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Accepted Manuscript
Synthesis, structural study, magnetic susceptibility and antimicrobial activity of the
first (μ-oxo)-bis(oxalato)-vanadium(IV) 1D coordination polymer
Hiba Sehimi, Badiaa Essghaier, Elisa Barea, Najla Sadfi-Zouaoui, Mohamed Faouzi
Zid
PII:
S0022-2860(18)31002-0
DOI:
10.1016/j.molstruc.2018.08.053
Reference:
MOLSTR 25574
To appear in:
Journal of Molecular Structure
Received Date: 21 June 2018
Revised Date:
12 August 2018
Accepted Date: 16 August 2018
Please cite this article as: H. Sehimi, B. Essghaier, E. Barea, N. Sadfi-Zouaoui, M.F. Zid, Synthesis,
structural study, magnetic susceptibility and antimicrobial activity of the first (μ-oxo)-bis(oxalato)vanadium(IV) 1D coordination polymer, Journal of Molecular Structure (2018), doi: 10.1016/
j.molstruc.2018.08.053.
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Synthesis, structural study, magnetic susceptibility and antimicrobial
activity of the first (µ-oxo)-bis(oxalato)-vanadium(IV) 1D coordination
polymer.
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Hiba Sehimia,b*, Badiaa Essghaierc, Elisa Baread, Najla Sadfi-Zouaouic, Mohamed
Faouzi Zida
a
Université de Tunis El Manar, Faculté des Sciences de Tunis, Laboratoire de Matériaux,
Cristallochimie et Thermodynamique appliquée, LR15ES01, El Manar II, 2092, Tunis,
Université de Gabès, Faculté des Sciences de Gabès, Campus Universitaire, Cité Erriadh
Zrig, 6072, Gabès, Tunisie
c
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b
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Tunisie
Université de Tunis El Manar, Faculté des Sciences de Tunis, Laboratoire Mycologie,
Pathologie et Biomarqueurs, El Manar II, 2092, Tunis, Tunisie
d
Departamento de Química Inorgánica, Universidad de Granada, Av. Fuentenueva, S/N,
18071, Granada, Spain
*
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Corresponding author:
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E-mail adress: hiba.shimi@fst.utm.tn; Tel: (00216)97241032
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Abstract:
Catena-poly[bis(guanidinium)(µ-oxo)-bis(oxalato)-vanadium(IV)], 1, a new 1D polymeric
V(IV) salt has been synthesized by slow evaporation at room temperature and its structure has
been elucidated by single-crystal X-ray diffraction, while bis[4-(dimethylamino)-pyridinium]
aquabis(oxalato)oxidovanadate(IV) dihydrate’s structure, 2, has been previously reported.
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In addition to elemental analysis, single crystal X-ray diffraction and structural data report of
1, both compounds 1 and 2 have been characterized by, FT-IR and UV-Vis spectroscopy,
Rietveld refinement, TG–DSC analysis. Magnetic susceptibility of the compounds had been
investigated and discussed in the context of their structures.
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The stability, availability and ease of synthesis of those compounds are an advantage for their
antimicrobial activity investigation against different Gram-positive and Gram-negative
bacteria, yeast and fungi microorganisms. Further investigations on the compounds’ lysozyme
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activity and spore germination inhibition aim to go deeper in their mechanism of actions.
Key words: Antimicrobial activity; Lysozyme activity; Fungal spore germination;
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Vanadium(IV) oxalate; Magnetic susceptibility.
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1. Introduction
Our world is facing numerous challenges related to health and the necessity of designing new
functional materials for therapeutic applications is more and more urgent. Since the success of
cis-diamminedichloroplatinum(II) in clinical use for human malignancies [1], organicinorganic hybrid compounds have become a major area of research and technological
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development owing to their biological applications as therapeutic relevant and worth studying
agents.
The ever-increasing interest in organic-inorganic coordination chemistry is also attributed to
the combination of dissimilar properties of organic and inorganic moieties when coordinated
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together on the molecular scale. Control over structures/properties are based on the nature of
ligand, counter-cation and metal center. Beginning with the choice of ligand that is extremely
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important when preparing metal hybrid salts, oxalate dianion is one of the ligands that easily
chelate the metal centers and create organic–inorganic hybrid materials, due to its good donor
ability resulting from the presence of four oxygen donors. Moreover, previously reported
studies on the oxalate dianion highlighted its impact in biological systems [2–4]. Literature
survey reveals that many metal ions, including the anti-arthritic gold, antibacterial silver,
antidiabetic vanadium and antiulcer bismuth [5], contribute to the creation of biochemically
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active compounds. In our work, we are particularly interested in vanadium, a known metal of
high physiological importance, based on the reports on its biological properties in treatment of
diabetes and cancer [6–9].
