Cobalt(II) Ions Connecting [CoII4] Helicates into a 2-D Coordination Polymer Showing Slow Relaxation of the Magnetization

Article abstract of DOI:10.1021/acs.inorgchem.7b01640

The reactions of cobalt(II) perchlorate with a diazine tetratopic helicand, H₄L, in the presence of sodium carbonate afford two coordination polymers constructed from tetranuclear anionic helicates as building blocks: _∞³[Co₄L₃Na₄(H₂O)₄]·4H₂O (1) and _∞²[Co₅L₃Na₂(H₂O)₉]·2.7H₂O·DMF (2). The tetranuclear triple-stranded helicates, {Co^II₄L₃}^4-, are connected in 1 by sodium(I) ions and in 2 by sodium(I) and cobalt(II) ions (H₄L results from the condensation reaction between 3-formylsalicylic acid and hydrazine). The crystal structures of the two compounds have been solved. In both compounds the anionic helicates interact with the assembling cations through the carboxylato oxygen atoms. Compound 2 features chains resulting from connecting the tetranuclear helicates through cobalt(II) ions. The analysis of the magnetic properties of compounds 1 and 2 evidenced a dominant antiferromagnetic coupling for 1, resulting in a diamagnetic ground state. In contrast, the magnetic behavior of 2 is dominated at low temperature by the Co^II ion which connects the antiferromagnetically coupled {Co^II₄} helical moieties. The ac magnetic measurements for 2 reveal the occurrence of slow relaxation of the magnetization that is due to the single, uncorrelated cobalt(II) ions, which are diluted in an essentially diamagnetic matrix of {Co^II₄} moieties (δE_eff = 26.7 ± 0.3 cm^-1 with τ₀ = (2.3 ± 0.2) × 10^-6 s).

Full text of DOI:10.1021/acs.inorgchem.7b01640

Article
pubs.acs.org/IC
Cobalt(II) Ions Connecting [CoII ] Helicates into a 2‑D Coordination
Polymer Showing Slow Relaxat4ion of the Magnetization
Paula Cucos,† Lorenzo Sorace,*,‡ Catalin Maxim,§ Sergiu Shova,∥ Delia Patroi,⊥ Andrea Caneschi,‡
and Marius Andruh*,§
†Ilie Murgulescu-Institute of Physical Chemistry of the Romanian Academy, Splaiul Independentei no. 202, 060021 Bucharest,
Romania
‡Department of Chemistry “U. Schiff” and INSTM RU, University of Florence, Via della Lastruccia 3, 50019 Florence, Italy
§University of Bucharest, Faculty of Chemistry, Inorganic Chemistry Laboratory, Str. Dumbrava Rosie no. 23, 020464 Bucharest,
Romania
∥Petru Poni, Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A, RO-700487 Iasi, Romania
⊥National Institute for R&D in Electrical Engineering INCDIE ICPE-CA, Splaiul Unirii no. 313, 030138 Bucharest, Romania
S
* Supporting Information
ABSTRACT: The reactions of cobalt(II) perchlorate with a diazine
tetratopic helicand, H4L, in the presence of sodium carbonate afford two
coordination polymers constructed from tetranuclear anionic helicates as
building blocks:
3 [Co4 L3 Na4 (H2 O)4 ]·4H2 O (1) and
∞
2[Co5L3Na2(H2O)9]·2.7H2O·DMF (2). The tetranuclear triple-
∞
stranded helicates, {CoII L3}4−, are connected in 1 by sodium(I) ions
4
and in 2 by sodium(I) and cobalt(II) ions (H4L results from the
condensation reaction between 3-formylsalicylic acid and hydrazine).
The crystal structures of the two compounds have been solved. In both
compounds the anionic helicates interact with the assembling cations
through the carboxylato oxygen atoms. Compound 2 features chains
resulting from connecting the tetranuclear helicates through cobalt(II)
ions. The analysis of the magnetic properties of compounds 1 and 2
evidenced a dominant antiferromagnetic coupling for 1, resulting in a diamagnetic ground state. In contrast, the magnetic
behavior of 2 is dominated at low temperature by the CoII ion which connects the antiferromagnetically coupled {CoII } helical
4
moieties. The ac magnetic measurements for 2 reveal the occurrence of slow relaxation of the magnetization that is due to the
single, uncorrelated cobalt(II) ions, which are diluted in an essentially diamagnetic matrix of {CoII } moieties (ΔEeff = 26.7 0.3
4
cm−1 with τ0 = (2.3 0.2) × 10−6 s).
