Dynamic Mechanisms for Ammonia Borane Thermolysis in Solvent

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Dynamic Mechanisms for Ammonia Borane Thermolysis in Solvent: Deviation from Gas-Phase Minimum-Energy Pathways Paul M. Zimmerman,† Zhiyong Zhang,‡ and Charles B. Musgrave*,§ †

Departments of Chemistry, and Chemical and Biological Engineering, University of California at Berkeley, ‡Stanford Nanofabrication Facility, Stanford University, and §Chemical and Biological Engineering, University of Colorado at Boulder

ABSTRACT The dynamic mechanisms involved in the dehydrogenation of ammonia borane are investigated using quasi-classical trajectory simulations. The effects of solvent and nuclear motion yield qualitatively different results compared to simulations where these considerations are neglected. Not only are rate-limiting barriers substantially different from the gas to solvent phase, trajectories leading from transition states branch to different products depending on the presence or lack of solvent. In addition, the formation of the diammoniate of diborane is shown to be noncompetitive in the gas phase due to the presence of a lower-barrier dehydrogenation pathway. The first comparative analysis of the pathways leading to the thermolysis of ammonia-borane is presented herein. SECTION Dynamics, Clusters, Excited States

he widespread interest in using ammonia borane (AB) for chemical storage of hydrogen is exhibited in the variety of techniques investigated for releasing H2 from AB.1-24 Although significant effort has been exerted to understand the dehydrogenation of solvent-phase AB,10,25-28 no comparative analysis of the relative activity of the various elementary mechanisms has been reported. An understanding of the detailed mechanisms is essential for enhancing the long-term stability of AB, whose decomposition follows a second-order rate law,10 which inhibits high-density hydrogen storage in solution. This study aims to describe the AB thermolysis mechanisms in detail to characterize the pathways that must be avoided to reduce unwanted AB decomposition and thus release H2 only at desired times. A key aspect of the uncatalyzed dehydrogenation of AB is the ease in which the N-B bond dissociates.27 Breaking AB's weak N-B bond can lead to the formation of species such as the ion pair isomer, diammoniate of diborane, (NH3BH2NH3)(BH4) (DADB).10,27 Previous theoretical studies have suggested that DADB releases two H2 molecules with relative ease to form two NH2BH2 molecules,25,28,29 which then rapidly oligomerize into the experimentally observed reaction products.28,30-36 Additional pathways for AB dehydrogenation have been described via catalytic assistance of BH3 or NH3.26 While these studies identified several possible reaction pathways, all employed gas-phase calculations that did not consider the effects of solvent or dynamics on reaction rates and product selectivity.37-41 Herein, we compare solution-phase simulations to gasphase simulations to understand the effects of both solvation and dynamics on AB reaction pathways. Static calculations employ the CCSD(T)/aug-cc-pVTZ coupled-cluster method42-44

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r 2011 American Chemical Society

and include conductor-like polarizable continuum model (CPCM) implicit solvent corrections45 calculated using MP2/ aug-cc-pVTZ,46,47 all performed within Gaussian03.48 We directly compare results from minimum-energy pathway (MEP) calculations using an intrinsic reaction coordinate (IRC) analysis with density functional theory (DFT)-based quasi-classical trajectory (QCT) simulations.49,50 QCT includes the zero-point motion of the orthogonal modes initiated with random phases and dynamically samples nuclear configurations in the reaction pathways, and unlike MEP, it can predict bifurcations in the reaction pathway. The form of QCT49 that we employ populates the modes with a random distribution of potential energy by displacement along each mode, with the remainder of the ZPE distributed as kinetic energy. The mode corresponding to the TS is initiated with a specified kinetic energy in the forward or reverse direction. These 0 K QCT simulations are performed using GAMESS-US51,52 with the M06-2X density functional53,54 and 6-31þþG** basis as described in detail in the Supporting Information. The detailed mechanism of AB dehydrogenation predicted by this theoretical study can be summarized as follows. First, although DADB has been assumed to be the major intermediate leading to dehydrogenation, we uncover an alternative competitive pathway that accounts for the observed low concentration of DADB.10 Second, the relative barriers in solvent for the rate-determining transition states (TS) in this chemistry are dramatically different from those in the gas Received Date: December 2, 2010 Accepted Date: January 13, 2011 Published on Web Date: January 24, 2011

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DOI: 10.1021/jz101629d |J. Phys. Chem. Lett. 2011, 2, 276–281

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Scheme 1. Summary of the Most Competitive Pathways Leading to the Dehydrogenation of the AB Dimer in the Gas Phase and Ether Solventa

Scheme 2. Important QCT Pathways in the Dehydrogenation of the AB Dimer in THF Solutiona

a

Energies are in kcal/mol, calculated using CCSD(T)/aug-cc-pVTZ, and include solvent and thermal (298 K) corrections. Reaction barriers are referenced to AB dimer, which is the overall lowest-energy intermediate. The reaction byproducts NH3 and BH3 combine with AB at high AB concentrations to yield 2 (AB;BH3;NH3), continuing the dehydrogenation cycle. NH2BH2 rapidly oligomerizes, as described previously.28

a Block arrows indicate active pathways with products predicted by QCT, while dashed arrows indicate active pathways with products predicted by MEP. Steps with barriers less than 6 kcal/mol are labeled small barrier. The reaction cycle begins with formation of an AB;AB hydrogen-bonded complex,1 which is the lowest-energy intermediate prior to reaction. Pathways proceeding through 1 f 2 f 3 f 5 f 4 or 1 f 2 f 4 are the most competitive in THF, and 1 f 6 f 7 is the most competitive in the gas phase. Previous gas-phase MEP simulations suggested that 1 f 2 f 3 f 5 f 4 and 1 f 6 f 7 were possibly competitive.25,26.

follow the gas-phase MEP. Instead, QCT shows that a proton from the NH3BH2þ cationic fragment of DADB combines with a hydride from the BH4- anion fragment to create H2 within 100 fs. Specifically, only 2 of the 43 QCT-generated trajectories resulted in formation of DADB, while the remainder resulted in H2 formation. This indicates that DADB formation is not likely in the gas phase because kinetic energy builds up as trajectories proceed from the TS toward the products. This excess kinetic energy leads the trajectory away from the MEP and instead results in fast H2 elimination. Consequently, QCT predicts that H2, NH3, BH3, and NH2BH2 are the major products in the gas phase through the same TS predicted by MEP to produce DADB (TS2-3). Similarly, QCT predicts that SN2 of AB via TS1-2 produces 6 (AB;BH3 and free NH3) due to kinetic energy transfer into NH3, although the MEP calculated by the IRC analysis predicts formation of 2. Furthermore, while the calculated MEP for DADB decomposition through TS3-5 predicts the formation of BH3, NH3BH2NH2, and H2, QCT predicts that NH3 dissociates from NH3BH2NH2 because this intermediate is only moderately stable. Therefore, following TS3-5, sufficient kinetic energy is deposited in the vibrational mode, leading to NH3 dissociation in which free NH3 and BH2NH2 are formed. We discovered a fifth gas-phase pathway for elimination of H2 from the AB dimer that has a lower barrier than the above four mechanisms. This mechanism proceeds through an 8-membered-ring TS (TS2-4) shown in Figure 1. This route is particularly favorable because NH3 operates as a proton shuttle, transferring its acidic hydrogen as its lone pair of electrons concomitantly accepts a proton from AB, whereas BH3 operates as a hydride shuttle, transferring its hydridic hydrogen as its empty p orbital concomitantly accepts a hydride from AB. This allows simultaneous transfer of a proton and hydride from NH3 and BH3 to form H2. In the

