Article pubs.acs.org/JPCA
Halide Abstraction from Halogenated Acetate Ligands by Actinyls: A Competition between Bond Breaking and Bond Making Phuong D. Dau and John K. Gibson* Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Transfer of halogen atoms from halogenated acetate ligands, CX3CO2 (X = F, Cl, Br), to actinyls, AnO22+ (An = U, Np, Pu) is stimulated by collision-induced dissociation (CID) in a quadrupole ion trap. CID of [AnO2(CF3CO2)3]− complexes results exclusively in F atom transfer, concomitant with elimination of CF2CO2, to produce [(CF3CO2)2AnO2F]−, [(CF3CO2)AnO2F2]−, and [AnO2F3]−. This contrasts with CID of transition metal fluoroacetates for which CO2-elimination to produce organometallics is an important pathway, a disparity that can be attributed to the differing bond dissociation energies (BDEs) of the created metal−carbon and metal−fluorine bonds. The dominant pathway for CID of [AnO2(CF3CO2)(CCl3CO2)(CBr3CO2)]− is Br-atom transfer to produce [(CF3CO2)(CCl3CO2)AnO2Br]−. The preferential formation of bromides, despite that the BDEs of An−F bonds are substantially greater than those of An−Br bonds, is attributed to the offsetting effect of higher BDEs for C−F versus C−Br bonds. The results for the trihaloacetates are similar for uranyl, neptunyl and plutonyl, indicating that for all three the An−X bond dissociation energies are sufficiently high that X atom transfer is overwhelmingly dominant. CID of [UO2(CH2XCO2)2(CX3CO2)]− (X = F, Cl, Br) resulted in F-transfer only from CH2XCO2, but Cl- and Brtransfer from both CH2XCO2 and CX3CO2, a manifestation of the characteristic increase in BDE[C−F] in CHx‑nFn species as n increases; the overall thermochemistry determines the observed CID processes, providing clear distinctions between fluorides and chlorides/bromides. The results of this work reveal the propensity of the actinides to form strong bonds with halogens, and suggest that there is not a large variation in actinyl−halogen BDEs between uranyl, neptunyl, and plutonyl.
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INTRODUCTION The actinide ions generally exhibit characteristics of “hard” electron acceptors,1−3 resulting in preferential complexation to “hard” donor ligands, such as fluoride.4 The bond dissociation energies (BDEs) in Tables 1 and 2 indicate that BDEs for
Table 2. Bond Dissociation Energies for U−X, HX2C−X, and H3C−X (X = F, Cl, Br).a
Table 1. Actinide−Fluorine and Carbon−Fluorine Bond Dissociation Energiesa bond
BDE
U−F Np−F Pu−F H3C−F HF2C−F
659 ± 22 624 ± 25 541 ± 10 468 538
U−X
HX2C−X
Δ[(U−X) − (HX2C−X)]b
H3C−X
F Cl Br
659 467 400
538 332 284
121 135 116
468 351 292
a
BDE in kJ/mol. The BDE[U-X] are from ref 59; the uncertainties are ca. ± 20 kJ/mol; the uncertainties in the C−X BDEs are not specified. The BDE[H3C−X] and BDE[HX2C−X] are from ref , except for the ΔHf298 K[CX2H] which is from ref 60. bDifference between the BDEs for U−X and HX2C−X.
to one or more halide: AnO2Xn(2‑n)+.2,7−16 One motivation for studying such simple complexes is that the effective electron donation of halides to the actinide metal center affects the axial actinide-oxygen bonds, generally weakening them.9−12 Uranyls, both UVI (unless otherwise specified the oxidation states hereafter are AnVI) and UV, have been shown to form various halide complexes in condensed and gas phases,2,17 including as representative examples UO2X3− (X = F, Cl, Br, I),10,11,18 UO2Fn(2‑n)+ (n = 1−4),19−22 UO2Br42−,23 UVO2X (X = Cl, I),24 and UO2X2 (X = Cl, Br).25 It has been established that the
a BDE in kJ/mol. The BDE[An−F] are from ref 59. The BDE[H3C− F] and BDE[HF 2 C−F] are from ref 53, except for the ΔHf298 K[CF2H] which is from ref 60. The uncertainties in the C−F BDEs are not specified.
actinide-fluorine bonds are generally greater than for carbon− fluorine bonds, and that the An−F bond strength decreases somewhat from An = U to Np to Pu. High actinide halide BDEs, BDE[An−X] (An = U, Np, Pu; X = F, Cl, Br), have been demonstrated by the ability of bare An+ ions to abstract halogens from halogenated hydrocarbons.5,6 A particular area of interest is complexes that comprise the actinyls, AnO22+, bound © 2015 American Chemical Society
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Received: January 29, 2015 Revised: March 12, 2015 Published: March 12, 2015 3218
DOI: 10.1021/acs.jpca.5b00952 J. Phys. Chem. A 2015, 119, 3218−3224
Article
The Journal of Physical Chemistry A
atoms in the acetate ligands provides an opportunity to examine differences in CID activation of the tethered halogenated acetate as a function of the actinyl (An = U, Np, Pu), the halogen (X = F, Cl, Br), and the number of halogen atoms in CH3‑nXnCO2−. Here we report the results for CID of [AnO 2 (CF 3 CO 2 ) 3 ] − and [AnO 2 (CF 3 CO 2 )(CCl 3 CO 2 )(CBr3CO2)]− for An = U, Np, and Pu, and for CID of [UO2(CX3CO2)(CH2XCO2)2]− for X = F, Cl, and Br. It was found that C−X activation with X atom transfer to the actinyl was dominant for all complexes. The results are interpreted in the context of the energetics of the bonds being broken and created, and clearly reveal the strong affinity of the actinides toward forming bonds with halogens, with no significant differences between the studied actinides.
