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Pt(II)‐based molecular catalysts stand as a prototypical system in hydrogen evolution reactions (HER) owing to their consistently elevated activity levels. Their integration into heterogeneous systems thus provides an ideal platform to develop catalytic materials with optimal atom economy. In this work, by rational molecular design, we have synthesized a novel two‐dimensional photoactive Covalent Organic Framework (COF), wherein the pore walls host a quinoline‐based α‐diimine ligand serving as a coordination site for anchoring a Pt(II) molecular catalyst. Thorough structural analyses, employing X‐ray photoelectron spectroscopy (XPS), infrared spectroscopy (FT‐IR), diffuse reflectance spectroscopy (DR), coupled with DFT calculations, distinctly confirm the Pt(II) complexation through the coordination site of the α‐diimine ligand. The Pt(II)‐metalated COF exhibits photocatalytic activity with a hydrogen evolution rate reaching up to 1300 μmol g−1 h−1. Nevertheless, the occurrence of platinum nanoparticles in post‐catalysis samples, along with reduced photocatalytic activity in the presence of chloride ions, suggests that Pt(II) anchored into the COF backbone might not be the primary catalytic site. [ABSTRACT FROM AUTHOR]

Mechanistic studies involving characterization of crucial intermediates are desirable for rational optimization of molecular catalysts toward CO2 reduction, while fundamental challenges are associated with such studies. Herein we present the systematic mechanistic investigations on a pyrene‐appended CoII macrocyclic catalyst in comparison with its pyrene‐free prototype. The comparative results also verify the reasons of the higher catalytic activity of the pyrene‐tethered catalyst in noble‐metal‐free CO2 photoreduction with various photosensitizers, where a remarkable apparent quantum yield of 36±3 % at 425 nm can be obtained for selective CO production. Electrochemical and spectroelectrochemical studies in conjunction with DFT calculations between the two catalysts have characterized the key CO‐bound intermediates and revealed their different CO‐binding behavior, demonstrating that the pyrene group endows the corresponding CoII catalyst a lower catalytic potential, a higher stability, and a greater ease in CO release, all of which contribute to its better performance. [ABSTRACT FROM AUTHOR]

The molecular complexes described herein use main‐group elements or transition metals to control the stoichiometric cleavage of N−H bonds of ammonia (NH3) and/or catalyze chemical and electrochemical NH3 oxidation to dinitrogen (N2). We highlight the phenomenon of coordination‐induced bond weakening and a variety of N−H bond cleavage mechanisms of NH3 including H atom abstraction, inter‐ and intra‐molecular deprotonation reactions, oxidative addition, and σ‐bond metathesis that have been demonstrated with molecular systems. We provide an overview of the molecular complexes reported for the rapidly developing field of NH3 oxidation catalysis to form N2. These systems exhibit several diverse structure types and innovative ligands to support transition metals capable of activating NH3 and mediating a challenging chemical transformation that requires breaking strong N−H bonds and forming an N−N bond en route to N2 formation. [ABSTRACT FROM AUTHOR]

Design tactics and mechanistic studies both remain as fundamental challenges during the exploitations of earth-abundant molecular electrocatalysts for CO2 reduction, especially for the rarely studied Cr-based ones. Herein, a quaterpyridyl CrIII catalyst is found to be highly active for CO2 electroreduction to CO with 99.8% Faradaic efficiency in DMF/phenol medium. A nearly one order of magnitude higher turnover frequency (86.6 s-1) over the documented Cr-based catalysts (<10 s-1) can be achieved at an applied overpotential of only 190 mV which is generally 300 mV lower than these precedents. Such a high performance at this low driving force originates from the metal-ligand cooperativity that stabilizes the low-valent intermediates and serves as an efficient electron reservoir. Moreover, a synergy of electrochemistry, spectroelectrochemistry, electron paramagnetic resonance, and quantum chemical calculations allows to characterize the key CrII, CrI, Cr0, and CO-bound Cr0 intermediates as well as to verify the catalytic mechanism. [ABSTRACT FROM AUTHOR]

The development of inexpensive and effective catalysts for the epoxidation of olefins to epoxides, which are key commodities for the chemical industry, is a continuing challenge. In this context, we present a supramolecular solution with the development of new host‐guest assemblies of sulfate ions and amidoammonium receptor cations that, for the first time, are shown to act as catalysts for olefin epoxidation by hydrogen peroxide under biphasic conditions. Analysis of the reaction mechanism shows that the reactive and oxidizing peroxymonosulfate is formed in the organic phase. Furthermore, a variety of readily available precursors may be used to form the supramolecular ion pairs (SIPs), which is enabling a large‐scale synthesis of the catalysts while maintaining catalytic control and effectiveness. [ABSTRACT FROM AUTHOR]

