New Mixed Ligand Cobalt(II), Nickel(II) and Copper(II) Complexes of 2,2'-Bipyridine-3,3'-Dicarboxylic Acid (bpdc) with 2-Mercapto-5-Phenyl-1,3,4-Oxadiazole (phozSH) and Their Antioxidant Activity

Introduction

Oxadiazoles have been recognized during a century, nevertheless the majority of research has focused on the symmetric 1, 2, 5- and 1, 3, 4-oxadiazoles¹.  1, 3, 4-oxadiazoles and their complexes have fascinated more consideration because of their biological behavior in both agrochemicals and pharmaceuticals.² For example, numerous derivatives of 1, 3, 4-oxadiazoles reveal analgesic,³ muscle relaxant, anti-malarial,⁴ anti-inflammatory,⁵ anti-tubercular,^6,7 fungicides,^8,9 anti-bacterial,^10,11 insecticidal, anti-oxidant¹² and anticancer.^2,13

In the present work, we give an account of the preparation and identification of new mixed ligand Co(II), Ni(II) and Cu(II) complexes comprising the two ligands of 2, 2′-bipyridine-3, 3′-dicarboxylic acid (bpdc) and 2-Mercapto-5-phenyl-1,3,4-oxadiazole(phozSH). We found that bpdc ligand behave as a bidentate chelate that connected to the metal ions by oxygen atom of the carboxyl group while the phozSH ligand work as a monodentate that link to the metal ions by nitrogen atom.                           

Materials and Methods

The (NiCl₂.6H₂O, CoCl₂.6H₂O, CuCl₂.2H₂O, KOH) materials are available in BDH and 2, 2′-bipyridine-3, 3′-dicarboxylic acid, 2-Mercapto-5-phenyl-1,3,4-oxadiazoleand 2, 2-diphenyl-1- picrylhydrazyl materials were commercially accessible from Yahoo Chem. China. The FT-IR spectra were registered on a Shimadzu IR- spectrophotometer in the range of 400-4000 cm^-1 by utilizing KBr discs. (¹H and ¹³C)-NMR spectra were performed on a Bruker 400 MHZ Ultra-shied. UV-Visible spectra were taken on a UV-Visible spectrometer, AE-UV1609 (UK) CO., LTD. The conductivity measurements were estimated on a conductivity meter type Senz µSiemen conductivity tester. CHNS analyses were executed on Euro EA 3000 Elemental Analyzer.

Synthesis of [Co(κ²-bpdc)(κ¹-phozSH)(H₂O)₃]  (1)

It was synthesized by the following stages

Stage One: Preparation of [Co(κ²-bpdc)(H₂O)₄]

2, 2′-bipyridine-3, 3′-dicarboxylic acid(bpdc) (0.3 millimole, 0.0732 gram) was dissolved in methanol (10 milliliter) and (5 milliliter) methanol solution of KOH (0.6 millimole, 0.0336 gram) was added. After refluxing for (30 min.), a solution of CoCl₂.6H₂O (0.3 millimole, 0.0713 gram) in methanol (5 milliliter) was added dropwise. The resulted solution was heated under reflux for 2 h. and filtered. The light brown precipitate was produced when the filtrate was evaporated at room temperature.  (Chemical formula: C₁₂H₁₀CoN₂O₆Cl₂; Yield: 0.09 gram, 65.7%; decomposition point: 296 °C; Color: light brown).

Stage two: Reaction of [Co(κ²-bpdc)(H₂O)₄] with (phozSH)

A warm solution of (phozSH) ligand (0.2 millimole, 0.0356 gram) in (10 milliliter) ethanol was added to a hot suspended solution of [CoCl₂(κ²-bpdc)] (0.2 millimole, 0.074 gram) in CH₂Cl₂ (10 milliliter). The mixture was heated under reflux for 2 h. and filtered. The yellow-green precipitate was attained when the solution was evaporated at room temperature.

