Electrocatalytic Water Oxidation by a Dinuclear Copper Complex in a Neutral Aqueous Solution

Electrocatalytic water oxidation using the oxidatively robust 2,7‐[bis(2‐pyridylmethyl)aminomethyl]‐1,8‐naphthyridine ligand (BPMAN)‐based dinuclear copper(II) complex, [Cu₂(BPMAN)(μ‐OH)]³⁺, has been investigated. This catalyst exhibits high reactivity and stability towards water oxidation in neutral aqueous solutions. DFT calculations suggest that the O? O bond formation takes place by an intramolecular direct coupling mechanism rather than by a nucleophilic attack of water on the high‐oxidation‐state Cu^IV?O moiety.     

Tracking an FeV(O) Intermediate for Water Oxidation in Water

High-valent iron-oxo species are appealing for conducting O−O bond formation for water oxidation reactions. However, their high reactivity poses a great challenge to the dissection of their chemical transformations. Herein, we introduce an electron-rich and oxidation-resistant ligand, 2-[(2,2′-bipyridin)-6-yl]propan-2-ol to stabilize such fleeting intermediates. Advanced spectroscopies and electrochemical studies demonstrate a high-valent Fe^V(O) species formation in water. Combining kinetic and oxygen isotope labelling experiments and organic reactions indicates that the Fe^V(O) species is responsible for O−O bond formation via water nucleophilic attack under the real catalytic water oxidation conditions.     

Some aspects of tetramethylammonium fluorochromate (VI) oxidations and identification of the reduced chromium species

The reduction product of tetramethylammonium fluorochromate, TMAFC, isolated after oxidation, has been identified from the results of chemical analyses, chemical determination of the oxidation state of chromium, magnetic susceptibility, cyclic voltametry, n.m.r. and i.r. spectral studies as (CH₃)₄N[CrO₂F], chromium(IV) species. The facile oxidation of triphenylphosphine to triphenylphosphine oxide by TMAFC in acetonitrile provides a clear-cut example of an oxygen transfer reaction.     

Electrochemical transfer of [18F]fluoride from [18O]water into organic solvents ready for labeling reactions

For labeling reactions [¹⁸F]fluoride has to be separated from [¹⁸O]water and transferred into an organic solvent suitable for nucleophilic substitutions. An electrolytical method is described for depositing [¹⁸F]fluoride on a vitreous carbon electrode and releasing it directly into CH₃CN or DMSO. In the presence of Et₃N×3HF, [¹⁸F]fluoride is almost quantitatively released into acetonitrile. When using n.c.a conditions, i.e., Et₃N^.HCl, desorption of the ¹⁸F activity is almost 70% and 60% in acetonitrile and DMSO, respectively, already within 5 minutes.     

Reaction of [HO(CH2)3]3P with α,β-Unsaturated Ketones Containing a Phenylpropanoid Backbone

Abstract

Interaction of 3,4-(MeO)₂-benzylideneacetone with [HO(CH₂)₃]₃P (THPP) was studied in CD₃OD by NMR to compare reactivity of a phenylpropanoid α,β-unsaturated ketone with a corresponding α,β-unsaturated aldehyde. In the presence of HCl, both the ketone and a related cinnamaldehyde first establish an equilibrium with the product formed by nucleophilic attack of the THPP at the C?O bond, [ArCH?CHCX(OD)PR₃]⁺Cl^?(X?H or CH₃, Ar?Ph or 3,4-(MeO)₂C₆H₃). The ketone salt then slowly transforms into [R₃PCH(Ar)CH(D)C(O)CD₃]⁺Cl^?, the phosphonium product of nucleophilic attack of THPP at the C?C bond, whereas the final product from the aldehyde is the (α-ether)phosphonium chloride [ArCH?CHCH(OCD₃)PR₃]⁺Cl^?. In aqueous media, in the absence of HCl, 4-HO-benzylideneacetone, which is similar to a lignin-type, α,β-unsaturated aldehyde model compound, interacts with THPP to afford a stable phosphonium zwitterion, in contrast to the previously studied aldehyde model, which forms dimeric, bisphosphonium products.     

