Pd(II) and Zn(II) Based Complexes with Schiff Base Ligands: Synthesis, Characterization, Luminescence, and Antibacterial and Catalytic Activities

Two new metal complexes involving Schiff base ligands, namely, [Pd(L1)2] (1) and [Zn(L2)2] (2), [HL1: 2,4-dibromo-6-((E)-(mesitylimino)methyl)phenol and HL2: 2-((E)-(2,6-diisopropylphenylimino)methyl)-4,6-dibromophenol], have been solvothermally synthesized and characterized by elemental analysis, IR-spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. Both 1 and 2 are mononuclear cyclometalated complexes with square planar and tetrahedral coordination geometry, respectively. 1 and 2 display photoluminescence in the solid state at 298 K (fluorescence lifetimes τ = 5.521 μs at 508 nm for 1; τ = 3.697 μs at 506 nm for 2). These Schiff base ligands and their metal complexes have been screened for antibacterial activity against several bacteria strains, and the results are compared with the activity of penicillin. Moreover, the Suzuki reaction of 4-bromoanisole with phenylboronic acid by 1 has also been studied.


Introduction
It is wellknown that Schiff bases are important compounds because of their wide range of biological activities [1] and as being ligands in conjunction with transition metals. These transition metals based compounds display diverse structural features and in some instances exhibit interesting reactivities, bacteriostasis, and photoluminescence [2][3][4][5][6]. Compared to platinum(II) complexes involving Schiff base ligands, whose applications as luminescent sensors are of considerable interest in inorganic photochemistry [7,8], relatively scant attention has been focused upon the luminescent characteristics of palladium(II) complexes [9]. Meanwhile, the palladium catalyzed formation of biaryls from aryl halides with arylboronic acids (the Suzuki reaction) has become a mainstay of modern synthetic organic chemistry for the formation of carbon-carbon bonds [10,11]. Moreover, Zn(II) complexes bearing salicylaldiminato ligands have been employed as blue, greenish-white, and red emitters in organic optoelectronics with better stability and efficiency [12,13]. In those zinc(II) complexes, the salicylaldiminato ligands are mainly salen ligands with two N and two O donor atoms. However, reports on the luminescent properties of bis(salicylaldiminato) zinc(II) complexes are still limited. On the basis of the above considerations, we here report the synthesis, characterization, luminescence, and antibacterial and catalytic activities of two new transition metal complexes based on Schiff base ligands.

Materials and Methods.
All chemicals were commercially available and used as received without further purification.

Physical Measurements.
Elemental analyses for C, H, and N were carried out using a Vario EL III Elemental Analyzer. Infrared spectra were recorded (4000-400 cm −1 ) as KBr disks on a Shimadzu IR-440 spectrometer. Thermogravimetric analyses (TGA) were performed on an automatic simultaneous thermal analyzer (DTG-60, Shimadzu) under a flow of N 2 at a heating rate of 10 ∘ C/min between ambient temperature and 800 ∘ C. Powder XRD investigations were carried out

Preparation of [Pd(L1
2 (0.112 g, 0.5 mmol) was dissolved in methanol (15 mL). HL1 (0.359 g, 1 mmol) was added, and the mixture was stirred at room temperature for 2 h under an anhydrous atmosphere. The resulting mixture was filtered under reduced pressure. The collected solid was washed with diethyl ether and dried in air to give yellow crystals that were purified by recrystallization from methylene chloride (10 mL) and hexane (10 mL). Yield 0.315 g (70%). Anal. For C 32 H 28

Antibacterial Activity Tests.
In vitro bacterial activities of the Schiff base ligands and their metal complexes were tested using the paper disc diffusion method. The chosen strains were G(+) S. aureus and B. cereus and Rhizopus and E. coli. The liquid medium containing the bacterial subcultures was autoclaved for 20 min at 15 lb pressure before inoculation. The bacteria were cultured for 24 h at 35 ∘ C in an incubator. Mueller Hinton broth was used for preparing basal media for the bioassay of the organisms. Nutrient agar was poured onto a Petri plate and allowed to solidify. The test compounds were dissolved in DMF and added dropwise to 10 mm diameter paper discs placed in the center of the agar plates. The plates were then kept at 5 ∘ C for 1 h and transferred to an incubator maintained at 35 ∘ C. The width of the growth inhibition zone around the disc was measured after 24 h of incubation. Four replicates were taken for each treatment. In order to clarify any participating role of DMF in the biological screening, separate control studies were carried out with the solutions of DMF alone, and they showed no activity against any bacterial strains.

