Synthesis, Crystal Structure, Fluorescent and Antioxidation Properties of Cerium(III) and Europium(III) Complexes with Bis(3-methoxysalicylidene)-3-oxapentane-1,5-diamine

Article abstract of DOI:10.1002/zaac.201600434

Two aliphatic ether Schiff base lanthanide complexes (Ln = Eu, Ce) with bis(3-methoxysalicylidene)-3-oxapentane-1,5-diamine (Bod), were synthesized and characterized by physicochemical and spectroscopic methods. [Eu(Bod)(NO₃)₃] (1) is a discrete mononuclear species and [Ce(Bod)(NO₃)₃DMF]_∞ (2) exhibits an inorganic coordination polymer. In the two complexes, the metal ions both are ten-coordinated and the geometric structure around the Ln^III atom can be described as distorted hexadecahedron. Under excitation at room temperature, the red shift in the fluorescence band of the ligand in the complexes compared with that of the free ligand can be attributed to coordination of the rare earth ions to the ligand. Moreover, the antioxidant activities of the two complexes were investigated. The results demonstrated that the complexes have better scavenging activity than both the ligand and the usual antioxidants on the hydroxyl and superoxide radicals.

Full text of DOI:10.1002/zaac.201600434

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201600434
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Synthesis, Crystal Structure, Fluorescent and Antioxidation
Properties of Cerium(III) and Europium(III) Complexes with
Bis(3-methoxysalicylidene)-3-oxapentane-1,5-diamine
Xia Tang,[a] Xinkui Shi,[a] Yuling Xu,[a] Kesheng Shen,[a] Shanshan Mao,[a] and Huilu Wu*[a]
Abstract. Two aliphatic ether Schiff base lanthanide complexes (Ln
= Eu, Ce) with bis(3-methoxysalicylidene)-3-oxapentane-1,5-diamine
temperature, the red shift in the fluorescence band of the ligand in the
complexes compared with that of the free ligand can be attributed to
(Bod), were synthesized and characterized by physicochemical and coordination of the rare earth ions to the ligand. Moreover, the antioxi-
spectroscopic methods. [Eu(Bod)(NO3)3] (1) is a discrete mono- dant activities of the two complexes were investigated. The results
nuclear species and [Ce(Bod)(NO3)3DMF]ϱ (2) exhibits an inorganic demonstrated that the complexes have better scavenging activity than
coordination polymer. In the two complexes, the metal ions both are both the ligand and the usual antioxidants on the hydroxyl and super-
ten-coordinated and the geometric structure around the LnIII atom can oxide radicals.
be described as distorted hexadecahedron. Under excitation at room
planar waveguide amplifiers, light-emitting diodes, and bioin-
1 Introduction
spired luminescent probes.[27–29] In general, of the various lu-
Rare earth elements (REEs) are widely distributed through-
minescent materials, LnIII are extensively studied due to their
out the earth’s crust in low concentrations. As it is known,
long-lived, millisecond lifetimes, narrow-width emission
lanthanide complexes have attracted widespread interest due
bands, and hypersensitivities to coordination environment.
to the wide variety of potential applications in magnetic mate-
However, direct excitation of LnIII is not efficient because of
rials, catalysts, photoelectric conversion material, and so
its small absorption cross-section. To overcome this problem,
on.[1–9] Comparatively less is known about the biological prop-
an organic chromophore, which serves as an antenna or sensi-
erties of lanthanide complexes.[10–12] Some rare-earth ions may
tizer, absorbing the excitation light and transferring the energy
participate in various physicochemical processes of the living
from its lowest triplet state energy level (T) to the resonance
systems.[13] In particular, some lanthanide complexes with
level of LnIII ion, is desired.[30,31] Such energy transfer is one
antimicrobial activities can be used as antibacterial and anti-
of the most important processes, which determine the fluores-
phlogistic agents, etc.[14–16] Schiff-base ligands with N, O do-
cence properties of LnIII complexes.
