Substitution Effects on the Water Oxidation of Ruthenium Catalysts: A

Article pubs.acs.org/JPCC

Substitution Effects on the Water Oxidation of Ruthenium Catalysts: A Quantum-Chemical Look Abu Md Asaduzzaman,*,†,‡ Derek Wasylenko,§ Curtis P. Berlinguette,§,∥ and Georg Schreckenbach*,† †

Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721, United States § Department of Chemistry, University of Calgary, Calgary, Alberta T2N 1N4, Canada ∥ Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada ‡

S Supporting Information *

ABSTRACT: Quantum chemistry has been used to investigate the oxidation of water by a family of seven catalysts based on [Ru(tpy)(bpy)(OH2)]2+ (tpy = 2,2′:6′,2′′-terpyridine, bpy = 2,2′bipyridine). The electron-donating −OMe and −NH2 groups (EDG) and electron-withdrawing −COOH and −NO2 groups (EWG) are installed in the catalyst by replacing hydrogen atoms on the bpy and tpy ligands. The EDG induces an increase in the electron density at the Ru center, whereas the EWG does the opposite. Reduced electron density at the metal center facilitates Ru(N+1)/Ru(N) reduction and thus a higher reduction potential. Catalytic evolution of one oxygen molecule from two water molecules using all catalysts is an exothermic process if driven by CeIV. The exothermicity increases from EDG to EWG via parents. Regarding intermediates, the singlet states of 7coordinated catalysts are slightly more stable than the triplet states of 6-coordinated catalysts for most catalysts. Only for a strong EWG (−NO2) containing catalyst, the triplet 6-coordinated states complex is the most stable. Calculated Ru−O and O−O distances suggest that oxygen will be liberated favorably from the triplet state of 6-coordinated complexes, whose stability increase (with respect to the singlet of 7-coordinated complexes) with increasing electron-withdrawing nature.

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polypyridyl, oxo-bridged Ru2 “blue dimer” was the first of the synthetic catalysts for water oxidation.9 Several inorganic compounds containing four to one metal centers have been synthesized and characterized with respect to their catalytic activity both experimentally and theoretically.10−39 It was recently demonstrated that a single ruthenium center is sufficient for catalytic water oxidation, sparking a new wave in the field of homogeneous water oxidation. This has resulted in numerous experimental and theoretical reports.10−12,17,22,23,32,40−51 More recently, other researchers have extended Meyer’s single-metal catalyst findings toward development of other relevant metal catalysts for water oxidation.11 Numerous ligand frameworks have been synthesized and characterized in addition to the exploration of different metal active sites (e.g., cyclometalated Ir,16 cyclopentadienyl Ir,52 polypyridyl Ru,10,15 macrocyclic Fe,53 and pentapyridine Co11). As it is seen in recent water oxidation studies, different active metal ions and different ligand groups control the activity of catalysts differently. Hence, it is conceivable to design the

INTRODUCTION There is increasing concern regarding the future supply of energy as the worldwide demand for energy is rising rapidly. The average global energy demand is currently estimated to be 17 TW1 and projected to rise to 30 TW by midcentury and almost 50 TW by the turn of the century.2−4 Anthropogenic factors causing global warming further direct the necessity of having a clean and sustainable energy economy. One of the most promising energy sources to meet the demand from both an availability and an environmental context is the sun.2,5 Utilization of sunlight in real-world applications, however, is not straightforward. Harnessing sunlight can be understood from the energyharvesting mechanism by green plants. Green plants use the CaMn4O4 oxygen-evolving complex in photosystem II to catalyze water oxidation, liberating the protons and electrons necessary to reduce CO2 to sugars with molecular oxygen as a byproduct.6−8 In photosystem II, the CaMn4O4 cluster has been identified as a catalyst in the water oxidation process. On the basis of nature’s photosynthetic process, one could envision a synthetic approach where water could be photocatalytically split into O2 and H2, where H2 could then be used as an energy carrier. Intense efforts have resulted in the synthesis and characterization of various catalysts for water oxidation. The © XXXX American Chemical Society

