Investigation of Uranyl Nitrate Ion Pairs Complexed with Amide

ARTICLE pubs.acs.org/JPCA

Investigation of Uranyl Nitrate Ion Pairs Complexed with Amide Ligands Using Electrospray Ionization Ion Trap Mass Spectrometry and Density Functional Theory Garold L. Gresham,† Adriana Dinescu,†,§ Michael T. Benson,† Michael J. Van Stipdonk,‡ and Gary S. Groenewold*,† † ‡

Idaho National Laboratory, Idaho Falls, Idaho, United States Department of Chemistry, Wichita State University, Wichita, Kansas, United States

bS Supporting Information ABSTRACT: Ion populations formed from electrospray of uranyl nitrate solutions containing different amides vary depending on ligand nucleophilicity and steric crowding at the metal center. The most abundant species were ion pair complexes having the general formula [UO2(NO3)(amide)n=2,3]þ; however, singly charged complexes containing the amide conjugate base and reduced uranyl UO2þ were also formed as were several doubly charged species. The formamide experiment produced the greatest diversity of species resulting from weaker amide binding, leading to dissociation and subsequent solvent coordination or metal reduction. Experiments using methyl formamide, dimethyl formamide, acetamide, and methyl acetamide produced ion pair and doubly charged complexes that were more abundant and less abundant complexes containing solvent or reduced uranyl. This pattern is reversed in the dimethylacetamide experiment, which displayed lower abundance doubly charged complexes, but augmented reduced uranyl complexes. DFT investigations of the tris-amide ion pair complexes showed that interligand repulsion distorts the amide ligands out of the uranyl equatorial plane and that complex stabilities do not increase with increasing amide nucleophilicity. Elimination of an amide ligand largely relieves the interligand repulsion, and the remaining amide ligands become closely aligned with the equatorial plane in the structures of the bis-amide ligands. The studies show that the phenomenological distribution of coordination complexes in a metalligand electrospray experiment is a function of both ligand nucleophilicity and interligand repulsion and that the latter factor begins exerting influence even in the case of relatively small ligands like the substituted methyl-formamide and methyl-acetamide ligands.

’ INTRODUCTION Uranium speciation can be defined as the envelope of chemical forms in which the element can exist and encompasses an extensive variety of oxidation states, oxides, salt precipitates, adsorbates, colloids, and ligand complexes in solution. It has been a topic of sustained study because speciation influences not only mobility in the geologic subsurface1,2 but also behavior in environmental remediation efforts and nuclear fuel reprocessing activities.3,4 Across the extensive panorama of uranium speciation, one of the most important chemical species is the uranyl dication UO22þ, which dominates uranium chemistry in solution. However UO22þ does not exist as a discrete entity in the environment; it functions as a strong Lewis acid, coordinating with electrondonating ligands that strongly influence its chemical behavior. The identity and number of equatorial ligands coordinated with UO22þ can vary significantly, strongly affecting the solubility, mobility, and extractability of the complexed actinide.57 In natural environments, speciation can be dominated by microbially produced ligands. A good example are the siderophores r 2011 American Chemical Society

such as desferrioxamine B (DEF), which forms strong complexes with metals, and has the potential to influence actinide mobility.8 DEF contains both amide and hydroxamate functional groups that can interact with Lewis acids such as uranyl. Molecules containing these functional groups are typical of those produced by fungi and bacteria in low-Fe environments for the purpose of metal sequestration,9,10 and they exhibit a high affinity for iron. However the ligands also display significant binding activity with other metals including the actinides.1116 Uranophilic ligands such as DEF and others like catechol derivatives could enhance actinide mobility, which would augment the geographic extent of contamination in the environment.8,1721 In industrial chemical processes, actinide chemistries can be quite complicated as well, due to the extractants used in nuclear fuel reprocessing22,23 and the presence of other metals and salt Received: October 8, 2010 Revised: March 2, 2011 Published: March 30, 2011 3497

dx.doi.org/10.1021/jp109665a | J. Phys. Chem. A 2011, 115, 3497–3508

The Journal of Physical Chemistry A contaminants.24 The Plutonium and Uranium Reduction Extraction (PUREX) process utilizes trin-butylphosphate (TBP), and hence it is technologically the most important extractant used in the reprocessing of spent nuclear fuels and radioactive wastes.24 Due to limitations in optimization and environmental waste considerations, alternative extractants are being investigated, including the bifunctional uranophile carbamoylmethylphosphine oxide (CMPO) which has the structural formula R2P(O)CH2C(O)NR2 and contains the aggressively binding phosphoryl and amide functional groups. CMPO is exploited in the trans-Uranium Extraction (TRUEX) and Universal Solvent Extraction (UNEX) processes for trivalent actinide separations.4,2426 CMPO binding is predominately bidentate via phosphoryl-oxygen and carbamoyloxygen atom coordination;27,28 however, CMPO can also act as a monodentate ligand28,29 where only the phosphoryl group interacts with the metal center.30 The CMPO ligands can provide efficient separation of the actinides;26,31 however, it is usually employed together with other ligating compounds to achieve desired efficiency. This complicates the coordination and partitioning chemistry, and hence the chemistry of CMPO in solvent extraction processes is still being investigated. A better understanding of the chemistry of the actinides in chemical processes and in the natural environment can be generated by examining reactivity of explicitly defined uranium complexes. System reactivity can be simplified by studying complexes in the gas phase environment of a mass spectrometer which enables reaction pathways to be examined one species at a time. Further simplification can be achieved by examining the reactions of small oxo-uranium cations with small molecules (as opposed to large multifunctional ligands). Early mass spectrometry studies focused on low oxidation state-species (Uþ and UOþ) reacting with organics3236 and undergoing oxidation by reaction with small molecules such as O2, CO2, N2O, and ethylene oxide.3740 Later, bombardment of solid UO3 by energetic ReO4 ions was used to generate sufficient quantities of monopositive uranium oxo cations for investigation of intrinsic hydration rates by ion-trap mass spectrometry (ITMS).41 The unique features of the ITMS allow for the investigation of complexes in a more reaction-discrete manner via isolation of specific ions of interest through the use of selected ion storage. A consequence of this powerful capability is that the ions are stored for relatively long periods of time in the mass analyzer. During ion storage, gas-phase ions will react with neutral molecules, which enable study of the initial solvation sphere.4143 These gas phase ion molecule reactions, i.e. solvation, afford the opportunity to investigate fundamental reaction pathways and kinetics of metal complexes by controlling the time between ion generation and ion scan-out and detection. More recently, electrospray ionization mass spectrometry (ESI) has proven to be an effective tool for generating metal ions in the gas phase and has allowed for the investigation of a wide range of metal ligand complexes,27,4446 metal-siderophore complexes,19,47,48 and more importantly for this research, the species dependent reactivity of a range of monopositive uranyl-ligand cations.42,43,4851 The drawback to investigating these systems using a strictly mass spectrometric strategy is that there is no direct structural information generated. However this limitation can be largely addressed using density functional theory (DFT) to provide structural and energetic insights for the gaseous species that are produced. Numerous studies by our group and others have shown the power of such an approach which combines gas-phase ion chemistry and DFT.52,53

