Tetrapositive Plutonium, Neptunium, Uranium, and Thorium

Mar 24, 2014 - The Pu4+, Np4+, and U4+ ions, which have large electron affinities of ∼34.6, ∼33.6, and ∼32.6 eV, respectively, were stabilized f...
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Tetrapositive Plutonium, Neptunium, Uranium, and Thorium Coordination Complexes: Chemistry Revealed by Electron Transfer and Collision Induced Dissociation Yu Gong, Guoxin Tian, Linfeng Rao, and John K. Gibson* Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: The Pu4+, Np4+, and U4+ ions, which have large electron affinities of ∼34.6, ∼33.6, and ∼32.6 eV, respectively, were stabilized from solution to the gas phase upon coordination by three neutral tetramethyl-3-oxa-glutaramide ligands (TMOGA). Both collision induced dissociation (CID) and electron transfer dissociation (ETD) of Pu(TMOGA)34+ reveal the propensity for reduction of Pu(IV) to Pu(III), by loss of TMOGA+ in CID and by simple electron transfer in ETD. The reduction of Pu(IV) is in distinct contrast to retention of Th(IV) in both CID and ETD of Th(TMOGA)34+, where only the C−Oether bond cleavage product was observed. U(TMOGA)34+ behaves similarly to Th(TMOGA)34+ upon CID and ETD, while the fragmentation patterns of Np(TMOGA)34+ lie between those of Pu(TMOGA)34+ and U(TMOGA)34+. It is notable that the gas-phase fragmentation behaviors of these exceptional tetrapositive complexes parallel fundamental differences in condensed phase chemistry within the actinide series, specifically the tendency for reduction from the IV to III oxidation states.

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INTRODUCTION Ligated tetrapositive metal ions are rare in the gas phase where such highly charged metal centers cannot be easily stabilized in the absence of solvent molecules.1,2 In the gas phase, the high fourth ionization energy (IE) of metals usually results in the formation of reduced charge products, such as by the ejection of a positively charged intact ligand or radical fragment. Many simple metal dications and some trications coordinated by protic and aprotic ligands have been observed in the gas phase,3−11 whereas tetrapositive metal ion complexes remain elusive,12,13 this being a significant gap between solution and gas phase coordination chemistry. Our recent studies of Th(L)34+ (L = TMOGA, tetramethyl-3-oxa-glutaramide; also known as TMDGA, N,N,N′,N′-tetramethyl diglycolamide, Figure 1) revealed that it is possible to stabilize a tetrapositive metal center in the gas phase using a multidentate ligand like TMOGA,14 which is known to form very stable solution

complexes with lanthanide and actinide metal cations (e.g., Figure 2).15−19 It is not necessarily unexpected that Th(L)34+

Figure 2. Structure of Pu(L)34+ (ref 19).

can be stabilized into the gas phase since the fourth IE of thorium (28.8 eV20) is lower than the third IEs of some transition metals for which gas-phase ligated trications have been observed.21,22 Thorium has the lowest fourth IE of all elements for which values have been measured or estimated.21 As the fourth IE increases, it should become more difficult to stabilize tetrapositive metal ion complexes in the gas phase. To probe the effects of increasing IE energy on the stability of tetrapositive ion complexes, new experimental results for gas Figure 1. Structure of the TMOGA ligand, C8H16N2O3 (mass = 188 Da). Red = oxygen; blue = nitrogen; gray = carbon; shaded gray = hydrogen. The fragments produced by C−Oether bond cleavage are identified. © 2014 American Chemical Society

Received: February 10, 2014 Revised: March 21, 2014 Published: March 24, 2014 2749