Dissociation energies (DC—H) of labile C—H bonds in radical adducts formed by coordination of thiyl radicals across the cyclohexadiene rings of 1,4-benzoquinone, N-phenyl-1,4-benzoquinone monoimine, and N,N′-diphenyl-1,4-benzoquinone diimine have been determined using composite methods of the Gaussian family and data on the enthalpies of formal homodesmotic reactions involving the adducts and simpler reference compounds. The DC—H values were found to be in the range of 140–170 kJ mol−1. These values are typical of the strength of bonds between atomic hydrogen and olefins. The weakness of the C—H bonds in both types of compounds is due to the compensation of bond breaking by the stabilization of the reaction products, viz., an olefin molecule or the cyclohexadiene ring of a quinoid. The proximity of the DC—H values in these adducts and the significant difference (∼50 kJ mol−1) between these values and the strength of the C—H bond in the adduct of atomic hydrogen with a benzene molecule confirms the fact that quinones and quinone imines are not aromatic compounds. The obtained DC—H values are required for the quantitative interpretation of the kinetic regularities found for the chain reactions of thiols with quinoids, as well as the catalysis of these reactions at the stage of chain propagation under the action of compounds with X—H groups (X = O, S, etc.). [ABSTRACT FROM AUTHOR]

Electrochemical ammonia oxidation reaction (eAOR) regains interest due to ammonia being an interesting alternative to hydrogen for fuel cell technologies. In the present review, we first discuss some of the most important findings on eAOR with solid catalysts, including mechanistic and feasibility aspects for practical implementation. We then examine the reports on molecular catalysis of eAOR that have recently emerged. We finally discuss immobilization strategies of these molecular catalysts, and discuss intrinsic advantages of those strategies, so as to guide the design of efficient catalytic systems able to compete with heterogeneous, solid catalysts. [ABSTRACT FROM AUTHOR]

The major limitations of photoelectrochemical (PEC) water splitting lies in the currently unsatisfying efficiency and stability of the semiconductor materials-based water splitting systems. By addressing these limitations, the immobilization of the molecular catalysts on semiconductor photoanodes to establish a hybrid inorganic-organic PEC system has attracted an increasing research attention. It is crucial to choose a suitable molecular catalyst and effectively couple it into a hybrid photoelectrode system. In this review, focusing on the water oxidation process, molecular catalysts integrated photoelectrochemical water oxidation systems are highlighted from the perspective of the roles of molecular catalysts and the integration strategies in the hybrid system. The most recent advances are summarized with various case studies presented, based on which perspectives are proposed to provide guidance toward the rational design of an integrated system for future development.

Abstract Electrochemical ammonia oxidation reaction (eAOR) regains interest due to ammonia being an interesting alternative to hydrogen for fuel cell technologies. In the present review, we first discuss some of the most important findings on eAOR with solid catalysts, including mechanistic and feasibility aspects for practical implementation. We then examine the reports on molecular catalysis of eAOR that have recently emerged. We finally discuss immobilization strategies of these molecular catalysts, and discuss intrinsic advantages of those strategies, so as to guide the design of efficient catalytic systems able to compete with heterogeneous, solid catalysts.

Nickel bisdiphosphine complexes bearing pendant amines form a unique series of catalysts (so‐called DuBois' catalysts) capable of bidirectional/reversible electrocatalytic oxidation and production of dihydrogen. This unique behaviour is directly linked to the presence of proton relays installed close to the metal center. We report here for the arginine derivative [Ni(P2CyN2Arg)2]6+ on a mechanistic model and its kinetic treatment that may apply to all DuBois' catalysts and show that it allows for a good fit of experimental data measured at different pH values, catalyst concentrations and partial hydrogen pressures. The bidirectionality of catalysis results from balanced equilibria related to hydrogen uptake/evolution on one side and (metal)‐hydride installation/capture on the other side, both controlled by concentration effects resulting from the presence of proton relays and connected by two square schemes corresponding to proton‐coupled electron transfer processes. We show that the catalytic bias is controlled by the kinetic of the H2 uptake/evolution step. Reversibility does not require that the energy landscape be flat, with redox transitions occurring at potentials up to 250 mV away for the equilibrium potential, although such large deviations from a flat energy landscape can negatively impacts the rate of catalysis when coupled with slow interfacial electron transfer kinetics. [ABSTRACT FROM AUTHOR]

The molecular nature and the structure of the real catalyst are matters of serious concern in homogeneous catalysis. Catalyst transformation in molecular water oxidation catalysis (WOC) is usually linked to catalyst decomposition with the generation of nanostructured materials. This is mainly due to the operational conditions that are conducive for the formation of metal oxides (MOx), which have a strong affinity for adsorbing onto the electrodes' surfaces. Recently, the exhibition of the molecular nature of the electrodeposits rather than MOx is a breakthrough in the field of WOC, suggesting that deposits in electrocatalysis are not constrained to metal oxides/hydroxides after molecular catalyst decomposition. Therefore, several points should be considered before deciding whether the deposits are aggregates of a molecular structure or simple metal oxides formed due to the decomposition of the original structure. Thus, the aim of this perspective is to showcase the examples of homogeneous WOCs where metal–organic species are electrodeposited instead of metal oxide‐based catalysts. The transformation of heterogenized molecular catalysts for WO into a more electroactive form while preserving their molecular identity, is also discussed. This summary provides an overview of the molecular nature of electrodeposits, how to identify their presence and decide about their nature. [ABSTRACT FROM AUTHOR]

