Channels with ordered water and bipyridine molecules in the porous coordination polymer {[Cu(SiF6)(C10H8N2)2]·2C10N2H8·5H2O}n

The structure of a [Cu(SiF6)(C10H8N2)2]n coordination polymer with ordered 4,4′-bipyridine and water molecule channels is described.


Chemical context
The title compound was obtained in an attempt to reproduce the synthesis of [Cu(-4,4 0 -bipy)(H 2 O) 2 (BF 4 ) 2 ]Á4,4 0 -bipy (Blake et al., 1997). A contamination with SiF 6 2À is, however, at the origin of the formation of {[Cu(SiF 6 )(C 10 H 8 N 2 ) 2 ]Á-2C 10 N 2 H 8 Á5H 2 O} n , whose framework was previously described by Noro et al. (2000Noro et al. ( , 2002. This framework has shown interesting gas adsorption properties in recent years (Burd et al., 2012;Yu et al., 2012;Fan et al., 2013). Several structures based on this porous framework have been published since its discovery [CSD refcodes: GORWUF (Noro et al., 2000), AFEKAX (Noro et al., 2002), HAPKOA (Burd et al., 2012)]. However, these structures which are reported in the tetragonal space group type P4/mmm are disordered: the framework bipyridines are disordered by symmetry whereas solvent molecules are not clearly identified within the pores. In this article, we show that this porous coordination polymer is capable of firmly stabilizing guest entities such as 4,4 0 -bipyridine and water molecules within its channels. The synthesis conditions thus seem a key factor in producing the ordering of guest molecules within this porous material.

Structural commentary
The asymmetric unit of the title compound ( Fig. 1) contains two copper(II) atoms, both lying on inversion centers; each of these two atoms is coordinated by N atoms of four symmetrically related 4,4 0 -bipyridine molecules (with one independent bipyridine for each copper atom), forming slightly distorted two-dimensional square grids parallel to (100). The ISSN 2056-9890 copper(II) atoms are both at the center of elongated octahedra (Table 1).
The basal plane is composed of four nitrogen atoms coming from the 4,4 0 -bipyridine molecules, whereas the apical positions are occupied by fluorine atoms belonging to the SiF 6 2À anions pillaring the structure (Fig. 2).
The 2D coordination grids are stacked along the [100] direction through the SiF 6 2À anions, leading to a threedimensional coordination polymer, which displays channels having a free aperture of 6.5 Â 6.9 Å parallel to [100] and smaller pores of 1.6 Â 5.3 Å along the [011] direction (as measured in projection in the plane perpendicular to the channels and using van der Waals radii). These interconnected pores are filled with two other 4,4 0 -bipyridine molecules and five water molecules (Figs. 3 and 4). In comparison, the The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level.
previously reported structures with this framework are described in the P4/mmm space group type, implying a squared Cu grid and channels; here, the Cu-Cu-Cu grid angle significantly deviates from 90 (96.62 ) and this may be related to the fact that, in the present compound, guest molecules fill the pores and interact significantly with the framework atoms (see below). The Si-F bond lengths in SiF 6 2À show some variations (Table 1), ranging from 1.6534 (11) to 1.7145 (10) Å . The two longest bond lengths are associated with opposite fluorine atoms bounded to a Cu II metal atom. Among the four remaining fluorine atoms, three of them form short hydrogen bonds with water molecules and display longer bond lengths than the last one, which only forms a weaker hydrogenbonding interaction with a 4,4 0 -bipyridine molecule (Table 2). A search for SiF 6 2À anions within the Cambridge Structural Database (CSD Version 5.36; Groom et al., 2016) leads to 241 hits (using options 'not disordered'" and 'no errors'); the reported Si-F bond lengths range from 1.577 to 1.748 Å with a mean of 1.684 Å and a standard deviation of 0.022 Å .

Synthesis and crystallization
An aqueous solution (5 cm 3 ) of hydrated copper(II) tetrafluoridoborate (47.43 mg, 0.2 mmol) was added to a refluxing acetonitrile solution (5 cm 3 ) of 4,4 0 -bipyridine (62.48 mg, 0.4 mmol). After filtration, Et 2 O vapor was diffused into the mother liquor for seven days, and then the solvent was allowed to evaporate very slowly. A mixture of blue and violet crystals was obtained; whereas the diffraction spots of the blue crystals could not be properly indexed, the violet crystals were of very good quality and led to the structure reported on herein. Hydrogen-bonding pattern of solvate water molecules, forming infinite chains along the [001] direction. All other atoms apart from those of the water molecules are omitted for clarity.

Figure 6
stacking pattern of the solvent 4,4 0 -bipyridine molecules in the [100] pores. All other atoms (except water molecules hydrogen-bonded to N atoms of these bipyridines) are omitted for clarity. Pyridyl centroids are displayed as green spheres. Table 3 Geometrical parameters (Å , ) for thestacking of the 4,4 0 -bipyridine molecules within the pores.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. All hydrogen atoms of water molecules were freely refined in an isotropic approximation. Aromatic hydrogen atoms were refined with riding coordinates and U iso (H) = 1.2U iso (C).  SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Poly[[bis(µ 2 -4,4′-bipyridine)(µ 2 -hexafluoridosilicato)copper(II)] 4,4′-bipyridine disolvate pentahydrate]
Crystal data 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.