Critical Hammett Electron-Donating Ability of Substituent Groups for Efficient Water Oxidation Catalysis by Mononuclear Ruthenium Aquo Complexes.

[Ru(Rtpy)(bpy)(H2O)]2+ (1R; bpy = 2,2'-bipyridine, and Rtpy = 2,2':6',2″-terpyridine derivatives) complexes with a variety of 4'-substituent groups on Rtpy were synthesized and characterized to reveal the effects of substituents on their structures, physicochemical properties, and catalytic activities for water oxidation. The geometric structures of 1R are not considerably influenced by the electron-donating ability of the 4'-substituent groups on Rtpy. Similar multistep proton-coupled electron transfer reactions were observed for 1R, and the redox potentials for each oxidation step tended to decrease with an increase in the electron-donating ability of the substituent, which is explained by the increased electron density on the Ru center by electron-donating groups, stabilizing the positive charge that builds up upon oxidation. This is consistent with the red-shift of the absorption bands around 480 nm assigned to the metal-to-ligand charge transfer transition for 1R due to the increased d orbital energy level of the Ru center. The turnover frequency (kO2) of 1R for water oxidation catalysis, however, depended greatly on the Rtpy ligands, varying from 0.05 × 10-2 to 44 × 10-2 s-1 (as the highest kO2 was observed for R = ethoxy) by a factor of 880. A critical electron-donating ability of the 4'-substituent groups with a narrow range of Hammett constants (σp = -0.27 to -0.24) found for the highest kO2 values is valuable for understanding the great difficulty in the search for efficient water oxidation catalysts. On another front, the kO2 values increased with a decrease in the redox potentials of RuIV═O/RuV═O for 1R, indicating that the potential of formation of RuV═O species for 1R is crucial for water oxidation catalysis under the employed conditions.

Mechanisms of photoisomerization and water-oxidation catalysis of mononuclear ruthenium(II) monoaquo complexes.

A ligation of Ru(tpy)Cl3 (tpy = 2,2':6',2"-terpyridine) with 2-(2-pyridyl)-1,8-naphthyridine) (pynp) in the presence of LiCl gave distal-[Ru(tpy)(pynp)Cl](+) (d-1Cl) selectively, whereas the ligation gave proximal-[Ru(tpy)(pynp)OH2](2+) (p-1H2O) selectively in the absence of halide ions. (The proximal/distal isomers were defined by the structural configuration between the 1,8-naphthyridine moiety and the aquo or chloro ligand.) An aquation reaction of d-1Cl quantitatively afforded distal-[Ru(tpy)(pynp)OH2](2+) (d-1H2O) in water, and d-1H2O is quantitatively photoisomerized to p-1H2O. The mechanism of the photoisomerization was investigated by transient absorption spectroscopy and quantum chemical calculations. The temperature dependence of the transient absorption spectral change suggests existence of the thermally activated process from the (3)MLCT state with the activation energy (ΔE = 49 kJ mol(-1)), which is close to that (41.7 kJ mol(-1)) of the overall photoisomerization reaction. However, quantum chemical calculations suggest another activation process involving the conformational change of the pentacoordinated distal structure to the proximal structure. Quantum chemical calculations provide redox potentials and pK(a) values for proton-coupled electron transfer reactions from Ru(II)-OH2 to Ru(IV)═O in good agreement with experiments and provide an explanation for mechanistic differences between d-1H2O and p-1H2O with respect to water oxidation. The calculations show that water nucleophilic attack (WNA) on d-[Ru(V)-O](3+) (the ruthenyl oxo species derived from d-1H2O, calculated ΔG(‡) of 87.9 kJ/mol) is favored over p-[Ru(V)-O](3+) (calculated ΔG(‡) of 104.6 kJ/mol) for O-O bond formation. Examination of the lowest unoccupied molecular orbitals in d- and p-[Ru(V)-O](3+) indicates that more orbital amplitude is concentrated on the [Ru-O] unit in the case of d-[Ru(V)-O](3+) than in the case of p-[Ru(V)-O](3+), where some of the amplitude is instead delocalized over the pynp ligand, making this isomer less electrophilic.

Substituent effects on core structures and heterogeneous catalytic activities of Mn(III)(µ-O)2Mn(IV) dimers with 2,2':6',2″-terpyridine derivative ligands for water oxidation.

