OâO Radical Coupling: From Detailed Mechanistic Understanding to

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

O−O Radical Coupling: From Detailed Mechanistic Understanding to Enhanced Water Oxidation Catalysis Yan Xie, David W. Shaffer, and Javier J. Concepcion* Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *

ABSTRACT: A deeper mechanistic understanding of the key O−O bond formation step of water oxidation by the [Ru(bda)(L)2] (bdaH2 = 2,2′bipyridine-6,6′-dicarboxylic acid; L is a pyridine or isoquinoline derivative) family of catalysts is reached through harmonious experimental and computational studies of two series of modified catalysts with systematic variations in the axial ligands. The introduction of halogen and electrondonating substituents in [Ru(bda)(4-X-py)2] and [Ru(bda)(6-X-isq)2] (X is H, Cl, Br, and I for the pyridine series and H, F, Cl, Br, and OMe for the isoquinoline series) enhances the noncovalent interactions between the axial ligands in the transition state for the bimolecular O−O coupling, resulting in a lower activation barrier and faster catalysis. From detailed transition state calculations in combination with experimental kinetic studies, we find that the main contributor to the free energy of activation is entropy due to the highly organized transition states, which is contrary to other reports. Previous work has considered only the electronic influence of the substituents, suggesting electron-withdrawing groups accelerate catalysis, but we show that a balance between polarizability and favorable π−π interactions is the key, leading to rationally devised improvements. Our calculations predict the catalysts with the lowest ΔG⧧ for the O−O coupling step to be [Ru(bda)(4-I-py)2] and [Ru(bda)(6,7-(OMe)2-isq)2] for the pyridine and isoquinoline families, respectively. Our experimental results corroborate these predictions: the turnover frequency for [Ru(bda)(4-I-py)2] (330 s−1) is a 10-fold enhancement with respect to that of [Ru(bda)(py)2], and the turnover frequency for [Ru(bda)(6-OMe-isq)2] reaches 1270 s−1, two times faster than [Ru(bda)(isq)2].

■

those of the OEC (20−1000 s−1).9 A key trait of these catalysts is that they undergo a bimolecular coupling between two RuV− oxos for the O−O bond formation step. Their activity is primarily dictated by the nature of the axial ligands and is relatively unresponsive to electronic changes in the bda2− ligand backbone.10 This is highlighted by the difference in catalytic activity between catalysts with 4-picoline as the axial ligand and those with isoquinoline, with TOF increasing from 32 to 303 s−1, presumably due to π−π interactions.9b On the other hand, only small changes are effected by the introduction of electronwithdrawing trifluoromethyl groups to the bipyridine ring of bda2−,10a despite significant changes in driving forces, as seen in the higher redox potentials for the RuIV/III and RuIII/II couples. This is especially noteworthy considering that the active site for these catalysts is in the equatorial plane occupied by the bda2− ligand. The closely related family of catalysts with the biisoquinoline dicarboxylate ligand biqa2−, [Ru(biqa)(L)2] (L is 4-picoline or 6-bromoisoquinoline), on the other hand, is almost totally unresponsive to axial ligand modifications. These complexes have been proposed to follow the single-site, water nucleophilic attack (WNA) mechanism, and therefore, axial ligand modifications are not expected to have a significant influence on O−O bond formation barriers.10b

INTRODUCTION Water oxidation catalysis is one of the key components of natural and artificial photosynthesis.1 It is the ideal source for the required redox equivalents and protons for solar fuels generation.2 In nature, this reaction takes place in photosystem II (PSII) and it is catalyzed efficiently by a Mn4CaO5 cluster known as the oxygen-evolving complex (OEC),3 with turnover frequencies (TOFs) of 100−1000 s−1 (100−400 s−1 in vivo and 1000 s−1 in vitro).4 However, water oxidation remains a challenge for artificial production of fuels with sunlight and electricity. The challenge originates from two aspects: water oxidation is thermodynamically uphill (ΔG = 4.92 eV) and it is mechanistically complicated, involving the transfer of four electrons and four protons coupled with the formation of an O−O bond.5 As a result, efficient and robust water oxidation catalysts (WOCs) are required to accumulate four oxidizing equivalents while concurrently removing electrons and protons to avoid the buildup of charge. Ruthenium complexes are among the most extensively studied molecular catalysts for water oxidation, from the blue dimer, the first well-defined molecular WOC,6 to the first examples of mononuclear WOCs,7 and to the recent progress with ruthenium-based molecular catalysts incorporating oxoacid-substituted bipyridine ligands.8 The [Ru(bda)(L)2] family, with a dianionic, tetradentate equatorial ligand, shows high water oxidation activity, reaching TOFs comparable to © XXXX American Chemical Society

Received: February 5, 2018

A

DOI: 10.1021/acs.inorgchem.8b00329 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthetic Route and Structures of the Complexes Investigated in This Work

4 and 4′ positions of the bda2− backbone had only a minor influence on catalytic activity when the axial ligands were kept unchanged (4-picoline and isoquinoline).10a The backbone substitutions had significant effect on redox potentials of the different couples but had little effect on the interaction energy between the axial ligands. In the present study, we focus our attention on the axial ligands, examining systematic changes throughout two series of halogen-substituted ligands (4-X-pyridine and 6-X-isoquinoline; X is H, F, Cl, Br, I) while keeping the parent bda2− backbone unchanged. We establish clear trends and provide and apply design guidelines based on a combination of detailed experimental studies and DFT calculations, including experimental determination of two rate constants and TS structure calculations for all complexes. Contrary to previous reports, we find that substituent groups affect catalytic activity based on the extent of their contribution to noncovalent interactions rather than their electron-withdrawing nature. Furthermore, we find that catalytic activity can be enhanced by electron-donating groups. Application of our design guidelines leads to TOFs of 330 s−1 for the fastest pyridine derivative and 1270 s−1 for the best isoquinoline derivative.

From the mechanistic point of view, [Ru(bda)(L)2] catalysts utilize a radical O−O coupling pathway (I2M) for the key O− O bond formation step between two RuV−oxo species with significant RuIV−oxyl radical character. Intermolecular noncovalent interactions such as π−π stacking interactions between the axial ligands can stabilize the transition state (TS) for O−O bond formation, lowering the activation barrier of this step. On the basis of these assumptions, many [Ru(bda)(L)2] catalysts with different axial ligands11 have been developed, including modifications to both pyridine12 and isoquinoline13 axial ligands. Nevertheless, these studies are inconclusive, with some changing several variables at once, and therefore not providing clear guidelines for new catalyst design. Ahlquist et al. reported DFT studies on [Ru(bda)(4-X-py)2] (X is H, Br, Me, COOEt, OMe, NMe2) and [Ru(bda)(6-X-isq)2] (X is H, F) and compared their findings with available experimental data for these catalysts from previous reports.14 On the basis of their calculations, it was found that there was no correlation between total energy activation barriers and the structure of the axial ligands. They also concluded that electron-withdrawing groups improved catalytic activity. It should be noted that the authors did not carry out TS calculations. Sun and coworkers also concluded that electron-withdrawing groups improved catalytic activity and that electron-donating groups had a negative effect on catalysis.11c Murata and coworkers performed a Hammett analysis for the series [Ru(bda)(4-X-py)2] (X is H, Br, Me, COOMe, OMe, CF3) and found no straightforward dependence on the electronic nature of the substituent group.15 Most of these studies compared catalysts using TOFs under specific conditions rather than rate constants. In addition, the interpretation of the data did not consider the existence of two kinetic regimes for these catalysts,10a which is the result of the possible rate-determining steps having different rate laws. The oxidative steps leading to the reactive RuVO are first order with respect to catalyst concentration, while the bimolecular coupling and the following steps are second order in catalyst. As a result, the I2M mechanism for these catalysts is ratelimited by a bimolecular step at low catalyst concentrations (rate ∝ [catalyst]2), but with increasing concentration this step becomes faster than one of the oxidation steps (rate ∝ [catalyst]). This will be discussed in this work in detail and is illustrated in Figure S17. We studied these catalysts with a systematic approach. As mentioned above, we previously found that the introduction of one or two electron-withdrawing trifluoromethyl groups in the

