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Novel dual-ionic imidazolium salts are shown to display excellent catalytic activity for cycloaddition of carbon dioxide and epoxides under room temperature and atmospheric pressure (0.1 MPa) without any solvent and co-catalyst leading to 96.1% product yield. It can be reused five times to keep the product yield over 90%. These intriguing results are attributed to a new reaction mechanism, which is supported by theoretical calculations along with the measurements of 13C NMR spectrum and Fourier transform infrared spectroscopy (FT-IR). The excellent catalytic activity can be traced to a CO2-philic group along with an electrophilic hydrogen atom. Our work shows that incorporation of CO2-philic group is an feasible pathway to develop the new efficient ionic liquids.

Isolation of RuIII-bda (17-electron specie) complex with an aqua ligand (2-electron donor) is challenging due to violation of the 18-electron rule. Although considerable efforts have been dedicated to mechanistic studies of water oxidation by the Ru-bda family, the structure and initial formation of the RuIII-bda aqua complex are still controversial. Herein, we challenge this often overlooked step by designing a pocket-shape Ru-based complex 1. The computational studies showed that 1 possesses the crucial hydrophobicity at the RuV(O) state as well as similar probability of access of terminal O to solvent water molecules when compared with classic Ru-bda catalysts. Through characterization of single-crystal structures at the RuII and RuIII states, a pseudo seven-coordinate “ready-to-go” aqua ligand with RuIII...O distance of 3.62 Å was observed. This aqua ligand was also found to be part of a formed hydrogen-bonding network, providing a good indication of how the RuIII-OH2 complex is formed. QC 20220811

Exploration of efficient water oxidation catalysts (WOCs) is the primary challenge in conversion of renewable energy into fuels. Here we report a molecularly well-defined heterogeneous WOC with Aza-fused, π-conjugated, microporous polymer (Aza-CMP) coordinated single cobalt sites (Aza-CMP-Co). The single cobalt sites in Aza-CMP-Co exhibited superior activity under alkaline and near-neutral conditions. Moreover, the molecular nature of the isolated catalytic sites makes Aza-CMP-Co a reliable model for studying the heterogeneous water oxidation mechanism. By a combination of experimental and theoretical results, a pH-dependent nucleophilic attack pathway for O-O bond formation was proposed. Under alkaline conditions, the intramolecular hydroxyl nucleophilic attack (IHNA) process with which the adjacent -OH group nucleophilically attacks Co4+=O was identified as the rate-determining step. This process leads to lower activation energy and accelerated kinetics than those of the intermolecular water nucleophilic attack (WNA) pathway. This study provides significant insights into the crucial function of electrolyte pH in water oxidation catalysis and enhancement of water oxidation activity by regulation of the IHNA pathway.

Electron transport through metal-organic frameworks by ahopping mechanism between discrete redox active sites is coupled to diffusion-migration of charge-balancing counter cations. Experimentally determinedapparent diffusion coefficients,Deapp, that characterize this form of chargetransport thus contain contributions from both processes. While this is wellestablished for MOFs, microscopic descriptions of this process are largelylacking. Herein, we systematically lay out different scenarios for cation-coupledelectron transfer processes that are at the heart of charge diffusion throughMOFs. Through systematic variations of solvents and electrolyte cations, it isshown that theDeappfor charge migration through a PIZOF-type MOF,Zr(dcphOH-NDI) that is composed of redox-active naphthalenediimide(NDI) linkers, spans over 2 orders of magnitude. More importantly, however,the microscopic mechanisms for cation-coupled electron propagation arecontingent on differing factors depending on the size of the cation and its propensity to engage in ion pairs with reduced linkers,either non-specifically or in defined structural arrangements. Based on computations and in agreement with experimental results, weshow that ion pairing generally has an adverse effect on cation transport, thereby slowing down charge transport. In Zr(dcphOH-NDI), however, specific cation-linker interactions can open pathways for concerted cation-coupled electron transfer processes thatcan outcompete limitations from reduced cationflux.

Replacing Ru by Fe in Ru(bda)(py)2 leads to drastically altered properties and loss of catalytic activity.

