Alternative Pathway of CO<sub>2</sub> Hydrogenation by Lewis-Pair-Functionalized UiO-66 MOF Revealed by Metadynamics Simulations

Heshmat, Mojgan

doi:10.1021/acs.jpcc.0c01088

Cited by 29 publications

(16 citation statements)

References 39 publications

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“… 7 − 9 The application of FLP chemistry has been expanded and, during the past decade, developed to include a broad range of new reactions and modifications of advanced chemical systems, including heterogeneous and solid-phase FLPs and frustrated radical pairs. 10 − 16 The mechanistic aspects of FLP systems have been subjected to theoretical investigations. The optimal distance of the LA/LB centers has been determined by modifying the LA and LB structures to increase the reactivity and the efficiency of FLPs.…”

Section: Introductionmentioning

confidence: 99%

Optimizing the Energetics of FLP-Type H₂ Activation by Modulating the Electronic and Structural Properties of the Lewis Acids: A DFT Study

Heshmat

Ensing

2020

J. Phys. Chem. A

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The great potential of frustrated Lewis pairs (FLPs) as metal-free catalysts for activation of molecular hydrogen has attracted increasing interest as an alternative to transition-metal catalysts. However, the complexity of FLP systems, involving the simultaneous interaction of three molecules, impedes a detailed understanding of the activation mechanism and the individual roles of the Lewis acid (LA) and Lewis base (LB). In the present work, using density functional theory (DFT) calculations, we examine the reactivity of 75 FLPs for the H 2 splitting reaction, including a series of experimentally investigated LAs combined with conventional phosphine-based ( t Bu 3 P) and oxygen-based (i.e., ethereal solvent) Lewis bases. We find that the catalytic activity of the FLP is the result of a delicate balance of the LA and LB strengths and their bulkiness. The H 2 splitting reaction can be changed from endergonic to exergonic by tuning the electrophilicity of the LA. Also, a more nucleophilic LB results in a more stable ion pair product and a lower barrier for the hydrogen splitting. The bulkiness of the LB leads to an early transition state to reduce steric hindrance and lower the barrier height. The bulkiness of the fragments determines the cavity size in the FLP complex, and a large cavity allows for a larger charge separation in the ion pair configuration. A shorter proton–hydride distance in this product complex correlates with a stronger attraction between the fragments, which forms more reactive ion pairs and facilitates the proton and hydride donations in the subsequent hydrogenation process. These insights may help with rationalizing the experimentally observed reactivities of FLPs and with designing better FLP systems for hydrogenation catalysis and hydrogen storage.

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Section: Introductionmentioning

confidence: 99%

Optimizing the Energetics of FLP-Type H₂ Activation by Modulating the Electronic and Structural Properties of the Lewis Acids: A DFT Study

Heshmat

Ensing

2020

J. Phys. Chem. A

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“…1a). The reaction was confirmed by 11 B NMR spectroscopy after dissolving the product in bromobenzene (Fig. 1b) which shows the 1 Hcoupled doublet resonance at -22.5 ppm characteristic of the ion pair.…”

Section: Nmr Spectroscopymentioning

confidence: 79%

Heterolytic Scission of Hydrogen Within a Crystalline Frustrated Lewis Pair

et al. 2020

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We report the heterolysis of molecular hydrogen under ambient conditions by the crystalline frustrated Lewis pair (FLP) 1-{2-[bis(pentafluorophenyl)boryl]phenyl}-2,2,6,6tetramethylpiperidine (KCAT). The gas-solid reaction provides an approach to prepare the solvent-free, polycrystalline ion pair KCATH2 through a single crystal to single crystal transformation. The crystal lattice of KCATH2 increases in size relative to the parent KCAT by approximately 2%. Microscopy was used to follow the transformation of the highly colored red/orange KCAT to the colorless KCATH2 over a period of 2 hours at 300 K under a flow of H2 gas. There is no evidence of crystal decrepitation during hydrogen uptake. Inelastic neutron scattering employed over a temperature range from 4 -200 K did not provide evidence for the formation of polarized H2 in a pre-cursor complex within the crystal at low temperatures and high pressures. However, at 300 K the INS spectrum of KCAT transformed to the INS spectrum of KCATH2. Computational methods suggest that the driving force is more favorable in the solid state compared to the solution or gas phase, but the addition of H2 into the KCAT crystal is unfavorable. Ab Initio methods were used to calculate the INS spectra of KCAT, KCATH2 and a possible precursor complex of H2 in the pocket between the B and N of crystalline KCAT. Ex-situ NMR showed that the transformation from KCAT to KCATH2 is quantitative and our results suggest that the hydrogen heterolysis process occurs via H2 diffusion into the FLP crystal with a rate-limiting movement of H2 from inactive positions to reactive sites.

