Influence of Countercation Hydration Enthalpies on the Formation of

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Influence of Counter-Cation Hydration Enthalpies on the Formation of Molecular Complexes: A Thorium-Nitrate Example Geng Bang Jin, Jian Lin, Shanna L. Estes, Suntharalingam Skanthakumar, and Lynda Soderholm J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09363 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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Journal of the American Chemical Society

Influence of Counter-Cation Hydration Enthalpies on the Formation of Molecular Complexes: A Thorium-Nitrate Example Geng Bang Jin,* Jian Lin, Shanna L. Estes, S. Skanthakumar and L. Soderholm* Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, United States

ABSTRACT: The influence of counter-cations (An+) in directing the composition of monomeric metal-ligand (ML) complexes that precipitate from solution are often overlooked despite the wide usage of An+ in materials synthesis. Herein we describe a correlation between the composition of ML complexes and A+ hydration enthalpies found for two related series of Th-nitrate molecular compounds obtained by evaporating acidic aqueous Th-nitrate solutions in the presence of A+ counter-ions. Analyses of their chemical composition and solid-state structures demonstrate that A+ not only affects the overall solid-state packing of the Th-nitrato complexes but also influences the composition of the Th-nitrato monomeric anions themselves. Trends in composition and structure are found to correlate with A+ hydration enthalpies, such that the A+ with smaller hydration enthalpies associate with less hydrated and more anionic Th-nitrato complexes. This perspective, broader than the general assumption of size and charge as the dominant influence of An+, opens a new avenue for the design and synthesis of targeted metal-ligand complexes.

INTRODUCTION Studies of the formation mechanisms of metal-ligand (ML, L = anions or neutral ligands) coordination compounds from solution have generally focused on the competition between ligand and solvent molecules for metal coordination.1 Other interactions, including those between metal complexes and other solutes, notably counter-ions, are generally less understood but are expected to contribute relatively little to the overall free-energy minimization that drives reaction outcomes.2,3 As such, counter-ions are widely used in ML complexation studies as relatively innocent charge-balancing species, which are added to provide control of ionic strength,4 metal-complex solubility,5 and the packing of ML moieties in the solid.6-10 Counter-ions are also often used to template the assembly of targeted oligomeric ML clusters,7,10 supramolecular entities,6 and extended ML networks11 from monomeric, molecular ML building units. In these roles, counter-ions are selected based on matching ionic charges and radii, geometry, and electrostatic influences, specifically hydrogen bonding. Any direct counter-ion influence on the composition of the monomeric ML unit itself is not considered, even though counter-ions readily form ion pairs and affect solvent structure.12-14 Herein, we demonstrate that the choice of counterion, in addition to influencing the packing of the molecular ions, also influences the specific constituents that comprise the basic ML moiety. Furthermore, we show that this influence on ML composition correlates with the counter-ion hydration enthalpy. To understand the interactions between metal complexes and counter-ions present in solution and how these interactions affect both the constituents and structures of metal complex precipitates, we systematically investigated the solids crystallized from acidic Th-nitrate solutions, which differed only in counter-ion selection. Thorium-nitrate solutions containing

