuclear Complexes

Article abstract of DOI:10.1002/ejic.201700046

Two Co^II–Co^III mixed-valance complexes of molecular formulas [Co₂(H₂L)₂(H₂O)₂][Co₂(H₂L)₂(H₂O)(m-phth)]·8(H₂O) {1; H₂

Full text of DOI:10.1002/ejic.201700046

A Journal of
Accepted Article
Title: Single ion magnet bevavior in Co(II)-Co(III) mixed valence
dinuclear and pseudo-dinuclear complexes
Authors: Soumen Manna, Apurba Bhunia, Soumen Mistri, Julia Vallejo,
Ennio Zangrando, Horst Puschmann, Joan Cano, and Subal
Chandra Manna
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700046
Link to VoR: http://dx.doi.org/10.1002/ejic.201700046

European Journal of Inorganic Chemistry
10.1002/ejic.201700046
Single ion magnet bevavior in Co(II)-Co(III) mixed valence dinuclear and
pseudo-dinuclear complexes
a
a
a
b
c
Soumen Manna, Apurba Bhunia, Soumen Mistri, Julia Vallejo, Ennio Zangrando, Horst
d
b, e
,a
Puschmann, Joan Cano,*, and Subal Chandra Manna*
a
Department of Chemistry and Chemical Technology, Vidyasagar University, Midnapore
7
21102, West Bengal, India, E-mail: scmanna@mail.vidyasagar.ac.in, Fax: (91) (03222)
75329
2
b
Institut de Ciència Molecular (ICMol), Universitat de València, 46980 Paterna, Valencia,
Spain
c
Department of Chemical and Pharmaceutical Sciences, University of Trieste, 34127 Trieste,
Italy
d
Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, U.K.
e
Fundació General de la Universitat de València (FGUV), Universitat de València, E-46980
Paterna, València, Spain
Two
Co(II)-Co(III)
mixed-valance
complexes
of
molecular
formula
and
{
{
[Co (H L) (H O) ][Co (H L) (H O)(m-phth)]∙8(H O)}
(1)
2
2
2
2
2
2
2
2
2
2
2
-
[Co (H L) (H O) (ppda)]∙2(dmf)∙3.2(H O)}
(2)
[H L =
2-((2-hydroxy-3-
m-phth 1,3-
4
2
4
2
2
2
2
methoxybenzylidene)amino)-2-(hydroxymethyl)propane-1,3-diolato;
=
benzenedicarboxylate; ppda = 1,4-phenylenediacrylate; dmf = N,N-dimethylformamide] have
been synthesized and characterized by single crystal X-ray diffraction and magnetic studies at
low temperature. The structural determination reveals that complex 1 is composed of
+
+
dinuclear ion pair, namely a cationic [Co (H L) (H O) ] (1 ) and an anionic
2
2
2
2
2
–
–
[
Co (H L) (H O)(m-phth)] (1 ) unit. In each of these ions, Co(II) and Co(III) centers present
2 2 2 2
a distorted octahedral geometry. Compound 2 is a centro-symmetric tetra-nuclear complex
III
II
comprising two symmetry related dinuclear Co -Co units bridged by the ppda anion.
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
AC/DC magnetic studies revealed that individual Co(III)-Co(II) unit exhibits field induced
slow magnetic relaxation consistent with the single ion magnets (SIMs) behavior. Ab initio
st
NEVPT2 calculations confirm large zero-field splitting (zfs) coming from a 1 order spin-
–
1
orbit coupling (SOC) in both complexes (D = –62.4, –95.8 and –101.9 cm and E/D = 0.219,
+
–
0
_
.216 and 0.234 for 1 , 1 and 2, respectively).
__________________________________________________________________________
Keywords: Co(II)-Co(III); Crystal structure; Magnetic properties; Single ion magnet
__________________________________________________________________________
_
Introduction
Paramagnetic compounds in which the individual molecules exhibit slow magnetic relaxation
1
and magnetic hysteresis are known as single-molecular magnets (SMMs). Since the
discovery of the first SMM (Mn OAc) in 1990, much significant work has been done on this
1
2
2
type of magnetic materials due to their potential application in information storage, quantum
3
4
5
computing, spintronics, and magnetic refrigeration. The essential requirement for a
compound to be a SMM is the presence of a spin-reversal energy barrier between the states
1
a
with the spin magnetic moments of opposite directions. The spin-reversal barrier is defined
2
2
as U=|D|S and U = |D| (S –1/4) for integer and half-integer spin moments (S) of the ground
states, respectively; D is the axial magnetic anisotropy which splits the M levels of the spin
S
1
a,6
ground state (GS) under zero magnetic field.
Transition metal-based mononuclear and
polynuclear complexes showing a SMM behavior have been widely reported in the last two
1
b, 7-9
decades.
The mononuclear compounds with one spin carrier on a molecule, large Ising-
type magnetic anisotropy and magnetic properties similar to polynuclear SMMs are usually
known as single-ion magnets (SIMs). Literature survey reveals that most of the reported SIMs
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
10
n
11
12
contain late lanthanide ions (4f , n>7) and only a few examples of Co(II), Mn(III),
1
3
7, 14
15
16
Ni(I), Fe(II),
and Fe(III), are reported as SIMs.