phase, and therefore, the relative competitiveness of potential pathways varies significantly between the two phases. Finally, dynamics simulations reveal that reaction trajectories from important TSs follow pathways that are not predicted by static MEP simulations. The major mechanistic conclusions of this study are outlined in Scheme 1. Gas-Phase Pathways. In the gas phase, four AB dehydrogenation pathways have been identified as potential low activation barrier routes.25-28 As illustrated in Scheme 2, two routes proceed from the AB dimer (1) through SN1 formation of NH3 and BH3, formation of adducts AB;BH3 (6) and AB;NH3 (8) and subsequent catalytic decomposition of adducts 6 and 8 via TS6-7 and TS8-9 to yield NH2BH2 and H2. An alternative pathway proceeds through TS1-2 and involves SN2 attack of AB at a second AB to yield NH3 and BH3 bound to AB (2). Complex 2 then either undergoes BH3catalyzed H2 elimination or transforms into the ion pair isomer, DADB (TS2-3). In the gas phase, DADB eliminates H2 via a significantly lower barrier than its barrier for formation. Furthermore, BH3-catalyzed H2 elimination from 2 and DADB formation through TS2-3 closely compete with one another in the gas phase, with barriers differing by approximately 2.0 kcal/mol.25 Our QCT gas-phase dynamics simulations indicate that NH3 or BH3 catalyzes AB dehydrogenation through TS6-7 and TS8-9, which produce NH2BH2 and H2, in agreement with MEP simulations. On the other hand, TS2-3 does not


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Table 1), although both reactions maintain a significantly lower barrier than uncatalyzed release of H2 from the AB monomer. We note, however, that catalysis by these species is always in competition with formation of AB by NH3 and BH3 recombination. The above results are illustrated in Scheme 2 and Figure 2, which show the favored pathways for AB dehydrogenation in solvent. The effects of dynamics and solvent on the mechanism of AB dehydrogenation are summarized in Scheme 1. The key conclusion is that predicted reactivities from simulations of both the solvent and dynamic effects differ considerably from gas-phase MEP. Furthermore, solvation significantly changes reaction barriers (by up to 10 kcal/mol), and these TSs lead to different products depending on the phase of the reaction. Therefore, predictive studies must incorporate both solvent and dynamics effects to accurately determine the active reaction pathways. The most active pathways for AB dehydrogenation must be evaluated relative to the relevant lowest-energy intermediates. For AB in vacuum, mechanisms like SN1 and SN2 are essentially irreversible because of the entropy gained by producing additional gas-phase molecules. Therefore, AB; NH3 or AB;BH3 is the proper reference point for the lowestenergy intermediates prior to dehydrogenation. However, in the solvent phase with significant AB concentration, the lowest-energy intermediate is the AB hydrogen-bonded dimer. Because rearrangement of AB;NH3 and AB;BH3 to an AB dimer and one AB can occur with barriers lower than that for AB;NH3 or AB;BH3 decomposition, these transition states must be referenced to the AB dimer and one AB (i.e., the reaction AB;NH3 þ AB;BH3 f AB dimer þ AB is exothermic). This exothermic rearrangement can occur with a small barrier and therefore dramatically increases the effective barriers for NH3 and BH3 catalysis compared to referencing to AB;NH3 or AB;BH3 as the zero-energy reference points (see Table 1). AB dehydrogenation proceeds through dissociation of a N;B bond,25,26 and this remains valid in solvent or when considering dynamical aspects. This leads to different reaction pathways in the gas and solvent phases; in the gas phase, AB;BH3 can form via SN1 or SN2 reactions, and AB;NH3 can form via SN1. Both BH3 and NH3 can serve as catalysts for AB dehydrogenation in the gas phase. Notably, the barrier for AB;BH3 decomposition in the gas phase is lower than the barrier for its formation, and indeed, this leads to BH3 catalysis being the most active gas-phase route for AB dehydrogenation. In solvent, translational motion of NH3 will be inhibited by solvent molecules, thus allowing the AB;BH3; NH3 complex to form via SN2. Therefore, in solvent, two competing mechanisms are active for AB dehydrogenation, one proceeding through DADB formation and decomposition (TS2-3 then TS3-5) and the other through direct decomposition (TS2-4). While NH3BH2NH2 is the predicted product of solvent QCT simulations for TS3-5, this species will be short-lived due to the small NH3 binding energy (less than 6 kcal/mol). Assuming that this dissociation occurs, the predicted products of the active dehydrogenation pathways in solvent (TS2-4 and TS3-5) are identical, NH2BH2, H2, NH3 and BH3.

