Crystal structure of MgK0.5[B6O10](OH)0.5·0.5H2O, poly[dimagnesium potassium bis(hexaborate) hydroxide monohydrate]

The solvothermal reaction of H3BO3, KCF3SO3, Mg(CF3SO3)2 and pyridine led to a new alkali- and alkaline-earth-metal borate. Its structure features an intricate three-dimensional framework built from [B6O13]8− clusters, thus resulting in a six-connected achiral net with high symmetry.

The solvothermal reaction of H 3 BO 3 , KCF 3 SO 3 , Mg(CF 3 SO 3 ) 2 and pyridine led to a new alkali-and alkaline-earth-metal borate, MgK 0.5 [B 6 O 10 ](OH) 0.5 Á0.5H 2 O. Its structure features an intricate three-dimensional framework built from [B 6 O 13 ] 8À clusters, thus resulting in a six-connected achiral net with high symmetry. Each [B 6 O 13 ] 8À building block is composed of three trigonal BO 3 and three tetrahedral BO 4 units, with these BO 4 units being further connected to neighboring BO 3 units, giving rise to an oxoboron cluster of the general formula [B 6 O 10 ] 2À .

Chemical context
As inorganic materials, borates are an important class of nonlinear optical crystals, mainly because they can easily crystallize in non-centrosymmetric space groups and such structures often show a large second-harmonic generation response Qui & Yang, 2021a). The combination of BO 3trigonal and BO 4 -tetrahedral units makes it possible to form a variety of isolated anionic clusters. Extended chains, layers and three-dimensional frameworks can be formed between clusters through the dehydration and condensation of the terminal hydroxyl groups of oxoboron clusters (Wang et al., 2017). In addition, negatively charged oxoboron clusters can also combine with a variety of counter-cations, making the structure of borates more complex and diverse. Here, single crystals of MgK 0.5 [B 6 O 10 ](OH) 0.5 Á0.5H 2 O with alkali-and alkaline-earth metals have been obtained under solvothermal conditions.

Structural commentary
The asymmetric unit of the title compound consists of 2 B, 10/3 O, 1/3 Mg, 1/6 K, 1/6 OH, and 1/6 H 2 O. The Mg, K, O4, O5 and O6 atoms are located on special positions with occupancy of 1/3 or 1/6, while the remaining B and O atoms are located at general positions with an occupancy of 1. Bond-valence-sum calculations show that Mg, K and B are consistent with the expected oxidation states (Brown & Altermatt, 1985;Brese & O'Keeffe, 1991

Supramolecular features
In the title compound, the Mg and K atoms are six-coordinated, with Mg-O and distances in the range 2.332 (1)-2.374 (1) Å and K-O = 2.845 (1) Å . The three-dimensional structure is stabilized by a water cluster formed by O5-H5Á Á ÁO5, O5-H5Á Á ÁO6 and O6-H6AÁ Á ÁO2 hydrogen bonds involving the water molecule, hydroxyl group and oxoboron cluster ( Table 1). The channels of the compound are filled with ions/molecules (Mg 2+ , K + , OH À and H 2 O). The title structure is similar to previously reported analogues NH 4 NaB 6 O 10 (Wang et al., 2014), K 0.5 [B 6 O 10 ]ÁH 2 OÁ1.5H 3 O , and NaRb 0.5 [B 6 O 10 ]Á0.5H 3 O , so the simultaneous use of NH 4 and Na or K or Na and Rb or Mg and K cations as templates has no effect on the crystallization of the oxoboron three-dimensional framework. However, after the introduction of Cl (Wu et al., 2011) or Br (Al-Ama et al., 2006, the new compounds crystallize in the trigonal space group R3m with a large second-harmonic generation response. The introduction of different anions can therefore play a key role in changing the crystalline structure to a non-centrosymmetric system. The asymmetric unit of the [B 6 O 13 ] 8À oxoboron cluster. Displacement ellipsoids are drawn at the 50% probability level.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms were positioned geometrically (O-H = 0.85 Å ) and refined as riding with U iso (H) 1.2U eq (O).

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. Refinement. The structure was solved by direct methods and refined by the full-matrix least-squares method on F 2 using the SHELXL programs (Bruker, 2006;Sheldrick, 2015a). All non-hydrogen atoms in the complex were refined anisotropically.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )