Disodium zinc bis(sulfate) tetrahydrate (zinc astrakanite) revisited

We present a new low-temperature refinement of disodium zinc bis(sulfate) tetrahydrate {systematic name: poly[tetra-μ-aqua-di-μ-sulfato-zinc(II)disodium(I)]}, [Na2Zn(SO4)2(H2O)4]n or Zn astrakanite, which is an upgrade of previously reported data [Bukin & Nozik (1974 ▶). Zh. Strukt. Khim. 15, 712–716]. The compound is part of an isostructural family containing the Mg (the original astrakanite mineral), Co and Ni species. The very regular ZnO(aqua)4O(sulfate)2 octahedra lie on centres of symmetry, while the rather distorted NaO(aqua)2O(sulfate)4 octahedra appear at general positions, linked into a three-dimensional network by the bridging water molecules and the fully coordinated sulfate groups.

The result of this intricate interconnectivity is the formation of broad two-dimensional structures parallel to (100) containing both types of polyhedra ( Fig.1) and internally linked by the two bridging aqua and O atoms O1, O2 and O3 from the sulfate anion.
These "planes", in turn, are interconnected along a single "strong" interaction, the O4-Na1 bonds between sulfate O4 atoms from a given layer and Na1 cations from their neighbours (Fig. 2).
It is worth noting that O1 and O4 act as the only (double) acceptors for H-bonding. In analyzing the S-O bond lengths, it appears that S1-O1 and S1-O4 present precisely the longest distances suggesting a slight weakening effect on the S -O covalent link due to the oxygen involvement in H interactions.
Even though the isostructural character of (I) with the rest of the strakanite family is obvious by inspection, the low precision with which the Mn and Co members have been reported leaves comparison with the Ni moiety as the only relevant one. In this respect, both structures are almost undistinguishable, as proved by the least squares fit of the extended group shown in Fig. 3, where the maximum departure amounts for less than 0.05Å for atom O2W.

S2. Experimental
The compound was obtained an an unintended product in a synthesis of Zn(II) complexes. Recently prepared anysaldehyde bisulfitic derivative (60 mg) were dissolved in 5 ml of water and mixed with an aqueous solution of Zn acetate (112 mg/5 ml). The aqueous mixture was left in a methanol atmosphere, until colourless cubic crystals were obtained.