■

RESULTS AND DISCUSSION Synthesis and Characterization. All the complexes in this work were synthesized from the easily accessible precursor [Ru(bda)(DMSO)2].16 In most cases, the desired compound was precipitated from the reaction solution and was isolated by filtration, yielding analytically pure product without the need of column chromatography. For simplicity, we refer to [Ru(bda)(py)2] as 1 and to [Ru(bda)(isq)2] as 2. Their corresponding derivatives are denoted as 1-X for [Ru(bda)(4-X-py)2] and 2-X for [Ru(bda)(6-X-isq)2] (2-(OMe)2 for [Ru(bda)(6,7-OMeisq)2]). Complexes 1,11c 1-Me,9,10,11c,12,15,17 1-Cl,11d,12a 1Br,11c 2,9b,10a,11d,13 2-F,13b and 2-OMe13a of the series have been reported previously, and complexes 1-I, 2-Cl, 2-Br, and 2(OMe)2 are new. Characterization by 1H NMR, 13C NMR, and mass spectrometry for all complexes is consistent with their proposed structures. Purity of new compounds was verified by elemental analysis. The synthetic route and structures of all the complexes are shown in Scheme 1. Detailed synthetic procedures and characterization are listed in the Supporting Information. Electrochemistry. The electrochemical properties of all the complexes were investigated by cyclic voltammetry (CV) and B


Article

Inorganic Chemistry square wave voltammetry (SWV). The RuIV/III and RuIII/II halfwave potentials for all complexes are each within 100 mV of one another. 2,2,2-Trifluoroethanol (TFE) was used as a cosolvent for all electrochemical measurements because of the poor solubility of complexes 2-F, 2-Cl, and 2-Br in both water and acetonitrile. The electrochemical data for all complexes at pH 1.0 shows the expected RuIII/II and RuIV/III couples, followed by a RuV/IV wave that is obscured by a large electrocatalytic water oxidation wave, as shown in Figures S8 and S9. The potentials of the RuIV/III and RuIII/II couples, listed in Table 1, are similar within

mechanism. At concentration higher than [Ru]x, the rate of coupling between two RuV−oxos is faster than an oxidation step due to the increased amount of catalyst available for dimerization. This behavior is represented graphically in Figure S17. We denote rate constants for the different steps as kx,y where x is the reaction order for the catalyst and y is the reaction order for CeIV. Table 2 shows the first order regime Table 2. Rate Constantsa for the Two Kinetic Regimes and Crossing Point Concentrations between the Two Regimes for All the Complexes

Table 1. Half-Wave Potentials (E1/2) for the RuIII/II and RuIV/III Couples and Onset Potential (Eonset) (V vs. NHE)a for All Complexes

a

L

RuIII/II

RuIV/III

Eonset

pic py 4-Cl-py 4-Br-py 4-I-py isq 6-F-isq 6-Cl-isq 6-Br-isq 6-OMe-isq 6,7-OMe-isq

0.63 0.65 0.70 0.70 0.69 0.64 0.66 0.66 0.66 0.62 0.61

1.21 1.20 1.19 1.14 1.14 1.15 1.15 1.15 1.15 1.15 1.16

1.53 1.49 1.48 1.43 1.36 1.30 1.38 1.28 1.28 1.28 1.30

a

L

k1,1 × 10−4 (M−1 s−1)

k2,0 × 10−5 (M−1 s−1)

[Ru]x (μM)

pic py 4-Cl-py 4-Br-py 4-I-py isq 6-F-isq 6-Cl-isq 6-Br-isq 6-OMe-isq 6,7-(OMe)2-isq

1.8 2.1 3.8 3.6 7.2 6.7 7.1 8.4 1.7 12.7 17.2

5.3 3.1 17.0 16.6 77 180 190 640 54 1210 3420

51 102 33 32 14 5.6 5.6 2.0 4.7 1.6 0.7

[CeIV] = 1.5 mM, 25 °C in 0.1 M HClO4 with 10% CH3CN.

rate constant (eq 1, k1,1), the second order regime rate constant (eq 2, k2,0), and [Ru]x for all the complexes in this study.

1 mM [Ru(bda)(L)2] in 0.1 M HClO4 with 50% TFE.

the pyridine or isoquinoline families as a function of substituted axial ligands. The potentials of RuV/IV couples are expected to show no significant differences due to the similar onset for water oxidation catalysis and similar RuIV/III and RuIII/II potentials. The results are indicative of little influence of the substituents in the axial ligands on redox potentials and it is in contrast with substitutions on the bda2− backbone,10a which significantly affect the RuIV/III and RuIII/II couples. Stopped-Flow Kinetics. Detailed stopped-flow kinetic studies at pH 1.0 using cerium ammonium nitrate (CeIV) as a sacrificial oxidant provided rate laws, rate constants, and TOFs for all the complexes. Solutions of 3.0−4.0 mM CeIV in 0.2 M HClO4 were rapidly mixed with 2.0−400 μM catalyst solutions in 20% aqueous acetonitrile, and the evolution of the water oxidation catalysis was followed by absorption spectroscopy in a stopped-flow instrument. As previously reported for [Ru(R-bda)(L)2] (R is one or two CF3 groups; L = 4-picoline or isoquinoline), the stopped-flow measurements revealed two kinetic regimes for all complexes, one with the rate being limited by an oxidation step and one being limited by a bimolecular step, both within the I2M mechanism.10a At lower catalyst concentrations, the rate showed a second order dependence on catalyst concentration, eq 1. This is consistent with the previously proposed bimolecular O−O coupling step by Sun and coworkers between two RuV−oxo species with significant RuIV−oxyl character.9b On the other hand, at higher catalyst concentrations, the rate law was first order in both CeIV and catalyst, eq 2. The crossing point catalyst concentration between the two regimes, [Ru]x (eq 3), varied within the two series and was lower for the isoquinoline series. It should be noted that it is not the mechanism that changes with the concentration of catalyst, but rather the rate limiting step within the same

rate = −

d[Ce IV ] = k 2,0[Ru]2 dt

(1)

rate = −

d[Ce IV ] = k1,1[Ru][Ce IV ] dt

(2)