Many enzymes use adaptive frameworks to preorganize substrates,accommodate various structural and electronic demands of intermediates,and accelerate related catalysis. Inspired by biological systems,a Ru-based molecular water oxidation catalyst containing a configurationallylabile ligand [2,2 ':6 ',2 ''-terpyridine]-6,6 ''-disulfonatewas designed to mimic enzymatic framework, in which the sulfonatecoordination is highly flexible and functions as both an electrondonor to stabilize high-valent Ru and a proton acceptor to acceleratewater dissociation, thus boosting the catalytic water oxidation performancethermodynamically and kinetically. The combination of single-crystalX-ray analysis, various temperature NMR, electrochemical techniques,and DFT calculations was utilized to investigate the fundamental roleof the self-adaptive ligand, demonstrating that the on-demand configurationalchanges give rise to fast catalytic kinetics with a turnover frequency(TOF) over 2000 s(-1), which is compared to oxygen-evolvingcomplex (OEC) in natural photosynthesis.

An iron(iii) salen complex has been developed as a new precatalyst for the acceptorless dehydrogenation of alcohols and mechanistic studies by experimental and theoretical methods indicated iron particles as the catalytically active species.

We present a highly efficient convergent asymmetric hydrogenation of E/Z mixtures of enamides catalyzed by N,P–iridium complexes supported by mechanistic studies. It was found that reduction of the olefinic isomers (E and Z geometries) produces chiral amides with the same absolute configuration (enantioconvergent hydrogenation). This allowed the hydrogenation of a wide range of E/Z mixtures of trisubstituted enamides with excellent enantioselectivity (up to 99% ee). A detailed mechanistic study using deuterium labeling and kinetic experiments revealed two different pathways for the observed enantioconvergence. For α-aryl enamides, fast isomerization of the double bond takes place, and the overall process results in kinetic resolution of the two isomers. For α-alkyl enamides, no double bond isomerization is detected, and competition experiments suggested that substrate chelation is responsible for the enantioconvergent stereochemical outcome. DFT calculations were performed to predict the correct absolute configuration of the products and strengthen the proposed mechanism of the iridium-catalyzed isomerization pathway.

A CoPc/CNT system has been only recently reported to transform CO2 to methanol via electrochemical reductions, despite the fact that catalyst has been studied extensively since the 1980s. The expla ...

Intermolecular radical coupling (also interaction of two metal centers I2M) is one of the main mechanisms for O–O bond formation in water oxidation catalysts. For Ru(bda)L2 (H2bda = 2,2′-bipyridine-6,6′-dicarboxylate, L = pyridine or similar nitrogen containing heterocyclic ligands) catalysts a significant driving force in water solution is the hydrophobic effects driven by the solvent. The same catalyst has been successfully employed to generate N2 from ammonia, also via I2M, but here the solvent was acetonitrile where hydrophobic effects are absent. We used a classical force field for the key intermediate [RuVIN(bda)(py)2]+ to simulate the dimerization free energy by calculation of the potential mean force, in both water and acetonitrile to understand the differences and similarities. In both solvents the complex dimerizes with similar free energy profiles. In water the complexes are essentially free cations with limited ion paring, while in acetonitrile the ion-pairing is much more significant. This ion-pairing leads to significant screening of the charges, making dimerization possible despite lower solvent polarity that could lead to repulsion between the charged complexes. In water the lower ion pairing is compensated by the hydrophobic effect leading to favorable dimerization despite repulsion of the charges. A hypothetical doubly charged [RuVIIN(bda)py2]2+ was also studied for deeper understanding of the charge effect. Despite the double charge the complexes only dimerized favorably in the lower dielectric solvent acetonitrile, while in water the separated state is more stable. In the doubly charged catalyst the effect of ion-pairing is even more pronounced in acetonitrile where it is fully paired similar to the 1+ complex, while in water the separation of the ions leads to greater repulsion between the two catalysts, which prevents dimerization. Graphic Abstract

Organic semiconductors provide significant potentials for the construction of photoelectrochemical (PEC) cells for solar hydrogen production because of their highly tunable properties. Herein, on carbon fiber paper (CFP) surface, pyridyl (Py), and 4,4'-bipyridin-1-ium (Py2+) groups were introduced into polythiophene (pTH) semiconductor by electrochemical copolymerization, respectively. After assembly with the Co(dmgBF2)2 type catalyst (CoB, dmgBF2 = difluoroboryldimethylglyoximate), the CoB@Py2+-pTH/CFP photocathode displayed nearly twice the photocurrent enhancement (550 μA cm-2 at 0.15 V vs reversible hydrogen electrode, RHE) comparing to that generated by the CoB@Py-pTH/CFP photocathode (290 μA cm-2 at 0.15 V vs RHE) for light-driven H2 generation under AM 1.5 solar illumination. Investigation of the mechanism revealed that the introduction of the positively charged pyridinium groups could improve the intrinsic Co(dmgBF2)2 catalyst activity for the H2 generation reaction. Meanwhile, the positively charged pyridinium groups serve as p-type dopants to increase the semiconductor bulk charge transfer rate and act as electron transfer mediators to promote the interfacial charge transfer kinetics between the catalyst and the pTH-based organic semiconductor.

