Synthesis, structure, antibacterial and spectroscopic properties of a zinc(II) complex with the ligand 4-Heptanoyl-pyrazol-5-one

Article abstract of DOI:10.1002/zaac.201200513

The long-chain ligand, 1-phenyl-3-methyl-4-heptanoyl-pyrazol-5-one (HL) and its zinc(II) complex ZnL₂ were synthesized. The structure and the properties of ZnL₂ were characterized by elemental analysis, IR spectroscopy, X-ray diffraction, and thermogravimetric analysis. The zinc ion is five-coordinated in a square-pyramidal environment by four oxygen atoms of the HL ligands in the equatorial plane and one water molecule in the axial position. The water molecule is directly bonded to Zn²⁺ and involved in intermolecular hydrogen bonding network. The complex and its corresponding ligand were screened in vitro against some strains of the human pathogenic bacteria. The metal complex exhibits higher antibacterial activity than its corresponding ligand. The complex exhibits purple effect emission as the result of fluorescence from the intraligand emission excited state. Copyright

Full text of DOI:10.1002/zaac.201200513

ARTICLE
DOI: 10.1002/zaac.201200513
Synthesis, Structure, Antibacterial and Spectroscopic Properties of a
Zinc(II) Complex with the Ligand 4-Heptanoyl-pyrazol-5-one
Li-Ying Xu,[a,b] Ning Li,*[a] Jia-Min Li,[c] and Wei Dong[b]
Keywords: Acylpyrazolonate complex; Crystal structure; Luminescence; Zinc; Antibacterial properties
Abstract. The long-chain ligand, 1-phenyl-3-methyl-4-heptanoyl-pyr-
azol-5-one (HL) and its zinc(II) complex ZnL2 were synthesized. The
structure and the properties of ZnL2 were characterized by elemental
is directly bonded to Zn2+ and involved in intermolecular hydrogen
bonding network. The complex and its corresponding ligand were scre-
ened in vitro against some strains of the human pathogenic bacteria.
analysis, IR spectroscopy, X-ray diffraction, and thermogravimetric The metal complex exhibits higher antibacterial activity than its corre-
analysis. The zinc ion is five-coordinated in a square-pyramidal envi-
ronment by four oxygen atoms of the HL ligands in the equatorial
plane and one water molecule in the axial position. The water molecule
sponding ligand. The complex exhibits purple effect emission as the
result of fluorescence from the intraligand emission excited state.
Perkin-Elmer Diamond TG-DTG thermal analyzer (6 mg sample was
placed in a platinum pan Ø 5 mm and heated from 28 to 776 °C at a
rate of 10 °C·min–1 in a nitrogen atmosphere). Fluorescence spectra
1 Introduction
Acylpyrazolones have attracted considerable interest in the
chemistry of chelate systems due to the fact that they are were obtained with a Perkin-Elmer LS55 luminescence spectropho-
tometer using a 150W xenon lamp as excitation source. UV/Vis spectra
were recorded with a HitachiU-3010 UV/Vis spectrophotometer made
in Japan.
widely used as solvent extractions of metal ions, laser materi-
als, and NMR shift-reagents.[1–3] Their metal complexes were
found to display catalytic performance, biological activity,[4,5]
and enhanced luminescence. Previously, 4-acylpyrazolone
derivatives and their transition metal complexes were re-
ported.[6–8] However, the long-chain ligand, 1-phenyl-3-
methyl-4-heptanoyl-5-pyrazolone, and its zinc(II) complex has
not been reported to date. In order to fill the gap of pyrazole-
containing 4-h-eptanoyl ligands, this paper focused on the syn-
thesis and characterization of a novel acylpyrazolone and its
zinc complexes. Furthermore, the antibacterial properties of
the complex were investigated.
2.2 Synthesis
The complex was prepared as translucent crystalline material by reac-
tion of HL (synthesized according to the method reported by March-
etti,[9] yield 71%) and Zn(NO3)2·6H2O in ethanol solution. The mix-
ture was stirred and refluxed at 78 °C for 6 h. The reaction procedure
is shown in Scheme 1. Single crystals for X-ray diffraction analysis
were obtained by evaporating slowly the solvent from a solution of
DMF at room temperature. Yield:69%. C32H42N4O5 Zn: calcd Zn 9.96;
C 58.89; H 6.44; N 8.58%; found: Zn 9.95; C 58.88; H 6.43; N 8.57%.
2. Experimental Section
2.1 Reactants and Physical Measurements
All chemical were commercially available and used without further
purification. Element analyses(C, H, and N) were performed with a
Vario ELIII elementar. Infrared spectra were recorded with a Perkin-
Elmer FTIR-1730 spectrophotometer (4000–400cm–1) with a crystal-
line sample in KBr pellets. Thermal analysis was performed with a
Scheme 1. Reaction leading to the complex.
* Prof. Dr. N. Li
Fax: +86-0451-86413721
E-Mail: lining1957@126.com
[a] School of Chemical Engineering and Technology
Harbin Institute of Technology
Harbin 150001, P. R. China
[b] School of Science
2.3 Crystallography
Diffraction data were measured with a Bruker SMART APEX CCD
diffractometer. The data were collected with graphite-monochromated
Mo-Kα radiation (λ = 0. 71073 Å) at 296 K. The structure was solved
by direct method and refined by full-matrix least-squares procedures
using the SHELXL-97 software package. Crystallographic data are
summarized in the literature.[10]
Harbin University
Harbin 150080, P. R. China
[c] School of Science
Northeast Forestry University
Harbin 150040, P. R. China
1800
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 639, (10), 1800–1803