[Ru]x =

k1,1 k 2,0

[Ce IV ] (3)

The lower [Ru]x for isoquinoline series limits our ability to collect reliable data for the bimolecular step, as shown in Figure S11 for two reasons: first, the region in which data can be collected is restricted by the smaller concentration regime. Second, at such low catalyst concentrations, the catalysts reach their maximum turnover number, making the delineation between the two regimes less clear and preventing us from observing slopes closer to the expected 2.0. This is also the case for [Ru(bda)(4-I-py)2]. The opposite is true for the slowest members of the pyridine series. For these, we can get excellent data in the second order regime with slopes very close to 2.0, but we can get fewer data points in the first order regime while keeping at least 5 turnovers. For a given substituent X, both k2,0 and k1,1 values of the isoquinoline series are larger than the corresponding analogues in the pyridine series. On average, k2,0 is 1−2 orders of magnitude larger, and k1,1 is about 2 times larger in the isoquinoline series. Higher values of k2,0 for the isoquinoline series can be explained in terms of stronger noncovalent interactions between the axial isoquinoline ligands (vide infra). The larger k1,1 values in the isoquinoline series are surprising, considering that the redox potentials for the various couples are very similar for isoquinoline and pyridine analogues, as discussed above. Presumably, the larger isoquinoline framework allows for better interactions with CeIV via noncovalent interactions and/or more delocalization of the C


Article

Inorganic Chemistry

calculated the complexation energies in the TS structure geometries by deleting all other atoms and then running a single-point energy calculation. We used counterpoise as implemented in Gaussian to account for base superposition errors.18 The counterpoise-corrected interaction energies between the axial ligands are denoted as Estab, which is calculated as the energy difference between four independent ligands and the complex, taking the four ligands as a whole molecule. All calculations were carried out using the M06 functional with a polarizable continuum solvation model with water as the solvent. This functional was used for locating TS structures for the O−O coupling step for all complexes because it has been parametrized for transition metals and is considered a good choice to capture noncovalent interactions.18 Earlier work used the hybrid functional M06-2X,9b which has not been parametrized for transition metals,18 and only partial optimizations were carried out without locating the actual TS.9b In a separate study, TS calculations were carried out in the gas phase with the local functional M06L.13a In that case, the reported free energies of activation were significantly higher than the ones reported in the present study due to increased repulsion between positively charged RuV−oxo moieties in the gas phase. The side and top views of the TS structure for 2, shown in Figure 1A and Figure 1B, differ significantly from the structure denoted as TS by Sun and coworkers for this complex.9b They reported an O−O distance of 2.038 Å, compared to 1.897 Å in our optimized TS structure. For the latter, the top view in Figure 1B clearly shows the nonparallel disposition of the two axial isoquinolines within each molecule with angles between the mean isoquinoline planes of 71.1° and 82.2° for the complex on the left- and right-hand side, respectively. This is in clear contrast with the coplanar disposition in the unoptimized TS structure reported by Sun and coworkers.9b The TS structures for 1 and 2 in Figure 1 show some resemblance to one another, but also some important differences. Perhaps the most significant is that the π−π stacking interactions clearly observed for 2 (Figure 2) are not

overall charge in positively charged intermediates. As anticipated from eq 3 based on the k1,1/k2,0 ratio, [Ru]x is inversely proportional to k2,0 and it is lower for the isoquinoline series. It is important to emphasize that the observed first order dependence on catalyst concentration indicates that this rate limiting oxidation step takes place before the bimolecular step. Sun and coworkers erroneously claimed that O2 evolution from a peroxo-bridged dimer is supposed to show first order dependence in catalyst concentration if the bimolecular step is not rate limiting.9b Any step involving a dimeric intermediate following the bimolecular step will necessarily have a second order dependence on [Ru]. Transition State Calculations. We carried out transition state (TS) searches on the three possible potential energy surfaces (PESs) for the oxo−oxo coupling reaction between two [RuV(bda)(isq)2(O)]+ molecules (triplet, closed-shell singlet, and open-shell singlet), and the search was successful only for the open-shell singlet, as previously observed for [RuV(bpa)(py)2(O)]− (bpaH4 is 2,2′-bipyridine-6,6′-diphosphonic acid).8c The methodology followed was to carry out a single point energy calculation on the TS guess geometry followed by a stability calculation with optimization of the wave functions. For the TS guess geometry, we used an O−O distance of 1.85 Å. We chose this number as a starting point based on the O−O distance of 1.95 Å in the TS for the equivalent reaction for [RuV(bpa)(py)2(O)]−.8c The lowest energy configuration from the stability calculation for the guess TS geometry corresponded to the weakly antiferromagnetically coupled singlet (open shell singlet). The optimized wave functions were used as a guess for the actual TS search and the same methodology was applied for all the complexes in Scheme 1. In the optimized TS structures, the O−O distance varied from ∼1.86 to ∼1.90 Å. Figure 1 shows the TS structures for 1 and 2 (1 was used for calculations of TS structures instead of 1Me for simplicity). Table S3 lists some geometrical parameters, including the O−O bond distances for all TS structures. To extract more precise information about noncovalent interactions between the axial ligands in the TS structures, we

Figure 2. Different views of the TS structure for 2 showing two types of π-stacking interactions. (A) Staggered sandwich overlap. (B) Edge overlap.

present for 1. This is not surprising considering that isoquinoline has a more extended π system than pyridine. There are two types of π−π stacking interactions for 2, which are highlighted in Figure 2. In one type, shown in Figure 2A, the more electron-rich benzene ring of one isoquinoline interacts with the more electron-deficient pyridine ring of the other one, with the two rings adopting a staggered arrangement. In the second type, shown in Figure 2B, the overlap is between the two opposite edges of the two isoquinoline ligands. The distance between the centroids of the two

Figure 1. Different views of the TS structures for 2 (top) and 1 (bottom). (A and C) Side views. (B and D) Top views. The O−O distance is 1.884 Å for 1 and 1.897 Å for 2. D


Article

Inorganic Chemistry

Figure 3. Reaction coordinates for O−O bond formation for 1 (A) and 2 (B) based on calculations. 2R = two [RuV(bda)(L)2(O)]+ monomers; Int = intermediate; TS = transition state; P = product.