An easily accessible class of helicands is represented by diazine
molecules, which are obtained from condensation reactions
between keto precursors and hydrazine.29−32 Such a ligand was
obtained by us from 3-formylsalicylic acid and hydrazine (H4L)33
and acts as a heterotetratopic helicand. That is, it contains four
binding domains: two with only O donor atoms and two others
with N and O donor atoms. The different coordination sites also
make this ligand appropriate for obtaining heterometallic
helicates. Indeed, the reaction of H4L, cobalt(II) perchlorate,
iron(III) perchlorate, and sodium carbonate afforded triple-
INTRODUCTION
■
The helicates represent an important chapter in metal-
losupramolecular chemistry.1−10 Their rational design consists
of metal-directed self-assembly processes using programmed
ligands (helicands), which are linear strands with repeating
complexation sites, separated by spacers characterized by a
limited flexibility.11,12 The helicands are twisted as a result of the
interaction with the metal ions. Beyond their beauty, the
metallohelicates display exciting physical properties, such as
luminescence13−15 and magnetism.16−24 In the field of crystal
engineering, numerous coordination polymers featuring heli-
coidal chains have been described.25 On the other hand, the
construction of coordination polymers using discrete helicates as
building blocks is limited to a few examples.26−28 In principle, the
helicates can be incorporated into a coordination polymer in two
ways: (a) through additional metal ions, that are coordinated by
the potentially bridging groups of anionic helicates, and (b)
through additional anions which bridge the terminal metal ions
of cationic helicates.
stranded tetranuclear anionic helicates, [L3CoII FeIII2]2−, which
2
are connected through sodium ions, resulting in infinite chiral
chains. In other words, the discrete anionic helicates can generate
coordination polymers if they are linked by another metal ion,
which is coordinated by the peripheral carboxylato oxygen
atoms. This result prompted us to check the possibility of
Received: June 27, 2017
© XXXX American Chemical Society
A
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Table 1. Crystallographic Data and Details of Data Collection and Structure Refinement Parameters
1
2
chem formula
formula wt (g/mol)
cryst syst
C48H40Co4N6Na4O26
1444.55
C51H54.4Co5N7Na2O30.7
1597.25
orthorhombic
Abm2
monoclinic
P21/n
space group
a (Å)
17.9377(3)
33.2486(4)
9.8599(1)
90.00
21.1009(11)
12.3163(12)
23.047(2)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
90.00
102.64(6)
90.00
γ (deg)
90.00
V (Å3), Z
temp (K)
calcd density (Mg m−3
5880.48(14), 8
293(2)
5844.9(8), 4
160
)
1.613
1.815
abs coeff (mm−1
)
1.227
1.508
no. of rflns collected
no. of indep rflns
21223
27152
5198 (Rint = 0.065)
5198/1/404
9960 (Rint = 0.1812)
9960/30/867
no. of data/restraints/params
final R indices (I > 2σ(I))
R1
0.0764
0.2006
0.0797
0.1121
wR2
R indices (all data)
R1
0.1031
0.1917
wR2
0.2316
0.1495
goodness of fit on F2
1.013
0.933
Δρmax and Δρmin (e Å−3
)
1.239 and −0.775
0.774 and −0.626
connecting the anionic helicates with paramagnetic metal ions, in
order to obtain extended networks with interesting magnetic
properties. Among these, we mention the possibility of observing
slow relaxation of the magnetization and then magnetic
bistability, as a consequence of either 1D Ising type magnetic
behavior (single-chain magnetism)34 or single-molecule magnet-
ism due to the anisotropy of a single ion, resulting in a barrier to
magnetization reversal, often termed single ion magnet
(SIM).35,36 Most of the helicates featuring the latter behavior
