Synthesis, characterization, crystal structure, and catalytic activity of an end-on azido-bridged polymeric copper(Ii) complex derived from 4-methyl-2- [(3-methylaminopropylimino)methyl]phenol

Article abstract of DOI:10.1080/24701556.2016.1241269

An end-on azido-bridged polymeric copper(II) complex, [CuL(μ_1,1-N₃)]_n (HL D 4-methyl-2- [(3-methylaminopropylimino)methyl]phenol), was synthesized and characterized by elemental analysis, IR spectra, and single-crystal X-r

Full text of DOI:10.1080/24701556.2016.1241269

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Synthesis, Characterization, Crystal Structure and Catalytic Activity of An End-On
Azido-Bridged Polymeric Copper(II) Complex Derived From
4-Methyl-2-[(3-methylaminopropylimino)methyl]phenol
Li-Jun Liu*
School of Chemistry and Chemical Engineering, Linyi University, Linyi Shandong 276005, P. R.
China
*Corresponding author. Email: 18553903986@163.com
Abstract
An end-on azido-bridged polymeric copper(II) complex, [CuL(μ1,1-N3)]n (HL
=
4-methyl-2-[(3-methylaminopropylimino)methyl]phenol), was synthesized and characterized by
elemental analysis, IR spectra and X-ray single crystal structure determination. X-ray structural
analysis indicates that the complex crystallizes in the monoclinic with space group Cc, with a =
13.375(1) Å, b = 16.922(2) Å, c = 6.7666(6) Å, β = 117.967(2)°, V = 1352.6(2) Å3, Z = 4. The
Cu atom is in square pyramidal coordination, with the phenolate oxygen, imino nitrogen and
amino nitrogen of the Schiff base ligand and one azido nitrogen defining the basal plane, and
with another azido nitrogen occupying the apical position. The complex show effective catalytic
property on the oxidation of benzyl alcohol derivatives to carbonyl compounds by using
hydrogen peroxide as the oxidant.
Keywords
Polymeric structure, crystal structure, copper, Schiff base, catalytic property
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INTRODUCTION
Schiff bases are considered as a very important class of organic compounds which have wide
applications in coordination chemistry and biological aspects [1-3]. Mono- and multinuclear
complexes of cobalt in its varied oxidation states have become a central theme of current
research because of their potentially applications in many fields [4-6]. In the last few years,
copper complexes with multi-dentate Schiff base ligands are reported to possess interesting
catalytic properties [7–9]. Among the N, O containing chelators, the tridentate mono-condensed
Schiff base ligands have received much attention for the synthesis of mono- and polynuclear
complexes. A literature search reveals that no complexes were reported with the Schiff base
4-methyl-2-[(3-methylaminopropylimino)methyl]phenol (HL; Scheme 1). In this paper, the
author describes the synthesis and characterization of a new end-on azido-bridged polymeric
copper(II) complex with the Schiff base HL as ligand. Crystal structure of the complex has been
investigated by single crystal X-ray diffraction. The catalytic oxidation of benzyl alcohol
derivatives to corresponding carbonyl compounds have been investigated for the complex by
using hydrogen peroxide as the oxidant.
EXPERIMENTAL
Materials and methods
All chemicals and solvents used for the synthesis and catalytic experiment were of analytical
grade, and purchased from Aldrich Chemical Company. Other chemicals were obtained from
commercial sources and used without further purification. Elemental analyses were performed on
a Perkin-Elmer 240C analytical instrument. The metal analysis was carried out by titration. IR
spectra in the 4,000–400 cm-1 range were measured with a Thermo Nicolet 320 FT-IR
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spectrometer using KBr discs. H NMR spectra were recorded on a Bruker 300 MHz
spectrometer.
Synthesis of the Schiff base HL
To a vigorously stirred methanol solution (30 ml) of 5-methylsalicylaldehyde (0.136 g, 1
mmol), a methanol solution (20 ml) of N-methylpropane-1,3-diamine (0.088 g, 1 mmol) was
added. The mixture was stirred for about 30 min and then refluxed for 30 min. The resulting
solution was evaporated under vacuum to remove the solvent to give a yellow gummy product.
Anal. Calc. for C12H18N2O: C, 69.87; H, 8.80; N, 13.58%. Found: C, 69.71; H, 8.92; N, 13.67%.
Synthesis of [CuL(μ1,1-N3)]n
To a methanol solution (10 ml) of copper nitrate trihydrate (24.1 mg, 0.1 mmol), a methanol
solution (10 ml) of HL (20.6 mg, 0.1 mmol) was added dropwise and the mixture was stirred for
10 min. Then to the mixture was added a minimum volume of aqueous solution of sodium azide
(6.5 mg, 0.1 mmol) under stirring. The resulting mixture was further stirred for 1 h at ambient
temperature to give a deep blue solution. The solution, on slow evaporation, gave blue single
crystals of the complex. The crystals were filtered off and dried in air. Yield 18.7 mg (60% based
on Cu). Anal. Calc. for C12H17CuN5O: C, 46.37; H, 5.51; N, 22.53; Cu, 20.44%. Found: C, 46.15;
H, 5.70; N, 22.31; Cu, 20.72%.
