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

Abstract

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 (U^VIO₂²⁺) 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)]⁻.

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Introduction

Electrospray ionization (ESI) provides easy access to a wide range of uranyl (UO₂²⁺) complexes for studies of gas-phase structure and reactivity in a species-specific fashion. For example, the use of ESI to transfer the uranium(V) dioxocation, U^VO₂⁺, from solution to the gas phase was first reported in 1992¹ and since then, advances in the fundamental understanding of uranyl coordination chemistry have been made using the ionization method combined with tandem mass spectrometry.^2–22 Most importantly, ESI has been used to generate doubly charged UO₂²⁺ complexes, such as [UO₂(L)_n]²⁺, with L = acetonitrile (acn) or acetone (aco) and n = 4 or 5, for collision-induced dissociation (CID) and ion–molecule reaction experiments.^12,13 Use of ESI and tandem mass spectrometry has since been expanded, primarily through the work of Gibson and coworkers, to explore the intrinsic dissociation and ion–molecule reactions of a range of transuranic species.^23–32

Focusing on U, recent investigations by our laboratory have shown that the MSⁿ capabilities of ion trap instruments can be used to generate novel gas-phase species for studies of intrinsic reactivity.^33–44 In one prior study³⁷ the CID pathways of homogeneous anionic uranyl complexes with general formula [UO₂(O₂C–R)₃]⁻, R = H (formate) or CH₃ (acetate) were determined using multiple-stage (MSⁿ) experiments in a linear ion trap (LIT) mass spectrometer. Use of the 2-D ion trap provided access to fragmentation pathways and reactions not observed in earlier studies with 3-D ion traps, which were complicated by a preponderance of product ions obviously generated by interactions with background H₂O.

Our previous study demonstrated that anionic complexes containing UO₂²⁺ and formate ligands fragmented by loss of CO₂, and elimination of CH₂=O, to create an oxo-hydride product [UO₂(O)(H)]⁻. Analogous complexes that contained acetate ligands instead showed an initial loss of acetyloxyl radical, CH₃CO₂˙, followed by decarboxylation and elimination of CH₄ to generate [UO₂(O)]⁻. The loss of CH₄ occurs by an intra-complex proton transfer process that generates UO₂⁺ coordinated by an acetate and acetate enolate ligand. A final CID step caused elimination of ketene (CH₂=C=O) to generate [UO₂(O)]⁻.

Here, we present an expanded study that includes anionic complexes that contain UO₂²⁺ coordinated by a mix of acetate and formate ligands. As outlined below, the mixed complexes dissociate to make new species, including several uranyl-hydride ions for which measurements of intrinsic reactivity are relatively rare. Of particular interest was the potential for some species to undergo intra-complex hydride attack steps that convert carboxylate ligands to aldehydes and oxide, which we assume were responsible for the loss of formaldehyde in our earlier investigation of the homogeneous UO₂–formate anions. In addition, the outcome of reactions with H₂O were determined for a range of dissociation product ions. In this case, our interest was in the tendency for the hydride species to form hydroxides by reaction with H₂O and elimination of H₂, the hydrolysis of novel UO₂–oxo–methide anions with H₂O, and the potential for hydrates of UO₂–oxo anions to rearrange to generate dihyroxides. To complement the experimental measurements, Kohn–Sham density functional theory (DFT) calculations were used to determine probable reaction pathways and relative energies.

Experimental methods

Sample preparation

Samples of uranyl formate, acetate and d₃-acetate were prepared by digesting ca. 2–3 mg of UO₃ Strem Chemicals, Newburyport MA with excess of formic or acetic acid individually (purchased from Sigma Aldrich, St. Louis MO) and 400 μL of deionized/distilled H₂O contained in a glass scintillation vial. Caution: uranium oxide is radioactive (α- and γ-emitter), and proper shielding, waste disposal and personal protective gear should be used when handling the material.

