**2. Photoisomerization of polypyridyl ruthenium(II) aquo complexes**

With respect to the photoisomerization of polypyridyl Ru(II) aquo complexes, the photoisomerization of *cis*-[Ru(bpy)2(OH2)2] 2+ (bpy = 2,2<sup>0</sup> -bipyridine) to its trans form in aqueous media was first reported by Meyer [24]. The mechanism of the photoisomerization reaction was later investigated by Planas et al. [41]. In this case, the *trans* form was present as a photostationary state and slowly went back to the original *cis* form. To the best of our knowledge, the *cis*-[Ru(bpy)2(OH2)2] 2+ had been the only one polypyridyl Ru(II) aquo complex that exhibits photoisomerization behavior before we presented the photoisomerization of *d*-**1** to *p*-**1** [21]. Furthermore, we reported the reversible photoisomerization equilibrium between *distal- and proximal-*[Ru(tpy)(pyqu)OH2] 2+ (*d*-**2** and *p*-**2**) isomers with a ligand of 2-(2-pyridyl)quinoline (pyqu) instead of pynp, in contrast to the irreversible photoisomerization of *d*-**1** to *p*-**1**. The aspect and mechanism of the irreversible photoisomerization of *d*-**1** are first described, followed by those of the reversible one of *d*�/*p*-**2** in this section.

#### **2.1 Irreversible photoisomerization of** *distal***-[Ru(Xtpy)(pynp)OH2] 2+ to the** *proximal***-isomer**

Photoisomerization behavior of *d*-**1** was investigated by <sup>1</sup> H-NMR and UV–Vis spectroscopy. Upon irradiation of visible light to a D2O solution of *d*-**1**, the NMR

*Mechanisms of Photoisomerization and Water Oxidation Catalysis of Ruthenium(II) Aquo… DOI: http://dx.doi.org/10.5772/intechopen.99730*

peak at 9.6 ppm due to *d*-**1** decreased with the irradiation time and disappeared completely after 25 min under the experimental conditions (**Figure 2A**) [21]. The new peak at 8.9 ppm assigned to *p*-**1** increased with the concomitant decrease of *d*-**1**. **Figure 2B** displays the concentration profile of *d*-**1** and *p*-**1**, which indicates that irreversible and stoichiometric photoisomerization from *d*-**1** to *p*-**1** proceeds in water by visible light irradiation (**Figure 1**). By contrast, no isomerization of *d*-**1** in water was found to occur under thermal treatment. The photoisomerization rate showed a first-order dependence on *d*-**1** concentration, and the kinetic analysis provided the observed rate constant ((*k*d-p)obs/s�<sup>1</sup> ) of photoisomerization (*distal* to *proximal*) to be 4.1 x 10�<sup>3</sup> s �<sup>1</sup> under the conditions employed (λ > 420 nm, 180 mW cm�<sup>2</sup> ). The (*k*d-p)obs values increased linearly with respect to the light intensity below 255 mW cm�<sup>2</sup> , indicating that the photoexcited state participated in photoisomerization under the employed conditions. Arrhenius plots of the photoisomerization gave a straight line in a range of 10 � 35°C, providing 41.7 kJ mol�<sup>1</sup> of activation energy (*E*a) for the photoisomerization (**Table 1**). Interestingly, the (*k*d-p)obs value decreased drastically at pD <sup>&</sup>gt; 7 (pD = �log [D+ ]), while it was unchanged over pD 1-7. The UV–Vis spectrophotometric pH titration of *d*-**1** gave p*K*<sup>a</sup> of 9.7 attributed to the deprotonation of an aquo ligand to form the hydroxo complex, *distal*-[Ru(tpy)(pynp)OH]<sup>+</sup> . The trend of (*k*d-p)obs change depending on pH corresponds to the fraction of *d*-**1** (aquo form) dissolved in the solution versus pH, suggesting that the hydroxo form of *d*-**1** is inert for the photoisomerization [21, 22].

The internal quantum yield (Φ) for photoisomerization, which is defined as the ratio of the number of the photoisomerized complexes to the number of incident photons of a given energy, was estimated from the UV–vis spectral change in the experiment under monochromatic light irradiation (520 nm, 26.4 mW cm�<sup>2</sup> ). The Φ values were calculated according to the following equation:

$$\Phi = \frac{hcN\_A k\_{pi} n\_{\text{int}}}{p\lambda A(1\text{-}T)} \tag{1}$$

where *h*, *c*, *NA*, *kpi*, *nint*, *p*, λ, *A* and *T* are Plank's constant, the speed of light, Avogadro's number, the rate constant for photoisomerization, initial amount of the complex, photon flux, wavelength, the irradiated area and the transmittance,

#### **Figure 2.**

*(A) Time course of <sup>1</sup> H NMR spectral changes during the photoisomerization of* d*-1 in D2O with 180 mW cm*�*<sup>2</sup> visible light irradiation (λ > 420 nm). (B) Kinetic profiles of* d*-1 (red circle) and of* p*-1 (blue square). (Reproduced with permission of American Chemical Society from ref. [21]).*


*a* c*Ru is concentrations of ruthenium complexes. The marks of "n.m." and "n.r." mean "not measured" and "no reaction", respectively. <sup>b</sup>*

*The filtered halogen lamp was used for visible light irradiation (λ > 420 nm, 180 mW cm*�*<sup>2</sup> ).*

#### **Table 1.**

*Summary of the observed rate constants and internal quantum yields of photoisomerization of* d�*/*p*-1,* <sup>d</sup>�*/*p*-Cl1,* <sup>d</sup>�*/*p*-<sup>2</sup> and* <sup>d</sup>�*/*p*-<sup>3</sup> isomers at 25°C.a*

respectively. The Φ values for photoisomerization from *d*-**1** to *p*-**1** are 1.5 and 0.31% at 0.088 and 3.9 mM, respectively, as shown in **Table 1**.

