Production and collision-induced dissociation of gas-phase, water- and alcohol-coordinated uranyl complexes containing halide or perchlorate anions

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.

Dinuclear Complexes of Uranyl, Neptunyl, and Plutonyl: Structures and Oxidation States Revealed by Experiment and Theory

Dinuclear perchlorate complexes of uranium, neptunium, and plutonium were characterized by reactivity and DFT, with results revealing structures containing pentavalent, hexavalent, and heptavalent actinyls, and actinyl-actinyl interactions (AAIs). Electrospray ionization produced native complexes [(AnO₂)₂(ClO₄)₃]^- for An:An = U:U, Np:Np, Pu:Pu, and Np:Pu, which are intuitively formulated as actinyl(V) perchlorates. However, DFT identified lower-energy structures [(AnO₂)(AnO₃)(ClO₄)₂(ClO₃)]^- comprising a perchlorate fragmented to ClO₃, actinyl(VI) cation An^VIO₂²⁺, and neutral AnO₃. For U:U and Np:Np, and Np in Np:Pu, the coordinated AnO₃ is calculated as actinyl(VI) with an equatorial oxo, [O_yl═An^VI═O_yl][═O_eq], whereas for Pu:Pu, it is plutonyl(V) oxyl, [O_yl═Pu^V═O_yl][-O_eq^•]. The implied lower stability of Pu^VI versus Np^VI indicates weaker Pu═O_eq versus Np═O_eq bonding. Adsorption of O₂ by the U:U complex suggests oxidation of U^V to U^VI, corroborating the assignment of perchlorate [(An^VO₂)₂(ClO₄)₃]^-. DFT predicts the O₂ adducts are [(An^VIO₂)(O₂)(An^VIO₂)(ClO₄)₃]^- with actinyls oxidized from +V to +VI by bridging peroxide, O₂^2-. In accordance with reactivity, O^2- addition is computed as substantially exothermic for U:U and least favorable for Pu:Pu. Collision-induced dissociation of native complexes eliminated ClO₂ to yield [(AnO₂)(O)₂(AnO₂)(ClO₄)₂]^-, in which fragmented O atoms bridge as oxyl O^-• and oxo O^2- to yield uranyl(VI) and plutonyl(VI), or as oxos O^2- to yield neptunyl(VII), [O_yl═Np^VII═O_yl]³⁺.

Collision-induced dissociation of [UO2 (NO3 )(O2 )]- and reactions of product ions with H2 O and O2

We recently reported a detailed investigation of the collision-induced dissociation (CID) of [UO₂ (NO₃ )₃ ]^- and [UO₂ (NO₃ )₂ (O₂ )]^- in a linear ion trap mass spectrometer (J. Mass Spectrom. DOI:10.1002/jms.4705). Here, we describe the CID of [UO₂ (NO₃ )(O₂ )]^- which is created directly by ESI, or indirectly by simple elimination of O₂ from [UO₂ (NO₃ )(O₂ )₂ ]^- . CID of [UO₂ (NO₃ )(O₂ )]^- creates product ions as at m/z 332 and m/z 318. The former may be formed directly by elimination of O₂ , while the latter required decomposition of a nitrate ligand and elimination of NO₂ . DFT calculations identify a pathway by which both product ions can be generated, which involves initial isomerization of [UO₂ (NO₃ )(O₂ )]^- to create [UO₂ (O)(NO₂ )(O₂ )]^- , from which elimination of NO₂ or O₂ will leave [UO₂ (O)(O₂ )]^- or [UO₂ (O)(NO₂ )]^- , respectively. For the latter product ion, the composition assignment of [UO₂ (O)(NO₂ )]^- rather than [UO₂ (NO₃ )]^- is supported by ion-molecule reaction behavior, and in particular, the fact that spontaneous addition of O₂ , which is predicted to be the dominant reaction pathway for [UO₂ (NO₃ )]^- is not observed. Instead, the species reacts with H₂ O, which is predicted to be the favored pathway for [UO₂ (O)(NO₂ )]^- . This result in particular demonstrates the utility of ion-molecule reactions to assist the determination of ion composition. As in our earlier study, we find that ions such as [UO₂ (O)(NO₂ )]^- and [UO₂ (O)(O₂ )]^- form H₂ O adducts, and calculations suggest these species spontaneously rearrange to create dihydroxides.