X-Ray Crystallography
Diffraction intensities for the complex were collected on a Bruker SMART 1000 CCD area
diffractometer equipped with graphite monochromated Mo Kα radiation with a radiation
wavelength of 0.71073 Å using the ω-scan technique at 298(2) K. Absorption corrections were
performed using the SADABS program [10]. The structure was solved by direct method and
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refined by full-matrix least-squares against F2 using SHELXTL [11]. All non-hydrogen atoms
were refined with anisotropic displacement parameters. Hydrogen atom isotropic displacement
parameters were all treated as riding on the parent atoms (1.2 or 1.5Ueq for the carbon H atoms).
All hydrogen atoms were generated geometrically and were included in the refinement in the
riding model approximation. Selected crystallographic data are summarized in the Table 1.
Coordinate bond lengths and angles are listed in Table 2.
Catalytic Oxidation Procedure
To an acetonitrile solution (10 mL) of benzyl alcohol derivative, the copper complex as the
catalyst (0.002 mmol) and H2O2 (V:V = 50%, 5 mL) were added and the mixture was refluxed at
80 °C for 2 h. The mixture was then poured into water and the product was extracted with
dichloromethane and dried over Na2SO4. The solvent was removed and the product was purified
by column chromatography. The product was characterized by IR and 1H NMR spectra.
RESULTS AND DISCUSSION
A 1:1 molar ratio of 5-methylsalicylaldehyde and N-methylpropane-1,3-diamine in methanol
was reacted to prepare the tridentate Schiff base HL. The complex was prepared by the reaction
of 1:1:1 molar ratio of HL, copper nitrate trihydrate and sodium azide in methanol. The air-stable,
moisture-insensitive complex was obtained as deep blue crystals, which are soluble in a range of
common organic solvents such as methanol, ethanol, dichloromethane, acetonitrile,
dimethylformamide and dimethylsulphoxide, but insoluble in water. The molar conductance
value of the complex measured in methanol at the concentration of 10-3 M is 27 Ω-1 cm2 mol-1,
indicating the non-electrolytic nature [12]. Microanalytical analyses of the complex are in
agreement with the formula proposed by the single crystal X-ray determination.
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Infrared and Electronic Spectra
Comparison of the IR spectra of the complex with that of the free Schiff base gives
information regarding bonding sites of the ligand. The weak and sharp bands indicative of the
N–H vibrations of HL and the complex are located at 3217 and 3241 cm-1, respectively. The
complex displays intense peak at 1627 cm-1 due to the azomethine (C=N) stretching [13,14],
which are shifted by 13 cm-1 to lower frequency compared to the free Schiff base. The shift of
the band towards lower frequency compared to that of the free Schiff base indicates the
coordination of the imino nitrogen to the metal center. For the complex, the strong stretching
related to the azide group is observed at 2047 cm-1 [15,16]. The presence of new bands in the
spectrum of the complex in low wavenumbers can be attributed to the Cu–O and Cu–N
stretching vibrations [17].
The electronic spectrum (Figure 1) of the complex in methanol exhibits obvious bands at 273,
300, and 380 nm owing to π–π* and n–π* transitions. The complex shows relatively a weak
absorption band centered at 640 nm due to ligand field transition.
Crystal Structure Description of the Complex
The single-crystal X-ray diffraction shows that the complex is an end-on azido-bridged
polymeric copper(II) complex (Figure 2). The Cu···Cu distance is 3.614(1) Å. Each Cu is in a
square-pyramidal coordination, with the phenolate oxygen, imino nitrogen and amino nitrogen of
the Schiff base ligand, and the terminal nitrogen of a bridging azide ligand defining the basal
plane, and with another terminal azido nitrogen occupying the apical position. The deviation of
the Cu atom from the least-squares plane defined by the basal donor atoms is 0.169(2) Å. The
azide ligands coordinate to two different but symmetry-related Cu atoms through the end-on
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bridging mode. Significant distortion of the square pyramid is revealed by bond lengths and
angles between the apical and basal donor atoms. The apical Cu–N bond length is much longer
than usual; the other bond lengths are within normal ranges and comparable to the corresponding
values observed in Schiff base copper(II) complexes [18,19]. The bridging azide groups are
nearly linear and show bent coordination with Cu atoms [N3–N4–N5 = 178.6(3)°, Cu1–N3–N4 =
120.0(2)°, Cu1A–N3–N4 = 118.3(2)°]. The chelate ring formed by the atoms Cu1, N1, C8-C10,
and N2 in the complex has chair conformation. The diagonally positioned atoms, Cu1 and C9,
are shifted from the least-squares plane defined by the atoms N1, N2, C8 and C10 by 0.439(2)
and -0.712(2) Å, respectively.
