Application of stopped flow techniques and energy dispersive EXAFS for investigation of the reactions of transition metal complexes in solution: activation of nickel beta-diketonates to form homogeneous catalysts, electron transfer reactions involving iron(III) and oxidative addition to iridium(I)

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

The dissipative translocation of the Zn²⁺ ion between two prototypical coordination complexes has been investigated by combining X-ray absorption and ¹H NMR spectroscopy. An integrated experimental and theoretical approach, based on state-of-the-art Multivariate Curve Resolution and DFT based theoretical analyses, is presented as a means to understand the concentration time evolution of all relevant Zn and organic species in the investigated processes, and accurately characterize the solution structures of the key metal coordination complexes. Specifically, we investigate the dissipative translocation of the Zn²⁺ cation from hexaaza-18-crown-6 to two terpyridine moieties and back again to hexaaza-18-crown-6 using 2-cyano-2-phenylpropanoic acid and its para-chloro derivative as fuels. Our interdisciplinary approach has been proven to be a valuable tool to shed light on reactive systems containing metal ions that are silent to other spectroscopic methods. These combined experimental approaches will enable future applications to chemical and biological systems in a predictive manner.

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

The satisfactory rationalization of complex reactive pathways in solution chemistry may greatly benefit from the combined use of advanced experimental and theoretical complementary methods of analysis. In this work, we combine X-Ray Absorption and ¹ H NMR spectroscopies with state-of-the-art Multivariate Curve Resolution and theoretical analyses to gain a comprehensive view on a prototypical reaction involving the variation of the oxidation state and local structure environment of a selected metal ion coordinated by organic ligands. Specifically, we investigate the 2-cyano-2-phenylpropanoic acid reduction of the octahedral complex established by the Cu²⁺ ion with terpyridine to the tetrahedral complex formed by Cu⁺ and neocuproine. Through our interdisciplinary approach we gain insights into the nature, concentration time evolution and structures of the key metal (XAS measurements) and organic (¹ H NMR measurements) species under reaction. We believe our method may prove to be useful in the toolbox necessary to understand the mechanisms of reactive processes of interest in solution.

Direct structural and mechanistic insights into fast bimolecular chemical reactions in solution through a coupled XAS/UV–Vis multivariate statistical analysis

A combined multivariate and theoretical analysis of coupled XAS/UV–Vis data was proven to be an innovative method to obtain direct structural and mechanistic evidence for bimolecular reactions in solution involving organic substrates.

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

In this work, we propose a method to directly determine the mechanism of the reaction between the nonheme complex Fe^II(tris(2-pyridylmethyl)amine) ([Fe^II(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 [Fe^III(TPA)(κ²-OOAc)]²⁺ and [Fe^IV(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.

A combined multivariate and theoretical XAS analysis was proven to be a powerful method to obtain direct evidence for the mechanism of the reaction between the nonheme complex Fe^II(tris(2-pyridymethyl)amine) ([Fe^II(TPA)(CH₃CN)₂]²⁺) and peroxyacetic acid. This approach allowed to determine the time evolution of the concentration profiles for all reaction intermediates and to quantitatively characterize their structures, suggesting the sixth coordinating ligand of the nonheme oxo complex [Fe^IV(TPA)(O)(X)]^+/2+ is X = AcO⁻ in opposition to X = CH₃CN.

Probing fast kinetics in complex fluids by combined rapid mixing and small-angle X-ray scattering

Stopped-flow mixing coupled to small-angle X-ray scattering (SAXS) is an established technique for investigating structural kinetics in solution down to the millisecond range. More recently, the emphasis has shifted to the sub-millisecond range using continuous flow microfluidic mixing devices. The aim of this article is to review the present status and limitations when applying mixing techniques to a wide range of soft matter and biological systems. In the case of SAXS, special consideration of the mixing quality is necessary for a quantitative description of the scattered intensity. This is demonstrated through two representative examples involving protein refolding and micellar self-assembly.

Brilliant opportunities across the spectrum

Third generation synchrotron light sources provide stable, tuneable light of energy up to the hard X-ray region. The gain of a trillion in brightness as compared to a conventional laboratory X-ray source transforms the opportunities for establishing structure-function relationships. The light may be quasi-continuous or pulsed, have controllable polarisation and have coherence lengths larger than the sample size. The high brightness provides a basis for adding time and spatial resolution to X-ray scattering and spectroscopy. It may also be used to identify very specific information about the magnetic properties of atoms within materials, element specific vibrations, and local structural descriptions identified with chemical speciation. More demanding scattering and diffraction problems can be solved such as weakly scattering materials, large unit cells and structural entities. The high collimation of the source also provides enhanced spectroscopic and diffraction resolution that gives more insight into molecular, extended and supramolecular structures. The length scales can be bridged from the atomic up to that of visible light microscopy and buried features within materials can be observed with the appropriate energy. With an increased emphasis on ease of use, such capabilities are open to exploitation for chemical challenges.

Reaction of hydroquinone with hematite

The rate of reaction of hematite with quinones and the quinone moieties of larger molecules may be an important factor in limiting the rate of reductive dissolution of hematite, especially by iron-reducing bacteria. It is possible that the rate of reductive dissolution of hematite in the presence of excess hydroquinone at pH 2.5 may be limited by the electron-transfer rate. Here, a reductive dissolution rate was measured and compared to electron-transfer rates calculated using Marcus theory. An experimental rate constant was measured at 9.5 x 10 (-6) s(-1) and the reaction order with respect to the hematite concentration was found to be 1.1. Both the dissolution rate and the reaction order of hematite concentration compare well with previous measurements. Of the Marcus theory calculations, the inner-sphere part of the reorganization energy and the electronic coupling matrix element for hydroquinone self-exchange electron transfer are calculated using ab initio methods. The second order self-exchange rate constant was calculated to be 1.3 x 10 (7) M(-1)s(-1), which compares well with experimental measurements. Using previously published data calculated for hexaquairon(III)/(II), the calculated electron-transfer rate for the cross reaction with hydroquinone also compares well to experimental measurements. A hypothetical reductive dissolution rate is calculated using the first-order electron-transfer rate constant and the concentration of total adsorbed quinone. Three different models of the hematite surface are used as well as multiple estimates for the reduction potential, the surface charge, and the adsorption density of hydroquinone. No calculated dissolution rate is less than five orders of magnitude faster than the experimentally measured one.

Reaction of hydroquinone with hematite

The reaction of hematite with quinones and the quinone moieties of larger molecules may be an important factor in limiting the rate of reductive dissolution, especially by iron-reducing bacteria. Here, the electrochemical and physical properties of hydroquinone adsorbed on hematite surfaces at pH 2.5-3 were investigated with cyclic voltammetry (CV), electrochemical-scanning tunneling microscopy (EC-STM), and X-ray photoelectron spectroscopy (XPS). An oxidation peak for hydroquinone was observed in the CV experiments, as well as (photo)reduction of iron and decomposition of the solvent. The EC-STM results indicate that hydroquinone sometimes forms an ordered monolayer with approximately 1.1 QH(2)/nm(2), but can be fairly disordered (especially when viewed at larger scales). XPS results indicate that hydroquinone and benzoquinone are retained at the interface in increasing amounts as the reaction proceeds, but reduced iron is not observed. These results suggest that quinones do not adsorb by an inner-sphere complex where adsorbate-surface interactions determine the adsorbate surface structure, but rather in an outer-sphere complex where interactions among the adsorbate molecules dominate.