Molecular docking and DFT studies on some nano-meter binuclear complexes derived from hydrazine-carbothioamide ligand, synthesis, thermal, kinetic and spectral characterization

Journal of Molecular Liquids 220 (2016) 311–323

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Molecular docking and DFT studies on some nano-meter binuclear complexes derived from hydrazine-carbothioamide ligand, synthesis, thermal, kinetic and spectral characterization Aisha Y. Al-Dawood a, Nashwa M. El -Metwaly a,b,⁎, Hoda A. El-Ghamry a,c a b c

Chemistry Department, College of Applied Sciences, Umm Al-Qura University, Makkah, Saudi Arabia Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt

a r t i c l e

i n f o

Article history: Received 5 April 2016 Received in revised form 17 April 2016 Accepted 18 April 2016 Available online xxxx Keywords: Schiff base Molecular docking TEM DFT X-ray and biological activity

a b s t r a c t New metal ion complexes were prepared using Cr(III), Co(II), Pd(II) and Cu(II) ions. The Schiff base ligand used is a multi-dentate chelating compound. The IR, 1HNMR and mass spectra are used to verify the structural and molecular formula of the organic ligand. The ligand coordinates as a neutral tetra-dentate towards Cr(III) and Pd(II) ions, whereas a bi-negative tetra-dentate towards Co(II) and Cu(II) ions in bi-nuclear complexes. The electronic spectral data were investigated to propose the geometry of four coordination numbers surrounding all metal ions except Cr(III) ion (six coordination). The covalency parameters were calculated for Cr(III) and Co(II) complexes using their electronic spectra. The EPR spectrum of Cu(II) complex introduces parameters supporting the electronic data. X-ray diffraction patterns are displaying the nano-crystalline character for the ligand, Co(II) and Cu(II) complexes. Their particle sizes calculated applying referenced equations propose the nano-sized feature isolated for them. TEM images also proposed the nano-meter appearance of the synthesized compounds. The DFT studies on the organic compound and its isolated complexes were carried out implementing Materials Studio package to calculate the characteristic parameters after geometry optimization. Molecular docking and theoretical statistics were done to expect the biological behavior of the organic compound and its complexes. The biological activity was studied against different organisms and the negative result is the main feature. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The coordination compounds of transition metal ions have a distinguish role in different application fields, especially with the polynuclear complexes which have special character based on the extensive presence of the metal ions [1–5]. Schiff base complexes are more pleasant than many others among various complexes [6]. This is due to their synthesis process being simple, having excellent characteristics and having various applications such as bioactivity, catalysis and in optical sensors [7–9]. Notable biological activities of the Schiff base metal complexes including antibacterial, antifungal, anti-flammatory, analgesic, anti-tubercular, anti-oxidant and antiviral effects were extremely displayed in the literature survey [10–16]. Also, ligands containing N_C\\C_N units display a strong chelating ability through the electron delocalization, which attached with extended conjugation. This may affect on the nature of the complex isolated. Also, mono or poly-nuclear complexes were prepared with multi-central Schiff base

⁎ Corresponding author. E-mail address: [email protected] (N.M. El -Metwaly).

http://dx.doi.org/10.1016/j.molliq.2016.04.079 0167-7322/© 2016 Elsevier B.V. All rights reserved.

ligands, some of which are biologically active [17,18]. Particularly, the first row of transition metal complexes represents wide applications in different areas [19]. These useful utilities cause the Schiff bases to play an important role in chemistry and are considered as target substance for many synthetic scientists. The studies of nano-structure compounds are extremely popular because of their unique properties that are completely different with respect to their presence in bulk phase, which is reflecting large numbers of surface molecules. Choosing the process of synthesis is an important factor to control the size of the materials at a submicrometer scale [20,21]. Here, in the study the ligand used have multi-central donors which may coordinate towards polynuclear metal ions. Such complexes may display a very good spectral data which deepen the structural characterization of the isolated complexes. Different spectral parameters will be calculated for the complexes and the spectral analysis will be supported by the DFT theoretical studies. The biological application will be our concern in this study. The protein–ligand docking has become a standard method used for the verification of biological studies. Also, it is widely used to predict protein–ligand [22] interaction feature and to cover large libraries for molecules that will modify the efficiency of a biological sensor.

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2. Experimental 2.1. Chemicals All chemicals used were of analytical reagent grade, commercially available from Fluka and used as purchased. Terphthalaldehyde, thiosemicarbazide and isatin are used to prepare the chelating organic. Some metal chloride salts (CoCl2·6H2O, CrCl3·6H2O, PdCl2 and, CuCl2·2H2O) represent the metal ions concerning complexation. 2.2. Synthesis process 2.2.1. Synthesis of N,N′-((1E,1′E)-1.4phenylenebis(methanylylidene))bis(hydrazinecarbothioamide),(A) In a round bottom flask, terephthalaldehyde (1.341 g, 0.01 mol) was dissolved in 40 ml ethanol. Thiosemicarbazide (1.823 g, 0.02 mol) was also dissolved in ethanol and then added slowly with stirring. The mixture was refluxed for ≈3 h and allowed to cool down. The yellowish white Schiff base product (A) was collected by filtration, washed with ether and re-crystallized in ethanol (m.p. 300 °C). 2.2.2. Synthesis of hydrazine carbothioamide ligand (H4PMOHC)(B) The ligand (B) was prepared (Scheme 1) using 0.01 mol of compound A (2.804 g), dissolved in 10 ml DMF solvent and then added slowly to an ethanolic solution of 0.02 mol isatin (2.943 g). The mixture was refluxed to 3–4 h and allowed to cool down. An orange precipitate was filtered off and washed with bi-distilled water and diethyl ether. The isolated ligand was re-crystallized in ethanol (240 °C m.p) and its elemental analyses are, C; 62.30 (calcd. 62.44%), H; 3.35 (calcd. 3.37%) and N; 20.80 (calcd. 20.81%). The formula of the ligand (C26H18N8O2S2) was verified by 1HNMR and mass spectral analysis. The distribution of the biological activity scores (version 2011.06) using Molinspiration strategy was calculated. GPCR ligands, kinase inhibitors, ion channel modulators, nuclear receptor ligands, protease inhibitors and other enzyme targets were compared with scores for about 100,000 average drug-like molecules. The score allows efficient separation of active and inactive molecules. The results show: GPCR (G protein-coupled receptor) ligand − 0.31, ion channel modulator −0.35, kinase inhibitor −0.04, nuclear receptor ligand −0.52, protease inhibitor −0.49 and enzyme inhibitor −0.11. Such theoretical data may expect an inhibition activity of the chelating organic. 2.2.3. Synthesis of the complexes All the complexes were prepared by equi-molar ratios (1: 1) inbetween the ligand and metal ions. 0.538 g, 1 mmol, of the ligand was

