Vibrational Characterization of Microsolvated Electrocatalytic Water

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Vibrational Characterization of Microsolvated Electrocatalytic Water Oxidation Intermediate: [Ru(tpy)(bpy)(OH)] (HO) 2+

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Erin M. Duffy, Jonathan M. Voss, and Etienne Garand J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05255 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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The Journal of Physical Chemistry

Vibrational Characterization of Microsolvated Electrocatalytic Water Oxidation Intermediate: [Ru(tpy)(bpy)(OH)]2+(H2O)0-4

Erin M. Duffy, Jonathan M. Voss, and Etienne Garand*

Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States

*Author to whom correspondence should be addressed email: [email protected]

1 ACS Paragon Plus Environment


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Abstract

The infrared predissociation spectra of the mass-selected electrocatalytic water oxidation intermediate [Ru(tpy)(bpy)(OH)]2+(H2O)0-4 are reported. The [Ru(tpy)(bpy)(OH)]2+ species is generated by passing a solution of [Ru(tpy)(bpy)(H2O)](ClO4)2 through an electrochemical flow cell held at 1.2 V and is immediately introduced into the gas phase via electrospray ionization (ESI). The microsolvated clusters are formed by reconstructing the water network in a cryogenic ion trap. Details of the hydrogen bonding network in these clusters are revealed by the infrared predissociation spectra in the OH stretch region. This improved method for capturing microsolvated clusters yielded colder complexes with much better resolved IR features than previous studies. The analysis of these spectra, supported by electronic structure calculations and compared to previous results on [Ru(tpy)(bpy)(H2O)]2+(H2O)0-4 clusters, reveals the nature of the Ru—OH bond and the effect of hydrogen bonding on facilitating the subsequent oxidation to [Ru(tpy)(bpy)(O)]2+ in the proposed catalytic cycle. Particularly, the hydrogen bonding interaction in [Ru(tpy)(bpy)(OH)]2+(H2O)1 is much weaker than that in the corresponding [Ru(tpy)(bpy)(H2O)]2+(H2O)1, and thus is less effective at activating the hydroxyl ligand for further oxidation via proton coupled electron transfer (PCET). Furthermore, the results here reveal that the Ru—OH bond, though formally described as an Ru3+/OH- interaction, has more covalent bond character than ionic bond character.


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I. Introduction Development of suitable catalysts is crucial for overcoming the high overpotential associated with oxygen evolution reactions from water,1-3 and is actively pursued in applications such as artificial photosynthesis.4-10 The precise mechanisms by which these catalysts operate are often uncertain, and are studied both experimentally and theoretically. Proposed catalytic cycles are typically supported by experimental electrochemical measurements, but these experiments do not provide direct structural information on the reaction intermediate species.6 This makes clear identification of reaction pathways and comparisons to theoretical results more difficult. Moreover, the active role of the solvent network in oxygen evolution reactions is important, as hydrogen-bonding can significantly influence an oxidation step via proton-coupledelectron-transfer (PCET) processes.11-14 However, these types of intermolecular interactions are difficult to capture in typical electrochemical measurements, and applications of theoretical treatments need proper benchmarking. Here, we present the experimental structural characterization of the water network at the active site of a reaction intermediate complex, [Ru(tpy)(bpy)(OH)]2+ (tpy = 2,2’:6’,2”-terpyridine, bpy = 2,2’-bipyridine, [Ru(OH)]2+ hereafter), formed during electrolysis of the homogenous water oxidation catalyst, [Ru(tpy)(bpy)( H2O)]2+ ([Ru(H2O)]2+ hereafter). Ruthenium-based mononuclear water oxidation catalysts have garnered much attention in recent years.15-26 They have also been the subject of several recent mass-selective spectroscopy studies, where the vibrational27-28 and electronic29-31 characterizations of a prototypical mononuclear water oxidation catalyst, [Ru(H2O)]2+, were carried out. Our group has recently studied the structures of reaction complexes involving this catalyst using cryogenic ion infrared vibrational spectroscopy (CIVS). Specifically, we probed the effect of solvation on the OH bond