In order to investigate influence of chloro substituent on photoisomerization of *d*-**1**, *distal*- and *proximal*-[Ru(Cl-tpy)(pynp)OH2] 2+ (*d*-**Cl1** and *p*-**Cl1**; Cl-tpy = 4<sup>0</sup> chloro-2,2<sup>0</sup> ;6<sup>0</sup> ,2″-terpyridine) complexes were prepared. When an aqueous solution of *d*-**Cl1** was irradiated with visible light (λ > 420 nm, 180 mWcm�<sup>2</sup> ), the stoichiometric photoisomerization of *d*-**Cl1** to *p*-**Cl1** was observed as it is for *d*-**1** (**Figure 1**). The rate constant for photoisomerization of *d*-**Cl1** was estimated to be (*k*d-p)obs = 4.9 x 10�<sup>3</sup> s �<sup>1</sup> [35], which is higher than that ((*k*d-p)obs = 4.1 x 10�<sup>3</sup> s �1 ) [22, 34] observed for *d*-**1** under the same conditions (**Table 1**). Additionally, the Φ value of *d*-**Cl1** (2.1%) is higher than that (Φ =1.5%) of *d*-**1**.

Several groups reported that some polypyridyl complexes undergo photosubstitution reactions *via* the triplet metal centered (3 MC) state from the <sup>3</sup> MLCT excited state [25–27]. According to the reports, a possible mechanism for the photoisomerization of *d*-**1** and *p*-**1** isomers was speculated as follows. The <sup>3</sup> MLCT excited state of *d*-**1** is generated by absorption of visible light. The photodissociation of the aquo ligand from the exited *d*-**1** proceeds through the thermal accessible <sup>3</sup> MC state, leading to the formation of the penta-coordinated [Ru(tpy) (pynp)]2+ intermediate. The *p*-**1** isomer is formed by re-coordination of a water molecule to the penta-coordinated intermediate from the opposite direction of a tpy plane. The temperature-dependent transient absorption spectroscopic measurements of *d*-**1** suggest existence of the thermally activated process from the <sup>3</sup> MLCT state with an *E*<sup>a</sup> of 49 kJ mol�<sup>1</sup> [22], which is close to those from the <sup>3</sup> MLCT state to the <sup>3</sup> MC state reported for various polypyridyl Ru complexes [42–45]. The agreement of the *E*<sup>a</sup> value (49 kJ mol�<sup>1</sup> ) with that (41.7 kJ mol�<sup>1</sup> ) calculated from the Arrhenius plot for the photoisomerization (**Table 1**) also supports the possibility that a main activation process of the photoisomerization reaction is the thermal transition from the <sup>3</sup> MLCT state to the <sup>3</sup> MC state. However, Density functional

*Mechanisms of Photoisomerization and Water Oxidation Catalysis of Ruthenium(II) Aquo… DOI: http://dx.doi.org/10.5772/intechopen.99730*

theory (DFT) calculations suggested a different activation process, where the *distal***-**penta-coordinated intermediate changes the conformation to *proximal***-**pentacoordinated intermediate for *p*-**1** while maintaining the otherwise octahedral structure of *d*-**1** [22].

To obtain deeper mechanistic insights into the irreversible photoisomerization, the <sup>3</sup> MLCT excited states of the *d*-**1** and *p*-**1** were characterized by the time-resolved infrared spectroscopy (TR-IR) [36]. The decay of the photoexcited <sup>3</sup> MLCT states for both isomers were investigated by the TR-IR analysis, and the lifetimes of the excited state for the *d*-**1** and *p*-**1** were determined to be 9.7 ns and 6.4 ps, respectively. In general, the decay of the <sup>3</sup> MLCT excited states of Ru polypyridyl complexes occurs on nanosecond timescale or above, because the transition from a triplet excited state to a singlet ground state is forbidden by spin selection rules. The very short excited lifetime (6.4 ps) for *p*-**1** imply that a non-radiative process from the <sup>3</sup> MLCT state was accelerated, so that *p*-**1** is inert for photoisomerization to *d*-**1**.

The large difference in lifetimes of the <sup>3</sup> MLCT state between *d*-**1** and *p*-**1** was interpreted by geometry optimization calculations using DFT of both *d*-**1** and *p*-**1** isomers in the singlet ground (S0) and <sup>3</sup> MLCT (T1) states. While both the structures of *d*-**1** and *p*-**1** isomers in the S0 states (indicated by lighter color atoms in **Figure 3**) show no considerable distortion, the aquo ligand was restricted by a hydrogen bond (1.48 Å) between its H atom and an N atom on the pynp ligand for *p*-**1**. The transition from S0 state to T1 state of *d*-**1** results in a significant change in the dihedral angle of the pynp plane to the tpy plane from 180° to 161°, together with bending at the bond between the naphthyridine and pyridine rings. The distortion for *d*-**1** is likely to originate from the steric hindrance between the extended π\* orbital of the pynp ligand and the π-orbital of the tpy ligand owing to the charge localization on the pynp ligand in the T1 state. In the case of *p*-**1**, on the other hand, no considerable distortion was observed in the transition (**Figure 3**), but the Ru-O (1.98 Å) and hydrogen bonds (1.06 Å) between the pynp and the water ligands were shorter compared with the Ru-O (2.12 Å) and hydrogen bonds (1.48 Å) in the S0 state. The shortened hydrogen bond is attributed to the charge localization on the pynp ligand in the T1 state.

