Redetermination of CaB8O11(OH)4 at low temperature

During the investigation of templated boroarsenate frameworks, single crystals of the known (Zayakina & Brovkin, 1978) title compound, (I) (Fig. 1), were obtained from a molten salt reaction of CaCl2, H3BO3 and NH4(H2AsO4). This redetermination at 120 (2) K offers a significantly better structural model and the H-atom positions and hydrogenbonding scheme have been established. There is also an isostructural strontium material, strontioborite, reported by Brovkin et al. (1975). The structure of (I) can be described in terms of linked triple six-rings of stoichiometry B6O12H with a pendant H3B2O5 group, as shown in Fig. 2. The three-coordinate O8 species (Table 1) is a distinctive feature of these units. Each of these triple-six-ring units have six O atoms that do not contribute to the ring formation. One of these forms a hydroxide grouping, four link to further similar units to form a sheet in the bc plane and the last bridges to an H3B2O5 unit that is located outside the plane. The triple six-ring unit has two of the rings in the bc plane, while the third is below this plane. The out-of-plane ring has the hydroxide group Received 31 August 2005 Accepted 15 September 2005 Online 27 October 2005


Comment
During the investigation of templated boroarsenate frameworks, single crystals of the known (Zayakina & Brovkin, 1978) title compound, (I) (Fig. 1), were obtained from a molten salt reaction of CaCl 2 , H 3 BO 3 and NH 4 (H 2 AsO 4 ). This redetermination at 120 (2) K offers a significantly better structural model and the H-atom positions and hydrogenbonding scheme have been established. There is also an isostructural strontium material, strontioborite, reported by Brovkin et al. (1975).
The structure of (I) can be described in terms of linked triple six-rings of stoichiometry B 6 O 12 H with a pendant H 3 B 2 O 5 group, as shown in Fig. 2. The three-coordinate O8 species (Table 1) is a distinctive feature of these units. Each of these triple-six-ring units have six O atoms that do not contribute to the ring formation. One of these forms a hydroxide grouping, four link to further similar units to form a sheet in the bc plane and the last bridges to an H 3 B 2 O 5 unit that is located outside the plane. The triple six-ring unit has two of the rings in the bc plane, while the third is below this plane. The out-of-plane ring has the hydroxide group The asymmetric unit of (I), showing 50% probability displacement ellipsoids and arbitrary spheres for the H atoms. The mixture of trigonal (B1, B2, B4, B6 and B8) and tetrahedral (B3, B5 and B7) B atoms and the three-coordinate O8 species are evident. attached, forming, along with the pendant H 3 B 2 O 5 unit, an extensive hydrogen-bonding network between the borate sheets (Table 2). There are six distinct hydrogen bonds per unit, with OÁ Á ÁO distances ranging from 2.585 (3) to 2.917 (4) Å . This network connects four adjacent B 8 O 11 (OH) 4 units to a central unit, as shown in Fig. 2.
The calcium ion sits in the centre of an 18-atom ring formed by four of the triple six-ring units (Fig. 3). Nine O atoms coordinate to the calcium cation, with Ca-O distances ranging from 2.482 (2) to 2.634 (2) Å ( Table 1). Six of these Ca-O bonds arise from the 18-atom ring, and two H 3 B 2 O 5 units that occur above and below the plane complete the Ca nine-coordination.

Experimental
Compound (I) was prepared using a molten salt technique. A typical reaction involved grinding H 3 BO 3 (0.4637 g, 7.5 mmol), NH 4 (H 2 AsO 4 ) (1.1923 g, 7.5 mmol) and CaCl 2 (0.5549 g, 5 mmol) in a pestle and mortar before placing the powder in a 23 ml Parr Teflonlined steel autoclave and heating to 513 K for 120 h. The product was washed with hot water to dissolve any remaining borate flux, leaving a white powder containing many colourless crystals of (I) in moderate yield (34% based on Ca). The material appears completely air-and water-stable.
The H atoms were found in a difference map and their positions and U iso values were freely refined.    Detail of (I), showing the Ca 2+ ion within its 18-atom ring. The H 3 B 2 O 5 units above and below the plane have been removed for clarity. Colour key: Ca green, other atom colours as in Fig. 2.