Gas-Phase Coordination Complexes of Dipositive Plutonyl, PuO22+: Chemical Diversity Across the Actinyl Series

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 [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.

Matrix-Assisted Ionization of Molecular Uranium Species

Matrix-assisted ionization (MAI) demonstrates high sensitivity for a variety of organic compounds; however, few studies have reported the application of MAI for the detection and characterization of inorganic analytes. Trace-level uranium analysis is important in the realms of nuclear forensics, nuclear safeguards, and environmental monitoring. Traditional mass spectrometry methods employed in these fields require combinations of extensive laboratory chemistry sample preparation and destructive ionization methods. There has been recent interest in exploring ambient mass spectrometry methods that enable timely sample analysis and higher sensitivity than what is attainable by field-portable radiation detectors. Rapid characterization of uranium at nanogram levels is demonstrated in this study using MAI techniques. Mass spectra were collected on an atmospheric pressure mass spectrometer for solutions of uranyl nitrate, uranyl chloride, uranyl acetate, and uranyl oxalate utilizing 3-nibrobenzonitrile as the ionization matrix. The uranyl complexes investigated were detectable, and the chemical speciation was preserved. Sample analysis was accomplished in a matter of seconds, and limits of detection of 5 ng of uranyl nitrate, 10 ng of uranyl oxalate, 100 ng of uranyl chloride, and 200 ng of uranyl acetate were achieved. The observed gas-phase speciation was similar to negative-ion electrospray ionization of uranyl compounds with notable differences. Six matrix-derived ions were detected in all negative-ion mass spectra, and some of these ions formed adducts with the uranyl analyte. Subsequent analysis of the matrix suggests that these molecules are not matrix contaminants and are instead created during the ionization process.

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.

Exploring the Coordination of Plutonium and Mixed Plutonyl-Uranyl Complexes of Imidodiphosphinates

The coordination chemistry of plutonium(IV) and plutonium(VI) with the complexing agents tetraphenyl and tetra-isopropyl imidodiphosphinate (TPIP^- and TIPIP^-) is reported. Treatment of sodium tetraphenylimidodiphosphinate (NaTPIP) and its related counterpart with peripheral isopropyl groups (NaTIPIP) with [NBu₄]₂[Pu^IV(NO₃)₆] yields the respective Pu^IV complexes [Pu(TPIP)₃(NO₃)] and [Pu(TIPIP)₂(NO₃)₂] + [Pu^IV(TIPIP)₃(NO₃)]. Similarly, the reactions of NaTPIP and NaTIPIP with a Pu(VI) nitrate solution lead to the formation of [PuO₂(HTIPIP)₂(H₂O)][NO₃]₂, which incorporates a protonated bidentate TIPIP^- ligand, and [PuO₂(TPIP)(HTPIP)(NO₃)], where the protonated HTPIP ligand is bound in a monodentate fashion. Finally, a mixed U(VI)/Pu(VI) compound, [(UO₂/PuO₂)(TPIP)(HTPIP)(NO₃)], is reported. All these actinyl complexes remain in the +VI oxidation state in solution over several weeks. The resultant complexes have been characterized using a combination of X-ray structural studies, NMR, optical, vibrational spectroscopies, and electrospray ionization mass spectrometry. The influence of the R-group (R = phenyl or ⁱPr) on the nature of the complex is discussed with the help of DFT studies.

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.

Divalent and trivalent gas-phase coordination complexes of californium: evaluating the stability of Cf(ii)

The divalent oxidation state is increasingly stable relative to the trivalent state for the later actinide elements, with californium the first actinide to exhibit divalent chemistry under moderate conditions. Although there is evidence for divalent Cf in solution and solid compounds, there are no reports of discrete complexes in which Cf(II) is coordinated by anionic ligands. Described here is the divalent Cf methanesulfinate coordination complex, Cf(II)(CH3SO2)3(-), prepared in the gas phase by reductive elimination of CH3SO2 from Cf(III)(CH3SO2)4(-). Comparison with synthesis of the corresponding Sm and Cm complexes reveals reduction of Cf(III) and Sm(III), and no evidence for reduction of Cm(III). This reflects the comparative 3+/2+ reduction potentials: Cf(3+) (-1.60 V) ≈ Sm(3+) (-1.55 V) ≫ Cm(3+) (-3.7 V). Association of O2 to the divalent complexes is attributed to formation of superoxides, with recovery of the trivalent oxidation state. The new gas-phase chemistry of californium, now the heaviest element to have been studied in this manner, provides evidence for Cf(II) coordination complexes and similar chemistry of Cf and Sm.