nor sets have often been used since they may assemble coordi-
On the other hand, an excess of activated oxygen species in
nation architectures directed by the lanthanide(III) ions,
the form of superoxide anion (O2–•) and/or hydroxyl radical
whereas the lanthanide ions can promote Schiff-base conden-
(•HO), generated by normal metabolic processes, may cause
sation and can give access to complexes of otherwise inaccess-
various diseases such as carcinogenesis, drug-associated
ible ligands.[17] Moreover, some lanthanide Schiff base com-
toxicity, inflammation, atherogenesis, and aging in aerobic or-
plexes, which are isolated from aqueous solution show excel-
ganisms.[32,33]
lent physical properties such as magnetism[18] and lumines-
Based on the above consideration, we describe the synthesis,
cence,[19,20] whereas other non-aqueous lanthanide complexes
characterization, spectroscopic and antioxidant activities of a
can serve as homogeneous catalysts to catalyze asymmetric
Schiff base ligand Bod and its complexes with EuIII and CeIII.
Aldol-Tishchenko reactions,[21] asymmetric epoxidation of
The results show that the compounds have good fluorescence
α,β-unsaturated ketones,[22] asymmetric nitro-Mannich reac-
properties and antioxidant activities.
tions,[23] other organic transformations,[24] and ring opening
polymerizations of ε-caprolactone and related cyclic es-
ters.[25,26]
2 Experimental Section
Highly luminescent lanthanide complexes are attracting at-
tention in a wide variety of photonic applications such as
2.1 Measurements and Methods
All chemicals were reagent grade and used without further purification.
The bis(2-chloroethyl)ether is highly toxic nature and should thus be
handled with care. The C, H and N elemental analyses were deter-
mined with a Carlo Erba 1106 elemental analyzer. The IR spectra were
measured in the 4000–400 cm–1 region with a Nicolet FT-VERTEX 70
spectrometer using KBr pellets. The fluorescence spectra were re-
corded with a LS-55 spectrofluorophotometer. Electronic spectra were
* Prof. Dr. H. L. Wu
E-Mail: wuhuilu@163.com
[a] School of Chemical and Biological Engineering
Lanzhou Jiaotong University
Lanzhou, Gansu, 730070, P. R. China
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201600434 or from the au-
thor.
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obtained in DMF, concentration of 10–5 M, at 25 °C with a Lab-Tech
UV Bluestar spectrophotometer. The antioxidant activities containing
ware.[37] The coordinates of the non-hydrogen atoms were refined an-
isotropically, while hydrogen atoms were included in the calculation
the hydroxyl radical and superoxide radical were performed in water- isotropically.
bath with 722sp spectrophotometer. Powder X-ray diffraction (PXRD)
In the complexes, all hydrogen atoms were found in different electron
investigations were carried out with a Rigaku D/Max-2500PC X-ray
maps and were subsequently refined in a riding-model approximation
with C–H distances ranging from 0.93 to 0.97 Å and Uiso (H) = 1.2
Ueq(C) or Uiso (H) = 1.5 Ueq(C). A summary of parameters for the
data collections and refinements is given in Table 1. Selected bond
lengths and bond angles of complexes 1 and 2 are listed in Tables S1
and S2 (Supporting Information).
diffractometer at 40 kV, 40 mA with Cu-Kα radiation (λ = 1.542 Å).
2.2 Preparation of 3-Oxapentane-1,5-diamine
The synthetic pathway to 3-oxapentane-1,5-diamine followed the pro-
cedure reported in the literature.[34,35] C4H12N2O (104.1): calcd. C
46.13; H 11.61; N 26.90%; found C 45.98; H 11.50; N 26.76. IR
(selected data, KBr): ν˜ = 1120 (C–O–C), 3340 (–NH2) cm–1.
Table 1. Crystallographic data and data collection parameters for com-
plexes 1 and 2.