opposed picoline ligands. Our TS calculations show no significant differences between the TS structures for 1 and 1Me with no twisting of the bda2− molecules. The vertical displacement along the axial axis of one bda2− molecule with respect to the other one is enough to avoid repulsion between the methyl groups on the picoline ligands. We carried out an intrinsic reaction coordinate (IRC) calculation in both directions from the TS structures of 2 and 1, followed by a full geometry optimization of the final points. The resulting reaction coordinate diagrams for 1 and 2 are shown in Figure 3. The forward direction leads to the expected O−O coupling product, and the reverse direction finds an intermediate with an O−O distance of 2.585 Å, longer than 1.896 Å for the TS structure. This structure differs significantly from the encounter complex (EC) described by Sun and coworkers for 2, which had an O−O distance of 3.22 Å.9b The structure of the intermediate reported here resembles that of the TS with the main differences located around the Ru−O− O−Ru core, as reflected on the similar values for Estab for both structures (Table 3). Intermediate formation is uphill from the two separate monomers with ΔG = +7.2 kcal/mol. The main contributor to the positive free energy change is the entropic term (−TΔS= +18.6 kcal/mol), a result of a high degree of organization in this intermediate compared to two noninteracting [RuV(bda)(isq)2(O)]+ molecules. This term is partially compensated by the favorable ΔH = −11.4 kcal/ mol, largely due to strong interactions between the axial isoquinoline ligands. Table 3 shows activation parameters (ΔG⧧, ΔH⧧, and ΔS⧧) and stabilization energies (Estab) for 2, using as the starting

staggered rings shown in Figure 2A is 3.74 Å, and the distance between the centroids of the four carbon atoms of each isoquinoline shown in Figure 2B with edge overlap is 3.49 Å. On the other hand, the distances between pyridine ring centroids are 4.30 and 4.41 Å, indicating less interaction. This is also reflected on the angles between the axial ligands: the angle between the two interacting isoquinolines in Figures 2A and B are 8.9° and 5.4°, respectively, while the analogous for the pyridine rings are 21.4° and 23.2°. There are also significant differences between 1 and 2 in the relative orientations of the bda2− ligands in the two catalyst molecules, which can be seen in Figure 1. For each of the two molecules in the TS dimer, we represent the open face of the complexes by defining a plane that is approximately perpendicular to the equatorial plane of the bda2− ligand using the Ru atom, the two axial N atoms, and the two carbon atoms of the carboxylate groups in the bda2− ligand. The angle between these two planes is 14.2° for 2 and 38.4° for 1, shown in Figures 1B and D, respectively, indicating the more symmetrical nature of the TS for 2. As can be seen in Figure 1A, the two bda2− ligands are almost parallel in the TS structure for 2 with an angle between the two mean planes defined by each bda2− ligand of 2.6° with an approximate distance of 1.52 Å between the two planes (taken as the average distance from each plane to the Ru centers). The equivalent angle between the bda2− ligands for 1 is 6.6° with an approximate distance of 2.18 Å, Figure 1C, very different from the more than 45° reported by Sun and coworkers for 1-Me. The authors attributed this twisting of the two bda2− molecules with respect to each other to repulsion between the methyl groups on E


Article

Inorganic Chemistry

An analysis of the enthalpic and entropic components of the ΔG and ΔG⧧ values for 1 and 2 illustrates the significance of the secondary interactions. The reaction coordinates for both catalysts in Figure 3 have similar profiles, and the values of the entropic contributions are also similar. By far, the most significant difference is in the magnitude of the enthalpic contributions, which can be attributed to favorable intermolecular interactions. It is informative to look at the evolution of the Mulliken atomic spin densities on Ru and O along the reaction coordinate for O−O bond formation for 1 and 2, Table 5.

Table 3. DFT-Calculated Activation Parameters for the O− O Coupling Transition State for 2 from a Weakly Bound Dimer Intermediate and from Two [RuV(bda)(isq)2(O)]+ Monomers at T = 298 K starting point 2 × [RuV(bda) (isq)2(O)]+ intermediate dimer

ΔG⧧ (kcal/mol)

ΔH⧧ (kcal/mol)

−TΔS⧧ (kcal/mol)

Estab (kcal/mol)

13.2

−7.2

20.4

−5.9

5.9

4.1

1.8

−5.2

point either two [RuV(bda)(isq)2(O)]+ molecules or the intermediate. Presumably, the experimentally observed second order rate limiting step would be the formation of the intermediate from the two monomers with ΔG = +7.2 kcal/ mol but with a favorable ΔH = −11.4 kcal/mol, but we were unable to locate a TS between the reactants and the intermediate. The formation of the product [(isq)2(bda)RuIVO−ORuIV(bda)(isq)2]2+ from the intermediate should be very fast, with ΔG⧧ = +5.9 kcal/mol and ΔG = −10.9 kcal/mol (overall ΔG = −3.7 kcal/mol from two isolated [RuV(bda)(isq)2(O)]+ molecules with −TΔS = +21.3 kcal/mol and ΔH = −25.0 kcal/mol, see reaction coordinate). Similar intermediates exist for the other members of the isoquinoline series, but for simplicity, we will focus our attention on overall activation parameters obtained using two [RuV(bda)(L)2(O)]+ molecules as reactants when comparing the rest of the complexes. As discussed below, there is excellent agreement between experimental results and DFT calculations following this approach. In the case of 1, there is an intermediate dimer as well, but with a much longer O−O distance of 3.208 Å (compared to 2.585 Å for the isoquinoline intermediate). The longer O−O distance is reflected in the positive complexation energy (Estab = +2.2 kcal/mol vs −5.2 kcal/mol for isoquinoline), indicative of less favorable interactions between the axial ligands for 1 than for 2. ΔG is +13.6 kcal/mol for the formation of the intermediate [(py)2(bda)RuIVO−ORuIV(bda)(py)2]2+ from the two monomers with a small favorable ΔH = −4.6 kcal/ mol. The latter probably arises from a weak interaction of the two RuV−oxos with significant RuIV−oxyl radical character. From the intermediate, ΔG⧧ is +6.2 kcal/mol, and ΔG is −10.3 kcal/mol for the formation of the peroxide [(py)2(bda)RuIVO− ORuIV(bda)(py)2]2+. The overall ΔG⧧ of 19.8 kcal/mol is consistent with the less favorable interaction between pyridines, compared to isoquinoline, as well as the smaller experimental rate constant, k2,0. Overall, O−O bond formation is slightly uphill for 1, with ΔG = +3.3 kcal/mol (−TΔS= +19.3 kcal/mol and ΔH = −16.0 kcal/mol) starting from two infinitely separated [RuV(bda)(py)2(O)]+ molecules. Calculated activation parameters for 1 are listed in Table 4, and a reaction coordinate is shown in Figure 3A.

Table 5. Evolution of the Mulliken Spin Densities on Ru and O along the Reaction Coordinate for O−O Bond Formation for 1 and 2

2 × [RuV(bda) (py)2(O)]+ intermediate dimer

ΔG⧧ (kcal/mol)

ΔH⧧ (kcal/mol)


Estab (kcal/mol)

19.8

2.3

17.5

0.7

6.2

6.9

−0.7

2.2

atom

monomer

intermediate

TS

1

Ru(1) Ru(2) O(1) O(2) Ru(1) Ru(2) O(1) O(2)

+0.414 +0.414 +0.669 +0.669 +0.441 +0.441 +0.637 +0.637

+0.408 −0.408 +0.667 −0.667 +0.386 −0.390 +0.662 −0.659

+0.129 −0.130 +0.519 −0.519 +0.135 −0.141 +0.532 −0.525

2

For both catalysts, the spin density in the [RuV(bda)(L)2(O)]+ monomer is shared between the Ru and the O atoms with a slightly larger spin density on the O atom. This is consistent with a significant oxyl radical character in this species. The distribution of the spin density in the intermediates for 1 and 2 is different, Figure 4 and Table 5.

