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

TL;DR

This study compares the reactivity of iron(iv)-imido and iron(iv)-oxo complexes, finding that both can be equally effective oxidants in certain reactions.

Contribution

The synthesis and characterization of a novel N4S ligated iron(iv)-imido complex as a mimic of a nitrogenase intermediate.

Findings

Iron(iv)-oxo species are generally more reactive than iron(iv)-tosylimido complexes.

The reverse reactivity trend is observed for the oxidation of specific para-substituted thioanisole substrates.

An equatorial sulfur group enhances the reactivity of thioanisole sulfoxidation.

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 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.

This work highlights the differences in structure, spectroscopic properties and reactivity of sulfur ligated iron(iv)-imido and iron(iv)-oxo complexes and how imido complexes can be as powerful oxidants as oxo complexes.

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