Elucidation of the Structure of Products of Triphenylphosphine Electrooxidation in the Presence of Camphene

The structure of the products of anodic oxidation of triphenylphosphine in the presence of camphene carried out in acetonitrile with sodium perchlorate as supporting electrolyte has been studied. The major product, triphenylphosphine oxide, has been isolated from the solution in the form of cocrystals of free triphenylphosphine oxide and its complex with sodium perchlorate. The molecular structure of the cocrystals has been studied by X-ray diffraction analysis. Triphenylcamphenylphosphonium perchlorate, bornylacetamide, and a terpene compound with triphenylphosphonium and acetamide substituents in the cycle have been detected by NMR ¹³C as the electrolysis side products.

     

Regioselectivity and regiospecificity of phosphorylation of (<Emphasis Type="Italic">Z</Emphasis>)- and (<Emphasis Type="Italic">E</Emphasis>)-4-chloromethylene-2-phenyl-5(4<Emphasis Type="Italic">H</Emphasis>)-oxazolones. Effect of solvation: Supramolecular approximation

By the methods of quantum chemistry in supramolecular approximation, are considered stereochemical and energetic features of phosphorylation of 4-chloromethylene-2-phenyl -5(4H)-oxazolone Z- and E-isomers in gas phase and their solvates with acetonitrile of 1:n composition where n varies from 1 to 10. On the MNDO-PM3 level the phosphorylation with triphenylphosphine proceeds endothermally in two steps: nucleophilic addition in the first step and elimination of chlorine anion with formation of phosphonium salt in the second step. Solvation with acetonitrile leads to stabilization of phosphonium intermediate and decrease in heat of conversion. On both semiempirical and nonempirical levels occur regioselectivity of nucleophilic attack at the double C=C, but not C=O, bond and regiospecificity of transformation without inversion of init configuration of the isomers owing to steric hindrances restricting rotation degree of freedom of -CHClP⁺Ph₃ group. Therewith, elimination of chlorine anion is characterized by low activation barrier and occurs with donation negative charge from π-orbital of carbon atom in the 4 position of heterocycle on antibonding σ*-orbital of carbon-chlorine bond; two orbitals become practically coplanar in the transition state.     

Effect of substituents on the rate of oxidation of anilines with peroxomonosulfate monoanion (HOOSO−3) in aqueous acetonitrile: A mechanistic study

Mechanistic studies on the oxidation of 18 meta‐, para‐, and ortho‐substituted anilines (Ans) by HOOSO⁻₃ in aqueous acetonitrile medium have been performed. The reaction can be characterized by the experimental rate equation, The addition of p‐toluenesulfonic acid (TsOH) retards the reaction. The increase in the reactivity of anilines as the medium is made more aqueous is interpreted. The reaction is enhanced by electron‐donating groups on the amine in the series consistent with the rate‐limiting nucleophilic attack of the amine on the persulfate oxygen. The proposed mechanism involves the conversion of phenylhydroxylamine to nitrosobenzene in a fast step. The ESR study reveals the absence of free radicals in the reaction. Various attempts have been made to analyze the experimental rate constants in terms of LFER plots. Improved correlations are obtained with σ⁻ values and the σ⁻ form of the Yukawa–Tsuno equation. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 649–657, 2005     

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.     

Oxidation of benzylic methylenes to ketones with Oxone-KBr in aqueous acetonitrile under transition metal free conditions

A green and highly efficient protocol for the oxidation of benzylic methylenes to their corresponding ketones with a combination of Oxone and KBr in aqueous acetonitrile is developed. The H₂¹⁸O labeling experiment demonstrated that the oxygen introduced into ketone originated from water. A plausible mechanism was also suggested.     

