Synthesis and Magnetic Properties of Two-Step-Coordination Schiff Base Clusters

Article abstract of DOI:10.1002/ejic.202100298

A new family of paramagnetic coordination compounds based on a diimine-pyridine pincer ligand has been prepared, using a two-step synthetic approach. The sequential introduction of identical or different transition metals (Co, Mn, Ni, Zn) afforded mono-,

Full text of DOI:10.1002/ejic.202100298

European Journal of Inorganic Chemistry
Accepted Article
Title: Synthesis and magnetic properties of two-step-coordination Schiff
base clusters
Authors: Laurens C. J. M. Peters, Hans Engelkamp, Uli Zeitler, Peter
C. M. Christianen, Paul Tinnemans, Jan-Kees Maan, and
Alan E. Rowan
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100298
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01/2020

10.1002/ejic.202100298
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis and magnetic properties of two-step-coordination
Schiff base clusters
Laurens C. J. M. Peters,[a, b] Hans Engelkamp,*[a, b] Uli Zeitler,[a, b] Peter C. M. Christianen,[a, b] Paul
Tinnemans,[b] Jan-Kees Maan,[a, b] Alan E. Rowan[b, c]
[a]
[b]
[c]
Laurens C. J. M. Peters, Dr. Hans Engelkamp, Prof. dr. Uli Zeitler, Prof. dr. Peter C. M. Christianen, Prof. dr. Jan-Kees Maan
High Field Magnet Laboratory (HFML – EMFL)
Radboud University
Toernooiveld 7, 6525 ED Nijmegen, The Netherlands
hans.engelkamp@ru.nl
Laurens C. J. M. Peters, Dr. Hans Engelkamp. Prof. dr. Uli Zeitler. Prof. dr. Peter C. M. Christianen, Dr. Paul Tinnemans, Prof. dr. Jan-Kees Maan, Prof. dr.
Alan E. Rowan
Institute for Molecules and Materials
Radboud University
Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
Prof. dr. Alan E. Rowan
Australian Institute for Bioengineering and Nanotechnology (AIBN)
The University of Queensland, Brisbane, Australia
Abstract: A new family of paramagnetic coordination compounds
based on a diimine-pyridine pincer ligand has been prepared, using a
two-step synthetic approach. The sequential introduction of identical
or different transition metals (Co, Mn, Ni, Zn) afforded mono-, di-, tri-
and tetranuclear clusters, whose crystal structure has been
determined. Magnetic studies reveal that the metals within the
trinuclear manganese cluster engage in a small ferromagnetic
exchange interaction (J = 0.15 K). These studies enable the design of
new clusters with specific magnetic properties.
coordination compounds have been investigated towards their
catalytic,[5] antibacterial,[6] radio therapeutic,[7] and their cytotoxic,
anti-inflammatory and antioxidant[8] activity. Recently, the
magnetic properties of dysprosium-based complexes using the
same ligand have been studied.[9] Our interest in this ligand
originates from its rigid five-fold coordination pocket. This
constrained coordination geometry is particularly interesting from
a magnetic point of view, since it enforces an approximate axial
D5h symmetry on the central ion, which has been shown to
generate a high degree of either in-plane or out-of-plane axial
anisotropy.[2c, 10] Therefore, this family of coordination compounds
is of interest for the potential formation of single molecule
Introduction
11]
magnets (SMMs).[9,
In addition, the oxygen atoms can
simultaneously bind a second metal ion on the outside of the
coordination pocket, to extent the magnetic system and facilitate
a Heisenberg super-exchange interaction between the metal ions.
This second (weaker) coordination site is in competition with the
(stronger) five-fold site with respect to the binding of metal ions.
This competition has been shown to produce hetero metallic
clusters,[12] which can possess magnetic ground states
(ferrimagnetism).
