Unravelling the Effect of Acid‐Driven Electron Transfer in High‐Valent Fe^IV=O/Mn^IV=O Species and Its Implications for Reactivity

Abstract

Metalloenzymes with metal‐oxo units usually involve outer‐sphere electron transfer (OSET). In contrast, biomimetic models exhibit inner‐sphere electron transfer (ISET) or OSET. This is evident in complexes [(N4Py)MnIV(O)]2+ (1) and [(N4Py)FeIV(O)]2+ (2) in the presence of Lewis/Bronsted acid with 1 showcases an OSET mechanism, while 2 exhibits an ISET mechanism. However, the precise reason for this puzzling difference remains elusive.This work unveils the origin of the perplexing inner‐sphere vs outer‐sphere electron transfer process (ISET vs OSET) in [(N4Py)MnIV(O)]2+ (1) and [(N4Py)FeIV(O)]2+ (2) species in the presence of acid. The calculations indicate that when the substrate (toluene) approaches both 1 and 2 that is hydrogen bonded with two HOTf molecules (denoted as 1‐HOTf and 2‐HOTf, respectively), proton transfer from one of the HOTf molecules to the metal‐oxo unit is triggered and a simultaneous electron transfer occurs from toluene to the metal centre. The absence of spin‐inversion and smaller reorganisation energy switch the mechanism to OSET for 1‐HOTf. Therefore, for the first time, the significance of spin‐state and spin‐inversion in the electron transfer process has been identified and demonstrated within the realm of high‐valent metal‐oxo chemistry.

Origin of Unprecedented Formation and Reactivity of FeIV═O Species via Oxygen Activation: Role of Noncovalent Interactions and Magnetic Coupling

“…Employing a slightly higher basis set (LANL2TZ for Fe and 6-311G** for H, C, O, and N) yields similar spin-state ordering, with the gap found to vary marginally for 1 (within 4 kJ/mol). The functional was chosen based on the earlier literature and benchmarking available on similar systems and the ability of B3LYP functional to interpret a correct ground state for the Fe IV O species. , A comprehensive benchmark investigation conducted by de Visser and colleagues, focusing on the non-heme Fe IV O-catalyzed sulfoxidation of thioanisole utilizing diverse DFT functionals (B3LYP, BP86, and PBE0) and basis sets revealed the superior ability of the B3LYP functional to accurately replicate both the experimental trends and absolute values of the free energy of activation . Our group has also investigated a series of oxidative transformations employing various heme/non-heme ferryl–oxo complexes and explored a spectrum of density functionals, including B3LYP, B97-D, ωB97X-D, and M06-2X. − Our analysis revealed that the B3LYP and ωB97X-D functionals consistently produce accurate ground states for all of the species examined.…”