1
2
Formula
EuC20H24N5O14
710.4
CeC23H31N6O15
771.66
2.3 Preparation of Bis(3-methoxysalicylidene)-3-
oxapentane-1,5-diamine (Bod)
Molecular weight /g·mol–1
Crystal system
Space group
triclinic
triclinic
¯
¯
P1
P1
The synthesis of Bod was carried out by the dropwise addition of o-
vanillin (12 mmol, 1.82 g) in EtOH (5 mL) to an EtOH (5 mL) solution
of 3-oxapentane-1,5-diamine (5 mmol, 0.52 g). After complete ad-
dition, the solution was stirred for an additional 4 h at 78 °C. After
cooling to room temperature, the precipitate was filtered. The product
was dried in vacuo, and a yellow crystalline solid was obtained. Yield:
1.59 g (68.0%). C20H24O5N2 (372): calcd. C 64.50; H, 6.50; N, 7.52%;
found C 64.30; H, 6.58; N, 7.51%. IR (selected data, KBr): ν˜ = 1629
(C=N), 1256 (C–O–C), 3440 (–OH) cm–1. UV/Vis (DMF): λ = 271,
330 nm. Λm (DMF, 297 K): 2 S·cm2·mol–1.
Unit cell dimensions
a /Å
b /Å
c /Å
α /°
β /°
9.449(5)
9.673(5)
15.181(8)
77.786(6)
84.835(6)
74.930(6)
1308(12)
2
11.120(14)
14.926(19)
15.023(19)
61.524(13)
72.430(14)
83.324(14)
2088(5)
γ /°
Volume /Å3
Z
T /K
2
296(2) K
1.803
296(2) K
1.227
1.148
D (calculated) /g·cm–3
Absorption coefficient /mm–1 2.476
F(000)
708
778
θ range for data collection /° 1.37 to 25.50
2.19 to 25.50
11910
2.4 Preparation of Complexes
Reflections collected
8945
The two complexes were prepared by utilizing a similar procedure. To
a stirred solution of Bod (372 mg, 1 mmol) in EtOH (10 mL) was
added Ln(NO3)3(H2O)6 (1 mmol; Eu, 446 mg; Ce, 434 mg) in EtOH
(10 mL). A yellow sediment was generated rapidly. The precipitate
was filtered off, washed with EtOH and absolute Et2O, and dried in
vacuo. The dried precipitate was dissolved in DMF to form a yellow
solution. The yellow block crystals of the LnIII complexes suitable for
X-ray diffraction studies were obtained by vapor diffusion of diethyl
ether into the solution for few weeks at room temperature.
Independent reflections
4722 [R(int) =
0.0251]
–11,11; –10,11;
–18,18
7583 [R(int) =
0.0401]
–13,13; –18,18;
–18,17
Index ranges(h,k,l)
Refinement method
Full-matrix least- Full-matrix least-
squares on F2
4722 / 0 / 363
1.123
squares on F2
7581 / 21 / 410
0.994
R1 = 0.0444,
wR2 = 0.1059
R1 = 0.0681,
wR2 = 0.1129
Data/restraints/parameters
Goodness-of-fit on F2
Finial R1 and wR2
[I Ͼ 2σ(I)]
R1 = 0.0359,
wR2 = 0.0944
R1 = 0.0445,
wR2 = 0.1084
R indices (all data)
[Eu(Bod)(NO3)3] (1): Yield: 476 mg (58%). EuC20H24N5O14 (710.4):
calcd. C 33.81; H, 3.41; N 9.86%; found C 33.79; H, 3.43; N, 9.85%..
UV/Vis (DMF): λ = 271, 353 nm. IR (selected data, KBr): ν˜ = 1075
(C–O–C), 1662 (C=N) cm–1. Λm (DMF, 297 K): 8 S·cm2·mol–1.
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-1006219 and CCDC-1006220 for complexes 1 and
2 (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
www.ccdc.cam.ac.uk).