Although vanadium complexes with oxalate ligand have been intensively studied and reported
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[10–14], the majority of them are about synthesis and structures. Few recent works are about
magnetic properties [15–17] and catalytic activities [18,19], while the study of the
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antimicrobial activity is relatively rare. Among those complexes, the oxovanadium core has
received considerable attention in biology compared to vanadium various coordination
environments [20–23] while the (µ-oxo)-vanadium core in vanadium-oxalato system was not
reported before according to our database survey. Whence the idea of having advantage of the
pharmacological properties of the (oxalato)oxovanadate moiety [24,25] resulting from direct
coordination of the oxalate dianion and the vanadyl ion and of comparing it to the unusual (µoxo)-bis(oxalato) vanadium moiety.
In view of these findings and in order to provide valuable insights into the impact of the
structure on the antimicrobial activity of transition metals oxalate structures tested as potential
biological agents, we are reporting herewith synthesis and description of the crystal structure
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of {(CH6N3)2[VO(C2O4)2]}n, 1, accompanied with spectroscopic, thermal and magnetic
characterization with antimicrobial activity of 1 and (C7H11N2)2[VO(C2O4)2(H2O)].2H2O, 2,
[26].
2. Materials and methods
2.1. Physical measurements
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All reagents and solvents were commercially available and used without further purification.
Elemental analyses for carbon, nitrogen and hydrogen were performed on a Flash 2000
Organic Elemental Analyser, CHNS-O analyser by Thermo Scientific (Centre of Scientific
Instrumentation of the University of Granada). An ICP-OES Perkin-Elmer Optima 8300
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Spectrometer (Centre of Scientific Instrumentation of the University of Granada) was used to
determine the metal content in the complex where the measurement was performed on a 1000
times diluted solution prepared from the solid sample, then the ppm result was converted to a
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percentage. Infrared spectra were recorded in the range 4000–400 cm-1 region on a Bruker
Tensor II FT-IR spectrometer (Department of Inorganic Chemistry of the University of
Granada) and while the UV–Vis spectra were recorded in the range 200–800 nm using UV1800-Shimadzu Scientific Instruments (Department of Inorganic Chemistry of the University
of Granada). Powder X-ray diffraction performed on a Bruker D2 Phaser X-ray diffractometer
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(Cu Kα) were used to verify the phase purity of the collected powder sample (Department of
Inorganic Chemistry of the University of Granada). Thermogravimetric analysis was
investigated using a Mettler-Toledo TGA/DSC1 Thermogravimetric Analyser (Centre of
Scientific Instrumentation of the University of Granada). The room temperature magnetic
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susceptibility of the of polycrystalline samples were measured using a Quantum Design
MPMS 5XL SQUID Magnetometer operating in the range of 2-300 K (Centre of Scientific
Instrumentation of the University of Granada). Empiric correction of -120.74×10-6 emu.mol-1
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and -317.32×10-6 emu.mol-1, estimated by Pascal’s constants for diamagnetic corrections [27],
were made for all the constituent atoms respectively for compounds 1 and 2.
2.2. Synthesis of {(CH6N3)2[VO(C2O4)2]}n (1)
Oxalic acid dihydrate (C2H2O4.2H2O / Prolabo, 99,5 %), vanadium pentoxide (V2O5 / Merck,
99 %) and guanidine chloride (CH5N3.HCl / Sigma Alrdich, ≥ 99 %) were used as starting
materials without any further purification.
Oxalic acid dihydrate (0.125 g, 1 mmol) dissolved in 10 cm3 of distilled water was added
dropwise to a stirring solution of vanadium pentoxide (0.090 g, 0.5 mmol) dissolved in 10
cm3 of distilled water under low temperature (70-80 °C). After stirring the mixture for almost
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10 min and a visual confirmation of the reduction of V5+ with the oxalic acid by a clear color
variation from strong yellow to pale green then blue, guanidine chloride (0.095 g, 1 mmol)
dissolved in 10 cm3 of distilled water was then added dropwise under continuous stirring and
heat for a further hour.
