Structure, Phase Transitions, and Isotope Effects in [(CH3)4N]2PuCl6

Jul 30, 2015 - Synopsis. The single-crystal structure of bis-tetramethylammonium hexachloroplutonate(IV) was determined and studied as a function of t...
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Structure, Phase Transitions, and Isotope Effects in [(CH3)4N]2PuCl6 Richard E. Wilson* Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: The single-crystal X-ray diffraction structure of [(CH3)4N]2PuCl6 is presented for the first time, resolving long-standing confusion and speculation regarding the structure of this compound in the literature. A temperature-dependent study of this compound shows that the structure of [(CH3)4N]2PuCl6 undergoes no fewer than two phase transitions between 100 and 360 K. The phase of [(CH3)4N]2PuCl6 at room temperature is Fd3̅c a = 26.012(3) Å. At 360 K, the structure is in space group Fm3̅m, with a = 13.088(1) Å. The plutonium octahedra and tetramethylammonium cations undergo a rotative displacement, and the degree of rotation varies with temperature, giving rise to the phase transition from Fm3̅m to Fd3̅c as the crystal is cooled. Synthesis and structural studies of the deuterated salt [(CD3)4N]2PuCl6 suggest that there is an isotopic effect associated with this phase transition, as revealed by a changing transition temperature in the deuterated versus protonated compound, indicating that the donor−acceptor interactions between the tetramethylammonium cations and the hexachloroplutonate anions are driving the phase transformation.

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INTRODUCTION Metal halides and their complex salts are technologically important materials spanning applications in photovoltaics,1 catalysis,2,3 and industrial metallurgy processes.4 In addition to their commercial applications, their straightforward compositions and generally high symmetry have made them ideal model compounds and starting materials in many seminal studies of the chemistry and physics of molecules and elements across the periodic table.5,6 Despite their relatively simple compositions, these compounds are known to possess a variety of phase behaviors, including first-order phase transitions and order− disorder transitions.7 Not surprisingly, such complex structural behavior has created confusion in the literature regarding their molecular and extended structures.8−13 Significant work has been conducted in studying these complex behaviors and their technological applications.8 Work from this laboratory and others has focused on the chemistry of the actinide and lanthanide ions with halides,14−22 pseudohalides,23 and other simple inorganic ligands24−28 in order to describe their thermodynamic properties,29,30 behavior in solution,22,31−33 reactivity,34,35 and spectroscopic14,36−38 and magnetic properties.26,39−42 The interpretation of such experimental data is complemented by computational methods and models that often rely on prior structural characterization of the studied molecules in order to arrive at meaningful and reasonable results.38,43,44 Significant earlier work has been focused on the preparation and study of actinide halide complexes such as Cs2UO2Cl4,36,45 the elpasolites such as Cs2NaAmCl6,46,47 and other alkylammonium hexahalometalates of the tetravalent actinides.17,41,42,48 Using these as model systems, valuable insight into the chemistry and physics of the actinide elements, particularly their spectroscopic and magnetic properties, can be obtained. It was therefore surprising that, © XXXX American Chemical Society

despite significant studies exploiting the salts (Me4N)2AnCl6 as model compounds in magnetic and spectroscopic studies of tetravalent actinides, little is known regarding the actual structures of these materials. The tetramethylammonium salts of the tetravalent actinide hexachlorides are understood from prior studies to be highsymmetry complexes, either cubic or tetragonal.48 However, it is likely that their structural properties and dynamics are also very similar to those of the transition metal and semimetal hexahalides.7 Work investigating the structures and properties of the transition metal and semimetal hexahalides has highlighted significant structural complexity, including site disorder and phase transitions in these materials arising from donor−acceptor interactions between the hexachlorometalate anion and the hydrogen atoms on the alkyl ammonium cations. Such complexity and phase transitions are encountered not only in the molecular species of interest here but also in extended solids, including the very important class of organic perovskites.49,50 Reported here is a study of the structure of [(CH3)4N]2PuCl6 and its isotopologue, [(CD3)4N]2PuCl6, highlighting a rotative-displacement transition that occurs between 120 and 350 K between Fd3̅c and Fm3̅m, and evidence for a first-order phase transition below 120 K, as previously observed for [(CH3)4N]2UCl6.9−11 The temperature of the Fd3c̅ to Fm3m ̅ transition is demonstrated to have an isotopic dependence, indicating donor−acceptor interactions between the tetramethylammonium cations and the hexachloroplutonate anion. Received: June 8, 2015

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DOI: 10.1021/acs.inorgchem.5b01288 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Caution! 242Pu is a long-lived alpha-emitting radionuclide. All sample preparations and manipulations were carried out in laboratory space specially equipped for handling such material, and strict radiological controls were enforced.

