Ion association with tetra-n-alkylammonium cations stabilizes higher-oxidation-state neptunium dioxocations

Extended-coordination sphere interactions between dissolved metals and other ions, including electrolyte cations, are not known to perturb the electrochemical behavior of metal cations in water. Herein, we report the stabilization of higher-oxidation-state Np dioxocations in aqueous chloride solutions by hydrophobic tetra-n-alkylammonium (TAA+) cations—an effect not exerted by fully hydrated Li+ cations under similar conditions. Experimental and molecular dynamics simulation results indicate that TAA+ cations not only drive enhanced coordination of anionic Cl– ligands to NpV/VI but also associate with the resulting Np complexes via non-covalent interactions, which together decrease the electrode potential of the NpVI/NpV couple by up to 220 mV (ΔΔG = −22.2 kJ mol−1). Understanding the solvation-dependent interplay between electrolyte cations and metal–oxo species opens an avenue for controlling the formation and redox properties of metal complexes in solution. It also provides valuable mechanistic insights into actinide separation processes that widely use quaternary ammonium cations as extractants or in room temperature ionic liquids.


Supplementary Methods.
Crystal Structure Solution and Refinement. For all structures, assignments of most non-hydrogen atoms were straightforward, based on their electron densities and coordination environments.
In the structure refinement of 1, substantial residual electron densities (~ 4 e·Å −3 ) were located in the difference Fourier maps mostly at < 1.0 Å from the closest Np positions. Attempts to include minor twin components or increase the absorption coefficient did not improve the refinement results. Cl (4) and Cl (5) atoms were found to be disordered with Ow (7) and Ow(8) (w = H2O), respectively, which are located at ~ 0.6 Å from the closest Cl positions. The constrained refinement with a unity occupancy for each set of two positions led to occupancies of 0.60(2)/0.40 (2) for Cl(4)/Ow (7) and 0.42(2)/0.58 (2) for Cl(5)/Ow (8). The refinement statistics did not change when the occupancies of Cl(4)/Ow (7) and Cl(5)/Ow (8) were fixed at 0.6/0.4 and 0.4/0.6 in the final refinement, so that charge balance of NpO2 + and Cl − ions is achieved. No supercell reflections were observed as demonstrated in the synthesized precession images (Supplementary Figure 8). Therefore, the distribution of Cl(4)/Ow (7) and Cl(5)/Ow (8) atoms within the structure of 1 is random. H atoms could not be identified from the difference Fourier maps, so they were not included in the structural refinements of 1. In the structure refinement of 2, the H atoms of methyl groups were identified in the difference Fourier maps and their positional and isotropic displacement parameters were refined without any constraints. The largest residual density (1.866 e·Å −3 ) in the difference Fourier maps is located at ~3.32 Å and ~3.28 Å from the closest chloride ligand and methyl carbon. Further examination of all residual electron density peaks suggests that the structure may include minor disorder, in which the O(1) and the Cl(1) atoms are exchanged. The largest density peak may then represent an alternative position for the [NMe4] + cation. However, this disorder could not be modeled with confidence, as anisotropic thermal parameters for the exchanged O(1) and Cl(1) atoms became non-positive definite and the alternative position of the [NMe4] + cation was unstable. Alternatively, the largest residual density peak may represent a solvent water bound to Cl − and [NMe4] + ions via hydrogen bonds; however its partial occupancy is too low to refine confidently. Similar behavior has been observed in the structure refinements of isostructural U and Pu compounds. 1 In the structure refinement of 3, two non-merohedral twin components were included in the final refinement, with the second twin component rotated from the first by 179.6°. The two twin domains are related by the twin law [-1.000 0.000 -0.003 -0.563 1.000 -0.482 0.012 0.002 -1.001]. All H atoms bonded to Ow and methyl C were located in the difference Fourier maps. H positions for the water molecules were refined using direct O−H and H−H distance restraints of 0.83 Å and 1.40 Å, respectively. H atoms coordinated to C were placed in idealized positions. Isotropic displacement parameters of hydrogen atoms were constrained at 1.20 times or 1.5 times the Ueq value of the Ow and C to which the hydrogen bonds.
The final refinement included anisotropic displacement parameters for all non-hydrogen atoms. The program STRUCTURE TIDY 2 was used to standardize the positional parameters. Additional experimental details are given in Supplementary Table 2 and in the CIFs.
