Generating high-valent iron with light: photochemical dynamics from femtoseconds to seconds

• Published in: International reviews in physical chemistry 2014-10, Vol.33 (4), p.521-553
• Main Authors: Torres-Alacan, Joel, Vöhringer, Peter
• Format: Article
• Language: English
• Subjects: Chemical bonds; Chemical synthesis; femtosecond spectroscopy; high-valent iron; infrared spectroscopy; Iron compounds; Oxidation; Photochemistry; photodissociation; reaction dynamics; recombination; Spectrum analysis; Temperature effects; transition metal complexes; vibrational relaxation
• ISSN: 0144-235X; 1366-591X
• DOI: 10.1080/0144235X.2014.973197

The most abundant oxidation states of iron are the ferrous and ferric states, +II and +III, respectively. High-valent iron compounds contain Fe centres at the extraordinary oxidation states, +IV, +V and +VI. Such species are of paramount importance for the catalysis of a number of biochemically or technologically relevant molecular transformations. Because of their unusual electronic structure, high-valent iron compounds are generally extremely reactive and hence, difficult to detect during their catalytic action. Therefore, a concerted research effort is currently undertaken worldwide to synthesise and analyse low molecular-weight model systems with the goal to better understand the reactivities of high-valent Fe-complexes and the long-term vision to exploit them as catalysts in important chemical transformations requiring inert bond activation. Particularly reactive are high-valent iron species that feature a terminal iron-nitrogen bond, so called nitridoiron compounds, and in which the metal is embedded in a fourfold-symmetrical coordination geometry like in haem proteins. Their isolation has not been accomplished to date, but very few non-haem model systems have been prepared by photochemical means and cryo-trapped in low-temperature solid matrices. Very recently, sophisticated time-resolved infrared (IR) spectroscopies have been successful at temporally trapping such elusive pseudo-octahedral nitridoiron species even in liquid solution and at room temperature, i.e. under biochemically and technologically relevant conditions, and to even allow for a preliminary study of their reactivity. This review will summarise our current state of understanding of the molecular-level mechanisms that are involved in the photochemical route to such high-valent iron species as seen through these powerful time-resolved IR spectroscopies.