The [sigma]-hole interaction of 1,3-bis(2,6-diisopropylphenyl)imidazolidine [SiPr] with iodine-containing small molecules

Abstract

Iodine-containing small molecules, such as R-CC-I and I-CC-I, can form halogen bonds, interactions that arise from the electrostatic forces between a halogen and Lewis base. This interaction arises from the presence of a [sigma]-hole, an area of relatively positive electrostatic potential on the outer tip of the halogen. Halogens also have the ability to act as both halogen bond donors and acceptors in polyhalide environments. Our interest in N-heterocyclic carbene [NHC] chemistry led us to explore the reactivity of these strong C-centered bases with several iodine-containing small molecules, in particular, diiodoacetylene [ICCI] and 1- iodo-2-(trimethylsilyl)acetylene [ICC(TMS)]. We have systematically studied the reactions between the NHC, 1,3- Bis(2,6-diisopropylphenyl)imidazolidine [SiPr], and both the iodoalkynes and we have identified and characterized a variety of interesting products. For instance, we isolated SiPr•ICC(TMS) from the reaction with the TMS substituted acetylene. Perhaps the most interesting species were isolated from the interaction of SiPr and ICCI in the presence of excess iodide. [SiPr-I]I•ICCI•I[I-SiPr] and [SiPr-I]I•I[subscript 2]CCI[subscript 2]•I[I-SiPr] are unprecedented hybrid polyhalide species. Single crystal X-ray data have been collected for all of the products isolated. The structural geometries have been used to investigate the halogen bonding, and other intermolecular interactions, in these novel compounds.