[1]
The introduction of coordination chemistry
has led to the
emergence of a wide range of new functional materials, which
have found their way into fields such as catalysis, electronics,
magnetism and medicine.[2] The design of coordination
compounds with a specific function is governed by our ability to
control their synthesis and understand their resulting electronic
behavior. The electronic and magnetic properties of coordination
compounds often strongly depend on their geometry. The
inherently flexible and versatile nature of their organic (ligand) and
inorganic (metal) building blocks therefore makes systematic
correlation studies and the design of functionality challenging. To
overcome this obstacle, rigid ligands can be used, which results
in more predictable coordination products. Examples of such
ligands are the so-called pincer ligands, which have at least three
adjacent, coplanar binding sites, forming a strong coordination
pocket. In this work, we explore the coordination compounds
based on the Schiff base pincer ligand H2L shown in Figure 1.
Figure 1. Molecular structure of the H2L ligand.
The ligand was synthesized by
a template-free double
condensation of 2,6-diformylpyridine and o-aminophenol.[3] Its
rigidity comes from its pyridine and phenol rings, which are linked
through two rigid imine bonds. Three nitrogen lone pairs and two
oxygen (covalent or donative) bonds bring about a five-fold
coordination pocket, which can host various metal ions. Since its
first publication, the coordination of H2L to a range of transition
and rare earth metals has been reported.[4] In addition, H2L based
We synthesized paramagnetic H2L-based coordination
compounds by the subsequent addition of a transition metal
chloride followed by an acetate salt, shown in Scheme 1. In
addition, we synthesized a compound by the direct addition of an
acetate salt only. These strategies afforded seven new
coordination compounds 1-7, of which we have investigated the
1
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10.1002/ejic.202100298
European Journal of Inorganic Chemistry
FULL PAPER
magnetic properties of compounds 3 and 5. Since magnetic
properties are very sensitive to minute structural changes,[13] we
have determined the crystal structures of these compounds to
correlate structure to magnetic behavior.
deprotonation of the H2Lligands, and four acetate ligands. M1 has
the expected (but slightly distorted) pentagonal bipyramidal
coordination, while M2 has a near octahedral coordination.
The addition of manganese(II) chloride or cobalt(II) chloride to H2L
afforded isostructural complexes 2 and 3 respectively (Figure 3).
These complexes consist of a single metal ion, located in the
coordination pocket of H2L. The ligand’s geometry enforces an
approximate five-fold rotational symmetry on the central ion,
which in case of cobalt cluster 3 results in equatorial bond angles
of 73.68° (O-M-O), 70.47° (O-M-N) and 72.67° (N-M-N). These
angles closely agree with the ideal pentagonal surrounding of
360/5 = 72°. Upon coordination the ligand remains protonated,
and the metal ion’s 2+ charge is compensated by both an axially
coordinated chloride ligand and a chloride ion in the second
coordination sphere. The 2+ oxidation state is confirmed by the
M-O and M-N bond distances for both metal ions (~2.3 Å for
manganese(II) and ~2.2 Å for cobalt(II)).[14] Interestingly, for the
manganese complex, a longer reaction time at much higher
concentration afforded a complex where both chloride ions reside
on the axial position of Mn(II).[5c]
Scheme 1. Overview of the synthesis route and reaction products of the
paramagnetic H2L-based coordination compounds.
Figure 3. Crystal structure of manganese complex 2. The crystal structure is
isostructural to cobalt complex 3. The thermal ellipsoids of the ORTEP
representation have been set at 50% probability. Carbon = grey, chloride =
yellow, manganese = green, nitrogen = light blue and oxygen = red. All non-
coordinated solvent molecules and hydrogen atoms have been omitted for
clarity.
The addition of nickel(II) chloride to H2L gave complex 4 (Figure
4). Although the nickel ion occupies the ligand’s coordination
pocket, it only binds to the three nitrogen donors. Instead, the ion
has an octahedral coordination geometry, with two chloride ions
as axial ligands and a coordinated THF molecule in the equatorial
plane. The phenol oxygen atoms do not coordinate to the metal
and therefore they rotate out of the ligand plane to reduce steric
hindrance.
