Structural Expansion of Chalcogenido Tetrelates in Ionic Liquids by Incorporation of Sulfido Antimonate Units

Abstract Multinary chalcogenido (semi)metalate salts exhibit finely tunable optical properties based on the combination of metal and chalcogenide ions in their polyanionic substructure. Here, we present the structural expansion of chalcogenido germanate(IV) or stannate(IV) architectures with SbIII, which clearly affects the vibrational and optical absorption properties of the solid compounds. For the synthesis of the title compounds, [K4(H2O)4][Ge4S10] or [K4(H2O)4][SnS4] were reacted with SbCl3 under ionothermal conditions in imidazolium‐based ionic liquids. Salt metathesis at relatively low temperatures (120 °C or 150 °C) enabled the incorporation of (formally) Sb3+ ions into the anionic substructure of the precursors, and their modification to form (Cat)16[Ge2Sb2S7]6[GeS4] (1) and (Cat)6[Sn10O4S20][Sb3S4]2 (2 a and 2 b), wherein Cat=(C4C1C1Im)+ (1 and 2 a) or (C4C1C2Im)+ (2 b). In 1, germanium and antimony atoms are combined to form a rare noradamantane‐type ternary molecular anion, six of which surround an {GeS4} unit in a highly symmetric secondary structure, and finally crystallize in a diamond‐like superstructure. In 2, supertetrahedral oxo‐sulfido stannate clusters are generated, as known from the ionothermal treatment of the stannate precursor alone, yet, linked here into unprecedented one‐dimensional strands with {Sb3S4} units as linkers. We discuss the single‐crystal structures of these uncommon salts of ternary and quaternary chalcogenido (semi)metalate anions, as well as their Raman and UV‐visible spectra.

Here, we report about the synthesis, structures, and optical absorption properties of the products of the mentioned reactions, (Cat) 16   with SbCl 3 in the ionic liquid (C 4 C 1 C 1 Im) [BF 4 ], in the ionic liquid mixture (C 4 C 1 C 1 Im) [BF 4 ]/(C 4 C 1 C 1 Im)Cl (1:1), or in (C 4 C 1 C 1 Im)Br,respectively.T he structures of the single-crystalline compounds were determined by means of single-crystal X-ray diffraction. [40] Compound 1 crystallizes as yellow blocks in the cubic crystal system,s pace group Fd3 m,w ith eight formula units in the unit cell.  16À }c onsistingo f seven chalcogenido (semi)metalate anions in sum ( Figure 1).

Results and Discussion
Each of the Sb1ÀSb1' bonds within the [Ge 2 Sb 2 S 7 ] 2À anions are located above one of the six S···S edges of the [GeS 4 ] 4À tetrahedra, at an S1···Sb1d istance of 3.337(3) ,a nd ad istance of the center of the Sb1ÀSb1' bond from the center of the S···S edges of the central tetrahedron of 2.420(1) .T he two subunits are thus not covalentlybonded. In order to explore whether they might undergo notable attractive dispersive interactions, we inspected the structural data in detail.
Within the [Ge 2 Sb 2 S 7 ] 2À anions, the terminal Ge2ÀS4 bond length is 2.107(5) ,w hile the bonds to the bridging m-S atoms, Ge2ÀS2 and Ge2ÀS3 , are 2.216 (5)  . [41] The only structural deviation that is notable at all concerns the terminal GeÀSb onds, which are slightly shorter in 1 (by % 3pm) than in the reference compound, which can be explained by aw eak additional interaction between the sulfur atom andahydrogen atom of the (Me 2 NH 2 ) + cations in the latter.
The Ge1ÀS1 bond in the central[ GeS 4 ] 4À anion exhibits a lengths of 2.215(6) ,i ng ood agreement with the literatureknown crystallographic data of Mn 2 [GeS 4 ]( GeÀS2 . 15-2.27 ). [42] The fact that the GeÀSb onds apparently are not influenced by neighboring [Ge 2 Sb 2 S 7 ] 2À cluster anions points at the absence of significant secondary interactions. The clustering of seven complex anions in 1 therefore seems to be driven by the nearly spherical shape of these assemblies, which are Scheme1.Overview of the synthesis of (Cat) 16 (2). The cations of compounds 1 and 2a are heavily disorderedi nthe crystal structures and could therefore not be determined by meanso fsingle-crystalX -ray diffraction, yet most probably,'Cat'r epresentst ris-alkylated imidazolium cations (C 4 C 1 C 1 Im) + ,a ccordingt os inglecrystal Raman spectroscopy(see below). This is indirectly supported by the absence of considerable amounts of Ka ccording to micro X-ray fluorescence (m-XRF)s pectroscopy (seethe Supporting Information) and by the presence of crystallographicallydetermined (C 4 C 1 C 2 Im) + cations in 2b (with' C 4 ', 'C 1 ', and 'C 2 'specifying the chainl engths of the butyl, methyl,a nd ethyl substituents of the imidazoliumr ing denoteda s' Im'). By-products( potassiumh alides and tetrafluoridoborate) are not indicated here; en = ethane-1,2-diamine. then packed in ad iamond-like superstructurei nt he crystal. Figure 2i llustrates the arrangemento ft he anions in the crystal structureo f1.