**Figure 4** shows the DFT-calculated energy diagram of *d*-**1** and *p*-**1** in the S0, T1 and putative <sup>3</sup> MC states. *p*-**1** in the S0 state is more stable than *d*-**1** by 65 kJ mol<sup>1</sup> because of hydrogen bond interaction between the pynp and the aquo ligand. The energy difference between the S0 and T1 states for *d*-**1** is 176 kJ mol<sup>1</sup> , which is remarkably higher than that (101 kJ mol<sup>1</sup> ) for *p*-**1**. The higher energy difference for *d*-**1** is mainly due to the significant distortion of the pynp ligand on the transition from the S0 to T1 states (**Figure 3**). As a result, the energy of *d*-**1** in the T1 state is higher than that of *p*-**1** in the T1 state by 140 kJ mol<sup>1</sup> . Considering the similar ligand field for both isomers, the energy difference in the <sup>3</sup> MC state between *d*-**1** and *p*-**1** is assumed to be not as much as that (140 kJ mol<sup>1</sup> ) in T1 states. For *p*-**1**, the 3 MC state is presumed to be located at much higher energies than that in the T1 state. This suggests the possibility of the different decay mechanism of *p*-**1** in the T1 state from the case of *d*-**1** (usual lifetime of 9.7 ns), exhibiting the non-radiative decay through the thermally populated <sup>3</sup> MC state. The possible mechanism is direct relaxation of the T1 to S0 states according to the energy-gap law: the decay rate decreases exponentially with increasing the energy gap between excited and ground states [46–48]. Considering almost the same geometries of *p*-**1** between the S0 and T1 states, the law can be applied as the weak coupling limit. The lower energy band gap of 101 kJ mol<sup>1</sup> between the S0 and T1 states for *p*-**1** could explain the short lifetime, at least qualitatively, based on the energy gap law. The direct relaxation mechanism is consistent with the non-thermal decay process suggested by the very quick decay (unusual lifetime of 6.4 ps) for *p*-**1** in the T1 state. The direct relaxation

#### **Figure 3.**

*Overlay of optimized structures of* d*-1 and* p*-1 in the S0 (lighter color) and T1 (deeper color) states. The top and bottom structures are illustrated from the horizontal directions of the pynp and tpy planes, respectively. The colored labels of atoms are as follows: Green, Ru; gray, carbon; red, oxygen; blue, nitrogen; white, hydrogen. (reprinted with permission from ref. [36] copyright 2015 Elsevier).*

process from the T1 states to the S0 state (not *via* the <sup>3</sup> MC state) could explain the inert photoisomerization of *p*-**1**.

#### **2.2 Reversible photoisomerization between** *distal***- and** *proximal***-[Ru(tpy) (pyqu)OH2] 2+ isomers**

The photoisomerization between *d*-**2** and *p*-**2** was examined to reveal the influence of structures of bidentate ligands on the photoisomerization of *d*-**1** (**Figure 5**) [34]. On irradiation of visible light to a D2O solution of *p*-**2**, the 8.9 ppm peak characteristic of *p*-**2** in the <sup>1</sup> H NMR spectrum (**Figure 6A**) decreased with increasing the 9.5 ppm peak assigned to the *d*-**2** isomer. The <sup>1</sup> H NMR spectral change reached saturation in 5 min, while the 8.9 ppm peak did not completely disappear. This result suggests that the photoisomerization of *p*-**2** to *d*-**2** reaches a photostationary state (**Figure 5**). This shows that irreversible photoisomerization alters to reversible one by the replacement of the pynp ligand to pyqu in mononuclear Ru aquo complexes [21, 22]. From the integrated peak areas at 8.9 and 9.5 ppm, the equilibrated concentration ratio of *p*-**2**:*d*-**2** was calculated to be 76:24 (**Figure 6B**). The kinetic profiles of the photoisomerization of *p*-**2** to *d*-**2** were well fitted with a reversible reaction model, giving the observed rate constants of the forward reaction (*proximal* to *distal*, (*k*p-d)obs/s<sup>1</sup> ) of 4.3 0.1 <sup>10</sup><sup>3</sup> <sup>s</sup> <sup>1</sup> and the back reaction (*distal* to *proximal*, (*k*d-p)obs/s<sup>1</sup> ) of 1.2 0.03 <sup>10</sup><sup>2</sup> <sup>s</sup> <sup>1</sup> under the conditions employed (λ > 420 nm, 180 mW cm<sup>2</sup> ), respectively (**Table 1**) [34]. The observed equilibrium constant were given to *K*obs (= (*k*p-d)obs/(*k*d-p)obs) = 0.34 0.01 under

*Mechanisms of Photoisomerization and Water Oxidation Catalysis of Ruthenium(II) Aquo… DOI: http://dx.doi.org/10.5772/intechopen.99730*

#### **Figure 4.**

*DFT-calculated energy diagram of* d*-1 and* p*-1 in the S0 and T1 states. The speculated energy levels of Tn 3 MC states were described. (see text) (reprinted with permission from ref. [36] copyright 2015 Elsevier).*

**Figure 5.**

*Photoisomerization equilibrium between* proximal*- and* distal*-[Ru(tpy)(pyqu)OH2] 2+ (*p*-2 and* d*-2) isomers.*

the employed conditions. Both (*k*p-d)obs and (*k*d-p)obs increased linearly with respect to light intensity up to 180 mW cm<sup>2</sup> , suggesting that the photoexcited states of *p*-**2** and *d*-**2** isomers are also involved in the forward and back reactions, respectively [34]. As shown in **Figure 7**, (*k*p-d)obs and (*k*d-p)obs decreased from pD = 8 to 12, and the photoisomerization did not take place at all above pD =12 [34]. This result implies that both hydroxo isomers, *proximal*- and *distal*-[Ru(tpy)(pyqu)OD]<sup>+</sup> are inert for the forward and back photoisomerization reactions, respectively, as observed in the case of *distal*-[Ru(tpy)(pynp)OH]<sup>+</sup> [21, 22]. The pD-dependent (*k*p-d)obs and (*k*d-p)obs analysis demonstrated inflection points at pD = 10.5 and 9.8 for the

#### **Figure 6.**

*(A) Time course of <sup>1</sup> H NMR spectral changes during the photoisomerization of* p*-2 (3.9 mM, pD = 8.4) with 180 mW/cm<sup>2</sup> visiblelight irradiation (λ > 420 nm) in D2O. (B) Kinetic profiles of* p*-2 (blue square) and* d*-2 (red circles). (reproduced from ref. [34] with permission of John Wiley & Sons, Inc.).*

