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Mechanism for O-O bond formation via radical coupling of metal and ligand based radicals – a new pathway Yulia Pushkar, Yuliana Pineda-Galvan, Alireza Karbakhsh Ravari, Tatiana Otroshchenko, and Daniel A Hartzler J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06836 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018
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Journal of the American Chemical Society
Mechanism for O-O bond formation via radical coupling of metal and ligand based radicals – a new pathway Yulia Pushkar*1, Yuliana Pineda-Galvan1, Alireza K. Ravari1, Tatiana Otroshchenko2, Daniel A. Hartzler1, ǂ Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette, IN, 47907 Leibniz-Institute for Catalysis at the University of Rostock, Albert-Einstein-Strasse 29a, D-18059, Rostock, Germany ǂ current address AECOM, National Energy Technology Laboratory, 626 Cochrans Mill Rd, Pittsburgh, PA 15236 1 2
Supporting Information Placeholder ABSTRACT: Artificial Photosynthesis carries promise to deliver abundant clean energy for the needs of a growing population. Deep mechanistic understanding is required to achieve rational design of fast and durable water oxidation catalysts. Here we provided first evidences for a new mechanism of the O-O bond formation via radical coupling of the oxidized metal=oxo of radicaloid character (RuIV=O) and ligand based radical ([ligand-NO]+ cation radical). O-O bond formation is facilitated via spin alignment and takes place via a virtually barrier less pathway inside the single metal complex. In situ reactive intermediate conversion was monitored by mass spectrometry, resonance Raman (RR) and EPR. Computational analysis have shown that the formation of [ligandNO]+ happens at a lower overpotential than the formation of the [RuV=O(ligand)]3+ intermediate. Overall, the presented paradigm for O-O bond formation opens new opportunities for rational catalyst design. In an effort to provide energy to an increasing population and find a substitute to polluting fossil fuels researchers strive to develop artificial photosynthesis. 1-2 A key catalytic reaction in this process is water oxidation and a lack of an efficient and durable water oxidation catalyst (WOC) is a significant limitation. A variety of transition metals oxides and hydroxides catalyze water splitting, however, the turnover per metal center is slow and the identity of the most catalytically active sites and their action mechanisms are still debated.3-5 The Mn4Ca cluster of Photosystem II achieves a high rate and ~60% efficiency.6-7 Ru based catalysts have been reported with improved rates and increased turnover numbers. 2, 8-11 Notably, all these catalysts contain polypyridine ligands but lack established structure activity relationships. Catalytic systems with Fe, Co and Cu have been reported12, but Ru complexes display faster O2 evolution kinetics and higher turnover numbers.13-14 Further development is critically dependent on understanding the mechanism of the O-O bond formation and identification of low barrier pathways for enhanced catalytic performance. There is an accepted view in the field that the O-O bond required to produce O2 can be formed via nucleophilic attack of water on a highly oxidized metal=oxo species, however, this pathway is expected to be slow due to high barriers to O-O bond formation, Figure 1A.2, 15-19 A few complexes achieve high turnover frequencies using only a single metal with a second-order rate dependence in the catalyst. These function via radical coupling
between two metal=oxo species. 13, 20 Unfortunately, such a mechanism is only effective in solution due to the bi-molecular nature of the transition state and leads to deactivation via dimer and trimer formation.21 As a result, complexes lose activity when functionalized into assemblies for artificial photosynthesis.22 This fact allowed the in situ characterization of the 7-coordinate [(isoq)2(bda)RuV=O]+ ( isoq = isoquinoline, bda = 2,2′-bipyridine6,6′-dicarboxylate) with RuV=O at ~1.75 Å.23 Attempts to design a radical coupling pathway in di-nuclear metal complexes resulted in slow O2 evolution likely due to a non-optimal transition state2425 or to rate limiting O release. 