Theoretical Study of the Mechanisms of Two Copper Water Oxidation

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Theoretical Study of the Mechanisms of Two Copper Water Oxidation Electro-catalysts with Bipyridine Ligands. Qiuyun Mao, Yun-Jie Pang, Xichen Li, Guangju Chen, and Hongwei Tan ACS Catal., Just Accepted Manuscript • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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Theoretical Study of the Mechanisms of Two Copper Water Oxidation Electro-catalysts with Bipyridine Ligands. Qiu-Yun Mao, Yun-Jie Pang, Xi-Chen Li*, Guang-Ju Chen*, Hong-Wei Tan College of Chemistry, Beijing Normal University, 100875, Beijing, China.

Table of Content (TOC) graphic

Abstract Hybrid density functional theory was employed to study reaction mechanisms of two homogeneous copper water oxidation catalysts (WOCs), [(L22−)CuII(OH2)2] (H2L2=6,6’dihydroxy-2,2’-bipyridine) (TOF = 0.4 s-1 at overpotential = 640 mV and pH 12.4) and [(L1) CuII(OH−)2] (L1=2,2’-bipyridine) (TOF = 100 s-1 at overpotential = 850 mV and pH 12.5). Interesting mechanistic insights were obtained by systematically exploring different oxidation and protonation states along reaction pathways. Two well-established protocols to compute thermodynamics of proton and electron release processes were employed and similar results were obtained. For the [(L22−)CuII(OH2)2] WOC, two distinct types of water oxidation mechanisms were found with similar rate-limiting barriers, Cosubstrate and Hydroxyl coupling, termed with the characteristics of O–O bond formation transition states. For the Cosubstrate mechanism, after the starting species is oxidized twice, O–O bond is formed between the copper-bound terminal oxyl radical ligand and its adjacent oxyanion substituent on the bipyridine radicaloid, showing an interesting mechanistic role of the electron-donating proton-responsive hydroxyl substituent. After the third oxidation of the complex, the organic peroxyl moiety is displaced into an inorganic superoxide by a

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free anionic hydroxide, which is the rate-limiting step in the Cosubstrate mechanism. For the Hydroxyl coupling mechanism, after two oxidations of the starting species, O–O bond formation occurs through coupling within the copper-bound (OHlig…OHfree)•- moiety, with hydrogen bonding stabilization from the 6,6’-oxyanions and the ligating and free water molecules. For the parent [(L1)CuII(OH−)2] WOC, O–O bond formation was suggested to occur through coupling between the copper-bound oxyl radical and hydroxide ligands. The results could be useful for further improvement of Cu WOCs. Keywords water oxidation, O–O bond formation, copper, cosubstrate, hydroxyl coupling, oxyl-hydroxide coupling, water nucleophilic attack, density functional theory

Introduction In nature, the oxygen-evolving complex (OEC) in photosystem II (PSII) catalyzes the oxidative splitting of two water molecules into one O2 molecule, releasing a total of four protons and four electrons.1 The fast turnover frequency (TOF) can be attributed to the low-barrier peroxide bond formation between MnIV-bound oxygen sites, as shown in Figure 1a2,3 and Figure 1b.4,5

Figure 1: O–O bond formation mechanisms proposed for: a. OEC;2,3 b. OEC;4,5 c. [(bda)(isoq)2R u];6 d. [(bda)(isoq)2Ru];7 e. [(tda−κ−N3O)(py)2RuIV(OH−)] + ;8 f. [(L1)CuII(OH−)2];9,10 g. [(L1)CuII (OH−)2];9,10 h. [(OPBAN)CuII]10,11 and [(L22−)CuII(OH2)2];10 i. [(L22−)CuII(OH2)2].


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Development of artificial water oxidation catalysts (WOCs) with competent efficiencies, with both noble and earth-abundant metals, is of crucial importance for generating clean and renewable energy.12,13 Significant progresses have been made for Ru WOCs. [(bda)(iso q)2Ru] (bda2-=2,2’-bipyridine-6,6’-dicarboxylic acid, isoq=isoquinoline) catalyzes water oxidation with a TOF > 300 s-1 with the chemical oxidant cerium ammonium nitrate (overpotential = 529 mV), comparable to the performance of the OEC in PSII.6 Two RuIV–O•moieties approaching each other, facilitated by strong dispersion attraction between the axial isoquinoline ligands, was proposed to be responsible for O–O bond formation promoted by chemical oxidant (Figure 1c). Under electrolysis conditions, O–O bond formation was suggested to occur after dimerization (Figure 1d).7 Even more extraordinary performance was reported for [(tda−κ−N3O)(py)2RuIV(OH−)] + (tda2-=[2,2’:6’,2’‘terpyridine]-6,6’’-dicarboxylate), which catalyzes water oxidation with a TOF of 8000 s-1 at pH 7 under applied potential of 1.25 V vs NHE (overpotential = 434 mV) and 50000 s-1 at pH 10 under the same voltage (overpotential = 611 mV).8 The pendant base effect provided by the dangling carboxylate was proposed to lower the activation barrier of O–O bond formation (Figure 1e). WOCs utilizing earth-abundant metals have been extensively investigated because of economic reasons.12 Copper-based WOCs are especially attractive due to copper’s extensive bio-mimetic chemistry with O2 and its rich oxidation states.9,14–23 Although not observed in biological systems,24,25 higher oxidation states of CuIII and even CuIV have been proposed in organometallic reactions.26,27 Water oxidation activity by simple CuII salts in neutral to weakly basic buffer solutions has been reported at relatively high overpotentials.21 Depending on specific reaction conditions, the actual catalysts were experimentally suggested to be single site or dinuclear copper complexes or even heterogeneous species.21 The mechanism for water oxidation in NaHCO3 solution was further described theoretically, where O–O bond formation occurs between the ligating hydroxide and one bicarbonate radicaloid ligand of the monocopper catalyst.28 The first homogenous Cu WOC, [(L1)CuII(OH−)2] (L1=2,2’-bipyridine), catalyzes water oxidation at an onset overpotential of 750 mV under pH 12.5 and reaches a maximum TOF of 100 s-1 (corresponding to 14.9 kcal/mol) at an overpotential of around 850 mV.9,23 A derived Cu WOC equipped with a redox non-innocent substituted bipyridine ligand, [(L22−)CuII(OH2)2] (H2L2=6,6’dihydroxy-2,2’-bipyridine) functions at an overpotential about 200 mV lower than that of the parent complex, and reaches an apparent TOF of 0.4 s-1 (corresponding to 18.1 kcal/mol) at an overpotential of 640 mV.15 For the latter system, the hydroxy-substituted bipyridine was proposed to lower the overpotential by not only providing a redoxaccessible ligand but also participating in the PCET processes via these hydroxyl moieties. Another related Cu WOC containing 1:2 Cu and 6,6’-dihydroxy-2,2’-bipyridine exhibits a similar TOF of 0.356 s-1 at an even lower overpotential of 477 mV.16 The mechanisms for [( L1)CuII(OH−)2] and [(L22−)CuII(OH2)2] at the molecular level were investigated theoretically recently.10 A step-wise single electron transfer mechanism for O–O bond formation was proposed for both WOCs. For water oxidation catalyzed by [(L1)CuII(OH−)2], a free anionic hydroxide first approaches the CuIII-bound oxyl radical to form a CuIII/II−O•− lig… −/• III/II and its O•- ligand, which is complex with antiferromagnetic coupling between Cu OHfree the rate-limiting step along the entire reaction pathway (Figure 1f). O–O bond is then


