Synthesis, Characterization, DFT Modeling and Antimicrobial Studies on the Ti(IV), Y(III) and Ce(IV) Ofloxacin Solid Complexes

Abstract

A new solid complexes of Ti(IV), Y(III) and Ce(IV) have been synthesized with ofloxacin. The formulae and structure of the complexes have been proposed in the light of analytical, spectral ($^1H$ NMR, IR and UV-Visible), magnetic, molar conductivities and thermal studies. The complexes are soluble in DMSO-$d_6$ and DMF. The measured molar conductance values indicate that, the three complexes are electrolyte in nature. The results support the formation of the complexes and indicated that ofloxacin reacts as a bidentate ligand chelate to the metal ion through the pyridone oxygen and one carboxylato oxygen. The kinetic parameters of thermogravimetric and its differential have been evaluated by using Coats Redfern (CR) and Horowitz-Metzeger (HM) methods. The thermodynamic data reflect the thermal stability for all complexes. The metal- ligand binding of the Ti(IV), Y(III) and Ce(IV) complexes is predicted using density funcational theory at the B3LYP-CEP-31G level of theory and total energy, dipole moment estimation of different Ti(IV), Y(III) and Ce(IV) ofloxacin structures. The biological activities of the ofloxacin, inorganic salts and their metal complexes were assayed against different bacterial species.

INTRODUCTION

Ofloxacin is a racemic mixture, which consists of 50% levofloxacin and 50% of its mirror image or enantiomer dextrofloxacin.12 Ofloxacin (HOfl) (Scheme 1) is one of the 4-quinolone synthetic antibiotics with high activity against a wide range of Gram-negative and Gram-positive bacteria.34

The coordination chemistry of fluoroquinolone drugs with metal ions of biological and pharmaceutical importance is of considerable interest. There have been several reports in the literature about the synthesis and crystal structure of metal complexes with ofloxacin.5−15

In continuation of our interest on synthetic and structural characterization of some ofloxacin complexes of transition metals,16 this work describes the synthesis of Ti(IV), Y(III) and Ce(IV) ofloxacin complexes. Characterization of the complexes was effected by melting points, magnetic properties, molar conductivity, elemental analysis, thermogravimetric and differential thermograimetric (TG and DTG) analyses, UV-visible, IR, 1H NMR and biological studies. Density functional theory (DFT) was used to compute the cation type influence on theoretical parameters of the Ti(IV), Y(III) and Ce(IV) complexes of ofloxacin and detect the exact structure of these complexes with different coordination numbers and show if the two oxygen atoms (Opyr and Ocar) of ofloxacin lie at trans or at cis positions so the two diastereoisomers of the studied complexes were calculated. Such computational characterization reduces time consuming experiments for biomedical and pharmaceutical studies of the drugs and its complexes. Profiles of the optimal set and geometry of these complexes were simulated by applying the GAUSSIAN 98W package of programs17 at B3LYP/CEP-31G18 level of theory.

Scheme 1.(RS)-9-fluoro-3-methyl-10-(4-methylpiperazin-1-yl)-7-oxo-3,7-dihydro-2H-[1,4]oxazino[2,3,4-ij]quinoline-6-carboxylic acid.

 

EXPERIMENTAL

All the chemicals and solvents were analytical reagent grade and were used as purchased without further purification. Ofloxacin was purchased from Egyptian Company for Chemicals ＆ Pharmaceuticals (ADWIA), Ti(SO4)2 and YCl3, (99.9%), Aldrich Chemical Co. Ce(SO4)2 (99.9%), NaOH and all solvents were purchased from Fluka Chemical Co.

Synthesis of the Complexes

The complexes have been prepared by direct reaction between HOfl and the corresponding metal cations in presence of NaOH solution in order to improve solubility of HOfl.

The Yellowish-white solid complex [Y(Ofl)2(H2O)2]Cl. 7H2O was synthesized as follows, Ofloxacin (361.37 mg, 1 mmol) and NaOH (40 mg, 1 mmol) were dissolved in ethanol (50 ml). After 40 min. of stirring we dropping wisely 0.5 mmol of YCl3 (195.4 mg) to the mixture. The reaction mixture was stirred for one day at room temperature. The solution was reduced in volume and left for slow evaporation. The Pale-brown precipitate was filtered off, washed several times by bidistilled water and dried under vacuum over CaCl2 in a dissector.

The pale-brown and buff solid complexes were prepared in a similar manner described above by using ethanol as a solvent and Ti(SO4)2 or Ce(SO4)2 metal salts. We did not manage to obtain a crystal of the complexes suitable for the structure determination with X-ray crystallography.

