Intramolecular Oxo Atom Migration to the cis Thiolate Sulfur of an Fe-Oxo Intermediate

Herein, we show for the first time that thiolate ligands can play an important role in promoting 2e- oxo atom transfer with nonheme iron. We examine the mechanism of oxo atom transfer to a cis thiolate sulfur for two structurally related iron complexes using low-temperature kinetics, spectroscopic, and computational methods. Intermediate oxo atom donor adducts, FeOIAr, are spectroscopically characterized and shown to have electronic spectral, EPR, and Mössbauer parameters, and kinetic barriers dependent on the nature of the oxo atom donor. More stable adducts containing pyridine N-oxide (PNO) were crystallographically characterized and computationally optimized to establish optimum functionals and basis sets. Oxo atom transfer is shown both experimentally and computationally to involve a stepwise, as opposed to a concerted, mechanism. A new metastable intermediate is observed by low-temperature Mössbauer and ⊥-mode EPR after the Fe-O_xIPh intermediate and prior to the final sulfenate Fe-OSR product. The DFT calculated minimum energy pathway is shown to contain a local minimum between the Fe-O₂IPh adduct and Fe-S(R)O product. The DFT optimized geometric and electronic structure of this intermediate is shown to be consistent with an S = 1 Fe(IV)═O that is antiferromagnetically coupled to an S = 1/2 radical delocalized over the two cis thiolate-sulfurs, analogous to the electronic configuration of P450 Cmpd I. Radical character on the thiolate sulfur adjacent to the oxo is shown to facilitate trapping of the high-valent Fe-oxo as a η²-SO-Fe sulfenate complex.