Synthesis and Characterization of Nanosized Uranyl Coordination Polymers Derived from Terephthalic Acid and Azoles

The structure of the complexes [UO2(TPA)(Azole)(H2O)].xH2O, TPA = 1,4-benzenedicarboxylic acid and azoles = 2-aminobenzothiazole, 2-aminothiazole, 2-amino-4-methylthiazole and 2-mercaptobenzothiazole has been prepared and characterized. The structure of the complexes has been assigned based on elemental analysis, IR, electronic spectral studies, magnetic measurement, molar conductance, Scanning electron microscope (S.E.M), X-ray powder diffraction techniques investigations and thermogravimetric analysis complete the characterization of the compound. Thermogravimetry(TG), derivative thermogravimetry (DTG) and differential thermal analysis (DTA) have been used to study the thermal decomposition of the complexes. The kinetic parameters have been calculated making use of the Coats-Redfern and Horowitz-Metzger. The scanning electron microscope SEM photographs and particle size calculations from the powder XRD data indicate the average size of the prepared UO2(II) (28-56 nm) supramolecular coordination polymers in the nanoscale range. The biological screening of the compounds was also tested.


INTRODUCTION
Currently, the coordination polymers span scientific fields such as organic and inorganic chemistry, biology, materials science, electrochemistry, and pharmacology, having many potential applications [1][2][3][4][5] .Terephthalic acid and its metal complexes have been a subject of interest in numerous studies because of their chemical and biological activities.They have a wide range of vehicle diverse characteristic properties attributed to their chelating ability with metal ions.Really, an important number of 1,4-benzenedicarboxylic acid being bidentate donors.Aromatic polycarboxylates, such as benzenedicarboxylate, have been well used to construct coordination frameworks 6 , by direct interaction with metal ions to form discrete polynuclear 7 , 1-, 2-and 3-D coordination networks 8 in a variety of coordination modes.Benzenedicarboxylate exhibits a variety of bridging modes and strong tendency to form large, tightly bound metal cluster aggregates and has been used as a building block to construct some porous coordination frameworks.In desiging 1-, 2-, 3-D extended porous coordination polymers, the selection of appropriate ligands is crucial to determining the structural outcome of target polymers.Carboxylates 9,10 tend to form relatively rigid frameworks due to the potential bidentate coordination of their carboxylate groups.Recently, mixed-linker systems of both carboxylates and another ligands have proved to be effective in preparation of coordination polymers [11][12][13][14][15][16] .The chemistry of uranium is the most developed, due to the low radioactivity of one of its isotopes and its wide use at industrial and laboratory scales 17 .Uranium(VI) tends to form linear cation with two oxygen atoms UO 2 2+ which can be further surrounded by a number of anions commonly laying in one plane, which is perpendicular to O-U-O structure [18].In this paper synthesis and characterization of uranyl(²²) coordination polymers containing 1,4-benzenedicarboxylate and azole ligands are described.

Preparation of the complexes
Preparation of the mixed ligand coordination polymers of TPA and azoles with UO 2 (acetate) follows essentially the same procedure, [UO 2 (TPA) (ABZ)(H 2 O)] is typical.To an ethanolic solution (15 ml) of UO 2 (acetate) (0.2 g , 4.7 mmol) a solution of TPA (0.078 g TPA in 15 ml 0.1M NaOH, 4.7 mmol) was added dropwise and with stirring, then ABZ solution (0.070 g in 15 ml ethanol, 2.4 mmol) was added to the mixture.The mixture was refluxed and then cooled to room temperature.Yellow precipitate separated, which filtered, washed with distilled water and EtOH and dried in over CaCl 2 anhydrous. .

Physical measurements
Stoichiometric analysis(C,H,N,S) were performed using Analyischer Funktionstest Vario El Fab-Nr.11982027elemental analyzer.The i.r spectra were recorded on a Shimadzu IR-470 spectrophotometer and electronic spectra were obtained using a Shimadzu UV-2101 PC spectrophotometer.Thermal studies were carried out in dynamic air on a Shimadzu DTG 60-H thermal analyzer at a heating rate 10 °C min

