Syntheses and Structural Characterizations of Anionic Borane-Capped Ammonia Borane Oligomers: Evidence for Ammonia Borane H₂ Release via a Base-Promoted Anionic Dehydropolymerization Mechanism

Abstract

Studies of the activating effect of Verkade's base, 2,8,9-triisobutyl-2,5,8,9-tetraaza-1-phosphabicyclo[3.3.3]undecane (VB), on the rate and extent of H(2) release from ammonia borane (AB) have led to the syntheses and structural characterizations of three anionic aminoborane chain-growth products that provide direct support for anionic dehydropolymerization mechanistic steps in the initial stages of base-promoted AB H(2) release reactions. The salt VBH(+)[H(3)BNH(2)BH(2)NH(2)BH(3)](-) (1) containing a linear five-membered anionic aminoborane chain was produced in 74% yield via the room-temperature reaction of a 3:1 AB/VB mixture in fluorobenzene solvent, while the branched and linear-chain seven-membered anionic aminoborane oligomers VBH(+)[HB(NH(2)BH(3))(3)](-) (2a) and VBH(+)[H(3)BNH(2)BH(2)NH(2)BH(2)NH(2)BH(3)](-) (2b) were obtained from VB/AB reactions carried out at 50 °C for 5 days when the AB/VB ratio was increased to 4:1. X-ray crystal structure determinations confirmed that these compounds are the isoelectronic and isostructural analogues of the hydrocarbons n-pentane, 3-ethylpentane, and n-heptane, respectively. The structural determinations also revealed significant interionic B-H···H-N dihydrogen-bonding interactions in these anions that could enhance dehydrocoupling chain-growth reactions. Such mechanistic pathways for AB H(2) release, involving the initial formation of the previously known [H(3)BNH(2)BH(3)](-) anion followed by sequential dehydrocoupling of B-H and H-N groups of growing borane-capped aminoborane anions with AB, are supported by the fact that 1 was observed to react with an additional AB equivalent to form 2a and 2b.

Anti and gauche conformers of an inorganic butane analogue, NH3BH2NH2BH3

“…It belongs to the monoclinic Pn space group (Table S1w). All N-B bond distances and bond angles are comparable with those observed in the gauche form and other reported B-N chain anion compounds, [14][15][16][17] and fall within the normal range (Table S2w). The striking feature of the molecular structure is that DDAB adopts an anti, rather than gauche, conformation (Fig.…”

“…17 In the structure of the anion V, a BH 3 NH 2 BH 2 NH 2 moiety adopts the anti conformation at one end of the anion and the gauche conformation at the other end. In the structure of the anion VI, the BH 3 NH 2 BH 2 NH 2 moiety at both ends adopts the gauche form but the NH 2 BH 2 NH 2 BH 2 moiety at the center adopts the anti form.…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…Under the same melt conditions, Fe 2 (CO) 9 was also found to catalyse the dehydrocoupling of H 3 B · PPh 2 H to form the same product (80% conversion in 15 h). The authors postulated that the loss of a carbonyl ligand at high temperatures allowed dehydrocoupling to occur in the vacant coordination site created [172].…”

“…It is the control of these processes, to either afford well-defined final products or the maximum rate and yield of hydrogen evolution, that makes catalytic routes attractive for dehydrocoupling. This is not the first time that dehydrocoupling of amine-boranes and phosphine-boranes has been reviewed, and there have been recent overviews dealing with their general chemistry and properties [1,2], role in hydrogen storage applications [4,8,9], as well as dehydrocoupling processes [5,7,10,11]. We do not attempt to review the extensive literature on the catalysed hydrolysis of amine-boranes to produce H 2 as the principal product of interest [3].…”

Metal‐Free Catalysis of Ammonia–Borane Dehydrogenation/Regeneration for a Highly Efficient and Facilely Recyclable Hydrogen‐Storage Material

“…[1] Light, safe, and compact storage of hydrogen for onboard applications remains the largest challenge in its implementation. [6] Nevertheless, poor recyclability, slow kinetics below 100 8C, and simultaneous liberation of excessive deleterious gases upon thermolysis (i.e., borazine, ammonia, and diborane) [7] are main challenges that need to be addressed before this material can be utilized in practical systems.Initial advances have demonstrated that the promoted rate, extent and purity of H 2 delivery required could be achieved from AB by using a variety of catalytic strategies, such as nanoscaffolds, [8] acids, [9] bases, [10] metal catalysts, [11] among others. [3] Ammonia borane (NH 3 BH 3 , AB) is considered as one of the most promising chemical hydrogen storage materials, [4] attributed to its high material storage density, low thermolysis temperature, [5] and recently realized regenerability.…”