Crystal structure of hexachlorothallate within a caesium chloride–phosphotungstate lattice Cs9(TlCl6)(PW12O40)2·9CsCl

The cystallization and capture of hexachlorothallium within the phosphotunsgate and caesium chloride matrix is described.


Chemical context
The Keggin ion, [�-XW 12 O 40 ] nÀ (X = B, Si, P, Ga, Ge, etc.), along with many other polyoxometalates (POMs), are renowned for their ability to lose [WO x ] moieties, yielding lacunary POMs (Pope, 1983).These lacunary derivatives of the Keggin, [XW 11 O 39 ] nÀ , [XW 10 O 36 ] nÀ , and [XW 9 O 34 ] nÀ , have been extensively studied as chelators for metal ions, in which they directly bind to cations, for example trivalent lanthanides and actinides (Wang et al., 2024).Recently, crystallization of microgram quantities of the radioactive element curium (Cm 3+ ) with [XW 11 O 39 ] nÀ , showcased the utility of the POM chelators (Colliard et al., 2022).However, not all metal ions have been able to coordinate to lacunary Keggin derivatives.In particular, some metals form highly stable complexes with smaller ions, like chlorides, impeding their potential interactions with POM chelators.As such, a new method to capture metal ions has been developed where the metal of choice can be captured in the lattice arrangement of the parent Keggin ion, [�-XW 12 O 40 ] nÀ , instead of direct interaction with lacunary Keggin ion ([XW 11 O 39 ] nÀ ).This allows for the POM-induced crystallization of the halide metal complex.

Structural commentary
This new crystal that incorporates thallium(III) into a caesium chloride and phosphotungstates lattice is fully formulated as Cs 9 (TlCl 6 )(PW 12 O 40 ) 2 •9CsCl and crystallizes in the cubic space group Fm3m with a volume of 12,166.8(4) A ˚3,  (Pope, 1983): W-OPO 3 of 2.347 (8) A ˚, W-O in the range 1.918 (2)-1.942(4) A ˚and W O of 1.713 (10) A ˚.The asymmetric unit that describes the Keggin ion is thus represented by the tungsten (W1), oxygen (O1, O2, O3, and O4), and phosphorous (P1) atoms.The tetrahedral symmetry of the Keggin ion thus arises from the tetrahedral symmetry of the central phosphate ion (atoms P1/O1-O4).Atom P1 is on the Wyckoff site 8c, corresponding to 4m3 symmetry, which then extends to O4 with a Wyckoff site of 32f with a symmetry of •3m.The remaining atoms W1, O1, O2, and O3 thus arrange themselves with the same tetrahedral symmetry, however, now with a Wyckoff site symmetry of ••m.The structure further features the sixfold coordinate Tl 3+ ion, making a [TlCl 6 ] 3À complex.The arrangement of the complex within the structure is discussed in the next section.Nevertheless, the asymmetric unit that describes the thallium complex is comprised of Tl1 and Cl1 and Wyckoff site symmetries of 4a and 24e, respectively.The symmetry around Tl1 i.e.Wyckoff site 4a is m3m, with Cl1 having 4mm symmetry.This results in an octahedral complex with six chlorides bound to Tl 3+ .However, the [TlCl 6 ] 3À ion features slightly longer bond lengths between Tl and Cl of 2.613 (12) A ˚compared to 2.423 A ˚in KTlCl 4 (ICSD 1527421;Glaser, 1980).What is unusual about the structure is the large excess of CsCl crystallizing -nine CsCl per [TlCl 6 ] 3À complex.The asymmetric unit only consists of two unique Cs atoms, Cs1 and Cs2 with Wyckoff sites of 48h and 24e, respectively.These caesium atoms can then thus be thought to coordinate to the other chlorides as well, Cl2, and Cl3.Nevertheless, the excess CsCl becomes significantly important when considering the relative arrangement of the two [PW 12 O 40 ] 3À and the [TlCl 6 ] 3À complex, Fig. 2. All caesium counter-ions are nine-coordinated with Cs-O distances ranging 3.179 (10)-3.221(7) A ˚and Cs-Cl ranging from 3.2081 (18)-4.139(12) A ˚.

