Synthesis, characterization and magnetic studies of dinuclear lanthanide complexes constructed with a Schiff base ligand

Article abstract of DOI:10.1080/00958972.2020.1759044

A series of seven isostructural homometallic dinuclear lanthanide metal complexes having the general formula [Ln₂(L)₂(NO₃)₄]·2CH₃ OH, where L = 6-methoxy-2-(2-pyridylmethyliminomethyl) phenol Ln = Tb

Full text of DOI:10.1080/00958972.2020.1759044

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Synthesis, characterization and magnetic studies
of dinuclear lanthanide complexes constructed
with a Schiff base ligand
Amin Khan, Olaf Fuhr, Muhammad Nadeem Akhtar, Yanhua Lan, Madhu
Thomas & Annie K. Powell
To cite this article: Amin Khan, Olaf Fuhr, Muhammad Nadeem Akhtar, Yanhua Lan, Madhu
Thomas & Annie K. Powell (2020): Synthesis, characterization and magnetic studies of dinuclear
lanthanide complexes constructed with a Schiff base ligand, Journal of Coordination Chemistry,
DOI: 10.1080/00958972.2020.1759044
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1759044
Synthesis, characterization and magnetic studies of
dinuclear lanthanide complexes constructed with a Schiff
base ligand
Amin Khana,c, Olaf Fuhrb, Muhammad Nadeem Akhtara,d, Yanhua Lana, Madhu
Thomase and Annie K. Powella,b
b
aInstitute of Inorganic Chemistry, Karlsruhe Institute of Technology, Karlsruhe, Germany; Institute of
Nanotechnology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany;
cDepartment of Chemistry, University of Science and Technology Bannu, Bannu, Pakistan;
dDepartment of Chemistry, Baghdad-ul-Jadeed Campus, The Islamia University of Bahawalpur,
e
Bahawalpur, Pakistan; Department of Industrial Chemistry, Addis Ababa Science and Technology
University, Addis Ababa, Ethiopia
ABSTRACT
ARTICLE HISTORY
Received 3 February 2020
Accepted 1 April 2020
A series of seven isostructural homometallic dinuclear lanthanide
metal complexes having the general formula [Ln2(L)2(NO3)4]ꢀ2CH3
OH, where L ¼ 6-methoxy-2-(2-pyridylmethyliminomethyl) phenol
Ln ¼ Tb (1), Dy (2), Ho (3), Er (4), Tm (5), Yb (6), and Lu (7), are
described. Compounds 1–7 were obtained from the reactions of
Schiff base ligand (L) with the respective metal nitrates in the pres-
ence of triethylamine in MeOH. The structure of 2 was determined
by single-crystal X-ray diffraction and the remaining compounds
were characterized from X-ray powder diffraction patterns and
found to be isostructural. The magnetic properties of 1–7 have
been investigated using dc and ac susceptibility measurements. All
these compounds show antiferromagnetic behavior between adja-
cent paramagnetic centers. Out of these seven compounds, only 2
displays an out-of-phase ac susceptibility signal which is attributed
to the presence of the anisotropic DyIII ions.
KEYWORDS
Single molecular magnet;
Schiff base;
lanthanide complexes
CONTACT Amin Khan
amin_khanpsh@yahoo.com
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2020.1759044.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
A. KHAN ET AL.
1. Introduction
Single molecule magnets (SMMs) in the field of research are attractive for both physi-
cists and chemists because of their promising applications in high-density information
storage, molecular spintronics and quantum computing [1, 2]. Large negative axial
magnetic anisotropy (D) and high spin values (S) are fundamental requirements and
useful to increase the anisotropic energy barrier (Ueff) to reversal of the molecular
magnetization [3]. Recently, attention has been diverted to lanthanide systems due to
the significant magnetic anisotropy of lanthanide ions arising from their large,
unquenched orbital angular momentum [4]. The significance of homonuclear lanthan-
ide complexes in molecular magnetism has increased with the discovery of the {Dy5}
[5] and {Dy4K2} [6] complexes which display the highest barriers of energy for reversal
of magnetization in any system so far. Up to now, various nuclearity lanthanide clus-
ters have been reported [7–11] but proper ligand design system to synthesis poly-
nuclear Ln(III) complexes with interesting properties is still a synthetic challenge.
Therefore, herein a tetradentate N,O-donor Schiff-base ligand, i.e. 6-methoxy-2-(2-pyri-
dylmethyliminomethyl) phenol (HL) (Scheme 1), synthesized by condensation of o-vaniline
and 2-(aminomethyl)pyridine, has been used to generate a series of isostructural dinuclear
Ln compounds. The synthesized ligand has only been used previously to report Cu [12]
and Pd [13] complexes. We are introducing this ligand for the first time in lanthan-
ide chemistry.
2. Experimental
2.1. Materials and methods
All chemicals and solvents were obtained from commercially available sources and
used as received without purification. o-Vanillin, 2-(aminomethyl)pyridine and
Ln(NO3)3ꢀ6H2O (Ln ¼ Tb, Dy, Ho, Er, Tm, Yb and Lu) were obtained from Sigma Aldrich
and methanol was received from Merck. All manipulations were performed under aer-
obic conditions.

