Crystal structure of poly[[diaquatetra-μ2-cyanido-platinum(II)iron(II)] methanol 4/3-solvate]: a three-dimensional Hofmann clathrate analogue

The crystal structure of the title polymeric coordination compound, {[FePt(CN)4(H2O)2]·1.33CH3OH} n , features a framework structure with pores in which disordered methanol solvent molecules are situated.

In the title polymeric coordination compound, {[FePt(CN) 4 (H 2 O) 2 ]Á-1.33CH 3 OH} n , the Fe II cation (site symmetry 4/mm.m) is coordinated by the N atoms of four cyanide anions (CN À ) and the O atoms of two water molecules, forming a nearly regular [FeN 4 O 2 ] octahedron. According the Fe-N and Fe-O bond lengths, the Fe II atom is in the high-spin state. The cyanide anions act in a bridging manner to connect the Fe II and Pt II atoms. The [Pt(CN) 4 ] 2moieties (Pt with site symmetry 4/mm.m) have a perfect square-planar shape. The latter anion is located perpendicular to the FeN 4 plane, thus ensuring the creation of a three-dimensional framework. The crystal structure features methanol solvent molecules of which 4/3 were located per Fe II cation. These solvent molecules are located in hexagonal pores; they interact with coordinating water molecules through weak hydrogen bonds. Other guest molecules could not be modelled in a satisfactory way and their contribution to the scattering was removed by a mask procedure.

Chemical context
Cyanide-based complexes form a large group of coordination compounds, which can offer numerous structures and functionalities. As a result of the ability of the cyanide anion to act in a bridging way, this group often links two different metal cations, enabling the formation of one-, two-or threedimensional frameworks. The beginning of the investigation of cyanide-based complexes dates back to the 18th century when Prussian blue was discovered (Dacarro et al., 2018). Since then, hundreds of cyanide-based complexes have been obtained and proven to be efficient as molecular magnets, in separation, condensation, storage, catalysis, polymer synthesis, switching, etc (Zakaria & Chikyow, 2017).
Among all cyanide-based complexes, Hofmann clathrate analogues attract considerable attention. This is a group of polymeric coordination complexes with general formula [M(L) x {M 0 (CN) y } z Án(guest/solvent) where M has an octahedral coordination environment with two L ligands in axial positions and four N atoms of bridging cyanide groups in equatorial positions, which link M and M 0 metals into infinite layers (Powell & Rayner, 1949;Hofmann & Kü spert, 1897). If the L ligand is bridging as well (e.g. pyrazine), the creation of a ISSN 2056-9890 three-dimensional framework is observed (Niel et al., 2001). In addition, the chemical composition of Hofmann clathrates can easily be modified by variation of the guest/solvent molecules.
One of the attractive properties of Hofmann clathrate analogues is the ability of some complexes of this class to undergo spin crossover under the influence of external stimuli (Carmen Muñ oz & Real, 2011;Kucheriv et al., 2021). The change of spin state can be observed in complexes of general formula [Fe(L) x {M 0 (CN) y } z ] where L = azine or azole ligand, M 0 = Cu, Ag, Au for y = 2, z = 2, and M 0 = Ni, Pt, Pd for y = 4, z = 1 (Shylin et al., 2020;Kuzevanova et al., 2021).
In this paper we report a {[FePt(CN) 4 (H 2 O) 2 ]Á4/3CH 3 OH} n coordination polymer with a non-classical Hofmann-type framework. The similar lengths of the Fe-O and Fe-N bonds provide an almost ideal octahedral environment. The Fe II -O and Fe II -N bond lengths indicate that, at the temperature of the diffraction study, Fe II is in the high-spin state. The cyanide anions connect the Fe II and Pt II atoms, whereby the latter (site symmetry 4/mm.m; Wyckoff position 3d) has a perfect squareplanar environment with a Pt1-C1 bond length of 1.953 (17) Å . Contrary to classical Hofmann clathrate arrangements , the tetracyanidoplatinate(II) anions in the title compound are located perpendicular to the FeN 4 plane, which ensures the creation of a three-dimensional framework (Fig. 2). As a result of the cubic symmetry of the crystal structure, no deviation from linearity is observed for the Fe-N-C-Pt fragments.

Structural commentary
The title compound incorporates 4/3 methanol solvent molecules per [FePt(CN) 4 (H 2 O) 2 ] unit, which are located in hexagonal pores (Fig. 3) and interact with the coordinating water molecules through O-HÁ Á ÁO hydrogen bonds ( Table 1). The framework features some additional highly disordered guest molecules, which could not be modelled satisfactorily. Their contribution to the scattering was removed with a mask procedure implemented in OLEX2 (Dolomanov et al., 2009). These disordered guest molecules reside in two types of void with total volumes of 138.3 and 20.3 Å 3 corresponding to 36.4 and 2.6 electrons, respectively.
In comparison, two similar coordination compounds, viz. View of the crystal structure of the title compound along the a axis showing the three-dimensional coordination framework. Hydrogen bonds are shown as red dashed lines. Hydrogen atoms of the methyl group of the methanol solvent molecules are omitted for clarity.

Synthesis and crystallization
Crystals of the title compound were grown by slow diffusion between three layers in a 3 ml tube. The first layer was a solution of K 2 [Pt(CN) 4 ] (0.02 mmol) in water (0.5 ml), the second was a mixture of water/methanol (1:1, 1.5 ml) and the third layer was a solution of Fe(OTs) 2 Á6H 2 O (0.02 mmol) (OTs = p-toluenesulfonate) in methanol (0.5 ml). After two weeks, colourless crystals grew in the middle layer; these were collected and maintained under the mother solution until measured.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms of water molecules and the methanol OH group were placed at calculated positions and refined as riding on the bonded O atom. The occupancy of methanol atoms was refined and found to be equal to approximately 0.5 and later restrained to half-occupancy. As a result of symmetry restrictions, H atoms of the water molecule are disordered over four positions and were constrained to have an occupancy of 1/4. The three H atoms of the methyl group are disordered over two sets of sites, and were refined as for an idealized methyl group and were allowed to rotate about the O-C bond. The H atom of the OH group is disordered over three sites. Its occupancy was restrained to coincide with half-occupancy of the complete molecule.

Figure 3
View of the crystal structure of the title compound showing the methanol solvent molecules, which are located in hexagonal pores. Hydrogen bonds are shown as red dashed lines.

Poly[[diaquatetra-µ 2 -cyanido-platinum(II)iron(II)] methanol 1.33-solvate]
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.