Following a Chemical Reaction on the Millisecond Time Scale by

Jun 12, 2017 - Figure 2B shows the absorbance evolution at λ = 524 nm recorded .... is further oxidized, at least in part, to [FeIV(TPA)(O)]2+, as wi...
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Following a Chemical Reaction on the Millisecond Time Scale by Simultaneous X‑ray and UV/Vis Spectroscopy Giorgio Olivo,† Alessia Barbieri,† Valeria Dantignana,† Francesco Sessa,† Valentina Migliorati,† Manuel Monte,‡ Sakura Pascarelli,‡ Theyencheri Narayanan,‡ Osvaldo Lanzalunga,*,†,§ Stefano Di Stefano,*,†,§ and Paola D’Angelo*,† †

Dipartimento di Chimica, Università di Roma “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy European Synchrotron Radiation Facility, 71, Avenue des Martyrs, 38000 Grenoble, France § Istituto CNR di Metodologie Chimiche (IMC−CNR), Sezione Meccanismi di Reazione, P.le A. Moro 5, 00185 Roma, Italy ‡

S Supporting Information *

ABSTRACT: An innovative approach aimed at disclosing the mechanism of chemical reactions occurring in solution on the millisecond time scale is presented. Time-resolved energy dispersive X-ray absorption and UV/vis spectroscopies with millisecond resolution are used simultaneously to directly follow the evolution of both the oxidation state and the local structure of the metal center in an iron complex. Two redox reactions are studied, the former involving the transformation of FeII into two subsequent FeIII species and the latter involving the more complex FeII−FeIII−FeIV−FeIII sequence. The structural modifications occurring around the iron center are correlated to the reaction mechanisms. This combined approach has the potential to provide unique insights into reaction mechanisms in the liquid phase and represents a new powerful tool to characterize short-lived intermediates that are silent to common spectroscopic techniques.

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electronic variations associated with a chemical transformation over a wide range of reaction times, but it conveys limited information on the structure of the species involved. On the other hand, X-ray absorption spectroscopy (XAS) is a very sensitive probe of both the local structure and oxidation state of an absorbing atom during a chemical reaction but is generally used to characterize isolated intermediates involved in biological as well as in synthetic reactions. The combination of UV/vis and time-resolved XAS spectroscopy can be a new powerful strategy to provide essential insight for mechanistic studies. In recent years, special interest has been devoted to C−H and CC oxidation reactions promoted by bioinspired nonheme iron catalysts, using hydrogen peroxide (H2O2) as the terminal oxidant.12−17 The high regio- and stereoselectivity in association with the environmentally friendly nature of the catalytic process has prompted a great effort to uncover the reaction mechanism leading to C−H and CC oxidations. The key feature of such a mechanism lies in a controlled O−O bond activation that does not generate free-diffusing radical intermediates (which undermine the oxidation selectivity) but closely resembles the pathway observed in heme and nonheme iron oxygenases.18−24 This pathway is made possible by the different oxidation states taken by iron during the catalytic cycle

ne of the ongoing challenges in chemistry is the unambiguous assignment of the transient intermediates formed in a chemical reaction. This insight is essential to elucidate mechanistic reaction pathways. Chemical reactions occur on a large range of time scales. Elementary steps involving valence electron dynamics and single-bond rearrangements usually take place on femtosecond to picosecond time scales (ultrafast chemistry). Conversely, some bimolecular processes including redox reactions in liquid solution occur on longer time scales (micro/milliseconds and above). Since the development of short-pulsed laser systems, time-resolved methods using infrared and Raman spectroscopy,1,2 electron diffraction,3,4 X-ray-based techniques at synchrotrons (X-ray diffraction5,6 and X-ray absorption7−10), and X-ray free electron laser11 methods have been extensively used to probe electronic and structural evolution during ultrafast chemical processes. By contrast, time-resolved structural dynamics techniques yielding information on the mechanism of chemical reactions involving cleavage and formation of covalent bonds on the millisecond time scale did not witness the same growth due to the rather demanding experimental setup. Here, time-resolved energy dispersive X-ray absorption spectroscopy (EDXAS) has been used, for the first time, in combination with UV/vis spectroscopy to directly characterize the intermediates formed during a chemical reaction, by monitoring the structural and electronic changes occurring in solution with time resolution in the millisecond range. UV/vis spectroscopy is the most used technique to follow the © XXXX American Chemical Society