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formed with a small barrier according to O–O distance scans on relevant electronic state surfaces. An alternative mechanism of O–O bond formation between the O•- ligand and the hydroxide ligand was computed to be higher in energy (Figure 1g). In the mechanism suggested for [(L22−)CuII(OH2)2], the reaction is similarly rate-limited by the initial step where a free anionic hydroxide approaches one of two CuIII-bound hydroxides to form a C uII−(HOlig…OHfree)•− complex (Figure 1h). Subsequent peroxide bond formation is accomplished with a small barrier according to O–O distance scans. For both Cu WOCs, the barriers obtained are considerably low compared to the experimental TOFs. Lower overpotential compared to [(L1)CuII(OH−)2] has also been achieved with other redox resistent ligand. For example, Cu(pyalk)2 (pyalk=2-pyridyl-2-propanoate) catalyzes water oxidation at an overpotential of 520–580 mV under basic conditions (pH > 10.4) with a TOF of around 0.7 s-1.17 The rate-limiting step of the reaction pathway was suggested to be O–O bond formation through water nucleophilic attack upon the CuIII–O•- species.17,29 Homogeneous dinuclear Cu WOCs have also been developed,18–20 although the performance is generally unsatisfactory, demanding further studies about copper-catalyzed water oxidation reaction mechanisms at the molecular level. In the present study, we have systematically re-investigated mechanisms for water oxidation by the [(L22−)CuII(OH2)2] and [(L1)CuII(OH−)2] WOCs, respectively. Similar to our previous investigations,30–35 efforts were made to search for optimal oxidation and protonation states for intermediates and transition states along the reaction pathways. Two well-established protocols to treat the thermodynamics of proton and electron release were employed. For the critical O–O bond formation, alternative mechanisms with bound intermediates were obtained. Localized molecular orbital analyses were performed, showing additional mechanistic insights.

Methods and models In the present study, the calculations were performed in three steps. Geometry optimizations were first done in solution phase using the SMD model with water as the solvent, using the hybrid density functional B3LYP36–39 including empirical dispersion correction computed with Grimme’s D3 formula (B3LYP-D3) and the 631+G(d)/LANL2TZ(+f) basis sets. Based on the coordinates optimized, frequency calculations were carried out to confirm the nature of equilibrium states and transition states, and to compute Gibbs free energy corrections after replacing frequencies lower than 50 cm−1 with 50 cm−1.40 Lastly, the electronic energies were re-computed with B3LYP*,41 which includes 15% of HF exact exchange, and the much larger cc-pVTZ(-f)/LANL2TZ(+f) basis sets. All calculations were performed in Gaussian.42 To obtain the energetics of the overall water oxidation pathways, two well-established protocols3,43–47 were employed to obtain the energetics of the electron release, proton release, or proton-coupled electron release, respectively, the details of which are given in the supporting information. The chemical models for the two copper complexes are given in the Figure 2. Since for both catalysts the peak current depends linearly on catalyst concentration at lower concentrations,9,15 only the monomeric species have been investigated in the present study.


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Figure 2: Chemical models for the two Cu WOCs studied in the present study. For the two Cu WOCs studied in the present study, the entire reaction pathways, including both the oxidative transitions and the chemical steps of O–O bond formation and O2 release, were investigated. To clarify the discussions of the results, the states along the reaction pathways are denoted with the corresponding ligand labels and the number of oxidations and charge. For example, L200 denotes the resting state for the [(L22−)CuII(OH2)2] WOC, while L2−2 specifies the state after removal of two electrons and three protons from the system. To account for strong polarization in the surroundings of the catalyst, for both Cu WOCs, three explicit water molecules were included in the second-sphere of the starting structures and kept throughout the reaction. These free water molecules were confirmed to bind by larger than 14 kcal/mol.3,48 Furthermore, relative energies are only slightly affected with more free water molecules. Lastly, when free anionic hydroxide is present in the chemical model, explicit water molecules were found necessary to stabilize the hydroxide, consistent with experimental observation of hydroxide hydration.49

Results and discussions The Results and discussions section will be divided into three subsections. In the first subsection, the results for the [(L22−)CuII(OH2)2] WOC will be described. An important aspect of the present study is the systematic exploration of the reaction pathway. For each state along the oxidative transitions, different protonation states and electron configurations were examined for the ligands and the copper center to find the structure with the lowest energy. For isomers with reactive oxyl or hydroxyl radical, transition states for O–O bond formation were optimized, which has not been done before. An interesting O– O bond formation mechanism was identified, where the unprotonated 6-oxyanion substituent on the pyridine plays an important role. The re-investigatation of water oxidation catalyzed by the parent [(L1)CuII(OH−)2] WOC also leads to interesting findings, which are discussed in the second subsection. The results for the two Cu WOCs are finally


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compared to those for natural water oxidation in the OEC and other artificial WOCs in the third subsection.

[(L22−)CuII(OH2)2] (H2L2=6,6’-dihydroxy-2,2’-bipyridine) Two distinct types of water oxidation mechanisms were obtained: Cosubstrate and Hydroxyl coupling, termed by the characteristics of O–O bond formation transition states. The mechanisms with O–O bond formed at L202 are sketched in Figure 3, emphasizing different reactive species at L202 and corresponding transition states for O–O bond formation. The corresponding energy diagrams are given in Figure 4.

Figure 3: Mechanisms for water oxidation by the [(L22−)CuII(OH2)2] WOC, with O–O bond formed at L202. Second-sphere water molecules are omitted for clarity. Transition states are marked with top-right double dagger sign. CuII and CuIII are shown within blue and magenta circles, respectively. Labels and values are shown in the same colors as in the energy diagrams.


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Figure 4: Energy diagram for water oxidation by the [(L22−)CuII(OH2)2] WOC at pH 12.4 and applied potential = 1.137 V, with values obtained using the protocol 1. Black: oxidative transitions before O–O bond formation; Red: the Cosubstrate mechanism, with organic peroxide formation at L202 and inorganic superoxide formation at L203. Blue: the Hydroxyl coupling mechanism, with inorganic peroxide formation at L202.

Cosubstrate mechanism, with organic peroxide formation at L202 and inorganic superoxide formation at L203 The Cosubstrate mechanism will be discussed first, with the energy diagram given in Figure 4 in black and red. The starting structure is the charge neutral [(HL2−)CuII(OH−)(O H2)] species at L200. The ligating hydroxide and water of CuII form strong hydrogen bonds (O…O < 2.5 Å) with the hydroxyl and unprotonated oxyanion substituents on the bipyridine, respectively. Even though the anionic conjugate base was instead suggested to be dominant based on titration,15 where excess (>20 equiv) of base was employed to generate a fully dissolved solution, it is 1.6 kcal/mol higher in energy than the neutral conjugate acid at pH 12.4. The first oxidative transition from the L200 state is removal of a (e-,H+)-couple, leading to the [(L2•−)CuII(OH−)(OH2)] species at an energy of -3.0 kcal/mol. The oxidation occurs for the substituted bipyridine rather than for the copper, leading to degenerate singlet and triplet states. The calculated reduction potential Ered[L2•−|HL2−] at pH 12.4 is 1.00 V, consistent with the experimental observation of ligand oxidation at ~1.12 V between pH 12.8 and 9.0.15 The second oxidative transition from the L201 state is removal of another (e-,H+)-couple to reach the doublet [(L2•−)CuIII(OH−)2] species at an energy of 3.5 kcal/mol. This oxidative transition has a large endergonicity of 6.5 = (3.5 - -3.0) kcal/mol,


