Impacts of Oxo Interactions on Np(V) Crown Ether Complexes

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

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Impacts of Oxo Interactions on Np(V) Crown Ether Complexes Madeline Basile, Erica Cole, and Tori Z. Forbes* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States S Supporting Information *

ABSTRACT: Intermolecular interactions between the oxo group of an actinyl cation and other metal cations (i.e., cation−cation interactions) are dependent on the strength of the actinyl bond. These cation−cation interactions are prominently observed for the neptunyl cation [Np(V)O2]+ and are sufficiently stable enough to explore using a variety of chemical techniques. Herein, we investigate these intermolecular interactions in the neptunyl 18-crown-6 system, because this macrocyclic ligand provides both stable coordination and the proper sterics to engage the oxo group in bonding with both low-valent metal cations and neighboring neptunyl units. We report the structural and spectroscopic characterization of five neptunyl, [Np(V,VI)O2]+,2+, compounds: Np1a ([NpO2(18-crown-6)]ClO4), Np1b ([NpO2(18-crown-6)]AuCl4), Na−Np ([Np(V)O2(18-crown-6)(Na(H2O)(18crown-6)][Np(VI)O2Cl4], Np−Np ([NpO2(18-crown-6)](NpO2Cl2NO3)], and Np−Cl (NpO2Cl(H2O)1.75). Each of these compounds were prepared from the ambient reactions of Np(V) in HX (where X = Cl, NO3) with the 18-crown-6 ether molecule. Structural information obtained from single-crystal X-ray diffraction data was paired with solid-state and solution Raman spectroscopy to provide information on the interaction of the neptunyl oxo atom with neighboring cations. Neptunyl (NpO) bond lengths are not perturbed upon interaction with the Na+ cation (Na−Np), but elongation is observed upon formation of a neptunyl−neptunyl interaction (Np−Np). This is also the first structurally characterized isolated, molecular complex that contains a simple T-shaped neptunyl−neptunyl interaction. Raman spectroscopy indicates little perturbation to the neptunyl bond until the formation of the neptunyl−neptunyl motif, which also results in activation of the ν3 asymmetric stretch. Additional spectroscopic studies indicated that the neptunyl 18-crown-6 inclusion complexes form in solution and persist in the presence of other low-valence cations.

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INTRODUCTION The chemistry of the higher-valence actinides is unique due to the formation of the chemically robust actinyl cation [An(V,VI)O2+,2+]. Both the hexavalent and pentavalent oxidation states for U, Np, Pu, and Am form the actinyl moiety, which is composed of the central metal cation bonded to two oxygen atoms that adopt a mutually trans arrangement.1 The strength of this An−Oyl bond is confirmed with relatively large bond enthalpies (403 kJ·mol−1 for PuO22+ to 529 kJ·mol−1 for UO22+), which includes both σ and π interactions between the 5f and 6d orbtials on the metal center and the O 2p orbitals.2,3 This actinyl moiety is prevalent in aqueous solutions under oxidizing conditions4−9 and can also be observed in organic solvents,10,11 ionic liquids,12−15 and molten salts.16−18 While the overall structure and persistence of the actinyl moiety is similar across the actinide series, there are subtle differences in the reactivity of the oxo group that can impart variability in intermolecular interactions (H-bonding, electrostatic, halogen). The U(VI)O22+ moiety exhibits the strongest actinyl (AnO) bond, which results in oxo groups that are relatively passivated and only weakly engage in intermolecular interactions.19 Reduction to U(V)O2+ weakens the uranyl bond, thus increasing the overall Lewis basicity of the oxo groups.20 This leads to additional bonding and reactivity for the © XXXX American Chemical Society

oxo atoms, but the instability of this species makes it difficult to isolate or study.19,20 Neptunyl cations (Np(VI)O22+ and Np(V)O2+) are similar to the uranyl moieties, but in this case, the pentavalent state is favored under ambient conditions.1 The relative stability of the Np(V)O2+ species allows for more detailed experimental examination, allowing for additional investigations regarding the interactions between the neptunyl oxo and neighboring molecules. Spectroscopic studies combined with computational analyses indicate that the Np(V)O2+ cation readily interacts with trivalent cations through both π- and σ-interactions.21 Unlike the uranyl cation, the Np(V)O2+ unit will often engage in “cation−cation” interactions, where the neptunyl oxo bonds to a neighboring neptunyl cation through the equatorial plane. This leads to significant variabilities in the structural features of Np(V) solids compared to U(VI) materials, and these types of interactions are thought to play an important role in redox chemistry and disproportionation reactions.1,19,22 These differences, both in reactivity and ability to engage in intermolecular interactions, are critically important for developing a better understanding of Received: February 26, 2018

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DOI: 10.1021/acs.inorgchem.8b00488 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

restricted to specialized laboratories and handled under appropriate regulatory controls and safe working practices. [Np(V)O2]+ Stock Solutions. 237Np (∼200 mg) was reprocessed from previous synthetic experiments, precipitated using saturated NaOH, and purified by a cation exchange column containing Dowex50-X8 resin. The resulting neptunyl hydroxide precipitate was washed three times with ultrapure H2O and dissolved in the smallest volume possible of 1.0 M HNO3 or HCl. The final concentration of Np(V) in each stock solution was determined using a Packard Tri-Carb Liquid Scintillation Counter. The oxidation state of the stock solution was confirmed with absorption and Raman spectroscopy. Reagents. 18-crown-6 (C12H24O6, Acros Organics), lithium chloride (LiCl, Fisher Scientific), sodium chloride (NaCl, Fisher Scientific), indium nitrate (In(NO3)3, Strem Chemicals), iron nitrate (Fe(NO3)3, Sigma-Aldrich), gold(III) chloride hydrate (HAuCl4· xH2O, Sigma-Aldrich), hydrochloric acid (HCl, Fisher Scientific, 38% w/w), and nitric acid (HNO3, Fisher Scientific) were reagent grade and used without further purification. Millipore-filtered ultrapure water (18.2 MΩ·cm) was provided by a ThermoScientific Barnstead EasyPure II, which was used in all the syntheses. All experiments were performed with approved safety operating procedures and performed in a designated fumehood/glovebox. Np1a ([NpO2(18-crown-6)]ClO4). 200 μL of 50 mM Np stock (10 μmol; 1 M HCl), 400 μL of 100 mM NaCl (40 μmol), and 400 μL of 100 mM 18-crown-6 (40 μmol) were combined in a 24-well polystyrene plate with a 3.5 mL working volume (Corning Inc.). The solution was allowed to slowly evaporate, until turquois crystals formed in the bottom of the well plate. Np1b ([NpO2(18-crown-6)]AuCl4). 150 μL of 50 mM Np stock (7.5 μmol; 1 M HCl), 225 μL of 100 mM HAu(III)Cl4 (22.5 μmol), and 225 μL of 100 mM 18-crown-6 (22.5 μmol) were combined in a 1.5 dr glass vial. Slow evaporation yielded dark yellow blades with an approximate average crystal dimension of 80 × 80 × 80 μm. Na−Np ([Np(V)O2(18-crown-6)(Na(H2O)(18-crown-6)][Np(VI)O2Cl4]. 250 μL of 50 mM Np stock (12.5 μmol; 1 M HCl or 1 M HNO3), 400 μL of 100 mM NaCl (40 μmol), and 400 μL of 100 mM 18-crown-6 (40 μmol) were combined in a 24-well polystyrene plate with a 3.5 mL working volume (Corning Inc.). Evaporation of the solution resulted in the formation of small, blocky green colored crystals within a fine-grained precipitate. Np−Np ([NpO2(18-crown-6)](NpO2Cl2NO3)]. 400 μL of 35 mM Np stock (14 μmol; 1 M HCl), 1.0 mL of 100 mM In(NO3)3 (100 μmol), and 1.0 mL of 100 mM 18-crown-6 (100 μmol) were combined in a 1.5 dr glass vial. A significant amount of fine-grained precipitate formed, but a small amount of single crystals could be observed within the multiphase system. Np−Cl (NpO2Cl(H2O)1.75). 500 μL of a 60 mM Np stock solution (1 M HCl) was loaded into a 3 × 3 Pyrex borosilicate crystallization well plate and allowed to slowly evaporate over several weeks to dryness. Small crystals were observed within the residual salts that were selected for additional characterization by single-crystal X-ray diffraction. Structural Characterization. Suitable single crystals were isolated from their mother liquor and mounted on an MiTeGen cryoloop (d = 50−150 μm) with mineral oil or NVH immersion oil (Cargille Laboratories). A Bruker D8 Quest single-crystal X-ray diffractometer equipped with Mo Kα radiation (λ = 0.710 73 Å) and an Oxford Systems low-temperature cryostream of gaseous N2 flow operating at 100 K. Data were collected with the Bruker APEX3 software package, and peak intensities were corrected for Lorentz, polarization, and background effects using Bruker APEX3 software.30 An empirical correction for absorption by the crystal was applied using the program SADABS.31,32 The structure solution was determined by intrinsic phasing methods and refined on the basis of F2 for all unique data using the SHELXTL version 5 series of programs.33 Np atoms were located by direct methods, and the C, O, Na, and Cl atoms were found in the difference Fourier maps calculated following refinement of the partial structure models. Hydrogen atoms associated with the 18crown-6 ligand were fixed using a riding model.

