Synthesis, crystal structure and properties of bis(isoselenocyanato-κN)tetrakis(pyridine-κN)nickel(II)

In the crystal structure of the title compound, discrete centrosymmetric complexes are observed, in which the Ni cations are octahedrally coordinated by two terminal N-bonded selenocyanate anions and four pyridine coligands.

The reaction of nickel chloride hexahydrate with potassium selenocyanate and pyridine in water leads to the formation of crystals of the title complex, [Ni(NCSe) 2 (C 5 H 5 N) 4 ], which were characterized by single-crystal X-ray diffraction. Its crystal structure consists of discrete complexes, located on centers of inversion, in which the Ni cations are sixfold coordinated by two terminal N-bonded selenocyanate anions and four pyridine ligands within a slightly distorted octahedral coordination. In the crystal, the complexes are connected by weak C-HÁ Á ÁSe interactions. PXRD investigations revealed that a pure crystalline phase has formed. In the IR and Raman spectra, the C-N stretching vibrations are observed at 2083 and 2079 cm À1 , respectively, in agreement with the presence of only terminally bonded anionic ligands. Upon heating, one well-resolved mass loss is observed, in which two of the four pyridine ligands are removed, leading to a compound with the composition Ni(NCSe) 2 (C 5 H 5 N) 2 . In this compound, the C-N stretching vibration is shifted to 2108 cm À1 (Raman) and 2115 cm À1 (IR), indicating the presence of -1,3bridging anionic ligands. In its PXRD pattern, very broad reflections are observed, indicating for poor crystallinity and/or very small particle size. This crystalline phase is not isotypic to its Co and Fe analogs.

Chemical context
Coordination compounds based on thiocanate anions are well investigated, which can partly be traced back to their versatile magnetic behavior, including antiferro-or ferromagnetic ordering as well as single-chain magnet behavior (Shurdha et al., 2013;Prananto et al., 2017;Mautner et al., 2018;Werner et al., 2014;Rams et al., 2020). In contrast, much less is known about the corresponding selenocyanate coordination compounds, which might be related to the fact that their synthesis is more difficult to achieve. This is especially the case if less chalcophilic metal cations are used and compounds with bridging anionic ligands are to be prepared. Therefore, only a very limited number of such compounds have been reported in the literature (Turpeinen, 1977;Vicente et al., 1993;Wö hlert et al., 2012). To overcome this problem, we developed a synthetic procedure that allows a more directed preparation of thio-and selenocyanate coordination compounds with bridging anionic ligands, which is based on thermal treatment of suitable precursor compounds in which the anionic ligands are only terminally bonded (Werner et al., 2015;Wriedt & Nä ther, 2010). Upon heating, the neutral coligands are usually stepwise removed, leading to the formation of the desired compounds with a bridging coordination as intermediates.
This procedure works perfectly for the synthesis of thiocyanates but can also be used for the synthesis of selenocyanates (Wö hlert et al., 2012).
In this context we have reported on compounds with the composition M(NCSe) 2 (pyridine) 4 (M = Fe, Co) that upon heating lose two of the pyridine coligands and transform into compounds with the composition M(NCSe) 2 (pyridine) 2 (M = Fe, Co), in which the metal cations are linked by pairs of -1,3bridging selenocyanate anions into chains (Boeckmann et al., 2012;. In the course of our systematic work we also became interested in the corresponding Ni compounds, which are not reported in the literature. The synthesis of the desired compound Ni(NCSe) 2 -(pyridine) 2 in solution was unsuccessful but we found that single crystals, as well as larger amounts of a microcrystalline powder with the composition Ni(NCSe) 2 (pyridine) 4 , can easily be prepared from solution. The CN stretching vibrations of the anionic ligand are observed at 2083 cm À1 in the IR and at 2079 cm À1 in the Raman spectrum, which indicates that the selenocyanate anions are only terminally bonded (Fig. S1 in the supporting information). A comparison of the experimental powder X-ray pattern with that calculated from singlecrystal data reveals that a pure crystalline phase has formed (Fig. 1). Measurements using differential thermal analysis and thermogravimetry coupled to mass spectrometry (DTA-TG-MS) show one well-resolved mass loss in which the pyridine ligands are emitted and that is accompanied with an endothermic event in the DTA curve at 140 C (Fig. 2). Upon further heating, the TG curve is poorly resolved and two additionally endothermic events are observed. The experimental mass loss of 26.4% in the first step is close to that calculated for the removal of half of the pyridine ligands (27.0%). Therefore, it can be assumed that in the first mass loss a compound with the composition Ni(NCS) 2 (pyridine) 2 is formed that, upon further heating, loses the remaining pyridine ligands and that this event cannot be separated from the decomposition of nickel selenocyanate at higher temperatures. For this residue, IR and Raman spectroscopy show that the CN stretching vibrations are located at 2115 cm À1 in the IR and at 2108 cm À1 in the Raman spectrum, indicating that -1,3-bridging selenocyanate anions are present (  PXRD investigations proved that the reflections of the precursor compound are absent but that a residue of poor crystallinity and/or very small particle size is obtained (Fig. S3). A comparison of the experimental powder pattern with that calculated for Co(NCSe) 2 (pyridine) 2 retrieved from literature shows that these compounds are not isotypic (Fig. S3). Indexing of this powder pattern failed.