Understanding the seven coordination and O−O coupling pathway of the distinguished Ru-bda catalysts is essential for the development of next generation efficient water-oxidation catalysts based on earth-abundant metals. This work reports the synthesis, characterization and catalytic properties of a monomeric ruthenium catalyst Ru-bnda (H2bnda = 2,2'-bi(nicotinic acid)-6,6'-dicarboxylic acid) featuring steric hindrance and enhanced hydrophilicity on the backbone. Combining experimental evidence with systematic density functional theory calculations on the Ru-bnda and related catalysts Ru-bda (H2bda = 2,2ʹ-bipyridine-6,6ʹ-dicarboxylic acid), Ru-pda ((H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid)), and Ru-biqa (H2biqa = (1,1ʹ-biisoquinoline)-3,3ʹ-dicarboxylic acid), we emphasized that seven coordination clearly determines presence of RuV=O with high spin density on the ORuV=O atom, i.e. oxo with radical properties, which is one of the necessary conditions for reacting through the O−O coupling pathway. However, an additional factor to make the condition sufficient is the favorable intermolecular face-to-face interaction for the generation of the pre-reactive [RuV=O···O=RuV], which may be significantly influenced by the secondary coordination environments. This work provides a new understanding of the structure-activity relationship of water-oxidation catalysts and their potential to adopt I2M pathway for O−O bond formation.

A molecular [Ru(bda)]-type (bda = 2,2’-bipyridine-6,6’-dicarboxylate) water oxidation catalyst with 4-vinylpyridine as the axial ligand (Complex 1) was immobilized or co-immobilized with 1-(trifluoromethyl)-4-vinylbenzene (3F) or styrene (St) blocking units on the surface of glassy carbon (GC) electrodes by electrochemical polymerization, in order to prepare the corresponding poly-1@GC, poly-1+P3F@GC, and poly-1+PSt@GC functional electrodes. Kinetic measurements of the electrode surface reaction revealed that [Ru(bda)] triggers the O–O bond formation via (1) the radical coupling interaction between the two metallo-oxyl radicals (I2M) in the homo-coupling polymer (poly-1), and (2) the water nucleophilic attack (WNA) pathway in poly-1+P3F and poly-1+PSt copolymers. The comparison of the three electrodes revealed that the second coordination sphere of the water oxidation catalysts plays vital roles in stabilizing their reaction intermediates, tuning the O–O bond formation pathways and improving the water oxidation reaction kinetics without changing the first coordination structures.

Photocarboxylation of alkyne with carbon dioxide represents a highly attractive strategy to prepare functionalized alkenes with high efficiency and atomic economy. However, the reaction mechanism, especially the sequence of elementary steps (leading to different reaction pathways), reaction modes of the H-transfer step and carboxylation step, spin and charge states of the cobalt catalyst, etc., is still an open question. Herein, density functional theory calculations are carried out to probe the mechanism of the Ir/Co-catalyzed photocarboxylation of alkynes. The overall catalytic cycle mainly consists of four steps: reductive-quenching of the Ir catalyst, hydrogen transfer (rate-determining step), outer sphere carboxylation, and the final catalyst regeneration step. Importantly, the cobalt catalyst can facilitate the H-transfer by an uncommon hydride coupled electron transfer (HCET) process. The pivotal electron delivery effect of the Co center enables a facile H-transfer to the α-C(alkyne) of the aryl group, resulting in the high regioselectivity for β-carboxylation.

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...

The reactivity of the same molecular electrocatalyst under homogenous and heterogeneous conditions can be dramatically different, highlighting that the reaction environment plays an important role in catalysis. For catalysis on solid electrodes, reactions take place in the electric double layer (EDL) where a strong electric field is experienced. In this work, empirical valence bond molecular dynamics (EVB-MD) was used to explore CO2 binding in the EDL. It allows explicit descriptions of the solvent, electrolyte, catalyst-reactant, and the electrode surface material, as well as an unbiased description of the applied electric field. The strong local electric field concentrates cations at the interface, which in turn stabilises the bound CO2. Furthermore, controlled computational experiments suggest that neither the electric field nor the cations alone can produce significant stabilisation, but that the combination led to a dramatic stabilisation of the CO2 bound state.

