Halogen Bonding as a Supramolecular Modulator of Crystal Packing and Exchange Interactions in Nitronyl Nitroxides

Through the interaction of a halogen bond donor (1,4-diiodotetrafluorobenzene; 1,4-FIB) with isomers of pyridyl-substituted nitronyl nitroxides (Py-NN), five novel cocrystals were constructed via halogen bonding. The cocrystals (pPy-NN)(n)1,4-FIB (n = 1, 2, 4) and (mPy-NN)(n)1,4-FIB (n = 1, 2) are represented by the discrete clusters Py-NN1,4-FIBNN-Py, cyclic tetrameric structures (pPy-NN)(4)(1,4-FIB)(4), or one-dimensional (1D) zigzag chains. Density functional theory calculations were used to analyze the nature of the halogen bonds and to compare the strengths of INPy and IONO interactions. The halogen bonds were shown to be quite strong, ranging from 6.4 to 9.0 kcal/mol, according to the QTAIM analysis. In addition, quantum chemical analyses revealed relatively weak exchange interactions of both ferromagnetic and antiferromagnetic nature, indicating a modulating effect of the halogen bonding on magnetic properties. These findings point to the potential usefulness of halogen bonding interactions for promoting the self-assembly of stable organic radicals for further use in the preparation of exchange-coupled organic spin systems.