Figure 2. Crystal structure of tetranuclear cobalt cluster 1. The thermal
ellipsoids of the ORTEP representation have been set at 50% probability.
Carbon = grey, cobalt = blue, nitrogen = light blue and oxygen = red. All non-
coordinated solvent molecules, a front facing acetate ligand on M2 (upper), and
hydrogen atoms have been omitted for clarity.
Adding a half equivalent of manganese(II) acetate to 2 gave
trinuclear manganese cluster 5 (Figure 5). A similar complex with
bridging acetates rather than chloride ligands was studied as a
catalase mimic.[5c] The cluster consists of two deprotonated
ligands, L, which still have manganese ions in their coordination
pockets (M1), but now share a third manganese ion (M2).
Therefore, the addition of manganese(II) acetate has effectively
“glued” two complexes 2 together. In doing so, both H2L ligands
are doubly deprotonated and both initial complexes 2 have lost a
chloride counter ion. The connecting ion, M2, binds the four
available oxygen atoms and an additional ethanol ligand, to adopt
Results and Discussion
Crystal structures
The direct addition of cobalt(II) acetate to H2L afforded
tetranuclear cluster 1 (Figure 2). The cluster contains two L
ligands (deprotonated), of which both the coordination pocket M1
and the outer position M2 are occupied. These two ligands stack,
while rotated 90°, to form a tetranuclear cluster. The four 2+
charges on the metal ions are compensated by the double
2
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10.1002/ejic.202100298
European Journal of Inorganic Chemistry
FULL PAPER
a slightly distorted trigonal bipyramidal coordination geometry.
The total 6+ charge of the three Mn(II) ions is compensated by
the chloride and deprotonated ligands, yielding an overall neutral
cluster.
is balanced by the fully deprotonated ligand and the chloride
ligand to result in an overall neutral cluster. Although the
additional Zn(II) ion is diamagnetic, and therefore has little
influence on the dominating paramagnetic magnetic behavior of
the clusters, it does demonstrate the controlled occupation of the
second coordination site. Particularly the controlled synthesis of
heterometallic clusters with unequal numbers of unpaired
electrons on M1 and M2 is of interest, since this can produce
ferrimagnetic ground states.
Figure 4. Crystal structure of nickel complex 4. The thermal ellipsoids of the
ORTEP representation have been set at 50% probability. Carbon = grey,
chloride = yellow, nitrogen = light blue, nickel = cyan, and oxygen = red. All non-
coordinated solvent molecules and hydrogen atoms have been omitted for
clarity.
Figure 6. Crystal structure of heterometallic manganese-zinc cluster 6. The
thermal ellipsoids of the ORTEP representation have been set at 50%
probability. Carbon = grey, chloride = yellow, manganese = green, nitrogen =
light blue, oxygen = red and zinc = brown. All non-coordinated solvent molecules,
and hydrogen atoms have been omitted for clarity.
Figure 5. Crystal structure of linear Mn3 cluster 5. The thermal ellipsoids of the
ORTEP representation have been set at 50% probability. Carbon = grey,
chloride = yellow, manganese = green, nitrogen = light blue and oxygen = red.
All non-coordinating solvent molecules, and hydrogen atoms have been omitted
for clarity. The ethanol oxygen atom on the central ion M2 can occupy two
different crystal sites, which are both shown in this figure.
Alternatively, adding an equivalent of zinc(II) acetate to 2 gave
heterometallic manganese-zinc cluster 6 (Figure 6). Similar to
starting complex 2, this cluster still contains a single ligand L with
a manganese ion in its coordination pocket (M1). However, the
second coordination site (M2) is now occupied by a Zn(II) ion.
