Slow magnetic relaxation in mononuclear gadolinium(III) and dysprosium(III) oxamato complexes

Article abstract of DOI:10.1016/j.poly.2019.04.056

A series of lanthanide(III) oxamate complexes of formula Na[Eu(4-HOpa)₄(H₂O)]?2H₂O (1), Na[Gd(4-HOpa)₄(H₂O)]?2H₂O (2), Na[Dy(4-HOpa)₄(H₂O)]?2H₂O (3), and Na

Full text of DOI:10.1016/j.poly.2019.04.056

Accepted Manuscript
Slow magnetic relaxation in mononuclear gadolinium(III) and dysprosium(III)
oxamato complexes
Tamyris T. da Cunha, Vitor M.M. Barbosa, Willian X.C. Oliveira, Carlos B.
Pinheiro, Emerson F. Pedroso, Wallace C. Nunes, Cynthia L.M. Pereira
PII:
S0277-5387(19)30303-1
https://doi.org/10.1016/j.poly.2019.04.056
POLY 13916
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
8 April 2019
28 April 2019
Please cite this article as: T.T. da Cunha, V.M.M. Barbosa, W.X.C. Oliveira, C.B. Pinheiro, E.F. Pedroso, W.C.
Nunes, C.L.M. Pereira, Slow magnetic relaxation in mononuclear gadolinium(III) and dysprosium(III) oxamato
complexes, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.04.056
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Manuscript Revised
Slow magnetic relaxation in mononuclear
gadolinium(III) and dysprosium(III) oxamato
complexes**
Tamyris T. da Cunha,† Vitor M. M. Barbosa,† Willian X. C. Oliveira,† Carlos B.
Pinheiro,‡ Emerson F. Pedroso,§ Wallace C. Nunes,|| Cynthia L. M. Pereira*,†
†Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas
Gerais. Av. Antônio Carlos 6627, Pampulha, Belo Horizonte, Minas Gerais, 31270-901
Brazil. e-mail: cynthialopes@ufmg.br
‡Departamento de Física, Instituto de Ciências Exatas, Universidade Federal de Minas
Gerais. Av. Antônio Carlos 6627, Pampulha, Belo Horizonte, Minas Gerais, 31270-901
Brazil;
§Centro Federal de Educação Tecnológica de Minas Gerais, Av. Amazonas 5523, Belo
Horizonte, Minas Gerais 30421-169, Brazil;
|| Instituto de Física, Universidade Federal Fluminense, Av. Gal. Milton Tavares de Souza,
s/nº, Campus da Praia Vermelha, 24210-346, Niterói, Rio de Janeiro, Brazil.
**This work is dedicated to Prof. Miguel Julve on the occasion of his 65th birthday and
for being an example of a generous and nice person and a brilliant scientist who has
inspired a generation of chemists worldwide. It is an enormous pleasure for us to
collaborate with him.
1

ABSTRACT
A series of lanthanide(III) oxamate complexes of formula Na[Eu(4-HOpa)4(H2O)]∙2H2O
(1), Na[Gd(4-HOpa)4(H2O)]∙2H2O (2), Na[Dy(4-HOpa)4(H2O)]∙2H2O (3), and Na[Ho(4-
HOpa)4(H2O)]∙2H2O (4) (4-HOpa= N-4-hydroxyphenyloxamate) were synthesized and
characterized. The crystal structures of complexes 1-4, as well as the oxamate ligand in
the ethyl ester acid form (Et[4-HOpa]), were elucidated by single crystal X-ray diffraction.
1-4 are isostructural, and they crystallize in the P21/n space group of the monoclinic
system. The crystal structures of 1-4 consist of mononuclear lanthanide(III) complex
anions, [Ln(4-HOpa)4(H2O)]– (Ln= Eu3+, Gd3+ Dy3+ and Ho3+), coordinated sodium(I)
,
,
countercations, and crystallization water molecules, resulting in a 2D NaILnIII
supramolecular coordination network. The coordination sphere of Ln3+ ions consists of
square-face capped square antiprism (SAPRS-9). Magnetic properties were investigated
for 1-4 in the 2-300 K temperature range using polycrystalline samples. Complex 2
exhibits an unusual and rare slow-relaxation of the magnetization for Gd3+ ions under DC
applied field of 1.0 kOe, behaving as a field-induced single-ion magnet as occurs for the
Dy3+ complex 3. The Eu3+ complex 1 has a nonmagnetic ground state, while the Ho3+
compound 4 behave as a paramagnet, and do not exhibit relaxation of the magnetization
with or without an applied field.
Keywords: oxamate ligands; lanthanide complexes; crystal structures; slow magnetic
relaxation; single-ion magnets
2

1. Introduction
Since the report of the first single-molecule magnet that exhibited magnetic bistability
with the potential for data-storage applications at temperatures of a few Kelvin [1],
several researchers in the world have directed their efforts to overcome this limitation,
increasing the temperature in compounds containing lanthanide ions for which this
phenomenon can be detected [2]. The discovery of magnetic relaxation of the
magnetization in mononuclear terbium(III)- and dysprosium(III)-bis(phthalocyaninato)
complexes at approximately [3] 40 K puts the strategy of using lanthanides ions for the
obtention of molecules containing only one magnetic ion, the so-called single ions
magnets (SIMs), in the spotlight. A factor in achieving such magnetic effects seems to be
the presence of a large single-ion magnetic anisotropy which depends on the type of
organic ligand involved. Thus, lanthanides(III) ions can be very promising in quantum
information processing and high-data storage at the molecular and atomic level [4].
In this direction, research groups worldwide have been invested their efforts in the
discovery of new lanthanide molecules containing different types of ligands with the
potential to become a magnetic storage device [5]. Recently, the discovery of
dysprosium(III)-based metallocenium cations of general formula [Dy(CpR)2]+ (CpR =
C5R5; R = H, alkyl or silyl) that contain a Dy3+ ion sandwiched between two
cyclopentadienyl-type aromatic π ligands, namely dysprosocenium, brings back this
research in evidence since the magnetic bistability phenomena in this compound was
detected at 60 K, is considered very high for such kind of molecule [6,7]. That being so,
it is essential to keep looking for new ligand types that can result in high-temperature
SIMs for potential data-storage applications.
In our research work we are dealing with N-substituted oxamate ligands, which
are very versatile ligands that present a large variety of binding modes combined with
innumerable possibilities of substituent groups, either aliphatic [8] or aromatic, such as
phenyl [9], phenylene [10] and polyaromatics [11]. This ligand versatility brings to these
plethora of complexes several different possibilities beyond magnetism [12–17], such as
metal organic frameworks for click chemistry catalysis [18] and decontamination of
organic pollutants for environmental purposes [19], hybrid materials based in carbon
nanotubes [20] and Vycor glass (PVG) [21] for magnetic studies, hybrid catalyst
3

