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Phenazine (1) and 5,10-dihydrophenazine (2) form not only a 1 : 1 complex (3), as expected from NÈHÉ É ÉN hydrogen bond requirements, but surprisingly also a 3 : 1 complex (4). Single crystal X-ray analyses of compound 2 and complexes 3 and 4 have been carried out. While the overall packing in complex 3 is as efficient as in pure 1 and 2, acidic CÈH groups, which contribute actively to the structural assembly in pure 2, are not involved in hydrogen bonding in 3. It is shown that the quest for hydrogen bonding by these CÈH groups steers the formation of complex 4 with an unprecedented NÈHÉ É ÉN and CÈHÉ É ÉN hydrogen bond mediated tape structure.

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1:1 cocrystals of oxalic and fumaric acid with 1,4-diazanaphthalene (quinoxaline) and 2,3-diazanaphthalene (phthalazine) were prepared. 1,5-Diazanaphthalene (naphthyridine) formed a 1:1 cocrystal only with oxalic acid. X-ray structural analyses are reported. The influence of the position of the two nitrogen atoms in these diazanaphthalenes on the crystal packing is analyzed. The gross crystal structure is governed by strong OH···N hydrogen bonds. The fine-tuning of the packing of the cocrystals of quinoxaline with oxalic and fumaric acid occurs via CH···O hydrogen bonds. The stabilization by the CH···O hydrogen bonds leads to a planar sheetlike structure in the cocrystal with fumaric acid, in contrast to the cocrystal with oxalic acid where the optimization of the CH···O hydrogen bonds results in a nonplanar bent structure. The geometrical arrangement of the nitrogen atoms in 1- and 5-positions in naphthyridine allows a planar sheetlike structure in the cocrystal with oxalic acid. In the case of phthalazine complete proton transfer from one carboxy group of oxalic and fumaric acid to a nitrogen atom of the heterocycle, generating ionic interactions is observed. The hydrogen atom of the second carboxy group of the acids is involved in a OH···OC hydrogen bond.

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