Yield = 0.09 gram, 81 %; Melting point: 174-176 °C; Color: yellow-green. Anal. Calc. for C₂₀H₁₅CoN₄O₇Cl₂S: C, 41.03; H, 2.56; N, 9.57; S, 5.48. Found: C, 41.66; H, 2.46; N, 8.60; S, 4.57 %. IR (ν _max/cm^-1): ν(H₂O) 3398; ν(C=O) 1718; ν(C=N) 1612; ν(C=N of bpdc) 1573; ν(N-N) 1446; ν(C-O-Co) 1076; ν(C-O-C) 1062; ν(C-S) 696; ν(M-H₂O) 630; ν(Co-O) 480; ν(Co-S) 418. ¹H NMR (295K, ppm, DMSO-d⁶): 7.65-8.9 (m, 11H, phenyl proton of bpdc and phozSH); 2.5 (s, DMSO proton); 4.14 (s, water proton in DMSO), ¹³C NMR (DMSO, 𝛿, 400 MHz): 175 C (carboxyl); 158 and 168 C (oxadiazole); 120.57-130.49 C (ph groups of bpdc and phozSH); 39.38 C (DMSO).

Synthesis of [Ni(κ²-bpdc)(κ¹-phozSH)(H₂O)₃] complex  (2)

It was synthesized by the following stages

Stage one: Preparation of [Ni(κ²-bpdc)( H₂O)₄]

A solution of KOH (0.6 millimole, 0.0336 gram) in methanol (5 milliliter) was placed to a solution of bpdc (0.3 millimole, 0.0732 gram) in methanol (10 milliliter) and refluxed for (30 minutes.). The subsequent mixture was refluxed for a further (2 h.) after addition of a methanol solution (5 milliliter) of NiCl₂.6H₂O (0.3 millimole, 0.0713 gram) and filtered. When the solution was evaporated at room temperature the dark-green solid was formed. (Chemical formula: C₁₂H₁₀NiN₂O₆Cl₂; Yield: 0.13 gram, 94.3%; Melting point: 230-232 °C; Color: dark-green).

Stage two: Reaction of [Ni(κ²-bpdc)(H₂O)₄] with (PhozSH)

To a warm stirred solution of [NiCl₂(κ²-bpdc)(H₂O)₄] (0.2 millimole, 0.0743 gram) in CH₂Cl₂ (10 milliliter), a hot ethanolic solution (10 milliliter) of phozSH (0.2 millimole, 0.0356 gram) was added and the resultant solution was heated under reflux for 2 h. and filtered. The green precipitate was obtained when the solution was evaporated at room temperature.  

Yield = 0.08 gram, 80 %; decomposition point: 210-212 °C; Color: green. Anal. Calc. for C₂₀H₁₇NiN₄O₈Cl₂S: C, 39.82; H, 2.82; N, 9.29; S, 5.31. Found: C, 39.15; H, 2.27; N, 8.33; S, 4.58 %. IR (ν _max/cm^-1):ν(H₂O) 3421;ν(C=O) 1724; ν(C=N) 1610; ν(C=N of bpdc) 1573, ν(N-N) 1446; ν(C-O-Ni) 1093; ν(C-O-C) 1076; ν(C-S) 696; ν(Ni-H₂O) 651; ν(Ni-O) 480; ν(Ni-S) 420. ¹H NMR (295K, ppm, DMSO-d⁶): 8.4-9.5 (m, 11H, phenyl roton of bpdc and phozSH); 2.5 (s, DMSO proton); 3.49 (s, water in DMSO), ¹³C NMR (DMSO, 𝛿, 400 MHz): 172 C (carboxyl); 150 and 160 C (oxadiazole); 123.96-130.18 C (ph groups of bpdc and phozSH); 39.32 C (DMSO).  

