Electroorganic Synthesis of Copper and Zinc Complexes with Novel Ligand Oxalic Acid

 

Shreya Gorai¹*, M. Alam², B. H Gorai³

¹St. Xavier’s College, Department of Chemistry, Ranchi.

²Redt. Professor of PG Department of Chemistry, Ranchi University, Ranchi.

³Redt. Associate Professor, R.L.S.Y College, Ranchi University, Ranchi.

*Corresponding Author E-mail: shreyagorai.gorai@gmail.com

 

 

INTRODUCTION:

The design and construction of new compounds with favourable properties is the main purpose of modern chemistry that is the core concept of electroorganic synthesis.  Electroorganic synthesis is the synthesis of organic compound by the use of universal chemical reagent “electrons” for current flow, regarded as non - polluting agent. This process is currently experiencing a renaissance. In this method it is possible that the green aspects of sustainable energy sources is transfer to the whole production process.

 

O- Donor ligands possess significant medical properties especially when they are connected to biologically important metals.^1,2,3

 

In the recent past a considerable attention was devoted to the preparation of copper and zinc complexes of macrocyclic ligands by electrochemical synthesis due to their increased potential in basic applied chemistry^4-5.

 

The design and synthesis of these systems are very useful for the achievement of specific chelate synthesis our research group has been concerned in the synthesis, determination of structural features and coordination behaviour of O- donor ligands based on oxalic acid with metal ions Zn and Cu. It has polymerising property to form metal complexes⁶. Oxalic acid is a conjugate acid of oxalate ion, C₂O₄²⁻ Fig 1. Due to the hydrogen bonding and polarity of oxalic acid the solubility was high in water as compared to other solvents like chloroform, acetone, methanol and ethanol⁷. It is a bidentate ligand forming 5- membered chelate ring complexes, when it binds to metal. Oxalic acid is a low-molecular-weight organic acid used as a wood bleaching agent, rust stains remover and waste water treatment^8,9. Transition metal chelated oxalates are used for synthesis of nanorods.¹⁰

 

Depending on the source of electricity, electrochemical methods can be inherently green and environmentally benign and can be easily controlled to achieve high levels of selectivity¹¹. We herein report the synthesis, structure and spectroscopic characterization of Zn and Cu derivatives.

 

2 electron pairs can form 2 coordinate covalent bonds in oxalate ion.

Fig 1: structure of oxalate ion

 

MATERIALS AND METHODS:

Materials:

Metals (Cu, Zn) rod, methanol, oxalic acid and LiClO4. 3H2O (Aldrich) were used as supplied. The electrosynthesis were carried out using a power supply 3.5 dc volt (rectifier).

 

Instrumentations:

Metal percentage were determined by Thermo Scientific ICE 3000 series Flame Mode atomic absorption spectrophtometery^12-13. C, H, N contents were determined by standard methods of organic analysis on a Carlo-Erba 1108 microanalyzer. IR spectra were recorded on a Shimadu-8201PC spectrophotometer. Perkin Elmer diamond TGA/DTA instrument was used for thermal analysis.

 

Experimental Procedure:

Electroorganic synthesis is the synthesis of organic compounds in electrochemical cell in which there are two electrodes, the anode (positive) and the cathode (negative) submerged in a solution phase containing ligand /substrate and organic aqueous solvent with a supporting electrolyte dissolved in it to assist in carrying the current through the cell

 

Electrochemical cell reaction can be schematically be represented by Fig: 2:

 

Fig 2: Electrochemical Cell

 

Cathode: 2HL + 2e → 2L- + H₂ (g)

Anode: 2L- + M → ML₂ + 2e

 

•       Anode: sacrificial metal electrode Cu,Zn

•       Cathode: platinum electrode

•       Solvent: Methanol, acetone

•       Electrolyte: LiClO₄

 

1)    Synthesis of Organocopper complex:

Electrochemical cell consisted of a 100ml tall form beaker in which two electrode: platinum as cathode and copper as anode are suspended in a liquid phase containing ligand: oxalic acid (2gm); solvent:  acetone (30ml); and supporting electrolyte: LiClO₄ (0.05gm dissolved in 5ml of methanol). The electrolysis was conducted at current strength of 20mA and voltage 3.5 v for 7 hours at room temperature. As the electrolysis proceeded, gas evolved at the cathode and a Bluish white colour product is formed at the anode. This material gradually deposited at the bottom of the cell. Resulting product was recovered by filtration, washed with acetone (5ml) and dried in a vacuo. Bluish white colour product was obtained and named as Cu01. The complex obtained in pure state was subjected to elemental, spectral and thermogravimetric analysis.

