Crystal structures of tetrakis(pyridine-4-thioamide-κN)bis(thiocyanato-κN)cobalt(II) monohydrate and bis(pyridine-4-thioamide-κN)bis(thiocyanato-κN)zinc(II)

The crystal structures of the title compounds consists of discrete octahedral (Co) or tetrahedral (Zn) complexes that are linked by intermolecular hydrogen-bonding interactions into three-dimensional networks.


Chemical context
Thio-and selenocyanate anions are useful ligands for the synthesis of new coordination compounds and polymers, because of their versatile coordination behaviour (Massoud et al., 2013;Mousavi et al., 2012;Prananto et al., 2017;Kabešová et al., 1995). In this regard, compounds with general composition [M(NCS) 2 (L) 2 ] n (M = Mn II , Fe II , Co II or Ni II ; L = neutral N-donor co-ligand) in which the metal cations are linked by these anionic ligands are of special interest, because magnetic exchange can be mediated (Palion-Gazda et al., 2015;Wö hlert et al., 2013a). In this context, we are especially interested in cobalt(II) compounds in which the metal cations are octahedrally coordinated by two neutral co-ligands and four anionic ligands, which link the central metal cations into chains by pairs of anionic ligands, as symbolized in Fig. 1. Some of these compounds show a slow relaxation of the magnetization, which in most cases can be traced back to single-chain magnetism (Rams et al., 2017a,b;Wö hlert et al., 2012Wö hlert et al., , 2013b. To study the influence of the neutral co-ligand on the magnetic properties, different pyridine derivatives substituted in the 4-position such as 4-benzoylpyridine, 4-vinylpyridine, 4-acetylpyridine, 4-ethylpyridine were investigated (Rams et al., 2017b;Werner et al., 2015;Wö hlert et al., 2014). It was found that all these compounds can be divided magnetically into two groups, even if the same Co(NCS) 2 chains are observed. In one group, the compounds exhibit an antiferromagnetic ground state and the relaxations observed in the magnetic measurements can be attributed to those of single chains. In the second group, the compounds show a ferromagnetic ground state and the relaxations observed at zero field do not correspond to single-chain relaxations. To gain a better insight into this behaviour, additional examinations of such chain compounds are required, which is of extraordinary importance for our project.
Therefore we became interested in the monodentate ligand 4-pyridinethioamide. In contrast to all ligands used previously, this ligand might be able to link the Co(NCS) 2 chains into layers by pairs of intermolecular hydrogen bonds between the amino H atoms and the thioamide S atom, which is observed, for example, in the crystal structure of the pure ligand (Colleter & Gadret, 1967;Eccles et al., 2014). It should be noted that only one such coordination polymer, namely with 4-pyridinethioamide and Cd, is reported in the literature (Neumann et al., 2016). Here the Cd II cations are linked by pairs of anionic ligands into a linear chain, which corresponds exactly to the structure we are interested in. However, irrespective of the ratio between Co(NCS) 2 and the co-ligand, a compound with composition Co(NCS) 2 (4-pyridinethioamide) 2 could not be obtained from solution. IR spectroscopic studies of all products showed bands for the CN stretching vibrations at about 2060 cm À1 , thus indicating only terminal N-coordinating anionic ligands. Therefore the formation of compounds with bridging anionic ligands can be excluded (Bailey et al., 1971), presumably because cobalt shows no high affinity to bond with sulfur atoms. Hence the formation of discrete complexes with only terminal N-bonding thiocyanate anions is preferred. The situation is reversed for cadmium, which shows a high affinity to sulfur, and this is obviously the reason why a cadmium compound with a chain structure can easily be obtained from solution. In an alternative approach we tried to synthesize discrete complexes with terminal Nbonding thiocyanate anions and with additional N-donor coligand in the coordination sphere, or mixed ligand complexes with 4-pyridinethioamide and other volatile ligands e.g. water. Such compounds can easily be transformed into compounds with anion bridges by thermal annealing, as shown previously (Suckert et al., 2017). In most of these cases, half of the Nbonding co-ligands are replaced by the sulfur atom of the (then bridging) thiocyanate anion, thus enabling the coordination number of 6 to be maintained. In the course of these investigations, crystals of [Co(NCS) 2 (C 6 H 6 N 2 S) 4 ]ÁH 2 O (1) were obtained from aqueous solution and characterized by single crystal X-ray diffraction, which revealed the formation of a discrete complex. Unfortunately, the powder pattern of all batches revealed multi-phase formation, and in several cases large amounts of the 4-pyridinethioamide ligand were present in the products (see Fig. S1 in the supporting information).
Co II sometimes forms discrete complexes with composition Co(NCS) 2 (L) 2 in which the cations are tetrahedrally coordinated by two terminal N-bonding thiocyanate anions and the N atoms of two neutral co-ligands. In several cases these complexes are isotypic with the corresponding zinc analogues, which enables a simple method for checking whether a tetrahedral Co complex might be present in the mixture. Hence we synthesized a compound with composition [Zn(NCS) 2 (C 6 H 6 N 2 S) 4 ] (1) that shows the expected tetrahedral coordination of zinc(II). However, the calculated X-ray powder diffraction pattern of 2 does not match with the additional reflections observed in some of the X-ray powder diffraction pattern of products obtained during synthesis of 1. Because of the unknown phase(s), no further investigations were performed.