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EXPERIMENTAL METHODS

Synthesis of [(CH3)4N]2PuCl6 and [(CD3)4N]2PuCl6. All reagents were purchased from Sigma-Aldrich and used as received, except for the 242Pu. To a solution of 25 mM 242Pu in 6 M HCl was added an excess of hydroxylamine hydrochloride to produce a blue solution of PuIII. The Pu-HCl solution was diluted with water, and the Pu was subsequently precipitated with ammonium hydroxide. The precipitate was washed several times with either H2O or D2O until the pH of the supernatant was neutral, as determined by indicator paper. The precipitate was then dissolved in either 11 M HCl or DCl (SigmaAldrich). A stoichiometric amount of tetramethylammonium chloride as either the H12 or D12 isotopologue (Sigma-Aldrich) was then added, and the solution was diluted with either water or deuterated water to dissolve any precipitates. The solutions were allowed to evaporate in ambient air (protonated) or in a desiccator (deuterated). After a few days, crystals suitable for X-ray diffraction measurements and spectroscopic measurements were deposited. Single-Crystal X-ray Diffraction. Crystals suitable for singlecrystal X-ray diffraction measurements were affixed to glass micropipettes using a quick drying two-part epoxy, and their diffraction patterns were collected on a Bruker SMART diffractometer equipped with an APEXII CCD detector using Mo Kα radiation. An Oxford Cryosystems 700 series cryostat was used for controlling the sample temperature between 100 and 360 K. Data were corrected for absorption using SADABS.51 Structure solutions and structure refinements on F2 were carried out using SHELXS and SHELXL, respectively.52 Raman Spectroscopy. Raman spectra of [(CH3)4N]2PuCl6 and [(CD3)4N]2PuCl6 were acquired using a Renishaw inVia Raman microscope using an excitation line of 532 nm. Samples were contained on a microscope slide with a concave cavity and covered with a glass coverslip affixed with epoxy. Spectra were collected using circularly polarized radiation between Δν 100 and 4000 cm−1.

Figure 1. Precession images of the 0kl reflections at 125 K (Fd3̅c; left) and 360 K (Fm3̅m; right) showing the weak reflections and supporting the assignment of Fd3̅c and Fm3̅m.

Table 1. Selected Structural Details for [(CH3)4N]2PuCl6 at 298 and 360 K [(CH3)4N]2PuCl6 FW = 602.99 g mol−1

Pu−Cl (Å) C−N (TMA-1) C−N (TMA-2) C−Cl (TMA-1) C−Cl (TMA-2)

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Fd3c̅ (No. 228) T = 298 K a = 26.012(3) Å Z = 32 V = 17600(3) Å3 ρ = 1.821 g cm−3 μ = 3.712 mm−1 λ = 0.71073 Å R1: 0.041, wR2: 0.067 GoF: 1.084 2.579(2) Å 1.49(1) Å 1.51(2) Å 3.69(1), 3.87(1) Å 3.38(6), 3.83(6) Å

Fm3m ̅ (No. 225) T = 360 K a = 13.088(2) Å Z=4 V = 2241.9(4) Å3 ρ = 1.787 g cm−3 μ = 3.642 mm−1 λ = 0.71073 Å R1: 0.031, wR2: 0.076 GoF: 1.336 2.577(3) Å 1.416(15) Å 3.786(1) Å