In the crystal structure of the complex, the [CuL(N3)] units are linked through end-on azido
bridging groups, forming polymeric chains running along the c axis. The chains are further
linked through intermolecular C–H···N hydrogen bonds [C4–H4···N5i: C4–H4 = 0.93 Å,
H4···N5i = 2.61 Å, C4···N5i = 3.4053(3) Å, C4–H4···N5i = 144(3)º; C11–H11B···N3:
C11–H11B = 0.96 Å, H11B···N3 = 2.34 Å, C11···N3 = 2.9539(3) Å, C11–H11B···N3 = 121(3)º;
symmetry code for i: x, – y, –1/2 +z], to form a three dimensional network (Figure 3). In addition,
there are π∙∙∙π stacking interactions among the adjacent benzene rings (Cg1···Cg5ii = 4.4865(4)
Å, Cg2···Cg5 = 4.4865(4) Å, Cg5···Cg5iii = 4.1074(4) Å, Cg5···Cg5ii = 4.10745(4) Å;
symmetry codes: ii: x, – y, 1/2 + z; ii: x, – y, –1/2 + z).
Oxidation of Benzyl Alcohols
The results of the oxidation reactions are listed in Table 3. The results are in consistent with
the literature that Schiff base copper complexes can oxidize benzyl alcohol and its derivatives
into corresponding aldehydes [9,20,21]. The oxidation reactions were carried out at normal
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pressure with 50% H2O2 and copper complex with catalytic amount in acetonitrile. Benzyl
alcohol oxidation was studied in different reactions to find out the optimized condition for the
oxidation process. In the presence of 4 mol% catalyst, H2O2 as oxidant and 10 mL acetonitrile at
80 °C for 2 h the best yield of benzaldehyde (90%) was obtained after the oxidation of
benzylalcohol. As comparison, the yield of the reaction was very low (< 10%) at room
temperature. A blank reaction was also performed in absence of catalyst which gave very low
yield of oxidation product. From Table 3, it can be seen that the presence of para-substituent
groups can decrease the yield of the oxidation reactions.
CONCLUSION
A
new
end-on
azido
bridged
polymeric
copper(II)
complex
with
4-methyl-2-[(3-methylaminopropylimino)methyl]phenol as ligand has been synthesized and
characterized by various physicochemical studies. Crystallographic study shows that the Cu
coordination sphere is square pyramidal. The complex can catalyze the oxidation of benzyl
alcohol derivatives to corresponding benzaldehydes in presence of H2O2 as oxidant under
atmospheric condition with high yields.
ACKNOWLEDGEMENTS
The author acknowledges the Experimental Center of Linyi Normal University for supporting
this work.
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Table 1. Crystallographic parameters of the complex
Parameters
Values
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C12H17CuN5O
310.85
298(2)
0.71073
monoclinic
Cc
Unit cell dimensions
a (Å)
13.375(1)
b (Å)
16.922(2)
c (Å)
6.7666(6)
β (°)
117.967(2)
1352.6(2)
V (Å3)
Z
4
ρcalc (g cm-3)
1.526
1.614
µ (mm-1)
F(000)
644
Crystal size (mm)
θ Range for data collection (º)
Max. and min. transmission
Reflections collected
Unique (Rint)
0.29 × 0.27 × 0.27
3.24-25.49
0.6518, 0.6697
6017
2271
Observed reflections [I > 2σ(I)]
Restraints
2209
3
Parameters
175
1.064
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0201, wR2 = 0.0515
R1 = 0.0210, wR2 = 0.0519
Largest diffraction peak and hole 0.275 and -0.184
(eÅ-3)
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Table 2. Selected bond distances (Å) and bond angles (°) for the complex
Bond lengths
Cu1-O1
Cu1-N2
Cu1-N3A
1.923(2)
2.057(2)
2.464(3)
Cu1-N1
Cu1-N3
1.987(2)
2.034(2)
Bond angles
O1-Cu1-N1
N1-Cu1-N3
N1-Cu1-N2
O1-Cu1-N3A
N3-Cu1-N3A
91.20(9)
158.08(9)
94.98(8)
85.69(8)
106.32(10)
O1-Cu1-N3
O1-Cu1-N2
N3-Cu1-N2
N1-Cu1-N3A
N2-Cu1-N3A
88.51(10)
173.66(10)
86.27(11)
95.50(9)
92.33(9)
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Table 3. Details of the catalytic oxidation reactions catalyzed by the complex
Substrate
Product
Yield (%)
93
CHO
CH2OH
89
88
85
90
81
F
CH2OH
F
CHO
Cl
CH2OH
CH2OH
Cl
Br
CHO
CHO
Br
O2N
CH2OH O2N
CHO
CH2OH
CHO
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1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
 (nm)
600
700
Figure 1. The electronic spectrum of the complex.
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Figure 2. Molecular structure of the complex drawn with 30% probability ellipsoids for
non-hydrogen atoms. Atoms with suffix A are related to the symmetry operation x, – y, 1/2 + z.
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Figure 3. The end-on azido bridged polymeric structure of the complex, viewed down the c axis.
Hydrogen bonds are shown as dashed lines.
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N
H
N
OH
Scheme 1. HL
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