Figure 1. The most energetically favorable mechanism for dehydrogenation of the AB dimer (TS2-4) in the gas phase. Concerted hydride and proton transfer makes this TS a relatively low barrier in gas and solution phases (see text). While the barrier for TS2-4 is 5.7 kcal/mol below the barrier for DADB formation (TS2-3) in the gas phase, it lies within 1.0 kcal/mol of TS2-3 in solvent. TS2-3 becomes competitive in solution due to dielectric stabilization of the highly polar TS TS2-3.

gas phase, the barrier for TS2-4 is 5.7 kcal/mol lower than the barrier for DADB formation via TS2-3, so that the lowestbarrier pathway for H2 elimination from the AB dimer proceeds through TS2-4. Solvent-Phase Pathways. In THF solvent (see Scheme 2 and Figure 2), the energetics for AB dehydrogenation change considerably. First, the N-B bond in solvent is significantly stronger due to favorable solvent interaction with AB's dipole. This leads to the N;B bond strength increasing to 33.7 kcal/ mol in THF, which is 6.6 kcal/mol stronger than that in the gas phase. This immediately suggests that the SN1 mechanisms will be less favorable, and therefore, the lowest-energy reference point for solvent-phase AB dehydrogenation is the hydrogen-bonded AB dimer (1). We now consider the pathways for H2 release starting from the AB dimer, which is favorable by 4.4 kcal/mol compared to separated AB in THF. Although gas-phase results predict that direct elimination of H2 from the AB dimer via TS2-4 is kinetically favored over DADB formation via TS2-3 by over 5 kcal/mol, in solution, TS2-3 is competitive because its barrier is significantly lower. This occurs because ionic species are considerably stabilized in polarizable dielectric media, and the TS for DADB formation exhibits high polarity. Although DADB is 12.1 kcal/mol uphill from the AB dimer in the gas phase, it is only 6.8 kcal/ mol uphill in THF. Furthermore, solvent-phase QCT simulations show that DADB formation is possible through TS2-3, and H2 is not directly eliminated as in the gas phase. Although DADB formation and direct H2 elimination via TS2-4 compete closely in solvent, BH3-catalyzed H2 elimination is not competitive. Because BH3-catalyzed H2 elimination from 2 proceeds without the formation of ions, it is not stabilized by solvent polarization and lies 13.5 kcal/mol higher in energy than TS2-3. Elimination of H2 from DADB via TS3-5 is not rate-limiting because TS3-5 is lower in barrier than TS2-3 in solvent. QCT predicts that the products of TS3-5 are BH3, NH3BH2NH2, and H2. The NH3;BH2NH2 bond strength is 5.3 kcal/mol, indicating that NH3 can dissociate with a small energetic cost to form NH2BH2. Although SN1 mechanisms will be disfavored in solvent, H2 release from the AB dimer yields NH3 and BH3, and therefore, catalytic dehydrogenation of AB by NH3 or BH3 remains possible. The barriers for catalysis by NH3 and BH3 increase compared to those in the gas phase (see TS8-9 and TS6-7 in


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Figure 2. Potential energy surface for the dehydrogenation of AB in THF including products predicted by QCT simulations. The paths through TS2-3 and TS2-4 are predicted to closely compete in solvent. Table 1. Energies (in kcal/mol) for the Important Reactions Leading to Dehydrogenation of AB Calculated Using CCSD(T)/aug-cc-pVTZa label

ΔE0K (gas)

ΔH298K (gas)

ΔG298K (THF)

-25.3

-27.1

-33.7

36.6

36.5

43.7

-17.1

-18.3

-16.9

AB þ BH3

37.6 23.3

37.8 23.0

47.8 29.6

AB dimer AB;BH3

-8.0

-8.4

-3.0

AB þ NH3

45.6

45.2

51.1

AB dimer

31.3

30.4

32.9

AB;BN3

-14.1

-14.3

-4.4

AB þ AB

26.3

27.0

27.7

AB dimer

13.8 -8.6

14.5 -8.7

16.8 -4.0

AB dimer AB;BH3;NH3 AB;BH3;NH3

reference

AB monomer N;B bond decomposition TS AB;BH3

AB;NH3

TS6-7

8

NH3 binding energy decomposition TS

AB dimer

TS8-9

1

AB;AB binding SN2 TS

TS1-2

AB;BH3;NH3

AB monomer

6

BH3 binding energy decomposition TS

NH3 þ BH3

2

formation NH3 binding energy

-17.6

-18.6

-18.3

decomposition TS

TS2-4

35.0

36.0

33.6

AB dimer

DADB formation TS

TS2-3

41.7

41.7

32.9

AB dimer

DADB

3

BH3 binding energy

formation decomposition TS a

TS3-5

12.1

11.9

6.8

AB dimer

31.4

31.4

25.3

AB dimer

All decomposition reactions result in H2 formation.

