Computational study of the carbonyl–ene reaction between formaldehyde and propylene encapsulated in coordinatively unsaturated metal–organic frameworks M3(btc)2 (M = Fe, Co, Ni, Cu and Zn)

Abstract

The carbonyl–ene reaction between encapsulated formaldehyde and propylene over the coordinatively unsaturated metal–organic frameworks M₃(btc)₂ (M = Fe, Co, Ni, Cu and Zn) has been investigated by means of density functional calculations. Zn₃(btc)₂ adsorbs formaldehyde strongest due to electron delocalization between Zn and the oxygen atom of the reactant molecule. The reaction is proposed to proceed in a single step involving proton transfer and carbon–carbon bond formation. We find the relative catalytic activity to be Zn₃(btc)₂ > Fe₃(btc)₂ ≥ Co₃(btc)₂ > Ni₃(btc)₂ > Cu₃(btc)₂, based on activation energy and turnover frequencies (TOF). The low activation energy for Zn₃(btc)₂ compared to the others can be explained by the delocalization of electron density between the carbonyl bond and the catalyst active sites, leading to a more stable transition state. The five MOFs are used to propose a descriptor for the relationship between activation energy on one side and electronic properties or adsorption energies on the other side in order to allow a quick screening of other catalytic materials for this reaction.