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Polypyridyl ruthenium(II) complexes have been widely researched as promising functional molecules. We have found unique photoisomerization reactions of polypyridyl ruthenium(II) aquo complexes. Recently we have attempted to provide insight into the mechanism of the photoisomerization of the complexes and distinguish between the distal−/proximal-isomers in their physicochemical properties and functions. Moreover, polypyridyl ruthenium(II) aquo complexes have been intensively studied as active water oxidation catalysts (WOCs) which are indispensable for artificial photosynthesis. The catalytic aspect and mechanism of water oxidation by the distal-/proximal-isomers of polypyridyl ruthenium(II) aquo complexes have been investigated to provide the guided thought to develop more efficient molecular catalysts for water oxidation. The recent progress on the photoisomerization and water oxidation of polypyridyl ruthenium(II) aquo complexes in our group are reviewed to understand the properties and functions of ruthenium complexes.
Ruthenium aquo complexes
Water oxidation catalysis
chapter and author info
Department of Materials Science and Technology, Faculty of Engineering, Niigata University, Niigata, Japan
Eman A. Mohamed
Department of Materials Science and Technology, Faculty of Engineering, Niigata University, Niigata, Japan
Zaki N. Zahran
Department of Materials Science and Technology, Faculty of Engineering, Niigata University, Niigata, Japan
Department of Materials Science and Technology, Faculty of Engineering, Niigata University, Niigata, Japan
*Address all correspondence to: firstname.lastname@example.org
From the Edited Volume
Ruthenium - an Element Loved by Researchers [Working Title]
Polypyridyl ruthenium(II) complexes have been widely researched as promising functional molecules due to appealing photochemical [1, 2, 3] and photophysical [4, 5, 6] properties as well as redox properties [7, 8], which enable them to exhibit a number of functions such as electrochromism [9, 10], proton-coupled electron transfer [11, 12, 13] and photocatalysis [14, 15]. As a result, the polypyridyl ruthenium(II) complexes have been applied to a large variety of devices including sensors , photovoltaic cells , displays  and artificial photosynthesis [19, 20].
We presented irreversible and stoichiometric photoisomerization of distal-[Ru(tpy)(pynp)OH2]2+ (d-1) (tpy = 2,2′;6′,2″-terpyridine, pynp = 2-(2-pyridyl)-1,8-naphthyridine) to proximal-[Ru(tpy)(pynp)OH2]2+ (p-1) as shown in Figure 1 [21, 22, 23], which had not been characterized previously for polypyridyl ruthenium(II) aquo complexes although various photochemical reactions of the ruthenium(II) complexes have been reported [24, 25, 26, 27, 28, 29, 30, 31, 32, 33]. We have attempted to provide insight into the mechanism of the photoisomerization of polypyridyl ruthenium(II) aquo complexes and distinguish between the distal−/proximal-isomers in their physicochemical properties and functions [21, 22, 23, 34, 35, 36]. We have also developed new synthetic strategy to form dinuclear ruthenium(II) complexes utilizing the photoisomerization [37, 38]. Moreover, polypyridyl ruthenium(II) aquo complexes have been intensively studied as active WOCs [21, 22, 34, 35, 37, 39, 40] which are indispensable for artificial photosynthesis. The catalytic aspect and mechanism of water oxidation by the distal-/proximal-isomers of polypyridyl ruthenium(II) aquo complexes have been investigated to provide the guided thought to develop more efficient molecular catalysts for water oxidation. In this chapter, we review the recent progress on the photoisomerization and water oxidation of polypyridyl ruthenium(II) aquo complexes in our group.
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′-bipyridine) to its trans form in aqueous media was first reported by Meyer . The mechanism of the photoisomerization reaction was later investigated by Planas et al. . In this case, the transform was present as a photostationary state and slowly went back to the original cisform. 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-1to p-1. Furthermore, we reported the reversible photoisomerization equilibrium between distal- and proximal-[Ru(tpy)(pyqu)OH2]2+ (d-2and p-2) isomers with a ligand of 2-(2-pyridyl)quinoline (pyqu) instead of pynp, in contrast to the irreversible photoisomerization of d-1to p-1. The aspect and mechanism of the irreversible photoisomerization of d-1are first described, followed by those of the reversible one of d−/p-2in this section.