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In the present study the combined analytical methods of ESI and ITMS were used to study the uranyl dication (UO22þ, a U(VI) species) and the reduced uranyl monocation (UO2þ, a U(V) species), complexed with one of six different ligands (L), where L represents formamide and acetamide, and their N-methyl- and N,N-dimethyl-derivatives. These ligands vary both in terms of their expected nucleophilicity and molecular size -- the latter factor may lead to steric repulsion within the inner coordination sphere and correspondingly weaker binding. Uranyl (U(VI)) complexes having the general formulas [UO2 (Anion)Ln(MeOH)m]þ and [UO2Ln(S)m]2þ were produced (where S = MeOH or H2O), together with reduced uranyl (U(V)) complexes [UO2Ln(MeOH)m]þ. The abundances of these families of ions varied depending on the amide used, and DFT calculations were employed to investigate these differences in terms of conformations of the uranyl complexes as well as reaction energetics. Experimental and computational studies of these simple amides are expected to eventually provide a better basis for understanding interactions of more complicated amidecontaining ligands like those mentioned above, e.g. DEF and CMPO.

’ EXPERIMENTAL METHODS ESI-MS Studies. Uranyl nitrate hexahydrate, UO2(NO3)2• 6H2O, was purchased from Fluka/Sigma-Aldrich (St. Louis, MO, USA) and used as received. The amide ligands (formamide, N-methyl formamide, N,N-dimethyl formamide, acetamide, N-methyl acetamide, and N,N-dimethylacetamide) were acquired from Sigma-Aldrich (St. Louis, MO, USA) and used as received, as was the HPLC grade methanol. A stock solution containing 2.26 103 M of UO2(NO3)2•6H2O was prepared by dissolving the appropriate amount of solid material in deionized H2O. Stock solutions containing 1.0 102 M of the amide ligands were prepared by dissolving the appropriate amount of ligand in methanol. The uranyl nitrate solution and amide ligand solutions were combined 1:1, such that the uranyl nitrate concentration was 1.0 103 M with a 5-fold molar excess of the amide ligand species. The amide concentration was selected because the most extensively coordinated species was expected to have no more than five amide ligands, in accord with prior results involving [UO2]2þ and acetone.50 In practice, the positive ion ESI mass spectrum did not appreciably change with increasing amide-uranyl ratios once that value exceeded a value of about two. ESI mass spectra were collected using a ThermoFinnigan LCQ Deca XP Plus ion-trap mass spectrometer (ThermoFinnigan Corporation, San Jose, CA USA) utilizing an orthogonal ESI/API geometry. The uranyl nitrate-amide water/methanol (1:1) solutions were infused into the ESI using the incorporated syringe pump at steady flow rates of 3 μL/min in all experiments. The API stack settings for the LCQ (lens voltages, quadrupole and octapole voltage offsets, etc.) were optimized for maximum ion transmission to the ion trap mass analyzer by using the autotune routine within the LCQ Tune program. Following the instrument tune, the spray needle voltage was maintained at þ5 kV with N2 sheath gas flows of 5 to 10 units (arbitrary to the LCQ instrument, corresponding to approximately 0.631 to 1.09 L/minute, respectively, as measured with a digital flow meter from a sealed ESI manifold). The capillary (desolvation) temperature was maintained at 125 °C, which provided the maximum yield of solvated metal cation 3498

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The Journal of Physical Chemistry A

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Table 1. Compositions of Coordination Complexes Generated by ESI of Uranyl Nitrate/Amide Solutionsa U(VI) ion pair monocation ligand (PA, kJ/mol) F (822)

m/z 467 454 436 423

MF (851)

þ

[UO2(FH)(F)2(MeOH)]

U(V) monocation m/z

formula

247.4

[UO2(F)5]2þ

225.1

2þ

[UO2(F)4]

2þ

211.4

[UO2(F)3(H2O)]

þ

[UO2(FH)(F)(MeOH)2]

formula

450

[UO2(F)4]þ

437

[UO2(F)3(MeOH)]þ

405

[UO2(F)3]þ

392

[UO2(F)2(MeOH)]þ

492

[UO2(MF)3(F)]þ

479

[UO2(MF)3(MeOH)]þ

þ

404

[UO2(FH)(F)2]þ

391 509

[UO2(FH)(F)(MeOH)]þ [UO2(NO3)(MF)3]þ þ

[UO2(NO3)(MF)2(MeOH)]

þ

282.4

[UO2(MF)5]2þ

253.1

2þ

[UO2(MF)4]

2þ

478

[UO2(MF-H)(MF)2(MeOH)]

232.3

[UO2(MF)3(H2O)]

450

[UO2(NO3)(MF)2]þ

223.7

[UO2(MF)3]2þ

311.8

[UO2(A)6]2þ

506

[UO2(A)4]þ

282.4

2þ

492

[UO2(A)3(F)]þ

479

[UO2(A)3(MeOH)]þ

521

[UO2(DMF)3(MeOH)]þ

534

[UO2(MA)3(F)]þ

521

[UO2(MA)3(MeOH)]þ

576 563

[UO2(DMA)3(F)]þ [UO2(DMA)3(MeOH)]þ

476

[UO2(DMA)2(MeOH)]þ

509

446 551

þ

[UO2(NO3)(A)3]

þ

[UO2(A-H)(A)2(MeOH)] þ

[UO2(NO3)(A)2]

[UO2(A)5]

2þ

253.1

þ

[UO2(A-H)(A)2] [UO2(NO3)(DMF)3]þ

[UO2(A)4]

2þ

232.4 317.4 þ

[UO2(A)3(H2O)] [UO2(DMF)5]2þ 2þ

510

[UO2(NO3)(DMF)2(MeOH)]

281.1

[UO2(DMF)4]

478

[UO2(NO3)(DMF)2]þ

253.5

[UO2(DMF)3(H2O)]2þ

244.7

[UO2(DMF)3]2þ

317.4

[UO2(MA)5]2þ

281.1

2þ

551 478

a

[UO2(NO3)(F)2(MeOH)]

[UO2(NO3)(F)(MeOH)]þ

437

DMA (908)

þ

[UO2(NO3)(F)2]

450

MA (889)

[UO2(NO3)(F)3]þ

409

478

DMF (888)

m/z

formula

422

482

A (864)

