Nucleophilic Attack Of Water Research Articles

Elucidating the Role of Aqueous Solvent in an Iron‐Based Water Oxidation System by DFT‐based Molecular Simulation

AbstractWater solvent plays an important role in catalytic water oxidation to dioxygen, in particular in the O−O bond formation process. In this work, we revisit the mechanism of O−O bond formation catalyzed by a mononuclear iron catalyst [Cl−FeIII(dpa)−Cl]+, in a DFT‐based molecular dynamics (DFT‐MD) study that incorporates explicit solvent and thermal fluctuations. Two possible mechanisms for the crucial O−O bond formation, namely water nucleophilic attack (WNA) and nitrate nucleophilic attack (NNA) on the high‐valent FeV‐oxo moiety, were considered and found to have similar barriers (15 kcal/mol vs 16 kcal/mol). Comparison with static DFT calculations demonstrated the important role of water solvent molecules, especially for the NNA pathway. For this mechanism, the interaction of the negatively‐charged nitrate with solvent molecules is substantial, giving rise to a free energy barrier increase of 7.7 kcal/mol compared with static DFT calculations. The study suggests that for molecular water‐oxidation catalysts, the local aqueous solvation structure and its thermal fluctuations plays a significant role in the crucial O−O bond formation step. The study also elucidates the role of the nitrate ion as a co‐catalyst, a notion that may serve as a potential design rule for developing improved water oxidation catalysts.

Switching the O–O bond-formation mechanism by controlling water activity

Switching the O–O bond-formation mechanism by controlling water activity

Mechanism of water oxidation catalyzed by vitamin B12: Redox non-innocent nature of corrin ligand and crucial role of phosphate

Vitamin B12 (macrocyclic cobalamin) has been recently reported to be capable of electrochemically catalyzing water oxidation in a neutral phosphate buffer solution. In this work, density functional calculations were employed to elucidate the water oxidation mechanism catalyzed by vitamin B12. The calculations showed that the catalytic cycle starts from the L•-CoII-OH2 complex 1. A proton-coupled electron transfer process then leads to the formation of a L•-CoIII-OH complex 2, followed by another proton-coupled electron transfer event to afford a corrin ligand radical cation intermediate 3 (L•-CoIII-O•). The redox non-innocent nature of the corrin ligand plays an essential role in the oxidation process. 3 is capable of triggering the O-O bond formation via a water nucleophilic attack mechanism, in which a hydrophosphate dianion functions as a base to accept a proton from the water nucleophile. A dioxygen molecule is released after the oxidation of the CoIII-OOH intermediate. The rate-determining step was calculated to be the O-O bond formation with a total barrier of 16.5 kcal/mol. While the use of water molecules as the proton acceptor was found to be less feasible for the O-O bond formation, with a barrier of 31.2 kcal/mol, further highlighting the crucial of phosphate in water oxidation.

Biomimetic Oxidative Bromination by cis‐Dioxidotungsten(VI) Complexes of Salan Type N,N’‐Capped Linear Tetradentate Amino Bisphenol