Structural commentary
The asymmetric unit of compound 1 consists of one Co II cation, two thiocyanate anions, one water molecule and four 4pyridinethioamide co-ligands. The Co II cations are sixfold coordinated by two terminal N-bonding thiocyanate anions and the N atoms of four 4-pyridinethioamide ligands, forming discrete octahedral complexes, in which all coordinating atoms are in trans-positions (Fig. 2). This corresponds to the most common arrangement for structures of compounds with general composition M(NCS) 2 (L) 4 , where M is a divalent 3d metal cation and L a monodentate N-donor co-ligand (Małecki, et al., 2011). In this context, it is noted that for bridging N-donor co-ligands, like pyrazine or 4,4 0 -bipyridine, two-dimensional networks are obtained, in which the anionic ligands are still terminal coordinating (Real et al., 1991;Lu et al., 1997). The Co-N bond lengths to the thiocyanate anions of 2.0944 (18) and 2.0956 (19) Å are significantly shorter than those to the pyridine N atoms of the 4-pyridinethioamide ligand [2.1640 (16) -2.1761 (16) Å ], which is in agreement with related coordination modes reported in the literature (Table 1; Goodgame et al., 2003;Prananto et al., 2017). The bond angles around the central metal cation deviate from the ideal values, indicating a slight distortion (Table 1). For each co-ligand, the thioamide group is rotated differently out of the pyridine ring plane, with dihedral angles of 11.8 (2), 55.5 (1), 40.1 (2) and 38.3 (1) .
In the structure of compound 2, the asymmetric unit consists of a Zn II cation that is located on a twofold rotation axis, and one thiocyanate anion as well as one 4-pyridinethioamide ligand in general positions. The Zn II cation is coordinated by the N atoms of two anionic and two neutral co- View of the asymmetric unit of compound 1 with the atom labelling and displacement ellipsoids drawn at the 50% probability level.
ligands within a slightly distorted tetrahedron (Fig. 3). Bond lengths and angles (Table 2) are in agreement with values retrieved from the literature. The dihedral angle between the thioamide group and the pyridine ring is 43.8 (4) .

Supramolecular features
In the crystal of compound 1, the discrete complexes are linked by centrosymmetric pairs of intermolecular N-HÁ Á ÁS hydrogen bonds between the amino H atoms and the thiocyanate S atoms into chains extending parallel to [100], which are further connected by additional N-HÁ Á ÁS hydrogen bonds into a three-dimensional network ( Fig. 4 and Table 3). By this arrangement, channels along the a axis are formed in which the water molecules are located (Fig. 4). These solvent molecules are linked to the network via intermolecular O-HÁ Á ÁS hydrogen bonding between the water H atoms and the thiocyanate S atoms ( Table 3). The water molecules additionally act as acceptors for N-HÁ Á ÁO hydrogen bonding to the amino H atoms. There are additional short contacts between some of the aromatic hydrogen atoms and the thiocyanate S atoms (Table 3).
In the crystal of compound 2, the discrete complexes are linked by intermolecular N-HÁ Á ÁS hydrogen-bonding interactions between the H atoms of the amino group and thioamide (S1) and thiocyanate (S11) S atoms, so forming a threedimensional hydrogen-bonded framework ( Fig. 5 and Table 4). There is also a weak C15-H15Á Á ÁS1 ii interaction present within the framework (Table 4.

Figure 4
Crystal structure of compound 1 viewed along the a axis with intermolecular hydrogen bonds shown as dashed lines.
over, there is one compound with cadmium, in which the Cd II cations are octahedrally coordinated by two terminal Nbonding pyridinethioamide ligands and four thiocyanate anions and linked by pairs of anionic ligands into linear chains (Neumann et al., 2016). Other coordination compounds with this ligand are unknown. Therefore, the title compound is the third structurally characterized coordination compound with 4-pyridinethioamide as a ligand. However, the pure 4-pyridinethioamide ligand is also known and in its structure the molecules are linked by pairs of hydrogen bonds between the amino H atoms and the thioamide S atom (Colleter & Gadret, 1967;Eccles et al., 2014). Finally, the protonated form with iodine as counter-anion was reported by Shotonwa & Boeré (2014).

Synthesis and crystallization
Co(NCS) 2 and 4-pyridinethioamide were purchased from Alfa Aesar. Zn(NCS) 2 was prepared by the reaction of equimolar amounts of Ba(SCN) 2 Á3H 2 O with ZnSO 4 ÁH 2 O in water. The white precipitate of BaSO 4 was filtered off, and the resulting clear solution was evaporated until complete dryness. The purity of the obtained Zn(NCS) 2 was checked by X-ray powder diffraction (XRPD) measurements.
Crystals of compound 1 were obtained by the reaction of 8.8 mg of Co(NCS) 2 (0.05 mmol) with 6.9 mg of 4-pyridinethioamide (0.05 mmol) in a mixture of 1 ml of methanol and 1 ml of water. The reaction mixture was heated to boiling and then slowly cooled to ambient temperature, leading to crystals of the title compound suitable for single crystal X-ray diffraction. XRPD revealed impurities by crystals of the employed 4-pyridinethioamide ligand as the major phase (see Fig. S1 in the supporting information). Some crystals were selected by hand to measure an infrared spectrum (see Fig. S2 in the supporting information). We also tried to obtain pure samples by using different amounts of Co(NCS) 2 and 4pyridinethioamide, however without any success.
For the synthesis of compound 2, 18.2 mg Zn(NCS) 2 (0.1 mmol) were reacted with 6.9 mg of 4-pyridinethioamide (0.05 mmol) in 1.0 ml of water which was then overlayed with 1.0 ml of chloroform. After a few days, crystals suitable for single crystal X-ray diffraction formed at the interface of the solvents.
were refined with U iso (H) = 1.2U eq (C) using a riding model.

Tetrakis(pyridine-4-thioamide-κN)bis(thiocyanato-κN)cobalt(II) monohydrate (1)
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.

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 )
x y z U iso */U eq