Structural commentary
Single-crystal structure determination proves that the title compound, Ni(NCSe) 2 (pyridine) 4 , is isotypic to its Co, Fe, Cd and Zn analogs already described in the literature . The asymmetric unit consists of one crystallographically independent Ni II cation that is located on a center of inversion as well as one selenocyanate anion and two pyridine ligands in a general position (Fig. 3). The Ni cations are sixfold coordinated by four pyridine coligands and two terminally N-bonded selenocyanate anions in trans-positions. Bond lengths are similar to those in the corresponding Fe and Co compounds, even if the Ni-N bond lengths are slightly shortened because of the lower ionic radii. From the bond lengths and angles (Table 1) it is obvious that the octahedra are slightly distorted.

Supramolecular features
In the crystal, the Ni(NCSe) 2 units are arranged in corrugated layers in the ac plane and the pyridine rings are arranged in columns that proceed along the crystallographic c-axis direction with no sign ofinteractions (Fig. 4). There are some C-HÁ Á ÁSe contacts, with angles above 150 , indicating weak hydrogen-bonding interactions (Table 2). There are additional C-HÁ Á ÁN contacts, but distances and especially angles indicate that they should not correspond to any significant interactions (Table 2) Crystal structure of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level. Symmetry code: (i) Àx + 3 2 , Ày + 3 2 , Àz + 1.  (8) Symmetry code: (i) Àx þ 3 2 ; Ày þ 3 2 ; Àz þ 1.

Figure 4
Crystal structure of the title compound with view along the crystallographic c-axis direction. C-HÁ Á ÁSe interactions are shown as red dashed lines.

Synthesis and crystallization
NiCl 2 Á6H 2 O and K(SeCN) 2 were purchased from Merck and pyridine was purchased from Alfa Aesar.

Synthesis:
Larger amounts of a microcrystalline powder were obtained by the reaction of 59.4 mg of NiCl 2 Á6H 2 O (0.25 mmol) and 72.0 mg (0.5 mmol) of KSeCN in a mixture of 1.5 ml of pyridine and 1.5 ml of water by stirring for 3 d at room temperature. The precipitate was filtered off and washed with a very small amount of water. Single crystals in the form of purple blocks were obtained under the same conditions but without stirring.
Experimental details: Differential thermal analysis and thermogravimetric (DTA-TG-MS) measurements were performed in a dynamic helium atmosphere in Al 2 O 3 crucibles using a Netzsch thermobalance with skimmer coupling and a Balzer Quadrupol MS. The XRPD measurements were performed by using a Stoe Transmission Powder Diffraction System (STADI P) equipped with a linear, position-sensitive MYTHEN detector from Stoe & Cie with Cu K radiation. The IR data were measured using a Bruker Alpha-P ATR-IR spectrometer and the Raman spectra were measured with a Bruker Vertex 70 spectrometer.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were positioned with idealized geometry (C-H = 0.95 Å ) and were refined with U ĩso (H) = 1.2U eq (C) using a riding model.

Bis(isoselenocyanato-κN)tetrakis(pyridine-κN)nickel(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.