Accommodation of Oversized In3+ in an Octahedra-Based Aluminoborate Framework Enables Enhanced Lewis Acidity and Epoxide Ring-Opening Catalysis.

The stabilization of oversized heteroatoms within microporous frameworks remains a longstanding challenge because of the geometric and thermodynamic limitations associated with framework substitution. Indium is a particularly intriguing case, as its large ionic radius and coordination preference generally prevent its incorporation into conventional tetrahedral microporous frameworks, where it typically exists as extra-framework species. Herein, In3+ can be successfully accommodated within the octahedra-based aluminoborate framework PKU-1 through isomorphous substitution of framework Al3+ sites, which is proved by comprehensive structural and compositional characterizations. More importantly, 27Al, 11B, and 1H MAS NMR results reveal that the incorporation of oversized In3+ induces pronounced local structural distortion and significant reorganization of the framework microenvironment. These structural changes lead to a nearly one-order-of-magnitude increase in acid-site density and the generation of abundant Lewis acid centers. Consequently, it exhibits outstanding catalytic performance in the ring-opening ammonolysis of epoxides with amines, affording β-amino alcohols in 99.8% yield with excellent selectivity. Correlation of structural, spectroscopic, and catalytic results establishes a clear framework distortion-Lewis acidity-catalytic activity relationship. This work provides a rare example of stabilizing oversized In3+ species within a microporous framework and demonstrates how framework distortion can be exploited to create highly active Lewis acid catalysts.