The solutions incubated on a hot plate overnight at 70 °C. 50 μL of the natural or labeled acetate solutions were combined with an equivalent volume of formate solution and diluted to 1 mL total volume using H₂O and CH₃CH₂OH for ESI. CH₃CH₂OH was used instead of CH₃OH for these experiments to avoid any ambiguity when identifying potential molecular O₂ adducts to product ions.^44–47

Mass spectrometry measurements

All experiments were performed using a ThermoScientific LTQ-XL LIT mass spectrometer (San Jose, CA) modified to allow the mixing of reagents with the helium buffer gas before introduction into the ion trap.⁴³ The uranyl–carboxylate spray solutions were directly infused into the instrument at a flow rate of 5 μl min⁻¹. The LTQ Tune program was used to optimize the atmospheric pressure ionization stack settings (for example, lens voltages, and the offset voltages for transfer quadrupole and octopole) and maximize the transmission of the singly charged ions [UO₂(O₂C–R)_n]⁻, R = H, CH₃ and CD₃, to the ion trap mass analyzer. Ultra-high purity He was used as the bath/buffer and collision gas.

For CID, precursor ions were isolated using an isolation width of 1.0 to 1.5 mass to charge (m/z) units centered on the ²³⁸U isotopic peak. The exact value was adjusted to provide maximum ion intensity while maintaining the isolation of a single isotopic peak. To probe CID behavior in general, the (mass) normalized collision energy (NCE, as defined by ThermoScientific) was set between 5 and 18% (approximately 0.075–0.27 V). The activation Q setting for all experiments was set at 0.30 and a 30 ms activation time was used.

To probe ion–molecule reactions with (neutral) H₂O, specific precursor ions were isolated using widths of between 1 and 2 m/z units and stored in the ion trap for reaction times ranging from 1 ms to 10 s. When examining ion–molecule reactions (IMRs), our intent was to identify the products generated by ion–molecule reactions rather than measure or report rates or rate constants. For both CID and IMR experiments, the mass spectra displayed were created by accumulating and averaging at least 30 isolation, dissociation, and ejection/detection steps.

Density functional theory calculations

While the objective was to generate experimental data regarding the gas-phase reactions of the anionic uranyl–carboxylate complexes and their dissociation products, supporting DFT calculations can provide important insight into reaction mechanisms and relative energetics. Our intent was not to rigorously assess the accuracy of DFT for determining reaction thermochemistry, bond lengths and angles, or bond-dissociation energies, but instead to identify probable reaction pathways “in silico” that reproduce the experimental observations in terms of dissociation or ion–molecule reactions. DFT has been used in several previous studies to probe the properties of gas-phase uranyl species.^48–59 For the reactions that were the focus of this study, the structures of the various isomers, reaction intermediates, and transition states, were optimized using the M06-L functional, which was chosen based on the good performance when investigating reaction pathways and energetics for uranyl species in our prior studies.^43,45,46 Transition states were identified using the QST2 and QST3 approaches^60–63 and were confirmed by means of intrinsic reaction coordinate calculations.

To minimize computational cost, initial geometry optimization was performed using the MDF60 pseudopotential and associated basis set on U and the cc-pvtz basis set on all other atoms. Single-point calculations were then performed on the minima and transition state structures (obtained at the M06L/MDF60/cc-pvtz level of theory) using the cc-pvdz-PP⁶⁴ basis of Peterson (with the MDF60 pseudopotential) on U and aug-cc-pvtz on all other atoms. The Gaussian 16 software package⁶⁵ was used for all calculations.

It should be noted that spin–orbit corrections were not explicitly included in our calculations. Although spin–orbit effects are not expected to significantly affect the energetics of processes in which there is no change in the formal oxidation state of the heavy metal, substantial changes can occur for reaction energies involving actinide atoms in different oxidation states.^66–68 While it is possible that the energies of specific species with U in different oxidation states could be affected by the inclusion of spin–orbit corrections, we feel that this caveat does not influence our interpretation of the DFT investigation of likely reaction mechanisms.

Results and discussion

The ESI mass spectrum generated from a mix of [UO₂(O₂C–CH₃)₂] and [UO₂(O₂C–H)₂] in H₂O/CH₃CH₂OH is shown in Fig. S1 of the ESI.† The dominant negative ions created by ESI were [UO₂(O₂C–H)₃]⁻ at m/z 405, [UO₂(O₂C–H)₂(O₂C–CH₃)]⁻ at m/z 419, [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻ at m/z 433 and [UO₂(O₂C–CH₃)₃]⁻ at m/z 447. The dissociation of [UO₂(O₂C–H)₃]⁻ (m/z 405) and [UO₂(O₂C–CH₃)₃]⁻ (m/z 447) were discussed in detail in our earlier report.³⁷ Our focus here was on the dissociation of heterogeneous complexes containing acetate and formate ligands.