Received: July 28, 2014 Revised: November 25, 2014

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dx.doi.org/10.1021/jp5075847 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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proposed catalytic cycle, we extend our analysis to include the nitro (−NO2) and amino (−NH2) groups as EWG and EDG examples, respectively, as shown in Figure 2.

desired catalytic complex by suitably choosing the active metal and ligand combination. However, first, one has to establish a detailed understanding of the characteristic properties of the constituent stakeholders. In fact, the water-splitting cycle has been investigated from various angles, e.g., O−O bond formation mechanism,18,23,46,51,54−56 electronic effects,43 ligand geometry,48 ligand type,32,49 etc. Berlinguette and co-workers10 reported an experimental study of water oxidation by a series of Ru-based catalysts. In their study, they reported a water oxidation cycle that follows the one proposed by Meyer and coworkers,13 but they extended this work to explore how the critical catalytic parameters (turnover numbers and frequency) change with varied electron density at the Ru center resulting from substituents on the ligand. While it is established that a single-site Ru catalyst is sufficient for water oxidation, the entire mechanism for water oxidation is not clear and is somewhat contradictory. For example, release of an O2 atom from the Ru catalyst (Ru−O2, f and g in Figure 1) is one of the most important steps in the

Figure 2. Schematic representation of Ru-based catalysts studied herein: [Ru(tpy)(bpy)(OH2)]2+ (1), [Ru(tpy)(4,4′-dimethoxy-2,2′bipyridine)(OH2)]2+ (2), [Ru(tpy)(4,4′-dicarboxy-2,2′-bipyridine)(OH2)]2+ (3), [Ru(4′-methoxy-2,2′:6′,2′′- terpyridine)(bpy)(OH)]2+ (4), [Ru(4′-carboxy-2,2′:6′,2′′-terpyridine)(bpy)(OH)]2+ (5), [Ru(4′amino-2,2′:6′,2′′-terpyridine)(bpy)(OH)]2+ (6), and [Ru(4′-nitro2,2′:6′,2′′-terpyridine)(bpy)(OH)]2+ (7), where tpy = 2,2′:6′,2′′terpyridine, bpy = 2,2′-bipyridine. Figure 1. Catalytic cycle for water oxidation.

While the methoxy and carboxylic groups are installed in both the bipyridine and the tripyridine ligands (in the 4 and 4′ positions of the bipyridine ligand and the 4′ position of the tripyridine ligand), amine and nitro groups are installed only in the tripyridine ligand (at the 4′ position of tripyridine). The structures of the seven systems studied are shown in Figure 2. We will discuss these systems from both a structural and an electronic point of view.

water oxidation cycle. The Ru−O2 complex has two isomers, i.e., the oxygen molecule can bind to the Ru atom through one oxygen atom (f in Figure 1), or both oxygen atoms can bind to the Ru atom and make a 7-coordinated (7c) complex (g in Figure 1). The stabilities of these two intermediates are reported contradictorily. Lin and co-workers55 reported that in the triplet state the 6-coordinate (6c) Ru complex (f) is more stable than the 7c complex, whereas Zhang and co-workers51 have not found any stable conformer for 6c in its triplet state. Moreover, Zhang and co-workers51 showed that the liberation of the oxygen molecule for the Ru−O2 complex depends on the types, orientations, and structures of ligands. All these previous reports lead to our belief that more study is necessary to pinpoint the finer mechanistic details of the water-splitting cycle. Toward that end, we have undertaken a detailed investigation where the electronic nature of the ligand would change. Specifically, we studied the mechanistic details of water oxidation catalyzed by [Ru(tpy)(bpy)(OH2)]2+ and investigated the response for changing the electronic nature of the ligand by installing electron-withdrawing (EWG) and electrondonating (EDG) groups in the bpy and tpy units of the catalyst. The catalytic water oxidation cycle reported by Berlingutte et al.10 is considered as the roadmap for this study (see Figure 1) using carboxylic acid (−COOH) and methoxy (−OCH3) groups as the respective EWG and EDG examples. In order to further explore the effects from EWG and EDG moieties on the

