Evidence for effect of macrocycle symmetry on radiation-induced ring opening in 18-crown-6 complexes with alkali-earth metal halogenides

The complexes of 18-crown-6 with alkali-earth metal halides were synthesized to simulate the macrocyclic component of crown-containing extractants selective for radionuclide 90 Sr and to investigate the effect of cationic and anionic factors on its radiation destruction at low-temperature (77 K) irradiation. The halide anion was found to protect the complexes from ring opening at the initial stages of radiolysis due to positive charge transfer from the primary macrocyclic radical cation to anion. This effect was found in 18-crown-6 complexes with SrCl 2 , SrBr 2 and BaCl 2 , where the macrocycle had D 3d symmetry. In contrast, the anionic protection was less efficient when the 18-crown-6 conformation was measurably distorted, as illustrated by the examples of complexes with SrI 2 , CaCl 2 and, especially, MgCl 2 with folded C 2 and D 2 symmetries. Based on the results obtained, it was concluded that preliminary conformational analysis of a macrocyclic complex could be a useful way to predict its resistance to radiation-induced polyether ring cleavage. • Radiation stability of 18-crown-6 complexes depends on the nature of cation and anions. • Halide-anions protect macrocycle from radiation-induced destruction. • Decrease in metal cation size promotes ring opening at early stage of radiolysis. • Distortion of macrocycle symmetry increases the degradation of the complex.