Tandem MS of [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻

The multiple-stage (MSⁿ) CID spectra generated by initially isolating [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻ at m/z 433 are shown in Fig. 1a–d and 2a and b. Related data generated using analogous species that contained d₃-acetate are provided in Fig. S2a–d and S3a and b of the ESI.† A summary of the MSⁿ dissociation pathways for the unlabeled and labeled complex anions are shown in Scheme 1a and b.

Fig. 1 Product ion spectra derived from MSⁿ CID of [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻: (a) CID (MS/MS stage) of [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻ at m/z 433, (b) CID (MS³ stage) of dissociation product ion at m/z 389, (c) CID (MS⁴ stage) of dissociation product ion at m/z 345, and (d) CID (MS⁵ stage) of dissociation product ion at m/z 301. In the spectra, the circles and arrows illustrate the MSⁿ pathway. In each spectrum, the bold peak label indicates the precursor selected for CID while labels in italics represent the products from dissociation or ion–molecule reactions as indicated in the text.

Fig. 2 Product ion spectra derived from MSⁿ CID of [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻ continued: (a) CID (MS³ stage) of product ion at m/z 387 generated by initial CID of [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻ at m/z 433, (b) CID (MS⁴ stage) of dissociation product ion at m/z 345. The bold peak labels indicate the precursor selected for CID while labels in italics represent the products from dissociation or ion–molecule reactions as indicated in the text.

Scheme 1 (a) Summary of the MSⁿ dissociation pathways for (unlabeled) [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻ and (b) summary of the MSⁿ dissociation pathways for (labeled) [UO₂(O₂C–H)(O₂C–CD₃)₂]⁻.

Initial CID (MS/MS stage, Fig. 1a) of [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻ generated product ions at m/z 389 and 387, corresponding to neutral losses of 44 and 46 mass units (Da) respectively. The elimination of 44 Da to create the ion at m/z 389 is attributed to decarboxylation, which is an effective way to generate organometallic species in the gas phase.⁶⁹ The loss of 46 Da to create the ion at m/z 387 suggests intra-proton transfer and elimination of neutral formic acid. Because of the low-mass cut-off of the LIT, we could not determine whether [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻ dissociates to create acetate (m/z 59) or formate (m/z 45) by elimination of neutral [UO₂(O₂C–H)(O₂C–CH₃)]⁻ or [UO₂(O₂C–CH₃)]⁻, respectively.

For CID of the d-labeled version of the anion, [UO₂(O₂C–H)(O₂C–CD₃)₂]⁻ (m/z 439, Fig. S1a, ESI†) the analogous product ions appear at m/z 395 and 392. Formation of the ion at m/z 395 involves elimination of 44 Da, consistent with decarboxylation, and the shift in product ion mass of 6 units (compared to the un-labeled analogue) indicates the retention of all deuterium atoms in the precursor complex. For the product ion at m/z 392, the shift in mass of 5 Da indicates retention of 5 deuterium atoms, and the shift of neutral loss to 47 Da identifies DO₂C–H as the neutral species ejected in the dissociation reaction.

The formation of the product ion at m/z 389 following CID of [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻ could involve decarboxylation of either a formate or acetate ligand. For the unlabeled complex, decarboxylation of the formate ligand would generate the hydride species [UO₂(H)(O₂C–CH₃)₂]⁻, while loss of CO₂ from an acetate ligand would create the organouranyl complex [UO₂(O₂C–H)(CH₃)(O₂C–CH₃)]⁻. To determine which dissociation product is generated, the ion at m/z 389 was isolated, without imposed collisional activation, in the ion trap for periods ranging from 1 ms to 1 s for reaction with H₂O (ca. 1 × 10⁻⁶ torr, Fig. S4a–d of the ESI†). Our hypothesis was that the hydride would react with H₂O to create [UO₂(OH)(O₂C–CH₃)₂]⁻ at m/z 405 by reaction (1), while the organouranyl species would generate [UO₂(OH)(O₂C–H)(O₂C–CH₃)]⁻ at m/z 391 by reaction (2). The product ion spectra in Fig. S4 (ESI†) show that an ion–molecule reaction product at m/z 405 is formed (net addition of 16 u to the precursor at m/z 389), consistent with reaction (1). Subsequent CID of the ion at m/z 405 (Fig. S4c, ESI†) caused the elimination of 60 Da, which is consistent with elimination of acetic acid to create [UO₂(O)(O₂C–CH₃)]⁻ at m/z 345. Overall, the ion–molecule reaction behavior strongly suggests that the dissociation product ion at m/z 389 (Fig. 1a) is [UO₂(H)(O₂C–CH₃)₂]⁻ ([UO₂(H)(O₂C–CD₃)₂]⁻ for the d-labeled precursor) generated by decarboxylation of a formate ligand.

