Redox properties of Tanaka's water oxidation catalyst: redox noninnocent ligands dominate the electronic structure and reactivity

[Ru(2)(OH)(2)(3,6-(t)Bu(2)Q)(2)(btpyan)](2+) ((t)Bu(2)Q, 3,6-di-tert-butyl-1,2-benzoquinone; btpyan, 1,8-bis(2,2':6',2'-terpyridyl)anthracene) is one of a handful of structurally well-defined homogeneous catalysts that can electrocatalytically oxidize water at room temperature. Unfortunately, the exact composition and the chemical properties of the redox intermediates leading to the catalytically competent species remains poorly resolved. On the basis of the UV-vis spectra the catalyst was previously speculated to lose two protons spontaneously to form an intermediate containing the key O-O bond in water. We evaluated this mechanistic scenario computationally and found that the associated pK(a) values are in the range of 21, much too high to justify spontaneous deprotonation under experimental conditions of pH = 4. In later work, the O-O bond formation was speculated to occur after removal of two protons and two electrons. Extensive exploration of the various oxidation and protonation states that the diruthenium complex may access during catalyst activation reveals surprisingly complex electronic structure patterns in several redox intermediates: the quinone and tpy ligands become redox noninnocent, i.e., they participate actively in the electron transfer processes by temporarily storing redox equivalents. On the basis of this new insight into the electronic structure we propose a novel alternative explanation of the spectroscopic observations reported previously and characterize the electronic structure of the key intermediates in detail. Finally, the redox potential for the first two-electron oxidation is evaluated based on our proposed intermediates and predicted to be 0.411 V, which compares well with the experimentally observed broad two-electron wave at ～0.32 V.     

Syntheses and redox properties of bis(hydroxoruthenium) complexes with quinone and bipyridine ligands. Water-oxidation catalysis

The novel bridging ligand 1,8-bis(2,2':6',2"-terpyridyl)anthracene (btpyan) is synthesized by three reactions from 1,8-diformylanthracene to connect two [Ru(L)(OH)]+ units (L = 3,6-di-tert-butyl-1,2-benzoquinone (3,6-tBu2qui) and 2,2'-bipyridine (bpy)). An addition of tBuOK (2.0 equiv) to a methanolic solution of [RuII2(OH)2(3,6-tBu2qui)2(btpyan)](SbF6)2 ([1](SbF6)2) results in the generation of [RuII2(O)2(3,6-tBu2sq)2(btpyan)]0 (3,6-tBu2sq = 3,6-di-tert-butyl-1,2-semiquinone) due to the reduction of quinone coupled with the dissociation of the hydroxo protons. The resultant complex [RuII2(O)2(3,6-tBu2sq)2(btpyan)]0 undergoes ligand-localized oxidation at E1/2 = +0.40 V (vs Ag/AgCl) to give [RuII2(O)2(3,6-tBu2qui)2(btpyan)]2+ in MeOH solution. Furthermore, metal-localized oxidation of [RuII2(O)2(3,6-tBu2qui)2(btpyan)]2+ at Ep = +1.2 V in CF3CH2OH/ether or water gives [RuIII2(O)2(3,6-tBu2qui)2(btpyan)]4+, which catalyzes water oxidation. Controlled-potential electrolysis of [1](SbF6)2 at +1.70 V in the presence of H2O in CF3CH2OH evolves dioxygen with a current efficiency of 91% (21 turnovers). The turnover number of O2 evolution increases to 33,500 when the electrolysis is conducted in water (pH 4.0) by using a [1](SbF6)2-modified ITO electrode. On the other hand, the analogous complex [RuII2(OH)2(bpy)2(btpyan)](SbF6)2 ([2](SbF6)2) shows neither dissociation of the hydroxo protons, even in the presence of a large excess of tBuOK, nor activity for the oxidation of H2O under similar conditions.     

An amide-linked chromophore-catalyst assembly for water oxidation

The synthesis and analysis of a new amide-linked, dinuclear [Ru(bpy)(2)(bpy-ph-NH-CO-trpy)Ru(bpy)(OH(2))](4+) (bpy = 2,2'-bipyridine; bpy-ph-NH-CO-trpy = 4-(2,2':6',2"-terpyridin-4'-yl)-N-[(4'-methyl-2,2'-bipyridin-4-yl)methyl]benzamide) assembly that incorporates both a light-harvesting chromophore and a water oxidation catalyst are described. With the saturated methylene linker present, the individual properties of both the chromophore and catalyst are retained including water oxidation catalysis and relatively slow energy transfer from the chromophore excited state to the catalyst.     

