A non-innocent salen naphthalene ligand and its Co2+, Ni2+ and Cu2+ metal complexes: Structural, electrochemical, and spectroscopic characterization and computational studies

Abstract Three mononuclear metal complexes of Co 2+ , Ni 2+ and Cu 2+ with the Salen type ligand (( 2,2′ -[( 1-methyl-1,2 - ethanediyl ) bis ( nitriloethylidyne )] bis ( 1-naphthalenol )), H 2 Naphpren or L ) were synthesized and structurally characterized. All metal complexes have square planar geometries and crystallize in the P2 1 /c space group. Structural characterization of the ligand shows contribution of the bis(ketone) tautomer in the crystalline phase, in agreement with density functional theory (DFT) computations. However, according to the UV-vis spectrum, there is a stronger influence of the bis(phenol) tautomer in solution. Cyclic voltammetry experiments showed that the ligand undergoes a single irreversible oxidation process at 0.434 V (vs Fc/Fc + ) while the complexes with Co 2+ , Ni 2+ and Cu 2+ suffer electroxidative process on the surface of the working electrode in addition to solution-assigned redox processes. The CoL oxidation is localized on the metal center, however, the CuL and NiL oxidations are considered as metal-ligand based processes. DFT calculations confirmed our experimental findings. The UV-vis spectra of the complexes illustrate ligand characteristic bands: in the case of the Cu 2+ complex, the metal-to-ligand charge transfer (MLCT) band is present at 385 nm, while in the case of Co 2+ complex this same type of MLCT band appears at 377 nm.