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

ABSTRACT Diborane(4) is a strong proton sponge when embedded in B 2 H 4 –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 [BH 2 ‐(η 1 ‐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.