Thermally activated slow magnetic relaxation is one of the essential requirements for a SMM
or SIM behaviour. But, from a mixing of the M levels promoted by the transverse zero-field
s
splitting (E) and hyperfine or dipolar interactions, this process may disrupt due to a quantum
tunneling magnetization (QTM). The mixing of ground degenerate M levels via transverse zfs
s
nd
is restricted, at least when it comes from a 2 order SOC, for a system with non-integer spin
system and negative D value. In this regard, mononuclear cobalt(II) complexes with non-
integer (S = 3/2) spin state, D< 0 and a forbidden mixing of the ground degenerate M levels
s
1
7
through transverse zfs (E) are suitable for a magnet-like behaviour. However, these rules are
not so clear when the zfs is derived from a first-order SOC, which is the case in octahedral
cobalt(II) complexes, and, consequently, the existence of cobalt(II) SIMs with positive D
values seems to be the norm. Also, mononuclear Co(II) can show substantial axial zfs (D)
that, however, can vanish in polynuclear complexes weather the zfs tensors of each metal ion
are not collinear, which is usual. This inconvenient can be the main reason why the searching
for molecule behaving as magnets is focused on mononuclear Co(II) complexes. But till date
1
1b, 18
only a few examples of mononuclear Co(II) compounds with pseudo-tetrahedral,
1
1c
11a
octahedral,
showing SIMs behavior.
In the present contribution we report two new Co(II)-Co(III) mixed valence complexes
and square-pyramidal,
are reported with SMMs properties are reported
{
{
[Co (H L) (H O) ][Co (H L) (H O)(m-phth)]∙8(H O)}
(1)
and
2
2
2
2
2
2
2
2
2
2
[Co (H L) (H O) (ppda)]∙2(dmf)∙3.2(H O)} (2) [where m-phth = 1,3-benzenedicarboxylate;
4
2
4
2
2
2
2
-
ppda = 1,4-phenylenediacrylate; H2L = 2-((2-hydroxy-3-methoxybenzylidene)amino)-2-
(hydroxymethyl)propane-1,3-diolato]. Detail magnetic study reveals single ion magnet
behavior for both the compounds.
3
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
Results and discussion
Synthesis of complexes 1 and 2. The complexes have been synthesized by adopting the
procedures schematically given in Scheme 1.
Scheme 1. Synthesis of complexes 1-2.
Crystal structure description
The reaction of cobalt acetate with H L in methanol and in the presence of triethylamine and
4
of m-phthalic acid (with molar ratio, cobalt acetate : H L : H phth = 1:1:1) leads to two ionic
4
2
+
dinuclear complexes, a cationic [Co (H L) (H O) ] and an anionic [Co (H L) (H O)(m-
2
2
2
2
2
2
2
2
2
−
phth)] species. Figure 1 shows a detail of the packing that accomplishes around a symmetry
center leading to a positional disorder. As described in the experimental part, a half
occupancy has been assigned to the m-phth anion, as well as to the coordinated aqua O(2w)
and lattice water molecules O(3w)-O(9w). So that the sixth coordination site of Co(2) is
occupied by a carboxylate m-phth oxygen atom and that of its symmetry related Co(2’) by a
water
molecule,
so
that
the
formulation
for
complex
1
is
[
Co (H L) (H O) ][Co (H L) (H O)(m-phth)]∙8(H O).
2
2
2
2
2
2
2
2
2
2
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
Figure 2 depicts an ORTEP view of the anionic fragment, where the m-phthalate anion (m-
phth) acts as a monodentate ligand. The oxidation states of the cobalt centers are confirmed
by considering the Co–O bond lengths and taken into account the total charge of the
coordinated ligand molecules. Co(1) has a +3 oxidation state and is chelated by two
2
−
polydentate H2L Schiff base ligands via the phenoxo oxygens O(1a/O1b), the imine
nitrogens N(1a/1b) and the deprotonated alcoholic –OH groups (O2a/2b). These chelating
ligands are such arranged that imine nitrogen donors are in trans position with phenolato
mean planes almost normal to each other, as already found in polynuclear cobalt complexes
1
9
containing comparable fragments.
The values of the Co(1)-N bond lengths are of 1.887(3) and 1.889(3) Å, while the Co(1)-O
distances fall in the range 1.885(3)-1.915(2) Å. On the other hand, the divalent Co(2) ion has
a O chromophore environment, being coordinated by the bridging alkoxido oxygen atoms
6
O(2a) and O(2b), by the hydroxyl groups O(3a) and O(3b), and by a water molecule (O(1w)).
The sixth position is occupied by an additional aqua (O(2w)) in the complex cation or by the
m-phth carboxylate oxygen donor O(1) in its anion counterpart. The Co(2)-O bond distances
are slightly longer, varying from 2.028(3) to 2.192(3) Å, but, due to the disorder, the
refinement led to Co(2)-O(2w)' and Co(2)-O(1) bond length values of low accuracy, being of
2
.06(2) and 2.05(2) Å, respectively.
The Co(2)-O bond lengths are significantly longer than those measured for Co(1), by 0.1–0.2
Å, consistent with the designation of the respective oxidation states of +2 and +3 for Co(2)
and Co(1), respectively. The alkoxo oxygen atoms O(2a) and O(2b) of the two H L ligands
2
III
II
bridge the two cobalt centers leading to an intermetallic Co -Co distance of 2.9847(7) Å.
On the other hand, by reacting cobalt acetate with H L in methanol, in the presence of
4
triethylammine and of 1,4-benzenediacrylic (H ppda) acid (with molar ratio, cobalt acetate :
2
H L : H ppda = 1:1:1), lead to the formation of a centro-symmetric tetranuclear complex of
4
2
5
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
formula [Co (H L) (H O) (ppda)]∙2(dmf)∙3.2(H O) (2). In fact the X-ray structural analysis of
4
2
4
2
2
2
III
II
the compound revealed that it comprises a crystallographic independent dinuclear Co Co
unit connected to a symmetry related one by the phenylenediacrylate anion located on a center
of symmetry. The molecular structure of complex 2 is illustrated in Figure 3 and a selection of
bond lengths and angles is reported in Table 1. The metals in the independent unit are six-
coordinated with a distorted octahedral geometry showing a close similarity to anionic
III
II
complex 1. The intermetallic distance in the Co Co unit is of 2.9654(3) Å (shorter by 0.02
Å compared to the value measured in 1), and the Co(2) ions bridged by the ppda anion, are
spaced by 15.225(1) Å. The same coordinating atom labels were assigned in 1 and 2 and data
of Table 1 highlight close comparable geometrical values in the two complexes. Due to this
similarity, we do not describe in detail the structure, being the Co-N and Co-O bond lengths
well within the ranges previous indicated for 1, but the coordination bond angles in the
present case indicate an octahedral geometry closer to ideal values.