Interestingly, the two chloride ions of starting complex 2 (both
axial and second coordination sphere) have migrated to the zinc
ion. Therefore, the zinc ion adopts a tetrahedral coordination
geometry. The axial position on the Mn(II) ion has been replaced
by a water ligand. The total 4+ positive charge of both metal ions
Figure 7. Crystal structure of tetranuclear cobalt cluster 7. The thermal
ellipsoids of the ORTEP representation have been set at 50% probability.
Carbon = grey, chloride = yellow, cobalt = blue, nitrogen = light blue and oxygen
= red. All non-coordinated solvent molecules, and hydrogen atoms have been
omitted for clarity.
The addition of an equivalent of cobalt(II) acetate to 3 gave
tetranuclear cobalt cluster 7 (Figure 7). The cluster consists of two
ligands L, each still having their Co(II) ions in their coordination
3
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10.1002/ejic.202100298
European Journal of Inorganic Chemistry
FULL PAPER
pockets (M1), and an additional Co(II) ion in the outer position (M2). of the four Co(II) ions is compensated by the chloride ligands and
This additional cobalt ion has a tetrahedral coordination geometry. two doubly deprotonated ligands, yielding an overall neutral
These two LM1M2 moieties are bridged by the axial chloride
ligands on M1, to give a tetranuclear cluster. The total 8+ charge
cluster. Details on the crystallographic data and the refinement
details are presented in Table 1.
Table 1. Crystallographic Data and Structure Refinement Details.
1
2
3
4
5
6
7
Formula
Weight
C46Co4H38N6O12
1102.56
C20ClH19MnN3O3
439.77
C20Cl2CoH17N3O3
511.26
C23Cl2H23N3NiO3
519.05
C44Cl2H45Mn3N6O7
1004.58
C20Cl2H19MnN3O4Zn
556.61
C44Cl4Co4H38N6O6
1124.35
Space
group
C 2/c
P -1
C 2/c
C 2/c
C 2/c
P 21/n
P 21/c
a (Å)
20.0548(9)
11.7054(6)
21.5865(15)
90
6.9184(3)
12.6804(9)
13.6337(8)
84.880(6)
83.552(4)
78.555(5)
1162.10(12)
2
28.7972(18)
6.8035(7)
21.938(3)
90
26.235(3)
13.6726(14)
15.1306(12)
90
25.714(5)
11.401(2)
21.561(4)
90
12.5181(13)
13.1870(7)
14.2358(14)
90
17.631(4)
13.245(3)
19.258(4)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
91.363(6)
90
92.111(7)
90
95.654(8)
90
119.81(3)
90
95.737(5)
90
98.52(3)
90
5066.0(5)
4
4295.2(8)
4
5400.9(9)
8
5485(2)
4
2338.2(4)
4
4549.4(18)
4
T (K)
208(2)
208(2)
208(2)
0.71073
1.078
208(2)
208(2)
0.71073
0.823
208(2)
208(2)
0.71073
1.723
λ (Å)
0.71073
1.569
0.71073
0.835
0.71073
0.952
0.71073
1.831
µ (cm-1)
R (Fo)[a]
Rw (Fo)[b]
0.0372
0.0964
0.0632
0.0425
0.0983
0.0434
0.1045
0.0632
0.1746
0.0281
0.0719
0.0412
0.0975
0.1576
[a] Residual factor for the reflections judged significantly intense and included in the refinement. [b] Weighted residual factors for significantly intense reflections
included in the refinement.
Magnetic properties
J12 = J21’ would be ferromagnetic and have a magnitude of
J = 0.22 K. A similarly small ferromagnetic interaction is found
with the cantilever measurement, for which the T = 4.2 K trace
overlaps with the simulated spin-only curve corresponding to a
coupling of J = 0.15 K.
