Sulfur-Ligated Iron(IV)-Imido and Iron(IV)-Oxo Complexes, Which One Is More Reactive?

Abstract

Mononuclear iron(IV)-oxo, iron(IV)-imido and iron(IV)-nitrido complexes are common catalytic cycle intermediates in enzymes, where the metal is typically linked to the protein through cysteinate or histidine sidechains. Enzymatic high-valent iron(IV)-imido and -nitrido intermediates have never been trapped and characterized hence there is uncertainty regarding their structure and function. Using biomimetic models, we have synthesized a novel N4S ligated iron(IV)-imido species as a faithful mimic of the corresponding intermediate in nitrogenase. The complex was characterized with a range of techniques, including UV-Vis absorption spectroscopy, electrospray ionization mass spectrometry, resonance Raman spectroscopy and XANES and EXAFS methodologies. A comprehensive investigation combining reactivity studies and computational analysis compares the oxidative reactivity and chemical properties with the iron(IV)-imido complex with its oxo-analogue. Although the iron(IV)-oxo species is, in general, more reactive than the iron(IV)-tosylimido counterpart, the reverse trend is observed for the oxidation of specific para-substituted thioanisole substrates. Furthermore, an equatorial sulfur group in the ligand framework is seen to enhance the reactivity of thioanisole sulfoxidation. These studies show that high-valent metal-imido groups can be as powerful oxidants as iron(IV)-oxo entities in atom/group transfer reactions.