Lithium and Sodium Ion Distributions in A2–x[W6I14] Structures

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

Lithium and Sodium Ion Distributions in A2−x[W6I14] Structures Thorsten Hummel,† Agnieszka Mos-Hummel,† Anna Merkulova,† Markus Ströbele,† Arun Krishnamurthy,‡ Scott Kroeker,*,‡ and Hans-Jürgen Meyer*,† †

Section for Solid State and Theoretical Inorganic Chemistry, Institute of Inorganic Chemistry, University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany ‡ Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada S Supporting Information *

ABSTRACT: Ag2[W6I14] and A2−x[W6I14] compounds with A = Na, Li were prepared from binary tungsten iodides (W3I12) and corresponding metal iodides. Their crystal structures are analyzed on the basis of X-ray diffraction data. 7Li and 23Na solid-state NMR measurements reveal that Li+ and Na+ ions are distributed over two sites in the respective structures. These results shed some new light on Ax[M6I14] with A = alkali and M = Mo, W compounds being reported with x = 1 and 2, which exhibit photophysical properties. The lithium compound is an exception in the series of A2−x[W6I14] compounds, because it is the only compound which is soluble in water.

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crystal structures reported for Ag[W6Br14 ] 12 and Na[W6Br14].11,13 However, the distribution of sodium ions in the structure of Na2−x[W6I14] is now the subject of a reinvestigation. A corresponding lithium compound was also mentioned to exist but a structure refinement remained difficult because positions of Li+ ions could not be determined from X-ray diffraction data. Herein, we reinvestigate the structures of A2−x[W6I14] compounds with A = Li and Na, using 7Li and 23 Na NMR to learn about cation distributions in these compounds, and introduce Ag 2 [W 6 I 14] as a reference compound.

INTRODUCTION Several binary1 and ternary2 tungsten iodide compounds can be prepared departing from W3I8·nI2 (n = 0, 1/2, 2). A preferred starting material is W3I12 (W3I8·2I2) because it can be produced on a large scale and in high yields. Reactions with this compound afford not only clusters with the trigonal W3I83 cluster core but also tetrahedral W4I10,4 square pyramidal W5I12,5 and octahedral W6I126 clusters. Octahedral cluster compounds A2[W6I14] with A = alkali metals are of current interest because they can serve as starting materials for the preparation of ligand (L) substituted A2[W6I8L6] clusters. Compounds with the [M6I8L6]2− ion (M = Mo, W) have shown remarkable photophysical properties with respect to phosphorescence and the generation of singlet oxygen.7 To this end, Cs2[Mo6I14] was the subject of several scientific reports, including the solvent-mediated purification of Cs2[Mo6I14] for increased luminescence efficiency.8 Recently, the preparation of a series of A2−x[W6I14] compounds was reported by solid-state reactions from W3I12 with appropriate amounts of alkali (A) metal iodide with A = Na, K, Rb, Cs. Crystal structures of Rb2[W6I14] and Cs2[W6I14] were refined isostructural to the reported structure of Cs2[Mo6I14].9 The cubic structure of K2[W6I14] is closely related to the crystal structure of the well-known Pb[Mo6Cl14] type.10,11 Both compounds crystallize in the space group Pn3̅, with the same arrangement of halide clusters, but with different distributions of counter cations. One K+ ion of K2[W6I14] occupies a 4c site (1/2, 1/2, 1/2), consistent with the position of Pb2+ in Pb[Mo6Cl14]. The second K+ ion occupies a 6d position (1/4, 3/4, 3/4) and was refined with a fractional occupation consistent with 2/3. The corresponding sodium compound was assigned as “Na[W6I14]”, isostructural to Pb[Mo6Cl14] and consistent with © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Preparations. Samples were prepared as previously reported, whereas several tungsten iodide compounds were employed as starting materials.1,3 Powders of A2[W6l14] were obtained from mixtures of a binary tungsten iodide (W3I12, W4I13, or W6I16) and AI (A = Li, Na, Ag). Metal iodides were used as purchased: LiI and AgI were from Sigma-Aldrich (both 99.99%) and NaI from ABCR (>99%). Starting mixtures (300 mg, following reactions 1−3) were homogenized under argon (glovebox) and fused into homemade silica ampules (length ≈ 6 cm, inner diameter = 0.7 cm) under vacuum. Ampules were heated for 24 h in a tube furnace at 550 °C for reactions with W3I12 or W4I13, and at 375 °C for W6I16, with heating and cooling rates of 2 °C/min. One part of the ampule containing the starting mixture was exposed to 550 °C, while the other side of the ampule was placed at one end of the furnace for the deposition of I2. Three reaction routes were successfully tested for the preparation of A2[W6I14], shown in the following reactions 1−3 with A = Li, Na, K, Rb, Cs, and Ag.