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COMPUTATIONAL PROCEDURE Calculations have been performed employing two computational packages, namely, Priroda (p6)57−59 and the Amsterdam Density Functional package (ADF)60−64 in the framework of density functional theory (DFT). The exchange and correlation functional has been treated with the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhoff (PBE).65 Geometries have been fully optimized with p6 in the gas phase using PBE. Frequency calculations have been used to verify that true local minima were obtained in each case. The gas-phase free energy on the optimized geometries is calculated by p6 at 298.15 K. Solvation free energies have been calculated using single-point energy calculations with ADF on the p6-optimized geometries in water using COSMO.66−68 Priroda applies a scalar four-component relativistic method with all-electron basis sets. Extensive correlation-consistent triple-ζ-polarized quality basis sets for all atoms were employed in the p6 calculations,58 together with the corresponding B



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Table 1. Calculated Reduction Potentials E1/2 (V) vs NHE for Ru Intermediates

kinetically balanced basis sets for the small component and appropriate fit basis sets. The TZ2P with a small frozen core basis set was used for all atoms in the ADF calculation. In the ADF calculations, scalar relativistic effects were included using the ZORA method.69 Both the four-component relativistic method and ZORA ensure the relativistic treatment of heavy elements like Ru. Previously,70,71 we found that these different methods give very similar results, provided that the same exchange correlation functional and comparable basis sets are used. The net free energy is calculated by adding the gas-phase free energy and the solvation free energy together. This solventcorrected free energy (ΔG) is presented in this article unless otherwise stated. This approach has been quite successful previously.72 The solvation free energy of the proton has been adopted from the work of Tawa et al.73 The reduction potential (E1/2) of Ru was calculated with the procedure described by Yang and Baik,74 with reference to the NHE (E = −4.24 V17,75). It is to be noted that calculation of the reduction potentials faces two challenges: solvation energies of highly charged species and underestimation of reduction potentials by DFT methods.76,77 Inclusion of an explicit solvation sphere using molecular mechanics followed by calculation of the solvation energy with implicit solvation models often provides a reasonable estimation of the solvation energy of such catalysts. Roy et al.77 suggested that the reduction potential can be calculated using the DFT functional if a reference is calculated first. In other words, the trend in the reduction potential can be estimated using DFT methods. In this work, we are particularly interested in comparing the parent compound and its modifications containing different substituents. Thus, using the same computational protocol throughout allowed for straightforward comparison of their properties as suggested by Roy et al.77

pair

6

4

2

1

3

5

7

b/a c/b d/c f/e

0.79 1.25 1.81 1.24

0.76 1.26 1.80 1.26

0.74 1.53 1.83 1.22

0.81 1.54 1.84 1.25

0.94 1.59 1.89 1.28

0.87 1.59 1.89 1.26

0.99 1.79 2.09 1.59

reduction potential is the modeling of the solvation energy. We tested the hypothesis of the necessity of explicit water molecules around the catalyst (parent only) as shown in Figure 3. It is to be noted that the two water molecules around

Figure 3. Parent complex with two explicit solvent molecules. The dark-cyan, red, blue, gray, and white spheres represent Ru, O, N, C, and H atoms, respectively.

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the water ligand of the main complex are placed based on pure chemical intuition. A systematic approach like molecular dynamics simulation using explicit solvent molecules would have provided a better structure. However, our intention is to check the validity of using an explicit solvent molecule rather than an accurate calculation. In any event, our relatively crude approach offers a reasonable result. Using two explicit water molecules, the reduction potentials we obtained for b/a, c/b, and d/c are 1.29, 1.34, and 1.67 V, respectively. This observation along with van Voorhis’s study17 confirms the need for more accurate solvation models for a quantitative estimation of reduction potentials. However, we would like to emphasize again that our goal is to evaluate the effect of EDG and EWG on the catalytic activity, i.e., to obtain a qualitative comparison between parent and substituted catalysts. Hence, we believe our presented results would adequately characterize the role of EDG and EWG in the catalyst. There are two trends emerging in the calculated values: (1) the E1/2 values increase in the order Ru (III/II) < R (IV/ III) < Ru (V/IV) for all catalysts and (2) the E1/2 values decrease for 2, 4, and 6 and increase for 3, 5, and 7 compared to the corresponding values for 1. Both of these trends are in agreement with the experimental work of Berlinguette10 (for catalysts 1−3) and computational work of Van Voorhis17 for similar catalysts. The differences in the E1/2 values for the various steps can be explained from the electronic environment of the Ru atom. At higher oxidation state, the electron density at the metal center is lower, and at lower oxidation state, the electron density at the metal center is correspondingly higher (see also Table 2 and