Influence of denticity and combined soft–hard strategy on the interaction of picolinic-type ligands with NpO2+

The denticity of the ligands and the combined hard–soft donor strategy work cooperatively in the coordination of NpO₂⁺ with ligands.

Gas Phase Reactions of Ions Derived from Anionic Uranyl Formate and Uranyl Acetate Complexes

The speciation and reactivity of uranium are topics of sustained interest because of their importance to the development of nuclear fuel processing methods, and a more complete understanding of the factors that govern the mobility and fate of the element in the environment. Tandem mass spectrometry can be used to examine the intrinsic reactivity (i.e., free from influence of solvent and other condensed phase effects) of a wide range of metal ion complexes in a species-specific fashion. Here, electrospray ionization, collision-induced dissociation, and gas-phase ion-molecule reactions were used to create and characterize ions derived from precursors composed of uranyl cation (U^VIO₂²⁺) coordinated by formate or acetate ligands. Anionic complexes containing U^VIO₂²⁺ and formate ligands fragment by decarboxylation and elimination of CH₂=O, ultimately to produce an oxo-hydride species [U^VIO₂(O)(H)]^-. Cationic species ultimately dissociate to make [U^VIO₂(OH)]⁺. Anionic complexes containing acetate ligands exhibit an initial loss of acetyloxyl radical, CH₃CO₂•, with associated reduction of uranyl to U^VO₂⁺. Subsequent CID steps cause elimination of CO₂ and CH₄, ultimately to produce [U^VO₂(O)]^-. Loss of CH₄ occurs by an intra-complex H⁺ transfer process that leaves U^VO₂⁺ coordinated by acetate and acetate enolate ligands. A subsequent dissociation step causes elimination of CH₂=C=O to leave [U^VO₂(O)]^-. Elimination of CH₄ is also observed as a result of hydrolysis caused by ion-molecule reaction with H₂O. The reactions of other anionic species with gas-phase H₂O create hydroxyl products, presumably through the elimination of H₂. Graphical Abstract ᅟ.

Collision-induced dissociation of uranyl-methoxide and uranyl-ethoxide cations: Formation of UO2 H(+) and uranyl-alkyl product ions

RATIONALE

The lower levels of adventitious H2 O in a linear ion trap allow the fragmentation reactions of [UO2 OCH3 ](+) and [UO2 OCH2 CH3 ](+) to be examined in detail.

METHODS

Methanol- and ethanol-coordinated UO2 (2+) -alkoxide precursors were generated by electrospray ionization (ESI). Multiple-stage tandem mass spectrometry (MS(n) ) and collision-induced dissociation (CID) were performed using a linear ion trap mass spectrometer.

RESULTS

CID of [UO2 OCH3 (CH3 OH)n ](+) and [UO2 OCH2 CH3 (CH3 CH2 OH)n ](+) , n = 3 and 2, causes loss of neutral alcohol ligands, leading ultimately to bare uranyl-alkoxide species. Comparison of 'native' to deuterium-labeled precursors reveals dissociation pathways not previously observed in 3-D ion trap experiments.

CONCLUSIONS

UO2 H(+) is generated from [UO2 OCH3 ](+) by transfer of H from the methyl group. Variable-energy and variable-time CID experiments suggest that the apparent threshold for production of UO2 H(+) is lower than for UO2 (+) , but the pathway is kinetically less favored for the former than for the latter. CID experiments reveal that [UO2 OCH2 CH3 ](+) dissociates to generate [UO2 CH3 ](+) , a relatively rare species with a U-C bond, and [UO2 (O = CH2 )](+) .

On the Highest Oxidation States of Metal Elements in MO4 Molecules (M = Fe, Ru, Os, Hs, Sm, and Pu)

Metal tetraoxygen molecules (MO4, M = Fe, Ru, Os, Hs, Sm, Pu) of all metal atoms M with eight valence electrons are theoretically studied using density functional and correlated wave function approaches. The heavier d-block elements Ru, Os, Hs are confirmed to form stable tetraoxides of Td symmetry in (1)A1 electronic states with empty metal d(0) valence shell and closed-shell O(2-) ligands, while the 3d-, 4f-, and 5f-elements Fe, Sm, and Pu prefer partial occupation of their valence shells and peroxide or superoxide ligands at lower symmetry structures with various spin couplings. The different geometric and electronic structures and chemical bonding types of the six iso-stoichiometric species are explained in terms of atomic orbital energies and orbital radii. The variations found here contribute to our general understanding of the periodic trends of oxidation states across the periodic table.

