Reversible coordination of dioxygen by tripodal tetraamine copper complexes incorporated in a porous silica framework

The present study reports the synthesis and rational design of porous structured materials by using a templating method. A tetraethoxysilylated tripodal tetraamine (TREN) was covalently incorporated in a silica framework with a double imprint: A surfactant template and a metal ion imprint. The presence of a cationic surfactant (CTAB) endowed the material with a high porosity, and the tripodal or square-pyramidal topology of the ligand was preserved thanks to the use of the silylated Cu(II) complex. After removal of the surfactant and de-metalation, the incorporated tetraamine was quantitatively complexed by CuCl(2) and the material has shown after thermal activation that a reversible binding of O(2) on the metal ions occurred. This chemisorption process was monitored by UV/Vis and EPR spectroscopies, and the Cu:O(2) adduct was postulated to be an end-on mu-eta(1):eta(1)-peroxodicopper(II) complex bridged by a chloride ion. The Cu(I)-active species, formed during the activation step, were fully recovered during several O(2) binding cycles. The high reactivity of the copper complexes and the room-temperature stability of the dioxygen adduct were explained by the fine adaptability of the tripodal ligand to different geometries, the confinement of the active sites in the hybrid silica that protect them from degradation by a control of the metal-ion microenvironment, as well as the short-range lamellar order of the copper complexes in the framework.