Entropic influence on the generation of Fe(iv)O species at mononuclear Fe(ii) sites in metal–organic frameworks

Abstract

We study the oxidation of mononuclear Fe(II) centers in the metal–organic framework MOF-74 in the presence of nitric oxide (NO), nitrogen dioxide (NO₂), nitrous oxide (N₂O), dinitrous dioxide (N₂O₂), oxygen (O₂), ozone (O₃), and hydrogen peroxide (H₂O₂) using static density-functional theory calculations and ab initio molecular dynamics simulations. We examine the effect of reaction entropies at room temperature on the Fe(II) oxidation barriers for these species. Singlet N₂O, O₂ and H₂O₂ exhibit large positive entropic contributions, which reduce the free energy barrier at room temperature compared to the enthalpy barriers. By contrast, large negative entropies are observed in the case of NO₂ and O₃, which indicate that the reactivity of these species decreases with temperature. We discuss the role of entropic effects on the mechanisms and energetics of the reactions examined.