In order to combine the advantages of molecular catalysts with the stability of solid-state catalysts, hybrid systems with catalysts immobilized on carbon nanotubes are prominent candidates. Here we explore our recent mechanistic proposal for Ru(tda)(py)

An improved understanding of the interactions of transition-metal (TM) nanoparticles with Lewis acids/bases will facilitate the design of more efficient catalysts. Therefore, Pt14, Pt13, Pt12, and ...

Adhering a cobalt porphyrin (Co(TPP)) catalyst on a carbon nanotube (CNT) supporting material greatly enhances its reactivity and enables catalysis in water, which is otherwise impossible. However, the effect of solvent as well as supporting materials on catalysis is still elusive. On the basis of computational results we found that water as a reaction medium lowers the reductive potential required due to the stabilization of intermediates and transition states, and provides higher availability of protons. To understand the effect of the support materials, we combine computations and experiments and illustrate that the curvature of the nanotubes plays an essential role in aggregation through the competition between the Ï-πinteractions between the porphyrin rings as well as between the Co(TPP) and the nanotube, providing an insight into lessening the degree of aggregation.

Ni-Catalyzed dicarboxylation of 1,3-dienes with CO2 provides a regioselective method for the construction of adipic acids. Here, density functional theory (DFT) calculations have been carried out to elucidate the detailed mechanisms of this reaction and the origin of regioselectivity. The computed results reveal that the catalytic cycle mainly proceeds via Ni(0)-catalyzed 1st carboxylation, followed by a reduction of the Ni(II) to Ni(I) intermediate and finally a Ni(I)-catalyzed 2nd carboxylation. The first and second carboxylations are rate- and regioselectivity-determining steps, respectively. In the first carboxylation, a new activation mode of CO2 by insertion into the uncoordinated alkene group has been found to be optimal. This is in contrast to other mechanisms described in the literature. Furthermore, we found that the interaction between the allyl group and nickel center in the alkyl Ni(I) intermediate significantly stabilizes the d9 electron configuration. This stabilizing interaction results in the preferential 1,4-dicarboxylation.

The outer coordination sphere of metalloenzyme often plays an important role in its high catalytic activity, however, this principle is rarely considered in the design of man-made molecular catalysts. Herein, four Ru-bda (bda=2,2 '-bipyridine-6,6 '-dicarboxylate) based molecular water oxidation catalysts with well-defined outer spheres are designed and synthesized. Experimental and theoretical studies showed that the hydrophobic environment around the Ru center could lead to thermodynamic stabilization of the high-valent intermediates and kinetic acceleration of the proton transfer process during catalytic water oxidation. By this outer sphere stabilization, a 6-fold rate increase for water oxidation catalysis has been achieved.

Asymmetric hydrogenation of alkenes is one of the most powerful tools for the preparation of optically active compounds. However, to achieve high enantioselectivity, the starting olefin in most cases needs to be isomerically pure in either the cis or the trans form. Generally, most olefination protocols provide olefins as isomeric mixtures that are difficult to separate, and in many cases also generate lots of waste. In contrast, the synthesis of racemic alcohols is straightforward and highly atom-efficient, with products that are easier to purify. Here, we describe a strategy that enables rapid access to chiral alkanes via enantioconvergent formal deoxygenation of racemic alcohols. Mechanistic studies indicate an Ir-mediated elimination of water and subsequent in situ hydrogenation. This approach allows rapid and efficient assembly of chiral intermediates and is exemplified in the total synthesis of antidepressant sertraline and σ2 receptor PB 28. The asymmetric hydrogenation of alkenes is a common route to optically active compounds, but alkene synthesis is often atom-inefficient, and the formation of isomers further complicates the procedure. Now the Ir-catalysed deoxygenation of racemic alcohols is shown to be a simple route to enantioenriched products.

