Synthesis, crystal structure, antioxidation and fluorescence of two lanthanide complexes with a noncyclic polyether Schiff base ligand

Article abstract of DOI:10.1080/00958972.2017.1320394

Two lanthanide (Sm and La) complexes with the Schiff base ligand bis(3-methoxysalicylidene)-3-oxapentane-1,5-diamine (Bod) have been synthesized and characterized by physico-chemical and spectroscopic methods. [Sm(Bod)(NO₃)₃] {bis(3-methoxysalicylidene)-3-oxapentane-1,5-diamine samarium(III) trinitrate} (1) is a discrete mononuclear species and [La(Bod)(NO₃)₃(DMF)]_n {bis(3-methoxysalicylidene)-3-oxapentane-1,5-diamine dimethylformamide lanthanum(III) trinitrate}_n (2) formed an inorganic coordination polymer. In the two complexes, the metal ions are both ten-coordinate and the geometric structure around the Ln(III) ions can be described as distorted hexadecahedral. An antioxidant assay in vitro shows that complexes 1 and 2 exhibit better scavenging activity than both the ligand and the usual antioxidants on hydroxyl and superoxide radicals. Under excitation at room temperature, a 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. Furthermore, 1 produced characteristic Sm(III) luminescence, which indicates the ligand Bod is a good organic chelator to absorb energy and transfer it to the Sm³⁺ ion.

Full text of DOI:10.1080/00958972.2017.1320394

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, crystal structure, antioxidation and
fluorescence of two lanthanide complexes with a
noncyclic polyether Schiff base ligand
Xinkui Shi, Shanshan Mao, Kesheng Shen, Huilu Wu & Xia Tang
To cite this article: Xinkui Shi, Shanshan Mao, Kesheng Shen, Huilu Wu & Xia Tang (2017):
Synthesis, crystal structure, antioxidation and fluorescence of two lanthanide complexes
with a noncyclic polyether Schiff base ligand, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2017.1320394
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Date: 25 April 2017, At: 11:19

Journal of Coordination Chemistry, 2017
https://doi.org/10.1080/00958972.2017.1320394
Synthesis, crystal structure, antioxidation and fluorescence of
two lanthanide complexes with a noncyclic polyether Schiff
base ligand
Xinkui Shi, Shanshan Mao, Kesheng Shen, Huilu Wu and Xia Tang
school of Chemical and Biological engineering, lanzhou Jiaotong university, lanzhou, P. r. China
ABSTRACT
ARTICLE HISTORY
received 20 october 2016
accepted 23 february 2017
Two lanthanide (Sm and La) complexes with the Schiff base ligand
bis(3-methoxysalicylidene)-3-oxapentane-1,5-diamine (Bod) have been
synthesized and characterized by physico-chemical and spectroscopic
methods. [Sm(Bod)(NO3)3] {bis(3-methoxysalicylidene)-3-oxapentane-
1,5-diamine samarium(III) trinitrate} (1) is a discrete mononuclear species
and [La(Bod)(NO3)3(DMF)]n {bis(3-methoxysalicylidene)-3-oxapentane-
1,5-diamine dimethylformamide lanthanum(III) trinitrate}n (2) formed
an inorganic coordination polymer. In the two complexes, the metal
ions are both ten-coordinate and the geometric structure around the
Ln(III) ions can be described as distorted hexadecahedral. An antioxidant
assay in vitro shows that complexes 1 and 2 exhibit better scavenging
activity than both the ligand and the usual antioxidants on hydroxyl
and superoxide radicals. Under excitation at room temperature, a 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. Furthermore, 1 produced characteristic Sm(III)
luminescence, which indicates the ligand Bod is a good organic chelator
to absorb energy and transfer it to the Sm3+ ion.
KEYWORDS
Bis(3-methoxysalicylidene)-
3-oxapentane-1,5-diamine;
lanthanide complex; crystal
structure; antioxidant;
fluorescence
CONTACT huilu Wu
wuhuilu@163.com, wuhl@mail.lzjtu.cn
supplemental data for this article can be accessed at http://dx.doi.org/10.1080/00958972.2017.1320394.
© 2017 informa uK limited, trading as taylor & francis Group