2W3I12 + 2AI → A 2 − x[W6I14] + 6I 2 + x AI

(1)

Received: November 21, 2017

A

DOI: 10.1021/acs.inorgchem.7b02948 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry 6W4I13 + 8AI → 4A 2 − x[W6I14] + 15I 2 + x AI

(2)

W6I16 + 2AI → A 2 − x[W6I14] + 2I 2 + x AI

(3)

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RESULTS AND DISCUSSION A(2+)[M6X14] compounds (M = Mo, W; X = Cl, Br, I) with divalent A ions (A = Pb, Hg, Cd, Fe, Co) are well-known to adopt the Pb[Mo6Cl14] structure with the space group Pn3̅.10,16−18 This structure can be derived from a hierarchical rock salt structure in which Pb2+ and [Mo6Cl14]2− adopt positions of Na+ and Cl−, whereupon the cluster occupies the centers of gravity of Cl− positions.10 The crystal structure of β-Cu2[W6Br14] can be considered as a structural derivative of the Pb[Mo6Cl14] type, crystallizing with the same cubic space group and the same face centered cubic (fcc) arrangement of [W6Br14]2− clusters (when considering their centers of gravity), but with a different distribution of Cu+ ions in the structure, where one type of Cu ion occupies octahedral sites similar to Pb2+ in Pb[Mo6Cl14] and another trigonal planar position in a distorted octahedral environment. A phase transition into α-Cu2[W6Br14] affords a rearrangement in the structure with two Cu+ ions situated in nearly trigonal planar environments of a distorted octahedral environment of iodide (space group Pbca).19 The same structure is evident for Cu2[Mo6I14], a compound that is related to Ag2[W6I14] and Li2−x[W6I14] as we will see later. A2[M6X14] compounds with A = Li, Na, Ag are synthesized by solid-state reactions in silica tubes when reacting metal iodide (AI) and W3I12. Crystal structures were refined on the basis of X-ray powder (A = Li, Na, Ag) diffraction data with some relevant data presented in Table S1. NMR Spectroscopy of Li2−x[W6I14] and Na2−x[W6I14]. Two Li sites are clearly observed in the 7Li MAS NMR spectrum: a narrow peak at −4.8 ppm (fwhm = 180 Hz) and a broader signal at −1.5 ppm (fwhm = 400 Hz) (Figure 1). On

