Synthesis and Characterization of Nanosized Uranyl Coordination Polymers derived from Terephthalic acid and Azoles

 Introduction

Currently, the coordination polymers span scientific fields such as organic and inorganic chemistry, biology, materials science, electrochemistry, and   pharmacology, having many  potential applications [1-5]. 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 [11-16]. 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 UO₂²⁺ 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(ІІ) coordination polymers containing 1,4-benzenedicarboxylate and azole ligands are described.

Figure 1: Structures of the ligands  Click here to View figure

 

Experimental

Material and methods

All chemical were of analytical grades. 1,4-Benzenedicarboxylic acid, 2-aminobenzo-thiazol, 2-aminothiazol, 2-amino-4-methyl-thiazol and 2-mercaptobenzothiazole were E. Merck grade. Were purchased and used without purification.

Preparation of the complexes

Preparation of the mixed ligand coordination polymers of TPA and azoles with UO₂(acetate)  follows essentially the same procedure, [UO₂(TPA)(ABZ)(H₂O)] is typical. To an ethanolic solution (15 ml) of UO₂(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 CaCl₂ anhydrous.

 Stoichiometric analysis(C,H,N,S) were performed using Analyischer Funktionstest Vario El Fab-Nr.11982027 elemental 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^-1. Molar susceptibilities were corrected for diamagnetism of the component atoms by the use of the Pascal ‘s constants . The calibrant used was Hg[Co(SCN)₄]. XRD Model PW 1710 control unit  Philips, Anode material Cu 40 K.V 30 M.A Optics: Automatic divergence slit.  Scanning electron microscope (JEOL JFC-1100E ION SPUTTERING DEVICE, JEOL JSM-5400LV SEM). SEM specimens were coated with gold to increase the conductivity. For the biological activity all microbial strains were kindly provided by the Assiut University Mycological Centre (AUMC), Egypt. These strains are common contaminants of the environment in Egypt and some of which are involved in human and animal diseases (Candida albicans, Geotrichum candidum, Scopulariopsis brevicaulis, Aspergillus flavus, Staphylococcus aureus), plant diseases (Fusarium oxysporum) or frequently reported from contaminated soil, water and food substances (Escherichia coli, Bacillus cereus, Pseudomonas aeruginosa and  Serratia marcescens). To prepare inocula for bioassay, bacterial strains were individually cultured for 48h in 100 ml conical flasks containing 30 ml nutrient broth medium. Fungi were grown for 7 days in 100 ml conicals containing 30 ml Sabouraud’s dextrose broth. Bioassay was done in 10 cm sterile plastic Petri plates in which microbial suspension (1 ml/plate) and 15 ml appropriate agar medium (15 ml/plate) were poured. Nutrient agar and Sabouraud’s dextrose agar were respectively used for bacteria and fungi. After solidification of the media, 5 mm diameter cavities were cut in the solidified agar (4 cavities/plate) using sterile cork borer. The chemical compounds dissolved in dimethyl sulfuxide (DMSO) at 2%w/v (=20 mg/ml) were pipetted in the cavities (20 ul /cavity). Cultures were then incubated at 28°C for 48 h in case of bacteria and up to 7 days in case of fungi. Results were read as the diameter (in mm) of inhibition zone around cavities.

Result and Discussion

The complexes were prepared by the reaction of TPA (1,4-benzenedicarboxylate) neutralized with NaOH, UO₂(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 : Azoles  to yield the corresponding compounds according to the following equation:

 

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).

Table 1: Colors, Elemental Analysis And Melting Points Of The Complexes.   Click here to View table

 

IR spectra

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 Δυ≤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 νC=N  for the complexes undergo no appreciable shift of ABZ 1640 cm^-1 [22]. 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(ІІ) ions [23]. The bands at 3410-3440  cm^-1 in the spectra of the complexes are assigned to νOH 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].

Table 2: Infrared Spectral Data Of Mixed Ligand Complexes.   Click here to View table

 

Figure 2: IR Spectrum of {[UO2(TPA)(MTBZ)(H2O)].H2O}n   Click here to View figure

 

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 π→π* within Azoles and TPA moieties, [27,23] respectively. In the visible spectra, there are characteristic bands attributed to the d-d transitions of the Uranyl complexes typical of  octahedral structures. Thus, the complexes 1, 2, 3 and 4 exhibit a d-d band in the range 15,188-18,825 cm^-1. Additionally, the magnetic moments of the compounds were measured and the uranyl complexes show diamagnetic values [28]. The results are shown in Table (3).

Table 3: Electronic Spectral Data Of The Complexes.   Click here to View table

 

Figure 3: Structure of the complex  [UO2(TPA)(ABZ)(H2O)]n   Click here to View figure

 

Thermal Studies

The thermal 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 UO₃ (calc. 47.48 %, found 46.12 %). Fig.(4).

Table 4: Thermal Decomposition Data Of The Complexes

 

 

Kinetic Analysis

Non-isothermal kinetic analysis of the complexes was carried out applying two different procedures: the Coats-Redfern [29] and the Horowitz-Metzger [30] methods.

Coats-Redfern equation

ln[1-(1-α)^1-n/ (1-n)T²] = M / T + B       for n ≠ 1                 (2)

ln[-ln(1-α)/ T²] = M / T + B                  for n = 1                 (3)

where α is the fraction of material decomposed, n is the order of the decomposition reaction and M = E/R and B = ZR/ΦE; E, R, Z and Φ are the activation energy, gas constant, pre-exponential factor and heating rate, respectively.

Horowitz-Metzger equation

ln[1-(1-α)^1-n/1-n] = lnZRT_s²/ΦE – E/RT_s+ Eθ/RT_s²  for n ≠ 1        (4)

ln[-ln(1-α)] = Eθ/RT_s²                           for n = 1                            (5)

where θ = T-Ts, Ts is the temperature at the DTG peak.

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 θ for equations (4) and (5). The kinetic parameters calculated according to the above two methods are cited in Table 5.

Table 5: Kinetic parameters of the thermal decomposition of the compounds.

---

This work is licensed under a Creative Commons Attribution 4.0 International License.