U(VI) dication

[UO2(NO3)(DMF)(MeOH)]

þ

þ

[UO2(NO3)(MA)3]

þ

[UO2(NO3)(MA)2]

593 506

[UO2(NO3)(DMA)3]þ [UO2(NO3)(DMA)2]þ

451

[UO2(NO3)(DMA)(MeOH)]þ

[UO2(MA)4]

2þ

253.5

[UO2(MA)3(H2O)]

309.1

[UO2(DMA)4]2þ

Proton affinity values (kJ/mol) are extracted from ref 75.

complexes. Helium gas, admitted directly into the ion trap, was used as the bath/buffer gas to improve trapping efficiency and as the collision gas for collision induced dissociation (CID)54 experiments. The IT mass analyzer was operated at a pressure of ∼1.5 105 Torr as measured using the ion gauge attached to the vacuum housing. The actual He pressure of the IT mass analyzer is on the order of 10 times higher than the measured pressure due to the relatively tight configuration of the IT. Ions stabilized in the quadrupole ion trap have a coherent oscillating ion motion in the ion trap derived from the trapping radio frequency and a superimposed random component derived from the thermal energy of the system. Collision induced dissociation (CID) results from hyperthermal collisions with the He bath gas that are induced by application of a potential on the endcaps at a frequency corresponding to the secular frequency of the motion of the ion. Single- and multiple-stage CID experiments were performed by setting the isolation width between 3 and 10 mass units (depending on the ion complex isolated), the activation Q (as labeled by instrument manufacturer, used to adjust the q value for the resonant excitation of the precursor ion during the CID portion of the experiment) was varied between 0.17 to 0.59, and the activation times for the CID experiments were held at 30 ms. The normalized collision energy55 (as defined by instrument manufacturer, used to automatically adjust the collision energy (Vpp) to compensate for the mass dependency of fragmentation efficiency) was varied between 10 and 40%.

DFT Studies. All calculations were performed with the Gaussian03 package.56 Geometry optimizations and vibrational frequency calculations employed the B3LYP hybrid exchangecorrelation functional.57,58 No symmetry constraints were imposed during the optimizations, and all structures were verified to be global minima with no imaginary frequencies. Starting geometries were based on those calculated for the free amides, assuming coordination through the carbonyl oxygen atom, and the amide oriented in the equatorial plane of the linear uranyl molecule. Relativistic effects for the uranyl complexes were included by employing the small-core (SC) effective core potential (RECP) developed by Stuttgart/Dresden groups.59 This SC-RECP replaces the 60 inner shell electrons (1s22s22p63s23p63d104s24p64d104f14) of uranium atom with a pseudopotential that accounts for scalar relativistic effects, while the remaining 32 “valence” electrons (5s25p65d105f36s26p66d17s2) are described by (12s,11p,10d,8f)/ [8s,7p,6d,4f] Gaussian basis functions. Due to linear dependence issues,60 the most diffuse s, p, d, and f functions were removed from the SC-ECP basis set. This approach has previously been applied to uranyl complexes.6164 Thus, the basis set employed in the calculations incorporated (11s,10p,9d,7f)/[6s,6p,5d,3f] Gaussian basis functions. Test calculations on [UO2(NO3) (formamide)n=2,3]þ complexes showed no significant structural or energetic differences between the [8s,7p,6d,4f] and [6s,6p,5d,3f] basis sets. All main group elements employed the Dunning aug-ccpVDZ basis set.65 3499



Figure 1. ESI mass spectra generated from H2O/MeOH solutions containing uranyl nitrate hexahydrate (a) formamide (F), (b) N-methylformamide (MF), (c) acetamide (A), (d) N,N-dimethylformamide (DMF), (e) N-methylacetamide (MA), and (f) N,N-dimethylacetamide (DMA). Ion designations: ( ion pair complexes, containing either nitrate of (amide-(Hþ)) anions; r reduced uranyl complexes. A complete listing of relevant ion species observed for all uranyl - amide ligand complexes is provided in Table 1. Proton affinity values (kJ/mol) are extracted from ref 75.

’ RESULTS AND DISCUSSION Families of Coordination Complexes Formed Directly by ESI. Solutions of uranyl with each of six different amides were

examined to evaluate the effects of amide nucleophilicity and interligand repulsion on the ion populations generated by ESI. Three families of ions were produced in all experiments: 1) uranylanion ion pair complexes [UO2(anion)(amide)n(solvent)m]þ; 2) doubly charged uranyl complexes [UO2(amide)n(solvent)m]2þ; and 3) reduced uranyl complexes [UO2(amide)n(solvent)m]þ. Ion compositions (Table 1) indicate that uranium is in the þ6 oxidation state in the first two families and in the þ5 oxidation state in the third.41,66 Figure 1(a)(f) shows the results of experiments using formamide (F), N-methyl formamide (MF), acetamide (A), dimethyl formamide (DMF), N-methyl acetamide (MA), and N,N-dimethylacetamide (DMA), and the mass spectra are arranged in order of increasing proton affinity of the amide ligand.67 For the [UO2(anion)(amide)n(MeOH)m]þ ion pair complexes, the anion can be either NO3 or the amide conjugate base (amide-(Hþ)), the number of amide ligands n can range from 1 to 3, and the number of methanol molecules is either zero or 1. The uranyl molecule is linear or nearly so, and thus the amide and solvent ligands are orthogonal to the uranyl OUO axis and termed equatorial. The number of equatorial ligands in these molecules does not exceed 4, i.e. (n þ m) = 3 in the

