A New Manganese(III) Complex from Bis(5-methylsalicylaldehyde)-3-oxapentane-1,5-diamine: Synthesis, Characterization, Antioxidant Activity and Luminescence

Article abstract of DOI:10.1002/jccs.201500121

A Schiff base ligand bis(5-methylsalicylaldehyde)-3-oxapentane-1,5-diamine (H₂L) has been prepared. Reaction of the shape-specific designed ligand with MnAc₂ afforded a new complex, namely, [Mn₂L₂Ac₂] 1. The ligand and complex 1 were characterized by physic-chemical and spectroscopic methods. The crystal structure of complex 1 has been determined by single-crystal X-ray diffraction. It reveals a centrosymmetric binuclear neutral entity, in which Mn(III) ion is bridged by two ligand L and six-coordinated with the coordination surround of distorted octahedron geometry. The antioxidant activities of the ligand and complex 1 were in vitro determined by superoxide and hydroxyl radical scavenging methods, which indicates that the complex 1 exhibits better scavenging activity of hydroxyl radical and superoxide radical than the ligand. Moreover, under the excitation at room temperature, luminescence studies have been carried out on solution-state samples. The result of their fluorescence properties shows that the strong and characteristic luminescence for the ligand and complex. The complex 1 exhibited intense shift in the fluorescence band compare to the ligand attributed to the coupled effect of the metal and the ligand.

Full text of DOI:10.1002/jccs.201500121

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CHEMICAL SOCIETY
Article
A New Manganese(III) Complex from Bis(5-methylsalicylaldehyde)-3-oxapentane-1,5-
diamine: Synthesis, Characterization, Antioxidant Activity and Luminescence
Huilu Wu,* Cuiping Wang, Fei Wang, Hongping Peng, Han Zhang and Yuchen Bai
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu, 730070, P.R. China
(Received: Mar. 31, 2015; Accepted: Sept. 21, 2015; Published Online: Oct. 30, 2015; DOI: 10.1002/jccs.201500121)
A Schiff base ligand bis(5-methylsalicylaldehyde)-3-oxapentane-1,5-diamine (H2L) has been prepared.
Reaction of the shape-specific designed ligand with MnAc2 afforded a new complex, namely, [Mn2L2Ac2]
1. The ligand and complex 1 were characterized by physic-chemical and spectroscopic methods. The crys-
tal structure of complex 1 has been determined by single-crystal X-ray diffraction. It reveals a
centrosymmetric binuclear neutral entity, in which Mn(III) ion is bridged by two ligand L and six-coordi-
nated with the coordination surround of distorted octahedron geometry. The antioxidant activities of the
ligand and complex 1 were in vitro determined by superoxide and hydroxyl radical scavenging methods,
which indicates that the complex 1 exhibits better scavenging activity of hydroxyl radical and superoxide
radical than the ligand. Moreover, under the excitation at room temperature, luminescence studies have
been carried out on solution-state samples. The result of their fluorescence properties shows that the
strong and characteristic luminescence for the ligand and complex. The complex 1 exhibited intense shift
in the fluorescence band compare to the ligand attributed to the coupled effect of the metal and the ligand.
Keywords: Bis(5-methylsalicylaldehyde)-3-oxapentane-1,5-diamine; Manganese(III); Crystal
structure; Antioxidant; Fluorescence.