RESULTS AND DISCUSSIONS The investigated catalytic cycle is shown in Figure 1. For the purpose of clarity, we represented the [Ru(tpy)(bpy)] part as [Ru], e.g., [Ru(tpy)(bpy)(OH2)]2+ = [Ru−OH2]2+ and [Ru(tpy)(bpy)(OH)]2+ = [Ru−OH]2+. Intermediate species are identified as a = [RuII−OH2]2+, b = [RuIII−OH]2+, c = [RuIV−O]2+, d = [RuV−O]3+, e = [RuIII−OOH]2+, f = [RuIV− OO]2+, g = [Ru7cIV−OO]2+, where the roman number (I−V) states the oxidation state on Ru and the subscript 7c defines Ru as being a 7-coordinated complex. It is to be noted that a similar representation is applicable for all other catalysts (2−7). In calculating the redox potentials for different Ru catalysts, we optimized all intermediate species. The catalytic activity of [Ru(tpy)(bpy)(OH2)]2+ is envisioned to be affected by the presence of EDG and EWG groups in 2−7. In a complete catalytic cycle, an oxygen molecule is evolved after four oxidation steps. The calculated reduction potentials for these four oxidation processes are presented in Table 1. The values in Table 1 correspond to the b/a, c/b, d/c, and f/e couples of Figure 1. Our calculated values for the reduction potentials are underestimated or overestimated compared to both experimental and calculated literature values.10,17 Accurate calculation of reduction potentials is a challenge for quantum chemistry.36,78,79 Van Voorhis17 discussed the factors that are responsible for the underestimation or overestimation of the reduction potential. As we discussed in the Computational Procedure section, the main reason for underestimating the C



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The change of electron density at the metal center due to having an EDG or EWG group would modify the interaction between the metal center and the ligands. The Ru−O bond is most important as it involves formation and decomposition in the process of oxygen evolution and water coordination. Therefore, we listed the Ru−O bond distances for the intermediates (a−e) of all catalysts in Table 3. It is to be noted that catalytic species f and g are the two most important steps in the water-splitting cycles as release of O2 molecules is happening in these steps, which is also the rate-limiting step.55 We thus wish to discuss them in the next section separately. The Ru−O bond distance is decreasing with increasing oxidation of the catalytic species (a−d). This can be understood from the simple chemical nature of the metal atom and ligand. For example, upon oxidation of [RuII−OH2]2+ intermediates, the pKa of the OH2 ligand decreases and a proton is subsequently liberated, resulting in a shorter and stronger Ru−O bond due to the anionic nature of the OH− ligand. It is worth noting that although the lowest energy configuration of intermediate a is singlet, it is triplet for intermediate c. In intermediate d, the total charge is higher (and hence Ru is more oxidized) than in intermediate c. The higher positive charge on the Ru atom exerts increased electrostatic interactions with the neighboring electronegative elements. These strong electrostatic interactions result in Ru− O being shorter in d than in c. After O−O bond formation, the oxygen atom in intermediate e has a similar Ru−O bond length to that of intermediate b. While it is true that the observed change in the Ru−O distance in the substituted catalysts (2−7) from the parent catalyst (1) is small, there is a clear trend in the bond distance among various catalysts. In the catalysts that contain EDG (2, 4, and 6), the Ru−O distance is generally longer than that of the parent catalyst. However, the opposite is true for catalysts containing EWG (3, 5, and 7). The longer and shorter Ru−O distance arises from the higher oxidation state (higher charge) on the metal center. This observation is in agreement with the charge analysis and reduction potential. While the structures and spin states of intermediates a−e are well characterized, the structure and spin state of species f and g are not. There is a lack of clarity in the literature regarding the coordination mode of the O2 moiety in these and related catalytic species. Some studies17 did not report the 7c complex as a reaction intermediate, whereas others10,14 did. Zhang and co-workers51 devoted their entire study to characterizing the structures of f and g containing various ligands. Lin and coworkers55 also reported various conformers of species f and g and their interconversion kinetics. Both Zhang co-workers51 and Lin and co-workers55 reported complex 6c in both its singlet and triplet states and complex 7c in its singlet state. They disagreed on the existence of complex 7c in its triplet state: While Lin et al.55 reported the 7c complex in its triple state, Zhang et al.51 reported that triplet 7c converts to triplet

