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

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.

Graphical abstractThe synthesis, structure and spectral and redox behaviour of copper(II) complexes of sterically constrained N3, N3S2 and N2S2 ligands are described. In the crystal structure of one of the complexes the CuN3Cl2 chromophore possesses a distorted square pyramidal coordination geometry. The replacement of isopropyl groups in this complex by the less sterically demanding methyl groups leads to a decreased geometric distortion. The interesting electrochemical behavior of the complexes has been explained based on a square scheme.Figure optionsDownload full-size imageDownload as PowerPoint slide