The design of a highly efficient system for CO2 photoreduction fully based on earth-abundant elements presents a challenge, which may be overcome by installing suitable interactions between photosensitizer and catalyst to expedite the intermolecular electron transfer. Herein, we have designed a pyrene-decorated Cu(I) complex with a rare dual emission behavior, aiming at additional π-interaction with a pyrene-appended Co(II) catalyst for visible light–driven CO2-to- CO conversion. The results of 1H NMR titration, time-resolved fluorescence/absorption spectroscopies, quantum chemical simulations, and photocatalytic experiments clearly demonstrate that the dynamic π–π interaction between sensitizer and catalyst is highly advantageous in photocatalysis by accelerating the intermolecular electron transfer rate up to 6.9 × 105 s−1, thus achieving a notable apparent quantum yield of 19% at 425 nm with near-unity selectivity. While comparable to most earth-abundant molecular systems, this value is over three times of the pyrene-free system (6.0%) and far surpassing the benchmarking Ru(II) tris(bipyridine) (0.3%) and Ir(III) tris(2-phenylpyridine) (1.4%) photosensitizers under parallel conditions. [ABSTRACT FROM AUTHOR]

An electrolyte engineering strategy was developed for CO2 reduction into formate with a model molecular catalyst, [Rh(bpy)(Cp*)Cl]Cl, by modifying the solvent (organic or aqueous), the proton source (H2O or acetic acid), and the electrode/solution interface with imidazolium‐ and pyrrolidinium‐based ionic liquids (ILs). Experimental and theoretical density functional theory investigations suggested that π+‐π interactions between the imidazolium‐based IL cation and the reduced bipyridine ligand of the catalyst improved the efficiency of the CO2 reduction reaction (CO2RR) by lowering the overpotential, while granting partial suppression of the hydrogen evolution reaction. This allowed tuning the selectivity towards formate, reaching for this catalyst an unprecedented faradaic efficiency (FEHCOO−) ≥90 % and energy efficiency of 66 % in acetonitrile solution. For the first time, relevant CO2 conversion to formic acid/formate was reached at low overpotential (0.28 V) using a homogeneous catalyst in acidic aqueous solution (pH=3.8). These results open up a new strategy based on electrolyte engineering for enhancing carbon balance in CO2RR. [ABSTRACT FROM AUTHOR]

A cobalt complex bearing a κ‐N3P2 ligand is presented (1+ or CoI(L), where L is (1E,1′E)‐1,1′‐(pyridine‐2,6‐diyl)bis(N‐(3‐(diphenylphosphanyl)propyl)ethan‐1‐imine). Complex 1+ is stable under air at oxidation state CoI thanks to the π‐acceptor character of the phosphine groups. Electrochemical behavior of 1+ reveals a two‐electron CoI/CoIII oxidation process and an additional one‐electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at Eonset=−1.6 V vs Fc/Fc+. In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1+ forms the cobalt hydride derivative CoIII(L)‐H (22+), which has been fully characterized. Further addition of 1 equiv of CoCp*2 (Cp* is pentamethylcyclopentadienyl) affords the reduced CoII(L)‐H (2+) species, which rapidly forms hydrogen and regenerates the initial CoI(L) (1+). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1+. [ABSTRACT FROM AUTHOR]

Ga-exchanged H-MFI zeolites are highly active for the dehydrogenation of light alkanes; however, both the nature of the active gallium species and the associated dehydrogenation mechanism have been difficult to establish. In this study, we examine the activity of Ga species in Ga/H-MFI by calculating the free energy landscapes on which all reactions occur. To this end, we use a hybrid quantum mechanics/molecular mechanics model for all electronic structure calculations. Quantum chemical calculations were carried out with a range-corrected functional and a good representation of dispersive interactions. The molecular mechanics part of our approach captures the long-range effects of Coulombic and dispersive interactions due to atoms in the extended framework. The rate-determining TS (RDTS) is identified by analysis of the free energy landscape for each mechanism, using the energetic span model. Our analysis reveals that, for reduced Ga/H-MFI, univalent and divalent gallium hydrides, [GaH2]+ and [GaH]2+, respectively, are more active for ethane dehydrogenation in comparison to H+ sites and Ga+ sites. [GaH]2+ sites consistently emerge as the most active sites for light alkane dehydrogenation, providing significant enthalpic stabilization to C-H cleavage TSs via alkyl and carbenium dehydrogenation routes. In contrast, carbenium-like C-H cleavage TSs occurring on Brønsted acid sites are enthalpically less favorable due to their limited electronic interactions with framework O atoms and H+ sites. Activation enthalpy barriers for dehydrogenation determined using the energetic span analysis are in good agreement with those measured experimentally. Accounting for entropy of activation reveals that constrained TSs become less favorable in free energy with increasing chain length. We also find that an increase in enthalpic favorability of the alkyl mechanism is observed with increasing chain length for the TS responsible for the second C-H cleavage step leading to alkene formation.

A carbazolide-bis(NHC) NiII catalyst (1; NHC, N-heterocyclic carbene) for selective CO2 photoreduction was designed herein by a one-stone-two-birds strategy. The extended π-conjugation and the strong σ/π electron-donation characteristics (two birds) of the carbazolide fragment (one stone) lead to significantly enhanced activity for photoreduction of CO2 to CO. The turnover number (TON) and turnover frequency (TOF) of 1 were ninefold and eightfold higher than those of the reported pyridinol-bis(NHC) NiII complex at the same catalyst concentration using an identical Ir photosensitizer, respectively, with a selectivity of ∼100%. More importantly, an organic dye was applied to displace the Ir photosensitizer to develop a noble-metal-free photocatalytic system, which maintained excellent performance and obtained an outstanding quantum yield of 11.2%. Detailed investigations combining experimental and computational studies revealed the catalytic mechanism, which highlights the potential of the one-stone-twobirds effect. [ABSTRACT FROM AUTHOR]