The 'accessible' void matches relatively well the volume of 16 (C 4 C 1 C 1 Im) + cationsp er formula unit, hence 128 cations within the unit cell ( % 28160 3 for an estimated 20 3 per C or Na tom), given that there are additional voids between the cation molecules. Although we cannote xclude with certainty the presence of K + cations as counterions betweent he anionic assemblies, we can excludet he presence of any cations between the [Ge 2 Sb 2 S 7 ] 2À and [GeS 4 ] 4À units of one such assembly,g iven the small interatomicd istances between them (see above).
Compound 2 was obtained in two different versions, one crystallizing from (C 4 C 1 C 1 Im)[BF 4 ]/Cl (1:1) as yellow blocks in the tetragonalc rystal system,s pace group P4 2 /mbc,w ith four formula units per unit cell (2a), and one crystallizing from (C 4 C 1 C 2 Im)Br as yellow blocks (see Figure S1) in the triclinic crystal system, space group P1 ,w ith two formula units per unit cell (2b). While the two compounds differ in their counterions (see below), they comprise the same anionic substructure. The lower crystallographic symmetry observed for compound 2b results from the fact that the symmetry-reducing counterions are localizable;h ence the high symmetry of 2a is likelya pseudo-symmetry,a st he heavily disordered cationsf orm an isotropic 'solid solution'.
The structural motifs of the related compounds that were quoted above forc omparison witht he substructures of compounds 1, 2a,and 2b are illustrated in Figure 4.
As the linkageo ft he two building units in the anionic substructure of 2 occurs via trans-edgeso ft he supertetrahedra, the entire strand possesses idealized D 2h symmetry (S 4 in the crystal,s ee above), and thus, the position of the two {Sb 3 S 4 } units alternates with respectt ot he twofold axis (Figure 5a). All strandsr un through the crystal in parallel fashion, in the direction of the crystallographic c axis. Neighboring strands along < 0,0,1 > and < 1 = 2 , 1 = 2 ,1 > are oriented perpendicularly to each other,w hich allows to optimize the positions of the {Sb 3 S 4 } groups ( Figure 5).
As provenb yt he crystal structure of 2b,t he cations are located between the strands, therebyf orming 'belts' aroundt he two alternating subunits (see Figure3c). In order to get an experimental hint for the actualp resence of imidazolium counterionsi n2a,w er ecorded Raman spectra of single-crystals of 2a,a nd of the pure ionic liquid that was used for the reaction (Figure 6).
Comparison of the two Ramans pectra indicate clearly the presence of the IL cation in compound 2a,a sv isible from the characteristic signals observed between 700 and 1600 cm À1 . Another signal group whichs temsf rom CÀHv alence vibrations can be found between 2700 and 3200 cm À1 (inset in Figure 6). The signals observed at lower wavenumbers( up to 400 cm À1 )a re assigned to vibrations of the anionic substructures. These regionso ft he spectra of all three compounds reportedh erein, 1, 2a,a nd 2b,a nd the Raman spectrumo btained from the related compound (Cat) 4 [Sn 10 O 4 S 16 (SMe) 4 ], [37] are presented togetherinF igure 7.
Some signals are only observed in the spectra of 2a, 2b, and the reference compound:aRamans ignal of medium strength at ca. 125 cm À1 ,astrong band at 174-180cm À1 , and as ignala tc a. 320 cm À1 in the spectrum of (Cat) 4 [Sn 10 O 4 S 16 (SMe) 4 ], which in the case of 2a and 2b appears slightly blue shifted( ca. 325 cm À1 )a sashoulder in the slightly red-shifted, strongest signal centereda round 345 cm À1 . As said signals are missing in the spectrum of 1,t hey likely stem from SnÀSa nd SnÀOv ibrations within the supertetrahedral oxo-sulfido stannate substructure.