Urgent solutions are needed to efficiently convert the greenhouse gas CO2 into higher-value products. In this work, fac-Mn(bpy)(CO)3Br (bpy = 2,2′-bipyridine) is employed as electrocatalyst in reductive CO2 conversion. It is shown that product selectivity can be shifted from CO toward HCOOH using appropriate additives, i.e., Et3N along with iPrOH. A crucial aspect of the strategy is to outrun the dimer-generating parent-child reaction involving fac-Mn(bpy)(CO)3Br and [Mn(bpy)(CO)3]- and instead produce the Mn hydride intermediate. Preferentially, this is done at the first reduction wave to enable formation of HCOOH at an overpotential as low as 260 mV and with faradaic efficiency of 59 ± 1%. The latter may be increased to 71 ± 3% at an overpotential of 560 mV, using 2 M concentrations of both Et3N and iPrOH. The nature of the amine additive is crucial for product selectivity, as the faradaic efficiency for HCOOH formation decreases to 13 ± 4% if Et3N is replaced with Et2NH. The origin of this difference lies in the ability of Et3N/iPrOH to establish an equilibrium solution of isopropyl carbonate and CO2, while with Et2NH/iPrOH, formation of the diethylcarbamic acid is favored. According to density-functional theory calculations, CO2 in the former case can take part favorably in the catalytic cycle, while this is less opportune in the latter case because of the CO2-to-carbamic acid conversion. This work presents a straightforward procedure for electrochemical reduction of CO2 to HCOOH by combining an easily synthesized manganese catalyst with commercially available additives.

Urgent solutions are needed to efficiently convert the greenhouse gas CO

The composition and structure of catalytic intermediates in the context of the Suzuki–Miyaura cross-coupling reaction cata-lyzed by Pd@MIL-101-NH2(Cr) has been investigated. Trimeric and tetrameric palladium species with formula [Br-Pd-Ar]nBr- (n = 3–4) have been identified by electrospray ionization mass spectrometry (ESI-MS) and density-functional theory (DFT) calculations, and their role in the transmetalation step has been studied. The weak nature of the bonds between Pd and the bridging halides in these species enables a very easy transmetalation step, with an estimated activation free energy of only 10 kcal/mol. Further experimental support for Pd speciation was obtained using scanning transmission electron micros-copy (STEM), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and fluorine-19 nuclear magnetic reso-nance spectroscopy (19F NMR).

The control of site selectivity in asymmetric mono‐hydrogenation of dienes or polyenes remains largely underdeveloped. Herein, we present a highly efficient desymmetrization of 1,4‐dienes via iridium‐catalyzed site‐ and enantioselective hydrogenation. This methodology demonstrates the first iridium‐catalyzed hydrogenative desymmetriation of meso dienes and provides a concise approach to the installation of two vicinal stereogenic centers adjacent to an alkene. High isolated yields (up to 96 %) and excellent diastereo‐ and enantioselectivities (up to 99:1 d.r. and 99 % ee) were obtained for a series of divinyl carbinol and divinyl carbinamide substrates. DFT calculations reveal that an interaction between the hydroxy oxygen and the reacting hydride is responsible for the stereoselectivity of the desymmetrization of the divinyl carbinol. Based on the calculated energy profiles, a model that simulates product distribution over time was applied to show an intuitive kinetics of this process. The usefulness of the methodology was demonstrated by the synthesis of the key intermediates of natural products zaragozic acid A and (+)‐invictolide., A highly efficient site‐selective desymmetric mono‐hydrogenation of 1,4‐dienes is reported. This protocol allows rapid access to a wide range of chiral allylic alcohols and amides bearing two vicinal chiral centers adjacent to the alkene. The utility of this method is further highlighted by the synthesis of the alkyl side chain of zaragozic acid A and the formal total synthesis of (+)‐invictolide.

Molecular electrocatalysts for CO2-to-CO conversion often operate at large overpotentials, the cleavage of C-O bond in the intermediate largely contributing to this phenomenon. Additional Lewis acids have been shown to aid in weakening the C-O bond. We herein present computational and experimental evidence, with ruthenium polypyridyl based CO2 reduction electrocatalysts, for a mechanistic route that involves one metal center acting as both Lewis base and Lewis acid at different stages of the catalytic cycle. The Lewis basic character of Ru is seen in the initial nucleophilic attack at CO2 to form [Ru-CO2]0, while its Lewis acid character allows the formation of a 5-membered metallacyclic intermediate, [Ru-CO2CO2]0,c, by intramolecular cyclization of a linear [Ru-CO2CO2]0 species that is formed from [Ru-CO2]0 and a second equivalent of CO2. [Ru-CO2CO2]0,c is crucial for energy-conserving turnover, as it allows for a third reduction at a more positive potential than that of the starting complex Ru2+. The calculated activation barrier for C-O bond cleavage in [Ru-CO2CO2]-1,c is dramatically decreased to 10.5 kcal mol-1 from 60 kcal mol-1, the latter required for C-O bond cleavage in the linear intermediate [Ru-CO2CO2]0. The intermediates are characterized experimentally by FT-IR and 13C NMR spectroscopy during electrocatalytic turnover and are corroborated by density functional theory (DFT).