JOURNAL OF COORDINATION CHEMISTRY
3
Scheme 1. 6-Methoxy-2-(2-pyridylmethyliminomethyl) phenol (HL).
2.2. Syntheses
The Schiff base ligand (HL) was obtained by condensation of o-vanillin (2-hydroxy-3-
methoxybenzaldehyde) with 2-(aminomethyl)pyridine in methanol [14].
2.2.1. [Tb2(L)2(NO3)4]ꢀ2CH3OH (1)
To a solution containing HL (0.1 mmol, 24.2mg), dissolved in 10 mL methanol, was added
triethylamine (0.1mmol, 10.1 mg) and the resulting mixture was stirred for about 5 min
before adding slowly 10mL of a methanolic solution containing Tb(NO3)3ꢀ6H2O (0.1 mmol,
45.3mg). After 1 h of stirring, the resultant solution was filtered and left undisturbed for
one week, yielding pale yellow rectangular-block crystals of the complex by slow evapor-
ation which were collected in ꢁ78% yield (0.086g). Elemental analysis (%) Found (calc. for
C30H32Tb2N8O18) (1110.48g molꢂ1) (1): C, 31.92 (32.44); H, 2.98 (2.90); N, 9.86 (10.09).
Selected IR data (KBr): t cmꢂ1 ¼ 3314 (br), 1644 (s), 1609 (m), 1565 (w), 1477 (m), 1424 (w),
1379 (vs), 1265 (w), 1229 (m), 956 (m), 841 (m), 745 (s), 655(w), 576 (m), 446 (w).
2.2.2. Preparation of 2–7
Compounds 2–7 were obtained by the same procedure using Dy(NO3)3ꢀ6H2O
(45.6 mg), Ho(NO3)3ꢀ6H2O (46.9 mg), Er(NO3)3ꢀ6H2O (46.1 mg), Tm(NO3)3ꢀ6H2O (46.3 mg),
Yb(NO3)3ꢀ6H2O (46.7 mg) and Lu(NO3)3ꢀ6H2O (46.9 mg) in place of Tb(NO3)3ꢀ6H2O,
respectively. Their IR spectra are nearly identical.
2.2.2.1. [Dy2(L)2(NO3)4]ꢀ2CH3OH (2). Yield: 70% (0.077 g); Elemental analysis (%) Found
(calc. for C30H32Dy2N8O18) (1117.64 g molꢂ1) (2): C, 31.82 (32.24); H, 2.98 (2.88); N, 9.80
(10.02). Selected IR data (KBr): t cmꢂ1 ¼ 3314 (br), 1644 (s), 1609 (m), 1565 (w), 1477 (m),
1424 (w), 1379 (vs), 1265 (w), 1229 (m), 956 (m), 841 (m), 745 (s), 655(w), 576 (m), 446 (w).
2.2.2.2. [Ho2(L)2(NO3)4]ꢀ2CH3OH (3). Yield: 83% (0.092 g); Elemental analysis (%) Found
(calc. for C30H32Ho2N8O18) (1122.49 g molꢂ1) (3): C, 31.84 (32.10); H, 2.76 (2.87); N, 9.84
(9.98). Selected IR data (KBr): t cmꢂ1 ¼ 3323 (br), 1631 (s), 1597 (m), 1538 (w), 1465 (m),
1421 (w), 1358 (vs), 1247 (w), 1228 (m), 946 (m), 845 (m), 727 (s), 637(w), 573 (m), 429 (w).
2.2.2.3. [Er2(L)2(NO3)4]ꢀ2CH3OH (4). Yield: 60.5% (0.073 g); Elemental analysis (%) Found
(calc. for C30H32Er2N8O18) (1127.15 g molꢂ1) (4): C, 31.91 (31.96); H, 2.74 (2.86); N, 9.68
(9.94). Selected IR data (KBr): t cmꢂ1 ¼ 3323 (br), 1635 (s), 1612 (m), 1546 (w), 1459
(m), 1432 (w), 1381 (vs), 1258 (w), 1225 (m), 942 (m), 843 (m), 721 (s), 643 (w), 564
(m), 422(w).