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structural formula. The observation of up to four equatorial ligands is consistent with the extent of coordination seen in the condensed phases: up to five equatorial H2O molecules have been identified in aqueous solution,68 and prior gas phase infrared studies have shown that four to five ligands are equatorially attached to the uranium atom in cationic complexes of this size formed by electrospray.53,69 The ion pair complexes are the most abundant species formed, and their high abundance strongly suggests their presence in solution; however, there is also a strong tendency for metal species to undergo charge reduction by ion pairing during electrospray.70 The ion pair complexes would be expected to become more prevalent as the solute concentration increases, which is what is occurring during ESI droplet evaporation. A second series of ion pair complexes is formed from the uranyl dication and the conjugate bases of the acidic amides and have the structural formula [UO2(amide-(Hþ))(amide)n]þ. As in the case of the nitrate-containing complexes, the number of equatorial ligands tends to be three or four. The conjugate basecontaining ion pair complexes are only seen for the experiments using amides with acidic protons and are not formed when DMF or DMA are used. Families of doubly charged uranyl complexes are observed at lower m/z values that have the structural formula [UO2 (amide)n]2þ. Complexes with n = 4 or 5 were the most prominent in the gas-phase spectra, an observation consistent with the fact that the dominant uranyl (VI) species in water is generally accepted to be the penta-aquo complex,50,71 although early experiments indicated only four ligands present.72 Studies by Van Stipdonk and co-workers50,51 of gas-phase complexes of [UO2]2þ ligated with acetone/acetonitrile showed penta-coordinated uranyl complexes. Shamov and Schreckenbach46 modeled aquo complexes using DFT that predicted stable penta- and hexa-aquo complexes and showed a structure for the hexa-aquo complex with H2O ligands bending out of the equatorial plane. In fact a hexa-coordinated acetamide complex, [UO2(A)6]2þ, is formed (Figure 1c); six A ligands can probably be accommodated by bending the ligands out of the equatorial plane or by stabilization of one or two A ligands in the second coordination sphere by hydrogen bonding. The final significant family of ions in the uranyl nitrate amide experiments contain the reduced uranyl cation [UO2]þ, ligated with up to several amide ligands. In these complexes, the oxidation state of the uranium metal is þ5 and is likely generated by reduction occurring in the electrospray process. Prior studies by our group have shown that uranyl is susceptible to reduction, particularly if the temperature of the ESI source is elevated above ambient.53 The complexes are particularly abundant in the formamide (F) experiments, with abundant complexes corresponding to [UO2(F)n=3,4]þ at m/z 405 and 450, respectively. Reduced uranyl complexes are also seen in the mass spectra of the other amides, although abundances are lower. Variations in Ion Distributions. Ligand nucleophilicity and steric repulsion are factors that control the distribution of uranylamide coordination complexes formed by electrospray of solutions of uranyl nitrate with each of the amide ligands. Variations seen in comparing the ESI-generated ions in the different amide experiments can be explained in part by the anticipated relative uranophilicities of the amides. Amide affinities for the uranyl dication or uranyl ion pairs are expected to follow the same trend seen in proton affinities (although there are instances where proton affinity does not correlate with basicity or metal nucleophilicity73). In the present case, addition of methyl groups 3500


The Journal of Physical Chemistry A to the amide moiety would be expected to donate more electron density to the carbonyl, increasing its proton affinity and nucleophilicity, and this expectation is supported by DFT calculations for [UO2(amide)]2þ complexes (vide infra). The least nucleophilic amide in this series is formamide F, and it generates the greatest diversity of gaseous coordination complexes. This is in part due to the fact that MeOH is energetically more competitive with F for coordination sites on the uranyl molecule during the electrospray process. This results in decreased stability of the [UO2(NO3)(F)3]þ complex and dissociation involving loss of a F ligand produces an undercoordinated uranium site that is then available for recoordination with neutrals in the electrospray plume, i.e., with MeOH. In addition, the more abundant undercoordinated complexes in the formamide experiments may be more susceptible to reduction, because open coordination sites around the uranium metal center would make the complex approachable by reducing agents. This would explain why the reduced coordination complexes are more abundant in the formamide experiments. These observations not withstanding, [UO2(NO3)(F)3]þ is still the most abundant ion in the mass spectrum at m/z 467. The abundances of the doubly charged complexes are lower in the formamide experiments compared to the other amides. This observation is also rationalized in terms of weaker coordination by the formamide ligand, which facilitates displacement by more strongly binding anions, converting dicationic complexes to ion pairs in the process. The extent of coordination is about the same as for the other amide complexes, in that the most abundant doubly charged complex contains four equatorial F ligands, although lower abundance tri- and penta-F complexes are also formed. The ensemble of ions produced by the N-methyl formamide (MF) experiment (Figure 1b) is much less complex compared to that of the formamide experiment. The ion pair complex [UO2(NO3)(MF)3]þ at m/z 509 accounts for the largest fraction of the ion population, although the bis-MF complex is also apparent at m/z 450. MF conjugate base ion pair complexes are also seen, e.g. [UO2(MF-H)(MF)2]þ at m/z 446, but their abundances are very low, as are the MeOH-containing ions. The fact that the conjugate base complexes are lower abundance reflects the fact that MF is expected to be a poorer acid compared to F, as a result of inductive e-donation to the nitrogen by the methyl group. Hence MF has a decreased ability to transfer a proton to nitrate anion, which is likely a prelude to elimination of HNO3 in the formation of the MF conjugate base complexes. Intramolecular H-transfer forming a departing HNO3 and a bound conjugate base has been observed previously;48 however, the (amide-(Hþ)) complexes may also be generated by another charge separation process occurring during the ESI process. Doubly charged MF complexes are present in higher abundances compared to the F-experiment, which is consistent with the expectation that MF is more strongly bound to the uranyl center. Formation of stable [UO2(MF)4,5]2þ complexes would effectively passivate the metal relative to charge reduction by nitrate (forming ion pairs) and oxidation state reduction (forming reduced uranyl complexes). And in fact, reduced uranyl complexes are much lower in abundance compared with the F-experiments. The fact that five MF ligands are accommodated by UO22þ is somewhat surprising given the bulky nature of MF, which would be expected to produce steric crowding and lower stability. However, this complex may also be a consequence of the stronger nucleophilicity of MF. Five equatorial ligands have been