2
X. SHI ET AL.
1. Introduction
Schiff bases are a very important class of compounds because of their ability to form stable
complexes with many different transition and rare-earth metal ions in various oxidation states
via N and O atoms [1–4]. Aguiari et al. have synthesized a series of flexible cyclic and acyclic
Schiff bases derived from functionalized polyamines to verify the influence of the different
coordinating moieties on metal ion recognition and to study the stability and the properties of
complexes containing d- or f-metal ions. In particular, by condensation in hot methanol of
2,3-dihydroxy-, 2-hydroxy-3-methoxy- or 2-hydroxy-3-ethoxy-benzaldehyde with 1,2-diami-
noethane, 1,3-diaminopropane, 1,5-diamino-3-oxapentane, 1,8-diamino-3,6-dioxaoctane and
1,11-diamino-3,6,9-trioxaundecane, macroacyclic ligands have been obtained. However, struc-
turally characterized hexadentate Schiff-base lanthanide complexes are very rare [5].
Mononuclear and homo- or heteropolynuclear complexes with compartmental acyclic or mac-
rocyclic Schiff bases have been widely studied because of their potential application in mag-
netism, luminescence and new functional materials, in which pore size, coordination forms, and
functionality could be varied [6–8].The Schiff base complexes have also been used as biological
models to understand the structures of biomolecules and biological processes [9, 10].
In recent decades, lanthanide complexes with organic ligands have been attracting great
attention as advanced optical materials for progressive technologies due to their useful
practical applications in telecommunications, OLED-devices, and solar energy conversion,
as well as in bio-sciences [11–14]. It was shown by Weissman that ligands in organolanthanide
complexes play the role of antenna to harvest and transmit the excitation energy to lantha-
nide ion [15]. The luminescent intensities of the lanthanide metal complexes are strongly
dependent on the efficiency of the organic ligand to absorb UV light, energy transfer from
the ligand to metal, and lanthanide metal luminescence [16–18].
−
An excess of activated oxygen species in the forms of superoxide anion (O2 ∙) and hydroxyl
radical (OH∙), generated by normal metabolic processes, may cause various diseases such
as carcinogenesis, drug-associated toxicity, inflammation, atherogenesis, and aging in aer-
obic organisms [19–21]. The potential value of antioxidants has prompted investigators to
search for the cooperative effects of metal complexes and natural compounds for improving
antioxidant activity and cytotoxicity [22, 23].
Our recent studies have focused on the use of a tetradentate Schiff base ligand, bis(N-sal-
icylidene)-3-oxapentane-1,5-diamine, to coordinate to Ln(III) ions to improve DNA-binding
behavior and antioxidant ability [24]. As part of our continuing studies, we herein report the
synthesis, crystal structure, antioxidant character and fluorescence of two lanthanide com-
plexes with a hexadentate Schiff-bases ligand Bod. We believe that by choosing Bod as a
chelating ligand it is possible to induce and enhance the fluorescent properties, and improve
antioxidant activity of the lanthanide complexes. Information obtained from this study will
be helpful in the development of some new antioxidants and optical materials.
2. Experimental
2.1. Materials and methods
All chemicals were of analytical grade. Nitroblue tetrazolium nitrate (NBT), methionine (MET),
o-vanillin and riboflavin (VitB2) were obtained from Sigma-Aldrich Co. (USA) and used without
purification. 3-Oxapentane-1,5-diamine was made based on a preparation described [25].