are based on strongly anisotropic lanthanide cations.13,14 In this
respect, it is tempting to try using CoII, a largely anisotropic ion,
as a building unit for helicate-based SIMs. We had some success
along this line by including tetrahedral CoII in o-vanillin-based
helicates,15 resulting in a dinuclear helicate showing slow
relaxation of the magnetization in applied field. On the other
hand, the use of octahedral CoII to obtain SIM helicates has not
been reported to date. Given the large anisotropy of this ion, this
might appear surprising at first sight. However, one should
consider that for octahedral CoII slow relaxation dynamics only
arises as a consequence of a subtle interplay of ligand field and
hyperfine and dipolar interactions.37 As such, the actual
mechanism of relaxation is often not easily determined, and
slow dynamics has been observed for systems characterized by
very different anisotropic features, ranging from easy plane to
rhombic and easy axis.38−46
and sodium carbonate to deprotonate the ligand.33 For the molar
ratio L4−:CoII = 3:4, we obtained a 3-D coordination polymer,
constructed from {CoII } anionic helicates, connected by sodium
4
ions: 3[Co4L3Na4(H2O)4]·4H2O (1). By increasing the
∞
amount of cobalt(II) ions, namely to a 3:5 L4−:CoII molar
ratio, we succeeded in connecting the tetranuclear helicates
through paramagnetic cobalt(II) ions, each cobalt ion replacing
two sodium ions: ∞2[Co5L3Na2(H2O)9]·2.7H2O·DMF (2).
EXPERIMENTAL SECTION
■
Materials. Reactions were carried out under a normal atmosphere.
The chemicals used were of reagent grade and were purchased from
commercial sources. H4L was prepared as previously described.33
Syntheses. 3[Co4L3Na4(H2O)4]·4H2O (1). An aqueous solution of
∞
Na2CO3·6H2O (0.171 g, 0.6 mmol) was added to a stirred suspension of
H4L (0.098 g, 0.3 mmol) in DMF (10 mL). The resulting yellow
solution was reacted with a solution of Co(ClO4)2·6H2O (0.146 g, 0.4
mmol) in a minimum amount of DMF. An excess of sodium carbonate
has been employed, in order to dissolve the helicand. The slow
evaporation of a reddish orange reaction mixture, at room temperature,
afforded red single crystals which were filtered, washed with ethanol, and
allowed to dry in air. Yield: 75% based on H4L. Anal. Calcd for
C48H40Co4N6Na4O26: C, 39.91; H, 2.79; N, 5.82. Found: C, 40.73; H,
2.39; N, 6.75. IR data (KBr, cm−1): 3434 (m), 1659 (m), 1600 (vs), 1549
(vs), 1457 (s), 1433 (s), 1378 (s), 1305 (m), 1226 (m), 879 (w), 764
(w), 660 (w). UV−vis−NIR diffuse reflectance spectrum (nm): 250
(sh), 450, 600 (sh), 1100 (Figure S1 in the Supporting Information).
2[Co5L3Na2(H2O)9]·2.7H2O·DMF (2). An aqueous solution of
Finally, the design of a chiral paramagnetic system is in
principle of peculiar interest to study the interplay between
chirality and magnetism, which are directly connected through
magneto-chiral dichroism (MχD), a phenomenon which arises
from the coexistence of spatial asymmetry and magnetization in a
material.47,48
In this paper, we report on the synthesis and structural and
magnetic characterization of two new octahedral CoII-based
helicates, obtained by using H4L, cobalt(II) assembling cations,
∞
Na2CO3·6H2O (0.171 g, 0.6 mmol) was added to a stirred suspension
of H4L (0.098 g, 0.3 mmol) in a DMF/H2O (2/1) mixture (10 mL). A
yellow solution was formed and further reacted with a solution of
Co(ClO4)2·6H2O (0.183 g, 0.5 mmol) in a minimum amount of water.
The slow evaporation of the resulting solution, at room temperature,
afforded red single crystals which were isolated by filtration, washed with
water (to remove the possible traces of water-soluble complex 1) and
isopropyl alcohol, and allowed to air-dry. Yield: 70% based on H4L. Anal.
