Trapping Iron(III)–Oxo Species at the Boundary of the “Oxo Wall”: Insights into the Nature of the Fe(III)–O Bond

Abstract

Terminal nonheme 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), correspond-ing 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 being stabilized by hydrogen bonding interac-tions. 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 ~70-cm-1 decrease in (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 (Fe–O) frequencies essentially unchanged from those of their parent iron(IV)-oxo complexes. The latter result suggests that the added electron does not occu-py a d orbital with Fe=O antibonding character, requiring an S = 3/2 spin assignment for the nascent FeIII–O– species. In the latter cases, water is found to hydrogen bond to the FeIII–O– unit, resulting in a change from quartet spin state to sextet. 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.