Room-Temperature Formate Ester Transfer Hydrogenation Enables an Electrochemical/Thermal Organometallic Cascade for Methanol Synthesis from CO.

The reduction of CO to synthetic fuels is a valuable strategy for energy storage. However, the formation of energy-dense liquid fuels such as methanol remains rare, particularly under low-temperature and low-pressure conditions that can be coupled to renewable electricity sources via electrochemistry. Here, a multicatalyst system pairing an electrocatalyst with a thermal organometallic catalyst is introduced, which enables the reduction of CO to methanol at ambient temperature and pressure.

Molecular catalyst coordinatively bonded to organic semiconductors for selective light-driven CO reduction in water.

The selective photoreduction of CO in aqueous media based on earth-abundant elements only, is today a challenging topic. Here we present the anchoring of discrete molecular catalysts on organic polymeric semiconductors via covalent bonding, generating molecular hybrid materials with well-defined active sites for CO photoreduction, exclusively to CO in purely aqueous media. The molecular catalysts are based on aryl substituted Co phthalocyanines that can be coordinated by dangling pyridyl attached to a polymeric covalent triazine framework that acts as a light absorber.

Circumventing Kinetic Barriers to Metal Hydride Formation with Metal-Ligand Cooperativity.

We report the two-electron, one-proton mechanism of cobalt hydride formation for the conversion of [CoCp(PN)(CHCN)] to [HCoCp(PN)]. This complex catalytically converts CO to formate under CO reduction conditions, with hydride formation as a key elementary step. Through a combination of electrochemical measurements, digital simulations, theoretical calculations, and additional mechanistic and thermochemical studies, we outline the explicit role of the PN ligand in the proton-coupled electron transfer (PCET) reactivity that leads to hydride formation.

Linear Free Energy Relationships Associated with Hydride Transfer From [(6,6'-R-bpy)Re(CO)H]: A Cautionary Tale in Identifying Hydrogen Bonding Effects in the Secondary Coordination Sphere.

Six rhenium hydride complexes, [(6,6'-R-bpy)Re(CO)H] (bpy = 2,2'-bipyridine, R = OEt, OMe, NHMe, Me, F, Br), were synthesized. These complexes insert CO to form rhenium formate complexes of the type [(6,6'-R-bpy)Re(CO){OC(O)H}]. All the rhenium formate species were characterized using X-ray crystallography, which revealed that the bpy ligand is not coplanar with the metal coordination plane containing the two nitrogen donors of the bpy ligand but tilted.

Fast Catalysis at Low Overpotential: Designing Efficient Dicationic Re(bpy)(CO)I Electrocatalysts for CO Reduction.

We report a series of isomeric, dicationic Re(bpy)(CO)I complexes with bpy (2,2'-bipyridine) modified by two phenyl-CH-(NMe) pendants with cations located at variable distances from the active site for electrocatalytic CO reduction in CHCN/2.8 M HO. The position of the cationic groups dramatically increases the rate of catalysis by ∼800-fold, from 1.

Reduction of CO to Methanol with Recyclable Organic Hydrides.

The reaction steps for the selective conversion of a transition metal carbonyl complex to a hydroxymethyl complex that releases methanol upon irradiation with visible light have been successfully quantified in acetonitrile solution with dihydrobenzimidazole organic hydride reductants. Dihydrobenzimidazole reductants have been shown to be inactive toward H generation in the presence of a wide range of proton sources and have been regenerated electrochemically or photochemically. Specifically, the reaction of -[Ru(bpy)(CO)] (bpy = 2,2'-bipyridine) with one equivalent of a dihydrobenzimidazole quantitatively yields a formyl complex, -[Ru(bpy)(CO)(CHO)], and the corresponding benzimidazolium on a seconds time scale.

Combined Effects of Hemicolligation and Ion Pairing on Reduction Potentials of Biphenyl Radical Cations.

Formal reduction potentials of highly oxidizing and short-lived radical cations of substituted biphenyls generated by pulse radiolysis in 1,2-dichloroethane (DCE) were measured using a redox equilibrium ladder method. The effect of halide ion-radical interactions on reduction potentials of biphenyls was examined by utilizing the ability of DCE to release Cl in the vicinity of the radical cation. The Hammett correlation of measured potentials across a range of over 700 mV shows saturation at high Hammett sigma values.

Predicting Mössbauer Parameters of Nonheme Diiron Complexes with Density Functional Theory.

Mössbauer spectroscopy provides significant insights into the electronic structure and environment of the metal centers. Herein, we investigate the electronic structures of a set of nonheme diiron complexes by evaluating two key parameters pertaining to Mössbauer spectroscopy, namely, the isomer shift (δ) and quadrupole splitting (|Δ|), using different levels of density functional theory (DFT). The diiron systems investigated here span diverse oxidation states, bridging motifs, and spin coupling patterns, which present a challenging case for theoretical predictions.

A Dicationic -Re(bpy)(CO)Cl for CO Electroreduction at a Reduced Overpotential.

