Luminescent Complexes of Co(II), Ni(II), Cu(II), Zn(II) and Bi(III) with 2-Aminothiazole based Ligand and their Biological Studies

3-(thiazol-2-yl carbamoyl) propanoic acid was synthesized by the reaction of 2-aminothiazole and succinic anhydride, which was employed as ligand (LH) for further reaction with Co(II), Ni(II), Cu(II), Zn(II) and Bi(III) chlorides at ambient conditions to afford respective metal complexes, denoted as Co-L2, Ni-L2, Cu-L2, Zn-L2 and Bi-L3. Both the ligand LH and metal complexes were characterized by techniques FT-IR, H-NMR, C-NMR, powder XRD, TGA and conductometry. FT-IR and NMR studies revealed that ligand (LH) behaved in anisobidentate fashion via carboxylate moiety. The ligand and metal complexes are also screened for their antibacterial and antifungal potential and found non-significant activities by most of the complexes, luminescent studies were also made and found Bi-L3 appreciably luminescent.

; m.p: 227°C), KOH were used without purification. Pyrex made oven dried glassware was utilized for synthesis and subsequent workup process. Gallon Kamp apparatus was used to measure melting point. FTIR spectra were recorded using Cary 630 FT-IR Agilent Technologies. UV-visible spectra were recorded by Perkin Elmer multimode T90+ model double beam spectrophotometer. Metal contents were determined by Atomic absorption spectrophotometer GBC model 400 XplorAA Dual and Conductivity Meter Cond 720. Thermal analysis was carried out on a TGA/DSC1 (Mettler Toledo AG, Analytical, Schwerzenbach, Switzerland). The TGA measurements were performed under nitrogen at a heating rate of 10 °C/min. XRD measurements were performed on a D2 Phaser X-ray diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.5418 Å).

Synthesis
LH was synthesized using previous protocol with slight modifications [25,26]. Drop wise addition of succinic anhydride solution in acetic acid (1 mmol, 0.1001g) was added dropwise into the acetic acidic solution of 2-aminothiozole (1 mmol, 0.10 g). The solution was stirred over night at room temperature The resulting solution was concentrated at 40 °C for few hours and straw colored precipitates were obtained. These precipitates were washed with small amount of methanol and n-hexane, recrystallized from hot methanol, dried and stored in desiccator for further use.

General method for preparation of metal complexes
Equimolar solutions of the prepared ligand (LH) (0.1 mmol, 10 mL) and corresponding metal salt (0.1 mmol, 5 mL) was prepared in methanol (0.1 mmol, 10 mL). In first step, deprotonation of LH was done by the addition of small volume of KOH solution gradually and pH was maintained at 7. Then respective metal salt solution was added along with continuous stirring. The resulting mixture was stirred at room temperature for 1 hour and then concentrated up to half. This concentration resulted precipitates, which were washed with small amount of methanol and n-hexane and then dried. For further analysis, these were stored in desiccator.

Results and discussions
The ligand LH was obtained in appreciable yield (42%) by reacting succinic anhydride and 2-aminothiazole at room temperature (Scheme 1). LH was subsequently deprotonated and reacted with respective metal salts to afford metal complexes in appreciable to good yields (45-82%). All the metal complexes as well as ligand L are quite stable towards air. Both the ligand L and metal complexes have good solubility in common organic solvents but remarkable solubility in DMSO and DMF. Conductometric studies of metal complexes (0.1 -0.5 S) revealed their non-electrolytic nature (Table  1.) [27,28]. The metal contents were determined on the basis of AAS/ICP-MS that showed Co-L2, Ni-L2, Cu-L2 & Zn-L2 and Bi-L3 type complexes. The prepared ligand LH is amisobidentate in behavior, explicated by FT-IR (detail below in FT-IR section).