2.1 Irreversible photoisomerization of distal-[Ru(Xtpy)(pynp)OH2]2+ to the proximal-isomer
Photoisomerization behavior of d-1was investigated by 1H-NMR and UV–Vis spectroscopy. Upon irradiation of visible light to a D2O solution of d-1, the NMR peak at 9.6 ppm due to d-1decreased with the irradiation time and disappeared completely after 25 min under the experimental conditions (Figure 2A) . The new peak at 8.9 ppm assigned to p-1increased with the concomitant decrease of d-1. Figure 2B displays the concentration profile of d-1and p-1, which indicates that irreversible and stoichiometric photoisomerization from d-1to p-1proceeds in water by visible light irradiation (Figure 1). By contrast, no isomerization of d-1in water was found to occur under thermal treatment. The photoisomerization rate showed a first-order dependence on d-1concentration, and the kinetic analysis provided the observed rate constant ((kd-p)obs/s−1) of photoisomerization (distalto proximal) to be 4.1 x 10−3 s−1 under the conditions employed (λ > 420 nm, 180 mW cm−2). The (kd-p)obs values increased linearly with respect to the light intensity below 255 mW cm−2, 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−1 of activation energy (Ea) for the photoisomerization (Table 1). Interestingly, the (kd-p)obs value decreased drastically at pD > 7 (pD = −log [D+]), while it was unchanged over pD 1-7. The UV–Vis spectrophotometric pH titration of d-1gave pKa of 9.7 attributed to the deprotonation of an aquo ligand to form the hydroxo complex, distal-[Ru(tpy)(pynp)OH]+. The trend of (kd-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-1is inert for the photoisomerization [21, 22].
Summary of the observed rate constants and internal quantum yields of photoisomerization of d−/p-1, d−/p-Cl1, d−/p-2and d−/p-3isomers at 25°C.a
cRu is concentrations of ruthenium complexes. The marks of “n.m.” and “n.r.” mean “not measured” and “no reaction”, respectively.
The filtered halogen lamp was used for visible light irradiation (λ > 420 nm, 180 mW cm−2).
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−2). The Φ values were calculated according to the following equation:
where h, c, NA, kpi, nint, p, λ, Aand Tare 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, respectively. The Φ values for photoisomerization from d-1to p-1are 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-Cl1and p-Cl1; Cl-tpy = 4′-chloro-2,2′;6′,2″-terpyridine) complexes were prepared. When an aqueous solution of d-Cl1was irradiated with visible light (λ > 420 nm, 180 mWcm−2), the stoichiometric photoisomerization of d-Cl1to p-Cl1was observed as it is for d-1(Figure 1). The rate constant for photoisomerization of d-Cl1was estimated to be (kd-p)obs = 4.9 x 10−3 s−1 , which is higher than that ((kd-p)obs = 4.1 x 10−3 s−1) [22, 34] observed for d-1under 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 photo-substitution reactions viathe triplet metal centered (3MC) state from the 3MLCT excited state [25, 26, 27]. According to the reports, a possible mechanism for the photoisomerization of d-1and p-1isomers was speculated as follows. The 3MLCT excited state of d-1is generated by absorption of visible light. The photo-dissociation of the aquo ligand from the exited d-1proceeds through the thermal accessible 3MC state, leading to the formation of the penta-coordinated [Ru(tpy)(pynp)]2+ intermediate. The p-1isomer 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-1suggest existence of the thermally activated process from the 3MLCT state with an Ea of 49 kJ mol−1 , which is close to those from the 3MLCT state to the 3MC state reported for various polypyridyl Ru complexes [42, 43, 44, 45]. The agreement of the Ea value (49 kJ mol−1) with that (41.7 kJ mol−1) 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 3MLCT state to the 3MC state. However, Density functional theory (DFT) calculations suggested a different activation process, where the distal-penta-coordinated intermediate changes the conformation to proximal-penta-coordinated intermediate for p-1while maintaining the otherwise octahedral structure of d-1.
To obtain deeper mechanistic insights into the irreversible photoisomerization, the 3MLCT excited states of the d-1and p-1were characterized by the time-resolved infrared spectroscopy (TR-IR) . The decay of the photoexcited 3MLCT states for both isomers were investigated by the TR-IR analysis, and the lifetimes of the excited state for the d-1and p-1were determined to be 9.7 ns and 6.4 ps, respectively. In general, the decay of the 3MLCT 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-1imply that a non-radiative process from the 3MLCT state was accelerated, so that p-1is inert for photoisomerization to d-1.