The use of water as an oxygen and hydrogen source for the paired oxygenation and hydrogenation of organic substrates to produce valuable chemicals is of utmost importance as a means of establishing green chemical syntheses. Inspired by the active Ni3+ intermediates involved in electrocatalytic water oxidation by nickel‐based materials, we prepared NiBx as a catalyst and used water as the oxygen source for the oxygenation of various organic compounds. NiBx was further employed as both an anode and a cathode in a paired electrosynthesis cell for the respective oxygenation and hydrogenation of organic compounds, with water as both the oxygen and hydrogen source. Conversion efficiency and selectivity of ≥99 % were observed during the oxygenation of 5‐hydroxymethylfurfural to 2,5‐furandicarboxylic acid and the simultaneous hydrogenation of p‐nitrophenol to p‐aminophenol. This paired electrosynthesis cell has also been coupled to a solar cell as a stand‐alone reactor in response to sunlight.

Manganese(iii) salen has been developed as a new catalytic motif for alcohol dehydrogenation and the mechanism has been elucidated., The first example of a manganese(iii) catalyst for the acceptorless dehydrogenation of alcohols is presented. N,N′-Bis(salicylidene)-1,2-cyclohexanediaminomanganese(iii) chloride (2) has been shown to catalyze the direct synthesis of imines from a variety of alcohols and amines with the liberation of hydrogen gas. The mechanism has been investigated experimentally with labelled substrates and theoretically with DFT calculations. The results indicate a metal–ligand bifunctional pathway in which both imine groups in the salen ligand are first reduced to form a manganese(iii) amido complex as the catalytically active species. Dehydrogenation of the alcohol then takes place by a stepwise outer-sphere hydrogen transfer generating a manganese(iii) salan hydride from which hydrogen gas is released.

Molecular electrocatalysts for CO2-to-CO conversion often operate at large overpotentials, the cleavage of C-O bond in the intermediate largely contributing to this phenomenon. Additional Lewis acids have been shown to aid in weakening the C-O bond. We herein present computational and experimental evidence, with ruthenium polypyridyl based CO2 reduction electrocatalysts, for a mechanistic route that involves one metal center acting as both Lewis base and Lewis acid at different stages of the catalytic cycle. The Lewis basic character of Ru is seen in the initial nucleophilic attack at CO2 to form [Ru-CO2]0, while its Lewis acid character allows the formation of a 5-membered metallacyclic intermediate, [Ru-CO2CO2]0,c, by intramolecular cyclization of a linear [Ru-CO2CO2]0 species that is formed from [Ru-CO2]0 and a second equivalent of CO2. [Ru-CO2CO2]0,c is crucial for energy-conserving turnover, as it allows for a third reduction at a more positive potential than that of the starting complex Ru2+. The calculated activation barrier for C-O bond cleavage in [Ru-CO2CO2]-1,c is dramatically decreased to 10.5 kcal mol-1 from 60 kcal mol-1, the latter required for C-O bond cleavage in the linear intermediate [Ru-CO2CO2]0. The intermediates are characterized experimentally by FT-IR and 13C NMR spectroscopy during electrocatalytic turnover and are corroborated by density functional theory (DFT).

Intercatalyst coupling has been widely applied in the functional mimics for binuclear synergy in natural metal enzymes. Herein, we introduce two facile and effective design strategies, which facilitate the coupling of two catalytic units via electrostatic interactions. The first system is based on a catalyst molecule functionalized with both a positively charged and a negatively charged group in the structure being able to pair with each other in an antiparallel manner arranged by electrostatic interactions. The other system consists of a mixture of two different of catalysts modified with either positively or negatively charged groups to generate intermolecular electrostatic interactions. Applying these designs to Ru(bda) (H2bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) water-oxidation catalysts improved the catalytic performance by more than an order of magnitude. The intermolecular electrostatic interactions in these two systems were fully identified by 1H NMR, TEM, SAXS, and electrical conductivity experiments. Molecular dynamics simulations further verified that electrostatic interactions contribute to the formation of prereactive dimers, which were found to play a key role in dramatically improving the catalytic performance. The successful strategies demonstrated here can be used in designing other intercatalyst coupling systems for activation and formation of small molecules and organic synthesis.