Catalytic four-electron oxidation of water by intramolecular coupling of the oxo ligands of a bis(ruthenium-bipyridine) complex

A bis(ruthenium-bipyridine) complex bridged by 1,8-bis(2,2':6',2'-terpyrid-4'-yl)anthracene (btpyan), [Ru(2)(μ-Cl)(bpy)(2)(btpyan)](BF(4))(3) ([1](BF(4))(3); bpy = 2,2'-bipyridine), was prepared. The cyclic voltammogram of [1](BF(4))(3) in water at pH?1.0 displayed two reversible [Ru(II),Ru(II)](3+)/[Ru(II),Ru(III)](4+) and [Ru(II),Ru(III)](4+)/[Ru(III),Ru(III)](5+) redox couples at E(1/2)(1) = +0.61 and E(1/2)(2) = +0.80?V (vs. Ag/AgCl), respectively, and an irreversible anodic peak at around E = +1.2?V followed by a strong anodic currents as a result of the oxidation of water. The controlled potential electrolysis of [1](3+) ions at E = +1.60?V in water at pH?2.6 (buffered with H(3)PO(4)/NaH(2)PO(4)) catalytically evolved dioxygen. Immediately after the electrolysis of the [1](3+) ion in H(2)(16)O at E = +1.40?V, the resultant solution displayed two resonance Raman bands at nu = 442 and 824?cm(-1). These bands shifted to nu = 426 and 780?cm(-1), respectively, when the same electrolysis was conducted in H(2)(18)O. The chemical oxidation of the [1](3+) ion by using a Ce(IV) species in H(2)(16)O and H(2)(18)O also exhibited the same resonance Raman spectra. The observed isotope frequency shifts (Δnu = 16 and 44?cm(-1)) fully fit the calculated ones based on the Ru-O and O-O stretching modes, respectively. The first successful identification of the metal-O-O-metal stretching band in the oxidation of water indicates that the oxygen-oxygen bond at the stage prior to the evolution of O(2) is formed through the intramolecular coupling of two Ru-oxo groups derived from the [1](3+) ion.     

Catalytic photooxidation of alcohols by an unsymmetrical tetra(pyridyl)pyrazine-bridged dinuclear Ru complex

The dinuclear complexes [(tpy)Ru(tppz)Ru(bpy)(L)](n+) (where L is Cl(-) or H(2)O, tpy and bpy are the terminal ligands 2,2':6',2'-terpyridine and 2,2'-bipyridine, and tppz is the bridging backbone 2,3,5,6-tetrakis(2-pyridyl)pyrazine) were prepared and structurally and electronically characterized. The mononuclear complexes [(tpy)Ru(tppz)](2+) and [(tppz)Ru(bpy)(L)](m+) were also prepared and studied for comparison. The proton-coupled, multi-electron photooxidation reactivity of the aquo dinuclear species was shown through the photocatalytic dehydrogenation of a series of primary and secondary alcohols. Under simulated solar irradiation and in the presence of a sacrificial electron acceptor, the photoactivated chromophore-catalyst complex (in aqueous solutions at room temperature and ambient pressure conditions) can perform the visible-light-driven conversion of aliphatic and benzylic alcohols into the corresponding carbonyl products (i.e., aldehydes or ketones) with 100% product selectivity and several tens of turnover cycles, as probed by NMR spectroscopy and gas chromatography. Moreover, for aliphatic substrates, the activity of the photocatalyst was found to be highly selective toward secondary alcohols, with no significant product formed from primary alcohols. Comparison of the activity of this tppz-bridged complex with that of the analogue containing a back-to-back terpyridine bridge (tpy-tpy, i.e., 6',6'-bis(2-pyridyl)-2,2':4',4':2',2'-quaterpyridine) demonstrated that the latter is a superior photocatalyst toward the oxidation of alcohols. The much stronger electronic coupling with significant delocalization across the strongly electron-accepting tppz bridge facilitates charge trapping between the chromophore and catalyst centers and therefore is presumably responsible for the decreased catalytic performance.     

Substituents dependent capability of bis(ruthenium-dioxolene-terpyridine) complexes toward water oxidation

The bridging ligand, 1,8-bis(2,2':6',2'-terpyrid-4'-yl)anthracene (btpyan) was synthesized by the Miyaura-Suzuki cross coupling reaction of anthracenyl-1,8-diboronic acid and 4'-triflyl-2,2':6'-2'-terpyridine in the presence of Pd(PPh(3))(4) (5 mol%) with 68% in yield. Three ruthenium-dioxolene dimers, [Ru(2)(OH)(2)(dioxolene)(2)(btpyan)](0) (dioxolene = 3,6-di-tert-butyl-1,2-benzosemiquinone ([1](0)), 3,5-dichloro-1,2-benzosemiquinone ([2](0)) and 4-nitro-1,2-benzosemiquinone ([3](0))) were prepared by the reaction of [Ru(2)Cl(6)(btpyan)](0) with the corresponding catechol. The electronic structure of [1](0) is approximated by [Ru(II)(2)(OH)(2)(sq)(2)(btpyan)](0) (sq = semiquinonato). On the other hand, the electronic states of [2](0) and [3](0) are close to [Ru(III)(2)(OH)(2) (cat)(2)(btpyan)](0) (cat = catecholato), indicating that a dioxolene having electron-withdrawing groups stabilizes [Ru(III)(2)(OH)(2)(cat)(2)(btpyan)](0) rather than [Ru(II)(2)(OH)(2)(sq)(2)(btpyan)](0) as resonance isomers. No sign was found of deprotonation of the hydroxo groups of [1](0), whereas [2](0) and [3](0) showed an acid-base equilibrium in treatments with t-BuOLi followed by HClO(4). Furthermore, controlled potential electrolysis of [1](0) deposited on an ITO (indium-tin oxide) electrode catalyzed the four-electron oxidation of H(2)O to evolve O(2) at potentials more positive than +1.6 V (vs. SCE) at pH 4.0. On the other hand, the electrolysis of [2](0) and [3](0) deposited on ITO electrodes did not show catalytic activity for water oxidation under similar conditions. Such a difference in the reactivity among [1](0), [2](0) and [3](0) is ascribed to the shift of the resonance equilibrium between [Ru(II)(2)(OH)(2)(sq)(2)(btpyan)](0) and [Ru(III)(2)(OH)(2)(cat)(2)(btpyan)](0).     

Chiral-at-metal ruthenium complex as a metalloligand for asymmetric catalysis

The mononuclear [Ru(bpy)(2)(bpym)][PF(6)](2) complex (bpy = 2,2'-bipyridine; bpym = 2,2'-bipyrimidine) has been prepared in its enantiopure Lambda form. Because of the chelating property of the bipyrimidine moiety, it is possible to use this chiral-at-metal complex as a chiral inorganic ligand for a second metal cation acting as a catalytic center. Here we report the synthesis and the structural characterization of a novel dinuclear Lambda-[(bpy)(2)Ru(bpym)RuCl(p-cymene)](3+) compound (1). The asymmetric-inducing properties of the enantiopure chiral-at-metal metalloligand have been probed during asymmetric transfer hydrogenation to ketones catalyzed by 1. This provides one of the very few illustrations of the potential of this original class of chiral inorganic ligands.     

Water oxidation catalysis: influence of anionic ligands upon the redox properties and catalytic performance of mononuclear ruthenium complexes

Aiming at highly efficient molecular catalysts for water oxidation, a mononuclear ruthenium complex Ru(II)(hqc)(pic)(3) (1; H(2)hqc = 8-hydroxyquinoline-2-carboxylic acid and pic = 4-picoline) containing negatively charged carboxylate and phenolate donor groups has been designed and synthesized. As a comparison, two reference complexes, Ru(II)(pdc)(pic)(3) (2; H(2)pdc = 2,6-pyridine-dicarboxylic acid) and Ru(II)(tpy)(pic)(3) (3; tpy = 2,2':6',2"-terpyridine), have also been prepared. All three complexes are fully characterized by NMR, mass spectrometry (MS), and X-ray crystallography. Complex 1 showed a high efficiency toward catalytic water oxidation either driven by chemical oxidant (Ce(IV) in a pH 1 solution) with a initial turnover number of 0.32 s(-1), which is several orders of magnitude higher than that of related mononuclear ruthenium catalysts reported in the literature, or driven by visible light in a three-component system with [Ru(bpy)(3)](2+) types of photosensitizers. Electrospray ionization MS results revealed that at the Ru(III) state complex 1 undergoes ligand exchange of 4-picoline with water, forming the authentic water oxidation catalyst in situ. Density functional theory (DFT) was employed to explain how anionic ligands (hqc and pdc) facilitate the 4-picoline dissociation compared with a neutral ligand (tpy). Electrochemical measurements show that complex 1 has a much lower E(Ru(III)/Ru(II)) than that of reference complex 2 because of the introduction of a phenolate ligand. DFT was further used to study the influence of anionic ligands upon the redox properties of mononuclear aquaruthenium species, which are postulated to be involved in the catalysis cycle of water oxidation.     

