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.     

Light-induced water oxidation by a Ru complex containing a bio-inspired ligand

The new Ru complex 8 containing the bio-inspired ligand 7 was successfully synthesized and characterized. Complex 8 efficiently catalyzes water oxidation using Ce(IV) and Ru(III) as chemical oxidants. More importantly, this complex has a sufficiently low overpotential to utilize ruthenium polypyridyl-type complexes as photosensitizers.     

单核钌催化剂化学催化和光催化水氧化反应

合成了一系列含有不同对位取代基团的吡啶轴向配体的单核钌化合物Ru(bda)(pic)₂ (H₂bda=2,2''-联吡啶-6,6''-二羧酸; pic=对甲基吡啶),对化合物的结构进行了核磁、质谱和X射线单晶衍射表征,并在中性和酸性条件下研究了这些化合物的电化学性质.以硝酸铈铵为氧化剂,对催化剂的催化活性进行了测试,并以[Ru(bpy)₃]²⁺为光敏剂,S₂O₈^2-为电子牺牲剂,在三组分体系中考察了这些化合物的光催化活性.研究发现,在化学法水氧化反应中,化合物1由于其轴向配体4,4''-联吡啶在酸性条件下能够发生质子化,从而增强了吸电子效应,因此表现出最高的催化活性,催化循环数达到4000.在光催化水氧化反应中,化合物2因其轴向配体具有最强的吸电子能力而表现出最高的催化活性,反应2h的催化循环数达到270.结果表明,轴向配体的吸电子能力明显提高了这类Ru催化剂催化水氧化反应活性.     

Nonaqueous catalytic water oxidation

The complex [Ru(Mebimpy)(bpy)(OH(2))](2+) [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy = 2,2'-bipyridine] and its 4,4'-(PO(3)H(2)CH(2))(2)bpy derivative on oxide electrodes are water oxidation catalysts in propylene carbonate and 2,2,2-trifluoroethanol (TFE) to which water has been added as a limiting reagent. The rate of water oxidation is greatly enhanced relative to that with water as the solvent and occurs by a pathway that is first-order in H(2)O; an additional pathway that is first-order in acetate appears when TFE is used as the solvent.     

Stability of a molybdena-silica catalyst enhanced by water in formaldehyde oxidation

A molybdena-silica catalyst showed rapid deactivation in the oxidation of formaldehyde to carbon oxides. The deactivation was greatly retarded by adding water in the feed. Thus, water coproduced in the partial oxidation of methanol to formaldehyde contributes to the relatively stable activity of the molybdena-silica catalyst in the partial oxidation reaction.     

Activation by oxidation and ligand exchange in a molecular manganese vanadium oxide water oxidation catalyst

Despite their technological importance for water splitting, the reaction mechanisms of most water oxidation catalysts (WOCs) are poorly understood. This paper combines theoretical and experimental methods to reveal mechanistic insights into the reactivity of the highly active molecular manganese vanadium oxide WOC [Mn₄V₄O₁₇(OAc)₃]³⁻ in aqueous acetonitrile solutions. Using density functional theory together with electrochemistry and IR-spectroscopy, we propose a sequential three-step activation mechanism including a one-electron oxidation of the catalyst from [Mn₂³⁺Mn₂⁴⁺] to [Mn³⁺Mn₃⁴⁺], acetate-to-water ligand exchange, and a second one-electron oxidation from [Mn³⁺Mn₃⁴⁺] to [Mn₄⁴⁺]. Analysis of several plausible ligand exchange pathways shows that nucleophilic attack of water molecules along the Jahn–Teller axis of the Mn³⁺ centers leads to significantly lower activation barriers compared with attack at Mn⁴⁺ centers. Deprotonation of one water ligand by the leaving acetate group leads to the formation of the activated species [Mn₄V₄O₁₇(OAc)₂(H₂O)(OH)]⁻ featuring one H₂O and one OH ligand. Redox potentials based on the computed intermediates are in excellent agreement with electrochemical measurements at various solvent compositions. This intricate interplay between redox chemistry and ligand exchange controls the formation of the catalytically active species. These results provide key reactivity information essential to further study bio-inspired molecular WOCs and solid-state manganese oxide catalysts.