Collision-induced dissociation of [UO2 (NO3 )3 ]- and [UO2 (NO3 )2 (O2 )]- and reactions of product ions with H2 O and O2

Electrospray ionization (ESI) can produce a wide range of gas-phase uranyl (UO₂ ²⁺ ) complexes for tandem mass spectrometry studies of intrinsic structure and reactivity. We describe here the formation and collision-induced dissociation (CID) of [UO₂ (NO₃ )₃ ]^- and [UO₂ (NO₃ )₂ (O₂ )]^- . Multiple-stage CID experiments reveal that the complexes dissociate in reactions that involve elimination of O₂ , NO₂ , or NO₃ , and subsequent reactions of interesting uranyl-oxo product ions with (neutral) H₂ O and/or O₂ were investigated. Density functional theory (DFT) calculations reproduce experimental results and show that dissociation of nitrate ligands, with ejection of neutral NO₂ , is favored for both [UO₂ (NO₃ )₃ ]^- and [UO₂ (NO₃ )₂ (O₂ )]^- . DFT calculations also suggest that H₂ O adducts to products such as [UO₂ (O)(NO₃ )]^- spontaneously rearrange to create dihydroxides and that addition of O₂ is favored over addition of H₂ O to formally U(V) species.

Formation and hydrolysis of gas-phase [UO2 (R)]+ : R═CH3 , CH2 CH3 , CH═CH2 , and C6 H5

The goals of the present study were (a) to create positively charged organo-uranyl complexes with general formula [UO₂ (R)]⁺ (eg, R═CH₃ and CH₂ CH₃ ) by decarboxylation of [UO₂ (O₂ C─R)]⁺ precursors and (b) to identify the pathways by which the complexes, if formed, dissociate by collisional activation or otherwise react when exposed to gas-phase H₂ O. Collision-induced dissociation (CID) of both [UO₂ (O₂ C─CH₃ )]⁺ and [UO₂ (O₂ C─CH₂ CH₃ )]⁺ causes H⁺ transfer and elimination of a ketene to leave [UO₂ (OH)]⁺ . However, CID of the alkoxides [UO₂ (OCH₂ CH₃ )]⁺ and [UO₂ (OCH₂ CH₂ CH₃ )]⁺ produced [UO₂ (CH₃ )]⁺ and [UO₂ (CH₂ CH₃ )]⁺ , respectively. Isolation of [UO₂ (CH₃ )]⁺ and [UO₂ (CH₂ CH₃ )]⁺ for reaction with H₂ O caused formation of [UO₂ (H₂ O)]⁺ by elimination of ·CH₃ and ·CH₂ CH₃ : Hydrolysis was not observed. CID of the acrylate and benzoate versions of the complexes, [UO₂ (O₂ C─CH═CH₂ )]⁺ and [UO₂ (O₂ C─C₆ H₅ )]⁺ , caused decarboxylation to leave [UO₂ (CH═CH₂ )]⁺ and [UO₂ (C₆ H₅ )]⁺ , respectively. These organometallic species do react with H₂ O to produce [UO₂ (OH)]⁺ , and loss of the respective radicals to leave [UO₂ (H₂ O)]⁺ was not detected. Density functional theory calculations suggest that formation of [UO₂ (OH)]⁺ , rather than the hydrated U^V O₂ ⁺ , cation is energetically favored regardless of the precursor ion. However, for the [UO₂ (CH₃ )]⁺ and [UO₂ (CH₂ CH₃ )]⁺ precursors, the transition state energy for proton transfer to generate [UO₂ (OH)]⁺ and the associated neutral alkanes is higher than the path involving direct elimination of the organic neutral to form [UO₂ (H₂ O)]⁺ . The situation is reversed for the [UO₂ (CH═CH₂ )]⁺ and [UO₂ (C₆ H₅ )]⁺ precursors: The transition state for proton transfer is lower than the energy required for creation of [UO₂ (H₂ O)]⁺ by elimination of CH═CH₂ or C₆ H₅ radical.