#### **Figure 7.**

*Plots of observed rate constants ((*k*p-d)obs and (*k*d-p)obs) vs. pD for the photoisomerization reactions shown in Figure 5. Visible light (λ > 420 nm, 180 mW/cm<sup>2</sup> ) was irradiated to the* p*-2 (3.9 mM) in D2O at 25°C. Inset shows diagram of observed equilibrium constants* K*obs vs. pD. (reproduced from ref. [34] with permission of John Wiley & Sons, Inc.).*

forward and back reactions, respectively. These values are close to the p*K*<sup>a</sup> of *p*-**2** (p*K*<sup>a</sup> = 10.5) and *d*-**2** (p*K*<sup>a</sup> = 9.4) (**Table 1**). The difference in p*K*<sup>a</sup> values (1.1) between *p*-**2** and *d*-**2** results in the markedly pD-dependent *K*obs values (inset of **Figure 7**). *K*obs increased above pH = 9, and reached its maximum value (*K*obs = 2.5) at pH = 12. This trend is consistent with the observation that the yield of *d*-**2** generated in the photoisomerization raised from 26–65% on increasing from pH 5.7 to 12.

The thermal dependent kinetics for (*k*p-d)obs and (*k*d-p)obs in a temperature range of 25 to 70°C gives *<sup>E</sup>*<sup>a</sup> = 30.6 2.9 and 23.3 2.1 kJ mol<sup>1</sup> for the forward and back reactions, respectively. The higher *E*<sup>a</sup> of the forward reaction ((*k*p-d)obs) compared to the back one ((*k*d-p)obs) is attributable to the unfavorable loss of the hydrogen bond (C-HO) between the H atom bonded to 8-C of the quinoline moiety of the pyqu ligand and the O atom of the aquo ligand upon water

*Mechanisms of Photoisomerization and Water Oxidation Catalysis of Ruthenium(II) Aquo… DOI: http://dx.doi.org/10.5772/intechopen.99730*

dissociation for photoisomerization. The van't Hoff plots for *K*obs gave <sup>Δ</sup>H = 7.7 2.7 kJ mol<sup>1</sup> , that is close to enthalpy for the hydrogen bond (5.4 kJ mol<sup>1</sup> ) of C-HO in the benzene-water complex [49].

The Φ values for the forward and back photoisomerization reactions were estimated to be 0.34% and 1.1% from the experiments using monochromatic light (520 nm, 26.4 mW cm<sup>2</sup> ) (**Table 1**). The higher Φ (1.1%) of *d***-2** compared to that (0.34%) of *p*-**2** could be attributed to the enthalpy of hydrogen bond (C-HO) of *p*-**2** (decreased Φ for *p*-**2**) and the steric repulsion between the H atom bonded to 8- C of the quinoline moiety of pyqu and the tpy plane for *d*-**2** (increased Φ for *d*-**2**). The Φ value of 1.1% for photoisomerization from *d*-**2** to *p*-**2** is 3.5 times higher than that (0.31%) for the photoisomerization from *d*-**1** to *p*-**1** under the same concentration [34]. This could also be ascribed to the dominant steric repulsion between the tpy ligand and the H atom bonded to 8-C of the quinoline ring of pyqu for *d*-**2**, compared to the repulsion between the tpy ligand and the uncoordinated nitrogen of the naphthyridine ring of pynp for *d*-**1**.

#### **2.3 New synthetic strategy for dinuclear ruthenium(II) complexes utilizing the photoisomerization**

The photoisomerization of Ru(II) aqua complexes was applied to the strategic synthesis of dinuclear Ru complexes that are difficult to be synthesized by conventional thermal reactions so far. Herein, we succeeded to newly synthesize several dinuclear Ru(II) complexes, *proximal*, *proximal*-[Ru2(tpy)2LXY]3+ (L = 5-phenyl-2,8-di(2-pyridyl)-1,9,10- anthyridine, X and Y = other coordination sites; denoted as *p*,*p*-**Ru2XY**) utilizing the photoisomerization of a mononuclear Ru aquo complex [37]. The tetradentate backbone ligand L was used to form a *proximal*, *proximal* dinuclear Ru(II) structure (**Figure 8**). The reaction of [Ru(tpy)Cl3] with L gave *distal*-[Ru(tpy)LCl]<sup>+</sup> (*d*-**3Cl**) selectively in ethanol in a 59% isolated yield. This selective formation of *d*-**3Cl** was possibly caused by the steric hindrance of L with a chloro ligand. *d*-**3Cl** was then converted to *distal*-[Ru(tpy)LOH2] 2+ (*d*-**3**) by chloro subtraction with a silver salt in water in a 90% isolated yield. The thermal reaction of *d*-**3** with a second ruthenium center [Ru(tpy)Cl3] for dimerization failed to give

#### **Figure 8.**

*Synthetic scheme of a μ-Cl bridged dinuclear ruthenium complex,* p*,*p*-Ru2(μ-Cl) using photoisomerization of* d*-3. (reproduced from ref. [37] with permission of American Chemical Society).*

**Figure 9.**

*Reversible bridging-ligand substitution reactions among* p*,*p*-Ru2(μ-Cl),* p*,*p*-Ru2(μ-OH)* p*,*p*-Ru2(OH) (OH2).*

