Dimeric and Trimeric Uranyl(VI)–Citrate Complexes in Aqueous Solution

Abstract

This research addresses a subject discussed controversially for almost 70 years. The interactions between the uranyl­(VI) ion, U­(VI), and citric acid, H3Cit, were examined using a multi-method approach comprising nuclear magnetic resonance (NMR), ultraviolet–visible (UV–vis), attenuated total reflectance Fourier-transform infrared (ATR FT-IR), and extended X-ray absorption fine-structure (EXAFS) spectroscopies as well as density functional theory (DFT) calculations. Combining 17O NMR spectroscopy and DFT calculation provided an unambiguous decision on complex configurations, evidencing for the first time that the dimeric complex, (UO2)2(HCit–H)2 2–, exists as two diastereomers with the syn-isomer in aqueous solution strongly favored over the anti-isomer. Both isomers interconvert mutually with exchange rates of ∼30 s–1 at −6 °C and ∼249 s–1 at 60 °C in acidic solution corresponding to an activation barrier of about 24 kJ mol–1. Upon increasing the pH value, ternary dimeric mono- and bis-hydroxo as well as trimeric complexes form, that is, (UO2)2(HCit–H)2(OH)3–, (UO2)2(HCit–H)2(OH)2 4–, (UO2)3(O)­(Cit–H)3 8–, and (UO2)3(O)­(OH)­(Cit–H)2 5–, respectively. Stability constants were determined for all dimeric and trimeric species, with log β° = −(8.6 ± 0.2) for the 3:3 species being unprecedented. Additionally, in the 6:6 sandwich complex, formed from two units of 3:3 species, the 17O NMR resonance of the trinuclear uranyl­(VI) core bridging μ3-O is shown for the first time. Species distribution calculations suggest that the characterized polynuclear U­(VI)–citrate species do not significantly increase uranium­(VI) mobility in the environment. Furthermore, we revise the misconceptions in the aqueous U­(VI)–citric acid solution chemistry, that is, structures proposed and repeatedly taken up, and outline generalized isostructural considerations to provide a basis for future U­(VI) complexation studies.

Europium(III) Meets Etidronic Acid (HEDP): A Coordination Study Combining Spectroscopic, Spectrometric, and Quantum Chemical Methods

“…[40] reported this pK a to be >14.6. Nevertheless, in the presence of appropriate Lewis acids, deprotonation was observed at pH values below the corresponding pK a value [41]. Consequently, such behavior was considered for the subsequent metal ion complexation studies.…”

2-Phosphonobutane-1,2,4,-Tricarboxylic Acid (PBTC): pH-Dependent Behavior Studied by Means of Multinuclear NMR Spectroscopy

“…Three sample series (S1-S3) were prepared for NMR titration. S1 comprises samples prepared using deuterated chemicals and pD adjusted as described elsewhere [39]. Therefore, original 50% (w/w) aqueous PBTC solution, batch 1, TCI Germany GmbH (see Supplementary Materials) was adjusted to pH~6 using NaOH prior to lyophilization.…”

“…Using Gaussian16 program [42], density functional theory (DFT) calculations on PBTC species in different protonation states were performed at the B3LYP level in water through the use of polarizable continuum model (PCM) [25,39,[43][44][45]. All-electron basis set of triple-zeta quality was used on all elements.…”

Insights at the molecular level into the formation of oxo-bridged trinuclear uranyl complexes

“…Finally, we believe that this result will be of considerable ongoing interest to those wishing to extend the use of 17 O-NMR spectroscopy to related uranyl chemistry investigations in the future. 32–35…”