Figure 4. Electron spin density isosurfaces along the reaction coordinate for O−O bond formation for 1 (top) and 2 (bottom). The products are not shown because their ground states are closedshell singlets.

The interaction between two O atoms in the intermediate is weak for 1, reflected from the largely unchanged spin density and similar O−O distances between two monomers and intermediate. For 2, a redistribution of the spin density has already begun at the intermediate. From the intermediates to TS structures, spin densities for both 1 and 2 are significantly distributed, consistent with the reduction of the Ru−O multiple bonding and the incipient formation of the O−O bond. The spin densities disappear completely in the O−O coupling products, for which the closed-shell singlet is the ground state. Calculation of Activation Parameters for HalogenSubstituted Catalysts. Given the agreement between experiment and theory for the results obtained for 1 and 2, we decided to carry out a systematic study with additional variants

Table 4. DFT-Calculated Activation Parameters for the O− O Coupling Step for 1 from a Weakly Bound Dimer Intermediate and from Two [RuV(bda)(py)2(O)]+ Monomers at T = 298 K starting point

catalyst

F


Article

Inorganic Chemistry of the axial ligands. We initially set our focus on the two halogen series [Ru(bda)(4-X-py)2] and [Ru(bda)(6-X-isq)2] (X = H, F, Cl, Br, I), with the goal of introducing noncovalent interactions involving the halogen atoms (halogen−aromatic interactions, dipole−induced dipole, London dispersion forces) in addition to π−π stacking interactions. The results obtained from DFT calculations were encouraging, especially for the pyridine series, where ΔG⧧ dropped 43%, from +19.8 kcal/mol for the parent pyridine complex to +11.2 kcal/mol for the 4-Ipy derivative (Table 6), as anticipated from the increased Table 6. DFT-Calculated Activation Parameters for the O− O Coupling Step, Stabilization Energies Due to Interaxial Ligand Interactions, and Experimental O−O Coupling Rate Constants for [Ru(bda)(L)2] with T = 298 K L

ΔG⧧ (kcal/mol)

ΔH⧧ (kcal/mol)


Estab (kcal/mol)

py 4-F-py 4-Cl-py 4-Br-py 4-I-py isq 6-F-isq 6-Cl-isq 6-Br-isq 6-I-isq

19.8 16.4 15.9 15.8 11.2 13.2 12.2 11.7 15.3 15.1

2.3 −1.4 −2.9 −4.0 −5.9 −7.2 −8.0 −8.8 −7.5 −6.6

17.5 17.8 18.8 19.8 17.1 20.4 20.2 20.5 22.8 21.7

0.7 −1.5 −3.5 −3.3 −3.8 −5.9 −7.3 −10.3 −9.8 −10.6

Figure 5. Plots of ln(k2,0) vs ΔG⧧ according to the Eyring equation for the two halogen-substituted series. ΔG⧧ is from DFT calculations, and k2,0 values are from experimental results. T = 298 K.

k2,0 × 10−5 (M−1 s−1) 3.1

different levels of accuracy with which the M06 functional captures those interactions. Faster Catalysts by Rationally Designed Derivatives. Given the success with the halogen series, we selected additional isoquinoline derivatives predicted to further stabilize the specific intermolecular interactions in the TS, thereby decreasing ΔG⧧ for the O−O coupling step. Inspection of the TS structure for 2 in Figures 2A and B reveals two types of π−π interactions. For the isq ligands in the plane of the page in Figure 2A, denoted “staggered sandwich”, the more electronrich benzene ring of one isoquinoline interacts with the electron-deficient pyridine ring of the adjacent isoquinoline. We anticipated that this interaction could be enhanced by introducing electron-donating groups on the benzene ring. For the isq ligands in the plane of the page in Figure 2B, denoted “edge overlap”, carbons C1, C8a, C8, and C7 (the number of carbons on isoquinoline ring is labeled in Scheme 1) on the N-containing edge (the more electron-deficient side) of one isoquinoline interact with carbons C4, C4a, C5, and C6 (the electron-rich side) of the other isoquinoline. We anticipated that this interaction could also be enhanced by introducing electron-donating groups on C6 of the donor isoquinoline. Considering that methoxy groups are strong electron donors and 6-methoxyisoquinoline (6-OMe-isq) and 6,7-dimethoxyisoquinoline (6,7-(OMe)2-isq) are commercially available compounds, [Ru(bda)(6-OMe-isq)2] (2-OMe) and [Ru(bda)(6,7-(OMe)2-isq)2] (2-(OMe)2) were predicted to be good candidates for further study. Our DFT calculations were encouraging with the TS geometries maintaining the same overlap patterns and ΔG⧧ values of +11.7 and +10.1 kcal/mol for the O−O coupling step for 2-OMe and 2-(OMe)2, respectively. These are the lowest values calculated in the isoquinoline series. Once again, the predictions of our DFT calculations were corroborated by the experimental results: the rate constants for the bimolecular O−O coupling step (k2,0, Table 7) for 2-OMe and 2-(OMe)2 are 1.21 × 108 and 3.42 × 108 M−1 s−1, respectively. These represent increases of 6.7 and 19 times compared to the parent isoquinoline catalyst. The negative ΔH⧧ values are the most negative of all the catalysts studied (−9.4 and −18.7 kcal/mol, respectively). This clearly originates from the stabilization energy due to interactions between the axial ligands (−8.1 and −13.3 kcal/mol), and it validates our hypothesis about the favorable influence of electron-donating

17.0 16.6 77 180 190 640 54

polarizability from hydrogen to iodine. The same trend is present in the stabilization energy due to interactions between the axial ligands Estab. In the isoquinoline series, ΔG⧧ dropped from +13.2 kcal/mol for X = H to +11.7 kcal/mol for X = Cl, but then increased to +15.3 and +15.1 kcal/mol for X = Br and X = I, respectively. As opposed to the 4-X-py series, the larger bromo and iodo substituents do not provide additional stabilization energy between the 6-X-isq ligands, possibly due to repulsive steric interactions in the more closely positioned isoquinoline ligands. This is reflected in the less negative values of ΔH⧧ that, together with the larger entropic term due to the larger volume of bromine and iodine substituents in a highly organized TS, leads to higher overall barriers. Illustrated in Table 6, the value of ΔG⧧ for the two halogensubstituted series is dominated by the entropic term, as expected for a highly organized TS in this bimolecular reaction, but the trends in ΔG⧧ are most clearly attributed to ΔH⧧. As discussed above and shown in Figure 3 for 1 and 2, ΔH⧧ is always negative and is significantly larger in the isoquinoline series than the corresponding values in the pyridine series. This is a reflection of a larger contribution of π−π interactions to TS stabilization in the isoquinoline series. As a result, the additional contribution due to the halogen is not as significant. For the pyridine series, the contribution of the halogens to the TS stabilization has a considerable effect. This is easily appreciated in Figure S18, which shows the relationship between Estab and ΔH⧧ for [Ru(bda)(4-X-py)2] and [Ru(bda)(6-X-isq)2]. We were pleased to see our DFT predictions validated by our experimental results. The inverse linear relationship between ΔG⧧ (calculated) and ln(k2,0) (experimental) predicted by the Eyring equation is observed for both the 4-X-pyridine and 6-Xisoquinoline series (Figure 5). The fact that the two series do not fit on the same line is due to the different nature of the dominant noncovalent interactions in the two series and the G