4
A. KHAN ET AL.
2.2.2.4. [Tm2(L)2(NO3)4]ꢀ2CH3OH (5). Yield: 54% (0.059 g); Elemental analysis (%) Found
(calc. for C30H32Tm2N8O18) (1130.5 g molꢂ1) (5): C, 31.73 (31.87); H, 2.68 (2.85); N, 9.83
(9.91). Selected IR data (KBr): t cmꢂ1 ¼ 3321 (br), 1645 (s), 1604 (m), 1535 (w), 1465
(m), 1420 (w), 1383 (vs), 1248 (w), 1234 (m), 946 (m), 838 (m), 737 (s), 661 (w), 565 (m),
438 (w).
2.2.2.5. [Yb2(L)2(NO3)4]ꢀ2CH3OH (6). Yield: 54% (0.06 g); Elemental analysis (%) Found
(calc. for C30H32Yb2N8O18) (1138.71 g molꢂ1) (6): C, 31.58 (31.64); H, 2.72 (2.83); N, 9.68
(9.84) Selected IR data (KBr): t cmꢂ1 ¼ 3314 (br), 1652 (s), 1607 (m), 1562 (w), 1468
(m), 1428 (w), 1377 (vs), 1256 (w), 1234 (m), 945 (m), 837 (m), 738 (s), 655 (w), 571 (m),
438 (w).
2.2.2.6. [Lu2(L)2(NO3)4]ꢀ2CH3OH (7). Yield: 54% (0.06 g); Elemental analysis (%) Found
(calc. for C30H32Lu2N8O18) (1142.56 g molꢂ1) (7): C, 31.47 (31.53); H, 2.66 (2.82); N, 9.74
(9.80). Selected IR data (KBr): t cmꢂ1 ¼ 3323 (br), 1646 (s), 1621 (m), 1557 (w), 1477
(m), 1421 (w), 1369 (vs), 1247 (w), 1232 (m), 961 (m), 844 (m), 738 (s), 647 (w), 580 (m),
451 (w).
2.3. Physical measurements
The elemental analyses (C, H, N) were performed using an Elementar Vario EL analyser.
FT-IR spectra were measured on a Perkin-Elmer Spectrum One spectrometer with sam-
ples prepared as KBr discs. X-ray powder diffraction patterns for 1–7 were measured
at room temperature using a Stoe STADI-P diffractometer with a Cu Ka radiation.
Single-crystal X-ray crystallographic data were collected for 2 at 130 K on a Stoe IPDS
II diffractometer using graphite-monochromated Mo Ka radiation. Crystallographic
data and details of measurement and refinement are summarized in Table 1. The
structure was solved using direct methods, followed by full-matrix least-squares refine-
ment against F2 (all data) using SHELX [15]. Anisotropic refinement was used for all
non-H atoms of 2; hydrogens were placed in calculated positions.
Magnetic susceptibility measurements were obtained using a Quantum Design
SQUID magnetometer, MPMS-XL. The magnetometer operates between 1.8 and 400 K
for dc applied fields of ꢂ7 to þ7 T. Measurements were performed on polycrystalline
samples of 9.6 mg (1), 22.7 mg (2), 8.9 mg (3), 7.8 mg (4), 29.9 mg (5), 22.7 mg (6), and
35.6 mg (7). Ac susceptibility measurements were measured with an oscillating ac field
of 3 Oe and ac frequencies ranging from 1 to 1500 Hz. The magnetic data were cor-
rected for the sample holder and the diamagnetic contribution referenced from the
Lu2 compound 7.
3. Results and discussion
3.1. Synthesis
The room temperature reaction of Ln(NO3)3ꢀxH2O, HL, and triethylamine with a molar
ratio of 0.1/0.1/0.1 in methanol led to formation of rectangular-block pale yellow crys-
tals of 1–7 in ca. 54–78% yield after one week by slow evaporation. Compounds 1–7