monovalent counter-cations provide an ideal system for this purpose. First, the large ThIV ion, which is redox inactive, exhibits flexible coordination to a wide range of ligands with coordination numbers of 4 to 15.15,16 Second, as the softest of the tetravalent metal cations, ThIV is least likely to undergo hydrolysis in moderately acidic solutions, decreasing the likelihood of observing competing formation of Th-hydroxo and oligomeric complexes.17 Third, previous research finds that the small NO3– oxoanion coordinates tetravalent metal cations (MIV) solely in a bidentate fashion, yielding MIV complexes with up to six coordinated bidentate NO3– ligands in both solutions and solids.18-29 This prevailing terminal bidentate bonding mode prevents NO3 from functioning as a bridging ligand between MIV centers. As a result, ThIV nitrates have only been observed as simple monomeric molecular [Th(NO3)x(H2O)y](x-4)− (x=0-6, y=0-10) complexes in acidic aqueous solutions and in solids. 22,23,30-32 In the present work, 14 A+/Th4+/NO3− (A+ = Li+, Na+, K+, Rb+, Cs+, NH4+, or NR4+ (R = Me, Et, Bu)) compounds, including 11 new phases, were crystallized by evaporation from acidic Thnitrate solutions under comparable conditions (Table 1).22,30,31 Two series of inorganic or organic counter-cations, with the same charge, were specifically chosen to assess the role of ionic radius, electrostatics, notably H-bonding, and hydration enthalpy on the formation and speciation of Th-nitrato complexes from aqueous solution. To augment this study, the influence of the A/Th molar ratio and the HNO3 concentration in the initial solutions are also reported. RESULTS AND DISCUSSION Samples were characterized for structure and composition using single-crystal X-ray diffraction and for phase purity using powder X-ray diffraction measurements. Each of the prepared compounds is found to contain one of two discrete molecular Th-nitrato anionic complexes, either [Th(NO3)5(H2O)2]− or

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Figure 1. Monomeric Th-nitrato complexes, isolated in solids by evaporating aqueous solutions, shown as a function of countercation hydration enthalpies. A second [Th(NO3)4(H2O)4] complex, formed by the evaporation of Th-nitrate solutions containing relatively high HNO3 concentrations, follows the same trend and is omitted from the figure for clarity. [Th(NO3)6]2−. ThIV in the [Th(NO3)5(H2O)2]− singly-charged anionic complex is coordinated by 12 O atoms from 5 bidentate NO3– ions and 2 H2O molecules, with a geometry that is best described as intermediate between a bicapped pentagonal prism and a bicapped pentagonal antiprism. In comparison, the ThIV in the [Th(NO3)6]2− dianion is coordinated by 12 O atoms from six bidentate NO3− ions, forming an icosahedral coordination geometry. The compounds formed from either of these pentanitrato or hexanitrato moieties are linked in the solid, via diverse connectivities with counter-ions (A+), H2O, and neutral HNO3 or anionic NO3−. Comparing all 18 compounds, including four previously reported phases, we observed that the presence of counter-cations (SI, Table S3),22,30,31,33 has a major impact on which of the Th-nitrato complexes is present in the precipitating solid. Depending on the counter-cation present in solution, four different monomeric Th-nitrato complexes have been isolated (Figure 1). Neutral [Th(NO3)4(H2O)3] and [Th(NO3)4(H2O)4] complexes form by the evaporation of Th-nitric acid solutions containing only protons as counter-ions.33 The presence in solution of Li+ or Na+ promotes crystallization of [Th(NO3)5(H2O)2]− complexes, whereas the presence of much larger Rb+, Cs+, or NR4+ (R = Me, Et, Pr, Bu) promotes crystallization of [Th(NO3)6]2− complexes.30 Interestingly, both anionic complexes can be found in compounds made from solutions containing either K+ or NH4+.22 It is noted that a range of Th-nitrato complexes including tetranitrato and hexanitrato complexes have been observed in solution, whereas the existence of Th-pentanitrato complexes in solution has not been established to date.23,32

(1.67 Å),34 but it behaves more like K+, forming compounds containing either penta- or hexanitrato ThIV complexes. Furthermore, reactions with the smallest cation, the proton, promote the formation of neutral tetranitrato, instead of pentanitrato, ThIV complexes as in the cases of Li+ or Na+ cations.33 Closer examination of counter-cation solvation properties shows a more consistent correlation between the composition of ML complexes and A+ hydration enthalpies than A+ ionic radii (Figure 1). Although hydration-enthalpies are highly correlated with ionic sizes, this correlation does not always hold.35 For example, monoatomic ions with the same charge, such as alkali-metal cations, exhibit a direct relationship between hydration enthalpy and ionic radii.35,36 In contrast, for polyatomic ions such as NH4+ and NR4+, energy variation related to the mutual structural and vibrational changes induced by hydrogen bonding can lead to substantial deviation from the above trend.35 In particular, the hydration enthalpies of NR4+ do not change monotonically, even as a function of the size of alkyl chains, possibly owing to variation in their manifestation of hydrophobicity.37 Employing hydration enthalpies, instead of the commonly used ionic radii, as a judge of the counter-ion's role in compound formation, eliminates all the exceptions discussed above. In addition, it sheds further light on the structure-directing role of the counter-ion.