Recently, it has been shown[15] that a single Co(II) ion in a
pentagonal surrounding can result in slow relaxation of its
magnetization (Single Ion Magnet behavior). Therefore, we have
performed low temperature AC susceptibility measurements on
Co(II) complex 3. The measurements have been taken at
temperatures between T = 1.8 and 50 K and at frequencies in the
range f = 0.1-1500 Hz at a drive field of 3.78 Oe. We did not
observe any out-of-phase signals, which means that ꢀ ≪ 1/2ꢁꢂ,
i.e. there is no indication of single ion magnet behavior.
The magnetization and susceptibility of linear Mn3 cluster 5 has
been determined using low-temperature high-field cantilever
magnetometry and SQUID measurements respectively. Figure 8
shows the parallel magnetization of a powdered sample of 5 (solid
black line), measured at T = 4.2 K at magnetic fields of B = 0 to
20 T through cantilever magnetometry. It shows a steep increase
of the magnetization at low fields, which saturates around B = 10
T. The inset shows the inverse molar susceptibility (solid
triangles) at a constant magnetic field of B = 0.1 T at a
temperature range of T = 1.8-300 K. It follows the Curie−Weiss
law down to low temperatures, with an extracted experimental
moment of µeff = 5.78 µB (spin-only gives µeff = 5.82 µB, see below)
per Mn(II) ion and a Weiss constant θW = 1.02 K (fit down to 40 K).
Since the Mn(II) ions are expected to be in a 6S ground state, we
ignore spin-orbit contributions and describe the system using a
spin-only Hamiltonian containing only Zeeman and Heisenberg-
exchange contributions. Concerning the exchange interaction, we
expect an open triplet spin topology based on the structure, with
identical exchange couplings between M1 and M2 (labelled J12)
and between M1' and M2 (labelled J21’) and a negligible exchange
between M1 and M1’. Based on the Weiss constant, the couplings
The coordination compounds in this work show that the two-step
approach employed here (subsequent addition of metal chloride
and -acetate) enables the synthesis of several new homo- and
heterometallic magnetic clusters. The controlled placement of
metal ions in sites M1 and M2 gives direct control over the
magnetic properties of the resulting clusters. However, this
method only works when the added metal acetate in the second
4
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10.1002/ejic.202100298
European Journal of Inorganic Chemistry
FULL PAPER
step has a lower affinity to the pentagonal coordination pocket
(M1) than the metal ion already present in the precursors complex.
These affinities have been determined using mass spectrometry,
where the abundant HLM+ fragment could be used to determine
the occupation of the pentadentate coordination pocket. The
HLM+ fragment of several first row transition metals are listed in
Table 2.
Their affinities are governed by their σ-acceptor (the ligand
donates five lone pairs) and π*-acceptor properties. The latter
interaction has been shown to be exceptionally strong in diimine-
pyridine ligands.[17] The relative affinities of the Zn(II), Mn(II) and
Co(II) ions to H2L resemble the Irving-Williams series,[18] with the
exception of Mn(II) binding stronger than Zn(II). This might be
explained by an increased back-bonding interaction of Mn(II)
compared to Zn(II).
Depending on the counter ions (acetate and/or chloride), we were
able to produce coordination compounds which contain 1, 2, 3 or
4 ions using H2L. Each of these compounds contain the rigid [LM]
fragment, which are organized in different ways in case of the
polymetallic clusters. This is exemplified by tetranuclear cobalt
clusters 1 and 7, which adopt a double-decker like structure of the
two [LM] fragments while in Mn3 cluster 5 the two fragments face
each other on a single axis. Moreover, cobalt cluster 1 is the only
compound in which the phenol oxygen atoms on the ligand are
used to bind three different metal ions, compared to two metal
ions in clusters 5-7. The extra bond results in a tetrahedral sp3
geometry of this oxygen atom. In contrast to the completely planar
ligands in compounds 2-7, this bends the ligand significantly
through a rotation around its Cphenol-Nimine bonds (~23 degrees).
