Advantages of Covalent Immobilization of Metal‐Salophen on Amino‐Functionalized Mesoporous Silica in Terms of Recycling and Catalytic Activity for CO<sub>2</sub> Cycloaddition onto Epoxides
Matthieu Balas, Sébastien Beaudoin, Anna Proust, Franck Launay, Richard Villanneau
Abstract
Abstract Ni II and Mn III Schiff base complexes (Salophen‐Ni and Salophen‐MnCl) bearing a pending carboxylic group were prepared and characterized. Both complexes were grafted onto a mesoporous amino‐functionalized SBA‐15 silica, by formation of an amide function between the propylamine groups of the support and the carboxylic acid functions of the salophen ligand (corresponding respective to 1.30 wt.% of Ni and 1.06 wt.% of Mn). The co‐catalytic behaviour of the free and grafted complexes was then evaluated in the CO 2 cycloaddition reaction onto styrene oxide, using tetra‐butylammonium bromide ( n− Bu 4 NBr) as the main catalyst. In homogeneous conditions, the Mn III Schiff base complex and the Ni II one, to a lesser extent, behave as efficient co‐catalysts for this reaction (styrene conversion of 100 % and 65 % respectively after 3 h at 120 °C, under 15 bars of CO 2 ). Upon immobilization at the surface of the amino‐functionalized SBA‐15, we showed that the co‐catalytic activity of the less efficient one, i. e. Ni 2+ salophen complex, could be enhanced (reaching a full conversion after 7 h), hence highlighting a potential synergistic effect between the unused amine functions of the support and the Ni 2+ salophen co‐catalyst. Both salophen complexes were successfully re‐used in homogeneous conditions or after their immobilization without any appreciable loss of activity. This work is only a first step towards a completely heterogeneous catalytic system, in which the tetraalkylammonium halide catalyst and the metal‐salophen co‐catalyst will both be covalently anchored on the same support.