[UO₂(O₂C–R)₂]⁻ → [UO₂(R)(O₂C–CH₃)]⁻ + CO₂ (1)

[UO₂(H)(O₂C–CH₃)₂]⁻ + H₂O → [UO₂(OH)(O₂C–CH₃)₂]⁻ + H₂ (2)

[UO₂(CH₃)(O₂C–H)(O₂C–CH₃)]⁻ + H₂O → [UO₂(OH)(O₂C–H)(O₂C–CH₃)]⁻ + CH₄ (3)

[UO₂(OH)(O₂C–CH₃)₂]⁻ → [UO₂(O)(O₂C–CH₃)]⁻ + CH₃COOH (4)

When using the unlabeled species, subsequent CID (MS³ stage) of the ion at m/z 389 (Fig. 1b) and the ion at m/z 387 (Fig. 2a) generated a fragment ion at m/z 345. Creation of the ion at m/z 345 by CID of the species at m/z 387 occurs through the elimination of 42 Da. CID experiments conducted with the analogous d-labeled ions show that the neutral loss shifts to 44 Da, consistent with the elimination of ketene (O=C=CH₂ for the unlabeled ions, O=C=CD₂ for the d-labeled analogues). The elimination of ketene was invoked in our earlier study to explain the MSⁿ fragmentation of [UO₂(O₂C–CH₃)₃]⁻,³⁷ and a similar process has been reported by O’Hair and coworkers for the catalytic dehydration of acetic acid.⁷⁰ In the present study, elimination of ketene from [UO₂(O₂C–CH₂)(O₂C–CH₃)]⁻ would create a product ion with composition [UO₂(O)(O₂C–CH₃)]⁻, and the assignment is consistent with the isotopic labeling experiments (the product ion at m/z 345 shifts to m/z 348 for the d-labeled precursor).

The loss of 44 Da from [UO₂(H)(O₂C–CH₃)₂]⁻ (Fig. 1b) to create the fragment ion at m/z 345 was initially attributed to a second decarboxylation step. However, CID of the analogous ion at m/z 395 derived from the d-labeled complex leads to a product ion at m/z 348, which represents a neutral loss of 47 Da. Therefore, the dissociation step cannot involve decarboxylation. The mass shift to the neutral loss suggests instead that the dissociation reaction involves elimination of acetaldehyde (reaction (4)), which we presume occurs by intra-complex hydride attack. A similar reaction was invoked in our earlier study, in which elimination of O=CH₂ (30 Da) was observed following CID of [UO₂(H)(O₂C–H)₂]⁻.

[UO₂(H)(O₂C–CH₃)₂]⁻ → [UO₂(O)(O₂C–CH₃)]⁻ + O=C(H)CH₃ (5)

When using the unlabeled species, subsequent CID (MS³ stage) of the ion at m/z 345 (MS⁴ stage, Fig. 1c and 2b) generated product ions at m/z 301 and 303. The former is created by elimination of 44 Da, suggesting formation of the oxyl-methide product [UO₂(O)(CH₃)]⁻ by decarboxylation of the acetate ligand. A neutral loss of 44 Da was also observed following CID of the d-labeled analogue, [UO₂(O)(O₂C–CD₃)]⁻, at m/z 348 (Fig. 2b), in this case to create a product ion at m/z 304. The shift in mass indicates the retention of 3 D atoms and is consistent with the composition assignment of [UO₂(O)(CH₃)]⁻ ([UO₂(O)(CD₃)]⁻ for the labeled precursor). The product ions at m/z 301 (unlabeled precursor) and m/z 304 (d-labeled analogue) were isolated for reaction with H₂O (Fig. S5 and S6 of the ESI†). Reaction of the ion at m/z 301 created the product ion at m/z 303, suggesting that the presence of the latter species in the spectrum shown in Fig. 1c is the result of hydrolysis reaction (5) to create [UO₂(O)(OH)]⁻. Isolation of [UO₂(O)(CD₃)]⁻ for reaction with H₂O also leads to formation of the product ion at m/z 303, consistent with a pathway like reaction (5), but with the elimination of CD₃H as the neutral species.