It is worth of note in both complexes a rather strong intramolecular hydrogen bond occurring
between the carboxylate oxygen O(2) and the adjacent coordinated water molecule O(1w) in
(O…O distance of 2.654(13) and 2.625(2) Å, in 1 and 2, respectively, see Figures 1 and 3),
which reinforces the link of the carboxylate group to the dinuclear cobalt unit. However, the
disorder of water molecules observed in both crystals does not allow a detailed analysis of the
H-bonding pattern. The volume of lattice water molecules and dmf in 2 accounts for the 19%
2
0
of the unit cell volume as derived by program Platon.
Magnetic Properties
The direct current (dc) magnetic properties of
1
and
2
in the form of the MT versus T
II
III
plot (M being the dc magnetic susceptibility per {Co Co } unit) were investigated in
2
detail (Figure 4). At room temperature, the MT values for
1
and 2, of 5.80 and 5.89
3
–1
7
cm mol K, respectively, are within the range expected for two isolated high-spin d
6
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
II
Co (S = 3/2) ion with some orbital momentum contribution. Upon cooling, the M
T
3
–1
value continuously decreases reaching 3.54 and 3.60 cm mol K for
1 and 2,
respectively at 2.0 K (Figures 4 and 5), revealing the occurrence of a significant spin-
orbit coupling (SOC). On the other hand, these temperature dependences of the
magnetic susceptibility confirm that the extended bridging ligands connecting the high-
II
spin Co ions in
1
and
2
are not able to transmit a magnetic coupling strong enough to
II
visualize its effects in the working temperature range, being the Co centres considered
entirely isolated. The magnetization data also support the presence of a spin-orbit
coupling or a zero-field splitting (zfs): i) even at 2 K, the saturation value of
magnetization (4.30 and 4.26 Nβ for
1 and 2, Figures 4 and 5) is well below the
expected value (Ms = 6.0 Nβ for g = 2.0); and ii) the M vs. H/T curves do not
superimpose (Figures S1 and S2). However, the latter curves are very close, featuring
a system with a very small or large zfs. In the last case, which is usual in high-spin
II
octahedral Co ions, the quartet ground state is split in well separated two Kramer’s
doublets, being at low temperatures practically only populated the Kramer doublet
ground state. Thus, changes in the temperature just cause tiny adjustments on the
population of the Kramer’s doublets and, in consequence, in the Mvs.H/T curves
(
Figures S1 and S2).
Although the magnetic data susceptibility should be analyzed with a model of a first order
spin-orbit coupling in case of significant SOC, a spin Hamiltonian corresponding to an
isolated S = 3/2 with large g-factors and zero-field splitting parameters (zfs) can be used as
2
z
2
x
2
well. In this case the spin Hamiltonian takes the form: H = D[S +S(S+1)/3] + E(S +S ) +
y
7
βHg(S +S +S ), where axial and rhombic distortion of the tetragonally distorted high-spin d
x
y
z
II
Co ion are taken into account through the D and E parameters, respectively. Whereas the E
parameter cannot be unambiguously evaluated from magnetic susceptibility data, the E and g
7
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
parameters are strongly correlated in the simulation of the magnetization data. However, these
problems can be solved when magnetic susceptibility and magnetization data are analyzed in
unison. Thus, the magnetic susceptibility data determines the value of the g-factor and, hence,
the magnetization allows a good estimation of the rhombic anisotropy value from the
determined g. However, the huge zfs leads to a collapse of the M vs. H/T curves that
represents a major constraint to achieve a good estimation of the D and E parameters. Thus,
we independently analyzed the thermal dependences of the magnetic susceptibility and the
2
2
magnetization. The agreement factor (F) can be defined as ∑[(P)exp – (P)calcd] /∑[(P) ] ,
exp
where P is the physical property under study. A minimum value of F for the magnetic
–
1
susceptibility is reached with the following values: g = 2.501(3) and |D| = 54.7(7) cm with F
–
5
–1
–5
=
3.6 10 for 1 and g = 2.515(2) and |D| = 60.8(6) cm with F = 2.5 10 for 2. The
theoretical curves well match the experimental data in the whole temperature range (solid
II
lines in Figures 4 and 5). Hence, the assumption that Co ions were magnetically isolated is
validated. In cobalt(II) SIMs, these large values of the axial zfs are typical, but both negative
1
1a,11b, 11c, 18
and positive have been reported.
On the other hand, the large axial zfs allows
simulating the magnetization data using a spin model that takes into account only the ground
Kramer’s doublet as an effective S = ½ spin moment. In this approach the magnetic
anisotropy is transferred to the g tensor, i.e., three different components should be considered.
Usually, only the parallel (g ) and perpendicular components (g ) are contemplated. This
||

model is summarized in the spin Hamiltonian: H = g βH S + g βH S + g βH S . Thus, the

x x

y y
||
z z
best fit of the magnetization data provides the following results: g = 7.519(10) and g =
||

–
4
–4
1
.901(9) with F = 3.0  10 for 1; and g = 7.349(8) and g = 1.950(7) with F = 3.6  10 for
||

2
(solid lines in Figures 4 and 5). These values, especially the large values of g , are those
||
II
clearly typical for octahedral Co ions with negative D values.