Anderson-type ([XM6O24]n−) polyoxometalates (POMs) are a class of polymetallic-oxygen cluster inorganic compounds with special structures and properties. They have been paid extensive attention by researchers now, due to their chemical modification and designability, which have been widely applied in the fields of materials, catalysis and medicine. In contemporary years, the application of Anderson-type POMs in catalytic organic oxidation reaction has gradually shown great significance for the research of green catalytic process. In this paper, we investigate the application of Anderson-type POMs in organic synthesis reaction, and these works are summarized according to the different structure of POMs. This will provide a new strategy for further investigation of the catalytic application of Anderson-type POMs and the study of green catalysis. [ABSTRACT FROM AUTHOR]

The employment of manganese complexes towards electrocatalytic water oxidation has gained immense interest among the scientific communities. In this present study, the reaction of Mn(CH3COO)2⋅4H2O with the novel ligand (2‐([2,2′:6′,2"‐terpyridin]‐4′‐yl)quinoline‐8‐ol) (8hq‐tpy) affords a dinuclear manganese complex, [MnII(8hq‐tpy)(H2O)]2(PF6)2 (Mn 1). Various spectroscopic techniques have been implemented for characterization purposes, and Mn 1 is employed as electrocatalysts towards water oxidation catalysis in the aqueous medium. Electrocatalytic responses in the aqueous medium at pH 6.0 lead to the electrodeposition of heterogeneous species (MnOX) which enables the oxidation event at an applied potential of 1.24 V vs. NHE with O2 liberation of 50.2 μmol/L. The XPS analysis performed after executing bulk electrolysis, provide evidence for the generation of MnIV=O as the active species formed by the catalyst precursor complex Mn 1, contributing to O2 evolution during the electrocatalytic water oxidation process. [ABSTRACT FROM AUTHOR]

We have evaluated six phenoxazine derivatives as visible light photosensitizers for the photochemical reduction of CO2 to CO with an iron porphyrin catalyst in organic media. The phenoxazine core was functionalized with electron‐donating or ‐withdrawing groups to modify the photophysical properties. Both singlet and triplet excited state potentials of the sensitizers spanned several hundred mV range and the ground state oxidation potentials spanned 250 mV. We observed that no correlation can be established between the production of CO and the excited state potential of the phenoxazine, which determines the driving force for electron transfer to the catalyst. On the contrary, a clear correlation can be made between the oxidation potentials of the phenoxazine and the production of CO. This observation indicates that the process was limited by photosensitizer regeneration and highlights the fact that electron transfers not directly related to the catalyst activation could play a key role in homogeneous photocatalytic systems. [ABSTRACT FROM AUTHOR]

Atomically precise metal clusters serve as a unique model for unraveling the intricate mechanism of the catalytic reaction and exploring the complex relationship between structure and activity. Herein, three series of water-soluble heterometallic clusters LnCu 6 , abbreviated as LnCu 6 -AC (Ln = La, Nd, Gd, Er, Yb; HAC = acetic acid), LnCu 6 -IM (Ln = La and Nd; IM = Imidazole), and LnCu 6 -IDA (Ln = Nd; H 2 IDA = Iminodiacetic acid) are presented, each featuring a uniform metallic core stabilized by distinct protected ligands. Crystal structure analysis reveals a triangular prism topology formed by six Cu 2+ ions around one Ln 3+ ion in LnCu 6 , with variations in Cu···Cu distances attributed to different ligands. Electrocatalytic oxygen evolution reaction (OER) shows that these different LnCu 6 clusters exhibit different OER activities with remarkable turnover frequency of 135 s -1 for NdCu 6 -AC, 79 s -1 for NdCu 6 -IM and 32 s -1 for NdCu 6 -IDA. Structural analysis and Density Functional Theory (DFT) calculations underscore the correlation between shorter Cu···Cu distances and improves OER catalytic activity, emphasizing the pivotal role of active-site distance in regulating electrocatalytic OER activities. These results provide valuable insights into the OER mechanism and contribute to the design of efficient homogeneous OER electrocatalysts., (© 2024 Wiley‐VCH GmbH.)

A new isolation protocol was recently reported for highly purified metallic Fullertubes D5h‐C90, D3d‐C96, and D5d‐C100, which exhibit unique electronic features. Here, we report the oxygen reduction electrocatalytic behavior of C60, C70 (spheroidal fullerenes), and C90, C96, and C100 (tubular fullerenes) using a combination of experimental and theoretical approaches. C96 (a metal‐free catalyst) displayed remarkable oxygen reduction reaction (ORR) activity, with an onset potential of 0.85 V and a halfway potential of 0.75 V, which are close to the state‐of‐the‐art Pt/C benchmark catalyst values. We achieved an excellent power density of 0.75 W cm−2 using C96 as a modified cathode in a proton‐exchange membrane fuel cell, comparable to other recently reported efficient metal‐free catalysts. Combined band structure (experimentally calculated) and free‐energy (DFT) investigations show that both favorable energy‐level alignment active catalytic sites on the carbon cage are responsible for the superior activity of C96. [ABSTRACT FROM AUTHOR]