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which means E[redCuIII|CuII] = 1.42 V at pH 12.4. The calculated difference between the first two reduction potentials of 0.42 V is roughly consistent with the experimental value of 0.17 = (1.35 - 1.18) V measured in the DMF/H2O mixed solvent.15,50 Structural isomer with one Cu–Npy coordination broken is higher in energy by 9.7 kcal/mol, because of interruption of hydrogen bonds between the hydroxide ligands of copper and the unprotonated oxyanion groups on the bipyridine. The lowest energy reactive complex at L202 is obtained by moving the proton of one ligating hydroxide to the other one in the [(L2•−)CuIII(OH−)2] species, leading to the doublet [(L2•−) CuII(O•−)(OH2)] species with a terminal oxyl radical ligand bound ferromagnetically to CuII (spin populations on atoms with radical characteristic are given in the supporting information), lying at an energy of 10.5 kcal/mol. The other component for the doublet state, with antiferromagnetic coupling between the oxyl radical and CuII, is slightly higher in energy. The lowest energy transition state for O–O bond formation is coupling between the oxyl radical ligand and its adjacent oxyanion substituent on the pyridine. As shown in Figure 5a, the O–O bond distance decreases from 2.57 Å to 1.96 Å, showing the partial formation of the peroxide bond. The Opy–C6py distance increases from 1.26 Å to 1.29 Å, and the Cu–N distance decreases from 2.05 Å to 1.98 Å. A unique imaginary frequency of 770.3 i cm-1 was confirmed. The up-spin population on the oxyl radical ligand decreases from 1.08 to 0.70, and the sum of down-spin populations on the substituted bipyridine decreases from 0.96 to 0.54, showing the partial electron transfer at the transition state. As depicted in Figure 5b for localized Pipek-Mezey localized MOs,51 α LMOe shows the α electron transfer from the unprotonated oxyanion to the unoccupied π orbital on the substituted bipyridine radicaloid, while α LMOO–O and β LMOO–O show the α and β electron share between the bonding oxygen sites. It can be noted that the oxyl radical ligand stays out of the pyridine plane to make α LMOe and α LMOO–O compatible in symmetry (the O–O–C6–N dihedral angle is 36°). The compositions of the localized MOs show that the α electron transfer and the α and β electron sharing are ongoing at the transition state. The TS is at an energy of 18.1 kcal/mol, which means an overall barrier of 21.1 = (18.1 - -3.0) kcal/mol. Organic peroxide intermediate is formed with a large exergonicity of 12.6 = (-2.1 - 10.5) kcal/mol. Water attack transition state for immediate isomerization into an inorganic peroxide at L202 has an overall barrier of 26.8 = (23.8 - -3.0) kcal/mol.


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Figure 5: The coupling transition state for O–O bond formation at L202, reached from the [(L 2•−)CuII(O•−)(OH2)] species. Pipek-Mezey localized MOs51 relevant to O–O bond formation, prepared using the Multiwfn52 and VMD53 packages, are shown in the lower panel.


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The third oxidative transition from the organic peroxide intermediate at L202 is removal of a (e-,H+)-couple, leading to a [(OO−L2•)CuII(OH−)] species with a peroxyl substituent, at an energy of -1.4 kcal/mol. The electron is removed from the pyridine not bearing the peroxyl moiety, the proton from the water ligand. This oxidative transition is endergonic by 0.7 kcal/mol. To displace the peroxyl substituent into an inorganic superoxide, another [(OO−L2•)CuII(O H2)] + species with an adjacent free anionic hydroxide is formed at an energy of 14.8 kcal/mol. The free hydroxide then attacks the C6py that bears the peroxyl group with an overall barrier of 24.0 = (21.0 - -3.0) kcal/mol, which is the rate-limiting step in the Cosubstrate mechanism. The overall barrier of 24.0 kcal/mol is thus 5.9 kcal/mol higher than the value of 18.1 kcal/mol corresponding to the experimental TOF of 0.4 s-1, beyond the typical errors of 3-5 kcal/mol of the present methodology. The reason for the discrepancy from the experimental kinetics could be attributed to accumulated endergonicities of oxidative transitions. As shown in the supporting information, for releasing each (e-,H+)-couple, the endergonicity calculated using the protocol 1 is larger than that using the protocol 2 by 2.4 kcal/mol, showing some uncertainty in the calculated barrier. As shown in Figure 6a for the transition state, the HO–C6py distance decreases to 2.00 Å, showing the formation of a fourth bond for C6py. The planarity of C6py is slightly distorted, showing the aromaticity of the pyridine attacked is being undermined at the TS. A unique imaginary frequency of 343.1 i cm-1 was confirmed. The Mulliken spin populations on the substituted bipyridine reside mainly on the pyridine attacked due to the broken aromaticity. The attacking hydroxide has a spin population of 0.31 at the TS. In the intermediate state, the sp3 C6py is fully formed. This step is endergonic by 11.4 kcal/mol counted from the [(OO−L2•)CuII(OH−)] species.


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Figure 6: The transition state for HO-–C6py bond formation at L203, reached from the [(OO•L2−) CuII(OH2)] + species with a free anionic hydroxide, and the subsequent transition state for O22–C6py bond cleavage.


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The inorganic superoxide is formed through O22-–C6py bond cleavage with an overall barrier of 19.5 = (16.5 - -3.0) kcal/mol. The transition state optimized is shown in Figure 6b. The O22-–C6py distance increases from 1.41 Å to 1.59 Å, showing the partial dissociation of the peroxyl moiety. The O–O distance decreases from 1.48 Å to 1.36 Å, showing the formation of superoxide upon O22-–C6py bond cleavage. A unique imaginary frequency of 1228.4 i cm-1 was confirmed. The sum of Mulliken spin populations on the dearomatized pyridine decreases from 1.05 to 0.41 , while the value on the peroxyl group increases from 0.14 to 0.79, showing the ongoing electron transfer from the peroxyl to the pyridine radicaloid upon the superoxide formation. In the intermediate state, a CuII-bound inorganic superoxide is formed and the aromaticity of pyridine is restored. The exergonicity is as large as 17.6 kcal/mol counted from the [(OO−L2•)CuII(OH−)] species, showing the low reduction potential of O2•-|O22-. The fourth oxidative transition completes the four-electron chemistry of water oxidation. Triplet oxygen molecule is formed from the CuII-bound superoxide. This step is exergonic by as large as 27.3 kcal/mol, showing the small reduction potential of O2|O2•-. It can be shown that two out of the total four oxidative transitions occur after O–O bond formation. The direct involvement of the ligand in the O–O bond formation is interesting but unexpected experimentally. Instead, water (hydroxide) was suggested to be the source of oxygen, based on the resemblance between UV–vis and NMR spectra collected before and after electrolysis, respectively.15 Nevertheless, as discussed above, the dihydroxyl bipyridine ligand is restored at the end of the Cosubstrate mechanism. Therefore, a realtime monitoring of the water oxidation reaction by UV–vis and/or NMR spectra would be necessary to test against the Cosubstrate mechanism. For comparison, the UV–vis spectra have been calculated for the long-lived species based on the energy diagram, which are given in the supporting information. Note that the spectra for [(HL2−)CuII(OH−)(OH2)] at L 200 is rather similar to the experimental spectra for the starting species. Furthermore, the spectra for [(OO−L2−)CuII(OH2)] at L202 is also similar to the experimental spectra. However, the spectra for [(OO−L2•)CuII(OH−)] at L203 has a much diminished intensity, which could be used as a criteria to test against. Isotope labeling, helpful to discriminate between the mechanisms but not carried out in the original experimental study, is discussed in the third subsection.