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logical containment glovebox.24 All the An(L)34+ (An = Pu, Np, U, Th) cations were produced by ESI of 1:1 An(ClO4)4 (200 μM) and TMOGA mixtures in acetonitrile with about 0.5% H2O. The Pu(ClO4)4 stock solution was prepared with about 72% yield by mixing PuO2(ClO4)2 and Pu(ClO4)3 in a 1:2 ratio, as described elsewhere.25 The Np(ClO4)4 and U(ClO4)4 stock solutions were made by adding zinc powder to solutions of NpO2(ClO4) and UO2(ClO4)2. The plutonium, neptunium, uranium, and thorium isotopes employed were Pu-242, Np237, U-238, and Th-232, which undergo alpha-decay with halflives of 3.7 × 105, 2.1 × 106, 4.5 × 109, and 1.4 × 1010 years, respectively. The MSn capabilities of the QIT/MS, which designates the ability to perform multiple (n) sequential mass spectrometry stages, allow isolation of ions with a particular mass-to-charge ratio, m/z, followed by CID, in which ions are excited and undergo energetic collisions with helium. The ions isolated inside the trap are at a temperature around 300 K.26 ETD was performed using the fluoranthene anion, C16H10−, as the electron donor from which an electron is transferred to a cation. The C16H10− was gated from the negative chemical ionization source into the ion-transfer optics and into the ion trap for reaction with trapped cations. The process associated with introducing anions into the trap may result in incomplete thermalization of the cation complexes in the trap; any such hyperthermal effects should be essentially constant and not appreciably affect the comparative results. The ETD reaction time was typically a few milliseconds. In high resolution mode, the instrument has a detection range of m/z 20−2200 with a mass width (fwhm) of m/z ≈ 0.3. Mass spectra were recorded in the positive ion accumulation and detection mode. The intensity distribution of ions in the mass spectra was highly dependent on instrumental parameters, particularly the RF voltage applied to the ion trap; the parameters are similar to those employed in previous experiments.14 The high-purity nitrogen gas for nebulization and drying in the ion transfer capillary was the boil-off from a liquid nitrogen Dewar. As has been discussed elsewhere,27,28 the background H2O and O2 pressures in the ion trap are estimated to be on the order of 10−6 Torr. The helium buffer gas pressure in the trap is constant at ∼10−4 Torr.

phase Pu(L)34+, Np(L)34+, and U(L)34+ are reported here. The fourth IEs increase significantly across the actinide (An) series beyond Th, from IE[Th3+] = 28.8 eV to IE[U3+] ≈ 32.6 eV, IE[Np3+] ≈ 33.6 eV, and IE[Pu3+] ≈ 34.6 eV, the last three having been estimated from thermodynamic cycles;20 the fourth IEs continue to increase beyond Pu in the actinide series (except Bk), with IE[Lr3+] estimated as 42.6 eV.20 Stabilization of gas phase Pu(L)34+, Np(L)34+, and U(L)34+ complexes also provides unique opportunities to probe the fragmentation chemistry of tetrapositive ions, particularly in comparison with that of Th(L)34+ because the condensed phase redox chemistries of Pu(IV), Np(IV), U(IV), and Th(IV) are quite disparate; in particular, whereas Th(IV) is the only common oxidation state, Pu, Np, and U have several readily accessible oxidation states, although their stabilities toward reduction or oxidation vary.23 The relative stabilities of the An(IV) oxidation states are revealed by their reduction potentials: −3.8 V (estimated) for Th(IV/III); −0.55 V for U(IV/III); +0.22 V for Np(IV/III); and +1.05 V for Pu(IV/III).23 The IV/III reduction potential increase sequentially from Th to U, U to Np, and Np to Pu by 3.3, 0.8, and 0.8 V, respectively. The difference between the reduction potentials of four actinides is roughly comparable to the corresponding sequential increase of the IEs by ∼3.8, ∼1.0, and ∼1.0 eV from Th3+ to Pu3+,20 illustrating the parallel between these fundamental solution and gas-phase properties. Transition metal and main group trications and dications are known to exhibit diverse metaldependent fragmentation patterns.22 Here we address the relationship between fragmentation patterns of gas-phase tetrapositive metal ion complexes and known condensed phase chemistry. A particularly unique aspect of this work is that both collision induced dissociation (CID) and electron transfer dissociation (ETD) were employed to examine the fragmentation of Pu(L)34+, Np(L)34+, U(L)34+, and Th(L)34+; previously only CID of Th(L)34+ had been studied.14 In CID, fragmentation occurs as a result of multiple ion−molecule collisions, which relatively gradually heat the ion complex during the course of several milliseconds. In ETD, which is applicable only to multiply charged positive ions, exothermic transfer of an electron from a fluoranthene anion deposits energy into the cation complex on a much shorter time scale, ≪1 ms, than that of CID. Another key difference between CID and ETD as applied to metal ion coordination complexes is that ETD necessarily results in charge-reduction, which can be accommodated by reduction of the metal center if lower oxidation states are accessible, or by fragmentation of a ligand into components that can form metal−radical bonds if lower oxidation states are not accessible. The new CID results for Pu(L)34+, Np(L)34+, and U(L)34+ are compared with those previously reported for Th(L)34+.14 The ETD results for Pu(L)34+, Np(L)34+, U(L)34+, and Th(L)34+ reported here are the first such studies of ETD applied to tetrapositive metal complexes and reveal fundamental chemical information that can be derived from this approach. Comparison between fragmentations of the four tetrapositive ions provide essential insights into the chemistries of the metal centers and demonstrate a direct connection between condensed and gasphase chemistry.