the actinyl moiety and to provide better chemical controls for these important species. Our research program aims to explore the subtle differences in An(VI)O22+ and An(V)O2+ chemistry, with particular interest in oxo atoms interactions within neptunyl systems. Studying Np(V) systems is important for nuclear waste reprocessing, but it is limited due to the radioactivity of the material. 237Np is produced in significant quantities during the nuclear fuel cycle and is considered a major contributor to the long-term radiotoxicity of nuclear fuel rods.23,24 The neptunium chemistry must be carefully controlled during separations processes (e.g., PUREX) and disposal, but there is an incomplete understanding of the behavior of Np(V)O2+ in complex matrices.23,25 This includes the formation of cation− cation interactions and how they may impact the overall chemical properties of the system. Our opening investigation evaluates Np(V)−cation interactions in the presence of 18-crown-6 ligands using X-ray crystallography to initially characterize the complexes and Raman spectroscopy to provide additional information on changes in the neptunyl bond strength. The 18-crown-6 molecule was chosen, because it was previously shown by Clark et al.26 to form an unusually stable inclusion complex with Np(V) cations and limit the formation of extended structural topologies. In addition, Shamov et al.27 performed density functional theory (DFT) calculations on the [Np(V)O2(18crown-6)]+ complex and reported that the bonding between the crown ether molecule and the neptunyl moiety were ionic in nature. This allows us to specifically evaluate changes in the neptunyl bond without needing to also consider significant destabilization effects of the equatorial ligand. Herein we provide the structural details of four neptunyl crown ether complexes that are either isolated within the crystalline lattice (Np1a ([NpO2(18-crown-6)]ClO4), Np1b ([NpO2(18-crown-6)]AuCl4)) or contain significant interactions with neighboring cations (Na−Np ([Np(V)O2(18-crown6)(Na(H 2 O)(18-crown-6)][Np(VI)O 2 Cl 4 ], Np−Np ([NpO2(18-crown-6)](NpO2Cl2NO3)]). In addition, we structurally characterized the neptunyl phase formed in the absence of the 18-crown-6 ligand (Np−Cl (NpO2Cl(H2O)1.75). Raman spectroscopy was utilized for the solid-state samples to provide a more comprehensive understanding of how these intermolecular interactions impact the bonding within the inclusion complex. Additional solution-phase Raman spectra were collected to evaluate the persistence of the neptunyl 18crown-6 complex in solution and probe the relevance of additional intermolecular interactions with alkali, alkali-earth, group 13, and transition-metal cations. These systems were chosen based upon previous literature precedent that suggested their possible intermolecular interaction with the neptunyl oxo group.21,28,29 One point of clarification: While the term cation−cation interactions has been used historically, we feel it is important to precisely distinguish the interaction in greater detail. Therefore, we will be using the term “neptunyl−cation” interaction to discuss interactions between the neptunyl oxo atom and other low-valence cations. The term “neptunyl−neptunyl” interaction will be used to specifically describe interactions between neighboring neptunyl units.

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EXPERIMENTAL METHODS

Synthesis. Caution! Neptunium-237 is a highly radioactive α-emitter and as such is considered a health risk. Research involving this isotope is B


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Inorganic Chemistry Table 1. Selected Crystallographic Parameters Np1aa −1

FW (g mol ) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalc (mg m‑3) μ (mm−1) F (000) crystal size (μm) θ range (deg) data collected

reflections collected/ unique GOF of F2 final R indices [I > 2σ(I)] R indices (all data) a

Np1bb

Na−Npc

Np−Npd

Np−Cle

632.57 I41/a 10.6082(3) 10.6082(3) 16.3993(8) 90 90 90 1845.48(14) 4 2.017 11.454 1000 100 × 120 × 160 3.681 to 28.337 −14 < h < 14 −13 < k < 13 −21 < l < 21 19 899/1143 [Rint = 0.0364] 1.253 R1 = 0.0474

872.08 C2/m 8.186(2) 11.704(3) 11.846(3) 90 108.910(7) 90 1073.7(5) 2 2.697 12.168 800 80 × 80 × 120 3.154 to 26.413 −10 < h < 10 −14 < k < 14 −14 < l < 14 13 674/1150 [Rint = 0.0332] 1.033 R1 = 0.0238

2526.82 P21/n 10.7219(13) 10.5113(13) 34.28(4) 90 97.240(4) 90 3833.0(8) 2 2.189 5.752 2404 30 × 90 × 100 2.28 to 25.45 −12 < h < 12 −12 < k < 12 −41 < l < 41 42 168/7226 [Rint = 0.0588] 1.197 R1 = 0.1192

