Coordination Structures of the Uranyl(VI)–Diamide Complexes: A

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Coordination Structures of the Uranyl(VI)−Diamide Complexes: A Combined Mass Spectrometric, EXAFS Spectroscopic, and Theoretical Study Xiuting Chen,†,‡ Qingnuan Li,† and Yu Gong*,† †

Department of Radiochemistry, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China School of Nuclear Science and Technology, University of Chinese Academy of Sciences, Beijing 100049, China

‡

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 03/25/19. For personal use only.

S Supporting Information *

ABSTRACT: The coordination structures of a series of uranyl−diamide complexes, namely UO2(TMGA) 22+ , UO2(TMTDA) 22+ , and UO 2(TMPDA) 22+ (TMGA: N,N,N′,N′tetramethylglutaramide; TMTDA: N,N,N′,N′-tetramethyl-3-thiodiglycolamide; TMPDA: N,N,N′,N′-tetramethylpyridine-2,6-dicarboxamide), in the gas phase and solution were studied by electrospray ionization (ESI) mass spectrometry, density functional theory (DFT) calculations, and extended X-ray absorption fine structure (EXAFS) spectroscopy. ESI of UO2Cl2−TMGA, UO2Cl2−TMTDA, and UO2Cl2−TMPDA solutions produced UO2(TMGA)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+ as the major complexes in the gas phase. The gas-phase fragmentation patterns of these three complexes upon collisioninduced dissociation (CID) are quite similar, and the products arising from (CH3)2N− COcarbonyl, CH2−COcarbonyl, Cpyridine−COcarbonyl, and CH2−CH2/S bond cleavages were identified, which agree with the degradation patterns of diamides upon γ irradiation in solution. DFT calculations revealed similar bidentate coordination modes of TMGA and TMTDA in the UO2(TMGA)22+ and UO2(TMTDA)22+ complexes in the gas phase. For UO2(TMPDA)22+, UO22+ is coordinated by two Ocarbonyl and one Npyridine from each TMPDA, which acts as a tridentate ligand. The results from EXAFS analysis indicate that the coordination structures of UO2(TMGA)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+ in solution are similar to those in the gas phase.

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over tetravalent or hexavalent ions.6−8 For the thiodiglycolamide ligands (Figure 1c), they can extract hexavalent, tetravalent, and trivalent actinides as well as trivalent lanthanides, but their extraction abilities are weaker than those of diglycolamides.6,9,10 The diamides of dipicolinic acid (Figure 1d) dissolved in organic diluents have a large capacity for uranium and effectively extract tetravalent and hexavalent actinide ions.11−13 These results demonstrate that the extraction properties of diamides strongly depend on their structures which affect the structures and stabilities of the coordination complexes formed between metal ions and diamide ligands. For UO22+, the coordination structures of its complexes with a series of neutral diamide ligands in the solid state have been determined using single crystal X-ray diffraction.14−21 The diglycolamides,14,15 dipicolinates, and the corresponding diamides16−18 act as tridentate chelating ligands in the uranyl complexes while glutaramides19,20 and thiodiglycolamides21 were found to bind the uranium center in bidentate fashions. Compared with the rich studies on the solid state structures of the uranyl−diamide complexes, limited information is available on the structures of these complexes in solution and gas phase. Extended X-ray absorption fine structure (EXAFS) has been

INTRODUCTION Diamide ligands have been of interest as promising extractants in spent fuel reprocessing for several decades due to their great affinities toward lanthanide and actinide ions as well as their environmentally benign characters.1−3 Extensive studies have been performed on the extraction properties of various substituted diamides. The family of glutaramide ligands (Figure 1a) shows excellent extraction ability to the hexavalent and tetravalent actinide ions but weak for the trivalent ions,4−6 while diglycolamides (Figure 1b) extract trivalent actinide ions

Figure 1. Structures of glutaramide (a), diglycolamide (b), thiodiglycolamide (c), and pyridine-2,6-dicarboxamide (d). © XXXX American Chemical Society

