Linear shaped hetero-metallic [Zn2Ln4] clusters with Schiff base ligand: Synthesis, characterization and magnetic properties

Article abstract of DOI:10.1016/j.ica.2021.120437

A series of isostructural 3d–4f coordination clusters (CCs) [Zn₂Ln₄(L1)₄(L2)₂(OAc)₂(N₃)₂] [Na₂(OH₂)₂(NO₃)_3.2(N₃)

Full text of DOI:10.1016/j.ica.2021.120437

Inorganica Chimica Acta 524 (2021) 120437
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Inorganica Chimica Acta
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Research paper
Linear shaped hetero-metallic [Zn2Ln4] clusters with Schiff base ligand:
Synthesis, characterization and magnetic properties
,
b
,
,
Amin Khana *, Muhammad Nadeem Akhtara c, Yanhua Lana, Christopher E. Ansona,
Annie K. Powella
a Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, Engesserstrasse 15, D-76131 Karlsruhe, Germany
b Department of Chemistry, University of Science and Technology Bannu, Bannu 28100, Pakistan
c Division of Inorganic Chemistry, Institute of Chemistry, Baghdad-ul-Jadeed Campus, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of isostructural 3d–4f coordination clusters (CCs) [Zn2Ln4(L1)4(L2)2(OAc)2(N3)2] [Na2(OH2)2(-
NO3)3.2(N3)0.8]∙2MeCN, where Ln = Dy (1), Ho (2), Er (3), Tm (4), and Yb (5), L1 = 6-Methoxy-2-(2-pyr-
idylmethyliminomethyl)phenol and L2 = (E)-2-methoxy-6-(2-((3-methoxy-2-oxidobenzylidene)amino)-1-oxido-
2-(pyridin-2-yl)ethyl)phenolate has been synthesised and structurally characterised. The magnetic properties of
1–5 have been investigated using dc and ac susceptibility measurements. The paramagnetic metal ions within the
clusters are weakly antiferromagnetically coupled, with the Er and Dy compounds displaying slow relaxation of
their magnetisation. This is the first report of this versatile ligand being used to target 3d–4f coordination
clusters.
Single molecular magnet
Schiff base ligand
Zn2Ln4 complexes
1. Introduction
triangular ring-like Zn6Ln24, [20] Salen-like Zn2Ln4 [21] and sandwich-
type Zn2Ln4 [22] because of their challenging design and controllable
Single molecule magnets (SMMs) in the field of research are attrac-
tive for both physicists and chemists because of their promising appli-
cations in high-density information storage, molecular spintronics and
quantum computing [1–3]. Initially, this work was originated from 3d
metal ions based polynuclear complex that behave as Single Molecule
Magnets (SMMs) [4] but with the passage of time, this research is
diverted to synthesis the 4f [5–7] and 3d-4f metal ions based clusters
[8–12]. Since Brennan and his co-workers reported 3d-4f heterometallic
clusters in 1995, hetero-metallic 3d-4f clusters containing ZnII–LnIII
metal ions with high-nuclearity have attracted increasing attention in
modern coordination chemistry primarily because of their interesting
magnetic and photo physical properties.[13,14] The magnetic proper-
ties of these compounds mainly arise from the intrinsic large spin ground
state and the strong uniaxial magnetic anisotropy of the LnIII ions,
whereas the incorporation of ZnII ions into the coordination system is a
good support in magnetic properties with LnIII ions which can offer as
multifunctional materials [15–22].
synthesis. In addition, Murry et al and Chandrasekhar et al well
contributed in Zn-Ln chemistry with some of them exhibits exciting
magnetic properties [23–26].
The synthesis of these clusters has been achieved by utilizing various
organic ligands containing alcoholic groups or amino alcohols, azide,
amine etc. on reacting with appropriate metal ions. Among them imine
alcohol containing ligands (Schiff base ligands), which could be syn-
thesized quite easily by condensing primary amine alcohols with alde-
hydes. O-vanillin containing imine alcohols based schiff bases were
found to be efficient building blocks for making both 3d, 4f and 3d-4f
homo or hetero metallic coordination clusters [27–30].
Two pocket containing Schiff base ligands looks efficient for the
synthesis of heterometallic (3d-4f) SMMs as one pocket can be use for 3d
and other one for 4f ions and moreover they could couple through oxide
or hydroxide or alkoxide bridges. Such SMMs are exchange coupled
systems, where both 3d and 4f ions contribute to the SMM behavior.