anticancer agent [22] and amphiphilic catalyst for biphasic process [23], among others
[24,25].
The design of oxamate ligands has been mainly focused on the building of
molecular edifices with transition metal ions. Some examples of lanthanide-containing
oxamate complexes were reported on the literature [26–31], the majority involving the
self-assembly of 3d and 4f paramagnetic metal ions. The first report of a compound
containing only oxamate ligands and lanthanides ions was described a few years ago [32].
It consisted of mononuclear dysprosium(III) complex with the N-(2,6-
dimethylphenyloxamate ligand, resulting in a supramolecular 2D hydrogen-bonded
polymer that exhibited a slow magnetic relaxation behavior typical of a SIM at low
temperatures [32].
Herein, we present the synthesis, crystal structures and magnetic properties of the
first series of mononuclear oxamate-based lanthanide coordination compounds of general
formula Na[Ln(4-HOpa)4(H2O)]∙2H2O [Ln= Eu3+ (1), Gd3+ (2), Dy3+ (3), and Ho3+ (4)],
where 4-HOpa stands for N-4-hydroxyphenyloxamate, obtained from the reaction of
corresponding LnX3 salts [X= Ac or Cl] with the sodium(I) salt of the proligand Na[4-
HOpa] (Scheme 1). For the first time, it is reported an unusual slow relaxation of the
magnetization for a gadolinium(III)-oxamato complex 2, behaving as single ion magnet
as the related dysprosium(III)-oxamato complex 3.
4

Scheme 1. The general chemical structure for the oxamate ligand derivatives Et[4-
HOpa] (R = Et) and Na[4-HOpa] (R = Na).
2. Experimental Section
2.1. General procedures
All chemicals and solvents used were of analytical grade and used without further
purification in the syntheses. Elemental analyses were carried out with a Perkin-Elmer
2400 analyzer, and IR spectra were recorded from KBr pellets in the range 4000 and 400
cm–1 on a Perkin Elmer 882 Spectrophotometer. NMR spectra of ligands were obtained
with a Bruker AVANCE-III 400 Nanobay spectrometer. Chemical shifts () are reported
in ppm concerning dmso-d6 as the internal standard. Melting points (m.p.) were
determined by the digital device MQAPF-302 12038DV from Microquímica. X-ray
powder diffraction (PXRD) intensities for polycrystalline samples of 1−4 were measured
at 293 K on a Shimadzu XRD-7000 X-RAY (Cu Kα, λ = 1.54056 Å) by scanning over
the range of 5−40° with steps of 2°/s. Simulated PXRD patterns of 1−4 were generated
with Mercury® software using crystal data retrieved from CIF file. Magnetic
measurements were performed using a Cryogenics SQUID magnetometer model S700X
with applied magnetic fields of 0.020 T (for DC measurements) and a Quantum Design
PPMS (for AC susceptibility measurements). The AC and DC measurements were
5

performed in samples placed in gelatin capsules with mineral oil to prevent crystals
movement. All diamagnetic corrections of the constituent atoms were estimated from
Pascal’s constants [33]. All experimental magnetic measurements were also corrected for
the sample holder.
2.2. Synthesis of Et[4-HOpa]
The proligand was prepared in a standard manner[34] from the condensation of 4-
aminophenol (1.08 g, 9.9 mmol) and ethyl oxalyl chloride (1.10 mL, 9.9 mmol) in the
presence of triethylamine (1.38 mL, 9.9 mmol) in tetrahydrofuran (THF), and it was
isolated as the ethyl ester acid derivative. Crystals of Et(4-HOpa), well-formed colorless
needles, were obtained in THF recrystallization. Yield: 57% (1.18 g). M.p. = 182 ºC.
Selected IR peaks (cm–1): 3340 (s), 1732 (s), 1678 (s), 1560 (w), 1516 (m), 1022 (m), 834
(m) cm–1. 1H NMR (dmso-d6), 400 MHz, 25 °C: δ = 1.3 (t, J = 7.1, 3H); 4.2 (q, J = 7.1,
2H); 6.7 (d, J = 8.9, 2H); 7.5 (d, J = 8.9, 2H); 9.4 (s, 1H); 10.5 ppm (s, 1H). 13C NMR
(dmso-d6, 101 MHz, 25°C): δ = 161.02, 155.13, 154.55, 129.05, 122.28, 115.22, 62.32,
13.90 ppm. Elemental analysis calcd (%) for C10H11NO4: C 57.40 H 5.30, N 6.70; found
C 57.65, H 5.10, N 6.94.
2.3. Synthesis of Na[4-HOpa]
The sodium(I) salt of the proligand was synthesized according to literature methods [34,
35]. NaOH (0.19 g, 4.78 mmol) was added to a methanolic solution (20.0 mL) of Et(4-
HOpa) (1.0 g, 4.78 mmol). The resulting mixture was refluxed at 80oC for 1 h under
continuous stirring. The resultant solid was filtered, washed with methanol and dried at
60oC for 1 h. Yield: 77% (0.747 g). Selected IR peaks (cm–1): 3408 (s), 3332 (s), 1670
(s), 1642 (s), 1540 (w), 1522 (m), 1258 (m), 820 (m), 804 (m) cm–1. 1H NMR (dmso-d6),
400 MHz, 25 °C: δ = 6.70 (d, J = 8.8, 2H); 7.55 (d, J = 8.9, 2H); 9.43 (s, 1H); 10.03 (s,
1H). 13C NMR (dmso-d6, 101 MHz, 25°C): δ = 163.87, 163.08, 153.65, 130.26, 121.02,
115.03 ppm. Elemental analysis calcd (%) for C8H6NNaO4: C 47.30 H 2.98, N 6.90;
found C 47.71, H 2.84, N 7.04.
2.4. Synthesis of Na[Eu(4-HOpa)4(H2O)]∙2H2O (1)
6

