Incorporation of palladium complexes into silsesquioxane−imine frameworks for the methoxycarbonylation of olefins

In this work, palladium-diphosphine complex ([Pd-L]) was incorporated into silsesquioxane-imine frameworks through one pot in-situ aldehyde-amine assembly concentration, leading to [Pd-L]@PSIF-X-0.5 (X = A, B and C) hybrids as catalysts for the methoxycarbonylation of olefins without any additives. Structural characterizations demonstrated that the position of active [Pd-L] loaded (either in the pores or on the surface), the concentration of [Pd-L] relative to the imine units, and the porosity of the hybrids are correlated to the specific aldehyde launched, thereby influencing the catalytic performance. Mechanistic experiments demonstrated that the methanolysis accounts for the rate-limiting step. Consequently, the incorporation of active [Pd-L] species on the surface is conductive to reduce the activation barrier by facilitating the adsorption and activation of MeOH (e.g., [Pd-L]@PSIF-A-0.5 and [Pd-L]@PSIF-B-0.5), while it partially loses the catalytic durability. In this regard, the intense adsorption of MeOH on the surface (e.g., [Pd-L]@PSIF-B-0.5) even leads to the dehydrogenative formation of formaldehyde and thereby accelerates the decomposition of molecular [Pd-L] species to Pd nanoparticles. By contrast, the active [Pd-L] sites encapsulated in the pores will be protected by the walls of the carriers (e.g., [Pd-L]@PSIF-C-0.5), which benefits the catalyst durability but partially loses activity due to the insufficient interaction with MeOH.