Mechanism of Water Oxidation Catalyzed by a Dinuclear Ruthenium Complex Bridged by Anthraquinone

We synthesized 1,8-bis(2,2′:6′,2”-terpyrid-4′-yl)anthraquinone (btpyaq) as a new dimerizing ligand and determined its single crystal structure by X-ray analysis. The dinuclear Ruthenium complex [Ru2(μ-Cl)(bpy)2(btpyaq)](BF4)3 ([3](BF4)3, bpy = 2,2′-bipyridine) was used as a catalyst for water oxidation to oxygen with (NH4)2[Ce(NO3)6] as the oxidant (turnover numbers = 248). The initial reaction rate of oxygen evolution was directly proportional to the concentration of the catalyst and independent of the oxidant concentration. The cyclic voltammogram of [3](BF4)3 in water at pH 1.3 showed an irreversible catalytic current above +1.6 V (vs. SCE), with two quasi-reversible waves and one irreversible wave at E1/2 = +0.62, +0.82 V, and Epa = +1.13 V, respectively. UV-vis and Raman spectra of [3](BF4)3 with controlled-potential electrolysis at +1.40 V revealed that [Ru(IV)=O O=Ru(IV)]4+ is stable under electrolysis conditions. [Ru(III), Ru(II)] species are recovered after dissociation of an oxygen molecule from the active species in the catalytic cycle. These results clearly indicate that an O–O bond is formed via [Ru(V)=O O=Ru(IV)]5+.

Therefore, water oxidation catalyzed by [2] 3+ proceeds via intramolecular coupling of two high-valent Ru=O species.However, the details of high-valent Ru=O coupling are not yet clear.In particular, the oxidation states of the active species in Ru=O coupling are not yet clear.
Scheme 2. Dinuclear ruthenium complex catalysts for water oxidation.

Syntheses of Bis(terpyridyl)anthraquinone and Dinuclear Ruthenium Complexes
A new bridging ligand, btpyaq, was prepared via a synthetic pathway (Scheme 3) similar to that for btpyan [51].
Catalysts 2017, 7, 56 3 of 16 structural differences between the anthracene and anthraquinone bridging ligands were investigated by spectroelectrochemical measurements and kinetic analysis.
Scheme 2. Dinuclear ruthenium complex catalysts for water oxidation.
Suitable crystals of btpyaq for X-ray analysis were obtained from a benzene solution (Figure 1).The dihedral angles between two terpyridine moieties and the anthraquinone skeleton are 65.59 and 59.85 • .The distance between N(2) and N(5) of the terpyridines is 7.061 Å, whereas that between the carbon atoms at 1,8-positions of anthraquinone is 5.077 Å, as the steric repulsion of the O(2) of anthraquinone lengthens the distance between the two terpyridine moieties.This structural feature of btpyaq differs from that of the anthracene analog, btpyan, in which the two terpyridines are close to each other by π-stacking interactions [53].
Catalysts 2017, 7, 56 4 of 16 carbon atoms at 1,8-positions of anthraquinone is 5.077 Å, as the steric repulsion of the O(2) of anthraquinone lengthens the distance between the two terpyridine moieties.This structural feature of btpyaq differs from that of the anthracene analog, btpyan, in which the two terpyridines are close to each other by π-stacking interactions [53].The distance between the two Ruthenium(II) atoms bridged by a single Cl − is typically within 5.2 Å and is shorter than that between N(2) and N(4) in the single crystal of btpyaq (Figure 1).This means that btpyaq has structural flexibility, and the distance between the two Ruthenium atoms is able to change from ca. 5 to 7 Å.

Reaction of btpyaq and two equivalents of RuCl
[Ru 2 Cl 2 (bpy) 2 (btpyaq)](SbF 6 ) 2 was prepared by the reaction of [Ru 2 Cl 6 (btpyaq)] with two equivalents of the bpy ligand in the presence of NEt 3 and purified using chromatographic techniques.The dinuclear Ruthenium complex [Ru 2 (µ-Cl)(bpy) 2 (btpyaq)](BF 4 ) 3 ([3](BF 4 ) 3 ) was prepared by Cl − abstraction reaction of the dichlorido complex, [Ru 2 Cl 2 (bpy) 2 (btpyaq)](SbF 6 ) 2 , with AgBF 4 (Scheme 4).The distance between the two Ruthenium(II) atoms bridged by a single Cl − is typically within 5.2 Å and is shorter than that between N(2) and N(4) in the single crystal of btpyaq (Figure 1).This means that btpyaq has structural flexibility, and the distance between the two Ruthenium atoms is able to change from ca. 5 to 7 Å.The distance between the two Ruthenium(II) atoms bridged by a single Cl − is typically within 5.2 Å and is shorter than that between N(2) and N(4) in the single crystal of btpyaq (Figure 1).This means that btpyaq has structural flexibility, and the distance between the two Ruthenium atoms is able to change from ca. 5 to 7 Å.

