Highly Soluble Supertetrahedra upon Selective Partial Butylation of Chalcogenido Metalate Clusters in Ionic Liquids

Abstract Supertetrahedral clusters have been reported in two generally different types so far: one type possessing an organic ligand shell, no or low charges, and high solubility, while the other cluster type is ligand‐free with usually high charges and low or no solubility in common solvents. The latter is a tremendous disadvantage regarding further use of the clusters in solution. However, as organic substituents usually broaden the HOMO–LUMO gaps, which cannot be overcompensated by the (limited) cluster sizes, a full organic shielding comes along with drawbacks regarding opto‐electronic properties. We therefore sought to find a way of generating soluble clusters with a minimum number of organic substituents. Here, we present the synthesis and full characterization of two salts of [Sn10O4S16(SBu)4]4− that are high soluble in CH2Cl2 or CH3CN, which includes first NMR and mass spectra obtained from solutions of such salts with mostly inorganic supertetrahedral clusters. The optical absorption properties of this new class of compounds indicates nearly unaffected band gaps. The synthetic approach and the spectroscopic findings were rationalized and explained by means of high‐level quantum chemical studies.


Introduction
Theu nique properties of materials based on supertetrahedral clusters,w hich are related to zeolites their general composition, have been documented in many reports in the past. [1][2][3][4][5][6][7][8] In some cases,the desired properties require linkage of the clusters into networks-for instance,i fc ertain pore sizes are needed for ion selectivity like in [H 3 [5] Forphotocatalytic properties,heterogeneous mixtures can be employed, for which the large diversity of open network structures like Na 14 [In 17 Cu 3 S 35 ]·x H 2 O [6] or HAEM 1. 6 [Ge 3.2 Zn 0.8 S 8 ] (HAEM = N-(2-aminoethyl)-morpholine-H + ) [7] are wonderful sources of tunable materials.Similar properties can also be achieved with salts of separate cluster units,a sr eported for the excellent photocatalytic activity of (C 4 C 1 C 1 Im) 9 [Cd 3 In 17 S 31 Cl 4 ]( C 4 C 1 C 1 Im = 1-butyl-2,3-dimethylimidazolium). [8] However, all of the named materials need to be used as suspensions,asthe relatively high charge of the anionic cluster substructures or the networks formed from them leads to an intrinsically low solubility in any common solvent. Consequently,a lso further derivatization or other chemical treatment has proven unsuccessful. This applies also to af ew compounds possessing solubility in highly polar solvents:Itwas shown that areplacement of Satoms with Se atoms in (C 4 C 1 C 1 Im) 9 [Cd 3 In 17 S 31 Cl 4 ]a llows solubility in DMSO (dielectric constant e % 41), [8] and compounds with supertetrahedral clusters of lower charges,l ike salts of [M 4 Sn 4 E 17 ] 10À (M = Co,F e, Mn, Zn;C d, Hg;E = S, Se, Te ), [9] dissolve in highly polar solvents like water (e % 80) or formamide (e = 109). In turn, clusters that are soluble in less polar solvents through shielding by organic group at bridging chalcogenolate ligands do usually show reduced opto-electronic features owing to significantly enhanced optical gaps, [10,11] unless the cluster sizes are getting huge,s uch as [Cd 54 S 32 (SPh) 48 (H 2 O) 4 ] 4À , [12] so that surface effects are overcompensated by asufficiently large cluster volume.
Thel ogical way of overcoming the named limitations would be the design and formation of compounds with molecular supertetrahedral clusters that exhibit as mall degree of organic shielding only-sufficient to solubilize the compounds in common organic solvents like CH 2 Cl 2 (e = 9.1) or CH 3 CN (e = 37.5), yet without affecting the HOMO-LUMO gaps,s imilar to recent reports in the field of polygermanide anions forming highly soluble derivatives [Ge 9 {Si(SiMe 3 ) 3 }] À , [13] [Ge 9 {Si(SiMe 3 ) 3 } 3 {SnPh 3 }], [14] or [RGe 9 -CH = CH À CH = CH-Ge 9 R] 4À (R = (2Z,4E)-7-amino-5aza-hepta-2,4-dien-2-yl). [15] Theo nly problem to be solved is thus finding ac orresponding synthetic approach for such target compounds.S of ar, all attempts to alkylate chalcogenido metalate clusters in ap ost-synthetic way failed-either due to the incompatibility of the alkylation reagents,like MeI or Meerwein salt, with the solvents from which the clusters were (irreversibly) obtained in crystalline form, or owing to the inherent insolubility of the cluster compounds in (organic) solvents appropriate for alkylation reactions.T he extremely low nucleophilicity of chalcogenido metalate clusters in general is another important aspect that prohibited success in this direction so far.
Recently,wereported first promising results by presenting am ethod that allowed to form salts of partially methylated chalcogenido metalate clusters,w hich were heretofore only known as purely inorganic and highly charged molecules. [16,17] Themethylation was achieved by ionothermal syntheses [18] in imidazolium-based ionic liquids (C l C m C n Im)X (C l ,C m ,C n = alkyl chains with l, m,orn carbon atoms in 1-, 2-, or 3-position of the imidazolium ring Im;X= Cl, Br,[ BF 4 ]). [19] With l or n being 1, thus ionic liquids with methyl substituents at one of the nitrogen atoms,t he methyl group was selectively transferred to the terminal chalcogenide ligands of the investigated clusters,w hich served to reduce the negative charge of the anion by the number of alkyl substituents-for instance from À8i n[ Sn 10 O 4 S 20 ] 8À [20][21][22] to À4i n[ Sn 10 O 4 S 16 (SMe) 4 ] 4À . [16] Interestingly,t he use of ionic liquids with ac ombination of any other alkyl groups and methyl substituents at the nitrogen atoms of the imidazolium ring also led to the selective transfer of the methyl group in all tested cases.H owever, as the methylation did not afford the desired increase in solubility, we continued to seek for away of selectively transferring alkyl groups with larger chain lengths in order to ultimately from readily soluble supertetrahedral clusters.
To avoid am ethyl transfer, the new studies started out with the synthesis of as ymmetrically butyl-substituted ionic liquid, (C 4 C 1 C 4 Im)Br (A), and its use in the cluster formation reaction. Indeed, as elective butylation of the discrete supertetrahedral clusters was achieved this way,a st he methyl group at the Ca tom in 2-position of the imidazolium ring is not transferred at all. This result is not only remarkable with respect to the synthetic approach that allows for abutylation of an extremely weak nucleophile,b ut finally allowed access to first salts of mainly inorganic supertetrahedral clusters that show high solubility in common organic solvents like CH 2 Cl 2 and CH 3 CN.H ence,w ed on ot only present the structural characterization of the products,but also show first NMR and mass spectra obtained from solutions of compounds with supertetrahedral cluster anions exhibiting purely inorganic bridging ligands.

Syntheses and Quantum Chemical Studies Regarding the Formation
Thetarget compound 1 was obtained in two different salts with the sum formulas (C 4 C 1 C 4 Im) 4+x [Sn 10 O 4 S 16 (SBu) 4 ]Br x with x = 3( 1a)a nd x = 2( 1b). 1a was prepared by in situ butylation during the cluster formation reaction, which was carried out by treatment of K 4 [SnS 4 ]·4 H 2 O [23] in (C 4 C 1 C 4 Im)Br (A) [24] in the presence of 2,6-dimethylmorpholine (DMMP) at 180 8 8C. Crystals of 1b form upon dissolving crystals of compound 1a in CH 3 CN and subsequently removing the solvent under reduced pressure.W ea ttribute this finding to the high tendencyofthe ionic liquids to remain in solution, such that the crystals of 1a lose one formula unit of it under these conditions.S cheme 1s ummarizes the synthetic steps towards compounds 1a and 1b.
Thestructures of 1a and 1b were determined by means of single-crystal X-ray diffraction. [25] Thep resence of butyl substituents and the ionic liquid cations were confirmed in the solid state by means of Raman spectroscopy,a nd in solution by mass spectrometry,w hich provided us with the first mass spectrum of this cluster type at all.
Corresponding reaction energies in kJ mol À1 for reactions (1)-(3) are À617.5 (1), À586.5 (2), À592.5 (3). All values point to exoenergetic processes.Acomparison of the energies of reactions (1) and (2) furthermore indicates ac lear preference of am ethylation (and not ab utylation) to take place if (C 4 C 1 C 1 Im) + is used. Finally,t he butylation becomes more exoenergetic with the use of (C 4 C 1 C 4 Im) + as butylation agent than with (C 4 C 1 C 1 Im) + .All of the calculated energies are thus in full agreement with our experimental findings.