Now-a-days SMMs are constructed more promptly extending the
concept of utilizing spin and anisotropy of metal ions. Several metal ions
were found (MnIII, FeII, CoII, DyIII, TbIII, UIII etc.) to be very anisotropic
and combining them into a coordination cluster, is a convincing way to
There are only a few reports of ZnII–LnIII coordination clusters with
high-nuclearity from kite-like ZnLn3, [15] square-shaped Zn2Ln2, [16]
cyclic Zn2Ln2,[17] cubane-like Zn2Ln2, [18] drum-like Ln4Zn8, [19]
* Corresponding author at: Department of Chemistry, University of Science and Technology Bannu, Bannu 28100, Pakistan.
E-mail address: amin_khanpsh@yahoo.com (A. Khan).
https://doi.org/10.1016/j.ica.2021.120437
Received 13 February 2021; Received in revised form 27 April 2021; Accepted 1 May 2021
Available online 5 May 2021
0020-1693/© 2021 Published by Elsevier B.V.

A. Khan et al.
Inorganica Chimica Acta 524 (2021) 120437
Table 1
Crystallographic data and structure refinement for compounds 1, 3–5.
Compound
(1)
(3)
(4)
(5)
Formula
C
108 H106
C108 H106 Er4
N30.2 Na2
O26.7 Zn2
3099.99
triclinic
C108 H106
N32 Na2 O24
Tm4 Zn2
3088.69
triclinic
P ꢀ 1
C108 H106
N31.6 Na2
O24.6 Yb4 Zn2
3109.12
triclinic
P ꢀ 1
Dy4 N25.6 Na2
O33.6 Zn2
3126.90
triclinic
P ꢀ 1
Formula weight
Crystal System
Space Group
a / Å
P ꢀ 1
11.5685(6)
15.4955(8)
16.4369(9)
93.057(4)
96.180(4)
104.104(4)
2831.4(3)
1
11.5500(12)
15.4242(14)
16.4132(16)
93.411(7)
96.845(8)
103.449(8)
2812.0(5)
1
11.5669(19)
15.440(2)
16.415(2)
93.654(12)
96.902(12)
103.605(12)
2815.9(7)
1
11.580(2)
15.416(3)
16.380(3)
93.401(14)
97.237(14)
103.215(14)
2812.5(8)
1
b / Å
c / Å
Scheme 1. 6-Methoxy-2-(2-pyridylmethyliminomethyl) phenol (HL1).
α
[o]
β[o]
γ[o]
diamagnetic contribution.
Volume [Å3]
Z
T / K
F(000)
180(2)
150(2)
150(2)
150(2)
2.2. Synthesis of the ligand (HL1)
1548
1533
1528
1534
D
calc [Mg mꢀ 3
]
1.834
1.831
1.821
1.836
μ(Mo-K
α
) /
3.124
3.468
3.632
3.807
The Schiff base ligand (HL1) was synthesized according to literature
[35] by condensing o-vanillin and 2 (amino- methyl)pyridine in meth-
anol in 1:1 M ratio Scheme 1.
mmꢀ 1
Diffractometer
Data Measured
Unique Data
Rint
Stoe IPDS II
21,909
10,584
0.0546
6669
Stoe IPDS II
30,955
12,838
0.0494
8742
Stoe IPDS II
31,437
11,434
0.0692
8970
Stoe IPDS II
19,858
10,013
0.0778
5253
2.3. Synthesis of [Zn2Dy4(L1)4(L2)2(OAc)2(N3)2]
Data with I ≥
2σ (I)
[Na2(OH2)2(NO3)3.2(N3)0.8]∙2MeCN (1)
Parameters
Restraints
709
709
709
709
0
0
0
0
To a 10 ml (MeOH/MeCN = 1:1 v/v) solution of HL1 (1 mmol, 0.242
g), triethylamine (1 mmol, 0.1 g) and NaN3 (0.240 g, 4.0 mmol) a so-
lution of Zn(OAc)2⋅4H2O (0.5 mmol, 0.1 mg) and Dy(NO3)3⋅6H2O (0.5
mmol, 0.228 g) in MeOH (10 ml) was added. After 1 h of stirring, the
resultant solution was filtered the reaction was left undisturbed for four
week, yielding orange rectangular-block crystals of complex by slow
evaporation which were collected in ~ 53% yield (based on Dy).
wR2, (all data)
0.0646
0.0326
0.936
0.673/
ꢀ 1.055
2,035,812
0.1427
0.0587
0.970
0.950/
ꢀ 2.902
2,035,814
0.1430
0.0552
0.978
1.276/
ꢀ 3.137
2,035,815
0.1440
0.0598
0.875
0.837/
ꢀ 2.687
2,035,816
R
1 [I ≥ 2 σ (I)]
S (all data)
Biggest diff.