Redox Behavior of [3](BF4)3
The cyclic voltammogram (CV) of an aqueous solution of [3](BF4)3 at pH 1.3 shows two quasireversible redox waves at E1/2 = +0.62 and +0.82 V (vs.SCE, Figure 4).On the basis of the rest potential of [3]  To assign the redox waves in the CV (Figure 4), we conducted UV-vis spectral measurements of [3](BF4)3 with controlled-potential electrolysis.An aqueous solution of [3](BF4)3 at pH 1.0 exhibited an absorption band at 485 nm arising from the metal-to-ligand charge transfer (MLCT) of the Ru(II)(bpy) moieties (Figure 5A).When the solution was electrolyzed at +0.81 V, which is the potential between the two quasi-reversible redox waves of the CV (Figure 4), the absorbance of the 485 nm band decreased to almost half that before electrolysis (Figure 5A).Re-reduction at +0.53 V resulted in the reversible recovery of the spectrum of [3](BF4)3; this result is consistent with the assignment of the wave at +0.62 V to a reversible [Ru(II)-Cl-Ru(II)] 3+ /[Ru(II)-Cl-Ru(III)] 4+ redox reaction.Further oxidation of the solution at +1.04 V changed the color of the solution from dark brown to dark green, and two-step spectral changes were clearly observed.First, the 485 nm band was reduced (blue lines in Figure 5B).Thereafter, the intensity of the 686 nm band gradually increased with a decrease in the 485 nm band (orange lines in Figure 5B).These spectral changes indicate that the one-electron oxidation of [Ru(II)-Cl-Ru(III)] 4+ to [Ru(III)-Cl-Ru(III)] 5+ is followed by cleavage of the Ru-Cl bond and coordination of OH − to give [Ru(III)-OH Cl-Ru(III)] 4+ , which is smoothly one-electron oxidized with a deprotonation to [Ru(IV)=O Cl-Ru(III)] 4+ .The corresponding redox wave at +0.62 V in the CV is quasi-reversible as Ru-Cl bond cleavage and coordination of water to Ru are quite slow compared to the CV scanning rate (Figure 4).The intensity of the 686 nm band arising from Ru(IV)=O is increased upon further oxidation of the solution at +1.40 V (spectra C of Figure 5).[Ru(IV)=O O=Ru(IV)] 4+ is produced via one-electron oxidation of the Cl-Ru(III) moiety with dissociation of Cl − and association of H2O, followed by double deprotonation.Surprisingly, the 686 nm band remained To assign the redox waves in the CV (Figure 4), we conducted UV-vis spectral measurements of [3](BF 4 ) 3 with controlled-potential electrolysis.An aqueous solution of [3](BF 4 ) 3 at pH 1.0 exhibited an absorption band at 485 nm arising from the metal-to-ligand charge transfer (MLCT) of the Ru(II)(bpy) moieties (Figure 5A).When the solution was electrolyzed at +0.81 V, which is the potential between the two quasi-reversible redox waves of the CV (Figure 4), the absorbance of the 485 nm band decreased to almost half that before electrolysis (Figure 5A).Re-reduction at +0.53 V resulted in the reversible recovery of the spectrum of [3](BF 4 ) 3 ; this result is consistent with the assignment of the wave at +0.62 V to a reversible [Ru(II)-Cl-Ru(II)] 3+ /[Ru(II)-Cl-Ru(III)] 4+ redox reaction.Further oxidation of the solution at +1.04 V changed the color of the solution from dark brown to dark green, and two-step spectral changes were clearly observed.First, the 485 nm band was reduced (blue lines in Figure 5B).Thereafter, the intensity of the 686 nm band gradually increased with a decrease in the 485 nm band (orange lines in Figure 5B).These spectral changes indicate that the one-electron oxidation of [Ru(II)-Cl-Ru(III)] 4+ to [Ru(III)-Cl-Ru(III)] 5+ is followed by cleavage of the Ru-Cl bond and coordination of OH − to give [Ru(III)-OH Cl-Ru(III)] 4+ , which is smoothly one-electron oxidized with a deprotonation to [Ru(IV)=O Cl-Ru(III)] 4+ .The corresponding redox wave at +0.62 V in the CV is quasi-reversible as Ru-Cl bond cleavage and coordination of water to Ru are quite slow compared to the CV scanning rate (Figure 4).The intensity of the 686 nm band arising from Ru(IV)=O is increased upon further oxidation of the solution at +1.40 V (spectra C of Figure 5).[Ru(IV)=O O=Ru(IV)] 4+ is produced via one-electron oxidation of the Cl-Ru(III) moiety with dissociation of Cl − and association of H 2 O, followed by double deprotonation.Surprisingly, the 686 nm band remained unaltered for over 24 h.This means that [Ru(IV)=O O=Ru(IV)] 4+ is relatively stable in acidic water at room temperature and is not the active species in the water oxidation cycle.Oxidation at +1.60 V, at which water oxidation proceeds, decreased the intensity of the 686 nm band (Figure 5D).After electrolysis was stopped, the MLCT band of Ru(II)(bpy) at 486 nm gradually increased and the absorbance became half that in the initial spectrum (Figure 5E).This result suggests that the [Ru(II) Ru(III)] species is recovered by releasing oxygen from the active species, which is expected to be the [Ru(III)-O-O-Ru(IV)] 5+ dimer generated by intramolecular coupling of [Ru(IV)=O O=Ru(V)] 5+ .In fact, the CV of the solution after electrolysis at +1.60 V showed a rest potential at +0.72 V between two redox waves (Figure S2).
Catalysts 2017, 7, 56 7 of 16 unaltered for over 24 h.This means that [Ru(IV)=O O=Ru(IV)] 4+ is relatively stable in acidic water at room temperature and is not the active species in the water oxidation cycle.Oxidation at +1.60 V, at which water oxidation proceeds, decreased the intensity of the 686 nm band (Figure 5D).After electrolysis was stopped, the MLCT band of Ru(II)(bpy) at 486 nm gradually increased and the absorbance became half that in the initial spectrum (Figure 5E).This result suggests that the [Ru(II) Ru(III)] species is recovered by releasing oxygen from the active species, which is expected to be the [Ru(III)-O-O-Ru(IV)] 5+ dimer generated by intramolecular coupling of [Ru(IV)=O O=Ru(V)] 5+ .In fact, the CV of the solution after electrolysis at +1.60 V showed a rest potential at +0.72 V between two redox waves (Figure S2).Insets of (B,C) are the enlarged spectra from 500 nm to 800 nm.