The structure was solved and refined in Fd3c̅ using the larger lattice parameter for both the protonated and deuterated complexes reported here. Selected metrical data regarding the structure and refinement are presented in Table 1. In Fd3̅c, the Pu is located on Wyckoff 32c, −3 site symmetry, and is coordinated by six chloride ions on the general position forming the octahedral hexachloroplutonate anion. The six Pu− Cl bond distances are 2.583(1) Å in [(CH3)4N]2PuCl6, both shorter than the sum of the ionic radii for six-coordinate tetravalent plutonium (Pu−Cl: 2.68 Å). Although this is on the shorter side of the range of bond distances reported, the Pu−Cl distance is consistent with those previously reported for other hexachloroplutonate salts.14,15,17,54 A projection down an axis of the unit cell highlights the rotation of the plutonium octahedra as well the tetramethyammonium cations with respect to each other, as shown in Figure 2. The torsion angle, as measured between Cl−Pu−Pu−Cl, between two octahedra down a face in the protonated structure is 9.48° at 300 K, and it changes with temperature and isotopic substitution, as will be discussed later, Figure 3. Such rotations of the chlorometalate octahedra are well-known for other hexachlorometalates and are classified as rotative-displacement transitions.7,12,55,56 Two crystallographically distinct tetramethylammonium cations (TMA-1 and TMA-2) are present in the structure. The TMA cations are on Wyckoff 48d (TMA-1) and 16a (TMA-2). Depending on the temperature, TMA-2 will order such that the methyl carbons will fall along the 3-fold rotation

RESULTS AND DISCUSSION Structure of [(CH3)4N]2PuCl6 in Fd3̅c. Prior descriptions of the crystallographic properties of [(CH 3 ) 4 N] 2 PuCl 6 indicated that there were no fewer than two phases that could be isolated under ambient conditions.48,53 While the first reports of this salt indicated that the most stable phase was a tetragonal phase, the tetragonal phase was not ever able to be produced in this laboratory.53 The preparation of a tetragonal phase is mentioned only among the earliest description of this salt and has never been subsequently reported. Instead, a cubic phase was reproducibly isolated from the synthetic procedure described above, producing large cubic crystals suitable for Xray diffraction measurements. Polarized light microscopy demonstrated that these crystals were optically isotropic, as previously observed, an indicator of cubic symmetry.48 Diffraction data collected at 300 K could be satisfactorily indexed to a face-centered cubic lattice, with a lattice parameter of 26.012 Å. Prior studies of [(CH3)4N]2PuCl6 indicated that the lattice parameter was ∼13.0 Å; however, inspection of the raw reflection data collected in this laboratory showed less intense reflections that supported the doubling of the lattice parameter to ∼26 Å, Figure 1 and Table 1. Additional data collected at variable temperature showed these reflections to extinguish at higher temperature; this observation, and an inspection of their intensities over all of the temperatures studied, leads to the conclusion that these were not due to λ/2 effects but, in fact, to a changing structure. B

DOI: 10.1021/acs.inorgchem.5b01288 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) Variation of the lattice parameter with temperature and (b) variation of the torsion angle between the PuCl62− octahedra with temperature.

C−Cl distances occurring in the unit cells are found on the disordered TMA-2 cation, being ∼3.3 Å (temperature independent) and ∼3.7 Å for TMA-1, shorter than the sum of their van der Waal’s radii of 3.80 Å, potentially indicative of a donor−acceptor type interaction occurring between the cations and the chlorides associated with the anion. Structure of [(CH3)4 N]2PuCl6 in Fm3̅m. As shown in Figure 1, at higher temperature (T > 350 K), the reflections giving rise to the larger 26 Å axis extinguish, and the remaining reflections can be indexed to a face-centered cubic cell in Fm3m ̅ . The refinement and structural parameters are presented in Table 1, and a packing diagram of the Fm3̅m phase is shown in Figure 2. In Fm3̅m, the Pu resides on the origin with Oh symmetry. The six chloride ions form an octahedron, and the Pu−Cl bond distances in the Fm3m ̅ structure are 2.577(3) Å. It is noteworthy that the Pu−Cl bond distances do not change with temperature or isotopic substitution in either Fm3̅m or Fd3̅c. There is one crystallographically unique tetramethylam-

Figure 2. Packing diagrams of [(CH3)4N]2PuCl6 in Fd3̅c (a) and Fm3̅m (b). PuCl62− is rendered as red polyhedra, and the tetramethylammonium cations are represented by ball and stick models.

axis of the tetrahedral ammonium sites at temperatures above 340 K in the protonated form (300 K in the deuterated) or will disorder about this site with the methyl carbons being refined to 1/3 occupancy on the general position about the 3-fold axis below these temperatures. The hydrogen atoms on the ordered methyl groups of the TMA cations were modeled using a standard riding model where both the distance and thermal parameters are constrained. Where the methyl carbons were disordered about the symmetry site, no attempt to model the hydrogen atoms was made. Therefore, the methyl−chlorine interactions are best compared using the C−Cl distances instead of the idealized C−H−Cl bond distances. The closest C

