Two-State Reactivity in Low-Valent Iron-Mediated C–H Activation and the Implications for Other First-Row Transition Metals

C–H bond activation/functionalization promoted by low-valent iron complexes has recently emerged as a promising approach for the utilization of earth-abundant first-row transition metals to carry out this difficult transformation. Herein we use extensive density functional theory and high-level ab initio coupled cluster calculations to shed light on the mechanism of these intriguing reactions. Our key mechanistic discovery for C–H arylation reactions reveals a two-state reactivity (TSR) scenario in which the low-spin Fe­(II) singlet state, which is initially an excited state, crosses over the high-spin ground state and promotes C–H bond cleavage. Subsequently, aryl transmetalation occurs, followed by oxidation of Fe­(II) to Fe­(III) in a single-electron transfer (SET) step in which dichloroalkane serves as an oxidant, thus promoting the final C–C coupling and finalizing the C–H functionalization. Regeneration of the Fe­(II) catalyst for the next round of C–H activation involves SET oxidation of the Fe­(I) species generated after the C–C bond coupling. The ligand sphere of iron is found to play a crucial role in the TSR mechanism by stabilization of the reactive low-spin state that mediates the C–H activation. This is the first time that the successful TSR concept conceived for high-valent iron chemistry is shown to successfully rationalize the reactivity for a reaction promoted by low-valent iron complexes. A comparative study involving other divalent middle and late first-row transition metals implicates iron as the optimum metal in this TSR mechanism for C–H activation. It is predicted that stabilization of low-spin Mn­(II) using an appropriate ligand sphere should produce another promising candidate for efficient C–H bond activation. This new TSR scenario therefore emerges as a new strategy for using low-valent first-row transition metals for C–H activation reactions.