Raman Spectra of the Intermediate
Raman spectroscopy is a powerful method for the assignment of intermediate species such as Ru=O and Ru-O-O-Ru, in water oxidation catalytic cycles.The Raman spectra of the green solutions after electrochemical oxidation of 16 O-water and 18 O-water solutions by [3](BF 4 ) 3 at +1.40 V, irradiated with a 532 nm laser, showed bands at 794 and 754 cm −1 , respectively (Figure 6).The difference between the two bands is −40 cm −1 , which is consistent with the calculated value for Ru=O (−41 cm −1 ).Values of 818 and 780 cm −1 were reported by Hurst et al. for the Raman bands of Ru(V)= 16 O and Ru(V)= 18 O, respectively, in cis,cis-[(bpy) 2 Ru(O)] 2 O 4+ [62].Therefore, the [Ru(IV)=O O=Ru(IV)] observed by Raman spectroscopy is not the active species.An O-O bond is expected to be formed at higher oxidation states of the catalyst, which is consistent with the CV results (Figure 4) and UV-vis spectral measurements with controlled-potential electrolysis (Figure 5).

Raman Spectra of the Intermediate
Raman spectroscopy is a powerful method for the assignment of intermediate species such as Ru=O and Ru-O-O-Ru, in water oxidation catalytic cycles.The Raman spectra of the green solutions after electrochemical oxidation of 16 O-water and 18 O-water solutions by [3](BF4)3 at +1.40 V, irradiated with a 532 nm laser, showed bands at 794 and 754 cm −1 , respectively (Figure 6).The difference between the two bands is −40 cm −1 , which is consistent with the calculated value for Ru=O (−41 cm −1 ).Values of 818 and 780 cm −1 were reported by Hurst et al. for the Raman bands of Ru(V)= 16 O and Ru(V)= 18 O, respectively, in cis,cis-[(bpy)2Ru(O)]2O 4+ [62].Therefore, the [Ru(IV)=O O=Ru(IV)] observed by Raman spectroscopy is not the active species.An O-O bond is expected to be formed at higher oxidation states of the catalyst, which is consistent with the CV results (Figure 4) and UV-vis spectral measurements with controlled-potential electrolysis (Figure 5).In contrast, we already reported that the Raman bands of the species formed by oxidation of [2](BF4)3 under the same conditions are observed at 442 and 824 cm −1 , which are shifted to 426 and 780 cm −1 , respectively, when the same electrolysis is conducted in H2 18   In contrast, we already reported that the Raman bands of the species formed by oxidation of [2](BF 4 ) 3 under the same conditions are observed at 442 and 824 cm −1 , which are shifted to 426 and 780 cm −1 , respectively, when the same electrolysis is conducted in H 2 18 O. is not the rate-determining step in the water oxidation by [3](BF 4 ) 3 .According to the crystal structure of btpyaq (Figure 1), [3](BF 4 ) 3 displays higher flexibility of the Ru−Ru distance compared to the anthracene analog, [2](BF 4 ) 3 .The structural flexibility of [3](BF 4 ) 3 would accelerate the release of an O 2 molecule from the [Ru-O-O-Ru] species and changes the rate-determining step in the catalytic cycle.

Catalytic Mechanism of Water Oxidation
