A Study of Metal Complexes of 2 - Picolinic Acid

Introduction                    

Picolinic acid is a chelating agent composed of elements in their human body. The zinc dipicolinate dietary additives are favored due to their articulate significance of instigating zinc into the body. The mineral manganese is essential for human’s life and it plays vital role for the plant growth. It act as a main energy source for the redox mechanism in photosystem (I) and (II). Deficiency of manganese leads to the osteoarthritis and osteoporosis. The hard ferromagnetic mineral is cobalt and well know cobalt complex is Vitamin-B₁₂. It is used in the treatment of anaemia because it enhance the production of red blood cells. The naturally abounded micro nutrient is nickel which increases the hormonal activities and also affects the lipid metabolisms. The copper mineral needed for human’s health which involves in the various natural process of human’s body. Copper based complexes are widely reported for wound healing and prevents the infection after the injury. Copper mostly stored in muscles and bones and its level can regulated by the liver. Its complexes commonly utilized for acne, Alzheimer and tooth plaque. The trace mineral zinc involves in the DNA creation, supports the body immunity, cell growth, protein synthesis and heals the damage tissues. Excess of zinc can reduce the absorption of copper and iron.

A literature review on the subject of picolinic acid coordinate to the metal through nitrogen and carboxylate oxygen are most prominent ^[1-4]. The antimicrobial activities ^[5,6] of some metal complexes of the mixed ligands were reported ^[7-15]. The electrochemical studies and the biological activities of the Cu (II) complex with picolinic acid and Co (II) complex imides were reported respectively ^[16] Similarly, the homogeneous H₂ evolution from the H₂O of cobalt complexed with picolinic acid ^[17] also reported. The homogeneous electrocatalytic reduction of water in aqueous medium of nickel (II) complex with picolinic acid ^[18] is  reported in the previous reviews. In subject to this paper, we have been examined the conglomeration, properties of cobalt, manganese, nickel, copper and zinc complexes with picolinic acid were studied and furthermore, the electrochemical behavior and anti-microbial activities also studied.

Materials and Methods

Materials

The chemicals utilized for the preparation of the five picolinic acid complexes are standard grade and are ethanol, diethyl ether, the ligand picolinic acid, five metal carbonates and the perchloric acid were procured from the Ponmani & Co Chemicals, Trichy, Tamilnadu, India.

Synthesis of five metal(II) perchlorate

The five different perchlorates of cobalt, manganese, nickel, copper and zinc were complexed by simple precipitation method. Ten grams of each of the metal carbonates (Cobalt(II) carbonate, Manganese(II) carbonate, Nickel(II) carbonate, Copper(II) carbonate,  and Zinc Carbonate) treated with fourteen milliliters of 70 % aqueous perchloric acid and then re-crystallized with water. All the complexes were concocted by treating the one mole of each metal (II) perchlorate and 3 moles of ligand are dissolved in ethanol and refluxed for six hours. The solutions are concentrated then cooled, the excess ligand in the reaction mixture is removed by washing with the diethyl ether. Finally synthesized five complexes are crystallized.

Characterization of Five Complexes and their Anti-microbial activity

The FT-IR spectra of synthesized five perchlorate complexes were screened by mixing five chlorates with the KBr in the shimadzu24 make of FT-IR 8400S. The electronic spectra of three complexes were screened in the ethanol and methanol solvent with make of a Hitachi U- 3400 spectrophotometer. Likewise, the ¹H-NMR spectrum of Zinc complex and ligand was transcripted with the solvents of DMSO-D₆ and CDCl₃. The TGA of five complexes were recorded between the ranges of 50-800°C under the inert nitrogen atmosphere with reference of α – Al₂O₃. By Guoy’s method the magnetic susceptibility of five perchlorates are measured and the electro chemical behaviors were investigated with the help of cyclic voltammetry instrument device VSP(SNO147). Tetrabutyl ammonium perchlorate is used as a supporting electrolyte.

Antimicrobial screening of the ligand and its five complexes have been investigated inimical to three fungal and four bacteria by using agar diffusion method.

Results and Validation

Physicochemical Studies

Molar conductance of the prepared complexes was assigned by the following composition. From the conductance studies, we came to know about the complexes be an adherent to 1:2 electrolyte ^[19] where the anions are present outside the coordination sphere. Magnetic moments,  and melting point of metal complexes are shown in Table 1.    of metal ions are confined to the predicted values.  On heating, the complexes decompose at temperature above 260°C. It revealed that all the complexes were stable.