A2−x[W6I14] compounds are formed at the hot section of the ampules while an excess of iodine is collected at the colder part of the ampule. All products were obtained as crystalline powders with green body color for the lithium compound, almost black for the sodium compound, and light brown for the silver compound in high yields (>95%) according to powder X-ray diffraction (XRD) patterns following reactions 1−3. Structure refinement plots did not reveal side phases (see the SI). In the synthesis of Na2−x[W6I14], an excess of NaI was used, to prevent the formation of W6I12 as a side phase. AI side phases became evident only when the amount of AI was increased to 10% excess (to A2.2[W6I14]) or more. X-ray Diffraction Studies. Powder XRD patterns of crystalline products were collected with an X-ray diffractometer (Stoe STADI-P, Ge monochromator) using Cu Kα1 radiation and a Mythen detector. Ag2[W6I14] was indexed isotypically to Cu2[Mo6I14] with the Ag positions freely refined. The lithium content of Li2−x[W6I14] was restrained to a site occupation factor of 1 for Li1 on 4a and restrained to 1/3 for Li2 on 8c to match with the ratio of the NMR measurement for a composition of Li1.66[W6I14]. The sodium content of Na1 on 4c was fixed to a site occupation factor of 1, while Na2 on 6d was freely refined, leading to a formal sodium content of 1.45. Crystal structures were refined by global refinement (Winplotr, Fullprof Suite).14 Further details of the crystal investigations can be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe. de), on quoting the deposition numbers CSD 433783 for Li1.66[W6I14], CSD 433784 for Na1.37[W6I14], and CSD 433782 for Ag2[W6I14]. Furthermore, CCDC 1584914, 1584915, and 1584918 contain the supplementary crystallographic data for this paper. NMR Measurements. 1D MAS NMR and 2D EXSY NMR experiments were carried out on a Varian 600 MHz (14.1 T) spectrometer equipped with a 3.2 mm double-resonance MAS NMR probe, and a Bruker Avance 500 MHz (11.74 T) spectrometer equipped with a 2.5 mm broadband MAS NMR probe. The samples were spun at the magic angle with a spinning speed of 14 kHz (±5 Hz) and 30 kHz (±2 Hz) at 14.1 and 11.74 T, respectively. The resonance frequencies at 14.1 and 11.74 T for 7Li were 194.37 and 233.23 MHz, and 132.29 and 158.74 MHz for 23Na, respectively. Bloch-decay experiments were carried out with pulse lengths and relaxation delays of 0.395 μs and 5 s for 7Li, and 0.375 μs and 5 s for 23 Na. The pulse lengths in all of these cases corresponded to a tip angle of 15° which would equally excite all sites with different quadrupolar coupling constants (CQ). The MAS NMR signal was averaged over 128 and 512 transients for 7Li and 23Na, respectively. Chemical shifts were referenced to 1 M LiCl and 0.1 M NaNO3. Relative intensities and spectral parameters were obtained by peak fitting using dmfit.15 Variable-temperature (VT) NMR spectra of the Li2[W6I14] complex were acquired at 14.1 T. The sample temperature was varied from −15 to 60 °C and returned to 0 °C. Sufficient time was given for the sample to reach the desired temperature and held constant using the VT gas controller setup from the instrument manufacturer. 2D EXSY NMR experiments were carried out at 11.74 T, at T = 258, 288, and 318 K. The spectral widths in the F1 and F2 dimensions were set to 15 and 80 ppm for 7Li and 23Na, respectively, and the number of increments in the indirect dimension was set to 128. Sixteen transients were collected for each increment with recycle delays of 5 s. The 90° pulse lengths (“solid 90”) were 3.32 and 4.25 μs for 7Li and 23Na, respectively. The mixing time was varied over a wide range (100 μs to 1.75 s) to obtain high-quality 2D EXSY NMR spectra. EDX Analyses. Energy dispersive X-ray spectroscopy (EDX) data were collected with a Hitachi TM303+ with a Bruker Quantax 70 EDX-System. Measurements were done with two pressed tablets of Na2−x[W6I14], leading to a composition of Na1.47[W6I14] and Na1.36[W6I14] (see the SI).

Figure 1. 7Li MAS NMR spectrum of Li2−x[W6I14].

the basis of the well documented relationship between cation chemical shifts and coordination environments,19 it may be concluded that the more shielded peak (i.e., −4.8 ppm) has a larger coordination number than the less shielded peak (i.e., −1.5 ppm). The relative integrated intensity of these peaks is about 40:60, respectively, indicating that the sites are not uniformly occupied. The narrower peak has no spinning sidebands, whereas the broader peak has spinning sidebands extending over about 100 kHz, implying that the latter possesses a non-negligible quadrupolar interaction arising from an asymmetric structural environment, whereas the former may be located in a more symmetrical structural environment. However, the narrow peak width may be primarily due to motional narrowing; see below. Subtle changes are observed with temperature variation. Upon increasing the temperature from −10 to +60 °C, the relative intensity of the two peaks shifts slightly in favor of the broader peak, with the narrow peak decreasing from 42% to 38%, mirrored by an increase in the broader peak from 58% to 62% of the total Li. The chemical shifts also change slightly (0.2 B

