The Structure and Characterization of 3,4,5-Triiodo-2-Methylthiophene: An Unexpected Iodination Product of 2-Methylthiophene

Abstract: We report the structure and characterization of 3,4,5-triiodo-2-methylthiophene, obtained as an unexpected, unreported, and significant side product from the iodination of 2-methylthiophene using iodine and iodic acid. Identity of this unexpected product was confirmed by X-ray crystallography and ¹H and ¹³C NMR. The compound crystallizes in the P2₁/c space group with unit cell parameters a = 16.4183(10) Å, b = 4.1971(3) Å, c = 14.3888(9) Å, β = 111.4442(14), Z = 4, and D_calc = 3.425 g cm⁻³. Analysis of residual electron density maps indicated the presence of crystallographic disorder between the 2-methyl and 5-iodo positions leading to a model of two distinct molecules of 3,4,5-triiodo-2-methylthiophene where the atoms of these two groups were exchanged. Non-covalent iodine–iodine and sulfur–iodine interactions are observed. Graphical Abstract: Three products, two of which are constitutional isomers, are possible when installing multiple iodine atoms on 2-methylthiophene; X-ray structural analysis and spectral characterization show that the 4,5-diiodo isomer is not formed and that the 3,4,5-triiodo isomer is unexpectedly obtained. Halogen and chalcogen bonding are clearly observed. [Figure not available: see fulltext.].