Proton sponges are extremely basic organic molecules. Their strong basicity arises from a rigid spatial structure.
The researchers have synthesized a new proton sponge, 1,1′,8,8′-tetraazabifluorenylidene (TABF), based on a bifluorenylidene backbone with pyridine units. This molecule can bind two protons intramolecularly and exhibits distinctive basicity properties.
Structures of the neutral base and doubly protonated dication were confirmed using single-crystal X-ray diffraction. UV–vis spectrophotometric titrations in acetonitrile revealed pKaH,2 values of 15.0–15.6 for the first deprotonation and pKaH,1 ≈ 18 for the second, indicating TABF is more basic than pyridine but less than the prototypical proton sponge DMAN.
The reduced basicity of TABF is due to antiaromatic destabilization in protonated forms, caused by positive charge delocalization into fluorenyl units. This research underscores the importance of electronic structure in designing organic superbases and opens new possibilities for applications in chemical and biological processes.
The group has published the results in the latest issue of "The Journal of Organic Chemistry".