Surprisingly Flexible Oxonium/Borohydride Ion Pair Configurations

Privalov

2018

J. Phys. Chem. B

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We have computationally evaluated water as an active Lewis base (LB) and introduced the borohydride/hydronium intermediate in the mechanism of B(CF)-catalyzed hydrogenation of carbonyl compounds with H in wet/moist ether. Our calculations extend the known frustrated Lewis pair mechanism of this reaction toward the inclusion of water as the active participant in all steps. Although the definition of the zero-energy point interweaves in comparison of the scenarios with and without water, we will be able to show that (i) water (hydrogen bonded to its molecular environment) can, in principle, act as a reasonably viable LB in cooperation with the borane Lewis acid such as B(CF) but relatively a strong borane-water complexation can be the hindering factor; (ii) the herein-proposed borohydride/hydronium intermediates with the hydronium cation having three OH···ether hydrogen bonds or a combination of the OH···ether/OH···ketone hydrogen bonds appear to be as valid as the previously considered borohydride/oxonium or borohydride/oxocarbenium intermediates; (iii) the proton-coupled hydride transfer from the borohydride/hydronium to a ketone (acetone) has a reasonably low barrier. Our findings could be useful for better mechanistic understanding and further development of the aforementioned reaction.

Section: Resultsmentioning

confidence: 99%

Water and a Borohydride/Hydronium Intermediate in the Borane-Catalyzed Hydrogenation of Carbonyl Compounds with H₂ in Wet Ether: A Computational Study

Privalov

2018

J. Phys. Chem. B

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“…Second, the H 2 activation is predominantly the rate-determining step in hydrogenation reactions, whereas the proton and hydride transfers from the product ion pair, LA–H (−) ··· (+) H–LB ( 1 ), to the substrates generally either have a small barrier or are entirely barrier-less. 19 , 39 , 44 , 45 …”

Section: Introductionmentioning

confidence: 99%

“…Second, the H 2 activation is predominantly the rate-determining step in hydrogenation reactions, whereas the proton and hydride transfers from the product ion pair, LA−H (−) ••• (+) H−LB (1), to the substrates generally either have a small barrier or are entirely barrierless. 19,39,44,45 The paper is organized as follows. First, we present the formation of the initial adduct between the LA and LB and the energetics and structural factors that affect this complexation.…”

Section: Introductionmentioning

confidence: 99%

“…The reaction energy of the hydrogen activation is based on the reaction Here, we focus our attention on this first step and therefore do not simulate the entire hydrogenation reaction process, including substrate molecules, because this step is common to all hydrogenation reactions. Second, the H 2 activation is predominantly the rate-determining step in hydrogenation reactions, whereas the proton and hydride transfers from the product ion pair, LA–H (−) ··· (+) H–LB ( 1 ), to the substrates generally either have a small barrier or are entirely barrier-less. ,,, …”

Section: Introductionmentioning

confidence: 99%

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

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.

“…In the present work, we investigate the reaction of CO 2 + H 2 ⇌ HCOOH, reaction 1 , inside the cavity of a Lewis pair (LP)-functionalized UiO-66 MOF. The recent findings obtained by advanced ab initio molecular dynamics (AIMD) simulations and the possibility of alternative pathways for the hydrogenation of carbonyl compounds suggest that the nature of complex reactions, e.g., FLP-mediated molecular activation, could be challenged in a dynamic picture. − This motivated us to investigate reaction 1 using the AIMD methodology, for the first time.…”

Section: Introductionmentioning

confidence: 99%

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

2020

J. Phys. Chem. C

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The reaction between H2 and CO2 catalyzed by an intramolecular frustrated Lewis pair, which is covalently bonded to a UiO-66 metal–organic framework (MOF), is considered in this work. Free energy surfaces (FESs) for this reaction are generated throughout finite-temperature density functional theory (DFT) metadynamics (MD) simulations. The simulated FESs indicate an alternative stepwise pathway for the hydrogenation of CO2. Furthermore, indications of weaker binding of CO2 than H2 to the Lewis pair centers have been observed via metadynamics simulations. These findings were unknown from the results of static-DFT calculations, which proposed a concerted reduction of CO2. The results of the present work may influence the design of new efficient heterogeneous Lewis pair (LP)-functionalized MOFs to catalyze capture and conversion of CO2 to high-value chemicals.