Dimensional Reduction of a Selenido Stannate Salt in Ionic Liquids to form 2D-K 2 Sn 2 Se 5 , a Direct Heavy Analogue of an Oxo Silicate

. The structural motifs of SiO 2 or silicates, on one hand, and their heavier homologues of group 14 (T) and group 16 (E) elements, on the other hand, commonly differ, as the strict adherence to corner-sharing is not necessary in the latter owing to larger interatomic distances. On the contrary: larger coordination numbers as well as edge-sharing of the coordination polyhedra are preferred in [T x E y ] subunits with T = Si, Ge, Sn and E = S, Se, Te. Hence, we were surprised to find a new modification of the selenido stannate K 2 Sn 2 Se 5 , which is comprised of exclusively corner-sharing [SnSe 4 ] tetrahedra in a layer-Introduction


Introduction
SiO 2 and oxo silicates formed under ambient pressure strictly follow the construction principle of corner sharing [SiO 4 ] units in their structures, owing to the electrostatic repulsion of two tetravalent Si atoms if being in closer contact than via one Si-O-Si bridge.However, this is not the case for heavier homologues owing to much larger interatomic distances.[6] In these cases, a higher density or more bonds per group 14 atom overcompensate the repulsive interaction.The latter is much lower here anyway, owing to the larger T-E bond lengths and the lower partial charge of the group 14 atoms.This holds especially for chalcogenides with much smaller differences in the electronegativity of T and E. For these compounds, corner-sharing [TE 4 ] units are rare: they are known from one-dimensional strands 1D-{[TE 3 ] 2-} in salts of the meta-metalates such as ((CH 3 ) 2 NH 2 )(NH 4 )SnS 3 , [7] yet 2D-layered structures that re-semble the subunits of oxo silicates have not been known, to the best of our knowledge.
A very prominent structural motif of tin chalcogenides, for instance, is a trigonal bipyramidal coordination of tin atoms that assemble in defect-heterocubane-type [Sn 3 E 4 ] moieties (d-HC) as a secondary building unit. Herein, we report a compound that does not follow any of these patterns, but rather behaves like an oxo silicate in spite of the presence of heavy congeners Sn and Se.The compound was accessed by ionothermal reactions of 3D-K 2 Sn 2 Se 5 (A) [14] in the presence of Lewis-acidic metal cations.
Compound 1 forms also (besides black rods of re-crystallized 3D-K 2 Sn 2 Se 5 ) when using ZnCl In all cases, compound 1 (or β-K 2 Sn 2 Se 5 ) crystallizes in the orthorhombic space group Pbca with 8 formula units in the unit cell.To our surprise, the crystal structure analysis did not reveal any variant of a [Sn 3 Se 4 ]-based structure, nor the known 3D structure of this composition (A; α-K 2 Sn 2 Se 5 ). [14]Instead, the atoms are arranged in a 2D-{[Sn 2 Se 5 ] 2-} inorganic framework consisting of exclusively [SnSe 4 ] TD building units which are all connected exclusively via corner-sharing.The structure of 1 exhibits a close relationship with silicates of the apophyllite family, K 0.84 Na 0.16 Ca 4 Si 8 O 20 (F,OH)•8H 2 O, [17] still with some differences in structural details, which is highlighted in the comparison given in Figure 1.Both, 1 and the apophyllite silicates possess 2D inorganic frameworks build from corner-sharing [ME 4 ] tetrahedra with layers parallel to (001).The tetrahedra are connected to two different types of rings, as shown in Figure 1 (a and e): (regular) four-membered rings on the one hand, and (distorted) eight-membered rings on the other hand.These are connected to form the well-known {4.8.8}net [18] (Figure 1b and f).The Sn/Se and Si/O structures, respectively, differ in the orientation of the tetrahedra with regard to the M-layer.In 1, the triangular base areas of the tetrahedra are rotated out of the ab plane.Two directly linked tetrahedra in each four-membered ring possess the same orientation.This way, zigzag chains are formed that extend parallel to the b axis, as illustrated in Figure 1a (dark and light gray shading).Thus, each four-membered rings contain two tetrahedra of each orientation, and the eight-membered rings consist of two inversely oriented sets of four tetrahedra with the same orientation.In the apophyllite structure, in contrast, tetrahedra that belong to one four-membered ring possess the same orientation, and such rings are linked in the ab plane, with alternate orientation (Figure 1e).Thus, two tetrahedra

SHORT COMMUNICATION
with the same orientation take turns in each eight-membered ring.As a consequence of the respective connection of the subunits, the Sn 2 Se 5 layers are shifted against each other in 1, thus covering the voids of every second layer (Figure 1c and  d), while in the apophyllites, they are congruent (Figure 1h), with the tetrahedra alternating in their orientation along c (Figure 1g).
The potassium cations in 1 are located between the selenido stannate layers and separate them from each other by filling the corresponding voids (see Figure 1c).In the (extended) asymmetric Sn 2 Se 7 subunit of 1 (Figure 2a), Sn-Se bond length ranging from 2.4268( 6 Notably, the composition of the starting material, 3D-K 2 Sn 2 Se 5 was not changed by this reaction, which therefore represents a mere dimensional reduction of the anionic framework from a three-dimensional to a two-dimensional substructure.A similar observation was made previously with the twodimensional vs. one-dimensional substructures of the related selenido stannates 2D-(C 4 C 1 C 1 Im) 4 [Sn 6 Se 14 ] and 1D-(C 4 C 1 C 1 Im) 4 [Sn 6 Se 14 ]. [12]In these compounds, a two-dimensional honeycomb structure was cut into one-dimensional double-strands, thereby reducing the coordination number of the involved third of the Sn atoms from 5 to 4. In 1, the change is more dramatic: (a) all Sn atoms turn from penta-coordination to tetra-coordination, and (b) all coordination polyhedra move from edge-sharing to corner-sharing, which has been unprecedented in layered selenido stannate structures to date.

Conclusions
The first selenido stannate salt with a layered anionic substructure comprising exclusively corner-sharing [SnSe 4 ] units, 2D-K 2 Sn 2 Se 5 (or β-K 2 Sn 2 Se 5 ; 1), was obtained upon ionothermal treatment of 3D-K 2 Sn 2 Se 5 (or α-K 2 Sn 2 Se 5 ) with a threedimensional anionic network structure, in (C 4 C 1 Im)[BF 4 ] or (C 2 C 2 Im)[BF 4 ] at 120 °C for 4 days.This is another example for dimensional reduction of a solid state compound by application of ionic liquids, as "solid state methods near room temperature".
X-ray Diffraction: X-ray data were collected with a Stoe StadiVari diffractometer using Cu-K α radiation (λ = 1.54186Å; T = 100 K) equipped with an Oxford Cryosystems module.Structure solution by dual space methods and full-matrix least-squares refinement against F 2 were carried out using using SHELXT15, SHELXL15, and OLEX2 software. [19]The non-hydrogen atoms were refined using anisotropic displacement parameters.
Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK.Copies of the data can be obtained free of charge on quoting the depository number CCDC-1996048 (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).