Water oxidation is a vital anodic reaction for renewable fuel generation via electrochemical- and photoelectrochemical-driven water splitting or CO 2 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 (pda 2– =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.

The catalytic activity of the bimolecular reaction was affected by many parameters. Although many efforts have been dedicated to investigate the influence of secondary interactions in pre-organizing catalysts, the hydrophobic effect on Ru-bda-type water oxidation catalysts remains unclear as a result of the lack of an ideal catalytic model. In this work, four catalysts 1–4 with variable hydrophobicity have been synthesized, and cerium(IV)-driven water oxidation results showed that the hydrophobic complexes 3 and 4 outperformed the hydrophilic complex 2. Steric mapping, nuclear magnetic resonance, and differential pulse voltammogram studies indicated that the increase in activity has no correlation with electronic and steric effects but has correlation with hydrophobicity. Molecular dynamics have shown that the modifications of the hydrophobicity on the axial pyridine ligands of the Ru-bda type of catalysts can improve the water oxidation catalytic activity by stabilizing the pre-reactive catalyst dimer. QC 20220314

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., Developing efficient molecular water oxidation catalysts for artificial photosynthesis is a challenging task. Here the authors introduce a ruthenium based complex with negatively charged sulfonate groups to effectively drive water oxidation under both acidic and neutral conditions.

The considerable rate enhancements along with the increase in pH values may be due to the direct involvement of hydroxide anion in attacking electrophilic [RuV(tda)(py)2O]+ (1; tda = [2,2′:6′,2″-terpyridine]-6,6″-dicarboxylate, py = pyridine). The enhanced reaction rate is well in agreement with the descending activation barriers in our calculation. The addition of four extra water molecules in the geometry optimization plays a key role in stabilizing hydroxide anion as well as building a reasonable hydrogen-bonding network, and three of these molecules are required to stabilize the OH as an anion instead of a radical.

This report describes all key steps in the O–O bond formation from two separated [RuV═O(bda)L2]+ cations to form the dinuclear [(bda)L2RuIV–O–O–RuIV(bda)L2]2+ in explicit solvent. The three steps involve the diffusion of the catalysts in the water phase, formation of the prereactive dimer, and the bond formation between the two catalysts. On the basis of the calculated parameters, we compute the rate constant of two catalysts with different L-ligands, isoquinoline and picoline, and the computed values are in excellent agreement with the experimental ones. The interaction of the axial ligands is key to the improved rates of the larger ligand, mainly by facilitating the formation of the prereactive dimer from the solvated monomer. By comparing the binding free energy of hydrophilic RuIV–OH and hydrophobic RuV═O, the hydrophobic driving force of RuV═O in this system has been estimated to 1 kcal mol–1.

Immobilization of molecular catalysts to electrode surfaces can improve the recyclability and electron transfer rates. The drawback is that most experimental techniques and theoretical methods are not applicable. Here we present results from a study of a ruthenium water oxidation catalyst [RuVO(bda)L2] in explicit water at a carbon nanotube water interface, forming the key O–O bond between two 128 atom catalysts, all fully dynamically. Our methodology is based on a recently developed empirical valence bond (EVB) model. We follow the key steps of the reaction including diffusion of the catalysts at the interface, formation of the prereactive dimer, and the bond formation between the two catalysts. On the basis of the calculated parameters, we compute the turnover frequency (TOF) at the experimental loading, in excellent agreement with the experiments. The key O–O bond formation was significantly retarded at the surface, and limiting components were identified that could be improved by catalyst modification.

The mononuclear ruthenium complex [Ru(pdc)L3] (H2pdc = 2,6-pyridinedicarboxylic acid, L = N-heterocycles such as 4-picoline) has previously shown promising catalytic efficiency toward water oxidation, both in homogeneous solutions and anchored on electrode surfaces. However, the detailed water oxidation mechanism catalyzed by this type of complex has remained unclear. In order to deepen understanding of this type of catalyst, in the present study, [Ru(pdc)(py)3] (py = pyridine) has been synthesized, and the detailed catalytic mechanism has been studied by electrochemistry, UV–vis, NMR, MS, and X-ray crystallography. Interestingly, it was found that once having reached the RuIV state, this complex promptly formed a stable ruthenium dimer [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+. Further investigations suggested that the present dimer, after one pyridine ligand exchange with water to form [RuIII(pdc)(py)2-O-RuIV(pdc)(py)(H2O)]+, was the true active species to catalyze water oxidation in homogeneous solutions.