[UO₂(O)(CH₃)]⁻ + H₂O → [UO₂(O)(OH)]⁻ + CH₄ (6)

Subsequent CID of the ions at m/z 301 (MS⁵ stage, Fig. 1d) caused elimination of 15 Da (CH₃) to create the terminal product ion at m/z 286. The same fragment ion was observed following CID of the analogous species derived from the d-labeled precursor (elimination of 18 Da, CD₃, Fig. S2b, ESI†). This observation is consistent with a composition assignment of [UO₂(O)]⁻ for the terminal product ion generated by MSⁿ CID of the [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻ precursor ion.

Tandem MS of [UO₂(O₂C–H)₂(O₂C–CH₃)]⁻

The multiple-stage (MSⁿ) CID spectra generated by initially isolating [UO₂(O₂C–H)₂(O₂C–CH₃)]⁻ at m/z 419 are shown in Fig. 3. Related data generated using analogous species that contained d₃-acetate are provided in Fig. S7 of the ESI.† A summary of the MSⁿ dissociation pathways for the unlabeled and labeled complex ions are shown in Scheme 2a and b.

Fig. 3 Product ion spectra derived from MSⁿ CID of [UO₂(O₂C–H)₂(O2C–CH3)]⁻: (a) CID (MS/MS stage) of [UO₂(O₂C–H)2(O₂C–CH₃)]⁻ at m/z 419, (b) CID (MS3 stage) of dissociation product ion at m/z 375, (c) CID (MS4 stage) of dissociation product ion at m/z 345. In the spectra, the circles and arrows illustrate the MSn pathway. In each spectrum, the bold peak label indicates the precursor selected for CID while labels in italics represent the products from dissociation or ion–molecule reactions as indicated in the text.

Scheme 2 (a) Summary of the MSⁿ dissociation pathways for (unlabeled) [UO₂(O₂C–H)₂(O₂C–CH₃)]⁻ and (b) summary of the MSⁿ dissociation pathways for (labeled) [UO₂(O₂C–H)₂(O₂C–CD₃)]⁻.

The initial CID (MS/MS stage, Fig. 3a) of [UO₂(O₂C–H)₂(O₂C–CH₃)]⁻ generated product ions at m/z 375 and 345. Formation of the species at m/z 375 corresponds to elimination of CO₂ (44 Da). The species at m/z 345 is consistent with elimination of 44 and 30 Da (74 total). For CID of the d-labeled version of the anion, [UO₂(O₂C–H)₂(O₂C–CD₃)]⁻ (m/z 422, Fig. S7a, ESI†) the analogous product ions appear at m/z 378 and 348. These product ions are also generated by the elimination of 44 and 74 (net) Da, respectively, indicating that the D-labels on the acetate ligand are retained the product ions.

The formation of the product ion at m/z 375 following CID of [UO₂(O₂C–H)₂(O₂C–CH₃)]⁻ could also involve decarboxylation of either a formate or acetate ligand, as considered above for the dissociation of [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻. Ion–molecule reactions (Fig. S8a–d of the ESI†) were again used to determine whether formation of the product ion at m/z 375 involves decarboxylation of a formate or acetate ligand. The reaction of the ion at m/z 375 with H₂O resulted in the generation of an ion–molecule reaction product at m/z 391 (net addition of 16 Da), consistent with the hydrolysis of a hydride species by reaction (6).