Synthesis of [Cu(κ²-bpdc)(κ¹-phozS)(H₂O)₃]Cl₂ complex  (3)

It was synthesized by the following stages

Stage One: Preparation of [Cu(κ²-bpdc)( H₂O)₄]

To a methanol solution (10 ml) of bpdc (0.3 millimole, 0.0732 gram), a methanol solution (5 milliliter) of KOH (0.6 millimole, 0.0336 gram) was added and refluxed for (30 min.) then a solution of CuCl₂.2H₂O (0.3 millimole, 0.0514 gram) dissolved in (5 milliliter) methanol was added and the resultant solution was refluxed for a further 2h. and filtered. The green solid was achieved when the solvent was evaporated at room temperature. (Chemical formula: C₁₂H₁₀CuN₂O₆Cl₂; Yield: 0.07 gram, 61.9 %; Melting point: 236-238 °C; Color: green).

Stage two: Reaction of [Cu(κ²-bpdc)(H₂O)₄] with (PhozSH)

To a hot suspended solution of [Cu(κ²-bpdc)(H₂O)₄] (0.2 millimole, 0.0752 gram) in dichloro methane (10 milliliter), a hot solution of phozSH (0.2 millimole, 0.0356 gram) in ethanol (10 milliliter) was added and heated under reflux for 2 h. then filtered. After the solvent was evaporated at normal temperature the light-green product was achieved.

Yield = 0.09 gram, 81 %; Melting point: 174-176 °C; Color: yellow-green. Anal. Calc. for C₂₀H₁₈CoN₄O₈S: C, 45.02; H, 3.37; N, 10.50; S, 6.01. Found: C, 45.66; H, 4.06; N, 10.10; S, 5.47 %. IR (ν _max/cm^-1): ν(H₂O) 3398; ν(SH) 2953; ν(C=O) 1718; ν(C=N) 1612; ν(C=N of bpdc) 1573; ν(N-N) 1446; ν(C-O-Co) 1076; ν(C-O-C) 1062; ν(C-S) 696; ν(M-H₂O) 630; ν(Co-O) 480; ν(Co-N) 443. ¹H NMR (295K, ppm, DMSO-d⁶): 7.65-8.9 (m, 11H, phenyl proton of bpdc and phozSH); 2.59 (s, SH proton); 4.14 (s, water proton in DMSO), ¹³C NMR (DMSO, 𝛿, 400 MHz): 175 C (carboxyl); 158 and 168 C (oxadiazole); 120.57-130.49 C (ph groups of bpdc and phozSH); 39.38 C (DMSO).

Scheme 1: Synthesis of [M(κ2-bpdc)(κ1-phozSH)(H2O)3], where M= Cobalt, Nickel or Copper(II). Click here to View Scheme

Results and Discussion

FT-IR Spectra for the Synthesized Complexes

The IR spectra of Cobalt(II), Nickel(II) and Copper(II) complexes, contain broad band at (3398 and 3421, 3088) cm^-1 and occurrence of a new weak peaks at (630 and 651 and 638) cm^-1 were ascribed to ѵ(O-H) of coordinated water molecule to the metal ions¹⁴. The (SH) stretching band at (2567) cm^-1 of (phozSH) was shifted to a higher frequency to (2953) cm^-1 in complex 1 and (2852) cm^-1 in complex 2 and 3^15-18.   A new weak band was appeared at (443, 513, 542) cm^-1 in the spectra of Cobalt(II), Nickel(II) and Copper(II) complexes were respectively indicated to the linkage of oxadiazole nitrogen to the metal ions. The broad band of (O-H) group of (bpdc) at (2576) cm^-1 was disappeared in all complexes and a new weak peak was observed at (480, 450 and 420) cm^-1 were indicated to O-coordination of (bpdc) ligand to (Cobalt(II), Nickel(II) and Copper(II)) metals correspondingly^19-22.

