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On the metal oxidation states for the O-O bond formation in the water oxidation catalyzed by a pentanuclear Iron complex Rong-Zhen Liao, Shigeyuki Masaoka, and Per E. M. Siegbahn ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02791 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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ACS Catalysis

On the metal oxidation states for the O-O bond formation in the water oxidation catalyzed by a pentanuclear Iron complex

Rong-Zhen Liao*[a], Shigeyuki Masaoka[b] and Per E. M. Siegbahn[c]

[a] Key Laboratory for Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medic, Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 (China) [b] Department of Life and Coordination-Complex Molecular Science, Institute for Molecular Science, Higashiyama 5-1, Myodaiji, Okazaki, Aichi 444-8787 (Japan) [c] Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm 10691 (Sweden) *E-mail: [email protected]

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Abstract Understanding the water oxidation mechanism, especially how the O-O bond formation takes place, provides crucial implication for the design of more efficient molecular catalysts for water oxidation in artificial photosynthesis. Density functional calculations have here been used to revisit the mechanism of O-O bond formation catalyzed by a pentanuclear iron complex. By comparing energetics for O-O bond formation at different oxidation states, it is suggested that the formally Fe5III,III,III,IV,IV state is the best candidate for the coupling of two oxo groups, with a barrier of 17.3 kcal/mol, rather than the previously suggested lower oxidation state of Fe5II,II,III,IV,IV. Importantly, the first water insertion into the Fe5III,III,III,III,III complex is associated with a barrier of 18.8 kcal/mol. The calculated barrier is somewhat overestimated as discussed in the text. Other possible reaction pathways, including water attack at the Fe5III,III,III,IV,IV state, coupling of oxo and hydroxide at the Fe5III,III,III,III,IV state, and coupling of two oxo groups at the Fe5III,III,IV,IV,IV state, were found to have much higher barriers.

Keywords: water oxidation; multinuclear iron catalyst; homogeneous catalysis; density functional calculations; O-O bond formation

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1. Introduction The harnessing of solar energy to split water into dioxygen and dihydrogen by an artificial photosynthetic machinery is one of the most promising technologies to solve the energy crisis and the environmental problem facing us in the future.[1,2,3] This machinery consists of five major processes, namely, photon absorption, charge separation, electron transfer, water oxidation, and proton reduction, all of which have to be very efficient to achieve high performance. The bottleneck is known to be the oxidation of water by releasing four electrons and four protons, which is both thermodynamically and kinetically unfavorable, with a relatively large energy penalty (E0=1.23 V vs the Standard Hydrogen Electrode (SHE) at pH=0). During the last few decades, extraordinary efforts have been dedicated to the development of homogeneous water oxidation catalysts using earth abundant transition metals, such as vanadium,[4] manganese,[5-8]

iron,[9-12] cobalt,[13,14] nickel,[15] and

copper.[16-19] Quite recently, the laboratory of one of the authors reported an exceptionally robust and efficient pentanuclear iron catalyst [FeII4FeIII(µ3-O)(µ-L)6]3+ (1, LH=3,5-bis(2-pyridyl)pyrazole, Figure 1) that mediates electrochemical water oxidation in acetonitrile/water solution.[20] The turnover frequency was measured to be 1900 s-1 with an applied potential of 1.42 V vs Fc+/Fc (1.88 V vs SHE). In complex 1, a triangular [Fe3(µ3-O)] core (Fe1, Fe2, and Fe3) is capsuled by two hexa-coordinated [Fe(µ-L)3] (Fe4 and Fe5) moieties. Fe1, Fe2, and Fe3 of the [Fe3(µ3-O)] core are all penta-coordinated, with a distorted trigonal bipyramidal geometry. These three iron ions were believed to bind water molecules and promote O-O bond formation during the oxidation of the complex. On the basis of preliminary density functional calculations in the gas phase, the O-O bond formation was suggested to take place by the coupling of two FeIV=O moieties in the 3


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Fe5II,II,III,IV,IV oxidation state. This would be very surprising since two ferrous ions are present in this state for the O-O bond formation even though the applied potential is as high as 1.88 V vs SHE.[21] In addition, the energetic requirement for the formation of this crucial intermediate leading to O-O bond formation was not considered, since redox potentials and pKa’s were not calculated.

Figure 1. Optimized structure of complex 1 (Fe5II,II,II,II,III). For clarity, hydrogen atoms were not shown.