DOI: 10.1021/acs.inorgchem.7b02948 Inorg. Chem. XXXX, XXX, XXX−XXX

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Hence, the 7Li NMR data for Li2−x[W6I14] indicate dynamic disorder between two crystallographically distinct sites, at least one of which is located in a distorted polyhedral environment. Their occupancy is temperature-dependent and somewhat reversible, although irreversible changes occur on the time scale of months, leading to complete occupancy of the more distorted lower-coordinate site. 23 Na MAS NMR of Na2−x[W6I14] reveals a spectrum which is remarkably similar to that found for the Li analogue (Figure 3,

ppm) over this temperature range, moving toward convergence and suggesting imminent coalescence. The change in the peak widths with increasing temperature is similar: the narrow peak gets broader while the broad peak gets narrower. Significantly, reducing the temperature produces the opposite trends, suggesting that any temperature-induced changes are nearly reversible. Exchange spectroscopy (EXSY) is particularly informative for defining ionic mobility (Figure 2). Off-diagonal “cross-peaks”

Figure 3. 23Na MAS NMR spectra of Na2−x[W6I14]. 1D MAS spectrum and 2D EXSY spectrum with a mixing time of 1.5 s.

top). The peak at −3 ppm is residual NaI from the synthesis.21 The other two peaks indicate that sodium is present in distinct chemical environments with a ratio of 38:62. The broad peak around +33 ppm (62% integrated intensity) is likely to have a lower coordination number and significant distortion from tetrahedral geometry, as indicated by the crystal structure. The position of the narrow peak at +7 ppm indicates a higher coordination number,21 possibly six, based on the crystal structure, and the lack of second-order quadrupolar broadening indicates very little deviation from octahedral point-group symmetry. The 2D 23Na EXSY spectrum (Figure 3, bottom) shows no evidence of exchange between these sites, even with long mixing times (up to 1.75 s). Significantly, the 1D 23Na MAS spectrum is invariant to temperature, in contrast to the 7 Li MAS spectrum of Li2−x[W6I14], where ionic hopping is present and influences the relative peak intensities at different temperatures. This could be expected because of the lower mobility of Na+ relative to Li+ due to size effects. The T1’s of the two non-NaI peaks are 2 and 7 s, the difference suggesting a greater degree of motion or “rattling” in the lower-coordinate, distorted site, even if ionic site exchange is not occurring. Crystal Structure of Ag2[W6I14]. Ag2[W6I14] was prepared and structurally characterized to serve as a reference compound for Li2−x[W6I14] because it was supposed to have the same structure. The crystal structure of Ag2[W6I14] was solved and refined in space group Pbca with some relevant data presented in Table S1. The crystal structure is isostructural to Cu2[Mo6I14]24 with silver ions on an 8c position in a nearly trigonal planar environment of iodide, with interatomic Ag−I distances equal to 2.637(9), 2.709(9), and 2.741(9) Å. Another possible position for silver ions in the structure of Ag2[W6I14] is on a 4a site, but remains unoccupied. Silver ions on this position would be surrounded by a strongly distorted

Figure 2. 7Li EXSY NMR spectra of Li2−x[W6I14], with mixing times of 1 ms (top) and 750 ms (bottom).

are observed with mixing times of greater than 100 ms (Figure 2, bottom), whereas no cross-peaks are observed for a short mixing time of 1 ms (Figure 2, top). The cross-peak intensity increases over the range of 100−750 ms, before leveling off at longer times, implying that Li ions are exchanging between these sites. Analyzing the ratio of the cross-peak and diagonalpeak intensities as a function of mixing time20 leads to an estimate of the Li ion exchange rate of 0.2 s−1 at 288 K. To verify that this apparent exchange rate is due predominantly to ionic hopping and not spin-diffusion, analogous EXSY experiments were carried out at temperatures of 258 and 318 K, confirming that the cross-peak intensities increase with temperature for a given mixing time. Further evidence of ionic exchange is provided by the different relaxation times of these sites: the narrow peak at −4.8 ppm has T1 values of 10, 5, and 2 s upon increasing the temperature from −10 to +60 °C, whereas the T1’s of the broader peak are roughly twice those values, at 20, 12, and 5 s, respectively. Normally, one might interpret this as meaning that these two peaks come from Li in different phases. However, the EXSY spectra show clearly that these sites are exchanging, confirming that they are in the same phase. On the basis of the relaxation data, peak shape, and lack of quadrupolar broadening, it is likely that the short spin−lattice relaxation time associated with the narrow peak is due to motion. C

DOI: 10.1021/acs.inorgchem.7b02948 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Unit cell of Li2−x[W6I14] with two crystallographically distinct Li+ positions Li1 (4a) and Li2 (8c) (left), and of Ag2[W6I14] with one Ag+ (8c) position (right). Iodide ligands are omitted for clarity.

