Supramolecular covalency of halogen bonds revealed by NMR contact shifts in paramagnetic cocrystals

Although supramolecular interactions such as halogen bonding are often classified as "non-covalent" interactions, computational methods have predicted that they possess a significant covalent component. In this article, we adopt a unique strategy that combines experimental solid-state NMR and relativistic DFT approaches to explore the electronic nature of previously postulated supramolecular covalency [Bora et al., Chem.-Eur. J., 2017, 23, 7315]. Our approach involves the analysis of hyperfine interactions and hyperfine shifts in the NMR spectra of halogen-bonded cocrystals containing a paramagnetic transition-metal complex. We demonstrate that the hyperfine interaction pertaining to the paramagnetic transition-metal center and observed at the probed nucleus of the cocrystallized (halogen-bonded) molecule is governed by the Fermi-contact mechanism. This contact mechanism originates in "through-bond" spin transmission and, therefore, unequivocally reports on the electron sharing between the halogen-bonded molecules, i.e., halogen-bond covalency.