Coupled X-ray Absorption/UV–vis Monitoring of Fast Oxidation Reactions Involving a Nonheme Iron–Oxo Complex

Abstract

Time-resolved X-ray absorption (XAS) and UV−vis spectroscopies with millisecond resolution are used simultaneously to investigate oxidation reactions of organic substrates by nonheme iron activated species. In particular, the oxidation processes of arylsulfides and benzyl alcohols by a nonheme iron−oxo complex have been studied. We show for the first time that the pseudo-first-order rate constants of fast bimolecular processes in solution (milliseconds and above) can be determined by time-resolved XAS technique. By following the Fe K-edge energy shift, it is possible to detect the rate of iron oxidation state evolution that matches that of the bimolecular reaction in solution. The kinetic constant values obtained by XAS are in perfect agreement with those obtained by means of the concomitant UV−vis detection. This combined approach has the potential to provide unique insights into reaction mechanisms in the liquid phase that involve changes of the oxidation state of a metal center, and it is particularly useful in complex chemical systems where possible interferences from species present in solution could make it impossible to use other detection techniques.

Two Faces of the Same Coin: Coupling X‐Ray Absorption and NMR Spectroscopies to Investigate the Exchange Reaction Between Prototypical Cu Coordination Complexes

“…For instance, recently X-ray Absorption Spectroscopy (XAS) has been coupled with UV-Vis in a stopped-flow experiment to follow transformations involving nickel, iridium and palladium complexes in solution [1,2] as well as to identify the reaction mechanisms and intermediates of a series of processes involving non-heme iron complexes, where the metal center changes its oxidation state during the reaction. [3][4][5][6] An advancement of this combined experimental approach is to couple XAS with 1 H NMR spectroscopy, a technique more informative although perhaps less straightforward if compared to UV-Vis. Indeed, the two spectroscopic methods can be considered complementary when a chemical process occurring between metal complexes and organic molecules is taken into account.…”

“…Indeed, the two spectroscopic methods can be considered complementary when a chemical process occurring between metal complexes and organic molecules is taken into account. On the one hand, XAS allows one to quantitatively monitor the evolution of the oxidation state of a given metal ion and of its local structural environment with an unrivaled degree of accuracy [3,7–10] …”

Activation of C–H bonds by a nonheme iron(iv)–oxo complex: mechanistic evidence through a coupled EDXAS/UV-Vis multivariate analysis

“…Notably, EDXAS offers the possibility of gaining simultaneous information on the time evolution of the oxidation state of the given photoabsorber and of its molecular surroundings within 5Å with a good degree of accuracy. [14][15][16] We have recently applied this technique in combination with UV-Vis spectroscopy for the identification of the succession of oxidation states and for the measurement of pseudo-first-order kinetic constants in bimolecular reactions involving nonheme iron complexes. 14,15,17 Herein, we combine multivariate statistical and theoretical analyses of timeresolved coupled EDXAS/UV-Vis data to gain a comprehensive mechanistic picture on the activation of C-H bonds in the substrates 9,10-dihydroanthracene (DHA) and diphenylmethane (Ph 2 CH 2 ) by the nonheme Fe IV -oxo complex [N4Py•Fe IV (O)] 2+ .…”

“…[14][15][16] We have recently applied this technique in combination with UV-Vis spectroscopy for the identification of the succession of oxidation states and for the measurement of pseudo-first-order kinetic constants in bimolecular reactions involving nonheme iron complexes. 14,15,17 Herein, we combine multivariate statistical and theoretical analyses of timeresolved coupled EDXAS/UV-Vis data to gain a comprehensive mechanistic picture on the activation of C-H bonds in the substrates 9,10-dihydroanthracene (DHA) and diphenylmethane (Ph 2 CH 2 ) by the nonheme Fe IV -oxo complex [N4Py•Fe IV (O)] 2+ . In particular, quantitative mechanistic and structural information for all the reaction intermediates is derived from the application of principal component analysis (PCA) and a strategy belonging to the multivariate curve resolution (MCR) family.…”

Following a Silent Metal Ion: A Combined X-ray Absorption and Nuclear Magnetic Resonance Spectroscopic Study of the Zn2+ Cation Dissipative Translocation between Two Different Ligands

“…Consequently, leveraging the insight provided by XAS to investigate Zn 2+ -based systems in solution may allow one to gain valuable and unrivaled information on the reactive, structural, and electronic properties of the Zn 2+ species at play. 23 − 27 Moreover, the combined use of different experimental techniques to monitor the advancement of a chemical process is often a powerful tool for a satisfactory understanding of the operating reaction mechanism. XAS has been successfully coupled with UV–vis to follow chemical reactions involving nickel, iridium, palladium, and iron complexes in solution.…”