Amorphous Cobalt Vanadium Oxide as a Highly Active Electrocatalyst for Oxygen Evolution

The water-splitting reaction provides a promising mechanism to store renewable energies in the form of hydrogen fuel. The oxidation half-reaction, the oxygen evolution reaction (OER), is a complex four-electron process that constitutes an efficiency bottleneck in water splitting. Here we report a highly active OER catalyst, cobalt vanadium oxide. The catalyst is designed on the basis of a volcano plot of metal–OH bond strength and activity. The catalyst can be synthesized by a facile hydrothermal route. The most active pure-phase material (<i>a-</i>CoVO_<i>x</i>) is X-ray amorphous and provides a 10 mA cm^–2 current density at an overpotential of 347 mV in 1 M KOH electrolyte when immobilized on a flat substrate. The synthetic method can also be applied to coat a high-surface-area substrate such as nickel foam. On this three-dimensional substrate, the <i>a-</i>CoVO_<i>x</i> catalyst is highly active, reaching 10 mA cm^–2 at 254 mV overpotential, with a Tafel slope of only 35 mV dec^–1. This work demonstrates <i>a-</i>CoVO_<i>x</i> as a promising electrocatalyst for oxygen evolution and validates M–OH bond strength as a practical descriptor in OER catalysis

Growth and Activation of an Amorphous Molybdenum Sulfide Hydrogen Evolving Catalyst

Amorphous molybdenum sulfide films, prepared by electrodeposition, are a class of highly active catalysts for hydrogen evolution. The growth mechanism of the films and the true active species were unclear. Herein, we report a study of the growth and activation of these films using Electrochemical Quartz Crystal Microbalance (EQCM) and X-ray photoelectron spectroscopy (XPS). Three processes, including oxidative deposition, reductive corrosion, and reductive deposition, are occurring during the growth of a molybdenum sulfide film. Deposition method, precursor concentration, and potential window are among the factors influencing the film growth. Regardless of deposition methods, all films exhibit similar catalytic activity on a per mass base. Potentiostatic oxidation (anodic electrolysis) is the method for fastest film growth; it produces a MoS₃ film precatalyst which can be electrochemically activated. The activity of the MoS₃ precatalyst scales with catalyst loading; at a loading of 0.2 mg/cm², the current density is 20 mA/cm² at an overpotential of 170 mV. Films differently deposited have different initial compositions, but the active catalysts in all films are the same MoS_2+<i>x</i> species, whose XPS characteristics are distinct from those of crystalline MoS₂. The activation process of a MoS₃ film precatalyst involves a reductive removal of slightly less than one equivalent of sulfide to form MoS_2+<i>x</i>

γ‑Selective Allylation of (<i>E</i>)‑Alkenylzinc Iodides Prepared by Reductive Coupling of Arylacetylenes with Alkyl Iodides

The first examples of Cu-catalyzed γ-selective allylic alkenylation using organozinc reagents are reported. (<i>E</i>)-Alkenylzinc iodides were prepared by Fe-catalyzed reductive coupling of terminal arylalkynes with alkyl iodides. In the presence of a copper catalyst, these reagents reacted with allylic bromides derived from Morita–Baylis–Hillman alcohols to give 1,4-dienes in high yields. The reactions are highly γ-selective (generally γ/α > 49:1) and tolerate a wide range of functional groups such as ester, cyano, keto, and nitro

Synthesis, Reactivity, and Catalytic Application of a Nickel Pincer Hydride Complex

The nickel­(II) hydride complex [(^MeN₂N)­Ni-H] (<b>2</b>) was synthesized by the reaction of [(^MeN₂N)­Ni-OMe] (<b>6</b>) with Ph₂SiH₂ and was characterized by NMR and IR spectroscopy as well as X-ray crystallography. <b>2</b> was unstable in solution, and it decomposed via two reaction pathways. The first pathway was intramolecular N–H reductive elimination to give ^MeN₂NH and nickel particles. The second pathway was intermolecular, with H₂, nickel particles, and a five-coordinate Ni­(II) complex [(^MeN₂N)₂Ni] (<b>8</b>) as the products. <b>2</b> reacted with acetone and ethylene, forming [(^MeN₂N)­Ni-O^<i>i</i>Pr] (<b>9</b>) and [(^MeN₂N)­Ni-Et] (<b>10</b>), respectively. <b>2</b> also reacted with alkyl halides, yielding nickel halide complexes and alkanes. The reduction of alkyl halides was rendered catalytically, using [(^MeN₂N)­Ni-Cl] (<b>1</b>) as catalyst, NaO^<i>i</i>Pr or NaOMe as base, and Ph₂SiH₂ or Me­(EtO)₂SiH as the hydride source. The catalysis appears to operate via a radical mechanism

Synthesis of <i>E</i>‑Alkyl Alkenes from Terminal Alkynes via Ni-Catalyzed Cross-Coupling of Alkyl Halides with B‑Alkenyl-9-bora­bicyclo­[3.3.1]­nonanes