Instrumentation

Elemental C, H, N and halogen analysis was carried out on a Perkin Elmer CHN 2400. The percentage of the metal ions were determined gravimetrically by transforming the solid products into oxide, and also determined by using atomic absorption method. Spectrometer model PYE-UNICAM SP 1900 fitted with the corresponding lamp was used for this purposed. IR spectra were recorded on FTIR 460 PLUS (KBr discs) in the range from 4000−400 cm−1, 1H NMR spectra were recorded on Varian Mercury VX-300 NMR Spectrometer using DMSO-d6 as solvent. TG-DTG measurements were carried out under N2 atmosphere within the temperature range from room temperature to 600 ℃ or 800 ℃ using TGA-50H Shimadzu. Electronic spectra were obtained using UV-3101PC Shimadzu. The solid reflection spectra were recorded with KBr pellets. Magnetic measurements were carried out on a Sherwood scientific magnetic balance using Gouy method and Hg[Co (SCN)4] as calibrant. Molar conductivities of the solutions of the ligand and metal complexes in DMSO at 10−3 M were measured on CONSORT K410. All measurements were carried out at ambient temperature with freshly prepared solutions.

Antimicrobial Investigation

Antibacterial activity of the ligand, metal salts and its metal complexes was investigated by a previously reported modified method of Beecher and Wong,19 against different bacterial species, such as S. aureus K1, B. subtilis K22, Br. otitidis K76, E. coli K32, P. aeruginosa SW1 and Klebsiella oxytoca K42. The tested microorganism’s isolates were isolated from Egyptian soil and identified according to the standard bacteriological keys for identification of bacteria as stock cultures in the microbiology laboratory, Faculty of Science, Zagazig University. The Müller-Hington agar (30.0% Beef extract, 1.75% Casein hydrolysate, 0.15% Starch and 1.7% Agar) was prepared and then cooled to 47 ℃ and seeded with tested microorganisms. After solidification 5mm diameter holes were punched by a sterile cork-borer. The investigated compounds, i.e., ligand, metal salts and their complexes, were introduced in holes (only 100 μL) after being dissolved in DMSO at 10−3 M. These culture plates were then incubated at 37 ℃ for 20 h. The activity was determined by measuring the diameter of the inhibition zones (in mm). Growth inhibition was calculated with reference to the positive control, i.e., ofloxacin.

 

RESULTS AND DISCUSSION

The isolated solid complexes were characterized by elemental analysis. The metal content was determined by atomic absorption and thermogravimetrically. The water content in the complexes also determined gravimetrically. The formation of the complexes was confirmed by IR, UV-vis and 1H NMR spectroscopic. Table 1 shows the data of the elemental analysis of the compounds obtained serving as a basis for the determination of their empirical formulae. Besides analytical data the molar conductivities, magnetic moments and melting points of the compounds are also reported and all the complexes are diamagnetic. The molar conductance values of all complexes were in the range of 73.2−85.7 S cm2 mol−1 indicating that the complexes electrolytes.

IR Spectral Studies

The mode of bonding of the ofloxacin to Ti(IV), Y(III) and Ce(IV) ions was elucidated by recording the infared spectra of the complexes as compared with those of the free ofloxacin. IR spectra of the compounds were recorded in solid state as KBr discs in the range from 4000−400 cm−1(Fig. 1). The IR spectra of the complexes exhibited characteristic band in the region 3406−3200 cm−1 which corresponding to ν(O−H) indicate that the complexes containing water molecules in their formulae. Also, the presence of coordinated water molecules in the metal complexes are indicated by the appearance of two weak bands in the region 844−815 and 745−700 cm−1 due to ν(O−H) rocking and wagging modes of vibrations, respectively.20

Table 1.Elemental analysis and Physico-analytical data for ofloxacin (HOfl) and its metal complexe

Figure 1.Infrared spectra for (A) ofloxacin, (B) [Ti(Ofl)2(H2O)2]SO4.3H2O, (C) [Y(Ofl)2(H2O)2]Cl.7H2O and (D) [Ce(Ofl)2(H2O)2]SO4.4H2O.

However, the absence of the absorption band at 1720 cm−1 arising from the carboxylic group (COOH) for under investigation complexes, states that the hydrogen ions in the ofloxacin molecules are substituted by the metal ions and the carboxylate group is coordinated with metal ions.21 The stretching asymmetric (νas) of carboxylate group at 1624 cm−1 and the symmetric vibrations (νs) at around 1405 cm−1 confirm the coordination of the metal ions via oxygen of carboxylate.22 The ν values for complexes fall around 219 cm−1 indicating a monodentate coordination mode of the carboxylato group of the ofloxacin ligand.112123 The characteristic band for the pyridone stretch ν(C=O) found at 1620 cm−1,2425 this band is shifted to around 1573 cm−1 upon coordination.23

