Disodium tricopper(II) tetrakis[selenate(IV)] tetrahydrate

The title compound, Na2Cu3(SeO3)4(H2O)4, has been prepared under hydrothermal conditions. The crystal structure contains a three-dimensional anionic framework made up from distorted [CuO4(H2O)2] octahedra ( symmetry), [CuO4(H2O)] square pyramids and trigonal-pyramidal SeO3 units sharing common corners. The connectivity among these units leads to four- and eight-membered polyhedral rings, which by edge-sharing interconnect into walls. A rhombus-like 16-membered polyhedral ring channel system with a longest length of approximately 14.0 Å and a shortest length of 5.3 Å is enclosed by such walls along the a axis. The water molecules attached to the Cu atoms, as well as the electron lone pairs of the SeIV atoms, protrude into these channels. The seven-coordinated Na+ counter-cations occupy the remaining free space of the 16-membered polyhedral ring channels. An intricate network of O—H⋯O hydrogen bonds further consolidates the three-dimensional structure.

The title compound, Na 2 Cu 3 (SeO 3 ) 4 (H 2 O) 4 , has been prepared under hydrothermal conditions. The crystal structure contains a three-dimensional anionic framework made up from distorted [CuO 4 (H 2 O) 2 ] octahedra (1 symmetry), [CuO 4 (H 2 O)] square pyramids and trigonal-pyramidal SeO 3 units sharing common corners. The connectivity among these units leads to four-and eight-membered polyhedral rings, which by edge-sharing interconnect into walls. A rhombus-like 16-membered polyhedral ring channel system with a longest length of approximately 14.0 Å and a shortest length of 5.3 Å is enclosed by such walls along the a axis. The water molecules attached to the Cu atoms, as well as the electron lone pairs of the Se IV atoms, protrude into these channels. The sevencoordinated Na + counter-cations occupy the remaining free space of the 16-membered polyhedral ring channels. An intricate network of O-HÁ Á ÁO hydrogen bonds further consolidates the three-dimensional structure.

Comment
Studies of hydrous copper selenites and selenates with three-dimensional frameworks have been reported previously, e.g.
In the asymmetric unit of (I) there are two crystallographically distinct copper atoms. The six-coordinated Cu1 site is a typical Jahn-Teller ion with a distorted, tetragonally elongated octahedral [Cu1O 4 (H 2 O) 2 ] coordination, whereas Cu2 is surrounded by five O-atoms, leading to a distorted square-pyramidal [Cu2O 4 (H 2 O)] environment. The two independent selenium atoms are coordinated by three oxygen atoms, forming the characteristic trigonal-pyramidal SeO 3 2anion ( Fig. 1). Cu2 as well as the electron lone-pairs of the selenium(IV) atoms protrude into the free space of this network (Fig. 2).
The basic features of the structure could also be described as the assemblage of linear chains of Cu and Se centres leading to 4-membered and 8-membered rings that interconnect by edge-sharing into two similar wavy layer packings extending along [011] and [011], respectively. Such layers intersect at the Cu(1) sites, eventually forming a rhombus-like 16-membered ring channel system extending along the a axis with the biggest length of approximately 14.0 Å and the smallest length of 5.3 Å (Fig.3).
The Na + counter cations are coordinated by seven oxygen atoms and occupy the central space of the 16-membered ring polyhedral channels to keep the structural stability and satisfy the charge balance. An intricate network of O-H···O hydrogen bonds further consolidates the three-dimensional structure (Table 2).