3d-4f Metallacrown complexes with a new sandwich core: Synthesis, structures and single molecule magnet behavior

Article abstract of DOI:10.1039/d0nj00292e

Two heterometallic metallacrown complexes, [Er{Cu4(butyrat)4}2]·Cl3·MeOH·26H2O (1) and [Yb{Cu4(butyrat)4}2]·Cl3·MeOH·26H2O (2) (H2butyrat = 3-aminobutyric hydroxamic acid), have been reported. X-ray crystallographic analysis reveals that the two complexes displayed nested-sandwich configurations with two 12-MC-4 metallacrown units capping LnIII in the centre. Magnetic studies revealed that 1-2 undergo antiferromagnetic coupling interaction. Magnetic measurements show that complex 2 displays frequency-dependent signals at a dc field of 1000 Oe, indicating field-induced single-molecule magnet behavior.

Full text of DOI:10.1039/d0nj00292e

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3d-4f metallacrown complexes with new sandwich core: synthesis,
structures and single molecule magnet behavior
Received 00th January 20xx,
Accepted 00th January 20xx
Wen-Wen Chang, Hua Yang, Hai-Quan Tian, Da-Cheng Li* and Jian-Min Dou
DOI: 10.1039/x0xx00000x
Two
heterometallic
metallacrown
complexes
[Er{Cu4(butyrat)4}2]·Cl3·MeOH·26H2O
(1)
and
[Yb{Cu4(butyrat)4}2]·Cl3·MeOH·26H2O (2) (H2butyrat = 3-aminobutyric hydroxamic acid) have been exhibited. X-ray
crystallographic analysis reveals two complexes displayed nested-sandwich configurations with two 12-MC-4
metallacrown units capping a LnIII in the centre. Magnetic studies revealed 1-2 have antiferromagnetic coupling interaction.
Magnetic measurements show that complex 2 displays the frequency-dependence signals at a dc field of 1000 Oe,
indicating field-induced single-molecule magnet behavior.
Our group has great interest in the synthesis of
metallacrowns and has reported
a large number of
Introduction
metallacrowns in the previous study.31-35 In order to continue
our study, we chose 3-aminobutyric hydroxamic acid as ligand
Metallacrowns (MCs), inorganic analogues of organic crown
ethers, are acknowledged with the metal and nitrogen atoms
replacing the methylene carbon atoms of crown ethers to
present [M-N-O] repeat units in metallacrown ring.1-2 Pecoraro
and Lah reported the first metallacrown examples in 1989,
since then, many of these complexes have been constructed
and characterized.3-5 Researchers have successively developed
various types of metalcrowns from 8-MC-4 to 60-MC-20 with
the connections of the rings consisting of [M-N-O], [M-N-N],
[M-N-C-O] and [M-N-C-N].6-8 The Ln-based complexes may
exist single-molecule magnets (SMMs) properties due to their
large magnetic anisotropy arising from the large, uquenched
and
synthesize
two
unprecedented
(1)
complexes
and
[Er{Cu4(butyrat)4}2]·Cl3·MeOH·26H2O
[Yb{Cu4(butyrat)4}2]·Cl3·MeOH·26H2O (2), which have been
characterized by X-ray single diffraction, Fourier transform
infrared spectroscopy and X-Ray Powder Diffracter. Their
magnetic properties were also explored in detail.
Experimental
Materials and synthesis
orbital angular momentum.9-14 Metallacrowns demonstrates a We first prepared 3-aminobutyric acid methyl ester
specific structural feature of a large amount of transition metal hydrochloride by adding 3-aminobutyric acid in methanol and
ions and rare earth ions located in a small volume, they would thionyl chloride solution under ice-water bath conditions, and
provides a large spin effect on the entire molecule to exhibit then, the above reaction solution was refluxed in an oil bath
interesting and better magnetic phenomena.15-23 The and kept a constant temperature for 24 h. Then an aqueous
combination of high anisotropy of 3d ions and large spin-orbit solution of hydroxylamine hydrochloride and sodium
of 4f ions in unique Metallacrown structure can provide a good hydroxide in a ratio of 1:3 was added into the above solution
way to obtain new molecular magnets. However, at present, with the stirring at room temperature for 6 h. Last, the
3d-4f MCs complexes with single molecular magnet properties solution was acidified using 10 ml of hydrochloric acid and
mainly focus on Mn-Ln MCs complexes, and the structural spin-evaporated to obtain the ligand. The synthesized method
feature only involved planar or warped MC structures.24-30 To of this ligand has not been reported in the literature.
take advantage of the MC structural feature and develop
intriguing configuration, it is still necessary to explore other
structural types of MC and further discuss the relationship of
structural properties.