In terms of geometry, manganese-zinc cluster 6 is the most
straightforward result we obtained. The addition of zinc acetate to
the manganese precursor complex 2 resulted only in the
occupation of the zinc ion on site M2. In principle, the tetranuclear
cobalt cluster 7 has a very similar structure, but in this case the
structure is extended by stacking two binuclear clusters on top of
each other. A similar structure has been observed for this ligand
before, by using a one-pot synthesis with cadmium acetate.[19]
The found ferromagnetic interaction between the metal ions in
Mn(II) cluster 5 is rather surprising. In an earlier publication on a
Mn(II) cluster highly similar to compound 1 (both in geometry and
ligand),[14] a small antiferromagnetic coupling of J12 = 1.1 K,
between ions M1 and M2 was found. A large difference between
these clusters is the local coordination geometry of M2, which in
cluster 5 is distorted trigonal bipyramidal and for the published
tetranuclear compound is octahedral. In the latter case, the
Goodenough-Kanamori-Anderson rule indeed predicts an
antiferromagnetic interaction. However, in case of the distorted
trigonal bipyramidal case these rules do not apply.
Figure 8. Magnetic properties of linear Mn3 cluster 5. The solid black line
represent the parallel magnetization, as measured by cantilever magnetometry
at T = 4.2 K between B = 0 and 20 T. The black and red dotted lines are
simulated traces which correspond to the uncoupled
J = 0 and the
ferromagnetically coupled J = 0.15 K situations. The inset shows the inverse
molar susceptibility (solid triangles) at a constant magnetic field of B = 0.1 T at
a temperature range of T = 1.8-300 K. Its linear shape up until low temperatures
corresponds to Curie-Weiss behavior, through which the effective moment (µeff
= 5.78 µB) and Weiss constant (θW = 1.02 K) are extracted by a linear fit down
to 40 K.
Table 2. Dominant LC/MS electrospray ion-trap mass fragments.
Experiment
m/z
Fragment
H[LMn(II)]+
H[LCo(II)]+
H[LNi(II)]+
H[LZn(II)]+
MnCl2 vs. CoCl2
CoCl2 vs. ZnCl2
Synthesis of 4
ZnCl2 vs. NiCl2
371.3
375.2
374.2
382.3
Although exchange interaction between M1 and M2 mainly
determines the magnetic ground state (magnetic/non-magnetic)
of the system, magnetic anisotropy is needed to induce SMM
behavior. A detailed analysis of the magnetic anisotropy (easy-
plane/axis) on coordination sites M1 and M2 in case of Mn(II) ions
have been determined by cantilever magnetometry on a single
crystal, and will be published separately.
The relative affinities were determined by simultaneously adding
equimolar quantities of two different metal chlorides to a solution
of H2L. In each case we observed dominating fragment species
containing only one of the two metal ions. From these
experiments we observe a bonding affinity to H2L of order: Ni(II) Conclusion
< Zn(II) < Mn(II) < Co(II). The lowest affinity of the Ni(II) ion to H2L
is easily explained by the fact that according to the Jahn-Teller
theorem, regular stereochemistry of seven-coordinate
We have developed a new two-step synthesis approach, through
a
which we were able to synthesize a family of paramagnetic
coordination compounds based on diimine-pyridine ligand H2L. By
sequentially introducing different transition metal salts, we can, to
a large extent controllably, insert (different) metal ions in the
coordination sites, M1 and M2. In addition, these metal ions are
shown to engage in an exchange interaction. This enables the
design of new clusters with specified magnetic properties.
pentagonal bipyramidal high-spin Ni(II) complex is unstable.[16]
Therefore, as seen in complex 4, the ion only engages in four
bonds with the ligand which reduces its affinity relative to a
pentadentate coordination. Attempting to insert a second metal
into M2 for this precursor would only be possible if a metal ion is
found with an even weaker affinity for M1. The remaining three
ions, Zn(II), Mn(II), and Co(II) all bind in a similar fashion to H2L.
