Yehoshoa Ben-David

Consecutive thermal H2 and light-induced O2 evolution from water promoted by a metal complex

Discovery of an efficient artificial catalyst for the sunlight-driven splitting of water into dioxygen and dihydrogen is a major goal of renewable energy research. We describe a solution-phase reaction scheme that leads to the stoichiometric liberation of dihydrogen and dioxygen in consecutive thermal- and light-driven steps mediated by mononuclear, well-defined ruthenium complexes. The initial reaction of water at 25°C with a dearomatized ruthenium (II) [Ru(II)] pincer complex yields a monomeric aromatic Ru(II) hydrido-hydroxo complex that, on further reaction with water at 100°C, releases H2 and forms a cis dihydroxo complex. Irradiation of this complex in the 320-to-420–nanometer range liberates oxygen and regenerates the starting hydrido-hydroxo Ru(II) complex, probably by elimination of hydrogen peroxide, which rapidly disproportionates. Isotopic labeling experiments with H217O and H218O show unequivocally that the process of oxygen–oxygen bond formation is intramolecular, establishing a previously elusive fundamental step toward dioxygen-generating homogeneous catalysis.

Low‐Pressure Hydrogenation of Carbon Dioxide Catalyzed by an Iron Pincer Complex Exhibiting Noble Metal Activity

A highly active iron catalyst for the hydrogenation of carbon dioxide and bicarbonates works under remarkably low pressures and achieves activities similar to some of the best noble metal catalysts. A mechanism is proposed involving the direct attack of an iron trans-dihydride on carbon dioxide, followed by ligand exchange and dihydrogen coordination.

Efficient Hydrogen Liberation from Formic Acid Catalyzed by a Well-Defined Iron Pincer Complex under Mild Conditions

Hydrogen liberation: An attractive approach to reversible hydrogen storage applications is based on the decomposition of formic acid. The efficient and selective hydrogen liberation from formic acid is catalyzed by an iron pincer complex in the presence of trialkylamine. Turnover frequencies up to 836 h⁻¹ and turnover numbers up to 100,000 were achieved at 40 °C. A mechanism including well-defined intermediates is suggested on the basis of experimental and computational data.

Iron Borohydride Pincer Complexes for the Efficient Hydrogenation of Ketones under Mild, Base‐Free Conditions: Synthesis and Mechanistic Insight

The new, structurally characterized hydrido carbonyl tetrahydridoborate iron pincer complex [(iPr-PNP)Fe(H)(CO)(η(1)-BH(4))] (1) catalyzes the base-free hydrogenation of ketones to their corresponding alcohols employing only 4.1 atm hydrogen pressure. Turnover numbers up to 1980 at complete conversion of ketone were reached with this system. Treatment of 1 with aniline (as a BH(3) scavenger) resulted in a mixture of trans-[(iPr-PNP)Fe(H)(2)(CO)] (4a) and cis-[(iPr-PNP)Fe(H)(2)(CO)] (4b). The dihydrido complexes 4a and 4b do not react with acetophenone or benzaldehyde, indicating that these complexes are not intermediates in the catalytic reduction of ketones. NMR studies indicate that the tetrahydridoborate ligand in 1 dissociates prior to ketone reduction. DFT calculations show that the mechanism of the iron-catalyzed hydrogenation of ketones involves alcohol-assisted aromatization of the dearomatized complex [(iPr-PNP*)Fe(H)(CO)] (7) to initially give the Fe(0) complex [(iPr-PNP)Fe(CO)] (21) and subsequently [(iPr-PNP)Fe(CO)(EtOH)] (38). Concerted coordination of acetophenone and dual hydrogen-atom transfer from the PNP arm and the coordinated ethanol to, respectively, the carbonyl carbon and oxygen atoms, leads to the dearomatized complex [(iPr-PNP*)Fe(CO)(EtO)(MeCH(OH)Ph)] (32). The catalyst is regenerated by release of 1-phenylethanol, followed by dihydrogen coordination and proton transfer to the coordinated ethoxide ligand.

Unprecedented iron-catalyzed ester hydrogenation. Mild, selective, and efficient hydrogenation of trifluoroacetic esters to alcohols catalyzed by an iron pincer complex

The synthetically important, environmentally benign hydrogenation of esters to alcohols has been accomplished in recent years only with precious-metal-based catalysts. Here we present the first iron-catalyzed hydrogenation of esters to the corresponding alcohols, proceeding selectively and efficiently in the presence of an iron pincer catalyst under remarkably mild conditions.

Evidence for a terminal Pt( iv )-oxo complex exhibiting diverse reactivity

Terminal oxo complexes of transition metals have critical roles in various biological and chemical processes. For example, the catalytic oxidation of organic molecules, some oxidative enzymatic transformations, and the activation of dioxygen on metal surfaces are all thought to involve oxo complexes. Moreover, they are believed to be key intermediates in the photocatalytic oxidation of water to give molecular oxygen, a topic of intensive global research aimed at artificial photosynthesis and water splitting. The terminal oxo ligand is a strong π-electron donor, so it readily forms stable complexes with high-valent early transition metals. As the d orbitals are filled up with valence electrons, the terminal oxo ligand becomes destabilized. Here we present evidence for a dn (n > 5) terminal oxo complex that is not stabilized by an electron withdrawing ligand framework. This d6 Pt(iv) complex exhibits reactivity as an inter- and intramolecular oxygen donor and as an electrophile. In addition, it undergoes a water activation process leading to a terminal dihydroxo complex, which may be relevant to the mechanism of catalytic reactions such as water oxidation.

