Trapping Iron(III)-Oxo species at the boundary of the “Oxo-Wall”

Today, Miquel Costas and co-workers reported in JACS on the trapping of Fe(III)=oxo species. Terminal non-heme iron(IV)-oxo compounds are among the most powerful and best studied oxidants of strong C-H bonds. In contrast to the increasing number of such complexes (>80 thus far), corresponding one-electron-reduced derivatives are much rarer and presumably less stable, and only two iron(III)-oxo complexes have been characterized to date, both of which are stabilized by hydrogen-bonding interactions. Herein we have employed gas-phase techniques to generate and identify a series of terminal iron(III)-oxo complexes, all without built-in hydrogen bonding. Some of these complexes exhibit á70 cm^-1 decrease in á(Fe-O) frequencies expected for a half-order decrease in bond order upon one-electron reduction to an S = 5/2 center, while others have á(Fe-O) frequencies essentially unchanged from those of their parent iron(IV)-oxo complexes. The latter result suggests that the added electron does not occupy a d orbital with FeáO antibonding character, requiring an S = 3/2 spin assignment for the nascent Fe^III-O^– species. In the latter cases, water is found to hydrogen bond to the Fe^III-O^– unit, resulting in a change from quartet to sextet spin state. Reactivity studies also demonstrate the extraordinary basicity of these iron(III)-oxo complexes. Our observations show that metal-oxo species at the boundary of the “Oxo Wall” are accessible, and the data provide a lead to detect iron(III)-oxo intermediates in biological and biomimetic reactions.

E. Andris, R. Navrátil, J. Jaáík, M. Puri, M. Costas, L. Que, and J. Roithová
“Trapping Iron(III)-Oxo Species at the Boundary of the “Oxo Wall”: Insights into the Nature of the Fe(III)-O Bond”
J. Am. Chem. Soc. 2018, 140, 14391-14400 [abstract]
DOI: 10.1021/jacs.8b08950