The coordination of the metal ions via oxygen atoms is confirmed by the ν(M−O) bands at 606 and 502 cm−1 for Ti(IV), 598, 517 and 486 cm−1 for Y(III) and 617 and 559 cm−1 for Ce(IV). According to the above discussion the ofloxacin is coordinated with the metal ions as a bidentate through the pyridone and the carboxylic group.2627 These results are consistant with the results we obtained from (V(IV), Zr(IV) and U(VI)).16

Electronic Spectroscopy of the Complexes

The UV-vis spectra of ligand have two types of absorption bands at λmax. 220 nm and 312, 369 nm, which can be assigned to π-π* and n-π* transitions within the organic molecule. While the UV-vis spectra of its their metal complexes has three types of absorption bands at λmax. in the regions of 217−275 nm, 301−407 nm and 512−657 nm, which can be, respectively, assigned to π-π* and n-π* transitions of the larger conjugated organic molecules and ligand to metal ions charge transfers (L→Mn+).28−31 The band shifts of λmax. to higher or lower and the presence of new bands for complexes in comparison with ligand indicate the formations of the complexes. Since the crystal structures of the metal complexes have not been obtained yet, we characterized the complexes and determined its possible structure by elemental analyses, melting point, molar conductivities, magnetic properties, IR, 1H NMR, UV-vis spectra and thermogravimetric analyses. The ligand and metal ions can form mononuclear complexes with 1:2 metal to ligand stoichiometry at the metal ions.

Table 2.as=strong, w=weak, sh=shoulder, v=very, br=broad, bν=stretching and δ=bending

Figure 2.Electronic reflection spectra for (A) ofloxacin, (B) [Ti(Ofl)2(H2O)2]SO4.3H2O, (C) [Y(Ofl)2(H2O)2]Cl.7H2O and (D) [Ce(Ofl)2(H2O)2]SO4.4H2O.

Table 3.UV-vis spectra of ofloxacin (HOfl.) and its metal complexes.

The 1H NMR Spectra

The 1H NMR spectra of ofloxacin and its metal complexes in DMSO-d6 are measured (Fig. 3) and analyzed to confirm the complexes formation (Table 4). A survey of the spectral data reveals chemical shifts of the proton in the complexes spectra relative to the free ligand. The ligand shows a peak at 15.12 ppm due to the carboxylic proton.10 This peak is absent in the spectra of the complexes due to the deprotonation of the carboxylic group. Hence, NMR results support the IR inferences and confirms the coordination of ofloxacin through oxygen of the carboxylic group. Also, according to the 1H NMR data for all complexes δ 4.35−4.95 [H, H2O] which not found in the free ofloxacin (Table 4), indicate the presence of water molecules in all complexes.2122

Figure 3.1H NMR spectra for (A) ofloxacin, (B) [Ti(Ofl)2(H2O)2]SO4.3H2O, (C) [Y(Ofl)2(H2O)2]Cl.7H2O and (D) [Ce(Ofl)2(H2O)2]SO4.4H2O. in DMSO, δTMS.

Table 4.1H NMR values (ppm) and tentative assignments for (A) ofloxacin, (B) [Ti(Ofl)2(H2O)2]SO4.3H2O, (C) [Y(Ofl)2(H2O)2]Cl.7H2O and (D) [Ce(Ofl)2(H2O)2]SO4.4H2O

Thermal Studies

Thermogravimetric (TG) and differential thermogravimetric (DTG) analysis were carried out under flow of N2 and at a heating rate of 10 ℃ min−1 over the temperature range of 25−800 ℃ in order to study the decomposition and structure of the ligand and the formed solid complexes. Fig. 2A-D show the thermograms of ofloxacin, [Ti(Ofl)2(H2O)2]SO4.3H2O, [Y(Ofl)2(H2O)2]Cl.7H2O and [Ce(Ofl)2(H2O)2]SO4.4H2O, respectively. Table 5 summarizes the thermogravimetric data for all compounds. The maximum temperature values along with the weight losses for each compound within the corresponding temperature ranges are listed in Table 5. These data support the proposed complexes formulae and also indicated that the decomposition of ofloxacin occurs in one main degradation step and is accompanied by a weight loss of 93.35% corresponding to the loss of 8C2H2+HF+NH3+2NO2.

The determined temperature ranges, percent mass losses, and thermal effects accompanying the changes in the coordination compounds on heating revealed the following findings. The TG curves of the three solid complexes show two decompostion ranges of 50−150, 50−200 and 150−600, 200−700, 200−600 ℃, respectively. The first estimated mass losses for all the complexes may be attributed to the loss of three, seven and four water of crystalization. The second estimated mass losses is attributed to the decomposition of two coordinated water and ofloxacin molecules.3233

The proposed structure formula on the basis of the results discussed in this paper located in Scheme 2:

Figure 4.TGA and DTG diagrams for (A) ofloxacin, (B) [Ti(Ofl)2(H2O)2]SO4.3H2O, (C) [Y(Ofl)2(H2O)2]Cl.7H2O and (D) [Ce(Ofl)2(H2O)2]SO4.4H2O.

