Diiodobutadiyne (1) and diiodohexatriyne (2) form cocrystals with bispyridyl oxalamides and ureas, based on the halogen bond between the alkynyl iodine and pyridine nitrogen. In each cocrystal, the oxalamide or urea host forms one-dimensional hydrogen-bonded networks, aligning the diiodopolyyne for potential topochemical polymerization with a repeat distance matching the host repeat.

Crystals have been designed for the purpose of carrying out the topochemical polymerization of two isomeric dipyridyldiacetylenes, 1,4-di(pyrdin-3-yl)buta-1,3-diyne (3-pda) and 1,4-di(pyrdin-4-yl)buta-1,3-diyne (4-pda). A successful polymerization depends on achieving a monomer spacing, d m , of 4.90 Å with a close contact R 1,4 , between the reacting atoms, C1−C4‘, of neighboring diacetylenes. A series of five oxalamide dicarboxylic acid host molecules were chosen for the study since they were known to form hydrogen-bond networks with spacings near the required 4.90 Å. Five host molecules and two diacetylene guests should have given 10 cocrystals, but only eight were successfully obtained. The best parameters were found for the cocrystal of the 4-pyridyl isomer, 4-pda, with the oxalamide of glycine, 1-ox. The repeat distance, d m , was 4.93 Å with an R 1,4 contact of 3.62 Å. The 4-pda:1-ox cocrystals were stable to heat, but ultraviolet irradiation polymerized the compound with an isolated yield of 61%.

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