Based on the above results, we propose possible mechanisms for the water oxidation catalyzed by [3](BF 4 ) 3 .Sequential one-electron oxidations of [3]    The other possibility is a nucleophilic-attack mechanism (Scheme 7), similar to that proposed for the water oxidation catalyzed by cis,cis-[(bpy)2Ru(O)]2O 4+ [63][64][65][66].A water molecule nucleophilically attacks the oxo ligand of Ru(V)=O more easily than that of Ru(IV)=O in [Ru(IV)=O O=Ru(V)] 5+ in acidic media.The Ru(IV)=O moiety acts as a base and redox center when an oxygen molecule is released from the peroxo complex.The other possibility is a nucleophilic-attack mechanism (Scheme 7), similar to that proposed for the water oxidation catalyzed by cis,cis-[(bpy)2Ru(O)]2O 4+ [63][64][65][66].A water molecule nucleophilically attacks the oxo ligand of Ru(V)=O more easily than that of Ru(IV)=O in [Ru(IV)=O O=Ru(V)] 5+ in acidic media.The Ru(IV)=O moiety acts as a base and redox center when an oxygen molecule is released from the peroxo complex.The other possibility is a nucleophilic-attack mechanism (Scheme 7), similar to that proposed for the water oxidation catalyzed by cis,cis-[(bpy) 2 Ru(O)] 2 O 4+ [63][64][65][66].A water molecule nucleophilically attacks the oxo ligand of Ru(V)=O more easily than that of Ru(IV)=O in [Ru(IV)=O O=Ru(V)] 5+ in acidic media.The Ru(IV)=O moiety acts as a base and redox center when an oxygen molecule is released from the peroxo complex.
filtration.The volume of the yellow toluene solution was reduced in vacuo to ca. 5 mL.Hexane (10 mL) was carefully layered on the toluene solution, which was allowed to stand for 1 d.Yellow needle crystals were obtained by filtration.Yield: 324 g (84%). 1 H-NMR (400 MHz, in CDCl 3 ): ∂ (ppm) 8.26 (dd, 2H, J 1 = 7.3 Hz, J 2 = 1.5 Hz, anthraquinone), 7.79 (dd, J 1 = 7.3 Hz, J 2 = 1.5 Hz, anthraquinone), 7.75 2H, J 1 = J 2 = 7.3 Hz anthraquinone), 3.86  A mixture of anthraquinone-1,8-diboronic acid (0.35 g, 1.1 mmol), 4 -[(trifluoromethylsulfonyl)oxy]-2,2 :6 ,2 -terpyridine (1.0 g, 2.7 mmol), Pd(PPh 3 ) 4 (0.28 g, 0.24 mmol, 10 mol %), and K 2 CO 3 (1.3 g, 9.4 mmol) in toluene (60 mL), ethanol (18 mL), and distilled water (8.2 mL) was stirred at 90 • C under N 2 for 3 d.After the reaction solution was cooled to room temperature, crude product was precipitated by addition of methanol (50 mL) to the solution.The crude product was obtained as a gray powder by filtration and dried in vacuo.The pure product was obtained by Soxhlet extraction with CHCl 3 .Yield: 0.63 g (74%).An ethanol solution (120 mL) of RuCl 3 •3H 2 O (340 mg, 1.30 mmol) was refluxed under N 2 for 5 min.After the solution was cooled to room temperature, btpyaq (300 mg, 0.45 mmol) was added to the solution, followed by reflux under N 2 for 3 h.The precipitated brown powder was filtered and washed three times with methanol (20 mL) and acetone (20 mL), respectively.The product was dried in vacuo.Yield: 548 g (99%).Anal The solution was refluxed for 10 h under N 2 .A saturated NaSbF 6 solution in water (1 mL) was then added and the resulting mixture evaporated to ca. 10 mL.The crude product was obtained as a brown powder by filtration, dried in vacuo, and separated by column chromatography with Alumina A, akt.I (MP Biomedicals Germany GmbH) with CH 2 Cl 2 /EtOH (95/5) as the eluent.An aqueous solution of NaSbF 6 was added to the dark purple fraction.Concentration of the solution under reduced pressure gave a dark purple powder that was then separated by filtration.The pure product was obtained as a dark purple powder by recrystallization from CH 3 CN/diethyl ether and dried in vacuo.Yield: 154 mg (27%

