Substituted Heterocyclic Naphthalene Diimides with Unexpected Acidity. Synthesis, Properties, and Reactivity

Naphthalene bisimides (NDIs) with a heterocyclic 1,4-dihydro-2,3-pyrazinedione moiety have been synthesized from both 2,6-dibromonaphthalene and 2,3,6,7-tetrabromonaphthalene bisanhydrides by means of a stepwise protocol including imidization, nucleophilic displacement of the bromine atoms by ethane-1,2-diamine, in situ reductive dehalogenation, and further oxidation. These heterocycles (R = n-pentyl, cyclohexyl) are yellow dyes with green emission in organic solvent, where the acid form dominates. The orange nonfluorescent conjugate base can be generated quantitatively by CH3COONBu4 addition in DMSO, where it exhibits a pKa=7.63. The coniugate base becomes the only detectable species (by UV-vis spectroscopy), in water solution, even under acid conditions (pH 1). In aqueous DMSO the acid/base equilibrium is a function of the DMSO/ water ratio. The unexpected acidity of these heterocyclic NDIs, which justifies the reactivity with CH2N2, has been rationalized by DFT computational means [PBE0/6-31+G(d,p)] in aqueous solvent (PCM models) as a result of a strong specific solvation effect, modeled by the inclusion of three water molecules.