Calcd for C51H54.4Co5N7Na2O30.7: C, 38.35; H, 3.43; N, 6.14. Found: C,
B
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38.81; H, 3.91; N, 6.31. IR data (KBr, cm−1): 3363 (m), 1656 (m), 1600
(vs), 1548 (vs), 1455 (s), 1432 (s), 1378 (s), 1300 (m), 1225 (m), 879
(w), 763 (w), 661 (w). UV−vis−NIR diffuse reflectance spectrum
(nm): 250 (sh), 460, 600 (sh), 1010 (Figure S1 in the Supporting
Information). The number of crystallization solvent molecules has been
confirmed by thermal analysis (Figure S2 in the Supporting
Information).
Physical Measurements. The IR spectra of 1 and 2 were recorded
on a FTIR Bruker Tensor V-37 spectrophotometer as KBr pellets in the
4000−400 cm−1 range at room temperature. The UV−vis−NIR spectra
were recorded, in the solid state, with a V-670 Jasco spectrophotometer
in the 2000−200 nm range. dc magnetic characterization of 1 and 2 was
performed by using a MPMS Quantum Design SQUID magnetometer,
applying a field of 1 kOe from 2 to 40 K and of 10 kOe from 30 to 300 K.
Raw magnetic data were corrected for the sample holder and
diamagnetic contributions to extract the paramagnetic molar magnet-
ization as a function of field and temperature. The susceptibility is then
evaluated as the ratio χM = MM/Hdc, where Hdc is the applied magnetic
field. Dynamic magnetic data were collected by using the ac
measurement system option of a PPMS Quantum Design setup.
Crystallographic Data Collection and Structure Determina-
tion. Details about data collection and solution refinement are given in
Table 1. X-ray diffraction measurements were performed on a STOE
IPDS II diffractometer operating with a Mo Kα (λ = 0.71073 Å) X-ray
tube with graphite monochromator for complex 1 and on an Xcalibur
Eos diffractometer operating with a Mo Kα (λ = 0.71073 Å) X-ray tube
with graphite monochromator for complex 2. The structures were
solved by direct methods using SHELXS-97 and refined by full-matrix
least squares on F2 with SHELXL-9749 with anisotropic displacement
parameters for non-hydrogen atoms, except those with partial sof. All H
atoms attached to carbon were introduced in idealized positions using
the riding model with their isotropic displacement parameters fixed at
120% of the value for their riding atom. Positional parameters of the H
atoms attached to coordinated water molecules were obtained from
difference Fourier syntheses and verified by the geometric parameters of
the corresponding hydrogen bonds. For compound 1 the structure has
been refined as two component twin (twin law: −100/010/00−1) in
space group Abm2. Crystallographic data for the structures have been
deposited in the Cambridge Crystallographic Data Centre, deposition
numbers CCDC 1557267 for 1 and CCDC 1557728 for 2. These data
can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Powder X-ray
diffraction (PXRD) measurements were carried out on a Bruker D8
Discover instrument (Cu Kα1 radiation, λ ≈ 1.5418 Å) equipped with a
Lynxeye 1D detector in parallel beam geometry. The samples were
mounted on a quartz low-background sample holder and scanned in the
range 5−50° (2θ) with a step size of 0.04°(2θ) and a dwell time of 1 s/
step.
Figure 1. View of the tetranuclear triple-stranded tetraanionic helicate in
1. Symmetry codes: i = −2 − x, −y, z.
distorted-octahedral geometry. The distances between the cobalt
ions are Co1···Co2 = 2.929 Å and Co2···Co2i = 3.586 Å. The
sodium ions are bridged by the carboxylato oxygen atoms and by
water molecules (Figure S5 in the Supporting Information),
forming layers paralleling the ac plane. The 3-D architecture of
the crystal can be described as being formed by parallel layers
made by sodium ions, connected by {Co4} pillars of opposite
chiralities (Figure 2).
