Olefin Substitution in (silox)3M(olefin) (silox = tBu3SiO; M = Nb, Ta): The Role of Density of States in Second vs Third Row Transition Metal Reactivity

This article discusses the role of density of states in second vs third row transition metal reactivity

A Better Way to Store Energy for Less Cost

Representing the Center for Molecular Electrocatalysis (CME), this document is one of the entries in the Ten Hundred and One Word Challenge. As part of the challenge, the 46 Energy Frontier Research Centers were invited to represent their science in images, cartoons, photos, words and original paintings, but any descriptions or words could only use the 1000 most commonly used words in the English language, with the addition of one word important to each of the EFRCs and the mission of DOE energy. The mission of CME to understand, design and develop molecular electrocatalysts for solar fuel production and use

Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen

Oxidation of H₂ (1 atm) is catalyzed by the manganese electrocatalysts [(P₂N₂)­Mn^I(CO)­(bppm)]⁺ and [(PNP)­Mn^I(CO)­(bppm)]⁺ (P₂N₂ = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane; PNP = (Ph₂PCH₂)₂NMe); bppm = (PAr^F₂)₂CH₂; Ar^F = 3,5-(CF₃)₂C₆H₃). In fluorobenzene solvent using 2,6-lutidine as the exogeneous base, the turnover frequency for [(P₂N₂)­Mn^I(CO)­(bppm)]⁺ is 3.5 s^–1, with an estimated overpotential of 700 mV. For [(PNP)­Mn^I(CO)­(bppm)]⁺ in fluorobenzene solvent using <i>N</i>-methylpyrrolidine as the exogeneous base, the turnover frequency is 1.4 s^–1, with an estimated overpotential of 880 mV. Density functional theory calculations suggest that the slow step in the catalytic cycle is proton transfer from the oxidized 17-electron manganese hydride [(P₂N₂)­Mn^IIH­(CO)­(bppm)]⁺ to the pendant amine. The computed activation barrier for intramolecular proton transfer from the metal to the pendant amine is 20.4 kcal/mol for [(P₂N₂)­Mn^IIH­(CO)­(bppm)]⁺ and 21.3 kcal/mol for [(PNP)­Mn^IIH­(CO)­(bppm)]⁺. The high barrier appears to result from both the unfavorability of the metal to nitrogen proton transfer (thermodynamically uphill by 9 kcal/mol for [(P₂N₂)­Mn^IIH­(CO)­(bppm)]⁺ due to a mismatch of 6.6 p<i>K</i>_a units) and the relatively long manganese–nitrogen separation in the Mn^IIH complexes

Absolute Estimates of Pd^II(η²‑Arene) C–H Acidity

Thermodynamic acidity is one of the most widely used quantities for characterizing proton transfer reactions. Measurement of these values for catalytically relevant species can be challenging, often requiring direct observation of equilibria. The C–H bonds of aromatic substrates are proposed to become substantially polarized during electrophilic activation, but quantifying the absolute acidity of the intermediate M­(η²-arene) complexes is highly challenging. Using a system that intercepts nascent protons at electrophilic Pd^II arene complexes, a combined experimental and computational study has demonstrated these C–H bonds to be far more acidic (p<i>K</i>_a^CH₃CN = 3–6) than many “nonbasic” substrates and additives that are present in electrophilic C–H activation catalysis, and the catalytic roles of these species may need to be reassessed

Rapid, Reversible Heterolytic Cleavage of Bound H₂

Heterolytic cleavage of dihydrogen into a proton and a hydride ion is a fundamentally important step in many reactions, including the oxidation of hydrogen by hydrogenase enzymes and ionic hydrogenation of organic compounds. We report the facile, <i>reversible</i> heterolytic cleavage of H₂ in a manganese complex bearing a pendant amine, leading to the formation of a manganese hydride and a protonated amine that undergo H⁺/H^– exchange at an estimated rate of >10⁷ s^–1 at 25 °C

