Resolving Heterogeneity Problems and Impurity Artifacts in Operationally Homogeneous Transition Metal Catalysts

Recyclable cobalt(0) nanoparticle catalysts for hydrogenations

Small Co(0) nanoparticles catalyze hydrogenations of alkenes, alkynes, imines, and heteroarenes; the magnetic properties enabled catalyst separation and multiple recyclings.

Efficient nickel-catalysed N-alkylation of amines with alcohols

A highly active and easy-to-prepare Ni based catalyst system is presented for the selective N-alkylation of amines with alcohols that is in situ generated from Ni(COD)₂ and KOH under ligand-free conditions.

Dehydrocoupling of dimethylamine–borane promoted by manganese(ii) m-terphenyl complexes

Low-coordinate m-terphenyl complexes are precatalysts for dehydrocoupling of dimethylamine–borane, where small changes in coordination environment effect significant mechanistic differences.

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

The design of efficient and robust water oxidation catalysts has proven challenging in the development of artificial photosynthetic systems for solar energy harnessing and storage. Tremendous progress has been made in the development of homogeneous transition-metal complexes capable of mediating water oxidation. To improve the efficiency of the catalyst and to design new catalysts, a detailed mechanistic understanding is necessary. Quantum chemical modeling calculations have been successfully used to complement the experimental techniques to suggest a catalytic mechanism and identify all stationary points, including transition states for both O-O bond formation and O₂ release. In this review, recent progress in the applications of quantum chemical methods for the modeling of homogeneous water oxidation catalysis, covering various transition metals, including manganese, iron, cobalt, nickel, copper, ruthenium, and iridium, is discussed.

Ligand-Promoted Palladium-Catalyzed Aerobic Oxidation Reactions

Palladium-catalyzed aerobic oxidation reactions have been the focus of industrial application and extensive research efforts for nearly 60 years. A significant transition occurred in this field approximately 20 years ago, with the introduction of catalysts supported by ancillary ligands. The ligands play crucial roles in the reactions, including promotion of direct oxidation of palladium(0) by O₂, bypassing the typical requirement for Cu salts or related redox cocatalysts to facilitate oxidation of the reduced Pd catalyst; facilitation of key bond-breaking and bond-forming steps during substrate oxidation; and modulation of chemo-, regio-, or stereoselectivity of a reaction. The use of ligands has contributed to significant expansion of the scope of accessible aerobic oxidation reactions. Increased understanding of the role of ancillary ligands should promote the development of new synthetic transformations, enable improved control over the reaction selectivity, and improve catalyst activity and stability. This review surveys the different ligands that have been used to support palladium-catalyzed aerobic oxidation reactions and, where possible, describes mechanistic insights into the role played by the ancillary ligand.

Synthesis of mesoporous iridium nanosponge: a highly active, thermally stable and efficient olefin hydrogenation catalyst

Three dimensional porous structures offer high specific surface areas and large pore volumes, which enhance substrate diffusion within the porous structures and provide a large number of surface active sites. Such types of structures find applications in catalysis. Herein, we report a simple synthetic strategy for the preparation of iridium nanosponges by the capping agent dissolution method. An Ir@BNH_x nanocomposite was prepared starting from different iridium precursors by a solid state reduction method using ammonia borane wherein iridium(0) nanoparticles are embedded in a BNH_x polymer. Capping agent (here, the BNH_x polymer) dissolution using water under ambient conditions resulted in the formation of a mesoporous iridium nanosponge. This iridium nanosponge exhibits a surface area of 33.5 m² g^-1. The iridium nanosponge was found to be catalytically active for hydrogenation of a variety of olefinic substrates including linear and cyclic alkenes and α,β-unsaturated esters under relatively mild conditions and exhibits high turnover frequencies. It was also found to exhibit much better catalytic activity as compared to other iridium based heterogeneous catalysts for olefin hydrogenation reactions. Additionally, catalyst recovery was achieved via simple filtration from the hydrogenation reaction mixture. The catalytically active surface area of iridium nanosponge was estimated using H₂-temperature programmed desorption (TPD) experiments. Moreover, the catalyst was found to be thermally quite robust. The catalyst was recyclable over seven cycles of styrene hydrogenation and was found to be capable of hydrogenating 99% of styrene to ethyl benzene after seven cycles.