INTRODUCTION
Schiff bases are considered as a very important class
biometric properties to mimic the enzyme activity.29-31
On the other hand, an excess of activated oxygen spe-
-
of organic compounds because of their ability to form sta-
ble complexes with many different transition metal and
rare-earth metal ions in various oxidation states via N and
O atoms.1-6 They have the potential to be used in different
areas such as electrochemistry, bioinorganic, catalysis, me-
tallic deactivators, separation processes, and environmen-
tal chemistry. Moreover they are becoming increasingly
important in the pharmacological, dye, and plastic indus-
tries as well as in the field of liquid crystal technology.7
The coordination chemistry of manganese has been
of considerable interest in recent years mainly due to the in-
creasing recognition of the role of this metal in biological
systems,8-11 being manganese the active site of numerous
enzymes,12 such as in catalase,13,14 superoxidase dismu-
tase15,16 and in the photosynthetic oxygen evolving com-
plex (OEC).17-19 In attempt to find artificial catalysts for the
decomposition of water20-22 or olefin oxygenation to alco-
hols,23 investigation on new manganese(III) complexes
with salen and its derivatives is a current area of inter-
est.24-25 In this respect manganese Schiff base complexes
are deeply studied due to its rich redox behavior,26-28 and
N/O donor polydentate ligand systems can present suitable
cies in the forms of superoxide anion (O2 ) and hydroxyl
radical (HO·), generated by normal metabolic processes,
may cause various diseases such as carcinogenesis, drug-as-
sociated toxicity, inflammation, atherogenesis, and aging in
aerobic organisms.32 Although a variety of (HO·) scavengers
are known, their application is limited by: (1) the require-
ment of unphysiologically high scavenger concentrations
which depend on the rate constant, (2) toxic side effects in-
cluding initiation of further radical chain reactions, or (3) the
instability of the compound in biological systems.33-35
Prompted by these applications, in this work, ligand
bis(5-methylsalicylaldehyde)-3-oxapentane-1,5-diamine
(H2L) and its Mn(III) complex were prepared and charac-
terized Besides the characterization of the complex, the an-
tioxidant activities of the ligand and its complex were in-
vestigated. The photoluminescence properties of the ligand
and the complex were explored in solution at room temper-
ature.
RESULTS AND DISCUSSION
General characterization
The synthetic routes to the multi-step procedures of
* Corresponding author. Email: wuhuilu@163.com
Supporting information for this article is available on the www under http://dx.doi.org/10.1002/jccs.201500121
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Antioxidant Activity and Luminescence of a Mn(III) Complex
the ligand H2L and complex 1 are shown in Scheme 1 and 2.
In the present investigations, the ligand and its new manga-
nese(III) acetate complex were synthesized and character-
ized by different physical and chemical techniques. The
complex 1 is soluble in polar aprotic solvents such as DMF,
DMSO and MeCN, slightly soluble in ethanol, methanol,
water, ethyl acetate, and chloroform and insoluble in Et2O
and petroleum ether. The elemental analysis shows that
composition is [Mn2L2Ac2], which was confirmed by the
crystal structure analysis.
the complex 1 crystallizes in monoclinic space group
P2(1)/c and has a centrosymmetric neutral homobinuclear
entity. The ORTEP structure of complex 1 with atom label-
ing is shown in Figure 1. The structure of complex 1 con-
sists of two trivalent Mn(III) ions, two fully deprotonated
ligands and two bidentate chelating acetate anions. An
illustration of the complex 1 shows that two adjacent
[Mn(L)2(Ac)2] moieties are bridged via two L groups. In
the m2-L bridged binuclear structure both Mn(III) centers
are six-coordinated by two oxygen atoms from a bidentate
acetate groups and the other two oxygen atoms and two
nitrigen atoms from the the different ligands L. The local
coordination environment is identical for both the centers
by symmetric and is best described as a distorted octa-
hedron geometry.
Synthetic route for ligand
Scheme 1
Two ligands form a typical rectangular cavity with
two manganese ions which are located at the top of the el-
lipse with the Mn···Mn separation of 7.761 Å (Fig. 2). It is
worth noting that intermolecular interactions have the po-
tential to assemble smaller and simpler fragments into de-
sired cavities under favorable conditions, which is impor-
tant in host-guest chemistry and has applications in chemis-
try, biology, and materials science. Selected bond lengths
and bond angles of complex 1 are listed in Table 1.
A reaction scheme for the complexation of
complex 1
Scheme 2
Constructional units of the complex are bridged by
hydrogen bonds from the C-atom of ligand and O atom of
Ac- from the other complex on the adjacent chain to form a
2-D layer in the bc plane (Fig. 3). At the same time, the 2-D
layers are connected by supramolecular interactions to
form the 3-D hydrogen bonding network. Selected hydro-
gen bonds are listed in Table 2.
The IR spectra of the free ligand and the complex 1
were compared. In the free ligand, a strong band is found at
1535 cm-1 together along with a weak band at 1091 cm-1.
By analogy with the assigned bands, the former can be at-
tributed to n(C=N), while the latter can be attributed to
n(C-O-C). These bands were shifted to the lower frequency
ca. 30~35 cm-1 in the complex.