peak/hole
CCDC number
Elemental analysis calc for
C104H96N18O22Zn2Dy4] [Na2(OH2)2(-
make nice SMMs. Therefore, herein tetradentate N, O-donor Schiff base
ligand, i.e. 6-Methoxy-2-(2-pyridylmethyliminomethyl) phenol (HL)
synthesized by condensation of o-vaniline and 2 (amino-methyl)pyri-
dine has been used to generate a series of isostructural hexanuclear
Zn2Ln4 series of compounds. The synthesised ligands has been used only
in synthesising Cu [31,32] and Pd [33] complexes. Here, we are intro-
ducing this ligand first time in zinc-lanthanide chemistry.
NO3)3.2(N3)0.8] ∙ 2MeCN⋅H2O (3144.9) gmol-1) (20) C 41.25, H 3.46, N
11.40; found C 41.16, H 3.52, N 11.43 Selected IR data (KBr):
υ
cmꢀ 1
=
3446(br), 2981(w), 2064(s), 1650(s), 1605(m), 1562(s), 1472(w), 1457
(m), 1441(w), 1385(w), 1340(w), 1316(w), 1279(m), 1244(m), 1220(s),
1169(w), 1054(m), 1011(w), 968(m), 968(m), 850(m), 738(s), 648(w).
2.4. Synthesis of compounds 2–5
2. Experimental
These compounds were obtained by the same procedure using, Ho
(NO3)3⋅6H2O, Er(NO3)3⋅6H2O, Tm(NO3)3⋅6H2O, Yb(NO3)3⋅6H2O in
place of Dy(NO3)3⋅6H2O in place of Dy(NO3)3•6H2O, respectively.
Their IR spectra are nearly identical.
2.1. Materials and physical methods
All chemicals and solvents were obtained from commercial sources
and used as received, without further purification. The reactions were
carried out under aerobic conditions. The elemental analyses (C, H, N)
were performed using an Elementar Vario EL analyser, FT-IR spectra
were measured on a Perkin-ElmerSpectrum Onespectrometer with
samples prepared as KBr discs. X-ray powder diffraction patterns for
1–5were measured at room temperature using a Stoe STADI-P diffrac-
2.4.1. Synthesis of [Zn2Ho4(L1)4(L2)2(OAc)2(N3)2]
[Na2(OH2)2(NO3)0.8(N3)3.2]∙2MeCN (2)
Yield: 53% based on Ho. Elemental analysis (%) Calc. for
[C104H96N18O22Zn2Ho4] [Na2(OH2)2(NO3)0.8(N3)3.2] ∙ 1CH3CN⋅5H2O
(3137.7 gmol-1), C 40.58, H 3.63, N 13.12; found C 40.52, H 3.58, N
13.19
tometer with a Cu-K
data were collected at 130 K on a Stoe IPDS II diffractometer using
graphite-monochromated Mo-K radiation. Crystallographic data and
α radiation. Single-crystal X-ray crystallographic
α
2.4.2. Synthesis of [Zn2Er4(L1)4(L2)2(OAc)2(N3)2]
[Na2(OH2)2(NO3)0.9(N3)3.1]∙2MeCN (3)
details of measurement and refinement are summarized in Table 1. The
structure was solved using direct methods, followed by full-matrix least-
squares refinement againstF2 (all data) using SHELX [34]. Anisotropic
refinement was used for all non-H atoms; organic hydrogen atoms 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 ꢀ 7 to +7 T. Measurements were performed on polycrystalline
samples of 1–5. 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 corrected for the sample holder and the
Yield: 54% based on Er. Elemental analysis (%) Calc. for
[C104H96N18O22Zn2Er4]
[Na2(OH2)2(NO3)0.9(N3)3.1]∙1CH3CN⋅6H2O
(3121.7 gmol-1), C 40.78, H 3.58, N 13.10; found C 40.79, H 3.44, N
13.11
2.4.3. Synthesis of [Zn2Tm4(L1)4(L2)2(OAc)2(N3)2] [Na2(OH2)2(N3)4]
∙2MeCN (4)
Yield: 51% Elemental analysis (%) Calc. for [C104H96N18O22Zn2Tm4]
[Na2(OH2)2(N3)4]∙1CH3CN⋅8H2O (3155.7 gmolꢀ 1), C 40.34, H 3.67, N
13.76; found C 40.30, H 3.40, N 13.76
2

A. Khan et al.
Inorganica Chimica Acta 524 (2021) 120437
Scheme 2. Structure and coordination modes of the ligand, (L1)ꢀ (A, B,C), structure and coordination mode of ligand (L2)3ꢀ (D, E).
where the ligand (L2)3ꢀ is formed during the reaction (Scheme 2D).