DOI: 10.1021/acs.inorgchem.5b01288 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry monium cation, and, unlike those encountered in Fd3c̅ , the methyl carbon atoms are ordered. Effect of Temperature and Deuteration on the Structure of [(CH 3 ) 4 N] 2 PuCl 6 . Prior studies on [(CH3)4N]2UCl6 and other hexahalogenometalates have demonstrated a temperature-dependent phase behavior as well as an isotope effect on the phase transition temperatures when the deuterated form of tetramethylammonium is used as the cation in these compounds.9−12,55 The uranium salt is reported to undergo a first-order phase transition at ∼125 K, from a centrosymmetric to acentric spacegroup, as indicated by the spectroscopic observation of pure electronic transitions of the U(IV) ion above 125 K, indicating a loss of inversion symmetry at higher temperature (both phases of the Pu salt described here are centrosymmetric).9 This phase transition is accompanied by a significant thermal hysteresis to the phase change behavior. Subsequent analysis using Raman spectroscopy confirmed these observations.10,11 Intrigued by these observations, and our own initial unsuccessful attempts to collect single-crystal diffraction data at 100 K for the plutonium salt, a standard procedure in this laboratory, an investigation of the thermal behavior of both [(CH 3 ) 4 N] 2 PuCl 6 and its deuterated isotopologue [(CD3)4N]2PuCl6 was conducted using single-crystal X-ray diffraction. Attempts to solve the collected diffraction data at 100 K were unsuccessful. The crystals appeared to have cracked during the cooling process, as indicated by the observation of multiply split reflections that could not be indexed to any cell. This was the first indication of a phase transition in this material occurring at lower temperature, as observed previously with the uranium salt.9 Subsequent data were collected by cooling the crystals from room temperature to 100 K in 25 K increments, with structure solutions and refinements being conducted at each temperature point. The lattice parameter for the material contracts as the temperature is lowered, as shown in Figure 3a. As well, the torsion angle between the hexachloroplutonate octahedra increases from ∼10° at 300 K to ∼12° at 125 K until approximately 120 K, Figure 3b, below which the diffraction data is unusable, as described above. The torsion between the octahedra and TMA cations gives rise to the doubled unit cell. The observed, but weak, reflections can be directly attributed to the chlorine and carbon atoms rotating out of their eclipsed geometry in Fm3̅m below 350 K and into the rotated geometry in Fd3̅c observed at lower temperatures. In Fd3̅c, the reflections that would give rise to the doubled axis can be attributed to the reflections of the chlorine and carbon atoms on the general position. The structural changes are also accompanied by an ordering of the carbon atoms associated with the tetramethylammonium cations of TMA-2 (16a). By 350 K, the diffraction data no longer support the doubled lattice parameter, and the data can be successfully indexed, solved, and refined in Fm3̅m. Inspection of the Pu−Cl bond distances across this temperature range show that the bond distance remains unchanged, and if any trend is to be extracted, the Pu−Cl bond distance changes inversely with temperature. The changing torsion angle between the octahedra, along with the invariant nature of the Pu−Cl bond distances with temperature, indicates that the typically weak donor−acceptor interactions between the tetramethylammonium cations and the hexachloroplutonate anions are responsible for the thermally induced structural changes of this compound and, consequently, the observation of the Fd3̅c to Fm3̅m transition.

A test of this hypothesis was conducted by synthesizing the deuterated isotopologue of the compound, [(CD3)4N]2PuCl6, and investigating its structural properties as a function of temperature. Because C−D has a lower zero point energy, ∼5 kJ mol−1 lower in energy than the C−H bond, we would expect an isotope effect in the transition temperatures if the donor− acceptor interactions between the methyl protons/deuterons and the chloride ions of the octahedra are responsible for the changes in phase.57−59 Generally, we would expect that any phase transition associated with the deuterated complex would occur at a higher temperature than that for the protonated complex based on the differences in zero point energies. Using the torsion angles of the plutonium octahedra as a function of temperature in the Fd3̅c to Fm3̅m transition as an indicator of the phase transition, a plot of the derivative shows that the transition from Fd3c̅ to Fm3m ̅ in the deuterated complex (as indicated by the inflection point of the torsion angle versus T curve; Figure S1, Supporting Information) occurs approximately 10 K higher in temperature in the deuterated complex versus the protonated complex. However, this result may be an oversimplification of the phase changes occurring since it is also observed that the TMA cations of the deuterated complex order lower in temperature (300 K) than do the protonated TMA cations (340 K). While these data suggest an isotope effect in the Fd3̅c to Fm3̅m transition, additional study using more sensitive methods such as neutron diffraction and calorimetry is necessary for a definitive confirmation of such an effect. Raman Spectra of [(CD3)4N]2PuCl6 and [(CH3)4N]2PuCl6. Because it is suspected that donor−acceptor interactions between the hexachloroplutonate and tetramethylammonium are occurring and driving these phase transitions, Raman spectra were collected for the two salts at room temperature, Figure 4. Dominating these spectra in the lowfrequency region 100−400 cm−1 are the vibrations associated with the PuCl62− octahedron. The symmetric stretching frequency v1 (PuCl62−) is observed at 300 cm−1, whereas the doubly and triply degenerate modes associated with the