AbstractReaction of [WVIO2(acac)2] (Hacac=acetylacetone) with salan‐type dibasic tetradentate ONNO donor Mannich bases derived from ethylenediamine, formaldehyde and 2,4‐di‐tert‐butylphenol (H2L1), 2‐tert‐butyl‐4‐methylphenol (H2L2), 2,4‐dimethylphenol (H2L3) and 2,4‐dichlorophenol (H2L4) in a 1 : 1 ([WVIO2(acac)2] : H2L) molar ratio in refluxing MeOH gave the corresponding cis‐dioxidotungsten(VI) complexes [WVIO2L1] (1), [WVIO2L2] (2), [WVIO2L3] (3) and [WVIO2L4] (4), respectively. Characterization by elemental analysis, various spectroscopic (FT‐IR, UV‐vis, 1H and 13C NMR) studies, DFT calculations and single‐crystal X‐ray analysis of 2 and 3 suggest six‐coordinated octahedral α‐cis (symmetric) isomeric form of the complexes where ligands coordinate through the two phenolate oxygen and two amine nitrogen atoms (in a cis‐α type symmetric binding mode) with one of the N atoms of the ligand and one of the terminal O atoms of the cis‐WO2 group in the axial position. These complexes are potential catalyst precursors for the oxidative bromination of thymol and styrene. Thymol upon bromination gave three products, namely, 2‐bromothymol, 4‐bromothymol, and 2,4‐dibromothymol; later one being the major product. Oxidative bromination of styrene resulted in 2‐bromo‐1‐phenylethanol and 1‐phenylethane‐1,2‐diol; the later one is the result of nucleophilic attack of water on the α as well as β carbons both of the initially formed 1,2‐dibromo‐1‐phenylethane.

Isolation of a Ru(IV) side-on peroxo intermediate in the water oxidation reaction.

The electrons that nature uses to reduce CO2 during photosynthesis come from water oxidation at the oxygen-evolving complex of photosystem II. Molecular catalysts have served as models to understand its mechanism, in particular the O-O bond-forming reaction, which is still not fully understood. Here we report a Ru(IV) side-on peroxo complex that serves as a 'missing link' for the species that form after the rate-determining O-O bond-forming step. The Ru(IV) side-on peroxo complex (η2-1iv-OO) is generated from the isolated Ru(IV) oxo complex (1iv=O) in the presence of an excess of oxidant. The oxidation (IV) and spin state (singlet) of η2-1iv-OO were determined by a combination of experimental and theoretical studies. 18O- and 2H-labelling studies evidence the direct evolution of O2 through the nucleophilic attack of a H2O molecule on the highly electrophilic metal-oxo species via the formation of η2-1iv-OO. These studies demonstrate water nucleophilic attack as a viable mechanism for O-O bond formation, as previously proposed based on indirect evidence.

Do multinuclear 3d metal catalysts achieve O–O bond formation via radical coupling or via water nucleophilic attack? WNA leads the way in [Co4O4]n+

Catalytic water oxidation is a required process for clean energy production based on the concept of artificial photosynthesis. Here, we provide in situ spectroscopic and computational analysis for the closest known photosystem II analog, [Co4O4]n+ ([Co4O4Py4Ac4]0, Py = pyridine and Ac = CH3COO-), which catalyzes electrochemical water oxidation. In situ extended X-ray absorption fine structure detects an ultrashort, CoIV=O (~1.67 Å) moiety, a crucial intermediate for O-O bond formation. Density function theory analyses show that the intermediate has two CoIV centers and a CoIV=O unit of strong radicaloid character sufficient to support a CoIV=O + H2O = Co-OOH + H+ transition, where the carboxyl ligand accepts the proton and the bridging oxygen stabilizes the peroxide via hydrogen bonding. The proposed water nucleophilic attack mechanism accounts for all prior spectroscopic evidence on the Co4O44+ core. Our results are important for the design and development of efficient water oxidation catalysts, which contribute to the ultimate goal of clean energy from artificial photosynthesis.

Iron‐Catalyzed Water Oxidation: O–O Bond Formation via Intramolecular Oxo–Oxo Interaction

AbstractHerein, we report the importance of structure regulation on the O−O bond formation process in binuclear iron catalysts. Three complexes, [Fe2(μ‐O)(OH2)2(TPA)2]4+(1), [Fe2(μ‐O)(OH2)2(6‐HPA)]4+(2) and [Fe2(μ‐O)(OH2)2(BPMAN)]4+(3), have been designed as electrocatalysts for water oxidation in 0.1 M NaHCO3solution (pH 8.4). We found that1and2are molecular catalysts and that O−O bond formation proceeds via oxo–oxo coupling rather than by the water nucleophilic attack (WNA) pathway. In contrast, complex3displays negligible catalytic activity. DFT calculations suggested that theantitosynisomerization of the two high‐valent Fe=O moieties in these catalysts takes place via the axial rotation of one Fe=O unit around the Fe‐O‐Fe center. This is followed by the O−O bond formation via an oxo–oxo coupling pathway at the FeIVFeIVstate or via oxo–oxyl coupling pathway at the FeIVFeVstate. Importantly, the rigid BPMAN ligand in complex3limits theantitosynisomerization and axial rotation of the Fe=O moiety, which accounts for the negligible catalytic activity.

