Synthesis, Structure, and Reactivity of Copper Dioxygen Complexes Derived from Molecular Receptor Ligands

Abstract

Three novel bis{(2-pyridyl)ethyl}amine (PY2) containing ligands and their corresponding Cu(I) complexes have been synthesized. The effect of varying Cu−Cu distances in these complexes on their ability to bind dioxygen was investigated, as was the reactivity of the resulting O2 complexes. Ligand 2, based on a diphenylglycoluril derived molecular receptor, was designed to bind dihydroxybenzenes in close proximity to a dinuclear metal center. To facilitate the identification of the chemistry of 2, the ligands 3 and 4, based on crown ethers, were also studied. Ligands 2−4 gave the air sensitive Cu(I) complexes 5−7. At −85 °C these formed the meta-stable O2 complexes 8−10 upon introduction of O2 into their solutions (8 = [2(CuII)2(O2)](ClO4)2, 9 = [{3(CuII)}2(O2)](ClO4)2, 10 = [4(CuII)2(O2)](ClO4)2). Spectroscopic studies showed that 8−10 are best described as peroxo-copper(II) species, most likely with a (bent) side-on μ-η2:η2-peroxo bridging ligand. A solvent dependence was found for the absorption spectra of 8 and 9, which is explained by geometric changes in the Cu2O2 core. XAS studies on complex 7 and 10 revealed a change in valence from Cu(I) to Cu(II) upon oxygenation of 7 and a change in the copper coordination sphere from 2 coordinating pyridines and 1 coordinating amine nitrogen in 7 to a coordination sphere that additionally includes 2 oxygens per copper in 10. Upon heating of 8 and 10, an oxidative N-dealkylation reaction was found to take place on the ligand. Oxidation of exogenous phenol substrates by 8 led mainly to polymeric products.