JOURNAL OF COORDINATION CHEMISTRY
3
C, H and N elemental analyses were determined using a Carlo Erba 1106 elemental analyzer.
IR spectra were recorded from 4000 to 400 cm−1 with a Nicolet FT-VERTEX 70 spectrometer using
KBr pellets. Conductance measurements were made with a DDS-307 type conductivity bridge
using 3 × 10−3 mol L−1 solutions in DMF at room temperature. The fluorescence spectra were
recorded on a LS-55 spectrofluorophotometer. Electronic spectra were taken on a Lab-Tech UV
Bluestar spectrophotometer. The antioxidant activities using the hydroxyl radical (OH∙) and
−
superoxide radical (O2 ∙) were measured in a water bath with a 722sp spectrophotometer.
2.2. Preparation of bis(3-methoxysalicylidene)-3-oxapentane-1,5-diamine (Bod)
o-Vanillin (1.82 g, 12 mmol) in EtOH (5 mL) was added dropwise to a 5 mL EtOH solution of
3-oxapentane-1,5-diamine (0.52 g, 5 mmol). After the completion of addition, the solution
was stirred for an additional 4 h at 78°C. After cooling to room temperature, the precipitate
was filtered off. The product was dried in vacuo and obtained as a yellow crystalline solid.
Yield: 1.59 g (68.0%). Anal. C20H24O5N2: Calcd C, 64.50; H, 6.50; N, 7.52%; found: C, 64.30; H,
6.51; N, 7.51%. Selected IR data (KBr, ν/cm−1), 1629(νC=N), 1256(νC–O–C), 1047(νOH). UV–vis
(DMF): λ = 271, 330 nm. Λm (DMF, 297 K): 1.7 S cm2 mol−1.
2.3. Preparation of [Sm(Bod)(NO3)3] {bis(3-methoxysalicylidene)-3-oxapentane-
1,5-diamine samarium(III) trinitrate} (1)
To a stirred solution of Bod (0.372 g, 1 mmol) in MeOH (5 mL) was added Sm(NO3)3(H2O)6 (0.1776 g,
1 mmol) in MeOH (5 mL). A yellow sediment was generated rapidly. The precipitate was filtered
off, washed with MeOH and absolute Et2O, and dried in vacuo.The dried precipitate was dissolved
in DMF to form a yellow solution. Yellow block crystals of [Sm(Bod)(NO3)3] suitable for X-ray dif-
fraction studies were obtained by vapor diffusion of diethyl ether into the solution for few weeks
at room temperature. Yield: 0.3121 g (56.8%). Anal. SmC20H24N5O14: Calcd C, 33.89; H, 3.41; N,
9.88%; found: C, 33.91; H, 3.39; N, 9.78%. Selected IR data: (KBr ν/cm−1), 1182 (νC–O–C), 1032(νOH),
1385(νNO3), 1654 (νC=N). UV–vis (DMF): λ = 271, 355 nm. Λm (DMF, 297 K): 7.6 S cm2 mol−1.
2.4. Preparation of [La(Bod)(NO3)3(DMF)]n {bis(3-methoxysalicylidene)-3-
oxapentane-1,5-diamine dimethylformamide lanthanum(III) trinitrate}n (2)
To a stirred solution of Bod (0.372 g, 1 mmol) in MeOH (5 mL) was added La(NO3)3(H2O)6 (0.1732 g,
1 mmol) in MeOH (5 mL). A yellow sediment was generated rapidly. The precipitate was filtered
off, washed with MeOH and absolute Et2O, and dried in vacuo.The dried precipitate was dissolved
in DMF to form a yellow solution. Pale yellow block crystals of 2 suitable for X-ray diffraction
measurement were obtained by vapor diffusion of diethyl ether into the solution for two weeks
at room temperature. Yield: 0.2933 g (53.8%). Anal. LaC23H31N6O15: Calcd C, 35.90; H, 3.93; N,
10.92%; found: C, 35.88; H, 3.95; N, 10.90%. Selected IR data: (KBr ν/cm−1), 1174 (νC–O–C), 1037(νOH),
1389(νNO3), 1655 (νC=N). UV–vis (DMF): λ = 279, 361 nm. Λm (DMF, 297 K): 6.9 S cm2 mol−1.
2.5. X-ray structure determination
A suitable single crystal was mounted on a glass fiber and the intensity data were collected
on a Bruker SMART APEX diffractometer with graphite-monochromated Mo Kα radiation