The large difference in lifetimes of the 3MLCT state between d-1and p-1was interpreted by geometry optimization calculations using DFT of both d-1and p-1isomers in the singlet ground (S0) and 3MLCT (T1) states. While both the structures of d-1and p-1isomers 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-1results 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-1is 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-1and p-1in the S0, T1 and putative 3MC states. p-1in the S0 state is more stable than d-1by 65 kJ mol−1 because of hydrogen bond interaction between the pynp and the aquo ligand. The energy difference between the S0 and T1 states for d-1is 176 kJ mol−1, which is remarkably higher than that (101 kJ mol−1) for p-1. The higher energy difference for d-1is 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-1in the T1 state is higher than that of p-1in the T1 state by 140 kJ mol−1. Considering the similar ligand field for both isomers, the energy difference in the 3MC state between d-1and p-1is assumed to be not as much as that (140 kJ mol−1) in T1 states. For p-1, the 3MC 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-1in the T1 state from the case of d-1(usual lifetime of 9.7 ns), exhibiting the non-radiative decay through the thermally populated 3MC 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, 47, 48]. Considering almost the same geometries of p-1between the S0 and T1 states, the law can be applied as the weak coupling limit. The lower energy band gap of 101 kJ mol−1 between the S0 and T1 states for p-1could 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-1in the T1 state. The direct relaxation process from the T1 states to the S0 state (not viathe 3MC 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-2and p-2was examined to reveal the influence of structures of bidentate ligands on the photoisomerization of d-1(Figure 5) . On irradiation of visible light to a D2O solution of p-2, the 8.9 ppm peak characteristic of p-2in the 1H NMR spectrum (Figure 6A) decreased with increasing the 9.5 ppm peak assigned to the d-2isomer. The 1H 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-2to d-2reaches a photo-stationary 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-2was calculated to be 76:24 (Figure 6B). The kinetic profiles of the photoisomerization of p-2to d-2were well fitted with a reversible reaction model, giving the observed rate constants of the forward reaction (proximalto distal, (kp-d)obs/s−1) of 4.3 ± 0.1× 10−3 s−1 and the back reaction (distalto proximal, (kd-p)obs/s−1) of 1.2 ± 0.03 × 10−2 s−1 under the conditions employed (λ > 420 nm, 180 mW cm−2), respectively (Table 1) . The observed equilibrium constant were given to Kobs (= (kp-d)obs/(kd-p)obs) = 0.34 ± 0.01 under the employed conditions. Both (kp-d)obs and (kd-p)obs increased linearly with respect to light intensity up to 180 mW cm−2, suggesting that the photoexcited states of p-2and d-2isomers are also involved in the forward and back reactions, respectively . As shown in Figure 7, (kp-d)obs and (kd-p)obs decreased from pD = 8 to 12, and the photoisomerization did not take place at all above pD =12 . This result implies that both hydroxo isomers, proximal- and distal-[Ru(tpy)(pyqu)OD]+ are inert for the forward and back photoisomerization reactions, respectively, as observed in the case of distal-[Ru(tpy)(pynp)OH]+ [21, 22]. The pD-dependent (kp-d)obs and (kd-p)obs analysis demonstrated inflection points at pD = 10.5 and 9.8 for the forward and back reactions, respectively. These values are close to the pKa of p-2(pKa = 10.5) and d-2(pKa = 9.4) (Table 1). The difference in pKa values (1.1) between p-2and d-2results in the markedly pD-dependent Kobs values (inset of Figure 7). Kobs increased above pH = 9, and reached its maximum value (Kobs = 2.5) at pH = 12. This trend is consistent with the observation that the yield of d-2generated in the photoisomerization raised from 26–65% on increasing from pH 5.7 to 12.
The thermal dependent kinetics for (kp-d)obs and (kd-p)obs in a temperature range of 25 to 70°C gives Ea = 30.6 ± 2.9 and 23.3 ± 2.1 kJ mol−1 for the forward and back reactions, respectively. The higher Ea of the forward reaction ((kp-d)obs) compared to the back one ((kd-p)obs) is attributable to the unfavorable loss of the hydrogen bond (C-H···O) 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 dissociation for photoisomerization. The van’t Hoff plots for Kobs gave ΔH = 7.7 ± 2.7 kJ mol−1, that is close to enthalpy for the hydrogen bond (5.4 kJ mol−1) of C-H···O in the benzene-water complex .