An aqueous solution (20.0 mL) of Eu(Ac)3·3H2O (0.33 mmol, 0.126 g) was added
dropwise to an aqueous solution of Na(4-HOpba) (1.0 mmol, 0.203 g) under continuous
stirring. The resulting light yellow solution was heated at 60 °C for 15 min. Colorless
prismatic single crystals were obtained after 5 days at room temperature. Yield: 68%
(0.160 g). Selected IR peaks (KBr/cm–1): 3338 (s), 1640 (s), 1552 (w), 1512 (m), 1370
(m), 1272 (m), 836 (m), 812 (w) cm–1. Elemental analysis calcd (%) for
C32H30EuN4NaO19: C 40.48, H 3.18, N 5.90; found C 40.17, H 2.82, N 6.20.
2.5. Synthesis of Na[Gd(4-HOpa)4(H2O)]∙2H2O (2)
An aqueous solution (20.0 mL) of Gd(Ac)3·4H2O (0.33 mmol, 0.134 g) was added
dropwise to an aqueous solution of Na(4-HOpba) (1.0 mmol, 0.203 g) under continuous
stirring. The resulting light yellow solution was heated at 60 °C for 15 min. Colorless
prismatic crystals were obtained after 5 days at room temperature. Yield: 44% (0.112 g).
Selected IR peaks (KBr/cm–1): 3350 (s), 1650 (s), 1560 (w), 1514 (m), 1386 (m), 1242
(m), 834 (m), 816 (w) cm–1. Elemental analysis calcd (%) for C32H30GdN4NaO19: C 40.25,
H 3.17, N 5.87; found C 40.80, H 3.17, N 5.83.
2.6. Synthesis of Na[Dy(4-HOpa)4(H2O)]∙2H2O (3)
An aqueous solution (20.0 mL) of Dy(Ac)3·3H2O (0.33 mmol, 0.129 g) was added
dropwise to an aqueous solution of Na(4-HOpba) (1.0 mmol, 0.203 g) under continuous
stirring. The resulting light pink solution was heated at 60 °C for 15 min. Light pink
prismatic single crystals were obtained after 5 days at room temperature. Yield: 82%
(0.195 g). Selected IR peaks (KBr/cm–1): 3354 (s), 1654 (s), 1638 (s), 1560 (w), 1514
(m), 1386 (m), 1242 (m), 836 (m), 810 (w). Elemental analysis calcd (%) for
C32H30DyN4NaO19: C 40.03, H 3.15, N 5.84; found C 39.97, H 3.30, N 5.73.
2.7. Synthesis of Na[Ho(4-HOpa)4(H2O)]∙2H2O (4)
An aqueous solution (20.0 mL) of HoCl3∙6H2O (0.33 mmol, 0.125 g) was added dropwise
to an aqueous solution of Na(4-HOpba) (1.0 mmol, 0.203 g) under continuous stirring.
The resulting light pink solution was heated at 60 °C for 15 min. Light pink prismatic
single crystals were obtained after 5 days at room temperature. Yield: 69 % (0.166 g).
Selected IR peaks (KBr/cm–1): 3354 (s), 1602 (s), 1554 (m), 1514 (m), 1370 (m), 1244
7

(m), 836 (m), 812 (w). Elemental analysis calcd (%) for C32H30HoN4NaO19: C 39.93, H
3.14, N 5.82; found C 39.29, H 3.29, N 5.77.
2.8. X-ray Crystallographic Measurements
Table 1 provides a summary of the crystal data, data collection and refinement parameters
for Et[4-OHpa] and 1-4. All diffraction data were collected on an Oxford-Diffraction
GEMINI-Ultra diffractometer at 298 K (Et[4-HOpa], 1, 3) and 150 K (2, 4) using Mo Kα
radiation (λ = 0.71073 Å). CRYSALIS Pro software [36] was used respectively for data
integration, scaling of the reflections and analytical absorption corrections. Space group
identification was done with XPREP [37], and structure solution was carried out by direct
methods using SUPERFLIP [38]. Refinements were performed using SHELXL-2018 [37]
based on F2 through the full-matrix least square routine. All non-hydrogen atoms were
refined with anisotropic displacement parameters. All hydrogen atoms were located in
difference maps and included as fixed contributions according to the riding model [39].
Values C–H = 0.97 Å and Uiso(H) = 1.2 Ueq(C) were used for aromatic and methylene
(R–CH2–R’) carbon atoms, C–H = 0.96 Å and Uiso(H) = 1.5 Ueq(C) for methyl carbon
atoms, N–H = 0.97 Å and Uiso(H) = 1.2 Ueq(N) for amide groups, O–H = 0.87 Å and
Uiso(H) = 1.5 Ueq(O) for the phenyl OH groups and O–H = 0.90 Å and Uiso(H) = 1.5
Ueq(O) for water molecules. Graphics were prepared with Mercury® [40] and VESTA
[41]. Crystallographic data for the reported structures have been deposited at the
Cambridge Crystallographic Data Centre with CCDC reference numbers 1840845 (Et[4-
HOpa]), 1840846 (1), 1840847 (2), 1840848 (3) and 1840849 (4).
Table 1. Summary of Crystallographic Data for Et[4-HOpa] and 1-4.
8

