Free COOH-tethered layered Co(ii) framework and flexible composite as a size-reliant, tandem and robust catalyst for mild and scalable synthesis of bioactive molecules

Abstract

Pore-functionalization in metal–organic frameworks (MOFs) through the immobilization of free carboxylic sites offers a promising strategy for designing high-performance materials with potential applications, including selective and benign chemical transformations. However, this feat is tricky because of their extreme tendency to coordinate with the concerned metal ions. Herein, we developed a layer-stacked and thermo-chemically stable two-dimensional MOF, encompassing flanked carboxylic acid and [Co₂(COO)₄] unit-decked porous channel, using a mixed-ligand approach. The guest-free structure serves as a one-of-a-kind superior heterogeneous catalyst for tricomponent Knoevenagel–Michael condensation, yielding a multitude of 2-amino-3-cyano-4H-pyrans with low catalyst loading, short duration and mild temperature compared to the majority of reported materials. The role of Lewis and Brønsted acidic sites in the MOF catalyst is comprehensively supported by control experiments, analyte-induced emission articulation, inferior activity of a task-specific site-truncated iso-skeletal framework, and density-functional theory results. Importantly, the MOF demonstrated the first-ever deacetalization multi-component reaction (MCR) with admirable and recyclable conversion under relatively green conditions. Besides covering twenty-two electronically diverse substrates, the MOF can synthesize nine bioactive pyrans with excellent yield and gram scale. Notably, fifteen 4H-pyrans are first-time characterized in their purest forms via X-ray crystallography besides other spectro-analytical studies. Larger-sized substrates failed to diffuse inside MOF's micropores and illustrate unprecedented molecular-dimension-mediated MCR. The in situ-grafted MOF inside melamine-foam (MF) yielded a reconfigurable composite that promotes this one-pot reaction with similar activity and reusability to that of the sole MOF and demarcates a paradigm shift toward cutting-edge sustainable catalysis over a practical platform.