Hydroxyl coupling mechanism, with inorganic peroxide formation at L202 The Hydroxyl coupling mechanism sketched in Figure 3 will be discussed next, with the energy diagram given in Figure 4 in black and blue. Starting from the lowest energy reactive [(L2•−)CuII(O•−)(OH2)] species with a CuII-bound terminal oxyl radical ligand, an inorganic peroxide bond could be formed by a nucleophilic water attack mechanism. However, the transition state approximated has a barrier of ~41 = (~38 - -3.0) kcal/mol. Because the attacking water should be oriented for both O–O bond formation and proton transfer at the same time, the coordination of copper and the hydrogen bonding network become distorted at the TS. Furthermore, the oxidant to accept an electron from the attacking water during O–O bond formation is L2•−, less powerful than


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CuIII, which also contributes to the high barrier. However, other solutions at L202 with a CuIII-bound terminal oxyl radical ligand were computed to be much higher in energy, raising barriers of O–O bond formation to be even higher. For example, the [(HL2−)CuIII(O H−)(O•−)] species, obtained by moving the proton of one ligating hydroxide in the [(L2•−)C uIII(OH−)2] species to the distant unprotonated oxyanion substituent on the pyridine, has an energy of 16.0 kcal/mol. Another type of oxygen radical at L202 can be obtained by moving one proton from a second-sphere water molecule to the terminal oxyl radical ligand in the [(L2•−)CuII(O•−)(O H2)] species, leading to the doublet {(L2•−)CuII[(OHlig…OHfree)•−](OH2)} species. The component for the doublet state with ferromagnetic coupling between the (OHlig…OHfree)•moiety and CuII is at an energy of 13.3 kcal/mol. The other component with antiferromagnetic coupling is similar in energy, showing that the coupling is weak. As shown in Figure 7a, the O–O distance in (OHlig…OHfree)•- is as short as 2.20 Å, resembling the immediate O–O bond cleavage intermediate in H2O2 disproportionation catalyzed by manganese catalases.54,55 Two second-sphere water molecules, together with the unprotonated oxyanion groups on the bipyridine, form an extensive hydrogen bonding network that stabilizes the (OHlig…OHfree)•- moiety. As shown in the figure, there is a significant distribution of up-spin population within the (OHlig…OHfree)•- moiety. Nevertheless, the sum of Mulliken atomic spin populations on L2•− is essentially the same as observed for the [(L2•−)CuIII(OH−)2] species or the [(L2•−)CuII(O•−)(OH2)] species, showing that the α electron transfer to L2•− during O–O bond formation has not occurred yet. As shown in Figure 7b for the Pipek-Mezey localized MOs, α LMO-2 and α LMO-20 are indeed isolated pO orbitals entirely located on the oxygen sites, respectively. Nevertheless, the composition of β LMO-3 shows that the β electron sharing between oxygen sites in O–O bond formation is essentially completed.


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Figure 7: The {(L2•−)CuII[(OHlig…OHfree)•−](OH2)} species at the L202 state. Starting from the {(L2•−)CuII[(OHlig…OHfree)•−](OH2)} species, an inorganic O–O bond could be formed by coupling within the (OHlig…OHfree)•- moiety. As shown in Figure 8a, the O–O distance in (OHlig…OHfree)•- decreases from 2.20 Å at the reactant state to 1.84 Å. In the


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vibrational mode corresponding to the unique imaginary frequency of 5189.3 i cm-1, the ligating hydroxyl transfers its proton to the adjacent unprotonated hydroxyl group on the bipyridine, while the hydroxyl moieties of (OHlig…OHfree)•- approach each other. The sum of the down-spin populations on L2•− decreases from 1.02 to negligible, while the sum of the up-spin populations on (OHlig…OHfree)•- decreases from 1.13 to 0.46, showing the significant electron redistribution at the transition state. As shown in Figure 8b for the localized MOs, α LMO and α LMO-23 show that the two hydroxyl moieties contribute collectively to the α electron transfer to the unoccupied π orbital on the substituted bipyridine. β LMO-22 retains the same composition as at the reactant state, but lies at a much lower energy level. The transition state is at an energy of 21.3 kcal/mol (Figure 4), which means an overall barrier of 24.3 kcal/mol. In the intermediate of O–O bond formation, a CuII-bound monoprotonated peroxide is formed at an energy of -1.7 kcal/mol.


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Figure 8: The coupling transition state for O–O bond formation, reached from the {(L2•−)CuII[( OHlig…OHfree)•−](OH2)} species at the L202 state. The third oxidative transition from the complex with an inorganic peroxide is removal of a (e-,H+)-couple, leading to a CuII-bound inorganic superoxide at an energy of -19.0 kcal/mol, joining the Cosubstrate mechanism. For completeness, the possibility to lose one additional proton before O–O bond formation was investigated. The schemes for the deprotonated Cosubstrate and Hydroxyl coupling mechanisms, as well as corresponding energy diagrams and detailed discussions, are given in the supporting information. The main findings are (1) the barrier for O–O bond formation is 5.2 kcal/mol lower in the deprotonated Cosubstrate mechanism than in the Hydroxyl coupling counterpart, and (2) both mechanisms have very similar rate-limiting barriers, 25.5 kcal/mol for the deprotonated Cosubstrate mechanism and 25.8 kcal/mol for the Hydroxyl coupling counterpart. It can be added that the relative energetics between the two distinct types of mechanisms remain with different values of the adjustable parameter in the protocol 1, which is verified in the supporting information. Relative energies were also computed using the protocol 2, with the energy diagram and discussions given in the supporting information. Compared to the findings obtained using the protocol 1, the main difference is that the overall rate-limiting barriers for Cosubstrate and Hydroxyl coupling mechanisms become 22.4 = (13.7 - -8.7) kcal/mol and 25.2 = (16.5 - -8.7) kcal/mol, respectively, both counted from the [(OO−L2•)CuII(OH−)] species at L203 along the Cosubstrate pathway.

[(L1)CuII(OH−)2] (L1=2,2’-bipyridine) The mechanisms investigated for water oxidation catalyzed by [(L1)CuII(OH−)2] are sketched in Figure 9, with the energy diagram given in Figure 10. The relative energies shown were computed using the protocol 1 with an overpotential of 850 mV, corresponding to the maximum TOF of 100 s-1 (corresponding to 14.9 kcal/mol). For the adjustable parameter in the protocol 1, the value was chosen so that the proton affinities of bulk surroundings for both WOCs are the same, given their very similar reaction solution pH. The main difference from the recent theoretical study is that O–O bond formation occurs at L12+ rather than at L102.


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Figure 9: Mechanisms for water oxidation catalyzed by the [(L1)CuII(OH−)2] WOC. Transition states are marked with double dagger sign. CuII and CuIII are shown within blue and magenta circles, respectively. Second-sphere water molecules are omitted for clarity.