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previously observed in gaseous uranyl complexes ligated with acetone and acetonitrile.50,51 The ensemble of complexes formed in the acetamide experiment (Figure 1c) was nearly identical to that seen in the MF experiment, an observation consistent with similar sizes, structures, and proton affinities of two ligands. An interesting difference between the spectra of the two amide isomers is that in the doubly charged A-containing ion envelope, the n = 5 complex is more abundant compared with that seen in the MF spectrum, and a low abundance n = 6 complex is even formed. The slightly stronger nucleophilicity of A may be responsible, however, in experiments employing the stronger nucleophiles DMF, MA, and DMA, the n = 4 complex is more abundant. It is likely that the n = 5 complexes are not favored in complexes containing these amides because interligand repulsion of the larger amides destabilizes the more extensively ligated complexes, as suggested by the DFT-generated structures discussed later. The ion ensembles generated from electrospray of solutions containing DMF and MA contained relatively few species, the most abundant of which were the tris-amide, nitrate ion pairs, [UO2(NO3)(DMF)3]þ and [UO2(NO3)(MA)3]þ both at m/z 551 (Figure 1d,e). The bis-amide ion pair complexes at m/z 478 were in higher abundance compared to the experiments with the smaller amides MF and A, which suggests that the tris-amide complexes of DMF and MA may be slightly less stable with respect to dissociation. Such a conclusion would run counter to expected relative nucleophilicities (i.e., the proton affinities of DMF and MA are greater than those of MF and A) but would be consistent with steric repulsion between the larger amide ligands. Complexes containing the reduced uranyl cation [UO2]þ have low abundances and do not accommodate four amides but instead only three amides and one MeOH (m/z 521). Obviously gas-phase [UO2]þ can accommodate four ligands, and such complexes have been observed previously.41 The absence of [UO2(amide)4]þ in this experiment indicates that it is not stable, which must be due to repulsion of the bulky DMF and MA ligands. Steric repulsion may also explain why the [UO2(amide)n=5]2þ complex is not as abundant as the n = 4 complex for both DMF and MA. Doubly charged complexes are very significant in both the DMF and MA experiments, and the combined abundances of these complexes are nearly as great as those of the ion pairs. This is consistent with the idea that DMF and MA are strongly nucleophilic and that doubly charged complexes containing these ligands do not readily dissociate to produce an undercoordinated uranyl metal center that would be susceptible to charge reduction or metal center reduction. The coordination complexes generated in the DMA experiment are also influenced by steric crowding of the ligands (Figure 1f). As seen in the spectra above, the most abundant species corresponds to the [UO2(NO3)(DMA)n=3]þ ion pair at m/z 593. However, the n = 2 ion pair (m/z 506) is more abundant compared to analogous n = 2 ions seen in the spectra of solutions containing the other amides, and similarly, the reduced uranyl complexes (notably m/z 563) are also more abundant. Conversely, the abundance of the doubly charged complexes is significantly lower. These observations suggest that steric crowding is significant in the DMA-containing complexes, resulting in more facile dissociation of DMA from the [UO2(NO3)(DMA)3]þ ion pair. The resulting n = 2 complex is undercoordinated but is still relatively crowded at the uranyl metal center, suggested by the low abundance of the methanol adduct [UO2(NO3)(DMA)2 (MeOH)]þ at m/z 538. The undercoordinated complex may 3501



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Figure 3. MS4 CID spectra of the [UO2(NO3)(MF)]þcomplex at m/z 391, derived from m/z 509, [UO2(NO3)(MF)3]þ. Peaks marked with a star indicate H2O/MeOH adducts formed from reaction of the CID products with adventitious H2O/MeOH in the IT-MS.

Figure 2. MSn sequence. Serial isolation and dissociation reactions of a. [UO2(NO3)(F)3]þ, m/z 467. b. [UO2(NO3)(F)2]þ, m/z 422. c. [UO2(NO3)(F)]þ, m/z 377. d. [UO2(NO3)]þ and the isobaric adduct [UO2(F)(H2O)]þ, m/z 332. e. [UO2(FH)]þ, m/z 314. Ions labeled with a star are solvent adducts formed from coordination complexes and water and or methanol.

nevertheless be susceptible to reduction, which would account for the enhanced abundance of the reduced uranyl complex [UO2(DMA)3(MeOH)]þ at m/z 563. As in the DMF and MA experiments, the reduced uranyl cation will not accommodate four amide ligands but is undercoordinated with only three, a situation that enables formation of a MeOH adduct even though methanol is a much weaker nucleophile. The bulkiness of the DMA ligands also decreases the stability in the dicationic complexes: interligand repulsion inhibits formation of a coordinatively saturated [UO2]2þ (note that the tetrakis-amide complex is lower in abundance, and the penta-amide complex does not form at all). The undercoordinated [UO2]2þ complexes are more susceptible to ion pairing reactions and metal center reduction, leading to overall lower abundance of the dicationic species. Dissociation and Association Reactions by MSn. A series of collision induced dissociation (CID) experiments was undertaken to compare dissociation behavior of the [UO2(NO3)(amide)3]þ complexes, which is of interest because ligand dissociation and association reactions occurring in the CID studies can be correlated with similar processes occurring in the ESI droplets. Generally, dissociation patterns did not provide insight into the comparative behavior of the amides seen in the direct electrospray experiments, but they did reveal unique dissociation reactions that were dependent on the presence of acidic protons or N-methyl moieties in the amide ligands. The CID studies of the [UO2(NO3)(F)3]þ complex provide a vehicle for describing the general pattern of the reactions, and they form a benchmark that can be used for comparison with the other five amide complexes. [UO2(NO3)(F)3]þ (m/z 467) undergoes elimination of a single F ligand to form an [UO2(NO3)(F)2]þ at

m/z 422 (Figure 2a). This undercoordinated complex and others formed from the CID reactions display a significant tendency to condense with adventitious H2O and MeOH that are present in the ion trap. Adducts are easily identified by isolating product ions and then allowing them to react in the quadrupole ion trap (30 ms in the present experiments), which furnishes condensation products that are higher than the isolated dissociation product ion by 18 u (þH2O), 32 u (þMeOH), 36 u (þ2 H2O), and 50 u (þH2O þ MeOH). Thus the bis-F complex at m/z 422 forms [UO2(NO3)(F)2(H2O)]þ at m/z 440 and a small amount of [UO2(NO3)(F2)(MeOH)]þ at m/z 454. These reactions refill the primary coordination spheres of the uranyl complexes (# equatorial ligands = 4) and produced spectra that were primarily dominated by H2O/MeOH adducts.41,48 Subsequent CID (MS3) of [UO2(NO3)(F)2]þ (m/z 422, Figure 2b) caused the elimination of a second F to produce [UO2(NO3)(F)]þ at m/z 377 and a less abundant loss of HNO3 to produce [UO2(FH)(F)]þ at m/z 359. The elimination of nitric acid involves the transfer of a proton from a coordinating F to the nitrate anion during the dissociation reaction, which is facilitated by stable conformations in which the acidic protons are in close proximity to the nitrate oxygen atoms. The reaction is consistent with analogous processes previously observed for gasphase Pb2þ- and UO22þ-alcohol nitrate complexes.48,74 CID (MS4) of [UO2(NO3)(F)]þ (m/z 377, Figure 2c) also resulted in elimination of F to produce [UO2(NO3)]þ at m/z 332 and a much more competitive elimination of HNO3 to form [UO2(FH)]þ at m/z 314, respectively. The [UO2(NO3)]þ complex is isobaric with the H2O adduct [UO2(FH)(H2O)]þ, and hence the ion at m/z 332 has two compositions. The contribution of the latter ion was confirmed by isolation of [UO2(FH)]þ followed by a 30 ms reaction interval that produced m/z 332 corresponding to [UO2(FH)(H2O)]þ. CID (MS5) of the isolated ion at m/z 332 (Figure 2d) produced five notable product ions that confirm the notion that the ion is a mixture of [UO2(FH)(H2O)]þ and [UO2 (NO3)]þ. The most prominent product is the loss of the H2O (18 u) forming [UO2(FH)]þ at m/z 314, consistent with a fraction of the ion population being [UO2(FH)(H2O)]þ. Product ions at m/z 288, 271, and 270 probably arise from dissociation of [UO2(NO3)]þ. [UO2]þ at m/z 270 is formed from the loss of 62 u, consistent with the loss of NO3•. We note that [UO2]þ could also arise via loss of (H2Oþ(FH)), but this reaction is not likely because CID of [UO2(FH)]þ does not eliminate (FH) radical (see below). Formation of [UO2]þ from [UO2(NO3)]þ has been observed in other studies.48 The ion at m/z 288 corresponds to [UO2(H2O)]þ which is produced from a reactive dissociation with [UO2(NO3)]þ in which H2O is added and NO3• is eliminated (Reaction 1).48 [UO2]þ can also 3502



