Emission spectroscopy of uranium(iv) compounds: a combined synthetic, spectroscopic and computational study

Abstract

Emission spectroscopy has been used for the first time in a spectroscopic study of a family of uranium(IV) halide complexes in non-aqueous media. The room temperature electronic absorption spectra of the simple coordination compounds [Li(THF)₄][UX₅(THF)] (X = Cl, Br, I), [Et₄N]₂[UCl₆] and UCl₄ in THF have been recorded and all transitions assigned with the aid of a comprehensive computational study using CASSCF and CASPT2 techniques. Excitation into a band of f-d and LMCT character followed by energy transfer into the 5f-orbital manifold accounts for the UV-visible radiative transitions observed in the emission spectra, which have been fully assigned as arising from transitions from the 5f¹6d¹ electronic configuration to envelopes of states arising from the ground state 5f² configuration. The bonding in [Li(THF)₄][UCl₅(THF)] has been further elucidated utilising NBO and AIM calculations which describe the nature of the U–Cl bond as predominantly ionic with some dative covalent character and substantial overlap between the Cl 3p orbitals and 5f and 6d orbitals on uranium. These studies indicate that the emission spectral fingerprint of simple U(IV) compounds of O_h, C_4v and C_2v symmetry are similar and characteristic and may be used as a diagnostic tool to assign U(IV) species in solution and by inference, in the environment, in the presence of [UO₂]²⁺.