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Figure 10: Energy diagram for water oxidation catalyzed by the [(L1)CuII(OH−)2] WOC at pH 12.5 and applied potential = 1.342 V, with values obtained using the protocol 1. Black: oxidative transitions; Red: bound mechanism for O–O bond formation at L12+ , via coupling between the hydroxide ligand and the oxyl radical ligand; Blue: bound mechanism for O–O bond formation at L102, via coupling between one ligating hydroxide and the oxyl radical ligand; Orange: unbound mechanism for O–O bond formation at L102, via coupling between the free hydroxide and the ligating oxyl radical. CuII and CuIII are shown within blue and magenta circles, respectively. The starting structure for water oxidation is the charge neutral [(L1)CuII(OH−)2] species, consistent with the experimental assignment.9,56 The first oxidative transition from L100 is removal of an electron, leading to the singlet [(L1)CuIII(OH−)2] + species at an energy of 0.0 kcal/mol. The oxidation occurs for the copper center. It should be noted that the exergonicity is dependent on the adjustable parameter in the protocol 1. The second oxidative transition from L11+ is removal of a (e-,H+)-couple, leading to the [(L1)CuIII(OH−)( O•−)] + species at L12+ at an energy of 9.2 kcal/mol. Both electron and proton are taken from one hydroxide ligand of CuIII, leading to a CuIII-bound terminal oxyl radical ligand. For O–O bond formation at L12+ , the coupling TS with structural similarities to that reported in the recent theoretical study was obtained. The local barrier of 6.5 kcal/mol is also similar to the literature value of 7.0 kcal/mol. The overall barrier of 15.7 kcal/mol is 0.8 kcal/mol higher than the value of 14.9 kcal/mol corresponding to experimental TOF of 100 s-1.


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For O–O bond formation at L102, the mechanism utilizing the free hydroxide, suggested in the recent theoretical study, was first optimized. It should be noted that the energies are much higher without the explicit water molecules included in the present study. The TS for hydroxide flipping has a barrier of 19.2 kcal/mol, while the subsequent peroxide bond formation via electronic state crossing has an approximated barrier of 17 kcal/mol. Another bound mechanism for O–O bond formation at L102 was investigated, since in the present study the lowest species at L102 was found to be the [(L1)CuIII(OH−)2(O•−)] species at an energy of 10.5 kcal/mol. The TS for O–O bond formation through coupling between the oxyl radical ligand and one of the two hydroxide ligands of CuIII is at an energy of 20.3 kcal/mol. The water attack TS for O–O bond formation is further higher in energy by 5.0 kcal/mol. It can be added that the findings essentially remain with relative energies obtained using the protocol 2, which is given in the supporting information.

Comparison to the OEC in the PSII and other artificial WOCs The most important finding in the present study is that [(L22−)CuII(OH2)2] may operate through two distinct types of mechanisms: Cosubstrate and Hydroxyl coupling. Even though the O–O bond formation step is faster in the Cosubstrate mechanism than in the Hydroxyl coupling mechanism, the rate-limiting steps have rather similar activation energy barriers. Experimental information is therefore necessary to discriminate between the mechanisms. Spectroscopy techniques, when performed in a real-time fashion, can characterize important reaction intermediates, thereby providing straightforward evidence to support or rule out a proposed reaction mechanism. For the two competing mechanisms studied, the [(OO−L2−)CuII(OH2)] species at L202 and the [(OO−L2•)CuII(OH−)] species at L203, both equiped with an organic peroxide moiety, are in particular attractive. As shown in Figure 4 for the energy diagram, both of them are sandwiched between two transition states with high activation barriers and therefore could be considered as long-lived representatives for the Cosubstrate mechanism. UV-vis spectra, calculated for important reaction intermediates including the above two species, are provided in the supporting information to be checked against. Isotope labeling is informative in discriminating between plausible mechanisms. Kinetic isotope effect, which is the ratio of rates observed for reactions where atoms relevant to bond cleavage/formation are substitued by corresponding isotopes,57 was examined for the two competing mechanisms studied. Assuming the oxidative transitions and the isomeric proton transfers at each oxidation state are so fast that the reaction kinetics could be determined by the energy diagram given in Figure 4, we have calculated solvent 1H|2H, solvent 16O|18O, and catalyst 16O|18O KIEs. The calculated solvent 1H|2H KIEs are 2.0 for the Cosubstrate mechanism and 2.6 for the Hydroxyl coupling mechanism. The value of 2.0 for the Cosubstrate mechanism shows that even the thermodynamics of releasing of (e,H+)-couples in the oxidative transitions contributes to the KIE. The slightly larger value of 2.6 for the Hydroxyl coupling mechanism could be attributed to the involvement of proton


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transfer in the rate-limiting O–O bond formation transition state. On the other hand, the calculated solvent 16O|18O KIEs are 0.943 for the Cosubstrate mechanism and 0.904 for the Hydroxyl coupling mechanism. Inverse solvent 16O|18O KIEs have been similarly observed in the electrochemical water oxidation by heterogenous iron oxide,58 where the inverse KIE suggests that O–O bond formation is rate-limiting rather than the oxidative transition to form the reactive ferryl oxo species. Statistical isotope abundances in the product O2 were further evaluated to aid in future verification of the involvement of 6/6’-hydroxyl substituent of 2,2’-bipyridine in O–O bond formation in the Cosubstrate mechanism. As shown in Table 1, for the reaction that starts from unlabelled catalyst and water with 15% 18O, the relative abundances of 16O16O, 16O18 O, and 18O18O after one run are 0.85, 0.00, and 0.15, respectively, for the Cosubstrate mechanism, and 0.72, 0.26, and 0.02, respectively, for the Hydroxyl coupling mechanism. On the other hand, as shown in Table 2, for the reaction that starts from 18O-labeled catalyst and unlabeled water, the relative abundances of 16O16O, 16O18O, and 18O18O after one run are 0.00, 1.00, and 0.00, respectively, for the Cosubstrate mechanism, and 1.00, 0.00, and 0.00, respectively, for the Hydroxyl coupling mechanism. Therefore, isotope abundance experiment with the latter starting condition would provide straightforward evidence to support or rule out the Cosubstrate mechanism proposed. Table 1: Statistical isotope abundances in the product O2 after one run of water oxidation reaction, which starts from unlabelled catalyst and water with 15% 18O. product Cosubstrate * Hydroxyl coupling* 16 16 0.72 O O 0.85 16 18

O O

0.00

0.26

18 18

0.02 O O 0.15 Table 2: Statistical isotope abundances in the product O2 after one run of water oxidation reaction, which starts from 18O-labeled catalyst and unlabeled water.

product Cosubstrate * Hydroxyl coupling* 16 16 1.00 O O 0.00 16 18

1.00

0.00

18 18

0.00

0.00

O O O O

The coexistence of two water oxidation mechanisms results in an interestingly delicate relationship between the applied potential and the overall barrier for the [(L22−)CuII(OH2)2 ] WOC. As shown in Figure 4 for the energy diagram, the overall barriers for the ratelimiting steps in the Cosubstrate mechanism and the Hydroxyl coupling mechanism are 24.0 kcal/mol and 24.3 kcal/mol, respectively. When the applied potential is increased by 3.0 kcal/mol (or 0.13 V), compared to the energies in Figure 4, L201 goes down by 3.0 kcal/mol to -6.0 kcal/mol. Similarly, [(L2•−)CuIII(OH−)2] at L202 descends by 6.0 = 3.0 * 2 kcal/mol to -2.5 kcal/mol, since it is oxidized twice from the starting species. [(OO−L2•)CuII (OH−)] at L203 lowers by 9.0 = 3.0 * 3 kcal/mol to -10.4 kcal/mol. Because O–O bond


