Zinc selenium oxochloride, β-Zn2(SeO3)Cl2, a synthetic polymorph of the mineral sophiite (2024)

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The synthesis and crystal structure determination of the new compoundβ-Zn₂(SeO₃)Cl₂, (I), a synthetic polymorph of the mineral sophiite(sem*nova et al., 1992), is a result of an ongoing investigation of thestructural chemistry of selenium and tellurium oxohalides. Transition metaloxohalides containing p-element cations, such as Se^IV or Te^IV withstereochemically active lone pairs, frequently show a low-dimensionalarrangement of the metal ions. In these compounds, the transition metals tendto bond to both oxygen and halogens, while the main group elements preferablyform bonds only with oxygen. A simple explanation is that the hard Lewis acidSe⁴⁺ prefers the hard Lewis base O^2-, while the softer Lewis acid Zn²⁺accepts both O^2- and Cl^- in an oxohalide environment. A consequence of thedifferent bonding preferences is that only the O atoms bond to both types ofcations and that the Se^IV lone pairs and the halogen atoms act as `chemicalscissors' by reducing the dimensionality of the crystal structure (Johnssonet al., 2000, 2003; Johnsson & Törnroos, 2003a,b). Theaim of this study was to test this concept of synthesis on the systemZnO₂–ZnCl₂–SeO₂.

The crystal structure of (I) consists of charge neutral layers connected onlyvia van der Waals interactions (Fig. 1). The Se atom has a typicalone-sided threefold coordination owing to the presence of its stereochemicallyactive 4s² lone pair (designated E), and its coordination polyhedronis therefore a slightly distorted [SeO₃E] tetrahedron. Atom Zn1 iscoordinated by two O atoms and two Cl atoms, forming a distorted [ZnO₂Cl₂]tetrahedron, and atom Zn2 is coordinated by four O atoms and two Cl atoms,completing a distorted [ZnO₄Cl₂] octahedron (Table 1). The Zn2—Cl2distance is 2.4306(4) Å while the Zn2—Cl1 distance is 2.7645(5) Å. Thelatter is quite long, but bond valence sum calculations (Brown & Altermatt,1985) suggest that atom Cl2 should be treated as coordinated by the Zn atom.

The three different building units, viz. the [Zn1O₂Cl₂],[Zn2O₄Cl₂] and [SeO₃E] groups, are connected to form infinite (010)layers. Each [ZnO₄Cl₂] polyhedron is linked to two others by cornersharing, forming infinite [001] chains within the layers. The chains areseparated by [ZnO₂Cl₂] and [SeO₃E] groups and each [ZnO₄Cl₂]polyhedron shares two corners and one edge with three [ZnO₂Cl₂] groups, aswell as two corners and one edge with three [SeO₃E] groups (Fig. 2).

The stereochemically active Se lone pairs are located between the layers,pointing in between the protruding Cl atoms of the opposite layer. Theshortest cation–anion distances between adjacent layers [Se···Cl1 = 3.5145(5) Å, Zn1···Cl1 = 3.6827(5) Å, Zn1···Cl2 = 4.7786(5) Å and Zn2···Cl2 =5.4849(7) Å] are similar to, or larger than, the cation–cation separationswithin the layers [Se···Zn2 = 2.9404(3) Å, Zn1···Zn2 = 3.2319(3) Å,Se···Zn1 = 3.3241(4) Å, Zn2···Zn2 = 3.8734(4) Å and Zn1···Zn1 = 4.1349(5) Å; symmetry codes as in Table 1]. The long interlayer distances imply thatthe layers are held together only by dispersion forces. Assuming an Se—Eradius of 1.22 Å (Galy et al., 1975), the fractional coordinates forthe lone pair E (x = 0.69, y = 0.47, z = 0.10)yield E···Cl1 and E···Cl2 contact distances of ~2.72 and ~ 2.71 Å,respectively.

The orthorhombic (Pccn) mineral sophiite shows the same connectivity ofthe building units within the layers as the synthetic form (I). The mainstructural difference between the two polymorphs is that every second layer inthe mineral structure is rotated 180° around the a axis; the layerrotation results in a doubling of the a axis.

Three other compounds are quite similar to the mineral sophiite, viz.CuZn(TeO₃)Cl₂ (Johnsson & Törnroos, 2003a), Zn₂(TeO₃)Cl₂(Johnsson & Törnroos, 2003b) and Co₂(TeO₃)Br₂ (Becker etal., 2006). They all crystallize in the orthorhombic space groupPccn. The main structural difference is that the octahedron around theatom corresponding to Zn2 is so distorted that it should rather be regarded asa square pyramid according to the idea that, for a ligand to be regarded asbonded, it should contribute more than 4% of the cation valence (Brown, 2002).A fourth related compound, Co₂(TeO₃)Cl₂ (Becker et al., 2006),crystallizes in the monoclinic space group P2₁/m. However,instead of tetrahedral and octahedral coordination of the metal cations, itcontains two types of distorted octahedra, resulting in a completely differentconnectivity within the layers.

For related literature, see: Becker et al. (2006);Brown (2002);Brown & Altermatt (1985);Galy et al. (1975);Johnsson & Törnroos (2003a, 2003b);Johnsson et al. (2000, 2003);sem*nova et al. (1992).

Compound (I) was synthesized by chemical transport reactions in sealed evacuatedsoda-glass tubes. ZnCl₂ (Avocado Research Chemicals Ltd, 98+%), ZnO (ABCR,99+%), and SeO₂ (ABCR, 99+%) were used as starting materials. Equimolaramounts of ZnCl₂ (0.135 g), ZnO (0.081 g) and SeO₂ (0.110 g) were mixed ina mortar and placed in a glass tube (length ~5 cm), which was evacuatedand heated at 700 K for 72 h in a muffle furnace. The product appeared ascolourless transparent plate-like single crystals, with a maximum size of 0.5 mm, and powder. The crystals are hygroscopic. The synthesis product wascharacterized in a scanning electron microscope (SEM, Jeol 820) with anenergy-dispersive spectrometer (EDS, LINK AN10000) on ten different singlecrystals. Analysis found: Zn 38.0 (17), Se 20.8 (7), Cl 41.2 (23)%. No Sioriginating from the glass tube was detected.

Data collection: SMART (Bruker, 1999); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 2001a); molecular graphics: SHELXTL/PC (Sheldrick, 2001b); software used to prepare material for publication: DIAMOND (Brandenburg, 2006) and PLATON (Spek, 2001).