1870.44 P21/n 10.7209(10) 18.4347(17) 11.6948(11) 90 90.096(3) 90 2311.3(4) 2 2.688 9.233 1708 40 × 80 × 120 2.575 to 25.349 −12 < h < 12 −22 < k < 22 −14 < l < 14 84 418/4111 [Rint = 0.0442] 1.093 R1 = 0.0262

1331.24 C2/c 20.1862(8) 8.4364(4) 23.2288(9) 90 91.951(2) 90 3953.5(3) 8 4.473 21.458 4485 20 × 30 × 40 2.617 to 25.258 −24 < h < 24 −10 < k < 10 −27 < l < 27 35 543/3580 [Rint = 0.0761] 1.042 R1 = 0.0279

wR2 = 0.1101 R1 = 0.0485 wR2 = 0.1106

wR2 = 0.0289 R1 = 0.0533 wR2 = 0.0561

wR2 = 0.2788 R1 = 0.1278 wR2 = 0.2788

wR2 = 0.0598 R1 = 0.0273 wR2 = 0.0604

wR2 = 0.0627 R1 = 0.0425 wR2 = 0.0663

[(NpO2)18-crown-6]ClO4. b[(NpO2)18-crown-6]AuCl4. (NpO2Cl2NO3)]. eNpO2Cl(H2O)1.75.

c

[NpO2(18-crown-6)(Na(H2O)(18-crown-6][NpO2Cl4].

Np1a was previously structurally characterized by Clark et al.26 but was included herein due to subtle differences in the synthetic conditions and completeness of the study. Significant twinning was observed for Na−Np, which led to higher R1 values for the finalized structure. Unit cell dimensions were confirmed with CELLNOW, and this program was also utilized to determine the appropriate twin law for the compound. The PLATON software34 was also used to confirm the space group assignment of Na−Np, Np−Np, and Np−Cl. Disorder was prevalent for the crown ether molecule, which is not unusual for crystallization of this ligand in other systems.35−37 In the case of Np1a, the disorder could be modeled by refining the positions of two different conformers. However, for Np1b, Na−Np, and Np− Np the disordered structure refinement (DSR) option for the 18crown-6 molecule was used to model the proper orientation of the ligand.38 This tool allows the user to find molecular fragments within the residual electron density map and then fit the correct isomer to the position of those fragments with bond restraints applied from the database. Select crystallographic details can be found in Table 1, and selected bond distances and angles for the compounds can be found in the Supporting Information section (Tables S1 and S2). Crystallographic information files for Np1b, Na−Np, Np−Np, and Np−Cl compounds can be found in the Cambridge Structural Database by requesting 1824932−1824935. Raman Spectral Analysis. Both solid-state and solution-phase Raman spectra were acquired on an SnRI high-resolution Sierra 2.0 Raman spectrometer equipped with 785 nm laser energy and 2048 pixels TE-cooled CCD. Laser power was set to the maximum output value of 15 mW, and the system was configured to acquire data by the Orbital Raster Scanning mode, giving the highest achievable spectral resolution of 2 cm−1. Experiments were conducted in dark conditions to mitigate fluorescence interference. Each sample was irradiated for an integration time of 60 s and automatically reiterated six times in MultiAcquisition mode. The average of the six Raman spectra acquired for a sample is reported as the final Raman spectrum. Raman spectroscopy was also used to analyze the solution phase before and after

d

[(NpO2(18-crown-6))-

crystallization of the solid-state species. To accurately process the Raman signals observed, the background was subtracted, multiple peaks were fit using the peak analysis protocol with Gaussian functions, and all the fitting parameters converged with a chi-squared tolerance value of 1 × 10−14 in the OriginPro 9.1.0 (OriginLab) 64-bit software. The Raman cross-section for all neptunyl species is assumed to be similar to uranyl; therefore, the relative abundance of each species can be estimated using the peak height of the Gaussian function. Full width at half-maximum (fwhm) values of ∼5−16 cm−1 depending on the nature of the molecular species were considered appropriate for accurate assignment of the spectral bands of the solidstate material. In aqueous solutions, the bands are broader, and thus larger values of 15−30 cm−1 are considered in the spectral analysis. Aqueous Raman Titration. A Raman spectral titration was qualitatively used to explore the formation of the Np(V)O2+−18crown-6 species in solution. Stock solutions of 100 mM Np(V) in 1 M HNO3 and 100 mM 18C6 were made in N2-purged, ultrapure water (18.2 MΩ·cm−1) and were used to prepare the working solutions of the appropriate Np(V)O2+ and 18-crown-6 concentrations for each Raman experiment. Following each aliquot addition of the titrant 100 mM 18C6, the Raman samples were allowed to equilibrate for 15 min prior to data acquisition. Throughout each Raman experiment, samples were kept in the dark to avoid stray light and fluorescence interferences. The analyte, 600 μL of 100 mM Np(V) (60 μmol) in 1 M HNO3, was loaded into a glass vial with a 4.0 mL working volume and analyzed. The titrated Raman solutions were subsequently prepared at varying concentrations via the addition of 100 mM 18crown-6 yielding 30, 60, 120, and 240 μmol 18-crown-6 solutions mixed with the initial 60 μmol Np(V)O2+ solution. Raman spectra were recorded for 60 s and duplicated six times with background measurements automatically collecting before each spectrum. The average of the six Raman scans acquired for each titration solution is reported here as the final Raman spectrum. Aqueous Cation Competition for 18C6 Encapsulation. The Raman vibrational modes of the neptunyl crown ether system were C


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Inorganic Chemistry

Figure 1. (left) The [NpO2(18-crown-6)]+ species in Np1a depicted by thermal ellipsoids with Np, O, and C atoms represented by green, red, and black ellipsoids, respectively. Because of disorder of the ligand, H atoms were not included in the model. (right) Molecular view along the equatorial plane provides information on the conformation of the 18-crown-6 ligand, which bends away from planar to form a C2 isomer.

Figure 2. Inclusion complex in Np1b (left) consists of a Np(V)O2+ cation within the 18-crown-6 molecule. Np, O, C, and H atoms represented by green, red, black, and light pink ellipsoids, respectively. A planar conformation (right) is observed for the 18-crown-6 molecule within the Np1b complex. further examined to ascertain the nature and extent of 18C6 size selectivity and Mn+ cation (Mn+ = Na+, K+, Sr2+, Al3+, Fe3+, In3+) complexation. Each sample was prepared in aqueous solution containing Np(V)O2+, Mn+, and 18C6 in a 1:3:6 molar ratio (20 μmol/60 μmol/120 μmol) and loaded into a 4.0 mL glass Raman spectroscopy vial. All experimental conditions for this solution Raman investigation were identical to the aqueous Raman titration protocol described in the previous paragraph.

