X-ray, FTIR, 1H and 13C NMR, PM3 and AM1 studies of (N—H⋯N)+ and (O—H⋯O)– hydrogen bonds in a complex of 1,8-diaminonaphthalene with maleic acid: proton cavity and basicity of proton sponges

Abstract

The crystal structure of the title complex has been determined by X-ray analysis. All H atoms were located and refined isotropically; R= 0.041 for 1595 observed reflections. The intramolecular hydrogen bonds are asymmetrical and are characterized by the following distances and angles: 2.670 (3)Å and 146 (3)°Å for the (NHN)⁺ bridge in the protonated 1,8-diaminonaphthalene cation (DAN⁺H) and 2.423 (2)Å and 177 (3)°Å for the (OHO)^– bridge in the hydrogen maleate anion (HM^–). The N⁺H₃ group is engaged in two intermolecular bonds with the anion [N⁺(H)⋯O distances 2.794 (2) and 2.856 (3)Å]. The interactions of the NH₂ group with the anion are weaker [N(H)⋯O distances 3.078 (3) and 3.225 (3)Å]. Most of the bond distances and angles in DAN⁺H·HM^– are very similar to those in the complex of 1,8-bis (dimethylamino)naphthalene with maleic acid, DMAN⁺H·HM^–. The deformations of the naphthalene skeleton in DAN⁺H·HM^– and DMAN⁺H·HM^– are also comparable. In DMAN⁺H, the proton is located in a cavity formed by four methyl groups, therefore, its exchange is slow on the NMR timescale. In DAN⁺H the proton is not inside a cavity and the exchange becomes rapid. Although spherical repulsions are not negligible, the cavity is of prime importance for the large basicity of DMAN. The PM3 method reproduces correctly the cation structure and its interactions with the anion but the O⋯O distance in the anion is too long. The AM1 method failed to reproduce the structure of the complex correctly.