Stepwise engineering of the pore environment within metal–organic frameworks for green conversion of CO2 and propargylic amines

Abstract

Achieving the green conversion of propargylic amines with CO₂ is highly desirable but challenging. Noble-metal catalysts, co-catalysts, harsh operating reaction conditions (high temperatures and high pressures) and toxic solvents are usually required to catalyze this reaction. Herein, a noble-metal-free composite material Cu₂O@MIL-101(Cr)-DABCO (DABCO = 1,4-diazabicyclo[2.2.2]octane) was rationally fabricated by successively introducing DABCO and Cu₂O into the metal–organic framework (MOF) MIL-101(Cr) via a stepwise assembly strategy. The resultant composite Cu₂O@MIL-101(Cr)-DABCO exhibits excellent solvent stability and can effectively catalyze the cyclization of propargylic amines and CO₂ with good reusability (≥5 times) as a result of the great confinement for Cu₂O nanoparticles (NPs) inside the MOF and the strong coordination interaction between DABCO and the MOF. Furthermore, the reaction mechanism was clearly illustrated by NMR, FTIR, ¹³C isotope labeling experiments and density functional theory (DFT) calculations, in which the synergistic effect between Cu₂O and DABCO within MIL-101(Cr) can greatly activate the CC and N–H bonds of propargylic amines and thereby boost the reaction. Importantly, this is the first report of a noble-metal free catalyst that can catalyze the green conversion of CO₂ with propargylic amines in the absence of a co-catalyst and solvent under room temperature and atmospheric pressure conditions.