26-27 It has been argued that radical 2 coupling in general provides a lower barrier for O-O bond formation, thus, new catalyst designs exploring radical coupling are of great importance. 28-30 Here we report the first demonstration of O-O bond formation via radical coupling inside a catalytic complex with a single metal center (intramolecular radical coupling), opening the opportunity to design novel classes of water oxidation catalysts based on low barrier O-O bond formation between an oxo group of the metal center and a cation radical group of the ligand, Figure 1B. Such kind of catalysts can be incorporated into a metal organic framework for improved stability without loss of function.31-32 We have long envisioned the possibility of an alternative pathway for O-O bond formation in some of the Ru based complexes. For instance, the nucleophilic attack hypothesis, Figure 1A, involving the [RuV=O(tpy)(bpy)]3+ intermediate, remains unverified. 16, 33-34 Reported peroxo species were later re-interpreted 15, 35 and the claimed rate limiting [RuV=O]3+ species in single site catalysts with neutral polypyridine ligands 15, 17-18, 36-38 was not observed under any experimental conditions. Note that water nucleophilic attack has a significant activation barrier of 0.57 to 1.1 eV according to some calculations.17-18, 39-42 Thus, to form an O-O bond via water nucleophilic attack on a RuV=O species, two significant activation barriers must be overcome: first to generate the RuV=O and second for RuV=O to react with water, Figure 1A. RuIV=O species are capable of fast oxidation of polypyridines with the formation of N-oxides.33 Even nitrogen centers coordinated to Ru as in the quaterpyridine [Ru(qpy)(L)2]2+ undergo facile oxidation.43 Liu et al. suggested that the [Ru(qpy)(L)2]2+ series are inactive unless the qpy-N,N”-dioxide ligand is formed.43 Computationally the [RuIV=O(tpy)(L)]2+ can convert to intermediates with coordinated N-oxide.44 Our DFT calculations (unpublished) have shown a G0 eV for the conversion of the
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[RuIV=O(tpy)(bpy)]2+ 16, 45 to [RuII(tpy-NO)(bpy)]2+ with tpy-Noxide (G=-0.005 eV) or bpy-N-oxide (G=+0.07 eV). Presently, all these observations
Figure 1. A) O-O bond formation via water nucleophilic attack. B) New O-O bond formation mechanism with oxidation of the bpyNO to the [ligand-NO]+ cation radical and its barrier-less radical coupling with RuIV=O. C) DFT analysis of the O-O bond formation. [RuIV=O(bpy)2(bpy-NO+)mono]3+ has significant spin density on oxygens and antiparallel alignment of unpaired electrons. D) Insert shows electronic energy as a function of the OO bond distance for the [RuIV=O(bpy)2(bpy-NO+)mono]3+. S=1/2 forms O-O bond in a virtually barrier-less (~0.1 eV) transition. leave the question open as to whether the oxygen atom of the Noxide polypyridine ligand is capable of O-O bond formation. To answer this question, we analyzed oxidation of a specially designed [RuII(bpy)2(bpy-NO)]2+ complex with a single N-oxide-bipyridine ligand, Figure 1C. We determined that oxygen of the bpy-NO group incorporates into produced O2. The [RuII(bpy)2(bpy-NO)]2+ was prepared and characterized by Uv-vis, NMR, mass spectrometry, FTIR and extended X-ray absorption fine structure (EXAFS) (see Methods in SI, Figures S110). Oxygen evolution was detected after addition of cerium ammonium nitrate (CeIV), Figure S3. Dissociation of the NO group of the bpy and binding of a water is needed for O2 evolution activity. With 20 equiv CeIV oxidant, ~10% of complexes enter the catalytic cycle while ~90% remain as [RuIII(bpy)2(bpy-NO)]3+ as detected by EPR (Figure 2, S3, S4). The O-O bond formation reaction was followed spectroscopically and by DFT to demonstrate O-O bond formation via radical coupling between the RuIV=O center and the oxygen of the [ligand-NO]+ cation radical. Oxidation of the complex with CeIV results in the formation of a mixture of the [RuIII(bpy)2(bpyNO)]3+ S=1/2, detectable by EPR (gxx=2.62, gyy=2.26, gzz=1.70 (Figure 2, S4A) and [RuIV=O(bpy)2(bpy-N-O)mono]2+ detected by resonance Raman (=442 nm) Figure 2, S6-9. Oxidation with 2 or more equiv of CeIV gives additional EPR signals at gxx=2.46 and
