Sulfur-ligated iron(iv)-imido and iron(iv)-oxo complexes, which one is more reactive?

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 a 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 of the iron(iv)-imido complex with those of its oxo-analogue. Although the iron(iv)-oxo species is, in general, more reactive than its 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.