Compound 2 illustrates that the anionic helicates can be
connected by paramagnetic metal ions as well, resulting in
coordination polymers, which are expected to show magnetic
properties more complex than those of the isolated helicates. The
triple-stranded helicate unit is similar to that observed in crystal
1, but in this case the four cobalt ions are crystallographically
independent (Figure 3). The fifth cobalt ion (Co5) connects two
helicates, interacting with the oxygen atoms from two
carboxylate groups belonging to two helicates with opposite
chiralities (Figure 4), resulting in infinite chains. The distances
between the cobalt ions are as follows: Co1···Co2 = 2.918 Å,
Co2···Co3 = 3.601 Å, Co3···Co4 = 2.887 Å, Co4···Co5 = 3.876
Å, Co5···Co1i = 5.348 Å (i = 0.5 + x, 0.5 − y, −0.5 + z). The Co5
ion shows a distorted-octahedral geometry: it is coordinated by
one carboxylato oxygen from a group bridging Co5 to Co1 (syn-
anti bridging mode), by a chelating carboxylate group that
connects Co5 to Co1i (monatomic bridging), and by three aqua
ligands (two of them make a double bridge between Co5 and
Na1). These chains are then connected by the chains made by
the sodium ions. Layers running along the crystallographic b axes
are thus formed (Figure 5). None of the two crystallographically
independent sodium ions interact directly with the oxygen atoms
of the helicates. They are bridged solely by aqua ligands. The Na1
ions are also doubly bridged by aqua ligands to the Co5 ions
(Figure S6 in the Supporting Information). Selected bond
distances and angles for compounds 1 and 2 are collected in
Table S1 in the Supporting Information.
RESULTS AND DISCUSSION
■
Description of Structures. The crystal structures of the two
compounds have been solved. Since the two compounds have
been obtained from the same starting materials, we checked their
purity by PXRD. The two diffractograms, compared with the
simulated measurements (Figures S3 and S4 in the Supporting
Information), confirm the purity of the crystalline phases.
Let us discuss briefly the structure of 1. It consists of
tetranuclear triple-stranded tetraanionic helicates (Figure 1),
connected through sodium ions. Within each anionic entity the
three ligands are twisted along the central N−N single bonds,
resulting in four compartments hosting the cobalt(II) ions. The
peripheral cobalt ions (Co1, Co1i, i = −2 − x, −y, z) are
coordinated by three carboxylato and three phenoxido oxygen
atoms (bond lengths varying between 2.037(10) and 2.120(10)
Å), with a distorted-trigonal-prismatic geometry. The inner
cobalt ions (Co2, Co2i, i = −2 − x, −y, z) are coordinated by
three phenoxido oxygens (2.050(10)−2.083(10) Å) and by
three nitrogen atoms (2.109(12)−2.135(12) Å), with a
Magnetic Properties. Static Magnetic Properties. The
room-temperature value of the χMT product for compound 1
(9.2 cm3 mol−1 K) falls at the lower end of the expected range for
four noncorrelated cobalt(II) ions, characterized by a non-
negligible unquenched angular momentum: this suggests a
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Figure 2. (left) Crystal packing of 1, viewed along the a crystallographic axis. (right) Representation of a layer constructed of [Co4L3]4− helicates,
superimposed over a layer of sodium(I) ions, along the b crystallographic axis. Color codes: cyan, P helicity; red, M helicity.
indeed clearly evidenced by both the χM vs T curve and the
isothermal M vs H curve measured at low temperature (Figures
S7−S9 in the Supporting Information). The latter could be fit by
assuming a Brillouin contribution for a species with an effective
Seff = 1/2 (due to the impurity) and the temperature-
independent paramagnetism contribution due to the four CoII
centers of the cluster, which accounts for the high-field linear
increase of the isothermal magnetization curves. The best-fit
parameters provided giso(impurity) = 4.29 0.07%IMP = 0.136
0.002 and χTIP = (5.3 0.6) × 10−2 cm3 mol−1. These parameters
strongly suggest that the paramagnetic impurity is due to a
monomeric CoII-containing species, possibly due to some cluster
fragmentation occurring during the synthetic procedure. The
estimated contribution of the paramagnetic impurity was then
subtracted from both the M vs H curve and the temperature
dependence of the susceptibility (corrected data are reported in
Figure 6) before attempting a fit of the data.
A quantitative analysis of the observed magnetic behavior is
nonetheless far from trivial in this case. We have in principle to
consider two structurally different cobalt centers, for each of
which at least three single-ion parameters have to be defined
(spin−orbit coupling, axial distortion, orbital reduction factor).
Further, two different exchange coupling constants are expected,
across phenoxido and diazine bridges, respectively. This makes a
total of eight parameters at least and, further, a dimension of the
Figure 3. (a) View of {Co5} repeating unit in 2. Symmetry codes: i = 0.5
+ x, 0.5 − y, −0.5 + z.
problem which is computationally expensive, as we have to
consider both spin and orbital degrees of freedom (124 × 124).
