Halogen and hydrogen bonding of 3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine and 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine

Seven new crystal structures of 3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine (2-bptz) and 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine (4-bptz) were determined with inorganic halogen donors (I₂ or I₃^-), organic halogen donors [1,4-diiodotetrafluroobenzene (p-F₄DIB) or 1,3,5-trifluoro-2,4,6-triiodobenzene (1,3,5-TFTIB)], and the hydrogen-bond donor hydroquinone to investigate how pyridyl orientation and donor type influence supramolecular assembly. Structures formed with molecular iodine, namely, 2,2'-(1,2,4,5-tetrazine-3,6-diyl)dipyridinium bis(triiodide)-3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine-diiodine (1/1/1), C₁₂H₁₀N₆²⁺·2I₃^-·C₁₂H₈N₆·I₂ (1), and 4-[6-(pyridin-4-yl)-1,2,4,5-tetrazin-3-yl]pyridinium pentaiodide, C₁₂H₉N₆⁺·I₅^- (2), are dominated by N-H...N hydrogen bonding, with pyridine protonation yielding distinct packing arrangements: 2-bptz forms stacked triads along the a axis, whereas 4-bptz assembles into chains of [H(4-bptz)]⁺ cations alternating with pentaiodide layers. Replacement of iodine with organoiodines in 3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine-1,4-diiodotetrafluorobenzene (1/1), C₆F₄I₂·C₁₂H₈N₆ (3), 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine-1,4-diiodotetrafluorobenzene (1/1), C₆F₄I₂·C₁₂H₈N₆ (4), and 3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine-1,3,5-trifluoro-2,4,6-triiodobenzene (1/2), C₆H₄N₃·C₆F₃I₃ (5), shifts the primary interactions to C-I...N halogen bonding, producing one-dimensional staggered chains in 2-bptz and linear motifs in 4-bptz, with bifurcated C-I...N interactions in 5 involving both pyridine and tetrazine N atoms. Halogen-halogen and halogen-fluorine interactions further modulate packing and stacking orientations. Reactions with hydroquinone in 3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine-hydroquinine (2/1), 2C₁₂H₈N₆·C₆H₆O₂ (6), and 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine-hydroquinine (1/1), C₁₂H₈N₆·C₆H₆O₂ (7), reveal the role of hydrogen bonding in long-range organization: 2-bptz forms discrete triads, whereas 4-bptz generates extended chains propagating through pyridine-phenol interactions. Pyridine-to-tetrazine plane-to-plane angles vary across structures, reflecting the influence of substitution pattern and donor type on molecular geometry. These results demonstrate that subtle changes in pyridyl orientation and the nature of halogen- or hydrogen-bond donors can direct the formation of distinct supramolecular architectures, providing a foundation for the rational design of tetrazine-based crystalline materials with tunable packing and noncovalent interactions.