Structural, spectroscopic, theoretical, and magnetic investigations of a novel cubane-like tetranuclear...

This study details the synthesis of a novel ligand, (E)-5-chloro-N'-(2-hydroxy-3-methoxybenzylidene) thiophene-2-carbohydrazide ligand (for short, H2L), and its tetranuclear Cu(II) complex (Cu4L4), together with their X-ray crystal structures and the magnetic properties and EPR spectra of Cu4L4 within the 4-300 K temperature range. Furthermore, we report the spectroscopic characterization (FTIR and UV-Vis) of the compounds and perform a Hirshfeld analysis of their non-covalent interactions, along with certain quantum chemical calculations. H2L crystallizes in the monoclinic space group Cc with Z = 8 molecules per unit cell and the Cu4L4 complex crystallizes in the tetragonal space group P4₁/a with Z = 4. The complex is at a crystal site of S₄ symmetry, conforming to a cubane-like Cu₄O₄ core. The main pathway for exchange interaction between neighboring copper ions in the core involves a relatively large overlap of the copper d(x² - y²) electron ground state orbital with the sp² lone-pair lobes of the bridging oxygen. Magnetic susceptibility in the 5-300 K range, mainly interpreted with the exchange Hamiltonian Ĥ_ex = J(Ŝ₁·Ŝ₂ + Ŝ₂·Ŝ₃ + Ŝ₃·Ŝ₄ + Ŝ₄·Ŝ₁), confirms the expected relatively strong antiferromagnetic (AF) character of the complex (J = -61.5(1) cm^-1). The powder room temperature Q-band EPR spectrum shows a very broad band (ΔB_pp = 1980 Gauss) corresponding to a gyromagnetic g-factor of 2.13. The band intensity decreases sharply with temperature, as expected for a Cu(II) tetramer with a well isolated spin singlet (S = 0) ground state.