SYNTHESIS, STRUCTURAL ELUCIDATION AND ELECTROCHEMICAL BEHAVIOR OF SOME OXIME-PHENYLALANINE MIXED LIGAND COMPLEXES

Four new mixed ligand complexes Me(II)/phenylalanine (phe)/HL [HL =4-(4-bromophenylaminoisonitrosoacetyl)biphenyl and Me = Co, Ni, Cu, Zn] were synthesized. These complexes are formulated as: [CoL(phe)(H2O)2], [NiL(phe)(H2O)2], [CuL(phe)(H2O)2] and [ZnL(phe)(H2O)2]. All the compounds were characterized by elemental analyses, FT-IR, magnetic susceptibility measurements, TG/DTA and cyclic voltammetry (CV) experiments. IR spectral data confirmed the coordination of the oxime ligand to the metal ions through the oxime and carbonyl oxygen. The geometrical structures of the complexes have been found to be octahedral. The measured molar conductance values of the complexes in DMF are in agreement with the nonelectrolytic nature ofthe complexes. The elemental analyses confirm a 1:1:1 [metal:HL:L(phenylalanine)] molar ratio. Thermal behavior of the compound was investigated by thermal gravimetric analysis (TG) and differential thermal analysis (DTA) techniques. All the complexes were transformed into metal oxides after thermal degradation. The electrochemical properties of both ligand and their complexes were analyzed by cyclic voltammetry (CV) using glassy carbon electrode in DMF solution containing 0.1 M TBAP as supporting electrolyte.

All chemicals used were of reagent grade. All chemicals were purchased from Merck or Aldrich and were used as received. Melting points of all the compounds determined on EZ-Melt Automated Melting Point Apparatus in open glass capillaries. Elemental (C, H, N) analyses were carried out by standard methods with a LECO, CHNS-932 analyzer. The IR spectra of the compounds were recorded on a Perkin Elmer Spectrum 100 FT-IR instrument with the samples in 4000-600 cm -1 range. The molar conductivity measurements (Ʌ m ) were taken in dimethylformamide (DMF) at 25 o C, using concentrations of 1.0x10 -3 M for the complexes. Thermal analyses (TG/DTA) were recorded on Shimadzu TG DTA 60 thermal analyzer with a heating rate of 10 K/min in dynamic N 2 atmosphere. 1 H NMR and 13 C NMR spectra were recorded on a Bruker Avance III 400 MHz FT-NMR. The proposed molecular structure of mixed ligand complexes are given in Figure 1.

Cyclic voltammetry measurements
Electrochemical measurements were conducted using an Autolab Potentiostat/Galvanostat PGSTAT-302N device controlled by Nova 2.1.4 program on a computer. A standard three electrode electrochemical cell was used for all measurements and main parts of this cell are a glassy carbon (GCE) as working electrode (2.0 mm diameter), platinum wire as counter electrode, and a Ag/AgCl as reference electrode. Previous to each experiment, GCE was polished with 1.0, 0.3 and 0.05 m alumina powder. Residual alumina particles were removed by an ultrasonic bath. Afterwards, the electrode was dried and washed with pure DMF before use. Cyclic voltammetry studies were performed in DMF containing 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte with an analyte concentration of 2.0 mM. All solutions were purged with nitrogen for 10 min before measurements, and the experiments were carried out at room temperature.
Synthesis and structural elucidation of some oxime Figure 1. Proposed molecular structure of the mixed Synthesis Synthesis of ligand (HL). 4-(4 according to previously published procedures mmol; 0.389 g) were dissolved in EtOH (50 mL) bromoaniline (3 mmol, 0.51 solution over 15 min with cooling. After that pe same temperature. Then it was allowed to stir at ambient temperature for 2 distilled water. The resulting precipitate was filtered and then recrystallized from ethanol. The product was filtered off, washed distilled water, cold ethanol and dried on P route of the ligand is shown in Scheme 1.