The correlation between A+ and associated Th-nitrato anions initially appears to be dependent on the A+ ionic radii, however there are exceptions. For example, NH4+ has a six-coordinate radius of 1.61 Å, between that of Rb+ (1.52 Å) and Cs+

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Figure 2. Selected coordination environments for alkali-metal cations. a) Li+(2) in the structure of Li[Th(NO3)5(H2O)2]·4.5H2O (1); b) K+(2) in the structure of K[Th(NO3)5(H2O)2]·5.5H2O (4); c) Cs+ in the structure of Cs2[Th(NO3)6] (9). How does the counter-cation hydration enthalpy influence the observed preferential selection of anionic Th-nitrato solid complexes from solution? To answer this question, we examined the detailed electrostatic interactions between counterions, Th-nitrato complexes, and hydration waters present in the compounds containing A+. For the alkali-metal cations, the hydration enthalpies decrease in the order: Li+ (−530 kJ/mol) > Na+ (−415 kJ/mol) > K+(−330 kJ/mol) > Rb+ (−305 kJ/mol) > Cs+ (−280 kJ/mol).35 Because of this variation in affinity to H2O, there are significant differences in the coordination environments of the alkali metals in the Th-nitrate compounds studied here (Figure 2). More specifically, at least half of the coordinated O atoms for the Li+ and Na+ compounds (1−3) are from H2O molecules, and the rest are from Th-coordinated NO3. In contrast, for Rb+ (8) and Cs+ (9) compounds, all coordinated O atoms are from NO3 in the Th complexes. The coordination behavior of K+ cations is the most complex and intermediate between the first and last two cations in the series, where the K+ coordination shell contains from 50% OW and 50% ON, as for K(1) in K[Th(NO3)5(H2O)2]·5.5H2O (4), to 0% OW and 100% ON, as in β-K2[Th(NO3)6] (6).

4.4218(8) Å. Considering the similar compositions in 1-4 and that K+ is larger and is surrounded by more anions in 4 than Li+ and Na+ in 1-3, it is surprising that the Th···K+ distance is actually much shorter than the corresponding Th···Na+ distance found in 3 (4.8778(9) Å), but is comparable to the Th···Li+ distance found in 1 (4.466(5) Å) and 2 (4.381(4) Å). These results demonstrate enhancement of ionic interactions between the less hydrated K+ cation and the [Th(NO3)5(H2O)2]− anion and are supported within the context of Brown’s bond-valence model,38 which is used as an estimate of the overall bonding strength of ionic interactions between A+ cations and molecular Th-nitrato anions. For example, the bond valence sum of A+−O bonds between a Th-nitrato complex and its surrounding A+ cations within Li+2···[Th(2)(NO3)5(H2O)2]− pairs in 1, K+4···[Th(1)(NO3)5(H2O)2]− pairs in 4, and Cs+12···[Th(NO3)6]2− pairs in 9 are 0.31, 0.39, and 0.89 respectively (Figure 3).

