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?

Inorganic Chemistry Frontiers Pub Date: 2022-03-04 DOI: 10.1039/D2QI00206J

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.

Graphical abstract: 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
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