Derivatizing immobilization methods for TEMPO radicals in metal–organic frameworks: Toward efficient aerobic oxidation and sequential reactions

Abstract

Metal–organic frameworks (MOFs) offer a versatile platform for immobilizing catalytically active species, enabling diverse organic transformations through their structural tunability and well-defined pore environments. This study investigates the covalent immobilization of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) radicals in Zr-based MOFs, synthesizing six distinct frameworks with varying linkages, including amides, esters, ethers, triazoles, and ureas. The catalytic efficiency of these MOFs was evaluated in the aerobic oxidation of secondary alcohols to ketones, highlighting the role of electronic effects, linker size, and TEMPO radical density. Furthermore, these systems enabled a sequential catalytic process for isoxazole synthesis via cycloaddition, yielding 16 isoxazoles with high efficiency. While Zr-based MOFs exhibited robust performance in the initial oxidation step, their crystallinity degraded under TBN-excess conditions during the cycloaddition step. Ce substitution on MOF structure partially mitigated this structural degradation, demonstrating potential for enhanced recyclability. These findings offer a comprehensive framework for designing TEMPO-immobilized MOFs with improved catalytic versatility and stability in oxidative transformations.