Factors That Affect the Nature of the Final Oxidation Products in “Peroxo‐Shunt” Reactions of Iron–Porphyrin Complexes

The present study focuses on the oxidation of the water‐soluble and water‐insoluble iron(III)–porphyrin complexes [Fe^III(TMPS)] and [Fe^III(TMP)] (TMPS=meso‐tetrakis(2,4,6‐trimethyl‐3‐sulfonatophenyl)porphyrinato, TMP=meso‐tetrakis(2,4,6‐trimethylphenyl)porphyrinato), respectively, by meta‐chloroperoxybenzoic acid (m‐CPBA) in aqueous methanol and aqueous acetonitrile solutions of varying acidity. With the application of a low‐temperature rapid‐scan UV/Vis spectroscopic technique, the complete spectral changes that accompany the formation and decomposition of the primary product of O? O bond cleavage in the acylperoxoiron(III)–porphyrin intermediate [(P)Fe^III? OOX] (P=porphyrin) were successfully recorded and characterized. The results clearly indicate that the O? O bond in m‐CPBA is heterolytically cleaved by the studied iron(III)–porphyrin complexes independent of the acidity of the reaction medium. The existence of two different oxidation products under acidic and basic conditions is suggested not to be the result of a mechanistic changeover in the mode of O? O bond cleavage on going from low to high pH values, but rather the effect of environmental changes on the actual product of the O? O bond cleavage in [(P)Fe^III? OOX]. The oxoiron(IV)–porphyrin cation radical formed as a primary oxidation product over the entire pH range can undergo a one‐ or two‐electron reduction depending on the selected reaction conditions. The present study provides valuable information for the interpretation and improved understanding of results obtained in product‐analysis experiments.     

Solvent effects on the oxidation of RuIV=O to O=RuVI=O by MnO4-. hydrogen-atom versus oxygen-atom transfer

The kinetics of the oxidation of trans-[RuIV(tmc)(O)(solv)]2+ to trans-[RuVI(tmc)(O)2]2+ (tmc is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, a tetradentate macrocyclic tertiary amine ligand; solv = H2O or CH3CN) by MnO4- have been studied in aqueous solutions and in acetonitrile. In aqueous solutions the rate law is -d[MnO4]/dt = kH2O[RuIV][MnO4-] = (kx + (ky)/(Ka)[H+])[RuIV][MnO4-], kx = (1.49 +/- 0.09) x 101 M-1 s-1 and ky = (5.72 +/- 0.29) x 104 M-1 s-1 at 298.0 K and I = 0.1 M. The terms kx and ky are proposed to be the rate constants for the oxidation of RuIV by MnO4- and HMnO4, respectively, and Ka is the acid dissociation constant of HMnO4. At [H+] = I = 0.1 M, DeltaH and DeltaS are (9.6 +/- 0.6) kcal mol-1 and -(18 +/- 2) cal mol-1 K-1, respectively. The reaction is much slower in D2O, and the deuterium isotope effects are kx/kxD = 3.5 +/- 0.1 and ky/kyD = 5.0 +/- 0.3. The reaction is also noticeably slower in H218O, and the oxygen isotope effect is kH216O/kH218O = 1.30 +/- 0.07. 18O-labeled studies indicate that the oxygen atom gained by RuIV comes from water and not from KMnO4. These results are consistent with a mechanism that involves initial rate-limiting hydrogen-atom abstraction by MnO4- from coordinated water on RuIV. In acetonitrile the rate law is -d[MnO4-]/dt = kCH3CN[RuIV][MnO4-], kCH3CN = 1.95 +/- 0.08 M-1 s-1 at 298.0 K and I = 0.1 M. DeltaH and DeltaS are (12.0 +/- 0.3) kcal mol-1 and -(17 +/- 1) cal mol-1 K-1, respectively. 18O-labeled studies show that in this case the oxygen atom gained by RuIV comes from MnO4-, consistent with an oxygen-atom transfer mechanism.     

