One dimensional halogen bond design: Br⋯N versus I⋯N with fluoroarenes

A series of co-crystallization experiments were performed using 1,4-dibromo- and 1,4-diiodotetrafluorobenzene (Br2F4bz and I2F4bz), as well as 4,4′-dibromo- and 4,4′-diiodooctafluorobiphenyl (Br2F8bph and I2F8bph), with nine N-heterocyclic diamines and the bicyclic, tertiary diamine 1,4-diazabicyclo[2.2.2]octane to elucidate trends between benzene and biphenyl-based halogen bond donors, as well as factors influencing the resulting halogen bond strength. The acceptors chosen contain nitrogen atom geometries resulting in the formation of chains and a total of 29 new crystal structures were obtained. In the majority of cases, cocrystals resulting from Br2F4bz and I2F4bz are isomorphous, as are the structures from Br2F8bph and I2F8bph, but the observed structures are different for benzene versus biphenyl based derivatives. Analysis of the halogen bond geometries indicates statistically significant correlations between halogen bond distance and both molecular electrostatic potential as well as percent buried volume, a measure of the steric encumbrance of the acceptor nitrogen atoms.