3.1. Description of the structure of compounds 1–5
A combination of X-ray powder diffraction and unit cell determina-
tion by single crystal X-ray diffraction indicates that all five compounds
crystallise isotypically (Fig. 1 and Table 1), and the full structures of
Zn2Dy4 (1), Zn2Er4 (3), Zn2Tm4 (4) and Zn2Yb4 (5) were determined by
single crystal X-ray diffraction. [31] The molecular structure of (1) is
described here in detail (Fig. 2). Compound (1) crystallizes in the
¯
triclinic space group Pı with Z = 1. Selected bond distances and angles
are listed in Table S1.
The
structure
contains
a
dicationic
hexanuclear
[Zn2Dy4(L1)4(L2)2(OAc)2(N3)2]2+ coordination cluster. The charge is
balanced
by
the
dianionic
dinuclear
sodium
complex
[Na2(OH2)2(NO3)3.2(N3)0.8
]
2ꢀ . The metal ions are linked together to
form a chain via four phenolate
μ
2–oxygens of the ligands (L1)ꢀ , two
Fig. 1. X-Ray powder diffraction patterns for compounds 1–5.
phenolate
μ
2–oxygens and two alkoxy μ2-oxygens of the ligands ((L2)3ꢀ
with Dy-O and Zn-O distances 2.329–2.429 Å and 2.049–2.079 Å
respectively.
2.4.4. Synthesis of Zn2Yb4(L1)4(L2)2(OAc)2(N3)2]
The deprotonated ligand (L1)ꢀ and the triply-deprotonated ligand
[Na2(OH2)2(NO3)0.9(N3)3.1]∙2MeCN (5)
(L2)3ꢀ adopt coordination modes (η1
:
η1
:
η2
:
η1
:μ2 (Scheme 2 B, C) and
Yield: 55%.Elemental analysis (%) Calc. for [C104H96N18O22Zn2Er4]
[Na2(OH2)2(NO3)0.2(N3)3.8]∙1CH3CN⋅7H2O (3158.1 gmolꢀ 1), C 40.31,
H 3.61, N 13.57; found C 40.34, H 3.67, N 13.62
η2
:
η1
:
η1
:
η2
:
η2
:
η1
:
μ4 (Scheme 2 E), respectively. Peripheral ligation is
provided by two terminal azide ligands and two bridging acetate
ligands.
The nine-coordinate DyIII ions are characterized by a distorted
bicapped trigonal prism environment. The five-coordinate ZnII ions have
a distorted square pyramidal geometry.
3. Results and discussion
Slow evaporation at room temperature of a methanol/acetonitrile
solution of HL1, Ln(NO3)3⋅6H2O, Zn(OAc)2⋅4H2O and NaN3 in the
presence of triethylamine afforded a series of hexanuclear complexes
[Zn2Ln4(L1)4(L2)2(OAc)2(N3)2] [Na2(OH2)2(NO3)3.2(N3)0.8]∙2MeCN,
3.1.1. Magnetic properties of compounds 1–5
The dc magnetic susceptibility data were collected for 1–5 in the
temperature range from 1.8 to 300 K in an applied field of 1000 Oe
3

A. Khan et al.
Inorganica Chimica Acta 524 (2021) 120437
Fig. 2. (A) Molecular structure of complex Zn2Dy4 (B) Central core of complex.
(Fig. 3). The dc magnetic data are summarized in Table 2. The observed
T products at room temperature are in good agreement with the ex-
pected values for non-interacting ions. On lowering the temperature, the
χ
χ
T products decrease steadily to 50 K and then undergo rapid decrease
down to 1.8 K. The overall behaviour suggests the presence of dominant
intramolecular antiferromagnetic (AF) interactions between adjacent
paramagnetic centres, although the thermal depopulation of the LnIII
excited states (Ln = Dy, Ho and Er) might also contribute to the decrease
in χT.
The field dependence of the magnetization of compounds 1–5 at low
temperatures shows that the magnetization increases smoothly with
increasing applied dc field without saturation even at 7 T) (Fig. S1). The
magnitude of magnetization at 2 K is given in Table 2. This behavior
indicates the presence of magnetic anisotropy and/or the lack of a well-
defined ground state suggesting the presence of low-lying excited states
that might be populated when a field is applied.