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lengths at the active site of the catalyst via a stepwise addition of water molecules to the isolated catalyst ion.27 We also showed that the product of the first oxidation step, the open-shell [Ru(OH)]2+ complex, can be isolated and characterized utilizing an in-line electrochemical cell with the electrospray source.32 Here, we extend our studies to probe the solvation structures of the first oxidation intermediate, [Ru(OH)]2+. This intermediate is isolated using our in-line electrochemical cell, and solvated clusters with 1-4 water molecules are controllably formed inside a cryogenic ion trap. Detailed structural information of these [Ru(OH)]2+(H2O)n clusters are obtained via CIVS, which yielded well-resolved vibrational features. Comparisons of these results with those from our previous experiments on [Ru(H2O)]2+(H2O)n highlight the evolution of catalyst-solvent interactions when the complex undergoes a single oxidation step, i.e. [Ru(H2O)]2+ [Ru(OH)]2+ + H+ + e-.

II. Experimental and Computational Details The [Ru(H2O)](ClO4)2 complex was synthesized from RuCl3·3H2O (Pressure Chemical Company) according to literature methods.33-34 All other reagents were purchased from Sigma Aldrich and used without further purification. The vibrational spectra of the [Ru(OH)]2+(H2O)n clusters with n=0-4 were acquired using a dual cryogenic ion trap infrared predissociation mass spectrometer described in detail previously.35 The [Ru(OH)]2+ intermediate species was generated using an inline electrochemical-electrospray (EC-ESI) setup, similar to those of Johnson36 and van Berkel37, and described previously.32 Briefly, a millimolar solution of [Ru(H2O)](ClO4)2 in water was pushed through an electrochemical flow cell equipped with a glassy carbon working electrode held at 1.2


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V with respect to a stainless steel counter electrode. The entire EC cell is floated at ESI voltage (~1 kV). The electrochemical products were directly electrosprayed into the mass spectrometer via a short (~25 mm) silica tip at the exit of the EC cell. This yielded a mixture of [Ru(H2O)]2+ and [Ru(OH)]2+ ions with overlapping masses due to their natural isotopic distributions. A pure ion beam of [Ru(OH)]2+ was obtained by controlled collisional activation, via increased voltages on the RF ion guide, in the high pressure region of the ion source, which fragmented the more weakly bound [Ru(H2O)]2+ ions. Microsolvated [Ru(OH)]2+(H2O)n clusters were formed by seeding water into the helium buffer gas inside a liquid nitrogen cooled (80 K) linear octupole ion trap. These clusters were gently transferred into a cryogenic 3D quadrupole ion trap held at 10 K, where further cooling occurred via collisions with buffer gas consisting of 10% D2 in He. The cold solvation complexes were size-selected inside the mass spectrometer prior to being mass gated and intersected with the output of a Nd:YAG pumped tunable OPO/OPA laser system (Laservision). Resonant absorption of a single photon in the 2800-3800 cm-1 range was sufficient to induce the photofragmentation and loss of a single water molecule from the cluster. Appearance of the [Ru(OH)]2+(H2O)n-1 photoproduct was monitored in a secondary time-offlight mass spectrometer stage. The photofragment intensity as a function of laser wavelength yielded the linear IR spectrum of the corresponding parent cluster. The vibrational spectrum of the bare [Ru(OH)]2+ ion was acquired by monitoring the D2 loss of the [Ru(OH)]2+·(D2)2 species formed in the 10K ion trap. The doubly tagged cluster was selected because it provided a better mass separation from the bare ion. To aid the assignment of the experimental spectra, calculations were performed using Gaussian 0938. Optimized structures and harmonic infrared spectra of each species were calculated using the ωB97XD functional, the Stuttgart-Dresden (SDD) pseudopotential for Ru,



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and the def2-TZVP basis set for all other atoms, which were found to have the best overall agreement with experimental results. Many DFT functionals were tested (See Supporting Information Fig. S1-S3 for details), and while some functionals yielded IR spectra in better agreement with the experiment, we ultimately chose ωB97XD because the predicted relative isomer energetics were in much better agreement with experimental observations. For comparisons to the experimental spectra, the harmonic frequencies of the CH stretches were scaled by a constant 0.9612, and the OH stretches were scaled linearly as described previously by Tabor et al.39 (see Supporting Information Figure S4 for details). All the reported energetics include unscaled zero-point energy correction.

