Redox chemistry of copper complexes with various salen type ligands

The two essential redox processes in Cu(II) salen (and salan) complexes Cu(II)/Cu(I) and [PhO]/[PhO[−]] were studied by electrochemical and spectroelectrochemical (UV–Vis-absorption or EPR) techniques on a series of complexes in which the salen (salan) type ligands bear various substituents R = H, F, Ph, tBu or CH3 on the linker C- or N-atoms or on the phenol core. The substitution pattern can be related to the stability of the phenoxy radicals. The geometry of the complexes (especially around the Cu atom) could be established by XRD for the new complex [(Me2salhexF4)Cu] {H2(Me2salhexF4) = (1R,2R)-N,N′-bis(2-hydroxy-3,5-di-fluoro-acetophenonylidene)-cyclohexane-1,2-diamine} in the solid and for all other derivatives by EPR in fluid solution. Also, the application of the complexes in oxidation catalysis was tested using the oxidation of benzyl alcohol.

Graphical abstractThe two essential redox processes in Cu(II) salen (and salan) complexes Cu(II)/Cu(I) and [PhO]/[PhO[−]] were studied by electrochemical and spectroelectrochemical (UV/Vis-absorption or EPR) techniques on a series of complexes in which the salen (salan) type ligands bear various substituents R = H, F, Ph, tBu or CH3 on the linker C- or N-atoms or on the phenol core.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Copper(II) salen complexes. ► structures from XRD and in solution (EPR). ► Electrochemistry, [PhO]/[PhO[−]] and Cu(II)/Cu(I) redox pairs. ► Spectroelectrochemistry, generation of phenoxyl radical complexes. ► Catalytic test reaction: benzyl alcohol oxidation.