Syntheses
[Er{Cu4(butyrat)4}2]·Cl3 · ·26H2O (1)
- MeOH
H2butyha (0.118 g, 1 mmol) was dissolved in MeOH (15 mL)
and CuCl2·2H2O (0.086 g, 0.5 mmol) in acetonitrile (5 mL),
followed by NEt3 (0.01g, 1mmol) added to the previous
solution. Then, Er(NO3)3·5H2O (0.045 g, 0.1 mmol) and NaCl
(0.058 g, 1 mmol) were added to the above solution after
stirring for 30 min. The color changed from dark green to navy
blue immediately. Then the reaction solution was stired for
a. Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell
Technology,School of Chemistry and Chemical Engineering, Liaocheng University,
Liaocheng 252059,P.R.China;E-mail address: dougroup@163.com ;
Tel.: +86-0635-8239298
†
Electronic Supplementary Information (ESI) available: CCDC 2009454,
2009455.For ESI and crystallographic data in CIF or other electronic format. See
DOI: 10.1039/x0xx00000x
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about 3 h. The crystals were obtained through slow Complexes 1-2 were synthesized by the reactionVoiefwHAr2tibclue tOynrlianet
evaporation of the solvent. Black block crystals were obtained ligand with CuII salts, LnIII salts (LnIII = ErIIDI,OYIb: 10).1a0n3d9/DN0aNJs0a0lt2s92inE
with yield 0.06 g, 39.0% based on Er salt. Elemental analysis stoichiometric ratio 10:5:1:10 in MeOH and MeCN mixed
calcd (%) for: C33H120Cl3Cu8ErN16O43: C, 17.90 %; H, 5.4 %; N, solution, and isolated by slow evaporation of the solvents at
10.13 %; found: C, 18.36 %; H, 5.29 %; N, 10.61 %. IR (KBr cm- room temperature (Scheme 1).
1): 3435 (s), 1580 (m), 1455 (vw), 1403 (w), 1341 (vw), 1270
III
I
(w), 1213 (vw), 1128 (w), 1037 (w), 1013 (w), 899 (vw), 851
(vw),746 (w), 620 (w), 443 (m).
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-
[Yb{Cu4(butyrat)4}2]·Cl3 ·MeOH·26H2O (2)
Complex 2 is synthesized in the same way as complex 1 with
Yb(NO3)3·5H2O (0.045 g, 0.1 mmol) and CuBr2 (0.112 g, 0.5
mmol) instead of Er(NO3)3·5H2O and CuCl2·2H2O. Yield: 0.062 g,
38.0% based on Yb salt. Elemental analysis calcd (%) for:
C33H120Cl3Cu8N16O43Yb: C, 17.86 %; H, 5.41 %; N, 10.10 %;
found: C, 17.99 %; H, 5.36 %; N, 10.59 %. IR (KBr cm-1): 3435
(s), 1578 (m), 1403 (w), 1341 (vw), 1270 (vw), 1128 (w), 1038
(w), 1013 (w), 899 (w), 852 (vw), 746 (w), 622 (w), 444 (w).
X-ray Crystallography
Single crystals of complexes 1-2 of the appropriate size were
placed on an Agilent Xcalibur Eos Gemini CCD plate
diffractometer at a certain temperature (complexes 1-2 were
measured at 132 (4) K and 130 (11) K, respectively) by using Cu
Kα radiation (λ = 1.54184 Å) measured by the graphite
monochromatic method as the measurement light source, and
the structural analysis of the complexes were completed by
the software package Shelxl-2018/3. The direct method is used
to solve the initial structure, the least-square method is used
to refine the coordinates and anisotropic parameters of all
non-hydrogen atoms using a full matrix program, and then the
hydrogen atoms are hydrogenated using the Fourier method,
followed by anisotropic purification. Crystallographic and
structure refinement data are listed in Table S1. The selected
bond distances and angles are summarized in Table S2−S3.
Scheme 1. Scheme of the way to obtain the [12-MCCuII, h2buyrat-4]3+ unit.