The electronic spectra of the ligand and the complex 1
were recorded in DMF solution at room temperature. The
UV bands of the ligand (268, 326 nm) are marginally red
shift of 3-14 nm in complex. Two absorption bands are as-
signed to p®p* (benzene) and p®p* (C=N) transitions.
The complex 1 shows one new band at 408 nm not found in
the ligand, assigned to L®Mn(III) charge transfer spectra.
X-Ray crystallography
The prepared pentadentate ligand contains strong do-
nors, namely phenoxo oxygen atoms as well as imine nitro-
gen atoms with excellent coordination ability for transi-
tion/inner-transition metal ions through its N2O3 donor set.
Single-crystal X-ray structure determination reveals that
Fig. 1. Molecular structure and atom numberings of
complex 1 with hydrogen atoms omitted for
clarity.
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Usually, mannitol and vitamin C are used as the standard
antioxidants for comparison.36 The 50% inhibitory concen-
tration (IC50) value of mannitol and vitamin C is about 9.6 ´
10-3 and 8.7 ´ 10-3 M. According to the antioxidant experi-
ments, the IC50 of the ligand and the complex are 1.15 ´
10-4 and 1.29 ´ 10-5 M in Fig. 4. The result suggests that the
complex 1 exhibits better scavenging activity than the
ligand, as well as mannitol and vitamin C, and the higher
abilities of complex 1 and the ligand for HO· possibly due
to the key roles of functional groups, -OH, which can react
with HO· to form stable macromolecular radicals by the
typical H-abstraction reaction.37 Due to the observed IC50
values, the ligand and complex 1 can be considered as a po-
tential drug to eliminate the hydroxyl radical.38-40
Fig. 2. A typical rectangular cavity of complex 1.
Superoxide radical scavenging activity
Table 1. Selected atomic distances Å and bond angles (°) for
complex 1
As another significant assay of antioxidant activity,
-
superoxide radical (O2 ) scavenging activity of the ligand
Bond distances
and complex 1 has been investigated. The concentration of
compound which causes 50% inhibition of NBT reduction
is reported as IC50. It is reported that the value of IC50 of vi-
tamin C for superoxide radical scavenging effect is 1.12 ´
10-3 M.41 As can be seen from Fig. 5, the IC50 values for the
ligand and complex 1 were determined by plotting the
graph of percentage inhibition of NBT reduction against
Mn(1)-O(2)
Mn(1)-O(3)
Mn(1)-N(2)
Bond angle
1.854(2)
1.856(2)
2.171(2)
Mn(1)-N(1)
Mn(1)-O(4)
Mn(1)-O(6)
2.058(2)
2.015(2)
2.504(3)
O(2)-Mn(1)-O(3) 178.57(10) O(2)-Mn(1)-O(4) 88.89(10)
O(3)-Mn(1)-O(4)
O(3)-Mn(1)-N(1)
O(2)-Mn(1)-N(2)
O(4)-Mn(1)-N(2)
O(2)-Mn(1)-O(6)
O(4)-Mn(1)-O(6)
N(2)-Mn(1)-O(6)
90.52(10) O(2)-Mn(1)-N(1) 90.12(9)
89.92(10) O(4)-Mn(1)-N(1) 157.39(9)
92.68(10) O(3)-Mn(1)-N(2) 88.65(9)
92.85(9)
N(1)-Mn(1)-N(2) 109.76(9)
89.11(10) O(3)-Mn(1)-O(6) 89.48(9)
56.15(9)
148.93(8)
Table 2. Selected hydrogen bonding distances (Å) and angles (°)
for complex 1
N(1)-Mn(1)-O(6) 101.25(9)
D
H×××A
d(D-H) d(H×××A) d(D×××A) Ð(DHA)
C(19)-H(19B)...O(3)#1 0.97
C(19)-H(19A)...O(4)#3 0.97
2.32
2.33
2.41
2.863(4) 114.9
3.286(4) 170.3
3.311(4) 162.0
Hydroxyl radical scavenging activity
Fig. 4 shows the plots of hydroxyl radical scavenging
effects (%) for the ligand and complex 1. The two com-
pounds show inhibitory activity and the suppression ratio
for OH· increases with increasing complex concentration.