UV-visible spectral analysis and Magnetic moment
UV-visible spectra of ligand LH and the complexes were obtained in DMF as solvent using 10 -3 concentration solutions. The electronic spectrum of Cu-L2 complex shows an absorption band at 1323.3 cm -1 that can be assigned to 2 T2  2 E transition of a 4-coordinate tetrahedral geometry while that of Ni-L2 complex exhibits two absorption bands at 1166.6 cm -1 , 1541.2 cm -1 assigned to 3 T1(F)  3 A2(F), 3 T1(F)  3 T1(P) transitions suggesting tetrahedral environment around it. Similarly, electronic spectrum of the Co-L2 complex depicts two allowed transitions at 1169.8 cm -1 , 1740.5 cm -1 assignable to 4 A2(F)  4 T1(F) and 4 A2(F)  4 T1(P) transitions respectively. These three transitions are in good agreement with tetrahedral stereochemistry [27,28].
The magnetic moment values for Cu-L2, Ni-L2, and Co-L2 complexes are 1.89 B.M., 3.31 B.M., 4.24 B.M respectively. These values also support 4-coordinate tetrahedral geometry around each ion as proposed by electronic spectroscopy [29,30]. Also, Zn-L2 complex has d 10 configuration and was found to be diamagnetic. The diamagnetic nature of this complex supports its tetrahedral geometry. Bi-L3 complex exhibited octahedral geometry as proposed on the basis of M-L ratio obtained from ICP-MS/AAS and corroborative with CHN analysis.

FT-IR Spectral analysis
FT-IR spectra were recorded for the ligand LH as well as metal complexes in the range 4000 -700 cm -1 and are shown in Figure 1. The FTIR spectrum of LH showed characteristic peak at 3105 cm -1 attributed to NH group. Also a clear peak at 1300 cm -1 is labeled as C-N. The presence of carboxylate moieties in LH, was confirmed by peaks at 1699 and 1364 cm -1 attributed to symmetric and asymmetric vibrations respectively. Both types of these peaks in LH support its proposed structure. The comparison between the frequencies of LH and its metal complexes showed a shift that indicates the contribution of COOmoiety in the formation of complexes [27]. The difference, (v), between v(COO)asym and v(COO)sym lies between 350 and 200 , that directs an intermediate state between bi and mono-dentate named as anisobidentate [31]. Hence it is concluded from FTIR studies that ligand LH behave in a monodentate as well as bidentate fashion.

Powder X-ray Diffraction Studies
In the XRD spectral analysis of the ligand (LH) and the complexes, it is found that the ligand and the prepared complexes yield well-defined peaks indicating the crystalline nature of the complexes and the ligand (Figure 3). The appearance of crystallinity in the complexes can be attributed to the inherent crystalline nature of the metallic compounds. The calculation of the grain size of the complexes was carried out using Debye-Scherer's formula employing the full width at half maximum of the XRD peaks.

= 0.9λ βCosθ
where 'λ' is the wavelength, 'β' is the full width at half maximum and 'θ' is the peak angle. The prepared complexes are found to possess the average crystallite size in the range of 11-68 nm. The obtained average crystallite size was found to be 68, 39.9, 11.3, 14.1, 21.3 nm for Co-L2, Ni-L2, Cu-L2, Zn-L2 and Bi-L3 respectively suggesting that the complexes are nano-crystalline in nature.

TGA analysis
The thermal stability of the prepared complexes is studied by subjecting it to thermogravimetric analysis ( Figure 4). For instance; it is revealed that decomposition results of Cu-L2 complex takes place in three stages. The first stage of decomposition is noticed in the temperature range 120-135°C ( Figure 4). It is due to the elimination of coordinated water molecule [32]. The mass loss observed in this step is 4.45% against the calculated loss of 4.37%. The second stage of decomposition occurs in the temperature range of 210-235°C, due to melting and partial decomposition of the ligand, this step bringing the weight loss of 17% (calc. 17.76%). The decomposition range (310-335°C) of weight loss in third stage gives 70% (calc. 70.21%). This is due to the decomposition of the ligand molecule. The complex (residual product) is present in the form of its metal oxide above 400°C.