Table 1: Physicochemical properties of metal complexes.

Complex	Conductance values Ohm-1cm2mol-1	Type of Electrolyte		Melting Point	Colour
[MnL3]) 2+(ClO4)2	171.89	1:2	6.02	260	White
[ CoL3] 2+(ClO4)2	186.6	1:2	4.9	260	Brown
[NiL3] 2+(ClO4)2	202.23	1:2	2.9	290	Light Blue
[CuL3] 2+(ClO4)2	193.07	1:2	1.9	310	Dark Blue
[ZnL3] 2+(ClO4)2	172.19	1:2	–	280	White

L=Picolinic acid

IR Spectroscopy

The spectra of ligand reveal a sharp band at 1742 cm^-1 is corresponds to the C=O stretch of carboxylic acid ^[20]. The stretch found at 1675 cm^-1 is attributed to the C=N of pyridine ring and the O-H stretch of the acid group is found at 3396 cm^-1. It was compared with the complexes and the shift changes to a lower frequency of about 1718 cm^-1 for the C=O of carboxylic acid and 1640 cm^-1 for C=N stretch of pyridine ring. The O-H stretch appears at a higher frequency was shifted suggesting that the acid functionality is not ionized. The C=C stretches of the ligand and its complexes which appears at 1600 cm^-1 is more or less similar.  The band found at 1087 cm^-1 and 627 cm^-1 is corresponds to the perchlorate anion ^[21] is present outside of the complex without any splitting. The new band is detected which occurs between the range of 551-524 cm^-1 are attributed to the M-O bonding (metal- oxygen) and other band in the range of 449 – 418cm^-1 is mainly because of metal- nitrogen bonding ^[22,23,24] 

Figure 1: Infrared Spectrum of [Mn(PA)3](ClO4)2 Complex. Click here to View figure

Figure 2: Infrared Spectrum of [Zn(PA)3](ClO4)2 Complex. Click here to View figure

Table 2: FT-IR of five metal complexes.

Compound	stretching cm-1	acid cm-1	cm-1	cm-1	stretching cm-1	cm-1	cm-1
Ligand	3396	1742	1675	1599	3099	–	–
[MnL3]2+(ClO4)2	3400	1705	1655	1598	3181	551	430
[ CoL3]2+(ClO4)2	3408	1724	1653	1602	3153	524	420
[NiL3]2+(ClO4)2	3404	1726	1653	1602	3153	524	418
[CuL3]2(ClO4)2	3427	1707	1654	1600	3157	551	449
[ZnL3]2+(ClO4)2	3497	1707	1643	1592	3242	530	441

 

Electronic Spectra of Metal Complexes ^[25]

 The copper perchlorate complex reveals the band at 19,607 cm^-1 is assigned to ⁴ T₁g (F)  ®  ⁴T₁g (P) transition having an octahedral geometry. The perchlorate complex of nickel also exhibits a band at 28,089 cm^-1 corresponds to ³A₂g   ®  ³T₁g (P) transition with an octahedral geometry. The copper complex gives a wide-ranging band at 15,290 cm^-1.  The complexes show absorption band in 600-900 nm regions. The octahedral complexes reveal a unique band due to ²Eg  ® ²T₂g transition, due to Jahn- Teller distortion. Similarly, copper perchlorates exhibits a band at 15,290 cm^-1 with a distorted octahedral geometry.

Table 3: Electronic spectra of metal complexes.

Complex	Medium	Assignment	Stereochemistry
[CoL3]2+( ClO4)2	Ethanol	19,607cm-1 4T1g(1F) ® 4T1g(P)	Octahedral
[NiL3]2+( ClO4)2	Ethanol	28,089cm– 3A2g ® 3T1g (P)	Octahedral
[CuL3]2+( ClO4)2	Ethanol	15,290 cm-1 2Eg ® 2T2g	Distorted octahedral

Figure 3: Electronic spectra of (i) Cobalt(II) perchlorate, (ii) Nickel(II) perchlorate, (iii)Copper (II) perchlorate complexes. Click here to View figure

 

¹H NMR Spectral studies

The spectrum of the ligand which has been taken in CDCl_3, shows different peaks at 7.6-8.8 ppm corresponding to the aromatic protons. The peak at 9.76 ppm was assigned to hydrogen of carboxylic acid group.

