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

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• 作者：William C. Ewing ; Allegra Marchione ; Daniel W. Himmelberger ; Patrick J. Carroll ; Larry G. Sneddon
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：October 26, 2011
• 年：2011
• 卷：133
• 期：42
• 页码：17093-17099
• 全文大小：890K
• 年卷期：v.133,no.42(October 26, 2011)
• ISSN：1520-5126

文摘

Studies of the activating effect of Verkade鈥檚 base, 2,8,9-triisobutyl-2,5,8,9-tetraaza-1-phosphabicyclo[3.3.3]undecane (VB), on the rate and extent of H₂ 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 H₂ release reactions. The salt VBH⁺[H₃BNH₂BH₂NH₂BH₃]^{鈭?/sup> (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(NH₂BH₃)₃]鈭?/sup> (2a) and VBH+[H₃BNH₂BH₂NH₂BH₂NH₂BH₃]鈭?/sup> (2b) were obtained from VB/AB reactions carried out at 50 掳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鈥揌路路路H鈥揘 dihydrogen-bonding interactions in these anions that could enhance dehydrocoupling chain-growth reactions. Such mechanistic pathways for AB H₂ release, involving the initial formation of the previously known [H₃BNH₂BH₃]鈭?/sup> anion followed by sequential dehydrocoupling of B鈥揌 and H鈥揘 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.}