Compound
Et(4-HOpa)
1
2
3
4
Formula
Fw /g mol–1
T /K
C10H11NO4
209.19
C32H30EuN4NaO19
949.54
C32H30GdN4NaO19
954.83
C32H30DyN4NaO19
960.08
C32H30HoN4NaO19
962.52
298
298
150
298
150
λ /Å
0.71073
Monoclinic
0.71073
0.71073
0.71073
0.71073
Crystal
System
Monoclinic
Monoclinic
Monoclinic
Monoclinic
Space group
P21/n
P21/n
P21/n
P21/n
P21/n
a / Å
b / Å
5.2065(5)
15.7243(15)
12.8790(11)
90
14.5377(12)
12.246(4)
12.246(4)
90
14.5944(11)
12.3188(11)
20.002(3)
90
14.5856(2)
12.3101(2)
20.0288(3)
90
14.5944(11)
12.3188(11)
20.002(3)
90
c / Å
α / o
β / o
95.782(9)
90
91.106(10)
90
91.356(9)
90
91.3570(10)
90
91.356(9)
90
γ / o
V / Å3
Z
1049.02(17)
4
3567.0(12)
4
3595.1(6)
4
3595.17(9)
4
3567.22(9)
4
ρ / mg m–3
μ /mm–1
F(000)
1.325
1.768
1.764
1.774
1.792
0.104
1.861
1.947
2.181
2.321
440
1904
1908
1916
1920
Crystal size/
0.69×0.13×0.09
0.47×0.46×0.20
0.30×0.10×0.06
0.22×0.15×0.10
0.88×0.59×0.26
mm3
Reflections
collected
(Rint)
6862 (0.064)
2148
18610 (0.0432)
7299
26011 (0.0700)
7351
129327 (0.0783)
7358
93721 (0.0468)
7293
Reflections
with I≥2σ(I)
Unique
1346
5115
4999
6616
6378
Reflections
Goodness-
of-fit on F2
1.032
1.067
1.064
1.063
1.075
Ra, wRb
0.0558, 0.1446
0.0899, 0.1446
0.0377, 0.0836
0.0695, 0.0991
0.0446, 0.0859
0.0815, 0.1017
0.0243, 0.0613
0.0286, 0.0642
0.0253, 0.0615
0.0330, 0.0668
Ra, wRb (all
data)
Larg. diff.
peak and
hole / e Å–3)
0.207, –0.206
1.651, –0.755
1.738, –1.160
0.993, –0.741
2.607, –0.678
aR = Σ ||Fo| – |Fc|| / Σ|Fo|. bwR [Σ (|Fo|2 – |Fc|2)2 / Σ|Fo|2]1/2.
9

3. Results and discussion
3.1. Synthesis and characterization
The proligand Et[4-HOpa] and the corresponding sodium(I) salt Na[4-HOpa]
were obtained in a similar way as described previously [34,35]. Crystal structure of Et(4-
HOpa) will be described for the first time in the present work. NMR data confirm the
molecular formula of both compounds Et(4-HOpa) and Na(4-HOpa). The IR spectra of
Et(4-HOpa) and Na(4-HOpa) are very similar, except for the presence of an intense
absorption peak at 1732 cm–1 in the spectrum of Et(4-HOpa) which is attributed to the
stretching vibration of the ethyl ester group. This result suggests that the basic hydrolysis
of Et(4-HOpa) resulted in Na(4-HOpa), replacing the ethyl substituent by sodium(I) ion
(see Scheme 1).
All complexes were synthesized following the same procedure, and single crystals
were obtained after five days at room temperature. The yield of products varies from 44
(2) to 82% (3). The analysis and refinement of single-crystal X-ray diffraction data
showed that all compounds are isostructural, and the IR spectra of 1-4 are very similar
between them also. The main stretching vibrations of 1-4 are observed in the following
regions: (i) 3338-3350 cm–1, related to N–H and O–H bonds; (ii) 1640-1654 cm–1,
attributed to the C=O bonds; 1370-1386 cm–1 reported to the C–N bonds. The PXRD
patterns of 1-4 are very similar (See SI, Figure S3). This feature corroborates that 1-4 are
isostructural, as revealed by single-crystal X-ray diffraction analysis.
3.2. Crystal structures
3.2.1. Crystal structure of Et[4-HOpa]
The proligand in the form of the ethyl ester Et[4-HOpa] crystallizes in the monoclinic
space group P21/n. The asymmetric unit consists of one Et[4-HOpa] molecule featuring
a 4-phenol substituent at the nitrogen atom of the ethyl ester oxamic acid moiety, as
shown in Figure 1(a). The oxamate group exhibits an s-trans conformation being almost
coplanar with the aromatic phenyl ring. The dihedral angle between the mean normal
vector plane of oxamate group and the normal vector plane of phenyl ring is
4.7(4)º, where the phenyl mean plane is defined by C3/C4/C5/C6/C7/C8, and the
oxamate means plane involves N1/C2/O3/C1/O1/O2 set of atoms.
10

Figure 1. (a) Representation of hydrogen-bonded dimers with ellipsoids at 50% of
probability level and (b) double layers interactions. Hydrogen bonds are represented in
dashed green lines along. (c) Packing view along the a axis in the spacefilling model and
hydrogen atoms were omitted for clarity. Color code: C in gray, N in light blue, O in red.
The oxamate groups from two centrosymmetrically-related Et(4-HOpa)
molecules form hydrogen-bonded dimers along crystallographic a axis , where the
amide N–H group interacts with the ester carbonyl oxygen atom in a double
symmetric motif [N1–H1∙∙∙O1ii = 2.55 Å; ii: 1–x, 2–y, –z] which is typical of oxamato
derivatives (Figure 1(a)) [42]. Additional hydrogen bonds between the carbonyl oxygen
atom of the amide group and the phenol group [O4−H4A∙∙∙O3i = 1.99 Å; i: 1/2+x, 5/2–
y,–1/2+z] of adjacent hydrogen-bonded dimers are responsible for the resulting
crystal packing of Et(4-HOpa) (Figure 1(b)). These hydrogen-bonding interactions
lead to interleaved dimers layers along the crystallographic b axis, as featured in
Figure 2(b). Besides, the double layers pack each other along the crystallographic
b axis direction driven by aliphatic ethyl to aromatic phenyl group interactions
(Figure 1(c)). A 3D supramolecular architecture is achieved taking into account the
combination of the interactions between dimers along the axis a and the double
layers interaction along b and c. Finally, no ∙∙∙ stacking interactions were
observed in Et(4-HOpa).
3.2.2. Crystal structures of Na[Ln(4-HOpa)4(H2O)]∙2H2O (Ln3+= Eu (1), Gd(2) Dy (3)
and Ho3+ (4)
11