Ordered heterogeneity is significant for molecular photosensitizers to enhance their practical applications. However, the ordered heterogeneity of molecular photosensitizers is still a great challenge. In this article, we describe a supramolecular assembly method for the heterogeneity of molecular photosensitizers, with which a mononuclear Zn(II) molecular photosensitizer in solution was orderly assembled in long range via p-p stacking interactions, affording a cheap, solid photocatalyst (p-1) with a porous structure. With Co(II), Fe(III), or Ni(II) as a cocatalyst, p-1 shows noticeably better photocatalytic activity for CO2 reduction than in a homogeneous system. The definite crystal structure and precise position of the catalytic center in p-1 were determined by singlecrystal X-ray diffraction combined with X-ray diffraction adsorption spectra, based on which the enhanced activity of p-1 for photocatalytic CO2 reduction was revealed by theoretical calculation. Thus, the reduced energy gap after ordered heterogeneity accelerates the electron transfer, greatly boosting the photocatalytic CO2 reduction activity. This work demonstrates a method for developing crystalline, heterogeneous photocatalysts with definite structures and enhanced, catalytic performance. [ABSTRACT FROM AUTHOR]

Benchmarking is a community-based and (preferably) community-driven activity involving consensus-based decisions on how to make reproducible, fair, and relevant assessments. In catalysis science, important catalyst performance metrics include activity, selectivity, and the deactivation profile, which enable comparisons between new and standard catalysts. Benchmarking also requires careful documentation, archiving, and sharing of methods and measurements, to ensure that the full value of research data can be realized. Beyond these goals, benchmarking presents unique opportunities to advance and accelerate understanding of complex reaction systems by combining and comparing experimental information from multiple, in situ and operando techniques with theoretical insights derived from calculations characterizing model systems. This Perspective describes the origins and uses of benchmarking and its applications in computational catalysis, heterogeneous catalysis, molecular catalysis, and electrocatalysis. It also discusses opportunities and challenges for future developments in these fields.

In the context of molecular catalysis of electrochemical reactions, the competition between reduction of the substrate and deactivation of the catalyst by a cosubstrate is investigated. It is a frequent situation because proton donors are ubiquitous cosubstrates in reductive electrochemical reactions and molecular catalysts, either transition metal complexes or organic aromatic molecules, and are often prone to electrohydrogenation. We provide a formal kinetic analysis in the framework of cyclic voltammetry, and we show that the response is governed by two parameters and that the competition does not depend on the scan rate. From this analysis a methodology is proposed to analyze such systems and then illustrated via the study of N2O to N2 electroreduction catalyzed by 4‐cyanopyridine in acetonitrile electrolyte with water as proton donor. Incidentally, new insights into the mechanism of 4‐cyanopyridine radical anion protonation are revealed. [ABSTRACT FROM AUTHOR]

Mechanistic studies involving characterization of crucial intermediates are desirable for rational optimization of molecular catalysts toward CO 2 reduction, while fundamental challenges are associated with such studies. Herein we present the systematic mechanistic investigations on a pyrene-appended Co II macrocyclic catalyst in comparison with its pyrene-free prototype. The comparative results also verify the reasons of the higher catalytic activity of the pyrene-tethered catalyst in noble-metal-free CO 2 photoreduction with various photosensitizers, where a remarkable apparent quantum yield of 36±3 % at 425 nm can be obtained for selective CO production. Electrochemical and spectroelectrochemical studies in conjunction with DFT calculations between the two catalysts have characterized the key CO-bound intermediates and revealed their different CO-binding behavior, demonstrating that the pyrene group endows the corresponding Co II catalyst a lower catalytic potential, a higher stability, and a greater ease in CO release, all of which contribute to its better performance., (© 2024 Wiley-VCH GmbH.)

Design tactics and mechanistic studies both remain as fundamental challenges during the exploitations of earth-abundant molecular electrocatalysts for CO 2 reduction, especially for the rarely studied Cr-based ones. Herein, a quaterpyridyl Cr III catalyst is found to be highly active for CO 2 electroreduction to CO with 99.8% Faradaic efficiency in DMF/phenol medium. A nearly one order of magnitude higher turnover frequency (86.6 s -1 ) over the documented Cr-based catalysts (<10 s -1 ) can be achieved at an applied overpotential of only 190 mV which is generally 300 mV lower than these precedents. Such a high performance at this low driving force originates from the metal-ligand cooperativity that stabilizes the low-valent intermediates and serves as an efficient electron reservoir. Moreover, a synergy of electrochemistry, spectroelectrochemistry, electron paramagnetic resonance, and quantum chemical calculations allows to characterize the key Cr II , Cr I , Cr 0 , and CO-bound Cr 0 intermediates as well as to verify the catalytic mechanism., Competing Interests: Competing interests statement:The authors declare no competing interest.