Figure 1: IR spectrum of bpdc ligand Click here to View Figure

Figure 2: IR spectrum of phozSH ligand Click here to View Figure

Figure 3:  IR spectrum of [Co(κ2-bpdc)( κ1-phozSH)(H2O)3]Cl2 complex Click here to View Figure

Figure 4: IR spectrum of [Ni(κ2-bpdc)( κ1-phozSH)(H2O)3] Click here to View Figure

Figure 5: IR spectrum of [Cu(κ2-bpdc)( κ1-phozSH)(H2O)3] Click here to View Figure

¹H-NMR Spectra for the Synthesized Complexes

The ¹H-NMR spectra of (1, 2) complexes have been recorded in DMSO-d⁶ and complex (3) in CDCl₃ solvent. The assignment of all signals have been attained by comparing the ¹H-NMR spectra of the ligands registered in literature. Each sort of signal has a particular chemical shift range that can be applied for initial assignment.

The ¹H-NMR spectra of complexes (1-3) displayed an unresolved multiplet signals in the region 𝛿(7.65-8.9), (8.4-9.5) and (7.67-8.4) ppm, were respectively ascribed to aromatic phenyl protons of both (bpdc) and (phozSH) ligands.

Furthermore, disappearance of the singlet carboxyl protons of (bpbc) ligand at (11.0) ppm indicated the coordination of O-atom of bpdc to Cobalt(II), Nickel(II) and Copper(II) metals. By contrast, the thiol proton of (phozSH) ligand was shifted to a higher chemical shift and appeared at (2.59, 3.49, 3.70) ppmwereassigned to N-bound coordination of oxadiazole nitrogen to Co, Ni and Cu(II) metals, respectively^23-26.

Figure 6: 1H-NMR of [Co(κ2–bpdc)(κ1-phozS)(H2O)3]Cl2 Click here to View Figure

Figure 7: 1H-NMR of [Ni(κ2-bpdc)( κ1-phozSH)(H2O)3] Click here to View Figure

Figure 8: 1H-NMR of [Cu(κ2-bpdc)( κ1-phozSH)(H2O)3] Click here to View Figure

¹³C-NMR Spectra for the Synthesized Complexes

The ¹³C-NMR spectral data of the prepared complexes were estimated in DMSO-d⁶ solvent.   In the spectra of (1-3) complexes, the carboxyl carbon atoms of (bpdc) ligand observed in the regions (175.53, 172 and 167) ppm respectively, while the two oxadiazole carbon atoms of (phozSH) ligand were occurred at 𝛿(158 and 168),  (150 and 160) and (152 and 162) ppm respectively. The aromatic phenyl carbon atoms of both bpdc and phozSH correspondingly occurred within (120.57-130.49), (123.96-130.18) and (122-132.08) ppm.^25-27

Figure 9: 13C-NMR of [Co(κ2-bpdc)( κ1-phozSH)(H2O)3] Click here to View Figure

Figure 10: 13C-NMR of [Ni(κ2-bpdc)( κ1-phozSH)(H2O)3] Click here to View Figure

Figure 11: 13C-NMR of [Cu(κ2-bpdc)( κ1-phozSH)(H2O)3] Click here to View Figure

Elemental Analysis for the Synthesized Complexes

The elemental analysis (C, H, N, S) data for all the synthesized complexes are listed in table 1. These numbers are coherent with the proposed stoicheometries. Other physical properties such as colors, molecular weight (M.Wt.) and melting points (M. P.) of the synthesized complexes are also written.

Table 1: Colors, M.Wt. , M.P. and  (CHNS) analysis for the synthesized complexes

Synthesized compounds	Colour	M.Wt	M.P.	(Calculated) Found %
		g/mol	(°C)	C             H             N                  S
1	yellow-green	533	174-176	(45.02)	(3.37)	(10.50)	6.01
				45.66	4.06	10.10	5.47
2	Green	532.75	d.p. 210-212	(45.04)	(3.37)	(10.51)	(6.01)
				45.15	3.27	10.33	4.58
3	light-green	537.60	d.p. 217-219	(44.64)	3.34	(10.41)	(5.96)
				44.20	3.40	8.51	5.51