To elucidate the reaction mechanism of water oxidation catalysed by this intriguing pentanuclear iron complex, we have now employed density functional calculations in combination with a continuum solvation model to calculate the redox potentials, pKa’s, various intermediates and transition states for the oxygen evolution. This protocol is well-established and has been successfully applied to the studies of many homogeneous water oxidation catalysts.[22,23,24] 4


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2. Computational details The density functional calculations presented herein were accomplished with the B3LYP-D3[25,26] functional as implemented in the Gaussian 09 software.[27] For the geometry optimizations, the SDD[28] pseudopotential was used for Fe, while 6-31G(d,p) for the C, N, O, H elements. Mulliken spin densities were calculated at the same level of theory as the geometry optimization for all structures. The large basis and solvation energies in acetonitrile were computed as single-points on the optimized structures using the SMD[29] continuum solvation model employing a larger basis set, where all elements, except Fe, were described by 6-311+G(2df,2p). Analytic frequency calculations were performed on all the optimized structures at the same level of theory, to identify the nature of all the stationary points and to obtain the Gibbs free energy corrections at 298.15 K. A concentration correction of 1.9 kcal/mol at room temperature [derived from the free energy change of 1 mol of an ideal gas from 1 atm (24.5 L/mol) to 1 M (1 mol/L in acetonitrile solution)] was added for all species except water and acetonitrile, for which the corresponding values are 2.8 kcal/mol and 3.6 kcal/mol, respectively. The reason is that the standard states of water and acetonitrile (volume ratio of acetonitrile/water 10:1) are approximately 5 mol/L and 17 mol/L, respectively. For the calculation of redox potentials and pKa’s in pure acetonitrile solution, the experimental absolute value is 4.52 V for the standard hydrogen electrode (SHE)[30], the experimental value is 0.46 V (vs SHE) for the Fc+/Fc couple,[21] and the experimental value is -258.4 kcal/mol for the solvation free energy of a proton.[22] In the case of redox potentials and pKa’s in pure water solution, the experimental reference potential is 4.281 V for the SHE, the experimental value is 0.40 V (vs SHE) for the Fc+/Fc couple,[31] and the experimental value is -264.0 kcal/mol for the 5


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solvation free energy of a proton.[32] Since a 5M water/acetonitrile solution was used in the electrochemical water oxidation reaction, a very critical concern is which values should be used for the absolute potential of the SHE (and the Fc+/Fc couple) and the solvation free energy of a proton. The use of an exponential average method ( Gsol   RT ln 

G G  sol WAT  sol  AN cWAT c AN e RT  e RT cWAT  c AN  cWAT  c AN

  , T is the temperature, R is 

the gas constant, cWAT is 5M for water and cAN is 17M for acetonitrile)[33] gave a value of -263.1 kcal/mol for the solvation free energy of a proton and an absolute reduction potential of 4.72 V for the Fc+/Fc couple. These two values are very close to those in a pure water solution, in which the corresponding values are -264.0 kcal/mol and 4.681 V, respectively. The reason is that both the proton and the Fc+(and Fc) are better solvated by pure water than solvated by pure acetonitrile.[34] The standard molar Gibbs energy of transfer of a proton from water to water + acetonitrile have been experimentally measured to be in the range of -1.7 to 1.6 kcal/mol with a molarity-scale of 80% (77.3% for the present case).[34]

For

simplicity, we just used the experimental values in pure water solution to set up the energy diagram as shown below. For comparison, we have also used the corresponding values in pure acetonitrile solution as the references, which are -258.4 kcal/mol for the solvation free energy of a proton and 4.98 V for the absolute reduction potential of the Fc+/Fc couple (see supporting information, pages S18-19). The use of different reference values indeed affects the total barriers somewhat; however, it does not change the main mechanistic conclusion for the present study. The uncertainties of the DFT/SMD methodology have been demonstrated to be about 0.2 V for redox potentials and three units for pKa’s.[22] For proton coupled electron transfer oxidations, the error for redox potentials is expected to be smaller, as the total charge of the system does not change during the oxidation and the solvation effect is much smaller. The initial pH of 4.8 was used in the controlled 6


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potential electrolysis in an unbuffered solution,[20] therefore, a pH of 4.8 was used as the reference to determine the redox potentials for the proton coupled electron transfer steps. It should be pointed out that a pH of 7.8 has also been used for electrolysis in borate buffer solution. The energy diagram of using pH=7.8 as reference is shown in the supporting information.