III. Results and Analysis The vibrational spectra of [Ru(OH)]2+(H2O)0-4 in the 2800-3800 cm-1 region are shown in Figure 1. The spectra display distinctive features in three spectral regions. The narrowwidth features above 3550 cm-1, highlighted in blue, correspond to the stretching modes of free OH, i.e., those not donating a hydrogen-bond (Hbond). The intense and broader Figure 1. Infrared predissociation spectra of [Ru(OH)]2+(H2O)n, with n = 0-4, in the 2800-3800 cm-1 region.

features between 3150 and 3500 cm-1,


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highlighted in red, correspond to the stretching modes of OH groups that are actively donating an H-bond. The magnitude of the redshift of these modes is roughly proportional to the square of the strength of the H-bond interaction.40-42 The appearance of such redshifted features with the addition of a water molecule is indicative of the formation of a H-bonded water network around the hydroxyl ligand. Lastly, very weak features are present between 3050 cm-1 and 3100 cm-1 in the bare [Ru(OH)]2+ and [Ru(OH)]2+(H2O) spectra, corresponding to C-H stretches of the bipyridine and terpyridine ligands. They are not visible in the spectra of the larger clusters due to higher relative intensities of the OH stretches. The spectrum of the bare [Ru(OH)]2+(D2)2 intermediate is relatively simple and shows a single narrow and intense feature at 3564 cm-1 in the free OH region. The addition of a water molecule in [Ru(OH)]2+(H2O)1 results in a broader peak at 3420 cm-1 that dominates the spectrum. The two peaks in the free OH region, at 3625 and 3712 cm-1, are comparatively much weaker. With the addition of a second water molecule, the spectrum of [Ru(OH)]2+(H2O)2 becomes more complex. The free OH region contains three peaks at 3632 cm-1, 3686 cm-1, and 3713 cm-1. The H-bonded region now has numerous broad features, where the most intense feature is also the most redshifted at 3320 cm-1. Additionally, there is a cluster of three peaks at 3435 cm-1, 3450 cm-1, and 3475 cm-1. The congestion in the [Ru(OH)]2+(H2O)3 spectrum in the H-bonded region is similar to that of [Ru(OH)]2+(H2O)2. There is a broad feature at 3250 cm-1 and a set of three peaks at 3360 cm-1, 3385 cm-1, and 3425 cm-1. Strikingly, a single, narrow feature is present in the free OH region at 3689 cm-1 and is the most intense feature in the spectrum. This is in sharp contrast to the spectra of the smaller clusters. Finally, with the addition of a fourth water molecule, the spectrum of [Ru(OH)]2+(H2O)4 becomes significantly more complicated, with an almost continuous progression of vibrational features between 3150 and



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3500 cm-1. In the free OH region, there is still a narrow and intense peak at 3689 cm-1, with additional two small peaks that surround it at 3638 and 3719 cm-1. Figure 2A shows the experimental spectrum of [Ru(OH)]2+(D2)2 with the corresponding calculated spectrum and geometry in Fig. 2B. The simplicity of the experimental and calculated spectra leads to straightforward assignments. The intense experimental feature at 3564 cm-1 is assigned to the stretch of the hydroxyl ligand. Note that the calculated spectrum overestimates the Figure 2. (A) Experimental and (B) calculated spectra of [Ru(OH)]2+(D2)2 with corresponding calculated geometry. (C) Experimental and (D) calculated spectra of [Ru(OH)]2+(H2O)1 with corresponding calculated geometry. L = ligand; S1 = 1st solvating water; subscript D = H-bond donor; subscript A = H-bond acceptor; s = symmetric; as = antisymmetric. For simplicity, the hydrogen atoms on the pyridine rings were omitted in the geometry figures.

frequency of this stretch by 56 cm-1. The additional smaller experimental peaks at 2970 and 3090 cm-1 show good agreement with the calculation and are assigned to the D2 and pyridyl

CH stretches, respectively. The experimental [Ru(OH)]2+(H2O)1 spectrum is compared with the calculation in Figure 2C and 2D, and the assignments are again relatively straightforward. The calculated geometry shows that the hydroxyl ligand acts as a H-bond donor to the H2O. This interaction results in a


