Monolayers of salen derivatives as catalytic planes for alkene oxidation in water

Monolayers at the gas/water interface have been used as an adjustable catalytic system in which the molecular density may be modified. Mn(III)-salen complexes bearing perfluoroalkyl substituents have been organized as a Langmuir film on an aqueous subphase containing a urea/hydrogen peroxide adduct (UHP, the oxidant) and cinnamyl alcohol (the substrate). The catalytic activity of the monolayer for the epoxidation of the alkene dissolved in water has been demonstrated and the reaction kinetic investigated. For a constant area per molecule of catalyst, the reaction rate exhibits first-order dependence on oxidant concentration and zero-order dependence on alkene concentration, in agreement with the reaction orders reported for Mn(III)-salen-catalyzed epoxidation reactions carried out in solution. Furthermore, kinetic experiments suggest an enhanced activity of the catalysts assembled in a Langmuir film relative to that observed in bulk reaction. Finally, varying the molecular density of the catalyst at the gas/water interface highlights an important dependence of the catalytic activity of the layer with the mean molecular area. A strong increase of the catalytic properties of the monolayer was observed for a mean molecular area of 140-145 A2, an increase which was supposedly related to a modification of the Mn(III)-salen complex orientation at the interface upon compression. This hypothesis was supported by PM-IRRAS (polarization modulation infrared reflection adsorption spectroscopy) experiments performed in situ on the monolayer. Such results demonstrate that a soft and adjustable molecular system like a Langmuir film can be used to better understand the reactivity in various heterogeneous and/or pseudohomogeneous (such as those based on dendrimers) catalytic systems.