Kinetic Data

The kinetic thermodynamic parameters were evaluated using the following methods and the results obtained by these methods are compared with one another.

Table 5.Thermogravimetric data of HOfl and their metal complexes

Scheme 2.The coordination mode of Ti(IV), Y(III) and Ce(IV) with ofloxacin.

The Coats-Redfern equation34

Where α and φ are the fraction of the sample decomposed at time t and the linear heating rate, respectively, R is the gas constant, and E* is the activation energy in KJ mol−1. A plot of left side against 1/T was found to be linear. From the slope, E*, was calculated and A (Arrhenius constant) can be deduced from the intercept. The enthalpy of activation, ΔH*, and Gibbs free energy, ΔG*, were calculated from

The Horowitz-Metzger equation35

Where θ=T−Ts, w𝛄=wα-w, wα=mass loss at the completion of the reaction; w=mass loss up to time t. The plot of log versus θ was drawn and E* was calculated from the slope. The pre-exponential factor, A, was calculated from the equation

The data are summarized in Table 6. The greater positive values of activation energies (E*) reflect the thermal stability of the complexes and the positive ΔH* values postulate an endothermic nature of the formed complexes. The entropy of activation was found to have negative values in all the complexes which indicate that the decomposition reactions proceeded with a lower rate than the normal ones and activated complexes have more ordered systems than reactants or the reactions are slow.3637

Biological Activities Test

The antibacterial studies suggested that, the metal complexes were found to be biologically active and showed significantly enhanced antibacterial nearly for all microbial species except for S. aureus K1 (non-detectable) (Table 7). A possible mode for increase in antibacterial activity may be considered in light of Overton's concept38 and Tweedy's chelation theory.39 Coordination reduces the polarity of the metal ion mainly because of the partial sharing of its positive charge with the donor groups4041 within the chelate ring system formed during coordination. This process, in turn, increases the lipophilic nature of the central metal atom, which favors its permeation more efficiently through the lipid layer of the micro-organism42 thus destroying them more aggressively.

Table 6.acorrelation coefficients of the Arrhenius plots and bstandard deviation.

Table 7.ND: non-detectable. i.e., the inhibition zones exceeds the plate diameter (-): no activity observed aginst microbial bacteria species Statistical significance PNS P not significant, P<0.05; P+1 P significant, P>0.05; P+2 P highly significant, P> 0.01; P+3 P very highly significant, P >0.001; student’s t-test (Paired)

A Comparison between Ti(VI), Y(III) and Ce(IV) Biological Results and Zr(IV),V(IV) and U(VI)16 Biological Results

The biological results we obtained of this paper16 showed that Ti(IV), Y(III) and Ce(IV) complexes had a strong effect on E. coli K32, P. aeruginosa SW1 and Klebsiella oxytocaK42 and a weak effect on S. aureus K1, B. subtilis K22, Br. otitidis K76, this was compared with the biological results of Zr(IV), V(IV) and U(VI) complexes we obtained.

Figure 5.Statistical representation for biological activity of ofloxacin and its metal complexes.

 

COMPUTATIONAL DETAILS

Computational Method

The geometric parameters and energies were computed by DFT at the B3LYP/CEP-31G level, using the GAUSSIAN 98W package of programs, on geometries that were optimized at CEP-31G basis set. The high basis set was chosen to detect the energies at a highly accurate level. The atomic charges were computed using the natural atomic orbital populations. The B3LYP is the hybrid functional,43 which is a linear combination of the gradient functionals proposed by Becke44 and Lee et al.,45 together with the Hartree-Fock local exchange function.46

Structural Parameters and Models

Ofloxacin

The biological activity of floroquinolones (ofloxacin) is mainly determined by its structure, the ofloxacin has many characteristic structural features. Table 8 gives the optimized geometry of ofloxacin as obtained from B3LYP/CEP-31G calculations. The molecule is a highly sterically-hindered. Values of geometric parameters (bond lengths and bond angles) of ofloxacin are calculated by using B3LYP/CEP-31G and compared with parameters experimentally obtained from crystal geometries.47 The methyl group and piperazine ring are out of plane of the molecule. This observation is supported by the values of calculated dihedral angles for: C4N5C11C26 and O13C12C11C26 are 86.93° and 95.81°, respectively, while the dihedral angles for C14C15N19C22 and C15N19C22C23 are −55.94° and 116.88°, respectively, where the values are neither zero nor 180. Fig. 6, shows the optimized structure of ofloxacin, the dihedral angles for C4C3C7O9 and C1C3C7O8 are −0.03° (almost 0.0) and 0.58° (almost 0.0). Also, C1C3C7O9 and C7C3C1O10 are −179.54° and −0.16° which confirms that the O8 and O10 lying in the same direction. The C7O8 bond lying in the same plane of C1O10 bond while O9 and O10 are lying in the opposite direction to each other so, the C1O10 of carbonyl group of pyridone not in the same plane of C7O9 of carboxlate group. The ionized bond lengths of C−H of piperazine and benzoxazine rings are calculated to range from 1.09 to 1.12 and from 1.102 to 1.104 as compared to the observed values of 1.422 and 1.440, respectively.

