Secondary Coordination Effects of Adjacent Metal Center in Metal-Nitrogen-Carbon Improve Scaling Relation of Oxygen Electrocatalysis.

Heterogenous single-atom catalysts (SACs) are reminiscent of homogeneous catalysts because of the similarity of structural motif of active sites, showing the potential of using the advantage of homogeneous catalysts to tackle challenges in hetereogenous catalysis. In heterogeneous oxygen electrocatalysis, the homogeneity of adsorption patterns of reaction intermediates leads to scaling relationships that limit their activities. In contrast, homogeneous catalysts can circumvent such limits by selectively altering the adsorption of intermediates through secondary coordination effects (SCEs).

Intermolecular O-O Bond Formation between High-Valent Ru-oxo Species.

Despite extensive research on water oxidation catalysts over the past few decades, the relationship between high-valent metal-oxo intermediates and the O-O bond formation pathway has not been well clarified. Our previous study showed that the high spin density on O in Ru=O is pivotal for the interaction of two metal-oxyl radical (I2M) pathways. In this study, we found that introducing an axially coordinating ligand, which is favorable for bimolecular coupling, into the Ru-pda catalyst can rearrange its geometry.

Removing the Barrier in O-O Bond Formation Via the Combination of Intramolecular Radical Coupling and the Oxide Relay Mechanism.

The Ru(tda) catalyst has been a major milestone in the development of molecular water oxidation catalysts due to its outstanding performance at neutral pH. The role of the noncoordinating carboxylate group is to act as a nucleophile, donating an oxygen atom to the oxo group, thereby acting as an oxide relay (OR) mechanism for O-O bond formation. A substitution of the carboxylates for phosphonate groups has been proposed, resulting in the Ru(tPaO) catalyst, which has shown even more efficient performance in experimental characterization.

O-O bond formation radical coupling in a dinuclear iron water oxidation catalyst with high catalytic activity.

The use of iron-based catalysts for the water oxidation reaction is highly attractive due to the high abundance of iron. While many molecular catalysts have been made, most show limited activity and short lifetimes. An exception with higher activity was presented by Thummel and co-workers in 2015.

The Effect of Conformational Freedom vs Restriction on the Rate in Asymmetric Hydrogenation: Iridium-Catalyzed Regio- and Enantioselective Monohydrogenation of Dienones.

Transition metal-catalyzed asymmetric hydrogenation constitutes an efficient strategy for the preparation of chiral molecules. When dienes are subjected to hydrogenation, control over regioselectivity still presents a large challenge and the fully saturated alkane is often yielded. A few successful monohydrogenations of dienes have been reported, but hitherto these are only efficient for dienes comprised of two distinctly different olefins.

Development and mechanistic investigation of the dehydrogenation of alcohols with an iron(III) salen catalyst.

The iron(III) salen complex (,)-,'-bis(salicylidene)-1,2-cyclohexanediamineiron(III) chloride has been developed as a catalyst for the acceptorless dehydrogenation of alcohols. The complex catalyzes the direct synthesis of imines in good yields from different primary alcohols and amines with the liberation of hydrogen gas. The mechanism has been investigated experimentally with labelled substrates and theoretically with density functional theory calculations.

Bioinspired Active Site with a Coordination-Adaptive Organosulfonate Ligand for Catalytic Water Oxidation at Neutral pH.

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 configurationally labile ligand [2,2':6',2″-terpyridine]-6,6″-disulfonate was designed to mimic enzymatic framework, in which the sulfonate coordination is highly flexible and functions as both an electron donor to stabilize high-valent Ru and a proton acceptor to accelerate water dissociation, thus boosting the catalytic water oxidation performance thermodynamically and kinetically. The combination of single-crystal X-ray analysis, various temperature NMR, electrochemical techniques, and DFT calculations was utilized to investigate the fundamental role of the self-adaptive ligand, demonstrating that the on-demand configurational changes give rise to fast catalytic kinetics with a turnover frequency (TOF) over 2000 s, which is compared to oxygen-evolving complex (OEC) in natural photosynthesis.

Computational comparison of Ru(bda)(py) and Fe(bda)(py) as water oxidation catalysts.

Ru(bda)(py) (bda = 2,2'-bipyridine-6,6'-dicarboxylate, py = pyridine) has been a significant milestone in the development of water oxidation catalysts. Inspired by Ru(bda)(py) and aiming to reduce the use of noble metals, iron (Fe) was introduced to replace the Ru catalytic center in Ru(bda)(py). In this study, density functional theory (DFT) calculations were performed on Fe- and Ru(bda)(py) catalysts, and a more stable 6-coordinate Fe(bda)(py) with one carboxylate group of bda disconnecting with Fe was found.

Microscopic Insights into Cation-Coupled Electron Hopping Transport in a Metal-Organic Framework.