formation (TS2-3) in the gas phase. The third reason is that QCT dynamics simulations show that TS2-3 yields DADB in the solvent phase, but this same TS leads to direct decomposition in the gas phase. In solution, the formation of DADB becomes feasible because AB;BH3;NH3 is more likely to form; the competing TS that leads to direct decomposition has

The current study indicates that DADB is unlikely to form in the gas phase for three reasons. The first is that SN2 of AB at AB yields AB;BH3 because NH3 dissociates and is unlikely to reassociate with the AB;BH3 complex. The second is that TS2-4 allows direct decomposition of AB;BH3;NH3 and has a barrier that is 5.7 kcal/mol smaller than that for DADB


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a similar barrier compared to DADB formation, and DADB can stably form in a polar medium. The comparison of gas- and solution-phase results clearly demonstrates that gas-phase simulations are inadequate representations of AB chemistry in solvent. The dynamic, solvent-phase simulations in this study describe several unexplained experimental observations. Experiments in glyme have shown the formation of a small amount of DADB during thermal dehydrogenation of AB and rapid decomposition when solid DADB is added to solution.10,30 The relatively low barrier height for DADB decomposition compared to that for formation agrees well with these results. In contrast, gas-phase simulations predict that DADB formation is neither competitive nor stable. In addition, because DADB formation (TS2-3) is in competition with a direct dehydrogenation pathway (TS2-4) in solvent, DADB formation is not the only pathway leading to dehydrogenation. However, this does not significantly change the predicted products because all dehydrogenation mechanisms eventually lead to NH2BH2 formation. Consequently, with or without DADB as an intermediate, the product distribution observed in experiment10 can be well-described by rapid oligomerization of NH2BH2 monomers.28 Thus far, no lowbarrier mechanism has been found to directly yield any of the experimentally observed products without proceeding through the intermediate NH2BH2. This observation and the results of the present and previous studies28 strongly suggest that NH2BH2 is a key intermediate, although its extremely short lifetime makes it essentially experimentally undetectable. We suggest that kinetic energy buildup during the formation and dehydrogenation of the oligomerization intermediate, cyclic-(N2B2H7)NH2BH3, could lead to borazine at an early stage of the reaction,10 but further simulations are necessary to validate this suggestion. Also, the second-order rate law for AB dehydrogenation in solvent10 is accounted for by the involvement of two AB molecules in all of our predicted low-barrier dehydrogenation mechanisms. Furthermore, our gas-phase simulations agree well with the observed formation of NH2BH2 in matrix isolation studies55,56 and predict that NH3 and BH3 catalyses are the operating gas-phase mechanisms that dehydrogenate AB.

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SUPPORTING INFORMATION AVAILABLE Additional computational details and the full ref 48. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

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Corresponding Author: *E-mail: [email protected]. (19)

REFERENCES (1) (2) (3)

The Hydrogen Economy: NRC and NAE; The National Academies Press: Washington, DC, 2004. The Hydrogen Initiative; The American Physical Society: Ridge, NY, 2004; http://www.aps.org/public_affairs/index.cfm. Basic Research Needs for the Hydrogen Economy; Office of Science at the U.S. DOE: Washington, DC, 2004; http://www. sc.doe.gov/bes/reports/files/NHE_rpt.pdf.