Electronic Spectra for the Prepared Complexes

The electronic spectra of (bpdc and phozSH) ligands were performed in ethanol and their synthesized complexes in DMSO solvent. The UV.-Vis. spectra of bpdc and phozSH ligands displayed two absorption peaks at (41666, 37037) and (40000, 33333) cm^-1, these transitions were correspondingly ascribed to π- π* and n- π* transitions. The spectrum of Co(II) complex (1), showed three bands in the UV and visible region at 14925, 16393 and 28571 cm^-1 , were due to ⁴T_1g(F) → ⁴T_2g , ⁴T_1g(F) → ⁴A_2g  and ⁴T_1g(F) → ⁴T_2g(P) transitions, respectively.

The spectrum of Ni(II) complex (2) exhibited two d-d transitions at 23809 and 30303 cm^-1 were respectively attributed to ¹A_1g  → ¹A_2g and ¹A_1g  → ¹B_1g , and the Cu(II) complex also exhibit two transition bands at 23809 and 29411 cm^-1 were due to ²B_1g  → ²A_1g  and ²B_1g  → ²B_2g  transitions. The electronic transitions for all metal complexes indicate an octahedral geometry (Table 2)^28,29.

Table 2: Electronic spectral bands of the ligands and their metal complexes

Compounds	Absorption band		Assignment Transition
	Nm	cm-1
bpdc	240	41666	π → π*
	270	37037	n → π*
phozSH	250	40000	π → π*
	300	33333	n → π*
1	350	28571 (ѵ3)	4T1g(F) → 4T2g(P)
	610	16393 (ѵ2)	4T1g(F) → 4A2g
	670	14925 (ѵ1)	4T1g(F) → 4T2g
2	330	30303 (ѵ2)	1A1g → 1B1g
	420	23809 (ѵ1)	1A1g → 1A2g
3	340	29411(ѵ2)	2B1g → 2B2g
	420	23809 (ѵ1)	2B1g → 2A1g

Figure 12: Electronic spectrum of bpdc ligand Click here to View Figure

Figure 13: Electronic spectrum of phozSH ligand Click here to View Figure

Figure 14. Electronic spectrum of [Co(κ2-bpdc)(κ1-phozS)(H2O)3]Cl2 (a) UV.  (b) Vis. Region Click here to View Figure

Figure 15: Electronic spectrum of [Ni(κ2-bpdc)( κ1-phozSH)(H2O)3] Click here to View Figure

Figure 16: Electronic spectrum of [Cu(κ2-bpdc)( κ1-phozSH)(H2O)3] Click here to View Figure

Molar Conductivity for the Synthesized Complexes 

The molar conductivities of the synthesized complexes (1-3) were taken for (10^-3 M) solution in DMSO at (25 °C). It was deduced that all the synthesized complexes are non-electrolyte as demonstrated in Table 3.

Table 3: Molar conductivity (cm². ohm^-1. mol^-1) of (10^-3 M) solution in DMSO for the synthesized complexes+

No.	Complexes		Molar conductivity (cm2. ohm-1. mol-1)
1	[Co(κ2-bpdc)(κ1-phozSH)(H2O)3]		17
2	[Ni(κ2-bpdc)(κ1-phozSH)(H2O)3]		25
3	[Cu(κ2-bpdc)(κ1-phozSH)(H2O)3]		20

Antioxidant Assay (DPPH free Radical Scavenging Activity)

The metal complexes were monitored for free radical scavenging activity by the 2,2-diphenyl-1- picrylhydrazyl (DPPH) method.

The different concentrations of the tested complexes (25, 50, 75 ppm) and standard vitamin-C were received in separate test tubes, and by addition of DMSO solvent the volume of each tube was settled to 2 milliliter. To all sample solution tubes in DMSO, a methanolic solution of DPPH (2 milliliter). The tubes were permitted to stand for 30 min. The control experiment was fulfilled by the same method but without the addition of test samples. The absorbance was taken at 517 nm. Radical scavenging activity was calculated by the subsequent formula.^{30, 31}

The results of free radical scavenger activity of the tested compounds at various concentrations are exhibited in Fig. 17. All complexes 1–3 showed comparable or slightly lower activity than the standard, ascorbic acid.