Catalytic Reactions.
A mixture of 4-bromotoluene (1.0 mmol), phenylboronic acid (1.2 mmol), organic solvents (6 mL), bases (2.0 mmol), and 0.5 mol% of catalyst was stirred at 80 ∘ C under air. After the reaction, the catalyst was separated by filtration. The filtrate was dried over Na 2 SO 4 and filtered. The products were quantified by GC-MS analysis (Shimadzu GCMS-QP5050A equipped with a 0.25 mm × 30 m DB-WAX capillary column). The typical GC-MS analysis program was as follows: set initial column temperature as 100 ∘ C, hold 2 min, ramp temperature to 280 ∘ C at 15 ∘ C/min, and hold for 5 min.

X-Ray Crystallography.
Single crystal X-ray diffraction analyses of complexes 1-2 were performed on a Bruker Apex II CCD diffractometer operating at 50 kV and 30 mA using MoK radiation ( = 0.71073Å). Data collection and reduction were performed using the APEX II software [15]. Multiscan absorption corrections were applied for all the data sets using SADABS, as included in the APEX II program [15]. The structure was solved by direct methods and refined by least squares on F 2 using the SHELXTL program package [16]. All nonhydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms attached to carbon and oxygen were placed in geometrically idealized positions and refined using a riding model. Crystallographic data are listed in Table 1. Selected bond lengths and angles and Hbonding parameters for compound 2 are given in Tables 2 and  3, respectively.

Results and Discussion
4.1. Structure of 1. Complex 1 crystallizes in the monoclinic space group P2 1 /n and contains one Pd(II) ion and two  crystallographically independent L1 ligands, as shown in Figure 1.  (Table 3).

Powder X-Ray Diffraction Analysis.
In order to check the purity of complexes 1-2, bulk samples were measured by X-ray powder diffraction at room temperature, as shown in Figure 4. Although the experimental patterns have a few unindexed diffraction lines and some are lightly broadened in comparison with those simulated from the single-crystal data using Mercury, it still can be well considered that the bulk synthesized materials and the crystals used for diffraction are homogeneous.

Infrared Spectra.
FT-IR spectra of HL1, HL2, 1, and 2 were recorded as KBr pellets (see Figure S1 in The absorption peaks of -CH=N-group in 1 and 2 are clearly blue shifted against the related ligands, perhaps due to the conjugative effect between metal ions and ligands.