Combined theoretical and experimental studies shed light on the initial steps of redox-activation of a molecular manganese vanadium oxide water oxidation catalyst.     

Photoinduced water oxidation by a tetraruthenium polyoxometalate catalyst: ion-pairing and primary processes with Ru(bpy)3(2+) photosensitizer

The tetraruthenium polyoxometalate [Ru(4)(μ-O)(4)(μ-OH)(2)(H(2)O)(4)(γ-SiW(10)O(36))(2)](10-) (1) behaves as a very efficient water oxidation catalyst in photocatalytic cycles using Ru(bpy)(3)(2+) as sensitizer and persulfate as sacrificial oxidant. Two interrelated issues relevant to this behavior have been examined in detail: (i) the effects of ion pairing between the polyanionic catalyst and the cationic Ru(bpy)(3)(2+) sensitizer, and (ii) the kinetics of hole transfer from the oxidized sensitizer to the catalyst. Complementary charge interactions in aqueous solution leads to an efficient static quenching of the Ru(bpy)(3)(2+) excited state. The quenching takes place in ion-paired species with an average 1:Ru(bpy)(3)(2+) stoichiometry of 1:4. It occurs by very fast (ca. 2 ps) electron transfer from the excited photosensitizer to the catalyst followed by fast (15-150 ps) charge recombination (reversible oxidative quenching mechanism). This process competes appreciably with the primary photoreaction of the excited sensitizer with the sacrificial oxidant, even in high ionic strength media. The Ru(bpy)(3)(3+) generated by photoreaction of the excited sensitizer with the sacrificial oxidant undergoes primary bimolecular hole scavenging by 1 at a remarkably high rate (3.6 ± 0.1 × 10(9) M(-1) s(-1)), emphasizing the kinetic advantages of this molecular species over, e.g., colloidal oxide particles as water oxidation catalysts. The kinetics of the subsequent steps and final oxygen evolution process involved in the full photocatalytic cycle are not known in detail. An indirect indication that all these processes are relatively fast, however, is provided by the flash photolysis experiments, where a single molecule of 1 is shown to undergo, in 40 ms, ca. 45 turnovers in Ru(bpy)(3)(3+) reduction. With the assumption that one molecule of oxygen released after four hole-scavenging events, this translates into a very high average turnover frequency (280 s(-1)) for oxygen production.     

Electrochemical evidence for catalytic water oxidation mediated by a high-valent cobalt complex

The pH-dependent electrochemical behavior for a Co(II) complex, [Co(Py5)(OH(2))](ClO(4))(2) (1; Py5 = 2,6-(bis(bis-2-pyridyl)methoxymethane)pyridine), indicates consecutive (proton-coupled) oxidation steps furnish a Co(IV) species that catalyzes the oxidation of water in basic media.     

Kinetics and mechanism of water catalytic oxidation by a Ru3+(bpy)3 complex in the presence of colloidal cobalt hydroxide

Kinetics of the catalytic oxidation of water to molecular oxygen by a tris(bipyridyl) Ru(III) complex is studied in the presence of colloidal cobalt hydroxide stabilized by starch. Oxidant consumption follows the first-order rate law with respect to the oxidant concentration. The dependence of the apparent rate constant of this process on the catalyst concentration, initial oxidant concentration, and initial concentration of its reduced form was determined. The dependence of the oxygen yield on H⁺ at pH 7–11 and a catalyst concentration of 10-⁷-10-³ mol¹ is studied. An intermediate product of the reaction was found, which is probably a bridged peroxo complex of cobalt. The kinetic scheme and mechanism of the reaction is proposed, which agree with experimental observations.     

