[M4Sn4Se17]10– Cluster Anions (M = Mn, Zn, Cd ... - ACS Publications

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[M4Sn4Se17]10− Cluster Anions (M = Mn, Zn, Cd) in a Cs+ Environment and as Ternary Precursors for Ionothermal Treatment Silke Santner and Stefanie Dehnen* Fachbereich Chemie and Wissenschaftliches Zentrum für Materialwissenschaften, Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, D-35043 Marburg, Germany S Supporting Information *

(ILs) possess excellent solvation properties, such as high thermal stability and negligible vapor pressure, and can act as both ionic solvents and structure-directing agents so that the presence of auxiliaries is not necessarily needed;16 however, the latter may serve to fine-tune the aggregation conditions. Reactions of binary precursors like [SnSe4]4− or [Ge4Se10]4− together with SnCl4· 5H2O and amines such as dimethylmorpholine (DMMP) or 1,2diaminoethane (en) in the IL [BM(M)Im][BF4] [BM(M)Im = 1-butyl-(2,)3-(di)methylimidazolium] led to the formation of a variety of materials with different compositions including binary or ternary systems, with topologies varying from a unique, discrete cluster through one-dimensional (1D) chains and twodimensional (2D)-layered structures to 3D extended frameworks.17 Having been interested in how ternary precursor anions, such as the P1 clusters in 1−3, would behave under the according conditions, we treated compound 1 with en in [BMIm][BF4], yielding [Mn(en) 2.5 (en-Me) 0.5 ][Sn 3 Se 7 ] (4; en-Me = H2NC2H4NHCH3) with a layered anionic 2D {[Sn3Se7]2−} substructure and in situ formed [Mn(en) 2.5 (en-Me)0.5 ]2+ counterion complexes. 1−4 were characterized by single-crystal X-ray diffraction and by UV/visible spectroscopy. 1 and 2 crystallize isotypically in the triclinic space group P1̅, with only one cluster anion within the asymmetric unit, while 3 possesses a larger triclinic unit cell with two cluster anions per asymmetric unit. As expected, the cell volumes of the three cesium compounds follow the trend of the ionic radii and expand with increasing size of the transition-metal ions and thus the cluster diameters. The cluster anion of 1 (M = Mn) is shown as an example in Figure 1 in comparison with the Mn/Sn/Se cluster anion observed within the according series of K+ salts.13d,e

ABSTRACT: Investigations on the transformation of selenidostannates in ionic liquids were extended by using ternary P1-type cluster precursors [M4Sn4Se17]10− [M = Mn (1), Zn (2), Cd (3)], which were synthesized for the first time as their Cs+ salts. Treatment of 1 with 1,2diaminoethane (en) in [BMIm][BF4] yielded two-dimensional-layered [Mn(en)2.5(en-Me)0.5][Sn3Se7] (4; en-Me = H2NC2H4NHCH3). 1−4 were characterized by singlecrystal X-ray diffraction and UV/visible spectroscopy.

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halcogenidometalates with large structural diversity, varying from isolated clusters to extended three-dimensional (3D) networks with large pores and cavities, combine the advantages of zeolite-like structures with the physical properties of chalcogenides such as semiconductivity or photoluminescence.1 This allows the synthesis of new functional materials and their applications as optoelectronic devices,2 photocatalysts,3 thin-film solar cells,4 or ion exchangers.5 Traditional synthesis methods involve solution reactions, solvothermal or hydrothermal approaches, and solid-state reactions to generate the products directly at high temperatures or by subsequent extraction.5c,6−8 First investigations with binary systems yielded supertetrahedral Tn-type clusters based on [In10S18] units.1b,5a,b,9 Expansion to other binary as well as new ternary and quaternary elemental combinations using group 13/14 and transition metals together with chalcogenides allowed for a fine-tuning of the physical properties and yielded ternary networks. Examples are [GaxSn4−xSe8]x− connected to [Zn4Ga16Se33]10− 10 or isolated Tn clusters like [M5Sn5S20]10−.6b,11 Also well-known in the literature is the pentasupertetrahedral Pn cluster family, including the P1 cluster anion [M4T4E17]10−, which could be synthesized both by flux syntheses, e.g., in K10[M4T4S17] (M = Mn, Fe, Co, Zn; T = Ge, Sn),12 and by a solution approach in protic solvents HOR (R = H, Me), which causes crystallization as respective solvate salts [A10(H2O)x(MeOH)y][M4Sn4E17] (A = Na, K, Rb; M = Mn, Zn, Cd, Hg; E = Se with A/x/y = K/16/0.5 and Na/34/ 0 or Te with A/x/y = K/20/0) with up to 34 crystal solvent molecules per formula unit.13 The title compounds presented herein complete this series by the addition of the so-far-missing Cs+ salts [Cs10(H2O)x][M4Sn4Se17], with M = Mn (x = 15.5, 1), Zn (x = 15, 2), and Cd (x = 17.5, 3). Elegant ways to synthesize crystalline chalcogenidometalates under mild conditions have been applied recently by the ionothermal14 and surfactant-thermal15 approaches. Ionic liquids © XXXX American Chemical Society

