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Iodoform – a forgotten halogen bond donor

Iodoform (CHI3) was one of the first halogen bond donors used for preparation of halogen bonded binary solids. It was one of halogen bond donors used by Odd Hassel in his seminal research of charge-transfer compounds during the early 1950- es. [1, 2] However, since the renaissance of halogen bond in 1990-es, very little attention was given to iodoform, so that its potential for crystal engineering has for the most part remained unknown. We have therefore decided to study the potential of iodoform as a halogen bond donor by preparing a series of cocrystals of iodoform with a wide range of neutral and ionic halogen bond acceptors. With neutral ditopic electron donors such as 4, 4'-bipyridine and 1, 4- diazabicyclo[2.2.2]octane iodoform was found to form cocrystals of 1:1 stoichiometry. In those cases iodoform is donor of two halogen bonds to two acceptor molecules, with the third iodine atom participating only in interhalogen (I∙∙∙I) contacts. An attempt to prepare an equivalent structure with N, N, N’, N’- tetramethylethylenediamine (temed) however yielded a somewhat unexpected cocrystal of iodoform with iodide salt of monoprotonated temed, with the iodide formed by decomposition of iodoform. The structure is characterized by a diamantoid network of iodoform molecules and iodide ions with each iodide acting as an acceptor of three halogen and one C–H∙∙∙I hydrogen bond from four separate iodoform molecules, with disordered cations filling the space within the network. This has prompted us to study the cocrystallisation of iodoform with organic iodides in more detail. We have found that iodoform forms cocrystals with many organic iodides such as N-methyl-3- methylpyridinium iodide, N-methyl-3- chloropyridinium iodide, N-methyl-3- bromopyridinium iodide, N-methyl-3- iodopyridinium iodide, N-methyl-4- methylpyridinium iodide, N, N, N, N’, N’, N’- hexamethylethylenediamonium diiodide, N, N’- dimethyl-1, 4- diazoniumbicyclo[2.2.2]octane diiodide and N- methyl-2, 4, 6 trimethylpyridine iodide. In all those cocrystals, the iodide anion acts as halogen (and occasionally C–H∙∙∙I hydrogen) bondacceptor forming halogen bonded anionic networks, the geometries and topologies of whichare determined by the steric properties of the organic counterion.

halogen bond, iodoform, cocrystals

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