Carbon-atom hybridization tunes the halogen-bond strength in the series of DABCO·C₂H_2nI₂ (n = 0-2) cocrystals

Two new cocrystals of 1,4-diazabicyclo[2.2.2]octane (DABCO, C6H12N2) with 1,2-diiodoethene (1,2-C2H2I2) and 1,2-diiodoethane (1,2-C2H4I2) complete a series of halogen-bond-assisted cocrystals which started with DABCO·C2I2 [Perkins et al. (2012). CrystEngComm, 14, 3033-3038]. The structural and computational analysis of this series illustrate the correlation between the polarization of the I atom and the hybridization of the C atom bound to it. The formation of a rather stable halogen bond by the alkylic iodide of saturated 1,2-C2H4I2 was unusual and respective cocrystals are formed only in nonpolar solvents, while, in the polar medium of acetonitrile, a very intense reaction of DABCO quaternization takes place resulting in 1-(2-iodoethyl)-4-aza-1-azoniabicyclo[2.2.2]octane triiodide, C8H16IN2+·I3- or [N(CH2CH2)3N-CH2CH2I][I3].