Figure 4. Selected coordination environments for NH4+ or NR4+ cations. H-bonding is shown with dashed lines. a) NH4+(1) cations in the structure of [NH4][Th(NO3)5(H2O)2]·3H2O (10); b) NMe4+ cations in the structure of [NMe4]2[Th(NO3)6] (11). Figure 3. Selected coordination environments for Th-nitrato complexes by alkali metal cations. a) [Th(2)(NO3)5(H2O)2]− complex in the structure of Li[Th(NO3)5(H2O)2]·4.5H2O (1); b) [Th(1)(NO3)5(H2O)2]− complex in the structure of K[Th(NO3)5(H2O)2]·5.5H2O (4); c) [Th(NO3)6]2− complex in the structure of Cs2[Th(NO3)6] (9). Completely naked (no coordinating water) Rb+ and Cs+ cations engage in more ionic interactions with Th-nitrato complexes than do the smaller alkali metals, which retain partial hydration shells in the Th-nitrate solid compounds. Ionic interactions between less-hydrated larger alkali-metal cations and ThIV hexanitrato dianions are stronger than the interactions between more hydrated smaller cations and pentanitrato ThIV anions in the structures studied. This trend is well demonstrated in the linkage between A+ and anionic ML complexes (Figure 2 and 3). More specifically, the Th···A+ distances between less-hydrated alkali metal and non-hydrated hexanitrato ThIV ions are shorter than those between more-hydrated cation-anion pairs in comparable structures, despite the larger ionic radii of A+ and greater number of neighboring ions in the former cases (SI, Table S4 and Crystallographic Studies). For example, the closest distance between K(2) and [Th(2)(NO3)5(H2O)2]− in K[Th(NO3)5(H2O)2]·5.5H2O (4) is

Compared to the alkali metal cations, the ammonium cations are more hydrophobic, particularly the NR4+ cations as demonstrated in the values of their hydration enthalpies: NH4+ (−325 kJ/mol) > NMe4+ (−203 kJ/mol) ≈ NEt4+ (−219 kJ/mol) ≈ NPr4+ (−206 kJ/mol) > NBu4+ (−182 kJ/mol).35,37 Similar to Rb+ and Cs+, smaller hydration enthalpies of NR4+ cations appear to dictate reaction outcomes, where non-hydrated NR4+ cations only associate with non-hydrated hexanitrato anions in the resulting solid. Hydrogen-bonding between cationanion pairs varies significantly as a function of carbon chain length in NR4+ cations, but does not affect the formation of the associated Th-nitrato complexes. Instead, the varied H-bonding only changes the overall packing of these Th-nitrato complexes in the solids. A similar conclusion can be reached for reactions with NH4+ cations. The hydration enthalpy of NH4+ is almost the same as that of K+, which explains their similar behavior in the Th-nitrate solids. Like K+ cations, NH4+ are found to be either hydrated or non-hydrated and associated with either hydrated penta- or non-hydrated hexanitrato ThIV anions, respectively, despite additional H-bonding between cation-anion pairs in the latter cases. More specifically, each NH4+ is coordinated via H-bonds to two H2O and two or three NO3– molecules in [NH4][Th(NO3)5(H2O)2]·3H2O (10) (Figure 4). In contrast, the NR4+ cations connect to only NO3– via H-

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bonding. The nude counter-cations in A2[Th(NO3)6] (A = NH4, NMe4, NEt4, NPr4, or NBu4) more actively participate in bonding interactions with the Th-nitrato complexes than NH4+ does in 10.22 Each [Th(NO3)6]2− attracts twelve NH4+ or six to twelve NR4+ cations, whereas each [Th(NO3)5(H2O)2]− anion in 10 only connects to two or three NH4+ cations (Figure 5). In addition, the closest distances between the two associated cation-anion pairs in the structure of [NH4]2[Th(NO3)6] (11) (4.723 Å) are substantially shorter than those found in the structure of 10 (5.245(3) Å).22 These results suggest that the non-hydrated NH4+ in 11 has greater ionic interactions with [Th(NO3)6]2− anions than the hydrated NH4+ cation in 10 has with [Th(NO3)5(H2O)2]− anions.

