Unraveling the Geometry-Driven C═C Epoxidation and C–H Hydroxylation Reactivity of Tetra-Coordinated Nonheme Iron(IV)–Oxo Complexes
Akhil Bhardwaj, Bhaskar Mondal
Abstract
The electronic structure and reactivity of tetra-coordinated nonheme iron(IV)–oxo complexes have remained unexplored for years. The recent synthesis of a closed-shell iron(IV)–oxo complex [(quinisox)Fe IV (O)] + ( 1 ) has set up a platform to understand how such complexes compare with the celebrated open-shell iron–oxo chemistry. Herein, using density functional theory and ab initio calculations, we present an in-depth electronic structure investigation of the C═C epoxidation [oxygen atom transfer (OAT)] and C–H hydroxylation [hydrogen atom transfer (HAT)] reactivity of 1 . Using a solvent-coordinated geometry of 1 ( 1′ ) and other potential tetra-coordinated iron(IV)–oxo complexes bearing rigid ligands ( 2 and 3 ), we established the geometric origin of spin-state energetics and reactivity of 1 . Complex 1 featuring a strong Fe–O bond exhibits OAT and HAT reactivity in its quintet state. The lowest quintet OAT pathway has a lower barrier by ∼4 kcal/mol than the quintet HAT pathway, corroborating the experimentally observed gas-phase OAT reactivity preference. A conventional HAT reactivity preference for 2 and a comparable OAT and HAT reactivity for 3 are observed. This further supports the geometry-driven reactivity preference for 1 . Noncovalent interaction analyses reveal a pronounced π–π interaction between the substrate and ligand in the OAT transition state, rationalizing the origin of the observed reactivity preference for 1 .