Single-crystal X-ray diffraction analysis reveals that complex
1 belongs to monoclinic crystal system and P2(1) space group.
The molecular structure contains a single-cap sandwich-like
core with each face comprising a 12-MC-4 unit(Fig. 1). Two 12-
MC-4 units (A and B) were linked together through a central
ErIII ion, which bonds eight oxygen atoms from two 12-MC-4
ring. In this complex, the unbound anions (three chloride
anions) are located around the structure of sandwich-like core.
The whole molecule exhibit electric neutrality. For [12-MCCu-4]
unit, four CuII ions are assembled by four deprotonated
polydentate ligands to form Cu−N−O connectivity ring. Each
CuII ion exhibits four-coordinate, planar square geometry
coordinating two N and two O atoms from the N2O2 pocket of
the ligand. Two [12-MCCu-4] as sandwich two sides all link to
the center ErIII ion with the help of the eight oxygen atoms
descended from oxime oxygens, which make ErIII ion locate
into the eight-coordinate square anti-prism polyhedron. The
plane distance between the upper and lower [12-MCCu-4]
planes is 3.552 Å. Cu-O-Er angles are 121.5(9)º for Cu1-O1-Er1,
122.0(9)º for Cu2-O3-Er1, 120.8(9)º for Cu3-O5-Er1, 122.9(9)º
for Cu4-O7-Er1, 120.6(9)º for Cu5-O9-Er1, 121.30(9)º for Cu6-
O11-Er1, 121.8(9)º for Cu7-O13-Er1 and 122.0(9)º for Cu8-
O15-Er1, respectively. The average bond lengths of Cu-O and
Cu-N on the equatorial plane are 1.94 Å and 1.976 Å,
respectively, which are in the normal range of 1.916-1.95 Å
and 1.894-2.022 Å. The chloride anion forms intermolecular
hydrogen bonds with the four N-H groups of the spatially
adjacent two complexes (N2-H2B···Cl2 = 2.620 Å, N8-H8B···Cl2
Physical Measurements
We performed elemental analysis of C, H and N on the
Elementar Vario EL analyzer. The infrared spectrum was
obtained on the Perkin-Elmer spectrum, and the samples were
prepared as KBr particles. Powder X-ray diffraction (PXRD) was
recorded on an XD-3 power diffractometer, which used a Cu-
Kα X-ray source (λ = 1.54Å) to operate at 36 kV and 20 mA.
Thermogravimetric analysis was performed on a PerkinElmer
TGA-7 thermogravimetric analyzer at the heating rate of 10 °C
min-1, from 25 °C to 780 °C, under a flow of N2 at 20.0 mL min-1.
Quantum Design MPMS-XL7SQUID magnetometer was used to
collect the magnetic susceptibility data of the sample under a
magnetic field of 1 kOe over a temperature range of 1.8-300 K.
The measurement of the ac magnetic susceptibility was carried
out by using an oscillation frequency in the range of 1−1000 HZ
with 0 and 1000 Oe applied dc field.
Results and discussion
Crystal Structure
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= 2.468 Å, N14-H14B···Cl2 = 2.624 Å, N16-H16B···Cl2 = 2.633 Å).
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The complexes are packed through the weak interactions of
four hydrogen bonds to form a one-dimensional chain (Fig. S3,
ESI†). As show in Fig. S4, ESI†, the complex 1 further forms a
two-dimensional network structure by weak interactions of
extra four hydrogen bonds (N4-H4B···Cl1 = 2.512 Å, N6-
H6B···Cl1 = 2.601 Å, N10-H10B···Cl1 = 2.526 Å and N12-
9
H12B···Cl1 = 2.540 Å) on one-dimensional chain foundation.36-
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The structure of complex 2 is similar to the structure of
complex 1 (Fig. S5, ESI†). In this complex, the unbound anions
(three chloride anions) are located around the structure of
sandwich-like core. The Yb-O bond lengths range from
2.256(15) to 2.328(14) Å. 1D coordination polymers can be
obtained through four kind of hydrogen bonds interactions
with the distances of 2.615 Å for N6-H6B···Cl1, 2.582 Å for N4-
H4B···Cl1, 2.558 Å for N12-H12B···Cl1 and 2.584 Å for N10-
H10B···Cl1) (Fig. S6, ESI † ). Plane distance of 2 between the
upper and lower [12-MCCu-4] planes is 3.526 Å. The
coordination sphere of the Ln(III) ions are completed by eight
equatorial oxygen atoms of ligand with the Yb-O bond lengths
in the range of 2.256-2.328 Å.