Received: January 7, 2019

A

DOI: 10.1021/acs.inorgchem.9b00047 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

of 3.5 GeV and a current between 150 and 210 mA. The photon energy was calibrated with the first inflection point of Y K-edge (17038 eV) using a Y metal foil. The pretreatment of EXAFS spectra and fitting of the k2-weighted EXAFS data were performed using ATHENA and ARTEMIS interfaces to IFEFFIT 7.0 software.33,34 The fitting of paths (i.e., U−Oyl, U−Ocarbonyl, U−Oether, and U− N p yr i d i n e ) was generated from the crystal structures of UO2(TMOGA)2(ClO4)2, UO2(TPriTDA)(NO3)2, and UO2(TMPDA)Cl2.14,18,21 Data analysis was performed in R space between 1 and 3 Å, and the k range used was 2−13.5 Å−1. The shift in the threshold energy (E0) was allowed in varying as a global parameter. In all fits, the amplitude reduction factor (S02) was fixed at 0.9, and the coordination number of the uranyl oxygen atoms (Oyl) was held at 2. DFT Calculations. DFT calculations on the UO2(TMGA)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+ complexes at the B3LYP level were performed by using the Gaussian 09 package.35−37 The 60electron core pseudopotential basis set was used for U,38 and the 631G(d) basis set was used for all the remaining atoms (C, H, O, N, and S).39−41 These calculations were performed on the same level of theory as we did for other transition metal, lanthanide, and actinide diamide complexes.25,29,42 All of the geometrical parameters were optimized, and the harmonic vibrational frequencies were obtained analytically at the optimized structures. Zero-point energy (ZPE) corrections were included in the relative energies. Natural bond orbital (NBO) analysis was performed on the optimized geometries (B3LYP) using the NBO6 program.43

used to provide information about the coordination structures of the uranyl complexes containing diglycolamide, diglycolate, and dipicolinamide ligands in solution.14,22,23 The gas-phase structures and reactivities of hexavalent, tetravalent, and trivalent actinide ions ligated by diglycolamide24−27 and trivalent lanthanide ions ligated by glutaramide, diglycolamide, and thiodiglycolamide27−29 have been studied under solvent and counterion free conditions by using mass spectrometry and theoretical calculations, which provide parallel results with those in solution. To systematically investigate the coordination chemistry of various diamide supported UO22+ complexes, we report a combined mass spectrometric, EXAFS spectroscopic and density functional theory (DFT) study of the UO22+ complexes containing N,N,N′,N′-tetramethylglutaramide (TMGA), N,N,N′,N′-tetramethyl-3-thiodiglycolamide (TMTDA), and N,N,N′,N′-tetramethylpyridine-2,6-dicarboxamide (TMPDA) ligands (Figure 1, R and R′ = CH3). The 1:2 complexes were observed via electrospray ionization (ESI) of the corresponding solutions, and their chemical properties in the gas phase were investigated by collision-induced dissociation (CID). DFT computations were employed to understand the gas-phase structures of these complexes, and EXAFS measurements were performed to probe the coordination structures of the UO22+−diamide complexes in H2O/CH3CN solution.

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RESULTS AND DISCUSSION Mass Spectrometric Studies of the UO22+−Diamide Complexes. Positive polarity mode ESI of 1:3 UO2Cl2 (200 μM) and diamide in acetonitrile resulted in the formation of a series of uranyl−diamide complex cations as shown in Figure 2. The dipositive charge can be easily identified based on the

