Five tripodal CMPO ligands with varying degrees of hydrophobicity: influence on f-element extraction ability and lanthanide luminescence

Abstract

Five tripodal ligands bearing carbamoylmethylphosphine oxide (CMPO) groups have been synthesized. These ligands vary in the length of the pendant alkyl chains bonded to the CMPO group (Et, Bu, Hex, 2-Ethyl hexyl), as well as the hydrophobicity of the tripodal cap (tertiary amine to methine). The effect of these structural changes on the ability of each ligand to extract Ln(NO₃)₃, Th(NO₃)₄ and UO₂(NO₃)₂ out of aqueous solutions of nitric acid into dichloromethane has been measured (Ln = all lanthanide ions except Pm). The ligand with the largest alkyl chains and tertiary amine cap demonstrated, overall, the best extraction ability with the highest percent extraction values of 13% for Yb³⁺, Lu³⁺, Th⁴⁺ and UO₂²⁺ nitrate. The ligand with the hydrophobic methine cap was the poorest extractant of the series, likely due to its inability to operate as a phase transfer agent. Solution studies (NMR, luminescence) were also carried out to probe the influence of the ligand structural changes on the complexes with Ln(NO₃)₃ in acetonitrile. ¹H and ³¹P NMR studies showed that at room temperature the Sm(NO₃)₃–ligand complexes experience fast exchange on both NMR timescales and are likely a dynamic mixture of complexes with differing stoichiometries and coordination geometries. Luminescence studies demonstrated that all five ligands sensitize the metal centered emission of Tb- and Eu(NO₃)₃, with decay lifetime values ranging from 1.97–2.09 ms and 1.08–1.34 ms for the Tb³⁺ and Eu³⁺ complexes, respectively.