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−2) (Table 1). The higher Φ (1.1%) of d-2compared to that (0.34%) of p-2could be attributed to the enthalpy of hydrogen bond (C-H···O) 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-2to p-2is 3.5 times higher than that (0.31%) for the photoisomerization from d-1to p-1under the same concentration . 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 . 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]+ (d-3Cl) selectively in ethanol in a 59% isolated yield. This selective formation of d-3Clwas possibly caused by the steric hindrance of L with a chloro ligand. d-3Clwas 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-3with a second ruthenium center [Ru(tpy)Cl3] for dimerization failed to give the proximal, proximal-dinuclear Ru(II) complex owing to the steric hindrance located between the tpy ligand on the d-3and the one on [Ru(tpy)Cl3]. However, if the photoisomerization of d-3to proximal-[Ru(tpy)LOH2]2+ (p-3) is utilized (Figure 8), the steric constraint of d-3for formation of the proximal,proximal-dinuclear Ru(II) species could be avoided. The photoisomerization of d-3stoichiometrically progressed to form p-3in water/ethanol mixture under visible light irradiation (λ > 420 nm). The subsequent thermal reaction of p-3with [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 . 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, 22, 23, 34, 35, 40, 50, 51, 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, 54, 55, 56]. In the WNAmechanism (Figure 10a), an electrophilic high-valent metal-oxo (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(n−2)+-OOH) species. Further oxidation and 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 WNAmechanism for water oxidation [21, 22, 23, 34, 35, 40, 51, 52, 57, 58]. In the I2Mmechanism (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′:6′,2″-terpyridine derivatives) complexes (4R) having a variety of 4′-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 (nO2/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) . The initial O2 evolution rates (vO2/mol s−1) were calculated from the initial slopes. The vO2 for 4EtOincreased with the CeIV concentration (cCe/M) and reached a saturation at cCe = 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 (vmax) of 1.5 (±0.08) x 10−1 mol s−1 (4EtOmol)−1 and the constant (Km = 1.2 (±0.06) mmol) in terms of the CeIV concentration for the half value of vmax. 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 4EtOis close to standard potential (E° = 1.47 V vs. SCE (1.71 V vs. NHE) ) of CeIII/IV. The redox equilibrium presumably leads to saturation of the vO2 with increase of the CeIV concentration (Figure 13). The vO2 increased linearly with the 4EtOconcentration (cRu/M) under the large excess CeIV conditions (cCe = 0.1 M, cRu = ∼ 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 4Ris 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 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 vO2 showed a first-order dependence on the complex amount for all the 4Rderivatives (Figure 14) , suggesting that O2 is produced by a unimolecular reaction of 4R. Turnover frequencies (kO2/s−1) were calculated from the slopes of the linear relationships. The kO2 values were largely affected by the Rtpy ligands, being variable from 0.05 to 44 x 10−2 s−1 (as the highest kO2 for 4EtO) by a factor of 880 (Table 2). The kO2 values were plotted with respect to Hammett constant σp of the 4′-substituents of Rtpy ligands in Figure 16A. However, the Me2N group is protonated to give Me2NH+ under the catalytic conditions (pH = 1.0), and the σp value of Me2NH+ was assumed as σp = 0.71 of the midpoint between σp = 0.60 for protonated amino group and σp = 0.82 for protonated trimethylamino group . The kO2 values were almost constant (3.4 ∼ 6.1 x 10−2 s−1) in a range of σp = −0.17 (R = Me) ∼ 0.23 (R = Cl), although the kO2 value (5 x 10−4 s−1) of 4Me2Nis lower than these values. The kO2 value increased sharply at σp = −0.24 (R = EtO) to 4.4 x10−1 s−1 as the maximum, and thereafter decreased with the σp decrease to −0.37 (R = OH). This result demonstrates that very critical Hammett constant (σp = −0.27 ∼ −0.24) exists for the high kO2 values. The very low kO2 values for 4OHand 4Me2Narise from the difficulty of RuV=O formation. Most likely, 4OHand 4Me2Nare considered to decompose through their ligand oxidation during water oxidation catalysis, as reported recently [65, 66].
Summary of kO2 by mononuclear Ru(II) aquo complexes.
The kO2 values were plotted with respect to redox potentials of RuIV=O/RuV=O in Figure 16B. The kO2 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 kO2 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 rate-determining step. The O-O bond formation process viawater nucleophilic attack could be no longer a rate-determining step singularly. One might deservedly expect that the kO2 values of 4H, 4Cl, and 4Mewith the higher redox potentials of RuIV=O/RuV=O are higher than those of 4MeO, 4PrOand 4EtOwith 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 kO2 values of 4H, 4Cl, and 4Mewere indeed lower than those of 4MeO, 4PrOand 4EtO, which could be explained by the slow oxidation rate (step I in Figure 15) from RuIV=O to RuV=O involved in a rate-determining step.
