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.