Copper (ii) dimers stabilized by bis(phenol) amine ligands: Theoretical and experimental insights

Using deprotonated forms of tetradentate bis(phenol) amine ligands 2-(((3,5-di-tert-butyl-2-hydroxybenzyl)(2-hydroxyethyl)amino)methyl)-4,6-di-tert-butylphenol (²LH₂) and newly synthesized methyl-2-(bis(3,5-di-tert-butyl-2-hydroxybenzyl)amino) propanoate (¹LH₂), dinuclear copper(ii) complexes were synthesized. These ligands yielded two dinuclear complexes of composition [(ⁿL)₂CuII2] [n = 1, (1) and n = 2, (2)], which have been characterized using X-ray crystallography, UV-Vis spectroscopy and magnetic susceptibility measurements. The phenolate moieties of the copper complexes were electrochemically oxidized to phenoxyl radicals. The antiferromagnetic exchange coupling of the two copper centers of these complexes was investigated by magnetic susceptibility (2-300 K) measurements. The weak exchange coupling constant (J) of the complexes, compared to the literature values, is attributed to the smaller Cu⋯Cu distance and Cu-O(Ph)-Cu angle. [¹LCu^II(py)] (4) and [²LCu^II(py)] (5) showed rhombic spectra typical of the d⁹ configuration. The dimer complex can be converted into the corresponding monomeric Cu(ii) complex, [¹L₂CuII2(X)] (X = py), by adding an exogenous ligand such as pyridine (py) into a CH₂Cl₂ solution of the dimer. All EPR spectra were simulated and analyzed. The cyclic voltammograms observed with ¹L₂Cu₂ and ²L₂Cu₂, revealed two oxidations with a similar minute difference in their redox potentials. Since copper(ii)-oxidation is not feasible at such low potentials, these redox processes were assigned to ligand-centered oxidation yielding a phenoxyl radical in the complex. Density Functional Theory (DFT) at the B3LYP level and Time-Dependent (TD)-DFT calculations rationalize the electronic structures of the complexes and throw light on the origin of the observed electronic transitions.