Synthesis of [CoL(phe)(H 2
Co(CH 3 COO) 2 ·4H 2 O (1.5 mmol, 0.37 g) in 50 mL of ethanol t 20 mL of hot methanol. The resulting mixture was refluxed for 30 min. To this solution, L phenylalanine (1.5 mmol, 0.24 g) dissolved in 4 mL water containing KOH (1.5 mmol, 0.24 g) and 10 mL methanol was added with consta solid product was collected by filtration, washed several times with cold water, ethanol, anhydrous ether and dried on P  (4-Bromophenylaminoisonitrosoacetyl)biphenyl (HL) was prepared according to previously published procedures [3,4]. First, 4-biphenylhydroximoyl chloride (1. mmol; 0.389 g) were dissolved in EtOH (50 mL) and the mixture was cooled 0 o C. Then 4 g) in EtOH (10 mL) was added dropwise to solution of first solution over 15 min with cooling. After that period, the reaction mixture was stirred 1 h at the same temperature. Then it was allowed to stir at ambient temperature for 2 h and diluted 100 mL distilled water. The resulting precipitate was filtered and then recrystallized from ethanol. The filtered off, washed distilled water, cold ethanol and dried on P 2 O 5 . The synthetic route of the ligand is shown in Scheme 1.

Microanalyses and molar conductance measurements
The synthesized ligand and the mixed ligand complexes were investigated using various physiochemical properties like melting point (m.p.), color, yield, micro analytical data and molar conductance value are given in the experimental part. The obtained C, H, and N analytical data of the synthesized metal(II) complexes agreed well with the calculated values which confirm a 1:1:1 (metal:HL:L-phenylalanine) molar ratio.
Molar conductivity measurements of the metal complexes were determined using freshly prepared solutions of the complexes in DMF at room temperature.The measured molar conductance values of 10 −3 M solutions of the mixed ligand complexes werefound to be in the range 2.3-7.2 S cm 2 mol −1 which is in agreement with the non-electrolytic nature ofthe complexes [17]. The results of the elemental analysis of the mixed ligand complexesare in good agreement with those required by the proposed formula.

NMR spectra of the oxime ligand (HL) and the mixed ligand Zn complex {[ZnL(phe)(H 2 O) 2 ]}
The oxime ligand in CDCl 3 was studied by 1 H and 13 C NMR spectroscopy. The deuteriumexchangeable proton of the (N-OH) group for the HL showed a chemical shift at 10.86 ppm as a singlet [4]. In the region of 7.32-8.12 ppm, chemical shifts for aromatic hydrogens were assigned as multiplet. The chemical shift of the aromatic amine proton of HL appeared at 6.76 ppm as a singlet. In 13 C NMR spectrum, the aromatic carbon resonances were shown in between δ = 127-139 ppm which is corresponding to the bromophenyl and biphenyl rings in the compound. The carbonyl and oxime carbons were clearly observed at δ = 183 and 147 ppm, respectively and more slightly deshielded in the spectra. The increasing electronegativity of oxygen and nitrogen atoms and different environment and conformations cause a deshielding effect for these signals. All of these values prove that the oxime ligand formed and are in good agreement with the values previously reported [4,24,25]. Since Co(II), Ni(II) and Cu(II) complexes are paramagnetic, their 1 H and 13 C spectra could not be obtained.
The 1 H NMR spectrum of free oxime ligand were compared with the diamagnetic mixed ligand Zn(II) complex taken in CDCl 3 medium. Unfortunately, the insolubility of phenylalanine in CDCl 3 makes it difficult to obtain 1 H NMR spectrum of phenylalanine to further clarify the binding of phenylalanine ligand to the metal ions. Zn(II) complex are well established in their predictable regions [26]. The absence of COOH proton of phe peak in the Zn(II) complex indicates that this proton is removed during chelation process and this prove that phe ligand is coordinated through bidentate mode with the Zn(II) ion via amino-N and deprotonated carboxylato-O atoms [26]. Also, the disappearance of proton peak of -NOH at 10.86 ppm for HL in the complex prove that oxime ligand is coordinated through bidentate mode with the Zn(II) ion via carbonyl and deprotonated oximato-O atoms. In the region of 7.34-8.23 ppm, chemical shifts for aromatic hydrogens were assigned as multiplet. The chemical shifts of the aromatic amine proton of HL and amine proton of phe appeared at 7.04 and 6.21 ppm as a singlet, respectively. In addition, the spectrum of Zn(II) complex shows a characteristic new peak centered at 5.82 ppm, which confirm the presence of coordinated water molecule [17,27]. The protons of the CH and CH 2 groups for phe in the Zn(II) complex showed chemical shifts at 4.41 and 4.08 ppm. Thus, the NMR studies reinforce the findings drawn from the vibrational spectral studies about the mode of binding.