Collision-induced dissociation of [UVI O2 (ClO4 )]+ revisited: Production of [UVI O2 (Cl)]+ and subsequent hydrolysis to create [UVI O2 (OH)]. Rapid Commun Mass Spectrom 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)]⁺ .

A 2:1 dicationic complex of tetraethyl methylenebisphosphonate with uranyl ion in acetonitrile and ionic liquids

A new 2:1 dicationic complex formed by TEMBP with uranyl ion in acetonitrile and two hydrophobic ILs, [BMIm][NTf(2)] and [N(4111)][NTf(2)], has been identified with combination of optical spectroscopic and mass spectrometric studies. With excess of TEMBP ligand (L/U > 2.0), the uranyl is completely coordinated by two ligands to form a dicationic complex [UO(2)(TEMBP)(2)](2+). The UV-vis spectra of [UO(2)(TEMBP)(2)](2+) in acetonitrile and in the two ILs are similar. The vibronic fine structures in UV-vis spectrum of [UO(2)(TEMBP)(2)](2+) show characters of tetragonal coordination in the uranyl equatorial plane. The symmetry of proposed structure of [UO(2)(TEMBP)(2)](2+) is D(2h), and its UV-vis spectrum is tentatively interpreted based on the structural similarity to the well studied [UO(2)Cl(4)](2-) complex. The luminescence emission spectrum of [UO(2)(TEMBP)(2)](2+) shows typical vibronic bands, having a mirror relationship with the 455-500 nm region of the corresponding absorption spectrum. The stoichiometry of [UO(2)(TEMBP)(2)](2+) is confirmed by electrospray ionization-ion trap mass spectrometry (ESI-ITMS) studies with acetonitrile as solvent. The "naked" dication (m/z 423) is characterized by the remarkable eight peaks with interval of 14 m/z units in its tandem mass spectra, representing the fragmentation of ligands by losing C(2)H(4) units from their ethoxy groups. However, the dication tends to exist as a weak adduct with either an additional ligand or an anion in the ESI mass spectrum. The adducts {[UO(2)(TEMBP)(2)](2+) + TEMBP} (m/z 567) and {[UO(2)(TEMBP)(2)](2+) + [ClO(4)](-)} (m/z 945) are favorable in pure acetonitrile, while only one adduct {[UO(2)(TEMBP)(2)](2+) + [NTf(2)](-)} (m/z 1126) is predominant in [BMIm][NTf(2)] (diluted with acetonitrile). The results of ESI-ITMS study are consistent with those of optical spectroscopic studies.

Unusual ion UO(4)(-) formed upon collision induced dissociation of [UO(2)(NO(3))(3)](-), [UO(2)(ClO(4))(3)](-), [UO(2)(CH(3)COO)(3)](-) ions

The following ions [UO(2)(NO(3))(3)](-), [UO(2)(ClO(4))(3)](-), [UO(2)(CH(3)COO)(3)](-) were generated from respective salts (UO(2)(NO(3))(2), UO(2)(ClO(4))(3), UO(2)(CH(3)COO)(2)) by laser desorption/ionization (LDI). Collision induced dissociation of the ions has led, among others, to the formation of UO(4)(-) ion (m/z 302). The undertaken quantum mechanical calculations showed this ion is most likely to possess square planar geometry as suggested by MP2 results or strongly deformed geometry in between tetrahedral and square planar as indicated by DFT results. Interestingly, geometrical parameters and analysis of electron density suggest it is an U(VI) compound, in which oxygen atoms bear unpaired electron and negative charge.

Vibrational Spectroscopy of Mass-Selected [UO2(ligand)n]2+ Complexes in the Gas Phase: Comparison with Theory

The gas-phase infrared spectra of discrete uranyl ([UO2]2+) complexes ligated with acetone and/or acetonitrile were used to evaluate systematic trends of ligation on the position of the O=U=O stretch and to enable rigorous comparison with the results of computational studies. Ionic uranyl complexes isolated in a Fourier transform ion cyclotron resonance mass spectrometer were fragmented via infrared multiphoton dissociation using a free electron laser scanned over the mid-IR wavelengths. The asymmetric O=U=O stretching frequency was measured at 1017 cm(-1) for [UO2(CH3COCH3)2]2+ and was systematically red shifted to 1000 and 988 cm(-1) by the addition of a third and fourth acetone ligand, respectively, which was consistent with increased donation of electron density to the uranium center in complexes with higher coordination number. The values generated computationally using LDA, B3LYP, and ZORA-PW91 were in good agreement with experimental measurements. In contrast to the uranyl frequency shifts, the carbonyl frequencies of the acetone ligands were progressively blue shifted as the number of ligands increased from two to four and approached that of free acetone. This observation was consistent with the formation of weaker noncovalent bonds between uranium and the carbonyl oxygen as the extent of ligation increases. Similar trends were observed for [UO2(CH3CN)n]2+ complexes, although the uranyl asymmetric stretching frequencies were greater than those measured for acetone complexes having equivalent coordination, which is consistent with the fact that acetonitrile is a weaker nucleophile than is acetone. This conclusion was confirmed by the uranyl stretching frequencies measured for mixed acetone/acetonitrile complexes, which showed that substitution of one acetone for one acetonitrile produced a modest red shift of 3-6 cm(-1).

Binding of Molecular O2 to Di- and Triligated [UO2]+

Gas-phase complexes containing dioxouranium(V) cations ([UO(2)](+)) ligated with two or three sigma-donating acetone ligands reacted with dioxygen to form [UO(2)(A)(2,3)(O(2))](+), where A is acetone. Collision-induced dissociation studies of [UO(2)(A)(3)(O(2))](+) showed initial loss of acetone, followed by elimination of O(2), which suggested that O(2) was bound more strongly than the third acetone ligand, but less strongly than the second. Similar behavior was observed for complexes in which water was substituted for acetone. Binding of dioxygen to [UO(2)](+) containing zero, one, or four ligands did not occur, nor did it occur for analogous ligated U(IV)O(2) or U(VI)O(2) ions. For example, only addition of acetone and/or H(2)O occurred for the U(VI) species [UO(2)OH](+), with the ligand addition cascade terminating in formation of [UO(2)OH(A)(3)](+). Similarly, the U(IV) species [UOOH](+) added donor ligands, which produced the mixed-ligand complex [UOOH(A)(3)(H(2)O)](+) as the preferred product at the longest reaction times accessible. Since dioxygen normally functions as an electron acceptor, an alternative mode for binding dioxygen to the cationic U(V)O(2) center is indicated that is dependent on the presence of an unpaired electron and donor ligands in the uranyl valence orbitals.

Investigating Bidentate and Tridentate Carbamoylmethylphosphine Oxide Ligand Interactions with Rare-Earth Elements Using Electrospray Ionization Quadrupole Ion Trap Mass Spectrometry

Electrospray ionization (ESI) quadrupole ion trap mass spectrometry (QIT-MS) and collisionally activated dissociation (CAD) were used to evaluate the rare-earth binding properties of two hydrophobic carbamoylmethylphosphine oxide (CMPO) ligands, the normal bidentate variety, (t-BuC6H4)2P(O)CH2C(O)N(i-Bu)2 (A), a new potentially tridentate extractant, (t-BuC6H4)2P(O)CH[CH2C(O)N(i-Bu)2]C(O)N(i-Bu)2 (B), and tributyl phosphate. The mass spectral results obtained from analysis of 1% HNO3/methanol solution containing the ligands and dissolved lanthanide salts reveal that the favorable stoichiometries of the ligand/metal/nitrate complexes are 2:1:2 for the bidentate ligand A, 1:1:2 for the tridentate ligand B, and 3:1:2 for the monodentate tributyl phosphate. These observed stoichiometries correlate with the number of available binding sites on each ligand as well as with potential steric effects. Energy-variable collisionally activated dissociation experiments showed that for the 2:1:2 complexes involving ligand A or B, as the ionic radius of the bound metal decreased, the removal of nitric acid required less energy and resulted in less extensive spontaneous solvent coordination. This experimental trend suggests that, as the ionic radius of the lanthanide ion decreases, a pair of the carbamoylmethylphosphine ligands is able to more completely solvate the bound metal ion thereby weakening the nitrate-metal interaction.