Comparative structural and magnetic studies on the binuclear Cu complexes of ¹ L(2−)/ ² L(2−) have been done and their electronic structures and observed electronic transitions (DFT and TD-DFT calculations) rationalized. The dimer complex can be converted into the corresponding monomeric Cu( ii ) complex, [ ¹ L ₂ CuII2(X)] (X = py), by adding an exogenous ligand such as pyridine (py).

Using the deprotonated forms of the tetradentate phenol amine ligands 2-((((1-methylbenzimidazol-2-yl)methyl)(pyridin-2-ylmethyl)amino)methyl)phenol (HL¹) and 2-(((pyridin-2-ylmethyl)(quinolin-2-ylmethyl)amino)methyl)phenol (HL²), dinuclear copper(II) complexes were synthesized. These ligands yielded two binuclear complexes with the composition [Cu₂(Lⁿ)₂](ClO₄)₂ [n = 1, (1) and n = 2, (2)] and two mononuclear complexes with the compositions [Cu(HL¹)(CH₃CN)](ClO₄)₂ (3) and [Cu(HL²) (CH₃CN)(OClO₃)](ClO₄) (4), which have been characterized using X-ray crystallography, UV-Vis, and magnetic susceptibility measurements. The magnetic susceptibility studies of 1 and 2 indicate moderate antiferromagnetic coupling between the Cu^II ions through the μ-phenoxo bridges [J = −38(2) and −145(1) cm⁻¹, respectively], which is described by the Bleaney–Bowers dinuclear model. In contrast, 3 and 4 are devoid of any significant magnetic interactions between the mononuclear units. 3 and 4 show axial spectra typical of the d⁹ (d_x²–y² as the ground state) configuration. The dimer complexes 1 and 2 can be converted into the corresponding monomeric Cu(II) complexes 3 and 4via adding two equivalents of an acid, such as perchloric acid (HClO₄), to a CH₃CN solution of the dimer. The dimeric core can be regenerated via adding an equivalent of a base, such as triethylamine (Et₃N). Time-dependent density functional theory (TD-DFT) calculations using the M06 functional were performed in order to rationalize the electronic structures of the complexes and to shed light on the origin of the observed electronic transitions. Additionally, DFT/B3LYP calculations helped us to unambiguously state the sign and magnitude of the magnetic coupling constants along with an approximation of the spin density distribution.

Using protonated forms of tridentate phenol amine ligand 2-((benzyl(pyridine-2-ylmethyl)amino)methyl)phenol (HL) mononuclear copper(II) complex was synthesized. This ligand yielded mononuclear complex of composition [Cu(HL)Cl₂] (1) which has been characterized by X-ray crystallography, UV–vis, and EPR measurements. Complex 1 shows axial spectra typical of d⁹ (d_x2-y2 as ground state) configuration. Using DFT/B3LYP/LANL2DZ and 6-311++G(d,p) level of theory, structural optimization, electronic and vibrational characteristics of the ligand and complex were examined. The experimental and theoretical parameters are quite well aligned. The assignments of the vibrational frequencies were performed by potential energy distribution analysis by using VEDA program. Molecular electrostatic potential and frontier molecular orbitals analysis have been carried out to understand the molecule reactivity. A TD-DFT computation is also started to replicate the UV–vis absorption spectrum and to determine various essential electronic parameters such as the HOMO–LUMO gap energy and electronic transitions.