Synthesis, structure and spectral and redox properties of new mixed ligand monomeric and dimeric Ru(II) complexes: predominant formation of the "cis-alpha" diastereoisomer and unusual 3MC emission by dimeric complexes

The tetradentate ligands 1,8-bis(pyrid-2-yl)-3,6-dithiaoctane (pdto) and 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (bbdo) form the complexes [Ru(pdto)(mu-Cl)](2)(ClO(4))(2) 1 and [Ru(bbdo)(mu-Cl)](2)(ClO(4))(2) 2 respectively. The new di-mu-chloro dimers 1 and 2 undergo facile symmetrical bridge cleavage reactions with the diimine ligands 2,2'-bipyridine (bpy) and dipyridylamine (dpa) to form the six-coordinate complexes [Ru(pdto)(bpy)](ClO(4))(2) 3, [Ru(bbdo)(bpy)](ClO(4))(2) 4, [Ru(pdto)(dpa)](ClO(4))(2) 5 and [Ru(bbdo)(dpa)](ClO(4))(2) 6 and with the triimine ligand 2,2':6,2'-terpyridine (terpy) to form the unusual seven-coordinate complexes [Ru(pdto)(terpy)](ClO(4))(2) 7 and [Ru(bbdo)(terpy)](ClO(4))(2) 8. In 1 the dimeric cation [Ru(pdto)(mu-Cl)](2)(2+) is made up of two approximately octahedrally coordinated Ru(II) centers bridged by two chloride ions, which constitute a common edge between the two Ru(II) octahedra. Each ruthenium is coordinated also to two pyridine nitrogen and two thioether sulfur atoms of the tetradentate ligand. The ligand pdto is folded around Ru(II) as a result of the cis-dichloro coordination, which corresponds to a "cis-alpha" configuration [DeltaDelta/LambdaLambda(rac) diastereoisomer] supporting the possibility of some attractive pi-stacking interactions between the parallel py rings at each ruthenium atom. The ruthenium atom in the complex cations 3a and 4 exhibit a distorted octahedral coordination geometry composed of two nitrogen atoms of the bpy and the two thioether sulfur and two py/bzim nitrogen atoms of the pdto/bbdo ligand, which is actually folded around Ru(II) to give a "cis-alpha" isomer. The molecule of complex 5 contains a six-coordinated ruthenium atom chelated by pdto and dpa ligands in the expected distorted octahedral fashion. The (1)H and (13)C NMR spectral data of the complexes throw light on the nature of metal-ligand bonding and the conformations of the chelate rings, which indicates that the dithioether ligands maintain their tendency to fold themselves even in solution. The bis-mu-chloro dimers 1 and 2 show a spin-allowed but Laporte-forbidden t(2g)(6)((1)A(1g))--> t(2g)(5) e(g)(1)((1)T(1g), (1)T(2g)) d-d transition. They also display an intense Ru(II) dpi--> py/bzim (pi*) metal-to-ligand charge transfer (MLCT) transition. The mononuclear complexes 3-8 exhibit dpi-->pi* MLCT transitions in the range 340-450 nm. The binuclear complexes 1 and 2 exhibit a ligand field ((3)MC) luminescence even at room temperature, whereas the mononuclear complexes 3 and 4 show a ligand based radical anion ((3)MLCT) luminescence. The binuclear complexes 1 and 2 undergo two successive oxidation processes corresponding to successive Ru(II)/Ru(III) couples, affording a stable mixed-valence Ru(II)Ru(III) state (K(c): 1, 3.97 x 10(6); 2, 1.10 x 10(6)). The mononuclear complexes 3-7 exhibit only one while 8 shows two quasi-reversible metal-based oxidative processes. The coordinated 'soft' thioether raises the redox potentials significantly by stabilising the 'soft' Ru(II) oxidation state. One or two ligand-based reduction processes were also observed for the mononuclear complexes.     