Finally, the literature on magnetostructural correlations for CoII
exchange coupled systems is extremely scarce, due to the ease of
parametrization and complexity of the treatment required for a
quantitative analysis of magnetic data of CoII centers. However,
the few reports on systems showing structural features similar to
those of 1 consistently point to weak antiferromagnetic
interactions mediated by both triple diazine33,51,52 and
phenoxido bridges.53,54
relatively large distortion from purely octahedral geometry.50 On
lowering the temperature (see Figure 6 and Figure S6 in the
Supporting Information) χMT decreases gradually until 150 K
and then more markedly, to reach 0.36 cm3 mol−1 K at 1.8 K.
While the high-temperature decrease can be traced back to
progressive depopulation of the excited doublets arising from the
combined effect of low-symmetry distortions and spin−orbit
4
coupling on the T1g state of octahedral CoII, the low-
To reduce the overparametrization of the problem, we then
considered a common set of single-ion parameters for the two
crystallographically independent cobalt(II) centers and further
assumed the diazine transmitted coupling to be negligible: this
amounts to consideration of the system as the sum of two
antiferromagnetically coupled dimers with no intradimer
temperature decrease strongly hints at the presence of
antiferromagnetic exchange coupling interactions mediated by
the phenoxido and diazine bridges. Since the ground state for
four linearly arranged antiferromagnetically coupled CoII ions is
diamagnetic, the nonzero value of the χMT product at 1.8 K can
be attributed to the presence of a paramagnetic impurity, which is
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Figure 4. Perspective view of the chains resulting from connecting {Co4} anionic helicates by cobalt(II) ions in 2. Color codes: cyan, P helicity; red, M
helicity. Symmetry codes: i = −0.5 + x, 0.5 − y, 0.5 + z; ii = 0.5 + x, 0.5 − y, −0.5 + z.
Figure 5. Crystal packing of 2, viewed from two different directions. Color codes: cyan, P helicity; red, M helicity (left). Schematic view of the {Co5}n
chains (purple) and of the chains constructed from sodium ions (yellow) -right.
interaction. While this is clearly a simplification, it should provide
an upper bound on the exchange coupling transmitted by the
phenoxido bridges. The Hamiltonian employed for fitting the
data55 was then
2
2
2
z2
̂
̂
̂
̂
H = βH[ge ∑ Si − α ∑ S′i] + Δ ∑ (S′i − 2/3)
i=1
i=1
i=1
2
̂
̂
̂
̂
− αλ ∑ SiS′i − 2JS1S2
(1)
i=1
where Si = 3/2 is the spin of CoII ions, Si′ = 1 is the fictitious spin
by which the orbital angular momentum L = 1 of the 4T1g state of
CoII is modeled, α takes into account both the orbital reduction
factor due to covalency and the T−P isomorphism, λ is the
multielectron spin−orbit coupling constant, Δ the axial
distortion parameter, and J is the isotropic exchange coupling
Figure 6. χMT vs T plot for 1 (empty squares) and 2 (full circles). Data
of 1 have been corrected for the paramagnetic impurity present in the
sample (see text for details). The continuous line is the best fit to the
data using the dimer model and parameters reported in the text.
E
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Figure 7. (a) Temperature dependence of χ″(ν) of 2 and best fit to Debye equation. (b) Corresponding Arrhenius plot using τ values extracted by a
Debye fit of the χ″(ν) curves and best linear fit with parameters reported in the text.
constant between the isotropic S = 3/2 spins.50 This provided as
a best fit the following parameters: Δ = −110 10 cm−1, λ =
−230 20 cm−1, α = 1.04 0.02, and J = −4.60 0.03 cm−1. It is
worth noting that Δ, λ, and α values are greatly correlated and the
obtained values should then be considered with much caution. At
any rate, the obtained picture points to a ground pseudodoublet,
split by about 2.0 cm−1, with a first excited doublet ca. 40 cm−1
above it (see Table S2 in the Supporting Information). The
obtained value for the phenoxido-transmitted coupling is quite
uncorrelated Co5 center, which is already quite well diluted in an
essentially diamagnetic matrix of Co4 moieties. This makes
dipolar-induced relaxation in zero field (i.e., quantum tunneling
type processes) very inefficient, allowing the observation of
dynamic behavior even in zero field (Figure S11). At the same
time, the constancy of the observed rate with field suggests that
the direct process is not playing a relevant role in the relaxation.