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RESULTS AND DISCUSSION ESI of Pu(L)34+, Np(L)34+, and U(L)34+. A series of plutonium containing species with different charge states and ligands were observed in the positive mode ESI mass spectrum of a 1:1 Pu(ClO4)4:TMOGA mixture in acetonitrile (Figure 3). The Pu(L)34+ complex was observed at m/z 201.4 that matches the actual m/z 201.5 to within the absolute accuracy of the spectrometer, which is estimated as m/z ca. 0.2 (the relative mass accuracy is better, m/z ca. 0.1); partially resolved lowerintensity peaks at higher m/z (Figure S1, Supporting Information), separated by m/z 0.25 from the dominant peak, are attributed to 13C isotopomers with z = 4+. Because of the 24 carbon atoms in Pu(L)34+ the predicted intensities of the first two isotopomer peaks are ca. 26% and 3% relative to that of the dominant fully 12 C peak, consistent with the experimental observations. CID of Pu(L)34+ mainly resulted in the m/z 205.8 peak, which is assigned to Pu(L)23+ on the basis of the m/z value and the 12C/13C splitting (m/z 0.33 indicating z = 3+). Minor CID products such as Pu(L)2(H2O)3+ and Pu(L)2(86)3+ were also observed. The appearance of trications upon CID further confirms the assignment of Pu(L)34+, the only tetrapositive ion observed in

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EXPERIMENTAL DETAILS The experiments were performed using an Agilent 6340 quadrupole ion trap mass spectrometer (QIT/MS) with the electrospray ionization (ESI) source located inside a radio2750

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tricapped trigonal prismatic geometry with plutonium coordinated by nine oxygen atoms (Figure 2).19 The gas-phase Pu(L)34+ complex likely possesses a similar structure in which the highly charged Pu4+ center is stabilized by three TMOGA ligands. A similar gas-phase structure was proposed for Th(L)34+ based on a recent computational study.14 Np(L)34+ and U(L)34+ are expected to have essentially the same structures as the other two actinide complexes. Figure 4

Figure 3. ESI mass spectrum of Pu(ClO4)4 and TMOGA (L) mixtures in acetonitrile.