4
X. SHI ET AL.
(λ = 0.71073 Å) at 296 K. Data reduction and cell refinement were performed using SAINT
programs [26]. The absorption corrections were carried out by the empirical method. The
structure was solved by direct methods and refined by full-matrix least-squares against F2
using SHELXTL software [27]. All hydrogens were found in difference electron maps and
were subsequently refined in a riding-model approximation with C–H distances 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 are given in table 1. Selected bond lengths and angles of
1 and 2 are listed in table 2 and table S1.
2.6. Hydroxyl radical scavenging activity
The hydroxyl radicals in aqueous media were generated through a Fenton-type reaction [28,
29]. Aliquots of the reaction mixture (3 mL) contained 1 mL of 0.1 mM aqueous safranin,
1 mL of 1.0 mM aqueous EDTA–Fe(II), 1 mL of 3% aqueous H2O2, and a series of quantitative
microadditions of solutions of the test compound. The mixture without the tested compound
was used as the control. The reaction mixtures were incubated at 37°C for 30 min in a water-
bath. Absorbance at 520 nm was measured and the solvent effect was corrected throughout.
The scavenging effect for HO∙ was calculated from the following expression:
Scavenging effect % = (Asample − Ar)∕(Ao − Ar) × 100%
Table 1. Crystal data and structure refinement for 1 and 2.
Complex
1
2
molecular formula
smC20h24n5o14
708.8
laC23h31n6o15
770.45
molecular weight (gm−1
)
Crystal system
space group
triclinic
P-1
triclinic
P-1
unit cell dimensions
a (Å)
b (Å)
c (Å)
α (°)
9.513(13)
9.720(13)
15.33(2)
77.541(16)
84.219(14)
11.14(2)
14.98(3)
15.09(3)
61.42(2)
72.620(19)
β (°)
γ (°)
74.545(16)
1333(3)
83.35(2)
2109(7)
Volume (Å3)
Z
2
2
T(K)
296(2)
1.766
2.281
702
296(2)
1.213
1.070
776
D (calculated) (g·cm−3
)
absorption coefficient (mm−1
F(0 0 0)
)
Crystal size (mm)
0.41 × 0.37 × 0.30
1.36 to 25.50
8985
0.39 × 0.36 × 0.32
2.19 to 25.50
13,477
θ range for data collection (°)
reflections collected
independent reflections
index ranges
4962 [R(int) = 0.0253]
−11 ≤ h ≤ 11
−8 ≤ k ≤ 11
−18 ≤ l ≤ 18
full-matrix least-squares on F2
4818 / 0 / 363
1.049
R1 = 0.0306, wR2 = 0.0737
R1 = 0.0389, wR2 = 0.0824
0.857 and −0.691
7652 [R(int) = 0.0600]
−13 ≤ h ≤ 13
−18 ≤ k ≤ 18
−18 ≤ l ≤ 15
full-matrix least-squares on F2
7652 / 367 / 410
0.934
R1 = 0.0576, wR2 = 0.1358
R1 = 0.1015, wR2 = 0.1483
1.071 and −0.8528
refinement method
data / restraints / parameters
Goodness-of-fit on F2
final R1 and wR2 [I > 2σ(I)]
R indices (all data)
largest diff. peak and hole (e Å−3
)

