Synthesis and Characterization of Water Stable Uranyl(V) Complexes

Abstract

The importance of uranyl(V) (UO 2 +)s pecies associated with environmental and geologic applications is becoming increasingly evident, but the tendency of the uranyl-(V) cation to disproportionate in water has prevented the isolation of stable complexes.Here we demonstrate that in the presence of the tridentate complexing dipicolinate (dpa 2À), aligand highly abundant in soil, the uranyl(V) species can be stabilized and isolated in anoxic basic water.Stable uranyl(V) dipicolinate complexes are readily formed from the reduction of the uranyl(VI) analogue both in organic solution and in basic water,a nd their solution and solid-state structure were determined. Ab is-dpa U V O 2 + complex was obtained from water at pH 10, while at higher pH values,atrinuclear monodpa cation-cation complex was isolated. These results present the second ever isolated water stable uranyl(V) complex. Moreover,w ed emonstrate that dipicolinate complexes of U VI O 2 2+ ,U V O 2 + and U IV are strongly luminescent with asignature characteristic of each oxidation state.This provides unique examples of luminescent U V and U IV compounds.

Photoluminescence of Pentavalent Uranyl Amide Complexes

“…By analogy with reported experimental and theoretical data of uranyl(V) 19 , 20 , 64 , 73 and neptunyl(VI) 47 5f 1 species (where the emission with An = O (yl) to actinide charge transfer character has been observed at 405, 440, and 438 nm respectively in experiments), the equivalent lowest energy U ← U=O (yl) LMCT transition is expected to lie in the UV and be blue-shifted with respect to that in uranyl(VI). In a complex of pseudo D 3 h symmetry, as in 2 (L) and 3 (L), this transition is predicted to be formally Laporte-forbidden but may be relaxed in part with respect to the free ion of higher symmetry ( D ∞ h ) due to the removal of the inversion symmetry by the inclusion of the equatorial ligands.…”

“… 9 , 16 , 17 Quite remarkably, they have recently been isolated and studied in aqueous media. 18 − 20 Since 2003, 21 there have been a number of reports on the structural and chemical properties of the once elusive [U V O 2 ] + cation. 9 , 16 , 17 , 22 − 29 Nevertheless, there is a remarkable paucity of information regarding the electronic structure and photophysical properties of [U V O 2 ] + .…”

“…Very recently, Mazzanti and co-workers reported the emission spectra of a water-stable dipicolinate [U V O 2 ] + complex. 20 However, the photophysical properties of the [U V O 2 ] + cation remain unassigned. We have previously shown that the application of photoluminescence techniques to [U VI O 2 ] 2+ can provide detailed information regarding the behavior of uranyl, particularly its coordination, speciation, and nuclearity including the existence of “cation–cation” interactions (CCIs).…”

Photoluminescence of Pentavalent Uranyl Amide Complexes

“…Very recently, the group of Mazzanti reported the emission properties of a water soluble uranyl(V) dipicolinic acid (dpa) complex [K(2.2.2.crypt)]2n{[KUO2(dpa)2]}n. 10 Here, the emission spectrum exhibits a broad profile with two maxima at 404 and 459…”

“…[5][6][7][8][9] Despite its inherent stability, uranyl can undergo a number of transformations in the environment, the cornerstone of which is the reduction of soluble uranyl(VI) to insoluble U 4+ . 10 This transformation has been postulated to proceed via a disproportionation mechanism, which involves the formation of transient uranyl(V) species, {[U V O2] + }n (n = 1 or 2). 11,12 The detection of this transient species in the environment and in biological systems remains a challenging goal due to its redox instability and lability.…”