these experiments. Binary trications and dications such as Pu(L)23+, Pu(L)33+, PuO(L)22+, and PuO2(L)22+ were also present in the parent ESI mass spectrum, some of which were more abundant than Pu(L)34+. Coordination of perchlorate anions to multiply charged plutonium cations resulted in several additional trications and dications. Gas phase Np(L)34+ and U(L)34+ complexes were produced by ESI of Np(ClO4)4/TMOGA and U(ClO4)4/TMOGA mixtures in acetonitrile (Figures S2 and S3, Supporting Information). The partially resolved isotopic peaks appearing at about m/z higher, as well as the appearance of trications upon CID, confirm the assignments of both tetrapositive ions. In addition, NpO(L)22+ and NpO2(L)22+ as well as UO(L)33+ and UO2(L)22+ complexes were also present together with Np(L)34+ and U(L)34+. UO2(L)22+ was the predominant species in the mass spectra from ESI of the U(ClO4)4/TMOGA mixture. The ESI mass spectrum of the Th(ClO4)4/TMOGA mixture is shown in ref 14. Compared with the species observed during ESI of Th(ClO4)4/TMOGA,14 the mass spectra of Pu(ClO4)4/ TMOGA, Np(ClO4)4/TMOGA, and U(ClO4)4/TMOGA exhibit greater species diversity, presumably due to the rich solution chemistry of these three actinides, which commonly exhibit oxidation states from III to VI.23 The ESI behavior of the Pu4+/TMOGA mixture particularly contrasts with that of Th4+/TMOGA where the dominant product was Th(L)34+.14 The high propensity for retention of Th4+ from solution to gas can be attributed to the extraordinary stability of the Th(IV) oxidation state and inaccessibility of Th(III).23 Species of oxidation states Np(III) and U(III) are negligible in the mass spectra, most likely due to the lower stability of these trivalent oxidation states toward oxidation. 23 The hexavalent PuO2(L)22+, NpO2(L)22+, and UO2(L)22+ complexes observed here were also the major species from ESI of PuO2(ClO4)2/ TMOGA, NpO 2 (ClO 4 ) 2 /TMOGA, and UO 2 (ClO 4 ) 2 / TMOGA mixtures.29 The predominance of the UO2(L)22+ peak in the mass spectrum of the U(ClO4)4/TMOGA mixture indicates that it is relatively facile for U(IV) to be oxidized to U(VI) during ESI. As in previous experiments,14 peaks due to Na(L)+ and Ca(L)22+, as well as HL+ and its fragments, were ubiquitous. Additionally, zinc containing species such as Zn(L)22+ and Zn(L)(H2O)x2+ were observed together with Np(L)34+ and U(L)34+ since zinc was added into both solutions as a reducing agent. CID of Pu(L)34+, Np(L)34+, and U(L)34+. The recently reported solid state structure of Pu(L)34+ exhibits a twisted

Figure 4. CID mass spectra of Pu(L)34+, Np(L)34+, U(L)34+, and Th(L)34+. A, AnIV(L)34+; B, AnIII(L)23+; C, AnIV(L)2(OH)3+; D, AnIV(L)2(L-86)3+. The asterisks denote secondary products of Pu(L)23+ and Np(L)23+ upon their isolation.

shows the CID spectra of Pu(L)34+, Np(L)34+, and U(L)34+. For comparison, a CID spectrum of Th(L)34+ is also included. There are two major differences in CID of the four tetrapositive ion complexes. The first major difference is that charge reduction products An(L)23+ (peak B in Figure 4) were only observed during CID of Np(L)34+ and Pu(L)34+ with the yield much higher for the latter (reaction 1), indicating that Pu(IV) and Np(IV) are directly reduced to Pu(III) and Np(III). This is in stark contrast to CID of Th(L)34+ and U(L)34+ which produced no detectable Th(L)23+ or U(L)23+. Although the fragmentation energies are not well established in multiple collision CID, the experiments were performed under the same CID conditions such that relative fragmentation abundances can be validly compared. From Figure 4, it is evident that CID of Th(L)34+ and U(L)34+ to give An(L)2(L-86)3+ (peak D) is less efficient than is elimination of an intact L+ from Pu(L)34+ (peak B); both fragmentation channels, B and D, are observed for Np(L)34+. The high CID yield of Pu(L)23+ suggests that Pu(L)34+ is inherently less stable toward reduction of the metal center than are Np(L)34+, U(L)34+, and Th(L)34+. Reduction of Th(IV) to Th(III) is very unfavorable, with a solution reduction potential of −3.8 V,23 this being in accord with the absence of Th(L)23+ upon CID of Th(L)34+. Although U(III) is 2751

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known in condensed phase,23 it is oxidized in aqueous solution to U(IV) with the evolution of H2, which is indicative of the low stability of U(III) and the U(IV/III) reduction potential of −0.55 V. Accordingly, no U(III) products were observed in the CID experiments. In contrast, Pu(IV) is known to be readily reduced to Pu(III) in the condensed phase, with a favorable Pu(IV/III) reduction potential of +1.05 V. The IEs noted above, 28.8 eV for Th3+, ∼32.6 eV for U3+, and ∼34.6 eV for Pu3+, similarly predict that Pu4+ should be inherently more susceptible to charge reduction via CID reaction 1. Both the IE of Np3+ and the Np(IV/III) reduction potential lie between those of plutonium and uranium,20,23 such that it is possible to stabilize the charge reduction product Np(L)23+, although not to the same extent as for plutonium. Charge reduction has also been observed for complexes of transition metals like iron, for which both the II and III oxidation states are accessible.22 An IV (L)3 4 + → An III(L)2 3 + + L+

[An = Pu, Np]

An IV (L)3 4 + → An IV (L)2 (86)3 + + (L‐86)+

As is evident in Figure 4, there are several other minor products in addition to those discussed above. Fragment peaks at m/z 211.8, 265.9, and 237.8 produced during CID of Pu(L)34+ also appeared when Pu(L)23+ was mass selected without subsequent application of a CID voltage, suggesting that these peaks result from secondary reactions of the primary CID product, Pu(L)23+. The first two of these three peaks correspond to Pu(L)2(H2O)3+ and Pu(L)(L-86)2+, while the abnormally broad peak at m/z 237.8 probably arises from fragmentation of fragile ions during resonant ejection from the ion trap.34 CID of Np(L)34+ also generated three minor products at m/z 209.8, 235.9, and 263.4, the latter two of which were observed when Np(L)23+ was mass selected without CID. Similar to the plutonium case, the m/z 263.4 peak is assigned to a secondary reaction product Np(L)(L-86)2+, and the origin of the abnormally broad m/z 235.9 peak is likely related to that of the m/z 237.8 peak in the case of plutonium. The m/z 209.8 peak corresponds to Np(L)2(OH)3+, a product of gas-phase hydrolysis with retention of the Np(IV) oxidation state. The CID spectrum of U(L)34+ shows the presence of U(L)2(OH)3+ at m/z 210.2, as well as the m/z 315.0 peak due to UO(L)22+, in addition to the major product, U(L)2(L-86)3+. Note that Pu(L)2(H2O)3+ is the only hydrate observed upon CID of all four An(L)34+, which can be attributed to the high yield of Pu(L)23+; the coordination sphere of this ligated complex is evidently sufficiently unsaturated to allow the addition of a water ligand. Given that Th(L)2(OH)3+, Np(L)2(OH)3+, and U(L)2(OH)3+ were minor products during CID of Th(L)34+, Np(L)34+, and U(L)34+, Pu(L)2(OH)3+ may have been a minor CID product from Pu(L)34+ as well, but it would be obscured by the more intense Pu(L)2(H2O)3+ at only 0.33 m/z higher. Neither An(L)24+ nor An(L)4+ was observed during CID of any of the An(L)34+ complexes, indicating that three ligands are necessary to stabilize Th4+, U4+, Np4+, and Pu4+. ETD of Pu(L)34+, Np(L)34+, U(L)34+, and Th(L)34+. The stabilization of ligated tetrapositive ions enables the first examination of gas-phase chemistry of such complexes by ETD. In ETD an electron is transferred from an anion to a multiply-charged cation; the resulting energetic charge-reduced cation typically undergoes fragmentations that may be different from those observed in CID. ETD has been extensively applied in recent years for structure analysis of peptides and proteins,35−37 whereas scant attention has been given to ETD of small metal ion complexes, and none to metal ion complexes with charges greater than 2+. Previously, ETD was used to probe the fragmentation chemistry of solvated uranyl and plutonyl dications, which necessarily revealed the formation of charge reduction products,24 this in contrast to CID that resulted mainly in charge conservation products.38,39 Figure 5 shows the ETD spectra of Pu(L)34+, Np(L)34+, U(L)34+, and Th(L)34+ with the fluoranthene anion, C16H10−, as the electron donor. Because EA[C16H10] = 0.6 eV32 is substantially smaller than the EA of most polycations, transfer of an electron from C16H10− to a multiply charged cation is substantially exothermic and thus generally results in cation fragmentation products. ETD of U(L)34+ and Th(L)34+ gave very similar products, UIV(L)2(L-86)3+ and ThIV(L)2(L-86)3+. Note that there is a very weak peak at m/z 267.2 in the ETD spectrum of U(L)34+, which can be assigned to U(L)33+. For Np(L)34+, the NpIV(L)2(L-86)3+ complex at m/z 238.2 is a major ETD product, but the m/z 266.9 peak due to Np(L)33+ is almost as intense as NpIV(L)2(L-86)3+. In the case of Pu(L)34+, the ETD spectrum is dominated by Pu(L)33+, while the yield of

(1)

[An = Pu] (2)

An IV (L)3 4 + → An IV (L)2 (L‐86)3 + + (86)+ [An = Np, U, Th]

(3)

The other major difference among the CID spectra of the four An(L)34+ complexes is that An(L)2(L-86)3+ (peak D) dominate for Th(L)34+, U(L)34+, and Np(L)34+, whereas Pu(L)2(L-86)3+ is not observed, with instead Pu(L)2(86)3+ being produced. The relatively minor peak at m/z 234.6 during CID of Pu(L)34+ is assigned to Pu(L)2(86)3+ (Figure 4), where (86) represents a ligand fragment with mass 86 Da that results from C−Oether bond cleavage of TMOGA (Figure 1) followed by loss of a monopositive ion with m/z 102, (L-86)+ (reaction 2). Such C−Oether bond cleavage was also an important gas phase fragmentation channel during CID of UO2(L)22+, NpO2(L)22+, and PuO2(L)22+,29 and it was considered the major radiolysis reaction without nitric acid in condensed phase.30,31 Np(L)34+, U(L)34+, and Th(L)34+ underwent the same C−Oether bond cleavage, but the products were Np(L)2(L-86)3+, U(L)2(L-86)3+, and Th(L)2(L-86)3+14 with the loss of a monopositive ion with mass 86 Da (reaction 3). Because both the (L-86) and (86) fragments are radical ligands that can form chemical bonds to the metal centers, Np(L)2(L86)3+, U(L)2(L-86)3+, Th(L)2(L-86)3+, and Pu(L)2(86)3+ all retain the formal An(IV) oxidation state. Two resonance structures of the (86) radical fragment ligand are H2C•− C(O)N(CH3)2 and H2CC(O•)NCH3. As metal−oxygen bonds are generally significantly stronger than metal−carbon bonds,32,33 it is likely that in the Pu(L)2(86)3+ product there is a Pu−O bond to the latter resonance structure of the (86) fragment ligand. The preference for a residual (L-86) ligand for thorium, uranium, and neptunium versus an (86) ligand for plutonium is unknown; it may be related to the decreasing ionic radii from Th4+ to Pu4+. Given that the ionic radii of Np(IV), 0.87 Å, and Pu(IV), 0.86 Å,21 differ only slightly, the (L-86) versus (86) ligand disparity upon CID of Np(L)34+ and Pu(L)34+ may instead reflect more complex underlying chemistry. On the basis of the CID results, it is concluded that stabilization of An4+ in the gas phase requires three neutral TMOGA ligands, but the stabilities of ligated complexes vary, with Pu(L)34+ particularly susceptible to reduction by L+ elimination. 2752

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An IV (L)3 4 + + C16H10− → An III(L)33 + + C16H10 [An = Pu, Np, U]