Iron-Catalyzed Water Oxidation: O-O Bond Formation via Intramolecular Oxo-Oxo Interaction.

Herein, we report the importance of structure regulation on the O-O bond formation process in binuclear iron catalysts. Three complexes, [Fe2 (μ-O)(OH2 )2 (TPA)2 ]4+ (1), [Fe2 (μ-O)(OH2 )2 (6-HPA)]4+ (2) and [Fe2 (μ-O)(OH2 )2 (BPMAN)]4+ (3), have been designed as electrocatalysts for water oxidation in 0.1 M NaHCO3 solution (pH 8.4). We found that 1 and 2 are molecular catalysts and that O-O bond formation proceeds via oxo-oxo coupling rather than by the water nucleophilic attack (WNA) pathway. In contrast, complex 3 displays negligible catalytic activity. DFT calculations suggested that the anti to syn isomerization of the two high-valent Fe=O moieties in these catalysts takes place via the axial rotation of one Fe=O unit around the Fe-O-Fe center. This is followed by the O-O bond formation via an oxo-oxo coupling pathway at the FeIV FeIV state or via oxo-oxyl coupling pathway at the FeIV FeV state. Importantly, the rigid BPMAN ligand in complex 3 limits the anti to syn isomerization and axial rotation of the Fe=O moiety, which accounts for the negligible catalytic activity.

Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalysts

Summary O–O bond formation is a key elementary step of the water-oxidation reaction. However, it is still unclear how the mechanism of O–O coupling depends on the applied electrode potential. Herein, using water-in-salt electrolytes, we systematically altered the water activity, which enabled us to probe the O–O bond-forming mechanism on heterogeneous Co-based catalysts as a function of applied potential. We discovered that the water-oxidation mechanism is sensitive to the applied potential: At relatively low driving force, the reaction proceeds through an intramolecular oxygen coupling mechanism, whereas the water nucleophilic attack mechanism prevails at high driving force. The observed mechanistic switch has major implications for the understanding and control of the water-oxidation reaction on heterogeneous catalysts.

Importance of the oxyl character on the IrO2 surface dependent catalytic activity for the oxygen evolution reaction

The oxygen evolution reaction catalyst optimization is hindered because in the desirable acidic conditions the sole active catalysts are RuO2 and IrO2. Thus, the understanding of the factors controlling the reactivity of these materials is mandatory. In this contribution, DFT (PBE-D2) periodic calculations are performed to analyze the catalytic activities of the main ((110), (011), (100) and (001)) IrO2 surfaces. Results show that the reaction only occurs if the Ir=O species on the surfaces exhibit an oxyl character. The water nucleophilic attack mechanism is the most favorable pathway on the (110), (100) and (001) surfaces. In contrast, for the (011) facet the oxo-coupling is preferred. The required overpotentials for the four IrO2 surfaces depend on the feasibility to oxidize the Ir-OH to Ir-O species and this is tuned by the coordination of the unsaturated iridium sites: the (100) and (001) surfaces appear to be more active than the (110) and (011).