All-Atom Explicit-Solvent Molecular Dynamics Simulations. All-atom MD simulations were performed using the package GROMACS (5.0.7) 3 using force field parameters for NpO2 2+ and NpO2 + as reported by Pomogaev et al. 4 using the SPC/E water model. The structures of the SPC/E water model were constrained using the SETTLE algorithm. 5 The force field parameters of H3O + were recently reported by van Keulen et al. 6 The force field parameters of all the other molecules are from the original CHARMM force field. 7,8 In the present work, two neptunyl ions were investigated, NpO2 2+ and NpO2 + , each in 5M LiCl and 5M NMe4Cl. All the simulations were performed with an H3O + concentration of 0.01 M, corresponding to the experimental condition of pH = 2. See Supplementary Table 6 for the compositions. The initial structures were built using the package Packmol. 9 The energy of the initial structure was first minimized using the steepest descent algorithm. An equilibration of 10 ps using the NTV ensemble (constant number of particles, temperature and volume) was followed by further equilibration of 0.1 ns using the NTP ensemble (constant number of particles, temperature and pressure). The production simulations were subsequently performed. Neighbor searching was done up to a cutoff distance of 1.2 nm. The short-range Coulomb interactions were calculated up to 1.2 nm with the smooth particle mesh Ewald (PME) method for the long-range electrostatic interactions with a grid real spacing of 0.12 nm and six order interpolation. 10,11 The Lennard-Jones 12-6 potential was switched off from 1.0 nm to 1.2 nm with the force switching method. No long-range dispersion corrections were applied for the energy and pressure. These parameters have been recommended for the accurate reproduction of the original CHARMM simulations on lipid systems. 12 The simulation integration time step of 2 fs was employed with all the hydrogen-involved covalent bond lengths constrained using the LINCS algorithm. 13 Each of the production simulations was performed for a duration of 50 ns using a saving frequency of 10 ps for data collection. The first 2 ns simulation trajectory was discarded for the final analysis, as suggested by the calculations in the convergence of the system densities and potential energies.

Supplementary Discussion.
Cyclic Voltammetry. Cyclic voltammograms collected for 5 mM Np V dissolved in 1 M and 5 M LiCl reveal increasing anodic and cathodic peak separations (ΔEp) with increasing scan rate (ν) (Supplementary Figure  2). Peak current ratios (ip,c/ip,a) in both the 1 M and 5 M LiCl solutions are less than one at all ν and decrease gradually with increasing ν. Together, these results indicate that the Np VI /Np V redox reaction in 1 M and 5 M LiCl is quasi-reversible and controlled by both charge-transfer and mass-transport. The DPV data (Supplementary Figure 2), which exhibit nearly symmetrical Gaussian peaks, confirm this finding. Furthermore, the Np VI /Np V electrode potentials, determined as the average half-wave potential (E1/2) over all scan rates, in 1 M and 5 M LiCl (Supplementary Table 1 Table 1). In fact, in 1 M NMe4Cl, the Np VI /Np V electrode potential is shifted cathodically by approximately 30 mV, indicating a moderate stabilization of Np VI compared to Np V . When the NMe4Cl concentration is increased to 5 M, the Np VI /Np V redox behavior changes even further (Supplementary Figure 4). For the 5 M NMe4Cl system, the Np VI /Np V redox behavior is complicated both kinetically, as indicated by the increasing ΔEp with increasing ν, and chemically, as indicated by the increasing ip,c/ip,a with increasing ν. Most importantly, the Np VI /Np V electrode potential measured in 5 M NMe4Cl is cathodically shifted by more than 200 mV compared with the electrode potentials measured in the Np V /LiCl systems (Supplementary Table 1). The DPV data further confirm the quasi-reversibility and potential shifts for the Np VI /Np V couple in the 1 M and 5 M NMe4Cl systems (Supplementary Figure 4). As [NMe4] + concentrations increase, from 1 M to 3 M to 5 M, the Np VI /Np V electrode potential shifts proportionately to more cathodic potentials. This effect is observed regardless of whether ionic strength is maintained (using LiCl) across the concentration range (Supplementary Table 1). Similar behavior, namely a cathodic shift in the Np VI /Np V electrode potential, is also observed in CV data for Np V dissolved in 3 M NEt4Cl (NEt4Cl concentration is limited by solubility) (Supplementary Figure 5).