relatively strong neptunyl bond that corresponds to the short NpO bond lengths observed in the crystalline lattice. Again, the electrostatic interaction provides an excellent reference point to explore the synergistic effects of the various intermolecular forces that influence the neptunyl oxo atoms without having to consider destabilization from ligated molecules about the equatorial plane. Crown ether ligands can exhibit significant disorder in the crystalline lattice, which creates difficulties in determining the exact conformational isomer using X-ray diffraction data. Our structural characterization for Np1a and the previous report by Clark et al.26 indicated disorder of the 18-crown-6 molecule. The electron density associated with the ligand can be modeled as two superimposed 18-crown-6 molecules, both of which exhibit C 2 point group symmetry. This confirms the interpretation made by Clark et al.26 that also suggested that the C2 configuration was observed within their solid-state coordination compound. The positively charged coordination complex is balanced with a perchlorate anion to form [Np(V)O2(18-crown-6)]ClO4. This perchlorate anion was found to be a contaminant within the Np stock solution, which may have occurred during the precipitation of the Np(V) from 1 M HClO4 (eluted from the column) and is conserved when the Np(V) is redissolved in 1 M HCl. However, we have some spectroscopic evidence from evaluating the HCl stock over time that the perchlorate concentration increases with aging. Our tentative hypothesis regarding the continued

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RESULTS AND DISCUSSION Isolated Np(V)-Crown Ether Complexes (Np1a and Np1b). As stated earlier, the isolated crown ether coordination complex [NpO2(18-crown-6)][ClO4] was initially reported by Clark et al.26 and reproduced here (Np1a) to serve as the baseline from which we can explore changes within the crown ether complexes (Figure 1). Np1a contains one crystallographically unique Np(V) atom that forms strong bonds with two O atoms to create the nearly linear NpO2+ moiety with an average NpO bond distance of 1.805(7) Å. The 18-crown-6 ligand encircles the neptunyl cation about the equatorial plane and bonds to the Np(V) cation through the six O atoms of the ether groups. Equatorial Np−O bond length ranges from 2.575(13) to 2.622(12) Å, which is slightly longer than what is typically observed for other pentavalent Np compounds (2.54(11) Å).22 As indicated previously, DFT calculations conducted by Shamov et al. reported that the interaction between the crown ether molecule and the neptunyl cation can be considered purely ionic.27 This interaction leads to a D


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Inorganic Chemistry

Figure 3. 18-crown-6 molecule encapsulates both Np(V)O2+ and Na+ cations, and these two units are linked through O4 atom. A second Np atom (Np1) charge balances the complex through the formation of the [Np(VI)O2Cl4]− anion. The Np, Na, C, O, and Cl atoms are depicted as green, purple, black, red, and blue ellipsoids, respectively.

to create planar D3d complexes when the charge balancing anions are [Cr(CO)5Cl]−,42 [HCl2]−,43 [I3]−,44 or perchlorotriphenylmethanide.45 Switching the counterion to nitrate results in the K+ cation positioned above the 18-crown-6 molecule in a “sunrise conformation”.37 Presence of hydronium in (H3O)[K(H2O)3(18-crown-6)]Cl2·H2O also results in buckling of the crown ether to create a distorted C 1 conformation. Similarly, the 18-crown-6 molecule in [Na(THF)2(18-crown-6)][CH3W(CO)5] (THF = tetrahydrofuran) exhibits a planar arrangement about the equatorial plane, but the slight size mismatch between the cationic radii and the ligand often results in lower symmetry conformations.37,46 Crown Ether Complexes with Neptunyl−Cation (Na− Np) and Neptunyl−Neptunyl (Np−Np) Interactions. Addition of sodium cations to the neptunyl crown ether system led to the formation of isolated molecular complexes that exhibit neptunyl−cation interactions (Figure 3). In the case of Na−Np, both the Np(V)O2+ and the Na+ cations are encapsulated by the crown ether ligand. Similar to the Np1a and Np1b structures, the 18-crown-6 ligand encircles the neptunyl moiety (Np2) about the equatorial plane with Np−O bond distances ranging from 2.59(2) to 2.68(2) Å. A second 18-crown-6 molecule also encapsulates the Na+ cation. Because of the size mismatch, only five of the six O atoms bond to the Na+ cation to create an asymmetric bonding arrangement (2.39(3)−2.72(3) Å) within the crown ether molecule. The sixth O atom (O10) is located 3.659(20) Å from the Na+ cation, which is much too long to be considered a Na−O bond. Additional bonding to the Na+ cation occurs through the axial positions. One water molecule (O5) is located below the sodium cation at a distance of 2.33(3) Å. The neptunyl oxo atom (O4) is located 2.38(3) Å above the complex to form a bridge to the Np(2) atom. Even with the added oxo interaction, the neptunyl bond remains symmetric with bond distances of 1.79(2) Å. Formation of the Na−Np crown ether complex results in conformational changes in the 18-crown-6 ligand encircling both metal centers. The close contact between the encapsulated Np(V)O2+ and Na+ cations results in deviations of the ligand from planarity and forms two puckered conformers that extend

ingrowth of perchlorate in this system is radiolysis of the hydrochloric acid. Changing the identity of the counteranion in this system leads to the crystallization of a Np(V) inclusion complex. In the case of Np1b, the perchlorate anion was replaced by [Au(III)Cl4]− to form the encapsulated NpO2+ species (Figure 2). This counterion was chosen, because we were investigating the reduction of other crown ether complexes in the presence of Au(I)Cl to form [Np(IV)L][Au(III)Cl4] complexes. These reduced complexes will be discussed in detail in future work, but herein we explored the addition of [Au(III)Cl4]− to the Np(V) crown ether system. In this case, the presence of the gold tetrachloro anion resulted in the crystallization of [Np(V)O2(18-crown-6)][Au(III)Cl4] in the C2/m space group. The neptunyl (NpO) bond lengths in this complex are 1.802(7) Å, with the equatorial bond distances ranging from 2.687(6) to 2.696(4) Å. The neptunyl bond length is similar to that observed in the perchlorate compound, but the equatorial bond lengths have increased by 0.1 Å in the presence of the [Au(III)Cl4]− anion. The Au(III)−Cl bond distance is 2.286(2) Å, which is typical for the Au(III) complex.39−41 Clark et al.26 also reported the crystallization of the triflate form of the inclusion complex, indicating the relative abundance of the Np(V) inclusion complex in the presence of a wide range of counteranions. Crystallization of the [Np(V)O2(18-crown-6)]+ complex with the [Au(III)Cl4]− anion results in the formation of a different conformational isomer for the crown ether molecule and provides additional details on the flexibility of the neptunyl inclusion complex. The 18-crown-6 ligand in Np1b exhibits a more planar arrangement than that observed in the perchlorate structure and assumes a D3d conformation. Planarity of the ligand explains the slightly longer bond distance observed in the equatorial plane of the neptunyl complex and compares well to the computational results from Shamov et al. (2.635−2.715 Å).27 The versatility of 18-crown-6 and the ability to change conformation upon variations in the counterion or crystalline packing are well-established in the literature, particularly for potassium and sodium inclusion complexes.37 The ionic radii of K+ (1.33 Å) provides a perfect “key and lock” complementarity E


