Dinuclear complexes of copper and zinc with m-xylene/cyclohexane-linked bis-aspartic acids: Synthesis, characterization, dioxygen activation, and catalytic oxidation of nitrobenzene in pure aqueous solution

Abstract

Two new m-xylene/cyclohexane-linked bis-aspartic acid ligands, L_b and L_c, were synthesized via Michael addition in basic aqueous solution. Their structures were characterized by elemental analysis, NMR and MS spectrometry. Both ligands react with Cu(II) and Zn(II) to form dinuclear complexes, with M₂L(OH)⁻ the major species in neutral/weak basic aqueous solution. To quantify the relative interaction strength between a Lewis acid and base, a new parameter σ = log K/14 was proposed which compares the stability constant with the binding constant between H⁺ and OH⁻. The dinuclear copper complexes (L_b–2Cu and L_c–2Cu) react with H₂O₂ in aqueous solution. The reaction in 0.020 M phosphate buffer at pH 7.5 is first-order for [L_c–2Cu], but second-order for [L_b–2Cu]. The oxidation products are oxygenated and/or dehydrogenated species. Radical trapping tests indicate that both complexes slightly scavenge the OH˙ radical, but generate the H˙ radical. L_c–2Cu generates the H˙ radical much more effectively than that of L_b–2Cu when reacted with H₂O₂. Both complexes are excellent catalysts for the oxidation of nitrobenzene in the presence of H₂O₂ in weakly basic aqueous solution. The oxidation follows the rate-law v = k[complex][nitrobenzene][H₂O₂]. The k values in pH 8.0 phosphate buffer at 25 °C are 211.2 ± 0.3 and 607.9 ± 1.7 mol⁻² L² s⁻¹ for L_b–2Cu and L_c–2Cu, respectively. The Arrhenius activation energies are 69.4 ± 2.2 and 70.0 ± 4.3 kJ mol⁻¹ for L_b–2Cu and L_c–2Cu, respectively, while the Arrhenius pre-exponential factors are 2.62 × 10¹⁴ and 1.06 × 10¹⁵, respectively. The larger pre-exponential factor makes L_c–2Cu more catalytically active than L_b–2Cu. These complexes are some of the most effective oxidation catalysts known for the oxidation of nitrobenzene.