8
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
On the other hand, these values of the D and E/D parameters agree with those found from
+
–
NEVPT2 calculations. Due to cationic (1 ) and anionic (1 ) unities coexist in 1, NEVPT2
calculations were done on both species to estimate the values of zfs parameters. In all used
II
III
models, Co and Co ions are present, and the unique metal ion showing a spin density (),
7
which corresponds to a partially delocalized high-spin d electronic configuration, is that was
II
assigned to a Co ion from the crystal structure [(Co) = 2.89 e]. Whereas D and E/D take
–
–1
–1
values similar in 1 (D = –95.8 cm , and E/D = 0.216) and 2 (D = –101.9 cm , and E/D =
+
–1
0
.234), these values are lower enough in 1 (D = –62.4 cm , and E/D = 0.219). These values
nd
obtained from a 2 -order perturbative approach qualitatively are similar to those reached
–
1
using an effective Hamiltonian (D = +55.9, –70.4 and –73.9 cm , and E/D = 0.294, 0.328 and
+
–
0
.331 for 1 , 1 and 2, respectively), which are however closer to those found from the
magnetometry. However, in our experience, the approach of effective Hamiltonian usually
overestimates the rhombicity in the zfs, being the E/D ratio close its maximum value (1/3) and
the meaning of the sign of D is thus lost. Mainly, these results confirm the negative sign of D
suggested from the experimental magnetization data. According to these calculations, the
second-order spin-orbit coupling is the main contribution to D parameter, being the
contribution of the spin-spin coupling negligible compared to the former one. Because the two
–
1
–
first quartet excited states are much closer to the quartet ground state (786 cm and 1519 cm
1
–
in 1 ) than other quartet and doublet excited states, the D value is largely given by the
–
1
contribution from these two excited states (–108.7 and +19.4 cm , respectively). Despite of
II
–
the fact that Co ions in 1 and 2 present the same coordination environment, the calculations
shows that complex 2 exhibits a higher zfs, that is probably related to a larger distortion of the
octahedral coordination sphere, supported by a major value of E/D. This provokes the two
–
1
–1
first excited quartet states to be slightly closer to the ground state (776 cm and 1449 cm )
–
–1
–1
than in 1 (786 cm and 1519 cm ) and they interact more strongly with it, leading to major
9
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
–1
contributions to the D parameter in 2 (–118.4 and +22.8 cm ). On the other hand, shape
+
–
measurements on the 1 , 1 and 2 complexes were done to check the involvement of structural
+
distortions on the different zfs suggested for 1 . Previous theoretical studies indicate that
octahedron geometries favor positive D parameters, but trigonal prismatic coordination
2
1
spheres provide negative D factors.
The shape measurements for our three complexes
suggest that they are very similar with coordination spheres close enough to an octahedron [S
+
–
=
18.4 (1 ), 21.1 (1 ), and 17.0 (2), being S the degree of conversion between an octahedron
and a trigonal prism, which takes values of 0.0 and 100 for the ideal octahedron and the
22
trigonal prism]. However, they are distorted and deviated from the path that connects an
octahedron and a trigonal prism; these deviations are also similar in the three species ( =
+
–
1
1.3, 7.6 and 9.4 for 1 , 1 and 2, respectively). Therefore, structural parameters are not able
+
–
to explain the different calculated axial zfs between 1 and the similar 1 and 2. On the other
2
1
hand, the negative D values for the three unities do not agree with an octahedral geometry.
On the contrary, high rhombicity, as it is proposed for complexes in 1 and 2, can inverse the
sign of D or just remove its physical meaning. Nevertheless, the zfs must be associated with
the electronic effects rather than to structural factors –although they can be coincident in some
cases, particularly in theoretical studies where only structural distortions are applied on a
specific model. In other situations, the ligands conforming the coordination sphere can display
different electronic structures, but geometrical factors influencing zfs are only supporting
–
actors. In the present case, in 1 and 2 the coordination sphere is similar, but a neutral water
+
molecule replaces one charged carboxylate group in 1 , provoking an apparent change in the
electronic structure of this last complex despite there is not a significant structural change.
–
This fact is the cause the D values for 1 and 2 were similar, and the found difference can be
+
associated only with structural features. But this is not the case for 1 , where the change of the
electronic state when the water molecule is coordinated shifts up the first excited quartet (874
1
0
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
–
1
cm ), the main contributor to the axial zfs in octahedral cobalt(II) complexes, decreasing its
input drastically. On the other hand, since these two excited quartet states contribute in
opposite way, the control of the symmetry of the electronic distribution allows stabilizing one
or other excited state and hence to obtain a positive or negative D value. Additionally, from
these calculations, an estimation of the three components of the g-factor gave: g = 1.719, g =
1
2
+
2
.607 (g = 2.208) and g = g = 7.307 for 1 , g = 1.517, g = 2.350 (g = 1.978) and g = g
 3 z 1 2  3 z
–
=
7.771 for 1 , and g = 1.510, g = 2.392 (g = 2.000) and g = g = 7.845 for 2, which are in
1 2  3 z
agreement with those previously obtained from the magnetization data.