Unsymmetrical Dinuclear RuII Complexes with Bridging Polydentate Nitrogen Ligands as Potential Water Oxidation Catalysts

AbstractMononuclear RuII complex [RuCl(κ3N‐tpm)(κ2N‐bptz)]Cl, [1]Cl [tpm=tris(1‐ pyrazolyl)methane; bptz=3,6‐di(2‐pyridyl)‐1,2,4,5‐tetrazine], and dinuclear complexes [RuCl(κ3N‐tpm)(μ‐κ2N:κ2N‐bptz)Ru(κ2N‐bipy)2][PF6]3, [3][PF6]3, [RuCl(η6‐p‐cymene)(μ‐κ2N:κ2N‐dpp)Ru(κ2N‐bipy)2][PF6]2, [4][PF6]2, and [RuCl(η6‐p‐cymene)(μ‐κ2N:κ2N‐dpp)Ru(κ2N‐biqn)2][PF6]3, [5][PF6]3 [dpp=2,3‐bis(2′‐pyridyl)‐pyrazine; bipy=2,2′‐bipyridine; biqn=2,2′‐quinoline], incorporating both potentially catalytic and photosensitive subunits, were synthesized and characterized by means of elemental analysis, mass spectrometry, and spectroscopic methods. The molecular structures of the new compounds were also investigated and compared by means of DFT calculations. The absorption spectra of all the compounds are dominated by metal‐to‐ligand charge‐transfer bands in the visible (which in most cases largely extend over the red portion of the spectrum) and ligand‐centered bands in the UV region. The oxidation behavior is based on metal‐centered RuII to RuIII oxidation processes, which in phosphate buffer solution are followed by a catalytic water oxidation wave for [1]Cl and [3][PF6]3. For these compounds, the mechanism of water oxidation is proposed to consist in water nucleophilic attack, according to chemical experiments with Ce(IV) salts, so demonstrating for the first time that the bptz ligand can be profitably used to build ruthenium(II) complexes with catalytic properties. On the contrary, no catalytic process is observed for 4 and 5, most likely due to the high positive potential for RuII oxidation induced by the presence of the p‐cymene moiety.

Highly Efficient Production of Tailored Monascus Pigments by Using a Biocompatible Chemical Reaction Interfacing with Microbial Metabolism

A biocompatible reaction interfacing with microbial metabolism is an efficient approach for production of desired chemicals. Monascus pigments are a traditional natural food colorant from a microbial source. The major color components are red Monascus pigments (RMPs) with various primary amine resides as well as some minor components of yellow ones (YMPs) and orange ones (OMPs). Herein, we elucidate the pathway of OMP degradation involving nucleophilic attack of water on OMPs for the first time. On the basis of OMP degradation and the conversion of OMPs into RMPs sharing a similar reaction mechanism, i.e., nucleophilic attack of water or primary amine on the activated OMPs, we design cascade reactions including biosynthesis of OMPs and biocompatible chemical conversion of OMPs into the stable RMPs with a single glycine residue using a flexible combining chemocatalyst (phosphate ion) and micellar catalyst (Triton X-100). The cascade reactions ensure that OMPs maintain a very low concentration during microbial fermentation by rapid chemical conversion of OMPs into the stable RMPs. In this way, the metabolite profile of microbial fermentation is tailored to RMPs with a single glycine residue as the major color components while free of OMPs and YMPs. Meanwhile, the possible OMP degradation as well as feedback inhibition is eliminated, and a high concentration of RMPs 3.2 mM (1.4 g/L) is also achieved. Designing cascade reactions also has potential for the efficient microbial fermentation of other azaphilone alkaloids.