A strategy to improve homogeneous molecular catalyst stability, efficiency, and selectivity is the immobilization on supporting surfaces or within host matrices. Herein, we examine the co‐immobilization of a CO2 reduction catalyst [ReBr(CO)3(4,4′‐dcbpy)] and a photosensitizer [Ru(bpy)2(5,5′‐dcbpy)]Cl2 using the isoreticular series of metal–organic frameworks (MOFs) UiO‐66, ‐67, and ‐68. Specific host pore size choice enables distinct catalyst and photosensitizer spatial location—either at the outer MOF particle surface or inside the MOF cavities—affecting catalyst stability, electronic communication between reaction center and photosensitizer, and consequently the apparent catalytic rates. These results allow for a rational understanding of an optimized supramolecular layout of catalyst, photosensitizer, and host matrix. [ABSTRACT FROM AUTHOR]

The benchmarking of the performance for H2 evolution of cobalt diimine‐dioxime catalysts is provided based on a comprehensive study of their catalytic mechanism. The latter follows an ECE'CC pathway with intermediate formation of a Co(II)‐hydride intermediate and second protonation possibly at a basic site of the ligand, acting as a proton relay. This suggests an intramolecular coupling between the hydride and protonated ligand as the proton concentration‐independent rate‐determining step controlling the turnover frequency for H2 evolution. [ABSTRACT FROM AUTHOR]

Anderson-type ([XM6O24]n−) polyoxometalates (POMs) are a class of polymetallic-oxygen cluster inorganic compounds with special structures and properties. They have been paid extensive attention by researchers now, due to their chemical modification and designability, which have been widely applied in the fields of materials, catalysis and medicine. In contemporary years, the application of Anderson-type POMs in catalytic organic oxidation reaction has gradually shown great significance for the research of green catalytic process. In this paper, we investigate the application of Anderson-type POMs in organic synthesis reaction, and these works are summarized according to the different structure of POMs. This will provide a new strategy for further investigation of the catalytic application of Anderson-type POMs and the study of green catalysis.

Heterogeneous molecular catalysts have attracted considerable attention as carbon dioxide reduction reaction (CO2RR) electrocatalysts. The π‐electron system of conjugated ligands in molecular catalysts may play an important role in determining the activity. In this work, by enlarging π‐conjugation through appending more aromatic substituents on the porphyrin ligand, altered π‐electron system endows the as‐prepared 5,10,15,20‐tetrakis(4‐(pyren‐1‐yl)phenyl)porphyrin CoII with high Faradaic efficiency (ca. 95 %) for CO production, as well as high turnover frequency (2.1 s−1 at −0.6 V vs. RHE). Density functional theory calculation further suggests that the improved electrocatalytic performance mainly originates from the higher proportion of Co dz2 orbital and the CO2 π* orbital in the HOMO of the (Co−porphyrin−CO2)− intermediate with larger π‐conjugation, which facilitates the CO2 activation. This work provides strong evidence that π‐conjugation perturbation is effective in boosting the CO2RR. [ABSTRACT FROM AUTHOR]

This review highlights the development of earth-abundant-metal complexes as the photosensitizers featuring Cu, Zn, Al, or B in molecular photocatalytic systems for CO 2 reduction. [Display omitted] • The progress in homogeneous, molecular photocatalytic systems for CO 2 reduction with earth-abundant-metal complexes as the photosensitizers were summarized. • The photophysical and photoredox properties of these earth-abundant-metal dyes were highlighted. • Their issues and corresponding operative strategies in photocatalytic CO 2 reduction were discussed in detail. Artificial photosynthesis involving CO 2 photoreduction is an appealing strategy for addressing environmental and energy-shortage issues. Exploration of cost-efficient photocatalytic systems for CO 2 reduction will substantially contribute to sustainable applications. However, the exploitation of earth-abundant alternatives to the extensively applied precious-metal-based photosensitizers for CO 2 reduction remains challenging. In this Review, the key development of earth-abundant-metal complexes as photosensitizers in molecular systems for CO 2 photoreduction is emphasized to offer an upcoming blueprint, including an analysis of the problems of these photosensitizers during photocatalytic CO 2 reduction and presenting potential solutions to overcome issues commonly found. [ABSTRACT FROM AUTHOR]

Several H−H bond forming pathways have been proposed for the hydrogen evolution reaction (HER). Revealing these HER mechanisms is of fundamental importance for the rational design of catalysts and is also extremely challenging. Now, an unparalleled example of switching between homolytic and heterolytic HER mechanisms is reported. Three nickel(II) porphyrins were designed and synthesized with distinct steric effects by introducing bulky amido moieties to ortho‐ or para‐positions of the meso‐phenyl groups. These porphyrins exhibited different catalytic HER behaviors. For these Ni porphyrins, although their 1e‐reduced forms are active to reduce trifluoroacetic acid, the resulting Ni hydrides (depending on the steric effects of porphyrin rings) have different pathways to make H2. Understanding HER processes, especially controllable switching between homolytic and heterolytic H−H bond formation pathways through molecular engineering, is unprecedented in electrocatalysis. [ABSTRACT FROM AUTHOR]

Dialkoxymethanes are becoming increasingly important as fuel additives, formaldehyde surrogates, and chemical intermediates, but the effective synthesis remains challenging. Herein, the catalytic synthesis of dialkoxymethane products using a molecular catalyst is reported. The catalytic system, comprising the [Ru(triphos)(tmm)] in combination with the Lewis acid Al(OTf)3, enables the direct synthesis of dialkoxymethane products with formic acid as C1 building block in high to excellent turnover numbers. [ABSTRACT FROM AUTHOR]

