Structural clarification of a tert-butyl-calix[4]arene-based 8-hydroxyquinoline complex with uranium(VI) in non-aqueous solution


Structural clarification of a tert-butyl-calix[4]arene-based 8-hydroxyquinoline complex with uranium(VI) in non-aqueous solution

Bauer, A.; März, J.; Barthen, R.; Jäschke, A.; Glasneck, F.; Schmeide, K.; Brendler, V.; Kersting, B.; Stumpf, T.

The actinides uranium and thorium are considered disturbing constituents in rare earth production. Thus they have to be removed, e.g. by extraction.[1] Due to their modifiable selectivity and solubility calix[n]arenes are interesting compounds for the extraction of uranium(VI).[2]
A new chalice-like tert-butyl-calix[4]arene-based 8-hydroxyquinoline ligand consisting of four phenolic units was synthesized. Tert-butyl substituents provide a hydrophobic character. Functionalizing of the phenolic hydroxyl groups by 8-hydroxyquinoline ensures the affinity to uranyl ions.[2]
For better process understanding we examined the mechanisms of uranyl complexation by the calix[4]arene derivative. The structure of the tert-butyl-calix[4]arene-based 8-hydroxyquinoline uranyl complex was unveiled by spectroscopy and microcalorimetry.
The complexation studies were performed in acetonitrile. Luminescence spectroscopic studies indicated the interaction of uranyl ions with the ligand. UV visible investigations evidenced the presence of a 1:1 and a 1:2 ligand uranyl complex with stability constants of log ß1:1 = 5.94 ± 0.016 and log ß1:2 = 6.33 ± 0.013.
Microcalorimetry provided the thermodynamic characterization of the complexation.
Single crystal X-ray analysis of the 1:1 complex revealed the coordination of the uranyl ion via a N2O2 donor set (Fig. 1). The charge is compensated by an additional coordinated nitrate ion. Due to the chelation reddish uranyl calix[4]arene complexes were formed as indicated by the absorption band at 525 nm. An increase of the peak up to the addition of two equivalents of uranyl ions suggested a complexation of both uranyl ions through chelation in acetonitrile.
The application of electrospray ionization time-of-flight mass spectrometry affirmed the coordination of two hexavalent uranyl nitrate ions in acetonitrile.
In addition, to improve the understanding of the ligand complexation properties the interaction with thorium is studied.

[1] Z. W. Zhu, Y. Pranolo, C.Y. Cheng, Miner Eng 2015, 77, 185-196.
[2] A. Jäschke, M. Kischel, A. Mansel, B. Kersting, 2016, in prep.

Keywords: SE-FLECX; rare earth production; calix[4]arene; uranium; complexation studies

  • Poster
    ANAKON 2017, 03.-06.04.2017, Tübingen, Deutschland

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Publ.-Id: 24332