Hydrate frameworks involving the pyridazino[4,5‐d]pyridazine unit as a multiple hydrogen‐bond acceptor

1,4,5,8‐Tetramethylpyridazino[4,5‐d]pyridazine trihydrate, C₁₀H₁₂N₄·3H₂O, (I), and 1,2,3,6,7,8‐hexahydrocinnolino[5,4,3‐cde]cinnoline tetrahydrate, C₁₂H₁₂N₄·4H₂O, (II), exhibit exceptional functionality of the condensed N₄‐heteroaromatic frame as a symmetric acceptor of four hydrogen bonds [N...O = 2.843 (2)–2.8716 (10) Å]. Thus, all the N atoms of the electron‐deficient and highly π‐acidic polynitrogen heterocycles function as lone‐pair donors. In (I), all the molecular components lie on or across special positions; the site symmetry is 2/m for the organic and m2m and m for the two water molecules. In (II), the organic polycycle lies across a crystallographic inversion center. Both structures involve a hydrogen‐bonded centrosymmetric water–pyridazine dimer as the basic supramolecular unit, which is integrated into two‐dimensional [in (I)] and three‐dimensional [in (II)] hydrate frameworks by hydrogen bonding with the additional water molecules [O...O = 2.744 (2)–2.8827 (19) Å]. The hydrate connectivity exists in the form of an (H₂O)₃ trimer in (I) and as a one‐dimensional zigzag (H₂O)_n chain in (II).