3.2 Comparison in catalytic activities between distaland proximalisomers of mononuclear ruthenium complexes
To explore the catalytic aspects of a series of distaland proximalisomers 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-1and CeIV, and nO2 increased linearly with time until 50 min (Figure 17). vO2 increased linearly with respect to the amount of the complex, and the slope of the linear relationship provides kO2 = 3.8 x 10−3 s−1 (Figure 18). The same chemical water oxidation experiments were carried out after visible light irradiation to the solution of d-1for 1 h to generate p-1completely (Figure 1). nO2 dramatically decreased compared with the case before light irradiation (Figure 17). The linear plot of vO2 with the catalyst amount provided kO2 = 4.8 x 10−4 s−1 for p-1, indicating that the observed kO2 value is decreased due to photoisomerization of d-1to p-1by 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-1to p-1when d-1is applied to photocatalytic systems.
The chemical water oxidation catalyzed by d-1Cland p-1Clwas investigated under the same conditions as the d−/p-1isomer system to understand the effect of 4′-chloro-substitusion on tpy ligand on the catalytic activity of the distal−/proximal-isomer complexes for water oxidation . vO2 increased linearly with respect to the complex amount for d-1Cland p-1Cl, as is the case of the d−/p-1isomer system (Figure 18). kO2 (6.3 × 10−3 s−1) for d-1Clwas 15 times higher than that (3.9 × 10−4 s−1) for p-1Cl. From a perspective of the effect of the chloro-substitution on tpy, kO2 for d-1Clwas also 1.6 times higher that (3.8 × 10−3 s−1) for d-1, while kO2 for p-1Clwas 1.2 times lower than that (4.8 × 10−4 s−1) for p-1. If assuming that the O-O bond formation viathe nucleophilic attack of water on the RuV=O intermediate is the rate-determining step, kO2 could increase by the chloro substitution because electrophilicity of RuV=O increased, as is the case for d-1Clrelative to d-1. However, another explanation is needed for the kO2 decrease for p-1Clrelative 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 kO2 value (1.7 × 10−3 s−1) of p-2was higher than that (1.0 × 10−3 s−1) of d-2by a factor of 1.7 (Table 2) . This result is in contrast to the d−/p-1and d−/p-1Clisomer systems, in which kO2 of distal-isomers are higher than those of the proximal-ones by one order of magnitude. The kO2 value of p-2is 3.5 times higher than that (4.8 × 10−4 s−1) of p-1, though the value of d-2is 3.8 times lower than that (3.8 x 10−3 s−1) of p-2under 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 (Figures8 and 9) for electrochemical water oxidation in homogeneous solution . 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−2 (the blank without the complex generates 0.31 mA cm−2) 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-3and p,p-Ru2(μ-Cl). Importantly, the current density value (3.5 mA cm−2 at pH 6.0) obtained from p,p-Ru2(OH)(OH2)was much higher than that (1.4 mA cm−2 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 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-[RuIII2(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 viaRuIII-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-3were examined in H2O and D2O media. A large H/D isotope effect (1.7) on electrocatalytic water oxidation by d-3was observed relative to the blank experiment (1.1). This result is consistent with the proton transfer-concerted O–O bond formation by the WNAmechanism. 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 O2viathe I2Mmechanism.
Recent progress on the aspects and mechanistic insights of photoisomerization of Ru(II) aquo complexes in our group was reviewed to unveil the photoisomerization reactions and its mechanism comprehensively. The controls of properties and functions of mononuclear Ru(II) aquo complexes by the photoisomerization were exemplified in terms of their water oxidation catalyses. An example of application of the photoisomerization is demonstrated; the employment of the photoisomerization enabled to synthesize the dinuclear Ru(II) complexes that have been difficult to be synthesized by conventional thermochemical processes. The synthesized dinuclear complexes also serve as efficient catalysts for water oxidation. New design and development of variety types of Ru complexes are desired to explore unique reactions, functions and application based on the photoisomerization for future molecular systems such as artificial photosynthetic devices.
Yuta Tsubonouchi, Eman A. Mohamed, Zaki N. Zahran and Masayuki Yagi (September 6th 2021). Mechanisms of Photoisomerization and Water Oxidation Catalysis of Ruthenium(II) Aquo Complexes [Online First], IntechOpen, DOI: 10.5772/intechopen.99730. Available from:
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