FTIR spectroscopy
The IR spectra of the investigated complexes ( Figure 3) are compared with those of the free ligand ( Figure 2) to determine the coordination sites. The basic theory involved is that the stretching modes of the ligands change upon complexation due to weaking/strengthening of the bonds involved in the bond formation resulting in subsequent change in the position of the bands appearing in the IR spectrum. The spectrum of the oxime ligand exhibits a characteristic band at 3425 cm −1 due to the stretching vibration of the OH group. On the other hand, the IR spectra of all the complexes show broad band at 3359-3332 cm −1 which have been assigned to υ(OH) stretching vibration of water molecules, in accordance with the results of the elemental analysis listed in Section 2.3 [11,[28][29][30].
The oxime ligand HL shows strong absorption band at 1667 cm −1 due to ʋ(C=O) and 1586 cm −1 due to ʋ(C=N). The (C=N) band is shifted to 1615-1599 cm −1 in the complexes indicating the involvement of the nitrogen atom of the azomethine group in the coordination to the metal ion [11,[28][29][30][31][32]. The (C=O) band is located in the same region (1675-1650 cm −1 ) with ʋ as (COO -) stretching bands of amino acidsor disappeared indicating the participation of carbonyl oxygen atom in coordination [26]. The strong absorption at 1052 cm −1 due to ʋ(N=O)of the oxime group in the free HL ligand is shifted towards lower frequency regions 1046-1033 cm −1 in all the complexes [3,33,34]. Allthese features indicate that the HL is coordinated to the metal ion through the oxygen of the oxime group.
The ʋ(NH 3 + ) of free amino acids is observed in the range 2933-3369 cm −1 [11]. In the complexes, NH 3 + gets deprotonated and binds to metal through the neutral NH 2 group. The NH 2 symmetric stretching is recorded at 3344 cm −1 in the IR spectrum of free phenylalanine [35]. The IR spectra of complexes show characteristic bands of ʋ(NH 2 ) in the region 3290-3248 cm −1 [11,36]. The spectra of amino acids display ʋ as (COO -) and ʋ s (COO -) frequency in the 1551-1634 cm −1 and 1379-1420 cm −1 range, respectively. In the metal complexes, ʋ as (COO -) and ʋ s (COO -) stretching bands are located in the region 1675-1650 cm −1 and 1399-1383 cm −1 , respectively [11,23]. The ligand L-phenylalanine coordinates with M(II) ions through amino-N of NH 2 group and deprotonated carboxylato-O of COOH group to form a stable 5-membered chelate ring [17,29,30]. The infrared spectra of the oxime ligand and the mixed ligand complexes are very much consistent with the structural data presented in this paper and these absorption data are in agreement with those previously reported for similar compounds [23,[28][29][30][31][32][33][34][35][36]. From this study, both the ligands HL and phe are bidentate which form stable metal chelates through the oxygen of deprotonated oxime group and carbonyl oxygen and amino-NH 2 and deprotonated carboxylato-O atoms, respectively.

Magnetic measurements
In a magnetic field, the paramagnetic compounds will be attracted while the diamagnetic compounds repelled. Therefore, paramagnetic substances will have positive susceptibilities. Thus, the magnetic susceptibility measurements provide information regardingthe geometric structure of the complexes. The room temperature magnetic moment measurements show that the Zn(II) complex is diamagnetic and are likely to octahedral. The Ni(II) complex is paramagnetic with magnetic susceptibility value of 2.70 B.M., which fits the d 8 metal ion in an octahedral structure, the two-spin value of 2.83 B.M. [3,36]. Cu(II) and Co(II) complexes are paramagnetic with magnetic susceptibilities of 1.75 B.M. and 1.33 B.M., respectively. Measured value of the magnetic moment for the cobalt complex, which corresponds to one unpaired electron and falls within the range normally observed for low-spin octahedral Co 2+ complexes. The Cu(II) complex fits the spin value of 1.73 B.M. indicating an octahedrald 9 -system [26,33,[36][37][38].