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“…This entropic contribution is unseen in the static‐DFT calculations, however, influences the ΔG ≠ calculated by dynamic‐DFT simulations. The notion that three‐dimensional flexibility of molecular pairing is connected with entropic gain has been considered in previous studies in FLP‐chemistry‐, [46‐,49] and is beyond the scope of the present work that addresses electronic properties of carbon and boron compounds as Lewis acids.…”

Section: Resultsmentioning

confidence: 99%

Lewis Acidity of Carbon in Activated Carbonyl Group vs. B(C₆F₅)₃ for Metal‐Free Catalysis of Hydrogenation of Carbonyl Compounds

Heshmat

2021

ChemPhysChem

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In this work, using DFT calculations, we investigated Lewis acidities of carbon (in activated carbonyl group) in comparison to the B(C 6 F 5 ) 3 in combination with dioxane as the Lewis base (LB) for metal-free catalysis of heterolytic H 2 splitting and hydrogenation of carbonyl compounds. We found that in case of carbon as the Lewis acid (LA) the reaction is controlled by frontier molecular orbital interactions between the H 2 and LA-LB fragments at shorter distances. The steric effects can be reduced by electrophilic substitutions on the carbonyl carbon. Synergic combination between stronger orbital interactions and reduced steric effects can lower the barrier of the H 2 splitting below 10 kcal/mol. With the B(C 6 F 5 ) 3 , the H 2 splitting is controlled by electrostatic interactions, which cause to form an early transition state. An advantage of employing Lewis acidity of the activated carbonyl carbon for hydrogenation is that the hydride-type attack and hydrogenation of the C=O bond occur in a single step throughout H 2 splitting. Hence, stronger Lewis acidity of the C(C=O) reinforces hydrogenation without prohibition of the hydride delivery.

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Alternative Pathway of CO₂ Hydrogenation by Lewis-Pair-Functionalized UiO-66 MOF Revealed by Metadynamics Simulations

Cited by 29 publications

References 39 publications

Optimizing the Energetics of FLP-Type H₂ Activation by Modulating the Electronic and Structural Properties of the Lewis Acids: A DFT Study

Optimizing the Energetics of FLP-Type H₂ Activation by Modulating the Electronic and Structural Properties of the Lewis Acids: A DFT Study

Heterolytic Scission of Hydrogen Within a Crystalline Frustrated Lewis Pair

Lewis Acidity of Carbon in Activated Carbonyl Group vs. B(C₆F₅)₃ for Metal‐Free Catalysis of Hydrogenation of Carbonyl Compounds

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Alternative Pathway of CO2 Hydrogenation by Lewis-Pair-Functionalized UiO-66 MOF Revealed by Metadynamics Simulations

Cited by 29 publications

References 39 publications

Optimizing the Energetics of FLP-Type H2 Activation by Modulating the Electronic and Structural Properties of the Lewis Acids: A DFT Study

Optimizing the Energetics of FLP-Type H2 Activation by Modulating the Electronic and Structural Properties of the Lewis Acids: A DFT Study

Heterolytic Scission of Hydrogen Within a Crystalline Frustrated Lewis Pair

Lewis Acidity of Carbon in Activated Carbonyl Group vs. B(C6F5)3 for Metal‐Free Catalysis of Hydrogenation of Carbonyl Compounds

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Alternative Pathway of CO₂ Hydrogenation by Lewis-Pair-Functionalized UiO-66 MOF Revealed by Metadynamics Simulations

Optimizing the Energetics of FLP-Type H₂ Activation by Modulating the Electronic and Structural Properties of the Lewis Acids: A DFT Study

Optimizing the Energetics of FLP-Type H₂ Activation by Modulating the Electronic and Structural Properties of the Lewis Acids: A DFT Study

Lewis Acidity of Carbon in Activated Carbonyl Group vs. B(C₆F₅)₃ for Metal‐Free Catalysis of Hydrogenation of Carbonyl Compounds