[UO₂(H)(O₂C–H)(O₂C–CH₃)]⁻ + H₂O → [UO₂(OH)(O₂C–H)(O₂C–CH₃)]⁻ + H₂ (7)

Subsequent CID of the ion–molecule reaction product at m/z 391 caused the elimination of 46 Da, consistent with elimination of formic acid to create [UO₂(O)(O₂C–CH₃)]⁻ at m/z 345. A minor product ion at m/z 347 corresponds to elimination of 44 Da (CO₂), which we assume generates [UO₂(O)(H)(O₂C–H)]⁻. These observations identify the dissociation product ion at m/z 375 (Fig. 3a) as [UO₂(H)(O₂C–H)(O₂C–CH₃)]⁻ ([UO₂(H)(O₂C–H)(O₂C–CD₃)]⁻ for the d-labeled precursor).

When using the unlabeled species, subsequent CID (MS³ stage) of the ion at m/z 375 (Fig. 3b) leads to a product ion at m/z 345 (loss of 30 Da). The elimination of 30 Da is attributed to loss of formaldehyde to generate [UO₂(O)(O₂C–CH₃)]⁻. As for the MSⁿ dissociation of [UO₂(O₂C–H)(O₂C–CH₃)₂]⁻ as described above, CID of the ion at m/z 345 caused decarboxylation to create [UO₂(O)(CH₃)]⁻ at m/z 301, which undergoes hydrolysis to generate [UO₂(O)(OH)]⁻ at m/z 303. As described above, subsequent CID of the ion at m/z 301 (MS⁵ stage, Fig. 1d) caused elimination of 15 Da (CH₃) to create the terminal product ion at m/z 286.

Computational investigation of intra-complex hydride attack

Of particular interest in this study was the apparent elimination of formaldehyde and/or acetaldehyde during CID of the anionic formate/acetate complexes. We initiated our computational investigation of the presumed hydride attack mechanism with the dissociation of [UO₂(H)(O₂C–H)₂]⁻, which was investigated experimentally in our previous study. Relevant minima and transition state structures for the dissociation of [UO₂(H)(O₂C–H)₂]⁻ are provided in Fig. S9 of the ESI.† The zero-point corrected electronic energies and free energies for the respective species are provided in Table S1 of the ESI.† A reaction energy diagram for the CID of [UO₂(H)(O₂C–H)₂]⁻ is shown in Fig. 4. The global minimum identified for [UO₂(H)(O₂C–H)₂]⁻ (structure I) features two bidentate formate ligands, and a pseudo-pentagonal bipyramidal coordination geometry. Intra-complex hydride attack requires conversion to an intermediate structure, II, which includes one monodentate formate ligand. This type of isomerization has been reported for other metal-carboxylate species^71,72 and a prior infrared multiple-photon dissociation spectroscopy study demonstrated variable denticity in anionic uranyl–carboxylate species.⁷³ From II, formation of an ion–molecule complex between [UO₂(O)(O₂C–H)]⁻ and formaldehyde (structure III) can occur through transition state structure TS II→III. Elimination of formaldehyde from III creates [UO₂(O)(O₂C–H)]⁻ (structure IV). The hydride attack pathway lies significantly lower in energy than the dissociation reaction in which formate would be generated by elimination of (neutral) [UO₂(H)(O₂C–H)].

Fig. 4 A reaction energy diagram for the CID of [UO₂(H)(O₂C–H)₂]⁻.

The product ion spectrum generated by CID of the analogous acetate complex [UO₂(H)(O₂C–CH₃)₂]⁻ (m/z 389) and the presumed elimination of acetaldehyde to create [UO₂(O)(O₂C–CH₃)]⁻ at m/z 345, was shown in an earlier section (Fig. 2a and Scheme 1). The relevant minima and transition state structures for the dissociation of [UO₂(H)(O₂C–CH₃)₂]⁻ are provided in Fig. S10 of the ESI.† The zero-point corrected electronic energies and free energies for the respective species are provided in Table S2 of the ESI.† As indicated in the reaction energy diagram for [UO₂(H)(O₂C–CH₃)₂]⁻ (Fig. S10 of the ESI†), the intra-complex hydride attack follows a pathway like the one identified and described above for the CID of [UO₂(H)(O₂C–H)₂]⁻, and ultimately generates [UO₂(O)(O₂C–CH₃)]⁻ (m/z 345, structure IV-ac in Fig. S9, ESI†). Here too, the hydride attack pathway is lower in energy than the decomposition to produce acetate anion and (neutral) [UO₂(H)(O₂C–CH₃)]⁻.