the *proximal*, *proximal*-dinuclear Ru(II) complex owing to the steric hindrance located between the tpy ligand on the *d*-**3** and the one on [Ru(tpy)Cl3]. However, if the photoisomerization of *d*-**3** to *proximal*-[Ru(tpy)LOH2] 2+ (*p*-**3**) is utilized (**Figure 8**), the steric constraint of *d*-**3** for formation of the *proximal*,*proximal*dinuclear Ru(II) species could be avoided. The photoisomerization of *d*-**3** stoichiometrically progressed to form *p*-**3** in water/ethanol mixture under visible light irradiation (λ > 420 nm). The subsequent thermal reaction of *p*-**3** with [Ru(tpy)Cl3] in water/ethanol mixture successfully generated *proximal*,*proximal*-[Ru2(tpy)2L(μ-Cl)]3+ (*p*,*p*-**Ru2(μ-Cl)** in a 67% isolated yield that was unambiguously characterized by X-ray diffraction [37]. The *p*,*p*-**Ru2(μ-Cl)** was converted to the *proximal*,*proximal*-[Ru2(tpy)2L(μ-OH)]3+ (*p*,*p*-**Ru2(μ-OH)**) in neutral or slightly basic aqueous medium via substitution of the μ-Cl bridge with an OH ion as shown in **Figure 9** [37, 38]. The *p*,*p*-**Ru2(μ-OH)** was then converted to *proximal*,*proximal*-[Ru2(tpy)2L (OH)(OH2)]3+ (*p*,*p*-**Ru2(OH)(OH2)**) via insertion of a water molecule to the central core to reach equilibrium between *p*,*p*-**Ru2(μ-OH)** (10%) and *p*,*p*-**Ru2(OH)(OH2)**.

## **3. Water oxidation catalysis by ruthenium(II) aquo complexes**

The mononuclear ruthenium polypyridyl aquo complexes are promising WOCs to understand water oxidation chemistry and provide the guided thought for efficient WOCs because of their simple structures, high catalytic activities, ease of chemical modification, and informative knowledge including catalytic mechanisms [13, 21–23, 34, 35, 40, 50–52]. Controlling the electron density on the metal center as an active site by alternating the substituent groups on their ligands is a common approach to reveal the mechanism and influencing factors for water oxidation catalysis.

The molecular WOCs take part in the process of the four electrons and four protons removal from two water molecules, either consecutively or concerted, to form the O-O bond. Understanding the mechanism of the O-O bond formation is vital in improvement of molecular WOCs. Two main classes of proposed reaction mechanisms at metal centers are shown in **Figure 10**; water nucleophilic attack (*WNA*) on metal-oxo centers (M = O) and interaction of two M-O centers (*I2M*) [53–56]. In the *WNA* mechanism (**Figure 10a**), an electrophilic high-valent metaloxo (Mn+ = O) species is formed via multiple consecutive oxidation steps. A nucleophilic attack of a water molecule on the Mn+ = O species occurs, that leads to formation of a hydroperoxide (M(n2)+-OOH) species. Further oxidation and

*Mechanisms of Photoisomerization and Water Oxidation Catalysis of Ruthenium(II) Aquo… DOI: http://dx.doi.org/10.5772/intechopen.99730*

#### **Figure 10.**

*Schematic representation of the two mechanistic pathways for O-O bond formation. (reproduced with permission from ref. [20]. Copyright 2019 Wiley).*

deprotonation steps generate a M(n�1)+-OO� intermediate, which releases O2 and converts to M(n�2)+-OH2 with incorporation of OH2. A lot of mononuclear complexes have been proposed to undergo the *WNA* mechanism for water oxidation [21–23, 34, 35, 40, 51, 52, 57, 58]. In the *I2M* mechanism (**Figure 10b**), the coupling of either two Mn+-O oxyl radicals or coupling of one Mn+-O oxyl radical with another Mn+-O unit of a non-radical character affords a peroxo Mn+-O-O-Mn+ intermediate, which releases O2 and returns to 2 M(n�2)+-OH2 with incorporation of OH2. It involves intramolecular [37, 59, 60] and intermolecular [61, 62] pathways.

In this section, we introduce our recent progress on chemical and electrochemical water oxidation catalyses by mono- and dinuclear ruthenium(II) aquo complexes in homogeneous systems. Firstly, substitution effects on the catalytic activity and mechanism of mononuclear ruthenium(II) aquo complexes for chemical water oxidation are described in Section 3-1. Secondly, the difference in the catalytic properties between *distal*�/*proximal*-isomers is explicated in Section 3-2. Finally, the electrocatalytic activities of a series of dinuclear ruthenium(II) complexes are discussed in Section 3-3.

#### **3.1 Substitution effect on the catalytic activities of mononuclear ruthenium complexes for water oxidation**

To investigate the catalytic aspects of [Ru(Rtpy)(bpy)(H2O)]2+ (Rtpy = 2,2<sup>0</sup> :60 ,2″-terpyridine derivatives) complexes (**4R**) having a variety of 4<sup>0</sup> substituent groups on Rtpy ligand (**Figure 11**), chemical water oxidation experiments were performed in a homogeneous aqueous solution using a CeIV as a

sacrificial oxidant [39, 40]. For **4EtO** (R = EtO), O2 was significantly evolved from the catalytic solution, and the amount (*n*O2/μmol) of O2 evolved increased linearly until 100 min and then saturated to 35 μmol at 4 h, which is 5 times higher than that for **4H** (**Figure 12**) [39]. The initial O2 evolution rates (*v*O2/mol s<sup>1</sup> ) were calculated from the initial slopes. The *v*O2 for **4EtO** increased with the CeIV concentration (*c*Ce/ M) and reached a saturation at *c*Ce = 1.0 M (**Figure 13**), indicating that O2 evolution is zero order with respect to CeIV under the large excess CeIV conditions. This behavior was well-analyzed by Michaelis–Menten-like kinetics to give the maximum catalytic rate (*<sup>v</sup>* max) of 1.5 (0.08) x 10<sup>1</sup> mol s<sup>1</sup> (**4EtO** mol)<sup>1</sup> and the constant (*K*<sup>m</sup> = 1.2 (0.06) mmol) in terms of the CeIV concentration for the half value of *v*max. The oxidation reaction from RuIV=O to RuV=O by CeIV could involve the redox equilibrium prior to oxygen evolution because the redox potential (1.45 V vs. SCE) of RuIV=O/RuV=O for **4EtO** is close to standard potential (E° = 1.47 V vs. SCE (1.71 V vs. NHE) [63]) of CeIII/IV. The redox equilibrium presumably leads to saturation of the *v*O2 with increase of the CeIV concentration (**Figure 13**). The *v*O2 increased linearly with the **4EtO** concentration (*c*Ru/M) under the large excess CeIV conditions (*c*Ce = 0.1 M, *c*Ru = 0.2 mM, 5.0 ml water) (**Figure 14**), showing that the O2 evolution is a first order process with respect to **4EtO**. This result is consistent with the case of earlier-reported mononuculear Ru(II) aquo complexes [50, 51].