Figure 4: 1H NMR spectrum of ligand. Click here to View figure

Zinc (II) perchlorate complex is extracted from DMSO-d₆.  Signals correspond to 7.5-9.07 ppm for aromatic protons. 13.48 ppm corresponds to carboxylic acid hydrogen. This group is involved in coordination. This peak in complex shifted to low field signal. The presence of carboxylic acid proton in the NMR spectrum of Zinc (II) complex reveals that the not ionization of –COOH (acid) group. The shift in the position of that proton in the spectrum of the complex in comparison with that of the ligand suggest that the electron density around the proton is changed due to the coordination of carbonyl oxygen (C=O) of the acid group and due to absence of hydrogen bonding.

Figure 5: 1H – NMR of Zinc (II) perchlorate complex. Click here to View figure

Table 4: ¹H – NMR of Ligand and Zinc(II)perchlorate complexes.

Compound	Aromatic ring proton (ppm)	Carboxylic acid proton (ppm)
Ligand2+	7.6-8.8	9.76
[ZnL3] (ClO4)2	7.5-9.07	13.48

L = Picolinic acid

Thermal Analysis

Manganese (II) perchlorate complex was stable up to 258.2°C which indicates that there is no water molecule in the complex^26,27. In the analysis of TGA curve, the moisture is removed of about 5.8% (cal 5.77%) at 258.2°C which is associated by an endothermic peak with 296°C on the DTA curve shows that the complex defrosts in prior to decomposition, which indicates that melting of the perchlorate ion. In the first stage of degradation, at 307.4°C it is observed that there is loss of perchlorate ions and decomposition of the pyridine ring with an experimental value of about 38.3% (cal 39.6%) thereby associated by an exothermic peak with 439.2°C and 510.1°C followed by decomposition of the ligands. In the second stage of degradation, at 657.5°C it correlated to the decomposition of the ligands with an experimental value of about 31.1% (cal 32%). The weight of the final residue is stable which is accompanied by Manganese oxalate with 21% (cal22.9%). The Cobalt (II) perchlorate complex is stable up to 197.6°C which indicates that there is no water molecule in the complex. An endothermic peak at 211.5°C corresponds to the melting of perchlorate ions. Decomposition occurs at an exothermic peak which was found at 311.8°C. There occurs a systematic drop in the weight of the complex at the terminal stage of about 537.4°C. The curve appears at Nickel (II) perchlorate complex intimates that the complex is stable up to 304.1°C which indicates that it has no water molecule. The initial stage of degradation arise at 304.1°C losses one perchlorate with moisture 17.3% (cal 15.9%), an exothermic peak with 335.3°C, 457.8°C and 503.5°C exhibits the ligand decompositions. The weight of the last residue that metal oxide is formed.

Figure 6: TGA and DTA of Manganese(II) perchlorate complex. Click here to View figure

Figure 7: TGA and DTA of Cobalt(II) perchlorate complex. Click here to View figure

Figure 8: TGA and DTA of Nickel(II) perchlorate complex. Click here to View figure

Figure 9: Structure of Metal(II) perchlorate complexes. Click here to View figure

