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• W2108851344 endingPage "17678" @default.
• W2108851344 startingPage "17673" @default.
• W2108851344 abstract "Liberating H2: Static and dynamic density-functional studies reveal that a metal-free Brønsted acid triggered dehydrocoupling of ammonia–borane initiates through the protonation of a BH bond. This protonation leads to the formation of an intermediate containing a pentacoordinate boron centre and results in the loss of H2 from the hypervalent boron centre (see figure). The catalytic release of dihydrogen (H2) from ammonia–borane (NH3BH3, AB) and related amine–borane compounds has received a lot of attention in recent years.1–7 These reactions have potential applications in hydrogen storage1–7 and in the synthesis of inorganic polymers.8 However, achieving sustainability for hydrogen storage through this route is still burdened with several challenges.1 Recent advances in recovering AB from dehydrogenated fuel have raised hopes within the hydrogen-storage community.6 AB, by its own virtue, is a fascinating molecule; AB belongs to a rare class of molecules that possess oppositely charged hydrogen atoms on adjacent atoms. This attribute can be exploited to facilitate catalytic H2 release from AB and from similar hydrogenated boron nitride materials.4 In fact, several elegant transition-metal catalysts have been devised to catalyse the dehydrogenation of AB under moderate-to-ambient conditions.3 Following these experiments, many theoretical studies have been carried out to elucidate the mechanistic details of this intriguing phenomenon.5 The current mechanistic understanding is that the dehydrogenation of AB propagates either through the concerted loss of a proton (from the N atom of AB) and loss of a hydride anion (from the B atom of AB)5a, 5b or through stepwise NH activation (proton loss from AB) followed by BH activation (hydride loss from AB) or the loss of hydrogen atoms in the reverse sequence.5c–5e Baker and co-workers made a significant breakthrough by showing that H2 release from AB can be triggered using non-metal agents, such as organic acids (for example, triflic acid) and electron-deficient boron-based Lewis acids.7 Their preliminary assumption was that the acid-triggered dehydrogenation of AB commences through hydride loss from AB.7 Notably, the current consensus is that AB dehydrocoupling only propagates through classical intermediates.5 On the contrary, our theoretical investigations into hydrogen release from AB (with a triflic acid solution in diglyme), reveals another possibility, in which AB dehydrocoupling propagates through a non-classical pentacoordinate boron-containing species, NH3BH4+. This species is an isolobal analogue of the hypervalent compound, BH5.9 Over several decades BH5 has generated a significant amount of interest among theoreticians and experimentalists. Olah and co-workers were the first to propose the existence of a BH5 species to explain the formation of copious amounts of H2, along with HD and D2, in the reaction of NaBH4 with D2SO4.10 This proposal was followed by several theoretical studies by the research groups of Kutzelnigg, Bartlett and Schaefer, studies that were not only focussed on the structural determination of this BH5 species, but also were directed towards understanding the origin of isotope scrambling in such species.11 Olah and co-workers have theoretically predicted the optimised geometries of similar hypervalent boron containing species, including NH3BH4+.9 However, to date there are no theoretical studies that have provided in-depth insights regarding the molecular events leading to the formation of such a non-classical pentacoordinate species in a reaction medium. Herein, by using a combined quantum chemical and Car–Parrinello molecular dynamics (MD) study we provide the details of AB dehydrogenation in the presence of explicit solvent molecules. This study shows, for the first time, two important molecular events: formation of a pentacoordinate boron species upon protonation of a BH bond of AB and the subsequent loss of a H2 molecule from the non-classical intermediate. In fact, this is the first ab initio simulation of the release of H2 from AB that emulates experimental AB dehydrocoupling. All gas-phase intermediates and transition-state structures were optimised using the M05-2X12 hybrid density functional and 6-31++g (d,p) family basis functions. Single-point solvent-phase calculation for diglyme was carried out employing the SMD solvent model,13 along with the M05-2X functional and 6-311++g (d,p) basis functions on each atom. The solvent-phase