dissolved in methanolic solution and then added to an aqueous solution of each metal chloride salt (0.238 g CoCl2·6H2O; 0.266 g CrCl3·6H2O; 0.177 g PdCl2 and 0.171 g CuCl2·2H2O). The mixtures were refluxed for 6–10 h, and the precipitate was filtered off, washed with methanol and diethyl ether and finally dried in a vacuum desiccator. 2.3. Antimicrobial activity The ligand and its complexes were screened for their antimicrobial and antifungal activities using cup-diffusion technique [23]. The investigation was carried out against Escherichia coli as G−, Staphylococcus aureus as G+ bacteria and Aspergillus flavus with Candida albicans as fungi. A 0.2 ml of tested substance (10 μg/ml) was placed in a specified cup made in nutrient agar medium on which a culture of the tested bacteria has been spread to produce a uniform growth. After 24 h incubation at 37 °C, the diameter of inhibition zone was measured as mm/mg. 2.4. Physical measurements Carbon, H and N were analyzed at the Microanalytical Unit of Cairo University. The metal and chloride contents were determined using standard methods [24]. The molar conductivities of freshly prepared 1.0 × 10−3 mol/cm3 DMSO solutions were measured for the soluble complexes using Jenway 4010 conductivity meter. The X-ray diffraction patterns (XRDs) were obtained on Rikagu diffractometer using Cu/Kα radiation. Transmittance electron microscopy (TEM) images were taken in Joel JSM-6390 equipment. The infrared spectra, as KBr discs, were recorded on JASCO FT-IR-4100 Spectrophotometer (200–4000 cm−1). The electronic and 1HNMR (200 MHz) spectra were recorded on UV2 Unicam UV/Vis, and a Varian Gemini Spectrophotometers. The effective magnetic moments were evaluated at room qffiffiffiffiffiffiffiffiffi temperature by applying μ eff = 2.828 x‘M T, where XM is the molar susceptibility corrected using Pascal's constants for the diamagnetism of all atoms in the ligand using a Johnson Matthey magnetic susceptibility balance. The thermal studies were carried out on a Shimadzu thermogravimetric analyzer at a heating rate of 10 °C min−1 under nitrogen in the temperature range of 20–900 °C. ESR spectrum of solid Cu(II) complex was obtained on a Bruker EMX Spectrometer working in the X-band (9.78 GHz) with 100 kHz modulation frequency. The microwave power was set at 1 mW, and modulation amplitude was set at 4 G. The low field signal was obtained after 4 scans with a 10 fold increase in the receiver gain. A powder spectrum was obtained in a 2 mm quartz capillary at room temperature. The biological study was carried out in Molecular Biology center at Cairo University Egypt.

Scheme 1. Stepwise equations for the synthesis of (Z)-N,N′-((1E,1′E)-1.4-phenylene bis (methanylylidene))bis(2-((Z)-2-oxoindolin-3-ylidene) hydrazine-1-carbothioamide) (H4PMOHC) ligand.

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313

Table 1 Analytical and physical data of H4PMOHC ligand and its metal complexes. Compound empirical formula (calcd./found)

Color

(1) [C28H18N8O2S2] (538.6/535) (2) [Cr2(Cl)6(C28H18N8O2S2)(H2O)2](891.35) (3) [Co2(Cl)2(C28H16N8O2S2)(H2O)2](761.39) (4) [Pd2(Cl)4(C28H18N8O2S2)](H2O)2(893.26) (5) [Cu2(Cl)2(C28H16N8O2S2)(H2O)2](770.65)

Elemental analysis (%) calcd./found

Yellow Brown Green Brown Yellow

C

H

N

M

Cl

62.44/62.34 35.04/35.10 41.01/41.11 34.96/34.95 40.52/40.54

3.37/3.36 2.49/2.48 2.65/2.65 2.03/2.02 2.61/2.57

20.81/20.79 12.57/12.57 14.72/14.72 12.54/12.53 14.54/14.50

– 11.67/11.67 15.48/15.50 23.83/23.83 16.45/16.44

23.86/23.86 9.31/9.32 15.87/15.87 9.20/9.22

2.5. Theoretical calculations 2.5.1. Kinetic studies The calculation of thermal analysis kinetics was carried out for all complexes. The order (n) and the energy of activation (E) of various decomposition stages were determined from TG curves. Different researchers [25–33] established the equations and discussed their advantages. The rate of decomposition is the product of two separate functions of temperature and conversion using: dα ¼ kðTÞf ðαÞ dT

ð1Þ

where, α is the fraction decomposed at time t, k(T) is the function of temperature dependent and f(α) is the conversion function. The rate constant temperature and dependent function k(T) is of Arrhenius type K ¼ Ae−E=RT

ð2Þ

where R is the gas constant in (J mol−1 k−1) substituting Eq. (2) into Eq. (1) we get this equation: dα ¼ dT



A



φe−E=RT

f ðαÞ

ð3Þ

where φ is the linear heating rate (dT/dt). From the integration and approximation, this equation can be obtained in the following form: lngðαÞ ¼

  −E AR þ ln  RT φE

ð4Þ

where g(α) is a function that depends on the mechanism of the reaction. The right hand side is known as temperature integral and has no close for solution. So, several techniques have been used to evaluate the temperature integral. The kinetic parameters for the ligand and its complexes are evaluated using Coats–Redfern [27] and Horowitz–Metzger methods [32].

2.5.2. Computational biological study Applying a theoretical study of Molinspiration strategy to calculate the score of the expected biological activity was done in comparing known drugs. It focuses on particular drug classes and development of specific activity score for each of these classes. The method implemented uses statistics to compare structures of representative ligands active on the particular target with structures of inactive molecules and to identify substructure features typical for active molecules. 2.5.3. Molecular modeling study The cluster calculations were performed using DMOL3 program [34] in Materials Studio package [34], which was designed for the realization of large scale density functional theory (DFT) calculations. DFT semicore pseudopods calculations (dspp) were implemented with the double numerical basis sets plus polarization functional (DNP). The DNP basis sets are of comparable quality to 6–31G Gaussian basis sets [35]. Kessi and Density. [36] showed that the DNP basis sets are more accurate than Gaussian basis sets of the same size. The Revised Perdew–Burke–Erenzrh of (RPBE) functional [37] that is so far the best exchange–correlation functional [37], based on the generalized gradient approximation (GGA), is employed to take account of the exchange and correlation effects of electrons. The geometric optimization is performed without any symmetry restriction. 2.5.4. Molecular docking Docking Server was used for carrying out the docking calculations using Gasteiger partial charges added to the ligand atoms. Non-polar hydrogen atoms were conjoined, and rotatable bonds were illustrated. The calculations were performed on the ligand–protein pattern. Auto Dock tools were implemented after the addition of; fundamental hydrogen atoms, Kollman united atom type charges, and solvation parameters [38]. Affinity (grid) maps of × × Å grid points and 0.375 Å spacing were created applying the Autogrid program [39]. Calculating van der Waals and the electrostatic terms was carried out using Auto Dock parameter set- and distance-dependent dielectric functions, respectively. Docking simulations were executed using the Solis & Wets local search method and the Lamarckian genetic algorithm (LGA) [40]. Initial position, orientation, and torsions of the ligand molecules were set indiscriminately. All rotatable torsions were emitted during docking. Each