3. Results and discussion We first investigated the redox behaviour of 1 in a pure acetonitrile solution, in which five reversible one electron oxidation/reduction waves at -0.55 V, 0.13 V, 0.30 V, 0.68 V and 1.08 V (vs Fc+/Fc), have been observed experimentally.[20] The calculated redox potentials at the B3LYP-D3[25,26], TPSSh-D3[35] and M06-D3[36] levels together with the corresponding experimental values are shown in Table 1. The lowest oxidation state of the pentanuclear iron complex observed experimentally corresponds to Fe5II,II,II,II,II (labelled as 0), and Fe5III,III,III,III,III (labelled as 5) for the highest one. For each structure, various spin states have been calculated to find the one with the lowest energy (see supporting information for details). The ferromagnetically-coupled states were considered for all structures in the present calculations, as they are more staightforward ones to be calculated. In addition, preliminary single-point calculations have also been performed on the antiferromagnetically-coupled state for selected structures, the spin-coupling could lower the energy by less than 5 kcal/mol. However, the difference on the relative energies is less than 1 kcal/mol (Table S1 and S2), which is so small that it can be disregarded. It should be pointed out that some species presented here may have some multiconfigurational character, and therefore may need a more accurate treatment. The calculated redox potentials agree reasonably well with experimental data. B3LYP-D3 gave the best results with a RMSD of 0.22 V, while M06-D3 (RMSD of 0.28 V) and TPSSh-D3 (RMSD of 0.46 V) yielded somewhat larger errors. It is 7


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pertinent to mention that the error increases with the increase of the total charge of the complex during the one electron oxidation. A possible reason is that the solvation energy increases significantly with the increase of the total charge (+7 for complex 5), and the use of the SMD solvation model results in somewhat larger errors. As the B3LYP-D3 functional gave the best performance, therfore it was applied for the study of the electrocatalytic water oxidation reaction in acetonitrile/water solution. Table 1. Comparison of calculated and experimental redox potentials (in V vs SHE) for the Fe5 complex with only one bridging oxo ligand in the acetonitrile solution. Redox couple B3LYP-D3 TPSSh-D3 M06-D3 Exp.[a] 22223[b](1)/22222(0) -0.15 -0.27 -0.24 -0.09 22233(2)/22223(1) 0.41 0.13 0.29 0.59 22333(3)/22233(2) 0.52 0.26 0.43 0.76 23333(4)/22333(3) 0.90 0.68 0.86 1.14 [c] 33333(5)/23333(4) 1.23 0.96 1.22 1.54 [a] The experimental absolute redox potentials of 4.52 V and 4.98 V were used for SHE and Fc+/Fc, respectively.[21,30] [b] The five numbers indicate the oxidation state of the five iron ions. [c] The addition of ClO4- as a conterion gave a potential of 1.10 V. With an applied potential of 1.42 V vs Fc+/Fc (1.82 V vs SHE), the formation of 5 by one electron oxidation of 4 (potential of 1.47 V) in an acetonitrile/water solution is exergonic by 8.1 kcal/mol (Figure 2). Complex 5 has a total charge of +7, and its oxidation state can be assigned as Fe5III,III,III,III,III. The high-spin 26-tet was calculated to be the lowest ferromagnetically-coupled spin state, and the spin densities on all five iron ions are around 4.2 (SFe=5/2). In 5, Fe1, Fe2, and Fe3 are all penta-coordinated, while Fe4 and Fe5 are both hexa-coordinated. This is followed by water insertion into the central triangular [Fe3(µ3-O)] core. The optimized transition state TS1W is shown in Figure 3. The barrier was calculated to be 18.8 8