Crystal structure analysis data revealed that complexes 1-4 are isostructural. They
crystallize in the monoclinic space group P21/n (Table 1). One crystallographically
independent LnIII ion, four 4-HOpa ligands and one sodium cation – playing the role of
charge balancing counterion – along with three water molecules (one of coordination and
the other two cocrystallized) compose the asymmetric unit, as illustrated in Figure 2 for
the Europium derivative 1 (see Figure S3 for 2-4). The nine-coordinated LnIII ion of 1-4
presents a square-face capped square antiprism (SAPRS-9) distorted geometry [43]
composed by four carboxylate oxygen atoms [O2, O6, O10, O14] and four carbonyl
amide oxygen atoms [O3, O7, O11, O15] from the oxamate groups in a bidentate
coordination mode, and also by one water molecule [O17] with very similar Ln−O bond
lengths (Table 2). The O2 atom occupies the capped position of the square face defined
by O6, O7, O11, and O15 atoms, the other square face being composed by O3, O10, O14
and O17 atoms, as illustrated in Figure 2(b) for the europium derivative 1. The average
value of the angles , the angle between the diagonals of the two squares faces (skew
angle) [2], are 39.05º (1), 39.17º (2), 39.14º (3), and 39.05º (4).
12

Figure 2. (a) Asymmetric unit of 1 with ellipsoids at 50% of probability level, hydrogen
atoms were omitted for clarity. Color code Eu (pink), Na (purple), C (gray), N (light blue),
O (red). (b) Different views of the coordination polyhedron of Eu1 in square-face capped
square antiprism distorted geometry.
The five-coordinated NaI ion coordination sphere of 1-4 is composed of three
carboxylate oxygen atoms [O6, O2ii, O14ii], one phenol oxygen atom [O8iii], and one
coordinated water molecule [O18] (ii: 1.5–x, 1/2+y, 1/2–z; iii: 2–x, 2-y, –z). The sodium
coordination polyhedron is better described as square a based distorted pyramid in an
anti-Berry conformation , as reflected by the value of the geometric parameter [44] τ of
0.28 [O6–Na1–O18 = 156.8(6)º and O8iii –Na1–O14ii = 140.2(2)º] [45–47]. The sodium
atom is lowered by 0.16 Å from the mean basal plane of the pyramid opposite to the apical
position occupied by the atom O2ii. Different views of the coordination polyhedral of 2-
4 are depicted in the Supporting Information, Figure S5.
13

Figure 3. Crystal packing views for 1, highlighting Eu1 (purple) and Na1 (yellow)
coordination polyhedrons with the ligand L (light green) along the (a) a axis and (b) b
axis, arrows in green and pink representing the layers of coordinate covalent bond
(Eu−O)/ion dipole (Na−O) and intermolecular interactions, respectively.
Table 2. Selected bond lengths and interatomic distances for 1-4.
Bond length or
interatomic distance / Å
Ln−O2
1
2
3
4
2.419(3)
2.387(3)
2.358(4)
2.460(3)
2.402(3)
2.429(3)
2.374(4)
2.423(3)
2.432(4)
7.255(2)
2.447(3)
2.422(3)
2.393(4)
2.486(4)
2.427(4)
2.452(3)
2.410(4)
2.454(3)
2.456(4)
7.3014(7)
2.434(2)
2.402(2)
2.376(2)
2.473(2)
2.406(2)
2.436(2)
2.382(2)
2.435(2)
2.435(2)
7.2881(4)
2.422(2)
2.389(2)
2.363(2)
2.460(2)
2.399(2)
2.424(2)
2.375(2)
2.423(2)
2.417(2)
7.255(2)
Ln−O3
Ln−O6
Ln−O7
Ln−O10
Ln−O11
Ln−O14
Ln−O15
Ln−O17
Ln∙∙∙Ln
A 2D NaILnIII supramolecular coordination framework composes the crystal
packing of 1-4, as illustrated in Figure 3 for the europium derivative 1. Along the b axis,
the NaI and LnIII ions are connected through oxygen atom bridges leading to a zig-zag
chain [Ln1−O6−Na1−µ−(O14i, O2i)−Ln1i; symmetry code (i)= 3/2–x, 1/2+y, 1/2+z]. The
shortest intrachain Ln···Ln distances are 7.255(2) (1), 7.3014(7) (2), 7.2881(4) (3), and
7.255(2) Å (4) values which are far enough so that no magnetic coupling was observed
in the magnetic measurements. Along the ac plane, the connection between the NaI and
LnIII ions is established using one unit of the ligand through carboxylate oxygen atom O6
and phenol oxygen atom O8 leading to layers, as shown in Figure 3(b). In addition, there
are intermolecular Van der Waals forces, mainly between the stacked phenol rings. Also,
the crystal packing of 1-4 is ruled by a complex hydrogen bond network between phenol
and carboxylate groups and water molecules maintaining the three-dimensional structure
(for compounds 1-4, see ESI tables S1-S4).
14