The photo carboxylation of the benzylic C(sp3)-H bond catalyzed by xanthone/nickel were examined by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. This study corroborates the previous proposal that light promotes the H-transfer from benzylic C(sp3)-H bond of the p-methoxytoluene to excited state of photocatalyst xanthone. Meanwhile, Ni(0) catalyst could mediate the H-transfer to occur via an electron-coupled-proton transfer manner, and then remarkably facilitates the carboxylation step (compared to the Ni-absent systems). After that, the generated Ni(I) intermediate and ketyl radical anion complete the carboxylation and electron transfer processes independently.

The coordination of a substrate water molecule on a metal centered catalyst for water oxidation is a crucial step involving the reorganization of the ligand sphere. This process can occur by substi ...

A ground-breaking empirical valence bond study for a soluble transition-metal complex is presented. The full reaction of catalyst monomers approaching and reacting in the Ru-V oxidation state were studied. Analysis of the solvation shell in the reactant and along the reaction coordinate revealed that the oxo itself is hydrophobic, which adds a significant driving force to form the dimer. The effect of the solvent on the reaction between the prereactive dimer and the product was small. The solvent seems to lower the barrier for the isoquinoline (isoq) complex while it is increased for pyridines. By comparing the reaction in the gas phase and solution, the proposed p-stacking interaction of the isoq ligands is found to be entirely driven by the water medium.

Many Ru water oxidation catalysts have been documented in the literature. However, only a few can catalyze the O–O bond formation via the radical coupling pathway, while most go through the water nucleophilic attack pathway. Understanding the electronic effect on the reaction pathway is of importance in design of active water oxidation catalysts. The Ru-bda (bda = 2,2′-bipyridine-6,6′-dicarboxylate) catalyst is one example that catalyzes the O–O bond formation via the radical coupling pathway. Herein, we manipulate the equatorial backbone ligand, change the doubly charged bda2– ligand to a singly charged tpc– (2,2′:6′,2″-terpyridine-6-carboxylate) ligand, and study the structure–activity relationship. Surprisingly, kinetics measurements revealed that the resulting Ru-tpc catalyst catalyzes water oxidation via the water nucleophilic attack pathway, which is different from the Ru-bda catalyst. The O–O bond formation Gibbs free energy of activation (ΔG⧧) at T = 298.15 K was 20.2 ± 1.7 kcal mol–1. The electro...

In the evaluation of systems designed for catalytic water oxidation, ceric ammonium nitrate (CAN) is often used as a sacrificial electron acceptor. One of the sources of failure for such systems is oxidative decay of the catalyst in the presence of the strong oxidant CAN (Eox = +1.71 V). Little progress has been made in understanding the circumstances behind this decay. In this study we show that a 2-(2′-hydroxphenyl) derivative (LH) of 1,10-phenanthroline (phen) in the complex [Ru(L)(tpy)]+ (tpy = 2,2′;6′,2″-terpyridine) can be oxidized by CAN to a 2-carboxy-phen while still bound to the metal. This complex is, in fact, a very active water oxidation catalyst. The incorporation of a methyl substituent on the phenol ring of LH slows down the oxidative decay and consequently slows down the catalytic oxidation. An analogous system based on bpy (2,2′-bipyridine) instead of phen shows much lower activity under the same conditions. Water molecule association to the Ru center of [Ru(L)(tpy)]+ and carboxylate don...

Two radicals can form a bond without an energetic barrier. However, the radical coupling mechanism in ruthenium-catalyzed water oxidation has been found to be associated with substantial activation energies. Here we have investigated the coupling reaction of [Ru═O(bda)L2]+ catalysts with different axial L ligands. The interaction between the two oxo radical moieties at the Ru(V) state was found to have a favorable interaction in the transition state in comparison to the prereactive complex. To further understand the existence of the activation energy, the activation energy has been decomposed into distortion energy and interaction energy. No correlation between the experimental rates and the calculated coupling barriers of different axial L was found, showing that more aspects such as solvation, supramolecular properties, and solvent dynamics likely play important roles in the equilibrium between the free RuV═O monomer and the [RuV═O···O═RuV] dimer. On the basis of our findings, we give general guidelines...