Purple thin needle crystals of 1, suitable for X-ray analysis were obtained from the solution
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after almost two weeks of slow evaporation. The elemental analytical results for carbon,
hydrogen, nitrogen and vanadium are close to the calculated values. Calculated: C: 19.82, H:
3.30, N: 23.13, V: 14.04; Found C: 19.49, H: 4.21, N: 23.52, V: 12.93.
2.3. X-Ray crystallography
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Suitable single crystal of compound 1 was carefully selected under a polarizing microscope
and glued to the end of glass fiber then mounted to the goniometer of an Enraf-Nonius CAD-4
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diffractometer [28]. Data were collected at room temperature. The reflection intensities were
collected in the monoclinic system with Cc space group up to θ = 27° with two standard
reflections for intensities and orientation control. The crystal structure was solved in the same
space group using direct methods using the SHELX-97 [29] computer program included in the
WinGX software package [30] and refined by full matrices least squares against F2 using
SHELXL-2014 [31].
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During the structure resolution, a type-B alert check, detecting new symmetry elements was
noticed; the space group transformation to the Fdd2 in the orthorhombic system was decided.
Before the transformation, empirical absorption corrections based on a ψ-scan have been
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made. After the transformation, the resolution of the structure has been finalized in the new
Fdd2 space group. Amine hydrogen atoms of the guanidinium cation were positioned
geometrically as riding atoms (N–H = 0.86 Å) using adequate HFIX instructions and refined
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with AFIX instructions. The structural graphics were created with the DIAMOND program
[32] and Mercury [33].
2.4. In vitro antimicrobial activity
2.4.1. Test microorganisms
The following list of microorganisms was used in the study, including Gram-positive and
Gram-negative bacteria (Escherchia coli, Staphylococcus aureus, Streptococcus agalactiae
and Enterobacter cloacae), three yeast species (Candida albicans, Candida parapsilosis and
Candida sake) and two fungi (Aspergillus niger and Penicillium spp).
2.4.2. Antimicrobial activity
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Before their use, each compound 1 and 2 was diluted in distilled water and sterilized by
filtration through a 0.22 µM pore size filter. Antibacterial and antifungal tests were performed
by agar well diffusion method [34]. A freshly cell suspension (0.1 ml) adjusted to 107 CFU/ml
for bacteria or yeast and 105 spores/ml for fungus were inoculated onto the surface of agar
plates. Thereafter, wells with 6 mm in diameter were punched in the inoculated agar medium
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and 50 µl of the compounds 1 and 2 were separately added to each well. Negative controls
consisted of 50 µl sterile water, used to dissolve the compound. The plate was allowed to
stand for 2 h to permit the compound diffusion followed by incubation at 37 °C for 24 h for
bacterial strains, 48 h for yeast and 4 days for fungi at 28 °C. The antimicrobial activity was
micro-organisms. All tests were repeated three times.
2.4.3. Lysozyme activity
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evaluated by measuring the zones of inhibition (clear zone around the well) against the test
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The Lysozyme activity of the compounds 1 and 2 were assayed turbidimetrically by
measuring the decrease in absorbance at 660 nm of a suspension of Staphylococcus aureus
and Streptococcus agalactiae [35].
2.4.4. Determination of Minimum Inhibitory Concentration (MIC)
The minimum inhibitory concentration (MIC) of the each compound was determined using
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the microdilution broth method [36]. MIC was estimated visually (absence of turbidity) and
were determined with three independent measurements.
2.4.5. Compounds effect’s on fungal spore germination
The surface of 10 days fungi culture was scraped gently with a sterile loop to release the
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spores. The resulting spore suspension was filtered through a sterile 30 µm filter to remove
any mycelia fragments. Conidial suspension of each fungus was adjusted to105 spores/ml by
using a hemocytometer. To examine the effects of the compounds 1 and 2 on spore
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germination, 50 µl of conidial suspensions (105 spores/ml) and 50 µl of the each compound at
4000 µg/mL, were pipetted into an Eppendorf tube containing 1 ml of 5 % glucose, then
incubated at 21 °C for 24 h. Control tubes were inoculated only with fungal spores of each
tested fungus. The percentage of spores germination inhibition (I %) was determined by
microscopic examination of spores in the presence of the compounds 1 and 2 (E), compared
to control tube containing only the spore suspensions as a formula: I % =
replicates were used for each treatment [37].