C(8)-H(8)...O(6)#4
0.93
Symmetry transformations used to generate equivalent atoms:
#1 = x, y, z; #2 = -x + 2, -y + 1, -z; #3 = -x + 2, y - 1/2, -z + 1/2;
#4 = -x + 2, y + 1/2, -z + ½.
Fig. 3. The hydrogen bonds of complex 1.
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Antioxidant Activity and Luminescence of a Mn(III) Complex
Fig. 4. Plots of hydroxyl radical-scavenging effect (%) for the ligand and complex 1. (a) Ligand (b) Complex 1.
Fig. 5. Plots of the superoxide radical-scavenging effect (%) for the ligand and complex 1. (a) Ligand (b) Complex 1.
the increase in the concentration of compound. The ligand
and complex 1 show the IC50 value of 5.69 ´ 10-4 and 4.15 ´
10-5 M in Fig. 5. The result of the superoxide radical scav-
enging activity show the ligand and the complex have
much better than the vitamin C. In addition, the complex 1
has better scavenging activity than the ligand. The ligand
and the complex exhibit good superoxide radical scaveng-
ing activity and may be an inhibitor to scavenge superoxide
trum of the complex 1 in solvent displays luminescence
bands at 475 nm (p-p*).
The red shift of the fluorescence band of the ligand in
the complex can be attributed to coordination of the metal
ions to the ligand, which resulting in increasing of the
delocalization of electrons and reducing the energy gaps
between the p-p* molecular orbitals of the ligand. Energy
transfer between the Schiff base ligand and the metal ion
appears to follow the well-known intramolecular energy
transfer mechanism exhibited by Schiff-base complexes.
-
radicals (O2 ) in vivo which needs further investigation.
Fluorescence studies
Fig. 6 shows the emission spectra of the ligand and
complex 1 in DMF solution (1 ´ 10-5 mol L-1) at the room
temperature. The emission peak of the ligand are observed
at 463 nm (p-p*) in solvent. The sensitized emission spec-
EXPERIMENTAL
The C, H and N elemental analyses were determined using
a Carlo Erba 1106 elemental analyzer. The IR spectra were re-
J. Chin. Chem. Soc. 2015, 62, 1028-1034
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solved in DMF to form a yellow solution. The brown block crys-
tals of the Mn(III) complex that suitable for X-ray diffraction
studies were obtained by vapor diffusion of diethyl ether into the
solution for few weeks at room temperature. Yield: 563 mg
(62.3%). Anal. Calcd for Mn2C44H50N4O10 (%): C, 58.41; H, 5.57;
N, 6.19. Found (%) C, 58.39; H, 5.55; N, 6.21. IR (selected data,
KBr, v/cm-1): 1126 v(C-O-C), 1568 v(C=N). UV-Visible (in DMF),
lmax (nm): 271, 340 and 408. Lm (DMF, 297 K): 12.2 S cm2 mol-1.
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-mono-
chromated Mo Ka radiation (l = 0.71073 Å) at 296 K. Date reduc-
tion and cell refinement were performed using SAINT programs.43
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 of data using SHELXTL
software.44 In the complex, All hydrogen atoms were found in dif-
ference electron maps and were subsequently refined in a riding-
model approximation with C–H distances are 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
are given in Table 3. Crystallographic data for complex 1 has been
deposited with the Cambridge Crystallographic Data Center as
supplementary publication No. CCDC 1030630. Copies of the
data can be obtained free of charge on application to The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223
336 033; e-mail: deposit@ccdc.cam.ac.uk).
Fig. 6. The photoluminescent emission spectra of the
ligand (Ex = 326 nm) and complex 1 (Ex = 400
nm) in DMF solvent.
corded in the 4000–400 cm-1 region with a Nicolet FT-VERTEX
70 spectrometer using KBr pellets. The fluorescence spectra were
recorded on a LS-45 spectrofluorophotometer. Electronic spectra
were recorded in DMF, concentration of 10-5 M, at 25 °C using a
Lab-Tech UV Bluestar spectrophotometer. The antioxidant activ-
ities containing the hydroxyl radical (OH·) and superoxide radical
-
(O2 ) were performed in water-bath with 722sp spectrophoto-
meter.