5
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10.1002/ejic.202100298
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis of Ni(II)H2LCl2(THF) (4). To a solution of H2L (0.10 g, 0.32 mmol)
in 100 mL methanol was added NiCl2·6H2O (0.08 g, 0.34 mmol). The
mixture was refluxed for two hours, after which the amount of solvent was
lowered (~20 mL) under reduced pressure. Upon addition of diethyl ether
(150 mL), the red product precipitated and was isolated by filtration. The
product was crystallized from a THF solution top-layered with diethyl ether
to yield brown crystals which were analyzed using single crystal X-ray
diffraction. ESI-MS MeOH m/z: [HL2Ni2]+, 754.9, 100%; [HLNi]+, 374.2,
79%.
Experimental Section
Commercially available reagents were used without further purification. All
reactions were carried out under atmospheric conditions unless mentioned
otherwise. Mass spectra were taken on a LCQ Advantage Max LC/MS
electrospray ion-trap mass spectrometer (positive ion mode, MeOH).
SQUID magnetization and susceptibility measurements in the ranges of
1.8 < T < 300 K and 0 < B < 70 kOe were taken on a Quantum Design
SQUID
magnetometer
(MPMS-XL).
Cantilever
magnetometry
measurements were performed on an in-house build cantilever
magnetometer at the High Field Magnet Laboratory in Nijmegen, The
Synthesis of Mn(II)3L2Cl2(EtOH)3 (5). To a solution of 2 (0.15 g, 0.31 mmol)
in 100 mL methanol was added a small excess of Mn(CH3COO-)2·4H2O
(0.08 g, 0.38 mmol). The red solution was stirred overnight, after which the
solvent was removed under reduced pressure. The orange powder was
dissolved in ~20 mL of methanol, and upon addition of diethyl ether (100
mL), the product precipitated and was isolated by filtration. The product
was crystallized from an ethanol solution top-layered with diethyl ether to
yield red-orange crystal bars which were analysed using single crystal X-
ray diffraction. ESI-MS MeOH m/z: [H(L)Mn]+, 371.3, 41%; [not attributed]
263.7, 100%.
Netherlands. Single-crystal diffraction was performed on
a Nonius
KappaCCD single crystal diffractometer θ and ω scan mode) using
graphite-monochromated Mo Kα radiation. Diffraction images were
integrated using Eval14.[20] Intensity data were corrected for Lorentz and
polarization effects. A semiempirical multiscan absorption correction was
applied (SADABS).[21] The structure was solved by the DIRDIF program
system using the program PATTY to locate the heavy atoms. Refinement
was performed with standard methods (refinement against F2 of all
reflections with SHELXL97) with anisotropic displacement parameters for
the non-hydrogen atoms.[22] All hydrogen atoms were placed at calculated
positions and refined riding on the parent atoms. CCDC 2086935 -
2086941 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/structures.
Synthesis of Mn(II)Zn(II)L(MeOH)(H2O) (6). To a solution of 2 (13.0 mg,
0.03 mmol) in 15 mL methanol was added an excess of
Zn(CH3COO)2·2H2O (7.70 mg, 0.04 mmol). The orange mixture was
stirred overnight, after which the solvent was removed under reduced
pressure. The product was crystallized from a methanol solution top-
layered with diethyl ether to yield red crystals which were analysed using
single crystal X-ray diffraction. ESI-MS MeOH m/z: [HL2MnZn]+, 749.9,
26%; [HL2Mn2]+, 741.1, 28%; [H(L)Mn(MeO-)]+, 402.1, 24%; [H(L)Mn]+,
371.2, 100%.
