Studies on Interactions of Transition Metals with Leucine and Isoleucine using Spectroscopic and Electrochemical Techniques

Abstract

The redox behavior of the metal ions, Cu(II), Zn(II) and Cd(II) in absence and presence of the two isomeric ligand leucine and isoleucine were studied in aqueous medium using cyclic voltammetry. It was found that the redox process of the respective metal ions is quasireversible and some chemical reaction also occurs at the electrode surface while electron transfers. The study also supports that maximum interaction of the metal ion occurs at 1:2 mole ratio with the anionic form of the ligand. Chronoamperometry and chronocoulometry were also performed to further investigate the electrochemical process. The result showed that after interaction the spike height is decreased, indicating towards a decrease in rate of electrolysis. The charge at τ is also decreased when ligand is mixed with the metal ion, which indicates that interaction occurs between metal ion and the ligand. The observations from the Anson plot indicate that adsorption of reactant or product occurs at the electrode surface in all cases. The solid products obtained from the reaction of the metal ions, Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II) with anionic ligand were characterized by different analytical methods. Both leucine and isoleucine forms 1:2 complexes with metal ion, which is confirmed by the elemental and metal analysis. Thermal analysis ensures that the Co and Ni complex of both leucine and isoleucine contains two molecules of crystalline water. The Cu complex of isoleucine contains one molecule and the Zn complex of isoleucine contains half molecule of crystalline water. The other complexes do not contain any water of crystallization. The compounds have high melting temperature and are mostly insoluble in the common solvents. But the Ni-leucine, Co-isoleucine, Ni-isoleucine and Cu-isoleucine complexes are soluble in methanol. The bonding nature of the complexes was characterized by spectroscopic study. IR spectral data of the complexes indicate that the metal-ligand bonding occurs through nitrogen atom of NH2 group and oxygen atom of COO- group. The shifting of the absorption bands, appearing of d-d transitions and charge transfer bands in the UV-Visible absorption spectrum also indicates the probability of forming M←L coordination bonds in the complexes. The UV-Visible diffuse reflectance spectral analysis shows that all the complexes have lower band gap energy, indicating their good conducting behavior. The nuclear magnetic resonance (NMR) spectral analysis demonstrated that the peaks overlap with each other. A peak for -COOH proton appears in the ligand but in the complex it is not seen. This may be due to complexation of ligand with the metal ion. The Differential Scanning Calorimetry (DSC) curve of the complexes is sharp endothermic. Therefore, the weight changes monitored by thermogravimetry involved absorption of energy. Magnetic susceptibility data of the complexes conclude that all of them are high spin paramagnetic complex excluding Zn, Cd and Hg complexes which are diamagnetic. Density functional theory (DFT) has been employed in calculating the equilibrium geometries and vibrational frequencies of the complexes at B3LYP level of theory using 6- 31G(d) and SDD basis sets. In addition, frontier molecular orbital and time-dependent density functional theory (TD-DFT) calculations are performed with CAM-B3LYP/6-31+G(d,p) and B3LYP/SDD level of theories. DFT calculation confirms that Co, Ni and Cu complexes form square planar structure whereas Zn, Cd and Hg adopt distorted tetrahedral structure. Cationbinding energy, enthalpy and Gibbs free energy values indicate that the complexes are thermodynamically stable. UV–Visible and TD-DFT studies reveal that these complexes demonstrate representative charge transfer and d–d transitions bands. The experimental IR vibrational frequencies and the absorption properties are very consistent with the calculated values. The HOMO-LUMO gap of the complexes is decreased from the parent ligand indicating that metal has a noticeable effect on the frontier molecular orbital energies.