The mechanism of water oxidation by **4R** is shown in **Figure 15** based on the WNA mechanism [50, 51, 56]. **4R** (abbreviated to RuII-OH2 as the oxidation state) is oxidized by CeIV to RuIII-OH2 by a 1-electron process, and subsequently oxidized to RuIV=O by two-proton/one-electron process at pH = 1.0 [39, 40]. RuIV=O is further oxidized to RuV=O, involving the above-mentioned redox equilibrium (step I, **Figure 15**). RuV=O undergoes water nucleophilic attack (step II, **Figure 15**) to

**Figure 11.** *Chemical structures of mononuclear ruthenium(II) aquo complexes.*

*Mechanisms of Photoisomerization and Water Oxidation Catalysis of Ruthenium(II) Aquo… DOI: http://dx.doi.org/10.5772/intechopen.99730*

#### **Figure 12.**

*Time courses of the amount (*n*O2/Mol) of O2 evolved in chemical water oxidation experiments in an aqueous solution at 25°C using a CeIV oxidant.* c*Ce = 0.1 M (0.5 mmol); complexes, 20 μM (0.1 μmol); pH = 1.0; liquid volume, 5.0 ml. (*○*) 4EtO, (*●*) 4H and (*□*) without complex. (reproduced with permission from ref. [39]. Copyright 2011 the Royal Society of Chemistry).*

**Figure 13.**

*Plots of* v*O2 versus the CeIV concentration (*c*Ce/M) in chemical water oxidation experiments using 0.2 mM 4EtO (5.0 ml water) at 25°C. the solid line is the simulated curve based on a Michaelis–Menten-like kinetic equation. (reproduced with permission from ref. [39]. Copyright 2011 the Royal Society of Chemistry).*

form an O-O bond in the RuIII-OOH intermediate. Finally, O2 is produced via further oxidation to RuIV-OO, with RuII-OH2 regenerated by incorporation of water.

The *v*O2 showed a first-order dependence on the complex amount for all the **4R** derivatives (**Figure 14**) [40], suggesting that O2 is produced by a unimolecular

**Figure 14.**

*Plots of initial O2 evolution rate (*v*O2/Mol s*�*<sup>1</sup> ) versus the amount of 4R complexes.* c*Ce = 0.1 M, 5.0 ml water, pH = 1.0. (reproduced with permission from ref. [40]. Copyright 2019 American Chemical Society).*

#### **Figure 15.**

*Mechanism of water oxidation by 4R complexes at pH = 1.0. (reprinted with permission from ref. [40]. Copyright 2019 American Chemical Society).*

reaction of **4R**. Turnover frequencies (*k*O2/s�<sup>1</sup> ) were calculated from the slopes of the linear relationships. The *k*O2 values were largely affected by the Rtpy ligands, being variable from 0.05 to 44 x 10�<sup>2</sup> s �<sup>1</sup> (as the highest *k*O2 for **4EtO**) by a factor of 880 (**Table 2**). The *k*O2 values were plotted with respect to Hammett constant *σ*<sup>p</sup> of the 4<sup>0</sup> -substituents of Rtpy ligands in **Figure 16A**. However, the Me2N group is protonated to give Me2NH<sup>+</sup> under the catalytic conditions (pH = 1.0), and the *σ*<sup>p</sup> value of Me2NH<sup>+</sup> was assumed as *σ*<sup>p</sup> = 0.71 of the midpoint between *σ*<sup>p</sup> = 0.60 for protonated amino group and *σ*<sup>p</sup> = 0.82 for protonated trimethylamino group [64]. The *<sup>k</sup>*O2 values were almost constant (3.4 � 6.1 x 10�<sup>2</sup> <sup>s</sup> �1 ) in a range of *σ*<sup>p</sup> = �0.17 (R = Me) � 0.23 (R = Cl), although the *<sup>k</sup>*O2 value (5 x 10�<sup>4</sup> <sup>s</sup> �1 ) of **4Me2N** is lower than these values. The *k*O2 value increased sharply at *σ*<sup>p</sup> = �0.24 (R = EtO) to 4.4 x10�<sup>1</sup> s �<sup>1</sup> as the maximum, and thereafter decreased with the *<sup>σ</sup>*<sup>p</sup> decrease to �0.37 (R = OH). This result demonstrates that very critical Hammett constant

*Mechanisms of Photoisomerization and Water Oxidation Catalysis of Ruthenium(II) Aquo… DOI: http://dx.doi.org/10.5772/intechopen.99730*


#### **Table 2.**

*Summary of* k*O2 by mononuclear Ru(II) aquo complexes.*

#### **Figure 16.**

*Plots of the turnover frequency (*k*O2/s*�*<sup>1</sup> )* versus *Hammett constant (*σ*p) of 4*<sup>0</sup> *-substituent groups (A) and* versus *the redox potentials of a RuIV=O/RuV=O pair (B). The Me2N group is protonated to a Me2NH+ group under the experimental conditions for water oxidation (pH = 1.0), and the* σ*<sup>p</sup> value of Me2NH<sup>+</sup> was assumed as* σ*<sup>p</sup> = 0.71 of the midpoint between* σ*<sup>p</sup> = 0.60 for protonated amino group and* σ*<sup>p</sup> = 0.82 for protonated trimethylamino group [64]. The redox potentials were estimated from* E*ap values, where* E*ap is an anodic peak potential corresponding to the RuIV=O/RuV=O couple in a cyclic voltammogram for each of 4R complexes. The red dashed line indicates the standard redox potential of CeIII/IV [63]. (Reproduced with permission from ref. [40]. Copyright 2019 American Chemical Society).*

(*σ*<sup>p</sup> = �0.27 � �0.24) exists for the high *k*O2 values. The very low *k*O2 values for **4OH** and **4Me2N** arise from the difficulty of RuV=O formation. Most likely, **4OH** and **4Me2N** are considered to decompose through their ligand oxidation during water oxidation catalysis, as reported recently [65, 66].