3. Results and discussion
3.1. Description of the structure of the compound 1
C-E
C
×100. Three
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The asymmetric unit of 1 contains one anionic complex [V1/2(C2O4)O1/2]-, considering that the
vanadium atom V1 and the oxygen atom O5 lie on a twofold rotation axis, and one
guanidinium cation (CH6N3)+ (Fig. 1). The -1 charge of the anionic moiety is requiring a
V(IV) ion, this oxidation state value is verified with the bond-valence sum calculation [38]
with a value of 4.21(2) valence units. Relevant crystallographic data and data collection
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parameters of the complex {(CH6N3)2[V(C2O4)2O]}n, (1), are summarized in Table 1.
Fig. 1. The asymmetric unit of 1, showing the atom-numbering scheme. Displacement
ellipsoids are drawn at the 50% probability level for non-H atoms.
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Table 1. Crystal data refinement results of 1.
Crystal data
Empirical formula
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Crystal system; Space group
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Unit cell dimensions
{(CH6N3)2[VO(C2O4)2]}n
Orthorhombic; Fdd2
a = 30.539(6) Å;
b = 24.040(4) Å;
c = 3.8324(8) Å
Volume; Z
2813.6(9) Å3; 8
Formula weight
363.16 g.mol-1
Absorption coefficient (µ)
0.76 mm-1
Crystal sharp; Color
Needle; Purple
Crystal size
0.42 × 0.24 × 0.18 mm
Data collection
Diffractometer
Enraf-Nonius CAD-4
Wavelength; Temperature
λMo Kα = 0.71073 Å; 298(2) K
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2.16°≤θ≤26.97°
Limiting indices
-38≤h≤10; -30≤k≤1; -4≤l≤4
Scan mode
ω/2θ
Reflections collected
1975
Independent reflections
1527 [Rint = 0.015]
Observed reflections [I > 2σ(I)]
1393
Refinement
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Theta range for data collection
Full-matrix least-squares on F2
Final R indices [I > 2σ(I)]
R(F) = 0.025; wR(F2) = 0.068
Reflections; Parameters
1527; 101
−3
0.26; -0.15
Goodness of fit (S)
1.08
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∆ρmax ; ∆ρmin (eÅ )
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Refinement method
In the structural unit of the compound 1, the vanadium center is coordinated by four O-atoms
(O1, O2, O1i, O2i) [Symmetry code: (i) −x+1, −y, z] corresponding to two chelating and
crystallographically dependent oxalate dianions in a trans geometry and one axial O-atom
(O5). The remaining coordination site among the hexa-coordinated vanadium is filled by the
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O5 symmetrically generated equivalent (O5ii [Symmetry code: (ii) x, y, z+1]), thus, a VO6
core with a distorted octahedral geometry is formed. The small bite V1—O5 bond [1.604(2)
Å (Table S1)] and the extended V1— O5ii bond [2.228(2) Å (Table S1)] account for this
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distortion.
Due to the oxygen bridge formed between the vanadium ions, the anionic structure may be
described as 1D parallel chains, formed through VO6 octahedra connected via corner sharing,
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running along the axis (Fig. 2). Hence, the resulting V—V distance measures 3.832(4) Å,
which may anticipate a probable spin coupling between the paramagnetic VIV centers,
mediated by the oxygen atom. A database survey revealed that this characteristic feature of
the 1D anionic {[V(C2O4)2O]}n chains was not reported before in any vanadium oxalate
complex, which makes the compound 1 the first (µ-oxo)-bis(oxalato)-vanadium(IV)
coordination polymer.
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Fig. 2. View of one anionic {[V(C2O4)2O]}n chain running along the axis.
The supramolecular structure of 1 is described as three different {[V(C2O4)2O]}n chains
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connected to each other’s via N—H···O hydrogen bonds (Table S2) engaging the three
nitrogen atoms of one guanidinium cation. Those lasts act as hydrogen-bond donor and
establish strong hydrogen bonds towards both coordination and terminal oxygen atoms with
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the chains, as illustrated in Fig. 3 with orange dashed lines.
