Facile Conversion of syn-[Fe^IV(O)(TMC)]²⁺ to its anti Isomer via Meunier's Oxo-Hydroxo Tautomerism Mechanism

Prakash, J.; Sheng, Y.; Draksharapu, A.; Klein, J. E. M. N.; Cramer, C. J.; Que, L., Jr.
Angew. Chem. Int. Ed. 2019, 58, 1995 (doi:10.1002/anie.201811454).

The syn and anti isomers of [Fe^IV(O)(TMC)]²⁺ (TMC = tetramethylcyclam) represent the first isolated pair of synthetic nonheme oxoiron(IV) complexes with the identical ligand topology and differing only in the position of the oxo unit bound to the iron center. Both isomers have previously been characterized by X-ray crystallography and various spectroscopic techniques. Herein we report that the syn isomer [Fe^IV(O_syn)(TMC)(NCMe)]²⁺ (2) converts to its anti form [Fe^IV(O_anti)(TMC)(NCMe)]²⁺ (1) in MeCN solution, an isomerization facilitated by water and monitored most readily by ¹H-NMR and Raman spectroscopy. Indeed, when H₂¹⁸O is introduced to 2, the nascent 1 becomes ¹⁸O-labeled. These results provide compelling evidence for a mechanism involving direct binding of a water molecule trans to the oxo atom in 2, which then undergoes oxohydroxo tautomerism to become incorporated as the oxo atom of 1. The nonplanar nature of the TMC supporting ligand makes this isomerization an irreversible transformation, unlike for their planar heme counterparts.