Breaking the Ceiling of Boron-Boron Bonds: Understanding and Fine-Tuning Basicity in Diborane(4) Complexes.

Diborane(4) is a strong proton sponge when embedded in B2H4-N-base complexes, but the still available electrodeficient site may attract a second N-base, making the BB bond even more basic. High-level G4 calculations show that [BH2-(η1-N-base)]2 species can have proton affinities (PAs) larger than that of 1,8-bis(dimethylamino)naphthalene by as much as 24 orders of magnitude in the equilibrium constant, depending on the N-base selected. Trends in basicity in complexes with a pair of nitrogenous bases are different from those with only one, due to critical structural changes during protonation. The PAs in complexes joining two different N-bases are very close to the average of those of the corresponding symmetric complexes, allowing a straightforward tuning of the basicity enhancement through the appropriate selection of the N-bases. The dissociation enthalpies have been analyzed in relation to the topological features of the BB bond, finding that values can be rationalized based on the distortion of the fragments involved in bond cleavage.