Fig. 3. View of N—H···O hydrogen bonds (orange dashed line) developed by the guanidinium
cation and connecting three different anionic chains.
Each guanidinium cation is separated from its symmetric by the distance of 3.832(1) Å, and
then it is engaged with it in a π- π stacking (Fig. 4). The resulting packing is a perfect
organized 3D network formed by alternation of corner-sharing VO6 chains linked by
guanidinium cations, which themselves are further linked with π- π interactions.
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in the structure of 1.
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3.2. Spectroscopic study
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Fig. 4. π– π stacking interactions (orange dashed lines) between adjacent guanidinium cations
Table 2. Propositions of attribution of the absorption bands of the two compounds 1 and 2.
Wavenumber (cm-1)
Attribution
Compound 2
3426, 3351, 3196
3482, 3373, 2935
ν(C-H); ν(N-H) [39]
-
3229, 3082
νs(O-H); νas(O-H) [17]
1639
νas(COO) [40]
1559
ν(C=C)
1397
νs(COO) [39]
1236
1236, 1212
ν(C-N) [17]
-
1061
δ(O-H) [41]
-
951
ν(V=O) [42,43]
909
891
δ(N-H)
808
802
γ(C-N); γ(C-C) [39]
536, 469, 424
530, 478
ν(V-O) [19]
1708, 1646
1560
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Compound 1
FT-IR spectroscopy was used to verify the functional groups present in both crystals. Both
FT-IR spectra of 1, in Fig. S1(Top) and of 2, in Fig. S1(Bottom) exhibit the same
characteristic features of the oxalate anion for νas(COO) [40], ν(C=C) and νs (COO) [39] as
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detailed in Table 2. Stretching vibrations of the N-H and C-H [39] bonds are also present in
both spectra witnessing the presence of organic cations.
The fact that compound 1 is anhydrate is confirmed by the absence of νs(O-H), νas(O-H) [17]
and δ(O-H) [41] in its FT-IR spectrum. Another major difference between the two compounds
is the vanadyl stretching mode ν(V=O) [42,43] can be observed at around 951 cm-1 which is
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in accordance with many previously reported results mentioning that the stretching
frequencies of metal-oxygen double bonds may generally be found between 900 and 1100 cm1
.
In the far IR, the absorption bands at around 530 and 536 cm-1 are attributable to of V-
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O(oxalate) stretching vibration [19] confirming the coordination of the metal center.
Electronic spectra were obtained from aqueous solutions of the compounds and are depicted
in Fig. S2. Both spectra show very intense bands in the UV region and are dominated by a
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strong absorption band centered at 210 nm can be assigned to 4-(dimethylamino)-pyridinium
and guanidinium π–π* transitions. The higher energy band at around 256 and 280 nm for 1
and 2 respectively is related to a charge transfer from the oxalate to central VIV.
3.3. Powder X-ray diffraction analysis
The powder diffraction patterns illustrated in Fig. 5(a) and Fig. 5(b) were recorded in ambient
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atmosphere under Bragg–Brentano geometry over the angular range 5-50 ° with step length of
0.02 °. Rietveld refinement was performed using the FullProf Suite program [44]. Pattern
indexing was carried out according to the single crystal structure data at 298 K for the
compound 1 and by means of the program TREOR [45] for the compound 2. Its purpose is to
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test the purity of the powder and to show that in the case of polymorphism, the powder
material corresponds to the same polymorph as the single crystals.
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The diffraction peaks on both calculated and observed patterns correspond well in position,
confirming the hypothesis and showing that the compounds are pure with final agreement
factors, Rp = 0.286, Rwp = 0.307, R(F2) = 0.065 and χ2 = 11.1 for 1 and Rp = 0.156, Rwp =
0.167, R(F2) = 0.093 and χ2 = 3.17 for 2, in the acceptable range from the refinement process.
The difference in reflection intensity between the patterns is due to a certain degree of
preferred orientation of the powder sample during data collection. In addition, the background
is linear, which is a qualitative indicator of the good crystallinity of the powder.
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(b)
(a)
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Fig. 5. Rietveld refinement plots for compounds 1 (a) and 2 (b) showing the observed,
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calculated and difference pattern. The Bragg reflections are indicated by vertical bars.
