Urea-engineering mediated hydrogen-bond donating Friedel–Crafts alkylation of indoles and nitroalkenes in a dual-functionalized microporous metal–organic framework with high recyclability and pore-fitting-induced size-selectivity

Abstract

As an effective alternative to Lewis-acid activation, hydrogen-bond donating (HBD) organo-catalysis represents a powerful construction tool for important classes of carbon–carbon bonds, wherein metal–organic frameworks (MOFs) alleviate issues like self-quenching, solubility and reactivity. However, size-selectivity is rather challenging in such catalysis, while the status quo is still unexplored when both H-bonding and open-metal sites (OMS) are present together in a single system. The pillar-bilayer Cd(II) MOF with a rare (3,8)-connected 2-nodal network upholds uni-directional microporous channels integrated with free –NH groups from the urea-moiety of the N,N-donor linker, and aqua-molecule bound [Cd₃(COO)₆] cluster. The activated framework allows the highly efficient Friedel–Crafts alkylation of indole and β-nitrostyrene under relatively mild conditions with low catalyst loading and no leaching. The strategically designed MOF exhibits unaltered activity over multiple catalytic cycles, and corroborates its effectivity towards a wide range of substituted electrophiles and nucleophiles. Importantly, suitably sized pores generated by two-fold interpenetration restrict the entry of a sterically encumbered substrate and result in poor conversion, demonstrating the rarest pore-fitting-induced size-selectivity. Given that this pore-engineered MOF contains both coordination unsaturated Cd(II) centres and unbound –NH groups as active interaction sites, explicit proof of the interaction of the MOF functionality with the –NO₂ group of the reactant is elaborated for the first time in light of the change in emission intensity of the framework in the presence of an electrophile, a judicious choice of substrate, and an in-depth comparison of the catalytic activity of an isostructural framework without a urea-moiety. These control experiments unprecedentedly authenticate urea-moiety-mediated two-point hydrogen bonding in the proposed catalytic route, and simultaneously exclude any major role for OMS at the SBU. Apart from pore-induced size-exclusive reactions, this MOF exemplifies site-specific Friedel–Crafts alkylation, and paves the way to tailor-made engineering of advanced functionalities in contemporary materials for unconventional HBD reactions at the interface of structure–property synergies.