Large Equatorial Ligand Effects on C-H Bond Activation by Nonheme Iron(IV)-oxo Complexes

In this article, we present density functional theory (DFT) calculations on the iron(IV)-oxo catalyzed methane C-H activation reactions for complexes in which the Fe-IV=O core is surrounded by five negatively charged ligands. We found that it follows a hybrid pathway that mixes features of the classical sigma- and pi-pathways in quintet surfaces. These calculations show that the Fe-O-H arrangement in this hybrid pathway is bent in sharp contrast to the collinear character as observed for the classical quintet sigma-pathways before. The calculations have also shown that it is the equatorial ligands that play key roles in tuning the reactivity of Fe-IV=O complexes. The strong pi-donating equatorial ligands employed in the current study cause a weak pi(FeO) bond and thereby shift the electronic accepting orbitals (EAO) from the vertically orientated O p(z) orbital to the horizontally orientated O p(x). In addition, all the equatorial ligands are small in size and would therefore be expected have small steric effects upon substrate horizontal approaching. Therefore, for the small and strong g-donating equatorial ligands, the collinear Fe-O-H arrangement is not the best choice for the quintet reactivity. This study adds new element to iron(IV)-oxo catalyzed C-H bond activation reactions.