Thermal studies
Thermal analysis (DTA/TG) for themetal complexes was carried out within the temperature range from ambient temperature up to 1000 o C. The correlations between the different decomposition steps of the complexes with the corresponding weight losses are discussed in terms of the proposed formula of the complexes. Initially, first weight losses correspond to the removal of coordinated water molecule. All the complexes (1)(2)(3)(4) in the first stage show the weight loss of 5.90-6.40% within the temperature range 21-238 o C. These weight losses correspond to the removal of water molecule from the complexes [17]. Amino acid ligand, Lphenylalanine moiety elimination occurred the second stage. In the final stage, decomposition of primary oxime ligand leading to the formation of MO (metal oxide) as residue [17,[39][40][41][42][43]. DTA/TG profiles of these complexes are given in Figures 4-7.
The Co(II) complex (1) with the general formula [C 29 H 28 N 3 O 6 BrCo] is thermally decomposed in two successive decomposition steps. The first step with estimated mass loss of 6.20% was found within the temperature range 21-182 o C and corresponds to loss of two H 2 O molecules (calculated mass loss = 5.51%). The second step occurs within the temperature range 182-850 o C with an estimated mass loss 81.28% (calculated mass loss = 83.02%) are reasonably accounted for the decomposition of the ligand molecules leaving CoO residue.  The Ni(II) complex (2), [C 29 H 28 N 3 O 6 BrNi], is thermally decomposed in two successive decomposition steps within the temperature range 21-920 o C. The first step with estimated mass loss of 6.12% was found within the temperature range      within the temperature range 21-238 o C. Corresponding to loss of two H 2 O molecules (calculated mass loss = 5.46%). The second step with anestimated total mass loss 81.63% which is due to loss of theligand molecules leaving ZnO residue occurring withinthe temperature range 238-840 o C (calculated mass loss = 82.20%).

Cyclic voltammetry
The application of electrochemical methods to study of the coordination of metal ion provides a helpful complement to the previously used methods of investigations such as spectroscopy. The electrochemical properties of ligands and their complexes were investigated by cyclic voltammetry. The potential was scanned in the range -2.0 V to +2.0 V at a scan rate 100 mVs -1 . The cyclic voltammogram of phenylalanine showed one anodic wave and one cathodic wave located at +0.75 V and -0.87 V. In the voltammogram of HL, one anodic wave is observed at +0.95 V which can be attributed to the oxidation of the oxime group [44]. The results of cyclic voltammetric investigations of all the complexes are summarized in Table 1 and the cyclic voltammogram of complex 2, HL and DMF are presented in Figure 8. The anodic wave seen around +0.95 V in the CVs for all metal complexes are presumed to be ligand based oxidation. The voltammograms of the metal complexes, in addition to the ligands peak, also have oxidation and reduction peaks. The peaks of the complexes confirm the presence of metal cations and ligands in the complexes.  Through the analysis of the CV voltammograms obtained at various scanning speeds, the transfer of electrons and the way of transporting the substance to the electrode surface are known. The cyclic voltammetric behaviors of the ligand and complexes were studied by varying the sweep rate from 20 mVs -1 to 500 mVs -1 . Linear correlations were obtained between square root of scan rate (v 1/2 ) and peak current (I pc) (Figure 9). On the other hand, the slope values of the correct equations between the logarithm of the potential scanning speed (log v) and the logarithm of the peak current (log I pc ) were found in values ranging from 0.75 to 1.0. These values confirmed that the process was adsorption controlled.

CONCLUSION
In conclusion, a new (4-(4-bromophenylaminoisonitrosoacetyl)biphenylligand (HL) and their mixed ligand Co(II), Ni(II), Cu(II) and Zn(II) complexes with L-phenylalanine were synthesized and were characterized by elemental analysis, magnetic susceptibility measurements, CV, FT-IR, and 1 H NMR. Elemental analysis, stoichiometric and spectroscopic data of the metal complexes indicated that the complexes can be formulated as [MeL(phe)(H 2 O) 2 ]. The results obtained by CV method pointed out that the electrode reaction was adsorption controlled.