Table 2 Assignments of the IR spectral bands (cm−1) of H4PMOHC ligand and its metal complexes. Compound

υOH υNH

δOH (in plane)

δNH

υC_N

υC_O

υC\ \O

υC_S(IV)

υC\ \S(IV)

υM\ \OH2υM\ \N

υM\ \S

(1) [C28H18N8O2S2]

3427 3168 3405 3158 3400 3166 – 3155 3429 3168

1364

1500

1595

1714

1093

811

680

–

–

1360

1536

1709

1095

756

–

243

1501

1710

1099

–

650

442

240

–

1542

1697

1090

800

–

440

250

1354

1521

1715

1093

–

619

825 560 820 540 – 538 825 537

531

1360

1596 1536 1595 1521 1599 1534 1599 1533

420

260

(2) [Cr2(Cl)6(C28H18N8O2S2)(H2O)2] (3) [Co2(Cl)2(C28H16N8O2S2)(H2O)2] (4) [Pd2(Cl)4(C28H18N8O2S2)] (5) [Cu2(Cl)2(C28H16N8O2S2)(H2O)2]

υM\ \Cl

314

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Fig. 1. The modeling structure of the tutomer forms of the H4PMOHC ligand.

docking experiment was derived from 10 different runs that were set to close after a maximum of 250,000 energy assessments. The population size was set to 150. During the search, a translational step of 0.2 Å and quaternion and torsion steps of 5 were applied.

3. Results and discussion 3.1. Analytical and physical data The analytical and physical data of H4PMOHC ligand and its Cr(III), Co(II), Pd(II) and Cu(II) complexes are listed in Table 1. The isolated complexes are stable in air, having high melting points, insoluble in H2O and most organic solvents but are completely soluble in DMSO and DMF. The coordination behavior of hydrazine carbothioamide towards the metal ions was investigated via the (IR and 1HNMR) spectral, thermal analyses and molar conductance. The elemental analysis of the complexes shows 2:1 (metal: ligand) stoichiometry for all complexes. The low molar conductance values (5.5–15.4 Ω−1 cm2 mol−1) of the complexes reveal their non-electrolytic features [41]. This is expected with chloride anion which favors its covalent attachment with the metal ions.

3.2. IR, 1HNMR and mass spectra Significant IR bands of the ligand and its metal ion complexes are tabulated in Table 2. The H4PMOHC free ligand displays the following essential bands: 3427, 1714, 811, 1595, 1093, 680, 1500, and 1364 cm− 1 which were attributed to these vibrations: υOH, υC_O, υC_S(IV), υC_N, υC\\O, υC\\S, δNHs and δOH [42]. This may propose the presence of its two tutomer forms (keto/enol ↔ enol/thiol) in conjugation with each other (Fig. 1). The arrangement stereo of these active sites inside the ligand molecular form makes it acceptable to view. This is also confirmed by the 1HNMR spectral data. The peaks appeared at: δ = 1.2 (s, 2H, 2SH); δ = 2.5 (s, DMSO); δ = 7.8 (s, 2H, 2 thioamide); δ = 7.8–8.1 (m, 12H, ph groups); δ = 8.1 (s, 2H, 2NH of isatin) and δ = 11.6 (s, 2H, 2OH) shifts. The analysis of mass spectrum corresponding to the ligand (Fig. 2) confirms the suggested molecular formula. The peaks are corresponding to the successive degradation for its molecular ion (Scheme 2). A lower intense peak that appeared at m/e = 535 (calcd. = 538.6) represents the complex molecular ion peak (M+− 3) by 9.8% abundance. The small abundance may reflect a high stability of the molecular ion. The abundant peak (base peak) with m/e = 59 (calcd. = 62) represents a stable part of the ligand (C5H2).

Fig. 2. The mass spectrum of H4PMOHC ligand.

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315

Scheme. 2. The fragmentation pattern of the H4PMOHC ligand.

The IR spectra of the investigated complexes displayed peaks assigned υOH, υC\\S, υC_O, υC_S and δOH vibrations which support with no doubt the presence of its tutomer forms. In [Cr2(Cl)6(C28H18N8O2S2)(H2O)2] and [Pd2(Cl)4(C28H18N8O2S2)] complex spectra, the ligand coordinates in the same manner as a neutral tetra-dentate inside the bi-nuclear complexes. This is based on the lower shifted appearance of one υC_N and υC_S(IV) bands, which considered the contributed sites towards Cr(III) and Pd(II) ions. Moreover, the un-shift observed for another band assigned for the second azomethine group (υC_N) suggests the contribution of only one inbetween. More or less un-shifted appearance for υC_O or υC\\O

proposes its ruling out from coordination. This is verified from the geometry optimization rearranging the ligand molecule in the most stable form attaching towards the metal atoms. In [Co2(Cl)2(C28H16N8O2S2)(H2O)2] and [Cu2(Cl)2(C28H16N8O2S2)(H2O)2] complex spectra, the ligand coordinates as bi-negative tetra-dentate towards the two metal atoms. The lower shifted appearance for υC_N and υC\\S bands proposed the four sites coordinating with the metal centers. The complete obscure for υC_S bands suggests its thioenolization during the complexation process with Co(II) and Cu(II) ions. The noncontributing behavior of C_O groups is also observed with the two complexes as the Cr(III) and Pd(II). This is may be

Table 3 The magnetic moment values and the electronic spectral data of paramagnetic complexes. Complexes

υ1 (cm−1)

aε1

105b 1

υ2 (cm−1)

ε2

105

[Cr2(Cl)6(H4PMOHC)(H2O)2] [Co2(Cl)2(H2PMOHC)(H2O)2] [Cu2(Cl)2(H2PMOHC)(H2O)2]

