Synthesis, X-ray structural characterization, and catalytic property of an end-to-end azido-bridged polymeric manganese(III) complex derived from N,N?-bis(5-fluorosalicylidene)-1,2-diaminoethane

Article abstract of DOI:10.1080/15533174.2016.1186070

A new end-to-end azido-bridged polymeric manganese(III) complex [MnL(μ_1,3-N₃)]_n with the Schiff base ligand N,N?-bis(5-fluorosalicylidene)-1,2-diaminoethane (H₂L), has been synthesized and structurally characterized by physicochemical methods and single-crystal X-ray determination. Crystal of the complex crystallizes in orthorhombic space group Pca2₁, with a=10.8211(5), b=13.4644(6), c=11.1811(5)?, V=1629.08(13)?³, Z=4, R₁=0.0255, wR₂=0.0653, GOOF=1.053. X-ray analysis indicates that the complex is an end-to-end azido-bridged polymeric manganese species. The Mn atom is coordinated in an octahedral coordination by two phenolate O and two imine N atoms from the Schiff base ligand, and two azido N atoms. The catalytic property for epoxidation of styrene by the complex was evaluated.

Full text of DOI:10.1080/15533174.2016.1186070

Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry
ISSN: 1553-3174 (Print) 1553-3182 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt20
Synthesis, X-ray Structural Characterization, and
Catalytic Property of an End-to-end Azido-bridged
Polymeric Manganese(III) Complex Derived from
N,N'-bis(5-Fluorosalicylidene)-1,2-diaminoethane
Yan Lei & Qiwen Yang
To cite this article: Yan Lei & Qiwen Yang (2016): Synthesis, X-ray Structural Characterization,
and Catalytic Property of an End-to-end Azido-bridged Polymeric Manganese(III) Complex
Derived from N,N'-bis(5-Fluorosalicylidene)-1,2-diaminoethane, Synthesis and Reactivity in
Inorganic, Metal-Organic, and Nano-Metal Chemistry, DOI: 10.1080/15533174.2016.1186070
To link to this article: http://dx.doi.org/10.1080/15533174.2016.1186070
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Synthesis, X-ray Structural Characterization, and Catalytic Property of an End-to-end
Azido-bridged
N,N’-bis(5-Fluorosalicylidene)-1,2-diaminoethane
Yan Lei1,2,*, Qiwen Yang2,3
Polymeric
Manganese(III)
Complex
Derived
from
1Key Laboratory of Water Environment Evolution and Pollution Control in Three Gorges
Reservior, Chongqing Three Gorges University, Chongqing 404000, P. R. China
2School of City Construction and Environmental Engineering, Chongqing University, Chongqing
400030, P. R. China
3China Huaxi Engineering Design & Construction Co., ltd, Sichuan 610000, P. R. China
*Corresponding author. E-mail: leiyan222@126.com
Abstract
A new end-to-end azido-bridged polymeric manganese(III) complex [MnL(μ1,3-N3)]n with the
Schiff base ligand N,N’-bis(5-fluorosalicylidene)-1,2-diaminoethane (H2L), has been synthesized
and structurally characterized by physico-chemical methods and single crystal X-ray
determination. Crystal of the complex crystallizes in orthorhombic space group Pca21, with a =
10.8211(5), b = 13.4644(6), c = 11.1811(5) Å, V = 1629.08(13) Å3, Z = 4, R1 = 0.0255, wR2 =
0.0653, GOOF = 1.053. X-ray analysis indicates that the complex is an end-to-end azido-bridged
polymeric manganese species. The Mn atom is coordinated in an octahedral coordination by two
phenolate O and two imine N atoms from the Schiff base ligand, and two azido N atoms. The
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catalytic property for epoxidation of styrene by the complex was evaluated.
Keywords
Manganese complex, Schiff base ligand, azide ligand, crystal structure, catalytic property
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INTRODUCTION
Schiff-base compounds represent one of the most attractive series of ligands in coordination
chemistry. The ligands bearing typical C=N groups and suitable O, N, or S donor atoms are
capable of binding various transition and rare earth metal atoms to form complexes with versatile
structures and properties [1-3]. To date, most Schiff base complexes have been reported to have
interesting catalytic properties, such as asymmetric epoxidation, oxidation of sulfides, and
various type of polymerization [4-8]. Among the Schiff base complexes, those with Mn centers
are of particular interest for their epoxidation of olefins [9-11]. Herein we report the synthesis,
X-ray crystal structure, and catalytic property of a new manganese complex, [MnL(μ1,3-N3)]n,
where L is the deprotonated form of N,N’-bis(5-fluorosalicylidene)-1,2-diaminoethane (see
Scheme 1).
EXPERIMENTAL
Materials
Manganese perchlorate, 3-fluorosalicylaldehyde, and N,N’-diaminoethane were purchased from
Aldrich. All other reagents with AR grade were used as received without further purification.
