Diborane(4) as a powerful acid–base transformer

Abstract

Conventional nitrogen bases such as ammonia, methanimine, hydrogen cyanide, and pyridine become very strong acids upon complexation with diborane(4), a very efficient electron donor whose structure was elucidated, among others, by Pople. The present study uses G4 high-level ab initio calculations and different chemical bonding tools to delve into the reasons why this fact occurs. We observe that the acidity of B₂H₄–N-Base complexes, in terms of the ionization constant, increases from 38 to 58 orders of magnitude compared to the corresponding free N-Bases, thus switching from different degrees of basicity to super acidic forms. Even though the formation of the complex involves breaking one of the characteristic (3c,2e) bonds of diborane(4), the neutral adduct is more stabilized the stronger the N-Base. The deprotonation of the N-Base significantly alters the structural and electronic landscape of the complex; in fact, the bridged B₂H₄ moiety is preserved for complexes with hydrogen cyanide and pyridine but fully rearranged with ammonia and methanimine. These latter rearrangements result in anionic global minima [BH₃–BHNH₂]⁻ and [BH₃–BHN=CH₂]⁻, whose very strong B-N bonds contribute substantially to their overall stabilization and are ultimately responsible for the huge acidity enhancement observed. In all cases, the estimated acidity is equal to or higher than that of phosphoric acid, but in particular, hydrogen cyanide becomes a stronger acid than perchloric acid, which is among the strongest superacids in the gas phase.