Reaction energies that are calculated for reactions that include differently high charges of reactants and products can be afflicted with systematic shifts from COSMO.A lthough these should be similar for the analogous reactions (1) to (3), thus rendering the trends reliable,wealso considered (hypothetical) exchange reactions (4) = (1)-(2) and (5) = (1)-(3), in which the charges of the involved species are the same on both sides of the reaction arrow with reaction energies in kJ mol À1 of À25.0 (4) and À31.0 (5). These numbers confirm that am ethylation in general is preferred over ab utylation. This rationalizes that the usage of an ionic liquid in which the nitrogen atoms of the imidazolium ring bear exclusively butyl substituents is essential for the formation of butylated clusters.W en ote in passing that corresponding calculations with Meerwein salt as methylation agent also yield (even more) negative reaction energies;y et such reactions cannot

Angewandte Chemie
Research Articles be practically carried out, as there is no solvent that is compatible to both the starting materials and Meerwein salt.

Crystal Structures
Compound 1a crystallizes in the triclinic space group type P1 ,w ith four formula units per unit cell. Theb utyl chains of both the anions and the cations are highly disordered, thus could not be fully localized on the difference Fourier map during the refinement. However,a so utlined below,o ther analytical techniques,t he analysis of the void space,a nd the comparison with compound 1b allowed to confirm the composition. Compound 1b crystallizes in the same space group type,b ut with two formula units only in the unit cell. Thecrystal structures are illustrated in Figure 1.
Besides the fact that the crystal structure of 1b is only slightly affected by disorder,asthe removal of one equivalent of (C 4 C 1 C 4 Im)Br obviously increased the lattice energy and thus crystallinity of the compound, the cluster molecules in both compounds are very similar (Figure 1a,b;for acomprehensive list of structural parameters,see Table S4). Sn À S terminal distances,which are indicative of an alkylation of the terminal Sligands,amount to 2.427(4)-2.448(4) in 1a and 2.429(4)-2.500(7) in 1b,while they were reported to be significantly shorter (2.355-2.374 )i nt he purely inorganic cousins. [20][21][22] Yet, the observed values are in good agreement with the SnÀ S terminal distances found for the methylated clusters (2.419(17)-2.511 (11) ). [16] An otable difference between the two compounds,h owever,i st he number x of co-crystallizing formula units of the ionic liquid. In 1b, x is unambiguously determined to be 2, while for 1a the corresponding number had to be derived from the refinement of atoms plus an analysis of the differences in void spaces.B yu sing the SQUEEZE tool [33] in OLEX2, [25] the void space volumes V VS were obtained by subtraction of the calculated volumes of the cluster units per unit cell from the unit cell volumes.T he difference in V VS between the two compounds, DV VS = V VS (1a)ÀV VS (1b), amounts to 494 3 ,w hich is in perfect agreement with the calculated volume of one formula unit of (C 4 C 1 C 4 Im)Br (497 3 )obtained with SQUEEZE from the structure of 1b. This comparison also supports the correctness of the entire formula of 1a,i nw hich the number of counterions is anticipated to be seven-in agreement with the number of cluster anions and Br atoms found on the difference Fourier map,w hile not all of the imidazolium cations could be localized this way.O bviously,t he different number of cocrystallizing (C 4 C 1 C 4 Im)Br units additionally affects the overall packing of anions and cations in the crystal (Figure 1c-f,a nd Figures S6,S7). Thec luster anions are evenly distributed in the crystal structure of 1b with center···center distances of 15.7714(2)-19.1377(3) .T he relative orientation of the cluster units is ab it different in 1a.H ere,t wo clusters show ac loser proximity,h ence forming pairs (cen-ter···center distance:1 2.0893(1) ), while the other center···center distances are in the range of 15.7675(2) to 22.1975-(3) .

Spectroscopy,Spectrometry,and Quantum Chemical studies of the Spectroscopic Findings
Single crystals of 1a and 1b dissolve well in organic solvents,w hich discriminates them from all reported salts of supertetrahedral chalcogenido metalate anions.S olutions in CH 3 CN-d3 were thus analyzed by 1 H-, 13 C-, and 119 Sn-NMR spectroscopy (Figure 2a nd Figure S8). Thep redominant peaks that are highlighted in the 1 H-NMR spectrum (Figure 2a)c onfirm the presence of the (C 4 C 1 C 4 Im) + cations (blue highlight). As the single crystals were covered by Structure diagrams with full labelling Scheme are given in the Supporting Information (Figures S6 and S7).
residues of the reaction medium that possesses ah oney-like consistence,w ec an also demonstrate the fate of the ionic liquid cation that served for the butylation:t he resulting species could be identified as minor component C 4 C 1 im in the spectrum (im = imidazole;r ed highlight in Figure 2a). The butyl signals of the alkylated cluster anions could not be assigned due to the very similar chemical shift of the other butyl substituents.