Mild and Regioselective Pd(OAc)2-Catalyzed C-H Arylation of Tryptophans by [ArN2]X, Promoted by Tosic Acid

A regioselective Pd-mediated C–H bond arylation methodology for tryptophans, utilizing stable aryldiazonium salts, affords C2-arylated tryptophan derivatives, in several cases quantitatively. The reactions proceed in air, without base, and at room temperature in EtOAc. The synthetic methodology has been evaluated and compared against other tryptophan derivative arylation methods using the CHEM21 green chemistry toolkit. The behavior of the Pd catalyst species has been probed in preliminary mechanistic studies, which indicate that the reaction is operating homogeneously, although Pd nanoparticles are formed during substrate turnover. The effects of these higher order Pd species on catalysis, under the reaction conditions examined, appear to be minimal: e.g., acting as a Pd reservoir in the latter stages of substrate turnover or as a moribund form (derived from catalyst deactivation). We have determined that TsOH shortens the induction period observed when [ArN₂]BF₄ salts are employed with Pd(OAc)₂. Pd(OTs)₂(MeCN)₂ was found to be a superior precatalyst (confirmed by kinetic studies) in comparison to Pd(OAc)₂.

Non-Pincer-Type Manganese Complexes as Efficient Catalysts for the Hydrogenation of Esters

Catalytic hydrogenation of carboxylic acid esters is essential for the green production of pharmaceuticals, fragrances, and fine chemicals. Herein, we report the efficient hydrogenation of esters with manganese catalysts based on simple bidentate aminophosphine ligands. Monoligated Mn PN complexes are particularly active for the conversion of esters into the corresponding alcohols at Mn concentrations as low as 0.2 mol % in the presence of sub-stoichiometric amounts of KOt Bu base.

Alkene Hydrogenations by Soluble Iron Nanocluster Catalysts

The replacement of noble metal technologies and the realization of new reactivities with earth-abundant metals is at the heart of sustainable synthesis. Alkene hydrogenations have so far been most effectively performed by noble metal catalysts. This study reports an iron-catalyzed hydrogenation protocol for tri- and tetra-substituted alkenes of unprecedented activity and scope under mild conditions (1-4 bar H2 , 20 °C). Instructive snapshots at the interface of homogeneous and heterogeneous iron catalysis were recorded by the isolation of novel Fe nanocluster architectures that act as catalyst reservoirs and soluble seeds of particle growth.

Alkene Metalates as Hydrogenation Catalysts

First-row transition-metal complexes hold great potential as catalysts for hydrogenations and related reductive reactions. Homo- and heteroleptic arene/alkene metalates(1-) (M=Co, Fe) are a structurally distinct catalyst class with good activities in hydrogenations of alkenes and alkynes. The first syntheses of the heteroleptic cobaltates [K([18]crown-6)][Co(η4 -cod)(η2 -styrene)2 ] (5) and [K([18]crown-6)][Co(η4 -dct)(η4 -cod)] (6), and the homoleptic complex [K(thf)2 ][Co(η4 -dct)2 ] (7; dct=dibenzo[a,e]cyclooctatetraene, cod=1,5-cyclooctadiene), are reported. For comparison, two cyclopentadienylferrates(1-) were synthesized according to literature procedures. The isolated and fully characterized monoanionic complexes were competent precatalysts in alkene hydrogenations under mild conditions (2 bar H2 , r.t., THF). Mechanistic studies by NMR spectroscopy, ESI mass spectrometry, and poisoning experiments documented the operation of a homogeneous mechanism, which was initiated by facile redox-neutral π-ligand exchange with the substrates followed by H2 activation. The substrate scope of the investigated precatalysts was also extended to polar substrates (ketones and imines).

Electrochemically Identified Ultrathin Water-Oxidation Catalyst in Neutral pH Solution Containing Ni2+ and Its Combination with Photoelectrode

Water oxidation electrocatalyzed by Ni²⁺ under neutral conditions was investigated using various electrochemical analyses. The addition of Ni²⁺ in a phosphate-buffered solution catalyzed the oxidation of water, as confirmed by the detection of oxygen generation via scanning electrochemical microscopy. A combination of cyclic voltammetry, coulometric titration, and electrochemical quartz microbalance measurements identified the catalysis as heterogeneous and the catalyst as a Ni-based ultrathin (<4 nm) layer ("Ni-Pi"). Analysis of the potential- and pH-dependency of the titrated amount of charge revealed that the catalyst was deposited only under anodic polarization conditions and was removed under unpolarized conditions; the catalyst may be Ni(III) oxide, and its formation and oxidation appeared to be chemically irreversible. The diffusion-limited nature of water oxidation catalyzed by Ni²⁺ was closely related to the phosphate ions involved in the catalyst formation and the accompanying catalysis. Although the catalytic performance of Ni²⁺ alone was not remarkable, it exhibited a synergetic effect with BiVO₄ for photoelectrochemical water oxidation, which can compete with Co-Pi-decorated BiVO₄.