free energies were computed after inclusion of entropic corrections to the ideal gas phase model. For carrying out quantum mechanics/molecular mechanics (QM/MM) MD simulations, an ammonia–borane molecule, a protonated diglyme molecule and two other neighbouring diglyme molecules were subjected to DFT using the plane-wave basis set and Perdew–Burke–Ernzerhof (PBE) functional. The remaining 265 diglyme molecules, all within a cubic-periodic box at their equilibrium density, have been subjected to the generalised AMBER force field.14 Dynamics of the QM part was performed using the Car–Parrinello MD technique.15 QM/MM metadynamics simulations were carried out to simulate the complete release of H2.16 Further details on the quantum chemical computations and molecular simulations are provided in the Supporting Information. In an experiment carried out by Baker et al. AB was added to a solution of triflic acid in diglyme.7 This solution is expected to contain protonated species because a strong acid, such as triflic acid in diglyme, is likely to transfer its proton to the solvent O atoms. Our computations reveal that ion pair 1, comprising of a triflate anion hydrogen bonded to a proton, trapped in diglyme, is a stable species that is formed initially. Interestingly, we found that through a series of low-barrier proton exchanges and ion-pair dissociations, an isolated protonated diglyme species, 2 (see Figure 2), along with composite anionic species 3, which consists of a triflate anion hydrogen bonded to triflic acid, can be formed from two ion pairs, 1, at a free-energy cost of 0.7 kcal mol−1 (see Figures 1 and 3).17 Hence, it is expected that species 2 can be found in abundance at room temperature, and slightly elevated temperatures in a solution of diglyme and triflic acid (the relative free-energy profile for formation of protonated diglyme is shown in Figure 3). Species 2 can form a hydrogen bond to AB, but cannot facilitate hydride loss from AB or the loss of H2. Species 2 can easily isomerise to three other low-lying protonated diglyme forms (see Figure 2). Of these protonated diglyme isomers, 2 b and 2 c readily react with AB to form 4 a and 4 b (Figure 4), respectively; intermediate 4 a is slightly more stable than intermediate 4 b. The proton attached to the O atom in species 2 b forms a hydrogen bond with the hydride attached to the boron end of AB, forming intermediate 4 a. The proton does not induce hydride loss from the AB molecule, as has been suggested earlier by Baker and co-workers.7 We were unable to locate any transition state corresponding to the loss of a hydride from AB. Rather, the proton gets transferred to the BH bond of the AB moiety, leading to the formation of non-classical species 5, through Ts[4a5], which is a proton-transfer transition state (see Figure 5). Species 5 is indeed non-classical pentacoordinate boron species NH3BH4+, which has a diglyme molecule in its primary solvation shell. The bond parameters (see Figure 5) of the solvated NH3BH4+ species suggest that this species is the same as that proposed by Olah and co-workers;9 however, in this case, the NH3BH4+ species is hydrogen bonded to a neighbouring diglyme molecule. The predicted free energy of activation for formation of 5 from 4a is 0.4 kcal mol−1. NH3BH4+ species 5 has a five-coordinate boron atom and is reminiscent of other non-classical entities, such as BH5 and CH5+.9 We found that AB dehydrogenation propagates through a similar non-classical species, the lowest-barrier route to dihydrogen release in this particular case. This non-classical species loses H2 through transition state Ts[5′6] (see Figure 5). The overall barrier for dehydrogenation through this unexpected route is predicted to be 13.1 kcal mol−1 (see Figure 6). To the best of our knowledge, instances of AB dehydrogenation propagating through a hypervalent pentacoordinate boron intermediate are extremely rare, or rather non-existent, in the scientific literature. Our unanticipated finding suggests that such species can be formed from AB in presence of a proton source and may trigger AB dehydrogenation. Optimised structures of four different conformers of protonated diglyme: 2, 2 a, 2 b and 2 c. Bond lengths are shown in Å. C and H atoms are shown in black and white, respectively. Formation of protonated diglyme 2 from a solution of diglyme and triflic acid. Relative free-energy profile for formation of protonated diglyme in a diglyme solution of triflic acid. Triflic acid, triflate anion and diglyme are denoted as Tf, Tfa and D, respectively. Optimised structures of different conformers of protonated diglyme