The role of the intermediate triplet state in iron-catalyzed multi-state C–H activation

Abstract

Efficient activation and functionalization of the C–H bond under mild conditions are of a great interest in chemical synthesis. We investigate the previously proposed spin-accelerated activation of the C(sp²)–H bond by a Fe(II)-based catalyst to clarify the role of the intermediate triplet state in the reaction mechanism. High-level electronic structure calculations on a small model of a catalytic system utilizing the coupled cluster with the single, double, and perturbative triple excitations [CCSD(T)] are used to select the density functional for the full-size model. Our analysis indicates that the previously proposed two-state quintet–singlet reaction pathway is unlikely to be efficient due to a very weak spin–orbit coupling between these two spin states. We propose a more favorable multi-state quintet–triplet–singlet reaction pathway and discuss the importance of the intermediate triplet state. This triplet state facilitates a spin-accelerated reaction mechanism by strongly coupling to both quintet and singlet states. Our calculations show that the C–H bond activation through the proposed quintet–triplet–singlet reaction pathway is more thermodynamically favorable than the single-state quintet and two-state singlet–quintet mechanisms.