JOURNAL OF COORDINATION CHEMISTRY
5
Table 2. selected bond distances (Å) for 1 and 2.
Selected bond lengths (Å)
1
2
ln–onitrate
sm(1)–o(2)
sm(1)–o(4)
sm(1)–o(5)
sm(1)–o(6)
sm(1)–o(8)
sm(1)–o(10)
sm(1)–o(13)
sm(1)–o(14)
sm(1)–o(11)
sm(1)–o(12)
2.557(4)
2.644(5)
2.552(5)
2.482(4)
2.553(5)
2.525(5)
2.322(4)
2.376(4)
2.740(4)
2.667(4)
la(1)–o(7)
la(1)–o(8)
2.638(6)
2.670(6)
2.671(6)
2.615(7)
2.616(7)
2.723(6)
2.454(5)
2.426(5)
2.794(6)
3.004(8)
2.534(6)
la(1)–o(10)
la(1)–o(11)
la(1)–o(13)
la(1)–o(14)
la(1)–o(2)#1
la(1)–o(4)
la(1)–o(5)
la(1)–o(1)#1
la(1)–o(15)
ln–ohydroxyl
ln–omethoxyl
ln–odmf
note: symmetry transformations used to generate equivalent atoms: #1 x + 1,y,z.
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 the tested compound and EDTA-Fe(II).
2.7. Superoxide radical scavenging activity
A non-enzymatic system containing 1 mL 9.9 × 10−6 M VitB2, 1 mL 1.38 × 10−4 M NBT and
−
1 mL 0.03 M MET was used to produce superoxide anion (O2 ∙), and the scavenging rate of
−
O2 ∙ under the influence of 0.1–1.0 μM of the tested compound was determined by moni-
toring the reduction in the rate of transformation of NBT to monoformazan dye. The specific
method proceeded following the procedure described [30]. The reactions were monitored
at 560 nm with a UV/Vis spectrophotometer, and the rate of absorption change was deter-
mined. The percentage inhibition of NBT reduction was calculated using the following equa-
tion [31]: percentage inhibition of NBT reduction = (1-k′/k) × 100, where k′ and k represent
the slopes of the straight line of absorbance values as a function of time in the presence and
absence of the SOD mimic compound (SOD is superoxide dismutase), respectively.
3. Results and discussion
Scheme 1 summarizes the multi-step procedure leading to the ligand Bod. In the present
investigations, Bod and its complexes were synthesized and characterized by different phys-
ical and chemical techniques. The complexes are soluble in polar aprotic solvents such as
DMF and DMSO, partially soluble in water, ethanol and methanol, but insoluble in Et2O and
petroleum ether. Elemental analysis shows that their compositions are [Sm(Bod)(NO3)3] and
[La(Bod)(NO3)3(DMF)]n, which was confirmed by the crystal structure analysis.
3.1. X-ray structure determination of complexes
3.1.1. Crystal structure of 1
Complex 1 crystallized in the triclinic space group P-1. Unexpectedly, two imine nitrogen
atoms of this ligand did not take part in coordinating with the metal ion and the complex
was observed to be a mononuclear structure. The asymmetric unit consists of discrete
[Sm(Bod)(NO3)3] species without any additional solvent molecules (figure 1). Each Sm(III) ion
adopted a distorted hexadecahedron geometry and is ten-coordinate with four oxygen

6
X. SHI ET AL.
Scheme 1. synthetic route for Bod.
Figure 1. (left) molecular structure and atom numberings of 1 with hydrogen atoms omitted for clarity.
(right) distorted hexadecahedron environment described by donor atoms around the central sm3+ atom
as illustrated in the polyhedral view.
atoms from two hydroxyl oxygen atoms and two methoxyl oxygen atoms of the ligand and
six oxygen atoms from three bidentate nitrate counterions. This is similar to other reports
on Schiff-base lanthanide complexes [Ln(H2L1)(NO3)3] (H2L1 = N,N′-bis-(3-methoxysalicylidene)
ethylene-1,2-diamine, Ln3+ = Sm3+, Nd3+, Eu3+, Pr3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+) [32–37],
[Ln(H2L2)(NO3)3] (H2L2 = N,N′-bis(3-methoxysalicylideneimino)diethylenetriamine, Ln3+ = La3+,
Nd3+) [38]. Bod acts as a deprotonated tetradentate ligand with the O2O2 set of donor atoms
capable of effective coordination. This is different from other reports on Schiff-base lantha-
nide complexes [Pr(H2L1)(NO3)3]n and [(L1)Pr(NO3)] [39]. The bond length ranges for Sm–Onitrate
,
Sm–Omethoxyl, and Sm–Ohydroxyl are 2.482(4)–2.644(5), 2.667(4)–2.740(4) and 2.322(4)–2.376(4) Å,
respectively. Of the three distinct bond types, Sm–Omethoxyl exhibits the longest bond length,
while Sm–Ohydroxyl has the shortest bond length and Sm–Onitrate possesses an intermediate
bond length.
As shown in figure 2, two benzene rings from two contiguous [Sm(Bod)(NO3)3] located
in the same line are in a parallel position, so the geometry of the complex ion is propagated
into an infinite 1-D chain via interligand π⋯π interactions (d = 3.750(0) Å). The non-classical
C–H⋯O hydrogen-bonding interactions play important roles in crystal packing in the com-
plex. Neighboring chains are connected by C(5)–H⋯O(2) [C(5)–O(2) 3.263(6) Å] hydrogen
bonds, thus generating an infinite 2-D layer.

