Evaluation of oxygen-containing pentadentate ligands with pyridine/quinoline/isoquinoline binding sites via the structural and electrochemical properties of mononuclear copper(ii) complexes

Eighteen mononuclear copper(II) complexes with oxygen-containing N4O1 pentadentate ligands were prepared. The ligand library consists of 2-aminoethanol derivatives (((ArCH2)-C-1)((ArCH2)-C-2)NCH2CH2OCH2Ar3) bearing three nitrogen-containing heteroaromatics (Ars) including pyridine, quinoline and isoquinoline via a methylene linker. Systematic replacements of pyridine binding sites with quinolines and isoquinolines reveal the general trends in the perturbation of bond distances and angles, the redox potential and the absorption maximum wavelength of the copper(II) complexes, depending on the position and number of (iso)quinoline heteroaromatics. The small effect on the redox potentials resulting from quinoline substitution at the Ar-3 position (near oxygen) of the ligand comes from the steric hindrance of the peri hydrogen atom in the quinoline moiety at this position, which removes the counter anion to enhance the coordination of quinoline nitrogen and ether oxygen atoms to the metal centre. In the absorption spectra of copper(II) complexes in the d-d transition region, the quinoline substitution at this site (Ar-3) exhibits an opposite effect to those at the Ar-1 and Ar-2 sites. The electronic and steric contributions of the heteroaromatic binding sites to the ligand properties are comprehensively discussed.