Modular, Active, and Robust Lewis Acid Catalysts Supported on a Metal−Organic Framework

Abstract

Metal-organic frameworks (MOFs) have shown promise as heterogeneous catalysts because of their high crystallinity, uniform pores, and ability to be chemically and physically tuned for specific chemical transformations. One of the challenges with MOF-based catalysis is few systems achieve all of the desired features for a heterogeneous catalyst, including high activity, robustness (recyclability), and excellent selectivity. Herein, postsynthetic modification (PSM) of a MOF is used to synthesize a series of MOF catalysts that are highly robust and active for epoxide ring-opening reactions. In the following study, four metalated MOFs (UMCM-1-AMInpz, UMCM-1-AMInsal, UMCM-1-AMFesal, and UMCM-1-AMCupz) are examined as catalysts for beta-azido and beta-amino alcohol synthesis with epoxides of varying sizes and shapes using two different nucleophiles (TMSN(3) and aniline). The four MOFs are isostructural, exhibit good thermal and structural stability, and display different catalytic activities based on the combination of metal ion and chelating ligand immobilized within the framework. In particular, UMCM-1-AMInpz and UMCM-1-AMInsal act as robust, single-site catalysts with distinct selectivity for ring-opening reactions with specific nucleophiles. More importantly, one of these catalysts, UMCM-1-AMInpz, selectively promotes the ring-opening of cis-stilbene oxide in the presence of trans-stilbene oxide, which cannot be achieved with a comparable molecular Lewis acid catalyst. The results show that PSM is a promising, modular, and highly tunable approach for the discovery of robust, active, and selective MOF catalysts that combine the best aspects of homogeneous and heterogeneous systems.