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Inorganic Chemistry away from the neighboring cation. This puckering lowers the overall point-group symmetry to C1 for the crown ether ligands. The linkage between the Na+ and neptunyl cations occurs through a single bridging oxo, which is not commonly observed within crown ether systems. Crown ether complexes containing two or more metal cations are scarce and usually occur within sandwich compounds or linked through two μ2-X (X = Cl, SCN) bridges.37 We are unaware of any 18-crown ether complexes that contain single oxo bridges connecting two inclusion complexes. Junk et al.47 reported an unusual Fe(III) aqua complex with a μ2-O bridge that occurs in the presence of crown ether molecules, but these macrocycles are observed in the outer coordination shell and are thought to stabilize the complex through extensive hydrogen bonding.47 The Na−Np dimeric coordination complex is positively charged and must be balanced with a counteranion to form the neutral solid-state compound. In this case, the [Np(V)O2(18crown-6)(Na(H2O)(18-crown-6)]2+ species is charge balanced with a [Np(VI)O2Cl4]2− anion. The Np(1)O bond lengths for this interstitial species are 1.71(2) and 1.75(2) Å, which are too short to be associated with Np(V)O2+ cation and indicates oxidation to Np(VI)O22+. Equatorial Np(VI)−Cl bonds range from 2.635(7) to 2.666(8) Å and are comparable to those reported for similar [Np(VI)O2Cl4]2− complexes.48,49 Formation of this mixed oxidation state compound was initially surprising given that the oxidation state of the initial stock solution was confirmed through Raman spectroscopy to be in the pentavalent state (Supporting Information Figures S1 and S2). Neptunium is subject to disproportionation reactions, which could result in the conversion of Np(V) to Np(IV) and Np(VI) by a one-electron transfer process.50 In addition, Cornet et al. noted that the [Np(VI)O2Cl4]2− was also produced as a counterion in the presence of phosphoruscontaining ligands (R3PX (R = Cy, Ph and X = O, NH) and commented on the stability of the complex in aprotic solvents.48 Attempts to crystallize a similar dimeric species with other positively charged cations (Li+, K+, Rb+, Cs+, Rb+, Sr2+, Ba2+, Fe3+, In3+, Ga3+) were not successful but did result in the isolation of a complex that exhibited neptunyl−neptunyl interactions (Figure 4). Crystallization occurred in the case where In3+ nitrate salts were introduced as starting reagents. The Np−Np compound contains Np(V)O2+ (Np1) cations encapsulated about the equatorial plane by the 18-crown-6 ligand, with an average Np−O bond distance of 2.609 Å. The 18-crown-6 molecule deviates from planarity to form the Ci conformer. The NpOyl bond in the inclusion complex is asymmetric with the free Np(1)−O(1) bond observed at 1.788(5) Å, and the Np(1)−O(2) bond involved in the neptunyl−neptunyl interaction is elongated to 1.850(5) Å. The Np(2)−O(2) bond length is 2.352(5) Å, which is typical for an equatorial O atom engaged in a neptunyl−neptunyl interaction.22,51,52 The neptunyl bond lengths for Np(2) are 1.742(7) and 1.743(7) Å, which again signifies the oxidation or disproportionation of the Np(V) stock solution upon evaporation. Two Cl atoms and a nitrate anion bond to Np(2) through the equatorial plane with average Np−Cl and Np−O distances of 2.658(2) and 2.490(7) Å, respectively. This leads to a neutral molecular complex with the chemical formula of [(Np(V)O2(18-crown-6))(Np(VI)O2Cl2NO3)]. The presence of the nitrate in the equatorial plane of Np(2) is due to the addition of indium nitrate as a synthetic starting material, as indium chloride was not originally available for the initial

Figure 4. Np−Np compound contains the Np(V) cation (Np1) encapsulated by the 18-crown-6 molecule and bonded to the second Np(VI) atom (Np2) through a neptunyl−neptunyl interaction to create the neutral molecule. The Np, N, C, O, and Cl atoms are depicted as green, light-blue, black, red, and blue ellipsoids, respectively.

synthesis. The use of other metal chloride salts did not lead to the isolation of the Np−Np compound. Bonding to the neighboring neptunyl unit takes place through the equatorial plane and provides the “T-shaped” neptunyl−neptunyl interaction with a Np(1)−O(2)−Np(2) angle of 170(1)°. Actinyl-actinyl interactions were first described by Sullivan et al.53 between Np(V)O2+ and U(VI)O22+ cations and has since been found to be a dominant species in concentrated Np(V) solutions.52,54 The exact structural nature of the neptunyl−neptunyl interactions has been classified as either T-shaped or a diamond confirmation, where the latter describes a situation where both metal centers engage in oxo interactions in a side-on fashion.55 Neptunyl− neptunyl interactions within isolated molecular complexes have been previously reported but contain either dimers with diamond interactions or tetrameric species with multiple Ttype linkages.56−58 Thus, the molecular complex within Np− Np represents the first report of an isolated complex that contains a simple T-shaped neptunyl−neptunyl interaction. While we have limited information regarding the stability of these two types of interactions in the neptunyl system, a computational assessment of uranyl−uranyl interaction chemistry was performed by Tecmer et al.55 This investigation found that, in general, the T-shaped interactions are more than 20 kcal·mol−1 lower in energy than the diamond linkages. Crystallization from Stock Solution (Np−Cl). The formation of the mixed oxidation state within both Na−Np and Np−Np compounds was somewhat unexpected; therefore, we decided to explore the importance of the crown ether molecule in the disproportionation of the pentavalent neptunyl cation. To investigate this aspect, the pure stock solution (in 1 M HCl) was subjected to a similar slow evaporation method used to create the 18-crown-6 coordination compounds. After several days of evaporation, crystals (Np−Cl) were obtained that were of high enough quality to be structurally characterized by single-crystal X-ray diffraction. Np−Cl is a hydrated neptunyl chloride [NpO2Cl(H2O)1.75] that is similar to that previously reported by Jin et al. [NpO2Cl(H2O)2]. While the topology is identical between the two compounds, variations in the hydration state lead to more disorder of the atoms in the F