EXPERIMENTAL SECTION

Mass Spectrometry. The acetonitrile solutions of UO2Cl2− TMGA, UO2Cl2−TMTDA, and UO2Cl2−TMPDA for ESI experiments were prepared by mixing 30 mM UO2Cl2 (in H2O/CH3CN, 1:2 by volume) and 0.1 M diamide (in CH3CN) stock solutions. The UO2Cl2 concentration in the mixture is fixed at 200 μM with the metal-to-ligand molar ratio varying between 2:1 and 1:10. The uranium isotope employed was 238U, which undergoes α-decay with a half-life of 4.5 × 109 years, and handling of α-emitting radionuclide 238 U requires proper shielding, waste disposal, and personal protective gear. The TMGA, TMTDA, and TMPDA ligands were synthesized according to the literature.17,30,31 The ESI and CID experiments were performed using a ThermoScientific (San Jose, CA) LTQ-XL linear ion trap mass spectrometer (LIT-MS) equipped with a Heated Ion Max Electrospray Ionization (HESI) source. ESI mass spectra were acquired in the positive polarity mode, and detailed instrumental parameters are listed in the Supporting Information. Mass selection and CID were performed using standard isolation and excitation procedures implemented in the LTQ software. The normalized collision energy (NCE) was set between 12% and 15% with activation Q value of 0.250 and activation time of 30 ms. High purity nitrogen (99.999%) gas was used for nebulization and drying in the ESI source, and helium (99.999%) was used as the collision gas. The pressure inside the ion trap is about 7.5 × 10−6 Torr during all the experiments. The accurate mass-to-charge ratios of the uranium-containing cations were obtained from a Bruker Daltonics (Bremen, Germany) SolariX XR 7.0 T Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS) with extreme resolution and ultrahigh mass accuracy. EXAFS Spectroscopy. For EXAFS measurements, the H2O/ CH3CN (1:2 by volume) solutions of UO2Cl2−TMGA, UO2Cl2− TMTDA, UO2Cl2−TMOGA, and UO2Cl2−TMPDA were prepared such that the concentration of UO2Cl2 is 30 mM, and the metal-toligand molar ratio is 1:20. All of the samples were sealed in 0.2 mL plastic centrifuge tubes (path length: 0.48 cm) at room temperature in an argon glovebox. The X-ray absorption data at the U LIII-edge (17166 eV) of the samples were recorded at room temperature in transmission mode using ion chambers at beamline BL14W1 of the Shanghai Synchrotron Radiation Facility (SSRF) in China.32 The station was operated with a Si(111) double-crystal monochromator. During the measurement, the synchrotron was operated at an energy

Figure 2. ESI mass spectra of 1:3 UO2Cl2 (200 μM) and diamide in acetonitrile. The asterisks denote the peaks of Na(TMGA)2+ and Na(TMTDA)2+.

separation of m/z 0.5 between the dominant peak and its neighboring peak with one 13C substitution. For TMGA, both UO2(TMGA)22+ and UO2(TMGA)32+ cations were observed in the ESI mass spectrum (top in Figure 2) although the intensity of the latter is much lower. Other peaks present in the spectrum include the protonated ligand HTMGA+ and common impurities such as Na(TMGA) 1,2 + and Fe(TMGA)22+.28,29 Neither UO2(TMGA)2+ nor B

DOI: 10.1021/acs.inorgchem.9b00047 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry UO2(TMGA)n2+ (n ≥ 4) was produced when the molar ratio of UO2Cl2/TMGA was changed from 2:1 to 1:10. The ESI spectrum of UO2Cl2 and TMTDA (middle in Figure 2) is quite similar to that of UO2Cl2 and TMGA, and it is dominated by UO2(TMTDA)22+ with UO2(TMTDA)32+ being a minor species. Different from the TMGA and TMTDA cases, UO2(TMPDA)22+ is the only uraniumcontaining species observed from ESI of the acetonitrile solution of UO2Cl2 and TMPDA, while the UO2(TMPDA)32+ complex which would appear at m/z 466.7 is absent (bottom in Figure 2) even if the molar ratio of UO2Cl2/TMPDA was increased to 1:10. This is similar to the TMOGA system which showed the existence of the UO2(TMOGA)22+ complex, but the UO2(TMOGA)32+ complex was not observed.24 The assignments of all the dipositive uranium-containing complexes observed in the mass spectra are in accord with the accurate m/z measurements (Table S1) taken by using FTICR-MS. The dipositive UO2(TMGA)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+ complexes were mass selected and subjected to CID to get insights into their fragmentation patterns (Figure 3). The major peaks observed upon CID of the three

Figure 4. Observed bond cleavage patterns (indicated by the dashed lines) upon CID of the uranyl−diamide complexes. The number in parentheses corresponds to the mass of the neutral fragment formed upon bond cleavage followed by hydrogen transfer.

