Magnetic, high-field EPR studies and catalytic activity of Schiff base tetranuclear CuII2FeIII2 complexes obtained by direct synthesis

Abstract

Two novel heterometallic complexes [Cu₂Fe₂(HL¹)₂(H₂L¹)₂]·10DMSO (1) and [Cu₂Fe₂(HL²)₂(H₂L²)₂]·2DMF (2) have been prepared using the open-air reaction of copper powder, iron(II) chloride and DMSO (1) or DMF (2) solutions of the polydentate Schiff base (H₄L¹, 1; H₄L², 2) formed in situ from salicylaldehyde (1) or 5-bromo-salicylaldehyde (2) and tris(hydroxymethyl)aminomethane. Crystallographic analysis revealed that both compounds are based on the centrosymmetric tetranuclear core {Cu^II₂Fe^III₂(μ-O)₆} where metal centres are joined by μ-O bridges from the deprotonated ligands forming a nonlinear chain-like arrangement. Variable-temperature (1.8–300 K) magnetic susceptibility measurements of 1 and 2 showed a decrease of the effective magnetic moment value at low temperature, indicative of antiferromagnetic coupling (J_Cu–Fe/hc = −10.2 cm⁻¹, J_Fe–Fe/hc = −10.5 cm⁻¹ in 1, J_Cu–Fe/hc = −10.5 cm⁻¹, J_Fe–Fe/hc = −8.93 cm⁻¹ in 2) between the magnetic centres in both compounds. They reveal an exceptionally high catalytic activity in the oxidation of cyclohexane with hydrogen peroxide under mild conditions, with the best observed yield/TON combined values of 36%/596 and 44%/1.1 × 10³ for 1 and 2, respectively.