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Inorganic Chemistry equatorial plane and lower the overall symmetry in the case of Np−Cl compound. Additional details on the crystal structure can be found in the Supporting Information section. Pure neptunyl chloride compounds of the formula Np(V)O2Cl(H2O)X have been identified for X = 1, 1.75, and 2.0. Grigorev et al. provided structural characterization of Np(V)O2Cl(H2O) and again observed extensive neptunyl−neptunyl interactions in a square net configuration.59 Unlike Np(V)O2Cl(H2O)1.75 (Np−Cl) and Np(V)O2Cl(H2O)2, the cationic networks in the monohydrate are linked through bridging Cl atoms instead of neptunyl−neptunyl interactions. With the addition of counter cations, lower-dimensional topologies are formed, including one-dimensional (1D) chains that still contain some neptunyl−neptunyl interactions but to a lesser extent than in the simple neptunyl chloride compounds.51,60 Mixed oxidation state compounds are also known, including Np(IV)/Np(V) ([Np(IV)(Np(V)O 2 ) 6 (H 2 O) 8 (OH)Cl 9 ]· H2O)61,62 and Np(V)/Np(VI)63 solid-state materials. This suggests that disproportionation is prevalent in neptunyl chloride systems, particularly for synthetic methods that utilize slow evaporation to concentrate the initial Np(V) stock solution. Solid-State Raman Analysis. Solid-state spectral analysis of the crown ether and neptunyl chloride materials will provide additional information regarding the strength of the neptunyl bond and the impact of cation−cation interactions. Locating the peak centroid can provide information regarding the relative red or blue shift of the symmetric vibrational frequency and can be linked back to stretching and interaction force constants through Hooke’s Law.64 Owing to the complex nature of the Raman traces acquired for this system, peak fitting was necessarily employed to determine the number of overlapping bands present in the spectra. Upon the basis of our previous analysis of uranyl materials we determined that the fwhm of the bands is typically between 5 and 20 cm−1, so we used this range to fit the spectral bands of the solid-state neptunyl compounds.65,66 We highlighted the major features in the spectra in Table 2, and the spectra can be found in the Supporting Information section (Figures S13−S21). The simplest spectra are those for the isolated inclusion complexes in Np1a and Np1b. For these coordination compounds, the υ1 symmetric stretching band is located at 777 and 789 cm−1 for Np1a and Np1b, respectively. The vibrational frequency for Np1a is identical to that reported by Clark et al.26 and represents a blue shift from the reported value for the penta-aqua species (767 cm−1). Again, the ionic nature of the interaction suggests that there is limited σ-donation in this system and that the bonding between the crown ether and the neptunyl cation is weaker than ligated water molecules. A larger blue shift is observed for Np1a than for Np1b, which could be due to subtle changes in the conformation of the crown ether. This is corroborated by the slight change in equatorial bond length in Np1b (2.687−2.696 Å) compared to Np1a (2.575−2.622 Å). Surprisingly, the neptunyl bond length in both compounds is observed at 1.80 Å, but this is likely due to symmetry constraints caused by the tetragonal space group assignment. For instance, if the symmetry constraints are removed within Np1a by determining the structure in the triclinic P1 space group then the average distance for the Np O bond is 1.812(8) Å. Conformational changes of the ligand and the difference in counteranion does result in variations in the bands observed in the Raman spectra. Al-Katani et al. have conducted an extensive

Table 2. Major Raman Bands Observed within the 700−900 cm−1 Range for Solid-State Compoundsa

Np1a Np1b Na− Np

Np− Np

peak centroid (cm−1)

fwhm (cm−1)

777 891 788 893 720

6.01

16.23

8.06

780 793 825 832

12.77 12.27 7.47 11.39

5.46 50.23 1.31 3.59

871 896 716

14.21 8.34 11.15

21.78 8.77 7.49

724 749 771 789

7.68 10.19 7.54 6.69

3.22 5.39 3.15 30.59

823 832

7.07 8.95

3.06 10.63

863 894

11.29 8.04

26.43 10.04

% area

6.08

assignment ν1 NpO2+ ν42 18-crown-6 ν1 NpO2+ ν42 18-crown-6 ν4 NO3− impurity ν1 NpO2+ ν44 18-crown-6 ν1 NpO22+/ ν104 18-crown-6 ν43 18-crown-6 ν42 18-crown-6 ν4 NO3− ν1 NpO2+ (Np−Np) ν4 NO3− ν3 NpO2+ (Np−Np) ν1 NpO2+ (isolated Npcrown ether) ν44 18-crown-6 ν1 NpO22+/ ν104 18-crown-6 ν43 18-crown-6 ν42 18-crown-6

a

Assignments for the bands associated with the 18-crown-6 molecule are based upon previous work by Al-Katani et al.67

computational analysis of 18-crown-6 conformers associated with alkali-metal cations and calculated the expected Raman bands for this system.67,68 We use this spectral interpretation to further understand the Raman data presented herein. The Raman spectrum for Np1a contains additional bands at 456, 540, 891, and 909 cm−1 that are associated with the 18-crown-6 ligand in a C2 configuration. In contrast, the spectral features associated with the crown ether molecule in Np1b are located at 551 and 893 cm−1 and can be related back to the highersymmetry D3d conformation. Other spectral features associated with the counteranion include a large peak at 934 cm−1 for the υ1 stretch of the perchlorate anion and the band at 334 cm−1 that is observed for the [Au(III)Cl4]− anion. Spectral complexity is induced by the occurrence of neptunyl−cation interactions in the case of Np−Na. There are multiple bands located between 900 and 1100 cm−1 and a band at 550 cm−1 that can again be assigned to the lowersymmetry18-crown-6 molecule. Peak fitting indicated that there are eight total bands located in the spectral window between 700 and 900 cm−1 with fwhm values ranging from 7 to 17 cm−1. As this is the spectral window of interest for interpretation of the neptunyl unit, these bands will be discussed in greater detail in the following paragraphs. The band at 895 cm−1 can been assigned to the 18-crown-6 molecule, and the presence of weaker spectral features at 871 cm−1 is associated with the lower-symmetry conformation.67 These bands can also be observed in the solid-state Raman spectrum of the 18-crown-6 starting material (Figure S3). There are two additional peaks located at 834 and 823 cm−1 that can also be observed in the spectrum of the free ligand, but this is also the region that we would also expect to see bands associated with the [Np(VI)O2Cl4]− counteranion. Previous G


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Inorganic Chemistry studies by Toshiyuki et al. observed the υ1 band at 834 cm−1 in concentrated calcium chloride solution.69 Therefore, we assign the peak at 832 cm−1 as related to both the Np(VI)O2+ symmetric stretch and a vibrational band (ν104) associated with 18-crown-6 molecule, and the small band at 823 cm−1 corresponds to the ν44 stretch of the 18-crown-6 ligand in a lower-symmetry conformation. The υ1 stretching band for the Np(V)O2+ cation is expected to occur between 700 and 800 cm−1, and there are no bands associated with the crown ether molecule within this spectral window. The major band in the Raman spectrum of Np−Na occurs at 793 cm−1 (50.23%) with a shoulder at 780 cm−1 (5.46%) and a minor band at 716 cm−1 (8.06%). The spectrum was collected on a sample that was isolated in the presence of HNO3; thus, we assign the 716 cm−1 band as the ν4 mode of NO3− associated with a secondary nitrate salt that forms as a fine-grained precipitate on the surface of the solid-state sample.70 This band can also be observed in other spectra isolated from the nitrate stock solution (see Figure S21 in Supporting Information section). The major band in the spectrum (793 cm−1) can be assigned to the υ1 symmetric stretch of the neptunyl crown ether complex. The position of the band is slightly blue-shifted to that of the Np1a and Np1b complexes. The absence of the ν3 asymmetric stretching vibration indicates that the interaction of the neptunyl oxo with the sodium cation does not result in activation of the band in the Raman spectrum. This again correlates well to the structural characterization of the complex, as there is no significant perturbation of the neptunyl bond due to this interaction. The shoulder at 780 cm−1 is unclear, but it may be the result of a small amount of a secondary 18-crown-6 conformer in the solid-state compound, as it is more similar to the signal observed in the Np1b conformer. This may account for the significant amount of disorder observed with within the crystalline lattice of the Na−Np compound. It was impossible to separate enough single crystals of Np− Np to collect a solid-state spectrum of the pure compound; however, we can investigate the bulk product to gain additional insight about the system. The crown ether bands are again observed, with the major peaks at 894 and minor ones between 900 and 1100 cm−1 and at 550 cm−1. There are now two major bands within the 800−870 cm−1 region. The band at 832 cm−1 can be assigned to the ν1 NpO22+/ ν104 18-crown-6, and the peak at 864 and 823 cm−1 can be assigned to the 18-crown-16 molecule.67 We assigned the large band at 789 cm−1 to the inclusion complex associated with an isolated NpO2[18-crown6]+ molecule, which occurs as a secondary phase within the solid-state mixture. The band at 716 and 749 cm−1 is associated with the nitrate anion within the Np−Np compound. The band at 749 cm−1 arises when the nitrate anion is bonded to the metal cation and the symmetry decreases to Cs, as previously reported for Y(NO3)3·6 H2O.70 This leaves the smaller bands at 724 and 771 cm−1 that are unassigned based upon our current understanding of the system. Therefore, we assign the band at 724 cm−1 to the ν1 of the Np−Np complex and the 771 cm−1 signal to the activated ν3 band. The presence of neptunyl−neptunyl interactions does change the symmetry of this moiety and results in the activation of vibrational modes in the Raman spectrum. Typically, U(VI)O22+ spectra can be analyzed by considering the unit as having D∞h symmetry, which leads to one active band (the υ1 symmetric stretching frequency) in the Raman spectra and two in the IR spectra (the υ2 and υ3 frequencies). If the bond is