Article

Inorganic Chemistry

Table 7. DFT-Calculated Activation Parameters for the O−O Coupling Step, Stabilization Energies Due to Interaxial Ligand Interactions, and Experimental O−O Coupling Rate Constants for [Ru(bda)(L)2] with T = 298 K L

ΔG⧧ (kcal/mol)

ΔH⧧ (kcal/mol)


Estab (kcal/mol)

k2,0 × 10−5 (M−1 s−1)

isq 6-OMe-isq 6,7-(OMe)2-isq

13.2 11.7 10.1

−7.2 −9.4 −18.7

20.4 21.1 28.8

−5.9 −8.1 −13.3

180 1210 3420

Figure 6. (A) O2 evolution vs time plots and TON for [Ru(bda)(4-X-py)2], [CeIV] = 0.365 M in 0.1 M HClO4, [Ru] = 62.5 μM. (B) O2 evolution vs time plots and TON for [Ru(bda)(6-X-isq)2], [CeIV] = 0.365 M in 0.1 M HClO4, [Ru] = 20.4 μM.

Table 8. TOF, TOFmax, and TONmax as well as O2 Conversion Efficiency for [Ru(bda)(L)2] with k2,0 and k1,1 Values Included for Comparison L

k2,0 × 10−5 (M−1 s−1)

k1,1 × 10−4 (M−1 s−1)

TOFa (s−1)

TOFmaxb (s−1)

TONmaxc

O2 yieldd (%)

pic py 4-Cl-py 4-Br-py 4-I-py isq 6-F-isq 6-Cl-isq 6-Br-isq 6-OMe-isq 6,7-(OMe)2-isq

5.3 3.1 17.0 16.6 77 180 190 640 54 1210 3420

2.21 2.06 3.80 3.6 7.2 6.7 7.1 8.4 1.7 12.7 17.2

26 9 33 77 143 114 316 258 73 401 403

34 22 62 101 334 668 790 364 88 1274 1034

1330 1142 3182 4942 5280 11 859 9822 26 992 7371 14 858 7215

97 83 93 90 96 98 90 98 67 100 99

[CeIV] = 0.365 M, [Ru] = 62.5 μM for pyridine series, and 20 μM for isoquinoline series. bThe maximum TOF obtained by increasing catalyst concentration from 16 to 143 μM for pyridine series and 4 to 85 μM for isoquinoline series, [CeIV] = 0.365 M. cMaximum TON obtained by decreasing catalyst concentration until excess CeIV remained, [CeIV] = 0.365 M. The specific conditions are listed in Table S2. dO2 conversion yield for TONmax measurements, the lower values indicate incomplete consumption of CeIV. a

The TOF values are mostly consistent with the trends in calculated activation parameters and experimental rate constants. For the pyridine series, the concentration of CeIV was 0.365 M and catalyst concentration was 62.5 μM. Under these conditions, the TOFs measured for the 4-X-pyridine series were 26, 9, 33, 77, and 143 s−1 for X = Me, H, Cl, Br, and I, respectively, calculated from the maximum slope of the pressure/time plot, as shown in Figure 6A. It is quite remarkable that more than 10-fold increase in TOF from 9 s−1 to 143 s−1 was achieved just by replacing hydrogen with iodine in the para position of pyridine. For the isoquinoline series, which undergo more turnovers, the concentration for CeIV was the same, but the catalyst concentration was 20 μM. The TOFs for the isoquinoline series were 114, 316, 258, 73, 401, and 403 s−1 for X = H, F, Cl, Br, OMe, and (OMe)2, respectively (Figure 6B). TOF increased when replacing hydrogen with fluorine as expected, though this enhancement

groups on such interactions. An additional benefit was the 1.9 and 2.6 times enhancements in the rate constant for the first order regime, k1,1 (1.27 × 105 M−1 s−1 for 2-OMe and 1.72 × 105 M−1 s−1 for 2-(OMe)2), Table 2. Benchmarking [Ru(bda)(X-L)2] Water Oxidation Catalysis. Turnover number (TON) and turnover frequency (TOF) values for all the complexes were obtained in a high concentration of CeIV acidic solution using a pressure transducer. For a typical measurement, a very small volume of catalyst was added to a solution of CeIV in 0.10 M perchloric acid with vigorous stirring. Gas generation and disappearance of the yellow color of CeIV were observed immediately. The evolution of O2 was measured as a pressure change of the headspace with a pressure transducer. Gas chromatography was used to corroborate the identity of the gas as O2 as well as to calibrate the amount of O2 generated. H


Article

Inorganic Chemistry

for ΔG⧧. Calculations further emphasized that the major contributor to the barrier is the entropy term, −TΔS⧧, a point that is intuitive for a bimolecular reaction, but often overlooked nonetheless. The faster catalysts are those with the most favorable ΔH⧧ term, which is dictated by the intermolecular ligand interaction energy (Estab). For the pyridine series, catalytic rates correlate with the size and polarizability of the 4-substituent, increasing from 9 s−1 for X = −H to 334 s−1 for X = −I. Inspection of the transition state geometry corroborates the premise that the key interaction is between the halogen substituent and the π system of the other molecule, which is most favorable for easily polarized iodine. The isoquinoline series is governed by π−π stacking interactions, and consequently, the effect of increasing substituent size is favorable to a point, but the gains in X-π interactions are offset by interference in the π−π overlap for the larger halogens (Br, I). That the RuIV/III and RuIII/II potentials are very close for all of these catalysts further indicates that the noncovalent interactions are the key to accelerating the bimolecular coupling, not electronic effects. This combination of experimental and theoretical tools has allowed us to increase TOF by an order of magnitude with simple ligand substitutions based on rational design that was informed by the specific interactions predicted in the calculated TS. One of the catalysts reached a TOF of 1270 s−1 for cerium(IV)-driven water oxidation. The results presented here provide a platform for future catalyst development for water oxidation catalysis, and the guiding principles are applicable to small molecule activation and catalysis in general. Additional studies are underway to extend the interligand interactions to electrostatic and hydrogen-bonding interactions.

