From Synthesis to Biological Impact of Pd (II) Complexes: Synthesis, Characterization, and Antimicrobial and Scavenging Activity

The Pd (II) complexes with a series of halosubstituted benzylamine ligands (BLs) have been synthesized and characterized with different spectroscopic technique such as FTIR, UV/Vis, LCMS, 1H, and 13C NMR. Their molecular sustainability in different solvents such as DMSO, DMSO : H2O, and DMSO : PBS at physiological condition (pH 7.2) was determined by UV/Vis spectrophotometer. The in vitro antibacterial and antifungal activities of the complexes were investigated against Gram-positive and Gram-negative microbes and two different fungi indicated their significant biological potential. Additionally, their antioxidant activity has been analyzed with DPPH• free radical through spectrophotometric method and the result inferred them as an antioxidant. The stronger antibacterial and antioxidant activities of the synthesized complexes suggested them as a stronger antimicrobial agent. Our study advances the biological importance of palladium (II) amine complexes in the field of antimicrobial and antioxidant activities.


Introduction
After a tremendous discovery of cisplatin, the synthesis and biological evaluation of new transition metal-based compounds are fields of growing interest [1]. The palladium (II) as nonplatinum metal complexes highly attracted the researchers because of its significant biological activity as well as lower side effects along with higher lipophilicity or solubility compared to cisplatin [2][3][4][5]. Palladium metal is a suitable candidate for metallodrugs because it displays structural properties similar to those of platinum and also exhibits promising in vitro cytotoxicity. Numerous Pd (II) complexes with different benzylamine ligands have been synthesized and their interesting in vitro biological activities have been reported [6,7]. The antimicrobial activity of different palladium (II) complexes on the growth and metabolism of various groups of microorganisms has been studied and reported elsewhere. Garoufis et al. reviewed numerous scientific papers on antiviral, antibacterial, and antifungal activity of Pd (II) complexes with different types of ligands (sulfur and nitrogen donor ligands, Schiff base ligands, and different drugs as ligands) [8]. There are other interesting works which are reported recently in the literature showing different intensities of palladium complex activity on various species of bacteria and fungi [9][10][11][12][13][14][15]. In view of the growing cases of drug resistance of microorganisms it is urgent to search for more biospecific antifungal, less toxic agents. Metal-based drugs might answer this claim, representing an alternative therapeutic route. In this context, the discovery of nonplatinum based metal complexes came into consideration [16].
The aim of this paper is to synthesize new series of palladium (II) complexes and evaluate their in vitro antibacterial and antifungal activities against different microbes. The main aim of this research is focused on the biological impact of the newly synthesized Pd (II) complexes on different microorganism like Gram-positive and Gram-negative bacteria and different fungi. Hence, our study is an attempt to get overcome from microbial disease up to some extent. So with the aforesaid objectives we have synthesized new Pd (II) with halo substituted BLs and analyzed in vitro antimicrobial, antifungal, and antioxidant activity.

Experimental Section
2.1. Materials and Methods. Palladium dichloride (PdCl 2 ), benzylamine ligands (BLs), DMSO, and ethanol (>99.5%) were purchased from Sigma Aldrich and used without further modifications. Elemental analysis was made with a Euro vector CHN analyzer, and UV/Vis spectra were recorded with a Spectro 2060 plus spectrophotometer over 200-600 nm in a 1 cm path length cuvette. FTIR (Perkin Elmer) spectra were taken with KBr palate where polystyrene thin film was used as a calibration standard. 1 H and 13 C NMR spectra were recorded in DMSO-d6 (NMR, 99.99%) with a Bruker-Biospin Avance-III 500 MHz FT-NMR spectrometer. Mass spectra were obtained with PE SCIEX API 165 with +ve ESI mode with ammonium acetate and acetonitrile in 1 : 9 v/v ratio as mobile phase. The molecular sustainability of Pd (II) complexes was determined by preparing a solution in DMSO, DMSO : water, and DMSO : phosphate buffer of pH 7.2. Buffer solution was prepared by adding 70 mL 0.1 M aqueous NaOH to 0.1 M aqueous KH 2 PO 4 solution. The pH of a resultant buffer was checked with RS-232 modelled CyberScan pH 2100, EUTECH pH meter.

General Consideration for Synthesis.
Initially, PdCl 2 and BLs (molar ratio 1 : 2, resp.) were separately dissolved in freshly prepared solvent (absolute ethanol and Milli-Q water in 1 : 1.5) using 1 MLH magnetic stirrer. Then, the BLs solutions were added dropwise to metal compound solution with continuous stirring at room temperature. After 10 h, the mixture turned from light red brown to greenish color and after 16 h, precipitates were formed. The precipitates were filtered off, washed several times with chilled water/ethanol in 1 : 1 ratio, and kept overnight in vacuum oven at room temperature for absolute dryness.

Biological Evaluation
is absorbance of DPPH • with a test compound and 0 is absorbance of DPPH • without a test compound. Absorbance data are presented as means ± SD of three determinations. Reaction Scheme. Synthesis of Pd (II) complexes is as follows:

Results and Discussion
The 3300 to 3119 cm −1 stretching frequencies inferred presence of NH 2 of benzylamine ligands in the complexes and similarly from 1497 to 1453 cm −1 predicted C=C in phenyl ring. The 495.92 to 438.78 cm −1 and 380-348 cm −1 bands indicate the Pd-N and Pd-Cl bands, respectively [17,18]. In 1 H NMR, 2H of -NH 2 and PhCH 2 -appeared at 4.03 to 4.10 and 3.58 to 3.80, respectively, with singlet for all complexes. The aromatic protons appeared with their specific peak from 7.32 to 7.59 (J = 7 to 8). In 13 C NMR, the benzyl carbon (PhCH 2 -) at 45.43 to 43.59 ranges for all the (PdCl(BLs) 2 ) complexes [19]. The aromatic ortho, meta, and para -Cl attached carbon appeared within 145.722 to 128.6 for the complexes. The carbon of -Cl at ortho, meta, and para appeared at 132.2, 128.5, and 129.69 ppm, respectively. The +ve ESI mass spectra of Pd complexes have found [M + 1] confirming their molecular mass. The UV/Vis absorption from 265 to 270 nm and 1 H NMR coupling constant between 5 and 9 MHz have confirmed their trans geometry (Figure 2) [20][21][22].

Absorption Spectroscopy.
To investigate a solid state structure retained in solution, the UV/Vis spectral behaviour was investigated in DMSO and DMSO + water as well as in DMSO + phosphate buffer for PdCl(BLs) 2 . The overall patterns of spectra for complexes solution were found similar to different mediums to ensure their molecular sustainability

Conclusion
Genome studies have provided a better understanding of the closer distance between the microbial kingdom and the human species. The synthesized PdCl(BLs) 2 complexes showed selective and moderate activity against different microbes and the interesting results were obtained for Grampositive species, which are common in the environment. Apart from their microbial studies, the complexes have also expressed significant free radical-scavenging activities acting as antioxidants and could be used for medicinal purposes. However, in present study, we have focused on their antimicrobial and antioxidant activities and their other activities relevant to medical, biophysical, and biochemical processes are being pursued. The anticancer activity on suitable cell line  against solid tumours, apoptosis, and DNA binding studies are under progress.