The *k*O2 values were plotted with respect to redox potentials of RuIV=O/RuV=O in **Figure 16B**. The *k*O2 values increased with the decrease of the redox potentials in an order of **4H**, **4Cl**, **4Me** < **4MeO** < **4PrO** < **4EtO**. This indicates that the potential for formation of RuV=O species is essential in the water oxidation catalysis under the employed conditions. This *k*O2 profile can be explained by the increased

fraction of RuV=O in the equilibrium. In this case, the oxidation rate (step I in **Figure 15**) from RuIV=O to RuV=O by CeIV could be comparable with the rate of the nucleophilic attack of water on RuV=O (step II in **Figure 15**) to be involved in a ratedetermining step. The O-O bond formation process *via* water nucleophilic attack could be no longer a rate-determining step singularly. One might deservedly expect that the *k*O2 values of **4H**, **4Cl**, and **4Me** with the higher redox potentials of RuIV=O/RuV=O are higher than those of **4MeO**, **4PrO** and **4EtO** with the lower redox potentials since water nucleophilic attack on RuV=O (step II, **Figure 15**) is assumed to accelerate because of the higher electrophilicity of the oxo of RuV=O for the formers. By contrast, the *k*O2 values of **4H**, **4Cl**, and **4Me** were indeed lower than those of **4MeO**, **4PrO** and **4EtO**, which could be explained by the slow oxidation rate (step I in **Figure 15**) from RuIV=O to RuV=O involved in a ratedetermining step.

### **3.2 Comparison in catalytic activities between** *distal* **and** *proximal* **isomers of mononuclear ruthenium complexes**

To explore the catalytic aspects of a series of *distal* and *proximal* isomers for the mononuclear Ru(II) aquo complexes, chemical water oxidation experiments were conducted in a homogeneous aqueous solution using a CeIV oxidant [21, 22]. O2 was significantly evolved from a mixed solution of *d***-1** and CeIV, and *n*O2 increased linearly with time until 50 min (**Figure 17**). *v*O2 increased linearly with respect to the amount of the complex, and the slope of the linear relationship provides *k*O2 = 3.8 x 10<sup>3</sup> s <sup>1</sup> (**Figure 18**). The same chemical water oxidation experiments were carried out after visible light irradiation to the solution of *d***-1** for 1 h to generate *p***-1** completely (**Figure 1**). *n*O2 dramatically decreased compared with the case before light irradiation (**Figure 17**). The linear plot of *v*O2 with the catalyst

#### **Figure 17.**

*Time courses of the amount (*n*O2/μmol) of O2 evolved in chemical water oxidation experiments in an aqueous solution at 25°C using a CeIV as a sacrificial oxidant.* c*Ce = 0.1 M (0.5 mmol); Ru complex, 1.0 μmol; pH = 1.0; liquid volume, 5.0 ml. (*●*)* <sup>d</sup>*-1, (*■*)* <sup>p</sup>*-1. (Reproduced with permission from ref. [21]. Copyright 2011 American Chemical Society).*

#### *Mechanisms of Photoisomerization and Water Oxidation Catalysis of Ruthenium(II) Aquo… DOI: http://dx.doi.org/10.5772/intechopen.99730*

amount provided *k*O2 = 4.8 x 10�<sup>4</sup> s �<sup>1</sup> for *p***-1**, indicating that the observed kO2 value is decreased due to photoisomerization of *d***-1** to *p***-1** by nearly an order of magnitude. This result tells us that we have to pay attention to the observed catalytic activity decrease due to photoisomerization of *d***-1** to *p***-1** when *d***-1** is applied to photocatalytic systems.

The chemical water oxidation catalyzed by *d*-**1Cl** and *p*-**1Cl** was investigated under the same conditions as the *d*�/*p***-1** isomer system to understand the effect of 40 -chloro-substitusion on tpy ligand on the catalytic activity of the *distal*�/*proximal*-isomer complexes for water oxidation [35]. *v*O2 increased linearly with respect to the complex amount for *d*-**1Cl** and *p*-**1Cl**, as is the case of the *d*�/*p*-**1** isomer system (**Figure 18**). *<sup>k</sup>*O2 (6.3 � <sup>10</sup>�<sup>3</sup> <sup>s</sup> �1 ) for *d*-**1Cl** was 15 times higher than that (3.9 � <sup>10</sup>�<sup>4</sup> <sup>s</sup> �1 ) for *p*-**1Cl**. From a perspective of the effect of the chlorosubstitution on tpy, *<sup>k</sup>*O2 for *<sup>d</sup>*-**1Cl** was also 1.6 times higher that (3.8 � <sup>10</sup>�<sup>3</sup> <sup>s</sup> �1 ) for *<sup>d</sup>*-**1**, while *<sup>k</sup>*O2 for *<sup>p</sup>*-**1Cl** was 1.2 times lower than that (4.8 � <sup>10</sup>�<sup>4</sup> <sup>s</sup> �1 ) for *p*-**1**. If assuming that the O-O bond formation *via* the nucleophilic attack of water on the RuV=O intermediate is the rate-determining step, *k*O2 could increase by the chloro substitution because electrophilicity of RuV=O increased, as is the case for *d*-**1Cl** relative to *d*-**1**. However, another explanation is needed for the *k*O2 decrease for *p*-**1Cl** relative to *p*-**1**. For instance, there are cases that the rate for oxidation of RuIV=O to RuV=O could be involved in a rate-determining step, or that the stabilities of the complexes are different, as pointed out in the Section 3-1.