14,706 11,481 12,420

336 – 270

210.5 – 383.2

20,230 12,634 16,500

315 272 253

421.05 125.12 430.6

a

2

υ2–υ1 (cm−1) 5524 1153 4080

2/ 1

2.00 – 1.12

μeff (BM) 3.34 4.11 0.41

Molar absorptivity. Oscillator strengths, , calculated using the following expression: = 4.6 × 10−9 εmax υ1/2, where; εmax is the molar absorptivity of the band maximum and υ1/2 is the band width at half-height expressed in wave numbers: C. J. Ballahusen, Prog. Inorg. Chem. 2.251(1960). b

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Fig. 3. a. The structural formula of Cr(III) and Co(II) complexes. b. The structural formula of Cu(II) and Pd(II) complexes.

due to the geometry distribution of multi-donor centers giving a chance for some of them to contribute and prevent the others. The appearance of bands at ≈820 and ≈630 cm−1 is assigned for δr(H2O) and δw(H2O) coordinating water molecules [43] in all complexes except Pd(II). Also, the appearance of vibration bands attributed to the metallic bonds (υM\\N, υM\\S and υM\\Cl) supports all the previous spectral data suggesting the mode of coordination.

the complex nucleus. The υ1 and υ2 values (11,481 and 12,634 cm−1) were calculated using the following equation: μ eff ¼ 3:87ð1−4λ=10DqÞ

λ ¼ −178 cm−1

υ1 ¼ 10Dq υ2 ¼ 15Dq þ 7:5−0:5√Y;

y2 ¼ 225B2 þ Δ2 −18ΔB:

3.3. Electronic spectra and magnetic measurements The electronic spectrum of 4.05 × 10−5 mol/L ligand (in ethanol) displays bands at 28,571 and 27,027 cm− 1 for π → π* and 25,641 cm−1 for n → π* intra-ligand transitions. The molar absorptivity values are ε1 = 2593, ε2 = 2346 and ε3 = 1852 mol−1 dm cm−1. The high values are completely attached with the intra-ligand transition bands especially with π → π* transitions. The high conjugation for chromophore groups in the ligand formula affects the position of charge transfer bands nearby the visible region and deepens the color of the compound. Electronic spectrum of [Cr2(Cl)6(H2O)2(H4PMOHC)] complex exhibits three absorption bands at 33,333, 20,230 and 14,706 (Table 3) assigned to 4A2g(F) → 4T1g(P) (υ3), 4A2g(F) → 4T1g(F) (υ2) and 4A2g(F) → 4T2g(F) (υ1) transitions, respectively. These transition bands are referring to an octahedral geometry around each central atom [Fig. 3a], also the magnetic moment value (3.34 BM) is found within the acceptable range for the geometry. The magnetic susceptibility value is slightly reduced, referring to strong interaction in-between the central atoms. The appreciable oscillator strengths of the electronic absorption bands are indicative of the degree of metal–ligand covalency [44]. The nephelauxetic parameter, β, is obtained using the relation: β = B(complex)/Bo(free ion). Whereas, B = (2 υ21 − 3 υ1 υ2 + υ22)/ (15 υ2 − 27 υ1) is found by 541.83 cm− 1 value and B (free ion) = 918 cm−1. The β value (0.59) is obtained from a consideration of lowenergy quartet–doublet transitions [44] and reflecting a strong covalency in the metal–ligand σ bonds also consistent with appreciable Cr–Sπ bonding in compounds containing Cr(III)S chromophores [45]. The electronic spectrum of green [Co2(Cl)2(H2PMOHC)(H2O)2] complex exhibits one characteristic band at 16,000 cm−1attributed to 4A2(F) → 4 T1(P) transition referring to tetrahedral geometry [46] [Fig. 3a]. The magnetic moment value (4.11 BM) suffers significant reduction from the range reported for such structure (4.4–4.7 BM). This verifies the presence of two Co(II) atoms interacting strongly inside

Also, the covalency parameters were calculated and tabulated in Table 4. Whereas, the Racah inter-electronic repulsion parameter B was calculated using the following equation: B = [4(υ3 − 15Dq)2 − 10Dq2]/[60(υ3 − 15Dq) − 18Δ]. The nephelauxetic parameter β value (0.463) reflects the high covalent character of the metal–ligand bonds. The decrease in β value is strongly associated with the reduction in the effective positive charge of metal cation to the lower oxidation state [47]. The Racah inter-electronic repulsion parameter values are varied for 3d transition metal complexes with changing Z and q values. Whereas, Z is the cationic charge and q is the occupation number of the dq shell. The Racah parameter is well-expressed by the relation: B (cm−1) = 384 + 58q + 124(Z + 1) − 540/(Z + 1), so the effective ionic charge of the complex was calculated (Table 4). The value of Cr(III) complex is found +1.02, considerably below the formal oxidation state of chromium ion(+3) and the value is within the range for Cr\\S, Cr\\N and M\\O bonds [48]. Whereas, the value of Co(II) complex is found − 0.399, which reflects the overcoming of the ligand anionic character on the metal cationic charge. Electronic spectrum of [Cu2(Cl)2(H2PMOHC)(H2O)2] complex displays bands at 12,420 and 16,500 cm−1 assigned for 2B1g → 2A1g (υ1) and 2B1g → 2Eg(υ2) transition, respectively in a square-pyramidal geometry [Fig. 3b]. The bands that appeared at 27,777 and 33,333 cm−1 are assigned for S → Cu and N → Cu charge transfer transitions [49]. A high reduction recorded for the magnetic moment value of the complex (μeff = 0.41 BM) suggests

Table 4 Spectroscopic and covalency parameters. Complex

Dq

B (cm−1)

β

Za

(2) (3)

1470.6 1148.1

541.83 449.53

0.5902 0.463

+1.02 −0.399

a

The effective charge of the metal in complex.

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317

Table 5 Spin Hamiltonian parameters of Cu(II) complex. Complex

g11

g⊥

go

A11 × 10−4

f

A⊥

Ao

p

2K11

2K⊥

k

G

α2

β2

(5)

2.195

2.095

2.13

160

137.15

60

93.33

76.657

−0.477

−0.919

1.343

2.073

0.716

0.669

its nearby diamagnetic appearance. This devotes us to expect the presence of a strong interaction in-between Cu(II) ions reaching to a relative bond in-between them. This may happen after a suitable rearrangement for the active sites in the ligand [50]. Electronic spectrum of [Pd2(Cl)4(C28H18N8O2S2)] complex displays charge-transfer bands for a square-planar geometry [Fig. 3b]. Intense band at 22,222 and 25,000 cm−1 could be assigned for S → M(II) and O → M(II) charge–transfer transition, respectively. A band at 33,500 cm− 1 is attributed to the combination of intra-ligand transitions for π → π* and n → π*.