Active Palladium Colloids via Palladacycle Degradation as Efficient Catalysts for Oxidative Homocoupling and Cross-Coupling of Aryl Boronic Acids

Active palladium colloids formed upon degradation of a palladacyclic complex (Herrmann-Beller 1) have been isolated for the first time and thoroughly characterized with techniques such as transmission electron microscopy (TEM), high-resolution TEM, X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure spectroscopy. The synthesized palladium colloids have been utilized as efficient catalysts for the oxidative homocoupling of aryl boronic acids. Cross-coupling of two different aryl boronic acids has also been made possible using these active palladium colloids. This is the first report of this kind of coupling between aryl boronic acids.

A Mild and Ligand-Free Ni-Catalyzed Silylation via C-OMe Cleavage

Metal-catalyzed transformations that forge carbon-heteroatom bonds are of central importance in organic synthesis. Despite the formidable potential of aryl methyl ethers as coupling partners, the scarcity of metal-catalyzed C-heteroatom bond formations via C-OMe cleavage is striking, with isolated precedents requiring specialized, yet expensive, ligands, high temperatures, and π-extended backbones. We report an unprecedented catalytic ipso-silylation of aryl methyl ethers under mild conditions and without recourse to external ligands. The method is distinguished by its wide scope, which includes the use of benzyl methyl ethers, vinyl methyl ethers, and unbiased anisole derivatives, thus representing a significant step forward for designing new C-heteroatom bond formations via C-OMe scission. Applications of this transformation in orthogonal silylation techniques as well as in further derivatizations are also described. Preliminary mechanistic experiments suggest the intermediacy of Ni(0)-ate complexes, leaving some doubt that a canonical catalytic cycle consisting of an initial oxidative addition of the C-OMe bond to Ni(0) species comes into play.

Silver Leakage from Ag/AgCl Reference Electrodes as a Potential Cause of Interference in the Electrocatalytic Hydrogen Evolution Reaction

The electrocatalytic hydrogen evolution reaction (HER) is of considerable interest for the production of H₂ from sustainable sources. Herein, we show that under conditions commonly employed in identifying new electrocatalysts for this reaction (using Ag/AgCl reference electrodes in 1 M H₂SO₄), silver ions can leak from the reference electrode into solution and then deposit on the working electrode as Ag(0), giving current densities for the HER of over 5 mA cm^-2 at ∼500 mV overpotential. This is well within the activity range reported for many electrocatalysts of the HER and calls into question the validity of any reports using Ag/AgCl reference electrodes which either fail to explicitly exclude silver as a cause of the electrocatalytic activity or else cannot demonstrate significantly superior activity to this baseline.

Thiols make for better catalysts: Au nanoparticles supported on functional SBA-15 for catalysis of Ullmann-type homocouplings

A strategy for arraying nanoparticles (NPs) on mesoporous supports modified with functional self-assembled monolayers is described. Au NPs supported on materials containing thiols exhibited thermal stability and catalytic competency well beyond that of unsupported, thiol-protected Au NPs of similar size.

Frontiers of water oxidation: the quest for true catalysts

Development of advanced analytical techniques is essential for the identification of water oxidation catalysts together with mechanistic studies.

Heterogeneous Metal-Free Hydrogenation over Defect-Laden Hexagonal Boron Nitride

Catalytic hydrogenation is an important process used for the production of everything from foods to fuels. Current heterogeneous implementations of this process utilize metals as the active species. Until recently, catalytic heterogeneous hydrogenation over a metal-free solid was unknown; implementation of such a system would eliminate the health, environmental, and economic concerns associated with metal-based catalysts. Here, we report good hydrogenation rates and yields for a metal-free heterogeneous hydrogenation catalyst as well as its unique hydrogenation mechanism. Catalytic hydrogenation of olefins was achieved over defect-laden h-BN (dh-BN) in a reactor designed to maximize the defects in h-BN sheets. Good yields (>90%) and turnover frequencies (6 × 10^-5-4 × 10^-3) were obtained for the hydrogenation of propene, cyclohexene, 1,1-diphenylethene, (E)- and (Z)-1,2-diphenylethene, octadecene, and benzylideneacetophenone. Temperature-programmed desorption of ethene over processed h-BN indicates the formation of a highly defective structure. Solid-state NMR (SSNMR) measurements of dh-BN with high and low propene surface coverages show four different binding modes. The introduction of defects into h-BN creates regions of electronic deficiency and excess. Density functional theory calculations show that both the alkene and hydrogen-bond order are reduced over four specific defects: boron substitution for nitrogen (B_N), vacancies (V_B and V_N), and Stone-Wales defects. SSNMR and binding-energy calculations show that V_N are most likely the catalytically active sites. This work shows that catalytic sites can be introduced into a material previously thought to be catalytically inactive through the production of defects.