Cu-N-O-Cu torsion angles lie within the range of 157.1-
162.3º in 1 and 159.1-161.9º in 2, respectively, indicating the
nearly planarity for two 12-MC-4 planes. The ligands are
concentrated above and below the xy plane and achieve an
extended axial coordination environment for the central LnIII
ion, which have the distinct difference with that in the
previous Cu-Ln complexes. Therefore, the comparison of our
complexes with the previous reported Cu-Ln complexes shows
the obvious difference. For the latter, they displayed the
various structural configurations, such as batter-like,3,7,42
linear,43 cube44, cycle,45-47 dumbbell,48 which distinguish the
sandwich conformation in the resulting complex. The two 12-
MC-4 units acting as faces of sandwich wrap one Ln ion in the
center through the linking of eight hydroxamate O atoms from
two 12-MC-4 units to accomplish the eight-coordination
square pyramid coordination geometry for Ln ion.
Figure 1. The structure of complex 1 with hydrogen atoms omitted for clarity (top).
Local coordination spheres of Cu1II ion and Er1III ion (middle) and coordination
geometries of core region (bottom).
Powder X-ray diffraction (PXRD)
The experimental RXRD patterns are in good agreement with
the corresponding calculated patterns, confirming the high
purity of the synthesized complexes (Fig. S1, ESI†).
Thermogravimetric analysis (TGA)
The thermogravimetric analysis (TGA) study revealed that
complexes 1-2 have a two-step weight loss between room
temperature and 780 °C (Fig. S2, ESI†). The first slowly weight
loss occurs at 30-250 °C, and the weight losses are 20.1% for 1
and 17.03% for 2, respectively, which are attributed to the loss
of free solvent molecules (calculated 22.0% for 1 and 21.9% for
2). As temperature increases, the frameworks of the two
complexes start to decompose.
Static magnetic properties
The
solid-state,
variable-temperature
dc
magnetic
susceptibility study of samples 1-2 were performed in the
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temperature range of 2−300 K under an applied magnetic field 1-2 display frequency dependence. For 1, owing tVoietwhAerticlilme Oitnlionef
of 1000 Oe. The collect data were plotted as χMT vs.T (Fig. 2). the instrument, the χ" maximum signDalOsI: w10e.1r0e39n/Do0tNJp0r0o2b9e2dE
Compounds 1-2 have room-temperature χMT values of 15.75 above 1.8 K (Fig. S9, ESI†) under the condition of 1000 Oe field.
and 7.44 cm3 mol-1 K. This is close to what is expected for eight The energy barrier (ΔEeff) and relaxation time (τ0) were only
CuII ions (S = 1/2, g=2) and one ErIII ion (S = 3/2, 4I15/2) of 14.48 calculated through Debye equation: ln(χ"/χ') = ln(ωτ0) + Eeff/kBT.
cm3 mo-1 K for 1 and eight CuII ions (S=1/2, g=2) and one YbIII The perfect fitting plots and results are given in Fig. S11, ESI†
ion (S = 1/2, 2F7/2) of 5.57 cm3 mo-1 K for 2. For complex 1, the and table. S4, ESI†. Complex 2 showed peak at 1000 Oe dc field
χMT values of 1 firstly decrease slowly from 14.48 cm3 mo-1 K at (Figure 3). This means the emergence of SMM behaviour.58-60
300 K to 5.14 cm3 mo-1 K at 5 K, and then drastically drop to For most of the reported Cu-Ln compounds, DyIII ion is the best
4.39 cm3 mo-1 K at 1.8 K. The χMT product for 2 steadily candidate to design and construct SMMs owing to the strong
decreases with decreasing temperature, reaching a minimum spin-orbit coupling and extremely oblate electron density.42-45
value of 1.41 cm3 mo-1 K at 1.8 K. This type of behavior is However, it is rarely seen for Cu-Yb compounds exhibiting
consistent with the antiferromagnetic relationship between single-molecule magnets. In this work, our Cu-Yb compound
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the metal centers.49-51
shows single-molecule magnet behavior. This may be due to
The magnetization data of complexes 1-2 were measured our sandwich construction give the YbIII ion an axially-
under magnetic field range 1-7 T and temperature range 1.8-8 coordinated ligand environment. Yb (III) has a more
K (Fig. S7, ESI†). For 1, the magnetization sharply increased at complicated energy splitting. When mJ =± 5/2, Yb (III) is mainly
