Physicochemical behaviour of a dinuclear uranyl complex formed with an octaphosphinoylated para-tert-butylcalix[8]arene. Spectroscopic studies in solution and in the solid state

Abstract

Spectrophotometric titrations of an octaphosphinoylated para-tert-butylcalix[8]arene (B8bL8) by uranyl nitrate and vice versa in anhydrous ethanol indicate that the species with 2:1 (uranyl:calixarene) stoichiometry is the major complex in solution. Based on these results, a synthesis route was designed to isolate this complex. The latter is an orange, non-hygroscopic polycrystalline powder, with chemical formula [(UO2)2(NO3)4(B8bL8) 2H2O]2(H2O). (Compd. 1), as ascertained by elemental analysis. Spectroscopic characterization of Compd. 1 in the solid and liquid states suggests that a neutral dinuclear uranyl calixarene complex was formed. MIR and FIR spectra indicate that, four phosphinoyl arms of the calixarene and two monodentate nitrates are bound to each 6-coordinate uranyl ion in the complex because no vibrational frequencies from un-coordinated O@P groups or from ionic nitrates are present; in addition the spectra reveal that water molecules form intramolecular hydrogen bonding with monodentate nitrates. The de-convoluted luminescence and XPS spectra obtained in the solid state point to a similar chemical environment around each uranyl ion, as confirmed by the mono-exponential decay of the luminescence. The more rigid conformation acquired by the calixarene in the complex and the non-symmetrical arrangement of the coordinated nitrates result in a particular feature of the emission spectra at 77 K. No evidence of cation-cation interaction was found. A rough approach to the molecular structure of the complex by molecular modelling based on the experimental findings yielded a molecule that was useful for understanding the physicochemical behaviour of Compd. 1.