Modulating Crystal Packing and Magnetic Properties of Nitroxide Free Radicals by Halogen Bonding

By
combining a halogen bonding donor of 1,4-diiodotetrafluorobenzene
(1,4-DITFB) or 1,2-diiodotetrafluorobenzene (1,2-DITFB) with 4-benzoyloxy-2,2,6,6-tetramethyl-piperidine-1-oxy
free radical (BTEMPO, 1), two novel cocrystals, BTEMPO·1,4-DITFB
(2) and BTEMPO·2­(1,2-DITFB) (3), featuring
a one-dimensional (1D) zigzag chain and a cyclic tetrameric structure,
respectively, were constructed via halogen bonding. Furthermore, by
tuning the halogen bonding between BTEMPO and halogen bonding donors,
the packing modes of organic free radicals in both 2 and 3 are successfully modulated to result in side-by-side 1D
mode in 2 and head-to-head dimer manner in 3. Magnetic studies reveal that in all compounds the organic radicals
are antiferromagnetically coupled; however, the magnetic coupling
in 2 and 3 is stronger than that in 1, indicating the modulating effect of halogen bonding on
magnetic properties that are further confirmed by quantum chemistry
calculations.