Electrodeficient Diborane (4) Converted into an Extraordinary Proton Sponge

TL;DR

Diborane(4) becomes an extremely strong proton acceptor when combined with nitrogen bases, surpassing known proton sponges in basicity.

Contribution

The study reveals that diborane(4) complexes with nitrogen bases exhibit unprecedented proton affinity, making them extraordinary proton sponges.

Findings

Diborane(4) complexes with nitrogen bases show proton affinities exceeding 1000 kJ·mol–1.

The B2H4–1,2,5-trimethylimidazole complex is 34 kJ·mol–1 more basic than the proton sponge.

The boron site in the complex is more basic than nitrogen in terms of enthalpy.

Abstract

High-level G4 calculations show that the interaction
of diborane(4)
(B2H4) with nitrogen bases not only stabilizes
the C

2v
isomer with respect
to the D

2d
one, but more
importantly retains and enhances the distinctive reactivity of the C

2v
isomer. The formation of
the complex results in a large enhancement of the donor ability of
the diborane subunit. As a first consequence, the boron site is by
far more basic than nitrogen in terms of enthalpy, leading to protonated
complexes that can be viewed as the association of the different bases
to the B2H5
+ cation. Further analysis
of the electron density redistribution upon complexation helps to
rationalize the key factors behind the drastic basicity enhancement
observed. The basicity of the B2H4–pyridine
complex falls within the range of gas-phase superbases, with a calculated
proton affinity (PA) exceeding 1000 kJ·mol–1. Moreover, complexes with stronger bases, such as guanidine and
methyl-substituted imidazoles, surpass the basicity of the prototypical
proton sponge and superbase 1,8-bis­(dimethylamino)­naphthalene. Precisely,
B2H4–1,2,5-trimethylimidazole is predicted
to be a boron base 34 kJ·mol–1 more basic than
the proton sponge, corresponding to an increase in the protonation
equilibrium constant of nearly 6 orders of magnitude.

Figures