gxx=2.73 which grow with the increase in CeIV concentration and with time after mixing, Figure S4B, C. gxx=2.73 corresponds to [RuIII(bpy)3]3+ while assignment of the signal at gxx=2.46 is currently unknown. RR of mixtures with 1-8 equiv of CeIV show the rise of a 799 cm-1 vibration beginning with the addition of 3 equiv and reaching a steady state by 5 equiv, Figure S6D. In H218O, the 799 cm-1 band experiences a downshift of 35 cm-1, Figure 2 and S6, consistent with the presence of RuIV=O according to our DFT calculations and previously reported values. 31, 33, 46-47 Since the ligand bound oxygen of bpy-NO was exclusively 16O, one would expect the vibrational band associated with RuIV=O to show little to no isotope sensitivity if the O2 formation pathway involved transfer of the ligand bound 16O to Ru. Thus, we can conclude that, since the relative intensities of the isotope split RuIV=O band so closely match the solvent water isotope ratio, the oxygen of RuIV=O originated from a solvent water, not the bpy-NO ligand. An additional isotope sensitive band at 832 cm-1, can be seen in the 18O / 16O difference spectrum of Figure 2, S6. Its associated isotope shift (-16 cm-1) can only be obtained via fits due to its low intensity and spectra interference from nearby vibrations, Figure S8, Table S2. Calculations indicate that the ~832 cm-1 band may be due to the Ru-18O bond of [RuIII(bpy)2(bpy-N18O)]3+, with a DFT predicted 849 cm-1 (-15 cm-1) isotope shift, or the Ru-18O bond in the [RuIIIOO-N-bpy(bpy)2]3+ peroxide 853 cm-1 (-15 cm-1), Figure S9. Additionally, the computed Raman difference spectra for [RuIIIOON-bpy(bpy)2]3+ and [RuIII(bpy)2(bpy-N18O)]3+ shows multiple oxygen isotope sensitive bands throughout the 500-800 cm-1 range that were not observed experimentally, probably due to low intensity or low resonance enhancement. To establish the origin of the oxygen atoms in the O2 formed as a result of [Ru(bpy)2(bpy-NO)]2+ oxidation, isotope labeling combined with gas chromatography/mass spectrometry was performed. Measurements 10 minutes after the addition of CeIV, Figure S10, detected 16O18O only for samples dissolved in H218O. The presence of 18O2 (m=36) can be explained by coordination of water to the [RuIII-OO-N-bpy(bpy)2]3+ in the O2 evolution step, Figure 2 and Table S4. In this event, the steps depicted in Figure 1 would repeat with labeled oxygen bound to the ligand and finishing with the release of 18O2. No products with m=34 or m=36 were detected when the compound was oxidized in H216O water. The reactions leading to O-O bond formation were also followed by DFT16, 33 to confirm the O-O bond formation via radical coupling between the RuIV=O center and the oxygen of the [ligandNO]+ cation radical, Table S3, S4, Figure 2. The DFT approach used is known to deliver good agreement with experiment for bond distances and oxidation potentials, however, higher errors are expected for energies of ligand disassociation.48 The initial [Ru(bpy)2(bpy-NO)]2+ complex cannot be oxidized by CeIV beyond the RuIII state (requiring ~2.88 eV, Table S4). However, dissociation of the N-O group and coordination of water to the Ru opens the PCET channel, allowing further oxidation to RuIV=O, Figures 1C, 2. The resulting [RuIV=O(bpy)2(bpy-NO)mono]2+ intermediate cannot yet form O-O via the peroxide ([RuII-OO-Nbpy(bpy)2]2+) due to a positive G, Table S4. Thus, it must undergo one more oxidation step of ~ +1.7 eV which results in the [RuIV=O(bpy)2(bpy-NO+)mono]3+ complex with a cation radical at the N-O group, Figure 2 insert. Onset of catalytic current in the range of 1.6-1.8 eV is often reported for Ru-based WOCs with neutral polypyridine ligands, however, its exact assignment to a particular molecular mechanism remains unconfirmed. 15, 17-18, 36-37 The computed redox potential for [RuIV=O(bpy)2(bpy-NO+)mono]3+ formation (+1.7 V) is in agreement with the reported pH independent onset of catalytic current in some Ru based catalysts 13, 36-37, 49 while oxidation to RuV=O might require at least ~2.0 eV according to calculations.16, 33 [RuIV=O(bpy)2(bpy-NO+)mono]3+
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Journal of the American Chemical Society can have S=1/2 and S=3/2 spin states which are almost equal in energy, Table S3, Figure 1D. Critically the [RuIV=O(bpy)2(bpyNO+)mono]3+ in the S=1/2 state displays spin alignment, Figure 1C, where unpaired electrons on the oxygen of the RuIV=O group
formed with Earth abundant Fe, Mn or Co and the ligand can be used in functional assemblies for photoelectrochemical water splitting.