Received: May 9, 2017 Accepted: June 12, 2017 Published: June 12, 2017 2958

DOI: 10.1021/acs.jpclett.7b01133 J. Phys. Chem. Lett. 2017, 8, 2958−2963

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The Journal of Physical Chemistry Letters (from FeII up to FeIV or FeV), but reaction conditions have great influence on the mechanistic pathway and the key intermediates are elusive to conventional spectroscopies.18−28 FeII(tris(2-pyridylmethyl)amine), [FeII(TPA)(CH3CN)2]2+, is a paradigmatic example of such nonheme iron complexes, and here its reaction with different oxidants has been used to exploit the potential of our simultaneous EDXAS-UV/vis technique in elucidating the structural and oxidation state evolution in the course of a chemical reaction. The present work aims at developing an innovative approach to follow the reaction pathway of redox processes taking place in solution on the millisecond time scale by directly probing the evolution of the oxidation state and local structure of a metal ion. To this end, we combined time-resolved EDXAS and UV/ vis spectroscopies carrying out chemical reactions in a stoppedflow apparatus that allowed fast injection of the reagents in the reaction cell. The principle of EDXAS is based on measuring the transmitted intensity from the sample, and therefore, the photoabsorber concentration has to be high enough for its detection in transmission (>30 mM). Very often, these concentrations are not compatible with UV/vis detection due to the high extinction coefficient values of the species under investigation, but it is possible to monitor the reaction evolution following the spectral changes on the peak tails. Figure 1 shows the comparison between the Fe K-edge EDXAS

around the metal center, without further changes in the iron oxidation state. The time-resolved EDXAS spectra were recorded every 40 ms and the reaction was followed for 2 s, although after 704 ms no significant changes were detected in the XANES spectra. Figure 2B shows the absorbance evolution at λ = 524 nm recorded simultaneously on the same reaction mixture. The increase of absorbance at this wavelength is consistent with the formation of the [FeIII(TPA)(CH3CN)(OOH)]2+ species (λmax = 538 nm).18,19,22,29 The subsequent absorbance decrease occurring between 104 and 704 ms illustrates the evolution of such an intermediate to the μ-oxo dimer [FeIII2(TPA)2(μ-O)(H2O)2]4+, as depicted in Figure 2D.19 After 704 ms, no changes are found in the EDXAS spectra or in the visible absorbance at 524 nm, indicating that the reaction is complete. The XANES spectra collected at t = 0, 104, and 1984 ms are shown in Figure 2C, while the edge region is magnified in the inset. Inspection of these spectra allows better detection of the evolution of both the iron oxidation state and local coordination geometry taking place during the reaction. The Fe K-edge position undergoes a shift toward higher energy of 0.7 eV after 104 ms, which is due to partial oxidation of the initial [Fe II (TPA)(CH 3 CN) 2 ]2+ complex to [FeIII(TPA)(CH3CN)(OOH)]2+. Consequently, the XANES structural oscillations are modified because an acetonitrile molecule coordinating the iron atom in [FeII(TPA)(CH3CN)2]2+ is replaced by an oxygenated ligand, leading to a different structural configuration. Note that the linear configuration of the acetonitrile molecule gives rise to strong multiple scattering contributions that are smeared out when one of the two acetonitrile molecules is replaced by a hydroperoxide ligand. The second step of the reaction does not involve any variation of the iron oxidation state, as clearly shown by the position of the absorption edge that remains unchanged until the end of the reaction. However, the XANES oscillations at around 7145 and 7170 eV move toward lower energies in the EDXAS spectra at the end of the reaction, pointing to an increase in the bond distance of some of the ligands. This is in agreement with the formation of a [FeIII2(TPA)2(μ-O)(H2O)2]4+ dimer where the iron atoms are connected through an oxygen bridge. In this case, the higher-frequency contribution could be due both to the presence of the Fe−Fe contact and to a slight increase of the Fe−O distances in the Fe−O−Fe configuration of [FeIII2(TPA)2(μ-O)(H2O)2]4+ as compared to the Fe−O and Fe−N distances in [FeIII(TPA)(CH3CN)(OOH)]2+. Therefore, the EDXAS analysis fully confirms the observations made with the UV/vis monitoring. The experimental body of evidence is compatible with the reaction mechanism depicted in Figure 2D.18,19,22,24,27−29 It is known that peroxyacetic acid is able to oxidize the [FeII(TPA)(CH3CN)2]2+ complex to a relatively stable iron(IV)−oxo complex, [FeIV(TPA)(O)]2+.27 Therefore, a second reaction of [Fe II (TPA)(CH 3 CN) 2 ] 2+ (35 mM) and CH3COO2H (35 mM) as the oxidant was carried out in CH3CN/CH3COOH (99.6:0.4 (v/v)) at 25 °C. Figure 3A shows the time-resolved XANES spectra collected each 40 ms during this reaction. The Fe K-edge position of the spectrum recorded after 24 ms is found to be shifted toward higher energy as compared to the initial [FeII(TPA)(CH3CN)2]2+ spectrum, as evident from the selected XANES spectra reported in Figure 3C (compare the black and blue traces in the inset). Within the following 264 ms, the edge position is further shifted to higher energy, and then, it moves back to lower