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144 cm-1 redshift of the hydroxyl ligand stretch to 3420 cm-1. We note that the calculation again slightly overestimates the frequency of the hydroxyl stretch. The experimental features at 3625 and 3712 cm-1 are assigned to symmetric and antisymmetric stretches of H2O, respectively. The isomer in which H2O acts as a H-bond donor to the hydroxyl ligand is calculated to be 8.8 kJ/mol higher in energy, and does not appear to contribute significantly to the experimental spectrum (see Figure S5). For the [Ru(OH)]2+(H2O)2 complex, two solvation motifs, shown in Figures 3B and 3C, are found to contribute to the experimental spectrum. Both isomers have the hydroxyl ligand donating a H-bond to a H2O, similar to [Ru(OH)]2+(H2O)1. This water molecule is denoted as S1 in the figures. In isomer 2I, the second H2O, S2, accepts a H-bond from S1. In isomer 2II, which lies 444 cm-1 (5.3

Figure 3. (A) Experimental and (B-C) calculated spectra of [Ru(OH)]2+(H2O)2 with corresponding calculated geometries. L = ligand; S1 = 1st solvating water; S2 = 2nd solvating water; D = H-bond donor; A = H-bond acceptor; s = symmetric; as = antisymmetric; d = donating; f = free.

kJ/mol) higher in energy, S2 instead acts as a H-bond donor to the hydroxyl ligand. The experimental feature at 3320 cm-1 is assigned to the hydroxyl ligand stretch of 2I, which exhibits a 100 cm-1 redshift from [Ru(OH)]2+(H2O)1. The cluster of features near 3450 cm-1, given the



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width and relative intensities of these features, likely have contributions from both isomers. It is difficult to assign them precisely given the overlapping calculated frequencies, labeled in Fig. 3B and 3C. Assignments in the free OH region is easier, because both isomers have three vibrations here at very similar frequencies. Therefore, the 3632 cm-1 and 3713 cm-1 experimental features are assigned to the symmetric and antisymmetric stretches of the H-bond accepting only H2O, and the 3686 cm-1 feature is assigned to the free OH stretch of the H-bond donating H2O. Because the 3320 cm-1 feature associates only with isomer 2I, and it dominates the experimental spectrum, this lower energy isomer (see Table S3) is expected to be the dominant contributor here.


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Figure 4 shows the experimental and calculated spectra for [Ru(OH)]2+(H2O)3. The presence of a single intense feature in the free OH region (3689 cm-1) indicates that all the water molecules here are in a singledonor, single-acceptor H-bonding configuration, pointing to a cyclic structure. This solvation motif, a fourmembered H-bonded ring, is shown in Fig. 4B. This structure is an extension of both 2I and 2II, with the additional water molecule completing the cyclic structure from either linear H-bonding network. The free OH stretches of the

Figure 4. (A) Experimental and (B) calculated spectra of [Ru(OH)]2+(H2O)3 with corresponding calculated geometry. D = H-bond donor; A = H-bond acceptor; ip = in phase; oop = out of phase; d = donating; f = free.



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water molecules are calculated to have approximately the same frequency, in good agreement with the single sharp experimental feature at 3689 cm-1. In the H-bonded region, the OH stretching modes are strongly coupled. The experimental features at 3360, 3385 and 3425 cm-1 are assigned to the out-of-phase OH stretches, and the experimental feature at 3250 cm-1 is assigned to the in-phase OH stretch. Note that the experimental relative intensity of the in-phase OH stretch differs significantly from calculation. Comparisons between different functionals showed that the calculated intensity of this vibration is highly sensitive to the coupling between the water molecules, and therefore highly dependent on the method and, interestingly, inclusion of dispersion correction. For example, B3LYP (Figure S1) without dispersion correction yields an in-phase OH stretch with similar intensity as the outof-phase stretches. Moreover, the overtone of water bending vibrations are expected to have a frequency of ~3200 cm-1, giving rise to further anharmonic coupling in this region. The experimental and Figure 5. (A) Experimental and (B-C) calculated spectra of [Ru(OH)]2+(H2O)4 with corresponding calculated geometries. L = ligand; S1-4 = 1st-4th solvating water; D = H-bond donor; A = H-bond acceptor; s = symmetric; as = antisymmetric; ip = in phase; oop = out of phase; d = donating; f = free.

calculated spectra of [Ru(OH)]2+(H2O)4 are shown in Figure 5. The presence of a strong feature at


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3689 cm-1 again suggests the dominant presence of a cyclic solvation motif similar to [Ru(OH)]2+(H2O)3. Isomer 4I has such a five-membered ring in which each water molecule and the hydroxyl ligand are both an H-bond donor and acceptor. Again, the H-bonded OH stretch modes are coupled, and the calculation underestimates the intensity, albeit less severely, of the in-phase mode. Note that although B3LYP (Figure S2) yields better agreement with the experimental intensity, its relative energies for the different isomers discussed here (Table S3) are less reasonable. The presence of two very weak features at 3638 cm-1 and 3719 cm-1 points to the minor presence of a second isomer, 4II, which is the extension of 3 with the additional water molecule on the outside of the four-membered ring. This isomer is calculated to be 683 cm-1 (8.2 kJ/mol) higher in energy than 4I.