The 119 Sn-NMR spectra (Figure 2b)s how two predominant signals for the butylated supertetrahedral cluster,one at À117.19 ppm (Sn A )a nd one at À518.74 ppm (Sn B ). Both signals exhibit satellites that we assign to 2 J(Sn À O/S À Sn) couplings:T he satellites of the first show coupling constants of 490 and 245 Hz, while the values for the latter are 376, 245, and 144 Hz. Theo bserved 119 Sn chemical shifts are in accordance with literature data for corresponding coordination numbers at the Sn IV atoms: [34][35][36][37] Sn A atoms are situated in between at etrahedral and at rigonal bipyramidal ligand environment, {SnS 4 ···O} (Sn···O 2.477(5)-2.550(4) ), Sn B atoms exhibit a pseudo-octahedral coordination, {SnS 4 O 2 } (SnÀO2 .077(5)-2.116(6) ). Furthermore,the coupling constants are in good agreement with data reported for [(PhCH 2 Sn) 12 (m 3 -O) 14 (m-OH) 6 ], for which 2 J values were found to be between 483 and 115 Hz. [38] As it is difficult to judge about the relative intensities with the naked eye (and as integration of the peaks is afflicted with high inaccuracies owing to different relaxation times in different chemical environments of the Sn atoms), we corroborated our assignment of the 119 Sn data by means of DFT calculations.W enote that the applied methodology for the calculation of coupling constants was implemented in the turbomole program suite only recently,and applied here for the first time:t he NMR shifts were calculated within the quasi-relativistic all-electron exact two-component (X2C) method [39] in its scalar-relativistic one-component variant, [40] coupling constants with two-component X2C. [41] Thef inite nucleus model [42] and the diagonal local approximation for the unitary decoupling transformation (DLU) [43] were used, and further triple-z basis sets [44] and integration grids optimized for all-electron calculations (size 4a). [45] Calculated chemical shifts (w.r.t. SnMe 4 )a re found at À5.8 ppm for Sn A and À411.4 ppm for Sn B .B oth are thus systematically shifted by % 115 ppm relative to the experimental values,but exhibit an excellent agreement of the shift difference Dd % 406 ppm with respect to the experimental spectrum (Dd % 401 ppm), which confirms our assignment. Measurements of indirect spin-spin coupling constants show acoupling constant of 245 Hz that is present in both Sn peaks (see Figure 2b), and thus most probably originates from the coupling 2 J(Sn A ÀSn B ). Fort he "inner" Sn B atoms,this is accompanied by asecond coupling constant that is lower by 101 Hz, and probably originates from the 2 J(Sn B À Sn B )c oupling.C alculated numbers overall are higher [mean values of 363.3 Hz for 2 J(Sn A ÀSn B )and 337.3 Hz for 2 J(Sn B ÀSn B )],but again, 2 J(Sn A ÀSn B )issignificantly larger than 2 J(Sn B ÀSn B ). We note that in view of the large range of values for Sn coupling constants,the comparably large shift in absolute number is not unexpected, similar as for the chemical shielding constants.T he weak satellites in the experimental spectrum at 499 Hz for Sn A and 376 Hz for Sn B cannot be attributed to 119 Sn coupling constants.
As an additional prove of the identity of the cluster units in 1a and 1b,Raman spectra were recorded on single crystals of both compounds and compared ( Figure S9). Both spectra exhibit the characteristic signal group of the Sn/O/S framework between 100 and 400 cm À1 ,w hile the signals at around 700 cm À1 and around 2900 cm À1 are clearly different from the signals observed for single crystals of the methylated anions. This confirms that the cluster anions in 1a are also butylated.