[Ce(Bod)(NO3)3·DMF]ϱ (2): Yield: 426 mg (53%). CeC23H31N6O15
(771.66): calcd. C 35.8; H, 4.05; N, 10.89%; found C 35.78; H, 4.02;
N, 10.90%.. UV/Vis (DMF): λ = 280, 358 nm. IR (selected data, KBr):
ν˜
=
1078 (C–O–C); 1652 (C=N) cm–1. Λm (DMF, 297 K):
7 S·cm2·mol–1.
2.5 X-ray Structure Determination of Complexes
2.6 Hydroxyl Radical Scavenging Activity
A suitable single-crystal was mounted on a glass fiber and the intensity The hydroxyl radicals were generated in aqueous media through the
data were collected with a Bruker SMART APEX diffractometer with
Fenton-type reaction.[38,39] Aliquots of the reaction mixture (3 mL)
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 296 K. contained 1 mL of 0.1 mM aqueous safranin, 1 mL of 1.0 mM aqueous
Date reduction and cell refinement were performed using SAINT pro-
EDTA-FeII, 1 mL of 3% aqueous H2O2, and a series of quantitative
grams.[36] The absorption corrections were carried out by empirical microadditions of solutions of the test compound. The sample without
method. The structure was solved by direct methods and refined by the tested compound was used as the control. The reaction mixtures
full-matrix least-squares against F2 of data using SHELXTL soft- were incubated at 37 °C for 30 min in a waterbath. Absorbance at
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520 nm was measured and the solvent effect was corrected throughout.
The scavenging effect for OH was calculated from the following ex-
pression:
3.1 X-ray Structure Determination of Complexes
•
3.1.1 [Eu(Bod)(NO3)3] (1)
¯
Complex 1 crystallized in the triclinic space group P1. Un-
expectedly, two nitrogen atoms of imine in this ligand did not
take part in coordinating with metal ion and the complex was
observed to be mononuclear. The asymmetric unit consists of
discrete [Eu(Bod)(NO3)3] species without any additional sol-
vent molecules (Figure 1). Each EuIII ion adopted a distorted
hexadecahedron arrangement and is ten-coordinated with four
oxygen atoms from two hydroxyl oxygen atoms and two meth-
oxyl oxygen atoms of the ligand, and six oxygen atoms from
three bidentate nitrate counterions. This is similar with other
reports on Schiff-base lanthanide complexes [Ln(H2L1)(NO3)3]
Scavenging effect (in %) = (Asample – Ar) / (Ao – Ar)ϫ100%
Where Asample is the absorbance of the sample in the presence of the
tested compound, Ar is the absorbance of the blank in the absence of
the tested compound, and Ao is the absorbance in the absence of both
the tested compound and EDTA-FeII.
2.7 Superoxide Radical Scavenging Activity
A nonenzymatic system containing 1 mL 9.9ϫ10–6 m VitB2, 1 mL
1.38ϫ10–4 m NBT, and 1 mL 0.03 m MET were used to produce su-
–•
peroxide anion. The scavenging rate of O2 under the influence of
[H2L1
=
N,NЈ-bis(3-methoxysalicylidene)ethylene-1,2-di-
0.1–1.0 μM of the tested compound was determined by monitoring
the reduction in the rate of transformation of NBT to monoformazan
dye.[40,41] The solutions of MET, VitB2 and NBT were prepared with
0.02 m phosphate buffer (pH 7.8) in the absence of light. The reactions
were monitored at 560 nm with a UV/Vis spectrophotometer, and the
amine, Ln3+ = Sm3+, Nd3+, Eu3+, Pr3+, Gd3+, Tb3+, Dy3+, Ho3+,
Er3+],[44–49] [Ln(H2L2)(NO3)3] [H2L2 = N,NЈ-bis(3-methoxy-
salicylideneimino)diethylenetriamine, Ln3+ = La3+, Nd3+].[50]
Ligand Bod acts as deprotonated tetradentate ligand with the
rate of absorption change was determined. The percentage inhibition O2O2 set of donor atoms capable of effective coordination
of NBT reduction was calculated using the following equation:[42,43]
percentage inhibition of NBT reduction = (1 – kЈ/k)ϫ100, where kЈ
and k represent the slopes of the straight line of absorbance values as
a function of time in the presence and absence of SOD mimic com-
pound (SOD is superoxide dismutase), respectively.