Figure 1. Comparison of the EDXAS spectrum of [FeII(TPA)(CH3CN)2]2+ 35 mM in CH3CN at 25 °C collected in 40 ms in the stopped-flow cell with the XANES spectrum of the same compound collected in transmission mode in 40 min.

spectrum of a 35 mM solution of [FeII(TPA)(CH3CN)2]2+ in acetonitrile collected in 40 ms in the stopped-flow cell used for the following reactions and the spectrum collected on the same sample in transmission mode in 40 min on a standard XAS beamline. The two spectra are almost identical, demonstrating the feasibility and reliability of the EDXAS experiment in the stopped-flow cell. Figure 2A shows the continuous evolution of the EDXAS spectra during the course of the reaction of [FeII(TPA)(CH3CN)2]2+ (35 mM) with H2O2 (70 mM) in CH3CN/H2O (99.8:0.2 (v/v)) at 25 °C. The normalized time-resolved recorded X-ray absorption near-edge structure (XANES) spectra show both a fast increase of the Fe K-edge position within the first 104 ms and a modification of the structural oscillations, in accordance with an initial oxidation of FeII to FeIII followed by a clear rearrangement of the local structure 2959

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The Journal of Physical Chemistry Letters

Figure 2. (A) Time evolution of the Fe K-edge EDXAS spectra of [FeII(TPA)(CH3CN)2]2+ (35 mM) after addition of H2O2 (70 mM) in CH3CN/ H2O (99.8:0.2 (v/v)) at 25 °C. (B) Time evolution of the absorbance at λ = 524 nm of the same reaction mixture of (A). EDXAS and UV/vis spectra have been recorded simultaneously in the stopped-flow cell. (C) Selected EDXAS spectra recorded at 0 (blue), 104 (black), and 1984 (green). A magnification of Fe K-edge region is shown in the inset. (D) Sequence of iron species compatible with the EDXAS-UV/vis spectral evolution.

