Complexation of CMPO with trivalent f-cations in ionic liquid medium: Solvent extraction, spectroscopic, EXAFS and DFT studies

Abstract

The complexation of octyl-(phenyl)-N,N-diisobutyl carbamoyl methylene phosphine oxide (CMPO), a well-known ligand for actinide partitioning, with trivalent actinides and lanthanides was investigated in a room temperature ionic liquid, viz. 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]). Solvent extraction studies in [C4mim][Tf2N] medium indicated the participation of three CMPO molecules in the extracted complex. Optical spectroscopic studies in [C4mim][Tf2N] confirmed that CMPO forms predominantly ML, ML2 and ML3 (where M = Nd3+, L = CMPO) types of complexes. The stability constants of the Nd3+/CMPO complexes were several orders of magnitude higher in [C4mim][Tf2N] than those observed in acetonitrile or methanol, which explains the higher extraction ability of CMPO for trivalent f-cations in [C4mim][Tf2N]. Luminescence spectroscopic investigations in [C4mim][Tf2N] also predicted the nature of the extracted species as ML3 (where, M = Eu3+, L = CMPO). DFT calculations indicated the formation of ML33+ as the most populated species, which was further confirmed by the EXAFS measurements.