Water Oxidation
An aqueous solution (5 mL) of (NH 4 ) 2 [Ce(NO 3 ) 6 ] (548 mg, 1.00 mmol) at pH 1.0, adjusted with HNO 3 , was added into a closed cell with a septum cap and an oxygen sensor.An Ar gas stream was passed through the solution for 15 min.A 2,2,2-trifluoroethanol solution (300 µL) of the catalyst (1.0 µmol) was added to the reaction cell.The concentration of oxygen in the gas layer was measured with an oxygen sensor.After the reaction, contamination of air was checked by GC-2014 (Shimazu Corp., Kyoto, Japan) with a molecular sieve 5A column.

Cyclic Voltammetry (CV) Measurements
CVs of aqueous solutions at pH 1.3 (adjusted with HNO 3 ) containing 0.2 mmol•L −1 of [3](BF 4 ) 3 and 100 mmol•L −1 of NaNO 3 as the electrolyte were measured at a scan rate of 50 mV•s −1 at 298 K using plate material evaluating cell (ALS Corp. Ltd., Tokyo, Japan) with an indium-tin-oxide (ITO) glass plate as the working electrode, a Pt wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode.The diameter of ITO electrode surface in contact with liquid was 7.8 mm.The test solutions were deoxygenated by passing a stream of Ar through them.

UV-Vis Spectral Measurements with Controlled-Potential Electrolysis
UV-vis spectral measurements with controlled-potential electrolysis were conducted with a UV cell (pass length = 10 mm) attached to an ITO electrode as the working electrode and a sample holder (ALS Co., Ltd.) containing SCE and Pt wire as the reference and counter electrodes, respectively.An aqueous solution at pH 1.3 (adjusted with HNO 3 ) containing [3](BF 4 ) 3 and NaNO 3 (100 mmol•L −1 ) was added to the optical cell and electrolyzed at different potentials.Spectral changes in the solution were observed by UV-vis spectral measurements.

Raman Spectral Measurements with Controlled-Potential Electrolysis
Electrolysis of H 2 16 O and H 2 18 O solutions at pH 1.0 (adjusted with HNO 3 ) containing 0.2 mol•dm −3 of [3](BF 4 ) 3 was conducted with a plate material evaluating cell (ALS Co., Ltd.) containing ITO as the working electrode and a sample holder (ALS Co., Ltd.) containing SCE and Pt wire as the reference and counter electrodes, respectively.After electrolysis of the solution at +1.6 V, the solution color changed from dark purple to dark green.Raman spectra of the dark green solutions were recorded at room temperature.
Scheme 1. Possible mechanisms of water oxidation.
1,8-positions of anthraquinone is 5.077 Å, as the steric repulsion of the O(2) of anthraquinone lengthens the distance between the two terpyridine moieties.This structural feature of btpyaq differs from that of the anthracene analog, btpyan, in which the two terpyridines are close to each other by π-stacking interactions[53].