Figure 5. Surroundings of Li+ ions in Li2−x[W6I14] with a nearly octahedral surrounding of Li1 (4a) (top left) and strongly distorted six-fold surrounding of Li2 (8c) (bottom left) which is in fact closely related to the three-fold (8c position) environment of Ag+ in Ag2[W6I14].

Lithium ions in the structure cannot be safely determined on the basis of XRD data. Therefore, we have analyzed the structure for most probable voids that may be occupied by lithium ions. 7Li NMR studies suggest there are two lithium environments, one with a higher coordination number and little distortion from a regular polyhedron, likely octahedral, and the other with a lower coordination number and distorted polyhedral environment. One likely void for lithium, assigned as Li1, corresponds to a 4a position (0, 0, 0) with a hexacoordinated environment of iodide forming a distorted octahedron. On this site, Li1 is surrounded by one apical and two inner iodide ligands of two [W6I8iI6a] clusters (i = inner; a

octahedron with Ag−I distances of 3.192(6) Å (2×), 3.199(5) Å (2×), and 2.618(6) Å (2×). Expected Ag−I distances in octahedral voids can be estimated at 3.0169(1) Å from rock salt structured AgI.22 Crystal structures of corresponding tungsten bromides were reported for Ag[W6Br14] with the space group Pn3̅, and for Ag 2 [W 6 Br 14 ] with the space group P2 1 /c, containing [W6Br14]n− cluster cores with n = 1 and 2.12 Crystal Structure of Li2−x[W6I14]. The crystal structure of Li2−x[W6I14] was solved and refined with the space group Pbca with a similar arrangement of tungsten iodide clusters as in Ag2[W6I14] (Figure 4). D

DOI: 10.1021/acs.inorgchem.7b02948 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry = outer or apical) with interatomic Li1−I distances of 2.710(4), 3.202(4), and 3.224(4) Å (Figure 5, top left). The average Li1− I distance (3.045(4) Å) is in good agreement with the corresponding distance in LiI (3.0129(2) Å).23 The position assigned for Li2 is a 8c site (0.341(4), 0.719(2), 0.256(2)) and was indicated in the difference Fourier map. The refinement in an environment with six iodide ions revealed Li2−I distances of 2.95(4), 2.96(4), 3.00(4), 3.48(6), 3.62(5), and 3.67(4) Å. We note that the herein proposed 8c position of Li2 in Li2[W6I14] is slightly shifted relative to the corresponding 8c position of Ag in Ag 2 [W 6 I 14 ] (0.2915(4), 0.6206(7), 0.2077(4)) and Cu in Cu2[Mo6I14] (0.28017(6), 0.63204(9), 0.20985(6)), in which Ag+ and Cu+ have trigonal planar iodide environments (Figure 5, bottom, right).24,25 Essentially two cations in A2[W6I14] (A = Ag, Cu) share one octahedral void by occupying an 8c position. As a result, they are situated near opposite trigonal faces of a distorted octahedron. The refined lithium position of Li2 is roughly in line with this picture with Li−Li separations of 5.98(5) Å. The overall distribution of lithium ions over these sites may be approximated from the 40:60 signal ratio of Li2:Li1 obtained in the 7Li NMR experiment. If the Li1 (4a) site is fully occupied, the corresponding number of lithium ions on the Li2 (8c) site is 2.6667 (site occupation of 1/3), leading to the composition Li1.66[W6I14]. However, variable 7Li NMR signal ratios from different preparations and handling of the compound suggest variable amounts of lithium ions, and therefore some degree of phase width for Li2−x[W6I14]. Li2−x[W6I14] is soluble in various solvents. A precise analysis of the Li+ content is difficult, in the light of that LiI was employed for the synthesis, although there has been no or a negligibly small LiI side phase detected in XRD patterns and none in NMR studies. Crystal Structure of Na2−x[W6I14]. The crystal structure of “Na[W6I14]” was recently reported to be isostructural to the structure of Pb[Mo6Cl14].2 Our current investigations, however, point to the presence of another sodium site in the structure, and thus to the composition Na2−x[W6I14]. As in the case with Li2−x[W6I14], there are two sodium positions indicated in our NMR studies, one with a lower and the other one with a higher coordination number. Octahedral [M6X8] type clusters, such as [(M6I8)I2I4/2] with M = Mo, W, usually occur with 24 electrons per cluster, consistent with [W6I14]2−. However, Na[W6Br14] with the Pb[Mo6Cl14] type structure and even neutral W6Br14, also discussed as Na[W6Br14], have been reported in the literature.12 To our knowledge, only two compounds are reported to contain [W6Br14] clusters having 23 electrons per cluster, namely, Na[W6Br14] and Ag[W6Br14] (space group Pbca). No significant differences are evident for W−W distances in the [W6X14]n− (n = 1, 2) clusters listed in Table 1. Only W−Br distances appear slightly shortened in comparison to iodide compounds. This is typically obtained as a result of the smaller radii of bromide ions surrounding the cluster core. Structural characterizations of these compounds are based on XRD studies, and it is clear that cation disordering in these compounds stresses the limits of X-ray diffraction techniques. Tungsten clusters in the structure of Na2−x[W6I14] are arranged similarly to those in the structure of Pb[Mo6Cl14], as shown in Figure 6. According to 23Na NMR results, there are two crystallographically distinct Na+ positions in the structure.

