Mechanism of B-H Redistribution during Reduction of Polyborazylene by Hydrazine

Abstract

Density functional theory has been used to elucidate the mechanistic underpinnings of the regeneration of ammonia-borane (H3B−NH3, AB) from polyborazylene (BxNxHx, PBz) in the presence of hydrazine (H2N−NH2, Hz). Herein, borazine (B3N3H6, Bz) is used as the simplest relevant model of PBz for the regeneration process. Digestion of Bz using Hz was found to occur by a string of Lewis acid base adduct (between B atoms of Bz and Hz molecule) formation and Hz assisted proton transfer processes. Later, B−H bonds of HB(NHNH2)2, the Bz digested product, are redistributed to form hydrazine-borane (H3B−NH2NH2, HzB) and B(NHNH2)3. Redistribution of B−H bonds occurs through hydroboration and concerted proton-hydride transfer. Another B−H redistributed product, B(NHNH2)3, produces HzB as a result of proton and hydride transfer from cis-diazene (Dz), the oxidized product of Hz in presence of O2.This work charts out the detailed mechanism of reduction of polyborazylene by hydrazine. Hydrazine initially helps to cleave the network of B−N bonds by acting as a Lewis base and proton shuttle. Later B−H redistribution between the B−H units formed after digestion of polyborazylene and further hydrogenation facilitated by diazene plays a vital role in regeneration of ammonia-borane (H3B−NH3).