Linear 2-Ethylhexyl Imidophosphoric Esters as Effective Rare-Earth Element Extractants

Nikolay S Bredov,Andrey S Esin,Maria D Ryabochenko,Mikhail V Gorlov,Anna A Bykovskaya,Oksana A Sinegribova,Vyacheslav V Kireev

doi:10.3390/app10041229

Abstract

Imidophosphoric organic esters containing phosphoryl groups are potential polydentate ligands and promising extractants of rare-earth elements. For their preparation, a monophosphazene salt [PCl3=N−PCl3]+[PCl6]− and short phosphazene oligomers of the general formula [Cl–(PCl2=N)n–PCl3]+[PCl6]−, where n = 4–7, were synthesized via living cationic polymerization of Cl3P=NSiMe3 and used as starting compounds. All phosphazenes were reacted with 2-ethylhexanol to obtain the corresponding esters of imidophosphoric acids (EIPAs). The formation of imidophosphoric acids occurs due to the phosphazene-phosphazane rearrangement of –P(OR)2=N– or –P(OH)(OR)=N– units, where R = 2-ethylhexyl. The prepared EIPAs were characterized by 1H, 31P NMR, and MALDI-TOF analyses and their extractive capacity towards lanthanide ions in aqueous solutions of nitric acid was examined. The EIPAs are mixtures of mono-, di-, and trifunctional compounds of the type HxA, where x = 1–3, which can form chelate complexes of lanthanide ions [Ln(A)z], where z = 3–6, depending on the chain length. The longer chain EIPAs are more suitable for collective rare-earth elements extraction. A comparison of the extraction properties of the EIPAs with the industrially used polyalkylphosphonitrilic acid (PAPNA) was drawn.

Highlights

Synthetic approaches to various derivatives of imidophosphoric acids are widely represented [12,13,14], and the effectiveness of these compounds as rare-earth elements (REE) extractants is well established [14,15,16,17]
esters of imidophosphoric acids (EIPAs) can be prepared by reacting linear phosphazenes with alcohols of various structures, in particular, 2-ethylhexanol
The number of phosphorus-containing units in the chain does not change during the substitution of chlorine atoms in linear oligophosphazenes with 2-ethylhexyl groups

Summary

Introduction

Synthetic approaches to various derivatives of imidophosphoric acids are widely represented [12,13,14], and the effectiveness of these compounds as rare-earth elements (REE) extractants is well established [14,15,16,17]. {OP(OR’)2 }NH{P(OR”) O}, where R’ and R” correspond to Et and n-Bu, Me and Ph, Et and Ph, Ph and p-methylphenyl, were prepared and described [18]. Most of these compounds are not even characterized by routine NMR spectroscopy or mass spectrometry.

Methods

Results

Conclusion