A Discrete Tetrahedral Indium Cage as an Efficient Heterogeneous Catalyst for the Fixation of CO2 and the Strecker Reaction of Ketones

A discrete tetrahedral indium cage, {[In12(μ3-OH)4(HCO2)24(tcma)4]} (In12-GL), was synthesized solvothermally by the reaction of indium nitrate with the tripodal tricarboxylic acid ligand N,N,N-tris{(2'-carboxy[1,1'-biphenyl]-4-yl)methyl}methylammonium chloride ([H3tcma]+Cl). This cage consists of four trimeric units [In3(μ3-OH)(μ2-CO2)3(μ2-HCO2)3] and four [tcma]2- ligands, which all perform as 3-connection nodes to bridge each other, resulting in a tetrahedral cage structure. The trimeric unit [In3(μ3-OH)(μ2-CO2)3(μ2-HCO2)3] is observed for the first time in the family of In-based metal-organic structures and can be considered as an evolution of a 6-connected [In3(μ3-O)(μ2-CO2)6] unit. Each In3+ is terminally coordinated by a μ1-HCO2 group. This cage contains potential Lewis acidic/basic active sites endowed by In3+ ions as Lewis acidic sites and the uncoordinated oxygen atoms of μ1-HCO2 moieties as Lewis basic sites and was explored as an effective heterogeneous catalyst in the cycloaddition of CO2 with epoxides and the Strecker reaction for amino nitriles. These catalytic reactions were deduced to happen on the surface of the In12-GL cage.