The fit of the frequency dependence of the ac susceptibility
measured at variable temperatures using the Debye equation58
allowed us to derive the thermal dependence of the relaxation
rate and its distribution, α, which decreases from 0.3 at 3.6 K to
0.05 at 9.8 K. This provided a linear Arrhenius plot, as expected
for a thermally activated Orbach process (Figure 7b). The fit
afforded an effective energy barrier for the reversal of the
magnetization, ΔEeff/k = 38.4 0.5 K (ΔEeff = 26.7 0.3 cm−1)
with τ0 = (2.3 0.2) × 10−6 s. We note here that the obtained
value for the effective barrier might not reflect the actual
existence of a real state at that energy,59 since it is much lower
than expected for a CoII center. This is a point often reported for
octahedral CoII-based complexes60−62 and prompted us to look
for the possible presence of other relaxation by including a
dominant Raman term, in combination with Orbach or quantum
tunneling processes (despite the fact that field-dependent
dynamics would tend to exclude this). None of these provided
fits as good as the Orbach fit (Figure S12 in the Supporting
Information), which is then favored by this analysis. The
apparent discrepancy, often reported for single-molecule
magnets, between the experimentally determined activation
energy and the energy of the first excited state has been recently
theoretically explained for a model S = 1 system by considering
the role of anharmonic phonons. In particular, it was shown that
“the finite line width of the phonons spectral distribution allows an
Orbach-type spin relaxation mechanism even when the phonon is not
resonant at the spin excitation”.63 While further spectroscopic and
theoretical investigation would be necessary to confirm the exact
mechanism by which slow relaxation occurs in 2, it is tempting to
assign the observation of a linear Arrhenius plot with ΔEeff largely
different from the energy of the 1s excited state to such a
phenomenon.
̀
close to those reported by Aromi and co-workers for structurally
similar systems, thus lending some support to the obtained
values.56
The temperature dependence of the χMT product for
compound 2 is also shown in Figure 6. Even in this case the
room-temperature value (11.8 cm3 mol−1 K) falls at the lower
end of the expected range for five uncorrelated CoII ions. Upon
cooling, χMT decreases gradually to 150 K and then more
abruptly, to reach 1.3 cm3 mol−1 K at 1.8 K. The nonzero value at
low temperature is consistent with the odd number of Kramers
ions per molecule, which can only give rise to a magnetic ground
state, even in the presence of antiferromagnetic interactions. This
is further confirmed by the M vs H curve, which is consistent with
expectation for a single CoII center at low temperature. This
behavior can be qualitatively explained by considering the strict
similarity of the structures of the magnetic cores of 1 and 2, which
is only broken by the extra CoII ion (Co5) in the latter. At low
temperature, the Co4 subunit of 2 is expected, on the basis of the
results of 1, to be in a diamagnetic ground state. If the interaction
of Co5 with the Co4 unit, mediated by the syn-anti carboxylato
bridge, is weaker than those mediated by diazine and phenoxido
bridges, the low-temperature magnetic properties of 2 are
expected to be essentially determined by those of the Co5 center.
In this respect it is however worth noting that the carboxylato
bridge is expected to transmit a ferromagnetic interaction of
magnitude comparable to the antiferromagnetic ones observed in
the Co4 unit, thus making a simple analysis based on the
properties of Co5 only, unfeasible.57
Dynamic Magnetic Properties. Dynamic magnetic
susceptibility was measured for both samples in zero and applied
field (Figure 7 and Figures S10 and S11 in the Supporting
Information). While no χ′′ signal was observed for 1, as expected
on the basis of the diamagnetic nature of its ground state, a clear
frequency-dependent signal was observed for 2. No relevant field
dependence could be observed for the dynamic behavior of 2 at 2
K (Figure S10). These observations may be traced back to the
fact that the observed dynamic behavior is due to the single,
CONCLUSIONS
■
In conclusion, we have shown that anionic helicates with
potentially bridging groups can be employed as building blocks in
constructing heterometallic coordination polymers. The two
compounds described herein have been obtained from the same
starting materials, but with different CoII:helicand molar ratios.