JOURNAL OF COORDINATION CHEMISTRY
7
3.1.2. Crystal structure of 2
Figure 3 shows the ORTEP structure of [La(Bod)(NO3)3(DMF)]n with atom labeling. Based
on the single-crystal X-ray analysis, 2 crystallizes in the triclinic space group P-1 and
possesses a 1-D ribbon framework built from an extended array of ten-coordinate La3+
centers and the ligand. Bod bridges each pair of La(III) ions via two La–O (hydroxy) and
one La–O (methoxyl) bonds, and one methoxyl O-atom is left uncoordinated. Each La(III)
ion, adopting a distorted hexadecahedral geometry, is 10-coordinate with two phenolic
oxygen atoms from different ligands, one methoxy oxygen atom, six oxygen atoms from
three bidentate nitrate ions and one DMF oxygen atom, while the nitrogen atoms of
Figure 2. Packing structure of 1.
Figure 3. (upper) molecular structure and atom numberings of 2 with hydrogen atoms omitted for clarity.
(lower) distorted hexadecahedral environment described by donor atoms around the central la3+ atom
as illustrated in the polyhedral view.

8
X. SHI ET AL.
the imines remain uncoordinated (figure 3). One methoxy oxygen atom coordinates to
the La(III) ion because of the shorter La–O (methoxy) bond length (2.794(6) Å), whereas
another methoxy oxygen atom remains uncoordinated to La(III) ion resulting in the
longer La–O(1) (3.004(8) Å) distance. The structure of 2 is similar with other reports on
Schiff-base lanthanide complexes [Sm(H2L1)(NO3)3(MeOH)] (H2L1 = N,N′-bis-(3-
methoxysalicylidene)ethylene-1,2-diamine) [37], [(H2L3)La(NO3)3(MeOH)] (H2L3 = N,N′-
bis(3-methoxysalicylidene)propane-1,2-diamine)) [40], but with slightly different from
the reported 1-D spiral Schiff base lanthanide complex [(H2L1)Ln(NO3)3·H2O] (Ln3+
= La3+, Pr3+) [39], in which it was proposed that the Ln(III) ion was only coordinated by
deprotonated phenolic oxygen atoms without complexation of the oxygen atom from
the neighboring methoxy group.
In figure 4, it is shown that each ligand and La(III) ion bridge together to form an inorganic
polymer. Hydrogen-bonding interactions are also very significant for 2. The C(8)–H(8)⋯O(13)
[C(8)–O(13) 3.258(9) Å] hydrogen bonding interactions between two chains led to the for-
mation of a 2-D supramolecular network as shown in figure 4.
We examined the known salen-type Schiff base lanthanide complexes and arrived at
the following conclusions. (i) Most of the lanthanide complexes with a hexadentate Schiff
base ligand (via reaction of o-vanillin and 1,2-diamines) are mononuclear complexes
[32–38], and only a small part of them are coordination polymers [37, 40]. (ii) In 1-D spiral
salen-type Schiff base lanthanide complexes, the coordination bridging mode of the lig-
ands is different. For tetradentate Schiff-base ligands (derived from salicylide and
1,2-diamines), two hydroxy oxygen atoms of each ligand act as a bidentate linker bridging
between two Ln3+ ions [33, 41], while for hexadentate Schiff-base ligands (derived from
o-vanillin and diamines), two hydroxy oxygen atoms and one methoxyl oxygen atom of
each ligand act as a tridentate linker bridging between two Ln3+ ions, and one methoxyl
oxygen atom of the ligand is left uncoordinated [37, 40]. (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 (La(III) 0.103 nm,
Sm(III) 0.0958 nm) might impact 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-oxapen-
tane-1,5-diamine [24].
Figure 4. Packing structure of 2.

