A p-tert-butylcalix[6]arene bearing phosphinoyl pendant arms for the complexation and sensitisation of lanthanide ions

Abstract

The new lower rim functionalised macrocycle 5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,41,42-hexakis(dimethylphosphinoylmethoxy)calix[6]arene (B6bL6) has been synthesised. Temperature dependent 1H and 31P NMR studies indicate a mixture of conformers with a time-averaged C6v symmetry at 405 K in dmso-d6; ΔG≠ values for conformational interconversion processes are equal to 68(1) and 75(2) kJ mol−1 and reveal a semi-flexible macrocycle with alternate in-out cone conformation, a fact confirmed by molecular mechanics and dynamics calculations. B6bL6 crystallises as a dimer where the two calixarenes are linked through hydrogen bonding and surrounded by water and toluene molecules in the lattice. UV-Vis spectrophotometric titration of B6bL6 with La(III) in acetonitrile yields stability constants logβ1 = 9.8 and logβ2 = 19.6 for the 1 ∶ 1 and 1 ∶ 2 (Ln ∶ B6bL6) species, respectively. The corresponding complexes with La, Eu, Gd and Tb have been isolated and characterised. Lifetime determinations of the Eu(III) and Tb(III) complexes in acetonitrile solution are consistent with no or little interaction of water molecules in the inner co-ordination sphere. The new ligand sensitises reasonably well the luminescence of the Tb(III) (Qabs = 4.8% , τf = 2.1 ms, 1 ∶ 1 complex) and Eu(III) (Qabs = 2.5%, τ = 2.0 ms, 1 ∶ 2 complex) ions.