Free-Energy Relationships between the Proton and Hydride Donor Abilities of [HNi(diphosphine)<sub>2</sub>]<sup>+</sup> Complexes and the Half-Wave Potentials of Their Conjugate Bases

Berning, Douglas E.; Miedaner, Alex; Curtis, Calvin J.; Noll, Bruce C.; DuBois, Mary C. Rakowski; DuBois, Daniel L.

doi:10.1021/om0100582

Cited by 121 publications

(225 citation statements)

References 35 publications

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“…Metal hydrides span a wide range of pK a values; considering only metal carbonyl hydrides shown in Table 7.1, the range exceeds 20 pK a units. As expected, a substantial decrease in acidity is 7.2 Stoichiometric Ionic Hydrogenations 157 [13][14][15][16][17][18][19]. Measurements of pK a values for a series of cobalt hydrides ( 2 ] + , which is much more acidic (pK a = 23.6).…”

Section: Transition-metal Hydrides As Proton Donorsmentioning

confidence: 77%

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Ionic Hydrogenations

Bullock¹

2006

The Handbook of Homogeneous Hydrogenation

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7 Ionic Hydrogenations 7 Ionic Hydrogenations 154 Scheme 7.1sis documents the early progress in this field; a book gives further details [2]. As shown in Eq. (2), proton transfer to the alkene generates a carbenium ion, and hydride transfer from the hydrosilane generates the product. 2Ionic hydrogenations of C=C bonds generally work well only in cases where a tertiary or aryl-substituted carbenium ion can be formed through protonation of the C=C bond. Alkenes that give a tertiary carbenium ion upon protonation include 1,1-disubstituted, tri-substituted and tetra-substituted alkenes, and each of these are usually hydrogenated by ionic hydrogenation methods in high yields.The success of stoichiometric ionic hydrogenations is due to achieving a fine balance that favors the intended reactivity rather than any of several possible alternative reactions. The acid must be strong enough to protonate the unsaturated substrate, yet the reaction of the acid and the hydride should avoid producing H 2 too quickly under the reaction conditions. The commonly used pair of CF 3 CO 2 H and HSiEt 3 meets all these criteria.The very strong acid, CF 3 SO 3 H (triflic acid, abbreviated as HOTf) can be used in conjunction with HSiEt 3 for the hydrogenation of certain alkenes [3]. These reactions proceed cleanly in 5 minutes at -50 8C. This discovery was surprising, since a review of the use of CF 3 CO 2 H and HSiEt 3 had stated that "F F F stronger acids cannot be used in conjunction with silanes because they react" [1]. Indeed, rapid evolution of H 2 does occur when HOTf is added to HSiEt 3 in the absence of an alkene. The order of addition is important in the use of HOTf/HSiEt 3 for hydrogenation of C=C bonds, to ensure that acid-induced formation of H 2 is minimized. The addition of HOTf to a solution containing the alkene and the hydrosilane results in rapid and clean hydrogenation, but the reaction is still subject to the limitation of forming a tertiary carbenium ion.Another potential mechanistic complication is capture of the intermediate carbenium ion by the conjugate base of the acid. When CF 3 CO 2 H is used as the acid, this would lead to trifluoroacetate esters. Kursanov et al. showed that, under the reaction conditions for ionic hydrogenations, trifluoroacetate esters can be converted to the hydrocarbon product (Eq. (3)). 7.2 Stoichiometric Ionic Hydrogenations 155 7 Ionic Hydrogenations 7.2 Stoichiometric Ionic Hydrogenations 161 Scheme 7.3 Thermodynamic cycle for determination of the hydricity using heterolytic cleavage of hydrogen. 38 7.3.6 Catalytic Hydrogenation of Iminium Cations by Ru ComplexesNorton and coworkers found that catalytic enantioselective hydrogenation of the C=N bond of iminium cations can be accomplished using a series of Ru complexes with chiral diphosphine ligands such as Chiraphos and Norphos [68]. Even tetra-alkyl-substituted iminium cations can be hydrogenated by this method. These reactions were carried out with 2 mol.% Ru catalyst and 3.4-3.8 bar H 2 at room temperature in CH 2 Cl 2 solvent (Eq....

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Section: Transition-metal Hydrides As Proton Donorsmentioning

confidence: 77%

“…DuBois et al carried out extensive studies on the thermodynamic hydricity of a series of metal hydrides [13,[15][16][17][18][19]. The determination of thermodynamic hydricity generally requires several measurements (coupled with known thermochemical data) to constitute a complete thermochemical cycle.…”

Section: Transition Metal Hydrides As Hydride Donorsmentioning

confidence: 99%

Ionic Hydrogenations

Bullock¹

2006

The Handbook of Homogeneous Hydrogenation

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“…11,21,22,28,29 Fig. 2 shows a plot of the hydride donor ability for a series of [HNi(diphosphine) 2 ] + complexes vs. the sum of the inductive and resonance parameters 30 for the different substituents R. A plot of the pK a values for this series of complexes versus the sum of the same ligand parameters shows less scatter and a higher r 2 value.…”

Section: Ligand Electronic Effects and Hydride Donor/acceptor Abilitiesmentioning

confidence: 99%

The roles of the first and second coordination spheres in the design of molecular catalysts for H₂production and oxidation

2009

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This tutorial review describes the development of discrete transition metal complexes as electrocatalysts for H 2 formation and oxidation. The approach involves the study of thermodynamic properties of metal hydride intermediates and the design of ligands that incorporate proton relays. The work is inspired by structural features of the H 2 ase enzymes and should be of interest to researchers in the areas of biomimetic chemistry as well as catalyst design and hydrogen utilization.

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“…The selected pK a THF values given in Table 4 [117][118][119][120][121][122][123][124][125][126][127]. Norton and coworkers examined the thermodynamic and kinetic acidity of a series of transition metal complexes [128][129][130][131][132][133].…”

Section: Metal-monohydride Species -"Hydride Ligands Can Be Acidic!"mentioning

confidence: 99%

The Activation of Dihydrogen

Tye

Darensbourg

Hall

2006

Activation of Small Molecules

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Free-Energy Relationships between the Proton and Hydride Donor Abilities of [HNi(diphosphine)₂]⁺ Complexes and the Half-Wave Potentials of Their Conjugate Bases

Cited by 121 publications

References 35 publications

Ionic Hydrogenations

Ionic Hydrogenations

The roles of the first and second coordination spheres in the design of molecular catalysts for H₂production and oxidation

The Activation of Dihydrogen

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Free-Energy Relationships between the Proton and Hydride Donor Abilities of [HNi(diphosphine)2]+ Complexes and the Half-Wave Potentials of Their Conjugate Bases

Cited by 121 publications

References 35 publications

Ionic Hydrogenations

Ionic Hydrogenations

The roles of the first and second coordination spheres in the design of molecular catalysts for H2production and oxidation

The Activation of Dihydrogen

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Product

Resources

About

Free-Energy Relationships between the Proton and Hydride Donor Abilities of [HNi(diphosphine)₂]⁺ Complexes and the Half-Wave Potentials of Their Conjugate Bases

The roles of the first and second coordination spheres in the design of molecular catalysts for H₂production and oxidation