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Intrinsic-Unsaturation-Enriched Biporous and Chemorobust Cu(II) Framework for Efficient Catalytic CO2 Fixation and Pore-Fitting Actuated Size-Exclusive Hantzsch Condensation with Mechanistic Validation

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posted on 2021-11-12, 14:10 authored by Nilanjan Seal, Subhadip Neogi
Carbon dioxide (CO2) utilization and one-pot Hantzsch condensation denote two important protocols pertinent to sustainable agenda because of the obvious advantages like reduction in chemical usage, short reaction time, and minimum waste generation. To this end, the astute combination of optimum-sized pore structure with built-in Lewis acid center in metal–organic frameworks (MOFs) can bring about such reactions under energetically favorable conditions and offer a step forward to size-exclusive catalysis. The chemoresistant and twofold interpenetrated Cu­(II) framework CSMCRI-13 (CSMCRI = Central Salt & Marine Chemicals Research Institute) is built from a C3-symmetric tricarboxylate ligand and an N,N′-donor linker that undergo incisive amalgamation of the paddle-wheel [Cu2(COO)4] secondary building unit (SBU) and the intrinsically unsaturated Cu­(II) node with four coordination. The microporous structure features a dual-pore containing cage-like network with free oxygen-atom-enriched cavities and exhibits appreciable CO2 adsorption with moderate MOF-CO2 interaction in activated form (13a). Benefitting from both, the coordinatively frustrated metal center containing MOF acts as a highly synergistic and solvent-free catalyst in CO2 cycloaddition reaction under an 8 bar CO2 pressure at 70 °C in 6 h. The catalyst furnished admirable reactivity and fair recyclability with a wide range of substrates, wherein sterically encumbered and long-chain epoxides produced poor conversion. This MOF further executes highly regenerable Hantzsch condensation reaction under mild condition with superior activity to contemporary materials, where most of the 1,4-dihydropyridine derivatives are additionally characterized through the single-crystal X-ray diffraction analysis. Importantly, mechanistic proof of the tricomponent condensation involving built-in Lewis acid sites is validated from several control experiments and in-depth analytical studies. To the best of the single-step multicomponent reaction, substrate molecules having incompatible molecular dimension to that of pore size of the framework resulted insignificant conversion and demonstrated the first-ever pore-fitting-induced size selectivity in Hantzsch condensation.

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