Nitroxide Radical Spin Probes for Exploring Halogen-Bonding Interactions in Solution.

The synthesis of 2,3,5,6-tetrafluoro-4-iodobenzyl tert-butyl nitroxide (2-I) and its use as spin probe for the detection of halogen-bond (XB) complexes by EPR is reported. Formation of a XB complex between 2-I and several XB acceptors was evidenced by a significant change in the value of the benzylic hyperfine splitting upon complexation. Thermodynamic parameters for the formation of XB complex with quinuclidine were obtained by recording EPR spectra in the temperature range 203-294 K. In addition, competitive experiments allowed for the measurement of the equilibrium constant of the XB complex with a chloride anion. The proposed procedure constitutes the first direct EPR methodology providing a reliable determination of the strength of the XB bond in solution.