IV. Discussion The frequency of the hydroxyl ligand stretch in the bare [Ru(OH)]2+ intermediate can be used to consider the nature of the Ru-OH bond. We have shown previously that the hydroxyl ligand stretch is a sensitive probe of the M-OH bond polarity through a vibrational Stark shift induced by the charged metal center.43 Specifically, the electric field produced by the positively charged metal center of a highly polar [M-OH]+ bond can induce a large blueshift of the hydroxyl stretch frequency,44-47 such that the hydroxyl frequency is linearly dependent on the strength of this electric field. In [Ru(OH)]2+(D2)2 (the effect of D2 on the hydroxyl stretch frequency is fairly minor, calculated to be 8 cm-1, see Table S11 for more detail), this vibration appears at 3564 cm-1, which is very close to the vibrational frequency of the free OH- hydroxide stretch (3556 cm-1) and free OH hydroxyl radical stretch (3570 cm-1),48 lower than those of the model [M-OH]+ complexes studied previously (it is ~20 cm-1 redshifted compared to



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[CuOH]+(H2O)(D2)). This frequency points to a relatively small Ru-OH bond polarity, despite the interaction being formally described as Ru3+-OH-. This is supported by Natural Population Analysis (NPA, Table S5) which shows that the Ru center has a +0.94 charge and the hydroxyl ligand has an overall -0.36 charge. Using these values and calculated geometry for the [Ru(OH)]2+(D2)2 complex, the hydroxyl ligand stretch is found to fall on the same M-OH trend determined previously, as shown in Figure S6. Therefore, all the results suggest that the hydroxyl ligand is not strongly anionic and that the Ru-OH interaction may be more aptly described as a polar covalent bond than a fully ionic interaction. A direct measurement of RuOH bond in the far-IR region may reveal more details of this interaction. It is of interest to compare the results obtained here on the solvation of the [Ru(OH)]2+ species with those of the [Ru(H2O)]2+ species27. These two complexes represent the initial and product species of the first water oxidation step promoted by [Ru(H2O)]2+. This oxidation step involves a proton-coupled electron transfer process, and thus, water network reorganization at the start and end is directly relevant to the reaction. In addition to the solvation structures determined from the present results, the frequencies of the OH stretches can directly convey information about the strength of H-bonding present in the solvent network. In general, a lower frequency OH stretch indicates a stronger H-bond; quantitatively, it has been shown that the Hbond strength is proportional to the root square of the OH redshift from the free OH stretch frequency.40-42 Figure 6 presents the comparison of selected clusters of [Ru(OH)]2+·(H2O)n with the corresponding [Ru(H2O)]2+·(H2O)n. The [Ru(H2O)]2+·(H2O)n spectra were described and analyzed previously.27 We note that the previous work was performed with the solvated clusters at higher temperatures, between 165 K and 185 K, which account for the broader appearance of


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the spectral features. We start by comparing the redshift induced by the addition of the first water onto both [Ru(OH)]2+ and [Ru(H2O)]2+, as displayed in Figure 6B. Interestingly, the [Ru(OH)]2+ species has a considerably smaller H-bond induced redshift (144 cm-1) compared to that of the [Ru(H2O)]2+ species (454 cm-1 relative to the free antisymmetric OH stretch). This experimental result indicates that the H-bonding strength in [Ru(OH)]2+(H2O) is much weaker than it is in [Ru(H2O)]2+·(H2O), consistent Figure 6. (A-C) Experimental spectra of [Ru(OH)]2+(H2O)n (black) overlaid on experimental spectra of [Ru(H2O)]2+(H2O)n (filled red). Optimized geometries (ωB97XD/def2tzvp/SDD) of dominant isomers of [Ru(H2O)]2+(H2O)n contributing to each spectrum, as assigned previously, are shown. (D) SCF density difference maps of [Ru(OH)]2+(H2O)1 (top) [Ru(H2O)]2+(H2O)1, (bottom).

