Thermodynamic and kinetic hydricities of metal-free hydrides

Ilić, Stefan; Alherz, Abdulaziz; Musgrave, Charles B.; Glusac, Ksenija D.

doi:10.1039/c7cs00171a

Cited by 134 publications

(199 citation statements)

References 218 publications

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“…[2] This unexpected PÀHb ond philicity leads to an inverted regioselectivity and opens new applications of phosphines in organic synthesis. Gudat and co-workers seminal work demonstrated that 1a is af orm of super-hydride, [3] yet with common features of organic reagents, [4] such as relatively low cost and good solubility in organic media. These advantages make synthetic applications of the PÀHh ydrides ab urgeoning field.…”

Section: Contrastingwiththewell-knownproticreactivityofconven-mentioning

confidence: 99%

A Nucleophilicity Scale for the Reactivity of Diazaphospholenium Hydrides: Structural Insights and Synthetic Applications

2019

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Nucleophilicity parameters (N,s N )o fagroup of representative diazaphospholenium hydrides were derived by kinetic investigations of their hydride transfer to as eries of reference electrophiles with knowne lectrophilicity (E) values, using the Mayrequation log k 2 = s N (N + E). The Nscale covers over ten Nunits,ranging from the most reactive hydride donor (N = 25.5) to the least of the scale (N = 13.5). This discloses the highest Nv alue ever quantified in terms of Mayrsn ucleophilicity scales reported for neutral transition-metal-free hydride donors and implies an exceptional reactivity of this reagent. Even the least reactive hydride donor of this series is still ab etter hydride donor than those of many other nucleophiles such as the C À H, B À H, Si À Ha nd transition-metal M À H hydride donors.S tructure-reactivity analysis reveals that the outstanding hydricity of 2-H-1,3,2-diazaphospholene benefits from the unsaturated skeleton.

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Section: Contrastingwiththewell-knownproticreactivityofconven-mentioning

confidence: 99%

A Nucleophilicity Scale for the Reactivity of Diazaphospholenium Hydrides: Structural Insights and Synthetic Applications

2019

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“…3 Recent interest in metal-free hydride donors as catalysts for hydrogen activation further solidies the importance of hydride transfer reactions. [4][5][6][7][8] Thermodynamic hydricity is the standard Gibbs free energy change for the dissociation of a hydride donor RH to R + and H À . These values are always positive and therefore, lower values indicate a better hydride donor.…”

Section: Introductionmentioning

confidence: 99%

“…These values are always positive and therefore, lower values indicate a better hydride donor. 6 Kinetic hydricity is usually expressed in terms of a nucleophilicity factor N, which is an empirical value obtained from rate constants for the reaction of hydride donors with reference acceptors. 6,9,10 Stronger hydride donors have a higher N value.…”

Section: Introductionmentioning

confidence: 99%

Kinetic hydricity of silane hydrides in the gas phase

Krajewski

Niu

et al. 2019

Chem. Sci.

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Herein, gas phase studies of the kinetic hydricity of a series of silane hydrides are described. An understanding of hydricity is important as hydride reactions figure largely in many processes, including organic synthesis, photoelectrocatalysis, and hydrogen activation. We find that hydricity trends in the gas phase differ from those in solution, revealing the effect of solvent. Calculations and further experiments, including H/D studies, were used to delve into the reactivity and structure of the reactants. These studies also represent a first step toward systematically understanding nucleophilicity and electrophilicity in the absence of solvent. Fig. 1 General hydride transfer reaction studied herein. † Electronic supplementary information (ESI) available: Cartesian coordinates for all calculated species, full citations for references with greater than 16 authors, and representative mass spectra are available. See

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“…Enthalpy and free energy measurements of organic hydride donors and a few transition hydrides at 298 K predict a decrease in ∆GH-with increasing temperature. 12,46,47 The new thermochemical methodology enabled the first experimental validation of this prediction and provide a quantitative estimate of the temperature dependence of hydricity.…”

Section: Temperature Effects On Thermodynamic Hydricity δGh-mentioning

confidence: 98%

Temperature and Solvent Effects on H2 Splitting and Hydricity: Ramifications on CO2 Hydrogenation by a Rhenium Pincer Catalyst

Hu¹,

Bruch²,

Miller³

2020

Preprint

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The catalytic hydrogenation of carbon dioxide holds immense promise for applications in sustainable fuel synthesis and hydrogen storage. Mechanistic studies that connect thermodynamic parameters with the kinetics of catalysis can provide new understanding and guide predictive design of improved catalysts. Reported here are thermochemical and kinetic analyses of a new pincer-ligated rhenium complex (tBuPOCOP)Re(CO)2(tBuPOCOP = 2,6-bis(di-tert-butylphosphinito)phenyl) that catalyzes CO2 hydrogenation to formate with faster rates at lower temperature. Because the catalyst follows the prototypical “outer sphere” hydrogenation mechanism, comprehensive studies of temperature and solvent effects on the H2splitting and hydride transfer steps are expected to be relevant to many other catalysts. Strikingly large entropy associated with cleavage of H2 results in a strong temperature dependence on the concentration of [(tBuPOCOP)Re(CO)2H]– present during catalysis, which is further impacted by changing the solvent from toluene to tetrahydrofuran to acetonitrile. New methods for determining the hydricity of metal hydrides and formate at temperatures other than 298 K were developed, providing insight into how temperature can influence the favorability of hydride transfer during catalysis. These thermochemical insights guided the selection of conditions for CO2 hydrogenation to formate with high activity (up to 364 h–1 at 1 atm or 3330 h–1 at 20 atm of 1:1 H2 CO2). In cases where hydride transfer is the highest individual kinetic barrier, entropic contributions to outer sphere H2 splitting lead to a unique temperature dependence: catalytic activity increases as temperature decreases in tetrahydrofuran (200-fold increase upon cooling from 50 to 0 °C) and toluene (4-fold increase upon cooling from 100 to 50 °C). Ramifications on catalyst structure-function relationships are discussed, including comparisons between “outer sphere” mechanisms and metal–ligand cooperation mechanisms.

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Thermodynamic and kinetic hydricities of metal-free hydrides

Cited by 134 publications

References 218 publications

A Nucleophilicity Scale for the Reactivity of Diazaphospholenium Hydrides: Structural Insights and Synthetic Applications

A Nucleophilicity Scale for the Reactivity of Diazaphospholenium Hydrides: Structural Insights and Synthetic Applications

Kinetic hydricity of silane hydrides in the gas phase

Temperature and Solvent Effects on H2 Splitting and Hydricity: Ramifications on CO2 Hydrogenation by a Rhenium Pincer Catalyst

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