is not as pronounced as the 10-fold achieved for the pyridine series. TOF dropped when the substituent was bromine, which is consistent with the slower rate constants in stopped-flow kinetics and higher ΔG⧧ in DFT calculations. However, only a qualitative correlation is expected considering the difference between the conditions used for TOF and rate constant measurements. At the concentrations used for TOF measurements, experimental rate constants predict the bimolecular step to be rate-determining for all the catalysts; however, we find that the TOFs track more closely with k1,1. Figure S21 shows plots of TOF vs k1,1 and TOF vs k2,0 for all the complexes. In the second order kinetic regime, an increase in catalyst concentration with constant CeIV concentration should lead to a higher TOF, and this was found to be the case up to a point where the trend reversed. Maximum TOF values (TOFmax) for all the complexes were measured at [CeIV] = 0.365 M by increasing catalyst concentrations until TOFs started to drop. For the fastest in each series of catalysts, [Ru(bda)(4-I-py)2] and [Ru(bda)(6-OMe-isq) 2], the TOFs versus catalyst concentration are shown in Figure S20. For [Ru(bda)(4-Ipy)2], TOFmax was 334 s−1 at [Ru] = 120 μM. The highest TOFmax of all the catalysts is 1274 s−1 for [Ru(bda)(6-OMeisq)2] at [Ru] = 30 μM. TOF, TOFmax, TONmax, and O2 conversion efficiency based on the initial amount of CeIV for all complexes are listed in Table 8. As with TOFmax, TONmax tracked fairly well with the experimental rate constants and calculated activation parameters, though the range was not as wide. This trend can be interpreted in terms of water oxidation competing with deactivation pathways. For faster catalysts, water oxidation can outcompete deleterious processes, resulting in more turnovers. TONmax is 2−5 times higher for the isoquinoline series compared to the corresponding analogues in the pyridine series. The highest TON obtained is for [Ru(bda)(6-Cl-isq)2] with TONmax = 27 000.

■

ASSOCIATED CONTENT

S Supporting Information *

■

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00329. Experimental details, 1H and 13C NMR spectra, MS and UV−vis spectra, kinetic data, oxygen evolution plots, and description of DFT calculations (PDF)

CONCLUSIONS Studies describing the defining aspects of the key O−O bondforming step in water oxidation by the remarkable [Ru(bda)(L)2] family of catalysts have been lacking. It has been known that the axial ligands play an important role, but previous work failed to provide a meaningful method for predicting and explaining reactivity. Whether this was primarily due to π−π stacking,9b hydrophobic,11c electrostatic,13a or other noncovalent interactions or electronic effects,11c was not entirely understood. Only recently was it shown that the electronics of the bda2− ligand backbone play just a minor role in the kinetics of cerium(IV)-driven water oxidation.10a Furthermore, some experimental and computational determinations of activation parameters have lacked rigor and fall short of describing experimental results.9b,13a,14 Guided by DFT computations of transition state structures and activation parameters, this systematic study of water oxidation catalyzed by [Ru(bda)(4-X-py)2] and [Ru(bda)(6-Xisq)2] quantifies the effects of noncovalent interligand interactions between the two types of axial ligands and among the series of halogen-substituted variants. Rate constants, which are independent of specific conditions (as opposed to TOFs), were determined for the slower oxidation step (k1,1) and the bimolecular step (k2,0), which each determined the catalytic rate at high and low catalyst concentrations, respectively. The experimental constants for the bimolecular step correlated well with the calculated values

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yan Xie: 0000-0003-0543-5479 David W. Shaffer: 0000-0002-8807-1617 Javier J. Concepcion: 0000-0002-9296-0943 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was carried out at Brookhaven National Laboratory and was supported by the U.S. Department of Energy, Office of Science, Division of Chemical Sciences, Geosciences, & Biosciences, Office of Basic Energy Sciences under Contract DE-SC00112704.

■

REFERENCES

(1) (a) Meyer, T. J. Chemical approaches to artificial photosynthesis. Acc. Chem. Res. 1989, 22, 163−170. (b) Bard, A. J.; Fox, M. A. Artificial

I


Article

Inorganic Chemistry photosynthesis: solar splitting of water to hydrogen and oxygen. Acc. Chem. Res. 1995, 28, 141−145. (c) Kaerkaes, M. D.; Johnston, E. V.; Verho, O.; Akermark, B. Artificial photosynthesis: from nanosecond electron transfer to catalytic water oxidation. Acc. Chem. Res. 2014, 47, 100−111. (d) Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Artificial photosynthesis: molecular systems for catalytic water oxidation. Chem. Rev. 2014, 114, 11863−12001. (e) Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W. Molecular catalysts for water oxidation. Chem. Rev. 2015, 115, 12974−13005. (2) (a) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar water splitting cells. Chem. Rev. 2010, 110, 6446−6473. (b) Kim, D.; Sakimoto, K. K.; Hong, D.; Yang, P. Artificial photosynthesis for sustainable fuel and chemical production. Angew. Chem., Int. Ed. 2015, 54, 3259−3266. (3) (a) Siegbahn, P. E. M. Water oxidation mechanism in photosystem II, including oxidations, proton release pathways, O-O bond formation and O2 release. Biochim. Biophys. Acta, Bioenerg. 2013, 1827, 1003−1019. (b) Yano, J.; Yachandra, V. Mn4Ca cluster in photosynthesis: where and how water is oxidized to dioxygen. Chem. Rev. 2014, 114, 4175−4205. (c) Barber, J. Mn4Ca cluster of photosynthetic oxygen-evolving center: structure, function and evolution. Biochemistry 2016, 55, 5901−5906. (d) Askerka, M.; Brudvig, G. W.; Batista, V. S. The O2-evolving complex of photosystem II: recent insights from quantum mechanics/molecular mechanics (QM/MM), extended X-ray absorption fine structure (EXAFS), and femtosecond X-ray crystallography data. Acc. Chem. Res. 2017, 50, 41−48. (4) Dismukes, G. C.; Brimblecombe, R.; Felton, G. A.; Pryadun, R. S.; Sheats, J. E.; Spiccia, L.; Swiegers, G. F. Development of bioinspired Mn4O4-cubane water oxidation catalysts: lessons from photosynthesis. Acc. Chem. Res. 2009, 42, 1935−1943. (5) Binstead, R. A.; Chronister, C. W.; Ni, J.; Hartshorn, C. M.; Meyer, T. J. Mechanism of water oxidation by the μ-oxo dimer [(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+. J. Am. Chem. Soc. 2000, 122, 8464−8473. (6) (a) Gersten, S. W.; Samuels, G. J.; Meyer, T. J. Catalytic oxidation of water by an oxo-bridged ruthenium dimer. J. Am. Chem. Soc. 1982, 104, 4029−4030. (b) Gilbert, J. A.; Eggleston, D. S.; Murphy, W. R.; Geselowitz, D. A.; Gersten, S. W.; Hodgson, D. J.; Meyer, T. J. Structure and redox properties of the water-oxidation catalyst [(bpy)2(OH2)RuORu(OH2)(bpy)2]4+. J. Am. Chem. Soc. 1985, 107, 3855−3864. (c) Yamada, H.; Siems, W. F.; Koike, T.; Hurst, J. K. Mechanisms of water oxidation catalyzed by the cis,cis-[(bpy)2Ru(OH2)]2O4+ ion. J. Am. Chem. Soc. 2004, 126, 9786−9795. (d) Liu, F.; Concepcion, J. J.; Jurss, J. W.; Cardolaccia, T.; Templeton, J. L.; Meyer, T. J. Mechanisms of water oxidation from the blue dimer to photosystem II. Inorg. Chem. 2008, 47, 1727−1752. (7) (a) Zong, R.; Thummel, R. P. A new family of Ru complexes for water oxidation. J. Am. Chem. Soc. 2005, 127, 12802−12803. (b) Concepcion, J. J.; Jurss, J. W.; Templeton, J. L.; Meyer, T. J. One site is enough. Catalytic water oxidation by [Ru(tpy)(bpm)(OH2)]2+ and [Ru(tpy)(bpz)(OH2)]2+. J. Am. Chem. Soc. 2008, 130, 16462−16463. (c) Chen, Z.; Concepcion, J. J.; Jurss, J. W.; Meyer, T. J. Single-site, catalytic water oxidation on oxide surfaces. J. Am. Chem. Soc. 2009, 131, 15580−15581. (d) Concepcion, J. J.; Jurss, J. W.; Norris, M. R.; Chen, Z.; Templeton, J. L.; Meyer, T. J. Catalytic water oxidation by single-site ruthenium catalysts. Inorg. Chem. 2010, 49, 1277−1279. (e) Concepcion, J. J.; Tsai, M.-K.; Muckerman, J. T.; Meyer, T. J. Mechanism of water oxidation by single-site ruthenium complex catalysts. J. Am. Chem. Soc. 2010, 132, 1545−1557. (f) Polyansky, D. E.; Muckerman, J. T.; Rochford, J.; Zong, R.; Thummel, R. P.; Fujita, E. Water oxidation by a mononuclear ruthenium catalyst: characterization of the intermediates. J. Am. Chem. Soc. 2011, 133, 14649−14665. (g) Lewandowska-Andralojc, A.; Polyansky, D. E.; Zong, R.; Thummel, R. P.; Fujita, E. Enabling light-driven water oxidation via a low-energy RuIV = O intermediate. Phys. Chem. Chem. Phys. 2013, 15, 14058−14068. (8) (a) Matheu, R.; Ertem, M. Z.; Benet-Buchholz, J.; Coronado, E.; Batista, V. S.; Sala, X.; Llobet, A. Intramolecular proton transfer boosts