3.3. Magnetic Properties
3.3.1. DC magnetic measurements
Considering the magnetic properties of trivalent lanthanides ions with a first-order
orbital momentum (4fn electronic configuration with n = 0-14), the spin-orbit coupling
(SOC) partially removes the degeneracy of the free ion 2S+1 ground term, where S and L
are respectively the quantum numbers associated with the spin and orbital angular
momenta and  is denoted as S (L = 0), P (L = 1), D (L = 2), F (L = 3), G (L = 4), H (L =
5), I (L = 6) and so on. This gives rise to 2S+1J states, where J is the quantum number
associated with the total angular momentum defined as J = L + S (Russel Saunders
coupling), with J varying by an integer value from |L - S| to (L + S).
In general, the 2S+1J ground state of the Ln3+ ion is well separated in energy from
the first excited state, so that only the ground state is populated at room temperature and
below, and the magnetic susceptibility ( (J)) for a mononuclear lanthanide(III) complex
obeys the Curie law given by Eq 1:
푁푔퐽2 2 퐽(퐽 + 1)

(Eq 1)
(퐽) =

3 푘푇
Where T is the absolute temperature, N is the Avogadro constant,  is the Bohr`s
magneton, k is the Boltzmann constant, and gJ is the Zeeman factor. The saturation
magnetization is then given by M = NgJJ, where the gJ value can be expressed by Eq 2:
3
2
[푆(푆 + 1) ‒ 퐿(퐿 + 1)]
+
2퐽 (퐽 + 1)
(Eq 2)
푔퐽 =
In most cases, however, the MT vs. T plot for mononuclear lanthanide(III)
complexes deviates from the predicted one from eq 1. This deviation can be understood
by analyzing two effects. The first one is the crystal field, which partially removes the 2J
+ 1 degeneracy of the 2S+1J ground state in zero field and the second one is due to the
thermal population of excited states.
In this latter case, the presence of excited states not so far in energy from the
ground state can add a significant temperature independent magnetic contribution. The
magnetic susceptibility considering the free ion approximation [48] can then be described
as shown in Eq 3:
푁푔퐽2 2 퐽(퐽 + 1)
2푁 2 (푔퐽 ‒ 1)(푔퐽 ‒ 2)


(Eq 3)
(퐽) =
+

3 푘푇
3

15

where  is the spin-orbit coupling parameter in the Hamiltonian H =  L · S ( > 0 for a
4fn electronic configuration with n < 7 and  < 0 for n > 7).
The magnetic properties of polycrystalline samples of 1-4 were investigated in the
temperature range of 2.0-300 K in the form of MT versus T plots, M being the molar
magnetic susceptibility and T temperature (Figure 4). The MT values at room
temperature for 2-4 are in agreement with the calculated values according to Eq 1, as
shown in Table 4. At room temperature (see Table 3), the MT for 1 is 1.33 emu K mol–
1, which is different from the expected value of zero. However, since the thermally
populated excited states located at least 300 cm1 are populated, the observed value is in
agreement with the expected value for one isolated Eu3+ ion (~ 1.5 emu K mol–1) in similar
situations [48–50]. As T is lowered, MT decreases and tends to a value very close to zero.
Table 3. Experimental MT values at room temperature for 1, 2, 3 and 4 and their expected
MT values.
Compound
Calculated
Experimental
Theory
L
MT(cm3 K mol-1)
MT(cm3 K mol-1)
gJ
S
Free ion
term
7F0
1
2
3
4
5
2
6/2
7/2
3
0
5
6
0
1.33
7.88
8S7/2
7.88
14.17
14.07
4/3 5/2
5/4 4/2
6H15/2
5I8
13.99
13.70
Since the 7F ground term of the Eu3+ ion is split by the SOC into seven states, 7FJ
with J taking values from 0 to 6, the energy of the states are given by Eq 4 [48]:
E (J) =  J (J+1)/2
(Eq 4)
Where E(J) is the energy of 7F0 ground state taken as the origin. In the case of Eu3+
ions, the ground state is nonmagnetic, but the thermal population of the excited states
leads to a nonzero and thermally dependent magnetic moment. Considering that  is small
enough for the first excited states to be thermally populated, the equation for the
calculation of the magnetic susceptibility of Eu3+ is:
16

퐽(퐽 + 1)
2푘푇
(2퐽 + 1) (퐽)푒푥푝 [ ‒ 
]
6
퐽 = 0
∑

(Eq 5)
=
푀
퐽(퐽 + 1)
2푘푇
퐽 = 0(2퐽 + 1) 푒푥푝 [ ‒ 
6
]
∑
All of the gJ factors are equal to 3/2, except g0 which is equal to 5 (2 + L = 5). The
full equation is given in the literature [48]. The least-square fit of magnetic susceptibility
data of 1 resulted in  equal to 355 cm–1. The agreement factor is defined by [(MT)calcd
-
(MT)obs]2/ [(MT)obs]2 and it is equal to 9.2 x 10–6, indicating that theoretical curve
reproduces quite well experimental data. The calculated  value is in agreement with that
expected for mononuclear Eu3+ compounds [49].
Figure 4. (a) MT versus T plot for 1 (H= 0.02 T). The solid line represents the best-fit
curve of experimental data. (b) MT versus T plot (o) for 2 (H= 0.02 T). In the inset, it is
shown the M versus H plot of 2 at 3.5 K. Solid line represents the best-fit curve of
experimental data through the Curie law. The solid line in the inset corresponds to the
Brillouin function. (c) MT versus T plot (o) for 3 (H= 0.02 T). In the inset, it is shown
the M versus H plot of 3 at 1.7, 3.0 and 3.5 K. The lines in the M versus H plots are just
eye guides. (d) MT versus T plot (o) for 4 (H= 0.02 T). In the inset, it is shown the M
versus H plot of 4 at 3.5, 4 and 6 K.
17