Synthesis of 2,6-bis(2-hydroxyphenyliminomethyl)pyridine (H2L). This
compound was synthesized according to a method published earlier [6]:
Boiling solutions of o-aminophenol (2.50 g, 22.9 mmol) in 270 mL water
and 2,6-diformylpyridine (1.55 g, 11.5 mmol) in 155 mL were mixed, and
refluxed for another 30 minutes. After being stored overnight in the
refrigerator, a yellow precipitate could be filtered off. The filtrate was
washed with hot water and recrystallized from methanol to afford 3.10 g of
product (yield 86%). Anal.: Calcd. for C19H15N3O2: C 71.94%; H 4.73%; N
13.24%. Found: C 71.87%; H 4.90%; N 13.11%.
Synthesis of Co(II)4L2Cl4(acetone)2 (7). To a solution of 3 (0.15 g, 0.31
mmol) in 100 mL methanol was added Co(CH3COO)2·4H2O (0.08 g, 0.31
mmol). The dark red-brown solution was stirred overnight, after which the
solvent was removed under reduced pressure. The brown powder was
dissolved in ~20 mL of methanol, and upon addition of diethyl ether (100
mL) the product precipitated and was isolated by filtration. The product
was crystallized from an acetone solution top-layered with diethyl ether to
yield dark brown crystal hexagonal bars which were analysed using single
crystal X-ray diffraction. ESI-MS MeOH m/z: [HL2Co2]+, 749.1, 79%;
[H(L)Co]+, 375.3, 100%.
Synthesis of Co(II)4L2(CH3COO-)4 (1). To a solution of 2,6-diformylpyridine
(0.1 g, 0.74 mmol) and o-aminophenol (0.16 g, 1.48 mmol) in 200 mL
methanol was added Co(CH3COO-)2·4H2O (0.262 g, 1.48 mmol). The
mixture was refluxed for 24 hours to obtain a dark brown solution. The
solvent was removed through rotary evaporation. Residual acetic acid was
stripped away through azeotropic evaporation of toluene. The strongly
hygroscopic powder was dissolved in ~50 mL of methanol, and upon
addition of diethyl ether (250 mL), the product precipitated and was
isolated by filtration. The product was crystallized from a dichloromethane
solution top-layered with n-pentane to yield dark brown crystal needles
which were analyzed using single crystal X-ray diffraction. ESI-MS MeOH
m/z: [HL2Co2]+, 748.9, 73%; [H(L)Co]+, 375.3, 100%.
Acknowledgements
This work is part of the research program of the 'Stichting voor
Fundamenteel Onderzoek der Materie (FOM)', which is financially
supported
by
the
'Nederlandse
Organisatie
voor
Synthesis of [Mn(II)H2LCl(CH3OH)]Cl (2). To a solution of H2L (0.10 g, 0.32
mmol) in 100 mL methanol was added MnCl2·4H2O (0.08 g, 0.42 mmol).
The mixture was refluxed for two hours, after which the amount of solvent
was lowered (to ~20 mL) under reduced pressure. Upon addition of diethyl
ether (150 mL), the red product precipitated and was obtained by filtration.
The brown-red product was crystallized from a methanol solution top-
layered with diethyl ether to yield red-brown crystals which were analyzed
using single crystal X-ray diffraction. ESI-MS MeOH m/z: [HL2Mn2]+, 741.1,
50%; [H(L)Mn(MeO-)]+, 402.1, 22%; [H(L)Mn]+, 371.3, 100%.
Wetenschappelijk Onderzoek (NWO)'. This work was supported
by HFML-RU/NWO-I, member of the European Magnetic Field
Laboratory (EMFL).
Keywords: Magnetic properties • Pincer ligand • Manganese •
Cobalt
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European Journal of Inorganic Chemistry
FULL PAPER
Entry for the Table of Contents
In the quest for single molecule magnets (SMMs), several new homo- and heterometallic (Co, Mn, Ni, Zn) paramagnetic coordination
compounds have been synthesized, based on a rigid diimine-pyridine pincer ligand. X-ray crystallography reveals their structure,
while magnetic studies reveal ferromagnetic exchange interactions in some of them.
8
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