fashion. This is different from other reports on Schiff-base lan-
thanide complexes, [Pr(H2L1)(NO3)3]n and [(L1)Pr(NO3)].[51]
The bond lengths of Eu–Onitrate, Eu–Omethoxyl, Eu–Ohydroxyl
Supporting Information (see footnote on the first page of this article):
Selected bond lengths and angles, simulated and experimental PXRD
patterns for complexes 1 and 2; photoluminescent emission spectra of
the ligand and the complexes in solid state and in DMF solution.
3 Results and Discussion
The multi-step procedures for the synthesis of ligand Bod
are summarized in Scheme 1. In this presented investigation,
ligand Bod and its new complexes were synthesized and char-
acterized by different physical and chemical techniques. The
complexes are soluble in polar aprotic solvents such as
DMF, DMSO, and MeCN; slightly soluble in ethanol, meth-
anol, water, ethyl acetate, and chloroform; and insoluble in
Et2O and petroleum ether. The elemental analyses
show that their compositions are [Eu(Bod)(NO3)3] and
[Ce(Bod)(NO3)3DMF]ϱ, which was further confirmed by crys-
tal structure analysis.
Figure 1. Molecular structure and atom numberings of complex 1
(hydrogen atoms are omitted for clarity).
Scheme 1. Synthetic route for ligand Bod.
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bonds are in the range of 2.469(4)–2.643(5) Å, 2.625(4)– ing distorted hexadecahedron arrangement, is 10-coordinated
2.701(4) Å, and 2.299(4)–2.353(4) Å, respectively. From the with two phenolic oxygen atoms from different ligands, one
three distinct bond types, Eu–Omethoxyl exhibits the longest methoxy oxygen atom, six oxygen atoms from three bidentate
bond length, whereas Eu–Ohydroxyl has the shortest bond length nitrate, and one DMF oxygen atom, whereas the nitrogen
and Eu–Onitrate possesses an intermediate bond length.
atoms of imine remain uncoordinated (Figure 3). One methoxy
The non-classical C–H···O hydrogen bonding interactions oxygen atom coordinates to CeIII ion because of the shorter
play important roles in crystal packing in the complex. As Ce–O (methoxy) bond length [2.793(4) Å], whereas another
shown in Figure 2, two benzene rings from two [Eu(Bod) methoxy oxygen atom remains uncoordinated to CeIII ion in
(NO3)3] units located on the same line are in a parallel posi- judgment of the longer distance of Ce–O(1) (3.014 Å).
tion, and the complex ion is propagated into an infinite 1D
chain via inter-ligand π···π interactions [d = 3.733(0) Å] and
the hydrogen bonding (Figure 2a). Neighboring chains are con-
nected by C–H···O hydrogen bonds, thus generating an infinite
2D layer (Figure 2b). Selected hydrogen bonds are listed in
Table 2.
Figure 3. Molecular structure and atom numberings of complex 2
(hydrogen atoms are omitted for clarity).
The structure of complex 2 is similar with other reports on
Schiff-base lanthanide complexes [Sm(H2L1)(NO3)3(MeOH)]
[H2L1
=
N,NЈ-bis(3-methoxysalicylidene)ethylene-1,2-di-
amine],[49] [(H2L3)La(NO3)3(MeOH)] [H2L3 = N,NЈ-bis(3-
methoxysalicylidene)propane-1,2-diamine]),[52] but slightly
different from the reported 1D spiral Schiff base lanthanide
complex [(H2L1)Ln(NO3)3·H2O] (Ln3+ = La3+, Pr3+),[51] which
was proposed that LnIII ion was only coordinated by deproton-
ated phenolic oxygen atoms without complexation of the oxy-
gen atom from the neighboring methoxy group.
