Magnetic, high-field EPR studies and catalytic activity of Schiff base tetranuclear Cu~(II) _2Fe~(III) _2 complexes obtained by direct synthesis

摘要

Two novel heterometallic complexes [Cu_2Fe_2(HL ~1)_2(H_2L~1)_2] ·10DMSO (1) and [Cu_2Fe_2(HL~2) _2(H_2L~2)_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 (H _4L~1, 1; H_4L~2, 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~(II) _2Fe ~(III) _2(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 × 10 ~3 for 1 and 2, respectively.