3.4. Thermal analysis
The thermal decomposition of both compounds 1 and 2 occurs in air under a heating rate of
10 °C.min−1 in a range between 36 °C and 950 °C, as depicted in Fig. 6(a) and Fig. 6(b)
respectively. This study was carried out in order to obtain preliminary information about their
thermal behavior. It is clear from the thermogravimetric profile that the dihydrate complex 2
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is stable up to 100 °C where the first mass loss corresponding to three water molecules
departure occurs, then undergoes decomposition to form a mixture of products. In fact, the
dehydration of the compound overlaps the loss processes of interstitial and coordinated water
molecules with a global weight loss of almost 10.4 %, which is in agreement with the
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theoretical loss of three water molecules, 9.94 % with a deviation being within the limits of
the method accuracy. Three exothermic peaks at 277.8 °C, 308.9 °C and 378.7 °C accompany
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the compound’s gradual decomposition between 250 and 400 °C.
Contradictorily, the anhydrate complex 1 is stable up to 300 °C where it starts decomposing
between 400 and 500 °C with two major thermogravimetric losses suggesting the
decomposition of the organic part. This result is in agreement with the structure
characterization, which reveals that the synthesized compound does not contain any water
molecule.
The remaining weights of respective theoretical percentages around 22.6 % and 15.25 % for 1
and 2 are in agreement with the experimental ones with percentages of 23.5 % and 15.3 %,
corresponding to the formation of the vanadium(IV) dioxide VO2.
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(a)
(b)
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3.5. Magnetic susceptibility measurement
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Fig. 6. TG-DSC thermogram of 1 (a) and 2 (b) from room temperature to 950 °C.
The importance of vanadium in emerging areas of molecular magnetism is increasing. Thus,
magnetic susceptibility per mol of vanadium(IV) for both compounds 1 and 2 was measured
in field of 5 kOe and in the temperature range 2–300 K. The thermal variations of and
are simultaneously plot in Fig. 7(a) and Fig. 7(b)Top respectively while is plot on
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Fig. 7(b)Bottom.
For the complex catena-poly[bis(guanidinium)(µ-oxo)-bis(oxalato)-vanadium(IV)], 1, the
magnetic moment (µeff) per formula at 300 K, determined from the equation
1
µeff = 2.282×(
) 2 , reaches a value of 1.91 µB, which is not far from the expected value of
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1.73 µB. The plot of the inverse of molar magnetic susceptibility as a function of temperature
(Fig. 7(a)Bottom) has an affine relation. It can be modeled by the Curie-Weiss equation
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= , where C and θ are the Curie–Weiss constant and magnetic coupling parameter,
respectively, with values of C = 0.49 emu mol−1 K and θ = -5.27 K.
Being a one-dimensional coordination polymer, the existence of an oxygen bridge between
the metal centers has influenced the magnetic character of the compound. First, the negative
value of θ indicates the presence of antiferromagnetic interactions between the V4+ ions
separated by approximately 3.832 Å. Secondly, the modeling of the experimental data by the
model of Bonner and Fisher (s = 1/2) (1) of the linear chains gave values of g = 2.24 and J = 0.25 cm-1 .
χ=
Nµ2
0.25+0.14995x+0.30094x 2
×
(1)
kBT 1+1.9862x+0.68854x 2 +6.0626x3
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Where x = |J|/kBT , kB is the Boltzman constant and N is the total number of spins. Although
the constant J is low in absolute value, but it confirms the existence of a magnetic coupling
between the metal centers. The curved branch of the plot = below 25 K (Fig.
7(a)Top) confirms this fact. Thus, the compound 1 may be classed among oxo-vanadium(IV)
compounds containing weakly coupled vanadium centers [46].
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For the complex bis[4-(dimethylamino)-pyridinium] aquabis(oxalato)oxidovanadate(IV)
dihydrate, 2, the magnetic moment (µeff) per formula at 300 K is 1.51 µB, which is in good
agreement with the spin-only expected value of 1.73 µB. This is consistent with the oxidation
state of the metal centers (VIV).
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As the temperature is reduced from 300 K, the magnetic susceptibility increases slowly with
decreasing temperature, and increases quickly below 50 K.