Spin-Enhanced O−H Cleavage in Electrochemical Water Oxidation

Herein we report the vital role of spin polarization in proton-transfer-mediated water oxidation over a magnetized catalyst. During the electrochemical oxygen evolution reaction (OER) over ferrimagnetic Fe₃O₄, the external magnetic field induced a remarkable increase in the OER current, however, this increment achieved in weakly alkaline pH (pH 9) was almost 20 times that under strongly alkaline conditions (pH 14). The results of the surface modification experiment and H/D kinetic isotope effect investigation confirm that, at weakly alkaline pH, during the nucleophilic attack of Fe^IV=O by molecular water, the magnetized Fe₃O₄ catalyst polarizes the spin states of the nucleophilic attacking intermediates. The spin-enhanced singlet O−H cleavage and triplet O−O bonding occur synergistically, which promotes the O₂ generation more significantly than the strongly alkaline case involving only spin-enhanced O−O bonding.     

Investigations on the oxidation of the carbamate pesticide aminocarb

The electron transfer stoichiometry and principal oxidation products of the carbamate pesticide Aminocarb (3-methyl-4-dimethylaminophenyl N-methylcarbamate) were investigated in methanol + water and acetonitrile + water media, using flow injection coulometry,¹H and ¹³C nuclear magnetic resonance spectrometry, and mass spectrometry. The four-electron oxidation was found to occur at aliphatic as opposed to aromatic sites, involving oxidation and nucleophilic attack at an N-methyl group to yield the hydroxy/methoxy analogs in water + methanol media, and yielding ultimately an aldehyde. The pathway in water + acetonitrile, as expected, was found to yield only the hydroxy analogs and the aldehyde.     

Chemistry and stereochemistry of the interaction of the water-soluble phosphine [HO(CH2)3]3P with cinnamaldehyde in aqueous media

To learn more about the bleaching action of pulps by (hydroxymethyl)phosphines, cinnamaldehyde was reacted with tris(3-hydroxypropyl)phosphine, [HO(CH2)3]3P (THPP), in aqueous solution at room temperature under argon. Self-condensation of the aldehyde into two isomeric products, 2-benzyl-5-phenyl-pent-2,4-dienal and 5-phenyl-2-(phenylmethylene)-4-pentenal, is observed; this implies initial nucleophilic attack of the phosphine at the beta-carbon of the alpha,beta-unsaturated aldehyde. Reaction in D2O gives the same products in which all but the phenyl and CHO protons are replaced by deuterons. NMR studies are consistent with carbanion formation and subsequent condensation of two phosphonium-containing aldehyde moieties to generate the products with concomitant elimination of phosphine oxide. In D2O in the presence of HCl, THPP reversibly attacks the aldehyde-C atom to form the (alpha-hydroxy)phosphonium derivative [PhCH=C(H)CH(OD)PR3]Cl (where R=(CH2)3OD), which slowly converts into the deuterated bisphosphonium salt [R3PCH(Ph)CD(H)CH(OD)PR3]Cl2 via the deuterated monophosphonium salt [R3PCH(Ph)CD(H)CHO]Cl. The phosphonium intermediates and phosphonium products in this chemistry, although having up to three chiral carbon centers, are formed with high stereoselectivity just in enantiomeric forms. In acetone-H2O (1:1 v/v), a cross-condensation of cinnamaldehyde with acetone to give 6-phenyl-3,5-hexadien-2-one is promoted by THPP via generation of OH-.     

A mass spectrometric and ab initio study of the pathways for dehydration of simple glycine and cysteine-containing peptide [M+H]+ ions

The gas phase fragmentation reactions of the [M+H]⁺ and [M+H?H²O]⁺ ions of glycylglycine, glycylcysteine, N-acetylglycine, N-acetylcysteine, their corresponding methyl esters, as well as several other related model systems have been examined by electrospray ionization (ESI) tandem mass spectrometry (MS ⁿ ) using triple quadrupole and quadrupole ion trap mass spectrometers. Two discrete gas phase fragmentation pathways for the loss of water from glycine-containing peptides, corresponding to retro-Koch and retro-Ritter type reactions were observed. Two pathways were also observed for the loss of water from C-terminal cysteine-containing peptides: a retro-Koch type reaction and an intramolecular nucleophilic attack at the carbonyl of the amide bond by the cysteinyl side chain thiol. Various intermediates involved in these reactions, derived from the [M+H?H²O]⁺ ions of N-formylglycine and N-formylcysteine, were modeled using ab initio calculations at the MP2(FC)/6-31G*//HF/6-31G* level of theory. These calculations indicate that: (i) the retro-Koch reaction product is predicted to be more stable than the product from the retro-Ritter reaction for N-formylglycine, and (ii) the intramolecular nucleophilic attack product is preferred over the retro-Koch and retro-Ritter reaction products for N-formylcysteine. The results from these ab initio calculations are in good agreement with the experimentally determined ion abundances for these processes.     