in this region.27−29 A significant increase of the absorbance is evident in the first phase of the reaction (0−264 ms), while a slight decrease is observed starting from 264 ms. The observed experimental evidence strictly reproduces the mechanistic sequence of events depicted in Figure 3D. The initial oxidation of [FeII(TPA)(CH3CN)2]2+ to [FeIII(TPA)(κ2-OOAc)]2+ is fast and occurs in the first 24 ms. In the following time range (24−264 ms), [FeIII(TPA)(κ2-OOAc)]2+ is further oxidized, at least in part, to [FeIV(TPA)(O)]2+, as witnessed by both the increase of the Fe K-edge energy and the sensible increase of the absorbance at λ = 722 nm. The FeIII to FeIV transformation has been indeed reported not to be quantitative in milder conditions (70% yield at −40 °C and lower concentration).23 The subsequent transformation of [FeIV(TPA)(O)]2+ to [FeIII2(TPA)2(μ-O)(μ-OAc)]3+ is made evident by the back motion of the Fe K-edge position (264−1964 ms).

energy during the subsequent 1700 ms. The time evolution of the Fe K-edge position is clearly visible in Figure 3C, where the XANES spectra collected at t = 0, 24, 264, and 1964 ms are reported. There is an edge shift of 0.7 eV in going from the [FeII(TPA)(CH3CN)2]2+ to [FeIII(TPA)(κ2-OOAc)]2+ species and a further shift of 0.7 eV when the [FeIV(TPA)(O)]2+ species is formed. Interestingly, the Fe K-edge positions of the XANES spectra collected at 24 and 1964 ms are perfectly coincident. As far as the structural oscillations are concerned, a clear change is observed during the first 24 ms, while less evident modifications of the XANES features are observed during the following reaction time course. The absorbance at 722 nm has been simultaneously monitored on the same reaction mixture, and its time evolution is shown in Figure 3B. This wavelength was chosen because the [FeIV(TPA)(O)]2+ complex has a diagnostic pale green chromophore at λmax = 720 nm, while none of its monomeric precursors show absorption 2960

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Figure 3. (A) Time evolution of the Fe K-edge EDXAS spectra of [FeII(TPA)(CH3CN)2]2+ (35 mM) after addition of CH3COO2H (35 mM) in CH3CN/CH3COOH (99.6:0.4 (v/v)) at 25 °C. (B) Time evolution of the absorbance at λ = 722 nm of the same reaction mixture of (A). EDXAS and UV/vis spectra have been recorded simultaneously in the stopped-flow cell. (C) Selected EDXAS spectra recorded at 0 (blue), 24 (black), 264 (red), and 1964 ms (green). A magnification of the Fe K-edge region is shown in the inset. (D) Sequence of iron species compatible with the EDXAS-UV/vis spectral evolution (X = OAc, CH3CN).

the EDXAS spectra collected at the end of the reaction. In this case, most probably the presence of the acetate bridge stretches the Fe−Fe distance in the [FeIII2(TPA)2(μ-O)(μ-OAc)]3+ dimer so that its contribution to the XANES spectrum becomes negligible. This study demonstrates that bimolecular reactions in solution occurring on the millisecond time scale can be profitably followed using a combined EDXAS-UV/vis spectroscopic analysis. It is possible to directly monitor the oxidation state evolution of an absorbing metal that is oxidized/reduced during the reaction. At the same time, the simultaneous collection of the XANES and UV/vis spectra allows one to follow the structural modification occurring in the reaction time course with millisecond time resolution. In particular, in the present investigation, we were able to directly follow two reaction mechanisms showing the high potential of this newly developed approach. In the former case, the transformation of an FeII species into two subsequent FeIII species was monitored,

However, this conversion is only partial, as witnessed by the slight decrease of the absorption at 722 nm within 2 s, due to the slow decay rate of [FeIV(TPA)(O)]2+ and the absorbance contribution of [FeIII2(TPA)2(μ-O)(μ-OAc)]3+ (this species exhibits an absorption band in the 650−800 nm region with an extinction coefficient 4 times smaller than that of [FeIV(TPA)(O)]2+).27,30 Furthermore, the reaction pathway is compatible with the variations of the XANES spectra. A clear modification of the structural oscillations is observed in the first phase of the reaction (0−24 ms) due to the change of the iron coordination sphere as two acetonitrile molecules in the first shell are replaced by a dioxygenated ligand. In the former case, the two acetonitrile ligands give rise to strong multiple scattering contributions that are not present in all of the other complexes formed along the reaction path. On the other hand the iron local coordination structures of the [FeIII(TPA)(κ2-OOAc)]2+ and [FeIV(TPA)(O)]2+ are very similar, and this explains the close resemblance of the XANES oscillations collected at 24 and 264 ms. The main XANES features are also maintained in 2961

