Structural, spectroscopic, theoretical, and magnetic investigations of a novel cubane-like tetranuclear copper( <scp>ii</scp> )-hydrazone complex

This study details the synthesis of a novel ligand, (<i>E</i>)-5-chloro-<i>N</i>'-(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 <i>Cc</i> with <i>Z</i> = 8 molecules per unit cell and the Cu4L4 complex crystallizes in the tetragonal space group <i>P</i>4₁/<i>a</i> with <i>Z</i> = 4. The complex is at a crystal site of <i>S</i>₄ 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(<i>x</i>² - <i>y</i>²) 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 <i>Ĥ</i>_ex = <i>J</i>(<i>Ŝ</i>₁·<i>Ŝ</i>₂ + <i>Ŝ</i>₂·<i>Ŝ</i>₃ + <i>Ŝ</i>₃·<i>Ŝ</i>₄ + <i>Ŝ</i>₄·<i>Ŝ</i>₁), confirms the expected relatively strong antiferromagnetic (AF) character of the complex (<i>J</i> = -61.5(1) cm^-1). The powder room temperature Q-band EPR spectrum shows a very broad band (Δ<i>B</i>_pp = 1980 Gauss) corresponding to a gyromagnetic <i>g</i>-factor of 2.13. The band intensity decreases sharply with temperature, as expected for a Cu(II) tetramer with a well isolated spin singlet (<i>S</i> = 0) ground state.