The alternating current (ac) magnetic susceptibility of 1 and 2 in the form of the  ' and
M
M'' versus T plots ( ' and  '' being the in-phase and out-of-phase ac magnetic
M
M
II
III
susceptibilities per {Co Co } unit) were measured in different applied static fields in the
2
range of 0.0–2.5 kG (Figures 6-7 and S3-S6). In zero dc magnetic field, no  '' signals can be
M
observed for 1 and 2 even for the highest frequency used ( = 10 kHz), suggesting that fast
zero-field quantum tunneling relaxation of the magnetization effects are present. However,
when such a small static dc field as 500 G is applied, strong frequency-dependent maxima
appear in both  ' and  '' below 10 K (Figures S3 and S5) in both cases. Additional ac
M
M
measurements for 1 and 2 under higher applied dc fields of 1000 and 2500 G are shown in the
ESI (Figures 6-7, S4 and S6), showing the same single strong frequency-dependent M''
maxima below 10 K. Relaxation times for 1 and 2 could be calculated from the maximum of
M'' at a given frequency ( = 1/2). These follow, at higher temperatures and for all applied
static dc magnetic field, the Arrhenius law characteristic of a thermally activated or Orbach
mechanism (see Figures 6d, 7d and S3d-S6d), which is determined for the first pre-
exponential factor ( ) and activation energy (E ). But, in all cases, a deviation of Arrhenius
0
a
law at lower temperatures is observed that is a definite indication of the presence of another
1
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
additional relaxation mechanism. Among the candidates there are the quantum tunneling,
direct and Raman relaxation mechanisms. The best simulation of the thermal dependence of
the relaxation times was obtained using only the Orbach and Raman mechanisms through the
–
1
–1
n

=  exp(–E /k T) +AT relation. The values of first pre-exponential factor ( ) and
0
a1
B
0
activation energy (E ) for 1 and 2 are consistent with those found for previously reported
a
1
1c
octahedral cobalt(II) SIMs (Tables 2 and 3). On the other hand, it is worth to note that, for
a particular applied magnetic field, Raman and direct mechanism are expressed in a similar
way and only the value of n changes, being unity in the last one and larger values for the
former, usually n = 7 and 9 for non-Kramer and Kramer ions, respectively. Lower values of n
are possible when phonons are considered. Thus, whereas a direct mechanism seems to be
present in 2, a relaxation mechanism through a Raman process that takes into account
phonons could be more appropriate in 1 (Tables 2 and 3). However, we like to note that
different sets of values for the parameters associated with Orbach and Raman processes, and
if it is also considered with the quantum tunneling, were able to reproduce the thermal
dependence of the relaxation time. Among of these sets, it can be outlined that with energy
–
1
barrier close to 100 cm , which is a value close to that provided by the D parameter.
However, the Raman A and n parameters unexpectedly ever depends on the applied static
magnetic field. In the only case n parameter seems to be independent of the magnetic field is
in 1, where this takes a value close to the unity as it is figured for a direct relaxation process.
In our experience in other families of cobalt(II) complexes, additional relaxation processes
together an Orbach mechanism do not provide satisfactory results because the simulations are
not good enough or the dependence with the applied magnetic field is not that expected. In
2
3
such cases, the inclusion of a second Orbach mechanism gives a good answer. In 1 and 2,
that can be also extended, obtaining values for the energy barriers and pre-exponential factors
similar to other previously reported (Table S1). However, at lower temperature that we have
1
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
values of relaxation times for all magnetic fields used (3.8 K), it seems there is a correlation
between  and H, therefore a direct relaxation process cannot be discarded.
It is not evident to know what unity (cationic or anionic) behaves as SIM in 1 or even
whether both do. Attending to the resemblance observed particularly in the structural and
–
electronic features between 1 and 2, but also in the SIM behavior (energy barrier and
–
relaxation times) reported for 1 and 2, it is only natural to think that the anionic species 1 is
the unique unity in 1 behaving as a SIM. Nevertheless, we cannot guarantee that the cationic
+
unity 1 don’t show this particular behavior even it is the only form of the three complexes
studied here that reveals a positive D value calculated from an effective Hamiltonian. On the
other hand, it is very usual in octahedral cobalt(II) complexes that the energy gap between the
ground and first excited Kramers doublet was much larger than that suggested by the energy
barrier obtained from the dynamic studies of the magnetization. In such cases, some authors
propose that an extra energy contribution from a network relaxation via a phonon can bring
down the energy that is necessary to overcome the barrier imposed by the zfs. The energy of
these phonons depends directly on the how the molecules are organized in the network and
the strength and nature of the contacts between them. Being the structural networks of 1 and 2
very different, it is not possible to establish a direct correlation between the found energy
barriers and D values.
The Cole–Cole plots at 6.0-8.0 K of 1 and 5.0-7.0 K of 2 and under different applied dc fields
of 500, 1000 and 2500 G gave almost perfect semicircles which could be fitted by the
2
4
generalized Debye model (solid lines in Figures 6c, 7c, S3c- S6c). The calculated low values
of the α parameter at the different applied dc fields (α = 0.02–0.12, Table 2) support a single
2
5
relaxation process and discard thus a spin-glass behavior (α = 0 for a Debye model) in both
compounds. We can think that 1, with two different cobalt(II) complexes, should present a
larger  value than 2 with one unique cobalt(II) unity. However, we have seen in the past that
1
3
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
both at the molecular level and in solid the metal complexes can display a relatively wide
range of D values at low temperatures (5 or 10 K) caused by the geometrical changes that
occur at these temperatures. Briefly, at low temperature the complexes are still in motion,
showing a Gaussian distribution of the geometries and of values of zfs parameters, which is
probably the cause of the presence of a distribution of relaxation processes and, therefore, a
2
6
non-null  value. The amplitude of these distributions depends on the molecular vibrational
frequencies, but in solid also on the nature and strength of the intermolecular contacts. To
establish an order of  value for 1 and 2 is not easy because the crystal structures of 1 and 2
different enough.
Conclusion
In summary, we have presented the synthesis, crystal structure, and low-temperature magnetic
behavior of two novel Co(II)-Co(III) mixed-valence complexes using multidentate Schiff base
and dicarboxylate ligands. AC/DC magnetic studies demonstrated that individual Co(II)-
Co(III) unit exhibits field induced slow magnetic relaxation consistent with single ion
magnets (SIMs) behavior. Ab initio NEVPT2 calculations ratify that large axial and rhombic
zfs on cobalt(II) unities are responsible for the particular magnetic behavior in both
compounds studied.