As discussed in an earlier section, the dissociation of [UO₂(H)(O₂C–H)(O₂C–CH₃)]⁻ (m/z 375, initially created by CID of the [UO₂(O₂C–H)₂)(O₂C–CH₃)]⁻ precursor at m/z 419, generated a product ion at m/z 345. The neutral loss in this process corresponds to 30 Da and is consistent with elimination of formaldehyde, and the single dissociation product suggests that elimination of formaldehyde is energetically favored over acetaldehyde in the mixed carboxylate/hydride species.

Relevant minima and transition state structures for the dissociation of [UO₂(H)(O₂C–H)(O₂C–CH₃)]⁻ are provided in Fig. S12 of the ESI.† The zero-point corrected electronic energies and free energies for the respective species are provided in Table S3 of the ESI.† A reaction energy diagram for the CID of [UO₂(H)(O₂C–H)(O₂C–CH₃)]⁻ is shown in Fig. 5.

Fig. 5 A reaction energy diagram for the CID of [UO₂(H)(O₂C–H)(O₂C–CH₃)]⁻.

The global minimum identified for [UO₂(H)(O₂C–H)(O₂C–CH₃)]⁻ (structure V, Fig. 5) again features bidentate coordination by the carboxylate ligands. Hydride attack upon the acetate ligand requires isomerization of V to structure VI, which includes a monodentate acetate ligand. Creation of the ion–molecule complex between [UO₂(O)(O₂C–H)]⁻ and acetaldehyde (structure VII) is achieved through transition state TS VI→VII. Elimination of neutral formaldehyde creates [UO₂(O)(O₂C–H)]⁻ (structure VIII).

Hydride attack upon the formate ligand of [UO₂(H)(O₂C–H)(O₂C–CH₃)]⁻ instead requires isomerization of V to structure IX. Creation of the ion–molecule complex between [UO₂(O)(O₂C–CH₃)]⁻ and formaldehyde (structure X) is achieved through transition state TS IX→X. Elimination of neutral formaldehyde creates [UO₂(O)(O₂C–CH₃)]⁻ (structure XI). While the post-attack complex VII, and formation of the terminal product VIII, is predicted to be lower in energy than the analogous structures X and XI on the formaldehyde loss pathway, the energy of TS IX→X is lower than the energy of TS VI→VII, consistent with the experimental observation that elimination of formaldehyde is favored over acetaldehyde following CID of [UO₂(H)(O₂C–H)(O₂C–CH₃)]⁻. We note again that both hydride attack pathways lie well below the energy required to generate either formate or acetate anion.

Computational investigation of reaction of [UO₂(O)(CH₃)]⁻ with H₂O

Our last objective was to investigate the hydrolysis of [UO₂(O)(CH₃)]⁻ to create [UO₂(O)(OH)]⁻ as shown in Fig. 1c and Fig. S5 (ESI†). Of interest here was the potential competition between hydrolysis reaction (5), and reaction (7), which involves formation of a hydrate, followed by isomerization to create a dihydroxide.

[UO₂(O)(CH₃)]⁻ + H₂O → [UO₂(OH)₂(CH₃)]⁻ (8)

We note that addition of H₂O, with subsequent isomerization to create the dihydroxide, would lead to a peak 18 Da higher in mass that the [UO₂(O)(CH₃)]⁻ ion (m/z 301), which was not observed in the product ion spectra shown in Fig. S3 (ESI†).

Relevant minima and transition state structures for the reactions of [UO₂(O)(CH₃)]⁻ (Fig. 6, structure XII) with H₂O are provided in Fig. S13 of the ESI.† The zero-point corrected electronic energies and free energies for the respective species are provided in Table S4 of the ESI.† A reaction energy diagram for hydrolysis of, or addition of H₂O to, [UO₂(O)(CH₃)]⁻ by reactions (5) or (6), respectively, is shown in Fig. 6.

Fig. 6 A reaction energy diagram for hydrolysis of, or addition of H₂O to, [UO₂(O)(CH₃)]⁻ by reactions (5) or (6), respectively, described in the text.