It is notable that Pu(L)33+ and Np(L)33+ were the major products during ETD of Pu(L)34+ and Np(L)34+ (Figure 5), while Pu(L)23+ and Np(L)23+ became predominant when Pu(L)34+ and Np(L)34+ were subjected to CID (Figure 4), which necessarily results in the formation of at least two fragment ions, Pu(L)23+/Np(L)23+ and L+. In contrast to CID, if the metal−ligand binding is adequately strong as for TMOGA, ETD can produce intact reduced charge cations, such as Pu(L)33+ and Np(L)33+ (and neutral C6H10). The observation of An(L)33+ and An(L)23+ for both plutonium and neptunium in the gas phase suggests that two TMOGA ligands are sufficient to stabilize Pu3+ and Np3+, this in contrast to Pu4+ and Np4+ that require three TMOGA ligands. Similarly, the minimum number of ligands to stabilize a dication in the gas phase is generally lower than for a trication.22,40 Several minor products were formed upon ETD of all four An(L)34+ together with the dominant An(L)33+ and An(L)2(L86)3+ species. ETD of Pu(L)34+ resulted in Pu(L)2(L-86)2+ (m/ z 360.0, Figure 5); this species also appeared when Pu(L)33+ was subjected to ETD, suggesting that it results from secondary ETD of the primary Pu(L)33+ product. ETD of Th(L)34+, U(L)34+, and Np(L)34+ yielded the minor secondary products, AnO(L)22+. Since the IE of fluoranthene is far below the third and fourth IEs of these four actinides20 (IE[fluoranthene] = 7.95 eV32), sequential two electron transfer process from the fluoranthene anion evidently occurs during ETD of An(L)34+, as indicated by the peaks due to C16H10+ (F in Figure 5). The result that ETD of Pu(L)33+ produces PuIII(L)2(L-86)2+ rather than PuII(L)32+ is in accord with the unfavorable reduction of Pu(III) to Pu(II).23 It should be noted that PuIII(L)2(L-86)2+ was observed during ETD of Pu(L)33+, while PuIV(L)2(86)3+ instead of PuIV(L)2(L-86)3+ was produced upon CID. The PuIII(L)2(L-86)2+ and PuIV(L)2(L-86)3+ complexes have the same formula, but the charges and plutonium oxidation states are different. The discrepancy in product formation could be related to the difference in ionic radii, which are 0.94, 0.89, 0.87, and 0.86 Å for Th(IV), U(IV), Np(IV), and Pu(IV).21 The radius of Pu(III) (1.00 Å) is larger than that of Th(IV), which could account for the formation of similar products. It is feasible that the (L-86) ligand, rather than the (86) ligand (Figure 1), can be accommodated only when the metal ion radius is greater than that of Pu(IV). Since the metal centers of Pu(L)34+, Np(L)34+, U(L)34+, and Th(L)34+ are coordinated and sheltered by nine oxygen atoms, no detectable hydrate was observed for any of them after 5 s reactions with residual H2O in the ion trap. The three TMOGA ligands also protect the An4+ metal centers from gas-phase hydrolysis, which would otherwise be efficient under these reaction conditions.41 While no significant reaction occurred in the cases of Np(L)34+, U(L)34+, and Th(L)34+, reduction of Pu(L)34+ to Pu(L)33+ was observed when the Pu(L)34+ cation was mass selected in the ion trap followed by reaction with background gases for 5 s, which is ∼1000× longer than the ETD exposure time of a few milliseconds. This reduction presumably occurs by electron-transfer from a neutral molecule in the ion trap. The most abundant impurities in the trap are H2O, O2, and N2, among which O2 has the lowest IE, 12.1 eV.32 The m/z of the potential H2O+, O2+, and N2+ electron-transfer products are below the low-mass detection limit. Reduction

Figure 5. ETD mass spectra of Pu(L)34+, Np(L)34+, U(L)34+, and Th(L)34+. A, AnIV(L)34+; B, AnIV(L)2(L-86)3+; C, AnIII(L)33+; F, C16H10+. The asterisks denote secondary ETD products of Th(L)2(L86)3+, U(L)2(L-86)3+, Np(L)2(L-86)3+, and Pu(L)33+.

PuIV(L)2(L-86)3+ is minuscule, which is completely different from the other three spectra. This distinct disparity among the ETD spectra of all four An(L) 3 4+ complexes clearly demonstrates the competition between two observed fragmentation channels: ligand fragmentation and charge reduction (reactions 4 and 5). Since one electron is transferred from C16H10− to An(L)34+ during ETD, the resulting products reveal whether it is facile for An(IV) to be reduced to a lower oxidation state, specifically An(III). On the basis of the An(IV)/An(III) reduction potentials, as well as the An4+ electron affinities, it is thermodynamically most favorable for Pu(IV) to be reduced followed by Np(IV) and U(IV), with reduction clearly least favorable for Th(IV). As a result, ThIII(L)33+, which is necessarily the initial product of ETD, is unstable and promptly fragments to ThIV(L)2(L-86)3+ to retain the favorable Th(IV) oxidation state. Both U(III) and Np(III) are common oxidation states in the condensed phase although both of them, especially U(III), are readily oxidized to U(IV) and Np(IV). Consistent with this chemistry, the simple charge reduction product, An(L)33+, is absent in the ETD spectrum of Th(L)34+, appears in very low yield for U(L)34+, and is substantially more abundant for Np(L)34+. Compared with Th(III), U(III), and Np(III), Pu(III) is more stable with the highest Pu(IV)/Pu(III) potential, such that ETD charge reduction of Pu(L)34+ to Pu(L)33+ is completely dominant, with no detected PuIV(L)2(L-86)3+. The ETD results clearly reveal the favorable reduction of Pu(IV) to Pu(III), followed by lesser reduction of Np(IV) and U(IV), and no reduction of Th(IV). These results are consistent with both gas-phase CID results and condensed phase redox chemistry.14,23 An IV (L)3 4 + + C16H10− → An IV (L)2 (L‐86)3 + + (86) + C16H10