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formation is more than two hundred times faster in the Cosubstrate mechanism than in the Hydroxyl coupling mechanism (an energy difference of 3.2 kcal/mol between O–O TSs), the overall barriers for the rate-limiting steps for both mechanisms should be counted from [(OO−L2•)CuII(OH−)] at L203. Consequently, with the applied potential increased by 3.0 kcal/mol, the overall barrier for the Cosubstrate mechanism decreases by 1.6 kcal/mol to 22.4 kcal/mol, while the value for the Hydroxyl coupling mechanism increases by 1.4 kcal/mol to 25.7 kcal/mol. It should be added that the overall barrier for the Cosubstrate mechanism will not decrease further even if the applied potential is increased even more. The relationship between the applied potential and the overall barrier for the [(L1)CuII(O H−)2] WOC is more straightforward than for [(L22−)CuII(OH2)2]. With the applied potential increased by 3.0 kcal/mol, as shown in the corresponding diagram given in the supporting information, the overall barriers for O–O bond formation TSs at L12+ and L102 both decrease by 3 kcal/mol, respectively. Both [(L22−)CuII(OH2)2] and the parent [(L1)CuII(OH−)2] WOC exhibit pH-dependence of the rate-limiting barriers, as similarly observed in other Cu WOCs.20 As shown in Figure 4 for the [(L22−)CuII(OH2)2] WOC, the rate-limiting TS in the Cosubstrate mechanism has an overall barrier of 24.0 kcal/mol, which includes the endergonicity of 6.5 = (3.5 - -3.0) kcal/mol for releasing the second (e-,H+)-couple. Similarly, as shown in Figure 10 for the parent [(L1)CuII(OH−)2] WOC, the overall barrier of 15.7 kcal/mol includes the endergonicity of 9.2 kcal/mol for releasing the second (e-,H+)-couple. Consequently, adjustment of the surroundings pH at the same external potential changes the overpotential, which in turn affects the endergonicity for a pH-dependent oxidation and the overall barrier. Compared to [(L1)CuII(OH−)2] with unsubstituted bipyridine,9 [(L22−)CuII(OH2)2] with 6,6’dihydroxyl-bipyridine lowers the overpotential for water oxidation by about 200 mV,15 which has been attributed to the redox non-innocence of substituted bipyridine.15,59 Result in the present investigation further elaborates the effect of 6,6’-dihydroxyl substituents on water oxidation reaction. As shown in Figure 4 and Figure S2 for the energy diagrams for [( L22−)CuII(OH2)2], the endergonicities for creating supported CuIII, CuII(O•−), and CuIII(O•−) moieties are 6.5 = (3.5 - -3.0) kcal/mol, 13.5 = (10.5 - -3.0) kcal/mol, and 12.8 = (9.8 - -3.0) kcal/mol, respectively, at the potential of 1.137 V at pH 12.4 (η = 640 mV). As shown in Figure 10 for the parent [(L1)CuII(OH−)2] WOC, the endergonicities for creating supported CuIII and CuIII(O•−) moieties are 0.0 kcal/mol and 9.2 kcal/mol, respectively, at the potential of 1.342 V at pH 12.5 (η = 850 mV), which corresponds to 4.8 kcal/mol and 18.8 kcal/mol, respectively, at the overpotential of 640 mV, showing the significant role of the hydroxylated bipyridine to stablize reactive species such as copper-bound terminal oxyl radical ligand at higher oxidation states. However, the introduction of 6,6’-dihydroxyl substituents results in a slower TOF.15 As discussed above, the coexistence of Cosubstrate and Hydroxyl coupling mechanisms imposes a minimum overall barrier of 22.4 kcal/mol even at higher applied potential. Our results further suggest that the hydroxylated bipyridine could directly participate in O– O bond formation. The involvement of pyridine-derived ligand framework in O–O bond


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formation has been suggested.59–61 For one case,60 free water molecule was suggested to attack an unsubstituted bipyridine ring, which then triggers the attaching of a second free water molecule. For the other study, a N–O moiety was proposed to promote the critical O– O bond formation.61 However, the entire pathway was not investigated, which could be crucial since the departure of the O2 molecule could be rate-limiting, as shown in the present study. Consequently, the specific bond formation pattern identified in the present study should provide important mechanistic insights. As shown in Figure 3 and Figure 4, and Figure S1 and Figure S2 for the [(L22−)CuII(OH2)2] WOC, O–O bond could be formed through coupling between the copper-bound terminal oxyl radical ligand and its adjacent unprotonated oxyanion substituent, whether the other redox active partner is the bipyridine radicaloid or CuIII. More importantly, computation of the entire water oxidation pathway reveals the difficulty in transforming the organic peroxyl moiety into the final product oxygen molecule.

Conclusions In the present study, hybrid density functional theory was used to systematically reinvestigate the water oxidation pathways catalyzed by two homogeneous copper water oxidation catalysts, [(L22−)CuII(OH2)2] (H2L2=6,6’-dihydroxy-2,2’-bipyridine), which catalyzes water oxidation with a TOF of 0.4 s-1 at overpotential = 640 mV and pH 12.4, and the parent [(L1)CuII(OH−)2] (L1=2,2’-bipyridine) complex, which catalyzes water oxidation with a TOF of 100 s-1 at overpotential = 850 mV and pH 12.5. Efforts were made to systematically explore the potential energy surface by examining constitutional isomers and electronic configurations for states along the reaction pathway. Two well-established protocols to obtain the energetics of the electron release, proton release, and protoncoupled electron release processes, respectively, were employed. For the [(L22−)CuII(OH2)2] WOC, two distinct types of water oxidation mechanisms with similar rate-limiting barriers were identified, the Cosubstrate mechanism and the Hydroxyl coupling mechanism, termed according to the characteristics of O–O bond formation intermediates. Based on the relative energies obtained using the protocol 1, in the Cosubstrate mechanism, the reactive species for O–O bond formation are likely to be the charge neutral [(L2•−)CuII(O•−)(OH2)] complex, reached after two (e-,H+)-couples are removed from the starting structure, and its conjugate base [(L22−)CuIII(OH−)(O•−)]− with one negative charge. O–O bond formation occurs between the copper-bound terminal oxyl radical ligand and its adjacent unprotonated oxyanion substituent, suggesting an interesting mechanistic role for the electron-donating proton-responsive hydroxyl substituent. The remaining states along the reaction pathways are in general with neutral charge. After the third oxidation of the system, the organic peroxyl moiety is displaced into an inorganic superoxide by a free anionic hydroxide through an aromatic substitution scheme, which is the rate-limiting step in the Cosubstrate mechanism. The overall barrier of 24.0 kcal/mol is 5.9 kcal/mol higher than the value of 18.1 kcal/mol corresponding to the experimental TOF of 0.4 s-1. In the Hydroxyl coupling mechanism, the reactive species for O–O bond formation is the charge neutral {(L2•−)CuII[(OHlig…OHfree)•−](OH2)}, reached after two (e-,H+)-couples are removed from the starting species. O–O bond is formed


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through coupling within the (OHlig…OHfree)•- moiety, with hydrogen bonding stabilization from the ligating and second-sphere water molecules and the 6,6’-oxyanions. Interestingly, even though the direct participation of the substituted bipyridine in the Cosubstrate mechanism lowers the barrier of O–O bond formation transition state, a second-sphere hydroxide is involved in the rate-limiting step in both types of mechanisms, different from the natural water oxidation in the OEC where the critical substrate derivatives are always bound. As shown in the diagrams, the introduction of the free hydroxide anion is disadvantageous in energy and thus contributes to the high activation barriers. For the parent [(L1)CuII(OH−)2] WOC, the reactive species was suggested to be the positively-charged [(L1)CuIII(OH−)(O•−)] + species, reached after two electrons and one proton are removed from the starting species. O–O bond is formed through coupling between the oxyl radical and the hydroxide anion, both ligating the planar CuIII center. The overall barrier of 15.7 kcal/mol is 0.8 kcal/mol higher than the value of 14.9 kcal/mol corresponding to the experimental TOF of 100 s-1. The mechanism where a free anionic hydroxide flips to approach the CuIII-bound oxyl radical to form O–O bond has a higher barrier of 19.2 kcal/mol, even with water molecules explicitly included to stabilize the free hydroxide with donating hydrogen bonds.

