Molecular adducts of 2,6-diaminopyridine with nitro-substituted aromatic carboxylic acids and the crystal structure of the 1:1 adduct of 2,6-diaminopyridine with 2-nitrobenzoic acid

The 1:1 adducts of 2,6-diaminopyridine (2,6-dap) with a series of nitro-substituted aromatic carboxylic acids have been prepared and the crystal structure of one of these, with 2-nitrobenzoic acid (2-nba), [(2,6-dap)(+) (2-nba)(-)] has been determined by X-ray crystallography and refined to a residual R 0.034 for 655 observed reflections. Crystals are monoclinic, space group P 2(1)/c, with four molecules in a cell of dimensions a 7.376(2), b 22.837(3), c 7.716(2) Angstrom, beta 90.01(2)degrees. The structure involves proton transfer with the resulting pyridinium proton and an amine proton combining with the carboxyl oxygens of the 2-nba molecule in a primary cyclic hydrogen-bonding association. Additional interactions, involving all the remaining amine protons of 2,6-dap with both carboxyl oxygens and one nitro oxygen of 2-nba, complete a chain polymer structure. All other members of this series have been confirmed by infrared spectroscopy as proton transfer adducts.