Figure 4. Raman spectrum of [(CH3)4N]2PuCl6 (black trace) and [(CD3)4N]2PuCl6 (red trace) highlighting the symmetric PuCl62− stretching (300 cm−1) and degenerate bending modes (130 cm−1). D

DOI: 10.1021/acs.inorgchem.5b01288 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry bending modes of PuCl62− are likely contained in the broad band around 130 cm−1. Of principal interest was the Pu−Cl symmetric stretching frequency and whether it was different between the D and H forms of the complex, indicating a perturbation to the vibrational frequency due to the donor− acceptor interaction from the cations. No such change in frequency was observed, with the PuCl62− symmetric stretching frequencies occurring at 300(1) cm−1 in both isotopologues. While this result does not exclude a donor−acceptor interaction in these complexes, such an effect does not seem to have the same influence as was observed in other donor− acceptor interactions that we have encountered previously.18,19,21 Although the diffraction data presented here clearly demonstrate a phase transformation within a face-centered cubic system, the energetics and underlying chemical causes are still unresolved. The indications that the transition temperature changes upon deuteration suggests that the phase transitions are principally driven by the donor−acceptor interactions between the TMA cations and the hexachloroplutonate. However, because the intensities of the reflections that arise from the twisting of the cations and the hexachloroplutonate contain information only regarding the carbon and chloride ions, they are consequently weak. Ideally, such a system should be studied using neutron diffraction and other spectroscopic probes sensitive to the environment of the light atoms and the nature of the metal−ligand interactions. Prior studies of transition metal and semimetal hexachloride systems using calorimetry, neutron diffraction, and NMR techniques have revealed quantitative information regarding the energetics and origins of such phase transitions.7,12,55,56,60−66 Not surprisingly, similar disorder types to those observed here were noted, including rotation of the cation coordination polyhedra with temperature and the metal-based octahedra. Correlations between anion and cation size and the transition temperature exist, and, interestingly, correlations also exist within the transition metals based on the d-electron configuration and resulting ligand field stabilization energy.7,13 This raises interesting questions regarding the phase transitions observed here and previously in the U(IV) compound with respect to the influence of the f-electron configuration of the metal and how it may be manifesting itself in the structural properties of these salts. Additional studies of the actinide(IV) hexachlorometalates (Th, Pa, U, and Np) are likely to yield interesting insights.

interesting insights into the chemical behavior of these complexes.

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ASSOCIATED CONTENT

* Supporting Information S

Raman spectra, crystallographic data in CIF format, and structure factor files. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01288.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work performed at Argonne National Laboratory, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357.

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REFERENCES

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CONCLUSIONS The single-crystal X-ray structures of [(CH3)4N]2PuCl6 and [(CD3)4N]2PuCl6 have been presented here for the first time, highlighting the rotative displacement of the hexachloroplutonate octahedra and the tetramethylammonium cations. This rotative displacement causes a change in structure from Fm3̅m at higher temperature (T > 350 K) to Fd3c̅ near room temperature and lower. Further cooling to ∼120 K indicates the occurrence of an additional phase transformation accompanied by the observation of multiply split X-ray reflections and possible crystal cracking. A similar phase transition was observed for [(CH3)4N]2UCl6, but it has not been explored further for that complex or the plutonium compound reported here. Data also indicate an isotope effect in the temperature of this phase transition, as indicated by the study of the deuterated isotopologues. Additional studies using powder neutron diffraction and NMR techniques to understand the dynamics of these phase transitions are warranted and likely to provide E

DOI: 10.1021/acs.inorgchem.5b01288 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b01288 Inorg. Chem. XXXX, XXX, XXX−XXX