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

A novel co-crystal, [(BTEMPO)(2)(2+) center dot 4I(2)center dot 2I(5)(-)] (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)(2)(2+) cations. The cationic dimers are held together by a pair of reversed C-H center dot center dot center dot O=C 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 I-5(-) 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 I-2 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 I-2 and organic electron donors, which are debated frequently, and fills the gaps in the reaction mechanism of I-2 with free radicals or analogues.