Table 2. Hirshfeld Charge on the Ru Atom in Various Intermediates in the Catalytic Cycle a b c d e f g

6

4

2

1

3

5

7

0.332 0.408 0.430 0.492 0.347 0.389 0.429

0.328 0.418 0.430 0.492 0.337 0.389 0.429

0.327 0.418 0.436 0.586 0.379 0.396 0.437

0.334 0.424 0.446 0.598 0.383 0.400 0.441

0.347 0.428 0.466 0.599 0.388 0.403 0.445

0.343 0.430 0.489 0.600 0.387 0.402 0.445

0.402 0.496 0.500 0.600 0.447 0.442 0.442

the accompanying discussion.) At lower electron density/ higher oxidation state, the Ru has a higher tendency to accept an electron than at lower oxidation state. This strong tendency results in a higher value of E1/2. This explains why we obtained increasing values from Ru(II) to Ru(V). Now, let us compare the E1/2 for the parent (1) catalyst to those of the substituted catalysts (2−7). The EDG and EWG modify the electron density at the central atom by donating and withdrawing electron density. The donated electron density increases the electron density at the metal center and hence reduces the tendency of the Ru atom to accept an electron. However, the EWG has the opposite effect, i.e., it increases the tendency to accept an electron. These effects thus result in, respectively, lower and higher E1/2 values compared to those of the corresponding parent catalyst. The type of EWG and EDG also plays an important role in the reduction of the metal center. For example, NO2 is a very strong EWG group, meaning it can oxidize the Ru atom the most. The most oxidized Ru atom thus would have the highest E1/2 values. From Table 1, it is clear that all E1/2 values for 7 are the highest among the corresponding values for all catalysts. From the strength of the electron-donating or electronwithdrawing capacity, one thus could expect lower or higher E1/2 in the water oxidation cycle. The different values for the reduction potential for different catalysts arise from the fact that, due to installation of EWG or EDG, the electron density at the metal center is varied with the type and strength of EWG and EDGs. Such variation of the electron density at the metal center can be verified from the charge analysis on the Ru atom. The Hirshfeld charges80 of the Ru atom in every intermediate for all catalysts are listed in Table 2. The Hirshfeld charge is, respectively, smaller and larger for catalysts 2, 4, and 6 and 3, 5, and 7 than those of 1. This observation is rationalized from the fact that while the EDG induces charge toward the Ru atom, the EWG pulls away charge from Ru. This observation is in agreement with experimental findings of Berlinguette and co-workers10 (for catalysts 1, 2, and 3). The charge on Ru for intermediates varies with the oxidation state (the higher the oxidation state, the higher the Hirshfeld charge) of the Ru atom. Table 3. Optimized Ru−O Bond Distances (Å)

catalysts intermediate

6

4

2

1

3

5

7

a b c d e

2.223 1.930 1.752 1.703 1.915

2.221 1.930 1.750 1.700 1.916

2.214 1.932 1.768 1.709 1.924

2.216 1.929 1.767 1.708 1.926

2.217 1.929 1.767 1.711 1.928

2.214 1.928 1.743 1.701 1.927

2.212 1.927 1.741 1.700 1.929

D



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Table 4. Ru−O1, Ru−O2, and O1−O2 Distances and Ru−O1−O2 Angles in 6- and 7-Coordinated Complexes of Various Catalystsa