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kcal/mol relative to 5 plus a separated water molecule. At TS1W, the critical Fe2-O2 and Fe3-O2 distances are both 3.12 Å. This leads to the formation of 5W, in which Fe2 is ligated to the water molecule and becomes hexa-coordinated. The pKa of 5W was calculated to be -6.6, therefore the deprotonation of 5W to generate 5W-Dp is exergonic by 15.6 kcal/mol using pH=4.8 as a reference.[20] In 5W-Dp, the hydroxide bridges Fe2 and Fe3, both of which are hexa-coordinated. Further deprotonation of 5W-Dp will not take place as its pKa is 7.2. We have also checked the effect of an additional water molecule to form a hydrogen bond to the bridging water molecule on the calculated pKa, which increases slightly from -6.6 to -4.5. The coordination of an acetonitrile molecule to 5 has also been taken into account (Figure S4). The barrier for this process was calculated to be 15.8 kcal/mol relative to 5 plus acetonitrile, and the binding is endergonic by 13.9 kcal/mol. This pathway is much less favourable than the water binding and deprotonation, which is close to isoenergetic. The insertion of a water molecule into 3 and 4 has also been investigated. The calculations showed that water binding to Fe2 in 3 to form 3W has a barrier of 17.9 kcal/mol, and is endergonic by 16.4 kcal/mol. The pKa of 3W was calculated to be 2.5, and its deprotonation is thus exergonic by 3.1 kcal/mol at pH=4.8. Therefore, the first water binding and deprotonation process for 3 is endergonic by 14.8 kcal/mol, which is much higher than that from 5 (endergonic by only 1.2 kcal/mol, Figure 2). Similarly, the water insertion into 4 to generate 4W is associated with a barrier of 19.0 kcal/mol, and this step is endergonic by 16.3 kcal/mol. Further deprotonation of 4W is exergonic by 7.8 kcal/mol as its pKa was calculated to be -0.9. Taking together, the first water binding and deprotonation process for 4 is endergonic by 8.5 kcal/mol. These calculations suggested that the first water binding preferentially takes place from 5.

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40

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Ref: E=1.82 V pH=4.8 33333O-O-O 3+ pKa=27.0

30

H+

52W-3Dp 32.4

20 33333O-O-OH 4+ 10

0

22333O 5+

3 23333O 6+ 0.0 1.14 V e H2 O

-10

-20

-30

4 -15.7

1.47 V e5 -23.8

33333O 7+

33333O-OH2 7+ 52W-2Dp pKa=21.1

TS1W

5W -7.0 pKa=-6.6 H+ H2 O 18.8 kcal/mol

H

-5.0

-22.6 5W-Dp 33333O-OH 6+

1.9

+

TS2WB -15.1

H+ -17.7 -20.2 52W pKa=2.9 52W-Dp

33333O-OH-OH2 6+

33333O-OH-OH 5+

Figure 2. Energy diagram (in kcal/mol) for the generation of the previously suggested catalytically competent species 52W-3Dp from 3. A reference potential of 1.82 V vs SHE (experimental value of 1.42 V for Fc+/Fc) and a pH of 4.8 were used. The blue texts indicate the formal oxidation state, the type of oxygen ligands, and the total charge. The energetics (Figure 2) was then calculated for the generation of the previously proposed Fe5II,II,III,IV,IV intermediate with two terminal oxo groups at Fe2 and Fe3, which was suggested to initiate O-O bond formation by the coupling of these two oxo groups.[19] The insertion of the second water molecule via TS2B (Figure S5) followed by the release of a proton to generate 52W-Dp, with two terminal hydroxide ligands bound to Fe2 and Fe3, is endergonic by 4.9 kcal/mol, which can be easily overcome. Quite unexpectedly, the further release of three protons from 52W-Dp is thermodynamically very unfavourable. The pKa of 52W-Dp was calculated to be as high as 21.1, and 27.0 for 52W-2Dp, which has an oxo ligand coordinated to Fe2 and a hydroxide ligated to Fe3. Consequently, the formation of 52W-3Dp from 5 is endergonic by an extremely high amount of 56.2 kcal/mol at pH=4.8. The endergonicity decreases to 44.4 kcal/mol with a pH of 7.8 (see Figure S15 in the 10


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supporting information). We have also considered all possible antiferromagnetically coupled states for 5 and 52W-3Dp (Table S1) to check their effects on the endergonicity. The endergonicity decreases slightly by 0.7 kcal/mol when the antiferromagnetic coupling is considered. This enormous energetic penalty can safely rule out the previously suggested Fe5II,II,III,IV,IV intermediate as a viable option for the O-O bond formation. Additional support can be seen from the oxidation potential for the generation of the Fe5II,II,III,IV,IV state, or equivalently Fe5III,III,III,III,III, the formation of which was calculated to be associated with a potential of 1.23 V (experimental value of 1.54 V) in pure acetonitrile. In acetonitrile/water solution, this potential increases slightly to 1.47 V, which is quite low compared with the experimentally applied potential of 1.88 V.