Table 8.Equilibrium geometric parameters bond lengths (Å), bond angles (o), dihedral angles (o) and charge density of ofloxacin ligand by using DFT/B3LYP/Cep-31G

Figure 6.The optimized geometrical structure of Ofloxacin by using B3LYP/CEP-31G.

The bond distance of C7−O8 and C7−O9 are 1.38Å and 1.26Å while, C1−O10 is 1.27Å. The value of bond angle C1C3C7 is 126.30° reflects a sp2 hybridization of C3, the same result is obtained with C1 and C7. The values of bond distances are compared nicely with that obtained from X-ray data.47

Charge distribution on the optimized geometry of ofloxacin is given in Table 8. There is a significant built up of charge density on the oxygen atoms which distributed over all molecule so ofloxacin molecule behaves as bidentate ligand (Opyr and Ocar) and molecule is not highly negative dipole μ=9.186D and the energy value of the optimized geometry of ofloxacin molecule is −222.06au.

The Yttrium-ofloxacin complexes

The Y(III) chelated with two molecules of ofloxacin through four oxygen atoms (Opyr and Ocar atoms) forming four coordinate bonds. The complex is six-coordinate with four coordinate bonds with two ofloxacin molecules and the other two coordinate bonds may be water and chloride ion or two water molecules. We studied [Y(Ofl)2(H2O)Cl] and [Y(Ofl)2(H2O)2]+.

Description of the structure of [Y(Ofl)2(H2O)Cl]:

The structure of complex with atomic numbering scheme is shown in Fig. 7. The complex consists of two units of ofloxacin molecule and one water molecule beside one chloride ion with metal ion Y(III). The complex is six-coor-dinate with distorted octahedral environment around the metal ion. The metal ion Y(III) is coordinated to one Opyr atom and one Ocar atom of ofloxacin ligand and one OH2O atom for water beside one chloride ion. The bond distance between Y−O3 and Y−O8 are 2.245Å and 2.238Å51 while the distance between Y−OH2O is 2.308Å52 and the distance between Y−Cl12 is 2.597 Å.5152 Also, the angles around the central metal ion Y(III) with surrounding oxygen atoms and chloride ion vary from 78.54° to 164.00°; these values differ legally from these expected for a distorted octahedron. The distances and angles in the quinolone ring system, as well as those of piprazin rings are similar to those found in reported structure of free ciprofloxacin.51 The energy of this complex is −596.53 au and total dipole moment is 18.84 D (Table 9).

Figure 7.Optimized geometrical structure of [Y(Ofl)2(H2O)Cl] complex by using B3LYP/CEP-31G.

Table 9.Equilibrium geometric parameters bond lengths (Å), bond angles (o) and charge density of [Y(Ofl)2(H2O)Cl] by using DFT/ B3LYP/CEP-31G

Description of the structure of [Y(Ofl)2(H2O)2]+:

The structure of complex with atomic numbering scheme is shown in Fig. 8. The complex consists of two units of ofloxacin molecule and two water molecules with Y(III). The complex is six-coordinate with distorted octahedral environment around the metal ion. The Y(III) is coordinated to one Opyr atom and one Ocar atom of ofloxacin ligand and two water molecules. The Y−O3 and Y−O8 bond lengths are 2.249Å and 2.193Å,38 while the Y−OH2O bond length is 2.321Å.52 Also, the angles around the central Y(III) with surrounding oxygen atoms vary from 76.62° to 154.45°; these values differ legally from these expected for a distorted octahedron. The distances and angles in the quinolone ring system, as well as those of piprazin rings are similar to those found in reported structure of free ciprofloxacin.51

Figure 8.Optimized geometrical structure of [Y(Ofl)2(H2O)2]+ complex by using B3LYP/CEP-31G.