low fields, and then slight increases to get saturation. Complex composed of oblate electron density with only small lobes
1 showed a steady increase and was saturated at 7 T. The extending toward the z-axis.61 Considering that our sandwich-
saturation values of the various field lines did not overlap type compound has a central cavity along the z-axis direction,
significantly, indicating the presence of magnetic anisotropy and the planar spacing of our Cu-Yb complex is 3.526 Å, which
and undetermined ground state in complex 1. The will be more conducive to axial extension. According to the
magnetization of 2 kept to increase at the whole field without fitting of Arrhenius equation τ = τ0 exp (ΔEeff / kBT) (where τ0 is
saturation. Complex 2 did not reach saturation at 7 T, its the pre-exponential factor, ΔEeff is the effective energy barrier
magnetization was 2.5473 μB. The variable field lines of to the relaxation of the magnetization, and kB is the Boltzmann
complex 2 almost coincided, indicating that there are a large constant), we can deduced ΔEeff =6.84 K and τ0 = 1.04×10-5 s
number of low excited states in complex 2.52-57
for complex 2, and we also compared the energy with that in
the reported Cu-Ln SMMs(Table S6).62-66
Figure. 2 χMT vs. T plots for complexes 1-2 over the temperature range 2-300 K under a
magnetic field of 1000 Oe
Figure 3. Temperature dependence (left) and frequency dependence (right) of the χ'
(top) and χ'' (bottom) susceptibility measured at several temperatures and 1 kOe dc
field for (2).
Dynamic magnetic properties
In order to further investigate the dynamic magnetic behavior
of three complexes, the temperature-dependent ac
susceptibilities of 1-2 were determined in the range of 2-20 K
under different dc fields. For complexes 1 and 2, the in-phase
(χ') and out-of-phase (χ") magnetic susceptibilities do not
exhibit temperature dependence under zero Oe dc field (Fig.
S8, ESI† and Fig. S10, ESI†). But an external dc field was applied,
The Cole–Cole plots we have fitted of YbIII (2) containing the χ'
and χ'' data show a relatively symmetrical semi-circular shape,
as shown in Fig. 4. At high temperatures, the Debye model
gives α parameter of 2 below 0.3, which mainly indicates a
single relaxation time in the present relaxation process (Table.
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S5). The magnetization relaxation time (τ) and plotted as ln(τ) measurements reveal antiferromagnetic coupling interactions
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vs. 1/T of 2 were shown in Fig. 5.
in complexes 1-2. The alternating magnetic susceptibility
measurements reveal that complex 2 show out-of-phase ac
susceptibility signal, a typical of SMMs. The energy barrier for
2 assessed from ac susceptibility data is 6.84 K and the
corresponding relaxation time is τ0 = 1.04×10-5 s.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21671093, 21271097 and 21701078).
Notes and references
1
M. S. Lah and V. L. Pecoraro, J. Am. Chem. Soc., 1989, 111 7258-
Figure 4. The Cole-Cole plots fitting of YbIII (2) between 3.0 K and 6.0 K
7259
2
R. Sessoli, H. L. Tsai, A. R. Schake, S. Y. Wang, J. B. Vincent, K.
Folting, D. Gatteschi, G. Christou and D. N. Hendrickson, J. Am.
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D. Moon, K. Lee, R. P. John, G. H. Kim, B. J. Suh and M. S. Lah.
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P. Happ, C. Plenk and E. Rentschler. Coord. Chem. Rev., 2015, 289-
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7
A. J. Stemmler, J. W. Kampf, M. L. Kirk, B. H. Atasi and V. L.
Figure 5. Magnetization relaxation time (τ), plotted as ln(τ) vs. 1/T for 2. The solid lines
Pecoraro. Inorg. Chem., 2015, 38, 2807-2817
being fits of the data using the experimental data.
8 C. M. Zaleski, E. C. Depperman, J. W. Kampf, M. L. Kirk and V. L.
Pecoraro. Inorg. Chem., 2006, 45, 10022-10024.
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P. Poddar and R. Mondal. Inorg. Chem., 2015, 54, 8197-8206
In conclusion, we have successfully synthesized two complexes
with sandwich construction. Magnetic susceptibility
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