[(OH(2))(R-terpy)Mn(µ-O)(2)Mn(R-terpy)(OH(2)) ](3+) (R-terpy = 4'-substituted 2,2':6',2″-terpyridine, R = butoxy (BuO), propoxy (PrO), ethoxy (EtO), methoxy (MeO), methyl (Me), methylthio (MeS), chloro (Cl)) have been synthesized as a functional oxygen-evolving complex (OEC) model and characterized by UV-vis and IR spectroscopic, X-ray crystallographic, magnetometric, and electrochemical techniques. The UV-vis spectra of derivatives in water were hardly influenced by the 4'-substituent variation. X-ray crystallographic data showed that Mn centers in the Mn(III)(µ-O)(2)Mn(IV) cores for derivatives with R = H, MeS, Me, EtO, and BuO are crystallographically indistinguishable, whereas the derivatives with R = MeO and PrO gave the significantly distinguishable Mn centers in the cores. The indistinguishable Mn centers could be caused by rapid electron exchange between the Mn centers to result in the delocalized Mn(µ-O)(2)Mn core. The exchange integral values (J = -196 to -178 cm(-1)) for delocalized cores were lower than that (J = -163 to -161 cm(-1)) for localized cores, though the Mn···Mn distances are nearly the same (2.707-2.750 Å). The half wave potential (E(1/2)) of a Mn(III)-Mn(IV)/Mn(IV)-Mn(IV) pair of the derivatives decreased with an increase of the electron-donating ability of the substituted groups for the delocalized core, but it deviated from the correlation for the localized cores. The catalytic activities of the derivatives on mica for heterogeneous water oxidation were remarkably changed by the substituted groups. The second order rate constant (k(2)/mol(-1) s(-1)) for O(2) evolution was indicated to be correlated to E(1/2) of a Mn(III)-Mn(IV)/Mn(IV)-Mn(IV) pair; k(2) increased by a factor of 29 as E(1/2) increased by 28 mV.

Stoichiometric photoisomerization of mononuclear ruthenium(II) monoaquo complexes controlling redox properties and water oxidation catalysis.

Although various reactions involved in photoexcited states of polypyridyl ruthenium(II) complexes have been extensively studied, photoisomerization of the complexes is very rare. We report the first illustration of stoichiometric photoisomerization of trans-[Ru(tpy)(pynp)OH(2)](2+) (1a) [tpy = 2,2':6',2''-terpyridine; pynp = 2-(2-pyridyl)-1,8-naphthyridine] to cis-[Ru(tpy)(pynp)OH(2)](2+) (1a') and the isolation of 1a and 1a' for X-ray crystallographic analysis. Polypyridyl ruthenium(II) aquo complexes are attracting much attention related to proton-coupled electron transfer and water oxidation catalysis. We demonstrate that the photoisomerization significantly controls the redox reactions and water oxidation catalyses involving the ruthenium(II) aquo complexes 1a and 1a'.

Distribution of manganese species in an oxidative dimerization reaction of a bis-terpyridine mononuclear manganese (II) complex and their heterogeneous water oxidation activities.

Heterogeneous water oxidation catalyses were studied as a synthetic model of oxygen evolving complex (OEC) in photosynthesis using mica adsorbing various manganese species. Distribution of manganese species formed in the oxidative dimerization reaction of [Mn(II)(terpy)2](2+) (terpy=2,2':6',2″-terpyridine) (1') with various oxidants in water was revealed. 1' was stoichiometrically oxidized to form di-µ-oxo dinuclear manganese complex, [(OH2)(terpy)Mn(III)(µ-O)2Mn(IV)(terpy)(OH2)](3+) (1) by KMnO4 as an oxidant. When Oxone and Ce(IV) oxidants were used, the further oxidation of 1 to [(OH2)(terpy)Mn(IV)(µ-O)2Mn(IV)(terpy)(OH2)](4+) (2) was observed after the oxidative dimerization reaction of 1'. The mica adsorbates with various composition of 1', 1 and 2 were prepared by adding mica suspension to the various oxidant-treated solutions followed by filtration. The heterogeneous water oxidation catalysis by the mica adsorbates was examined using a Ce(IV) oxidant. The observed catalytic activity of the mica adsorbates corresponded to a content of 1 (1ads) adsorbed on mica for KMnO4- and Oxone-treated systems, indicating that 1' (1'ads) and 2 (2ads) adsorbed on mica do not work for the catalysis. The kinetic analysis suggested that 1ads works for the catalysis through cooperation with adjacent 1ads or 2ads, meaning that 2ads assists the cooperative catalysis by 1ads though 2ads is not able to work for the catalysis alone. For the Ce(IV)-treated system, O2 evolution was hardly observed although the sufficient amount of 1ads was contained in the mica adsorbates. This was explained by the impeded penetration of Ce(IV) ions (as an oxidant for water oxidation) into mica by Ce(3+) cations (generated in oxidative dimerization of 1') co-adsorbed with 1ads.

Highly active and tunable catalysts for O2 evolution from water based on mononuclear ruthenium(II) monoaquo complexes.

The catalytic activity of [Ru(tpy)(bpy)OH(2)](2+) (tpy = 2,2':6',2''-terpyridine and bpy = 2,2'-bipyridine) increased by a 4'-substituted ethoxy group on the tpy ligand by more than one order of magnitude to give 1.1 × 10(-1) s(-1) of catalyst turnover frequency, which is comparable with the hitherto-reported champion data.

An artificial model of photosynthetic photosystem II: visible-light-derived O2 production from water by a di-µ-oxo-bridged manganese dimer as an oxygen evolving center.

Visible-light-derived O(2) production was yielded by conjugating water oxidation catalysis by [(OH(2))(terpy)Mn(µ-O)(2)Mn(terpy)(OH(2))](3+) as an oxygen evolving center model and photo-sensitization of [Ru(bpy)(3)](2+) as a photoexcitation center model at an interlayer of mica.