The FeIVO• oxyl unit as a key intermediate in water oxidation on the FeIIIhydroxide: DFT predictions

AbstractThe OO coupling process in water oxidation on the gamma FeOOH hydroxide catalyst is simulated by means of density functional theory using model iron cubane cluster Fe4O4(OH)4. A key reactive intermediate is proposed to be the HOFeIVO• oxyl unit with terminal oxo radical. The “initial” vertex FeIII(OH) moiety forms this intermediate at the calculated overpotential of 0.93 V by adding one water molecule and withdrawing two proton–electron pairs. The OO coupling goes via water nucleophilic attack on the oxyl oxygen to form the OO bond with a remarkably low barrier of 11 kcal/mol. This process is far more effective than alternative scenario based on direct interaction of two ferryl FeIVO sites (with estimated barrier of 36 kcal/mol) and is comparable with the coupling between terminal oxo center and three‐coordinated lattice oxo center (12 kcal/mol barrier). The process of hydroxylation of terminal oxygen inhibits the OO coupling. Nevertheless, being more effective for ferryl oxygen, the hydroxylation in fact enhances selectivity of the OO coupling initiated by the oxyl oxygen.

From Ru-bda to Ru-bds: a step forward to highly efficient molecular water oxidation electrocatalysts under acidic and neutral conditions

Significant advances during the past decades in the design and studies of Ru complexes with polypyridine ligands have led to the great development of molecular water oxidation catalysts and understanding on the O−O bond formation mechanisms. Here we report a Ru-based molecular water oxidation catalyst [Ru(bds)(pic)2] (Ru-bds; bds2− = 2,2′-bipyridine-6,6′-disulfonate) containing a tetradentate, dianionic sulfonate ligand at the equatorial position and two 4-picoline ligands at the axial positions. This Ru-bds catalyst electrochemically catalyzes water oxidation with turnover frequencies (TOF) of 160 and 12,900 s−1 under acidic and neutral conditions respectively, showing much better performance than the state-of-art Ru-bda catalyst. Density functional theory calculations reveal that (i) under acidic conditions, the high valent Ru intermediate RuV=O featuring the 7-coordination configuration is involved in the O−O bond formation step; (ii) under neutral conditions, the seven-coordinate RuIV=O triggers the O−O bond formation; (iii) in both cases, the I2M (interaction of two M−O units) pathway is dominant over the WNA (water nucleophilic attack) pathway.

Switching the O-O Bond Formation Pathways of Ru-pda Water Oxidation Catalyst by Third Coordination Sphere Engineering.

Water oxidation is a vital anodic reaction for renewable fuel generation via electrochemical- and photoelectrochemical-driven water splitting or CO2 reduction. Ruthenium complexes, such as Ru-bda family, have been shown as highly efficient water-oxidation catalysts (WOCs), particularly when they undergo a bimolecular O-O bond formation pathway. In this study, a novel Ru(pda)-type (pda2– =1,10-phenanthroline-2,9-dicarboxylate) molecular WOC with 4-vinylpyridine axial ligands was immobilized on the glassy carbon electrode surface by electrochemical polymerization. Electrochemical kinetic studies revealed that this homocoupling polymer catalyzes water oxidation through a bimolecular radical coupling pathway, where interaction between two Ru(pda)–oxyl moieties (I2M) forms the O-O bond. The calculated barrier of the I2M pathway by density-functional theory (DFT) is significantly lower than the barrier of a water nucleophilic attack (WNA) pathway. By using this polymerization strategy, the Ru centers are brought closer in the distance, and the O-O bond formation pathway by the Ru (pda) catalyst is switched from WNA in a homogeneous molecular catalytic system to I2M in the polymerized film, providing some deep insights into the importance of third coordination sphere engineering of the water oxidation catalyst.

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.

Observation of a Potential-Dependent Switch of Water Oxidation Mechanism on Co-Oxide-Based Catalysts

O-O bond formation is a key elementary step of the water oxidation reaction. However, it is still unclear how the mechanism of O-O coupling depends on the applied electrode potential. Herein, using water-in-salt electrolytes, we systematically altered the water activity, which enabled us to probe the O-O bond forming mechanism on heterogeneous Co-based catalysts as a function of applied potential. We discovered that the water oxidation mechanism is sensitive to the applied potential: At relatively low driving force, the reaction proceeds through an intramolecular oxygen coupling mechanism, whereas the water nucleophilic attack mechanism prevails at high driving force. The observed mechanistic switch has major implications for the understanding and control of the water oxidation reaction on heterogeneous catalysts.

