- Probing the second dehydrogenation step in ammonia-borane dehydrocoupling: Characterization and reactivity of the key intermediate, B-(cyclotriborazanyl)amine-borane
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While thermolysis of ammonia-borane (AB) affords a mixture of aminoborane- and iminoborane oligomers, the most selective metal-based catalysts afford exclusively cyclic iminoborane trimer (borazine) and its B-N cross-linked oligomers (polyborazylene). This catalysed dehydrogenation sequence proceeds through a branched cyclic aminoborane oligomer assigned previously as trimeric B-(cyclodiborazanyl)amine-borane (BCDB). Herein we utilize multinuclear NMR spectroscopy and X-ray crystallography to show instead that this key intermediate is actually tetrameric B-(cyclotriborazanyl)amine-borane (BCTB) and a method is presented for its selective synthesis from AB. The reactivity of BCTB upon thermal treatment as well as catalytic dehydrogenation is studied and discussed with regard to facilitating the second dehydrogenation step in AB dehydrocoupling. This journal is
- Kalviri, Hassan A.,G?rtner, Felix,Ye, Gang,Korobkov, Ilia,Baker, R. Tom
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- Catalytic Dehydrogenation of Amine-Boranes using Geminal Phosphino-Boranes
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The reaction of the intramolecular frustrated Lewis pair (FLP) tBu2PCH2BPh2 with the amine-boranes NH3·BH3 and Me2NH·BH3 leads to the formation of the corresponding FLP-H2/
- Boom, Devin H. A.,de Boed, Ewoud J. J.,Nicolas, Emmanuel,Nieger, Martin,Ehlers, Andreas W.,Jupp, Andrew R.,Slootweg, J. Chris
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p. 586 - 592
(2020/02/11)
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- Dehydrogenation of ammonia borane through the third equivalent of hydrogen
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Ammonia borane (AB) has high hydrogen density (19.6 wt%), and can, in principle, release up to 3 equivalents of H2 under mild catalytic conditions. A limited number of catalysts are capable of non-hydrolytic dehydrogenation of AB beyond 2 equivalents of H2 under mild conditions, but none of these is shown directly to derivatise borazine, the product formed after 2 equivalents of H2 are released. We present here a high productivity ruthenium-based catalyst for non-hydrolytic AB dehydrogenation that is capable of borazine dehydrogenation, and thus exhibits among the highest H2 productivity reported to date for anhydrous AB dehydrogenation. At 1 mol% loading, (phen)Ru(OAc)2(CO)2 (1) effects AB dehydrogenation through 2.7 equivalents of H2 at 70 °C, is robust through multiple charges of AB, and is water and air stable. We further demonstrate that catalyst 1 has the ability both to dehydrogenate borazine in isolation and dehydrogenate AB itself. This is important, both because borazine derivatisation is productivity-limiting in AB dehydrogenation and because borazine is a fuel cell poison that is commonly released in H2 production from this medium.
- Zhang, Xingyue,Kam, Lisa,Williams, Travis J.
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p. 7672 - 7677
(2016/05/24)
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- Zirconium-Catalyzed Amine Borane Dehydrocoupling and Transfer Hydrogenation
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κ5-(Me3SiNCH2CH2)2N(CH2CH2NSiMe2CH2)Zr (1) has been found to dehydrocouple amine borane substrates, RR′NHBH3 (R = R′ = Me; R = tBu, R′ = H; R = R′ = H), at low to moderate catalyst loadings (0.5-5 mol %) and good to excellent conversions, forming mainly borazine and borazane products. Other zirconium catalysts, (N3N)ZrX [(N3N) = N(CH2CH2NSiMe2CH2)3, X = NMe2 (2), Cl (3), and OtBu (4)], were found to exhibit comparable activities to that of 1. Compound 1 reacts with Me2NHBH3 to give (N3N)Zr(NMe2BH3) (5), which was structurally characterized and features an η2 B-H σ-bond amido borane ligand. Because 5 is unstable with respect to borane loss to form 2, rather than β-hydrogen elimination, and 2-4 do not exhibit X ligand loss during catalysis, dehydrogenation is hypothesized to proceed via an outer-sphere-type mechanism. This proposal is supported by the catalytic hydrogenation of alkenes by 2 using amine boranes as the sacrificial source of hydrogen.
- Erickson, Karla A.,Stelmach, John P. W.,Mucha, Neil T.,Waterman, Rory
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p. 4693 - 4699
(2015/10/28)
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- Mechanistic studies of ammonia borane dehydrogenation catalyzed by iron pincer complexes
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A series of iron bis(phosphinite) pincer complexes with the formula of [2,6-(iPr2PO)2C6H 3]Fe(PMe2R)2H (R = Me, 1; R = Ph, 2) or [2,6-(iPr2PO)2-4-(MeO)C6H 2]Fe(PMe2Ph)2H (3) have been tested for catalytic dehydrogenation of ammonia borane (AB). At 60 °C, complexes 1-3 release 2.3-2.5 equiv of H2 per AB in 24 h. Among the three iron catalysts, 3 exhibits the highest activity in terms of both the rate and the extent of H2 release. The initial rate for the dehydrogenation of AB catalyzed by 3 is first order in 3 and zero order in AB. The kinetic isotope effect (KIE) observed for doubly labeled AB (kNH3BH3/k ND3BD3 = 3.7) is the product of individual KIEs (k NH3BH3/kND3BH3 = 2.0 and kNH3BH3/k NH3BD3 = 1.7), suggesting that B-H and N-H bonds are simultaneously broken during the rate-determining step. NMR studies support that the catalytically active species is an AB-bound iron complex formed by displacing trans PMe3 or PMe2Ph (relative to the hydride) by AB. Loss of NH3 from the AB-bound iron species as well as catalyst degradation contributes to the decreased rate of H2 release at the late stage of the dehydrogenation reaction.
- Bhattacharya, Papri,Krause, Jeanette A.,Guan, Hairong
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supporting information
p. 11153 - 11161
(2014/08/18)
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- Thermally induced dehydrogenation of amine-borane adducts and ammonia-borane by group 6 cyclopentadienyl complexes having single and triple metal-metal bonds
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Treatment of solutions of ammonia-borane (NH3·BH 3, AB) with catalytic amounts (5 mol-%) of the singly bonded dimers [M2Cp2(CO)6] [M = Cr (1a), Mo (1b), W (1c); Cp = cyclopentadienyl] under mild thermal activation (333 K) led to the progressive dehydrogenation of the adduct and quantitative conversions were achieved after 12, 24, and >34 h, respectively. At the initial stages of these reactions (low conversions), the major products were cyclic and branched oligomers of aminoborane (NH2=BH2). However, at longer reaction times (high conversions), the major products were, in all cases, borazine, [HNBH] 3, and polyborazylene, [NBHx] (x 3] [M = Cr (2a), Mo (2b), W (2c)], which are supposed to be the catalytically active species in these processes, as also supported by similar catalytic activity exhibited by pure samples of the dihydride [Mo2Cp2(H)2(μ-Ph 2PCH2PPh2)(CO)2] (2b′). Under similar conditions, 1a-c were also active catalysts for the dehydrogenation of adducts derived from substituted amines (tBuH2N·BH3 and Me2HN·BH3), although the rate of dehydrogenation was significantly lower than that of AB. This lower activity follows from deprotonation of hydrides 2 by the free amines, which are in turn generated through B-N bond-cleavage processes. The dehydrogenation products of tBuH2N·BH3 are also derived from oligomerization processes of the corresponding aminoborane (tBuHN=BH2), which in this case was identified in the reaction mixtures, but even at long reaction times, the formation of the borazine-like product was not complete, and the reaction mixture contained significant amounts of (poorly defined) soluble polymeric materials. For Me2HN·BH3, the major product obtained in all of the reactions was cyclic dimer [Me2N=BH 2]2. Similar studies were performed with triply bonded complexes [Mo2Cp2(CO)4] (3b) and [Mo 2Cp2(μ-Ph2PCH2PPh 2)(CO)2] (3b′), which displayed similar catalytic activity while remaining essentially unperturbed along the reactions, and these complexes yielded product distributions that were similar to those observed for singly bonded dimers 1a-c. Readily accessible group 6 binuclear cyclopentadienyl complexes having single and triple M-M bonds are efficient catalysts for the dehydrogenation of a range of amine-borane adducts, including ammonia-borane, under mild thermal activation (333 K). Copyright
- Garcia-Vivo, Daniel,Huergo, Estefania,Ruiz, Miguel A.,Travieso-Puente, Raquel
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p. 4998 - 5008
(2013/10/21)
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- Iron complex-catalyzed ammonia-borane dehydrogenation. A potential route toward B-N-containing polymer motifs using earth-abundant metal catalysts
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Ammonia-borane (NH3BH3, AB) has garnered interest as a hydrogen storage material due to its high weight percent hydrogen content and ease of H2 release relative to metal hydrides. As a consequence of dehydrogenation, B-N-containing oligomeric/polymeric materials are formed. The ability to control this process and dictate the identity of the generated polymer opens up the possibility of the targeted synthesis of new materials. While precious metals have been used in this regard, the ability to construct such materials using earth-abundant metals such as Fe presents a more economical approach. Four Fe complexes containing amido and phosphine supporting ligands were synthesized, and their reactivity with AB was examined. Three-coordinate Fe(PCy3)[N(SiMe3)2]2 (1) and four-coordinate Fe(DEPE)[N(SiMe3)2]2 (2) yield a mixture of (NH2BH2)n and (NHBH)n products with up to 1.7 equiv of H2 released per AB but cannot be recycled (DEPE = 1,2-bis(diethylphosphino)ethane). In contrast, Fe supported by a bidentate P-N ligand (4) can be used in a second cycle to afford a similar product mixture. Intriguingly, the symmetric analogue of 4 (Fe(N-N)(P-P), 3), only generates (NH2BH2)n and does so in minutes at room temperature. This marked difference in reactivity may be the result of the chemistry of Fe(II) vs Fe(0).
- Baker, R. Tom,Gordon, John C.,Hamilton, Charles W.,Henson, Neil J.,Lin, Po-Heng,Maguire, Steven,Murugesu, Muralee,Scott, Brian L.,Smythe, Nathan C.
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p. 5598 - 5609
(2012/05/20)
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- An investigation on the synthesis of borazine
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Borazine is a promising precursor for boron nitride. A detailed investigation on the reaction of sodium borohydride and ammonium sulfate from 40 °C to 120 °C for synthesis of borazine was performed. The reaction was monitored by means of 11B nuclear magnetic resonance (11B NMR) and Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), mass spectroscopy (MS). The reaction produces mainly ammonia borane (AB), but not borazine at temperatures below 60 °C. Increases of temperature promote yield of borazine, which reaches the maximum around 110 °C. Whereas further increased temperature causes severe polymerization of borazine, and hence holds back yields of borazine.
- Li, Jun-Sheng,Zhang, Chang-Rui,Li, Bin,Cao, Feng,Wang, Si-Qing
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p. 173 - 176
(2011/03/21)
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- An improved synthesis of borazine with aluminum chloride as catalyst
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Borazine is an excellent precursor for boron nitride. However the existing synthetic methods for the preparation of borazine have drawbacks such as relatively high reaction temperatures, side reactions, long reaction times, and low yields. An improved synthesis procedure was disclosed, which involved the use of aluminum chloride as a catalyst in the reaction of sodium borohydride with ammonium sulfate. The aluminum chloride catalyst: brought the reaction temperature down from 120-140 °C to 45 °C. Improved yields of borazine were obtained in comparison to the reaction without a catalyst. In addition, the reaction process was investigated in detail by 11B NMR spectroscopy and Fourier transform, infrared spectroscopy (FTIR). It was found that aluminum, borohydride formed in very small quantity when aluminum chloride was introduced, which plays an important role in the reaction.
- Li, Jun-Sheng,Zhang, Chang-Rui,Li, Bin,Cao, Feng,Wang, Si-Qing
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p. 1763 - 1766
(2010/08/13)
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- Development of rhenium catalysts for amine borane dehydrocoupling and transfer hydrogenation of olefins
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Five-coordinated rhenium(I) hydride complexes of the type [Re(Br)(H)(NO)(PR3)2] (R = Cy 2a, iPr 2b) were prepared from [Re(Br)2(NO)(PR3)2(η2- H2)] (R = Cy la,. iPr lb) via deprotonation of the η2-H2 ligands with various bases. Filling the vacant site of 2a or 2b by various less bulky two-electron donors produced the 18-electron complexes [Re(Br)(H)(NO)(PR3)2(L)] (L = O 2 3, CH2=CH2 4, acetylene 5, H2 6, CO 7, CH3CN 8). The influence of the trans-coordinated ligand on the Re-H bond was examined. The 1H NMR chemical shift of the hydride depends on L in the order O2 > acetylene > CH 2=CH2 > H2 > CO > CH3CN. The reactions of 2a or 2b with the IMes or SIMes ligands afforded the five-coordinated complex [Re(Br)(H)(NO)(PR3)(NHC)] (NHC = IMes 9 (IMes = 1,3bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), SIMes 10 (SIMes = l,3-bis(2,4,6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)) via replacement of one phosphine. The reaction of 2a or 2b with n-BuLi leads to the formation of the n-butene-coordinated dihydride complexes [Re(H)2(NO)(PR 3)2(η2-n-CH2=CHC 2H5)] (R = Cy 12a, iPr 12b). Species 1a and 1b reacted also with NaNMe2BH3, affording the tetrahydride complexes [Re(H)4(NO)(PR3)2] (R = Cy 14a, iPr 14b) via the intermediacy of 2a and 2b. The molecular structures of complexes 8b, 10a, and 10b were established by single-crystal X-ray diffraction studies. The five-coordinated rhenium(I) hydride complexes 2a, 2b, 9a, and 9b catalyzed the dehydrocoupling of Me2NHBH3 and the transfer hydrogenation of olefins using Me2NHBH3 as a hydrogen donor, which showed high activities. Mechanistic studies were carried out indicating that these rhenium(I) hydride catalyses allowed formation of dihydrogen hydride complexes. A plausible catalytic cycle for both dehydrocoupling and transfer hydrogenation was proposed, which implies the ability of rhenium(I) complexes to activate B-H and N-H bonds by the facile redox interplay of Re(I) and Re(III) species.
- Jiang, Yanfeng,Blacque, Olivier,Fox, Thomas,Frech, Christian M.,Berke, Heinz
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p. 5493 - 5504
(2009/12/25)
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- Interaction of lithium hydride and ammonia borane in THF
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The two-step reaction between LiH and NH3BH3 in THF leads to the production of more than 14 wt% of hydrogen at 40 °C. The Royal Society of Chemistry.
- Xiong, Zhitao,Chua, Yong Shen,Wu, Guotao,Xu, Weiliang,Chen, Ping,Shaw, Wendy,Karkamkar, Abhi,Linehan, John,Smurthwaite, Tricia,Autrey, Thomas
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p. 5595 - 5597
(2009/04/13)
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