Oxygen binding and activation by the complexes of PY2- and TPA-appended diphenylglycoluril receptors with copper and other metals

The copper(I) complexes of diphenylglycoluril basket receptors 1 and 2, appended with bis(2-ethylpyridine)amine (PY2) and tris(2-methylpyridine)amine (TPA), respectively, and their dioxygen adducts were studied with low-temperature UV-vis and X-ray absorption spectroscopy (XAS). The copper(I) complex of 1, [1·Cu(I)2] or 1a, forms a μ-η2:η2 dioxygen complex, whereas the copper(I) complex of 2, [2·Cu(I)2] or 2a, does not form a well defined dioxygen complex, but is oxidized to Cu(II). Dioxygen is bound irreversibly to 1a and the formed complex is stable over time. The coordination geometries of the above complexes were determined by XAS, which revealed that pyridyl groups and amine N-donors participate in the coordination to Cu(I) ions in the complexes of both receptors. The catalytic activities of various metal complexes of 1 and 2, that were designed as mimics of dinuclear copper enzymes that can activate dioxygen, were investigated. Phenolic substrates that were expected to undergo aromatic hydroxylation, showed oxidative polymerization without insertion of oxygen. The mechanism of this polymerization turns out to be a radical coupling reaction as was established by experiments with the model substrate 2,4-di-tert-butylphenol. In addition to Cu(II), the Mn(III) complex of 1 and the Fe(II) complex of 2 were tested as oxidation catalysts. Oxidation of catechol was observed for the Cu(II) complex of receptor 1 but the other metal complexes did not lead to oxidation.