Direct Mechanistic Evidence for a Nonheme Complex Reaction through a Multivariate XAS Analysis.

Francesco Tavani,Andrea Martini,Osvaldo Lanzalunga,Paola D’Angelo,Giorgio Capocasa,Stefano Di Stefano

doi:10.1021/acs.inorgchem.0c01132

Abstract

In this work, we propose a method to directly determine the mechanism of the reaction between the nonheme complex FeII(tris(2-pyridylmethyl)amine) ([FeII(TPA)(CH3CN)2]2+) and peracetic acid (AcOOH) in CH3CN, working at room temperature. A multivariate analysis is applied to the time-resolved coupled energy-dispersive X-ray absorption spectroscopy (EDXAS) reaction data, from which a set of spectral and concentration profiles for the reaction key species is derived. These “pure” extracted EDXAS spectra are then quantitatively characterized by full multiple scattering (MS) calculations. As a result, structural information for the elusive reaction intermediates [FeIII(TPA)(κ2-OOAc)]2+ and [FeIV(TPA)(O)(X)]+/2+ is obtained, and it is suggested that X = AcO– in opposition to X = CH3CN. The employed strategy is promising both for the spectroscopic characterization of reaction intermediates that are labile or silent to the conventional spectroscopic techniques, as well as for the mechanistic understanding of complex redox reactions involving organic substrates.

Highlights

The full understanding of a given reaction mechanism, defined as the sequence of elementary steps leading reactants to products, is vital for chemical knowledge
We employed time-resolved energydispersive X-ray absorption spectroscopy (EDXAS) to qualitatively identify the sequence of oxidation states during the reaction between the nonheme iron complex FeII(tris(2pyridylmethyl)amine) ([FeII(TPA)(CH3CN)2]2+) and peroxyacetic acid (AcOOH) in CH3CN/AcOH (99.6:0.4 (v/v)) at 25 °C.5. Investigating this transformation at −40 °C, a seminal study showed that AcOOH oxidizes [FeII(TPA)(CH3CN)2]2+ to the relatively stable oxo-complex [FeIV(TPA)(O)(X)]+/2+, which in turn decays upon warming to the μ-oxo dimeric product [Fe2III(TPA)2(μ-O)(μ-OAc)]3+
Multivariate Curve Resolution (MCR) techniques have been increasingly applied to time- and space-resolved X-ray absorption near edge structure (XANES) with studies investigating doped V2O5 lithium batteries,[45] ZnO Q-dot formation,[46] degradation of chlorine layered double hydroxide (LDH) upon heating,[47] and a variety of catalytic systems in the solid phase.[48−53] To the best of our knowledge, we report the first application of the MCR approach to XANES spectra pertaining to a bimolecular reaction in solution on organic substrates evolving on the millisecond time scale

Summary

Introduction

The full understanding of a given reaction mechanism, defined as the sequence of elementary steps leading reactants to products, is vital for chemical knowledge. Nonheme iron complexes are a class of bioinspired catalysts that are gaining special interest for their capacity of oxidizing C−H and C C bonds with high regio- and stereoselectivity.[1−3] A special attention has been dedicated to the use of the environmentally friendly H2O2 oxidant in association with acetic acid, which is able to increase both catalytic activity and reaction selectivity Under these conditions a metal-based oxidant is formed rather than freediffusing radical species with the iron center that assumes different oxidation states during the reaction cycle.[4] In a previous investigation, we employed time-resolved energydispersive X-ray absorption spectroscopy (EDXAS) to qualitatively identify the sequence of oxidation states during the reaction between the nonheme iron complex FeII(tris(2pyridylmethyl)amine) ([FeII(TPA)(CH3CN)2]2+) and peroxyacetic acid (AcOOH) in CH3CN/AcOH (99.6:0.4 (v/v)) at 25 °C.5. We employed time-resolved energydispersive X-ray absorption spectroscopy (EDXAS) to qualitatively identify the sequence of oxidation states during the reaction between the nonheme iron complex FeII(tris(2pyridylmethyl)amine) ([FeII(TPA)(CH3CN)2]2+) and peroxyacetic acid (AcOOH) in CH3CN/AcOH (99.6:0.4 (v/v)) at 25 °C.5 Investigating this transformation at −40 °C, a seminal study showed that AcOOH oxidizes [FeII(TPA)(CH3CN)2]2+ to the relatively stable oxo-complex [FeIV(TPA)(O)(X)]+/2+, which in turn decays upon warming to the μ-oxo dimeric product [Fe2III(TPA)2(μ-O)(μ-OAc)]3+.6 The complex [FeIV(TPA)(O)(X)]+/2+ was studied through a combination

Methods

Results

Conclusion