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Scheme 1. Proposed Mechanism for Elimination of 78 u from [UO3(NO3)(d6-MA)]þ

Table 2. Dissociation Energies (kJ/mol) for Elimination of an Amide from the [UO2(NO3)(amide)3]þ and [UO2(amide)]2þ Complexes, Calculated at B3LYP/SCRECP/aug-cc-pVDZ Levela dissociation energies, kJ/mol [UO2(amide)]2þ amide proton affinity

41

be hydrated by an ion molecule reaction; however, the reaction rates are too slow to account for the significant abundance of [UO2(H2O)]þ. The CID product at m/z 271 corresponds to [UO(OH)]þ which is probably also formed by a reactive dissociation of [UO2(NO3)]þ (Reaction 2). This product ion was also seen in earlier CID experiments of [UO2(NO3)]þ, but its occurrence was not specifically addressed48 ½UO2 ðNO3 Þþ þ H2 O f ½UO2 ðNO3 ÞðH2 OÞþ f ½UO2 ðH2 OÞþ þ NO3 •

ð1Þ

½UO2 ðNO3 Þþ þ H2 O f ½UO2 ðNO3 ÞðH2 OÞþ f ½UOOHþ þ HOONO2 þ

ð2Þ 5

CID of [UO2(FH)] at m/z 314 (a parallel MS reaction, Figure 2e) resulted in elimination of HCN which furnished [UO2(OH)]þ at m/z 287. Formation of the [UO2(OH)]þ species must occur by transfer of a proton from the amide N to the ligating O atom and elimination of HCN. No elimination of the (FH) radical occurred. The multiple-stage dissociation pathways identified for the F-containing complexes can now be qualitatively compared with those seen in the other five amides. For each of the [UO2(NO3)(amide)3]þ complexes, the dominant MS2 dissociation is elimination of an intact amide. The bis-amide product ions slowly add a single solvent ligand, which suggests that a similar set of reactions occurring in the ESI process are responsible for water- and MeOH-containing complexes. Dissociation of the [UO2(NO3)(amide)2]þ complexes occurring in the MS3 experiments principally occur by loss of another amide ligand; however, a low abundance loss of HNO3 is observed in all four amides that have acidic protons. Loss of HNO3 does not occur from DMF or DMA in either the MS3 or subsequent CID stages. Similar behavior is seen in the dissociation of the [UO2(NO3)(amide)]þ complexes in the MS4 experiments: elimination of both the intact amide and HNO3 occurs; however in these processes, the two reactions are competitive (i.e., the abundances of the product ions are similar). A new reaction is observed from [UO2(NO3)(amide)]þ complexes at the MS4 stage that accounts for the loss of 76 u for those complexes containing amides that have N-methyl moieties; loss of 76 u was not observed in uranyl coordination complexes containing either A or F. For example, an abundant product ion at m/z 315 is formed in the CID of [UO2(NO3)(MF)]þ (m/z 391) (Figure 3), and ions corresponding to loss of 76 u are also prominent in the spectra of analogous MA-, DMA-, and DMF-

a

ΔE0

[UO2(NO3)(amide)3]þ ΔE0

ΔH298

ΔG298

F

822

460.2

133.7

133.3

86.5

MF

851

508.6

138.6

137.4

90.7

DMF

888

537.0

129.6

127.7

85.9

A

864

505.8

138.5

138.6

86.6

MA

889

536.9

128.6

128.2

74.4

DMA

908

568.8

127.4

125.7

78.1

Proton affinity values are included for comparison.

containing complexes. The elimination of 76 u suggests the possible elimination of methylene together with NO3, and perdeuterated MA was used to test the postulation. Dilution of d7-Nmethylacetamide in the uranyl-MeOH-H2O ESI solution replaced the acidic deuterium with a proton prior to ion formation, resulting in the formation of d6-N-methylacetamide (d6-MA, structure of coordinated ligand provided in Scheme 1). ESI resulted in formation of [UO2(NO3)(d6-MA)3]þ, and two consecutive CID events furnished [UO2(NO3)(d6-MA)]þ at m/z 411 (Scheme 1). CID of this complex resulted in the loss of 78 u, consistent with the elimination of CD2, together with NO3, to form an ion at m/z 333 that corresponds to [UO2 ((d6-MA)-CD2)]þ. The elimination of CD2 (CH2 from the unlabeled experiment) can be rationalized by insertion of the undercoordinated uranium metal center into the CN bond, followed by rearrangement to form an intermediate in which an O atom is bound to the migrating methyl group (Scheme 1). Subsequent D rearrangement (H rearrangement in the unlabeled experiment) would produce a second intermediate from which NO2 and formaldehyde could readily be eliminated. Of course neither the structure of the product neutral(s) nor the ion are known at this time; however, such a scheme would account for the elimination from all of the N-methylcontaining amide complexes and the deuterium labeling. Structures and Energies of the Ion Pair Complexes by DFT. Since the dominant dissociation reaction of the [UO2(NO3) (amide)3]þ complexes is the elimination of an amide, dissociation energies for reactions 3 - 8 were calculated using B3LYP/SCRECP/aug-cc-pVDZ (Table 2). The calculated dissociation energies (ΔE0) display very modest variation (a total range of only 11.1 kJ/mol from the most strongly held ligand (MF) to the weakest (DMA)), despite the likely differences in nucleophilicities, which were hypothesized to follow trends in proton affinities.75 Dissociation energies for the [UO2(amide)]2þ complexes were calculated to evaluate this assumption (Table 2, Figure 4) since these complexes will not be affected by interligand repulsion. The ΔEo values for the [UO2(amide)]2þ complexes increased in a regular fashion with increasing amide proton affinities, as expected. Drawing comparisons from the [UO2(NO3)(formamide)3]þ dissociation energies of the formamide derivatives, MF is slightly more strongly bound compared to F, as expected. However, DMF, which should be the most strongly bound of the three, actually produces the smallest dissociation energy value. The comparison of the acetamide derivatives is even more surprising: ΔE0 values 3503



