Three examples of naphthalene proton sponges with extreme or unusual structural parameters. General view on factors influencing proton sponge geometry

X-ray measurements of four naphthalene proton sponges have been performed. They include 2,7-di(4-tolylethynyl)-1,8-bis(dimethylamino)naphthalene 4 together with its cation, 2,4-bis(trifluoroacetyl)-1,8-bis(dimethylamino)naphthalenes 16 and 2,6-difluoro-1,3,4,5,7,8-hexakis(dimethylamino)naphthalene 15. It has been disclosed that among all known proton sponge derivatives, compound 16 possesses the largest N⋯N distance and the strongest deviation of the NMe2 groups from the average ring plane while for compound 15 the related parameters are record small. A peculiarity of 2,7-diacetylene 4 is its unusually enhanced basicity contrasting with electron acceptor nature of 4-tolylethynyl group. An origin of the latter phenomena is discussed.

► Four new naphthalene proton sponges were structurally analyzed. ► Proton accepting properties and conformation of NMe2 groups are generalized. ► Ethynyl groups may raise proton sponge’s basicity if adjacent to the basic centers. ► Effective conjugation as stimulus for severe aromatic ring distortions.