Crystal structure of poly[tetra-μ2-cyanido-1:2κ8 N:C-bis(dimethyl sulfoxide-1κO)diargentate(I)iron(II)]

Cyanide anions bridge FeII and AgI cations to form a two-dimensional polymeric compound.

In the title polymeric complex, [Fe{OS(CH 3 ) 2 } 2 {Ag(CN) 2 } 2 ], the Fe II cation is located at an inversion centre and is coordinated by four cyanide (CN À ) anions and two dimethyl sulfoxide molecules in a slightly compressed N 4 O 2 octahedral geometry, the Ag I cation is C-coordinated by two CN À anions in a nearly linear geometry. The CN À anions bridge the Fe II and Ag I cations to form a twodimensional polymeric structure extending parallel to (102). In the crystal, the nearest AgÁ Á ÁAg distance between polymeric sheets is 3.8122 (12) Å . The crystal studied was a twin with a contribution of 0.2108 (12) for the minor component.

Chemical context
Metal-organic frameworks (MOFs), also known as porous coordination polymers, form a group of compounds that consist of metal ions and organic ligand linkers (Zhou & Kitagawa, 2014). MOFs have attracted considerable attention over the past decades due to the ability to tune their porosity, structure and other properties by a rational choice of the metal and linkers. Despite the fact that the most investigated properties of MOFs are gas storage and separation, it has been shown that the incorporation of corresponding building blocks or guests into MOFs can provoke specific functional magnetic, chiral, catalytic, conductive, luminescence and other properties.

ISSN 2056-9890
Hofmann clathrate analogues represent a huge group of MOFs. The first prototype clathrate of this family was [Ni(NH 3 ) 2 {Ni(CN) 4 }] reported by Hofmann & Kü spert (1897), however its structure was only obtained by Powell & Rayner (1949). The structure analysis showed that the coordination framework of this complex is supported by bridging squareplanar tetracyanidonickelate ligands, and the octahedral coordination sphere of Ni II is completed by two NH 3 molecules. The layers in this clathrate are separated by $8 Å , which leads to the formation of guest-accessible cavities. This has allowed a series of clathrates to obtained with different aromatic guests such as benzene, phenol, aniline, pyridine, thiophene and pyrrole. Later, the group of Hofmann clathrate analogues was expanded to [M(L) 2 {M 0 (CN) 4 }] where M = Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ and Mn 2+ , M 0 = Ni 2+ , Pd 2+ , Pt 2+ and L is either a unidentate or bridging ligand to form two-or three-dimensional coordination frameworks, respectively.
More importantly, due to the rational choice of ligand, Kitazawa et al. (1996) succeeded in obtaining the first Hofmann-type complex [Fe(py) 2 {Ni(CN) 4 }] that exhibited spin-crossover behavior. This phenomenon is a spectacular ability of some 3d metals to exist in two different spin states. This discovery has led to multiple attempts to modify this compound in order to obtain other spin-crossover materials. Here we offer a new Hofmann-like coordination compound with general formula [Fe(dmso) 2 {Ag(CN) 2 } 2 ] in which the Fe II atoms are stabilized in a high-spin state.

Structural commentary
The crystal structure of the title compound was determined from 243 K data. The Fe II cation is located at an inversion centre and coordinated by four CN À anions and two dimethylsulfoxide molecules in a slightly compressed N 4 O 2 octahedral environment (Fig. 1). The Ag I cation is C-coordinated by two CN À anions in a nearly linear mode [C1-Ag-C2 = 173.0 (3) ]. The CN À anions bridge the Fe II and Ag I cations to form a two-dimensional polymeric structure. In the structure, the equatorial Fe-N bonds [2.166 (4) and 2.176 (4)  coordination environments of the Fe II and Ag I atoms in the structure of the title compound, showing the atom-labelling scheme, with displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes:  by 0.03 Å with respect to non-coordinating dmso; the average S-C bond of 1.774 (6) Å is shorter than in those in noncoordinating dimethylsulfoxide. This is a typical value for Obonded dimethylsulfoxide complexes (Calligaris, 2004). The torsion angles around the Fe-O bond are Fe1-O1-S1-C3 = 96.3 (3) and Fe1-O1-S1-C4 = À159.2 (3) . The polyhedral distortion which is described by the deviation from an octahedral geometry is AEFe|90 À Â| = 9.86 (16) where Â is the N-Fe-N or O-Fe-N angle in the coordination environment of the metal; however, this value is slightly lower than expected for a high-spin Fe II complex.

Supramolecular features
The coordination framework is connected by bridging dicyanidoargentate moieties into a two-dimensional grid that propagates along the (102) plane (Fig. 2a). The short interlayer AgÁ Á ÁAg distance of 3.8122 (12) Å indicates argentophilic interactions that propagate along the c-axis direction. A similar type of intermolecular bonding between seemingly closed-shell metal atoms has previously been reported for many Ag-and Au-containing Hofmann-type structures, e.g.

Database survey
The title compound has never been obtained before. A database survey reveals numerous Fe-Ag CN-bridged frameworks supported by various co-ligands axially bound to the iron atoms.

Synthesis and crystallization
Crystals of the title compound were obtained by the slowdiffusion method within three layers in 10 ml tubes: the first layer was a solution of Fe(ClO 4 ) 2 (0.1 mmol, 26 mg) in dimethylsulfoxide (2 ml); second one was a dimethylsulfoxideethanol mixture (1:1, 5 ml); the third was a solution of K[Ag(CN) 2 ] (0.1 mmol, 20 mg) in an ethanol-water mixture (9:1 ratio v/v, 2 ml). After two weeks, orange crystals grew in the second layer; they were collected and kept under the mother solution prior to the measurements.

Poly[tetra-µ 2 -cyanido-1:2κ 8 N:C-bis(dimethyl sulfoxide-1κO)diargentate(I)iron(II)]
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.