Figure 1. Molecular structure of the P1-type [Mn4Sn4Se17]10− anions in 1 (left, thermal ellipsoids at 50% probability) and in the respective K+ salt (center, ball-and-stick representation)13d and an overlay of the projections along the 21 axis (right, Cs+ salt 1 in green and K+ salt in red). Received: October 29, 2014

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DOI: 10.1021/ic5026087 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

As a first attempt to use ternary precursors in ionothermal reactions, crystals of compound 1 were treated as synthesized in [BMIm][BF4], being heated within a sealed pyrex ampule in the presence of en for 5 days at 150 °C (VIL/Ven = 0.25 mL/0.05 mL). All three components of the P1 cluster anion, Mn, Sn, and Se, have been transferred into the product [Mn(H2NC2H4NH2)2.5(H2NC2H4NHCH3)0.5][Sn3Se7] (4), which crystallizes in the monoclinic space group P21/c. It is worth noting that, under the reaction conditions, en was methylated, in part, by reaction with the IL cation, and the product of the latter was incorporated into the coordination sphere of the Mn2+ cation. What was first considered to be an error of the crystal structure analysis and then to be a singular exception during the first synthesis of 4 turned out to occur regularly during this reaction, as has been previously observed under solvothermal conditions.18 As illustrated in Figure 2 (bottom), and in Figures

The cluster anions possess nearly ideal tetrahedral symmetry, reflected in the slight difference of the edge lengths of the anions measured as the distance between two terminal selenium atoms and relatively small deviations from tetrahedral angles around the metal atoms (see Table S2 in the Supporting Information, SI). As for the reported compounds, all metal−Se bond lengths are in the expected range. It is worth noting that the different cations in the two known series (Cs+ versus K+) affect the cluster geometry of the anions. While the crystal structure of the corresponding K+ salts shows higher symmetry because the cation arrangement allows for a tetragonal space group (P41212 or P43212), the opposite can be observed concerning the symmetry of the cluster molecules: the P1-type arrangement in 1−3 (although possessing local C1 symmetry) is by far less distorted than that within the corresponding K+ salts, although the latter possess local point group symmetry C2. The M−Se−M angles around the μ4-selenium atoms in 1−3 differ only slightly from the ideal value for tetrahedral coordination [108.1(6)−111.1(7)°], and the terminal [SnSe4] groups are only slightly twisted around the μ4Se···Sn axes. Whereas this differs from the situation in the K+ salts of the M/Sn/Se anions, it is similar to the observation made for the homologous [M4Sn4Te17]10− series (M = Mn, Zn, Cd). However, in these cases, low distortion comes along with crystallization in the cubic crystal system, which is intuitive. In retrospect, the mentioned distortion of the anions of the K+ salts cannot be explained, but the crystal symmetries that result from details in the cation arrangements are apparently plausible. Generally, the arrangement of the cations around the clusters is similar in the K+ and Cs+ salts. Figure S3 highlights the coordination of adjacent Cs+ cations. The crystal structures of 2 (M = Zn) and 3 (M = Cd) are provided in the SI. The six edges of the supertetrahedal clusters are capped by six crystallographically independent cations (disregarding disorder) via four A+···Se contacts. Overall, this leads to a distorted octahedral arrangement of the respective cations. However, because of the smaller ionic radius of K+ compared to that of Cs+, the K+ positions show heavy disorder on these positions. In both the Cs+ and K+ salts, up to three cluster anions are linked through the cations, which leads to the formation of a 3D coordination network. In addition, Cs1, Cs2, and Cs9 and the disordered positions Cs20 and Cs21 are arranged in chains bridging two to four cluster anions so that each Cs+ cation possesses four to six Cs···Se contacts. The coordination spheres around the cations are completed with up to six water molecules, some of which are also disordered. The optical absorption behavior of 1−3 was investigated by UV/visible spectroscopy of suspensions of the single crystals in Nujol oil (Figure S12 in the SI). The color of the crystals is well reflected in the excitation energies that have been gathered from the spectra. Replacement of Mn2+ by Cd2+ and Zn2+ leads to a blue shift in the quoted order of M (1, 1.9 eV; 3, 2.2 eV; 2, 2.3 eV), similar to that observed for the K+ salts.13d Besides the expected impact of a variation of M on the excitation energies, the exchange of cations is also visible, with a slight red shift (0.22−0.37 eV) observed for the Cs+ compounds in comparison with the K+ salts. This is due to the slightly smaller amount of crystal solvent molecules in the Cs+ salts (15−17.5 vs 16−16.5 in the K+ analogues) along with a larger coordination number of the Cs+ ions, which causes more Cs···Se contacts (47 Cs+···Se contacts on average per cluster vs 23 K+···Se contacts), although the shortest intercluster distances are larger (μ4-Se···μ4-Se: 12.1 vs 10.0 Å on average for Cs+ vs K+ salts). This indicates clearly that the observed absorption energies are not exclusively molecular-based.