Figure 3. Transition state structures for the first water insertion from 5 (TS1W) and the second from 6W (TS1W). Distances are given in Angstrom, and spin densities on selected atoms are displayed in red italic. The imaginary frequencies are also shown.

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20

10

0

kcal/mol

Ref: E=1.82 V pH=4.8

22333O 5+

3 23333O 6+ 0.0 1.14 V H2 O e

-10 4 1.47 Ve -15.7 -20

-30

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33334O-O-OH2 6+ TS1W

pKa=6.1

5W -7.0 pKa=-6.6 H+ 18.8 kcal/mol -22.6 5W-Dp

33333O-OH

TS1B -2.9

H+

-5.0

5 -23.8 33333O 7+

33334O-O-OH 5+ H2 O

33333O-OH2 7+

H2 O

TS2W -14.3

62W -15.3

TS1WA 11.1 TS1C 3.4 2.32 V e-

TS1 62W-Dp -10.2 -13.5 72W -19.0 17.3 kcal/mol + 2H e 1.66 V

H+ e- -27.5 6+ 1.60 V 6W

82W -7.5

Int1B -29.9

Int1WA -31.6

33344O-O-O 5+

-40 33334O-O 6+ -50

Int1 -47.8 O2

-60

-70

3 -80.6

-80

Figure 4. Energy diagram (in kcal/mol) for water oxidation by 3. A reference potential of 1.82 V vs SHE (experimental value of 1.42 V for Fc+/ Fc) and a pH of 4.8 were used. The blue texts indicate the formal oxidation state, the type of oxygen ligands, and the total charge. As an alternative, we find that a further proton-coupled electron transfer (PCET) oxidation of 5W-Dp to produce 6W is quite facile, with a potential of 1.60 V. This is somewhat lower than the applied potential of 1.88 V, and this oxidation is thus exergonic by 4.9 kcal/mol. The oxidation of 5W-Dp is thermodynamically much more favourable than the insertion of water into 5W-Dp. The formal oxidation state of 6W is Fe5III,III,III,III,IV, and the lowest ferromagnetically-coupled energy state is 25-tet. In 6W, there are four high spin ferric ions (SFe=5/2 for Fe1, Fe3, Fe4, and Fe5) and a high spin Fe2IV=O (SFe2=2) moiety. The spin densities on Fe2 and O2 are 3.14 and 0.63, respectively. During the oxidation, the bridging hydroxide becomes a terminal oxo group bound to Fe2. Therefore, Fe3 returns to be penta-coordinated, which is capable of binding the second water molecule. Subsequently, the second water 12


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insertion to generate 62W takes place via TS2W (Figure 3), which is associated with a barrier of 13.2 kcal/mol. The insertion of the second water molecule is thus more favourable than that of the first one, due to the more exposed structure of 6W and also the formation of a hydrogen bond between the incoming water and O2. The crucial Fe3-O3 distance is 3.08 Å at TS2W. The deprotonation of 62W to form 62W-Dp is endergonic by 1.8 kcal/mol (Figure 4) as the pKa of 62W was calculated to be 6.1. Further deprotonation of 62W-Dp is very unlikely at pH=4.8 as 62W-Dp has a pKa of 20.0. The subsequent oxidation of 62W to produce 72W (oxidation state of Fe5III,III,III,IV,IV) is associated with the release of two protons, as 72W-Pt (protonated form of 72W) has a pKa of 1.9. This process has a potential of 1.66 V, which is reasonably close to the applied potential of 1.88 V.[20] During the oxidation, both protons are removed from the Fe3-bound water molecule, while an electron is removed from Fe3. The lowest ferromagnetically-coupled energy state of 72W is a 26-tet, in which Fe1, Fe4 and Fe5 are all high spin ferric ions (S=5/2), ferromagnetically-coupled with a high spin Fe2III-O• (SFe2=5/2 and SO=1/2) and a high spin Fe3IV=O (SFe3=2). Compared with the 26-tet, the 24-tet state is 4.9 kcal/mol higher, and the main difference between these two states is that the 24-tet state has a low spin Fe3III-O• (SFe2=1/2 and SO=1/2), but a high spin Fe3IV=O (SFe3=2) in the 26-tet. Other lower spin states have also been considered, but with even higher energies. In addition, single-point calculations have been performed on the 26-tet structure to estimate the effect of antiferromagnetic coupling on the energies. 16 possible spin-coupling states have been located in total (Table S2). The antiferromagnetic coupling lowers the energy by 3.5 kcal/mol. In addition, Fe1 prefers to be antiferromagnetically-coupled with both Fe2 and Fe3. From 72W, O-O bond formation takes place via the direct coupling of the Fe3IV=O and the Fe2IV=O moieties with significant oxyl radical characters. O-O bond 13


