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

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(mu(1,3)-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(mu(1,3)-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(mu(1,3)-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 logK(11) = 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.