via R2N−COcarbonyl, CH2−COcarbonyl, and CH2−Oether/S bond cleavages.44,45 This implies the relevancy of CID results of gasphase uranyl−diamide complexes to the radiolysis behavior of diamide ligands in solution. Note that the peak which is 18 m/ z lower than the m/z of UO2(TMGA)22+ is assigned to the dehydration product UO2(TMGA)(TMGA-18)2+. Loss of H2O occurs within the TMGA ligand as proved by the CID result of U18O2(TMGA)22+ (Figures S1 and S2), which is not particularly relevant to the solution chemistry. In the higher m/ z region, loss of L+ to form charge reducing product UVO2(L)+ was observed for all of the three complexes with the relative yield being lowest in the UO2(TMTDA)22+ case, during which U(VI) was reduced to U(V). Reactions of UO2(L)22+ and trace amount of water in the ion trap resulted in the formation of the minor UVIO2(L)(OH)+ product in some cases. The CID results of UO2(TMGA)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+ are quite similar but different from those of UO2(TMOGA)22+ considering the formation of charge conserving products.24 In addition to UO2(TMOGA)+ and UO2(TMOGA)(OH)+, the major CID product is UO2 (TMOGA)(TMOGA-86)+ where TMOGA-86 is a carbonyl alkoxy group resulting from the cleavage of the CH2−Oether bond. The preference for UO22+ to form the U−O bond with the alkoxy group rather than form the U−C or U−S bond with either R-CH2 or R-S under similar CID conditions is consistent with the hard character of UO22+,46 which accounts for the absence of the UO2(TMGA)(TMGA-86)+ and UO2(TMTDA)(TMTDA-86)+ products. Instead, dipositively charged UO2(TMGA)(TMGA-87)2+ and UO2(TMTDA)(TMTDA-87)2+ were formed, and the neutral TMGA-87 and TMTDA-87 ligands are presumably bound to uranium through Ocarbonyl. The CID spectra of UO2(TMGA)32+ and UO2(TMTDA)32+ are completely different from those of UO2(TMGA)22+ and UO2(TMTDA)22+ (Figures S3 and S4). Loss of one neutral ligand to form UO2(TMGA)22+ and UO2(TMTDA)22+ is the only reaction observed in the experiments. The simple fragmentation patterns of UO2(TMGA)32+ and UO2(TMTDA)32+ are in line with those of Ln(TMGA)43+ and Ln(TMTDA)43+.28,29 In fact, the intensities of the UO2(TMGA)32+ and UO2(TMTDA)32+ peaks are very sensitive to the ESI conditions such as sheath gas flow rate and capillary temperature. The 1:3 complexes almost disappeared when either the flow rate or the temperature was increased while the intensities of the 1:2 complexes were barely affected at the same time, indicating the third ligand in the uranyl complexes most likely resides in the second coordination sphere. Gas-Phase Structures of the UO22+−Diamide Complexes. To understand the gas-phase coordination structures of these UO2(L)22+ (L = TMGA, TMTDA, and TMPDA)

Figure 3. CID mass spectra of mass selected UO2(L)22+ (L = TMGA, top; TMTDA, middle; TMPDA, bottom) complexes. a, b, c, c1, d, and e denote UVIO2(L)(L-87)2+, UVIO2(L)(L-73)2+, UVIO2(L)(L-45)2+, UVIO2(L)(L-45)(H2O)2+, UVO2(L)+, and UVIO2(L)(OH)+, respectively. The asterisk denotes the UVIO2(TMGA)(TMGA-18)2+ peak.

UO2(L)22+ complexes are due to the charge-conserving dipositive products which retain the VI oxidation state of uranium. For UO2(TMGA)22+ and UO2(TMTDA)22+, their dipositive products are formed via the cleavage of the (CH3)2N−COcarbonyl, CH2−COcarbonyl, and CH2−CH2/S bonds (Figure 4). In the case of UO2(TMPDA)22+, cleavage occurs for the (CH3)2N−COcarbonyl and Cpyridine−COcarbonyl bonds while Cpyridine−Npyridine bond cleavage was not observed since such a fragmentation pattern would require the breaking of the stable pyridine ring in the ligand. All these dipositive ions contain nonradical fragments bound to UO22+ such that the oxidation state of uranium remains VI. The fragmentation patterns associated with the formation of these dipositive cations are consistent with the degradation patterns of thiodiglycolamide and diglycolamide upon γ irradiation in solution which showed the formation of degradation products C

DOI: 10.1021/acs.inorgchem.9b00047 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. NPA Charges and Sequential Binding Energies ΔE (kcal/mol) Computed at the B3LYP Level of Theorya

complexes, theoretical calculations were performed by using the DFT method which has been employed in combination with infrared multiphoton dissociation spectroscopy to establish the structures of UO22+ complexes.47,48 As shown in Figure 5, the optimized geometries of UO2(TMGA)22+ and