3.4. EPR spectrum of Cu(II) complex The spin Hamiltonian parameters and the G value of solid Cu(II) complex are calculated [Table 5] from its EPR spectrum [Fig. 4]. The axially symmetric g tensor parameter is g11 N g⊥ N 2.0023 indicating the d2x–2y orbital as a ground state [51]. The G-factor is expressed by, G = (g11 − 2.0023)/(g⊥ − 2.0023) = 4, which measures the exchange interaction between copper centers in the solid. According to Hathaway [52], the greater the G value than 4, the negligible exchange interaction between copper(II) centers. Whereas, the lower the value than 4, the high exchange interaction exists between the metal centers in the solid complex. Here, the G value is very low (2.073) indicating that a strong exchange coupling between copper(II) centers must be in polynuclear complex [53]. The value of g11 b 2.3 for covalent character of the M–L bond and g11 N 2.3 for ionic feature was reported [54]. Applying this criterion the covalent character of the metal–ligand bond is shinned and the g11 (2.195) value matching with O\\M and S\\M bonds [55]. The tendency of A11 (160 × 10−4 G) to decrease with increasing g11 is an index for the tetrahedral distortion (f = g11/A11) in the coordination sphere [56,57]. Whenever the value in-between 105 and 135 for square planar geometry and the value will be increased with the presence of tetrahedral distortion, the calculated value (137.15 cm− 1) proposes the presence of a significant tetrahedral distortion in the xy-plane.

3.4.1. Calculation of molecular orbital coefficients The σ2 (covalence of the in-plane σ-bonding) and β2 (covalence of the in plane π-bonding) were calculated using the following equations: α2 ¼



 A== =0:036 þ g== –2:0023 þ 3=7ðg⊥ −2:0023Þ þ 0:04



β2 ¼ g== –2:0023 E=–8λα2 ; where λ (spin–orbital coupling) = −828 cm−1 for the free copper ion and E (16,500 cm−1) is the electronic transition energy. The α2 (0.716) and β2 (0.669) values indicate that the in-plane σ-bonding and the inplane π-bonding are highly covalent [58]. The lower the β2 value in comparing α2 the more covalent the in-plane π-bonding than the inplane σ-bonding. This data is completely matched with the covalency parameters of electronic spectra. The orbital reduction factors viz., K// and K⊥ are also calculated using the following equations: 2

2



K== ¼ g== −2:00277 E=8λ K⊥ ¼ ðg⊥ −2:00277Þ E=2λ

where, 2K// = α2β2. For pure σ-bonding K// ≈ K⊥ ≈ − 0.77 while, 2K // (− 0.477) b 2K ⊥ (− 0.919) signifies in-plane π-bonding, with 2K⊥ b 2K// accounting for out of plane π-bonding [59]. 3.4.2. Calculation of MO coefficients and bonding parameters The g value observed is so far from the value (2.0023). This is related to the spin orbit interaction of the ground state dxy level with low-lying excited states. The isotropic and anisotropic (g and A) parameters have been calculated from Eqs. Ao ¼ ðA11 þ 2A⊥ Þ⁄3

go ¼ ðg11 þ 2g⊥ Þ=3:

Taking A11 and A⊥ to be negative values the expression for K is. K ¼ −ðAo =pÞ–ðge −go Þ: Thus K (Fermi — contact term) can be evaluated (1.34). The Fermi contact term, k, is a measure of the polarization produced by the uneven distribution of d-electron density on the inner core s-electron. 3.4.3. Dipolar term (P) The P can be evaluated using the following equation: P = 2 γCu βo βN (r−3). γCu is the magnetic moment of copper, βo is the Bohr magneton, βN is the nuclear magneton and r is the distance from the central nucleus to the electron. 3.5. X-ray powder diffraction of some complexes

Fig. 4. EPR spectrum of Cu(II) complex.

X-ray powder diffraction patterns were carried out in the 10° b 2θ b 90° range for the free ligand and its complexes [Fig. 5] to give an obvious view about the lattice dynamics of the solid compounds. The patterns display the absence of contamination with starting materials through the comparison of the patterns with the reactants and

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Fig. 5. X-ray diffraction pattern of H4L ligand, Co(II) and Cu(II) complexes, respectively.

also represent a definite restricted structure for each compound. This identification was done by known methods [60]. Some patterns display overlapping contributed peaks suggesting an amorphous appearance, whereas sharp separate peaks appeared in the ligand, Co(II) and Cu(II) complexes suggesting nano-crystalline appearance [61,62]. This may be attributed to the formation of well-defined distorted crystalline structures. The θ, d values, full width at half maximum (FWHM) of prominent intensity peak, relative intensity (%) and particle size of compounds were presented in Table 6. The crystallite size was calculated by applying FWHM of the characteristic peaks using Deby–Scherrer equation: B = 0.94 λ/(S cos θ), where S is the crystallite size, θ is the diffraction angle, B is the line width at half maximum height, and Cu/Kα (λ) = 1.5406 Å. The inner crystal plane d-spacing values were determined by using Bragg equation: nλ = 2dsin(θ) at n = 1.

micrographs display nano-sized particles by different sizes, but appear mostly in spherical shape [63]. The particles with a diameter range of 21.6–47.6 nm are matching with X-ray diffraction calculations. The spherical shape of the nano-particles in complexes may be attributed to highly symmetric spherical chlorido groups. These topologies of the complexes suggest that the metal ion present in the complex has a

Table 7 Thermogravimetric data of the investigated complexes. Complex

Steps

Temp. range/°C

Decomposed assignments

Weight loss/found (calcd. %)

(2)

1st 2nd 3rd 4th Residue 1st 2nd 3rd 4th Residue 1st 2nd 3rd 4th Residue 1st 2nd 3rd 4th Residue

70.1–210.3 210.3–400.1 400.1–550.2 550.2–780.2

−2H2O + 2 Cl2 -Cl2 + (C8H5N3O) -(C8H5NO) -(C10H8N4S2) 2Cr −2H2O -Cl2 + (C8H5O) -(C8H5N3O) -(C10H6N5) 2CoS −2 Cl2 -Cl2 + (C8H5N3O) -(C8H5N3O) -(C10H8N2) 2PdS −2H2O + Cl2 -(C8H5N3O) -(C8H5N3O) -(C6H6N2S2) 2Cu + 4C