 

Cell reaction represented by equation (1)

Cu ₍₊₎ | oxalic acid   + acetone + LiClO₄ | Pt _(-)     …._.(1)

 

2)    Synthesis of Organozinc complex:

Electrochemical synthesis was carried out in an undivided cell equipped with two electrode: platinum as cathode and zinc as anode suspended in a liquid phase containing ligand: oxalic acid (2gm); solvent:  acetone (30ml); and supporting electrolyte: LiClO₄ (0.05gm dissolved in 5ml of methanol. The electrolysis was conducted at current of 20mA and voltage 3.5v for 7hours at room temperature. As the electrolysis proceeded, gas evolved at the cathode and a white colour product is formed at the anode. This material gradually deposited at the bottom of the cell. At the end of the experiment, it was collected by filtration, washed with acetone (5ml) and dried in a vacuo. White colour product was obtained and named as ZnO₂. The complex obtained in pure state was subjected to elemental, spectral and thermogravimetric analysis

Cell reaction represented by equation (2)

 

Zn ₍₊₎ | acetone + acetone + LiClO₄ | Pt _(-)           ……...(2)

 

RESULT AND DISCUSSION:

The preparative results show that the direct electrochemical oxidation of the metals in the presence of a ligand solution is a one-step process and represents a convenient and simple route to a variety of transition metal complexes.

 

1.     Elemental analysis:

Table I: Copper Complex

O= observed data; E= experimental data

 

Molecular formula: H₂Cu (C₂O₄)

Elemental analytical data of sample Cu01 are very close to the theoretical values (observed data) as shown in Table I. Metal percentage was obtained by atomic absorption spectroscopy method. Elemental analysis show that the metal to ligand ratio is 1:2 and the composition of metal complex is CuC₄H₂O₈

 

Table II : Zinc Complex

O= observed data; E= experimental data

 

Molecular formula: H₂Zn (C₂O₄) ₂

On the basis of elemental analysis data of zinc complex molecular formula of the complex is formulated as H₂Zn (C₂O₄) ₂

 

2.     FTIR spectral analysis:

Copper Complex:

 

Graph: 1: The FTIR spectrum of copper complex with oxalic acid as a ligand

 

The spectrum exhibit the following characteristic absorption shown in table III.

 

Table III. IR ABSORPTION PEAKS. Cu01

νs = symmetric vibration; νa= antisymmetric vibration

 

The oxalate ion (C₂O₄)^2-   Is a bidentate dibasic ligand; having two bonding points for the metal ion occurs through its both negatively charged O-atoms. The complexation of metals with ligands can drastically change the Physico–chemical and biological properties of the metal species.

 

The infrared spectrum showed frequencies corresponding to the carboxylate group, Hydroxyl group, metal –oxygen etc. are given in Table III. The infrared spectrum of CuC₄H₂O₈ showed a broad band at 3576  cm^-1  due to (-OH) stretching ,a broad band at 1662cm^-1 and 1512cm^-1 due to ᶹasy(C=O) and band at 1442 cm^-1, 1361.74 cm^-1  and 1323.17 cm^-1 due to ᶹsy (C-O) of coordinated oxalate group^15,16. It is known that the coordination of oxalate anion leads to the band shift of symmetric and antisymmetric C=O stretching frequencies. The symmetric stretching frequency will decrease with the covalent bond formation between the metal cation and oxygen.^{17, 18}. The IR strong band at 825 cm-' has been assigned to M-O band where M = (Cu). The bidentate linkage to be more favorable of the oxalate group with the metal was confirmed on the basis of the difference between the antisymmetric and symmetric (C=O) stretching frequencies^{19, 20}. It is reported that In the divalent metal series as Cu(II), υ(C=O) absorbs at higher frequency than υ (C-O), this supports our spectral data  findings.²¹   Bands at 1442cm^-1 is assigned to c-c stretching vibration. The Above FTIR spectral analysis supports the proposed formulation shown in Fig :3

 

Fig 3: Proposed formulation of copper complex

 

Zinc Complex:

 

Graph: 2. The FTIR spectrum zinc complex with oxalic acid as a ligand,

 

The spectrum exhibit the following characteristic absorption:

 

Table IV. IR absorption peaks Zn02

νs = symmetric vibration; νa = antisymmetric vibration

 

The infrared spectrum of Zn0 showed frequencies corresponding to the carboxylate group, Hydroxyl group, metal – oxygen etc. are given in Table IV.

