Ci-Symmetry, [2 × 2] grid, square copper complex with the N4,N5-bis(4-fluorophenyl)-1H-imidazole-4,5-dicarboxamide ligand: structure, catecholase activity, magnetic properties and DFT calculations

Abstract

A new square tetranuclear copper complex of formula Cu₄(LH)₄·4DMF with the dinucleating amide ligand N⁴,N⁵-bis(4-fluorophenyl)-1H-imidazole-4,5-dicarboxamide (LH₃) is reported herein. This ligand and the complex have been characterized by elemental analysis, FTIR, NMR, mass and UV-visible spectroscopy, as well as magnetic and electrochemical measurements. The single crystal X-ray diffraction study showed each Cu centre in a distorted square pyramidal environment, the square plane being formed by the extended coordination of two sets (N,N and N,O) of donor atoms from pairs of different ligands. The Cu₄ unit has a C_i symmetry and crystallises in the P space group in between DMF layers. The electronic spectrum of the complex exhibits a d–d transition at 676 nm. The complex also displays mild catecholase activity in DMF solution by using 3,5-di-tert-butylcatechol as substrate. Variable temperature magnetic measurements reveal an antiferromagnetic interaction between the copper centres with two different coupling constants of −143.4 and −169.0 cm⁻¹. DFT calculations made on a single Cu₄ unit from the crystal structure corroborated the antiferromagnetic coupling, the spin density of the lower-energy broken-symmetry state being consistent with an alternating +−+− singlet state operating at low temperature.