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

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 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]⁻ (<b>1</b>) 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]⁻ (<b>2a</b>) and VBH⁺[H₃BNH₂BH₂NH₂BH₂NH₂BH₃]⁻ (<b>2b</b>) 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 <i>n</i>-pentane, 3-ethylpentane, and <i>n</i>-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 H₂ release, involving the initial formation of the previously known [H₃BNH₂BH₃]⁻ 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 <b>1</b> was observed to react with an additional AB equivalent to form <b>2a</b> and <b>2b</b>.