Lanthanide Complexes with a Calix[8]arene Bearing Phosphinoyl Pendant Arms

The phosphinoyl-substituted macrocyclic receptor B8bL8, derived from p-tert-butylcalix[8]arene, was synthesized and its structure was studied in solution. According to temperaturedependent 1H and 31P NMR spectroscopic data, the calix[8]arene adopts a so-called in–out cone conformation. To investigate the influence of the narrow rim substitution, interaction with trivalent lanthanide ions, LnIII (Ln = La, Eu, Tb, Lu), was probed by MS, UV/Vis and NMR spectroscopic titrations. Although both 1:1 (in the presence of triflate) and 2:1 (in the presence of nitrate) Ln:B8bL8 complexes could be isolated in the solid state, the major species present in methanol is the 1:1 complex, irrespective of the anion. NMR spectroscopic data point to a common conformation for the 1:1 complexes in solution, the lanthanide ions being coordinated by four of the eight phosphinoyl arms, with a coordination sphere completed by methanol molecules or by nitrate ions, as ascertained by IR and MS spectra. The ligand displays a weak absorption at 360 nm that can be assigned to an intraligand charge-transfer (ILCT) state, which is very sensitive to coordination. Photophysical data of the EuIII 2:1 complex point to both metal ion sites providing a very similar chemical environment for the lanthanide ions, with no coordinated water, which is contrary to what is observed for the 1:1 complex. The use of optical electronegativity to predict the energy of the charge-transfer states in the lanthanide systems with inequivalent ligands is discussed and extensive analysis of the vibronic satellites of the Eu(5D0->7FJ) transitions ascertains conclusions drawn on the EuIII coordination.