Cyclic Voltammetry studies

Figure 10: Cyclic voltammogram of [CuL3]2(ClO4)2   Click here to View figure

Figure 11: Cyclic voltammogram of [ZnL3]2+(ClO4)2 Click here to View figure→

The cyclic voltammogram of ligand and all metal complexes are recorded in 0.001M electrolyte containing DMF.  All the metal(II) perchlorate were scanned in the limit of -2V to +2V at the scan rate 20 mV/s, except manganese(II) perchlorate  complex. The scan rate of this complex is 10 mV/s. The measurable parameter of a quasi-reversible system is ∆Ep. This would be greater than 0.59V for quasi reversible process. The ligand exhibits irreversible reduction peak around at -0.69V with irreversible oxidative peak at 1.31V. Manganese (II) perchlorate complex exhibits one electron reduction corresponding to Mn(II) ® Mn(I) at E_1/2 value of   -0.784V. (=65mV) and irreversible oxidation peak at 1.152V reveals that Mn(II) → Mn(III) oxidation^[23].   Cobalt(II) perchlorate complex exhibits  an irreversible reduction at -0.4699Vcorresponding to  Co(II) ® Co(I) On the oxidative response, it exhibits an irreversible  oxidation at 0.8485Vreveals that Co(II) ® Co(III) oxidation. Nickel(II) perchlorate complex exhibit quasi reversible^[28] one electron reduction corresponding to  Ni(II) ® Ni(I) at E_1/2 values of -0.534V ∆E_p = 129 mV),and an irreversible oxidative response  at  E_pa = 0.769V reveals that Ni(II) ® Ni(III) oxidation²³.  Copper (II) perchlorate complex exhibits irreversible reduction at  = -0.4019 and -1.7525V corresponding to Cu(II) ® Cu(I) an Cu(I) ® Cu(0) reduction and an irreversible oxidation occurs with the peak value of  = 1.133V attributed to Cu(II) ® Cu(III) oxidation²⁴.  Zinc(II) perchlorate complex exhibits quasi reversible one electron reduction corresponding to Zn(II) ® Zn(I) at E_1/2 values of -0.840V, – (∆E_p = 77 mV)  and an irreversible oxidation  peak at  = 1.022V attributed to  Zn(I) ® Zn(0) We concluded that , most of the complexes display one electron reduction and irreversible oxidation potential range 0.76 V to 1.1V.The reduction waves with E_1/2  values in the range of 0.53V to 0.84V is assigned to the M(II) ) ® M(I) reduction. Therefore, the cyclic voltammogram of all complexes show metal centered process. The potentials are summarized in the table 6.

Table 5: CV study of metal complexes.

Compound	(V)	(V)	(V)		(mV)	(V)
Ligand		-0.690				1.310
[MnL3]2+(ClO4)2	-0.752	-0.817	-0.784		65	1.152
[CoL3]2+(ClO4)2		-0.4699	–			0.8485
[NiL3]2+(ClO4)2	-0.4702	-0.599	-.0.534		129	0.769
[CuL3]2(ClO4)2	–	0.4019 1.7525	–		–	1.119
[ZnL3]2+(ClO4)2	-0.8786	-1.022  0.8016		-0.840	77	–

 

Antifungal Activity

Table 6: Antifungal activity of metal complexes.

Name of organism	Ligand	[ CoL3]2+(ClO4)2	[NiL3]2+(ClO4)2	[CuL3]2(ClO4)2
Aspergillus niger	11	4	–	9
Aspergillus flaves	9	10	–	3
Aspergillus terreus	9	11	–	28

It has been investigated that the antifungal activity of the mixture of metal complexes was inimical to 3 fungal pathogens by the assay of disc- diffusion and their outcomes are shown in table 6. Comparative inhibitions of the non-complexed ligand with their complexes concludes that the Cobalt (II) perchlorate complex has less activity against Aspergillus niger. Copper (II) complex has very less activity against Aspergillus niger and Aspergillus flaves, but having a good activity against Aspergillus terreus.

Figure 12: Antifungal activity of metal complexes. Click here to View figure

Antibacterial Activity

Table 7: Antibacterial activity of metal complexes

Name of organism	Ligand	[ CoL3]2+(ClO4)2	[NiL3]2+(ClO4)2	[CuL3]2(ClO4)2
K. pneumonia	13	–	12	17
E. coli	14	14	25	11
Bacillus anthrax	–	10	27	–
Sta. aurerus	–	9	20	16

From the anti-bacterial analysis, a moderate activity is imparted. Whereas, in Nickel(II) perchlorate complex it exhibits a higher activity in opposition to three bacterial microorganism. The Cobalt(II) complex has moderate activity against three pathogens. Copper(II) complexes has good activity against K.Pneumonia and S.aurerus.

Figure 13: Antibacterial activity of metal complexes Click here to View figure

Conclusion

The spectral studies of the metal(II) perchlorate complexes revealed that the ligand exhibits a mode of coordination through the oxygen of carbonyl and nitrogen of the pyridine ring. The spectral studies evidence that the octahedral geometry of the complex is manifested.   A thermal study revealed that all the complexes have no water molecule. The CV studies revealed that the complex of nickel exhibits the quasi-reversible one electron transfer reduction process. Manganese(II) and Zinc(II) complexes reported the reversible one electron  reduction process  and  Copper(II) and Cobalt(II) complexes exhibit an irreversible reduction and oxidation process The ligand and  complexes possesses better activity against fungal and bacterial strains.

Acknowledgement

Both the authors would like to thank the parent institution Holy Cross College (Autonomous), Trichy to do this research work in their laboratory.

Conflict of Interest

Both the authors declares no conflict of interests.

Funding Sources

There is no funding source.

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