Crystal structure of pyridinium tetraisothiocyanatodipyridinechromium(III) pyridine monosolvate

The crystal structure of the title compound consists of discrete negatively charged [Cr(NCS)4(pyridine)2]− complexes that are charge balanced by pyridinium cations and contains additional pyridine solvent molecules that are linked by weak C—H⋯S hydrogen bonding into a three-dimensional network.


Chemical context
Coordination compounds with paramagnetic transition metals are of great interest because of their diverse magnetic properties (Cirera et al., 2009;Giannopoulos et al., 2014;Glaser, 2011;Yuan et al., 2007). Those in which the metal cations are linked by small-sized ligands that can mediate magnetic exchange are of special importance because co-operative magnetic phenomena can be expected. Prominent examples for this class of ligands are azides, oxalates and cyanides (Wang et al., 2005(Wang et al., , 2008Zhang et al., 2012;Manson et al., 2005;Ding et al., 2012). In this context also, thiocyanate ligands are useful because they show a large variety of coordination modes, with the -1,3-bridging mode as the most important (Jochim et al., 2018;Mautner et al., 2016Mautner et al., , 2017Shurdha et al., 2013;Mekuimemba et al., 2018;Wö hlert et al., 2014a;Werner et al., 2015). It is noted that these compounds are frequently difficult to prepare because terminal N-coordination is usually preferred for 3d metal cations. Nevertheless, in recent years, an increasing number of bridging compounds have been reported, which might be traced back to the fact that several of them were prepared by thermal decomposition of precursors that contain terminal anionic ligands . In this context, we and others have reported on several new thiocyanate coordination polymers based on transition-metal thiocyanates, in which the metal cations are linked by bridging anionic ligands into chains (Rams et al., 2017;Baran et al., 2019;Wö hlert et al., 2013Wö hlert et al., , 2014bMautner et al., 2018). Most of these compounds contain Mn II , Fe II , Co II , Ni II or Cu II cations, whereas no bridging compounds are reported with chromium.
There is only one compound in which alternating Cr III and K + cations are bridged by -1,3-coordinating thiocyanate anions into chains in which each cation is octahedrally sourrounded by two bridging thiocyanate anions and four pyridine ligands (Kitanovski et al., 2007). Therefore, we decided to investigated if similar compounds are available with chromium. Hence, CrCl 2 was reacted with NH 4 NCS to prepare Cr(NCS) 2 in situ, which is similar to the procedure we frequently use for the synthesis of thiocyanate coordination polymers with other metal cations. Initially, pyridine was selected as the N-donor ligand, because most of our compounds are based on pyridine derivatives as co-ligands. In this reaction, crystals were obtained that were identified by single-crystal X-ray diffraction. This proved that a discrete cationic complex had formed.

Structural commentary
The asymmetric unit of the title compound consists of one half of the cation, namely a Cr III ion, two pyridine ligands which lie on a crystallographic mirror plane and two isothiocyanate anions that occupy general positions, as well as one pyridinium cation and a pyridine molecule that are also located on a crystallographic mirror plane (Fig. 1). The Cr III ion is sixfold coordinated by four N-bonding isothiocyanate anions and two pyridine ligands, within a slightly distorted octahedral geometry (Figs. 1 and 2). The Cr-N bond lengths (Table 1) to the pyridine ligands (N11 and N21) are slightly longer than that to the isothiocyanate anions (N1 and N2). The distortion of the octahedron is also obvious from the mean quadratic elongation of 1.0015 and the octahedral angle variance of 0.9447 calculated according to Robinson et al. (1971). The four isothiocyanate anions are located in the basal plane of the octahedron, whereas the pyridine ligands are in apical positions with the pyridine ring planes rotated by 90 (Fig. 2). Charge balance is achieved by a pyridinium cation that is hydrogen bonded to a pyridine solvent molecule via N-HÁ Á ÁN hydrogen bonding, forming pyridinium-pyridine dimers (Fig. 3). Several models were tested, but in the final refinement, a split model was used, in which the pyridinium H atom is disordered over two sites in a ratio of 70:30. Presumably, because of sterical reasons, the pyridine-ring planes are perpendicular to each other (Fig. 3). The molecular structure of the title compound with the atom labelling and displacement ellipsoids drawn at the 50% probability level. The pyridinium N-bound H atom is disordered over two sets of sites.

Figure 2
View of the coordination sphere of the Cr III ion.