perturbed in a way that lowers the symmetry to C2v, C2, or C∞v then all three vibrational bands associated with the actinyl unit become active in both the Raman and IR spectra.71,72 In the case of U(VI)O22+, this may occur with bending of the uranyl moiety to create C2v symmetry.73 Uranyl−uranyl interactions are quite rare, but there is also evidence that these interactions can also activate the υ3 band in Raman spectra.72 We can also consider the relationship between the asymmetric and symmetric stretching bands to provide additional support for our assignment of the neptunyl bands associated with the Np−Np compound. Within uranyl systems, the force constant (k1) is usually between 6 and 7.5 mdyn·Å−1, and the interaction force constant (k12) is very small (−0.1 to −0.3 mdyn·Å−1). Because of this relationship and assuming harmonic vibrations characteristic of a linear ion, we can relate the two vibrational modes by ν3 = ν1(1 + 2MO/MNp)1/2

This translates to a ν3/ν1 ratio of 1.065 in the case of the neptunyl ion. This ratio holds relatively well for uranyl; however, it is dependent on identity of the equatorial ligands and other factors that may influence the interaction force constant. In the case of our Np−Np compound, the ν3/ν1 ratio is 1.065, which is expected based upon the established relationship. Evaluating the Raman spectra of the Np−Cl salt provides an opportunity to compare our molecular structures to extended topologies. The spectral window of interest has been increased in this case to include the Raman vibrational modes between 600 and 900 cm−1. Three bands at 622.5, 668.5, and 678.4 cm−1 are observed within the spectrum and correspond to the extended cationic net observed in the evaporate that forms Np−Cl compound and similar solids. Bands at 624, 671, and 675 cm−1 were previously reported for neptunyl chloride hydrates and were assigned to the ν1 symmetric stretching bands for the neptunyl cation.52 Smaller bands at 724, 736, and 752 cm−1 in Np−Cl are also located in the same region as the secondary bands (711, 714, 717, and 743 cm−1) in the previous study. In addition, Jin52 reported bands at 821 and 846 cm−1 for NpO2Cl(H2O)2 that correspond well to the peaks at 820 and 846 cm−1 in Np−Cl. The larger red shift in the ν1 bands for the neptunyl chloride system is due to the presence of extended neptunyl−neptunyl nets that result in extensive elongation of the neptunyl bond (1.826(7)−1.852(7) Å). With the formation of the extended cationic nets, we also note that the ν3/ν1 ratio is 1.18, which is larger than what was observed in the isolated molecular system in Np−Np. A larger value for the ν3/ν1 ratio may be related to differences in the interaction force constants for the neptunyl cation compared to the uranyl ion.71 Additional studies are necessary to investigate the activation and position of the ν3 within the vibrational spectra for neptunyl systems and explore how the ν3/ν1 ratio is impacted by neptunyl−cation interactions in a range of chemical systems. Solution Phase Spectra. The solid-state spectra provide information regarding relative band positions for the molecular complexes, and we utilize solution spectra to interrogate the presence and importance of these neptunyl 18-crown-6 complexes in aqueous solutions. We begin this analysis with the Raman spectra for the stock solutions in both HCl and HNO3 stock solutions (Figures S1 and S2). Again, Np(V) stock for the reactions were initially prepared in 1 M HCl, but we also noted a small band at 938 cm−1 that corresponds to a small H


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Inorganic Chemistry

band at 860 cm−1 appears, and the intensity of this peak increases as more ligand is titrated into the solution. We assign the Raman signal at 860 cm−1 to the free 18-crown-6 ligand present in solution, and this was confirmed by obtaining a spectrum of the original stock solution (Figure S4). Thus, we assign the band at 877 cm−1 to the formation of a neptunyl crown ether complex. A Raman band at 893 cm−1 appears in all of the solid neptunyl crown ether complexes, which may signify a 16 cm−1 red shift of the vibrational frequency in aqueous solution. If a similar red shift occurs for the ν1 stretch of the neptunyl crown ether complex, then this would potentially overlap with the penta-aqua species. This may explain why we do not observe a band between 780 and 790 cm−1 for the neptunyl crown ether inclusion complex in the solution Raman spectra. Time-dependent studies also demonstrated little variation in the spectra after two weeks of aging, suggesting that the solution is relatively stable and there is no ingrowth of neptunyl−neptunyl complexes in the system (Figure S19). Next, we explored the impact of counter cations on the Raman vibrational modes of the neptunyl 18-crown-6 solution. Presence of counter cations in the solution creates more variability within the Raman spectra and suggests that some cations may compete with the Np(V) for inclusion into the crown ether molecule (Figure 6). The spectrum of the Na+−

amount of perchlorate anion. The v1 band for Np(V) is observed at 766 cm−1 and has previously been assigned to the [NpO2(H2O)5]+ species. Within the 1 M HNO3 stock solution the major band at 1048 and 717 cm−1 can be assigned to the ν1 and ν4 modes of the nitrate anion. The 765 cm−1 frequency is also present in this sample and can be again assigned to the [Np(V)O2]+ complex. Previous studies on neptunyl vibrational signatures confirm the presence of these signals in aqueous solution. Guillaume et al.54 prepared Np(V) in dilute solutions of 0.1 M Np(V)O2+(aq) and reported a large band at 767 cm−1 associated with the penta-aqua species. A second band at 738 cm−1 was observed when the concentration increased to 1.0 M Np(V)O2+(aq) and was assigned as a neptunyl−neptunyl interaction that occurs in the solution. These dimeric complexes have subsequently been observed in lower concentrations of Np(V) (1 M HClO4;0.4 M NpO2+) using high-energy X-ray scattering.74 At higher concentrations of Np(V) (>1.0 M), additional bands at 685, 712, 783, and 820 cm−1 were also reported by Guillaume et al.54 but were not specifically assigned to a known neptunyl species. This again pairs well with the study performed by Jin,52 where the Raman spectrum of a concentrated neptunyl chloride stock solution contained broad peaks at 689, 752, 803, and 843 cm−1. Jin52 suggested that the presence of the major band at 689 cm−1 indicates the development of larger oligomers that are formed through neptunyl−neptunyl interactions. First, we evaluated the changes in the Raman spectrum of the HNO3 stock solution with the addition of the crown ether ligand (Figure 5). The major peak associated with Np(V)

