Halogen-bonded complexes with neutral nitroxide radicals as the Lewis base have been investigated in liquid and frozen solutions by multifrequency CW and pulse EPR spectroscopies, including ENDOR and ELDOR-detected NMR (EDNMR) techniques. The non-covalent interaction with iodopentafluorobenzene as halogen-bond donor is shown to affect a variety of EPR parameters of the stable nitroxide radicals. In liquid solution, only bulk effects on the EPR signal, i.e. isotropic g value, isotropic 14N hyperfine coupling and linewidth, are observed. Experiments on frozen solutions allow for a more in-depth dissection of complexing effects. W-band EPR spectra at cryogenic temperatures exhibit multiple signal components of different 14N hyperfine interactions and spectral widths. This demonstrates the coexistence of several halogen-bonded complexes that differ in donor-acceptor binding geometries. These complexes have different relaxation properties, which allow for their spectral discrimination. 19F ENDOR experiments prove the origin of these effects to be different specific intermolecular interactions rather than a consequence of changes in the solvation environment. The EPR spectra yet reveal a strong influence of solvent composition on the amount of the various complexes formed. The introduced methodology for the characterization of such adducts improves our understanding of halogen bonding and could be helpful in the development of tailor-made donors and complexes for specific applications.
Dedicated to: Kev Salikhov on the occasion of his 80th birthday.
This work was supported by the Max-Planck-Gesellschaft and the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft (DFG).
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