RESULT AND DISCUSSION
The complexes were prepared by the reaction of TPA (1,4-benzenedicarboxylate) neutralized with NaOH, UO 2 (acetate), 2-amino-benzothiazole(ABZ), 2-aminothiazole (AZ), 2-amino-4-methyl-thiazole (AMTZ) and 2-mercaptobenzothiazole (MBZT) (dissolved in EtOH).The prepared complexes were found to react in the molar ratio 1: 1: 1 metal : TPA  The compounds are air stable, insoluble in common organic solvents but partially soluble in DMF and DMSO .The conductivity values, measured in DMSO at room temperature, fall in the range for non-electrolytes 19 .The compositions of the complexes are supported by the elemental analysis recorded in Table (1).
The main IR frequencies can be seen in Table (2).The IR spectra of the prepared compounds show two bands in the ranges 1540-1568 and 1374-1400 cm -1 characteristic of the asymmetric and symmetric stretching vibrations of the carboxylic groups of the BDC coordinated to the metal center.Furthermore, the IR spectra of the complexes 1 -4 showed a separation value u∆≤190 cm -1 indicating a bidentate mode of coordination for the carboxylate group 20 .It is found that the CSC band of azoles at ~740 cm -1 is almost unchanged in the respective complexes, indicating that the thiazol-S is not involved in the bonding 21 .The stretching vibration of íC=N for the complexes undergo no appreciable shift of ABZ 1640 cm -122 .Furthermore, the stretching vibration of the amino group in free ABZ observed at 3220 cm -1 is shifted to a lower wave number and appears in the range 3150-3190 cm -1 in the complexes suggesting coordination of the amino nitrogen to the metal(²²) ions 23 .The bands at 3410-3440 cm -1 in the spectra of the complexes are assigned to íOH of the crystalline water molecules 24,25 .Metal-oxygen and metal-nitrogen bonding are manifested by the appearance of bands in the 520-536 cm -1 and 410-440 cm -1 regions respectively 26 .

Electronic Spectra and Magnetic Moments
The UV-Vis spectra of the complexes have been recorded in DMSO.The spectra display two distinct bands in the ranges 36,654-38,760 and 21,458-24,382 cm -1 which attributed to interaligand transitions and p !p* within Azoles and TPA moieties, [27,23] respectively.In the visible spectra, there are characteristic bands attributed to the  Additionally, the magnetic moments of the compounds were measured and the uranyl complexes show diamagnetic values 28 .The results are shown in Table (3).

Thermal Studies
The ther mal decomposition of the complexes has been investigated in dynamic air from ambient temperature to 750°C.As a representative example the thermogram of the complex 1 shows three decomposition steps.In the first step the coordinated water molecule is released (calc.2.99 %, found 3.02 %).The DTG curve displays this step at 92 °C and an exothermic peak appears at 94 °C in the DTA trace.The second and third steps correspond to the decomposition of ABZ and TPA.The final product was identified on the basis of mass loss consideration as UO 3 (calc.47.48 %, found 46.12 %).where a is the fraction of material decomposed, n is the order of the decomposition reaction and M = E/R and B = ZR/fE; E, R, Z and f are the activation energy, gas constant, preexponential factor and heating rate, respectively.The correlation coefficient r is computed using the least squares method for equations (2), ( 3), ( 4) and ( 5).linear curves were drawn for different values of n ranging from 0 to 2. The value of n, which gave the best fit, was chosen as the order parameter for the decomposition stage of interest.The kinetic parameters were calculated from the plots of the left hand side of equations ( 2), (3), against 1/T and against è for equations ( 4) and ( 5).The kinetic parameters calculated according to the above two methods are cited in Table 5.

X-ray powder diffraction
The X-ray powder diffraction patterns of the complexes were recorded.The crystal lattice parameters were computed with the aid of the computer program TREOR.The crystal data are recorded in Table (6) (Fig. 5).The nanosized of these coordination polymers were 28.56 nm.
The crystal data for the three compounds belong to the crystal system triclinic.The significant broadening of the peaks indicates that the particles are of nanometer dimensions (XRD of the compound 3 is depicted in Figure 5).Scherrer's equation was applied to estimate the particle size of the coordination polymers: where K is the shape factor, l is the X-ray wavelength typically 1.54 Å, b is the line broadening at half the maximum intensity in radians and q is Bragg angle and D is the mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size.The crystal data together with particle size are collected in Table (6).The average size of the particles lies in the range 28-56 nm for compounds.

Biological Activity
Terephthalic acid and some ternar y coordination polymers were tested against some five bacterial and six fungal species (Tables 7).The data showed that in some cases the compounds have a similar antimicrobial activity than the selected standards (chloramphenicol and clotrimazole ).These bacteria and fungi strains are common contaminants of the environment in Yemen e.g.some of which are frequently reported from contaminated soil, water and food, or involved in human and animal diseases.

Scanning Electron Micrographs ( S.E.M)
The scanning electron micrograph of UO 2 complex 1 is depicted in Fig. 6.The figure shows the spherical shape of the nanoparticles which are aggregated to some extent.

CONCLUSIONS
A number of new one-dimensional nanosized UO 2 (II) metal supramolecular coordination polymers of TPA (1,4-benzenedicarboxylate) and azoles has been prepared and characterized.