Electrodeficient Diborane (4) Converted into an Extraordinary Proton Sponge

High-level G4 calculations show that the interaction of diborane(4) (B₂H₄) with nitrogen bases not only stabilizes the <i>C</i>_<i>2v</i> isomer with respect to the <i>D</i>_<i>2d</i> one, but more importantly retains and enhances the distinctive reactivity of the <i>C</i>_<i>2v</i> 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.