NPA charges q(U) UO22+ TMGA UO2(TMGA)2+ UO2(TMGA)22+ TMTDA UO2(TMTDA)2+ UO2(TMTDA)22+ TMPDA UO2(TMPDA)2+ UO2(TMPDA)22+

Figure 5. Optimized structures (distances in Å) of the UO2(TMGA)22+ (a), UO2(TMTDA)22+ (b), and UO2(TMPDA)22+ (c) complexes at the B3LYP level of theory (C: gray; N: blue; O: red; S: yellow; U: cyan). All of the hydrogen atoms are omitted for clarity.

q(Oc)

q(S)

q(Np)

ΔE

2.51 1.82 1.61 1.94 1.68 1.85 1.67

−0.65 −0.69 −0.67 −0.63 −0.71 −0.69 −0.63 −0.66 −0.63

−207.6 −85.1 0.20 0.33 0.29

−231.3 −105.6 −0.47 −0.48 −0.45

−259.6 −112.3

Oc and Np denote Ocarbonyl and Npyridine, respectively. ΔE is the binding energy calculated based on the following reaction: UO2(L)n−12+ + L → UO2(L)n2+ (n = 1, 2).

a

upon coordination of the first TMTDA and TMPDA to the uranium center while coordination of the second ligand only resulted in a smaller charge reduction of 0.26 e (TMTDA) and 0.18 e (TMPDA), respectively. Note that the charge on Npyridine is slightly more negative in UO2(TMPDA)2+ than in bare TMPDA, which should be due to the electron transfer from other parts of TMPDA to the Npyridine atom when it donates electrons to the uranium center. Such change on charge distribution upon coordination is the same as that in the Zr(TMPDA)x4+ system.42 The sequential binding energies are also listed in Table 1. Because of the flexibility of these diamide ligands, geometry optimizations were performed on all the possible conformations, and the results are summarized in Figure S5. The energy differences between conformations are less than 6.5 (TMGA), 3.3 (TMTDA), and 1.7 kcal/mol (TMPDA), suggesting the ligand’s flexibility has a negligible impact on the gas-phase binding energies considering the fact that the energy released upon ligand addition is >100 kcal/mol in general. The binding energy of the first diamide ligand is about twice as large as that of the second one. The less exothermicity upon coordination of the second ligand results from the reduction of natural charge on uranium and the increase of steric hindrance due to the presence of the first ligand around uranium. It is also obvious that the binding energy of UO2(TMPDA)2+ is 28.3 and 52.0 kcal/mol larger than those of the UO2(TMTDA)2+ and UO2(TMGA)2+ complexes, respectively, in line with the fact that TMPDA acts as a tridentate ligand in UO2(TMPDA)2+ while both TMTDA and TMGA are bidentate. Compared with UO2(TMGA)2+, the larger binding energy of UO2(TMTDA)2+ is consistent with the shorter U− Ocarbonyl distance in UO2(TMTDA)2+ (2.209 Å) than in UO2(TMGA)2+ (2.272 Å). Structures of the UO22+−Diamide Complexes in Solution. EXAFS spectroscopy was used to probe the structures of the uranyl-TMGA, uranyl-TMTDA, and uranylMPDA complexes in H2O/CH3CN solution. The spectra of the UO2Cl2−TMOGA sample are also included for comparison. The k2-weighted EXAFS functions [χ(k)] extracted using autobk and the corresponding Fourier transform (FT) results are shown in Figure 6. The FT peaks below 1 Å (without phase shift) are artifacts of the spline removal and are not associated with any coordination distance. The best-fit results of four UO2Cl2−diamide samples are listed in Table 2.