Physical measurements
Infrared spectra (4000-400 cm-1) were recorded as KBr discs with a FTS-40 BioRad FT-IR
spectrophotometer. Microanalyses (C, H, N) of the complex were carried out on a Carlo-Erba
1106 elemental analyzer. Solution electrical conductivity was measured at 298K using a DDS-11
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conductivity meter. GC analyses were performed on a Shimadzu GC-2010 gas chromatograph.
X-ray crystallography
Crystallographic data of the complex were collected on a Bruker SMART CCD area
diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 298(2) K.
Absorption corrections were applied by using the multi-scan program [12]. The structure was
solved by direct methods and successive Fourier difference syntheses (SHELXS-97), and
anisotropic thermal parameters for all nonhydrogen atoms were refined by full-matrix
least-squares procedure against F2 (SHELXL-97) [12]. All non-hydrogen atoms were refined
anisotropically. The crystallographic data and experimental details for the structure analysis are
summarized in Table 1, and the selected bond lengths and angles are listed in Table 2.
Preparation of H2L
Hot methanol solutions of 5-fluorosalicylaldehyde and N,N’-diaminoethane (2:1, v/v) were
stirred under reflux for 1 h, and cooled to room temperature. The yellow precipitate was
collected by filtration and dried in vacuo. Yield: 91%. Anal. calcd. for C16H14F2N2O2 (%): C,
63.2; H, 4.6; N, 12.5. Found (%): C, 63.0; H, 4.7; N, 12.3.
Preparation of the manganese complex
To a stirred solution of H2L (0.30 g, 1 mmol) and sodium azide (0.065 g, 1 mmol) in 20 mL
methanol was added Mn(ClO4)2·6H2O (0.36 g, 1 mmol). The resulting mixture was refluxed for
1 h. The deep brown reaction solution was filtered and the solvent removed under reduced
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pressure, yielding brown solid of the complex. Yield: 55%. Well-shaped single crystals suitable
for X-ray diffraction were obtained by recrystallization of the solid from methanol. Anal. calcd.
for C16H12F2MnN5O2 (%): C, 48.1; H, 3.0; N, 17.5. Found (%): C, 48.2; H, 3.1; N, 17.7.
Styrene epoxidation
The epoxidation reaction was carried out at room temperature in acetonitrile under N2
atmosphere with constant stirring. The composition of the reaction mixture was 2.00 mmol of
styrene, 2.00 mmol of chlorobenzene (internal standard), 0.10 mmol of the complex (catalyst)
and 2.00 mmol iodosylbenzene or sodium hypochlorite (oxidant) in 5.00 mL freshly distilled
acetonitrile. When the oxidant was sodium hypochlorite, the solution was buffered to pH 11.2
with NaH2PO4 and NaOH. The composition of reaction medium was determined by GC with
styrene and styrene epoxide quantified by the internal standard method (chlorobenzene). All
other products detected by GC were mentioned as others. For each complex the reaction time for
maximum epoxide yield was determined by withdrawing periodically 0.1 mL aliquots from the
reaction mixture and this time was used to monitor the efficiency of the catalyst on performing at
least two independent experiments. Blank experiments with each oxidant and using the same
experimental conditions except catalyst were also performed.
RESULTS AND DISCUSSION
Synthesis
The Schiff base was readily prepared by condensation reaction of 5-fluorosalicylaldehyde with
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N,N’-diaminoethane in methanol. The stoichiometric reaction of the Schiff base and sodium
azide with manganese perchlorate in refluxing methanol yield the corresponding manganese
complex (see Scheme 2). The reaction progress is accompanied by an immediate color change of
the solution from yellow to deep brown. We have attempted to prepare and cultivate diffraction
quality crystals of the complexes from various solvents; yet, only methanol and ethanol are
suitable. The molar conductivity (ΛM = 23 Ω–1 cm2 mol–1) is consistent with the values expected
for non-electrolyte [13].
Description of the structure of the complex
Single-crystal X-ray analysis reveals that the compound is an end-to-end azido-bridged
polymeric manganese complex. The ORTEP plot of the complex is shown in Figure 1. The
manganese atom is in a distorted octahedral geometry, which is coordinated by the N2O2 donor
atoms from a Schiff base ligand in the equatorial plane and two N donor atoms from two azide
ligands in the axial positions. The distortion of the octahedral coordination of the structure can be
observed from the bond angles (Table 2) related to the Mn atoms. The cis- and trans- angles
related to the Mn atom are in the range of 86.81(9)–94.45(7)º and 173.28(7)–173.52(8)º,
respectively. The bond lengths of Mn–O and Mn–N (Table 2) are close to those in other
Mn-salen complexes [14,15]. As expected, the bond lengths in the axial positions are elongated
due to a Jahn-Teller distortion effect. The space packing of the complex along the c axis is shown
in Figure 2, in which azide ligands link the manganese complex subunits into one-dimensional
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chains.
