Electron transfer dissociation of dipositive uranyl and plutonyl coordination complexes

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)]-.

Unusual Actinyl Complexes with a Redox-Active N,S-Donor Ligand

Understanding the fundamental chemistry of soft N,S-donor ligands with actinides across the series is critical for separation science toward sustainable nuclear energy. This task is particularly challenging when the ligands are redox active. We herein report a series of actinyl complexes with a N,S-donor redox-active ligand that stabilizes different oxidation states across the actinide series. These complexes are isolated and characterized in the gas phase, along with high-level electronic structure studies. The redox-active N,S-donor ligand in the products, C₅H₄NS, acts as a monoanion in [U^VIO₂(C₅H₄NS^-)]⁺ but as a neutral radical with unpaired electrons localized on the sulfur atom in [Np^VO₂(C₅H₄NS^•)]⁺ and [Pu^VO₂(C₅H₄NS^•)]⁺, resulting in different oxidation states for uranium and transuranic elements. This is rationalized by considering the relative energy levels of actinyl(VI) 5f orbitals and S 3p lone pair orbitals of the C₅H₄NS^- ligand and the cooperativity between An-N and An-S bonds that provides additional stability for the transuranic elements.

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⁺.

Lanthanide Complexes Containing a Terminal Ln═O Oxo Bond: Revealing Higher Stability of Tetravalent Praseodymium versus Terbium

We report on the reactivity of gas-phase lanthanide-oxide nitrate complexes, [Ln(O)(NO₃)₃]^- (denoted LnO²⁺), produced via elimination of NO₂^• from trivalent [Ln^III(NO₃)₄]^- (Ln = Ce, Pr, Nd, Sm, Tb, Dy). These complexes feature a Ln^III-O^• oxyl, a Ln^IV═O oxo, or an intermediate Ln^III/IV oxyl/oxo bond, depending on the accessibility of the tetravalent Ln^IV state. Hydrogen atom abstraction reactivity of the LnO²⁺ complexes to form unambiguously trivalent [Ln^III(OH)(NO₃)₃]^- reveals the nature of the oxide bond. The result of slower reactivity of PrO²⁺ versus TbO²⁺ is considered to indicate higher stability of the tetravalent praseodymium-oxo, Pr^IV═O, versus Tb^IV═O. This is the first report of Pr^IV as more stable than Tb^IV, which is discussed with respect to ionization potentials, standard electrode potentials, atomic promotion energies, and oxo bond covalency via 4f- and/or 5d-orbital participation.

Characterization of Uranyl Coordinated by Equatorial Oxygen: Oxo in UO3 versus Oxyl in UO3. J Phys Chem A 2021;125:5544-5555. [PMID: 34138571 DOI: 10.1021/acs.jpca.1c03818]

Uranium trioxide, UO₃, has a T-shaped structure with bent uranyl, UO₂²⁺, coordinated by an equatorial oxo, O^2-. The structure of cation UO₃⁺ is similar but with an equatorial oxyl, O^•-. Neutral and cationic uranium trioxide coordinated by nitrates were characterized by collision induced dissociation (CID), infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory. CID of uranyl nitrate, [UO₂(NO₃)₃]^- (complex A1), eliminates NO₂ to produce nitrate-coordinated UO₃⁺, [UO₂(O^•)(NO₃)₂]^- (B1), which ejects NO₃ to yield UO₃ in [UO₂(O)(NO₃)]^- (C1). Finally, C1 associates with H₂O to afford uranyl hydroxide in [UO₂(OH)₂(NO₃)]^- (D1). IRMPD of B1, C1, and D1 confirms uranyl equatorially coordinated by nitrate(s) along with the following ligands: (B1) radical oxyl O^•-; (C1) oxo O^2-; and (D1) two hydroxyls, OH^-. As the nitrates are bidentate, the equatorial coordination is six in A1, five in B1, four in D1, and three in C1. Ligand congestion in low-coordinate C1 suggests orbital-directed bonding. Hydrolysis of the equatorial oxo in C1 epitomizes the inverse trans influence in UO₃, which is uranyl with inert axial oxos and a reactive equatorial oxo. The uranyl ν₃ IR frequencies indicate the following donor ordering: O^2-[best donor] ≫ O^•-> OH^-> NO₃^-.