(20)

(21)

280

Stephens, F. H.; Pons, V.; Baker, R. T. Ammonia-Borane: The Hydrogen Source Par Excellence?. Dalton Trans. 2007, 2613–2626. Marder, T. B. Will We Soon Be Fueling Our Automobiles with Ammonia-Borane? Angew. Chem., Int. Ed. 2007, 46, 8116–8118. Smythe, N. C.; Gordon, J. C. Ammonia Borane as a Hydrogen Carrier: Dehydrogenation and Regeneration. Eur. J. Inorg. Chem. 2010, 509. Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. B;N Compounds for Hydrogen Storage. Chem. Soc. Rev. 2009, 38, 279. Kathmann, S. M.; Parvanov, V.; Schenter, G. K.; Stowe, A. C.; Daemen, L. L.; Hartl, M.; Linehan, J. C.; Hess, N. J.; Karkamkar, J.; Autrey, T. Experimental and Computational Studies on Collective Hydrogen Dynamics in Ammonia Borane: Incoherent Inelastic Neutron Scattering. J. Chem. Phys. 2009, 130, 024507. Heldebrant, D. J.; Karkamkar, A.; Hess, N. J.; Bowden, M.; Rassat, S.; Zheng, F.; Rappe, K.; Autrey, T. The Effects of Chemical Additives on the Induction Phase in Solid-State Thermal Decomposition of Ammonia Borane. Chem. Mater. 2008, 20, 5332–5336. Shaw, W. J.; Linehan, J. C.; Szymczak, N. K.; Heldebrant, D. J.; Yonker, C.; Camaioni, D. M.; Baker, R. T.; Autrey, T. In Situ Multinuclear NMR Spectroscopic Studies of the Thermal Decomposition of Ammonia Borane in Solvent. Angew. Chem., Int. Ed. 2008, 47, 7493–7496. Bluhm, M. E.; Bradley, M. G.; Butterick, R.; Kusari, U.; Sneddon, L. G. Amineborane-Based Chemical Hydrogen Storage: Enhanced Ammonia Borane Dehydrogenation in Ionic Liquids. J. Am. Chem. Soc. 2006, 128, 7748–7749. Stephens, F. H.; Baker, R. T.; Matus, M. H.; Grant., D. J.; Dixon, D. A. Acid Initiation of Ammonia-Borane Dehydrogenation for Hydrogen Storage. Angew. Chem., Int. Ed. 2007, 46, 746–749. Kalidindi, S. B.; Indirani, M.; Jagirdar, B. R. First Row Transition Metal Ion-Assisted Ammonia-Borane Hydrolysis for Hydrogen Generation. Inorg. Chem. 2008, 47, 7424–7429. Forster, T. D.; Tuonomen, H. M.; Parvez, M.; Roesler, R. Characterization of β-B-Agostic Isomers in Zirconocene Amidoborane Complexes. J. Am. Chem. Soc. 2009, 131, 6689–6691. Dallanegra, R.; Chaplin, A. B.; Weller, A. S. Bis(sigman-amineborane) Complexes: An Unusual binding Mode at a Transition-Metal Center. Angew. Chem., Int. Ed. 2009, 48, 6875. Chaplin, A. B.; Weller, A. S. Amine- and Dimeric AminoBorane Complexes of the {Rh(PiPr3)2}þ Fragment and Their Relevance to the Transition-Metal-Mediated Dehydrocoupling of Amine-Boranes. Inorg. Chem. 2010, 49, 1111. Rousseau, R.; Schenter, G. K.; Fulton, J. L.; Linehan, J. C; Engelhard, M. H.; Autrey, T. Defining Active Catalyst Structure and Reaction Pathways from ab Initio Molecular Dynamics and Operando XAFS: Dehydrogenation of Dimethylaminoborane by Rhodium Clusters. J. Am. Chem. Soc. 2009, 131, 10516–10524. Paul, A.; Musgrave, C. B. Catalyzed Dehydrogenation of Ammonia-Borane by Iridium Dihydrogen Pincer Complex Differs from Ethane Dehydrogenation. Angew. Chem., Int. Ed. 2007, 46 (43), 8153–8156. Zimmerman, P. M.; Paul, A.; Zhang, Z.; Musgrave, C. B. The Role of Free N-Heterocyclic Carbene (NHC) in the Catalytic Dehydrogenation of Ammonia-Borane in the Nickel NHC System. Angew. Chem., Int. Ed. 2009, 48, 2201. Zimmerman, P. M.; Paul, A.; Musgrave, C. B. Catalytic Dehydrogenation of Ammonia Borane at Ni Monocarbene and Dicarbene Catalysts. Inorg. Chem. 2009, 48, 5418. Miller, A. J. M.; Bercaw, J. E. Dehydrogenation of AmineBoranes with a Frustrated Lewis Pair. Chem. Commun. 2010, 46, 1709–1711.