Figure 2. Packing structure of complex 1.
Table 2. Selected hydrogen bonding distances /Å and angles /° for
complexes 1 and 2.
The constructional units of polymer bridged by CeIII ions of
neighboring units are connected by oxygen atoms to form an
infinite 1D chain along a direction (Figure 4). The 1D double
chain along the b direction are linked by hydrogen bonds from
D–H···A
d(D–H)
d(H···A) d(D…A)
DHA
1
C(2)–H(2B)···O(9)#1
C(3)–H(3)···O(11)#2
C(11)–H(11B)···O(14)#3 0.97
C(13)–H(13)···O(13)#4
2
0.97
0.93
2.82
2.40
2.48
2.49
3.284(9)
3.249(7)
3.360(8)
3.244(7)
110.1
151.4
151.0
138.2
–
the C atom of the ligand and O atom of NO3 from the other
complex on the adjacent chain to form a 2D layer in the ab
plane.
0.93
C(8)–H(8)···O(11)#5
C(15)–H(15)···O(8)#6
0.93
0.93
2.44
2.55
3.227(7)
3.348(8)
142.5
144.3
Symmetry transformations used to generate equivalent atoms: #1 = x,
y–1, z; #2 = –x+1, –y+1, –z; #3 = x+1, y–1, z; #4 = –x+1, –y+1, –z+1;
#5 = –x+2, –y, –z; #6 = –x+2, –y, –z.
3.1.2 [Ce(Bod)(NO3)3DMF]ϱ (2)
Figure 3
shows
the
ORTEP
structure
of
[Ce(Bod)(NO3)3(DMF)]ϱ with atom labeling. Based on the
Figure 4. Packing structure of complex 2.
single-crystal X-ray analysis, complex 2 crystallizes in the tri-
¯
clinic space group P1 and shows a 1D ribbon framework built
We summarized our findings on salen-type Schiff base lan-
from an extended array of ten-coordinated central Ce3+ atoms thanide complexes and get the following conclusions: (i) Most
and the ligand. The ligand Bod bridges each two CeIII ions by of the lanthanide complexes with a hexadentate Schiff base
two Ce–O (hydroxy) and one Ce–O (methoxyl) bonds, and one ligand (by reacting o-vanillin and 1,2-diamines) are mononu-
methoxyl O atom is left uncoordinated. Each CeIII ion, adopt- clear complexes,[44–50] and only a small part of them are coor-
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dination polymers.[49,52] (ii) In 1D spiral salen-type Schiff base complexes. Two absorption bands are assigned to πǞπ*
lanthanide complexes, the coordination bridging mode of the (benzene) and πǞπ* (C=N) transitions.[58]
ligands is different. For tetradentate Schiff-base ligand (de-
rived from salicylide and 1,2-diamines), two hydroxy oxygen
atoms of each ligand act as a bidentate linker bridging between
two Ln3+ ions,[45,53] whereas for hexadentate Schiff-bases li-
3.3 Fluorescence Studies
In recent years, there has been an increasing interest in lumi-
gand (derived from o-vanillin and diamines), two hydroxy
oxygen atoms and one methoxyl oxygen atoms of each ligand
act as a tridentate linker bridging between two Ln3+ ions, and
one methoxyl oxygen atoms of ligand is left uncoordi-
nated.[49,52] (iii) From the above research content, it can also
be found that the same ligand with different rare earth ions
formed the different structures of the complexes. The size of
the rare earth ions (CeIII 0.102 nm, EuIII 0.0947 nm) might im-
pact the structure of the complexes. The heavy rare earth ions
form mononuclear complexes, and the light rare earth ions
form polynuclear complex. This is mainly attributed to the
“lanthanide contraction effect”. This conclusion is consistent
with our previous findings in lanthanide complexes with bis(N-
salicylidene)-3-oxapentane-1,5-diamine.[54,55]
To determine whether the crystal structures are truly repre-
sentative of the bulk materials used for property studies, pow-
der X-ray diffraction (PXRD) experiments were carried out
for the complexes. The PXRD experimental and as-simulated
patterns are shown in Figure S1 (Supporting Information). The
crystalline samples of complexes 1 and 2 gave a positive match
between the experimental and simulated PXRD patterns, indi-
cating that the bulk synthesized material and the crystals are
homogeneous.