UO2(TMTDA)22+ at the B3LYP level of theory reveal that the UO22+ center in both complexes is coordinated by four Ocarbonyl atoms in the equatorial plane. The U−S distance of UO2(TMTDA)22+ is 4.847 Å, which is in accord with the values (4.8−4.9 Å) obtained from the UO22+−thiodiglycolamide crystals.21 Note that this U−S distance is much longer than the sum of single-bond radii of U and S (2.73 Å),21,49 suggesting there is no bonding interaction between S and uranium. Hence, both TMTDA and TMGA act as bidentate chelating ligands when they form complexes with UO22+. The UO2(TMTDA)22+ complex was computed to possess a D2 symmetry with the TMTDA ligand being distorted in a zigzag geometry while a C2h symmetry was obtained for the UO2(TMGA)22+ complex in which the TMGA ligand possesses a planar geometry. The Ocarbonyl−Ocarbonyl distance of TMTDA (3.154 Å) in the UO2(TMTDA)22+ complex is shorter than that of TMGA in UO2(TMGA)22+ (4.010 Å). This makes it more favorable for the Ocarbonyl lone pair electrons of TMTDA to be donated to uranium, leading to a shorter U−Ocarbonyl distance (2.348 Å) than that in the UO2(TMGA)22+ complex (2.469 Å). For the UO2(TMPDA)22+ complex (Figure 5c), the uranyl moiety is coordinated by four Ocarbonyl atoms with a U−Ocarbonyl distance of 2.482 Å, close to the values (2.38−2.41 Å) in the solid state 1:1 uranyl tetraalkylpyridine-2,6-dicarboxamide complex.17,18 The U−Npyridine distance was computed to be 2.781 Å, which is comparable with the U−Oether distance (2.759 Å) of the UO2(TMOGA)22+ complex calculated at the same level of theory. The existence of weak interactions between Oether and uranium in UO2(TMOGA)22+ is indicative of the presence of similar interactions between uranium and Npyridine in the UO2(TMPDA)22+ complex.24 This calculated U−Npyridine distance is also comparable with the experimental values of the U−N bonds in which nitrogen is part of the aromatic ring. 5 0 Therefore, the uranyl center in UO2(TMPDA)22+ is coordinated by four Ocarbonyl and two Npyridine atoms, and the pyridine ring of TMPDA is deviated from the ligand plane forming C2 symmetry. NBO analysis reveals that the natural population analysis (NPA) charge on uranium (2.51 e) decreases as the number of ligand increases (Table 1). For example, it decreases from 2.51 to 1.82 e in UO2(TMGA)2+ and to 1.61 e in UO2(TMGA)22+. Similarly, a charge reduction of 0.57 and 0.66 e was observed D

DOI: 10.1021/acs.inorgchem.9b00047 Inorg. Chem. XXXX, XXX, XXX−XXX

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the presence of two diamide ligands around uranyl makes it much more difficult for acetonitrile to approach the uranium center in solution. The coordination structures for the UO2(TMGA)22+ and UO2(TMTDA)22+ complexes are very similar in solution. The uranium center is coordinated by four Ocarbonyl atoms while the central CH2 and S moieties of TMGA and TMTDA do not participate the uranium−ligand bonding. The experimental U−Oyl (1.76/1.77 Å) and U−Ocarbonyl distances (2.40−2.41 Å) are consistent with those in the crystal structures of some uranyl−diamide compounds (U−Oyl: 1.74−1.78 Å; U− Ocarbonyl: 2.36−2.40 Å).19−21 It should be noted that the coordination mode of TMGA in the UO2(TMGA)22+ complex is quite different from that of TBGA (N,N,N′,N′-tetrabutylglutaramide) or TEGA (N,N,N′,N′-tetraethylglutaramide) in the UO22+−TBGA and UO22+−TEGA crystals where two uranium atoms are bridged by two Ocarbonyl atoms of either TBGA or TEGA ligand. The EXAFS fitting (Figure 6) shows there is no peak around 4 Å which would arise from dimeric uranium species, 51,52 suggesting the amount of UO 2 2+ complexes with bridging TMGA ligands is negligible in the solution of UO2Cl2 and TMGA. This is also consistent with the absence of dimeric UO22+−TMGA complexes in the ESI experiment, although the ESI results are not always valid for inferring solution speciation.53,54 Different from the TMGA and TMTDA cases, both TMPDA and TMOGA ligands are coordinated to UO22+ in tridentate fashions, which are the same as the coordination modes of similar ligands in the UO22+ complexes in solutions and crystals.14−18,22 The U−Ocarbonyl (2.42−2.43 Å), U−Oether (2.64 Å), and U−Npyridine (2.66 Å) distances are within the range of those in the solid state (U− Ocarbonyl: 2.38−2.42 Å; U−Oether: 2.61 Å; U−Npyridine: 2.64 Å).14−18 On the basis of the EXAFS results, the coordination structures of the 1:2 uranyl−diamide complexes in solution are consistent with those predicted in the gas phase, and the computed geometric parameters of these complexes such as U−Oyl, U−Ocarbonyl, and U−Npyridine are in good agreement with the best-fit parameters obtained from U LIII-edge EXAFS analysis. For example, the U−O carbonyl and U−N pyridine distances of UO2(TMPDA)22+ were calculated to be 2.482 and 2.781 Å, respectively, which are close to the U−Ocarbonyl and U−Npyridine distances (2.43 and 2.66 Å) obtained in solution. The differences between theoretical and experimental results are within the typical values between EXAFS data fitting and DFT computations.14,24,55−57