When [ 3 ]
(BF4)3 was used as the catalyst, the initial reaction rate was directly proportional to the concentration of the catalyst and independent of the oxidant concentration (Figure3A,B, respectively).Thus, the reaction does not proceed via intermolecular coupling of two catalyst molecules.Sakai and Masaoka proposed a mechanism involving O-O formation through the coupling of Ru=O and Ce(IV)-OH as the typical oxidant for water oxidation.If the rate-determining step in the catalytic cycle is the coupling of Ru=O and Ce(IV)-OH, the reaction rate of water oxidation is directly proportional to the Ce(IV) concentration[59][60][61]. In our catalytic system, Ce(IV) does not participate in the rate-determining step of the catalytic cycle.

When [ 3 ]
(BF4)3 was used as the catalyst, the initial reaction rate was directly proportional to the concentration of the catalyst and independent of the oxidant concentration (Figure3A,B, respectively).Thus, the reaction does not proceed via intermolecular coupling of two catalyst molecules.Sakai and Masaoka proposed a mechanism involving O-O formation through the coupling of Ru=O and Ce(IV)-OH as the typical oxidant for water oxidation.If the rate-determining step in the catalytic cycle is the coupling of Ru=O and Ce(IV)-OH, the reaction rate of water oxidation is directly proportional to the Ce(IV) concentration[59][60][61]. In our catalytic system, Ce(IV) does not participate in the rate-determining step of the catalytic cycle.
O.These bands are assigned to Ru-O and O-O vibrations because the isotope shift values (−16 and −44 cm −1 ) are identical to the calculated value.The rate-determining step in the catalytic cycle is not the O-O bond formation, but the release of O2 from the [Ru-O-O-Ru] species.When [3](BF4)3 was used as the catalyst, the [Ru-O-O-Ru] species was not observed by Raman spectral measurements.These results clearly indicate that evolution of O2 from [Ru-O-O-Ru] is not the rate-determining step in the water oxidation by [3](BF4)3.According to the crystal structure of btpyaq (Figure 1), [3](BF4)3 displays higher flexibility of the
These bands are assigned to Ru-O and O-O vibrations because the isotope shift values (−16 and −44 cm −1 ) are identical to the calculated value.The rate-determining step in the catalytic cycle is not the O-O bond formation, but the release of O 2 from the [Ru-O-O-Ru] species.When [3](BF 4 ) 3 was used as the catalyst, the [Ru-O-O-Ru] species was not observed by Raman spectral measurements.These results clearly indicate that evolution of O 2 from [Ru-O-O-Ru] (BF 4 ) 3 produce [Ru(III)-Cl-Ru(III)] 5+ , to which OH − is slowly added.The resulting [Ru(III)-OH Cl-Ru(III)] 4+ undergoes oxidation coupled with deprotonation (PCET reaction) at the same potential of the second oxidation of [3](BF 4 ) 3 .[Ru(IV)=O O=Ru(IV)] 4+ is produced by PCET reaction of [Ru(IV)=O Cl-Ru(III)] 4+ in the same manner (Scheme 5).Catalysts 2017, 7, 56 9 of 16 Ru−Ru distance compared to the anthracene analog, [2](BF4)3.The structural flexibility of [3](BF4)3 would accelerate the release of an O2 molecule from the [Ru-O-O-Ru] species and changes the ratedetermining step in the catalytic cycle.

Scheme 5 .
Scheme 5. Mechanism for Ru(IV) species in the water oxidation catalytic cycle.

Scheme 5 .
Scheme 5. Mechanism for Ru(IV) species in the water oxidation catalytic cycle.

Scheme 5 .
Scheme 5. Mechanism for Ru(IV) species in the water oxidation catalytic cycle.For the oxygen evolution mechanism after the production of [Ru(IV)=O O=Ru(IV)] 4+ , two possibilities cannot be ruled out.From the results of UV-vis and Raman spectral measurements with controlled-potential electrolysis, [Ru(IV)=O O=Ru(IV)] 4+ is stable and does not form an O-O bond.It is at least necessary that [Ru(IV)=O O=Ru(IV)] 4+ loses an electron and is oxidized to [Ru(IV)=O O=Ru(V)] 5+ .There are considerable contributions from the oxo radical species, Ru(IV)-O −. , as a resonance form in the electronic state of Ru(V)=O.The radical character of Ru(IV)-O −. induces [Ru(IV)=O O=Ru(V)] 5+ to form [Ru(III)-O-O-Ru(IV)] 5+ .An oxygen molecule is released by substitution of two water molecules to give [Ru(II)-OH HO-Ru(III)] 3+ , as observed by CV and UVvis spectral measurements (intramolecular coupling mechanism, Scheme 6).