with the calculated binding energies for the first solvating water, 46.3 kJ/mol for [Ru(OH)]2+ and 62.8 kJ/mol for [Ru(H2O)]2+. As described in Ref. 27,

in [Ru(H2O)]2+(H2O), the strength of the H-bond correlates with the Ru-OH2 interaction, which forms a stronger Ru-O bond as the proton is pulled away. This concerted change can be visualized in the SCF density difference plot upon binding of the first water, as shown in Figure 6D. For [Ru(H2O)]2+, the addition of the first solvent water increased the polarization of the water ligand, as well as induced an increase in electron density in the Ru-O bond. This initiates the formation of the more covalent Ru-OH interaction observed here in the [Ru(OH)]2+ complex. In contrast, the addition of the first solvent water onto [Ru(OH)]2+ only increased polarization of



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the OH ligand, with minimal change at the Ru-O bond. This indicates that proton removal from the hydroxyl ligand is not as strongly activated as it is for the water ligand. For solvated clusters with four solvent water molecules, the IR spectra of [Ru(OH)]2+ and [Ru(H2O)]2+ are quite similar in the H-bonding region, as shown in Fig. 6C. In both cases, we find geometries containing 4- and 5-membered rings as the preferred solvent arrangements. Moreover, the similar OH stretch frequencies indicate similar H-bonding strengths. Again, this is supported by calculations which find average binding energies of 45.3 kJ/mol and 48.1 kJ/mol for the last water in [Ru(OH)]2+(H2O)4 and [Ru(H2O)]2+(H2O)4 clusters, respectively. The origin for the similar H-bonding strength in the larger solvation clusters can be traced back to different anti-cooperative effects in [Ru(OH)]2+and [Ru(H2O)]2+. When a second solvent water molecule binds to the water ligand in [Ru(H2O)]2+, a symmetric network forms in which the water ligand is a double H-bond donor. This creates a strong anti-cooperative effect that reduces the strength of the first H-bond as a result of forming the second H-bond. This double donor solvation motif leads to a blueshift of the OH stretch by ~150 cm-1 from [Ru(H2O)]2+(H2O) to [Ru(H2O)]2+(H2O)2.27 This effect is minimal in [Ru(OH)]2+ because the hydroxyl ligand can only donate one H-bond. Moreover, the hydroxyl ligand can act as a H-bond acceptor for the second solvent water molecule (conformer 2II). This induces only a small ~35 cm-1 blueshift of the hydroxyl ligand stretch with respect to [Ru(OH)]2+(H2O), indicating that the strength of the first H-bond is not significantly perturbed. This lack of an anti-cooperative effect in [Ru(OH)]2+ counterbalances the stronger interaction observed in the [Ru(H2O)]2+(H2O), such that the overall interactions are nearly equivalent in the larger clusters.

V. Conclusion


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Spectroscopic interrogation of the stepwise microsolvation of [Ru(OH)]2+ enabled us to examine the effect of H-bonding on the active site of the first intermediate in the proposed [Ru(H2O)]2+-catalyzed water oxidation cycle. By probing the OH stretching region of the infrared spectrum, we gleaned information about the strength of the H-bonding interactions with increasing solvation. Combined with our previous microsolvation study of the initial catalyst, [Ru(H2O)]2+, we showed that the [Ru(OH)]2+ species forms significantly weaker H-bonds with the first solvent molecule. Consequently, the hydroxyl ligand is not as strongly activated by the solvent as the water ligand in the initial catalyst. This suggests that the [Ru(OH)]2+ intermediate is less reactive, making a second oxidation step in the catalytic cycle more energetically demanding than the first. However, for the larger solvated clusters, we find very similar H-bond network structures in [Ru(H2O)]2+ and [Ru(OH)]2+, suggesting minimal solvent reorganization between the first two steps of water oxidation promoted by the [Ru(H2O)]2+ complex.

Acknowledgments This work was supported by the National Science Foundation under grant number CHE1454086. The computational resources used in this work are supported by National Science Foundation Grant CHE-0840494.