Bond dissociation energies of low-valent lanthanide hydroxides: lower limits from ion-molecule reactions and comparisons with fluorides

Despite that bond dissociation energies (BDEs) are among the most fundamental and relevant chemical properties they remain poorly characterized for most elementary lanthanide hydroxides and halides. Lanthanide ions Ln+ = Eu+, Tm+ and Yb+ are here shown to react with H2O to yield hydroxides LnOH+. Under low-energy conditions such reactions must be exothermic, which implies a lower limit of 499 kJ mol-1 for the Ln+-OH BDEs. This limit is significantly higher than previously reported for YbOH+ and is unexpectedly similar to the BDE for Yb+-F. To explain this apparent anomaly, it is considered feasible that the inefficient hydrolysis reactions observed here in a quadrupole ion trap mass spectrometer may actually be endothermic. More definitive and broad-based evaluations and comparisons require additional and more reliable BDEs and ionization energies for key lanthanide molecules, and/or energies for ligand-exchange reactions like LnF + OH ↔ LnOH + F. The hydroxide results motivated an assessment of currently available lanthanide monohalide BDEs. Among several intriguing relationships is the distinctively higher BDE for neutral LuF versus cationic LuF+, though quantifying this comparison awaits a more accurate value for the anomalously high ionization energy of LuF.

Proton affinities of pertechnetate (TcO4-) and perrhenate (ReO4-)

The anions pertechnetate, TcO4-, and perrhenate, ReO4-, exhibit very similar chemical and physical properties. Revealing and understanding disparities between them enhances fundamental understanding of both. Electrospray ionization generated the gas-phase proton bound dimer (TcO4-)(H+)(ReO4-). Collision induced dissociation of the dimer yielded predominantly HTcO4 and ReO4-, which according to Cooks' kinetic method indicates that the proton affinity (PA) of TcO4- is greater than that of ReO4-. Density functional theory computations agree with the experimental observation, providing PA[TcO4-] = 300.1 kcal mol-1 and PA[ReO4-] = 297.2 kcal mol-1. Attempts to rationalize these relative PAs based on elementary molecular parameters such as atomic charges indicate that the entirety of bond formation and concomitant bond disruption needs to be considered to understand the energies associated with such protonation processes. Although in both the gas and solution phases, TcO4- is a stronger base than ReO4-, it is noted that the significance of even such qualitative accordance is tempered by the very different natures of the underlying phenomena.

Gas-Phase Complexes of Americium and Lanthanides with a Bis-triazinyl Pyridine: Reactivity and Bonding of Archetypes for F-Element Separations

Bis-triazinyl pyridines (BTPs) exhibit solution selectivity for trivalent americium over lanthanides (Ln), the origins of which remain uncertain. Here, electrospray ionization was used to generate gas-phase complexes [ML₃]³⁺, where M = La, Lu, or Am and L is EtBTP 2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)-pyridine. Collision-induced dissociation (CID) of [ML₃]³⁺ in the presence of H₂O yielded a protonated ligand [L(H)]⁺ and hydroxide [ML₂(OH)]²⁺ or hydrate [ML(L-H)(H₂O)]²⁺, where (L-H)^- is a deprotonated ligand. Although solution affinities indicate stronger binding of BTPs toward Am³⁺ versus Ln³⁺, the observed CID process is contrastingly more facile for M = Am versus Ln. To understand the disparity, density functional theory was employed to compute potential energy surfaces for two possible CID processes, for M = La and Am. In accordance with the CID results, both the rate determining transition state barrier and the net energy are lower for [AmL₃]³⁺ versus [LaL₃]³⁺ and for both product isomers, [ML₂(OH)]²⁺ and [ML(L-H)(H₂O)]²⁺. More facile removal of a ligand from [AmL₃]³⁺ by CID does not necessarily contradict stronger Am³⁺-L binding, as inferred from solution behavior. In particular, the formation of new bonds in the products can distort kinetics and thermodynamics expected for simple bond cleavage reactions. In addition to correctly predicting the seemingly anomalous CID behavior, the computational results indicate greater participation of Am 5f versus La 4f orbitals in metal-ligand bonding.