The bond distances between Y(III) and surrounded oxygen atoms of ofloxacin in water complex are shorter than that in chloride complex, so that the Y(III) metal ion is bonded strongly with surrounded oxygen atoms of ofloxacin in water complex more than that in chloride complex. Also, the charge accumulated on Ocar are −0.375 and −0.376 and for Opyr are −0.333 and −0.332, for water complex while, for the chloride complex the charge on Ocar are −0.378 and −0.389 and Opyr are 0.374 and 0.413 (Table 10). There is a strong interaction between central Y(III) metal ion which has become charge equal 0.155 and more negative oxygen atoms in water complex greater than that in chloride complex. The energy of this complex is −549.149 au and total dipole is 12.25D. For all these reasons the water complex is more stable than chloride complex and Y(III) favor coordinated with two water molecules more than water and one chloride ion to complete the octahedron structure. These data agree quite well with the experimental data which indicate that the complex is electrolyte and the chloride ion found outside the complex sphere. These data agree quite well with the experimental data which indicate that the complex is electrolyte and the chloride ion found outside the complex sphere.

Table 10.Equilibrium geometric parameters bond lengths (Å), bond angles (o) and charge density of [Y(Ofl)2(H2O)2]+ by using DFT/ B3LYP/CEP-31G

The Titanium (IV)-ofloxacin complexes:

The Ti(IV) chelated with two molecules of ofloxacin through two coordinate bonds (Opyr and Ocar atoms) from each molecule. The experimental data set that the result complex is six-coordinate so, the complex consists of four coordinate bonds with two ofloxacine molecules and there are other two coordinated bonds may be with water molecules or with two oxygen atoms of sulphato group. In this part we study theoretically the two structures of [Ti(Ofl)2(SO4)] and [Ti(Ofl)2(H2O)2]2+.

Description of the structure of [Ti(Ofl)2(SO4)]

Fig. 9 shows the optimized geometrical structure of the complex with the atomic numbering scheme selected bond distances and angles are given in Table 11.

Figure 9.Optimized geometrical structure of trans-isomer of [Ti(Ofl)2(SO4)] complex by using B3LYP/CEP-31G.

The titanium ion, at a crystallographic inversion center, is in a distorted octahedral environment. In the equatorial plane the metal ion is coordinated by four oxygen atoms (Opyr and Ocar) of two ofloxacin ligand at the distances vary from 1.934Å to 2.091Å, these bond lengths are similar to those observed in related compounds.55−59 The difference in the carboxylate O58−C2 and O3−C2 (1.349Å and 1.211Å),61 confirms the formation of bond between the ionic carboxylate oxygen atom and titanium ion. The octahedral coordination environment is completed by two oxygen atoms of sulphato group. The bond distance between Ti−O58 is 1.938Å29−31 and Ti−O6 is 2.091Å,5859 while the distance between Ti−O of sulfato group are 1.843 and 1.852Å.4748 The bond angles around the central metal ion Ti(IV) vary from 70.35° to 177.10°; these values differ significantly from these expected for a regular octahedron.

Table 11.Equilibrium geometric parameters bond lengths (Å), bond angles (o) and charge density of [Ti(Ofl)2(SO4)] by using DFT/B3LYP/CEP-31G

In the equatorial plane the titanium ion bonded with two oxygen atoms (O27 and O32) of one ofloxacin molecule in the same plane which perpendicular to the other plane occupied by other two oxygen atom (O6 and O58) of the second ofloxacin molecule. The bond angle O6−Ti−O27 is 90.89° and O56−Ti−O32 is 88.02°. The sulphato group not lying in the same plane but out of plane in twisting form, the bond angle O56−Ti−O6 is 169.24°, so the oxygen atom of sulphato group lying trans respect to one oxygen atom (Opyr) of one ofloxacin molecule, while the angle O57−Ti−O27 is 176.05°, so the other oxygen atom of sulphato group lying trans respect to the oxygen atom (Ocar) of other ofloxacin molecule. The dihedral angle O54−S53− O57−Ti is 108.12° and O55−S53−O57−Ti is −110.74, which means that the two oxygen atoms of sulphato group (O54 and O55) lying in opposite direction to each other and out of plane occupied by other atoms.

The energy of this complex is −591.844 au and the dipole moment is weak (12.351D) so, this complex is less stable.

Description of the structure of [Ti(Ofl)2(H2O)2]2+

Table 12 lists selected inter atomic distances and angles. The structure of complex with atomic numbering scheme is shown in Fig. 10. The complex consists of two ofloxacin molecule and two water molecules with metal ion Ti(IV). The complex is six-coordinate with distorted octahedral environment around the metal ion. The Ti(IV) is coordinated to one Opyr atom and one Ocar atom of ofloxacin ligand and two OH2O atoms for water. The Ti−O3 and Ti−O29 bond lengths are 1.895Å and 1.889Å, respectively, which are shorter than that Ti−O8 and Ti−O34 (1.934Å and 1.951Å, respectively). Also, the angles around the central metal ion Ti(IV) with surrounding oxygen atoms vary from 68.98° to 178.38°; these values agree with a distorted octahedron. The two ofloxacin molecules are perpendicular to each other they are not lying in the same plane the bond angle O8−Ti−O34 is 89.69° and O3−Ti−O34 is 88.87°, which confirm that the two ofloxacin molecules not exist in the same plane. The two water molecules bonded with Ti(IV) not exist in trans position to each other but exist as cis to each other, the bond angle O2−Ti−O55 is 90.68°.