Preparation of 3-oxapentane-1,5-diamine: 3-oxapent-
ane-1,5-diamine was synthesized following the procedure in
ref.42 Anal. Calcd for C4H12N2O (%): calcd. C, 46.13; H, 11.61; N,
26.90. Found C, 45.98; H, 11.50; N, 26.76. IR (selected data, KBr,
Table 3. Crystallographic data and data collection parameters for
complex 1
v/cm-1): 1120 n(C-O-C), 3340 n(-NH2)
.
Complex
Empirical formula
Formula weight
[Mn2L2Ac2]
C44H50Mn2N4O10
904.76
Preparation of bis(5-methylsalicylaldehyde)-3-oxapent-
ane-1,5-diamine (H2L): For the synthesis of H2L, 5-methyl-
salicylaldehyde (12 mmol, 1.63 g) in EtOH (5 mL) was added
dropwise to a 5 mL EtOH solution of 3-oxapentane-1,5-diamine
(5 mmol, 0.52 g). 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. The product was dried in
vacuo, and obtained yellow crystalline solid. Yield: 1.59 g
(68.0%). Anal. Calcd for C20H24O3N2 (%): C, 70.17; H, 7.11; N,
8.23. Found C, 69.92; H, 6.92; N, 8.36. IR (selected data, KBr,
v/cm-1): 1091 n(C-O-C), 1538 n(C=N). UV-Visible (in DMF), lmax
(nm): 268 and 326.
Crystal system
Space group
Monoclinic
P2(1)/c
a/Å
10.747(4)
b/Å
13.007(4)
c/Å
15.560(5)
b/°
95.655(4)
2164.6(12)
V/Å3
Z
2
Calculated density/(g·cm–3)
F(000)
1.388
944
Crystal size/mm3
h/k/l (max, min)
0.41 ´ 0.37 ´ 0.30
-13 <= h <= 13,
-12 <= k <= 15,
-18 <= l <= 18
1.90 to 25.50
R1 = 0.0472, wR2 = 0.1221
R1 = 0.0593, wR2 = 0.1318
0.797 and -0.562 e×Å–3
Preparetion of Mn(III) complex: To a stirred solution of
H2L (340 mg, 1 mmol) in EtOH (10 mL) was added MnAc2 (1
mmol, 467 mg) in EtOH (10 mL). The yellow sediment generated
rapidly. The precipitate was filtered off, washed with EtOH and
absolute Et2O, and dried in vacuo. The dried precipitate was dis-
q range for data collection/°
Final R1, wR2 indices [I > 2s(I)]
R1, wR2 indices (all data)
Largest differences peak and hole
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Antioxidant Activity and Luminescence of a Mn(III) Complex
Antioxidation study methods: The hydroxyl radicals
were generated in aqueous media through the Fenton-type reac-
tion.45,46 The 3 mL reaction mixtures contained 1.0 mL of 40
mg/mL aqueous safranin, 1 mL of 1.0 mmol aqueous EDTA-
Fe(II), 1 mL of 3% aqueous H2O2, and a series of quantitatively
microadding solutions of the tested compound. The sample with-
out 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 absorbance was then measured at 520
nm. The scavenging effect for OH· was calculated from the fol-
lowing expression:47
ACKNOWLEDGEMENTS
The present research was supported by the National
Natural Science Foundation of China (Grant No.
21367017), the Fundamental Research Funds for the
Gansu Province Universities (212086), National Natural
Science Foundation of Gansu Province (Grant No.
1212RJZA037), and ‘Qing Lan’ Talent Engineering Funds
for Lanzhou Jiaotong University.
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CONCLUSIONS
In conclusion, a Schiff base ligand bis(5-methyl-
salicylaldehyde)-3-oxapentane-1,5-diamine (H2L) and its
Mn(III) complex have been synthesized, structurally char-
acterized and properties. In addition, the Mn(III) complex
exhibited potential antioxidant activities against HO· and
O2- radicals in vitro studies. These findings indicate that
the Mn(III) complex has many potential practical applica-
tions for the development of nucleic acid molecular probes
and new therapeutic reagents for diseases on the molecular
level. However, their pharmacodynamical, pharmacologi-
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