Experimental section
Physical measurements. Elemental analyses (carbon, hydrogen and nitrogen) were
performed using a Perkin–Elmer 240C elemental analyzer. IR spectra were recorded as KBr
-1
pellets on a Bruker Vector 22 FTIR spectrophotometer operating from 400 to 4000 cm .
Electronic absorption spectra were obtained with a Shimadzu UV-1601 UV–Vis
3
spectrophotometer at room temperature. Quartz cuvettes of 1 cm path length and 3 cm
1
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
volume were used for all measurements. ESI-MS spectra of the compounds in methanol were
recorded on an Agilent Q-TOF 6500 mass spectrometer by using the Mass hunter software for
1
13
their analysis. H and C NMR spectra were recorded on a Varian 400 MHz instrument.
Crystallographic data collection and refinement. Data collections for complexes 1-2 were
carried out at 120(2) K with Mo-Kα radiation (λ = 0.71073 Å) on a Xcalibur, Sapphire3
diffractometer equipped with CCD. Cell refinement, indexing and scaling of the data sets
2
7
were done by using the CrysAlisPropackage. The structures were solved by direct methods
28
29
using Olex2 1.2 software and subsequent Fourier analyses and refined by the full-matrix
2
29
least-squares method based on F with all observed reflections. Hydrogen atoms were
placed at calculated positions, except those of some water molecules that were detected on the
difference Fourier map, although a suitable H-bonding scheme among these was difficult to
inspect for the disordered situation detected in the crystal packing. In fact in complex 1 most
of the lattice water molecules and the 1,3-benzenedicarboxylate anion share the same site at
half occupancy. Crystal data and details of refinements are given in Table 4.
Magnetic measurements. Variable temperature (2.0 – 300 K) direct current (dc) magnetic
susceptibility measurements under an applied field of 250 G (T < 30 K) and 0.5 T (T ≥ 30 K),
and field-dependence (0–5.0 T) magnetization measurements at low temperatures in the range
of 2.0–10.0 K were carried out with a Quantum Design SQUID magnetometer. Variable-
temperature (2.0–14.0 K) alternating current (ac) magnetic susceptibility measurements under

4.0 G oscillating field at frequencies in the range of 1.0–10.0 kHz were carried out under
different applied static fields in the range of 0.0–2.5 kG with a Quantum Design Physical
Property Measurement System (PPMS). The magnetic susceptibility data were corrected for
the diamagnetism of the constituent atoms and the sample holder. Dc and ac magnetic
measurements were performed on powdered micro-crystals (18.69 and 13.07 mg for 1 and 2,
1
5
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
respectively) and restrained the sample with n-eicosane (27.84 and 28.47 mg for 1 and 2,
respectively) in order to avoid any field induced torquing.
II
Computational details. Since the magnetic coupling between Co ions through extended
dicarboxylate ligands is negligible, all calculations were performed on the experimental
II
III
II
geometries of {Co Co } unities, i.e., only one of the two Co ions was considered. Thus, all
II
III
ligands of a{Co Co } unity has been fully taken into account, but in order to gain efficiency
without loss of accuracy, the dicarboxylate bridging ligands were simplified in the distant
II
region from the Co ion. A view of the molecular models used in these calculations is shown
in Figure S7. The parameters that determine the axial (D) and rhombic (E) zfs were estimated
from calculations based on a second order N-electron valence state perturbation theory
(NEVPT2) applied on a wave function, which was previously obtained from a complete
active space (CAS) calculation. These calculations were carried out with the ORCA program
3
0
31
version 3.0
using the TZVP basis set proposed by Ahlrichs and the auxiliary TZV/C
3
2
nd
Coulomb fitting basis sets. The 2 order contributions to zfs from 10 quartet and 20 doublet
excited states were generated from an active space with seven electrons in the d orbitals. The
g-tensor was calculated for the ground and excited Kramer’s pairs using Multi-Reference
Configuration Interaction (MRCI) wave functions with a first-order perturbation theory on the
33
SOC matrix.
Materials. High-purity 1,4-phenylenediacrylic acid (H ppda) (97%) and cobalt(II) acetate
2
tetrahydrate (98%) were purchased from the Aldrich Chemical Co. Inc. and used as received.
All other chemicals were of AR grade and the reactions were carried out under aerobic
conditions. Solvents used for spectroscopic studies were purified and dried by standard
3
4
procedures before use.
Synthesis
1
6
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
2
-((2-hydroxy-3-methoxybenzylidene)amino)-2-(hydroxymethyl)propane-1,3-diol (H L). To
4
a methanolic solution (50 mL) of 2-hydroxy-3-methoxybenzaldehyde (0.608 g, 4 mmol) and a
methanolic solution (50 mL) of tris(hydroxymethyl)amino methane (0.484 g, 4 mmol) was
added under stirring condition at room temperature (27 °C). The stirring was continued for
next 2 h. The filtered solution was allowed to evaporate at room temperature and solid yellow
compound was obtained after a few days. Yellow crystals of H L were obtained by re-
4
crystallization of the compound in methanol. Yield: 0.714 g (70%). C H NO (255.27): ESI-
1
2
17
5
+
MS (m/z, ion): [M + H] , m/z, 256.118, calcd.: m/z, 255.27 (100%). Anal. Calc.C, 56.46; H,
1
6
.71; N, 5.48 %; Found: C, 56.44; H, 6.70; N, 5.51 %. H NMR (DMSO-d , 400 MHz, δ,
6
ppm): 8.456 (s, 1H, imine), 6.944, 6.924, 6.890, 6.871 (double doublet, 1H, Ar, J = 21.6, 8
Hz), 6.581, 6.563, 6.542 (t, 1H, Ar, J = 7.6 Hz), 4.907 (s, phenolic –OH ), 3.611 (s, 6H,
1
3
CH ), 3.718 (s, 3H, OMe). C NMR (DMSO-d , 400 MHz, δ, ppm): 164.266 (−CH=N−), 159
2
6
(Ar-C-OMe), 149.754 (Ar-C-OH), 124.343 (Ar-C-imine), 114.551-117.174 (Ar-C), 66.520 (-
–
1
CH -OH), 61.311 (tertiary carbon), 55.881 (-OCH ). FT-IR (KBr, cm ): 3425 (br), 2988 (w),
2
3
2
1
949 (w), 2916 (w), 2832 (w), 2606 (w), 1952 (w), 1880 (w), 1818 (w), 1646 (vs), 1545 (vs),
467 (s), 14149 (vs), 1372 (vs), 1300 (s), 1232 (vs), 1167 (s), 1072 (s), 1023 (s), 962 (m), 913
(
(
(
m), 877 (w), 851 (s), 727 (w), 662 (w), 613 (w), 528 (s), 414 (vs). UV-Vis spectra [λmax, nm
–
1
–1
4
4
4
ε, L mol cm )] (CH OH solution) 202 (6.77 × 10 ), 241 (5.56 × 10 ), 294 (4.15 × 10 ), 420
3
4
1.77 × 10 ).