Table 1. Average Interatomic Distances (Å) in A2−x[W6I14] Compounds Containing Eight Inner (i) and Six Outer (a) Ligands compound

W−W

W−Ii

W−Ia

Li2−x[W6I14] Na2−x[W6I14] K2[W6I14]2 Rb2[W6I14]2 Ag2[W6I14] Cs2[W6I14]2 (TBA)2[W6I14]26 Na[W6Br14]11 Ag[W6Br14]12 Ag2[W6Br14]12

2.653(4) 2.685(2) 2.669(2) 2.675(5) 2.690(6) 2.664(9) 2.657(6) 2.6513(4) 2.648(2) 2.633(6)

2.790(5) 2.783(2) 2.799(3) 2.804(6) 2.783(7) 2.800(1) 2.784(8) 2.6119(9) 2.616(4) 2.624(2)

2.813(5) 2.799(2) 2.839(3) 2.784(5) 2.861(7) 2.832(2) 2.857(5) 2.5657(7) 2.575(6) 2.613(2)

Na1 occupies a 4c position (1/2, 0, 0) and is situated in a slightly distorted octahedral environment of apical iodide ligands of clusters with an average Na1−I distance of 3.179(1) Å, which is fully consistent with the position of Pb2+ in Pb[Mo6Cl14]. The position of Na2 could not safely be determined by means of powder XRD techniques. A likely position for Na2 is the 6d (1/4, 3/4, 3/4) site, which would result in a [4 + 4] coordination with four shorter (3.321(2) Å) and four longer (3.600(3) Å) Na−I distances (Figure 7). This parallels the structure of K2[W6I14], where K+ ions reside on 4c and 6d sites, with average K−I distances of 3.507(2) Å with six iodide ligands, and 3.568(2) Å for eight nearest iodide neighbors. The composition of the sodium compound could be determined as Na1.45[W6I14] with sodium ions being distributed over two distinct positions, by NMR, EDX, and Rietveld refinement (see the SI). Still, there can be a phase width, as already mentioned for the corresponding lithium compound.

