Conformational variability exhibited by mellitic acid anions (MA-n) with organic amine cations:: Structures with an abundance of NH•••OC hydrogen bonds and a paucity of hydrogen bonds

Benzenehexacarboxylic acid (mellitic acid) is an ideal substrate for the study of radially arranged ionic interactions with organic bases. Earlier work has shown that the net charge in MA (-4, -3, or -2) influences its assembly, respectively, either as ribbons or sheets. In the former case the cations intersperse the ribbons forming a tight network. In the MA sheets from the latter, the cations are arranged in orthogonal stacks. In the MA-phenanthroline complex such stacks are arranged exactly parallel resulting in significant pi-pi interactions. The present work relates to the modification of the phenanthroline motif to flexible structures. The reported ability of 2,3-bis-(2'-pyridyl) pyrazine (P2), prepared from 2,2'-pyridyl by reaction with ethylenediamine (EDA) followed by dehydrogenation of the resulting 2,3-bis-(2'-pyridyl)-5,6-dihydropyrazine, to form helical structures made it an obvious choice for complexation studies with MA. These resulted not only in the formation of the desired 7 MAP2 complex but also, surprisingly, with the dihydroprecursor complex with EDA 3 (MAEDA), a compound that eluded direct preparation with MA and EDA. The crystal structure of MAEDA reports the first complex of MA where MA(-5) ions are encountered. The 15 hydrogen bonds, three for each N, result in hard packing. The complex with P2 7 (MAP2) showed the common features of MA(-2) sheets. Unlike phenanthroline, the nonplanar disposition of the pyridine rings made it possible for both pyridine protons to be available for salt formation, resulting in the arrangement of the cations in layers interposed between the sheets of MA(-2) ions. The pyrazine ring does not participate in hydrogen bonding or stacking in the crystal.