{
[Co (H L) (H O) ][Co (H L) (H O)(m-phth)]∙8(H O)} (1). A methanolic solution (20 mL)
2 2 2 2 2 2 2 2 2 2
containing H L (0.5 mmol, 0.128 g) and triethylamine (Et N) (1mmol, 0.101 g) was added to
4
3
a methanolic solution (20 mL) of Co(CH COO) ∙4H O (0.5mmol, 0.124 g) and stirred for 1 h.
3
2
2
To the resulting deep brown color solution, a mixture of m-phthalic acid (0.5mmol, 0.083g)
and Et N (1mmol, 0.101 g) in methanolic solution (20 mL) was added dropwise and the
3
1
7
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
stirring was continued for additional 2 h. Then the deep brown solution was filtered and the
filtrate was allowed to evaporate at room temperature. After 2 days a brown colored
crystalline compound was collected by filtration. Yield: 0.523 g, 65%. Anal. Calcd. for
C H Co N O (1611.00): C, 41.75; H, 5.38; N, 3.47 %. Found: 41.72; H, 5.41;N, 3.49 %.
56
86
4
4
35
–
1
FT-IR (KBr, cm ): 3431 (br), 2982 (s), 2946 (w), 2154 (w), 1639 (vs), 1551 (vs), 1467 (vs),
1
9
413 (vs), 1372 (w), 1300 (vs), 1245 (s), 1219 (w), 1171 (w), 1138 (w), 1078 (vs), 1017 (s),
–1
–1
78 (s), 881 (w), 738 (w), 629 (w). UV-Vis spectra [λmax, nm (ε, L mol cm )] (CH OH
3
4
3
3
solution) 250 (3.13 × 10 ), 313 (3.91 × 10 ), 393 (2.71 × 10 ).
[Co (H L) (H O) (ppda)]∙2(dmf)∙3.2(H O)} (2). Complex 2 was synthesized by following
{
4
2
4
2
2
2
the same procedure as adopted for 1, using 1,4-benzenediacrylic acid (0.5 mmol, 0.109 g)
instead of m-phthalic acid. The filtrate was left for slow evaporation at room temperature.
After a few days, a brown coloured compound was collected and it was dissolved in DMF and
filtered. Brown crystals suitable for X-ray analysis were obtained after 4 weeks. Yield: 0.511
g, 60%. Anal. Calcd. For C H Co N O (1704.77): C, 46.49; H, 5.46; N, 4.92 %.
31.20
6
6
92.40
4
6
–
1
Found: C, 46.51; H, 5.44; N, 4.95%.FT-IR (KBr, cm ): 3428 (br), 2983 (s), 2946 (w), 2151
(
w), 1642 (vs), 1551 (vs), 1467 (vs), 1411 (vs), 1373 (w), 1300 (vs), 1245 (s), 1219 (w), 1170
w), 1138 (w), 1076 (vs), 1014 (s), 979 (s), 882 (w), 729 (w), 632 (w). UV-Vis spectra [λmax
(
,
–
1
–1
4
4
3
nm (ε, L mol cm )] (CH OH solution) 250 (2.37 × 10 ), 312 (1.26 × 10 ), 398 (2.40 × 10 ).
3
CCDC numbers are 1518364 and 1518365 (for complexes 1 and 2, respectively) contain the
supplementary crystallographic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information. Results of FT-IR, electronic absorption and mass spectral study,
Figures and Tables for magnetic analysis are provided as supplementary material.
Acknowledgements
1
8
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European Journal of Inorganic Chemistry
10.1002/ejic.201700046
The authors gratefully acknowledge the financial assistance given by the CSIR, Government
of India, to Dr. Subal Chandra Manna (Project No.01 (2743) / 13 / EMR-II). J. V. thanks the
Ministerio de Economía y Competitividad for predoctoral contract. Financial support by the
Spanish MICINN (Project CTQ2013-44844-P) and the Generalitat Valenciana (Projects
PROMETEOII/2014/070 and ISIC/2012/002).
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3748; (b) C. H. Woodall, G. A. Craig, A. Prescimone, M. Misek, J. Cano, J. Faus, M. R.
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European Journal of Inorganic Chemistry
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Lett. 1995, 242, 652-660; (c) K. Eichkorn, F. Weigend, O. Treutler, H.Ohm and R. Ahlrichs,
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2
5
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European Journal of Inorganic Chemistry
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Table 1. Coordination bond lengths (Å) and angles (°) for complexes 1-2.