6c during optimization. We carefully optimized both conformers (complexes 6c and 7c) in both spin states. The structures for 6c and 7c of 1 are shown in Figure 4. Confirming

distance (Ǻ ) catalysts

conformers

spin state

Ru− O1

Ru− O2

O1− O2

Ru−O1− O2

1

6c

S T S T S T S T S T S T S T S T S T T T S T S T S T S T

1.891 1.999 2.001 2.055 1.884 1.993 2.003 2.048 1.892 2.000 2.001 2.058 1.885 1.998 2.002 2.049 1.893 2.000 2.001 2.057 1.883 1.997 2.002 2.047 1.896 2.004 2.001 2.265

2.786 3.044 2.001 2.589 2.784 3.042 2.003 2.582 2.787 3.045 2.001 2.592 2.784 3.042 2.001 2.586 2.787 3.045 2.001 2.589 2.786 3.04 2.002 2.588 2.878 3.046 2.001 2.265

1.278 1.260 1.379 1.285 1.280 1.263 1.381 1.289 1.277 1.259 1.378 1.284 1.281 1.262 1.379 1.287 1.277 1.259 1.379 1.284 1.282 1.264 1.379 1.288 1.276 1.258 1.378 1.288

121.8 137.0 69.8 99.0 121.9 136.6 69.9 99.1 121.8 137.0 69.9 99.0 122.0 136.6 69.9 99.1 121.8 137.0 69.9 99.0 122.1 136.5 69.9 99.3 121.9 136.9 69.9 73.5

7c 2

6c 7c

3

6c 7c

Figure 4. Ball and stick representation of (a) the 6-coordinated 1f and (b) the 7-coordinated [RuIV−OO]2+ complex 1g (1 = compound; f, g = intermediate). The presentation is similar to Figure 3.

4

6c 7c

the results of Zhang et al.,51 optimization of 7c in the triplet state leads to a new local energy minimum type complex 6c for most catalysts. However, the optimized 7c triplet structure is not the same structure as that of the 6c triplet. The distance and angle related to Ru and the two O atoms in various catalysts are summarized in Table 4. There are a few common structural features observed in all 6c and 7c complexes for all catalysts (Table 4). The structural parameters in the singlet and triplet states are different for the 6c and 7c complexes for all catalysts. The singlet 6c has shorter Ru−O1 and Ru−O2 bond distances than that the triplet state. However, the O1−O2 bond distance is longer in the singlet state than the triplet state for the 6c complex. The extent of lengthening of the Ru−O distance is much higher (∼0.1 Å for Ru−O1 and ∼0.25 Å for Ru−O2) than the shortening (∼0.02 Å) of the O1−O2 distance from the singlet to the triplet state for 6c conformers. For the 7c complexes, both Ru−O distances are the same for all catalysts in their singlet state. In the triplet state, however, the 7c conformers optimized to a new type of structure, where the two Ru−O distances are different. While both of the Ru−O distances are lengthening, the Ru−O2 lengthening is much higher (∼0.60 Å) compared to the Ru− O1 lengthening (∼0.05 Å). Similar to the 6c conformers, the O1−O2 distance is shortened in the triplet state. The reported 7c conformers in the triplet state are very much similar to the one reported by Lin et al.55 with the exception of 7. On the other hand, it is possible that Zhang el al.51 might not have considered conformers containing two unequal Ru−O distances for the 7c complexes. We would like to emphasize the fact that the 7c conformers in their triplet state are not in the same geometrical shape as in their singlet state, e.g., while both Ru−O distances are equal at the singlet state, the two Ru−O distances are quite different at the triplet state. However, these two unequal Ru−O distances are totally different from those of the 6c conformers in their triplet state. The only exception to having different structural features in the 7c complex in its triplet state is catalyst 7. Upon optimization, the geometrical shape of the 7c conformer is similar in both singlet and triplet states. The only difference is in the Ru−O and O−O distances. The Ru−O and O−O distances are, respectively, longer and shorter in the triplet state than those in the singlet state. The longer Ru−O distance in the triplet state can be

angle (deg)