Figure 2. Fragment of the 2D {[Sn3Se17]2−} layers in 4 (top). Red or green polyhedra denote up or down orientations of the [Sn3Se4] units with regard to the layer of tin atoms. Cation complexes that are situated below the layer are given in transparent. Asymmetric unit (bottom center) and illustration of the interaction of the en-Me H atoms at the connection of the Sn/Se substructure (bottom left and right).

S9−S11 in the SI, the additional methyl group might additionally contribute to the stabilizing cation···anion interactions and also to a specific variation of the Sn/Se network: the layers represent a distorted variation of the well-known honeycomb network of defect-heterocubane [Sn3Se4] units, each of which are linked by two μ-selenium bridges per tin atom to three further units. The resulting 2D {[Sn3Se7]2−} substructure in 4 is shown along with the asymmetric unit in Figure 2. All tin atoms are arranged in layers parallel to {−102}, with the selenium atoms located below and above. The 2D {[Sn3Se7]2−} layers are packed in a hexagonal manner so that each second layer is congruent. The orientations of the defect-heterocubane units (red and green polyhedra in Figure 2) alternate with regard to the layer of tin atoms. The composition and general topology of the anionic substructure of 4 has been reported for several further compounds in the literature.7b,14g,19,20 However, most of them, such as [BMIm]2[Sn3Se7],16 comprise anionic layers with regular hexagonal rings (mean atom-to-atom diameter of 11.33 Å). In B

DOI: 10.1021/ic5026087 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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contrast, the hexagonal rings in 4 are heavily distorted, being elongated because of a folding of the [Sn2Se2] rings that connect the defect-heterocubane units. As a result, minimum and maximum diameters are measured at 7.77 and 13.87 Å, respectively. This kind of distortion has only been reported for one further compound, [Mn(peha)2][Sn3Se7],20 with an Mn2+ cation coordinated by a hexadentate pentaethylenehexaamine (peha) molecule in a trigonal-prismatic manner. However, different from the synthesis of 4, the quoted compound was not generated by a ternary anion but by treatment of elemental tin with selenium and MnCl2·4H2O in a polyamine/glycol mixture in a Teflon-lined steel vessel at 190 °C for 1 week. In compound 4, the layers are separated by the metal complexes that show nearly perfect octahedrally coordinated Mn2+ cations by three (N-methyl)ethylenediamine molecules. Two complexes are located below and above each ring, thus blocking each cavity by four cationic complexes. Several N−H··· Se and C−H···Se hydrogen bonds are found between the cationic complexes and the anionic substructure to result in the quoted distortion and in a 3D cross-linking of the layers (Figures 2, bottom, and S9−S11 in the SI).

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ASSOCIATED CONTENT

S Supporting Information *

CIF, details on syntheses, single-crystal XRD, and UV/visible spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft within the framework of SPP 1708.

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REFERENCES

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DOI: 10.1021/ic5026087 Inorg. Chem. XXXX, XXX, XXX−XXX