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

A facile synthesis of cyclic aminodiborane (NH<sub>2</sub>B<sub>2</sub>H<sub>5</sub>, ADB) from ammonia borane (NH<sub>3</sub>·BH<sub>3</sub>, AB) and THF·BH<sub>3</sub> has made it possible to determine its important characteristics. Ammonia diborane (NH<sub>3</sub>BH<sub>2</sub>(μ-H)­BH<sub>3</sub>, AaDB) and aminoborane (NH<sub>2</sub>BH<sub>2</sub>, AoB) were identified as key intermediates in the formation of ADB. Elimination of molecular hydrogen occurred from an ion pair, [H<sub>2</sub>B­(NH<sub>3</sub>) (THF)]<sup>+</sup>[BH<sub>4</sub>]<sup>−</sup>. Protic-hydridic hydrogen scrambling was proved on the basis of analysis of the molecular hydrogen products, ADB and other reagents through <sup>2</sup>H NMR and MS, and it was proposed that the scrambling occurred as the ion pair reversibly formed a BH<sub>5</sub>-like intermediate, [(THF)­BH<sub>2</sub>NH<sub>2</sub>]­(η<sup>2</sup>-H<sub>2</sub>)­BH<sub>3</sub>. Loss of molecular hydrogen from the ion pair led to the formation of AoB, most of which was trapped by BH<sub>3</sub> to form ADB with a small amount oligomerizing to (NH<sub>2</sub>BH<sub>2</sub>)<sub><i>n</i></sub>. 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 (B<sub>2</sub>H<sub>6</sub>, DB). The B–H–B bridge is opened when ADB and NaH react to form sodium aminodiboronate, Na­[NH<sub>2</sub>(BH<sub>3</sub>)<sub>2</sub>]. The structure of the sodium salt as its 18-crown-6 ether adduct was determined by X-ray single crystal analysis.