• Additive-free layering of water-insoluble Fe(II) complexes on ITO. • Sharply different behavior depending on the heterocyclic ligand. • The benzimidazole variant is suitable electrocatalyst to produce O 2. Herein, we compare the electrochemical and electrocatalytic properties of two selected, water-insoluble Fe(II) coordination complexes made with the non-symmetric, bidentate ligands, 2-(2′-pyridyl)benzimidazole (PBI) in [Fe(PBI) 3 ](OTf) 2 (1 , OTf− = trifluoromethyl sulfonate anion) and 2-(2′-pyridyl)benzoxazole (PBO) in [Fe(PBO) 2 (OTf) 2 ] (2). Cyclic voltammetry in water/acetonitrile mixture indicates considerable activity for both compounds. However, only 1 acts as homogeneous catalyst. The complexes have been successfully immobilized on indium-tin-oxide (ITO) electrode surface. The hydrophobic ligands allowed for a simple dip-coating and drop-casting of 1 and 2 onto ITO. Both 1 /ITO and 2 /ITO showed increased activity in electrocatalytic O 2 evolution in borate buffer at pH 8.3. According to scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), moreover, re-dissolution tests, the Fe remains in complex with PBI during electrolysis in the drop-casted, nano-porous films of 1 /ITO. In contrast, the PBO complex in 2 /ITO undergoes a rapid in situ decomposition yielding a mineralized form that is responsible for catalysis. [ABSTRACT FROM AUTHOR]

Herein, we present the synthesis of a cobalt complex with a pentadentate ligand platform and its application in the electrochemical hydrogen evolution reaction (LCF3=2,6-bis(methoxybis (4-trifluormethyl-1H-imidazol-2-yl)methyl)pyridine). The complex is active in neutral to acidic solvent mixtures containing 50% water. Electronic tuning in the ligand backbone, that is introduction of electron-withdrawing CF3-groups, shifts the onset potential of the catalytic wave to less negative potentials. NMR and UV/Vis studies indicate that the CoIII complex is deprotonated twice in the ligand backbone under neutral conditions. Reduction is accompanied by protonation, as indicated by pH-dependent CV measurements, and the CoI state initiates catalysis. The result suggests that the pentadentate ligand is an ideal platform for cobalt catalysts in the oxidative and reductive water splitting reaction. [ABSTRACT FROM AUTHOR]

In this work, we report the electrochemical response of a family of Co(II) complexes, [CoII(L)3]2+ and [CoII(L’)2]2+ (L = 2,2’-bipyridine, 1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline, 5,6-dimethyl-1,10-phenanthroline, and 4,7-diphenyl-1,10-phenanthroline; L’ = terpyridine and 4-chloro-terpyridine), in the presence and absence of CO2 in order to understand the role of the redox potential and molecular structure on the molecular catalysis of CO2 reduction. The tris chelate complexes exhibited three electron transfer processes [CoII(L)3]2+ ⇄ [CoIII(L)3]3+ + 1e−, [CoΙΙ(L)3]2++1e− ⇄ [CoΙ(L)3]+, and [CoΙ(L)3]+ + 2e- ⇄ [CoΙ(L)(L−)2]−. In the case of complexes with 1,10-phen and 2,2-bipy, the third redox process showed a coupled chemical reaction [CoΙ(L)(L−)2]− → [CoΙ(L−)2]− + L. For bis chelate complexes, three electron transfer processes associated with the redox couples [CoΙΙ(L)2]/[CoIII(L)2]3+, [CoΙΙ(L)2]2+/[CoΙ(L)2]+, and [CoΙ(L)2]+/[CoΙ(L)(L−)] were registered, including a coupled chemical reaction only for the complex containing the ligand 4-chloro-terpyridine. Foot to the wave analysis (FOWA) obtained from cyclic voltammetry experiments allowed us to calculate the catalytic rate constant (k) for the molecular catalysis of CO2 reduction. The complex [Co(3,4,7,8-tm-1,10-phen)3]2+ presented a high k value; moreover, the complex [Co(4-Cl-terpy)3]2+ did not show catalytic activity, indicating that the more negative redox potential and the absence of the coupled chemical reaction increased the molecular catalysis. Density functional theory (DFT) calculations for compounds and CO2 were obtained to rationalize the effect of electronic structure on the catalytic rate constant (k) of CO2 reduction.

Colloidal dye-sensitized photocatalysis is a promising route toward efficient solar fuel production by merging properties of catalysis, support, light absorption, and electron mediation in one. Metal-organic frameworks (MOFs) are host materials with modular building principles allowing scaffold property tailoring. Herein, we combine these two fields and compare porous Zr-based MOFs UiO-66-NH2(Zr) and UiO-66(Zr) to monoclinic ZrO2 as model colloid hosts with co-immobilized molecular carbon dioxide reduction photocatalyst fac-ReBr(CO)3(4,4′-dcbpy) (dcbpy = dicarboxy-2,2′-bipyridine) and photosensitizer Ru(bpy)2(5,5′-dcbpy)Cl2 (bpy = 2,2′-bipyridine). These host-guest systems demonstrate selective CO2-to-CO reduction in acetonitrile in presence of an electron donor under visible light irradiation, with turnover numbers (TONs) increasing from ZrO2, to UiO-66, and to UiO-66-NH2 in turn. This is attributed to MOF hosts facilitating electron hopping and enhanced CO2 uptake due to their innate porosity. Both of these phenomena are pronounced for UiO-66-NH2(Zr), yielding TONs of 450 which are 2.5 times higher than under MOF-free homogeneous conditions, highlighting synergistic effects between supramolecular photosystem components in dye-sensitized MOFs.