19.94 (19.95) 25.82 (25.81) 15.46 (14.71) 26.89 (27.86) 11.89 (11.67) 4.77 (4.73) 24.68 (24.69) 20.10 (20.90) 26.81 (25.77) 23.64 (23.90) 7.91 (7.94) 25.77 (25.76) 18.11 (17.82) 17.68 (17.49) 30.53 (31.01) 13.88 (13.87) 20.62 (20.65) 21.10 (20.65) 21.40 (22.09) 23.00 (22.73)

3.6. Transmission electron microscopy(TEM) (3)

TEM has become a widely employed method for the elucidation of the particle size and shape. High resolution transmission electron micrographs of the complexes have been observed (Fig. 1S). The (4)

Table 6 XRD spectral data of the H4L ligand and its nanocrystalline complexes. Compound

Size of particles (nm)

θ

Intensity

d spacing (Å)

FWHM

H4L Co(II)–H2L Cu(II)–H2L

0.72 0.58 1.14

14.17 5.68 7.44

584.6 1769.2 13,086

3.147 7.783 5.949

0.208 0.250 0.128

(5)

60.6–125.8 125.8–310.1 310.1–505.4 505.4–790.6 80.4–145.1 145.1–393.2 393.2–510.5 510.5–780.8 70.2–155.4 155.4–320.6 320.6–510.5 510.5–740

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319

Table 8 Kinetic parameters using the Coats–Red fern (CR) and Horowitz–Metzger (HM) operated for the complexes. Comp.

Step

Method

(2)

2nd

(3)

2nd

CR HM CR HM CR HM CR HM CR HM

3rd (4)

2nd

(5)

2nd

Kinetic parameters E (J mol−1)

A (S−1)

ΔS (J mol−1 K−1)

ΔH (J mol−1)

ΔG (J mol−1)

r

6.40E+04 1.04E+04 1.48E+05 1.20E+05 1.39E+05 1.07E+05 2.69E+04 3.85E+04 1.50E+05 1.68E+05

2.54E−02 4.57E−03 4.78E−02 2.24E+07 2.81E−02 2.92E+04 5.54E−03 2.29E+01 2.78E−02 5.95E+07

−2.82E+02 −2.96E+02 −2.77E+02 −1.11E+02 −2.83E+02 −1.68E+02 −2.93E+02 −2.24E+02 −2.84E+02 −1.05E+02

5.85E+04 4.85E+03 1.43E+05 1.14E+05 1.32E+05 1.00E+05 2.25E+04 3.41E+04 1.43E+05 1.60E+05

2.47E+05 2.03E+05 3.27E+05 1.88E+05 3.65E+05 2.39E+05 1.80E+05 1.54E+05 3.93E+05 2.53E+05

0.9993 0.9991 0.9993 0.9991 0.9993 0.9991 0.9993 0.9991 0.9993 0.9991

significant influence on the formation of the nano-particles [64]. The appearance of moderately strong diffraction spots rather than diffraction rings confirms the formation of moderately single crystalline cube of complexes [65,66]. In all the micrographs, the dark areas are related to the high concentration of the particles with aggregate nature. The spherical shapes are observed in the dark area in the micrographs of all complexes, signifying that these morphologies are constituted by a discrete accumulation of several individual particles in polycrystalline nature. The nano-sized appearance may improve the properties in current application in biological activity area with respect to bulk analogue.

3.8.1. Coats–Redfern equation The equation is a typical integral method, represented as: Zα 0

The plausible degradation behavior of the decomposition stages was translated from the data presented in Table 7 for the thermogravimetric analysis of the investigated complexes. The first decomposition stage in all TG curves is attributed to the removal of water molecules and/or chlorine molecules. All the TG curves display four decomposition stages starting the decomposition at relatively high temperature which reflects the relative thermal stability of the complexes under investigation. The organic ligand starts to decompose mainly in the second step at the temperature range ~ 300 to ~ 400 °C. A part of the ligand far from the coordination chalet was firstly expelledinto two sequenced steps (second and third). An observable difference inbetween the calculated and found weight loss was displayed in step three which may indicate the overlapping in-between two follower steps. The decomposition process for all the complexes ended at a range of 740–790 °C. The final residue mainly includes the two metal atoms in free state or attached with sulfur atoms. The only polluted residue was observed with Cu(II) complex. The presence of carbons with copper atoms may be due to the relatively lower temperature limit (740 °C) of the TG curve in which the residue was recorded. 3.8. Kinetic studies The order n and the heat of activation E of the various decomposition stages were calculated from the TG and DTG, in order to assess the effect of the metal ion on the complex thermal behavior.

ZT2

  −E exp dt: RT

ð5Þ

T1

For convenience of integration the lower limit T1 is usually taken as zero. This equation on integration gives: ln

3.7. Thermal analysis

dα A ¼ ð1−αÞn φ

  − ln ð1−αÞ 2

T

 ¼ ln

 AR E − : φE RT

ð6Þ

 (LHS) against 1/T was drawn. E* is the energy of A plot of ln ½ ln ð1−αÞ T2

activation in J mol−1 and calculated from the slope and A is (S−1) from the intercept value (Fig. 1S). The entropy of activation S* in (J K−1 mol−1) was calculated by using the equation:  ΔS ¼ R ln

AH KB Ts

 ð7Þ

where kB is the Boltzmann constant, h is the Plank's constant and Ts is the DTG peak temperature [27]. 3.8.2. Horowitz–Metzger equation The authors derived the relation [32]: In½‐Inð1‐α Þ ¼

E Θ RT m

ð8Þ

where α is the fraction of the sample decomposed at time t and Θ =T − Tm. A plot of ln[−ln(1 − α)] against Θ (Fig. 2S) was found to be linear, from the slope of which E, was calculated and Z can be deduced from the relation: Z¼

  E exp RTm RT2m Eφ

ð9Þ

Table 9 The calculated quantum chemical parameters of the ligand and its complexes. Comp.

EHOMO

ELUMO

x (eV)

μ (eV)

η (eV)

S (eV−1)

ω (eV)

(eV)

(1) Ligand (keto/thion) (2) Ligand (enol/thiol) (3) Cr(III)–H4L (4) Co(II)–H2L (5) Pd(II)–H4L (6) Cu(II)–H2L

−4.48 −4.99 −5.15 −3.73 −4.26 −5.60

−3.10 −3.68 −4.83 −3.68 −4.23 −5.37

3.789 4.335 4.99 3.705 4.245 5.4885

−3.789 −4.335 −4.99 −3.705 −4.245 −5.4885

0.689 0.655 0.16 0.025 0.015 0.1155

0.344 0.3275 0.08 0.0125 0.0075 0.05775

10.423 14.345 77.813 274.541 600.668 130.405

1.452 1.527 6.250 40.00 66.667 8.658

(EL–EH) (eV) 1.38 1.31 0.32 0.05 0.03 0.23

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Table 10 Some energetic properties of ligand and its complexes calculated by DMOL3 using DFT-method. Comp.