JOURNAL OF COORDINATION CHEMISTRY
5
Table 1. Crystallographic data and structure refinement for 2.
Formula
C30H32Dy2N8O18
1117.64
0.15 ꢃ 0.13 ꢃ 0.10
Yellow
FW [g molꢂ1
]
Crystal dimensions [mm3]
Color
Crystal system
Space group
T [K]
Triclinic
P-1
180(2)
a [Å]
b [Å]
9.7117(19)
9.907(2)
11.010(2)
71.96(3)
68.20(3)
88.48(3)
930.5(4)
1
c [Å]
a [ꢄ]
b [ꢄ]
c [ꢄ]
V [Å3]
Z
qcalcd [g cmꢂ3
]
1.995
4.074
544
m(Mo-Ka) [mmꢂ1
F(000)
]
Data measured
Unique data
Rint
11633
3779
0.0492
Data with I > 2r(I)
3690
Parameters/restraints
wR2 (all data)
255/0
0.0565
1.108
0.0202
Gof on F2 (all data)
R1 [data with I > 2r(I)]
Largest difference peak/hole [e Åꢂ3
CCDC number
]
0.897/ꢂ1.248
823654
are stable, retain their crystallinity on exposure to air and are soluble in common
organic solvents such as methanol, acetone, acetonitrile, tetrahydrofuran and N,N0-
dimethylformamide.
3.2. Description of the structure
X-ray powder diffraction indicates that all seven compounds are isostructural (Figure
1), thus only the full structure of 2 was determined by single crystal X-ray diffraction.
Dinuclear complex 2 crystallizes in the triclinic space group P-1 with Z ¼ 1 with two
molecules of lattice bound methanol per formula unit. Selected interatomic distances
are listed in Table 2. A perspective view of the dinuclear unit is depicted in Figure 2.
The complex consists of two dysprosium(III) ions which are linked by phenoxo oxygen
atoms of two Schiff base ligands with Dy–O distances Dy(1)–O(1) ¼ 2.3036(19) Å and
Dy(1)–O(1a) ¼ 2.357(2)
Å
and
a
Dy(1)–O(1)–Dy(1a) angle of 105.86(8). The
Dy(1) … Dy(1a) distance is 3.7186(12) Å. The dinuclear aggregate is centrosymmetric.
Each Dy(III) ion is nine-coordinate by three oxygens and two nitrogens from the Schiff
base ligand and four oxygens belonging to two nitrate anions in the bidentate coord-
ination mode. The Dy–O bond lengths are in the range of 2.3036(19)–2.428(2) Å. The
Dy–N bond lengths are 2.42(2) and 2.907(3) Å. The bond angles and bond lengths are
shown in Table 2. The coordination polyhedron of the nine-coordinate Dy(III) ions can
be approximated as a distorted three-capped trigonal prism (Figure 3). A literature
review indicates that various [Dy2] clusters with their bond lengths and bond angles
are comparable with our synthesized compound. For example, Powell et al. reported

