Reactions of gas-phase uranyl formate/acetate anions: reduction of carboxylate ligands to aldehydes by intra-complex hydride attack

In a previous study, electrospray ionization, collision-induced dissociation (CID), and gas-phase ion-molecule reactions were used to create and characterize ions derived from homogeneous precursors composed of a uranyl cation (UVIO22+) coordinated by either formate or acetate ligands [E. Perez, C. Hanley, S. Koehler, J. Pestok, N. Polonsky and M. Van Stipdonk, Gas phase reactions of ions derived from anionic uranyl formate and uranyl acetate complexes, J. Am. Soc. Mass Spectrom., 2016, 27, 1989-1998]. Here, we describe a follow-up study of anionic complexes that contain a mix of formate and acetate ligands, namely [UO2(O2C-CH3)2(O2C-H)]- and [UO2(O2C-CH3)(O2C-H)2]-. Initial CID of either anion causes decarboxylation of a formate ligand to create carboxylate-coordinated U-hydride product ions. Subsequent CID of the hydride species causes elimination of acetaldehyde or formaldehyde, consistent with reactions that include intra-complex hydride attack upon bound acetate or formate ligands, respectively. Density functional theory (DFT) calculations reproduce the experimental observations, including the favored elimination of formaldehyde over acetaldehyde by hydride attack during CID of [UO2(H)(O2C-CH3)(O2C-H)]-. We also discovered that MSn CID of the acetate-formate complexes leads to generation of the oxyl-methide species, [UO2(O)(CH3)]-, which reacts with H2O to generate [UO2(O)(OH)]-. DFT calculations support the observation that formation of [UO2(O)(OH)]- by elimination of CH4 is favored over H2O addition and rearrangement to create [UO2(OH)2(CH3)]-.

Gas-phase synthesis of [OU-X]+ (X = Cl, Br and I) from a UO22+ precursor using ion-molecule reactions and an [OUCH]+ intermediate

Difficulty in the preparation of gas-phase ions that include U in middle oxidation states(III,IV) have hampered efforts to investigate intrinsic structure, bonding and reactivity of model species. Our group has used preparative tandem mass spectrometry (PTMS) to synthesize a gas-phase U-methylidyne species, [OUCH]+, by elimination of CO from [UO2(CCH)]+ [M. J. van Stipdonk, I. J. Tatosian, A. C. Iacovino, A. R. Bubas, L. Metzler, M. C. Sherman and A. Somogyi, J. Am. Soc. Mass Spectrom., 2019, 30, 796-805], which has been used as an intermediate to create products such as [OUN]+ and [OUS]+ by ion-molecule reactions. Here, we investigated the reactions of [OUCH]+ with a range of alkyl halides to determine whether the methylidyne is a also a useful intermediate for production and study of the oxy-halide ions [OUX]+, where X = Cl, Br and I, formally U(IV) species for which intrinsic reactivity data is relatively scarce. Our experiments demonstrate that [OUX]+ is the dominant product ion generated by reaction [OUCH]+ with neutral regents such as CH3Cl, CH3CH2Br and CH2CHCH2I.

Collision-induced dissociation of [UVI O2 (ClO4 )]+ revisited: Production of [UVI O2 (Cl)]+ and subsequent hydrolysis to create [UVI O2 (OH)]. RAPID COMMUNICATIONS IN MASS SPECTROMETRY : RCM 2018;32:1085-1091. [PMID: 29645301 DOI: 10.1002/rcm.8135] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

RATIONALE

In a previous study [Rapid Commun Mass Spectrom. 2004;18:3028-3034], collision-induced dissociation (CID) of [U^VI O₂ (ClO₄ )]⁺ appeared to be influenced by the high levels of background H₂ O in a quadrupole ion trap. The CID of the same species was re-examined here with the goal of determining whether additional, previously obscured dissociation pathways would be revealed under conditions in which the level of background H₂ O was lower.

