Structural and spectroscopic characterization of two new layered uranyl(VI) p-xylenediphosphonate compounds synthesized via ionothermal method

Two new uranyl phosphonates were synthesized by using p-xylenediphosphonic acid (pmbH4) to react with uranyl salts in Emim or BMMim ([EMim][PF6] = 1-ethyl-3-methylimidazolium hexafluorophosphate; [BMMim]Cl = 1-butyl-2,3-dimethylimidazolium chloride) at elevated temperature, namely [EMim][UO2(pmbH2)0.5(pmbH2)0.5(ox)0.5] (1) and [BMMim]2[(UO2)2(pmbH2)(pmb)] (2). Both compounds adopt two dimensional structures, consisting of an anionic layer of uranyl phosphonate, with cations filled in the interlayer spaces. The spectroscopic properties of the compounds were studied extensively using UV–Vis absorption, temperature dependent fluorescence, Raman and IR spectroscopies. The UV–Vis absorption spectrum contains typical peaks of uranyl complexes at around 320 nm and 420 nm, which are originated from the charge transfer transitions from O 2p orbitals to the U 5f/6d orbitals. Both compounds adopt five strong emission peaks, whose intensities increase with the decrease of temperature, as observed in the temperature dependent fluorescence spectra. In Raman and IR spectra, the peaks corresponding to the stretch of C–H/O–H, P–O/P O, and O U O bonds could all be located and assigned.