Zinc(II) Complex with the Ligand 4-Heptanoyl-pyrazol-5-one
2.4 Biological Studies
The complex and the ligand were individually tested against patho-
genic bacteria (Gram negative and Gram positive), namely Escherichia
coli (ATCC 8739), Staphylococcus aureus (ATCC 6538), Bacillus sub-
tilis, and Staphylococcus epidermidis. The organisms were obtained
from the Department of Microbiology, Harbin University, China. Anti-
bacterial activities of the ligand and metal complex were carried out
using the disc diffusion method described by Bauer et al.[11] Bacteria
were maintained on Mueller-Hinton nutrient agar at 4 °C. Molten
Mueller Hinton agar was inoculated with a broth culture of the respec-
tive bacterial strains and poured over sterile 90 mm Petri dishes. Both
metal complex and ligand were dissolved in DMF to a final concentra-
tion of 10 mg·mL–1. Sterile Whatman No. 1 (6 mm) disc were sepa-
rately impregnated with each sample to be tested at 5.0 mg·mL–1 and
placed on the inoculated agar. The plates were incubated at 37 °C for
24 h and zones of inhibition were measured at the end of the incubation
period. The standard antibiotic reference drug used is chloramphenicol.
Figure 1. Molecular structure of the complex showing 50% prob-
ability displacement ellipsoids and hydrogen atoms as spheres of arbi-
trary radii.
Crystallographic data (excluding structure factors) for the structure 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 number CCDC-803939 (Fax: +44-1223-336-033; E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk)
3 Results and Discussion
3.1 Crystal Structure
In the zinc complex, the central atom displays square-py-
ramidal coordination, which is strongly distorted towards tri-
gonal-bipyramidal coordination (Figure 1 and Figure 2). Char-
acteristic of the distortion are the angles about the zinc atom:
104.14(9)°
(O1–Zn1–O5),
98.08(11)°
(O5–Zn1–O3),
89.97(8)° (O1–Zn1–O3), and 151.27(10)° O1–Zn1–O2. The
Zn–O bond lengths were found in the range of 1.9723(18)–
Figure 2. Crystal packing of the complex, viewed along the c axis.
2.070(2) Å. The C–O and C–C bond lengths within the chelate Hydrogen atoms were omitted for clarity.
rings are nearly equal, indicating delocalized 7r electrons. Both
chelate rings are folded about the oxygen donor atoms, by
angles of 1.276(3)° and 1.252(3)°. The planes formed by the
3.2 IR and UV/Vis Spectroscopy
chelate and pyrazolone rings are nearly parallel to each other.
There are conspicuous differences between the complex and
The double bonds N2=C9 1.316(4) Å and N4=C26 1.308(4) Å free ligand in the IR spectra.[12] The medium-intensity band at
are characteristic for the pyrazolone ring. The planes of the 3210 cm–1, assigned to ν(OH) of diketone tautomer, and one
phenyl rings are slightly twisted with respect to the pyrazolone strong band around 1631 cm–1, assigned to (C=O) of the lateral
planes (characterized by torsion angles of 11.26°). The com- chain, are absent in the complex.[13] In the complex, there are
plex shows pseudo twofold symmetry, with the Zn and O5 three new bands at 1077 cm–1, 2950 cm–1, and 432 cm–1, as-
atoms positioned on the non-crystallographic twofold axis. signed to ν(C–O–), water molecules, and νs(Zn–O), respec-
Intermolecular hydrogen bonds were found between atoms O5 tively.[14] From these observations, it is concluded that the eno-
and N2 via H2w, as well as between atoms O5 and N4 via lic proton of the ligand is replaced by zinc(II) in the complex.
H1w. Thus, each molecule is linked to two others, forming
UV/Vis absorption spectra of the title ligand and complex
infinite chains. The H1w···N4 bonds are located nearly parallel in ethanol solution were tested in the wavelength range of 190–
to the b axis, but the H2w···N2 bonds are oriented parallel to 500 nm. Three absorptions of the ligand at 205, 230, and
the ab plane. The structure is closely related to that of aqua 267 nm are attributed to π–π* transitions of the aryl ring, carb-
bis(acetylacetonato)zinc, which was also described as trigonal onyl, and n–π* transition of carbonyl, respectively.[15] Com-
bipyramidal, whereas bis(benzoylacetonato)(ethanol)zinc was pared with the free ligand, the absorption at 205 and 230 nm
observed to be more square-pyramidal. However, any similar disappeared in the complex. Moreover, the band at 267 nm of
zinc chelates including the pyrazolone moiety are not known.
HL emerged blue shifting to 263 nm in the complex. The result
Z. Anorg. Allg. Chem. 2013, 1800–1803
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.zaac.wiley-vch.de 1801

