TABF as a Double Proton Sponge: Steric Preorganization versus Antiaromatic Destabilization

Proton sponges are a distinctive class of highly basic organic molecules whose basicity is governed by molecular architecture. We report the synthesis of a proton sponge scaffold, 1,1',8,8'-tetraazabifluorenylidene (TABF), based on a bifluorenylidene backbone incorporating pyridine units and capable of binding two protons intramolecularly. Single-crystal X-ray diffraction established the structures of the neutral base and the doubly protonated dication. Acid-base properties were determined by UV-vis spectrophotometric titrations in acetonitrile, giving pK_aH,2 = 15.0-15.6 for the first deprotonation of the dication and pK_aH,1 ≈ 18 for the second deprotonation step. These values exceed those of pyridine but are lower than those of the prototypical proton sponge 1,8-bis(dimethylamino)naphthalene (DMAN). The attenuated basicity of TABF is attributed to antiaromatic destabilization in the protonated forms arising from delocalization of positive charge into the fluorenyl units, underscoring the critical role of electronic structure and charge distribution in the design of new organic superbases.