6
A. KHAN ET AL.
Figure 1. Comparison of simulated powder X-ray diffraction (PXRD) pattern of 2 (Dy2) with experi-
mental pattern of 1–7.
[16] three centrosymmetric dinuclear Dy complexes with the Dy–O (bridged) bond
lengths: Dy1–O1a ¼ 2.255(5) and Dy1–O1 ¼ 2.277 Å for [Dy2(HL1)2(NO3)4], where
HL1 ¼ 2-[(2-hydroxy-ethyl)-pyridin-2-ylmethylamino]ethanol; Dy1–O1a ¼ 2.259(7) and
Dy1–O1 ¼ 2.266(7) Å for [Dy2(L2)2(NO3)4], where HL2 ¼ 2-(bis-pyridin-2-ylmethylamino)-
ethanol; Dy1–O1a ¼ 2.257(17) and Dy1–O1 ¼ 2.265(17) Å for [Dy2(HL3)2(NO3)4], where
HL3 ¼ 2-(bis-pyridin-2-ylmethylamino)-propane-1,3-diol. While their Dy1–O–Dy1a bonds
angles are: 109.83(18)ꢄ, 109.71(15)ꢄ, and 110.65(7)ꢄ, respectively. Similarly, Baskar et al.
[17] reported [Dy2(L)2(Ph2acac)2(OH2)2]3 cluster using Schiff base ligand with bond
lengths, Dy1–O1 ¼ 2.325(4) and Dy1–O2 ¼ 2.254(4)
Å
and bond angles,
Dy1–O1–Dy1a ¼ 105.6(14)ꢄ.
3.3. Magnetic properties
Variable-temperature dc magnetic susceptibility data for 1–7 were collected in the
temperature range 1.8–300 K under an applied field of 0.1 T (Figure 4). Of this series of
compounds, Lu2 (7) is a diamagnetic compound; thus its magnetic susceptibility (vLu2
¼ vdia ¼ ꢂ3.9 ꢃ 10ꢂ4 cm3 molꢂ1) was used for diamagnetic correction for the other
complexes (1–6). The dc magnetic data of 1–6 are summarized in Table 3. The
observed vT products at room temperature are in good agreement with the expected
values for two non-interacting LnIII ions. On lowering the temperature, the vT products
decrease steadily down to 30 K and then undergo a more abrupt decrease down to
1.8 K. The continuous decrease of vT product mainly results from the thermal depopu-
lation of the Stark sublevels of the anisotropic LnIII (Tb, Dy, Ho, Er, Tm, and Yb) ions
since the LnꢀꢀꢀLn interactions are probably insignificant by comparison to other Ln
complexes with similar superexchange bridges [18–20].
The field dependence of the magnetization of 1–6 at low temperatures shows that
the magnetization increases smoothly with increasing applied dc field without satur-
ation even at 7 T (Figure S1). The magnitude of magnetization at 1.8 K is also given in
Table 3. This behavior indicates the presence of magnetic anisotropy and/or the lack

JOURNAL OF COORDINATION CHEMISTRY
7
Table 2. Selected bond distances (Å) and angle parameters (ꢄ) for 2.
Dy1–O1
Dy1–O1a
Dy1–O2a
Dy1–N2
Dy1–O4
Dy1–O7
Dy1–N1
2.3036(19)
2.357(2)
2.428(2)
2.428(2)
2.454(2)
2.457(2)
2.489(3)
Dy1–N4
Dy1–N3
Dy1–Dy1a
O1–Dy1a
O2–Dy1a
Dy1–O3
Dy1–O6
2.882(3)
2.907(3)
3.7186(12)
2.357(2)
2.428(2)
2.497(2)
2.448(2)
Dy1–O1–Dy1a
O1–Dy–N2
O1–Dy–O1a
O1i–Dy1–O2a
O1i–Dy1–O2a
O1–Dy1–O2a
N2–Dy1–N1
105.86(8)
77.82(8)
74.14(8)
66.88(7)
66.88(7)
140.30
67.79(9)
Symmetry code: (i) ꢂx þ 1, ꢂy þ 1, ꢂz.
Figure 2. Thermal ellipsoid structure of [Dy2(L)2(NO3)4]ꢀ2CH3OH (a) nitrate ligands relative to the
cluster cores (b).
Figure 3. Distorted three-capped trigonal prism environment.
of a well-defined ground state, suggesting the presence of low-lying excited states
that might be populated when a field is applied.
The magnetic relaxation was tested for all compounds using ac susceptibility meas-
urements under zero dc field. We found that only 2 exhibits a non-zero out-of-phase
component below 10 K, indicating slow relaxation of its magnetization under these
conditions (Figure 5). The in-phase component of ac susceptibility is weakly frequency