ASSOCIATED CONTENT Supporting Information Additional supporting information is available on Materials and Methods, mass spectrometry, EPR, resonance Raman, EXAFS data and DFT calculations. The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION Corresponding Author *
[email protected] ACKNOWLEDGMENT
Figure 2. DFT (UB3LYP/DGDZVP) derived Latimer−Frost diagram for oxidation of [RuII(bpy)2(bpy-NO)]2+ under standard conditions (pH = 0). Spectroscopic signals of the detected intermediates are shown with EPR for S=1/2 =[RuIII(bpy)2(bpyNO)]3+ and RR for [RuIV=O(bpy)2(bpy-N-O)mono]2+. Spin density is shown for [RuIV=O(bpy)2(bpy-N-O+)mono]3+ S=1/2 to demonstrate radicaloid character of the RuIV=O unit and cation radical. (O=+0.87) and the oxygen of the [ligand-NO]+ cation radical (O=-0.61) have antiparallel alignment. Spin alignment was introduced as a criteria for low barrier O-O bond formation via radical coupling in the Photosystem II oxygen evolving complex.30, 50 While the energy of the S=3/2 state increases with decrease of the O-O distance, S=1/2 shows a virtually barrier-less transition (~0.1 eV) into the peroxo intermediate, Figure 1D. Thus, the [RuIV=O(bpy)2(bpy-NO+)mono]3+ S=1/2 intermediate can produce the O-O bond via what we have termed a “ligand recoil” mechanism. It happens when the [ligand-NO]+ cation radical recollides with a RuIV=O fragment to form the O-O bond, facilitated by spin alignment between the two oxygen centers with significant spin densities. Overall the [RuIV=O(bpy)2(bpy-NO+)mono]3+ to [RuIII-OO-N-bpy(bpy)2]3+ conversion has a small, negative G, Table S4. The formed peroxo species has a strong driving force of G-1.9 eV to expel oxygen with formation of [Ru(bpy)3]3+, however, other pathways such as further oxidation of the peroxide or water coordination onto the Ru center with concomitant O2 evolution can be envisioned as well. A model compound [RuII(bpy)2(bpy-NO)]2+ has been designed to demonstrate the fundamentally new O-O bond formation mechanism. Intra-molecular coupling between the metal center oxo and a ligand radical eliminates the need for aligning two supramolecular complexes in the correct orientation and overcoming steric and solvation barriers to form the transition state. It has been argued that radical coupling in general provides a lower barrier for O-O bond formation 29, 50 and our data reinforce this assertion. While the data presented here are the first demonstration of the direct involvement of the organic (ligand) in the step of O-O bond formation, it has been established that in the H-H bond formation by hydrogenase and in biomimetic complexes the nitrogen of the ligand plays a crucial role providing hydrogen for coupling. 51 Catalysts accomplishing O-O bond formation via radical coupling between the metal which can also be represented by oxo species
This material is based upon work supported by the U.S. Department of Energy, Office of Sciences, Office of Basic Energy Sciences under grant numbers DE-FG02-10ER16184 (Y.P.). Access to EPR was provided by the Amy Instrumentation Facility, Department of Chemistry under the supervision of Dr. Michael Everly. Authors would like to thank Connie Bonham for assistance with gas chromatography/mass spectrometry measurements (measurements were performed at Purdue Campus-Wide Mass Spectrometry Center under the supervision of Dr. Karl Wood), Dr Roman Ezhov for assistance with NMR measurements and Dr Mark Palenik for helpful discussion. The use of the Advanced Photon Source, an Office of Science User Facility operated by the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract DE-AC0206CH11357. The PNC/XSD (Sector 20) facilities at the Advanced Photon Source and research at these facilities were supported by the U.S. Department of Energy, Basic Energy Science and the Canadian Light Source.
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