5

6c 7c

6

6c 7c

7

6c 7c

a

The numbering of oxygen atoms is in accordance with Figure 4.

understood from the fact that the charge on Ru in the triplet state is lower than that in the singlet state. The lower charge at the Ru center results in weaker Ru−O interactions, and thus, a longer Ru−O distance is observed. The longer Ru−O distance in the triplet state is compensated by a shorter O−O distance in both the 6c and the 7c catalysts. This observation has implications for the release of the O2 molecule. The gaseous O2 molecule has a triplet ground state, whereas complex a has a singlet ground state. Presumably triplet O2 is released, thus, during the liberation of the O2 molecule, either of a or the O2 molecule has to flip its spin. Our calculated bond distances suggest that the O2 molecule can easily be liberated from the triplet state of 6c or 7c, since the weaker Ru−O bond will be easier to break. This process might help to retain the triplet state of O2 in its gaseous form. Further, release of O2 in its triplet state would facilitate formation of a in its singlet state since the ground state of water is singlet. Regarding the trend of Ru−O and O−O distances with different EDG and EWG, while the variation is very subtle, there is a clear trend in the Ru−O and O−O distances across the catalysts. The Ru−O distances are slightly shorter in the catalysts containing EDG and slightly longer in the EWGcontaining catalysts than those in the parent catalyst. The opposite trend is observed for the O−O distance, i.e., the O−O bond is longer in EDG-containing catalysts and shorter in EWG-containing catalysts compared to the parent catalyst (1). E



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endothermic) from EWG (2, 4, and 6) to 1 to EDG (3, 5, and 7). Again, this finding is in agreement with the observation of other properties (bond distance, Hirshfeld charge, redox potential E1/2) that are all related to the charge density at the metal center. Among the EDGs and EWGs studied in this paper, −NO2 has the strongest81 electron-withdrawing capability. Accordingly, its impact on the catalytic behavior of the water oxidation cycle is the strongest. Specifically, it has altered the structural and energetic characteristics of the Ru−O2 complexes from the regular pattern seen in all other catalysts. The altered structural and energetic features suggest that the liberation of the O2 molecule would be easier from the Ru−O 2 complex thermodynamically. The activation barrier for the oxygen release process will provide further clarification on the effect of EWG and EDG on the catalytic water oxidation process, which we envision to study in the future. Nevertheless, our calculations on the effect of the EDG and EWG provide mechanistic details of how pushing/pulling of electron density by EDG/EWG ultimately control the catalytic activity of the water oxidation cycle.

The slightly longer Ru−O bond suggests that liberation of the O2 molecule would be slightly easier in EWG-containing catalysts. The 6c complexes in their triplet state are more stable than in their singlet state. However, the opposite is observed for the 7c complexes, i.e., the singlet state is more stable than the triplet state. Among complexes 6c and 7c with two different spin states, the 7c conformer in its singlet state is the most stable conformer for all catalysts except 7, where the 6c conformer with the triplet state is the most stable. Zhang et al.51 reported the stability of different complexes of catalyst 1. Our findings on catalyst 1 are in agreement with those of Zhang et al.51 This agreement further validates our results for other catalysts. The calculated free energy difference between 6c triplet and 7c singlet is 1.1−5.0 kcal/mol for all catalysts. The free energy difference is, respectively, higher (4.1−4.5 kcal/mol) and lower (1.1−2.6 kcal/mol) than for the parent catalyst (3.5 kcal/mol) for EDG- and EWG-containing catalysts. The only exception is 7, where the 6c triplet state is 5.1 kcal/mol more stable than the 7c singlet. The relative stability of the 6c triplet and 7c singlet states shows that the stability of the 6c triplet state increases with increasing electron-withdrawing nature of substituents and flips at catalyst 7. Lin et al.55 suggested that liberation of O2 from Ru−O2-type complexes is much faster (much lower activation barrier) in the triplet state than in the singlet state and suggested that a Ru−O2 complex in the triplet state is needed for O2 liberation. Our findings regarding the stability and structural features suggest that catalysts with a strong EWG would be thermodynamically more favorable for O2 liberation than those with an EDG, which is in agreement with Lin et al.55 The stability of the triplet state of 6c is due to the fact that the oxygen in 6c is closer to its atomic structure, which is a triplet in the ground state. On the other hand, the oxygen is in a triangular arrangement with Ru in 7c and has two Ru−O bonds. The linear arrangement of the oxygen in 6c leads to an imbalance in the charge density between the two oxygens, whereas a symmetrical triangular arrangement in 7c leads to equal sharing of the charge density between the two oxygens. The symmetrical charge balance and an extra Ru−O bond in 7c result in a higher stabilization energy for 7c. The trend for the free energy difference is in accordance with the charge on Ru for having EDG and EWGs. The evolution of one oxygen molecule involves two water molecules. The following reaction can be summarized from the catalytic cycle