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formation via the direct coupling of two M=O species has also been suggested for a number of dinuclear Ru water oxidation catalysts on the basis of density functional calculations.[37-43] The optimized transition state TS1 is displayed in Figure 5. The barrier was calculated to be only 8.8 kcal/mol in the 24-tet (11.5 kcal/mol for 26-tet) relative to 72W. TS1 has been confirmed to have only one imaginary frequency of 204.6i cm-1, which corresponds to O-O bond formation. At TS1, the nascent O-O bond distance is 2.08 Å. Importantly, the two oxygen atoms have opposite spin densities, being 0.43 on O2 and -0.38 on O3, which favours the formation of a single bond between the two oxygen atoms.

The antiferromagnetic coupling has a very

minor effect on the O-O bond formation barrier, which increases by only 0.3 kcal/mol (Table S2). Downhill from TS1, an intramolecular electron transfer from the O2 moiety to Fe1 takes place, leading to the formation of the superoxide intermediate Int1 (Fe5II,III,III,III,III-O2•-), which lies at -28.8 kcal/mol relative to 72W. The formation of a superoxide at Int1 is evidenced by the short O2-O3 distance of 1.30 Å and also a total spin density of 1.38 on the O2 moiety. The final dioxygen release to regenerate 3 is very facile, and the whole reaction is exergonic by 80.6 kcal/mol. The total barrier for the O-O bond formation is thus 17.3 kcal/mol (from 6W to TS1, Figure 2), which is slightly lower than that for the first water insertion (18.8 kcal/mol). Therefore, it is difficult to assign which one of them is rate-limiting, and both steps could contribute to the rate for water oxidation. The calculated barrier seems to be somewhat overestimated as compared with the experimental value of about 13 kcal/mol, which can be obtained by converting the experimental rate constant of 1900 s-1 using classical transition state theory.[20] This value corresponds to a unimolecular reaction, while the whole catalytic cycle for water oxidation is believed to be much more complicated. The concentration of the catalyst and various intermediates could also play an important role. It should be pointed out that the kinetics for all electrochemical steps was not modelled 14


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explicitly. We cannot rule out the possibility that one of the electrochemical steps, for example, the one electron/two proton oxidation of 62W/72W, is rate-limiting. We have also tested using one additional water molecule to form a hydrogen bond with the incoming water molecule in TS1W (Figure S14), and the barrier becomes slightly higher, 21.0 kcal/mol, suggesting that this is a less favourable pathway. However, it is possible that the first water insertion and proton release takes place in a concerted step in the acetonitrile solution, which may lower the barrier by a number of kcal/mol. In additional, the harmonic entropy was used for the Gibbs free energy correction, which may lead to an error of several kcal/mol. Here, if the quasiharmonic approximation[44,45] was used to for the estimation of entropies, the barrier decreases slightly to 18.5 kcal/mol.

Figure 5. Transition state structures for O-O bond formation from 72W (TS1: direct coupling; TS1WA: water attack). Distances are given in Angstrom, and spin densities on selected atoms are displayed in red italic. The imaginary frequencies are also shown. Other possible O-O bond formation pathways have also been considered. First, a water nucleophilic attack pathway was explored, which is commonly found in many 15


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other water oxidation catalysts.[34,46-56] The attack of a water molecule on one of the oxo groups from 72W is coupled with a proton transfer from the water to the other oxo group (TS1WA, Figure 5). This is associated with a barrier of 38.6 kcal/mol relative to 6W, which is too high to be accessible. Second, O-O bond formation from 72W-Pt via the coupling of an oxo and a hydroxide (TS1Pt, Figure S9) has a barrier of 23.1 kcal/mol relative to 6W, which is 5.8 kcal/mol higher than that from 72W. Third, O-O bond formation at a lower oxidation state of Fe5III,III,III,III,IV, namely from 62W-Dp, via the coupling of an oxo and a hydroxide (TS1B, Figure S8) has a barrier of 24.6 kcal/mol relative to 6W. This is 7.3 kcal/mol higher than that from 72W. Lastly, one additional electron oxidation of 72W to produce 82W (oxidation state of Fe5III,III,IV,IV,IV) has a potential of 2.32 V, and the following O-O bond formation via the coupling of two oxo groups (TS1C, Figure S11) also has a much higher barrier than that from 72W (30.9 kcal/mol vs 17.3 kcal/mol). These results further substantiate the O-O bond formation at the Fe5III,III,III,IV,IV oxidation state via the direct coupling of two oxo groups as the most viable option under the experimental conditions.