Crown ether complexes of uranyl, neptunyl, and plutonyl: hydration differentiates inclusion versus outer coordination

The structures of actinyl-crown ether complexes are key to their extraction behavior in actinide partitioning. Only UO2(18C6)(2+) and NpO2(18C6)(+) (18C6 = 18-Crown-6) have been structurally characterized. We report a series of complexes of uranyl, neptunyl, and plutonyl with 18-Crown-6, 15-Crown-5 (15C5), and 12-Crown-4 (12C4) produced in the gas phase by electrospray ionization (ESI) of methanol solutions of AnO2(ClO4)2 (An = U, Np, or Pu) and crown ethers. The structures of 1:1 actinyl-crown ether complexes were deduced on the basis of their propensities to hydrate. Hydration of a coordinated metal ion requires that it be adequately exposed to allow further coordination by a water molecule; the result is that hydrates form for outer-coordination isomers but not for inclusion isomers. It is demonstrated that all the actinyl 18C6 complexes exhibit fully coordinated inclusion structures, while partially coordinated outer-coordination structures are formed with 12C4. Both inclusion and outer-coordination isomers were observed for actinyl-15C5 complexes, depending on whether they resulted from ESI or from collision-induced dissociation. Evidence for the formation of 1:2 complexes of actinyls with 15C5 and 12C4, which evidently exhibit bis-outer-coordination structures, is presented.

Mass spectrometric characterization and quantification of Pu(VI) hydrolysis products

In the present work the first direct measurement of hydrolysis products of Pu(VI) was achieved by means of nano-electrospray ionization time-of-flight mass-spectrometry. The results indicate that monomeric PuO2(OH) + and dimeric (PuO2)2(OH) 2 2+ species are present in solution. A trimeric species does not appear within the detection limit of the experiment, in contrast to U(VI) hydrolysis. The relative abundances of the Pu(VI) hydrolysis species in the ESI mass spectra are in good agreement with the published formation constants.

Electron transfer dissociation of dipositive uranyl and plutonyl coordination complexes

Reported here is a comparison of electron transfer dissociation (ETD) and collision-induced dissociation (CID) of solvent-coordinated dipositive uranyl and plutonyl ions generated by electrospray ionization. Fundamental differences between the ETD and CID processes are apparent, as are differences between the intrinsic chemistries of uranyl and plutonyl. Reduction of both charge and oxidation state, which is inherent in ETD activation of [An(VI) O(2) (CH(3) COCH(3) )(4) ](2+) , [An(VI) O(2) (CH(3) CN)(4) ](2) , [U(VI) O(2) (CH(3) COCH(3) )(5) ](2+) and [U(VI) O(2) (CH(3) CN)(5) ](2+) (An = U or Pu), is accompanied by ligand loss. Resulting low-coordinate uranyl(V) complexes add O(2) , whereas plutonyl(V) complexes do not. In contrast, CID of the same complexes generates predominantly doubly-charged products through loss of coordinating ligands. Singly-charged CID products of [U(VI) O(2) (CH(3) COCH(3) )(4,5) ](2+) , [U(VI) O(2) (CH(3) CN)(4,5) ](2+) and [Pu(VI) O(2) (CH(3) CN)(4) ](2+) retain the hexavalent metal oxidation state with the addition of hydroxide or acetone enolate anion ligands. However, CID of [Pu(VI) O(2) (CH(3) COCH(3) )(4) ](2+) generates monopositive plutonyl(V) complexes, reflecting relatively more facile reduction of Pu(VI) to Pu(V).

Gas-phase coordination complexes of U(VI)O2(2+), Np(VI)O2(2+), and Pu(VI)O2(2+) with dimethylformamide

Electrospray ionization of actinyl perchlorate solutions in H(2)O with 5% by volume of dimethylformamide (DMF) produced the isolatable gas-phase complexes, [An(VI)O(2)(DMF)(3)(H(2)O)](2+) and [An(VI)O(2)(DMF)(4)](2+), where An = U, Np, and Pu. Collision-induced dissociation confirmed the composition of the dipositive coordination complexes, and produced doubly- and singly-charged fragment ions. The fragmentation products reveal differences in underlying chemistries of uranyl, neptunyl, and plutonyl, including the lower stability of Np(VI) and Pu(VI) compared with U(VI).