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The Journal of Physical Chemistry Letters while in the latter case, the more complex FeII−FeIII−FeIV−FeIII sequence was demonstrated to occur.13,25 We envisage that such an approach will provide unique insights into chemical reaction mechanisms involving transition metals for which the direct and unambiguous assignment of the metal oxidation state as well as the definition of its first coordination sphere is a crucial step to discriminate among different mechanistic hypotheses. Moreover, this combined technique represents a powerful tool to characterize intermediates that are silent to common spectroscopic techniques. A definite improvement of this approach will be the adoption of a stopped-flow apparatus able to reach low temperatures when working with organic solvents. This will allow a slowdown of the reaction kinetics with the opportunity to detect and quantitatively monitor the elusive species.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01133. Experimental details and Figures S1−S3, showing EDXAS spectra and the time evolution of the Fe Kedge EDXAS spectra of [FeII(TPA)(CH3CN)2]2+ (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.D.). *E-mail: [email protected] (S.D.S.). *E-mail: [email protected] (O.L.). ORCID

Valentina Migliorati: 0000-0003-4733-6188 Theyencheri Narayanan: 0000-0003-1957-1041 Osvaldo Lanzalunga: 0000-0002-0532-1888 Stefano Di Stefano: 0000-0002-6742-0988 Paola D’Angelo: 0000-0001-5015-8410 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the University of Rome “La Sapienza” (Progetti Ateneo 2015 C26H159F5B). ESRF is acknowledged for the provision of synchrotron beam time.

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REFERENCES

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The Journal of Physical Chemistry Letters (24) Oloo, W. N.; Que, L., Jr. Bioinspired Nonheme Iron Catalysts for C−H and CC Bond Oxidation: Insights into the Nature of the Metal-Based Oxidants. Acc. Chem. Res. 2015, 48, 2612−2621. (25) Nam, W.; Lee, Y.-M.; Fukuzumi, S. Tuning Reactivity and Mechanism in Oxidation Reactions by Mononuclear Nonheme Iron(IV)-oxo Complexes. Acc. Chem. Res. 2014, 47, 1146−1154. (26) Bryliakov, K. P.; Talsi, E. P. Active Sites and Mechanisms of Bioinspired Oxidation with H2O2, Catalyzed by Non-Heme Fe and Related Mn Complexes. Coord. Chem. Rev. 2014, 276, 73−96. (27) Lim, M. H.; Rohde, J.-U.; Stubna, A.; Bukowski, M. R.; Costas, M.; Ho, R. Y. N.; Münck, E.; Nam, W.; Que, L., Jr. An FeIV=O Complex of a Tetradentate Tripodal Nonheme Ligand. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 3665−3670. (28) Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.; Bukowski, M. R.; Stubna, A.; Münck, E.; Nam, W.; Que, L., Jr. Crystallographic and Spectroscopic Characterization of a Nonheme Fe(IV)-O Complex. Science 2003, 299, 1037−1039. (29) Kim, C.; Chen, K.; Kim, J.; Que, L., Jr. An FeIV=O Complex of a Tetradentate Tripodal Nonheme Ligand. J. Am. Chem. Soc. 1997, 119, 5964−5965. (30) It is important to note that the partial conversion of [FeIV(TPA) (O)]2+ to [FeIII2(TPA)2(μ-O)(μ-OAc)]3+, less noticeable in the UV− vis spectrophotometric analysis, is evident from the Fe K-edge energy shift of the XANES spectra. As a matter of fact, the two techniques have a different sensitivity towards the interconversion between the reactive intermediates, and a clear picture of the reaction pathway can be gained only by the combination of the two techniques. On the other hand, it should be stressed that our investigation is limited to a qualitative description at this stage due to the present experimental setup.

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