4. Conclusion In summary, we have performed density functional calculations to revisit the water oxidation mechanism catalyzed by an unprecedented pentanuclear iron complex. A revised mechanism (Scheme 1) has been suggested with the critical O-O bond formation taking place at the formally Fe5III,III,III,IV,IV state, while the previously suggested oxidation state of Fe5II,II,III,IV,IV for O-O bond formation can be safely ruled out on the basis of the very high energetics for the formation of the proposed active species. The reaction starts with complex 3, which has an oxidation state of Fe5II,II,III,III,III. Two sequential one electron oxidations of 3 leads to the formation of 5 with an oxidation state of Fe5III,III,III,III,III. This is followed by the insertion of the first water 16


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molecule to generate 5W, which is associated with a barrier of 18.8 kcal/mol. The deprotonation of 5W is exergonic by 15.6 kcal/mol at pH=4.8. Then, a proton-coupled electron transfer oxidation of 5W-Dp takes place with a potential of 1.60 V. This leads to the generation of 6W with an oxidation state of Fe5II,II,III,III,IV, in which a Fe2IV=O is produced. Subsequently, the second water substrate is inserted into 6W to form 62W, the barrier for which was calculated to be 13.2 kcal/mol. Further one electron oxidation of 62W to generate 72W was found to be coupled with the release of two protons from the Fe3 bound water molecule, which has a potential of 1.66 V. O-O bond formation proceeds from 72W via the coupling of two terminal oxo groups, and the barrier was calculated to be only 8.8 kcal/mol relative to 72W , but 17.3 kcal/mol from 6W to TS1, see Figure 4. This results in the formation of a superoxide intermediate Int1, from which a dioxygen molecule can be released easily, coupled with the regeneration of the starting species 3 and the closing of the catalytic cycle. The first water insertion (barrier of 18.8 kcal/mol) has a slightly higher barrier than the O-O bond formation (17.3 kcal/mol). It is therefore difficult to assign which one of them is rate-limiting and both may contribute to the rate of the water oxidation. The calculated barrier of 18.8 kcal/mol is somewhat overestimated compared with the experimental kinetic rate constant of 1900 s-1, which gave a barrier of about 13 kcal/mol using classical transition state theory. O-O bond formation via water attack and at other oxidation states, namely Fe5III,III,III,III,IV and Fe5III,III,IV,IV,IV have all been shown to be unlikely. These new results may be helpful for experimentalists to improve the catalysts and design more efficient catalysts.

17


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Fe4III

Fe1II

e-

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Fe4III

O O2

Fe4III

O Fe3III

Fe2II

Fe3III

Fe2III

Fe5III

3

Fe1II

Fe5III

4

eFe4III

Fe1III

O

O Fe3III

Fe2III O

Fe2III

Fe3III

5

Fe5III H 2O

O Fe5III

Int1

Fe1III

TS1 O-O bond formation

H+ Fe4III Fe1III

Fe4III Fe1III O Fe2IV O

72W

O Fe3IV

O H Fe5III

O Fe5III 2H+ e-

Fe3III

Fe2III

Fe4III

Fe1

Fe4III

III

O Fe3III

Fe2IV O H 2O

62W

Fe5III

5W-Dp Fe1

III

H+ e-

O H 2O

Fe3III

Fe2IV O

6W

Fe5III

Scheme 1. Suggested water oxidation mechanism on the basis of present calculations.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Figures, Tables, Energies, and Coordinates.

Author information Corresponding author *E-mail: [email protected] ORCID 18


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Rong-Zhen Liao: 0000-0002-8989-6928 Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the National Natural Science Foundation of China (21503083, 21873031) and the Fundamental Research Funds for the Central Universities (2017KFKJXX014), the Swedish Research Council, the Knut and Alice Wallenberg Foundation. Computer time was generously provided by the Swedish National Infrastructure for Computing.

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