The *<sup>k</sup>*O2 value (1.7 � <sup>10</sup>�<sup>3</sup> <sup>s</sup> �1 ) of *<sup>p</sup>*-**<sup>2</sup>** was higher than that (1.0 � <sup>10</sup>�<sup>3</sup> <sup>s</sup> �1 ) of *d*-**2** by a factor of 1.7 (**Table 2**) [34]. This result is in contrast to the *d*�/*p*-**1** and *d*�/*p*-**1Cl** isomer systems, in which *k*O2 of *distal*-isomers are higher than those of the *proximal*-ones by one order of magnitude. The *k*O2 value of *p*-**2** is 3.5 times higher

#### **Figure 18.**

*Plots of initial rate (*v*O2 / mol s*�*<sup>1</sup> ) of O2 evolved vs. the amount (μmol) of* <sup>d</sup>*-1Cl (*■*),* <sup>p</sup>*-1Cl (*●*),* <sup>d</sup>*-<sup>1</sup> (*□*),* <sup>p</sup>*-<sup>1</sup> (*○*). Conditions:* <sup>c</sup>*Ce = 0.1 M (0.5 mmol); liquid volume, 5.0 mL; pH = 1.0. (reproduced with permission from ref. [35]. Copyright 2015 Elsevier).*

than that (4.8 <sup>10</sup><sup>4</sup> <sup>s</sup> 1 ) of *p*-**1**, though the value of *d*-**2** is 3.8 times lower than that (3.8 x 10<sup>3</sup> s 1 ) of *p*-**2** under the same conditions.

#### **3.3 Electrocatalytic activities of a series of dinuclear ruthenium complexes for water oxidation**

We investigated catalytic activities of *p*,*p*-**Ru2(OH)(OH2)** and the related mono- and dinuclear Ru(II) complexes (**Figures 8** and **9**) for electrochemical water oxidation in homogeneous solution [37]. The cyclic voltammogram (CV) of *p*,*p*-**Ru2(OH)(OH2)** displayed a higher anodic current after 1.2 V vs. SCE attributed to water oxidation (**Figure 19**). The catalytic current density increased to 3.5 mA cm<sup>2</sup> (the blank without the complex generates 0.31 mA cm<sup>2</sup> ) at a potential of 1.4 V and pH 6.0, that is 4.8 and 9.2 times higher compared to those of *d*-**3** and *p*,*p*-**Ru2(μ-Cl)**. Importantly, the current density value (3.5 mA cm<sup>2</sup> at pH 6.0) obtained from *p*,*p*-**Ru2(OH)(OH2)** was much higher than that (1.4 mA cm<sup>2</sup> at pH 9.0) obtained for *p*,*p*-**Ru2(μ-OH)**, under even thermodynamically unfavorable pH conditions. These results suggest that the *proximal*,*proximal*-dinuclear Ru(II) core structure with vicinal aquo and hydroxo groups is inevitably essential for efficient electrocatalytic water oxidation. Bulk electrolysis was performed in a nearly neutral phosphate buffer solution (pH 6.0) for *p*,*p*-**Ru2(OH)(OH2)** at 1.3 V vs. SCE. A higher charge amount of 2.1 C compared to the that (0.55 C) of the blank without the complex was obtained, and a 4.2 μmol (Faradaic efficiency 76–80%) of O2 was

#### **Figure 19.**

*CVs of 1 mM* p*,*p*-Ru2(OH)(OH2) (red),* d*-3 (blue),* p*,*p*-Ru2(μ-Cl) (green), and blank (black dots) in a 0.1 M phosphate buffer (pH 6.0) at a scan rate of 50 mV s<sup>1</sup> . CV of* p*,*p*-Ru2(μ-OH) (black) was measured at pH 9.0 because it gradually converts to* p*,*p*-Ru2(μ-Cl) at acidic conditions. Inset shows CV of 0.5 mM* p,p*-Ru2(OH)(OH2) in a 0.1 M phosphate buffer (pH 7.0) at a scan rate of 20 mV s<sup>1</sup> . (Reprinted with permission from ref. [37] with permission of American Chemical Society).*

*Mechanisms of Photoisomerization and Water Oxidation Catalysis of Ruthenium(II) Aquo… DOI: http://dx.doi.org/10.5772/intechopen.99730*

produced after 1 h electrolysis. This evolved O2 amount obtained by *p*,*p*-**Ru2(OH) (OH2)** corresponds to 5.3 equivalent of the total *p*,*p*-**Ru2(OH)(OH2)** amount (0.6 μmol) in the electrolyte solution, ensuring the catalytic water oxidation. The UV–Visible absorption spectrum of the electrolyte solution after the electrolysis displayed an intense band at 694 nm, assigned to *proximal*,*proximal*-[RuIII 2(tpy)2L (OH)2] 4+ (abbreviated to RuIII-OH:RuIII-OH as a oxidation state). This observation suggests that RuIII-OH:RuIII-OH is involved in the catalytic cycle. For the proposed electrocatalytic cycle for water oxidation under neutral conditions, *p*,*p*-**Ru2(OH) (OH2)** was electrochemically oxidized by the proton-coupled electron transfer reactions *via* RuIII-OH:RuIII-OH and RuIV=O:RuIV–OH states most possibly to the RuV=O:RuV=O state, which could oxidize water to O2 together with the regeneration of RuIII-OH:RuIII-OH.

In order to provide mechanistic insights into O–O bond formation for O2 production, the H/D isotope effect on electrocatalytic water oxidation by *p*,*p*-**Ru2(OH) (OH2)** and *d*-**3** were examined in H2O and D2O media. A large H/D isotope effect (1.7) on electrocatalytic water oxidation by *d*-**3** was observed relative to the blank experiment (1.1). This result is consistent with the proton transfer-concerted O–O bond formation by the *WNA* mechanism. On the other hand, the isotopic effect (1.1) on the electrocatalysis by *p*,*p*-**Ru2(OH)(OH2)** was comparable with the blank (1.1). The lower isotope effect indicates proton-non-concerted chemical reaction process in the electrocatalytic cycle, and *p*,*p*-**Ru2(OH)(OH2)** is likely to produce O2 *via* the *I2M* mechanism.
