Lanthanide Complexes of a Calix[4]arene Ligand with Dangling Phosphonate and Picolinamide Arms: Synthesis, Crystal Structures, and Extraction Properties

Abstract

The coordination behavior of the neutral calix[4]arene‐based ligand L exhibiting dangling phosphonate ester and picolinamide groups in 1,3‐ and 2,4‐distal positions towards a series of lanthanide ions was examined. Reaction of L with Ln(NO3)3 in MeOH afforded mononuclear [LnL(µ1,3‐NO3)2]NO3 complexes [Ln = La (6), Pr (7), Eu (8), Gd (9)]. X‐ray crystallography for 6 and 7 revealed tenfold coordination by phosphoryl O, amide O, and pyridine N atoms from L and four O atoms from two chelating nitrato coligands. Treatment of the [LnL(µ1,3‐NO3)2]NO3 salts with NaBPh4 leads to metathesis of the counterion and, in the case of Pr, Eu, and Gd, to substitution of one nitrato coligand by a methanol molecule yielding [LnL(µ1,3‐NO3)2–n(MeOH)n](BPh4)n+1 for n = 0, Ln = La (10) and n = 1, Ln = Pr (11), Eu (12), Gd (13). X‐ray crystallography of complexes 11–13 revealed an isostructural series of ninefold‐coordinated lanthanide complexes in which one nitrato coligand is exchanged by a methanol molecule. UV/Vis batch titration experiments showed the formation of 1:1 complexes of L with trivalent lanthanides (La3+, Pr3+, Eu3+, Gd3+, Yb3+). The stability constants of the complexes determined by UV/Vis spectroscopic titrations are log K11 = 6.44 ± 0.02 (La), 5.96 ± 0.01 (Pr), 6.37 ± 0.01 (Eu), 7.38 ± 0.3 (Gd), and 9.6 ± 0.2 (Yb). Solvent‐extraction experiments with lanthanide picrates revealed that L is able to extract both La3+ and Eu3+ in 1:1 stoichiometry from aqueous media into chloroform.