Vis-NIR Spectroscopy. The electronic spectrum of the uncomplexed, hydrated Np V O2 + cation exhibits an intense 5f → 5f transition at 980.2 nm. 14 Changes in the symmetry of the Np V O2 + unit, either through coordination to the yl-O atom or through coordination in the equatorial plane, can significantly alter both the intensity (molar absorptivity) and energy (wavelength) of this transition. For example, the formation of CCI complexes with the uranyl, UO2 2+ , cation 15,16 or the formation of [O=Np V =O···M n+ ] complexes (M n+ = highly-charged metal cation) 17 causes a decrease in the intensity and energy (red-shift) of the Np V 980.2 nm absorption band. Replacement of water in the equatorial plane of the Np V O2 + cation with various ligands also typically causes a red-shift of the 980.2 nm band. However, if complexation in the equatorial plane creates inversion symmetry about the Np center, the 5f → 5f transition becomes Laporte forbidden 14 and the complex will be optically silent in the Vis-NIR spectral range. Furthermore, calculated absorption spectra for several [Np V O2(Cl)x] 1-x (x = 0, 1, 3, 5) complexes support that the intensity of the Np V 5f → 5f transition is greatest for complexes containing five equatorial ligands. 14 With these considerations, the absorption spectra for Np V dissolved in 1 M , 3 M, and 5 M NMe4Cl (Supplementary Figure 6) compared with that of the 0.1 M Np V stock solution reveal that the coordination environment around the Np V O2 + cation changes with increasing concentrations of NMe4Cl, as evidenced by the small, but significant, blueshift of the 980.2 nm band. Although the presence of optically silent Np V O2 + complexes cannot be excluded, continued observation of intense 5f → 5f transitions for Np V /NMe4Cl solutions further suggests that any new complexes formed do not possess inversion symmetry and likely retain five coordinating ligands in the Np V O2 + equatorial plane. Despite the significant decrease in the measured Np VI /Np V electrode potential in 5 M NMe4Cl, the corresponding absorption spectrum provides no evidence for the presence of Np in other oxidation states, not even as Np VI . This indicates that although Np VI is more stable in 5 M NMe4Cl solutions than in, for example, 1 M HClO4, the induced free-energy change (ΔΔG = -22.2 kJ mol -1 (see Supplementary Table 8) is not sufficient under these dilute concentrations to promote Np V oxidation to Np VI . However, increasing Np V concentrations during evaporative syntheses in the presence of O2 provide a sufficient thermodynamic driving force to promote oxidation to Np VI . (H2O)7 (1) crystallizes in the monoclinic space group C2/c. As shown in Supplementary Figure 9, the structure of 1 contains four crystallographically unique nearly linear NpO2 + cations, each of which is coordinated by five ligands in the equatorial plane in a pentagonal bipyramidal geometry. More specifically, each NpO2 + cation is coordinated by two Oyl (yl = actinyl), zero to three Ow, and zero to three Cl atoms. The Np−Oyl distances range from 1.837(5) Å to 1.853(5) Å and Oyl−Np−Oyl angles range from 176.4(2) and 179.1(2)º (Supplementary Table 3 (3) to 4.2612(2) Å) found within "cationic square nets" of NpO2 + cations in the structure of (NpO2)Cl(H2O)2. 18 In comparison, each Np(3)O2 + cation in the structure of 1 is further connected to one Np(4)O2 + cation through a bridging Cl(3) − anion in addition to the CCI. The Np−Np distances within these edge-sharing dimers of neptunyl(V) pentagonal bipyramids are 3.8589(4) Å, which are close to the value (3.885 (1) Å) found between two Np centers bridged by one Oyl and one Cl atoms in the structure of (NpO2)Cl(H2O). 19 The chemical formula of 1 can be rewritten as (NpO2)Cl(H2O)7/4, which is close to previously reported (NpO2)Cl(H2O)2. 19 In fact, the structures of these two compounds are also closely related. Both structures adopt a 3-D CCI network of neptunyl(V) cations with open channels filled by chloride anions and water molecules, where each NpO2 + unit involves four CCIs with neighboring units in a similar square geometry. The main difference between the two structures lies in the bonding at the equatorial plane of NpO2 + cations. Compared to 1, neptunyl(V) pentagonal bipyramids in the dihydrate consist of more H2O molecules and less Cl − anions on average, consistent with the higher water content per Np in the formula. More specifically, half of NpO2 + cations in the dihydrate structure are coordinated by three Ow and two Oyl atoms and the other half are coordinated by one Ow, two Cl, and two Oyl atoms. Furthermore, all chloride anions in the dihydrate are terminal, as such the neptunyl units in the dihydrate are only connected through CCIs. In contrast, some of chloride anions (Cl(3) in Supplementary Figure 9) in 1 are bridging between two Np V cations, providing additional equatorial connection between neighboring NpO2 + units.  [NMe4]Cl[NpO2Cl(H2O)4] (3) crystallizes in the triclinic space group P1 � . The structure of 1 consists of one crystallographically unique NpO2 + cation, two Cl − anions, four water molecules, and one [NMe4] + cation (Supplementary Figure 12). Cation NpO2 + is coordinated by one Cl(2) − anion and four water molecules in the equatorial plane in a pentagonal bipyramidal geometry to form an unprecedented hydrated actinyl mono-chloride complex. Among four coordinating water molecules, three of them (1, 2, 4) have hydrogens sit above and below the equatorial plane, whereas those of the H2O (3)  Raman Spectroscopy. Neptunyl (NpO2 n+ ) cations exhibit three fundamental Np-Oyl vibrational modes, including ν1 (symmetric stretching, Raman active), ν2 (doubly degenerate bending, infrared active), and ν3 (asymmetric stretching, infrared active). These vibrational modes are sensitive to the neptunyl coordination environment, which includes the inner-sphere coordination of ligands in the equatorial plane or the outersphere coordination of other species to the yl-O atoms. In particular, the formation of CCI complexes, in which the yl-O atom of one neptunyl cation is coordinated within the inner-sphere equatorial plane of a second neptunyl cation, can make both neptunyl stretching modes Raman active and can shift the stretching bands toward lower frequencies by changing the site symmetry of the neptunyl units and Np−Oyl interactions. 25,26 The ν1 and ν3 modes of NpO2 + units in solutions and solids have been observed in the region of 630−770 cm -1 and 770−850 cm -1 , respectively. [25][26][27][28][29][30] In comparison, the ν1 of NpO2 2+ units have been observed in the region of 802−863 cm −1 in several Raman spectra of neptunyl(VI) aqueous solutions and solid compounds without CCIs. 28,30,31 The bending bands of neptunyl cations are less certain due to their much lower intensity and potential overlap with ligand modes.
Raman spectra of 1−3 and select green amorphous products (X) from Np V /LiCl reactions in the region of 600−900 cm −1 are presented in Supplementary Figure 13. At first glance, the Raman spectra of 1 and X are more complex than those of 2 and 3 that lack CCIs as expected. Most of the vibrational bands of 1 and X in the regions of neptunyl stretching modes are comparable to those observed for green products from evaporating acidic Np V chloride solutions including (NpO2)Cl(H2O)2, mixed-valent NaxNp IV (Np V O2)6(OH)1+xCl9(H2O)8-x (0<x≤1) and other unidentified green phases. 18,26 More specifically, Raman bands of 1 located at 672, 708 (vw = very weak), 744 (vw), 806 (w = weak), and 847 (w) cm -1 match with most of bands for (NpO2)Cl(H2O)2 (675, 800 (w), and 846 (w) cm -1 ) and other unidentified green products (671/672, 744 (vw), 805 (w), 825 (w), and 846 (w) cm -1 ) from slow evaporation reactions of Np V /HCl. 26 This is consistent with X-ray structural analyses, which reveal comparable Np−ON distances and similar CCI connectivities for 1 and (NpO2)Cl(H2O)2. 26,32 Raman spectra of X depend on the measured samples, which suggests the formation of a mixture of Np containing products. Most of Raman bands of X resemble those of green products from rapid evaporation reactions of Np V /HCl. 26 For example, broad bands in the 680−730 cm −1 and 780−820 cm −1 regions are similar to those of NaxNp IV (Np V O2)6(OH)1+xCl9(H2O)8-x (0<x≤1). 26 The strong vibration band located at 633 cm −1 is close to the lowest reported frequency (635 cm −1 ) of the ν1 modes of NpO2 + units observed for NaNpO2(OH)2. 33 A similar strong band located at 624 cm −1 has been observed for some of green products from rapid evaporation reactions of Np V /HCl. 26 The intense peak at 797 cm −1 in the spectrum of 2 and 741 cm −1 in the spectrum of 3 can be assigned to the ν1 mode of O=Np=O moieties, whereas the weak bands at 754 cm −1 in the spectrum of 2 and 757 cm −1 in the spectrum of 3 can be assigned to the ν1 mode of NMe4 + units. The ν1 frequency of neptunyl(VI) cations in compound 2 is comparable to the value observed for Cs2[NpO2Cl4] (802 cm −1 ) and [NBu4]2[NpO2Cl4] (800 cm −1 ). 20,21 The ν1 band of neptunyl(V) cations in compound 3 shifts to a lower frequency compared to typical values observed for those that do not involve in CCIs and is close to those for CCI dimers in solution (738 cm −1 ). For example, the ν1 band of discrete NpO2 + cations has been observed at ca. 773 cm -1 in the Raman spectra of Na3(NpO2)(SeO4)2(H2O). 32 This is consistent with the trend of Np−Oyl distances observed in the structures of 3, Na3(NpO2)(SeO4)2(H2O), and Np(V) CCI compounds.   (6) 11.0308 (7) c, Å 23.2897 (9) 11.3998 (7) 11.1054 (7