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CONCLUSION A quantum-chemical approach has been applied to investigate the effects of the electron density at the metal center of a ruthenium catalyst in the catalytic water oxidation cycle. The variable electron density at the metal center has been achieved by substituting hydrogen atoms by −OMe, −NH2, −COOH, and −NO2 in the pyridine ligand of the Ru catalyst. The electron-donating −OMe and −NH2 and electron-withdrawing −COOH and −NO2 groups, respectively, increase and decrease the electron density at the Ru center, which is seen directly in the calculated Hirshfeld charges. While electron deficiency facilitates the reduction process, increased electron density at the metal center has the opposite effect. The 6-coordinated Ru−O2 complexes (f) are more stable in their triplet state, whereas 7c complexes are more stable in their singlet state than those of their corresponding counterpart, with the exception of catalyst 7, where 6c in its triplet state is the most stable. The relative stability of the 6c triplet and the 7c singlet increases with increasing electron-withdrawing nature of the substituents and flips at catalyst 7. The Ru−O and O−O distances are, respectively, longer and shorter in the triplet state than those at the singlet state. The longer Ru−O distance and the triplet nature of 6c would facilitate the liberation of an O2 molecule in its triplet state and regeneration of the catalyst. Thus, in a thermodynamic context, a catalyst with enhanced EWG nature would facilitate the O2 liberation process, which is believed to be the rate-limiting step.

4Ce(IV) + 2H 2O → 4Ce(III) + O2 (g) + 4H+

Our calculated data reveals that the evolution of one oxygen molecule from a complete catalytic cycle is an exothermic process for all catalysts, being slightly more (∼1 kcal/mol) exothermic in the order EDG > parent > EWG. Analyzing the individual steps in Figure 1, it follows that two of them are endothermic. Reactions of [RuV−O]3+ and the 7-coordinated [Ru7cIV−OO]2+ with H2O are both endothermic processes. Interestingly, other than the structural rearrangement from f to g, these are the only two steps that do not involve any Ce. The reason for these two reactions being endothermic is that they involve formation and breaking of a Ru/O bond. In the [RuV− O]3+ case, the strong Ru−O (shorter, see Table 1) bond is replaced by a relatively weaker Ru−O (longer, see Table 1) bond. The same holds true for [Ru7cIV−OO]2+. These processes are energetically not favorable, and hence, we obtained that these reactions are endothermic. The free energy of reaction for these two reactions decreases (becomes less

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ASSOCIATED CONTENT

S Supporting Information *

Optimized coordinates of all intermediates. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. F



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ACKNOWLEDGMENTS We would like to acknowledge funding from The EJLB Foundation (http://www.ejlb.qc.ca/), the Natural Sciences and Engineering Research Council of Canada (NSERC), and the University of Manitoba (University Research Grants Program, URGP). Some of the calculations were performed using WestGrid computing resources. Westgrid is funded in part by the Canada Foundation for Innovation, Alberta Innovation and Science, BC Advanced Education, and the participating research institutions. We would also like to thank two unknown reviewers for their insightful comments that were instrumental in the improvement of the paper.

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