Thermogravimetric Analysis of 1-2.
The TG and DTA curves of 1-2 are shown in Figure 5. Both structures show good thermal stability as no clean weight-loss step occurs below 280 ∘ C for 1 and 320 ∘ C for 2. The DTA trace of 2 shows superior endotherm to 1 indicating that 2 has better thermal stability than 1. The weight-loss step above 280 ∘ C and 320 ∘ C corresponds to decomposition of the framework structures. Finally, 1 and 2  4.6. Luminescent Properties. As part of a continuing program dedicated to luminescent transition systems, the spectroscopic behavior of complexes 1-2 is presented. The solidstate emission luminescence spectra of 1 at room temperature upon excitation at 340 nm are shown in Figure 6. The emission spectrum shows two bands at max = 508 and 766 nm, respectively. The solid-state excitation spectra for 1 reveal the presence of low-energy ligand-field states in the 300-400 nm spectral region. The high-energy structured band of 1 is assigned to a transition to 3 IL excited state [19]. Complex 2 emits strong blue light in the solid state at room temperature with maximal emission wavelength at 506 nm (excitation wavelength 354 nm, Figure 6). The coordinated zinc atom plays a dual role in the luminescence of 2 as pointed out in the literature for coordination complexes [20]. First, the formation of covalent bonds between the Zn and O atoms via -donation of a lone-pair electrons on the O atom to the Zn atom changes the emission energy due to the lowering of the energy gap between * and . Second, the coordination of the ligands with the Zn atom increases the rigidity of the ligands, which can diminish the loss of energy via vibrational motions and increase the emission efficiency. Compared to the free ligands, the emission maxiumu of 1-2 is obviously red-shifted (see Figure S2) in the solid state, which may be assigned as metal-to-ligand charge transfer [21]. The luminescence lifetimes of solids 1-2 using an Edinburgh FLS S920 phosphorimeter with a 450 W xenon lamp as excitation source show lifetimes for 1 of = 5.521 s at 508 nm and for 2 of = 4.023 s at 506 nm (Figure 7). It should be noted that complexes 1 and 2 possess longer fluorescence lifetime than other palladium and zinc complexes involving Schiff bases [2,22] and so could be used for lightemitting luminescent materials.

Biological Activity
Tests. The susceptibility of certain strains of bacterium towards the ligands HL1-HL2 and their metal complexes 1-2 was judged by measuring the size of their bactericidal diameters (vide supra). The results are given in Table 4. The effect against Staphylococcus aureus of ligands and complexes was found to be close to that of sodium penicillinate. All of the ligands and complexes showed inhibition diameters larger than sodium penicillinate against Bacillus cereus . Compared to HL1-HL2 and 1-2    good activities, the penicillin is ineffective against Escherichia coli. However, similar to sodium penicillinate, the four tested samples showed no appreciable activity. Moreover, we found that the complexes were more effective than the ligands. It is possible that the ligand may be activated by the metal ion [23].
El-Sherif reported a series of palladium complexes that show antibacterial activities against S. pyogenes and E. coli bacteria at different concentrations 1, 2.5, and 5 mg/mL in DMSO in which the activities were smaller in the complexes than in ligands [24]. Also the results obtained for 1 and HL show better antibacterial activities compared to the platinum(II) and palladium(II) complexes based on Schiff bases [20].

Suzuki
Reaction Catalysis of 1. Pd(II) based complexes are well known for their catalytic activities [2,5]. The Suzuki reaction of 4-bromoanisole with phenylboronic acid by 1 is reported here. The results reveal that the base and solvent for the Suzuki reaction greatly influence catalytic activity ( Table 5). The reaction temperature has lower effect on the catalytic activity of 1. Among six different organic solvents, DMF is found to the best solvent for this catalytic system. In other organic solvents, for example, ethanol, ethanol/water (2 : 2), ethylene glycol, acetonitrile, and toluene, relatively low yields of coupling products were obtained. Among five different bases investigated for these reactions, NaOAc The Scientific World Journal 7   was found to be the most effective (Table 5, entry 1); K 2 CO 3 , Na 2 CO 3 , CH 3 ONa, and NaOH were substantially less effective. KF failed to promote the reaction (Table 5, entry 13). Compared with other Pd(II) complexes based on Schiff base ligands, the catalytic activities of the present complexes for the Suzuki reaction proved to be highly effective [25].

Conclusion
In summary, two new transition metal complexes based on Schiff base ligands have been successfully synthesized under solvothermal conditions. Both 1 and 2 show good thermal stability and exhibit photoluminescence in the solid state at room temperature ( = 5.521 s at 508 nm for 1, = 3.697 s at 506 nm for 2), suggesting utility as light-emitting luminescent materials. The antibacterial activity tests showed that the ligands and complexes exhibited superior biological activity against Staphylococcus aureus, Bacillus cereus, and Escherichia coli. Moreover, complex 1 displays highly catalytic activities in the Suzuki coupling reaction of 4-bromoanisole with phenylboronic acid, which are also very sensitive to the choice of base and solvent.

8
The Scientific World Journal