A Calix[4]arene-Based Cyclic Dinuclear Ruthenium Complex for Light-Driven Catalytic Water Oxidation

A cyclic dinuclear ruthenium(bda) (bda: 2,2’-bipyridine-6,6’-dicarboxylate) complex equipped with oligo(ethylene glycol)-functionalized axial calix[4]arene ligands has been synthesized for homogenous catalytic water oxidation. This novel Ru(bda) macrocycle showed significantly increased catalytic activity in chemical and photocatalytic water oxidation compared to the archetype mononuclear reference [Ru(bda)(pic)₂]. Kinetic investigations, including kinetic isotope effect studies, disclosed a unimolecular water nucleophilic attack mechanism of this novel dinuclear water oxidation catalyst (WOC) under the involvement of the second coordination sphere. Photocatalytic water oxidation with this cyclic dinuclear Ru complex using [Ru(bpy)₃]Cl₂ as a standard photosensitizer revealed a turnover frequency of 15.5 s⁻¹ and a turnover number of 460. This so far highest photocatalytic performance reported for a Ru(bda) complex underlines the potential of this water-soluble WOC for artificial photosynthesis.     

Dinuclear asymmetric ruthenium complexes with 5-cyano-1,10-phenanthroline as a bridging ligand

New dinuclear asymmetric ruthenium complexes of the type [(bpy)(2)Ru(5-CNphen)Ru(NH(3))(5)](4+/5+) (bpy = 2,2'-bipyridine; 5-CNphen = 5-cyano-1,10-phenanthroline) have been synthesized and characterized by spectroscopic, electrochemical, and photophysical techniques. The structure of the cation [(bpy)(2)Ru(5-CNphen)Ru(NH(3))(5)](4+) has been determined by X-ray diffraction. The mononuclear precursor [Ru(bpy)(2)(5-CNphen)](2+) has also been prepared and studied; while its properties as a photosensitizer are similar to those of [Ru(bpy)(3)](2+), its luminescence at room temperature is quenched by a factor of 5 in the mixed-valent species [(bpy)(2)Ru(II)(5-CNphen)Ru(III)(NH(3))(5)](5+), pointing to the occurrence of intramolecular electron-transfer processes that follow light excitation. From spectral data for the metal-to-metal charge-transfer transition Ru(II) --> Ru(III) in this latter complex, a slight electronic interaction (H(AB) = 190 cm(-1)) is disclosed between both metallic centers through the bridging 5-CNphen.     

Synthesis and structural,electrochemical, and optical properties of Ru(II) complexes with azobis(2,2'-bipyridine)s

Two new ditopic ligands, 5,5"-azobis(2,2'-bipyridine) (5,5"-azo) and 5,5"-azoxybis(2,2'-bipyridine) (5,5"-azoxy), were prepared by the reduction of nitro precursors. Mononuclear and dinuclear Ru(II) complexes having one of these bridging ligands and 2,2'-bipyridine terminal ligands were also prepared, and their properties were compared with previously reported Ru(II) complexes having 4,4"-azobis(2,2'-bipyridine) (4,4"-azo). The X-ray crystal structure showed that 5,5"-azo adopts the trans conformation and a planar rodlike shape. The X-ray crystal structure of [(bpy)(2)Ru(5,5"-azo)Ru(bpy)(2)](PF(6))(4) (Ru(5,5"-azo)Ru) showed that the bridging ligand is in the trans conformation and nearly planar also in the complex and the metal-to-metal distance is 10.0 A. The azo or azoxy ligand in these complexes exhibits reduction processes at less negative potentials than the terminal bpy's due to the low-lying pi level. The electronic absorption spectra for the complexes having 5,5"-azo or 5,5"-azoxy exhibit an extended low-energy metal-to-ligand charge-transfer absorption. The ligands, 5,5"-azo and 5,5"-azoxy, and the mononuclear complex, [(bpy)(2)Ru(5,5"-azo)](2+), isomerize reversibly upon light irradiation. The low-energy MLCT state sensitizes the isomerization of the azo moiety in this complex. While [(bpy)(2)Ru(4,4"-azo)Ru(bpy)(2)](PF(6))(4) exhibits light switch properties, namely, significant electrochromism and a large luminescence enhancement, upon reduction, Ru(5,5"-azo)Ru does not show these properties. The radical anion formation upon reduction of these complexes has been confirmed by ESR spectroscopy.     

