Crystal structure of Rb6[B12O18(OH)6]·2H2O

The solvothermal reaction of H3BO3, sodium tert-butoxide, Rb2CO3 and pyridine led to a new alkaline metal borate hexarubidium hexahydroxydodecaborate dihydrate. Its structure contains a large cyclic dodecaoxoboron cluster, [B12O18(OH)6]6−, formed by six {B3O3} rings. In the crystal, O—H⋯O hydrogen bonds between the components lead to the formation of a three-dimensional supramolecular framework.


Chemical context
In recent years, borates have made excellent contributions to the development of nonlinear optical (NLO) materials and so they are the focus of material chemists (Bashir et al., 2018;Qiu et al., 2021a;Wei et al., 2016). Scientists have found that alkaliand alkaline-earth-metal borates often exhibit a short ultraviolet cut-off edge due to no d-d and f-f electron transition in the ultraviolet region with wide transparency ranges (Shi et al., 2019;Tang et al., 2019). Generally, boron has two kinds of coordination modes: either BO 3 trigonal or BO 4 tetrahedral, and they further bond to each other through common O atoms forming different oxoboron clusters, which can further polymerize into isolated clusters, one-dimensional chains, twodimensional layers or three-dimensional frameworks. Here, single crystals of Rb 6 [B 12 O 18 (OH) 6 ]Á2H 2 O with alkali metal atoms and isolated oxoboron clusters have been obtained under solvothermal conditions.

Structural commentary
There are 13.5 independent atoms in the asymmetric unit of the title compound, including 3 B, 9/2 O, 3/2 OH, 3/2 Rb, and 1/ 2 H 2 O. It should be noted that the Rb1, Rb2, B2, B4, O4, O6 and O8 atoms are located on special positions with occupancy of 0.25 or 0.5, while the remaining Rb, B and O atoms are located at general positions with an occupancy of 1. Bondvalence-sum calculations show that Rb and B are consistent with the expected oxidation states (Brown & Altermatt, 1985;Brese & O'Keeffe, 1991). Six trigonal BO 2 (OH) units [B-O(av

Supramolecular features
In the title compound, each [B 12 O 18 (OH) 6 ] 6À cluster is connected to other clusters by O1-H1Á Á ÁO6, and O6-H6Á Á ÁO1 hydrogen bonds, resulting in a three-dimensional supramolecular framework (Fig. 2 (Zhang et al., 2004), and Na 2 Cs 4 Ba 2 [-B 12 O 18 (OH) 6 ]Á4OH (Zhang et al., 2016). Both compounds crystallize in the non-centrosymmetric Pmn2 1 space group and their supramolecular structures are different from that of the title compound. Therefore, the use of different alkali metals as templates may affect the crystallization of the oxoboron supramolecular structure.

Figure 1
The
The weight loss of 8.6% (cal. 8.4%) in the temperature range 350-950 K for the compound is attributed to the loss of the water molecules and the removal of dehydration of the hydroxyl groups. The compound has almost no weight loss after 950 K. The ultraviolet visible diffuse reflectance spectrum of the title compound is shown in Fig. 3b. The band gap obtained by extrapolating the linear part of the rising curve to zero for the compound is 5.59 eV.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen-atom coordinates were refined without any constraints or restraints. Their U iso values were set to 1.2U eq of the parent atoms.     Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT2018/3 (Sheldrick, 2015a); program(s) used to refine structure:

Funding information
SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008 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.