Hydrophobic/Hydrophilic Directionality Affects the Mechanism of Ru-Catalyzed Water Oxidation Reaction

From the study of supramolecular dimers of [RuVO(pda)]+ (pda = 1,10-phenanthroline-2,9-dicarboxylic acid) and [RuVO(bda)]+ (bda = 2,2′-bipyridine-6,6′-dicarboxylate) complexes, the O–O bond-forming intermediates in water oxidation, we found orientational distinction induced by the pda and bda ligands. The bda complex prefers the front-to-front geometry, while the pda complex favors the front-to-back geometry in the formation of prereactive geometry. In the bda complex, the hydrophobic oxo will point at another oxo with the bda directed toward water, which favors the I2M mechanism. In the pda complex, the hydrophobic oxo instead is directed toward a more hydrophobic phenanthroline moiety of the pda of another species, which disfavors I2M. The binding free energy of the nonproductive front-to-back of pda is 3 kcal mol–1 more stable than that of the prereactive dimer. This incorrect orientation leads to an additional rearrangement required before the O–O bond can be formed. Estimation of the rate constant shows 2 orders of magnitude lower reactivity for the I2M mechanism of the pda complex relative to the bda complex, which makes the water nucleophilic attack mechanism competitive.

Palladium-Promoted Carbon–Nitrogen Bond Cleavage in 1,3,5-Triazinane Derivatives

trans-Dichloropalladium(II) complexes with amino acid esters PdCl2L2 have been synthesized via selective hydrolytic cleavage of the nitrogen–carbon bond in 1,3,5-tris(ethoxycarbonylmethyl)- and 1,3,5-tris[(S)-1′(methoxycarbonyl)ethyl]-1,3,5-triazinane under the action of palladium(II) in methanol or in water–acetone mixture. The nucleophilic attack of water or methanol results in selective cleavage of the 1,3,5-triazinane ring in the complexes of palladium(II) with the starting triazinanes, affording the dichloropalladium(II) complexes with amino acids esters.

Cobalt Metal-Organic Framework Based on Layered Double Nanosheets for Enhanced Electrocatalytic Water Oxidation in Neutral Media.

A new cobalt metal-organic framework (2D-Co-MOF) based on well-defined layered double cores that are strongly connected by intermolecular bonds has been developed. Its 3D structure is held together by π-π stacking interactions between the labile pyridine ligands of the nanosheets. In aqueous solution, the axial pyridine ligands are exchanged by water molecules, producing a delamination of the material, where the individual double nanosheets preserve their structure. The original 3D layered structure can be restored by a solvothermal process with pyridine, so that the material shows a "memory effect" during the delamination-pillarization process. Electrochemical activation of a 2D-Co-MOF@Nafion-modified graphite electrode in aqueous solution improves the ionic migration and electron transfer across the film and promotes the formation of the electrocatalytically active cobalt species for the oxygen evolution reaction (OER). The so-activated 2D-Co-MOF@Nafion composite exhibits an outstanding electrocatalytic performance for the OER at neutral pH, with a TOF value (0.034 s-1 at an overpotential of 400 mV) and robustness superior to those reported for similar electrocatalysts under similar conditions. The particular topology of the delaminated nanosheets, with quite distant cobalt centers, precludes the direct coupling between the electrocatalytically active centers of the same sheet. On the other hand, the increase in ionic migration across the film during the electrochemical activation stage rules out the intersheet coupling between active cobalt centers, as this scenario would impair electrolyte permeation. Altogether, the most plausible mechanism for the O-O bond formation is the water nucleophilic attack to single Co(IV)-oxo or Co(III)-oxyl centers. Its high electrochemical efficiency suggests that the presence of nitrogen-containing aromatic equatorial ligands facilitates the water nucleophilic attack, as in the case of the highly efficient cobalt porphyrins.