Synthesis, metal binding and spectral properties of novel bis-1,3-diketone calix[4]arenes

Abstract

New bis-1,3-diketone derivatives of calix[4]arene (3–5) have been synthesized with good yields by the addition of a sodium salt of acetylacetone, 1-benzoylacetone and dibenzoylmethane to 5,17-bis-(bromomethyl)-25,26,27,28-tetrahydroxycalix[4]arenes. The structural properties of the obtained compounds and their complexes have been established by means of IR, UV-Vis, NMR spectroscopy and quantum-chemical calculations. The complex ability of bis-1,3-diketones towards Al³⁺, Ni²⁺, Cu²⁺ and lanthanide ions (Nd³⁺, Eu³⁺, Tb³⁺) has been investigated by using a liquid–liquid extraction method. The UV-Vis data indicate thermodynamically favorable 1 : 1 complex formation of the ligands with Tb³⁺ in alkaline DMF, although the time required for the equilibration reveals the difference between calix[4]arenes bearing acetylaceton-, benzoylaceton- and dibenzoylmethane-substituents. The steric hindrance effect on keto–enol transformation is the reason for the difference. The ligand-centered emissions of Gd³⁺ complexes with benzoylaceton- and acetylaceton-substituted calix[4]arenes reveal them both as more convenient antennae for red and infra-red than for green lanthanide luminescence. Indeed, the benzoylaceton-substituted counterpart sensitizes Yb³⁺-centered luminescence to a good extent. Nevertheless, the luminescence of Tb³⁺ is sensitized by the acetylaceton-substituted counterpart to a better extent than that of Yb³⁺, while only poor red Eu³⁺ emission is observed under sensitization by both the ligands.