Magnetic, high-field EPR studies and catalytic activity of Schiff base tetranuclear (Cu2Fe2III)-Fe-II complexes obtained by direct synthesis

Two novel heterometallic complexes [Cu2Fe2(HL1)(2)(H2L1)(2)]center dot 10DMSO (1) and [Cu2Fe2(HL2)(2)(H2L2)(2)]center dot 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 {(Cu2Fe2III)-Fe-II(mu-O)(6)} where metal centres are joined by mu-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 x 10(3) for 1 and 2, respectively.