Herein a transition‐metal catalyst system for the selective synthesis of cyclic and linear acetals from the combined utilization of carbon dioxide, molecular hydrogen, and biomass derived diols is presented. Detailed investigations on the substrate scope enabled the selectivity of the reaction to be largely guided and demonstrated the possibility of integrating a broad variety of substrate molecules. This approach allowed a change between the favored formation of cyclic acetals and linear acetals, originating from the bridging of two diols with a carbon‐dioxide based methylene unit. This new synthesis option paves the way to novel fuels, solvents, or polymer building blocks, by the recently established "bio‐hybrid" approach of integrating renewable energy, carbon dioxide, and biomass in a direct catalytic transformation. [ABSTRACT FROM AUTHOR]

The conversion of carbon dioxide (CO 2) into value-added chemicals under photochemical conditions has attracted increasing attention in recent years. One of the great challenges is to develop novel active catalysts under visible light irradiation with sustained lifetime and high activity. In this regard, herein, we report a highly efficient, stable and recyclable photocatalyst by embedding photoactive rhenium complex (Re(CO) 5 Cl) into porous, crystalline, bipyridine-based covalent organic frameworks (COFs). The rhenium post-metallated COFs exhibits salient photocatalytic activity towards CO 2 reduction into CO under visible light. The quantity of the CO produced on Re-functionalized COFs is twice higher than that produced on the famous Re(bpy)(CO) 3 Cl (bpy = 2,2′-bipyridine) molecular photocatalyst under similar reaction conditions. Image 1 • Covalent organic framework with monodispersed metallic catalytic site was synthesized. • It served as photocatalyst for CO 2 reduction with sustained lifetime up to 12 h. • Molecular catalysis was realized in heterogeneous catalysis with a higher efficiency. • Mechanism of photocatalytic performance was explored by a new in-situ FTIR reactor. [ABSTRACT FROM AUTHOR]

Transfer hydrogenation using molecular catalysts has become a powerful tool in synthetic chemistry and a wide range of unsaturated substrates can be reduced with this protocol. Whereas reactions using iso‐propanol as hydrogen donor are already well established, recent examples demonstrate the possibility to use linear alcohols from renewable resources. Herein, the first effective transfer hydrogenation of the challenging substrate carbon dioxide (CO2) directly to methanol is described, applying a molecular ruthenium catalyst and linear alcohols as the hydrogen source. In neat ethanol, TONs up to 121 are achieved under moderate pressures of CO2. Moreover, systematic investigations enable to propose initial acceptorless dehydrogenation of the alcohol, followed by the reduction of CO2 to methanol via ethyl formate, as mechanistic basis. [ABSTRACT FROM AUTHOR]

As an accompaniment to the current renaissance of synthetic organic electrochemistry, the heterogeneous and space-dependent nature of electrochemical reactions is analyzed in detail. The reactions that follow the initial electron transfer step and yield the products are intimately coupled with reactant transport. Depiction of the ensuing reactions profiles is the key to the mechanism and selectivity parameters. Analysis is eased by the steady state resulting from coupling of diffusion with convection forced by solution stirring or circulation. Homogeneous molecular catalysis of organic electrochemical reactions of the redox or chemical type may be treated in the same manner. The same benchmarking procedures recently developed for the activation of small molecules in the context of modern energy challenges lead to the establishment and comparison of the catalytic Tafel plots. At the very opposite, redox-neutral chemical reactions may be catalyzed by injection (or removal) of an electron from the electrode. This class of reactions has currently few, but very thoroughly analyzed, examples. It is likely that new cases will emerge in the near future. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Nickel-based molecular catalysts for electrocatalytic and photocatalytic hydrogen evolution reaction. • Nickel-based molecular catalysts for electrocatalytic and photocatalytic oxygen evolution reaction. • Nickel-based molecular catalysts for electrocatalytic and photocatalytic carbon dioxide reduction. Abstract One of the most appealing strategies to address the problems of global warming and energy shortage is to develop clean and sustainable energy source. To this end, considerable efforts have been devoted to designing well-defined catalysts for energy conversion. As an earth-abundant transition metal, nickel has been regarded to be an important catalytic center in molecular catalysts for redox reactions related to energy conversion. This Review focuses on the properties of nickel complexes as molecular catalysts in energy conversion driven by electricity or light, including water splitting and carbon dioxide (CO 2) reduction. [ABSTRACT FROM AUTHOR]

The present work reports, the new route for the synthesis of diarylselenides by coupling of aryltriethoxysilanes with diaryldiselenides. Various ligands were screened for the said coupling in presence of a copper salt. Cu(II)/bpy in DMSO showed excellent activity for the coupling of aryltriethoxysilanes with various diaryldiselenides. In the present work, no fluoride source was used for the activation of aryltriethoxysilanes and also a possible mechanism was proposed for the said reaction. Moreover, diaryldisuphides and diarylditellurides were also effectively coupled with aryltriethoxysilanes using proposed catalytic system. Importantly, use of non‐precious metal based catalytic system, makes the protocol interesting. New route for the synthesis of diarylselenides by coupling of aryltriethoxysilanes and diaryldiselenides was developed. Various ligands were screened for the said coupling in presence of a copper salt where Cu(II)/bpy in DMSO showed excellent activity for the coupling of aryltriethoxysilanes with various diaryldiselenides. The given route was also found effective for the coupling aryltriethoxysilanes with diaryldisuphides and diarylditellurides. No fluoride source was used for the activation of aryltriethoxysilanes moreover, possible mechanism was also proposed for the given synthesis. [ABSTRACT FROM AUTHOR]