1
1.889(3)
1.887(3)
1.885(3)
1.899(3)
1.915(2)
1.894(3)
2.05(2)
2
Co(1)-N(1a)
Co(1)-N(1b)
Co(1)-O(1a)
Co(1)-O(1b)
Co(1)-O(2a)
Co(1)-O(2b)
Co(2)-O(1)
1.8861(15)
1.8899(15)
1.8749(12)
1.9005(13)
1.9123(12)
1.8870(12)
2.0213(14)
2.0379(13)
2.0235(12)
2.1971(14)
2.1895(14)
2.0904(15)
-
Co(2)-O(2a)
Co(2)-O(2b)
Co(2)-O(3a)
Co(2)-O(3b)
Co(2)-O(1w)
Co(2)-O(2w)'
Co(1)-Co(2)
2.028(3)
2.050(3)
2.192(3)
2.155(3)
2.078(3)
2.06(2)
2.9847(7)
2.9654(3)
N(1a)-Co(1)-N(1b)
O(1a)-Co(1)-O(2a)
O(1b)-Co(1)-O(2b)
O(3a)-Co(2)-O(3b)
177.43(13)
175.19(11)
176.11(11)
176.75(12)
176.69(6)
176.35(5)
175.52(6)
177.72(6)
173.69(6)
170.38(5)
−
O(2b)-Co(2)-O(1w) 179.74(11)
O(2a)-Co(2)-O(1) 162.4(4)
O(2a)-Co(2)-O(2w)' 173.5(4)
Co(1)-O(2a)-Co(2)
Co(1)-O(2b)-Co(2)
98.36(11)
98.30(11)
97.25(5)
98.57(5)
Symmetry code for O(2w)' in 1 = −x+1, −y+1, −z+1.
2
6
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European Journal of Inorganic Chemistry
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Table 2. Selected ac magnetic data for 1 at different dc applied fields.
a
b
b
b
b
c
c
c
H (G)
 
Ea
A
n
α
χS
3
χT (temp)
0
−1
3
−1
(
cm mol ) (cm mol )
0
.07
0.26
0.24
0.20
0.74 (5.0 K)
5
00
4.29
2.81
7.71
16.4
19.0
15.3
2019
1682
1341
1.08
1.19
1.05
0.02
0.62 (6.0 K)
0.54 (7.0 K)
0
0
.05
.12
0.10
0.10
0.10
0.72
0.62
0.54
(5.0 K)
(6.0 K)
(7.0 K)
1
2
000
500
0.07
.04
0
0
.08
0.09
.06
0.04
0.04
0.02
0.72
(5.0 K)
0.61 (6.0 K)
0.52 (7.0 K)
0
a
b
Applied dc magnetic field. The values of the pre-exponential factor (τ ) and activation
0
–
1
–1
n c
energy (E ) are calculated through the Arrhenius law [ =  exp(–E /k T) + A T ]. The
a
0
a
B
values of the  parameter, adiabatic ( ) and isothermal ( ) susceptibilities are calculated
S
T
from the experimental data through the generalized Debye law (see text).
2
7
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European Journal of Inorganic Chemistry
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Table 3. Selected ac magnetic data for 2 at different dc applied fields.
a
b
7
b
b
b
c
c
c
H (G)   10
Ea
A
n
α
χS
χT
(temp)
0
3
−1
3
−1
(
cm mol ) (cm mol )
0
.11
0.20
0.18
0.16
0.59
0.52
0.46
(6.0 K)
5
00
4.61
0.82
0.50
22.3 481.0 2.34
32.3 227.5 2.67
0.09
(7.0 K)
(8.0 K)
0
0
.06
.12
0.09
0.09
0.08
0.60 (6.0 K)
0.52 (7.0 K)
1
2
000
500
0.09
0
.07
0.46 (8.0 K)
0
.05
0.05
0.05
0.05
0.59 (6.0 K)
0.51 (7.0 K)
0.45 (8.0 K)
35.1 100.6 3.08
0.08
.05
0
a
b
Applied dc magnetic field. The values of the pre-exponential factor (τ ) and activation
0
–
1
–1
n c
energy (E ) are calculated through the Arrhenius law [ =  exp(–E /k T) + A T ]. The
a
0
a
B
values of the  parameter, adiabatic ( ) and isothermal ( ) susceptibilities are calculated
S
T
from the experimental data through the generalized Debye law (see text).
2
8
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European Journal of Inorganic Chemistry
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Table 4. Crystal Data and Structure Refinement for complexes 1-2.
Complex
1
2
Empirical formula
C H Co N O
C H Co N O
56
86
4
4
35
66 92.40
1704.77
Triclinic
4
6
31.20
–
1
Formula mass, g mol
Crystal system
Space group
a, Å
1611.00
Triclinic
P
1
P
1
9.0902(4)
10.2821(4)
19.6120(8)
97.720(3)
99.862(3)
105.991(4)
1703.91(12)
1
8.9043(2)
10.0665(2)
21.2197(5)
77.346(2)
80.303(2)
77.212(2)
1795.36(7)
1
b, Å
c, Å
α, deg
β, deg
γ, deg
3
V, Å
Z
–
3
D(calcd), g cm
(Mo-K), mm
F(000)
Theta range, deg
1.570
1.577
–
1

1.054
838
1.002
888
2.84 - 27.88
26400
1.98 - 30.51
57856
No. of collected data
No. of unique data
Rint
7878
0.0515
10969
0.0454
9262
Observed reflections [I> 2ζ(I)] 5506
Parameters refined
Goodness of fit (F )
553
1.030
555
1.039
2
[
a]
R1, wR2 (I >2ζ(I))
R1, wR2 (all data)
0.0607, 0.1397
0.0933, 0.1616
1.649, –0.566
0.0405, 0.0996
0.0506, 0.1055
0.776, –0.535
–
3
Residuals, e Å
[
a]
2
2
2
2
2 2 ½
R1(Fo) = Fo–Fc / Fo, wR2(Fo ) = [w (Fo – Fc ) / w (Fo ) ]
2
9
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