Figure 6. k2-weighted U LIII-edge EXAFS spectra (left) and the corresponding FT results (right) of UO2Cl2 and diamide (1:20) in water/acetonitrile solution. Black lines represent the experimental data, and red lines represent the model fit.

Two predominant peaks were observed in the FTs of all the four samples. The first peak at ∼1.39 Å (before phase shift correction) can be well fitted with the two U−O single scattering paths at 1.76−1.78 Å corresponding to the two Oyl atoms in UO22+. The second peak at 1.80−1.83 Å (2.40−2.43 Å after phase shift correction) arises from the backscattering of four Ocarbonyl atoms from two diamide ligands of which the first coordination sphere of uranyl is composed. In addition to these common characters, two shoulders were partially resolved in the FT results of UO2Cl2−TMOGA and UO2Cl2−TMPDA. For the peak located at 2.04 Å in the TMOGA case, the best fit reveals two direct coordinated Oether atoms at a distance of 2.64 Å. Similarly, the shoulder located at 2.08 Å in the FT of UO2Cl2 and TMPDA mixture fits the U− Npyridine interaction with a distance of 2.66 Å. No analogous peak was observed in this region in the UO22+−TMGA case, which agrees with the fact that the central CH2 moiety in TMGA is uncoordinated. According to the best-fit results listed in Table 2, it is reasonable to assign a 1:2 stoichiometry to the complexes formed between UO22+ and diamide ligands in solution. We tried to fit the 1:1 complexes with two or three water molecules in the equatorial plane as well as the 1:3 complexes, but there are always some unreasonable parameters in the fitting results, suggesting these structures are not responsible for the EXAFS signals. The UO2(TMGA)32+ and UO2(TMTDA)32+ complexes present in the mass spectra are most likely formed during the ESI process. Inclusion of acetonitrile in the fit also gave poor results, which indicate that

Table 2. Best-Fit Results for the U LIII-Edge EXAFS Spectra of the UO2Cl2−Diamide Samples sample UO2Cl2−TMGA UO2Cl2−TMOGA

UO2Cl2−TMTDA UO2Cl2−TMPDA

N

shell Oyl Ocarbonyl Oyl Ocarbonyl Oether Oyl Ocarbonyl Oyl Ocarbonyl Npyridine

a

2 3.9 2a 3.8 1.9 2a 4.0 2a 3.9 1.8

± 0.2 ± 0.2 ± 0.1 ± 0.3 ± 0.2 ± 0.1

R (Å) 1.76 2.41 1.77 2.42 2.64 1.77 2.40 1.78 2.43 2.66

± ± ± ± ± ± ± ± ± ±

0.01 0.02 0.01 0.02 0.03 0.03 0.04 0.01 0.02 0.01

ΔE0 (eV) 4.88 10.92 7.11 9.75 15.32 3.21 9.46 7.83 9.60 13.42

± ± ± ± ± ± ± ± ± ±

2.26 1.26 2.08 1.36 2.89 1.83 1.27 2.14 2.10 2.38

σ2 (Å2)

