Diammonium tricadmium tris(sulfate) dihydroxide dihydrate

The title compound, (NH4)2Cd3(SO4)3(OH)2(H2O)2, has been obtained serendipitously. It is isotypic with the heavier alkali analogues M 2Cd3(SO4)3(OH)2(H2O)2 (M = K, Rb, Cs). The structure contains two Cd2+ ions, one in a general position and one with site symmetry m. The former Cd2+ ion is coordinated by three O atoms of three SO4 groups, two hydroxide O atoms and one water O atom, the latter Cd2+ ion by four O atoms of four SO4 groups and two hydroxide O atoms, both in a distorted octahedral coordination geometry. This arrangement leads to the formation of a layered framework extending parallel to (100), with the ammonium cations situated in the voids. O—H⋯O hydrogen bonds involving the water molecules, hydroxide groups and sulfate O atoms, as well as N—H⋯O hydrogen bonds between ammonium cations and sulfate O atoms consolidate the crystal packing.


Comment
The title compound, (NH 4 ) 2 Cd 3 (SO 4 ) 3 (OH) 2 (H 2 O) 2 , (I), formed accidentally under hydrothermal reaction conditions. Our intended target product was to synthesis a coordination compound from 1H-benzimidazole-5,6-dicarboxylate and CdSO 4 . 8/ 3H 2 O. The presence of ammonium ions in the finally obtained compound points to an internal redox process that presumably has caused a (partly) reduction of the nitrogen atoms of 1H-benzimidazole-5,6-dicarboxylate or the nitrate anions. The asymmetric unit of (I) is illustrated in Fig. 1. The crystal structure of (NH 4 ) 2 Cd 3 (SO 4 ) 3 (OH) 2 (H 2 O) 2 is made up from two different Cd 2+ ions (one on a general position (Cd1), one with site symmetry m (Cd2)), two sulfate ions (likewise one on a general position and the other with site symmetry m), two hydroxide groups, one water molecule and one NH 4 + cation. Both Cd 2+ cations are six-coordinated in an octahedral coordination geometry. Cd2 is coordinated by four sulfate ions and two hydroxide ions, while Cd1 is coordinated by three sulfate ons, two hydroxide anions and one water molecule.
There are four types of oxygen atoms in the crystal structure of the title compound. The O3 atom of one SO 4 2anion is solely bound to the S atom, O8 and O9 represent the oxygen atoms of hydroxide groups shared by three Cd atoms, O10 is the water O atom bound to one Cd atom and all other O atoms represent sulfate O atoms coordinated to only one Cd atom.
As can be seen in Fig. 2, the Cd(1)O 6 polyhedra are connected by sharing edges of OH groups. Cd(2)O 6 octahedra and SO 4 tetrahedra are linked to these dimers via common corners, thus forming a two-dimensional network extending parallel to the bc plane. The NH 4 + cations are situated in the voids of the layers. Through formation of N-H···O and O-H···O hydrogen bonds a three-dimensional structure is formed. Since all water and hydroxide groups and most of the sulfate O atoms are involved in hydrogen bonding, the resulting network can be considered as relatively stable (  Fig. 1. The asymmetric unit of (I), with displacement parameters shown at the 30% probability level. Fig. 2. View of the two-dimensional network structure of (I) parallel to (100) in the polyhedral representation.

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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.