2,5-Diiodothiophene: A Versatile Halogen Bonding Synthon for Crystal Engineering

Relative to the common diiodobenzene-based halogen bond donors, little is known about the halogen bonding of iodine-substituted thiophene compounds. A series of co-crystallization experiments were performed using the sulfur-atom-containing halogen bond donor 2,5-diiodothiophene with N-heterocyclic diamines, as well as ammonium iodide salts. These results reveal a wide range of halogen bonding motifs tied to the halogen bond donor/acceptor stoichiometry. While, in acceptor-rich stoichiometries, discrete units prevail through C-I center dot center dot center dot I halogen bonds, donor-rich stoichiometries encourage the formation of two-dimensional halogen bonding networks, often with the additional involvement C-I center dot center dot center dot S and S center dot center dot center dot S interactions. Halogen bonding through the C-I iodine atom predominates in all structures. The measured halogen bond distances fall between those typically observed for 1,4-diiodobenzene and 1,4-diiodotetrafluorobenzene, reaffirming the role of fluorination in strengthening the halogen bond while also expanding the library of halogen bonding synthons to donors having intermediate electrostatic potentials.