Anionic and neutral 2D indium metal–organic frameworks as catalysts for the Ugi one-pot multicomponent reaction

Abstract

Two metal–organic frameworks (MOFs) made of indium and 1,3,5-tris(4-carboxyphenyl)benzene (H₃btb) and having a layered structure have been synthesized under solvothermal conditions: [In(btb)(H₂O)(DMF)]·L (InPF-50) and [In₂(btb)₂Cl₂]²⁻·[(CH₃)₂NH₂]₂²⁺·L (InPF-51). The structures of both materials have been determined by single crystal X-ray diffraction. The synthetic study which has been carried out demonstrates the influence of the selected indium salt in obtaining each MOF. The structure of both materials consists of pairs of catenated layers, where the metal atoms display coordinated solvent ligands that provide potential open metal sites. The accessibility to these sites along with the presence of Lewis basic sites in the form of uncoordinated oxygen atoms make InPF-50 and -51 efficient catalysts for the four-component Ugi reaction. We attribute this high activity not only to the presence of both acid and basic sites, but also to their convenient locations in the MOF structures. This is further supported by the comparison with [In₃O(btb)₂(HCO₂)]·L, InPF-110, a highly porous indium MOF that only displays Lewis acid sites, and shows lower activity.