complexation affinities of uranyl by halides decreases in the order F− > Cl− > Br−.26 The reported synthesis of neptunyl and plutonyl halide complexes is substantially more limited and has focused primarily on the more highly Lewis basic fluoride and chloride ligands.27,28 Among the reported neptunyl halide complexes are NpO2F2,29−31 NpVO2F,29,32 NpVO2F2−,32 and NpO2Cl42−.33 In the case of plutonyl, reported complexes include PuO 2 F 2 , 3 1 , 3 4 PuO 2 Cl 2, 3 5 , 3 6 Pu V O 2 Cl, 3 6 and PuO2Cl42−.37 Organofluorine materials have long attracted interest due to novel properties; among the most notable, and ubiquitous, is polytetrafluoroetheylene, Teflon, which was discovered in 1938. Organochloride compounds are also of importance, perhaps most notoriously as the toxic pesticide dichlorodiphenyltrichloroethane, DDT. Applications of organobromine compounds are less common, though polybrominated diphenyl ethers, PBDEs, are widely used as flame retardants. Organofluorine compounds continue to attract substantial interest as pharmaceuticals and in new materials applications.38,39 The importance of organohalogen compounds has resulted in importance of the activation of C−X bonds by metal centers. Mazurek and Schwarz have discussed activation of C−F bonds facilitated by metal ions.40 Actinyls, which have metal centers that can strongly bind to halides, present an opportunity to study fundamental aspects of metal-mediated carbon−halogen bond activation. In particular, activation processes in the gas phase are often sufficiently straightforward that insights can be gained that are difficult to evaluate in more complex condensed phase environments. Carbon−halogen bond activation can furthermore offer a means to access new actinyl halide species, including for the less studied cases of neptunyl and plutonyl. Rijs and O’Hair have reported on the competition between decarboxylation and C−F bond activation for trifluoroacetate, [CF3CO2]−, coordinated to Cu+, Ag+, and Au+.41 The observation of C−F bond activation as a significant pathway in collision-induced dissociation (CID) of [M(CF3CO2)2]− contrasts with dominant decarboxylation for unbound [CF3CO2]− to yield [CF3]− and neutral CO2,42−44 revealing the significant effect of the metal center on the observed chemistry, in this case an enhancement in C−F activation enabled by F atom transfer to the metal center. It is known that the linear actinyls ions, [O AnO]2+ can coordinate three acetate ligands in the equatorial plane,45 such that it is possible to prepare gas-phase actinyl acetate complexes, [AnO2(CH3CO2)3]− by electrospray ionization (ESI).10 Shown in Figure 1 is a possible structure of the linear actinyl ion coordinated by three trihalide acetate ligands. Since the third carboxylate ligand can bind to the center actinide either by mono- or bidentate coordination,46 the structure in Figure 1 exhibits only the prevalent bidentate mode. As seen in Figure 1, substitution of one or more halogen
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EXPERIMENTAL METHODS The general experimental approach has been described previously,47 with some details summarized here. Methanol solutions of 200 μM AnO22+ (An = U, Np, Pu) with the desired acetate ligands were prepared from the following actinyl stock solutions: 10 mM UO 2(ClO4 )2 at pH ≈ 4, 28 mM NpO2(ClO4)2 at pH ≈ 1, and 8 mM PuO2(ClO4)2 at pH ≈ 1. The actinide isotopes were U-238, Np-237 and Pu-242, which are all radioactive and must be handled with proper radiological controls. The acetate ligands were added as 50 mM aqueous solutions of the following salts/acids: CX3CO2Na (X = F, Cl), CBr3CO2H, CH2XCO2Na (X = F, Cl), CH2BrCO2H. The actinyl:ligand ratios in the ESI solutions were ca. 1:4 for U, 1:8 for Np and 1:16 for Pu in the [AnO2(CF3CO2)3]− experiments, where these ratios were empirically identified as providing optimal yields of the desired complex ions. The actinyl:ligand ratios were 1:4:4:4 for the [AnO2(CF3CO2)(CCl3CO2)(CBr3CO2)]− experiments, and 1:4:4 for the [UO2[(CH2XCO2)2(CX3CO2)]− experiments. The ESI mass spectrometry experiments were performed using an Agilent 6340 quadrupole ion trap mass spectrometer (QIT/MS) with CID capabilities, also referred to as MSn. The ESI source region of the QIT/MS is inside of a radiological-containment glovebox, as described in detail elsewhere.48 In high resolution mode, the instrument has a detection range of 50−2200 m/z and a resolution of ∼0.25 m/z (fwhm). Mass spectra were recorded in the negative ion accumulation and detection mode. All spectra were acquired with the following instrumental parameters: solution flow rate, 60 μL/min; nebulizer gas pressure, 15 psi; capillary voltage and current, 6000 V, 80 nA; end plate voltage offset and current, −500 V and 800 nA; dry gas flow rate, 3 l/min; dry gas temperature, 325 °C; capillary exit, −50.0 V; skimmer, −36.3 V; octopole 1 and 2 DC, −10.88 and −3.00 V; octopole RF amplitude, 190 Vpp; lens 1 and 2, 10.0, and 91 V; trap drive, 55. High-purity nitrogen gas for nebulization and drying in the ion transfer capillary was supplied from the boil-off from a liquid nitrogen dewar. As has been discussed elsewhere, the background water pressure in the ion trap is estimated as ∼10−6 Torr;48 reproducibility of hydration rates of UO2(OH)+49 confirms that the background water pressure in the trap remains constant to within
