Anticooperativity and Competition in Some Cocrystals Featuring Iodine-Nitrogen Halogen Bonds

Phenomena such as anticooperativity and competition among non-covalent bond donors and acceptors are key considerations when exploring the polymorphic and stoichiomorphic landscapes of binary and higher-order cocrystalline architectures. We describe the preparation of four cocrystals of 1,3,5-trifluoro-2,4,6-triiodobenzene with N-heterocyclic compounds, namely acridine, 3-aminopyridine, 4-methylaminopyridine, and 1,2-di(4-pyridyl)ethane. The cocrystals, which are characterized by single-crystal and powder X-ray diffraction experiments, all show moderately strong and directional iodine center dot center dot center dot nitrogen halogen bonds with reduced distance parameters ranging from 0.79 to 0.92 and carbon-iodine center dot center dot center dot nitrogen bond angles ranging from 165.4(3) to 175.31(7)degrees. The cocrystal comprising 1,3,5-trifluoro-2,4,6-triiodobenzene and acridine provides a relatively rare example where all three halogen bond donor sites form halogen bonds with three acceptor molecules, overcoming an anticooperative effect. This effect manifests itself through the lengthening of non-halogen-bonded C-I bonds, weakening their potential to form halogen bonds. The effect is only observed once two halogen bonds have been formed to 1,3,5-trifluoro-2,4,6-triiodobenzene; one such bond does not appear to be adequate. Among the four cocrystals studied, competition between the pyridyl nitrogen atoms and the amine nitrogen atoms suggests that the former are the preferred halogen bond acceptors. Analysis by Hirshfeld fingerprint plots and C-13 and F-19 magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy provides additional insights into the prevalence of various short contacts in the crystal structures and into the spectral response to halogen-bond-induced cocrystallization.

Automated summary

This paper discusses phenomena such as anticooperativity and competition among non-covalent bond donors and acceptors in binary and higher-order cocrystalline architectures. Four cocrystals of 1,3,5-trifluoro-2,4,6-triiodobenzene with N-heterocyclic compounds were prepared and characterized using X-ray diffraction experiments. The cocrystals exhibit moderately strong and directional iodine-nitrogen halogen bonds and show interesting effects such as overcoming anticooperative effects and competition between different nitrogen acceptors. Analysis using Hirshfeld fingerprint plots and solid-state NMR spectroscopy provides further insights into the crystal structures and spectral response to halogen-bond-induced cocrystallization.