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.     

Dehydration of peptide [M + H]+ ions in the gas phase

The loss of water from protonated peptides was studied using [¹⁸O]-labeling of the C-terminal carboxyl group. The structures (including the location of the isotopic label) of first-generation product ions were examined by sequential product ion scanning (MS³ and MS⁴) using a hybrid sector/quadrupole mass spectrometer. Water loss may involve carboxylic acid groups, side-chain hydroxyls, or peptide backbone oxygens. Although one of these three pathways often predominates, more than one dehydration route can be operative for a single peptide structure. When peptide backbone oxygen is lost, the dehydration can occur at one or two primary sites along the backbone, with the location of the site(s) varying among peptides. When water loss involves the C-terminal carboxyl group, the resulting ion may undergo extensive intraionic oxygen isotope exchange. This evidence for complex intraionic interactions further emphasizes the significance of gas-phase conformation in determining the fragmentations of peptide ions.     

A Cobalt(III)−Hydroxo Complex Bearing a Pentadentate Amidate Ligand as an Electrocatalyst for Water Oxidation

A Co(III)−hydroxo complex, [Co^III(dpaq)OH]ClO₄ ( 1-OH ) bearing a pentadentate ligand, H-dpaq, (H-dpaq=(2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate]) catalyses water oxidation in mildly alkaline medium (pH 8.0) at a potential of 1.4 V_NHE with an average Turn-Over-Frequency (TOF_max) of 2.8×10⁴ s⁻¹ and faradaic efficiency of 88 %. Post-electrolysis characterization of the electrode rules out the formation of any heterogeneous electroactive species. Electrochemical results and theoretical calculations confirm the occurrence of both metal and ligand centered PCET processes during anodic scanning. The resulting formally Co(V)−oxo/oxyl intermediate undergoes water nucleophilic attack to install the O−O bond. The role of axial ligand in water oxidation by Co(III)−dpaq system has been examined by comparing the reactivity of the Co-hydroxide complex ( 1-OH ) with that of its chloride-ligated counterpart, [Co^III(dpaq)Cl]Cl ( 1-Cl ). The results confirm the ability of the Co-dpaq complexes to bind water/or water derived ligands over chloride or non-aqueous solvents. The interplay of ligand redox non-innocence and σ-donating ability of the N5-carboxamido ligand helps to store oxidizing equivalents and triggers O−O bond formation.     

Catalysis of water oxidation in acetonitrile by iridium oxide nanoparticles

Water oxidation catalysed by iridium oxide nanoparticles (IrO₂ NPs) in water–acetonitrile mixtures using [Ru^III(bpy)₃]³⁺ as oxidant was studied as a function of the water content, the acidity of the reaction media and the catalyst concentration. It was observed that under acidic conditions (HClO₄) and at high water contents (80% (v/v)) the reaction is slow, but its rate increases as the water content decreases, reaching a maximum at approximately equimolar proportions (≈25% H₂O (v/v)). The results can be rationalized based on the structure of water in water–acetonitrile mixtures. At high water fractions, water is present in highly hydrogen-bonded arrangements and is less reactive. As the water content decreases, water clustering gives rise to the formation of water-rich micro-domains, and the number of bonded water molecules decreases monotonically. The results presented herein indicate that non-bonded water present in the water micro-domains is considerably more reactive towards oxygen production. Finally, long term electrolysis of water–acetonitrile mixtures containing [Ru^II(bpy)₃]²⁺ and IrO₂ NPs in solution show that the amount of oxygen produced is constant with time demonstrating that the redox mediator is stable under these experimental conditions.