F
DOI: 10.1021/acs.inorgchem.7b01640
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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4
metalloligand toward both sodium and cobalt ions. This
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4
the slow relaxation of the magnetization for the single CoII ions,
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ASSOCIATED CONTENT
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(13) Habib, F.; Murugesu, M. Lessons learned from dinuclear
lanthanide nano-magnets. Chem. Soc. Rev. 2013, 42, 3278−3288.
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Gheorghe, R.; Caneschi, A.; Hillebrand, M.; Andruh, M. Magnetic and
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S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01640.
Diffuse reflectance spectra, selected bond distances and
angles, thermal analysis, PXRD data, packing diagrams,
molar magnetic susceptibility vs temperature data,
magnetization vs field data, and energy levels (PDF)
(16) Floquet, S.; Ouali, N.; Bocquet, B.; Bernardinelli, G.; Imbert, D.;
Bunzli, J. C. G.; Hopfgartner, G.; Piguet, C. The first self-assembled
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trimetallic lanthanide helicates driven by positive cooperativity. Chem. -
Eur. J. 2003, 9, 1860−1875.
Accession Codes
CCDC 1557267 and 1557728 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(17) Albrecht, M. Homodinuclear f−f and heterodinuclear f−p
lanthanide helicates. Z. Anorg. Allg. Chem. 2010, 636, 2198−2204.
(18) Albrecht, M.; Osetska, O.; Bunzli, J. C. G.; Gumy, F.; Frohlich, R.
̈
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Homo- and heterodinuclear helicates of Lanthanide(III), Zinc(II) and
Aluminium(III) based on 8-hydroxyquinoline ligands. Chem. - Eur. J.
2009, 15, 8791−8799.
(19) Vandevyver, C. D. B.; Chauvin, A.-S.; Comby, S.; Bunzli, J. C. G.
̈
Luminescent lanthanide bimetallic triple-stranded helicates as potential
AUTHOR INFORMATION
■
cellular imaging probes. Chem. Commun. 2007, 1716−1718.
Corresponding Authors
*E-mail for L.S.: lorenzo.sorace@unifi.it.
*E-mail for M.A.: marius.andruh@dnt.ro.
́
(20) Fernandez-Moreira, V.; Song, B.; Sivagnanam, V.; Chauvin, A.-S.;
Vandevyver, C. D. B.; Gijs, M.; Hemmila, I.; Lehr, H.-A.; Bunzli, J. C. G.
̈
̈
Bioconjugated lanthanide luminescent helicates as multilabels for lab-
on-a-chip detection of cancer biomarkers. Analyst 2010, 135, 42−52.
(21) Chauvin, A.-S.; Comby, S.; Baud, M.; De Piano, C.; Duhot, C.;
ORCID
Lorenzo Sorace: 0000-0003-4785-1331
Marius Andruh: 0000-0001-8224-4866
Notes
Bunzli, J. C. G. Luminescent lanthanide helicates self-assembled from
̈
ditopic ligands bearing phosphonic acid or phosphoester units. Inorg.
Chem. 2009, 48, 10687−10696.
(22) Elhabiri, M.; Scopelliti, R.; Bunzli, J. C. G.; Piguet, C. Lanthanide
The authors declare no competing financial interest.
̈
helicates self-assembled in water: A new class of highly stable and
luminescent dimetallic carboxylates. J. Am. Chem. Soc. 1999, 121,
10747−10762.
ACKNOWLEDGMENTS
■
The financial support of the Italian and Romanian foreign affair
secretariat through the project Multifunctional Molecular
Nanosystems is gratefully acknowledged.
(23) Stomeo, F.; Lincheneau, C.; Leonard, J. P.; O’Brien, J. E.; Peacock,
R. D.; McCoy, C. P.; Gunnlaugsson, T. Metal-directed synthesis of
enantiomerially pure dimetallic lanthanide luminescent triple-stranded
helicates. J. Am. Chem. Soc. 2009, 131, 9636−9637.
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