Co(II), Cu(II) complexes have demonstrated a good free radical scavenging activity with (IC50= 0.920, 5.075 𝛍 mol dm^-1) were the most effective than Ni(II) complex, Fig. 18. Whereas Nicelectrolytekel(II) complex with (IC50 = 31.683 𝛍 mol dm^-1) has showed less activity. The metal complexes of Copper(II), Cobalt(II) were displayed higher scavenging activity than the standard, while Ni(II) complex has lower activity than ascorbic acid. The synthesized complexes scavenged the DPPH radical in a concentration dependent manner.

Table 4: Absorbance of compounds at different concentration at 517 nm

Com. No.	Compound	Concentration (𝛍g/ml)
		25	50	75
	Ascorbic acid (AA)	0.857	0.837	0.816
Com. 1	[Cu(κ2-bpdc)(κ1-phozS)(H2O)3]Cl2	0.791	0.679	0.571
Com. 2	[Ni(κ2-bpdc)(κ1-phozS)(H2O)3]Cl2	0.884	0.860	0.824
Com. 3	[Co(κ2-bpdc)(κ1-phozS)(H2O)3]Cl2	0.722	0.589	0.413

Table 5 : Antioxidant activity of the metal complexes at different concentration using DPPH assay

Com. No.	Compound	Concentration (𝛍g/ml)
		25	50	75
	Ascorbic acid (AA)	51.195	52.334	53.53
Com. 1	[Cu(κ2-bpdc)(κ1-phozS)(H2O)3]Cl2	54.954	61.332	67.482
Com. 2	[Ni(κ2-bpdc)(κ1-phozS)(H2O)3]Cl2	49.658	51.025	53.075
Com. 3	[Co(κ2-bpdc)(κ1-phozS)(H2O)3]Cl2	58.883	66.457	76.48

Table 6: Superoxide dismutase activity of the prepared complexes

Com. No.	Complexes	IC50 (𝛍 mol dm-1)
Com. 1	[Cu(κ2-bpdc)(κ1-phozS)(H2O)3]Cl2	5.075818
Com. 2	[Ni(κ2-bpdc)(κ1-phozS)(H2O)3]Cl2	31.683
Com. 3	[Cu(κ2-bpdc)( κ1-phozSH)(H2O)3]	0.92

Figure 17: Scavenging activity of compounds 1–3 for DPPH Click here to View Figure

Figure 18: IC50 of compounds 1–3 for DPPH. Click here to View Figure

Conclusion

This work comprises synthesis of new mixed ligand Cobalt(II), Nickel(II) and Copper(II) complexes with 2-Mercapto-5-phenyl-1,3,4-oxadiazoleand 2,2′-bipyridine-3,3′-dicarboxylic acid ligands. Based upon Infrared, UV-Visible, ¹H-NMR, ¹³C-NMR and (CHNS) analysis, we concluded that all the prepared complexes have an octahedral shape in which (bpdc) ligand coordinated to the metals through oxygen atoms, whereas the phozSH ligand connected to the metals via oxadiazole nitrogen atom.  In accordance with the molar conductivity data, it has been recommended that all complexes are non-electrolyte.Finally, on the basis of Antioxidant assay,in which metal complexes were screened for free radical scavenging activity by the 2,2-diphenyl-1- picrylhydrazyl (DPPH) method, it can be conclude that all the Cobalt(II), Nickel(II) and Copper(II) complexes have a good free radical scavenging activity.

Acknowledgement

We would like to thank the University of Salahaddin, College of Education for partial support of this work..

Conflict of Interest

There is no conflict of interest.

Funding Source

There is no funding source.

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