Figure 6. Overlayed Raman spectra of the neptunyl 18-crown-6 solution in the presence of Na+, K+, Sr2+, and Al3+ cations. The NpO2+/metal cation/18-crown-6 ratio in this system is 1:3:6.

Np(V)O2+−18-crown-6 sample again contains a large band at 861 cm−1, corresponding to the free 18-crown-6 ligand and a smaller shoulder at 876 cm−1 that was assigned to the inclusion complex. Within the Al3+ system, the shoulder at 876 cm−1 is barely observable due to the prominence of the band at 861 cm−1. Moving to the K+ and Sr2+ cations reveals a more pronounced band that has blue-shifted to 880 cm−1 and a decrease in the intensity of the peak associated with the free crown ether molecule. These differences are likely due to the competition between the neptunyl and the countercation for inclusion into the 18-crown-6 molecule. The size of the K+ and

Figure 5. Overlayed Raman spectra of the titration of a 100 mM 18crown-6 stock solution into a Np(V) (in 1 M HNO3) solution.

aqueous complex is located at 765 cm−1, and peak centroid does not significantly change with the addition of 18-crown-6 ligand to form NpO2+/18-crown-6 ratio of 2:1, 1:1, 1:2, and 1:4. More significant variations in peak intensities can be observed in the spectral window between 840 and 880 cm−1. At a Np(V)/18-crown-6 ratio of 2:1, a peak located at 877 cm−1 appears in the spectrum. With the addition of more ligand, a I


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Inorganic Chemistry

and 740 cm−1, which indicates that neptunyl−neptunyl interaction is not present under these conditions. This may be due to lower concentrations of Np(V) that do not favor the formation of these interactions. Unfortunately, the amount of solution needed to collect the Raman spectra prevented the level of evaporation needed to form the neptunyl−neptunyl interactions in this system. Similar experiments were performed on Fe3+ and Ga3+ systems (Figures S21 and S22), but the spectral signals were not significantly different than those observed for the mono- and divalent cations.

Sr2+ atoms is ideal for encapsulation within the crown ether ligand; thus, they compete with the Np(V)O2+ cation.37 Variation in the intensity likely results from differences in the stability constants, where Sr2+ forms a more stable complex in aqueous solution and variations in Raman scattering for different complexes.75,76 The shoulder at 876 cm−1 in the solutions containing Na+ and Al3+ indicates that the Np(V)O2+ inclusion complex is present and is likely favored due to the size mismatch that occurs between the crown ether molecule and the smaller Na+ and Al3+ cations. There is no observable change to the major band at 767 cm−1 in the presence of these cations. This observation is not totally unexpected, as neptunyl−cation interactions are most favored with higher-valence cations.21 Freiderich et al.21 also demonstrated that the interaction strength was much weaker in the case of Al3+ compared to In3+ and Fe3+ cations. The solution-phase Raman spectroscopy in the presence of In3+ was also explored, as it is expected to exhibit more significant neptunyl−cation interactions21 and was also present in the formation of the Np−Np compound (Figure 7). Initially,

■

CONCLUSIONS We have explored the neptunyl 18-crown-6 system to investigate the impacts of intermolecular cation−neptunyl and neptunyl−neptunyl interactions on the neptunyl (NpO) bond. From this work, we have crystallized isolated complexes (Np1a and Np1b) and those with additional interactions to the neptunyl oxo group (Na−Np and Np−Np). This includes the characterization of the first isolated molecular complex to display a single T-shaped neptunyl−neptunyl interaction. The Raman spectroscopy revealed a stronger neptunyl bond for the isolated [Np(V)O2(18-crown-6)]+ complexes compared to the penta-aqua complex. This is corroborated by the bond distances obtained by single-crystal X-ray diffraction and is the result of the ligand engaging in ionic bonding with the NpO2+ cation. When the sodium cation participates in a neptunyl−cation interaction, the solid-state Raman band is further blue-shifted, suggesting additional strengthening from the bond. Formation of a neptunyl−neptunyl interaction resulted in a significant red shift in the spectrum (∼40 cm−1) and an asymmetric elongation of the neptunyl cation engaged in the intermolecular interaction. Solution-phase Raman data suggested that the [Np(V)O2(18-crown-6)]+ inclusion complex forms in solution and is not significantly impacted by the presence of mono- and divalent cations. In the case of In3+, the subtle blue shift of the major band associated with the ν1 symmetric stretch may indicate a cation−neptunyl interaction in this system. These studies represent the initial investigations into the impact of intermolecular interactions with the neptunyl oxo groups. Isolation and characterization of diamond-shaped neptunyl−neptunyl interactions and further explorations of different ligand systems are also necessary to further understand the impacts of geometrical arrangements and σ-donation on the neptunyl bond. Additional studies are also necessary to further explore the complex Raman signals generated in these systems and to provide a chemical signature of these species in aqueous systems.

Figure 7. Overlayed Raman spectra of the Np(V)O2+−In3+−crown ether solution that underwent slow evaporation for 50 d.

the spectrum was similar to those collected with the mono- and divalent cations; a major band at 861 cm−1 is associated with the free crown molecule and the shoulder at 874 cm−1 corresponds to the inclusion complex. The solution was allowed to slowly evaporate over the course of 50 d, until ∼50% of the original volume remained in the reaction vial. Intensity of the bands at 874 and 861 cm−1 varies with evaporation, which suggests that these species are not at equilibrium. In addition, the band at 832 cm−1 increases significantly, which could be due to conformational changes in the crown ether molecule or the oxidation of the Np(V)O2+ to Np(VI)O22+. The band at 767 cm−1 broadens and begins to form a subtle shoulder centered at 775 cm−1. This could signify an interaction between the In3+ and the neptunyl inclusion complex, as the molecular species in the Na−Np also exhibited a subtle blue shift. Spectral bands are not observed between 720

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00488. Select bond distances and angles, structural images and Raman spectra (PDF) Accession Codes

CCDC 1824932−1824935 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The J


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AUTHOR INFORMATION

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*E-mail: [email protected]. ORCID

Tori Z. Forbes: 0000-0002-5234-8127 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge the Department of Energy Early Career Award (DE-SC0013980) for supporting this work. REFERENCES

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