Table 12.Equilibrium geometric parameters bond lengths (Å), bond angles (o) and charge density of [Ti(Ofl)2(H2O)2]2+ by using DFT/B3LYP/CEP-31G

The metal ion Ti(IV) is bonded strongly with surrounded oxygen atoms of ofloxacine in water complex more than that in sulfate complex. Also, the charge accumulated on Ocar (−0.441) and Opyr (−0.389), in water complex while, Ocar (−0.439) and Opyr (−0.334), in sulphate complex. There is a strong interaction between central metal ion Ti(IV) which become has charge equal +0.937 and more negative oxygen atoms in water complex greater than that in sulfate complex, at which Ti(IV) becomes has less positively charge (+0.895) in sulfate complex. The energy of this complex is −609.183 au and highly dipole 21.394D. For all these reasons the water complex is more stable than sulfate complex and Ti(IV) favor coordinated with two molecules of water more than one sulphato group to complete the octahedron structure.

Figure 10.Optimized geometrical structure of trans-isomer of [Ti(Ofl)2(H2O)2]2+ complex by using B3LYP/CEP-31G.

The Cerium(IV)-ofloxacin complexes:

The Ce(IV) chelated with two ofloxacin molecules through four oxygen atoms (Opyr and Ocar atoms) forming four coordinate bonds. The experimental data shows that the complex is six-coordinate so, the complex consists of four coordinate bonds with two ofloxacin molecules and the other two coordinated bonds may be water molecules or sulphato group. In this part we study theoretically the two structures of [Ce(Ofl)2(SO4)] and [Ce(Ofl)2(H2O)2]2+.

Description of the structure of [Ce(Ofl.)2(SO4)]

Fig. 11 shows the optimized structure of complex with atomic numbering schema. Table 13 lists selected inter atomic distances and angles. The complex consists of two ofloxacin molecule and one sulphate group with Ce(IV) ion. The complex is six-coordinate with distorted octahedral environment around the metal ion. The Ce(IV) metal ion is coordinated to one Opyr atom and one Ocar atom of two ofloxacin molecules in equatorial plane. In the equatorial plane the metal ion is coordinated by four oxygen atoms (Opyr and Ocar) of two ofloxacin molecules at the distances vary from 2.311Å to 2.357Å, these bond lengths are similar to those observed in related compounds.62−66 The difference in the bond length for the carboxylate O58−C2 and O3−C2 (1.349Å and 1.212Å),60 confirms the formation of bond between the ionic carboxylate oxygen atom and Cerium ion. The octahedral coordination environment is completed by two oxygen atoms of sulfato group. The bond distance between Ce−O58 and Ce−O6 are 2.314Å6566 and 2.355Å63−65 while the distance between Ce−O of sulphato group is 1.992−1.993Å.67−69 The bond angles around the central metal ion Ce(IV) vary from 87.52° to 166.13°; these values differ significantly from these expected for a distorted octahedron.

Figure 11.Optimized geometrical structure of trans-isomer of [Ce(Ofl)2(SO4)] complex by using B3LYP/CEP-31G.

Table 13.Equilibrium geometric parameters bond lengths (Å), bond angles (o) and charge density of [Ce(Ofl)2(SO4)] by using DFT/B3LYP/CEP-31G

In the equatorial plane the cerium ion bonded with two oxygen atoms (O27 and O32) of one ofloxacin molecule in the same plane which perpendicular to the other plane occupied by other two oxygen atom (O6 and O58) of ofloxacin molecule. The bond angle O6−Ce−O27 and O56− Ce−O32 are 91.08° and 87.52°. The sulphato group not lying in the same plane but out of plane in twisting form, the bond angle O56−Ce−O6 is 166.13°, so the oxygen atom of sulphato group lying trans respect to one oxygen atom (Opyr) of one ofloxacin molecule, while the angle O57−Ce−O27 is 174.31°, so the other oxygen atom of sulfato group lying trans respect to the oxygen atom (Ocar) of other ofloxacin molecule. The dihedral angle O54−S53− O57−Ce and O55−S53−O57−Ce are 105.31° and −109.99°, which means that the two oxygen atoms of sulphato group O54 and O55 lying in opposite direction to each other and out of plane occupied by other atoms. The energy of this complex is −517.05 au and the dipole moment is weak 8.431D, so this complex is less stable.

