Single crystal EPR spectroscopy, magnetic studies and catalytic activity of a self-assembled [2 x 2] Cu4II cluster obtained from a carbohydrazone based ligand

A new tetranuclear [2 x 2] cluster of Cu(II) with a symmetric carbohydrazone based ligand, [Cu4L4] (NO3)(4)center dot 1.6(H2O) (1), (where HL donates bis-[(E)-N'-(1-(pyridin-2-yl)ethylidene)]carbohydrazide} was synthesized and characterized by spectroscopic methods and X-ray analysis. The EPR spectra were performed on single crystals of complex 1 at various temperatures and allowed the identification and separation of two types of magnetic objects contributing to magnetism: single atoms of Cu(II) and a tetranuclear Cu-4 cluster. The main values of the g-factor and hyperfine structure were determined for single ions of Cu(ll). The copper atoms in the tetramer are coupled antiferromagnetically with an isotropic antiferromagnetic exchange J= 215 K (149.4 cm(-1)). A small anisotropic exchange of the order of 0.06 K (0.04 cm(-1)) is responsible for the initial zero-field splitting of the energy levels in the tetramer spectrum. Magnetic measurements of complex 1 confirmed the existence of a strong antiferromagnetic exchange coupling between four Cu(II) ions. Complex 1 showed high potential for the catalytic and selective oxidation of cis-cyclooctene with aqueous H2O2. (C) 2014 Elsevier Ltd. All rights reserved.