Another peculiarity of the butylated supertetrahedra and their significant solubility is the possibility to transfer them into the gas phase without decomposition. By means of electrospray-ionization mass spectrometry (ESI-MS) in negative ion mode,wewere able to detect the butylated anion-as am ono-anionic aggregate along with three (C 4 C 1 C 4 Im) + cations,{(C 4 C 1 C 4 Im) 3 [Sn 10 O 4 S 16 (SBu) 4 ]} À .The overview spectrum and the high-resolution of the molecular peak is shown in Figure 3. Thefull version of the inset with all m/z values of the individual peaks is provided in the Supporting Information ( Figure S10). We would like to emphasize that this is the first spectrum of asupertetrahedral cluster that possesses such asmall number of organic substituents.
To probe to which extent the four organic groups would affect the electronic properties of the clusters,w er ecorded optical absorption spectra of the solid compounds as well as their solutions,a nd compared them to the spectrum of the methylated and the ligand-free analogs.A si llustrated in Figures 4a and b, both solid compounds exhibit very similar values for their onset of absorption at around 3.5 eV in accordance with their colorless appearance.T he salt of the methylated cluster [16] exhibits avery similar excitation energy (3.35 eV). Corresponding values of the salts of the purely inorganic cluster anion [Sn 10 O 4 S 20 ] 8À were not reported, but for compounds with at wo-dimensional linkage of such clusters,t he value was determined to be 2.98 eV,w hich we Figure 2. NMR spectra of asolution of single crystals of 1b in CH 3 CN-d3 with residues of attached reaction medium (causing minor signals). a) 1 H-NMR spectrum.b ) 119 Sn-NMR spectrum with zoom into the signals assigned to the two different Sn atomic sites (cf. Figure 1a: Sn A ,red;Sn B ,b lue). Note that the relative intensities do not reflect the relative abundance owing to the measurementtechnique. take as al ower limit of the value of the separated cluster molecules. [46] This suggests that the partial alkylation only slightly affects the electronic structure of the cluster,and that the chosen alkyl group makes no difference in this regard. As expected, the spectrum recorded in solution (Figure 4c) indicates as lightly blue-shifted onset of absorption for the individual clusters-by % 0.1 eV to 3.62 eV.
Thecalculated absorption energies (and respective onsets of absorption) are in excellent agreement with the data measured on the individual clusters in solution, shifted by only % 0.1 eV to higher energies.W ithin the calculated spectra, the alkylated clusters show as light blue-shift of % 0.35···0.4 eV with respect to the purely inorganic cluster, hence in perfect agreement with the blue-shift of % 0.35···0.5 eV observed in the experimental spectra. This blue-shift can be understood upon inspection of the involved orbitals.T he lowest energy transition is dominated by charge transfer from S(3p) orbitals of the corner Sl igands (red contours in Figure 5) into orbitals of the cluster core (blue contours in Figure 5). As one of these S(3p) orbitals per S atom is involved in the SÀCb ond to the alkyl group,t hus lowered in energy,its excitation requires more energy than in the purely inorganic case.This explains,why the overall effect is notably smaller than for fully protected clusters,w here all surface chalcogenide ligands are involved in chalcogen À ligand bonds.Hence,while we significantly vary the solubility of the super-tetrahedra, we keep their opto-electronic properties comparably constant (with as light tendency towards smaller energy differences between the first and second excitation bands) when going from ligand-free clusters to methylated, and finally,butylated versions.

Conclusion
Reactions of K 4 [SnS 4 ]·4 H 2 Oi na nd with the ionic liquid 1,3-dibutyl-2-methylimidazolium bromide,(C 4 C 1 C 4 Im)Br, allowed for the straight-forward synthesis of salts of partially butylated supertetrahedral cluster anions [Sn 10 O 4 S 16 (SBu) 4 ] 4À . In contrast to methylated or purely inorganic analogs,t he compounds are fully soluble in common organic solvents CH 2 Cl 2 and CH 3 CN.W ec ould thus record first NMR and mass spectra of solutions of this cluster type,w hich are presented in this work. Remarkably,the partial and selective alkylation of the supertetrahedrast erminal Sl igands does only slightly affect the electronic structures of the compounds, which possess only slightly blue-shifted excitation energies in regard to the purely inorganic salts.D FT calculations confirmed that the methylation in general is preferred over the butylation, and they allowed for the assignment of shifts and coupling constants to inner/outer Sn atoms.T imedependent DFT calculations showed that the lowest electronic excitation band is dominated by charge transfer from 3p orbitals of the corner Sl igands into orbitals of the cluster core.O verall, the findings provided us with au nique perspective of ar ich follow-up chemistry and, ultimately,u se of soluble supertetrahedral clusters for opto-electronic applications in the future.