Acknowledgements We thank Prof. Per E M Siegbahn and Prof. Feliu Maseras for valuable discussions. This work was supported by grants from the National Natural Science Foundation of China (Grant 21503018, 21571019, 21573020), the Knut and Alice Wallenberg Foundation, and the Swedish Research Council. Computer time was provided by the Swedish National Infrastructure for Computing.

Supporting information The supporting information contains energies, coordinates and figures for the structures in the figures and diagrams.

References

(1) Umena, Y.; Kawakami, K.; Shen, J.-R.; Kamiya, N. Crystal Structure of Oxygen-Evolving Photosystem II at a Resolution of 1.9Å. Nature 2011, 473, 55. (2) Siegbahn, P. E. M. O–O Bond Formation in the S4 State of the Oxygen-Evolving Complex in Photosystem Ii. Chemistry – A European Journal 2006, 12, 9217–9227. (3) Siegbahn, P. E. M. Water Oxidation Mechanism in Photosystem II, Including Oxidations, Proton Release Pathways, O―O Bond Formation and O2 Release. Biochimica et Biophysica Acta (BBA) - Bioenergetics 2013, 1827, 1003–1019.


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(4) Corry, T. A.; O’Malley, P. J. Evidence of O–O Bond Formation in the Final Metastable S3 State of Nature’s Water Oxidizing Complex Implying a Novel Mechanism of Water Oxidation. The Journal of Physical Chemistry Letters 2018, 9, 6269–6274. (5) Pushkar, Y.; Davis, K. M.; Palenik, M. C. Model of the Oxygen Evolving Complex Which Is Highly Predisposed to O–O Bond Formation. The Journal of Physical Chemistry Letters 2018, 9, 3525–3531. (6) 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. Nature Chemistry 2012, 4, 418. (7) Concepcion, J. J.; Zhong, D. K.; Szalda, D. J.; Muckerman, J. T.; Fujita, E. Mechanism of Water Oxidation by [Ru(bda)(L)2]: The Return of the “Blue Dimer”. Chemical Communications 2015, 51, 4105–4108. (8) 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. Journal of the American Chemical Society 2015, 137, 10786–10795. (9) Barnett, S. M.; Goldberg, K. I.; Mayer, J. M. A Soluble Copper–Bipyridine Water-Oxidation Electrocatalyst. Nature Chemistry 2012, 4, 498. (10) Funes-Ardoiz, I.; Garrido-Barros, P.; Llobet, A.; Maseras, F. Single Electron Transfer Steps in Water Oxidation Catalysis. Redefining the Mechanistic Scenario. ACS Catalysis 2017, 7, 1712–1719. (11) Garrido-Barros, P.; Funes-Ardoiz, I.; Drouet, S.; Benet-Buchholz, J.; Maseras, F.; Llobet, A. Redox Non-Innocent Ligand Controls Water Oxidation Overpotential in a New Family of Mononuclear Cu-Based Efficient Catalysts. Journal of the American Chemical Society 2015, 137, 6758–6761. (12) Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W. Molecular Catalysts for Water Oxidation. Chemical Reviews 2015, 115, 12974–13005. (13) Liao, R.-Z.; Siegbahn, P. E. M. Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis. ChemSusChem 2017, 10, 4236–4263. (14) Ghosh, T.; Ghosh, P.; Maayan, G. A Copper-Peptoid as a Highly Stable, Efficient, and Reusable Homogeneous Water Oxidation Electrocatalyst. ACS Catalysis 2018, 8, 10631– 10640. (15) Zhang, T.; Wang, C.; Liu, S.; Wang, J.-L.; Lin, W. A Biomimetic Copper Water Oxidation Catalyst with Low Overpotential. Journal of the American Chemical Society 2014, 136, 273– 281. (16) Gerlach, D. L.; Bhagan, S.; Cruce, A. A.; Burks, D. B.; Nieto, I.; Truong, H. T.; Kelley, S. P.; Herbst-Gervasoni, C. J.; Jernigan, K. L.; Bowman, M. K.; Pan, S.; Zeller, M.; Papish, E. T. Studies of the Pathways Open to Copper Water Oxidation Catalysts Containing Proximal


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Hydroxy Groups During Basic Electrocatalysis. Inorganic Chemistry 2014, 53, 12689– 12698. (17) Fisher, K. J.; Materna, K. L.; Mercado, B. Q.; Crabtree, R. H.; Brudvig, G. W. Electrocatalytic Water Oxidation by a Copper(II) Complex of an Oxidation-Resistant Ligand. ACS Catalysis 2017, 7, 3384–3387. (18) Su, X.-J.; Gao, M.; Jiao, L.; Liao, R.-Z.; Siegbahn, P. E. M.; Cheng, J.-P.; Zhang, M.-T. Electrocatalytic Water Oxidation by a Dinuclear Copper Complex in a Neutral Aqueous Solution. Angewandte Chemie International Edition 2015, 54, 4909–4914. (19) Dong, Z.; Cramer, H. H.; Schmidtmann, M.; Paul, L. A.; Siewert, I.; Müller, T. Evidence for a Single Electron Shift in a Lewis Acid–Base Reaction. Journal of the American Chemical Society 2018. (20) Koepke, S. J.; Light, K. M.; VanNatta, P. E.; Wiley, K. M.; Kieber-Emmons, M. T. Electrocatalytic Water Oxidation by a Homogeneous Copper Catalyst Disfavors Single-Site Mechanisms. Journal of the American Chemical Society 2017, 139, 8586–8600. (21) Chen, Z.; Meyer, T. J. Copper(II) Catalysis of Water Oxidation. Angewandte Chemie International Edition 2013, 52, 700–703. (22) Terao, R.; Nakazono, T.; Parent, A. R.; Sakai, K. Photochemical Water Oxidation Catalyzed by a Water-Soluble Copper Phthalocyanine Complex. ChemPlusChem 2016, 81, 1064–1067. (23) Shen, J.; Wang, M.; Gao, J.; Han, H.; Liu, H.; Sun, L. Improvement of Electrochemical Water Oxidation by Fine-Tuning the Structure of Tetradentate N4 Ligands of Molecular Copper Catalysts. ChemSusChem 2017, 10, 4581–4588. (24) Solomon, E. I.; Heppner, D. E.; Johnston, E. M.; Ginsbach, J. W.; Cirera, J.; Qayyum, M.; Kieber-Emmons, M. T.; Kjaergaard, C. H.; Hadt, R. G.; Tian, L. Copper Active Sites in Biology. Chemical Reviews 2014, 114, 3659–3853. (25) Kaim, W.; Rall, J. Copper—a “Modern” Bioelement. Angewandte Chemie International Edition in English 1996, 35, 43–60. (26) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Structure and Spectroscopy of Copper−dioxygen Complexes. Chemical Reviews 2004, 104, 1013–1046. (27) Cramer, C. J.; Tolman, W. B. Mononuclear Cu–O2 Complexes: Geometries, Spectroscopic Properties, Electronic Structures, and Reactivity. Accounts of Chemical Research 2007, 40, 601–608. (28) Winikoff, S. G.; Cramer, C. J. Mechanistic Analysis of Water Oxidation Catalyzed by Mononuclear Copper in Aqueous Bicarbonate Solutions. Catalysis Science & Technology 2014, 4, 2484–2489.