JOURNAL OF COORDINATION CHEMISTRY
9
To determine whether the crystal structures are truly representative of the bulk materials
used for property studies, powder X-ray diffraction (PXRD) experiments were carried out for
the complexes. The PXRD experimental and as-simulated patterns are shown in figure S1.
The crystalline samples of 1 and 2 gave a positive match between the experimental and
simulated PXRD patterns, indicating that the bulk synthesized material and the crystals are
homogeneous.
3.2. IR and electronic spectra
In the free ligand Bod, a strong band is found at 1629 cm−1 along with a weak band at
1255 cm−1. By analogy with the previously assigned bands, the former can be attributed to
ν
(C=N), while the latter can be attributed to ν(C-O-C). These bands shifted ca. 20 ~ 80 cm−1 in the
complexes, which can be attributed to coordination of the ligand. Bands at 1385–1389 cm−1
in the complexes indicate bidentate nitrate [42], in agreement with X-ray diffraction.
The electronic spectra of the ligands and the metal complex were recorded in DMF solu-
tion at room temperature. The UV bands of Bod (271, 330 nm) are marginally shifted in the
complexes. Two absorption bands are assigned to π → π* (benzene) and π → π* (C=N)
transitions.
3.3. Hydroxyl radical scavenging activity
The plots of hydroxyl radical scavenging effects (%) of the ligand and complexes have been
depicted in figure 5. The complexes show inhibitory activity and the suppression ratio for
OH∙ increases with increasing complex concentration. Usually, mannitol and vitamin C are
used as the standard antioxidants for comparison [43]. The 50% inhibitory concentration
(IC50) values of mannitol and vitamin C are about 9.6 × 10−3 and 8.7 × 10−3 M, respectively.
According to the antioxidant experiments, the IC50 of the ligand and 1 and 2 are 8.60 × 10−5,
6.82 × 10−6 and 8.7 × 10−6 M in figure 5(a–c). The results suggest that the complexes exhibit
better scavenging activity than the ligand, as well as mannitol and vitamin C. Due to the
observed IC50 values, 1 and 2 can be considered as a potential drugs to eliminate the hydroxyl
radical [44, 45].
3.4. Superoxide radical scavenging activity
−
As another significant assay of antioxidant activity, superoxide radical (O2 ∙) scavenging
activity of the title complexes has been investigated. The complexes and the ligand have
good superoxide radical scavenging activity. The concentration of the complex which causes
50% inhibition of NBT reduction is reported as IC50. As can be seen from figure 5, the IC50
values for the complexes were determined by plotting the graph of percentage inhibition
of NBT reduction against the increase in the concentration of the complexes. The ligand and
1 and 2 show IC50 values of 2.05 × 10−4, 3.78 × 10−5 and 3.47 × 10−5 M, respectively, in figure
5(d–f). The value of IC50 of vitamin C for superoxide radical scavenging effect is 1.12 × 10−4 M.
The complexes have better superoxide radical scavenging activity than the ligand and vita-
min C. The results indicate that 1 and 2 also exhibit good superoxide radical scavenging
activity and may be inhibitors (or drugs) to scavenge superoxide radical (O2−·) in vivo which
needs further investigation.

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X. SHI ET AL.
Figure 5. Plots of antioxidation properties for the ligand and the complexes. (a) ligand (b) [sm(Bod)
(no3)3] (c) [la(Bod)(no3)3(dmf)]n of the hydroxyl radical-scavenging effect (%); (d) ligand (e) [sm(Bod)
(no3)3] (f) [la(Bod)(no3)3(dmf)]n of the superoxide radical-scavenging effect (%).
The lower IC50 values observed in hydroxyl radical and superoxide radical scavenging
assays did demonstrate that 1 and 2 have good scavenging effects for hydroxyl radical (OH∙)
−
and superoxide radical (O2 ∙).
3.5. Fluorescence studies
The ability to transfer energy from ligand-centered to metal-centered states is important in
the design of lanthanide(III) photonic devices [46, 47].
Presented in table 3 are the data from the fluorescence emission spectra of the ligand
and its complexes in DMF solution (1 × 10−5 mol L−1) and in the solid state, which were

JOURNAL OF COORDINATION CHEMISTRY
11
Table 3. luminescence spectra data of the schiff base ligand Bod and its complexes in dmf at room
temperature.
Solvent
Stokes
λem (nm) shift (nm)
Solid
Stokes
λem (nm) shift (nm)
Assign-
ment
Assign-
ment
Compound
λex (nm)
λex (nm)
ligand
Complex 1
371
357
460
476
566
599
645
464
89
π–π*
π–π*
200
355
338
490
138
135
π–π*
π–π*
119
209
242
288
115
4G5/2 → 6h5/2
4G5/2 → 6h7/2
4G5/2 → 6h9/2
π–π*
Complex 2
349
392
521
129
π–π*
Figure 6. the photoluminescent emission spectra (solid and solvent in dmf) of the ligand and the
complexes. (a) ligand (b) [sm(Bod)(no3)3] (c) [la(Bod)(no3)3(dmf)]n in dmf solvent; (d) ligand (e)
[sm(Bod)(no3)3] (f) [la(Bod)(no3)3(dmf)]n in solid-state.

