Synthesis of a para-tert-octylcalix[4]arene fitted with phosphinoyl pendant arms and its complexation properties towards f-elements

Abstract

A new functionalised calixarene, para-tert-octylcalix[4]arene, fitted with phosphinoyl pendant arms (B4boctL4) 1, was synthesised and fully characterised. The procedure using xylene via a Williamson reaction between the tetra sodium derivative of the calixarene and dimethyl-chloromethyl-phosphine oxide decreased the reaction time from 96 to 15h and increased the reaction yield from 46% to 89% with respect to a reported procedure using toluene for the synthesis of a phosphinoylated calixarene with para-tert-butyl substituents. Spectrophotometric titrations of B4boctL4 with La(III), Lu(III), Th(IV) and UO22+ and luminescence titrations of Tb(III) and UO22+ were performed in CH3CN in the presence of 0.01M (CH3CH2)4NCl. The results suggest the formation of two predominant species with 1:1 and 1:2 metal:ligand ratios. The former had a log β11 values of 8.4 and 4.5 for La(III) and UO22+ ions, respectively, and the latter had a log β12 values of 13.8 and 7.4, respectively. Based on this result, a 1M:2L molar ratio was used with the aim of isolating Eu(III) and UO22+ complexes with 1M:2L stoichiometry. However, the characterisation of the complexes indicated 1M:1L stoichiometry. The luminescence properties, such as emission and excitation spectra, lifetimes and quantum yields, of the UO22+ (2) and Eu(III) (3) isolated complexes were analysed. The molecular modelling of the europium complex molecule based on the experimental results suggested a coordination number equal to 9 and a distorted tri-capped trigonal prismatic geometry as the most probable structure. The extraction ability of the titled calixarene in CHCl3 towards UO22+, Th(IV), La(III) and Eu(III) ions from 1M HNO3/3.5M NaNO3 aqueous medium was investigated. Tributylphosphate (TBP) (2.4×10−4M) added to the calixarene ligand (2.4×10−4M) (v/v) both in CHCl3 caused a significant synergistic effect on the uranyl extraction, increasing the extraction from 48% to 96%. Under our conditions, TBP in CHCl3 extracted only 11%. The capability and loading capacity of B4boctL4 towards Th (IV) ions were studied.