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(29) Rudshteyn, B.; Fisher, K. J.; Lant, H. M. C.; Yang, K. R.; Mercado, B. Q.; Brudvig, G. W.; Crabtree, R. H.; Batista, V. S. Water-Nucleophilic Attack Mechanism for the CuII(pyalk)2 Water-Oxidation Catalyst. ACS Catalysis 2018, 8, 7952–7960. (30) Li, X.; Chen, G.; Schinzel, S.; Siegbahn, P. E. M. A Comparison Between Artificial and Natural Water Oxidation. Dalton Transactions 2011, 40, 11296–11307. (31) Li, X.; Siegbahn, P. E. M. Water Oxidation Mechanism for Synthetic Co–Oxides with Small Nuclearity. Journal of the American Chemical Society 2013, 135, 13804–13813. (32) Liao, R.; Li, X.; Siegbahn, P. E. M. Reaction Mechanism of Water Oxidation Catalyzed by Iron Tetraamido Macrocyclic Ligand Complexes – a DFT Study. European Journal of Inorganic Chemistry 2014, 2014, 728–741. (33) Li, X.; Siegbahn, P. E. M. Alternative Mechanisms for O2 Release and O-O Bond Formation in the Oxygen Evolving Complex of Photosystem II. Physical Chemistry Chemical Physics 2015, 17, 12168–12174. (34) Li, X.; Siegbahn, P. E. M. Water Oxidation for Simplified Models of the Oxygen-Evolving Complex in Photosystem II. Chemistry – A European Journal 2015, 21, 18821–18827. (35) Siegbahn, P. E. M.; Li, X. Cluster Size Convergence for the Energetics of the Oxygen Evolving Complex in PSII. Journal of Computational Chemistry 2017, 38, 2157–2160. (36) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Physical Review A 1988, 38, 3098–3100. (37) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Physical review B 1988, 37, 785. (38) Becke, A. D. A New Mixing of Hartree–Fock and Local Density-functional Theories. The Journal of Chemical Physics 1993, 98, 1372–1377. (39) Becke, A. D. Density-functional Thermochemistry. III. The Role of Exact Exchange. The Journal of Chemical Physics 1993, 98, 5648–5652. (40) Ryu, H.; Park, J.; Kim, H. K.; Park, J. Y.; Kim, S.-T.; Baik, M.-H. Pitfalls in Computational Modeling of Chemical Reactions and How to Avoid Them. Organometallics 2018, 37, 3228– 3239. (41) Reiher, M.; Salomon, O.; Artur Hess, B. Reparameterization of Hybrid Functionals Based on Energy Differences of States of Different Multiplicity. Theoretical Chemistry Accounts 2001, 107, 48–55. (42) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada,


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J.; Williams. Gaussian 09 Rev. E.01, 2016. (43) Siegbahn, P. E. M.; Blomberg, M. R. A. Modeling of Mechanisms for Metalloenzymes Where Protons and Electrons Enter or Leave. In Computational modeling for homogeneous and enzymatic catalysis: A knowledge-base for designing efficient catalysis; Morokuma, K., Musaev, D. G., Eds.; 2008; pp 57–81. (44) Siegbahn, P. E. M.; Blomberg, M. R. A. Energy Diagrams for Water Oxidation in Photosystem Ii Using Different Density Functionals. Journal of Chemical Theory and Computation 2014, 10, 268–272. (45) Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen, M. H.; Earhart, A. D.; Coe, J. V.; Tuttle, T. R. The Proton’s Absolute Aqueous Enthalpy and Gibbs Free Energy of Solvation from Cluster-Ion Solvation Data. The Journal of Physical Chemistry A 1998, 102, 7787–7794. (46) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. Aqueous Solvation Free Energies of Ions and Ion−water Clusters Based on an Accurate Value for the Absolute Aqueous Solvation Free Energy of the Proton. The Journal of Physical Chemistry B 2006, 110, 16066–16081. (47) Truhlar, D. G.; Cramer, C. J.; Lewis, A.; Bumpus, J. A. Molecular Modeling of Environmentally Important Processes: Reduction Potentials. Journal of Chemical Education 2004, 81, 596. (48) Blomberg, M. R. A.; Borowski, T.; Himo, F.; Liao, R.-Z.; Siegbahn, P. E. M. Quantum Chemical Studies of Mechanisms for Metalloenzymes. Chemical Reviews 2014, 114, 3601– 3658. (49) Robertson, W. H.; Diken, E. G.; Price, E. A.; Shin, J.-W.; Johnson, M. A. Spectroscopic Determination of the Oh− Solvation Shell in the Oh−(H2O)n Clusters. Science 2003, 299, 1367–1372. (50) Porras, S. P.; Kenndler, E. Capillary Electrophoresis in N,N-Dimethylformamide. ELECTROPHORESIS 2005, 26, 3279–3291. (51) Pipek, J.; Mezey, P. G. A Fast Intrinsic Localization Procedure Applicable for Ab Initio and Semiempirical Linear Combination of Atomic Orbital Wave Functions. The Journal of Chemical Physics 1989, 90, 4916–4926. (52) Lu, T.; Chen, F. Multiwfn: A Multifunctional Wavefunction Analyzer. Journal of Computational Chemistry 2012, 33, 580–592.


Page 28 of 29

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ACS Catalysis

(53) Humphrey, W.; Dalke, A.; Schulten, K. VMD: Visual Molecular Dynamics. Journal of Molecular Graphics 1996, 14, 33–38. (54) Siegbahn, P. E. M. A Quantum Chemical Study of the Mechanism of Manganese Catalase. Theoretical Chemistry Accounts 2001, 105, 197–206. (55) Yang, X.-X.; Mao, Q.-Y.; An, X.-T.; Li, X.-C.; Siegbahn, P. E. M.; Chen, G.-J.; Tan, H.-W. Theoretical Study of the Mechanism of the Manganese Catalase Katb. JBIC Journal of Biological Inorganic Chemistry 2019, 24, 103–115. (56) Garribba, E.; Micera, G.; Sanna, D.; Strinna-Erre, L. The Cu(II)-2,2-Bipyridine System Revisited. Inorganica Chimica Acta 2000, 299, 253–261. (57) Cramer, C. J. Essentials of Computational Chemistry, 2nd ed.; John Wiley & Sons, Inc.: The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England, 2004. (58) Haschke, S.; Mader, M.; Schlicht, S.; Roberts, A. M.; Angeles-Boza, A. M.; Barth, J. A. C.; Bachmann, J. Direct Oxygen Isotope Effect Identifies the Rate-Determining Step of Electrocatalytic Oer at an Oxidic Surface. Nature Communications 2018, 9, 4565. (59) Lyaskovskyy, V.; Bruin, B. de. Redox Non-Innocent Ligands: Versatile New Tools to Control Catalytic Reactions. ACS Catalysis 2012, 2, 270–279. (60) Cape, J. L.; Hurst, J. K. Detection and Mechanistic Relevance of Transient Ligand Radicals Formed During [Ru(bpy)2(OH2)]2O4+-Catalyzed Water Oxidation. Journal of the American Chemical Society 2008, 130, 827–829. (61) Pushkar, Y.; Pineda-Galvan, Y.; Ravari, A. K.; Otroshchenko, T.; Hartzler, D. A. Mechanism for O–O Bond Formation via Radical Coupling of Metal and Ligand Based Radicals: A New Pathway. Journal of the American Chemical Society 2018, 140, 13538– 13541.


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