Spectral characterization
The IR spectrum of the complex is shown in Figure 3. The absorption at 1633 cm-1 is assigned
to the azomethine group, ν(C=N) [16]. This band undergoes a negative shift of 15 cm-1 when
compared to that of the free Schiff base. The presence of a single band for ν(C=N) in the
spectrum of the complex indicates participation of both the azomethine nitrogen atoms in
coordination bond formation with the metal ion. The phenolic ν(C-O) appears at 1286 cm-1. The
typical absorption for the azide ligand is observed at 2030 cm-1.
The UV-Vis spectrum of the complex in acetonitrile is shown in Figure 4. The band at 340 nm
is attributed to the azomethine chromophore π-π* transition. The band at higher energy (280 nm)
is associated with the benzene π-π* transition [17]. The band observed at 415 nm may be due to
the d→d transitions.
Catalytic epoxidation results
The percentage of conversion of styrene, selectivity for styrene oxide, yield of styrene oxide and
reaction time to obtain maximum yield using both the oxidants are given in Table 3. The data
reveals that the complex as a catalyst converts styrene most efficiently in the presence of both
oxidants. Nevertheless, the catalyst is selective towards the formation of styrene epoxides despite
of the formation of by-products which have been identified by GC-MS as benzaldehyde,
phenylacetaldehyde, styrene epoxides derivative, alcohols etc. From the data it is also clear that
the complex exhibits excellent efficiency for styrene epoxide yield. When the reactions are
carried out with PhIO and NaOCl, styrene conversions were about 82% and 73%, respectively. It
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is evident that between PhIO and NaOCl, the former acts as a better oxidant with respect to both
styrene conversion and styrene epoxide selectivity. The epoxide yields for the complex are 75%
using PhIO as an oxidant and 60% using NaOCl as an oxidant.
CONCLUSION
An end-to-end azido-bridged polymeric manganese complex with a Schiff base ligand has been
prepared and structurally characterized in this paper. Structural analysis shows that the Mn atom
is in an octahedral coordination. The Schiff base ligand coordinates to the Mn atom through the
phenolate oxygen and imino nitrogen atoms. The complex show effective catalytic epoxidation
on styrene.
ACKNOWLEDGEMENTS
The Project was Supported by Scientific and Technological Research Program of Chongqing
Municipal Education Commission (Grant No. KJ1401025), Key Laboratory of Water
Environment Evolution and Pollution Control in Three Gorges Reservior (Chongqing Three
Georges University) (Grant No. 0969802), and the Youth Foundation of Three Georges
University (13QN13).
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Table 1 Crystallographic data for the single crystal of the complex
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C16H12F2MnN5O2
399.25
298(2)
Orthorhombic
Pca21
10.8211(5)
13.4644(6)
11.1811(5)
1629.08(13)
4
b (Å)
c (Å)
V (Å3)
Z
F(000)
808
Data/restraints/parameters 2328/1/236
Goodness-of-fit on F2
R indices [I > 2σ(I)]
R indices (all data)
1.053
R1 = 0.0255, wR2 = 0.0653
R1 = 0.0291, wR2 = 0.0681
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Table 2 Selected bond distances (Å) and bond angles (º) for the complex
(1)
Bond distance
Mn1-O1
1.8914(16)
1.986(2)
2.333(2)
Mn1-O2
Mn1-N2
Mn1-N5A
1.8809(16)
1.9913(18)
2.290(2)
Mn1-N1
Mn1-N3
Bond angle
O2-Mn1-O1
O1-Mn1-N1
O1-Mn1-N2
O2-Mn1-N5A
N1-Mn1-N5A
O2-Mn1-N3
N1-Mn1-N3
N5A-Mn1-N3
94.45(7)
91.93(8)
173.28(7)
92.04(8)
86.81(9)
91.07(9)
90.13(9)
176.86(9)
O2-Mn1-N1
O2-Mn1-N2
N1-Mn1-N2
O1-Mn1-N5A
N2-Mn1-N5A
O1-Mn1-N3
N2-Mn1-N3
173.52(8)
91.58(7)
82.11(8)
89.83(9)
92.92(8)
89.57(9)
87.35(8)
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Table 3 Catalytic epoxidation results
Time (hour)
Oxidant
Conversion
Epoxide yield Selectivity
(%)
82
(%)
75
(%)
85
2
3
PhIO
NaOCl
73
60
81
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Figure 1. ORTEP diagram of the complex (30% thermal ellipsoid).
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Figure 2. Molecular packing structure of the complex linked by azido bridges.
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1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Figure 3. IR spectrum of the complex.
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0.8
280
0.6
0.4
340
415
0.2
0.0
300
400
500
600
700
800
 (nm)
Figure 4. UV-Vis spectrum of the complex.
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N
N
F
O
NH2
+
H2N
F
HO
F
OH
OH
Scheme 1. The preparation of the Schiff base.
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NNN
N
N
N
O
N
NNN
F
Mn
O
Mn(ClO4)2
+
NaN3
+
F
F
HO
F
OH
Scheme 2. The preparation of the complex.
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