Evidence for Indirect Action of Ionizing Radiation in 18-Crown‑6 Complexes with Halogenous Salts of Strontium: Simulation of Radiation-Induced Transformations in Ionic Liquid/Crown Ether Compositions

Ionic
liquid/crown ether compositions are an attractive alternative to traditional
extractants in the processes for spent nuclear fuel and liquid radioactive
wastes reprocessing. These compositions are exposed to ionizing radiation,
and their radiation stability, especially in the presence of metal
salts, is a crucial issue. In the present study, the macrocyclic 18C6·Sr­(BF4)2 and 18C6·Sr­(PF6)2 complexes simulating the components of metal loaded ionic liquid/crown
ether extractants were synthesized and their structures were characterized
by FTIR spectroscopy and single-crystal X-ray diffraction analysis.
Inclusion of Sr2+ cation into the 18C6 cavity resulted
in more symmetric D3d conformations of the macrocycle.
The structural transformations of the crown ether were accompanied
by an elongation of polyether CO bonds that could increase
the possibility of radiolytic cleavage of the macrocycle. However,
EPR study of the synthesized compounds subjected to X-ray irradiation
revealed predominant formation of macrocyclic −CH2–ĊH–O– radicals. This result demonstrated
an evidence for indirect action of ionizing radiation on individual
components of the complexes and was reasonably described by a positive
“hole” transfer from primary macrocyclic radical cation
to fluorous anion at the primary stages of radiolysis and a subsequent
interaction of fluorine atom with 18C6 macrocycle in secondary radical
reactions. The observed effects may be partially responsible for enhanced
sensitivity of the ionic liquid/crown ether extractants to ionizing
radiation due to chemical blocking of the crown ether with radiolytic
HF, radiation-chemical degradation of the 18C6, and precipitation
of a low-soluble SrF2.