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The plot of inverse magnetic susceptibility versus temperature shows a linear relation at low
temperatures with a little dispersion above 200 K. It is fitted by the Curie–Weiss equation to
yield values for C and θ as 0.29 emu.mol−1.K and 5.31 K respectively. This positive θ value
indicates probable ferromagnetic interactions between V4+ ions. Thus, we tried to confirm the
result reproducing the measured magnetic susceptibility using the Bonner and Fisher s=1/2
linear chain model (1). The solid curve is the best fit with g = 1.94 and J = -0.007 cm-1. The
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non-coupling between adjacent vanadium centers V4+ ions separated by relatively long distances
around 7.689 Å and 8.287 Å (Fig. 8) was confirmed by the almost zero value of the coupling
constant and the data do not reveal any long-range magnetic order, which may be due to the
absence of possible pathways mediating the magnetic coupling. Hence, the compound exhibits
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simple paramagnetic behavior. The fact that the paramagnetic centers remain non-coupled is
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also confirmed by the plot of = versus T showing values almost constant [47].
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(b)
Fig. 7. Thermal dependence of and (Top) and (Bottom) for compounds 1 (a)
and 2 (b) and their corresponding theoretical curves.
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Fig. 8. Arrangement of VIVO6 polyhedra , in the structure of 2, in linear chains contained in
parallel layers through the connection of O-H…O hydrogen bonds (orange dashed lines) with
3.6. Antimicrobial activities
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different distances (purple and blue dashed lines).
Table 3. Antimicrobial activities detection and MIC values (µg/ml).
Compound 1
Microorganisms
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Presence of
activity
Bacteria
Escherichia coli
+
Streptococcus agalactiae
MIC
2000
Compound 2
Presence of
activity
+
MIC
31
-
Staphylococcus aureus
-
-
Enterobacter cloacae
-
+
500
Candida albicans
-
+
1000
Candida parapsilosis
+
Candida sake
-
+
Aspergillus niger
-
-
Penicillium spp
-
-
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-
Yeasts
250
-
Fungi
1000
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As the balance between the therapeutic potential and toxic side effects of a compound is very
important when evaluating its usefulness as a pharmacological drug, in vitro cytotoxicity of
the compounds were conducted using colorimetric microculture MTT assay against A2780
and A278cis human ovarian carcinoma and found to be non-significant.
The in vitro antibacterial evaluation results reveal that the compound 1 exhibited antibacterial
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activities against E.coli with MIC values of 2000 µg/mL while the compound 2 exhibited
higher antibacterial activities especially against E.coli and Enterobacter cloacae bacteria with
MIC values of 31 and 500 µg/mL respectively (Table 3). Both compounds have antimicrobial
activities against the E.coli bacteria with a higher effectiveness for bis[4-(dimethylamino)-
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pyridinium] aquabis(oxalato)oxidovanadate(IV) dihydrate complex, 2. Similar activity was
noticed in many other metal oxalate compounds such as potassium diaquabis(oxalato)
cobaltate(II) [48], potassium tris(oxalato) ferrate (III) trihydrate and sodium bis(oxalato)
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cuprate (II) dehydrate [49] and 1,5-naphthyridine trans-diaquadioxalatochromate(III)
dehydrate [50], although the activity was not quantified by the MIC value to be considered as
a comparison reference.
Notably, Fig. 9 shows high lysozyme activity (against Staphylococcus aureus) expressed by
both compounds of about 4.9 and 3.8 UA/mL for 1 and 2 respectively. This activity remains
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higher than the one exhibited by another synthetic compound, tetraaqua bismaleato iron(II)
complex, previously reported by our laboratory team [51]. The compound 1 shows
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9
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Lysozyme activity (UA mL-1)
great lysozyme activity against Streptococcus agalactiae of about 8.2 UA/mL.
8.2
7
6
4.9
5
3.8
4
3
2
0
1
0
Compound (1)
Staphylococcus aureus
Compound (2)
Streptococcus agalactiae
Fig. 9. Lysozyme activities (UA mL-1) of 1 and 2 against Staphylococcus aureus and
Streptococcus agalactiae.
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On surface agar plates, the compounds reveal contradictory Candida growth inhibition and
similar behavior as failed to inhibit fungi growth. While the compound 1 shows Candida
parapsilosis growth inhibition with MIC value of about 250 µg/ml, the compound 2 shows
Candida albicans and Candida sake growth inhibition with MIC values of about 1000 µg/ml.
