Cp* non-innocence and the implications of (η4-Cp*H)Rh intermediates in the hydrogenation of CO2, NAD+, amino-borane, and the Cp* framework – a computational study

Pal, Shrinwantu

doi:10.1039/d2dt03611h

Cited by 6 publications

(5 citation statements)

References 58 publications

(108 reference statements)

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“…It is noteworthy that alcohols as a necessary evil (i.e., uneconomical B–H protonolysis leading to in situ borane formation) are not required when either a better electrophile or a stronger hydride donor is involved; indeed, the uncatalyzed reduction of CO 2 (a better electrophile) with BH 4 – as reported by Knopf and Cummins and the reduction of cyclohexanone with triethylborohydride (aptly named superhydride owing to its superior hydride donating ability) both occur in aprotic solvents and, as calculated in this work, involve kinetically accessible barriers associated with intermolecular hydride transfers.…”

Section: Introductionmentioning

confidence: 99%

Protonolysis of BH₄^– Leads to Intermediacy of BH₃-σ(H₂) That Evolves H₂ and Furnishes Borane as the Key Reducing Agent

Pal

2023

Organometallics

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Borohydride (BH 4 − ) is a ubiquitous reducing agent in chemistry despite its apparent inability to transfer hydride fragments without assistance. In protic solvents such as methanol, B−H protonolysis and H 2 evolution furnish borane, which serves as the key hydride delivery agent in the reduction of cyclohexanone. Contrasting with widely accepted but inaccessible mechanisms involving direct (intermolecular) hydride transfer from BH 4 − , in this work, DFT calculations performed in close conjunction with H/D exchange studies unambiguously justify the use of alcohol as a necessary evil that renders substantial kinetic advantages in intramolecular hydride transfer, albeit requiring excess BH 4 − to compensate for the protonolytic loss of available B−H fragments as H 2 .

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Section: Introductionmentioning

confidence: 99%

Protonolysis of BH₄^– Leads to Intermediacy of BH₃-σ(H₂) That Evolves H₂ and Furnishes Borane as the Key Reducing Agent

Pal

2023

Organometallics

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“…17,20,21 Clearly the reactivity of these complexes is strongly influenced by the supporting bidentate chelating ligand, but it is not apparent what ligand features (e.g., donor atom identity, metal-ligand bite angle, or steric bulk) promote or hinder catalysis. Although previous computational studies have investigated the mechanism for H 2 generation, 22,23 this study tries to understand in detail what the role of the ligand is, which may provide key insights for the design of better catalysts. The ligand-based features of these structures have then been systematically varied to ascertain their effect on the reaction energy of each step.…”

Section: Introductionmentioning

confidence: 99%

Computational Insights into Ligand Influences on Hydrogen Generation with [Cp*Rh] Hydrides

Balduf

Blakemore

Caricato

2023

Preprint

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This work reports a computational investigation of the effect of ancillary ligands on the activity of a Rh catalyst for hydrogen evolution based on the [Cp*Rh] motif (Cp* = η5-pentamethylcyclopentadienyl). Specifically, we investigate why a bipyridyl (bpy) ligand leads to H2 generation but diphenylphosphino-based (dpp) ligands do not. We compare the full ligands to simplified models and systematically vary structural features to ascertain their effect on the reaction energy of each catalytic step. The calculations, based on density functional theory, show that the main effect on reactivity is the choice of linker atom, followed by its coordination. In particular, P stabilizes the intermediate Rh-hydride species by donating electron density to the Rh, thus inhibiting the reaction towards H2 generation. Conversely N, a more electron withdrawing center, favors H2 generation at the price of destabilizing the hydride intermediate, which cannot be isolated experimentally and makes determining the mechanism for this reaction more difficult. We also find that steric effects of bulky substituents on the main ligand scaffold can lead to large effects on the reactivity, which may be challenging to fine-tune. On the other hand, structural features like the bite angle of the bidentate ligand have a much smaller impact on reactivity. Therefore, we propose that the choice of linker atom is key for the catalytic activity of this species, which can be further fine-tuned by a proper choice of electron-directing groups on the ligand scaffold.

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“…23,26,27 Clearly the reactivity of these complexes is strongly influenced by the supporting bidentate chelating ligand, but it is not apparent what ligand features (e.g., donor atom identity, metal−ligand bite angle, or steric bulk) promote or hinder catalysis. Although previous computational studies have investigated the mechanism for H 2 generation, 28,29 this study tries to understand in detail what the role of the ligand is, which may provide key insights for the design of better catalysts. The ligand-based features of these structures were then systematically varied to ascertain their effect on the reaction energy of each step.…”

Section: Introductionmentioning

confidence: 99%

“…On the other hand, if the bpy ligand is replaced with bis(diphenylphosphino) methane ( dppm ), bis(diphenylphosphino)benzene ( dppb ), or bis(diphenylphosphino)-9,9′-dimethylxanthene ( dppx ), the Rh(III)H species can be generated but is stabilized significantly, implying from experimental work that Reaction 2 is no longer favorable and cannot proceed, even upon exposure of the Rh(III)H species to strong acid(s). ,, Clearly the reactivity of these complexes is strongly influenced by the supporting bidentate chelating ligand, but it is not apparent what ligand features (e.g., donor atom identity, metal–ligand bite angle, or steric bulk) promote or hinder catalysis. Although previous computational studies have investigated the mechanism for H 2 generation, , this study tries to understand in detail what the role of the ligand is, which may provide key insights for the design of better catalysts. The ligand-based features of these structures were then systematically varied to ascertain their effect on the reaction energy of each step.…”

Section: Introductionmentioning

confidence: 99%

Computational Insights into the Influence of Ligands on Hydrogen Generation with [Cp*Rh] Hydrides

2023

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This work reports a computational investigation of the effect of ancillary ligands on the activity of an Rh catalyst for hydrogen evolution based on the [Cp*Rh] motif (Cp* = η5-pentamethylcyclopentadienyl). Specifically, we investigate why a bipyridyl (bpy) ligand leads to H2 generation but diphenylphosphino-based (dpp) ligands do not. We compare the full ligands to simplified models and systematically vary structural features to ascertain their effect on the reaction energy of each catalytic step. The calculations based on density functional theory show that the main effect on reactivity is the choice of linker atom, followed by its coordination. In particular, P stabilizes the intermediate Rh-hydride species by donating electron density to the Rh, thus inhibiting the reaction toward H2 generation. Conversely, N, a more electron-withdrawing center, favors H2 generation at the price of destabilizing the hydride intermediate, which cannot be isolated experimentally and makes determining the mechanism of this reaction more difficult. We also find that the steric effects of bulky substituents on the main ligand scaffold can lead to large effects on the reactivity, which may be challenging to fine-tune. On the other hand, structural features like the bite angle of the bidentate ligand have a much smaller impact on reactivity. Therefore, we propose that the choice of linker atom is key for the catalytic activity of this species, which can be further fine-tuned by a proper choice of electron-directing groups on the ligand scaffold.

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Cp* non-innocence and the implications of (η⁴-CpH)Rh intermediates in the hydrogenation of CO₂, NAD⁺, amino-borane, and the Cp framework – a computational study

Abstract: In hydrogenation mediated by half-sandwich complexes of Rh, CpRh(III)-H intermediates are critical hydride-delivery agents. For the bipyridine-supported complexes, a unique transformation dubbed ‘Cp non-innocence’ leads to the observation of (Cp*H)Rh(I)...

Cited by 6 publications

References 58 publications

Protonolysis of BH₄^– Leads to Intermediacy of BH₃-σ(H₂) That Evolves H₂ and Furnishes Borane as the Key Reducing Agent

Protonolysis of BH₄^– Leads to Intermediacy of BH₃-σ(H₂) That Evolves H₂ and Furnishes Borane as the Key Reducing Agent

Computational Insights into Ligand Influences on Hydrogen Generation with [Cp*Rh] Hydrides

Computational Insights into the Influence of Ligands on Hydrogen Generation with [Cp*Rh] Hydrides

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Cp* non-innocence and the implications of (η4-Cp*H)Rh intermediates in the hydrogenation of CO2, NAD+, amino-borane, and the Cp* framework – a computational study

Abstract: In hydrogenation mediated by half-sandwich complexes of Rh, Cp*Rh(III)-H intermediates are critical hydride-delivery agents. For the bipyridine-supported complexes, a unique transformation dubbed ‘Cp* non-innocence’ leads to the observation of (Cp*H)Rh(I)...

Cited by 6 publications

References 58 publications

Protonolysis of BH4– Leads to Intermediacy of BH3-σ(H2) That Evolves H2 and Furnishes Borane as the Key Reducing Agent

Protonolysis of BH4– Leads to Intermediacy of BH3-σ(H2) That Evolves H2 and Furnishes Borane as the Key Reducing Agent

Computational Insights into Ligand Influences on Hydrogen Generation with [Cp*Rh] Hydrides

Computational Insights into the Influence of Ligands on Hydrogen Generation with [Cp*Rh] Hydrides

Contact Info

Product

Resources

About

Cp* non-innocence and the implications of (η⁴-CpH)Rh intermediates in the hydrogenation of CO₂, NAD⁺, amino-borane, and the Cp framework – a computational study

Abstract: In hydrogenation mediated by half-sandwich complexes of Rh, CpRh(III)-H intermediates are critical hydride-delivery agents. For the bipyridine-supported complexes, a unique transformation dubbed ‘Cp non-innocence’ leads to the observation of (Cp*H)Rh(I)...

Protonolysis of BH₄^– Leads to Intermediacy of BH₃-σ(H₂) That Evolves H₂ and Furnishes Borane as the Key Reducing Agent

Protonolysis of BH₄^– Leads to Intermediacy of BH₃-σ(H₂) That Evolves H₂ and Furnishes Borane as the Key Reducing Agent