Supramolecular features
The supramolecular assembly of the crystal is particularly interesting and departs from the typical structures observed with the Keggin ion.The [PW 12 O 40 ] 3À anion herein behaves like a super-atom.Super-atoms are nano-sized structures that mimic atomic behavior, in particular in the lattice formations (Colliard et al., 2020).In this structure, the [PW 12 O 40 ] 3À anion can be thought to arrange itself in a cubic close packing within the unit cell.The Wyckoff letter of P1 (8c) and the single phosphorous per Keggin reveals there are eight Keggin ions
A thallium(III) nitrate solution was prepared by dissolving the corresponding Tl(NO 3 ) 3 in 0.1 M HCl.After this, the Tl 3+ solution was added to a 1 mL 200 mM Na 9 PW 9 O 34 •7H 2 O solution in 0.1 M acetate buffer at pH 5.5 at a 1:2 stoichiometry.For crystallization, 6 M CsCl was titrated in 5-50 mL to 1:2 stochiometric solutions (10 to 100 mL, at pH 5.5, 100 mM acetate buffer) with a final pH of 5.5 during crystallization.After 1-5 days at ambient conditions, several cube-shaped single crystals of PW 12 -TlCsCl were visible to the naked eye.XRD-quality crystals were then mounted and characterized, while the rest were characterized through Raman microscopy.
Raman spectra were collected using a Senterra II confocal Raman microscope (Bruker), equipped with high resolution gratings (1200 lines mm À 1 ) and a 532 nm laser source (operated at 15 mW), and a TE-cooled CCD detector.Reported spectra are the average of at least 2-5 different spots per sample, each spot analysis consisting of 2 binned 16 scans.The integration time was set to 2000 ms per scan.No damage to the sample was observed due to the laser irradiation.Infrared spectra were collected using a Cary 630 FTIR instrument (Agilent Technologies) equipped with an attenuated total reflectance (diamond ATR) cell.Selected Raman data (cm À 1 ):  Computer programs: CrysAlis PRO (Rigaku OD, 2019), SHELXT (Sheldrick, 2015a), SHELXL2019/3 (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.All atoms were refined anisotropically.The only issue resulting from the high Z for tungsten and Cs was that high residual Q-peaks of 10% of Z A À 3 remained (Massa & Gould, 2004); the highest residual Q-peak at 3.9 located at ( 1 2 , 1 2 , 1 2 ) could not reasonably be assigned to any of the elements already present (or those present during synthesis).

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.
Fig. 1.The crystal features the parent Keggin structure, [�-PW 12 O 40 ] 3À , which arises from the successive hydrolysis and condensation reactions of [WO 4 ] 2À in the presence of [PO 4 ] 3À ions as the pH is lowered (ca lower than 7).Briefly, twelve octahedral [WO 6 ] units can be grouped into four trimer sets [W 3 O 13 ] 8À .Each trimer is linked by the central phosphate anion and then to each other, keeping the overall tetrahedral symmetry of the central [PO 4 ] 3À .The W-O bond lengths are all consistent with reported values for other POMs

Figure 2
Figure 2 Ball-and-stick representation of [�-PW 12 O 40 ] 3À and [TlCl 6 ] 3À , showcasing the connectivity with the excess CsCl.W in maroon, O in red, P in blue, Cl in green and Tl in brown.

Figure 3
Figure 3 Ball-and-stick representation of the unit cell viewed along (111) showing the cubic close packing of [�-PW 12 O 40 ] 3À by additionally overlapping the blue spheres to see the ABC layers.The [TlCl 6 ] 3À ion thus fills half the octahedral voids.W in maroon, O in red, P in blue, Cl in green and Tl in brown.
(Balboni & Burns, 2014)llow other elements in the molecule and no results were obtained.As such, the search was expanded to another formula search for any structures with W, O, and Tl, none of which consisted of phosphotungstates and/or thallium compounds.Only one compound containing K, W, O, and Tl was found, but this additionally contains uranium and is not comprised of the Keggin structure(Balboni & Burns, 2014).
� C).Na 9 PW 9 O 34 •7H 2 O was prepared by dissolving 12 g of Na 2 WO 4 •2H 2 O in 15 mL of H 2 O. 0.4 mL of 85% H 3 PO 4 was added dropwise.Afterwards the pH was adjusted to 7-7.5 with glacial acetic acid (2.25 mL).During the addition, a white solid formed immediately.The solid-solution slurry was left to stir for an hour, after which the solid was filtered under vacuum.[PW9 O 34 ] 9À converts to[PW 11 O 39

Table 1
Experimental details.