Keto/thion Enol/thiol Cr(III)–H4L Co(II)–H2L Pd(II)–H4L Cu(II)–H2L

Sum of atomic energies

Kinetic energy

−1.49 × 106 −1.36 × 106 −3.45 × 106 −1.79 × 106 −2.85 × 106 −2.44 × 106

−9.06 × 103 −8.49 × 103 −1.21 × 104 −1.15 × 104 −1.23 × 104 −1.29 × 104

Energy components (kcal/mol) Electrostatic energy

Exchange-correlation energy

Spin polarization energy

Total energy

−2.45 × 103 −2.83 × 103 −1.01 × 103 −6.93 × 102 −5.06 × 102 −4.96 × 102

2.85 × 103 2.78 × 103 3.41 × 103 3.58 × 103 3.12 × 103 3.07 × 103

2.067 × 103 2.08 × 103 2.41 × 103 2.35 × 103 2.06 × 103 2.19 × 103

−1.29 × 105 −1.29 × 105 −3.56 × 106 −1.80 × 106 −2.86 × 106 −2.45 × 106

where φ is the heating rate, the order of reaction, n, can be calculated from the relation: n¼

33:64758−182:295αm þ 435:9073α2m −551:157α3m þ 357:3703α4m −93:4828α5m

ð10Þ

where αm is the fraction of the substance decomposed at Tm. The enthalpy of activation ΔH* and Gibbs free energy was calculated from using ΔH* = E* − RT and ΔG* = ΔH* − TΔS*, equations. Also the entropy of activation ΔS* was calculated from Eq. (7). The data were tabulated in Table 8. The following observations were recorded: (i) the stepwise increase in the activation energy E values may reflect the thermal rigidity of the remaining part, (ii) the negative values of ΔS* may indicate that the fragment has ordered structures, (iii) the positive values of ΔH* reflect the endothermic decomposition process, and (iv) the positive values of ΔG* reveal that the free energy of the final residue is higher than that of the initial compound, and the decomposition stages are non-spontaneous. The upraising of TΔS* values (by negative singe) from one step to another override the ΔG* values may reflect that the rate of removal of the subsequent species will be lower than that of the precedent one [67].

Binding energy (kcal/mol)

Dipole moment (D)

EHOMO/eV

ELUMO/eV

−6.62 × 103 −6.46 × 103 −7.33 × 103 −6.22 × 103 −6.67 × 103 −7.18 × 103

4.40 2.78 14.44 10.51 14.44 8.14

−4.477 −4.992 −5.152 −3.73 −4.258 −5.60

−3.987 −3.681 −4.828 −3.68 −4.230 −5.37

3.9. Molecular modeling study 3.9.1. Molecular parameters Quantum chemical parameters of the organic compound obtained from the calculation of, EHOMO, ELUMO, ΔE (separation energies), electronegativity (χ), chemical potential (μ), global hardness (η), global softness (S), global electrophilicity index (ω) and absolute softness ( ) were estimated according to the following equations [68,69]: χ ¼ −1=2ðELUMO þ EHOMO Þ

ð1Þ

μ ¼ −χ ¼ 1=2ðELUMO þ EHOMO Þ

ð2Þ

η ¼ 1=2ðELUMO −EHOMO Þ

ð3Þ

S1=2η

ð4Þ

ω ¼ μ 2 =2η

ð5Þ

6 ¼ 1=η:

ð6Þ

Electrophilicity index (ω) is one of the most important quantum chemical descriptors in describing toxicity and the reactivity of various

Fig. 6. Molecular modeling forms of the complexes applying the DFT method.

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321

Fig. 7. The ligand in interaction with receptor 3t88. Fig. 8. HB plot of interaction between the ligand and receptor 3t88.

selective sites. The electrophilicity may quantify the biological activity of drug receptor interaction. Also, this index measures the stabilization energy when the system acquires extra negative charge from the environment. η and indexes, are the measure of the molecular stability and reactivity also, their concepts are related to each other. The softness indexes are the vice versa image for global hardness [70]. The data calculated are presented in Table 9 and reflect the following notes about the free ligand: i) The data of two tutomer forms of the organic ligand are comparable which may give the same chance for them with expected priority of enol/thiol form in biological activity based on high ω value. ii) S and are the softness indexes while η is for hardness indication, a hard molecule has a high stability due to its high energy difference in-between the EHOMO and ELUMO than the soft molecule. So, the soft molecule is the reactive one having flexible donation towards the metal ions. Accordingly, the investigated molecule is soft towards the coordination with the relatively favor of its enol/thiol form [70]. iii) The positive electrophilicity index (χ) value and the negative electronic chemical potential (μ) value indicate that the molecule capable of accepting electrons from the environment and its energy must decrease upon accepting electronic charge. Therefore, the electronic chemical potential must be negative.

iii) The lower total and binding energies of the complexes than the ligand forms indicate the comparable stability form for the complexes (Table 10). iv) The EHOMO and ELUMO values are calculated and showed an increase in EHOMO with Cr(III) and Cu(II) complexes than the free ligand which may be accompanied with the weakness of metal–ligand bonds due to elongation. Whenever, the other two EHOMO values are lower than that of the ligand which represents the strength of M–L shorter bonds. v) Their negative values indicate the stability of the molecules [71]. vi) The energy gap (ΔE) in-between an important index of stability was applied to develop the theoretical model for explaining the structure and conformation barriers in many molecular systems. vii) The HOMO level is mostly localized on the azomethine nitrogen and sulfur of thioamide group in the ligand, which indicates the preferable sites for nucleophilic attack to the central metal atoms (Table 10).

The data of the complexes are displayed in the following:

3.9.2. Geometry optimization with DFT method The geometry optimization applying the DFT for the ligand and its related complexes produces the most stable structural forms (Fig. 6). From the analysis of the data calculated for the bond lengths and angles, one can conclude the following remarks:

i) All the complexes are soft with a very small energy gap. ii) The complexes show high values of dipole moments than the free ligand, especially with Pd(II) and Cr(III) complexes, in which the high dipole moment form of the ligand (keto/thion) is coordinating.

i) There is an elongation in the coordination bonds after complexation, which correlates with the experimental IR frequencies. Moreover, the elongation of the bonds is conjugated with the lower vibration frequency needed.