impact on the solid-product constituents was observed, particularly for reactions containing counter-cations with intermediate hydration enthalpies, K+ and NH4+. With more flexibility in coordination to water and the two anionic Th-nitrato complexes, K+ and NH4+ reactions are the most influenced by the solution conditions and have produced the most A+/Th4+/NO3− products among all reactions studied (SI, Bulk Product Characterization). For example, fitting X-ray powder diffraction patterns obtained on bulk precipitates reveals that increasing the A/Th molar ratio in the initial solutions promotes the formation of hexanitrato ThIV dianions, a result only observed in the case of K+ and NH4+. Increasing the HNO3 concentration decreased the number of lattice H2O per Th in Li+ and particularly K+ structures, a finding similar to that seen for the thorium-nitrate reactions with only H+ counter-cations.33 Reactions with higher HNO3 concentrations also promoted the incorporation of HNO3 into K3[Th(NO3)6](NO3)(HNO3)3·3H2O (7). For reactions with the same A+, increasing the HNO3 concentration prolongs crystalformation time; however, there is no observable influence of the HNO3 concentration or evaporation time on which of the monomeric Th-nitrato complexes are observed in the solids for any of the reactions, except those with K+ and NH4+, as discussed in the text.

Figure 5. Selected coordination environments for Th-nitrato complexes by NH4+ or NR4+ cations. a) [Th(2)(NO3)5(H2O)2]− complex in the structure of [NH4][Th(NO3)5(H2O)2]·3H2O (10); b) [Th(NO3)6]2− complex in the structure of [NMe4]2[Th(NO3)6] (12). Hydrogen atoms and H-bonding are omitted for clarity.

All these results suggest a significantly negative impact of ion hydration on the electrostatic association between cation-anion pairs in the solid state. This is reminiscent of the competition between ion hydration and ion association that has been well documented in solution.12,41,42 Therefore, crystallization of the more hydrophobic Rb+, Cs+, and NR4+ cations with hexanitrato Th anions results from stronger ion association (weaker ion hydration) in solution, whereas crystallization of Li+ and Na+ with pentanitrato Th anions results from weaker ion association (stronger ion hydration) in solution. The ion-hydration and ion-association interactions for K+ and NH4+ cations are more comparable in solution, which could explain the observed solution-dependent formation of products with either hexa- or pentanitrato Th complexes. For the proton, ion association with Th-nitrato complexes is too weak compared to proton hydration to have any noticeable influence on the formation of Th-nitrato/water complexes from solution.

The proton is an end-member example of A+, considering its much larger hydration enthalpy (−1091 kJ/mol) compared with the other A+ (−(182–530) kJ/mol). Because of its high affinity to water, the proton is generally regarded as existing in the form of a solvated oxonium ion [H(OH2)n]+ (n = 1 to ≈20). These oxonium ions interact weakly with other ionic species, primarily via hydrogen bonding.39,40 As such, it is not surprising that the proton does not promote the formation of anionic Th-nitrato complexes. In addition, most protons, in the form of HNO3, are lost during the evaporative syntheses, reducing the counter-cations available to charge-balance anionic ThIV complexes and necessitating the formation of neutral [Th(NO3)4(H2O)n] complexes. For reactions containing residual HNO3 towards the end of evaporation, the proton’s influence on the formation of Th-nitrato complexes may be limited to affecting Th hydration solely via H2O-HNO3 H-bonding. This behavior is well demonstrated in the structure of K3[Th(NO3)6](NO3)(HNO3)3·3H2O (7), where incorporated HNO3 forms strong H2O-HNO3 H-bonds but has no direct contact with the [Th(NO3)6]2− anion (SI, Crystallographic Studies). This perspective is further supported by experimental observations that the HNO3 concentration in the starting solutions only affects the number of hydration and solvent H2O but not the number of Th-coordinating nitrate in the solid products obtained from Th(NO3)4/HNO3 solutions containing only H+ counter-cations.33 In addition to the results discussed above, which were obtained using similar initial-solution conditions, studies were also conducted in which the Th/A molar ratio and/or the HNO3 concentration in the initial solutions was varied. Some