Synthesis of Peptides and Pyrazines from β‐Amino Alcohols through Extrusion of H2 Catalyzed by Ruthenium Pincer Complexes: Ligand‐Controlled Selectivity

Peptides constitute one of the most important families of compounds in chemistry and biology. Short peptides have found intriguing biological and synthetic applications. For example, the conformational rigidity of cyclic peptides makes them attractive for drug discovery and biomedical research. Several cyclic peptides that show intriguing biological activity are found in nature. Cyclic peptides have been discovered that are novel antibiotics, enzyme inhibitors, and receptor antagonists. Among them are the smallest cyclopeptides, 2,5diketopiperazines derivatives, which are commonly found as natural products. These compounds exhibit high-affinity binding to a large variety of receptors and show a broad range of biological acitivities, including antimicrobial, antitumoral, antiviral, and neuroprotective effects. 2,5-diketopiperazine derivatives are synthesized in solution or on the solid phase from commercially available and appropriately protected chiral a-amino acids in processes that are usually not atom-economical and generate considerable amounts of waste. Large libraries of cyclic peptides are accessible through solid-phase split-and-pool synthesis, and various methods were developed for their syntheses. Very recently, the synthesis of diketopiperazines from amino acids under microwave irradiation was reported. Green, atom-economical methods for the generation of peptides are highly desirable. We have developed several reactions catalyzed by PNN and PNP Ru pincer complexes based on pyridine, bipyridine, 13] and acridine and have discovered a new mode of metal–ligand cooperation based on ligand aromatization–dearomatization. For example, the PNN Ru pincer complex 1 (Scheme 1) catalyzes the direct synthesis of amides from alcohols and amines with liberation of H2 [17] (Scheme 2, Eq. (1)). Several reports on amide formation by dehydrogenative coupling of amines with alcohols appeared later. Unlike complex 1, the analogous PNP complex 2 (or complex 3 in the presence of an equivalent of base) catalyzes the coupling of amines with alcohols to form imines rather than amides with liberation of H2 and H2O (Scheme 2, Eq. (2)). Herein we report a novel method for peptide synthesis, which involves dehydrogenative coupling of b-amino alcohols with extrusion of H2 catalyzed by complex 1. This environmentally benign and atom-economical reaction proceeds under neutral reaction conditions without the use of toxic reagents, activators, condensing agents, or other additives. With the analogous PNP complex 2, a strikingly different reaction takes place, which leads to pyrazines with extrusion of H2 and H2O. Initially, we were interested to see whether coupling of bamino alcohols with amines can be accomplished and whether racemization would be involved. Reaction of (S)-2-amino-3phenylpropan-1-ol (4), benzylamine, and 1 mol% of the catalyst 1 in toluene at reflux for six hours led to (S)-2-aminoN-benzyl-3-phenylpropanamide 5 in 58% yield after column chromatography (Scheme 3). The specific rotation of amide 5 obtained from the catalysis is essentially the same as reported (+ 16.08). The neutral reaction conditions likely help to prevent racemization. Scheme 1. PNNand PNP-type pincer ruthenium complexes.

Selective Hydrogenation of Nitriles to Primary Amines Catalyzed by a Cobalt Pincer Complex.

The catalytic hydrogenation of nitriles to primary amines represents an atom-efficient and environmentally benign reduction methodology in organic chemistry. This has been accomplished in recent years mainly with precious-metal-based catalysts, with a single exception. Here we report the first homogeneous Co-catalyzed hydrogenation of nitriles to primary amines. Several (hetero)aromatic, benzylic, and aliphatic nitriles undergo hydrogenation to the corresponding primary amines in good to excellent yields under the reaction conditions.

A New Mode of Activation of CO2 by Metal–Ligand Cooperation with Reversible CC and MO Bond Formation at Ambient Temperature

Team work: Although CO(2) binding to metal centers usually involves π coordination to a C=O group or σ bonds to the carbon or oxygen atom of the CO(2) molecule, a new mode of metal-ligand cooperative activation of CO(2) to a ruthenium PNP pincer complex involving aromatization/dearomatization steps is presented in experimental and theoretical studies (see scheme).

Direct synthesis of pyridines and quinolines by coupling of γ-amino-alcohols with secondary alcohols liberating H2 catalyzed by ruthenium pincer complexes

A novel, one-step synthesis of substituted pyridine- and quinoline-derivatives was achieved by acceptorless dehydrogenative coupling of γ-aminoalcohols with secondary alcohols. The reaction involves consecutive C-N and C-C bond formation, catalyzed by a bipyridyl-based ruthenium pincer complex with a base.