A dicationic Re bipyridine-type complex, -Re(6,6'-(2-((trimethylammonio)-methyl)phenyl)-2,2'-bipyridine )(CO)Cl hexafluorophosphate (), has been synthesized, and its electrochemical behavior under Ar and CO has been investigated. The presence of pendent tetra-alkylammonium cations induces an anodic shift in the electrocatalytic potential for CO reduction relative to structurally similar model complexes. The electrochemical mechanisms in anhydrous CHCN and in the presence of weak acids (water or trifluoroethanol) have been analyzed using cyclic voltammetry assisted by infrared spectroelectrochemistry and theoretical calculations.

Heterogeneous Electrochemical Ammonia Oxidation with a Ru-bda Oligomer Anchored on Graphitic Electrodes via CH-π Interactions.

Molecular catalysts can promote ammonia oxidation, providing mechanistic insights into the electrochemical N cycle for a carbon-free fuel economy. We report the ammonia oxidation activity of carbon anodes functionalized with the oligomer {[Ru(bda-κ- )(4,4'-bpy)](4,4'-bpy)}, , where bda is [2,2'-bipyridine]-6,6'-dicarboxylate and 4,4'-bpy is 4,4'-bipyridine. Electrocatalytic studies in propylene carbonate demonstrate that the Ru-based hybrid anode used in a 3-electrode configuration transforms NH to N and H in a 1:3 ratio with near-unity faradaic efficiency at an applied potential of 0.

Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO Reduction by Manganese Bipyridyl Complexes.

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph)bpy vs mesbpy ligands) and Lewis basic ((Ph)bpy vs (MeOPh)bpy vs (MeSPh)bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO reduction mechanisms with [Mn(Rbpy)(CO)(CHCN)] precatalysts, and on their respective transition-state geometries/energies for rate-determining C-OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph)bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mesbpy-based precatalysts. Cyclic voltammetry conducted in CO-saturated electrolyte for the (Ph)bpy-based precatalyst confirms that CO binding occurs at the two-electron-reduced activated catalyst in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO binding.

Peroxide-Selective Reduction of O at Redox-Inactive Rare-Earth(III) Triflates Generates an Ambiphilic Peroxide.

Metal peroxides are key species involved in a range of critical biological and synthetic processes. Rare-earth (group III and the lanthanides; Sc, Y, La-Lu) peroxides have been implicated as reactive intermediates in catalysis; however, reactivity studies of isolated, structurally characterized rare-earth peroxides have been limited. Herein, we report the peroxide-selective (93-99% O) reduction of dioxygen (O) at redox-inactive rare-earth triflates in methanol using a mild metallocene reductant, decamethylferrocene (Fc*).

Reorganization Energy of Electron Transfer in Ionic Liquids.

Bandshape analysis of charge-transfer optical bands in room-temperature ionic liquids (ILs) was performed to extract the reorganization energy of electron transfer. Remarkably, the reorganization energies in ILs are close to those in cyclohexane. This result runs against common wisdom in the field since conducting ILs, which are characterized by an infinite static dielectric constant, and nonpolar cyclohexane fall to the opposite ends of the polarity scale based on their dielectric constants.

Mechanistic investigation of the aerobic oxidation of 2-pyridylacetate coordinated to a Ru(II) polypyridyl complex.

A new ruthenium polypyridyl complex, [Ru(bpy)(acpy)] (acpy = 2-pyridylacetate, bpy = 2,2'-bipyridine), was synthesized and fully characterized. Distinct from the previously reported analog, [Ru(bpy)(pic)] (pic = 2-pyridylcarboxylate), the new complex is unstable under aerobic conditions and undergoes oxidation to yield the corresponding α-keto-2-pyridyl-acetate (acpyoxi) coordinated to the Ru center. The reaction is one of the few examples of C-H activation at mild conditions using O as the primary oxidant and can provide mechanistic insights with important implications for catalysis.

An amide-based second coordination sphere promotes the dimer pathway of Mn-catalyzed CO-to-CO reduction at low overpotential.

The [-Mn(bpy)(CO)Br] complex is capable of catalyzing the electrochemical reduction of CO to CO with high selectivity, moderate activity and large overpotential. Several attempts have been made to lower the overpotential and to enhance the catalytic activity of this complex by manipulating the second-coordination sphere of manganese and using relatively stronger acids to promote the pathway. We report herein that the complex [-Mn(bpy-CONHMe)(CO)(MeCN)] ([]; bpy-CONHMe = -methyl-(2,2'-bipyridine)-6-carboxamide) as a pre-catalyst could catalyze the electrochemical reduction of CO to CO with low overpotential and high activity and selectivity.

Calcium-Ion Binding Mediates the Reversible Interconversion of Cis and Trans Peroxido Dicopper Cores.