Electron transport through metal-organic frameworks by a hopping mechanism between discrete redox active sites is coupled to diffusion-migration of charge-balancing counter cations. Experimentally determined apparent diffusion coefficients, , that characterize this form of charge transport thus contain contributions from both processes. While this is well established for MOFs, microscopic descriptions of this process are largely lacking.

Promoting Proton Transfer and Stabilizing Intermediates in Catalytic Water Oxidation via Hydrophobic Outer Sphere Interactions.

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.

Combined Theoretical and Experimental Studies Unravel Multiple Pathways to Convergent Asymmetric Hydrogenation of Enamides.

We present a highly efficient convergent asymmetric hydrogenation of / mixtures of enamides catalyzed by N,P-iridium complexes supported by mechanistic studies. It was found that reduction of the olefinic isomers ( and geometries) produces chiral amides with the same absolute configuration (enantioconvergent hydrogenation). This allowed the hydrogenation of a wide range of / mixtures of trisubstituted enamides with excellent enantioselectivity (up to 99% ).

Promoting Selective Generation of Formic Acid from CO Using Mn(bpy)(CO)Br as Electrocatalyst and Triethylamine/Isopropanol as Additives.

Urgent solutions are needed to efficiently convert the greenhouse gas CO into higher-value products. In this work, -Mn(bpy)(CO)Br (bpy = 2,2'-bipyridine) is employed as electrocatalyst in reductive CO conversion. It is shown that product selectivity can be shifted from CO toward HCOOH using appropriate additives, i.

Molecular Engineering of Photocathodes based on Polythiophene Organic Semiconductors for Photoelectrochemical Hydrogen Generation.

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 (Py) groups were introduced into polythiophene (pTH) semiconductor by electrochemical copolymerization, respectively. After assembly with the Co(dmgBF) type catalyst (CoB, dmgBF = difluoroboryldimethylglyoximate), the CoB@Py-pTH/CFP photocathode displayed nearly twice the photocurrent enhancement (550 μA cm at 0.

Site- and Enantioselective Iridium-Catalyzed Desymmetric Mono-Hydrogenation of 1,4-Dienes.

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.

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 CO 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 =1,10-phenanthroline-2,9-dicarboxylate) molecular WOC with 4-vinylpyridine axial ligands was immobilized on the glassy carbon electrode surface by electrochemical polymerization.

Electrostatic Interactions Accelerating Water Oxidation Catalysis via Intercatalyst O-O Coupling.

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.

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)] (Ru-bds; bds = 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 under acidic and neutral conditions respectively, showing much better performance than the state-of-art Ru-bda catalyst.

Pivotal Electron Delivery Effect of the Cobalt Catalyst in Photocarboxylation of Alkynes: A DFT Calculation.

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.

Oxide Relay: An Efficient Mechanism for Catalytic Water Oxidation at Hydrophobic Electrode Surfaces.

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), the oxide relay mechanism, in a hybrid system from an experimental study. It reacts with the same efficiency but with increased stability compared to the homogeneous molecular catalyst.

The Carboxylate Ligand as an Oxide Relay in Catalytic Water Oxidation.

Carboxylate groups have diverse functionalities in ligands of transition metal catalysts. Here we present a conceptually different function of the carboxylates: the oxide relay. It functions by providing an intramolecular nucleophilic oxygen close to the oxo group to facilitate O-O bond formation and at a later stage a remote electrophilic center to facilitate OH nucleophilic attack.

Paired Electrocatalytic Oxygenation and Hydrogenation of Organic Substrates with Water as the Oxygen and Hydrogen Source.

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 Ni intermediates involved in electrocatalytic water oxidation by nickel-based materials, we prepared NiB as a catalyst and used water as the oxygen source for the oxygenation of various organic compounds. NiB 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.

Development and mechanistic investigation of the manganese(iii) salen-catalyzed dehydrogenation of alcohols.

The first example of a manganese(iii) catalyst for the acceptorless dehydrogenation of alcohols is presented. ,-Bis(salicylidene)-1,2-cyclohexanediaminomanganese(iii) chloride () 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.

Dynamics and Reactions of Molecular Ru Catalysts at Carbon Nanotube-Water Interfaces.

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 [RuO(bda)L] in explicit water at a carbon nanotube water interface, forming the key O-O bond between two 128 atom catalysts, all fully dynamically.

Mechanistic Studies on NaHCO Hydrogenation and HCOOH Dehydrogenation Reactions Catalysed by a Fe Linear Tetraphosphine Complex.

We present a theoretical extension of the previously published bicarbonate hydrogenation to formate and formic acid dehydrogenation catalysed by Fe complexes bearing the linear tetraphosphine ligand tetraphos-1. The hydrogenation reaction was found to proceed at the singlet surface with two competing pathways: A) H association to the Fe-H species followed by deprotonation to give a Fe(H) intermediate, which then reacts with CO to give formate. B) CO insertion into the Fe-H bond, followed by H association and subsequent deprotonation.

Capturing the Role of Explicit Solvent in the Dimerization of Ru (bda) Water Oxidation Catalysts.

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 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.