A p-tert-butylcalix[4]arene functionalised at its lower rim with ether-amide pendant arms acts as an inorganic–organic receptor: structural and photophysical properties of its lanthanide complexes

Abstract

The new narrow-rim functionalised receptor 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis[2-(diethylcarbamoylmethoxy)ethoxy]calix[4]arene (A₄bL⁴) has been synthesised. NMR spectra show it to adopt a cone conformation in solution with a time-averaged C_4v symmetry. A₄bL⁴ reacts with trivalent lanthanide ions in acetonitrile to yield 1 ∶ 1 complexes with logK₁ in the range 8.6–9.6, as demonstrated by ES-MS, ¹H- and ¹³C-NMR data. Complexes with Ln = Eu, Gd, Tb, and Lu have been isolated and the crystal structure of [Lu(A₄bL⁴)(H₂O)](CF₃SO₃)₃·2Et₂O shows the metal ion encapsulated in the cavity formed by the four arms. Lu^III is 9-co-ordinated, being bound to the four ether and four carbonyl functions and to a water molecule itself H-bonded to the phenolic ether functions, which rigidifies the cavity formed by the pendant arms. Additionally, an ether molecule is inserted into the hydrophobic cavity defined by the aromatic rings. Both NMR (La, Lu) and luminescence (Eu, Tb) data point to a fairly high local symmetry at the metal centre, while lifetime determinations are consistent with the co-ordination of an inner-sphere water molecule. The new ligand sensitises the luminescence of the Tb^III ion (Q_abs = 5.8% in acetonitrile, τ_F  = 1.42 ms) while the probable presence of a LMCT state severely limits such a process for Eu^III: Q_abs = 2.0%, τ_F  = 0.73 ms.