Copper(II)-Mediated Transformation of a Hexadentate Non-Innocent Ligand into a Pentadentate Non-Innocent Ligand

Abstract

A potentially hexadentate Schiff base ligand with redox-active properties, represented as H2L [2-((Z)-(2-(2-(2-((Z)-3,5-di-tert-butyl-2-hydroxybenzylideneamino)phenylthio)ethylthio) phenylimino)methyl)-4,6-di-tert-butylphenol], has been used in the reaction with Cu-II(NO3)(2).3H(2)O in CH3OH in the presence of air. A black crystalline solid with the formula [Cu(HL')(ONO2)] (1) has been isolated, where H2L' represents (Z)-2-((2-(2-(2-aminophenylthio)ethylthio)phenylimino)methyl)-4,6-di-tert-butylphenol). This transformation leads to partial hydrolysis of one of the imine-phenolate arm present in parent ligand. When analyzed through cyclic voltammetry (CV), compound 1 has been found to exhibit a single quasi-reversible response at a potential of E-1/2 = 0.82 V, with a peak-to-peak separation of Delta E-p = 160 mV measured against the saturated calomel electrode (SCE). In its solid-state form, the parent complex displays an axial spectrum characteristic of a normal tetragonal Cu(II) environment, featuring two g values (g parallel to and g perpendicular to), both higher than the free electron g value. In contrast, at a lower temperature of 80 K, the electron paramagnetic resonance (EPR) spectra of both the solid and solution states reveal three distinct g values, indicating a penta-coordination environment around the Cu(II) ion with a geometry intermediate between a trigonal bipyramid and a square pyramid. Structural analysis confirmed that compound 1 is a molecular mononuclear complex. Within this complex, each Cu(II) ion is situated in a distorted octahedral environment consisting of CuN2O2S2 coordination, involving two nitrogen atoms, one oxygen atom, and two sulfur atoms from the ligand, as well as one oxygen atom from the nitrate. Geometrical parameters have been calculated using Density Functional Theory (DFT), specifically the B3LYP method with LANL2DZ basis sets. Molecular Electrostatic Potential (MEP) surface analysis has been employed to visualize the charge distribution within the molecule. Furthermore, the energy levels of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) have been determined, revealing significant charge transfer within the molecule. Additionally, Hirshfeld surface analysis has been conducted from both three-dimensional (3D) and two-dimensional (2D) perspectives to investigate the electron distribution on the molecular surface.