The first Ni-catalyzed Suzuki–Miyaura coupling of alkyl halides with alkenyl-(9-BBN) reagents is reported. Both primary and secondary alkyl halides including alkyl chlorides can be coupled. The coupling method can be combined with hydroboration of terminal alkynes, allowing the expedited synthesis of functionalized alkyl alkenes from readily available alkynes with complete (<i>E</i>)-selectivity in one pot. The method was applied to the total synthesis of (±)-Recifeiolide, a natural macrolide

Manganese-Mediated Reductive Transamidation of Tertiary Amides with Nitroarenes

Amides are an important class of organic compounds, which have widespread industrial applications. Transamidation of amides is a convenient method to generate new amides from existing ones. Tertiary amides, however, are challenging substrates for transamidation. Here we describe an unconventional approach to the transamidation of tertiary amides using nitroarenes as the nitrogen source under reductive conditions. Manganese metal alone mediates the reactions and no additional catalyst is required. The method exhibits broad scope and high functional group tolerance

Bimetallic Oxidative Addition in Nickel-Catalyzed Alkyl–Aryl Kumada Coupling Reactions

The mechanism of alkyl–aryl Kumada coupling catalyzed by the nickel pincer complex Nickamine was studied. Experiments using radical-probe substrates and DFT calculations established a bimetallic oxidative addition mechanism. Kinetic measurements showed that transmetalation rather than oxidative addition was the turnover-determining step. The transmetalation involved a bimetallic pathway

Mild and Phosphine-Free Iron-Catalyzed Cross-Coupling of Nonactivated Secondary Alkyl Halides with Alkynyl Grignard Reagents

A simple protocol for iron-catalyzed cross-coupling of nonactivated secondary alkyl bromides and iodides with alkynyl Grignard reagents at room temperature has been developed. A wide range of secondary alkyl halides and terminal alkynes are tolerated to afford the substituted alkynes in good yields. A slight modification of the reaction protocol also allows for cross-coupling with a variety of primary alkyl halides

Synthesis, Reactivity, and Catalytic Application of a Nickel Pincer Hydride Complex

The nickel­(II) hydride complex [(^MeN₂N)­Ni-H] (<b>2</b>) was synthesized by the reaction of [(^MeN₂N)­Ni-OMe] (<b>6</b>) with Ph₂SiH₂ and was characterized by NMR and IR spectroscopy as well as X-ray crystallography. <b>2</b> was unstable in solution, and it decomposed via two reaction pathways. The first pathway was intramolecular N–H reductive elimination to give ^MeN₂NH and nickel particles. The second pathway was intermolecular, with H₂, nickel particles, and a five-coordinate Ni­(II) complex [(^MeN₂N)₂Ni] (<b>8</b>) as the products. <b>2</b> reacted with acetone and ethylene, forming [(^MeN₂N)­Ni-O^<i>i</i>Pr] (<b>9</b>) and [(^MeN₂N)­Ni-Et] (<b>10</b>), respectively. <b>2</b> also reacted with alkyl halides, yielding nickel halide complexes and alkanes. The reduction of alkyl halides was rendered catalytically, using [(^MeN₂N)­Ni-Cl] (<b>1</b>) as catalyst, NaO^<i>i</i>Pr or NaOMe as base, and Ph₂SiH₂ or Me­(EtO)₂SiH as the hydride source. The catalysis appears to operate via a radical mechanism

Oxidatively Electrodeposited Thin-Film Transition Metal (Oxy)hydroxides as Oxygen Evolution Catalysts

The electrolysis of water to produce hydrogen and oxygen is a simple and attractive approach to store renewable energies in the form of chemical fuels. The oxygen evolution reaction (OER) is a complex four-electron process that constitutes the most energy-inefficient step in water electrolysis. Here we describe a novel electrochemical method for the deposition of a family of thin-film transition metal (oxy)­hydroxides as OER catalysts. The thin films have nanodomains of crystallinity with lattice spacing similar to those of double-layered hydroxides. The loadings of these thin-film catalysts were accurately determined with a resolution of below 1 μg cm^–2 using an electrochemical quartz microcrystal balance. The loading–activity relations for various catalysts were established using voltammetry and impedance spectroscopy. The thin-film catalysts have up to four types of loading–activity dependence due to film nucleation and growth as well as the resistance of the films. A zone of intrinsic activity has been identified for all of the catalysts where the mass-averaged activity remains constant while the loading is increased. According to their intrinsic activities, the metal oxides can be classified into three categories: NiO_<i>x</i>, MnO_<i>x</i>, and FeO_<i>x</i> belong to category I, which is the least active; CoO_<i>x</i> and CoNiO_<i>x</i> belong to category II, which has medium activity; and FeNiO_<i>x</i>, CoFeO_<i>x</i>, and CoFeNiO_<i>x</i> belong to category III, which is the most active. The high turnover frequencies of CoFeO_<i>x</i> and CoFeNiO_<i>x</i> at low overpotentials and the simple deposition method allow the fabrication of high-performance anode electrodes coated with these catalysts. In 1 M KOH and with the most active electrode, overpotentials as low as 240 and 270 mV are required to reach 10 and 100 mA cm^–2, respectively