New "proton sponges" 14 - Isomeric tetrakis(dimethylamino)naphthalenes: Synthesis, structure-dependence of basicities, crystal structures, and physical properties

For comparison to the recently described 2,3,6,7-tetrakis(dimethylamino)nap hthalene (1) the three isomers 2, 3, and 4 were synthesized. The basicities of this group of isomers are strongly dependent upon the different mutual orientations of the pairs of dimethylamino substituents: only the isomers 3 and, partially, 4, both with dimethylamino groups in adjacent peri-positio ns of the naphthalene, are strong "proton sponges". For the isomers 1 and 2 with the same number and kind of twofold dimethylamino substituents in nei ghbouring ortho-positions, however, no significant basicity increase is obs erved. To explain this difference between the two groups of isomers it is s uggested that in the ortho-pairs of 1 and 2 the C-N bonds diverge considera bly, leading to an increased N ... N distance and consequently to less stab le [N ... H ... N](+) hydrogen bonds in contrast to the parallel C-N bonds in the peri-substituted isomers 3 and 4. X-ray crystal structure analyses o f the bases and of some of the salts derived therefrom were solved and are discussed. Cyclic voltammetry indicates that 1 to 4 are strong electron don ors, reacting easily to radical cations or dications which with suitable ac ids have been obtained as salts.