and ammonia–borane adduct, 4 a and 4 b. Bond lengths are shown in Å. C and H atoms are shown in black and white, respectively. Optimised structures of Ts[4a5], 5, 5′ and Ts[5′6]. Bond lengths are shown in Å. C and H atoms are shown in black and white, respectively. Relative free-energy profile for hydrogen generation from ammonia–borane (AB) in the presence of triflic acid in diglyme solution. We have also conducted QM/MM MD simulations to investigate the viability of the events traced by the quantum chemical computations in the presence of explicit solvent molecules. Such simulations could incorporate thermal fluctuations and finite solvent effects. We started the QM/MM simulation from thermally equilibrated structure 4 a.17 During the canonical-ensemble MD simulation, we observed that the proton is transferred from the protonated diglyme molecule, in the AB primary solvation sphere, to a hydridic BH bond within 1 picosecond at 300 K (see Figure 7).18 This observation indicates that the barrier for proton transfer from diglyme to AB is less than 1 kBT at room temperature. Thus, the QM/MM simulation further strengthens the existence of a transient solvated NH3BH4+. However, we did not observe H2 release from this pentacoordinate species upon carrying out the dynamics for a longer time. Hence, we turned to metadynamics, which is useful for capturing plausible rare events and for estimating the associated free-energy barriers.17 Desorption of H2 from the complex required a free energy of approximately 6.7 kcal mol−1 (see Figure 8). The static QM calculations carried out by using M05-2X, however, predicted a 0.4 kcal mol−1 free-energy barrier for Ts[5′6] (3.4 kcal mol−1 by using the PBE functional). This minor disagreement could be due to either the difference in the functionals or the absence of explicit solvent molecules and finite temperature effects in the static calculations. Change in BH distance (red) and OH distance (blue) during the QM/MM canonical-ensemble simulations. Representative snapshots along the proton-transfer process are shown in the lower panel. Solvent molecules are represented by thin sticks. Reconstructed free-energy surface for the H2 release metadynamics simulation. Representative structures corresponding to minima on the free-energy surface are shown in the lower panel. The two H atoms that are released are indicated in pink. After release of stoichiometric H2 from AB by triflic acid (TfOH), the strong Lewis acidic B atom of the NH3BH2+ cation promptly reacts with a diglyme O atom to form [NH3BH2(L)]+, 6 (see Figure 9), in which L is solvent, that is, diglyme. The BO bond leads to the trapping of the NH3BH2+ species through a highly exoergic reaction (ΔG=−22.7 kcal mol−1). Baker et al. proposed, through their preliminary computational study, that NH3BH2+ is formed through a highly endothermic process.7 Although Baker et al. have confirmed the formation of 6 by using 11B NMR spectroscopy, they have not explored the role of 6 in releasing H2 from AB.7 Our theoretical prediction successfully explains the formation of 6 and simultaneous stoichiometric H2 generation on the addition of AB to a solution of a Brønsted acid. However, the process described above is not catalytic and only explains stoichiometric H2 release from AB on addition to a diglyme solution of triflic acid. The experimental findings of Baker and co-workers suggest that more equivalents of H2 are released when an excess of AB is reacted with triflic acid in diglyme.7 Optimised structures of 6, Ts67, 7 and 7′. Bond lengths are shown in Å. C and H atoms are shown in black and white, respectively. The Lewis acid–base complex between diglyme and NH3BH2+, 6, can serve as a proton source. Hence, we explored the possibility of cation 6 being a catalytic agent for H2 release from the remaining AB in the medium, (i.e. AB that had not reacted with TfOH). Species 6 can protonate another AB molecule (via Ts7′8) and generate, non-classical NH3BH4+, 5′, and simultaneously releases NH2BH2. The NH2BH2 formed can rapidly oligomerise giving rise to cyclic, linear and branched oligomers as observed by Baker et al.7 This step has an overall rate-determining free-energy barrier of 26.0 kcal mol−1. Species 5′ releases H2, regenerating catalytic agent 6 in the process. Hence, a catalytic cycle can operate for AB dehydrogenation at a free-energy barrier of 26.0 kcal mol−1 (see Figure 10). The predicted free-energy barrier for the catalytic cycle shown in Figure 11 suggests that the cycle can operate at the experimental temperature reported by Baker et al.7 (see Figure 12 for optimised structures of Ts7′8 