Rapid, Reversible Heterolytic Cleavage of Bound H₂

Heterolytic cleavage of dihydrogen into a proton and a hydride ion is a fundamentally important step in many reactions, including the oxidation of hydrogen by hydrogenase enzymes and ionic hydrogenation of organic compounds. We report the facile, <i>reversible</i> heterolytic cleavage of H₂ in a manganese complex bearing a pendant amine, leading to the formation of a manganese hydride and a protonated amine that undergo H⁺/H^– exchange at an estimated rate of >10⁷ s^–1 at 25 °C

Rapid, Reversible Heterolytic Cleavage of Bound H₂

Heterolytic cleavage of dihydrogen into a proton and a hydride ion is a fundamentally important step in many reactions, including the oxidation of hydrogen by hydrogenase enzymes and ionic hydrogenation of organic compounds. We report the facile, <i>reversible</i> heterolytic cleavage of H₂ in a manganese complex bearing a pendant amine, leading to the formation of a manganese hydride and a protonated amine that undergo H⁺/H^– exchange at an estimated rate of >10⁷ s^–1 at 25 °C

Iron Complexes Bearing Diphosphine Ligands with Positioned Pendant Amines as Electrocatalysts for the Oxidation of H₂

The synthesis and spectroscopic characterization of Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)­Cl, <b>[3-Cl]</b> (where C₅F₄N is a tetrafluoropyridyl substituent and P^<i>t</i>Bu₂N^Bn₂ = 1,5-dibenzyl-3,7-di­(<i>tert</i>-butyl)-1,5-diaza-3,7-diphosphacyclooctane), are reported. Complex <b>3-Cl</b> and [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂)­Cl], <b>4-Cl</b>, are precursors to intermediates in the catalytic oxidation of H₂, including Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)H <b>(3-H)</b>, Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂)H (<b>4-H)</b>, [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)]­BAr^F₄ (<b>[3]­(BAr</b>^<b>F</b>_<b>4</b>), [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂)]­BAr^F₄ (<b>[4]­(BAr</b>^<b>F</b>_<b>4</b>), [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^Bn₂)­(H₂)]­BAr^F₄ (<b>[3-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>), and [Cp^C₅F₄NFe­(P^<i>t</i>Bu₂N^<i>t</i>Bu₂<i>H</i>)<i>H</i>]­BAr^F₄ (<b>[4-Fe</b><i><b>H</b></i><b>(N</b><i><b>H</b></i><b>)]­BAr</b>^<b>F</b>_<b>4</b>). All of these complexes were characterized by spectroscopic and electrochemical studies; <b>3-Cl</b>, <b>3-H</b>, and <b>4-Cl</b> were also characterized by single crystal diffraction studies. <b>3-H</b> and <b>4-H</b> are electrocatalysts for H₂ (1.0 atm) oxidation in the presence of an excess of the amine bases <i>N</i>-methylpyrrolidine, Et₃N or ^<i>i</i>Pr₂EtN. Turnover frequencies at 22 °C for <b>3-H</b> and <b>4-H</b> with <i>N</i>-methylpyrrolidine as the base are 2.5 and 0.5 s^–1, and overpotentials at <i>E</i>_cat/2 are 235 and 95 mV, respectively. Studies of individual chemical and electrochemical reactions of the various intermediates provide important insights into the factors governing the overall catalytic activity for H₂ oxidation

A Cobalt Hydride Catalyst for the Hydrogenation of CO₂: Pathways for Catalysis and Deactivation

The complex Co­(dmpe)₂H catalyzes the hydrogenation of CO₂ at 1 atm and 21 °C with significant improvement in turnover frequency relative to previously reported second- and third-row transition-metal complexes. New studies are presented to elucidate the catalytic mechanism as well as pathways for catalyst deactivation. The catalytic rate was optimized through the choice of the base to match the p<i>K</i>_a of the [Co­(dmpe)₂(H)₂]⁺ intermediate. With a strong enough base, the catalytic rate has a zeroth-order dependence on the base concentration and the pressure of hydrogen and a first-order dependence on the pressure of CO₂. However, for CO₂:H₂ ratios greater than 1, the catalytically inactive species [(μ-dmpe)­(Co­(dmpe)₂)₂]²⁺ and [Co­(dmpe)₂CO]⁺ were observed