MIC against Candida albicans is four time the value reported by similar amino-
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chloropyridinium diaqua dioxalato iron (III) compound [52]. Also, previous reports have
demonstrated similar simultaneous antibacterial and anti-candida activities, as example, the
tris(ethylenediamine)cobalt(III)chloride oxalate trihydrate compound [53].
To better understand their mode of action, we have evaluated the compounds’ effect on spore
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germination and results were given in Fig. 10. Markedly, high percentage of spore
germination inhibition was observed in the presence of the compound 2 with more than
91.75% against two tested fungi species compared to the control conditions (spores without
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the compound) and even compared the compound 1 where its spore germination inhibition do
not exceed 56.67%.. Thus, the compounds proved specify action on microorganisms due to
their structural difference, notable antibacterial, anti-candida and anti-fungal activities
especially on fungal spore germination inhibition were given by each compound.
Various methods have been used to measure the sensitivity of filamentous fungi to antifungal
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compounds. Germination assays are also very useful for evaluating the mechanism of action
of the antifungal compounds. In this work, both compounds fail to attack mycelial growth but
the germination assays proved their high fungitoxic role. This behavior is similar to
previously reported results exhibited by the complex di(4-sulfamoyl-phenyl-ammonium)
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sulphate [54], which is extremely efficient to inhibit spore germination but it fail to affect
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mycelial growth of some fungal species.
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93.4 91.75
90
80
70
56.67
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60
50
32.41
40
30
20
10
0
0
0
Compound (1)
Compound (2)
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Control
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Spores germination inhbition (%)
100
Aspergillus
Penicillium
Fig. 10. Compounds 1 and 2 adjusted at 4000 µg/ml effects on fungal spore germination
inhibition, after incubation for 24 h at 21°C, compared to control tube containing fungal spore
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suspension without the compounds.
4. Conclusions
The successful synthesis of the first (µ-oxo)-bis(oxalato)-vanadium(IV) 1D coordination
polymer, compound 1, was described and the compound characterization was compared to a
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previously reported one, compound 2. Thermogravimetric analysis allowed speculating that
the decomposition occur in different steps starting from 100 °C for 2 while the compound 1
remains stable until 300 °C. Magnetic study revealed a simple paramagnetic behavior for the
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complex 2 compared to a weak antiferromagnetic coupling between the metal centers for the
complex 1 due to the oxygen bridge.
The two compounds revealed antibacterial activities against Gram-negative bacteria, antiCandida activities and lysozyme activities against Gram-positive bacteria. They fail to attack
mycelial growth yet show high fungitoxic role with eminent inhibition action on the fungal
spore germination.
Acknowledgments
Financial support from the Ministry of Higher Education and Scientific Research of Tunisia is
gratefully acknowledged. H. S. also thanks Dr. Elisa Barea (Department of Inorganic
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Chemistry of the University of Granada) for support and advice during her short-term stay in
the University of Granada, where most of the characterization of the reported compounds
were conducted.
Supplementary materials
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Crystallographic data and full lists of bond lengths and angles have been deposited with the
Cambridge Crystallographic Data Centre, CCDC No 1844523. Copies of this information
may be obtained free of charge from The Director, CCDC, 12 Union Road, CAMBRIDGE,
CB2
1EZ,
UK
(fax:
+44-1223-336-033;
email:deposit@ccdc.cam.ac.uk
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http://www.ccdc.cam.ac.uk).
or
Available supplementary information: Selected bond lengths (Å) of 1 (Table S1). Hydrogenbond geometry (Å,°) of 1 (Table S2). FT-IR spectra of the compounds 1 and 2 (Fig. S1). UV–
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Vis absorption spectra of the compounds 1 and 2. (Concentration, 4×10−3 M, path length, 1
cm) (Fig. S2).
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•
The compound 1, the first reported (µ-oxo)-bis(oxalato)-vanadium(IV) 1D
coordination polymer, was synthesised and showed antiferromagnetic behaviour (J=0.25 cm-1).
Both compounds 1 and 2 prove significant antibacterial activities against Gramnegative bacteria Escherchia coli and high lysozyme activities against Gram-positive
bacteria Staphylococcus aureus and Streptococcus agalactiae.
•
Both compounds fail to attack mycelial growth but the germination assays proved
their eminent fungitoxic role on the fungal spore germination inhibition of the two
fungi Aspergillus niger and Penicillium spp.
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•
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