As trong signal that is found exclusively in the Ramans pectrum of compound 1 (ca. 140 cm À1 ), in turn, can be unambiguously assigned to GeÀSo rS b ÀSb vibrations of the [Ge 2 Sb 2 S 7 ] 2À and [GeS 4 ] 4À anionsthat occur exclusively in this compound.
As outlined in the introduction, we aimed at affectingn ot only the development of structuralm otifs, but also the electronic structures by introducing Sb III into group 14 chalcogenido (semi-)metallate substructures. To probe the impact on the electronic properties, optical absorption spectra were recorded, which served to illustrate the presence of building units with We cannot explain the differenceo ft he band gap energies of 2a and 2b with certainty,b ut we assumet hat the highly disordered cationsi n2a allow for ad enser packing of the anions. This is in agreement with as lightly smaller unit cell volumeo bservedf or 2a (11098.4(17) 3 )a sc ompared to the unit cell in 2b (11431.8(12) 3 ), see Table S4.
Ac omparison witht he band gaps of the formally underlying binary compounds, GeS 2 (3.43 eV,d irecta llowed transition), [48] SnS 2 (2.38 eV,d irecta llowed transition), [6] and Sb 2 S 3 (1.78 eV, direct allowed transition), [49] clearlyr eflect the multinary elemental composition of the anionic substructures:i n1,t he relatively large amount of Sb III leads to as ignificant narrowing of the band gap, which is distinctly smaller than that of bulk GeS 2 .T he comparably small degree of Sb III incorporation into compound 2,i nc ontrast, is reflected by ab and gap that is relativelyc lose to (a bit larger than) that of SnS 2 -yet one needs to keep in mind that (a) this is ac lusters ubstructure, not bulk SnS 2 ,a nd that (b) the cluster composition is furthermore optically 'diluted' by SnÀOu nits. Beyond this background, ab and gap that is similart ot he one in SnS 2 also indicates an otable effect of Sb III incorporation. Thisi sa dditionally supported by the fact that similarly-sized crystals of (Cat) 4  O wing to ah ight endency for disorder of the (C 4 C 1 C 1 Im) + molecules, the counterions could only be located in the crystal structure of 2b,c omprising (C 4 C 1 C 2 Im) + .H owever,R aman spectra served to corroboratet he presence of ionic liquid cations in the other salt. Raman spectra of all three compounds in comparison with that of recently reported Figure 8. UV-visible spectra (left hand side) and Ta uc plots generated using the Kubelka-Munkfunction (F(R 1 )hn) 1/g with g = 0.5 (righth and side) of single-crystalline 1 (a), 2a (b), and 2b (c). The measurement was performed under inert conditionse mploying aP raying Mantisa ccessory. [50][51][52] (Cat) 4 [Sn 10 O 4 S 16 (SMe) 4 ]a llowed to assign the most relevant bands. Finally,o ptical absorption spectra clearly demonstrate the effect of af ormal admixture of Sb III to the sulfido (semi)metalate compounds, whichl eads to significant narrowing of the correspondingband gaps.
Deposition Numbers 2024786 (1), 2024787 (2a), and 2024788 (2b) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures. UV-visible spectroscopy:O ptical absorption spectra were recorded on aV arian Cary 5000/UV/Vis/NIR spectrometer in the range of 200-800 nm in diffuser eflectance mode employing aP raying Mantis TM accessory (Harrick). For ease of viewing, raw data was transformed from %Reflectance Rt oAbsorbance Aa ccording to A = log (1/R). [50] The recorded diffuser eflectance spectra were converted in Ta uc plots by using the Kubelka-Munk function (K-M) to estimate the indicated band gap energies of allowed (in)direct transitions [51,52] [Eq. (1)]: where k is the K-M absorption coefficient, R1 is the diffuse reflection, and s is the K-M scattering coefficient. [54,55] Ta uc plots were generated by plotting ðFR ðÞ hnÞ 1=g as af unction of the photon energy hn. The power coefficient might be g = 1 = 2 , 2 = 3 ,2or 3, depending on the nature of the transition, which corresponds to direct allowed, direct forbidden, indirect allowed, or indirect forbidden transitions, respectively. E g is estimated from the intercept with the x axis of the linear fit from the corresponding region. [56] Raman spectroscopy:R aman data was collected on an S&I Mono-Vista CRS + device. The measurements were performed with a laser wavelength of 633 nm and ag rating of 300 and 1200 grooves mm À1 .Each measurement had ad uration of 5swith 10 coadditions and 10 sw ith 25 coadditions.