8
A. KHAN ET AL.
Figure 4. Temperature dependence of the vT products for 1–6 at 0.1 T (with v being the molar
susceptibility defined as M/H).
Table 3. Summary of the dc magnetic data for 1–6.
1
(Tb)
2
(Dy)
3
(Ho)
4
(Er)
5
(Tm)
6
(Yb)
Compounds
Ground state term of LnIII ion
S
L
7F6
3
3
6H15/2
5/2
5
5I8
2
6
4I15/2
3/2
6
3H6
1
5
2F7/2
1/2
3
g
3/2
4/3
5/4
6/5
11.5
7/6
7.15
8/7
2.57
C (cm3 K molꢂ1) for each Ln ion9
11.82 14.17 14.07
vT (cm3 K molꢂ1) expected value for 2 non-interacting Ln2 at RT 23.64 28.34 28.14 23.00 14.30 5.14
vT (cm3 K molꢂ1) experimental value for Ln2 at RT
vT (cm3 K molꢂ1) experimental value for Ln2 at 1.8 K
Magnetization (mB) observed at 7 T and 1.8 K
24.44 28.15 27.65 23.09 14.85 4.99
6.49
11.4
9.70
11.0
3.10
9.4
9.04
9.8
4.28
7.1
2.20
4.0
Figure 5. Temperature dependence of the in-phase (v0) and out-of-phase (v00) components of the
ac magnetic susceptibility at indicated frequencies for 2 under zero dc field.
dependent but that of out-of-phase component is strongly frequency dependent, sug-
gesting that this compound is a SMM. However, the energy barrier for the relaxation
process cannot be determined from this set of data because the maxima of the v00
peaks are out of the ranges available (lowest temperature 1.8 K and highest frequency
1500 Hz) on the SQUID. The barrier for the reversal of the magnetization is generated
by the presence of magnetic anisotropy of the Dy3þ ions as is often observed in many
such compounds [18–20], but it is impossible to discuss it more without any further

JOURNAL OF COORDINATION CHEMISTRY
9
measurements, for instance hysteresis measurements using
magnetometer.
a
micro-SQUID
In order to study the relaxation behavior further and check for quantum tunneling
effects above 1.8 K, the frequency dependence of the ac susceptibility of 1 and 2 at
1.8 K was also measured with application of small dc fields up to 2000 Oe. In SMMs
with a relaxation partially influenced by quantum effects, the application of a dc field
removes the ground state degeneracy and thus the possibility of quantum tunneling,
inducing a slowing down of the magnetization relaxation. For Tb compound 1, there
is no field-induced magnetic relaxation appearing under an external field (Figure S2);
however, for Dy compound 2, the slow relaxation process is not significantly influ-
enced by applying an external dc field (Figure S3), indicating that no appreciable
quantum tunneling effect is observed above 1.8 K.
4. Conclusion
A series of dinuclear lanthanide complexes formulated as [Ln2(L)2(NO3)4]ꢀ2CH3OH,
where Ln is Tb (1), Dy (2), Ho (3), Er (4), Tm (5), Yb (6), and Lu (7), was constructed
from Schiff base ligand. Magnetic studies of these compounds revealed antiferromag-
netic behavior between adjacent paramagnetic centers and Dy containing compound
exhibit slow relaxation. This type of ligand, therefore, provides an avenue to obtain a
dinuclear compound having interesting magnetic properties.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
Financial support from the University of Science and Technology Bannu, Pakistan is gratefully
acknowledged.
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