The MT versus T plot for 2 is shown in Figure 4(b). At room temperature, the
MT value is 7.88 emu K mol–1, which is equal to the expected value for one Gd3+ ion
with no first-order orbital momentum. The magnetic susceptibility is constant and obeys
1
Curie law over a wide range of temperature. The curve of M versus T plot (see SI,
Figure S6) depicted a straight line. The value of C is 7.87 cm3 K mol1. In this compound,
the molecules are very well isolated, and Gd3+ do not present orbital contribution. The
8
crystal field effects do not perturb the S7/2 ground state, and the first excited state was
located at some 104 cm1 high in energy [48]. Below 10 K, a small increase of MT can
be associated with very weak ferromagnetic dipolar interactions or with the reorientation
of the crystals in the presence of magnetic field [48,49]. The M versus H curve at 3.5 K
presents a saturation value of 6.75 N at 60 kOe, being a characteristic curve of isotropic
compounds with S = 7/2.
The MT versus T plot for 3 is shown in Figure 4 (c). At room temperature, MT
value is 13.99 cm3 K mol–1, close to the expected value for one paramagnetic Dy3+ ion
(MT = 14.17 cm3 K mol–1) using a free ion approximation [49–51]. When the system is
cooled down from room temperature, the MT value smoothly reduces until it reaches the
value of 10.94 cm3 K mol–1 at 5 K. Below 5 K, a small increase of MT value appears,
probably due to the weak ferromagnetic dipolar interactions. The reduction of the MT
product for 3 from room temperature to 5 K, is related with the thermal depopulation of
the mJ levels of the free-ion 6H15/2 ground state that arrive from spin-orbit coupling that
couples the ground state with the excited ones, besides the crystal field splitting effects
into these levels [52–55]. The M versus H measurements in different temperatures show
a strong magnetization increase until approximately 5 kOe, and above this value, M
increases more slowly, and no effects of saturation are verified. At 60 kOe, the
magnetization saturation values are 6.02 N, 5.45 N e 5.92 N at 1.7, 3.0 e 3.5 K,
respectively. The expected value of saturation magnetization for 3 considering gj= 4/3
and J=15/2 is 10 N. The resultants values are different from the expected ones, indicating
the presence of anisotropy and strong crystal field splitting of different components of mJ
[51,56]. The crystal field effects eliminate the degeneracy of the 2S+1J ground state at the
lanthanide(III) ions [52]. Moreover, the M versus HT–1 curves for 3 at distinct
temperatures does not superimpose (see Figure S7), corroborating with the presence of
magnetic anisotropy [52] in 3.
18

At room temperature, the MT value for 4 is 13.70 cm3 K mol–1, which is in
agreement with the expected one for one isolated holmium(III) ion (14.07 cm3 K mol–1)
[53]. The MT value remains slightly constant until 100 K, and below this temperature
the MT product reduces until reaching a value of 8.68 cm3 K mol–1 at 3.0 K. This
reduction of these values is associated with the splitting of energy levels caused by both
spin-orbit coupling and crystal field splitting, as occurs in 3. The M versus T curve (inset
of Figure 4(d)) shows a strong increase of the magnetization until approximately 10 kOe,
and above this value, M increases more slowly. The saturation of the M is not reached,
and at 60 kOe the following values are obtained: 5.58 (3.5 K), 5.98 (4.0 K) and 6.03 (6.0
K) N These values are smaller than the expected for holmium(III) ion (10.0 N). As
explained for 3, this occurs due to the presence of magnetic anisotropy. As the
temperature is lowered, the reduction of the saturation magnetization values can be
explained by the increase of the thermal population of energy levels with small J values.
The anisotropy exhibited by 4 is confirmed by the absence of superposition of the M
versus HT–1 curves at 3.5, 4.0 and 6.0 K (see Figure S8).
3.3.2. AC magnetic measurements
The dynamic magnetic behavior for 2-4 was analyzed through AC magnetic
susceptibilities measurements under an external DC field of 1.0 kOe. For all compounds,
both in-phase (’) and out-of-phase (’’) components of the AC molar susceptibility
exhibited a dependence with the frequency of the alternating AC field of 5 Oe in the
temperature range below 5.0 K. Under zero applied dc field, however, there was no
frequency dependence of the AC magnetic susceptibility for 2-4, more likely because of
the occurrence of a fast quantum-tunneling magnetization (QTM) relaxation.
Figure 5 shows the frequency dependence of both M’T and M"T for 2 at 1.0 kOe.
Frequency dependence of  ’ and  ’’ data obtained at different temperatures were
fitted by using the generalized Debye equation [57], resulting in a reasonably good
agreement between the calculated and experimental data (Figures 5(a) and (b) and Table
S5, SI). The temperature dependence of the relaxation times () afforded from the fit was
used to construct an Arrhenius plot (Figure 5(d)). The deviations between experimental
data and the linear behavior expected from the Arrhenius law is indicative of the existence
of several relaxation processes. The Cole-Cole plots constructed from the M"T vs. M'T
19

data at different temperatures and fitted curves show semicircular shapes with  values
within the range 0.14-0.21, which corroborate the existence of several relaxation modes
in 2 ( = 0 for a single relaxation process).
Figure 5. (a) Frequency dependence of M’T and (b) M"T for 2 under an applied field of
1.0 kOe in the frequency range of 10-10000 Hz. The solid lines represent the best fit using
the generalized Debye equation. (c) Cole-Cole plots for complex 2 under an applied field
of 1.0 kOe in the frequency range of 50-10000 Hz. The solid lines represent the best fit
based on the generalized Debye equation. (d) Temperature dependence of the average
relaxation time for complex 2 at 1kOe dc field. The red line is the best fits obtained
according to Eq. 5.
To investigate the temperature dependence of relaxation time of 2 in detail, the
experimental data were fitted considering multiple relaxation processes. Since for the
externally applied field of 1 kOe, the quantum-tunneling magnetization is expected to be
negligible, we adjusted the 1 vs. T data by using Eq. 5, which considers the single-
20