CONCLUSIONS In summary, identities of counter-cation-Th-nitrato pairs across the alkali metal cation and NR4+ cation series reported herein demonstrate a well-defined trend of preferential coordination of counter-cations to specific monomeric Th-nitrato complexes in solids, which exhibits a more consistent correlation with counter-cation hydration enthalpies than with their sizes. This phenomenon of preferential stabilization of anionic metal-ligand solid complexes using counter-cations does not appear to be limited to this simple Th-nitrate system. Although scattered evidence supporting a broader base for this phenomenon has been reported, it has not been systematically investigated to reveal the underlying chemical principles. For example, the constituents of CuII-chloride complexes present in solids formed by evaporating ACl-CuCl2-H2O solutions varies as a function of the hydration enthalpy of A+ (A+ = Li+, K+, Rb+, Cs+, NH4+, NMe4+).43-45 Similar to the trend found in the Th-nitrate system, A+ with smaller hydration enthalpies (e.g. Cs+ and NMe4+) are less hydrated and tend to associate with

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non-hydrated anionic [CuCl4]2− complexes in the resulting compounds.44 Our results also show substantially enhanced ionic interactions between non-hydrated counter-cations and ThIV-hexanitrato dianions as the counter-cation hydration enthalpies decrease. These results point out the importance of direct interactions between counter-cations and Th-nitrato complexes and indicate that, as a function of counter-cation hydration enthalpy, these electrostatic interactions can direct the formation of solids with specific anionic Th-nitrato complexes. These findings expand the current understanding of the relative influences of counter-ion size and electrostatic, ion-paring interactions, which are generally expected to influence the overall packing of monomeric ML complexes into oligomers or bulk structures, but not to impact the constituents of the basic ML unit itself. Similarly, it has been suggested that direct interactions between counter-ions and macromolecules play a larger role in the Hofmeister effect than the ability of counter-ions to “make” or “break” bulk water structure.46,47 Overall, our studies suggest that counter-ions can function as more than innocent charge-balancing species – a finding that impacts the quest to design materials with targeted structures.

EXPERIMENTAL SECTION Caution! 232Th is an α-emitting radioisotope, and standard precautions for handling radioactive materials should be followed when working with the quantities used in the syntheses that follow. Table 1. Synthesis details for 1 − 14. A

Li

A/Th Ratio

HNO3 Conc. (M)

Product

1:1

0

Li[Th(NO3)5(H2O)2]·4.5H2O (1)

2:1

1

Li[Th(NO3)5(H2O)2]·4.5H2O (1)

2:1

7.5

Li[Th(NO3)5(H2O)2]·3H2O (2)

1:1

0

2:1

0, 5, 7.5, 10, 15.6

1:1

0

K[Th(NO3)5(H2O)2]·5.5H2O (4)

2:1

0, 5

K[Th(NO3)5(H2O)2] (5)

2:1

0, 1, 15.6

β-K2[Th(NO3)6] (6)

2:1

1, 2.5, 5, 7.5, 10, 15.6 K3[Th(NO3)6](NO3)(HNO3)3·3H2O (7)

1:1

0

Na

Na[Th(NO3)5(H2O)2]·5.72H2O (3)

K

A

A2[Th(NO3)6] (A = Rb (8), Cs(9)) 2:1

0, 5, 10, 15.6

1:1

0

[NH4][Th(NO3)5(H2O)2]·3H2O (10)

2:1

0, 5, 10, 15.6

[NH4][Th(NO3)5(H2O)2]·3H2O (10)

NH4

2:1

0, 5, 10, 15.6

1:1

0

2:1

0, 5, 10, 15.6

[NH4]2[Th(NO3)6] (11)

[NR4]2[Th(NO3)6]

NR4 (R = NMe4 (12), NEt4 (13), NBu4 (14))