A molecular light-driven water oxidation catalyst

Two mononuclear Ru(II) complexes, [Ru(ttbt)(pynap)(I)]I and [Ru(tpy)(Mepy)(2)(I)]I (tpy = 2,2';6,2"-terpyridine; ttbt = 4,4',4"-tri-tert-butyltpy; pynap = 2-(pyrid-2'-yl)-1,8-naphthyridine; and Mepy = 4-methylpyridine), are effective catalysts for the oxidation of water. This oxidation can be driven by a blue (λ(max) = 472 nm) LED light source using [Ru(bpy)(3)]Cl(2) (bpy = 2,2'-bipyridine) as the photosensitizer. Sodium persulfate acts as a sacrificial electron acceptor to oxidize the photosensitizer that in turn drives the catalysis. The presence of all four components, light, photosensitizer, sodium persulfate, and catalyst, are required for water oxidation. A dyad assembly has been prepared using a pyrazine-based linker to join a photosensitizer and catalyst moiety. Irradiation of this intramolecular system with blue light produces oxygen with a higher turnover number than the analogous intermolecular component system under the same conditions.     

Mono- and dinuclear ruthenium(II) 1,6,7,12-tetraazaperylene complexes

We report the synthesis of free 1,6,7,12-tetraazaperylene (tape). Tape was obtained from 1,1'-bis-2,7-naphthyridine by potassium promoted cyclization followed by oxidation with air. Mono- and dinuclear ruthenium(II) 1,6,7,12-tetraazaperylene complexes of the general formulas [Ru(L-L)(2)(tape)](PF(6))(2), [1](PF(6))(2)-[5](PF(6))(2), and [{Ru(L-L)(2)}(2)(μ-tape)](PF(6))(4), [6](PF(6))(4)-[10](PF(6))(4), with{L-L = phen, bpy, dmbpy (4,4'-dimethyl-2,2'-bipyridine), dtbbpy (4,4'-ditertbutyl-2,2'-bipyridine) and tmbpy (4,4'5,5'-tetramethyl-2,2'-bipyridine)}, respectively, were synthesized. The X-ray structures of tape·2CHCl(3) and the mononuclear complexes [Ru(bpy)(2)(tape)](PF(6))(2)·0.5CH(3)CN·0.5toluene, [Ru(dmbpy)(2)(tape)](PF(6))(2)·2toluene and [Ru(dtbbpy)(2)(tape)](PF(6))(2)·3acetone·0.5H(2)O were solved. The UV-vis absorption spectra and the electrochemical behavior of the ruthenium(ii) tape complexes were explored and compared with the data of the analogous dibenzoeilatin (dbneil), 2,2'-bipyrimidine (bpym) and tetrapyrido[3,2-a:2',3'-c:3',2'-h:2',3'-j]phenazin (tpphz) species.     

Water oxidation by mononuclear ruthenium complexes with TPA-based ligands

The synthesis, characterization, and water oxidation activity of mononuclear ruthenium complexes with tris(2-pyridylmethyl)amine (TPA), tris(6-methyl-2-pyridylmethyl)amine (Me(3)TPA), and a new pentadentate ligand N,N-bis(2-pyridinylmethyl)-2,2'-bipyridine-6-methanamine (DPA-Bpy) have been described. The electrochemical properties of these mononuclear Ru complexes have been investigated by both experimental and computational methods. Using Ce(IV) as oxidant, stoichiometric oxidation of water by [Ru(TPA)(H(2)O)(2)](2+) was observed, while Ru(Me(3)TPA)(H(2)O)(2)](2+) has much less activity for water oxidation. Compared to [Ru(TPA)(H(2)O)(2)](2+) and [Ru(Me(3)TPA)(H(2)O)(2)](2+), [Ru(DPA-Bpy)(H(2)O)](2+) exhibited 20 times higher activity for water oxidation. This study demonstrates a new type of ligand scaffold to support water oxidation by mononuclear Ru complexes.     