phonon direct process (parameters A and m), Raman process (parameters C and n), and
two-phonon Orbach process (parameters U and 0) [58].
‒ 1 = 퐴퐻푚푇 + 퐶푇푛 + 휏0 ‒ 1exp ( ‒ 푈/푘퐵푇)
(Eq 5)
휏
To avoid over-parametrization, we fixed the values of m = 1 (direct processes)
and n = 9 (Kramer ion). Best-fits results gave the following set of parameters (values in
parentheses are the standard errors): A = 2.9 (1), C = 2(1) x 105, U = 13 (1) K, and 0 =
6 (1) x 106 s. These values suggest that the relaxation time at a low-temperature region
is dominated mainly by the overcome of direct and Orbach processes.
Since the Gd3+ ion is very isotropic, only gadolinium(III) complexes with
particular coordination environments can behave as single-molecule magnets at very low
temperatures [59,60], as reported previously by Murugesu and coworkers for the GdIII-
EDTA complex (EDTA= ethylenediaminetetracetic acid)[61], which shows distinct
relaxation processes that each dominate in different regimes. They observed the absence
of magnetic relaxation in zero DC field due to the fast QTM promoted by internal dipolar
fields by removing Kramers degeneracy. For a relevant applied DC field, the QTM is
suppressed, and the slow relaxation of magnetization is observed associated with
interlevel spin-phonon transitions of direct and Raman mechanisms on the GdIII site [61].
These behaviors seem to be related with the axial symmetry of the coordination sphere of
the gadolinium(III) complex, which is nonacoordinated as observed for 2.
The AC magnetic properties of 3 in the form of in-phase (’) and out-of-phase
(’’) components of AC molar susceptibility per mononuclear unit evidence the
occurrence of field-induced slow magnetic relaxation effects typical of SIMs. At zero DC
magnetic field, both ’ and ’ show no dependence effects with the frequency.
However, when a field of 1.0 kOe was applied, relaxation of magnetization phenomena
was observed, as has been reported for other dysprosium(III)-based SIMs [32,62]. Figure
6 shows the frequency dependence of both  ’T and  ’’T for 3 at 1.0 kOe. Besides the
maximum in  ’’ observed at high frequency, a sort of plateau can be observed in the
low-frequency region. This behavior might be associated with two different relaxation
modes slightly separated in frequency. The existence of these low relaxation modes is
more evident when analyzing the Cole-Cole plots of 3, that shows a semicircular shape
followed by a plateau at lower frequencies (Figure 6(c)). Therefore, the  ’=f() and
21

 ’’=f() data were fitted by taking into account only the high-frequency region of
semicircular shape (200-10000 Hz). Quantitative determination of the relaxation rate for
this frequency range was obtained by fitting  ’=f() and  ’’=f() data obtained at
different temperatures with the generalized Debye equation, which gave  values (width
distribution of ) within the range 0.21-0.33, suggesting a narrow distribution of
relaxation rates (Table S6, SI). The temperature dependence of  obtained for 3 is shown
as the Arrhenius plot in Figure 6(d). Fitting these data using the Equation 10 (assuming
that the QTM is effectively quenched under relevant fields), we obtain a negligible value
for A, C =2(5), U = 16 (1) K, and 0 = 1.4(9) x 108 s. Therefore, there are only Raman
and Orbach processes associated with the high-frequency region (200 - 10000Hz). On the
other hand, the second relaxation mode occurring at the low-frequency region can be
associated with collective behavior, such as spin-glass-like phase since the process is
frequency independent and dominant at low temperature.
22

Figure 6. (a) Frequency dependence of M’T and (b) M’T for 3 under an applied field of
1.0 kOe in the frequency range of 10-10000 Hz. The solid lines represent the best fit of
experimental results. (c) Cole-Cole plots for complex 3 under an applied field of 1.0 kOe
in the frequency range of 50-10000 Hz. The solid lines represent the best fit based on the
generalized Debye equation. (d) Temperature dependence of the average relaxation time
for complex 3 at 1 kOe dc field. The red line represents the best-fit curve obtained
according to Eq. 5.
The holmium(III) derivative 4 displays no frequency dependence of AC magnetic
susceptibility, neither without an applied DC field or with a DC field up to 2 kOe.
Consequently, it does not exhibit a SIM behavior. The presence of slow relaxation of
magnetization in holmium(III) compounds is rare, even if a few examples are described
in the literature [63,64].
4. Conclusion
In this work, we described the synthesis and crystal structures of a new proligand, namely
4-hydroxyphenyl-N-oxamic acid ethyl ester, and the sodium salts of its corresponding
oxamato complexes containing lanthanide ions [Ln = Eu3+ (1), Gd3+ (2), Nd3+ (3), and
Ho3+ (4)]. Compounds 1-4 are isostructural and consist of mononuclear lanthanide(III)
complexes with four bidentate oxamato ligands coordinated to one metal ion, through the
carbonyl amide and carboxylate oxygen atoms, and one water solvent molecule are
completing the coordination sphere of lanthanide ions, resulting in square-face capped
square antiprism (SAPRS-9) coordination geometry. The supramolecular structure of all
complexes consists of an intricate two-dimensional NaILnIII coordination network.
Magnetic properties were investigated in the form of AC and DC magnetic
measurements. Thus, we discovered the unique features of Gd3+ complex 2. In the zero-
field, the Gd3+ complex behaves as a paramagnetic ion. However, under an applied field,
we observed the frequency-dependent magnetic phenomena characteristic of anisotropic
field-induced SIMs. The interpretation of this result seems to be the same described by
Murugesu et al. [61], in which occurs the interlevel spin–phonon transitions that are not
separated by any blocking barrier in Gd3+ ions. The Dy3+ complex 3 also exhibited the
relaxation of the magnetic susceptibility under an applied field, behaving as a SIM. The
Eu3+ compound 1 has a nonmagnetic ground state, while the Ho3+ compound 4 act as a
23

paramagnet, and do not exhibit relaxation of the magnetization with or without an applied
field.
ACKNOWLEDGMENTS
This work was supported by Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Minas Gerais
(FAPEMIG), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-
Finance code 001) and RQ-MG (Rede Mineira de Química). Authors are grateful to
Laboratório de Magnetismo from Departamento de Química, for the use of SQUID
magnetometer from Universidade Federal de Minas Gerais.
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