Crystal structure of bis(1-ethylpyridinium) dioxonium hexacyanidoferrate(II)

The crystal structure of the title compound comprises a segregated columnar structure of cations and anions extending parallel to the b axis

The present X-ray crystallographic analysis of the title salt, (Etpy) 2 (H 3 O) 2 [Fe(CN) 6 ] (Etpy + is 1-ethylpyridinium), (I), was performed in order to elucidate the crystal packing of a charge-transfer hexacyanidoferrate(II) anion with a monocationic organic acceptor and an oxonium ion.

Supramolecular features
The projection of the crystal structure of (I) along the b axis is shown in Fig. 2.
The [Fe(CN) 6 ] 4À electron-donor anions and the Etpy + electron-acceptor cations stack separately in columns parallel to the b axis whereby both types of columns are alternately arranged in the a-and c-axis directions.
In the crystal of (I), the oxonium ions and [Fe(CN) 6 ] 4À ions form a three-dimensional O-HÁ Á ÁN hydrogen-bonding network (Table 1). A pair of Etpy + cations is enclosed in the hydrogen-bonding cage formed by six [Fe(CN) 6 ] 4À ions and six oxonium ions (Fig. 3). Two pyridinium rings of the Etpy + cations are arranged in parallel and the ethyl groups are alternating with each other. The centroid-to-centroid distance (4.147 Å ) and the face-to-face distance of the least-square planes (3.731 Å ) between two pyridinium rings indicate that interactions are not developed. The crystal packing of compound (I) in a view along the b axis. H atoms have been omitted for clarify; the probability function is as in Fig. 1. Table 1 Hydrogen-bond geometry (Å , ). (15) 173.6 (18) Symmetry code: (i) x þ 1 2 ; Ày þ 1 2 ; Àz þ 1 2 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. In the final refinement of the title compound, three reflections, viz. (0 17 1), (2 16 0) and (5 15 2), were omitted owing to poor agreements between observed and calculated intensities. H atoms of the Etpy + cation were, at first, located in a difference Fourier map, but finally placed in geometrically calculated positions and refined as riding, with C(methylene)-H = 0.92 Å , C(methyl)-H = 0.98 Å and C(aromatic)-H = 0.95 Å , all with U iso (H) = 1.5U eq (C). H atoms of the oxonium ion were located in a difference Fourier map and their positions refined with U iso (H) = 1.5U eq (O). The maximum and minimum electron density peaks are located 1.00 Å from atom C1 and 0.71 Å from atom Fe1, respectively.

Bis(1-ethylpyridinium) dioxonium hexacyanidoferrate(II)
Crystal data (C 7  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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )