Copper(II)-salicylaldehydate/-methoxy(pyridine-2-yl)methanolate complexes via in-situ hydrolysis of Schiff bases

Abstract

Reaction of copper(II) nitrate with N-2-(R-pyridyl)salicylaldimine affords the mononuclear bis[salicylaldehydato-κO,O′]copper(II) (1) via acidic hydrolysis of the aldimine ligand back to the salicylaldehyde. Similarly, reaction with (R)-N-1-(p-BrC6H4)ethyl-salicylaldimine or (R)-1-(p-BrC6H4)-N-((pyridin-2-yl)methylene)ethanamine in presence of 2,2′-bipyridine (bipy) provides the mixed-ligated dinuclear bis[(µ-salicylaldehydato-κO,O′:O′)(2,2′-bipyridine)nitrato-copper(II)] (2) or bis[{µ-methoxy(pyridine-2-yl)methanolato-κN,O:O}(methanol)nitrato-copper(II)] (3). The structures of 2 and 3 consist of a dinuclear unit involving two (bipy)(salicylaldehydate)- or {methoxy(pyridine-2-yl)methanolate}(methanol)-coordinated copper(II) cations and two nitrate anions. The coordination sphere of copper is of N2O4-type with an elongated octahedron due to the Jahn-Teller effect in 2. The axial positions of the square bipyramid are occupied by the oxygen atoms from NO3− and the phenoxo group of the nearest salicylaldehydate molecule. The coordination sphere of copper is of NO4-type in 3, with the alkoxido group of methoxy(pyridine-2-yl)methanolate bridging between the Cu atoms. Cyclic voltammograms for 1 demonstrate two quasi-reversible one electron charge transfer processes for [Cu(L)2]2−/[Cu(L)2]− and [Cu(L)2]−/[Cu(L)2] (L=salicylaldehydato), respectively in acetonitrile. DSC analyses reveal an irreversible phase transformation from solid to solid-liquid mixture phase, and then to an isotropic liquid phase for 1. The excited state properties of 1 studied by DFT/TDDFT are comparable to the experimental electronic spectrum.