Description of the structure of [Ce(Ofl)2(H2O)2]2+

Table 14 lists selected bond distances and angles. The optimized geometrical structure of complex with atomic numbering schema is shown in Fig. 12. The complex consists of two ofloxacin molecule and two water molecules with metal ion Ce(IV) ion. The complex is six-coordinate with distorted octahedral environment around the metal ion. The Ce(IV) is coordinated to one Opyr atom and one Ocar atom of ofloxacin ligand and two OH2O atoms for water. The Ce−O3 and Ce−O29 bond lengths are (2.309Å and 2.315Å, respectively) are shorter than that Ce−O8 and Ce−O34 (2.347Å and 2.351Å, respectively). Also, the angles around Ce(IV) with surrounding oxygen atoms vary from 86.48° to 178.57°; these values agree nicely with a regular octahedron. The two ofloxacin molecules are perpendicular to each other they are not lying in the same plane the bond angle O8−Ce−O29 is 86.48° and O3−Ce−O34 is 88.88°, which confirm that the two ofloxacin molecules not exist in the same plane. The two water molecules bonded with Ce(IV) not exist in trans position to each other but exist as cis to each other, the bond angle O2−Ce−O55 is 91.31°.

Table 14.Equilibrium geometric parameters bond lengths (Å), bond angles (o) and charge density of [Ce(Ofl)2(H2O)2]2+ by using DFT/B3LYP/CEP-31G

Figure 12.Optimized geometrical structure of trans-isomer of [Ce(Ofl)2(H2O)2]2+ complex by using B3LYP/CEP-31G.

The Ce(IV) is bonded strongly with surrounded oxygen atoms of ofloxacin in water complex more than that in sulphato complex. Also, the charge accumulated on Ocar (−0.461) and Opyr (−0.368), in water complex while, Ocar (−0.423) and Opyr (−0.321), in sulphato complex. There is a strong interaction between central metal ion Ce(IV) which become has charge equal +0.889 and more negative oxygen atoms in water complex greater than that in sulphato complex, at which Ce(IV) becomes has less positively charge (+0.738) in sulphato complex. The energy of this complex is −593.519 au and highly dipole 19.34D. For all these reasons the water complex is more stable than sulphato complex and Ce(IV) favor coordinated with two molecules of water more than one sulphato group to complete the octahedron structure.

A Comparison Between Ti(IV), Y(III) and Ce(IV) and Zr(IV), V(IV) and U(VI) [16] in Theory and Experiment

As for Spectroscopic, thermal analyses, structural and antibacterial studies on the interaction of some metals with ofloxacin,16 we studied if the two oxygen of atoms of ofloxacin ligand of [VO(Ofl.)2(H2O)].5H2O and [ZrO(Ofl.)2(H2O)].4H2O were in cis or trans position. We found the energy of the most stable trans Oc isomer [ZrO(Ofl.)2(H2O)].4H2O complex is −1009.915 au and the dipole moment was 21.09 D and for [UO2(Ofl.)2(H2O)].2H2O complex the energy was −552.623 au and the total dipole moment is 12.81 D. So that Zr(IV) and V(IV) preferred trans isomer (trans Oc) which were more stable than cis Oc.

On the other hand in this paper we studied SO4−2 or Cl− of [Ti(Ofl)2(H2O)2]SO4.3H2O, [Y(Ofl)2(H2O)2]Cl.7H2O and [Ce(Ofl)2(H2O)2]SO4.4H2O complexes were located outside or inside the complex sphere. We experimentally found that SO4−2 or Cl− ions are located outside the complex sphere so we tried to prove this in theory which indicated that SO4−2 or Cl− are located outside the complex sphere for the three complexes.

 

CONCLUSION

The complexes of Y(III), Ti(IV) and Ce(IV) with ofloxacinin the solid state were synthesized. Infrared and 1H NMR spectra of the complexes indicate bonding of the metal ions to ofloxacin forming complexes through two pyridone and two carboxylic oxygen atoms of two ofloxacin molecules. Thermal analysis indicates that water molecules are included as lattice and coordinated water in the crystal structure. Magnetic susceptibility measurements and molar conductivities show that the complexes are diamagnetic and electrolyte in nature. The results of the biological activities of the complexes indicate that the complexes showed activity compared with ofloxacin against all bacterial species except S. aureus KI.

The ofloxacin has two donating centers Opyr and Ocar when chelated with Yttrium ion Y(III) there are six-coordinated bonds are formed four with two ofloxacin molecules and other two with two water molecules. The water complex is more stable than chloride complex for some reasons (i) bonds between Y(III) with surrounded oxygen atoms in water complex are shorter and stronger than that others in chloride complex. (ii) in water complexe there are more negative charges are accumulated over oxygen atoms and large positive charge is formed over central metal ion. (iii) water complex is highly dipole greater than chloride complex. The product complex is treated as distorted octahedral complex. Also for the same reasons Ce(IV) and Ti(IV) ions favor coordinated with two water molecules more than one sulphato group to complete the octahedron structure.