Selective Recognition of Thallium(I) by 1,3-alternate Calix[4]arene-bis(crown-6 Ether): A New Talent of the Known Ionophore

Abstract

1,3-Alternate calix[4]arene-bis(crown-6) (1), designed originally for selective separation of Cs⁺ from other alkali metal cations, has been found to possess even higher affinity for a soft electron acceptor Tl⁺. Thallium(I) picrate is extracted by 1 from an aqueous solution into chloroform in 1:1 metal-to-ligand stoichiometry, with the extraction constant being 10-fold higher than that found for the extraction of caesium(I) picrate under otherwise identical conditions. The coordination mode of Tl⁺ in its complex with 1 in solution has been probed by ¹H NMR (CDCl₃) and compared with the analogous complexes of Rb⁺ and Cs⁺. In addition, X-ray crystal structures of the dinuclear complexes of 1 with TlPic, RbPic and CsPic have been determined. As evident from the structural data in solution and solid state, π-coordination of Tl⁺ with the calixarene aromatic framework of 1 plays an important role in the enhanced complexing ability of the calix-biscrown towards this metal ion.

Keywords:

• thallium(I)
• 1,3-alternate calix[4]arene-bis(crown-6)
• extraction selectivity
• π-coordination
• solution and crystal structures

Acknowledgements

This research was supported by Howard University (Grant NF05/14). R.J.B. wishes to acknowledge the NSF-MRI program for funds to purchase the X-ray diffractometer.

Notes

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¹ Earlier [23], we demonstrated that the extraction selectivities of benzo-substituted CEs for AMC may be affected by the picrate anion due to the Pic^–-ligand π–π interactions. However, no ‘picrate effect’ on the metal extraction by 1 is envisioned, since no evidence of π–π stacking of Pic⁻  with the calixarene aromatic units is found in the crystal as well as solution structures of the corresponding picrate complexes vide infra.

² In this work, assignment of the signals for protons b–e in the spectra of the MPic-complexes of 1 was made based on the published NMR data for the closest analogues [10,18], with account for the vide infra crystallographic information for Tl₂ 1(Pic)₂, Rb₂ 1(Pic)₂ and Cs₂ 1(Pic)₂, in particular, the M–O bond distances.

³ Crystal structures of CsPic-complexes with the related mono-crown calix[4]arene derivatives in 1,3-alternate conformation are described in [8,9].

⁴ The CE conformations in the complexes A, B and C, similar to the geometry of crown 2 in D, are distorted from the conformation described by the sequence of O–C–C–O torsion angles as g ⁺ g ^– g ⁺ g ^– g ⁺ [25] (g stands for gauche angle), which is considered to be most energetically beneficial for 18-crown-6 ring. A detailed information on the CE conformations in these complexes is provided in Tables S1 and S2 in the Supplementary information.

Additional information

Notes on contributors

Ebony D. Roper

† † [email protected]

Vladimir S. Talanov

‡ ‡ [email protected]

Raymond J. Butcher

¶ ¶ [email protected]