Coupled dinuclear copper oxygen cores (Cu O ) featured in type III copper proteins (hemocyanin, tyrosinase, catechol oxidase) are vital for O transport and substrate oxidation in many organisms. μ-1,2-cis peroxido dicopper cores ( P) have been proposed as key structures in the early stages of O binding in these proteins; their reversible isomerization to other Cu O cores are directly relevant to enzyme function. Despite the relevance of such species to type III copper proteins and the broader interest in the properties and reactivity of bimetallic P cores in biological and synthetic systems, the properties and reactivity of P Cu O species remain largely unexplored.

Synthesis, Characterization, and Water Oxidation Activity of Isomeric Ru Complexes.

The synthesis and characterization of the isomeric ruthenium complexes with the general formula and [Ru(trpy)(qc)X] (trpy is 2,2':6',2″-terpyridine, qc is 8-quinolinecarboxylate, and , X = Cl, = 0; and , X=OH, = 1) with respect to the relative disposition of the carboxylate and X ligands are reported. For comparison purposes, another set of ruthenium complexes with general formula and [Ru(trpy)(pic)(OH)] (pic is 2-picolinate (-, -)) have been prepared. The complexes with a qc ligand show a more distorted geometry compared to the complexes with a pic ligand.

Methane Generation from CO with a Molecular Rhenium Catalyst.

The atomic-level tunability of molecular structures is a compelling reason to develop homogeneous catalysts for challenging reactions such as the electrochemical reduction of carbon dioxide to valuable C-C products. Of particular interest is methane, the largest component of natural gas. Herein, we report a series of three isomeric rhenium tricarbonyl complexes coordinated by the asymmetric diimine ligands 2-(isoquinolin-1-yl)-4,5-dihydrooxazole (), 2-(quinolin-2-yl)-4,5-dihydrooxazole (), and 2-(isoquinolin-3-yl)-4,5-dihydrooxazole () that catalyze the reduction of CO to carbon monoxide and methane, albeit the latter with a low efficiency.

Efficient Iridium Catalysts for Formic Acid Dehydrogenation: Investigating the Electronic Effect on the Elementary β-Hydride Elimination and Hydrogen Formation Steps.

We report herein a series of Cp*Ir complexes containing a rigid 8-aminoquinolinesulfonamide moiety as highly efficient catalysts for the dehydrogenation of formic acid (FA). The complex [Cp*Ir(L)Cl] (HL = -(quinolin-8-yl)benzenesulfonamide) displayed a high turnover frequency (TOF) of 2.97 × 10 h and a good stability (>100 h) at 60 °C.

Oxygen Atom Transfer as an Alternative Pathway for Oxygen-Oxygen Bond Formation.

Fundamental understanding of catalytic mechanisms of water oxidation is a prerequisite for the design and development of efficient and rugged water oxidation catalysts. In this work, a detailed mechanistic study of the water oxidation mechanism of the [Ru(npm)(4-pic)(HO)] (npm = 4--butyl-2,6-di(1',8'-naphthyrid-2'-yl)-pyridine, pic = 4-picoline) complex, , reveals oxygen atom transfer from highly reactive ruthenium oxo intermediates to noncoordinating nitrogen atoms of the ligand as a novel route for oxygen evolution via storage of oxidizing equivalents as N-oxide groups on the ligand framework. Theoretical calculations show that the initial complex, , is transformed to a di-N-oxide complex upon oxidation via facile OAT steps from species and that represents the most likely reactive species for the critical O-O bond formation.

Second Coordination Sphere Effects in an Evolved Ru Complex Based on Highly Adaptable Ligand Results in Rapid Water Oxidation Catalysis.

A new Ru complex containing the deprotonated 2,2':6',2''-terpyridine-6,6''-diphosphonic acid (HtPa) and pyridine (py) of general formula [Ru(HtPa-κ-NO)(py)], , has been prepared and thoroughly characterized by means of spectroscopic and electrochemical techniques, X-ray diffraction analysis, and density functional theory (DFT) calculations. Complex presents a dynamic behavior in the solution that involves the synchronous coordination and the decoordination of the dangling phosphonic groups of the tPa ligand. However, at oxidation state IV, complex becomes seven coordinated with the two phosphonic groups now bonded to the metal center.

An Investigation of Electrocatalytic CO Reduction Using a Manganese Tricarbonyl Biquinoline Complex.

The subject of this study [-Mn(bqn)(CO)(CHCN)] (bqn = 2,2'-biquinoline), is of particular interest because the bqn ligand exhibits both steric and electronic influence over the fundamental redox properties of the complex and, consequently, its related catalytic properties with respect to the activation of CO. While not a particularly efficient catalyst for CO to CO conversion, generation and activity measurements of the [-Mn(bqn)(CO)] active catalyst allows for a better understanding of ligand design at the Mn center. By making direct comparisons to the related 2,2'-bipyridyl (bpy), 1,10-phenanthroline (phen), and 2,9-dimethyl-1,10-phenanthroline (dmphen) ligands via a combination of voltammetry, infrared spectroelectrochemistry, controlled potential electrolysis and computational analysis, the role of steric vs.