Syntheses. 232Th(NO3)4(H2O)3·2H2O (Fisher Scientific, > 99.0%), LiNO3 (Mallinckrodt, > 99.0%), NaNO3 (Sigma-Aldrich, > 99.0%), KNO3 (Fisher Scientific, > 99.0%), RbNO3 (Aldrich, ≥ 99.7%), CsNO3 (Aldrich, 99.99%), NH4NO3 (Mallinckrodt, > 99.0%), [NMe4]NO3 (Aldrich, 96%), [NEt4]NO3 (Acros Organics, 99%), [NBu4]NO3 (Eastman Kodak Co.), and HNO3 (Fisher Scientific, 69.3%) were used as obtained. Thorium-nitrate solutions were prepared by dissolving Th(NO3)4(H2O)3·2H2O in deionized H2O and HNO3, whereas ANO3 (A = Li, Na, K, Rb, Cs, NH4, NMe4, NEt4, and NBu4) solutions were prepared by dissolving ANO3 in deionized H2O. Solutions containing 0.5 mL Th(NO3)4 (0.1 M) with different concentrations of HNO3 (0 − 15.6 M) were mixed with 0.1 mL ANO3 (1 M or 2 M) in 8 mL shell glass vials (Table 1). After combining reactants, the vials were covered by Parafilm with pinholes to permit gradual evaporation of the solutions under ambient laboratory conditions. Colorless crystals of 14 counter-cation containing Thnitrate compounds formed after a few days to a few months of evaporation, depending on the choice of A+ and particularly the concentrations of HNO3. In general, NR4+-containing crystals formed faster than crystals containing alkali metal cations. For reactions with the same counter-cation, the evaporation time, taken as the total time required for crystal formation, typically increased with increasing concentrations of HNO3. Crystallographic Studies. Single-crystal X-ray diffraction data for all compounds were collected using graphite-monochromatized MoKα radiation (λ = 0.71073 Å) at 100 K on a Bruker APEXII diffractometer. The crystal-to-detector distance was 5.00 cm. Data were collected by a scan of 0.5º in ω in groups of 360 frames at φ settings of 0º, 90º, 180º, and 270º. The exposure time was either 10 s, 20 s, or 30 s depending on crystal size. The collection of intensity data, cell refinement, and data reduction were carried out using the program APEX2.48 Absorption corrections, as well as incident beam and decay corrections, were performed using the program SADABS.49 The structures were solved with the direct-methods program SHELXS and refined with the least-squares program SHELXL.50 Selected crystallographic information is listed in Table S2. Powder X-ray Diffraction Measurements. Room temperature powder X-ray diffraction patterns were obtained from solid reaction products using a Scintag X1 diffractometer with Cu Kα radiation (λ = 1.5418 Å). The results were quantitatively analyzed for crystalline sample constituents through Rietveld refinements.51-53 Structures obtained from the single-crystal studies were used to inform the modeling. Further information is provided in the SI. Raman Spectroscopy. Raman spectra were collected at room temperature on at least three single-crystal or powdered samples for each of 1 – 14, [NH4]NO3, [NMe4]NO3, [NEt4]NO3, [NBu4]NO3, and Th(NO3)4(H2O)32H2O using a Renishaw inVia Raman Microscope with a circularly polarized excitation line of 532 nm. A complete listing of all vibrational bands for each of these samples is given in Table S5. The identity of each sample used for Raman spectroscopy measurements was confirmed by X-ray diffraction.

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ASSOCIATED CONTENT Supporting Information Experimental details and results including single-crystal and powder X-ray diffraction, and Raman spectroscopy (PDF); Crystallographic files in cif format for 1 – 14.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected]. ORCID G.B.J.: 0000-0002-6125-208X L.S.: 0000-0003-4435-2721

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was performed at Argonne National Laboratory, operated by UChicago Argonne LLC for the United States Department of Energy under contract number DE-AC02-06CH11357 and was supported by the DOE Office of Basic Energy Sciences, Chemical Sciences, Heavy Elements Chemistry.

ABBREVIATIONS NMe4, tetramethylammonium; NEt4, tetraethylammonium; NPr4, tetra-n-propylammonium; NBu4, tetra-n-butylammonium.

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Influence of Countercation Hydration Enthalpies on the Formation of

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