Solar CO₂ reduction using a molecular Re(I) catalyst grafted on SiO₂via amide and alkyl amine linkages

Heterogenized molecular catalysts have shown interesting activities in different chemical transformations. In our previous studies, a molecular catalyst, Re(bpy)(CO)₃Cl where bpy is 2,2'-bipyridine, was covalently attached to silica surfaces via an amide linkage for use in photocatalytic CO₂ reduction. Derivatizing the bpy ligand with electron-withdrawing amide groups led to detrimental effects on the catalytic activity of Re(bpy)(CO)₃Cl. In this study, an alkyl amine linkage is utilized to attach Re(bpy)(CO)₃Cl onto SiO₂ in order to eliminate the detrimental effects of the amide linkage by breaking the conjugation between the bpy ligand and the amide group. However, the heterogenized Re(I) catalyst containing the alkyl amine linkage demonstrates even lower activity than the one containing the amide linkage in photocatalytic CO₂ reduction. Infrared studies suggest that the presence of the basic amine group led to the formation of a photocatalytically inactive Re(I)-OH species on SiO₂. Furthermore, the amine group likely contributes to the stabilization of a surface Re(I)-carboxylato species formed upon light irradiation, resulting in the low activity of the heterogenized Re(I) catalyst containing the alkyl amine linkage.