Stabilization of the trivalent oxidation state of copper by tridentate imine–oxime–amine ligands

Two tridentate imine–oxime–amine ligands have been synthesized and their corresponding copper(II) complexes have been isolated. These copper(II) complexes are readily oxidized both chemically and electrochemically to give relatively stable copper(III) complexes. In the pH range 1.5–3.0 the electron transfer process is electrochemically reversible with ΔEp = 60 mV and ipa/ipc ∼ 1. Plots of E1/2 versus pH are linear with a slope = −60 indicating the involvement of one proton in the electron transfer process. Aqueous solutions of copper(III) complexes have high molar absorption at λmax with ε > 104 M−1 cm−1. Solid samples of the complexes are diamagnetic consistent with a d8 square planar geometry. It seems that only imine–oxime nitrogens are coordinated to copper(II) with the NH2 group being free as indicated by i.r. spectra. Substitution of a –CH3 group on the carbon atom adjacent to the oxime group by the more electron donating group –CH(CH3)2 lowers electrode potential by more than 90 mV. This is consistent with an earlier observation that electron-donating substituents on the carbon atom adjacent to the oxime group lower the potential of CuIII/CuII couples and stabilize the higher oxidation state.

Graphical abstractTwo tridentate imine–oxime–amine ligands HL1 and HL2 and their corresponding CuII complexes were synthesized. These complexes are readily oxidized to give diamagnetic d8CuIII complexes.Figure optionsDownload full-size imageDownload as PowerPoint slide