For the detection of slow relaxation of magnetization, ac suscepti-
bility measurements were performed in absence of an external dc
magnetic field as in-phase susceptibility (χ′) and out-of-phase suscepti-
bility (χ′′) vs. T for different frequencies of a small ac modulated mag-
netic field. Compounds 2–5 do not show any in-phase and out-of-phase
Fig. 3. Temperature-dependence of the χT product under dc field of 1000 Oe
for compounds 1–5 (compound codes are indicated in the inset of figures).
Table 2
Magnetic data summarized from the dc measurements for 1–5.
Compounds
Curie constant for each Ln
χ
T expected for non-interacting ions
χ
T measured at 300 K per
χ
T measured at 1.8 K per
Magnetization at 2 K
[36] (cm3Kmolꢀ 1
)
per complex (cm3Kmolꢀ 1
)
complex (cm3Kmolꢀ 1
)
complex (cm3Kmolꢀ 1
)
and 7 T (µB)
Zn2Dy4 (1)
Zn2Ho4 (2)
Zn2Er4 (3)
Zn2Tm4 (4)
Zn2Yb4 (5)
14.17
14.07
11.5
56.68
56.28
46.0
55.66
56.16
45.34
27.76
9.97
34.24
25.06
19.40
5.51
20.9
20.8
19.3
15.0
7.9
7.15
28.60
10.28
2.57
5.53
4

A. Khan et al.
Inorganica Chimica Acta 524 (2021) 120437
Fig. 4. Temperature dependence of the in-phase (χ′) and out-of-phase (χ′′) components of the ac magnetic susceptibility at indicated frequencies for (1) under zero
dc field.
4. Conclusions
Table 3
Comparison of magnetic study with similar reported system.
The first successful use of 6-Methoxy-2-(2-pyridylmethylimino-
Compounds
Structural Motif
SMM
yes
Ref.
methyl) phenol in 3d–4f chemistry in the presence of base is reported.
This resulted in hexanuclear heterometallic compounds formulated as
{[DyZn3L2(bpdc)2](ClO4).2MeOH}n,
L= N,N’-bis(3-methoxysalicylidene)-
1,3-diaminopropane, and H2bpdc =
2,2′-bipyridine-6,6′-dicarboxylic
acid.
1D chain
[37]
[Zn2Ln4(L1)4(L2)2(OAc)2(N3)2]
[Na2(OH2)2(NO3)3.2(N3)0.8]∙2MeCN
(Ln = Dy (1), Ho (2), Er (3), Tm (4) and Yb (5). Magnetic studies showed
dominant antiferromagnetic interactions within the CCs with possible
SMM behaviour for the [Zn2Dy4] (1) and [Zn2Er4] (3) analogues.
Further synthetic studies varying the molecular ratio of the reactants are
in progress.
[Dy2Zn3L2(bpdc)4]_2MeOH_3.5H2O,
L= N,N’-bis(3-methoxysalicylidene)-
1,3-diaminopropane, and H2bpdc =
2,2′-bipyridine-6,6′-dicarboxylic
acid.
1D chain
1D chain
yes
[37]
[Zn(
μ
-L)2Ln(NO3)3]5.5H2O (Ln: Gd, Tb,
No
No
No
[38]
[39]
[39]
Declaration of Competing Interest
Dy and Ho), L=bis(2-
pyridylcarbonyl)amine
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[Zn2Nd(ac)2(HL)2]NO3.3H2O, N,N’-bis
(5-bromo-3-methoxysalicylidene)-
1,3-diamino-2-propanol
Linear or Bent
conformation
[Zn2Sm(ac)2(HL3)2]
Linear or
NO3.3CH3OH⋅0.3H2O, N,N’-bis(5-
bromo-3-methoxysalicylidene)-1,3-
diamino-2-propanol
Bentconformation
Acknowledgements
[Zn2Ln(ac)2(HL)2]NO3.5.33H2O, Ln =
Eu, Tb, Dy, N,N’-bis(5-bromo-3-
methoxysalicylidene)-1,3-diamino-2-
propanol
Linear or
No
[39]
The financial support from DFG Centre for Functional Nano-
structures and University of Science and Technology Bannu, Pakistan for
(AK), are thankfully acknowledged.
Bentconformation
[Zn2Dy4(L1)4(L2)2(OAc)2(N3)2]
[Na2(OH2)2(NO3)3.2(N3)0.8]∙2MeCN
Linear
SMM
present
work
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120437.
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