Protonation effect on catalytic water oxidation activity of a mononuclear Ru catalyst containing a free pyridine unit

Two efficient single-site Ru water oxidation catalysts [Ru(bda)(pic)(Ln)](bda = 2,2'-bipyridine-6,6'-dicarboxylic acid, pic = picoline, L1 = 4,5-bipyridine-2,7-di-tert-butyl-9,9-dimethylxanthene, L2 = 4-pyridine-5-phenyl-2,7-di-tert-butyl-9,9-dimethylxanthene) were only synthesized containing different xanthene ligands at the axial site. These complexes have been thoroughly characterized by spectroscopic(UV-vis, NMR) and electrochemical(CV and DPV) techniques. Kinetic analysis proved that the mechanism of water oxidation comprises the water nucleophilic attack process on high-valence ruthenium species.It is found that the catalyst 1 displayed higher activity than catalyst 2 on water oxidation, caused by the protonation of the axial ligand L1 with a free pyridine.     

Electronic structure of 2,2'-bipyridine organotransition-metal complexes. Establishing the ligand oxidation level by density functional theoretical calculations

A density functional theoretical (DFT) study (B3LYP) has been carried out on 20 organometallic complexes containing η(5)- and/or η(3)-coordinated cyclopentadienyl anions (Cp(-)) and 2,2'-bipyridine (bpy) ligand(s) at varying oxidation levels, i.e., as the neutral ligand (bpy(0)), as the π-radical monoanion (bpy(?-))(-), or as the diamagnetic dianion (bpy(2-))(2-). The molecular and electronic structures of these species in their ground states and, in some cases, their first excited states have been calculated using broken-symmetry methodology. The results are compared with experimental structural and spectroscopic data (where available) in order to validate the DFT computational approach. The following electron-transfer series and complexes have been studied: [(Cp)(2)V(bpy)](0,+,2+) (1-3), [(Cp)(2)Ti(bpy)](-,0,+,2+) (4-7), [(Cp)(2)Ti(biquinoline)](0,+) (8 and 9), [(Cp*)(2)Ti(bpy)](0) (10) (Cp* = pentamethylcyclopentadienyl anion), [Cp*Co(bpy)](0,+) (11 and 12), [Cp*Co(bpy)Cl](+,0) (13 and 14), [Fe(toluene)(bpy)](0) (15), [Cp*Ru(bpy)](-) (16), [(Cp)(2)Zr(bpy)](0) (17), and [Mn(CO)(3)(bpy)](-) (18). In order to test the predictive power of our computations, we have also calculated the molecular and electronic structures of two complexes, A and B, namely, the diamagnetic dimer [Cp*Sc(bpy)(μ-Cl)](2) (A) and the paramagnetic (at 25 °C) mononuclear species [(η(5)-C(5)H(4)(CH(2))(2)N(CH(3))(2))Sc((m)bpy)(2)] (B). The crystallographically observed intramolecular π-π interaction of two N,N'-coordinated π-radical anions in A leading to an S = 0 ground state is reliably reproduced. Similarly, the small singlet-triplet gap of ~600 cm(-1) between two antiferromagnetically coupled (bpy(?-))(-) ligands in B, two ferromagnetically coupled radical anions in the triplet excited state of B, and the structures of A and B is reproduced. Therefore, we are confident that we can present computationally obtained, detailed electronic structures for complexes 1-18. We show that N,N'-coordinated neutral bpy(0) ligands behave as very weak π acceptors (if at all), whereas the (bpy(2-))(2-) dianions are strong π-donor ligands.     