 

The infrared spectrum of (H₂Zn (C₂O₄) ₂ showed a broad band at 3398 cm^-1 due to (-OH) stretching, a broad band at 1681 cm^-1 due to ᶹasy(C=O) and band at 1261 cm^-1 and1346 cm^-1 due to ᶹsy (C-O) of coordinated oxalate group. An increase in oxalate resonance leads to single-bond character in the carboxyl group, which is observed as a lowering of the frequency of the compounds which corresponds to coordination of oxygen atom of carboxyl group to the metal ions.^22,23,24 Bands at 486 cm^-1 attribute to nM-O bond. The Above FTIR spectral analysis supports the proposed formulation shown in Fig.: 4

 

Fig 4: Proposed Formulation of Copper Complex

 

3.Thermogravimetric analysis:

Copper complex:

Thermogravimetric loss pattern of: Cu01

The sequence of decomposition reactions as deduced from TGA and DTA studies in Graph :3 and Graph :4  are  summarized below in table V

 

Table V. DTA CURVE OF Cu01

GRAPH 3: TGA CURVE OF Cu01

 

GRAPH 4: DTA CURVE OF Cu01

 

Scheme1: Illustrating the Formulation Sequence DTA Curve of Copper Complex

 

The above results are in contrast to the reported thermogravimetric analysis (TG) measurements Graph :3 and differential thermal analysis (DTA) Graph: 4, where the data were interpreted as reduction of CuC₄H₂O₈ to Cu somewhere between 50°C and 400°C. TGA curve shows two step decomposition (scheme 1). The first step is observed in the temperature range 250°C -300°C and is accompanied with 48.51% loss. This is attributed to loss of water molecule, carbon dioxide and two molecule of carbon monoxide. The second step occurs in the temperature range 350°C – 400°C showing weight loss of 24.02% against theoretical loss of 24.84% shown in Table: V. This is attributed to loss of one molecule of carbonate which corresponds to the complete conversion of CuC₄H₂O₈ to Cu. The weight loss in the temperature range between 250°C -300°C give endothermic DTA peaks which occurred at same temperature region in TGA curve. It was verified that the presence of endothermic peak at 281.30°C corresponding to fusion and thermal decomposition respectively. Literature studies supports our TGA data findings^{25,26, 27, 28, 29}. The proposed formulation Fig: 3 is supported by FTIR peaks and thermogravimetric loss pattern. 

 

Zinc complex:

Thermogravimetric loss pattern of: ZnO₂

The sequence of decomposition reactions as deduced from TGA   and D TA studies Graph: 5 and Graph:6 are summarized below in table VI:

 

Graph 5: TGA curve of ZnO₂

 

Graph 6: DTA curve of ZnO₂

 

TABLE VI. DTA CURVE ZnO₂

 

Scheme 2: Formulation Sequence of DTA Curve 

 

The above results are in contrast to the reported thermogravimetric analysis (TG) Graph:5 and differential thermal analysis (DTA) measurements Graph 6, where the data were interpreted as reduction of ZnC₄H₂O₈ to ZnO somewhere between 25°C and 500°C. TGA curve shows two step decomposition. Scheme: 2. The first step is observed in the temperature range 25°C -100°C and is accompanied with 28.86% loss. This is attributed to loss of one molecule of carbon dioxide and carbon monoxide. The second step occurs in the temperature range 150°C – 200°C showing weight loss of 68.83 % against theoretical loss of 73.98% (Table VI). This is attributed to loss of two molecule of carboxylic acid group which corresponds to the complete conversion of ZnC₄H₂O₈ to ZnO. The weight loss in the temperature range between 25°C-300°C give two endothermic DTA peaks which occurred at same temperature region in TGA curve. It was verified that the presence of endothermic peak at corresponding to fusion and thermal decomposition respectively^30,31. The proposed Fig: 4 formulation is supported by thermogravimetric loss pattern.

 

CONCLUSION:

The results shows that it is possible to carry out the electrosynthesis of metal oxalates complexes with almost quantitative yields in the mild condition. The anodic electrolytic procedure is an alternative route for the synthesis of oxalate complex which needs only a current and can be easily scaled up for production. Electrochemical process is non- polluting, product specific and one pot synthesis than multistage conventional chemical process. In recent years, organic electrosynthesis has been recognized as new reactor for industrial revolution. Organic electrosynthesis, known for decades, has now gained enormous momentum and stretched its wings to other branches of chemistry, thus making it an “in situ –interdisciplinary research”. Finally, from a viewpoint of practical application, the present study describes a process which can be easily scaled up for the production of zinc and copper oxalate powder.

 

CONFLICT OF INTEREST:

no conflicts of interest regarding this investigation.

 

ACKNOWLEDGMENTS:

We thank the head of the University Department of Chemistry, Ranchi University Professor Dr. Hari Om Pandey for providing necessary facilities and encouragement. We are grateful to acknowledge IIT Mumbai, CIF, Bit Meshra Ranchi and SAIF Tezpur University for providing the facility to carry out thermal, elemental and FTIR analysis.

 

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Received on 26.05.2022                    Modified on 15.06.2022

Accepted on 30.06.2022                   ©AJRC All right reserved