Crystal structure of K6[Zn(CO3)4]

The crystal structure of the title compound comprises a tetracarbonatozincate(II) anion, [Zn(CO3)4]6−, with point-group symmetry 2 where the ZnII atom is surrounded in a distorted tetrahedral manner by four monodentate carbonate groups.


Chemical context
Oxidotellurates(IV) exhibit a multifarious crystal chemistry (Christy et al., 2016) that can be attributed to the different coordination numbers of Te IV (usually between 3 and 5) in an oxidic environment and, particularly, to the stereoactive nonbonding 5s 2 electron lone pair at the Te IV atom (Galy et al., 1975). The space requirement of the lone pair leads to unilateral coordination polyhedra [Te IV O x ] with rather low point-group symmetries. From a crystal-engineering point of view, [Te IV O x ] units are promising building blocks for the construction of new ferro-, pyro-or piezoelectric compounds or materials exhibiting non-linear optical behaviour like second-harmonic generation, as such compounds need to crystallize in non-centrosymmetric space groups with polar axes (Ok et al., 2006).
In the quest to obtain new transition-metal oxidotellurates(IV) modified by addition of alkali cations, we developed syntheses under pseudo-hydrothermal conditions where water does not act as a typical solvent but rather as a mineralizer Eder et al., , 2023. Characteristic for this kind of preparation method, only a few drops of water are added to the reaction mixture instead of the few millilitres typically used in a hydrothermal experiment. In an alternative route employed also for the present study, water is not added at all to the reaction mixture but originates from the initial decomposition of one of the educt(s) in the closed reaction container where it then acts as a mineralizing agent. Simultaneously, the employed oxidotellurate(VI) phase can be reduced under these conditions to an oxidotellurate(IV). In this sense, solid K 2 CO 3 , ZnO and H 6 TeO 6 (as the source for water) were treated thermally under these conditions. However, the reaction did not result in an intended potassium zinc oxidotellurate(IV) phase. Instead, K 6 [Zn(CO 3 ) 4 ] was one of the obtained products, and its crystal structure is reported in the present communication.

Structural commentary
Of the 13 atoms (4 K, 1 Zn, 2 C, 6 O) in the asymmetric unit of K 6 [Zn(CO 3 ) 4 ], three are located on the twofold rotation axis (Zn1, K3, K4; Wyckoff position 4 e) of the space group C2/c. The remaining ten all are located on the general 8 f position. The most peculiar structural feature in the crystal structure is the tetracarbonatozincate(II) anion, [Zn(CO 3 ) 4 ] 6À , for which bond lengths and angles are given in Table 1. The Zn II atom is surrounded in a slightly distorted tetrahedral manner by two pairs of monodentately binding carbonate groups (Fig. 1). The mean Zn-O distance of 1.976 Å conforms with the value of 1.952 (31) Å for Zn with a coordination number (CN) of 4 (Gagné & Hawthorne, 2020). The deviation from the ideal tetrahedral shape is small (Table 1), as indicated by the 4 index of 0.92 ( 4 = 1 for an ideal tetrahedron; Yang et al., 2007). In the carbonate groups, the mean C-O bond lengths of 1.290 (25) Å for C1 and 1.285 (25) Å for C2 are in very good agreement with the grand mean bond length of 1.284 (20) Å calculated from 389 individual carbonate groups (Gagné & Hawthorne, 2018). In the title compound, the longest C-O bond of ' 1.315 Å occurs for the O atoms that are bonded to the Zn II atom. The angular distortions of the carbonate groups are minute (Table 1), with an angular sum of 360 in each case. However, both CO 3 2À groups in the [Zn(CO 3 ) 4 ] 6À anion are aplanar, with the C atoms slightly shifted out of the plane of the three O atoms [C1 by À0.008 (2) Å from the plane defined by O1, O2, O3 and C2 by À0.006 (3) Å from O4, O5, O6]. Such a deviation from planarity is a frequently observed phenomenon for carbonate groups (Zemann, 1981;Winkler et al., 2000).
The charge of the [Zn(CO 3 ) 4 ] 6À anion is compensated by large potassium cations. Since coordination numbers of large cations are not always simple to derive because there is no clear boundary for longer bonds and the corresponding (weak) interactions between the central atom and the ligand atom (Gagné & Hawthorne, 2016), we defined a threshold of 3.0 Å for K-O interactions as being significant in Bond-valence sums (Brown, 2002) were calculated with the values provided by Brese & O'Keeffe (1991). Individual values (in valence units) are collated in the following list and research communications Acta Cryst. (2023). E79, 718-721 Eder and Weil K 6 [Zn(CO 3 ) 4 ] 719 Table 1 Selected geometric parameters (Å , ).

Figure 2
The crystal structure of K 6 [Zn(CO 3 ) 4 ] in a projection along [100]. Carbonate groups are shown as flattened red polyhedra and [ZnO 4 ] units as blue tetrahedra. All atoms are drawn as spheres of arbitrary radii (K green, O white, Zn blue, C red).

Figure 1
The  (Fig. 2).  (Hans et al., 2015). This makes K 6 [Zn(CO 3 ) 4 ] the phase with the highest quantity of an alkali metal. Except for the two copper(II) compounds where Cu II shows a square-planar coordination by carbonate O atoms, the coordination numbers of all other transition metals are higher than 4.
In the crystal structure of LiZn(CO 3 )(OH), the Zn II atom is tetrahedrally coordinated by two O atoms of monodentate carbonate groups and two bridging OH groups, leading to

Synthesis and crystallization
All employed educts were obtained from commercial sources and were chemically pure. Solid ZnO, H 6 TeO 6 and K 2 CO 3 were thoroughly mixed in the molar ratio 2:3:10 (original sample weights 0.0584 g, 0.2486 g, 0.4498 g, respectively) and locked in a Teflon container with an inner volume of about 3 ml. The container was sealed and placed in a steel autoclave that was heated for one week at 483 K. The obtained solid product was colourless, comprising the title compound in the form of a few colourless crystals with a plate-like form. Powder X-ray diffraction (PXRD) revealed K 6 [Zn(CO 3 ) 4 ], K 2 CO 3 Á1.5H 2 O (Skakle et al., 2001), KTeO 3 OH (Lindqvist, 1972) and the starting material ZnO as product phases with approximate contingents (in mass percentages) of 45%, 40%, 10% and <5%, respectively, together with some unassigned reflections of low intensities. K 6 [Zn(CO 3 ) 4 ] could also be synthesized by slow evaporation of a solution containing Zn(NO 3 ) 2 Á6H 2 O and K 2 CO 3 in a molar ratio of 1:5, resulting in an increased yield of the title compound (70%), together with K 2 CO 3 Á1.5H 2 O (25%) and ZnO (<5%) as by-products, as determined by phase analysis on basis of PXRD data.

Hexapotassium tetracarbonatozincate(II)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )