Formation Mechanisms, Structure, Solution Behavior, and Reactivity of Aminodiborane

Abstract

A facile synthesis of cyclic aminodiborane (NH2B2H5, ADB) from ammonia borane (NH3·BH3, AB) and THF·BH3 has made it possible to determine its important characteristics. Ammonia diborane (NH3BH2(μ-H)BH3, AaDB) and aminoborane (NH2BH2, AoB) were identified as key intermediates in the formation of ADB. Elimination of molecular hydrogen occurred from an ion pair, [H2B(NH3) (THF)](+)[BH4](-). Protic-hydridic hydrogen scrambling was proved on the basis of analysis of the molecular hydrogen products, ADB and other reagents through (2)H NMR and MS, and it was proposed that the scrambling occurred as the ion pair reversibly formed a BH5-like intermediate, [(THF)BH2NH2](η(2)-H2)BH3. Loss of molecular hydrogen from the ion pair led to the formation of AoB, most of which was trapped by BH3 to form ADB with a small amount oligomerizing to (NH2BH2)n. Theoretical calculations showed the thermodynamic feasibility of the proposed intermediates and the activation processes. The structure of the ADB·THF complex was found from X-ray single crystal analysis to be a three-dimensional array of zigzag chains of ADB and THF, maintained by hydrogen and dihydrogen bonding. Room temperature exchange of terminal and bridge hydrogens in ADB was observed in THF solution, while such exchange was not observed in diethyl ether or toluene. Both experimental and theoretical results confirm that the B-H-B bridge in ADB is stronger than that in diborane (B2H6, DB). The B-H-B bridge is opened when ADB and NaH react to form sodium aminodiboronate, Na[NH2(BH3)2]. The structure of the sodium salt as its 18-crown-6 ether adduct was determined by X-ray single crystal analysis.

Designing an Effective Metal‐Free Lewis Acid Catalyst for Ammonia‐Borane Dehydrogenation: A DFT Investigation on Triarylboranes

“…So molecules of 1 can serve as the nucleophile to facilitate ion‐pair formation from the hydride shared complex. The nucleophilic role of the B−H bond of 1 has been implicated in previous studies by others and also by our group . Musgrave et al.…”

“…In both the cases, we studied the associated thermodynamics and kinetics for the formation of ion pairs and the ease of H 2 release from these ion pairs. Additionally, we re‐evaluated the free‐energy profile of the THF⋅BH 3 ‐catalyzed dehydrogenation of 1 reported by Li et al . with the theoretical methods used in this work (see Supporting Information for details).…”

“…Both B(C 6 H 5 ) 3 and BH 3 can plausibly act as catalysts with rate‐determining barriers that can give reasonable rates of dehydrogenation under mild conditions. Although BH 3 has a slightly lower rate‐limiting barrier for catalysis than B(C 6 H 5 ) 3 , the presence of excess hydride in the case of BH 3 may lead to unwanted byproducts such as μ‐amminodiborane that may further hamper the catalytic process. The absence of B−H hydrides in B(C 6 H 5 ) 3 makes it a suitable dehydrogenation catalyst.…”

“…They predicted that BH 3 can act as a bifunctional catalyst that dehydrogenates 1 by concerted proton and hydride abstraction . On the contrary, recent theoretical studies revealed that BH 3 facilitates dehydrogenation with assistance from the ether solvent, in which BH 3 acts as a Lewis acid to abstract hydride from 1 to yield a ion pair, [(THF⋅BH 2 NH 3 )(BH 4 )] . This type of solvent‐assisted hydride transfer has been reported previously for transition‐metal catalysts .…”

“…To solve these problems, a comprehensive theoretical study was performed on the triarylborane‐catalyzed dehydrogenation of 1 to provide mechanistic insights for the future development and rational design of metal‐free triarylborane‐based catalysts. We chose triarylborane catalysts over other boranes because of the absence of a hydroborating B−H hydride, which may hinder the dehydrogenation process by the formation of unwanted byproducts such as μ‐amminodiborane . Additionally, the triphenylborane skeleton provides ample opportunity to tune the electronic effects on the Lewis acid center through judicious substitutions on the phenyl groups.…”