water oxidation catalyzed by a Ru complex. J. Am. Chem. Soc. 2015, 137, 10786−10795. (b) Fan, T.; Duan, L.; Huang, P.; Chen, H.; Daniel, Q.; Ahlquist, M. S. G.; Sun, L. The Ru-tpc water oxidation catalyst and beyond: water nucleophilic attack pathway versus radical coupling pathway. ACS Catal. 2017, 7, 2956−2966. (c) Xie, Y.; Shaffer, D. W.; Lewandowska-Andralojc, A.; Szalda, D. J.; Concepcion, J. J. Water oxidation by ruthenium complexes incorporating multifunctional bipyridyl diphosphonate ligands. Angew. Chem., Int. Ed. 2016, 55, 8067−8071. (9) (a) Duan, L.; Fischer, A.; Xu, Y.; Sun, L. Isolated sevencoordinate Ru(IV) dimer complex with [HOHOH]− bridging ligand as an intermediate for catalytic water oxidation. J. Am. Chem. Soc. 2009, 131, 10397−10399. (b) Duan, L.; Bozoglian, F.; Mandal, S.; Stewart, B.; Privalov, T.; Llobet, A.; Sun, L. A molecular ruthenium catalyst with water-oxidation activity comparable to that of photosystem II. Nat. Chem. 2012, 4, 418−423. (10) (a) Shaffer, D. W.; Xie, Y.; Szalda, D. J.; Concepcion, J. J. Manipulating the rate-limiting step in water oxidation catalysis by ruthenium bipyridine−dicarboxylate complexes. Inorg. Chem. 2016, 55, 12024−12035. (b) Scherrer, D.; Schilling, M.; Luber, S.; Fox, T.; Spingler, B.; Alberto, R.; Richmond, C. J. Ruthenium water oxidation catalysts containing the non-planar tetradentate ligand, biisoquinoline dicarboxylic acid (biqaH2). Dalton Trans. 2016, 45, 19361−19367. (11) (a) Duan, L.; Araujo, C. M.; Ahlquist, M. S. G.; Sun, L. Highly efficient and robust molecular ruthenium catalysts for water oxidation. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15584−15588. (b) Wang, L.; Duan, L.; Stewart, B.; Pu, M.; Liu, J.; Privalov, T.; Sun, L. Toward controlling water oxidation catalysis: tunable activity of ruthenium complexes with axial imidazole/DMSO ligands. J. Am. Chem. Soc. 2012, 134, 18868−18880. (c) Duan, L.; Wang, L.; Inge, A. K.; Fischer, A.; Zou, X.; Sun, L. Insights into Ru-based molecular water oxidation catalysts: electronic and noncovalent-interaction effects on their catalytic activities. Inorg. Chem. 2013, 52, 7844−7852. (d) Wang, L.; Duan, L.; Tong, L.; Sun, L. Visible light-driven water oxidation catalyzed by mononuclear ruthenium complexes. J. Catal. 2013, 306, 129−132. (12) (a) Jiang, Y.; Li, F.; Huang, F.; Zhang, B.; Sun, L. Chemical and photocatalytic water oxidation by mononuclear Ru catalysts. Chin. J. Catal. 2013, 34, 1489−1495. (b) Liu, Z.; Gao, Y.; Yu, Z.; Zhang, M.; Liu, J. Effects of Br substituent on catalytic performance of Ru-bda (H2bda = 2,2’-bipyridine-6,6’-dicarboxylic acid) catalysts for water oxidation. Chin. J. Catal. 2015, 36, 1742−1749. (13) (a) Richmond, C. J.; Matheu, R.; Poater, A.; Falivene, L.; BenetBuchholz, J.; Sala, X.; Cavallo, L.; Llobet, A. Supramolecular water oxidation with Ru−bda-based catalysts. Chem. - Eur. J. 2014, 20, 17282−17286. (b) Wang, L.; Duan, L.; Wang, Y.; Ahlquist, M. S. G.; Sun, L. Highly efficient and robust molecular water oxidation catalysts based on ruthenium complexes. Chem. Commun. 2014, 50, 12947− 12950. (14) Fan, T.; Zhan, S.; Ahlquist, M. S. G. Why is there a barrier in the coupling of two radicals in the water oxidation reaction? ACS Catal. 2016, 6, 8308−8312. (15) Sato, Y.; Takizawa, S.-y.; Murata, S. Substituent effects on physical properties and catalytic activities toward water oxidation in mononuclear ruthenium complexes. Eur. J. Inorg. Chem. 2015, 2015, 5495−5502. (16) Gao, Y.; Ding, X.; Liu, J.; Wang, L.; Lu, Z.; Li, L.; Sun, L. Visible light driven water splitting in a molecular device with unprecedentedly high photocurrent density. J. Am. Chem. Soc. 2013, 135, 4219−4222. (17) Tong, L.; Duan, L.; Xu, Y.; Privalov, T.; Sun, L. Structural modifications of mononuclear ruthenium complexes: a combined experimental and theoretical study on the kinetics of rutheniumcatalyzed water oxidation. Angew. Chem., Int. Ed. 2011, 50, 445−449. (18) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215−241.

J


OâO Radical Coupling: From Detailed Mechanistic Understanding to

Recommend Documents

OâO Radical Coupling: From Detailed Mechanistic Understanding to