Ethane C–H bond activation on the Fe(iv)–oxo species in a Zn-based cluster of metal–organic frameworks: a density functional theory study

Abstract

We first investigate the feasibility of designing a Fe–oxo complex for the activation of alkane C–H bonds by (a) incorporating an Fe ion into a Zn-based cluster derived from a metal–organic framework (MOF) and (b) creating the Fe–oxo complex via decomposition of N₂O over a Fe²⁺-substituted Zn-based cluster (Fe–Zn₃O(pyrazole)₆). From the energy profile, it turns out that both steps should be feasible and that the resulting Fe–oxo complex is stable. In the main step, we then investigate the reactivity of this Fe–oxo cluster for the C–H bond cleavage of ethane by calculating the reaction energy profile and analyzing the electronic structure along the relevant steps. Two mechanisms, namely the σ and π pathways on the triplet and quintet potential energy surfaces, were unraveled for this study of catalytic activity. It is shown that the σ pathway on the quintet surface is kinetically and thermodynamically favorable with an energy barrier of 22.5 kcal mol⁻¹. The π pathway on the quintet and triplet surfaces has activation energies of 26.9 kcal mol⁻¹ and 24.9 kcal mol⁻¹, respectively. An alternative unusual pathway called the δ mechanism on the triplet surface is also observed with an energy barrier of 12.6 kcal mol⁻¹. It is, however, thermodynamically at a disadvantage compared to the σ pathway on the quintet surface. Favorable d–d interaction on the Fe center and less steric hindrance from the equatorial ligands at the transition state are the key factors that cause the σ pathway on the quintet surface to have the lowest activation energy. All our calculations are of the cluster type and have been performed at the B3LYP-D3/def2-TZVP level of theory.