Supporting Information Computational details including tables and figures comparing different DFT methods, derivation of the linear scaling factors, comparison of calculated isomers for [Ru(OH)]2+(H2O)1, comparison of [Ru(OH)]2+ with [MOH]+, and complete ref. 38 are included. 17 ACS Paragon Plus Environment


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(41) Rozenberg, M.; Loewenschuss, A.; Marcus, Y. An Empirical Correlation Between Stretching Vibration Redshift and Hydrogen Bond Length. Phys. Chem. Chem. Phys. 2000, 2, 2699-2702. (42) Rozenberg, M.; Shoham, G.; Reva, I.; Fausto, R. A Correlation Between the Proton Stretching Vibration Red Shift and the Hydrogen Bond Length in Polycrystalline Amino Acids and Peptides. Phys. Chem. Chem. Phys. 2005, 7, 2376-2383. (43) Marsh, B. M.; Zhou, J.; Garand, E. Charge Transfer in MOH(H2O)+ (M = Mn, Fe, Co, Ni, Cu, Zn) Complexes Revealed by Vibrational Spectroscopy of Mass-Selected Ions. Phys. Chem. Chem. Phys. 2015, 17, 25786-25792. (44) Hermansson, K. Ab initio Calculations of the Fundamental OH Frequency of Bound OH− Ions. J. Chem. Phys. 1991, 95, 3578-3588. (45) Ojamäe, L.; Hermansson, K. Water Molecules in Different Crystal Surroundings: Vibrational O–H Frequencies from ab initio Calculations. J. Chem. Phys. 1992, 96, 9035-9045. (46) Hermansson, K. Redshifts and Blueshifts of OH Vibrations. Int. J. Quantum Chem 1993, 45, 747-758. (47) Hermansson, K. O-H Bonds in Electric Fields: Electron Densities and Vibrational Frequency Shifts. Chem. Phys. Lett. 1995, 233, 376-382. (48) Shimanouchi, T., Molecular Vibrational Frequencies. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Linstrom, P. J.; Mallard, W. G., Eds. National Institute of Standards and Technology: Gaithersburg, MD.


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TOC graphic



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Infrared predissociation spectra of [Ru(OH)]2+(H2O)n, with n = 0-4, in the 2800-3800 cm-1 region. 82x112mm (300 x 300 DPI)

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Figure 2. (A) Experimental and (B) calculated spectra of [Ru(OH)]2+(D2)2 with corresponding calculated geometry. (C) Experimental and (D) calculated spectra of [Ru(OH)]2+(H2O)1 with corresponding calculated geometry. L = ligand; S1 = 1st solvating water; subscript D = H-bond donor; subscript A = H-bond acceptor; s = symmetric; as = antisymmetric. For simplicity, the hydrogen atoms on the pyridine rings were omitted in the geometry figures. 82x112mm (300 x 300 DPI)



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Figure 3. (A) Experimental and (B-C) calculated spectra of [Ru(OH)]2+(H2O)2 with corresponding calculated geometries. L = ligand; S1 = 1st solvating water; S2 = 2nd solvating water; D = H-bond donor; A = H-bond acceptor; s = symmetric; as = antisymmetric; d = donating; f = free. 82x112mm (300 x 300 DPI)


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Figure 4. (A) Experimental and (B) calculated spectra of [Ru(OH)]2+(H2O)3 with corresponding calculated geometry. D = H-bond donor; A = H-bond acceptor; ip = in phase; oop = out of phase; d = donating; f = free. 82x112mm (300 x 300 DPI)



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Figure 5. (A) Experimental and (B-C) calculated spectra of [Ru(OH)]2+(H2O)4 with corresponding calculated geometries. L = ligand; S1-4 = 1st-4th solvating water; D = H-bond donor; A = H-bond acceptor; s = symmetric; as = antisymmetric; ip = in phase; oop = out of phase; d = donating; f = free. 82x112mm (300 x 300 DPI)


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Figure 6. (A-C) Experimental spectra of [Ru(OH)]2+(H2O)n (black) overlaid on experimental spectra of [Ru(H2O)]2+(H2O)n (filled red). Optimized geometries (ωB97XD/def2tzvp/SDD) of dominant isomers of [Ru(H2O)]2+(H2O)n contributing to each spectrum, as assigned previously, are shown. (D) SCF density difference maps of [Ru(OH)]2+(H2O)1 (top) [Ru(H2O)]2+(H2O)1, (bottom). 82x69mm (300 x 300 DPI)



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TOC graphic 82x44mm (300 x 300 DPI)


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Vibrational Characterization of Microsolvated Electrocatalytic Water

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