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follow the trend A > MA > DMA, precisely the opposite of what would be expected. The fact that the two most nucleophilic ligands (DMF and DMA) are calculated to have the lowest ΔE0 values in the [UO2(NO3)(amide)3]þ complexes suggests that interligand repulsion is weakening binding, consistent with the fact that these are also the most bulky of the amides studied ½UO2 ðNO3 ÞðFÞ3 þ f ½UO2 ðNO3 ÞðFÞ2 þ þ F

ð3Þ

½UO2 ðNO3 ÞðMFÞ3 þ f ½UO2 ðNO3 ÞðMFÞ2 þ þ MF

ð4Þ

½UO2 ðNO3 ÞðDMFÞ3 þ f ½UO2 ðNO3 ÞðDMFÞ2 þ þ DMF ð5Þ ½UO2 ðNO3 ÞðAÞ3 þ f ½UO2 ðNO3 ÞðAÞ2 þ þ A ½UO2 ðNO3 ÞðMAÞ3 þ f ½UO2 ðNO3 ÞðMAÞ2 þ þ MA

ð6Þ ð7Þ

½UO2 ðNO3 ÞðDMAÞ3 þ f ½UO2 ðNO3 ÞðDMAÞ2 þ þ DMA

Figure 4. Amide binding energies plotted versus proton affinity values. Circular data points represent complexes containing formamide derivatives (F, MF, DMF), while square points represent complexes containing acetamide derivatives (A, MA, MDA). Points in the upper box represent [UO2(amide)]2þ complexes, while points in the lower box represent [UO2(NO3)(amide)3]þ box.

ð8Þ Insight into possible ligand repulsion is provided by comparing bond distances (Table 3) and nitrateUOC dihedral

Table 3. UranylAmide Bond Distances (Å) for [UO2(NO3)(amide)3]þ and [UO2(NO3)(amide)2]þ Complexes complex family tris-formamides

tris-acetamides

bis-formamides

bis-acetamides

complexes UO2(NO3)(F)3

þ

UOcis

UOcis

UOtrans

Onitrate 3 3 3 H

OuranylH 2.840

2.389

2.390

2.434

2.027, 2.027

UO2(NO3)(MF)3þ

2.377

2.378

2.418

2.011, 2.011

2.816

UO2(NO3)(DMF)3þ

2.388

2.405

2.416

-

2.682

UO2(NO3)(A)3þ

2.367

2.368

2.407

2.003, 2.003

2.157

UO2(NO3)(MA)3þ

2.364

2.374

2.405

1.978, 1.969

-

UO2(NO3)(DMA)3þ

2.370

2.371

2.398

-

-

UO2(NO3)(F)2þ

2.347

2.347

-

2.217, 2.218

-

UO2(NO3)(MF)2þ UO2(NO3)(DMF)2þ

2.331 2.325

2.331 2.326

-

2.206, 2.206 -

-

UO2(NO3)(A)2þ

2.329

2.329

-

2.179, 2.180

-

UO2(NO3)(MA)2þ

2.323

2.323

-

-

-

UO2(NO3)(DMA)2þ

2.314

2.314

-

-

-

Table 4. UranylAmide Dihedral Angles for [UO2(NO3)(amide)3]þ and [UO2(NO3)(amide)2]þ Complexes complex family tris-formamides

tris-acetamides

bis-formamides

bis-acetamides

cis amide 1 NnitrateUOamideC

cis amide 2 NnitrateUOamideC

trans amide Ocis amideUOamideC

UO2(NO3)(F)3þ

12.3

12.3

89.4

UO2(NO3)(MF)3þ UO2(NO3)(DMF)3þ

13.6 57.2

13.6 59.1

89.5 85.6

UO2(NO3)(A)3þ

24.2

24.2

88.7

UO2(NO3)(MA)3þ

27.1

14.5

75.9

UO2(NO3)(DMA)3þ

46.9

42.5

83.3

UO2(NO3)(F)2þ

0.7

0.0

-

UO2(NO3)(MF)2þ

0.0

0.0

-

UO2(NO3)(DMF)2þ

8.9

4.8

-

UO2(NO3)(A)2þ UO2(NO3)(MA)2þ

0.3 10.0

0.3 0.4

-

UO2(NO3)(DMA)2þ

9.3

11.2

-

complexes

3504



Figure 5. Structural conformations calculated for the constituents of reactions 3 - 8.

angles (Table 4) of the [UO2(NO3)(amide)3]þ complexes. The lowest energy conformers of the [UO2(NO3)(amide)n=23]þ complexes (based on zero-point corrected electronic energies, E0) show that the nitrate ligand coordinates UO22þ in a bidentate fashion in all complexes (Figure 5). In general, DFT calculations show that donor ligands prefer to be oriented in the equatorial plane of the uranyl molecule to the greatest degree possible. This is also the case for the nitrate ligands in all of the complexes studied and is also indicated by the very small dihedral nitrateUOC angles predicted for the bis-amide complexes (see lower half of Table 4 and numerous prior DFT and spectroscopic studies).53,7678 However, for the tris-amide complexes, all of the amide ligands are significantly rotated out of the equatorial plane. In general, the bulkier the amide, the greater the rotation. In the [UO2(NO3)(formamide)3]þ complexes, the F and MF ligands that are cis- to the nitrate are rotated 1214° out of the equatorial plane, while this value increases to ∼58° in the tris DMF complex (Table 4). And the trans amide is very nearly orthogonal to the equatorial plane for all three formamide derivatives. UOequatorial bond distances for the cis ligands of the tris-F and -MF complexes are ∼2.382.39 Å, while the distance for the trans-ligands is modestly longer at 2.422.43 Å. The UOequatorial bond distances for the tris-DMF complex are not a lot different from those seen in the tris-F and -MF containing complexes, which again is not in accord with expectations based on a stronger nucleophilicity. The tris-DMF complex showed significant differences between UOequatorial bond

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Figure 6. Comparison of trends in the UOequatorial bond distances for the tris-amide complexes with those calculated for the bis-complexes. Bond distances are plotted versus amide nucleophilicities as represented by proton affinity values, for [UO2(NO3(amide)n=2,3]þ complexes. a. Complexes containing formamide derivatives. b. Complexes containing acetamide derivatives. Square data points are derived from tris-amide complexes. Circular data points are derived from bisamide complexes.

