Binding of pyrazine-functionalized calix[4]arene ligands with lanthanides in an ionic liquid: thermodynamics and coordination modes

Abstract

The complexation of representative lanthanides with three calix[4]arenes functionalized with four pyrazine pendent arms containing different substituents such as carbamoyl dioctyl (), diisopropyl phosphonate (), and diphenyl phosphoryl () was investigated in water-saturated 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BumimTf2N) by absorption spectroscopy, luminescence spectroscopy, and microcalorimetry. All three ligands form 1 : 1 ML complexes (M = Eu(3+) and L = ligand), and the stability constants (log β) follow the order: (-1.38 ± 0.66) ≪ (3.71 ± 0.02) < (7.47 ± 0.03), similar to the trend in the metal distribution coefficients in solvent extraction using these ligands as extractants. The enthalpy of complexation, determined by microcalorimetry, shows that the complexation of lanthanides with these bulky ligands is exothermic, and proceeds via replacement of water molecules from the primary coordination spheres. The 1 : 1 stoichiometry of the ML complexes was confirmed by electrospray ionization mass spectrometry. Results from optical absorption, luminescence and (31)P-NMR spectroscopy suggest that, out of four pendent arms on the rigid calixarene platform, only two arms coordinate with the lanthanide ion and each arm is tridentate. The influence of structural features of the ligand on the complexation of lanthanides is explained with the help of thermodynamic parameters.

Lanthanide phosphine oxide complexes

“…Whilst the presence of the chloroalkyl groups reduced the extraction efficiency marginally compared to the unsubstituted calix [4]arene, it offers the potential of tethering the extractant to a solid substrate. The coordination of the pyrazine functionalised (67) (X = t Bu, Y = (68), n = 4) in the ionic liquid 1-butyl-3methylimidazolium bis(trifluoromethanesulfonyl) imide (69) has been studied [142]. Lanthanide ions form 1:1 complexes in which two arms bind via CO, aromatic N-atom and PO whilst the other two remain uncoordinated.…”

Hydroxyquinoline–Calix[4]arene Conjugates as Ligands for Polynuclear Lanthanide Complexes: Preparation, Characterization, and Properties of a Dinuclear EuIII Complex

“…(4) There are only a few examples in the literature in which quinoline or 8‐hydroxyquinoline units are tethered to a calix[4]arene backbone,, and none of these have been structurally characterized . (5) The macrocyclic platform can be used for preorganization of multicoordinate and chelating groups in two‐phase solvent extraction for the separation of rare earth elements …”

Role of Tf2N− anions in the ionic liquid–water distribution of europium(iii) chelates

“…Time-resolved laser-induced uorescence spectroscopy (TRLFS) has been widely used as an analytical technique for the speciation of uorescent metal ions such as lanthanides(III) and actinides(III). [32][33][34][35][36][37][38][39] Because the uorescence is sensitive to the inner-sphere coordination environment of the metal ion, the uorescence intensity, spectral shape, and uorescence lifetime change are dependent on the solvation or coordination structure of the metal chelate. Especially, the uorescence lifetime, s, gives the hydration state in the rst coordination sphere of the metal ion.…”