SEM Analysis
Scanning electron microscopy has been widely used to elucidate the morphological features of metal complexes. It has been observed that coordination of metal ion with ligand significantly changes the surface morphology of resulting metal complexes, the same was investigated by SEM analysis [33]. The SEM images of LH and its respective metal complexes are shown in Figure 5. The differences are seen in surface morphology of the ligand and metal complexes which can be used as corroborative evidence to support the formation of metal complexes. For instance; the ligand LH showed irregular sponge shaped morphology with porous nature, while Co-L2 showed shiny particles as an irregular aggregation. Similarly, the SEM micrograph of Ni-L2 showed small irregular shaped particles, entrapped in bigger one to constitute a cotton shape structure. Cu-L2 showed distribution of particles as clusters of irregular shape having some crystalline type particles. The SEM image of the Zn-L2 exhibited a polyhedral appearance with only a few particles that are aggregated. Bi-L3 showed distribution of particles arranged as rod shape. Biological studies Antibacterial studies: Antibacterial activity of LH and metal complexes was investigated against various gram positive and gram negative bacterial stains i.e. gram positive bacterial stains Bacilus subtilis, Staphylococcus aurenus and gram negative bacterial stains Escherichia coli, Pseudomonas aeruginosa, Salmonela typhi by disc diffusion method using 200 g/mL as minimum inhibitory concentration (MIC) [34]. The results presented in the table 2 depict that the ligand LH and most of the prepared metal complexes displayed non-significant activity, however the Co-L2 and Bi-L3 exhibited relatively significant activity (58.6% & 57.6%, respectively) only against E. coli. This enhanced antibacterial activity of metal complexes compared to LH can be explained on the basis of chelation theory, which says that upon chelation polarity of the complex decreases compared to ligand and also the π electron become more polarized. All this phenomenon facilitates the penetration of complexes into the bacterial cell that ultimately disturbs the internal system of cells and control their growth [35,36]. Overall this activity was much less than that of standard drug i.e Arythromycin (89.6%) ( Table 2).
Antifungal studies: Antifungal activity of LH and metal complexes against six fungal stains, (Candida albicans, Trichyton rubrum, Aspergilus niger, Microsporum canis, Fusarium lini and Candida glabrata) was investigated by agar tube dilution protocol [37]. MIC (minimum inhibition concentration) of samples was 400 g/mL. Overall the complexes exhibited non-significant activity against various strains except cobalt complex which showed appreciable inhibition (85%) that is comparable to that of standard drug miconazole (97.8%), against C. albicans (Table 3). This inhibition value can be justified on the basis of partial sharing and delocalized electron on metal and ligand in the complex that enhances the lipophilic property of complex and helps in lipid layer penetration that ultimately destroys the internal system of fungal stain [36,37].   The photoluminescence measurements of ligand LH and its complexes were carried out. The micromolar solution in methanol was sonicated for 2 min to form a homogeneous solution. The samples were excited at 350 and 420 nm to observe the photoluminescence phenomena. It has been observed that metal complex (Bi-L3, Zn-L2) shows enhanced PL intensity as compared to ligand ( Figure 6) that can be attributed to the π ̶ π * transitions present in the complexes [38,39]. The Zn-L2 complex too displayed mild fluorescence, however it was observed to be less than the ligand LH, itself. These shows quite different emission pattern when the PL data was measured at 420 nm. Comparative to ligand LH and other complexes, bismuth complex showed highest emission intensity which is actually a function of high SOC [40]. The emission peaks of ligand L and metal complexes were found in the range 410-425 nm, which can be attributed to the π ̶ π * transitions. The increase in emission intensity indicates an exciting opportunity of employing the complexes Bi-L3 and Zn-L2 for various photochemical applications.

Conclusions
A carboxylate containing ligand LH and its five metal complexes have been synthesized and structurally characterized. Ligand LH behaved in anisobidentate fashion via carboxylate moiety thus resulting four tetrahedral and one octahedral complex. All the metal complexes are nano-crystalline in nature as evident from PXRD studies. All of these complexes exhibited appreciable luminescence activity. Keeping in view their straightforward synthesis, these can be used as inexpensive precursors for their possible applications in the development of OLED materials, specifically the bismuth complex which is relatively less toxic compared to its neighboring atoms and high spin orbit coupling constant, a prerequisite for triplet harvesting.