pubs.acs.org/JPCL

(22)

(23)

(24)

(25)

(26)

(27)

(28)

(29) (30) (31)

(32)

(33)

(34)

(35)

(36)

(37)

(38)

(39)

(40)

Luo, Y.; Ohno, K. Computational Study of Titanocene-Catalyzed Dehydrocoupling of the Adduct Me2NH Center BH3: An Intramolecular, Stepwise Mechanism. Organometallics 2007, 26, 3597–3600. Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. Base Metal Catalyzed Dehydrogenation of Ammonia-Borane for Chemical Hydrogen Storage. J. Am. Chem. Soc. 2007, 129, 1844–1845. Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. Efficient Catalysis of Ammonia Borane Dehydrogenation. J. Am. Chem. Soc. 2006, 128, 12048–12049. Nguyen, V. S.; Matus, M. H.; Grant, D. J.; Nguyen, M. T.; Dixon, D. A. Computational Study of the Release of H2 from Ammonia Borane Dimer (BH3NH3)2 and Its Ion Pair Isomers. J. Phys. Chem. A 2007, 111, 8844–8856. Nguyen, M. T.; Nguyen, V. S.; Matus, M. H.; Gopakumar, G.; Dixon, D. A. Molecular Mechanism for H2 Release from BH3NH3, Including the Catalytic Role of the Lewis Acid BH3. J. Phys. Chem. A 2007, 111, 679–690. Dixon, D. A.; Gutowski, M. Thermodynamic Properties of Molecular Borane Amines and the [BH4-][NH4þ] Salt for Chemical Hydrogen Storage Systems from ab Initio Electronic Structure Theory. J. Phys. Chem. A 2005, 109, 5129. Zimmerman, P. M.; Paul, A.; Zhang, Z.; Musgrave, C. B. Oligomerization and Autocatalysis of NH2BH2 with Ammonia-Borane. Inorg. Chem. 2009, 48, 1069–1081. Nutt, W. R.; McKee, M. L. Theoretical Study of Reaction Pathways to Borazine. Inorg. Chem. 2007, 46, 7633–7645. Mayer, E. Symmetrical Cleavage of Diborane by Ammonia in Solution. Inorg. Chem. 1972, 11, 866–869. Boddeker, K. W.; Shore, S. G.; Bunting, R. K. Boron-Nitrogen Chemistry. 1. Syntheses and Properties of New Cycloborazanes, (BH2NH2)n. J. Am. Chem. Soc. 1966, 88, 4396–4401. Wang, J. S.; Geanangel, R. A. 11B NMR Studies of the Thermal Decomposition of Ammonia Borane in Solution. Inorg. Chim. Acta 1988, 148, 185–190. Stowe, A. C.; Shaw, W. J.; Linehan, J. C.; Schmid, B.; Autrey, T. In Situ Solid State 11B MAS-NMR Studies of the Thermal Decomposition of Ammonia Borane: Mechanistic Studies of the Hydrogen Release Pathways from a Solid State Hydrogen Storage Material. Phys. Chem. Chem. Phys. 2007, 9, 1831– 1836. Chen, Y.; Fulton, J. L.; Linehan, J. C.; Autrey, T. In Situ XAFS and NMR Study of Rhodium-Catalyzed Dehydrogenation of Dimethylamine Borane. J. Am. Chem. Soc. 2005, 127, 3254–3255. Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Transition Metal-Catalyzed Formation of Boron-Nitrogen Bonds: Catalytic Dehydrocoupling of Amine-Borane Adducts to Form Aminoboranes and Borazines. J. Am. Chem. Soc. 2003, 125, 9424–9434. Clark, T. J.; Russell, C. A.; Manners, I. Homogeneous, Titanocene-Catalyzed Dehydrocoupling of Amine-Borane Adducts. J. Am. Chem. Soc. 2006, 128, 9582–9583. Ess, D. H.; Wheeler, S. E.; Iafe, R. G.; Xu, L.; Celebi-Olcum, N.; Houk, K. N. Bifurcations on Potential Energy Surfaces of Organic Reactions. Angew. Chem., Int. Ed. 2008, 47, 7592–7601. Xu, L.; Doubleday, C. E.; Houk, K. N. Dynamics of 1,3-Dipolar Cycloaddition Reactions of Diazonium Betaines to Acetylene and Ethylene: Bending Vibrations Facilitate Reaction. Angew. Chem., Int. Ed. 2009, 48, 2746–2748. Katori, T.; Itoh, S.; Sato, M.; Yamatake, H. Reaction Pathways and Possible Path Bifurcation for Schmidt Reaction. J. Am. Chem. Soc. 2010, 132, 3413–3422. Takata, T.; Taketsugu, T.; Hirao, K.; Gordon, M. S. Ab Initio Potential Energy Surface by Modified Shepard Interpolation:


(41)

(42) (43)

(44)

(45)

(46)

(47)

(48)

(49) (50)

(51)

(52)

(53)

(54)

(55)

(56)

281

Application to the CH3þH2fCH4þH Reaction. J. Chem. Phys. 1998, 109, 4281. Oyola, Y.; Singleton, D. A. Dynamics and the Failure of Transition State Theory in Alkene Hydroboration. J. Am. Chem. Soc. 2009, 131, 3130–3131. Bartlett, R. J.; Musial, M. Coupled-Cluster Theory in Quantum Chemistry. Rev. Mod. Phys. 2007, 79, 291. Dunning, T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. 1. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007. Kendall, R. A.; Dunning, T. H.; Harrison, R. J. Electron Affinities of the First-Row Atoms Revisited. Systematic Basis Sets and Wave Functions. J. Chem. Phys. 1992, 96, 6796. Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. Energies, Structures, And Electronic Properties of Molecules in Solution with the C-PCM Solvation Model. J. Comput. Chem. 2003, 24, 669. Møller, C.; Plesset, M. S. Note on an Approximation Treatment for Many-Electron Systems. Phys. Rev. 1934, 46, 618– 622. Pople, J. A.; Binkley, J. S.; Seeger, R. Theoretical Models Incorporating Electron Correlation. Int. J. Quantum Chem. 1976, 10, 1–10. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. See Supporting Information for full citation. Porter, R. N.; Raff, L. M. In Dynamics of Molecular Collisions, Part B; Miller, W. H., Ed.; Plenum Press: New York, 1976. Stewart, J. J. P.; Davis, L. P.; Burggraf, L. W. Semi-Empirical Calculations of Molecular Trajectories: Method and Applications to Some Simple Molecular Systems. J. Comput. Chem. 1987, 8, 1117. Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. General Atomic and Molecular Electronic Structure System. J. Comput. Chem. 1993, 14, 1347. Gordon, M. S.; Schmidt, M. W. Advances in Electronic Structure Theory: GAMESS a Decade Later. In Theory and Applications of Computational Chemistry: The First Forty Years; Dykstra, C. E., Frenking, G., Kim, K. S., Scuseria, G. E., Eds.; Elsevier: Amsterdam, The Netherlands, 2005; pp 1167-1189. Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215. Zheng, J.; Zhao, Y.; Truhlar, D. G. The DBH24/08 Database and Its Use to Assess Electronic Structure Model Chemistries for Chemical Reaction Barrier Heights. J. Chem. Theory Comput. 2009, 5, 808. Carpenter, J. D.; Ault, B. S. Matrix-Isolation Study of the Mechanism of the Reaction of Diborane with Ammonia ; Pyrolysis of the H3BHNH3 Adduct. Chem. Phys. Lett. 1992, 197, 171. Carpenter, J. D.; Ault, B. S. Infrared Matrix-Isolation Characterization of Aminoborane and Related Compounds. J. Phys. Chem. 1991, 95, 3502–3506.


Dynamic Mechanisms for Ammonia Borane Thermolysis in Solvent

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