and 8). Species 6 can be converted into species 7 through a nucleophilic substitution process with a 13.8 kcal mol−1 free-energy activation barrier (see Figures 9 and 10). The free energy of formation of species 7 from species 6 is favourable by only 0.4 kcal mol−1, a result that could indicate that these species are in equilibrium. However, given that there is a large excess of solvent in the reaction medium compared with the reactants, the concentration of species 6 is likely to be much higher than that of 7. In the previous study by Baker et al. it was suggested that excess H2 is liberated by a non-catalytic BN chain-growth route from species 7 by simultaneous BN bond formation and hydrogen liberation, although suitable transition states for the proposed pathway were not identified.7 In contrast to the mechanistic proposal by Baker et al., we found that the proposed mechanism is unlikely to operate because the activation barrier (ΔG≠ is 56.8 kcal mol−1) along that route is insurmountable under the reported experimental conditions.17 We have also explored the possibility of species 7 playing a similar catalytic role to species 6. However, such a catalytic cycle is predicted to have a much higher activation barrier (ΔG≠ is 30.6 kcal mol−1).17 From our theoretical model calculations, we can conclude that after stoichiometric H2 release from AB (with an equivalent amount of TfOH), excess H2 (ca. 0.6 equivalents) is liberated solely by the catalytic action of 6, not through the dehydrocoupling pathway suggested by Baker et al.7 Relative free-energy barrier for hydrogen generation from AB by catalyst 6. Schematic representation of AB dehydrogenation pathway using triflic acid in diglyme solution. Optimised structures of Ts7′8 and 8. Bond lengths are shown in Å. CH hydrogen atoms are omitted for clarity. C atoms are in shown in black. Baker et al. reported that formation of μ-aminodiborane is dependent on the amount of catalyst added to the solution and suggested that μ-aminodiborane is possibly responsible for inhibiting H2 release from AB by an acid.7 We found that μ-aminodiborane formation occurs by a pathway that is different from the one proposed by Baker et al.7 We have previously shown that a BN butane analogue could be a possible by-product of NH2BH2 oligomerisation.19 This analogue may not form by the pathway suggested in the study by Baker et al.7, 17 This deactivation pathway [Eq. (1)] explains that μ-aminodiborane formation is dependent on the concentration of acid-initiated catalyst 6. This deactivation pathway may not be operative at low catalyst concentration, hence Baker et al. observed 1.3 equivalents of H2 production at a low concentration of TfOH.7 In the presence of less acid, species 6 can further react with cyclic oligomers or other oligomeric species of NH2BH2 to produce excess H2, and can generate a new proton source, which is inactive or is not a dehydrogenation catalyst under the reaction conditions. The mechanistic picture outlined in this work is in good agreement with the experimental findings of Baker et al. for triflic acid in diglyme reacting with AB. In summary, we present a convincing testimony for an unprecedented route for H2 release from AB under metal-free conditions that propagates through a non-classical pentacoordinate boron species. The suggested catalytic cycle is unique because it operates through formation of a non-classical pentacoordinate boron containing species as an intermediate. The use of boronium salts as catalysts for the dehydrogenation of AB is rare. Our findings strongly suggest that protonation of the BH bond can be explored to design metal-free catalysts for H2 release from AB under mild conditions. A. Paul would like to thank the DST FASTRACK (NO-SR/FT/CS-118/2011) and INT/NOR/RCN/P-03/2011 projects for providing research funds. S. B. and A. B. would like to acknowledge Council of Scientific and Industrial research (CSIR). As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article." @default.
• W2108851344 created "2016-06-24" @default.
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• W2108851344 date "2013-12-09" @default.
• W2108851344 modified "2023-10-18" @default.
• W2108851344 title "Ammonia-Borane Dehydrogenation by Means of an Unexpected Pentacoordinate Boron Species: Insights from Density Functional and Molecular Dynamics Studies" @default.
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• W2108851344 doi "https://doi.org/10.1002/chem.201303263" @default.
• W2108851344 hasPubMedId "https://pubmed.ncbi.nlm.nih.gov/24323946" @default.
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