Figure 3
View of the pyridinium-pyridine dimer, with N-HÁ Á ÁN hydrogen bonding shown as dashed lines. The pyridinium N-bound H atom is disordered over two sets of sites. Table 1 Selected geometric parameters (Å , ).

Supramolecular features
In the crystal, discrete complexes and pyridinium cations are arranged in alternating layers parallel to the bc plane (Fig. 4, bottom). The discrete complexes are linked by pairs of C-HÁ Á ÁS hydrogen bonds between the thiocyanate S atoms of one complex and two H atoms of one of the pyridine ligands of a neighbouring complex into chains, that elongate along the crystallographic b axis (Fig. 4, top). The bond lengths and angles of these hydrogen bonds show that this is only a very weak interaction (Table 2). These chains are further linked by additional very weak C-HÁ Á ÁS interactions between the thiocyanate S atoms that are not involved in chain formation and one H atom of the pyridinium-pyridine dimers (Fig. 4, bottom, and Table 2). Finally, further C-HÁ Á ÁS interactions link all building blocks into a three-dimensional network. It is noted that both the discrete complexes, as well as the pyridinium-pyridine dimers, point in the same direction along the crystallographic c axis, clearly showing the presence of a polar structure (Fig. 4, bottom).

Database survey
There are four structures published in the CSD (Version 5.4, Update 1, February 2019; Groom et al., 2016) that consist of chromium(II) and thiocyanate anions. In all of them, the Cr II cations are square-planar coordinated by two isothiocyanate anions and two co-ligands (Jubb et al., 1989(Jubb et al., , 1991Shurdha et al., 2012Shurdha et al., , 2013. With chromium(III) there are two structures in which the cations are octahedrally coordinated by only two terminal isothiocyanate anions and by two 4,4 0 -dimethyl-2,2 0pyridine ligands and the positive charge is neutralized by iodide or triiodide anions (Walter & Elliott, 2001). In most of the reported structures with Cr III , the cations are surrounded by four or six isothiocyanate anions and the positive charges are neutralized by protonated solvent or ligand molecules. There is also one discrete complex with pyridine as co-ligand, in which the Cr III cations are coordinated by three isothiocyanate anions and three pyridine ligands (Malecki, 2016). A similar structure is also known with 4-methylpyridine (Kitanovski et al., 2007). Finally, there is one structure reported that is comparable to that of the title compound with 4-methylpyridine, in which the Cr III cations are coordinated by two 4-methylpyridine ligands and four N-terminal thiocyanate anions. Charge balance is achieved by one 4-methylpyridinium cation that is hydrogen bonded to one 4-methylpyridine solvent molecule (Young et al., 2011). In contrast to the title compound, the N-H distances are not symmetrical (N-H = 1.16 Å and NÁ Á ÁH = 1.5 Å ), but the N-HÁ Á ÁN hydrogen-bond distance is comparable (2.686 Å ) to that in the title compound (2.684 Å ).

Synthesis and crystallization
CrCl 2 (0.5 mmol, 66.5 mg) was reacted with NH 4 NCS (1.0 mmol, 76.1 mg) in 2.0 ml pyridine. The precipitate was filtered off and the filtrate was stored at room temperature. After a few days, crystals of the title compound suitable for single-crystal x-ray diffraction were obtained.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms were positioned with idealized geometry and refined with U iso (H) = 1.2U eq (C) using a riding model. The pyridinium H atom was located in a difference map and was initially freely refined. In this case, it is located nearly in the middle between both pyridine N atoms, leading to very long N-H bonds of 1.32 (16)  View of a chain (top) and the crystal structure of the title compound viewed along the crystallographic b-axis and with the intermolecular hydrogen bonding shown as dashed lines. Table 2 Hydrogen-bond geometry (Å , ).  , 1980, 1982). Therefore, the pyridinium H atom was placed at an ideal distance and the displacement parameter was refined. In this case, there is a strong indication that the H atom is disordered and therefore a split model was used with the site-occupation factor for each H atom in a ratio of 70:30, which leads to similar isotropic displacement parameters that are lower than that obtained by the refinement of a symmetrical hydrogen bond. In the final refinement, both H atoms were placed in ideal positions and were refined with U iso (H) = 1.2U eq (N) using a riding model.   (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: XP in SHELXTL (Sheldrick, 2008) and DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

Pyridinium tetraisothiocyanatobis(pyridine)chromium(III) pyridine monosolvate
Crystal data (C 5  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.