Protonation of naphthalene proton sponges containing higher N-alkyl groups. Structural consequences on proton accepting properties and intramolecular hydrogen bonding

1,8-Bis(dipropylamino)naphthalene and 1,8-di(pyrrolydin-1-yl)naphthalene as well as monoprotonated forms of the latter and of 1,8-di(piperidin-1-yl)naphthalene were structurally analyzed and compared with known data for 1,8-bis(dimethylamino)naphthalene and its protonated form. The N-alkyl groups enlarging the N…N distance (up to ∼2.9 Å) and changing the degree of conjugation between the NAlk2 and naphthalene moieties modify in a complex way the basicity (pKa values of conjugated acids) as well as the properties of intramolecular hydrogen bonds both in the solid state and in solution for a set of 1,8-bis(dialkylamino)naphthalenes.