Carbon–sulfur bond strength in methanesulfinate and benzenesulfinate ligands directs decomposition of Np(v) and Pu(v) coordination complexes

Adjusting intra-ligand bond strengths in actinide sulfinate complexes directs towards alternative cleavage of carbon–sulfur or actinide–sulfinate bonds.

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.

Pentavalent Curium, Berkelium, and Californium in Nitrate Complexes: Extending Actinide Chemistry and Oxidation States

Pentavalent actinyl nitrate complexes An^VO₂(NO₃)₂^- were produced by elimination of two NO₂ from An^III(NO₃)₄^- for An = Pu, Am, Cm, Bk, and Cf. Density functional theory (B3LYP) and relativistic multireference (CASPT2) calculations confirmed the AnO₂(NO₃)₂^- as An^VO₂⁺ actinyl moieties coordinated by nitrates. Computations of alternative An^IIIO₂(NO₃)₂^- and An^IVO₂(NO₃)₂^- revealed significantly higher energies. Previous computations for bare AnO₂⁺ indicated An^VO₂⁺ for An = Pu, Am, Cf, and Bk, but Cm^IIIO₂⁺: electron donation from nitrate ligands has here stabilized the first Cm^V complex, Cm^VO₂(NO₃)₂^-. Structural parameters and bonding analyses indicate increasing An-NO₃ bond covalency from Pu to Cf, in accordance with principles for actinide separations. Atomic ionization energies effectively predict relative stabilities of oxidation states; more reliable energies are needed for the actinides.

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]

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.

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 )](+) .

Activation of carbon dioxide by a terminal uranium-nitrogen bond in the gas-phase: a demonstration of the principle of microscopic reversibility

Activation of CO2 is demonstrated by its spontaneous dissociative reaction with the gas-phase anion complex NUOCl2(-), which can be considered as NUO(+) coordinated by two chloride anion ligands. This reaction was previously predicted by density functional theory to occur exothermically, without barriers above the reactant energy. The present results demonstrate the validity of the prediction of microscopic reversibility, and provide a rare case of spontaneous dissociative addition of CO2 to a gas-phase complex. The activation of CO2 by NUOCl2(-) proceeds by conversion of a U[triple bond, length as m-dash]N bond to a U[double bond, length as m-dash]O bond and creation of an isocyanate ligand to yield the complex UO2(NCO)Cl2(-), in which uranyl, UO2(2+), is coordinated by one isocyanate and two chloride anion ligands. This activation of CO2 by a uranium(vi) nitride complex is distinctive from previous reports of oxidative insertion of CO2 into lower oxidation state U(iii) or U(iv) solid complexes, during which both C-O bonds remain intact. This unusual observation of spontaneous addition and activation of CO2 by NUOCl2(-) is a result of the high oxophilicity of uranium. If the computed Gibbs free energy of the reaction pathway, rather than the energy, is considered, there are barriers above the reactant asymptotes such that the observed reaction should not proceed under thermal conditions. This result provides a demonstration that energy rather than Gibbs free energy determines reactivity under low-pressure bimolecular conditions.

Synthesis and hydrolysis of gas-phase lanthanide and actinide oxide nitrate complexes: a correspondence to trivalent metal ion redox potentials and ionization energies

Gas-phase hydrolysis of lanthanide/actinide MO₃(NO₃)₃⁻ions relates to the stabilities of the M^IVoxidation states, which correlate with IV/III solution reduction potentials and 4th ionization energies.

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.