Crystal structure determination of Rb[SbO(OH)4] – a hitherto new structural set-up for alkali metal hydroxidoantimonates(V)

Abstract

Due to their comparatively good solubility in water, oxidic alkali metal hydroxidoantimonates(V) are important precursor compounds for the synthesis of oxidic antimony compounds under hydrothermal conditions. Our knowledge of the family of alkali metal hydroxidoantimonates(V) is restricted to the phases K2[Sb(O)(OH)5], Na[Sb(OH)6], Cs[Sb(OH)6] and Cs2[Sb2(μ-O)2(OH)8] [1]. A corresponding rubidium salt has not been reported so far, which motivated us to start phase formation studies in the system Rb/SbV/O/OH. Indeed, it was possible to obtain a previously undescribed salt with the composition Rb[SbO(OH)4] and to determine its crystal structure. Rb[SbO(OH)4] was obtained hydrothermally at 210 °C, starting from RbOH, Te(OH)6 and Sb2O3 at pH ~10. Under these conditions, Te(OH)6 oxidizes Sb2O3 under formation of elementary tellurium. The grown colourless Rb[SbO(OH)4] crystals had a needle- to block-like form and showed extensive intergrowth, which complicated crystal structure determination and refinement due to the presence of multiple domains. For the final structure solution, a data set originating from three domains was used. The diffraction pattern showed pseudo-orthorhombic symmetry (a ~ 5.66 Å, b ~ 7.29 Å, c ~ 5.67 Å, β ~ 90°), with the crystals in fact being monoclinic (P21/m, Z = 2). The asymmetric unit of Rb[SbO(OH)4] comprises of one Sb atom (site symmetry 1), one Rb atom (m), one O atom (m), two O atoms belonging to hydroxyl groups (1) and two H atoms (1). The SbV exhibits a slightly distorted octahedral [Sb(OH)4O2] coordination environment with a mean Sb–O distance of ~1.98 Å. By corner-sharing the oxygen atoms, straight [SbO2/2(OH)4/1]−∞ chains extending parallel to [010] are formed. The Rb+ ions are situated between these chains and have contacts to 12 oxygen atoms within a range between 3.00 and 3.40 Å. O–H...O hydrogen-bonding interactions of medium strengths consolidate the crystal packing.