METHODS

Water- and methanol-coordinated [U^VI O₂ (ClO₄ )]⁺ precursor ions were generated by electrospray ionization. Multiple-stage tandem mass spectrometry (MSⁿ ) for CID and ion-molecule reaction (IMR) studies was performed using a linear ion trap mass spectrometer.

RESULTS

Under conditions of low background H₂ O, CID of [U^VI O₂ (ClO₄ )]⁺ generates [U^VI O₂ (Cl)]⁺ , presumably by elimination of two O₂ molecules. Using low isolation/reaction times, we found that [U^VI O₂ (Cl)]⁺ will undergo an IMR with H₂ O to generate [U^VI O₂ (OH)]⁺ .

CONCLUSIONS

With lower levels of background H₂ O, CID experiments reveal that the intrinsic dissociation pathway for [U^VI O₂ (ClO₄ )]⁺ leads to [U^VI O₂ (Cl)]⁺ , apparently by loss of two O₂ molecules. We propose that the results reported in the earlier CID study reflected a two-step process: initial formation of [U^VI O₂ (Cl)]⁺ by CID, followed by a very rapid hydrolysis reaction to leave [U^VI O₂ (OH)]⁺ .

Influence of Background H2O on the Collision-Induced Dissociation Products Generated from [UO2NO3]<sup/>

Developing a comprehensive understanding of the reactivity of uranium-containing species remains an important goal in areas ranging from the development of nuclear fuel processing methods to studies of the migration and fate of the element in the environment. Electrospray ionization (ESI) is an effective way to generate gas-phase complexes containing uranium for subsequent studies of intrinsic structure and reactivity. Recent experiments by our group have demonstrated that the relatively low levels of residual H₂O in a 2-D, linear ion trap (LIT) make it possible to examine fragmentation pathways and reactions not observed in earlier studies conducted with 3-D ion traps (Van Stipdonk et al. J. Am. Soc. Mass Spectrom. 14, 1205-1214, 2003). In the present study, we revisited the dissociation of complexes composed of uranyl nitrate cation [U^VIO₂(NO₃)]⁺ coordinated by alcohol ligands (methanol and ethanol) using the 2-D LIT. With relatively low levels of background H₂O, collision-induced dissociation (CID) of [U^VIO₂(NO₃)]⁺ primarily creates [UO₂(O₂)]⁺ by the ejection of NO. However, CID (using He as collision gas) of [U^VIO₂(NO₃)]⁺ creates [UO₂(H₂O)]⁺ and UO₂⁺ when the 2-D LIT is used with higher levels of background H₂O. Based on the results presented here, we propose that product ion spectrum in the previous experiments was the result of a two-step process: initial formation of [U^VIO₂(O₂)]⁺ followed by rapid exchange of O₂ for H₂O by ion-molecule reaction. Our experiments illustrate the impact of residual H₂O in ion trap instruments on the product ions generated by CID and provide a more accurate description of the intrinsic dissociation pathway for [U^VIO₂(NO₃)]⁺. Graphical Abstract ᅟ.

Complexation of Manganese with Glutarimidedioxime: Implication for Extraction Uranium from Seawater

The molecule of glutaroimidedioxime, a cyclic imidedioxime moiety that can form during the synthesis of the poly(amidoxime)sorbent and is reputedly responsible for the extraction of uranium from seawater. Complexation of manganese (II) with glutarimidedioxime in aqueous solutions was investigated with potentiometry, calorimetry, ESI-mass spectrometry, electrochemical measurements and quantum chemical calculations. Results show that complexation reactions of manganese with glutarimidedioxime are both enthalpy and entropy driven processes, implying that the sorption of manganese on the glutarimidedioxime-functionalized sorbent would be enhanced at higher temperatures. Complex formation of manganese with glutarimidedioxime can assist redox of Mn(II/III). There are about ~15% of equilibrium manganese complex with the ligand in seawater pH(8.3), indicating that manganese could compete to some degree with uranium for sorption sites.

The enhanced uranyl–amidoxime binding by the electron-donating substituents

This work describes the enhanced binding between uranyl and amidoxime ligands due to the electron-donating effect of the substituents.