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CONCLUSIONS Nearly fcc arrangements of clusters (with respect to their centers of gravity) are most common for A2 [M 6X 14] compounds, where cations reside in appropriate voids. Generally, this arrangement of clusters offers octahedral and tetrahedral voids to be occupied by cations. Depending on the orientation of [M6X14]2− clusters, these voids may offer variable space and coordination numbers for cations. Crystalline samples of Li2−x[W6I14], Na2−x[W6I14], and Ag2[W6I14] were synthesized and structurally characterized. Ag2[W6I14] crystallizes isotypic to Cu2[W6I14]. The presence of two crystallographically distinct cation positions for lithium and sodium ions in the structures of Li2−x[W6I14], Na2−x[W6I14] were clearly identified by 7Li and 23 Na NMR studies. The amount of alkaline metal in A2[W6I14] for A = Na, Li was determined to be 1.66 for Li and 1.45 for Na. Lithium ions in Li2−x[W6I14] are considered to reside in nearly octahedral voids (Li1, 4a), and strongly distorted octahedral voids (Li2, 8c) similar to Ag+ in Ag2[W6I14]. Partial occupations of these sites by Li+ ions make up a maximum occupation of 8/12 sites for the composition Li2[W6I14], whereas a composition Li1.66[W6I14] may be assumed in our samples. NMR studies reveal a temperature-dependent dynamic disorder of lithium ions between both crystallographically distinct sites. Sodium ions in the structure of Na2−x[W6I14] were also assigned to two crystallographically distinct sites. Na1 ions were E

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Figure 6. Crystal structure of Na2−x[W6I14] with two Na+ positions Na1 (4c, blue) and Na2 (6d, crossed, blue) (left). Arrangement of Pb (4c) in Pb[Mo6Cl14] (right). Iodide and chloride ligands are omitted for clarity.

pattern, calculated pattern, Bragg positions, and difference curve. Table S3: Atomic coordinates, Wyckoff sites, SOF, and equivalent isotropic displacement parameters for Na1.37W6I14. Table S4: Results of the EDX measurements of pressed pills of Na2−x[W6I14]. Table S5: Summary of the composition of Na2−x[W6I14]. Figure S2: Rietveld refinement pattern of Na2[W6I14] with observed powder XRD pattern, calculated pattern, Bragg positions, and difference curve. Table S6: Atomic coordinates, Wyckoff sites, SOF, and equivalent isotropic displacement parameters for Ag2W6I14. Figure S3: Rietveld refinement pattern of Ag2[W6I14] with observed powder XRD pattern, calculated pattern, Bragg positions, and difference curve (PDF)

Figure 7. Octahedral surrounding of Na1 (left) and eight-fold [4 + 4] coordination of Na2 (right) in Na2−x[W6I14].

Accession Codes

CCDC 1584914, 1584915, and 1584918 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]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

refined on distorted octahedral voids (4c) similar to positions of Pb2+ in Pb[Mo6Cl14]. Na2 (6d) is supposed to occupy a distorted eight-fold [4 + 4], or rather tetrahedral [4 + 0] environment. Partial occupations of these sites with Na+ ions offer a 8/10 occupation for Na2[W6I14]. The composition of our sample was determined as Na1.45[W6I14] by EDX and XRD. No ionic mobility is observed for Na+ ions in the structure under ambient conditions. The crystal structure is closely related to that of K2[W6I14]. The influence of the reaction conditions (cooling rate) on the cation distributions in structures is not known, but NMR studies show that Li ions tend to completely occupy the lowercoordinate site (4a) over long-term exposure to air. A = Li is an exception among A2[W6I14] compounds with A = Na, K, Rb, Cs, and Ag, due to the fact that it is soluble in water and other polar solvents.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.-J.M.). *E-mail: [email protected] (S.K.). ORCID

Markus Ströbele: 0000-0002-5147-5677 Hans-Jürgen Meyer: 0000-0003-2450-4011

ASSOCIATED CONTENT

S Supporting Information *

Funding

This research was supported by the Deutsche Forschungsgemeinschaft (Bonn) via grant ME 914/27-1. S.K. is grateful to the Canada Foundation of Innovation for infrastructure support and the Natural Sciences and Engineering Research Council of Canada for continued funding.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02948. Table S1: Crystal structure and structure refinement data of A2[W6I14]. Table S2: Atomic coordinates, equivalent isotropic displacement parameters, Wyckoff sites, and SOF for Li1.66W6I14. Figure S1: Rietveld refinement pattern of Li2[W6I14] with observed powder XRD

Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS The authors would like to thank Johannes R. Giebel and PhD Dr. Thomas Wenzel (University of Tübingen) for the EDX measurement.

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DOI: 10.1021/acs.inorgchem.7b02948 Inorg. Chem. XXXX, XXX, XXX−XXX