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

Abstract

AbstractThe phosphinoyl‐substituted macrocyclic receptor B8bL8, derived from p‐tert‐butylcalix[8]arene, was synthesized and its structure was studied in solution. According to temperature‐dependent 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)