As shown in Fig. 6, hydrolysis by reaction (5), and H₂O addition and isomerization by reaction (6) proceed through H₂O adduct structures that differ in the relative position of orientation of the H₂O ligand. For the hydrolysis pathway (reaction (5)), the adduct structure XIII the H₂O and CH₃ ligands occupy adjacent equatorial coordination sites. In this case, proton transfer to generate the ion–molecule complex between [UO₂(O)(OH)]⁻ and CH₄ (structure XIV) occurs through transition state structure TS XIII→XIV, and loss of CH₄ is computed to proceed spontaneously to generate [UO₂(O)(OH)]⁻ (structure XV). For the H₂O addition pathway with isomerization (reaction (6)), the adduct structure XVI includes the oxo and H₂O ligands that occupy adjacent equatorial coordination sites. Proton transfer to generate the terminal dihydroxide product [UO₂(OH)₂(CH₃)]⁻ (structure XVII) occurs through transition state structure TS XVI→XVII.

Overall, the computed energies suggest that formation of either terminal product (XIV and XVII) should be energetically favorable. However, the energy of TS XVI→XVII is nearly at the energy of reactants (XII and H₂O), and significantly higher than the proton transfer step of hydrolysis reaction (5) (TS XIII→XIV), which suggests that reaction (6) is kinetically less favored. The DFT calculations therefore support the experimental observation that the hydrolysis of [UO₂(O)(CH₃)]⁻ to create [UO₂(O)(OH)]⁻ is spontaneous, indicating that the process is significantly favored over adduct formation with subsequent isomerization to make [UO₂(CH₃)(OH)₂]⁻.

Conclusions

To summarize, we have conducted a follow-up study of the MSⁿ CID of [UO₂(O₂C–CH₃)₂(O₂C–H)]⁻ and [UO₂(O₂C–CH₃)(O₂C–H)₂]⁻, and the reactions of product ions with H₂O. Reaction pathways were determined with the aid of deuterium labeling. Regardless of the precursor ion examined, initial CID causes decarboxylation of a formate ligand to create UO₂-hydride with carboxylate ligands. Using d-labeled acetate, we determined that subsequent CID of the hydride complexes causes elimination of acetaldehyde or formaldehyde, depending on the precursor complex. The elimination of aldehyde as neutral species is consistent with reactions that include intra-complex hydride attack upon bound acetate or formate ligands. Density functional theory calculations reproduce the experimental observations, including favored elimination of formaldehyde over acetaldehyde by hydride attack during CID of [UO₂(H)(O₂C–CH₃)(O₂C–H)]⁻. To the best of our knowledge, this represents the first experimental and computational investigation of the intrinsic reactivity of this type of uranyl-hydride species. The results presented here are interesting given the desire to develop strategies for the selective conversion of carboxylic acids to aldehydes.^74–76

We also discovered that MSⁿ CID of the acetate–formate complexes leads to generation of the oxyl-methide species, [UO₂(O)(CH₃)]⁻. The formation of the methide was confirmed using isotopically labeled acetate. Reaction of [UO₂(O)(CH₃)]⁻ with H₂O generates [UO₂(O)(OH)]⁻. DFT calculations confirm that formation of [UO₂(O)(OH)]⁻ by elimination of CH₄ would be spontaneous and favored over H₂O addition and rearrangement to create the dihydroxide species [UO₂(OH)₂(CH₃)]⁻.

Author contributions

ARB: investigation, methodology and validation writing – review and editing. IJT: investigation and methodology. AI: investigation and methodology. TAC: conceptualization, methodology, writing – review and editing. MJV: conceptualization, methodology, writing – original draft, project administration.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

M. V. S and T. A. C. acknowledge support for this work from the School of Science and Engineering (SOSE) at Duquesne University. Laboratory space renovation was made possible through support by the National Science Foundation through grant CHE-0963450. A. R. B and A. I. acknowledge SOSE, Duquesne University and the National Science Foundation (CHE-1659823 and CHE-1950585) for support of their summer undergraduate research. Quantum mechanical calculations were performed using the resources of the Duquesne University Center for Computational Studies, and the National Science Foundation (CHE-1726824). This work was also supported in part by the Robert Dean Loughney Faculty Development Endowment of Duquesne University.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp00823e

‡ Present address: Department of Chemistry, University of Utah, Salt Lake City, UT, 84112 USA.

§ Present address: American Preparatory Academy, Draper, UT, 84020 USA.

¶ Present address: PPG Industries, Allison Park PA, 15101 USA.

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