[An = Np, U, Th]

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under these thermal conditions must be exothermic such that the occurrence of spontaneous electron transfer suggests that EA[Pu(L)34+] ≥ 12.1 eV and that the energy deposited in Pu(L)34+ in ETD is at least 11.5 eV, IE[O2] − EA[C10H16]. Reduction of Th(L)34+ to Th(L)33+ by reaction with background gases was not observed under similar conditions. This difference indicates that EA[Th(L)34+] < EA[Pu(L)34+], in accord with the higher IE[Pu3+] and greater stability of the tetrapositive thorium complex. It cannot necessarily be inferred that EA[Th(L)34+] < 12.1 eV because there are generally large Coulomb barriers to electron transfer reactions that result in charge separation into two positively charged ions.1,42,43

readily amenable to condensed phase experiments, including the actinides beyond plutonium.

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ASSOCIATED CONTENT

* Supporting Information S

Partially resolved isotopic profile for Pu(L)34+; ESI mass spectra of Np(ClO4)4/TMOGA and U(ClO4)4/TMOGA mixtures in acetonitrile. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(J.K.G.) E-mail: [email protected].

CONCLUSIONS The Pu4+, Np4+, and U4+ tetracations were stabilized in the gas phase upon coordination by three neutral TMOGA ligands. The Pu(L)34+, Np(L)34+, and U(L)34+ complexes were made by ESI of 1:1 mixtures of Pu(ClO4)4, Np(ClO4)4, or U(ClO4)4 and TMOGA in acetonitrile. In view of the substantially higher IE of Pu3+, Np3+, and U3+ (34.6, 33.6, and 32.6 eV), as compared to Th3+ (28.8 eV), this significantly extends the realm of tetrapositive metal ion stabilization from solution to gas. CID of U(L)34+ mainly resulted in ligand fragmentation through C−Oether bond cleavage to form UIV(L)2(L-86)3+, this being similar to the fragmentation pattern observed for Th(L)34+, in which the Th(IV) oxidation state is retained. In contrast, it is much more favorable for Pu(L)34+ to be reduced to Pu(L)23+ via L+ loss, which reflects the inherently lower stability of the ligated tetrapositive plutonium complex. Both C−Oether bond cleavage and charge reduction products were observed during CID of Np(L)34+, which thus lies between U(L)34+ and Pu(L)34+ in its redox behavior. C−Oether bond cleavage occurred as well for Pu(L)34+ although as a minor pathway, and the resulting Pu(L)2(86)3+ complex differs in composition from the An(L)2(L-86)3+ complex produced in the cases of neptunium, uranium, and thorium. ETD of Pu(L)34+, Np(L)34+, U(L)34+, and Th(L)34+ revealed two fragmentation channels. Pu(L)33+ was observed as the major product via simple one electron reduction, consistent with the relatively stable Pu(III) oxidation state. In contrast, Th(L)2(L-86)3+ and U(L)2(L-86)3+ were dominant upon ETD of Th(L)34+ and U(L)34+, which is quite similar to the CID results; the Th(IV) and U(IV) oxidation states are preferentially retained. A very weak peak due to U(L)33+ was observed during ETD of U(L)34+. Both Np(L)2(L-86)3+ and Np(L)33+ were significant ETD products of Np(L)34+, again revealing the intermediate behavior of the Np complex, consistent with condensed phase chemistry. The observation of Pu(L)34+, Np(L)34+, and U(L)34+ in the gas phase revealed that tetrapositive metal ions can be stabilized upon coordination by simple ligands under solvent free conditions, even if the fourth IE of the metal is as high as 34.6 eV; accommodating such a large energy in a small gasphase coordination complex is unprecedented. The gas-phase fragmentation chemistry of the tetrapositive ion complexes clearly depends on the essential chemistry of the actinide metal centers, effects revealed in both condensed phase and gas phase. The use of TMOGA ligands enables studying the gasphase chemistry of a series of tetrapositive metal ions, important but rarely explored systems. Gas-phase chemistry such as that reported here offers the possibility to explore the fundamental chemical behavior of elements that might not be

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Heavy Element Chemistry, at LBNL under Contract No. DE-AC02-05CH11231.

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