Rfactor

± ± ± ± ± ± ± ± ± ±

0.02

0.002 0.010 0.001 0.010 0.014 0.003 0.009 0.001 0.04 0.016

0.001 0.002 0.001 0.004 0.005 0.001 0.002 0.001 0.002 0.007

0.01

0.02 0.01

a

Fixed parameters. E

DOI: 10.1021/acs.inorgchem.9b00047 Inorg. Chem. XXXX, XXX, XXX−XXX

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CONCLUSIONS

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ASSOCIATED CONTENT

S Supporting Information *

ESI mass spectrometry, DFT calculations, and EXAFS spectroscopy were employed to study the coordination s t r u c t u r e s o f t h r e e u r a n y l− d i a m i d e c o m p l e x e s , UO2(TMGA)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+, in the gas phase and solution. ESI of UO2Cl2−TMGA, UO 2 Cl2 −TMTDA, and UO 2 Cl 2 −TMPDA solutions in acetonitrile produced dipositive UO 2 (TMGA) 2 2 + , UO2(TMTDA)22+, and UO2(TMPDA)22+ complexes as the predominant species in the gas phase. Similar fragmentation behaviors were found upon CID of these cations which revealed the formation of dipositive products via (CH3)2N− COcarbonyl, CH2−COcarbonyl, Cpyridine−COcarbonyl, and CH2− CH2/S bond cleavages. Loss of one ligand cation to form UVO2(TMGA)+, UVO2(TMTDA)+, and UVO2(TMPDA)+ was observed as well. The CID results of these three complexes are quite different from that of UO2(TMOGA)22+.24 In addition, the fragmentation patterns of UO2(TMGA)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+ are in agreement with the degradation behaviors of diamide ligands upon γ irradiation, indicating the relevancy between gas-phase fragmentation chemistry and radiolysis behavior in solution. Different from the 1:2 complexes, CID of the minor species, UO2(TMGA)32+ and UO2(TMTDA)32+, only resulted in the formation of UO2(TMGA)22+ and UO2(TMTDA)22+ via neutral ligand loss. Theoretical calculations at the B3LYP level revealed that the uranyl moiety in either UO2(TMGA)22+ or UO2(TMTDA)22+ is coordinated by four Ocarbonyl from two ligands with the U− Ocarbonyl distances of 2.469 and 2.348 Å, respectively, while the S and central CH2 moieties of TMTDA and TMGA do not interact with UO22+. Different from these two ligands, TMPDA acts as a tridentate ligand which is coordinated to UO22+ through the Ocarbonyl and Npyridine atoms. The U−Ocarbonyl and U−Npyridine distances were computed to be 2.469 and 2.781 Å, which are similar to those of the UO2(TMOGA)22+ complex characterized before.24 The larger first binding energy of the UO2(TMPDA)2+ complex supports the tridentate character of TMPDA in UO2(TMPDA)2+ while both TMTDA and TMGA are bidentate ligands. The EXAFS spectra demonstrated that the UO2(TMGA)22+ and UO2(TMTDA)22+ complexes are composed of two bidentate ligands bound to the uranyl center via four Ocarbonyl atoms with the distances of 2.41 and 2.40 Å. The U−Ocarbonyl and U−Npyridine distances in the UO2(TMPDA)22+ complex were found to be 2.43 and 2.66 Å, which confirms that UO22+ is coordinated by two tridentate TMPDA ligands in solution. Similarly, TMOGA was characterized as a tridentate ligand as well when UO22+ forms a 1:2 complex with TMOGA. It is clear that the coordination structures of UO2(TMGA)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+ derived from the EXAFS results are in agreement with those predicted by DFT calculations based on the observation of the 1:2 complexes in the mass spectra. Gas-phase studies in combination with theoretical calculations can provide fundamental understanding on the coordination chemistry of neutral diamide complexes of UO22+ in solution.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00047. Detailed instrumental parameters used in the ESI experiments, 18O-labeled experimental results, observed and calculated m/z of the species from the ESI mass spectra of the UO2Cl2 and diamide mixtures, CID mass spectra of UO2(TMGA)32+ and UO2(TMTDA)32+, optimized geometries (Cartesian coordinates) of UO2(TMGA)22+, UO2(TMTDA)22+, and UO2(TMPDA)22+ (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yu Gong: 0000-0002-8847-1047 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21771189), the Strategic Priority Research Program and Frontier Science Key Program (Grants XDA02030000 and QYZDY-SSW-JSC016) of the Chinese Academy of Sciences, and the Young Thousand Talented Program. The authors thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

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REFERENCES

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