Copper(II) complexes of sterically hindered schiff base ligands: synthesis, structure, spectra and electrochemistry

Abstract

The synthesis, structure and spectral and redox behaviour of copper(II) complexes of sterically constrained N₃, N₃S₂ and N₂S₂ ligands are described. The ligands 2,6-bis(2,4,6-trimethylphenyliminomethyl)pyridine (L1), 2,6-bis(2,6-diisopropylphenyliminomethyl)pyridine (L2), 2,6-bis(2-methylthiophenyliminomethyl)pyridine (L3), 1,3-bis(2-methylthiophenyliminomethyl)benzene (L4), 1,3-bis(2,4,6-trimethylphenyliminomethyl)benzene (L5) and 1,3-bis(2,6-diisopropyl-phenyliminomethyl)benzene (L6) have been synthesized. The 1:1 copper(II) complexes of L1-L5 have been isolated and characterized by spectral and electrochemical techniques. The X-ray crystal structure of the complex [Cu(L2)Cl₂] (2) has been successfully determined and is found to possess a distorted square pyramidal coordination geometry. The replacement of isopropyl groups in this complex by the less sterically demanding methyl groups as in [Cu(L1)Cl₂] leads to a decreased geometric distortion. The complexes show relatively high positive redox potentials with a low reorganizational energy barrier for electron transfer involving the Cu(II)/Cu(I) couple. The redox cycle of the complexes 1 and 2 has been explained by assuming a square scheme. The spectral and electrochemical properties of CuN₃S₂ and CuN₂S₂ complexes generated by the incorporation of thioether sulfurs into L1/L2 and L4/L5, respectively, are also discussed. Both the complexes show positive redox potentials suggesting the coordination of at least one thioether sulfur to copper(II). The incorporation of a pyridine nitrogen donor into the CuN₂S₂ complexes leads to higher LF field strengths but with lower absorptivities. The coordination of Cl^- ions to the CuN₃S₂ and CuN₂S₂ complexes significantly alter the spectral and redox properties of the complexes.