Fluoride-promoted oxidation of Fischer alkoxy carbene complexes: stoichiometric and catalytic conditions

The utility of fluoride anion as promoter of the oxidation of Fischer carbene complexes is presented. Two different and complementary methods that allow the fast and convenient preparation of carbene-derived esters in good yields have been developed using stoichiometric or catalytic quantities of fluoride ion.     

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.     

Electrochemical water oxidation by photo-deposited cobalt-based catalyst on a nano-structured TiO2 electrode

A cobalt-based catalyst was directly photo-deposited on the surface of a widely used n-type nano-structured semiconductor(TiO 2).Different thicknesses of the TiO 2 films as well as different time of photo-deposition of the Co-based catalyst on TiO 2 films have been optimized.It was found that the electrode with 3 layers of TiO 2 film(in 8 m thickness) and 1 hour photo-deposition of the cobalt-based catalyst by light irradiation from a 500 W Xenon lamp gave the highest current density(～5 mA/cm 2).Using this cobalt-modified TiO 2 film as a working electrode in an electrochemical device,highly efficient water oxidation has been demonstrated in a pH 7.0 aqueous solution with low overpotential.     

Oxygen kinetic isotope effects upon catalytic water oxidation by a monomeric ruthenium complex

Oxygen isotope fractionation is applied for the first time to probe the catalytic oxidation of water using a widely studied ruthenium complex, [Ru(II)(tpy)(bpy)(H(2)O)](ClO(4))(2) (bpy = 2,2'-bipyridine; tpy = 2,2';6",2"-terpyridine). Competitive oxygen-18 kinetic isotope effects ((18)O KIEs) derived from the ratio of (16,16)O(2) to (16,18)O(2) formed from natural-abundance water vary from 1.0132 ± 0.0005 to 1.0312 ± 0.0004. Experiments were conducted with cerium(IV) salts at low pH and a photogenerated ruthenium(III) tris(bipyridine) complex at neutral pH as the oxidants. The results are interpreted within the context of catalytic mechanisms using an adiabatic formalism to ensure the highest barriers for electron-transfer and proton-coupled electron-transfer steps. In view of these contributions, O-O bond formation is predicted to be irreversible and turnover-limiting. The reaction with the largest (18)O KIE exhibits the greatest degree of O-O coupling in the transition state. Smaller (18)O KIEs are observed due to multiple rate-limiting steps or transition-state structures which do not involve significant O-O motion. These findings provide benchmarks for systematizing mechanisms of O-O bond formation, the critical step in water oxidation by natural and synthetic catalysts. In addition, the measurements introduce a new tool for calibrating computational studies using relevant experimental data.     

磷钨酸季铵盐催化氧化汽油深度脱硫

以十六烷基三甲基溴化铵和磷钨酸为原料制备了磷钨酸季铵盐催化剂,并对催化剂进行了红外光谱和SEM表征。研究了磷钨酸季铵盐为催化剂,双氧水为氧化剂,催化氧化法生产低硫汽油技术。考察了萃取剂以及氧化条件和萃取条件对脱硫效果的影响。结果表明,在汽油10 mL,双氧水0.01 mL,催化剂0.0016 g,氧化温度30℃,氧化时间60 m in的条件下,采用复合溶剂LJ-1进行萃取,萃取温度20℃,静置时间15 m in,剂油比为1时,直溜汽油中的硫含量由179.3 mg/L降至10.8 mg/L,脱硫率达94.0%。氧化萃取时的脱硫率比未经氧化直接萃取时的脱硫率高45.6%,氧化脱硫效果显著。     

Hydroaminomethylation of olefins using a rhodium carbene catalyst

Hydroaminomethylation of terminal as well as internal aliphatic and aromatic olefins with various amines is described in the presence of [Rh(cod)(Imes)Cl] as a catalyst. In general good to excellent yields and high chemoselectivity were obtained in THF at 85-105°C using 0.1 mol% of catalyst.