distances for the two cis-DMF ligands and a less elongated distance for the trans-DMF. Ligand rotation is evidence for interligand repulsion, which evidently counteracts bond shortening expected from more nucleophilic ligands. This is illustrated by comparing trends seen in plots of UOequatorial distances versus formamide nucleophilicity as estimated by the proton affinity. In the case of the bis-formamide complexes (Figure 6a, circular points), UOequatorial distances decrease with increasing amide nucleophilicity. In contrast, no discernible trend is apparent in the tris-formamide complexes (Figure 6a, square points). Part of the apparent stabilization seen in the less nucleophilic formamide complexes might be due to hydrogen bonding between the acidic protons and the nitrate O-atoms. OnitrateH bond distances of ∼2.012.03 Å suggest a moderately strong hydrogen bond,79 that would stabilize [UO2(NO3)(F)3]þ and [UO2(NO3)(MF)3]þ. Interestingly, these bonds lengthen somewhat in the bis-complexes [UO2(NO3)(F)2]þ and [UO2(NO3)(MF)2]þ. This is somewhat surprising because the amides in the bis-complexes are very nearly exactly in the equatorial plane, which should enable a closer approach of the acidic protons. However, this is compensated by a slight expansion of the nitrate-U-amide angle compared to the tris-formamide complexes. The hydrogen bonding interactions in the F and MF complexes (and the A and MA complexes) are consistent 3505


The Journal of Physical Chemistry A with significant CID elimination of HNO3 seen in the MS3 and MS4 CID experiments. The ligated acetamide derivatives are also rotated out of the equatorial plane to a significant extent in the [UO2(NO3) (acetamide)3]þ complexes (Table 4). The trans-acetamide ligands are nearly orthogonal to the equatorial plane in each of the A-, MA- and DMA-containing complexes, while the cisamides display quite a bit of variability even within the same complex. The structure of the acetamide complex [UO2(NO3) (A)3]þ resembles that of the analogous tris-F complex in that the trans-A is nearly orthogonal to the equatorial plane, and both of the cis-A ligands are rotated to the same extent (24.2°). The tris-MA complex is much less symmetrical, with the two cis-amides rotated by different degrees, 27.1 and 14.5°, respectively. In the case of the tris-DMA complex, both of the cis-ligands are at angles of 4247° out of the equatorial plane, the trans-ligand is also significantly rotated. These dihedral angles reflect significant interligand repulsion even in the case of acetamide, which is the least bulky acetamide derivative. As in the comparisons of the three tris-formamide complexes, the UOequatorial bond distances do not become noticeably shorter with increasing nucleophilicity in the tris-ligated complexes of the acetamide derivatives. This trend is observed in plots of bond distances versus acetamide PA values for the bis-ligated complexes [UO2(NO3)(acetamide)2]þ (Figure 5b, circular points): distances decrease following the trend A > MA > DMA. In the tris-ligated complexes, a modest decrease is seen in the UOequatorial distance for the trans-DMA ligands, but other than that, no appreciable changes are calculated. Thus the expected bond strengthening resulting from increased nucleophilicity is offset by interligand repulsion as indicated by the dihedral angles and the equatorial bond distances. The tris complexes containing acetamide derivatives with acidic hydrogen atoms are also stabilized by hydrogen bond interactions with the nitrate O atoms. OnitrateH distances of ∼2.00 and 1.98 Å were calculated for the A and MA-containing complexes, indicative of a moderately strong hydrogen bond79 between nitrate and the cis-ligands. A second potential hydrogen bond was suggested, this one between the trans-acetamide and the uranyl O atom, with a calculated distance of 2.157 Å. In the bis-acetamide complex [UO2(NO3)(A)2]þ both A ligands are oriented in the equatorial plane, suggesting that interligand repulsion is not playing a role. Acidic hydrogen atoms are located about 2.18 Å from the nitrate O atoms, close enough to suggest hydrogen bonding interactions. In the bis-MA complex, the MA ligands are much closer to the equatorial plane (10.0° and 0.4°), as would be expected after removing the steric repulsion of the trans ligand, but surprisingly, the acidic protons are now rotated away from the nitrate O atoms. The DFT results clearly indicate that this is a stable structure and suggests that there is repulsion between the acetyl methyl and the amide N-methyl, that is relieved by rotation, and at the cost of stabilizing H-bonding. In the calculated structure of the [UO2(NO3)(DMA)2]þ both of the amide ligands approach an equatorial coplanar orientation (out of the equatorial plane by 9.3° and 11.2°), which indicates that most, but not all, of the interligand repulsion has been relieved.

’ CONCLUSIONS The distributions of ion species in electrospray ionization experiments of uranyl nitrate and various amide ligands are

ARTICLE

strongly influenced by both the amide nucleophilicity and by interligand repulsion. The weakly nucleophilic formamide produces ion pair and doubly charged complexes that are prone to dissociate, resulting in formation of species containing solvent ligands and reduced uranyl UO2þ. These tendencies are lessened in complexes containing stronger nucleophiles such as methyl formamide, acetamide, dimethyl formamide, and methyl acetamide: experiments with these amides produced far less abundant species containing either solvent or UO2þ and much more abundant ion pair and doubly charged species. Experiments using dimethyl acetamide show the effect of interligand repulsion, resulting in more abundant undercoordinated ion pair- and UO2þ-complexes, while the abundance of the doubly charged complexes are dramatically reduced. These phenomenological trends are supported by DFT modeling, which shows that for [UO2(NO3)(amide)3]þ complexes, stability with respect to elimination of an amide is relatively unaffected by amide nucleophilicity. This is because this factor is effectively balanced by interligand repulsion, which causes increasing ligand rotation out of the uranyl equatorial plane when bulkier amides are used, despite the fact that they are more nucleophilic. All of the trisamide complexes [UO2(NO3)(amide)3]þ are affected by interligand repulsion although this factor does not begin causing significant changes to the distribution of ion species until dimethyl acetamide is utilized. These observations should help provide qualitative guidance for interpreting distributions of metal ion species formed from electrospray ionization of solutions containing ligands of varying size and nucleophilicity. The results also provide a starting point for assessing amide binding in more complex systems, such as those existing in separations or environmental processes, where ligands are encountered that have multiple functional groups of varying nucleophilicity and varying degrees of steric repulsion within the inner coordination sphere.

’ ASSOCIATED CONTENT

bS

Supporting Information. The coordinates for the structures calculated for the complexes and free amides. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses §

Department of Chemistry, Wilkes University, Wilkes-Barre, PA 18766.

’ ACKNOWLEDGMENT The INL authors acknowledge support by the U.S. Department of Energy, Environmental Systems Research Program, under contract DE-AC-07-99ID13727. M.V.S. acknowledges support for this work by a grant from the National Science Foundation (CAREER-0239800), a First Award from the Kansas Technology Enterprise Corporation/Kansas NSF EPSCoR program, and a subcontract from the U.S. Department of Energy through the INL. The authors would like to thank T. Crain, A. Custer, and J. S. Barklund for their assistance in generating the ESI data. 3506



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