Anionic 3D cage networks self-assembled by iodine and V-shaped pentaiodides using dimeric oxoammonium cations produced in situ as templates

Abstract

A novel co-crystal, [(BTEMPO)₂²⁺·4I₂·2I₅⁻] (BTEMPO⁺ = 4-benzoyloxy-2,2,6,6-tetramethylpiperidinyl-1-oxoammonium cation), was successfully constructed using iodine and 4-benzoyloxy-2,2,6,6-tetramethylpiperidinyl-1-oxy free radical (BTEMPO) as starting materials and was well characterized by XRD, Raman and calculation. The co-crystal possesses a fascinating 3D anionic cage structure formed by V-shaped-pentaiodides and iodine via multiple halogen bonding and on a template of dimeric (BTEMPO)₂²⁺ cations. The cationic dimers are held together by a pair of reversed C–H⋯OC hydrogen bonds and stabilized the 3D cage structure by C–H⋯I hydrogen bonds between methyl-protons of BTEMPO⁺ and iodine in the framework. The reaction mechanism of producing BTEMPO⁺ and I₅⁻ is proposed and verified by UV-Vis spectroscopy and ESI-MS, which initially goes through a halogen bonding complex between iodine and BTEMPO free radical and then Milliken inner charge transfer and charge separation reaction. UV-Vis absorption spectroscopy confirms the halogen bonding complex between I₂ and BTEMPO with a formation constant of 6.94 M⁻¹ and a 1 : 1 stoichiometry in chloroform. The ESI-MS directly led to observation of the less stable intermediates in the mechanism. It is believed that the mechanism proposed here is helpful in understanding the interactions between I₂ and organic electron donors, which are debated frequently, and fills the gaps in the reaction mechanism of I₂ with free radicals or analogues.