nescent lanthanide (Ln) complexes because of their attractive
emission properties such as long lifetime, large stokes shift,
and line-like emission.[44]
The luminescence spectra of all compounds were recorded
in solid state and in DMF solution (1ϫ10–5 mol·L–1) at room
temperature, as shown in Figure S2 (Supporting Information).
All compounds exhibit luminescence spectra characteristic of
the ligand. The details of the emission characteristic of the
compounds are listed in Table 3. The emission peaks of the
ligand are observed at 460 nm (πǞπ*) in solvent and 338 nm
(πǞπ*) in solid state. The sensitized emission spectrum of the
[EuBod(NO3)3] complex in solvent displayed luminescence
bands at 475 nm owing to the (πǞπ*) transitions, respectively.
The intense emission peaks of the [Eu(Bod)(NO3)3] in solid
state appeared at around 506 nm (πǞπ*). The fluorescent in-
tensities observed in the EuIII complex were very weak (spec-
tra not shown).[59–61] Although the ligand is a good chelating
agent, europium complexes do not exhibit ionic characteristic
fluorescence, which may be due to the mismatch of the orbital
energy levels between the ligand and europium ions. Only the
fluorescence peak of the ligand appeared. The intense emission
peak of the ligand for the [Ce(Bod)(NO3)3DMF]ϱ was ob-
served at 466 nm (πǞπ*) in solvent and 501 nm (πǞπ*) in
solid state.
The red shift in the fluorescence band of the ligand in the
complexes compared with the free ligand can be attributed to
coordination of the rare earth ions to the ligand, which results
in increasing of the delocalization of electrons and reducing
the energy gaps between the π–π* molecular orbitals of the
ligand.[62–64]
3.2 IR and UV Spectroscopy
The IR spectroscopic data for the free ligand and LnIII com-
plexes (Ln = Eu and Ce) with their relative assignments were
studied to characterize their structures in the region 4000–
400 cm–1.[56] In the free ligand Bod, a strong band is found at
1629 cm–1, together along with a weak band at 1255 cm–1. By
analogy with the assigned bands, the former can be attributed
to ν(C=N),[57] whereas the latter can be attributed to
ν(C–O–C). The locations of the two bands were slightly
shifted for complex 1, the band at 1629 cm–1 was shifted to
1662 cm–1, while the band at 1255 cm–1 is shifted to
3.4 Antioxidant Activity
3.4.1 Hydroxyl Radical Scavenging
Figure 5 depicts the inhibitory effect of Bod and the com-
•
1228 cm–1. These can be attributed to the coordination of the plex on OH radicals. It can be seen that the inhibitory effects
ligand to the central metal atom. Similar shifts also appeared of the tested complex on •OH radicals are concentration related
in complexes 2 (1652, 1278 cm–1), which gives the same con- and the suppression ratios increase with the increasing of sam-
clusion, in agreement with X-ray diffraction.
ple concentrations in the range of the tested concentrations.
DMF solutions of ligands and complexes show, as expected, Moreover, we compared the abilities of the compounds to
almost identical UV spectra at room temperature. The UV scavenge hydroxyl radicals with those of the well-known natu-
bands of ligand (271, 330 nm) are marginally red-shifted in ral antioxidants mannitol and vitamin C, using the same
Table 3. Luminescence spectra data of the Schiff base ligand and complexes 1 and 2 in ethanol at room temperature.
