Hydrogen Bonding in the 2,2'-Bipyridinium Salt of 1,2,4,5-Benzenetetracarboxylic Acid (Pyromellitic Acid)

Abstract

The 2,2′-bipyridinium salt of pyromellitic acid (PMA) (1,2,4,5-benzenetetracarboxylic acid), 2,2′-bipyridinium hemi[1,2,4,5-benzenetetracarboxylate(2-)] hemi(1,2,4,5-benzenetetracarboxylic acid), C10H8N2H+.1/2[C6H2(COO)4H2O]2-.1/2[C6H2(COO)4H4], has been prepared and studied by X-ray diffraction and IR spectroscopy. 2,2′-Bipyridine acts as a proton sponge by accepting a proton from pyromellitic acid. The transfer of protons results in strong asymmetric intramolecular hydrogen bonds: H(N)⋯N 2.603 (2) A in the bipyridinium cation which has syn conformation and H(O)⋯O 2.396 (2) A in the pyromellitate anion. The N-H⋯N hydrogen bond is part of a three-centred hydrogen bond with a carbonyl group of the neutral pyromellitic acid: N—H⋯N,O with H(N)⋯O 2.805 (2) A. Each neutral PMA molecule is connected by hydrogen bonds with two cations and four anions forming a two-dimensional network with plane indices (112). The IR spectrum also suggests the presence of strong hydrogen bonds.