Table 11 Molecular docking energy values obtained for the ligand with receptor of Escherichia coli protein 3t88. Receptors

Est. free energy of binding (kCal/mol)

Est. inhibition constant (Ki) (uM)

vdW + bond + dissolved energy (kCal/mol)

Electrostatic energy (kCal/mol)

Total intercooled energy (kCal/mol)

Frequency

Interact surface

3 t88 4 m01 4ynt 1zap

−6.41 −4.95 −9.85 −6.27

19.92 235.87 60.57 25.20

−8.36 −6.89 −12.05 −7.76

−0.13 −0.30 −0.02 −0.08

−8.49 −7.19 −12.07 −7.84

20% 20% 30% 10%

1118.732 852 1340.234 1026.438

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Fig. 9. 2D plot of interaction between the ligand and receptor 3t88.

ii) The bond angles lie in the range reported for the geometry proposed for all complexes with a little shift. This may somewhat represent distortion from the ideal form which was expected for such metal ion.

3.10. Molecular docking The docking complexes showed a mild interaction between the inhibitor and receptors of E. coli (3t88), S. aureus (4m01), A. flavus (4ynt) and C. albicans (1zap) (Figs. 7, 3S). The calculated energies are listed in Table 11. The energies reflect the relative stability of docked complexes for all receptors except for 4m01 which has a lower stability. Binding energies are most widely used as a measuring vector for the ligand binding affinity. Thus, decrease in binding energy due to mutation will increase the binding affinity of the ligand towards receptors

[22]. In accordance with the results obtained, HB plot curves display that, the ligand interacts to 3t88, 4ynt and 1zap receptors with multihydrogen bonds. The interaction decomposition energies in-between inhibitor and receptors were shown in Figs. 8, 4S. 2D-docking plot curves are shown in Figs. 9, 5S. This interaction could deactivate or apoptosis the microorganisms. The characteristic feature of the ligand was represented in the presence of several active sites available for hydrogen bonding interaction. This theoretical proposes the high biological activity of the organic ligand towards different bacteria or fungi. 3.11. Biological activity The biological investigation was carried out against E. coli as G−, S. aureus as G+ bacteria and A. flavus with C. albicans as fungi. All the investigated compounds have no significant effect against the organisms except that the free organic ligand inhibits the growth of E. coli, A. flavus and C. albicans by 9, 6 and 7 mm/mg diameter zones in

A.Y. Al-Dawood et al. / Journal of Molecular Liquids 220 (2016) 311–323

agreement with the docking results. The negative results may be attributed to different causes: 1) The inability of the complex to diffuse into the bacterial cell membrane. This is supported by the high dipole moment, which suggests the insolubility of the compounds in the lipid cell organisms. So, it prohibits the chance of direct attachment with the enzymes. As well as the calculated effective charge of some complexes (Cr(III) and Co(II)) goes by the way to support the previous postulation. 2) The inability of the compounds to interfere with the biological systems inside the cell may be due to the blocking for essential active sites by coordination with the metal ions. 3) The compounds can diffuse and inactivate by unknown cellular mechanism inside the organisms. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2016.04.079. References [1] D. Dive, C. Biot, Chem. Med. Chem. 3 (2008) 383. [2] F. Han, N. Shioda, S. Moriguchi, Z.-H. Qin, K. Fukunaga, Neuroscience 151 (2008) 671. [3] L. Ronconi, P.J. Sadler, Coord. Chem. Rev. 251 (2007) 1633. [4] P.C.A. Bruijnincx, P.J. Sadler, Curr. Opin. Chem. Biol. 12 (2008) 197. [5] B. Desoize, Anticancer Res. 24 (2004) 1529. [6] A.K.M. Nur Alam Siddiki, Shahidur Rahman, Arifur Rahman, Abdus Salam, Abu Yousuf, Farhadul Islam, Azharul Arafat, Bang. Pharm. J. 15 (1) (2012) 83. [7] M. Shebl, S.M.E. Khalil, S.A. Ahmed, H.A.A. Medien, J. Mol. Struct. 980 (2010) 39. [8] S.K. Bharti, G. Nath, R. Tilak, S.K. Singh, Eur. J. Med. Chem. 45 (2010) 651. [9] G. Kumar, V. Singh, K. Singh, I. Ahmad, D.S. Yadav, A. Kumar, N. Shishodia, J. Pharm. 2 (2012) 45. [10] R.S. Hunoor, B.R. Patil, D.S. Badiger, R.S. Vadavi, K.B. Gudasi, V.M. Chandrashekhar, I.S. Muchchandi, Appl. Organomet. Chem. 25 (2011) 476. [11] D. Kovala-Demertzi, D. Hadjipavlou-Litina, M. Staninska, A. Primikiri, C. Kotoglou, M.A. Demertzis, J. Enzyme Inhib. Med. Chem. 24 (2009) 742. [12] N. Nishat, S. Hasnain, T. Ahmad, A. Parveen, J. Therm. Anal. Calorim. 105 (2011) 969. [13] K.C. Gupta, A.K. Sutar, Coord. Chem. Rev. 252 (2008) 1420. [14] P.G. Cozzi, Chem. Soc. Rev. 33 (2004) 410. [15] H.F. Abd El-Halim, F.A. Nour El-Dien, G.G. Mohamed, N.A. Mohamed, J. Therm. Anal. Calorim. 109 (2012) 883. [16] A. Jarrahpour, D. Khalili, E. De Clercq, C. Salmi, J.M. Brunel, Molecules 12 (2007) 1720. [17] F.M. Belicchi, F. Bisceglie, G. Pelosi, J. Inorg. Biochem. 83 (2001) 169. [18] N.M. El-Metwally, R.M. El-Shazly, I.M. Gabr, A.A. El-Asmy, Spectrochim. Acta A 61 (2005) 1113. [19] E.M. Jouad, A. Riou, M. Allian, M.A. Khan, G.M. Bovet, Polyhedron 20 (2001) 67. [20] F. Kim, S. Connor, H. Song, T. Kuykendall, P.D. Yang, Angew. Chem. Int. Ed. 43 (2004) 3673. [21] H. Zhang, D.R. Yang, D.S. Li, X.Y. Ma, S.Z. Li, D.L. Que, Cryst. Growth Des. 5 (2005) 547. [22] T. Cheng, Q. Li, Z. Zhou, Y. Wang, S. Bryant, AAPS J. 14 (2012) 133. [23] S.D. Dhmwad, K.B. Gudasi, T.R. Goudar, Indian J. Chem. 33 (1994) 320. [24] A.I. Vogel, Text Book of Quantitative Inorganic Analysis Longman, London, 1986. [25] G.A. Bain, J.F. Berry, J. Chem. Educ. 85 (2008) 532. [26] E.S. Freeman, B. Carroll, J. Phys. Chem. 62 (1958) 394. [27] W. Coats, J.P. Redfern, Nature 201 (1964) 68.

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