Compound
Solvent
Solid
λex /nm
λem /nm
Stokes shift /nm Assignment
λex /nm
λem /nm
Stokes shift /nm
Assignment
Ligand
1
2
371
355
349
460
475
466
89
120
117
πǞπ*
πǞπ*
πǞπ*
200
389
379
338
506
501
138
117
122
πǞπ*
πǞπ*
πǞπ*
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Journal of Inorganic and General Chemistry
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Figure 5. Plots of antioxidation properties for the ligand and the complexes. (a) Ligand (b) complex 1 (c) complex 2 of the hydroxyl radical-
scavenging effect (%); (d) Ligand; (e) complex 1; (f) complex 2 of the superoxide radical-scavenging effect (%).
method as reported in a previous paper.[65,66] The 50% inhibi- As can be seen, Figure 5 shows the plots of superoxide radical
tory concentration (IC50) value of mannitol and vitamin C is scavenging effects (in %) for the compounds, which are also
about 9.6ϫ10–3 and 8.7ϫ10–3 m. According to the antioxi-
dant experiments, the IC50 of the ligand, EuIII and CeIII com-
concentration-dependant, but both the plots slightly blend to-
gether. The ligand and complexes 1 and 2 show IC50 values of
plexes are 8.60ϫ10–5, 6.82ϫ10–6, and 8.7ϫ10–6 m (Fig-
ure 5a–c). The result suggests that the complexes exhibit better
scavenging activity than the ligand, as well as mannitol and
vitamin C. Due to the observed IC50 values, the complexes
can be considered as potential drugs to eliminate the hydroxyl
radical.[61,67]
2.05ϫ10–4, 3.78ϫ10–5, and 3.47ϫ10–5 m (see Figure 5d–f).
The complexes have better superoxide radical scavenging ac-
tivity than the ligand. The ligand and the complexes exhibit
good superoxide radical scavenging activity and may act as
inhibitors to scavenge superoxide radicals in vivo upon further
investigation. The value of IC50 of vitamin C for superoxide
radical scavenging effect is 1.12ϫ10–4 m. The complexes have
better superoxide radical scavenging activity than the ligand
and vitamin C. The results imply that the complexes also have
the preferable ability to scavenge superoxide radical than vita-
min C in vivo.
3.4.2 Superoxide Radical Scavenging Activity
As another significant assay of antioxidant activity, superox-
ide radical (O2–•) scavenging activity of the title complexes
was investigated. The concentration of the complex, which
causes 50% inhibition of NBT reduction is reported as IC50. radicals scavenging assays demonstrate that the LnIII com-
The lower IC50 values observed in hydroxyl and superoxide
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4 Conclusions
Lanthanide (EuIII and CeIII) nitrate complexes with a Schiff
base ligand (Bod) were synthesized and characterized. The size
of the rare earth ions impose evident influences on the coordi-
nation arrangement leading to two different types of structure.
In addition, the LnIII complexes exhibited potential antioxidant
activities against HO and O2 radicals in in vitro studies.
Furthermore, only the fluorescence characteristic of the ligand
has been shown in the complexes. These findings indicate that
the rare earth Schiff base complexes have many potential prac-
tical applications for the development of luminescence and
new therapeutic reagents for diseases on the molecular level.
•
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Acknowledgements
The present research was supported by the National Natural Science
Foundation of China (Grant No. 21367017).
Keywords: Bis(3-methoxysalicylidene)-3-oxapentane-1,5-
diamine; Lanthanides; Crystal structure; Fluorescence;
Antioxidant
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Received: November 22, 2016
Published Online: ᭿
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X. Tang, X. Shi, Y. Xu, K. Shen, S. Mao, H. Wu* ............ 1–9
Synthesis, Crystal Structure, Fluorescent and Antioxidation
Properties of Cerium(III) and Europium(III) Complexes with
Bis(3-methoxysalicylidene)-3-oxapentane-1,5-diamine
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