12
X. SHI ET AL.
studied at room temperature. As shown in figure 6, the emission peak of the ligand is
observed at 460 nm (π–π*) in solvent (figure 6(a)) and 338 nm (π–π*) in the solid state (figure
6(d)). Among the complexes studied, the emission spectrum of the Sm(III) complex in DMF
was measured for excitation at 357 nm. The emission spectrum is shown in figure 6(b).
Emission bands were observed at 476, 566, 599 and 645 nm and are attributed to π–π*
4
4
transition, f-f transitions G5/2 → 6H5/2 (zero–zero band: forbidden transition), G5/2 → 6H7/2
(magnetic dipole transition) and G5/2 → 6H9/2 (electric dipole transition), respectively.
4
4G5/2 → 6H9/2 transition intensity was 1.5 times larger than 4G5/2 → 6H7/2 transition intensity.
The full width at half-maxim (FWHM) of the G5/2 → 6H9/2 transition was 7 nm. The narrow
4
and strong emission of the 4G5/2 → 6H9/2 transition is due to the asymmetric ten-coordinate
structure related to the special odd parity [48]. Based on the data we can learn the emission
peaks which appear near 566, 599 and 645 nm are the emission peaks of Sm(III) [49]. The
emission of 1 produced characteristic Sm(III) luminescence, which indicated an effective
match of the orbital energy levels between the ligand and Sm(III) ion. The intense emission
peaks of 1 in the solid state appear near 490 nm (π–π*) in figure 6(e) and the fluorescence
of the Sm(III) ion was not observed due to the effect of diffuse reflection of the metal ions.
The intense emission peak of 2 is observed at 464 nm (π–π*) in solvent (figure 6(c)) and
521 nm (π–π*) in solid state (figure 6(f)). For 2, La(III) has no 4f electrons and has a closed-
shell electron configuration. Therefore, the energy absorbed by Bod cannot be transferred
to La(III) ions by an intramolecular energy transfer processes, however it relaxes through
its own lower energy levels, due to the mismatch of the orbital energy levels between the
ligand and La(III) ions.
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 an increase of the delocalization of electrons and reduction of the energy gaps between
the π–π* molecular orbitals of the ligand [50–53]. Energy transfer between the Schiff base
ligand and the rare earth ion appears to follow the well-known intramolecular energy transfer
mechanism exhibited by lanthanide Schiff-base complexes [54].
4. Conclusion
The lanthanide (Sm(III) and La(III)) nitrate complexes of the Schiff base ligand bis(3-methox-
ysalicylidene)-3-oxapentane-1,5-diamine (Bod) have been synthesized and structurally
characterized by elemental analysis, molar conductivity, IR spectra and UV–vis spectra.
−
Complexes 1 and 2 exhibited potential antioxidant activity against OH∙ and O2 ∙ radicals in
in vitro studies. These findings clearly indicate that the Ln(III) complexes have potential prac-
tical applications of antioxidants, which warrants further in vivo experiments and pharma-
cological assays. In addition, 1 exhibits the characteristic luminescence of Sm(III) ions whereas
2 shows only the fluorescence characteristics of Bod. This indicates that Bod is a good organic
chelator to absorb and transfer energy to Sm(III) ions.
Supplementary material
Crystallographic data (excluding structure factors) for 1 and 2 have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication CCDC 998536 and 998537. Copies of the
data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2
1EZ, UK.

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13
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the National Natural Science Foundation of China [grant number
21367017]; Natural Science Foundation of Gansu Province [grant number 1212RJZA037]; Graduate
Student Innovation Projects of Lanzhou Jiaotong University.
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