L.-Y. Xu, N. Li, J.-M. Li, W. Dong
ARTICLE
of the shifting is that the coordination reaction reduced the 3.5 Biological Properties
auxochromic effect of the ligand.
Antibacterial activity of the ligand and complex were tested
in vitro against 4 human pathogenic bacteria. The activities
were compared with that of chloramphenicol, a standard
broad-spectrum antibiotic for bacterial strains. Zones of inhibi-
tion against the growth of various microorganisms are listed in
Table 1. It was found that the inhibitory effects of the HL and
the complex differed with the species of bacteria. HL and the
complex were active against all the bacterial strains tested,
with inhibitory zones of 10–13 and 13–19 mm, respectively.
The results show that the complex exhibited higher zones on
inhibition than its corresponding ligand against all the bacterial
strains tested. ZnL2 showed the highest zone of inhibition of
19 mm against E. coli, The increased activity of the zinc com-
plex can be explained on the basis of Overtones concept[17]
and Tweedy’s chelation theory.[17,18] The complex disturbs the
respiration process of the cell and thus blocks the synthesis of
proteins, which restricts further growth of the organism.
3.3 Fluorescence Spectroscopy
The fluorescence spectra of the title complex and the ligand
in ethanol solution at room temperature were investigated. As
shown in Figure 3, the complex emits an intense purple fluo-
rescence centered at 406 nm upon an excitation wavelength
of 338 nm. The emission of the complex is assigned to the
fluorescence from the intraligand emission excited state. Com-
pared with the ligand (440 nm), the emission wavelength of
the complex is blue-shifted by 34 nm. Generally, multiple π–
π* stacking interactions and protonated ligands contribute to
blue shift of emission band and enhancement of fluorescence
efficiency.[16] The complex could be anticipated as potential
fluorescent materials in visible light.
Table 1. Antibacterial activities of the ligand HL and its zinc(II) com-
plex.
Diameter of inhibition zone of bacteria in different compounds /mm
Bacterial strains
HL
ZnL2
Chloramphenicol
B. subtilis
E. coli
S. epidermidis
S. aureus
11
10
13
10
13
19
17
18
21
20
22
20
4 Conclusions
An 4-acylpyrazolone derivative ligand and its zinc(II) com-
plex were synthesized and characterized. Experimental data
showed that the zinc ion is coordinated by four oxygen atoms
of HL ligands in the equatorial plane and one water molecule
in axial positions. The solid-state structure is stabilized by hy-
drogen bonding involving hydrogen atoms of water and nitro-
gen atoms of the pyrazole ring. A comparative study of the
HL and its complex showed that the complex exhibited higher
antibacterial activity than the ligand.
Figure 3. Excitation and emission spectra for the complex at room
temperature. Inset shows the excitation and emission spectra of the
ligand.
3.4 Thermogravimetric Analysis
Acknowledgments
The TG and DTG curves of Zn(L)2·(H2O) show the decom-
position of the complex in three regions. The first region at
176 °C with mass loss of 2.8% (weight loss of calcd. 2.76%)
Financial supports from Scientific Research Foundation of the Educa-
tion Department of Heilongjiang Province (No. 12513059), Youth
Foundation of Harbin University (No. HXKQ200610), the Fundamen-
is the evolution of one water molecule. Meanwhile, in this tal Research Funds for the Central Universities (No. DL11BB28), and
Harbin University Students Innovative Training Program are greatly
acknowledged.
temperature range, the DTA curve shows an endothermic peak.
This behavior is typical of the energy absorption and the con-
sequent energy release due to the structural rearrangement of
the complex undergoing the loss of a chain involved in the
coordination. The second decomposition at 317 °C with a mass
loss of 65.16% is close to the theoretical mass loss of the
complex stepwise decomposed to gas production. The final
products are ZnO and carbon, the experimental value
(12.53%) is in good agreement with the theoretical mass (12.
2%).
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M
=
652. 11, translucent block,
63, 4398–4446.
0.48ϫ0.48ϫ0.40 mm3, triclinic, space group P1,
a
=
¯
10.1007(10) Å, b = 12.8152(12) Å, c = 14.2981(13) Å, α =
Received: November 26, 2012
Published Online: June 4, 2013
113.9°, β = 92.3°, γ = 98.3°, V = 1663.88(3) Å3, Z = 2, D =
Z. Anorg. Allg. Chem. 2013, 1800–1803
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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