Photoelectrochemistry on Ru(II)-2,2'-bipyridine-phosphonate-derivatized TiO2 with the I3-/I- and quinone/hydroquinone relays. Design of photoelectrochemical synthesis cells

Photocurrent measurements have been made on nanocrystalline TiO2 surfaces derivatized by adsorption of a catalyst precursor, [Ru(tpy)(bpy(PO3H2)2)(OH2)]2+, or chromophore, [Ru(bpy)2 (bpy(PO3H2)2)]2+ (tpy is 2,2':6',2' '-terpyridine, bpy is 2,2'-bipyridine, and bpy(PO3H2)2 is 2,2'-bipyridyl-4,4'-diphosphonic acid), and on surfaces containing both complexes. This is an extension of earlier work on an adsorbed assembly containing both catalyst and chromophore. The experiments were carried out with the I3-/I- or quinone/hydroquinone (Q/H2Q) relays in propylene carbonate, propylene carbonate-water mixtures, and acetonitrile-water mixtures. Electrochemical measurements show that oxidation of surface-bound Ru(III)-OH2(3+) to Ru(IV)=O(2+) is catalyzed by the bpy complex. Addition of aqueous 0.1 M HClO4 greatly decreases photocurrent efficiencies for adsorbed [Ru(tpy)(bpy(PO3H2)2)(OH2)]2+ with the I3-/I- relay, but efficiencies are enhanced for the Q/H2Q relay in both propylene carbonate-HClO4 and acetonitrile-HClO4 mixtures. The dependence of the incident photon-to-current efficiency (IPCE) on added H2Q in 95% propylene carbonate and 5% 0.1 M HClO4 is complex and can be interpreted as changing from rate-limiting diffusion to the film at low H2Q to rate-limiting diffusion within the film at high H2Q. There is no evidence for photoelectrochemical cooperativity on mixed surfaces containing both complexes with the IPCE response reflecting the relative surface compositions of the two complexes. These results provide insight into the possible design of photoelectrochemical synthesis cells for the oxidation of organic substrates.     

1,10]-Phenanthrolin-2-yl ketones and their coordination chemistry

A synthetic protocol involving the Friedl?nder reaction of 8-amino-7-quinolinecarbaldehyde followed by potassium dichromate oxidation was applied to 2,3,4-pentanetrione-3-oxime and 1-(pyrid-2'-yl)propane-1,2-dione-1-oxime to provide the ligands di-(phenathrolin-2-yl)-methanone (1) and phenanthrolin-2-yl-pyrid-2-yl-methanone (8), respectively. Ligand 1 complexed as a planar tetradentate with Pd(II) to form [Pd(1)](BF4)2 and with Ru(II) and two 4-substituted pyridines (4-R-py) to form [Ru(1)(4-R-py)2](PF6)2 where R = CF3, CH3, and Me2N. With [Ru(bpy)2Cl2], the dinuclear complex [(bpy)2Ru(1)Ru(bpy)2](PF6)4 was formed (bpy = 2,2'-bipyridine). Ligand 8 afforded the homoleptic Ru(II) complex [Ru(8)2](PF6)2, as well as the heteroleptic complex [Ru(8)(tpy)](PF6)2 (tpy = 2,2';6,2'-terpyridine). The ligands and complexes were characterized by their NMR and IR spectra, as well as an X-ray structure determination of [Ru(1)(4-CH3-py)2](PF6)2. Electrochemical analysis indicated metal-based oxidation and ligand-based reduction that was consistent with results from electronic absorption spectra. The complexes [Ru(1)(4-R-py)2](PF6)2 were sensitive to the 4-substituent on the axial pyridine: electron donor groups facilitated the oxidation while electron-withdrawing groups impeded it.     

Structural study on highly oxidized states of a water oxidation complex [RuIII(bpy)2(H2O)]2(mu-O)4+ by ruthenium K-edge X-ray absorption fine structure spectroscopy

The oxidation-induced structural change of a water-oxidizing diruthenium complex, [(bpy)(2)(H(2)O)Ru(III)(micro-O)Ru(III)(OH(2))(bpy)(2)](4+) (bpy = 2,2'-bipyridine), was investigated by means of X-ray absorption spectroscopy. Ru K-edge XANES (X-ray absorption near-edge structure) spectra from the acidic solution and solid precipitates obtained by oxidation showed that the absorption edge shifts toward higher energy with a preedge feature slightly more enhanced than those of the lower oxidation states. This indicates that the higher oxidation state has a lower symmetry due to shortening of the Ru-O bonds that originated from the water ligands. The EXAFS (extended X-ray absorption fine structure) spectra were similar to those of the lower oxidation states, whose analysis revealed the existence of short Ru-O double bonds and an almost linear Ru-O-Ru angle (169 +/- 2 degrees ). Ab initio EXAFS simulations for several possible structural models suggest that the dimeric structure is maintained during the water oxidation reaction.