Crystal structure, synthesis and thermal properties of bis(acetonitrile-κN)bis(4-benzoylpyridine-κN)bis(isothiocyanato-κN)nickel(II)

In the crystal structure of the title compound, the NiII cations are octahedrally coordinated into discrete solvate complexes, that upon heating loses the acetonitrile ligands and transforms into an unknown modification of [Ni(NCS)2(4-benzoylpyridine)2].

In the crystal structure of the title compound, [Ni(NCS) 2 (CH 3 CN) 2 (C 12 H 9 NO) 2 ] or Ni(NCS) 2 (4-benzoylpyridine) 2 (acetonitrile) 2 , the Ni II ions are octahedrally coordinated by the N atoms of two thiocyanate anions, two 4-benzoylpyridine ligands and two acetonitrile molecules into discrete complexes that are located on centres of inversion. In the crystal, the discrete complexes are linked by centrosymmetric pairs of weak C-HÁ Á ÁS hydrogen bonds into chains. Thermogravimetric measurements prove that, upon heating, the title complex loses the two acetonitrile ligands and transforms into a new crystalline modification of the chain compound [Ni(NCS) 2 (4-benzoylpyridine) 2 ], which is different from that of the corresponding Co II , Ni II and Cd II coordination polymers reported in the literature. IR spectroscopic investigations indicate the presence of bridging thiocyanate anions but the powder pattern cannot be indexed and, therefore, this structure is unknown.

Chemical context
In most cases, the synthesis of new coordination compounds is performed in solution, which in some cases leads to inhomogenous samples or some, e.g. metastable compounds, formed by kinetic control which can easily be overlooked. There are, however, some alternative routes, like synthesis via molecular milling, molten flux synthesis, solid-gas reactions or thermal decomposition of suitable precursor compounds (Braga et al., 2005(Braga et al., , 2006Nä ther et al., 2013;Zurawski et al., 2012;Hö ller et al., 2008;Den et al., 2019). These methods can have several advantages because, in most cases, they are irreversible, the products are obtained in quantitative yield, no solvent is needed and sometimes metastable isomeric or polymorphic modifications can be obtained. This is especially the case for thiocyanate coordination polymers prepared by thermal decomposition of suitable precursor compounds that consist of complexes in which the anionic ligands are only terminally bonded and additionally coordinated by neutral N-donor coligands (Wö hlert et al., 2014;Werner et al., 2015). Upon heating, the co-ligands are stepwise removed, leading to new compounds in which the metal cations are linked by thiocyanate anions into chains or layers (Neumann et al., 2019). In this context, we have reported on coordination polymers based on 4-benzoylpyridine. In [M(NCS) 2 (4-benzoylpyridine) 2 ] (M = Co and Ni) prepared in solution, a rare cis-cistrans coordination is observed, in which the thiocyanate N and S atoms are each in cis positions, whereas the co-ligand is trans (Rams et al., 2017;Jochim et al., 2018). This is in contrast to all other linear chain compounds, in which the coordinating atoms always show an all-trans coordination. Surprisingly, this coordination is found in [Cd(NCS) 2 (4-benzoylpyridine) 2 ] . Therefore, the question arose if this form can be prepared with Ni by thermal decomposition using a suitable Ni II precursor compound. One discrete complex with methanol has already been reported in the literature, but this compound cannot be prepared pure (Wellm & Nä ther, 2019a). In the course of this project, we were able to prepare crystals from acetonitrile, which were characterized by singlecrystal structure analysis, which proves that the title compound consists of discrete complexes with the composition Ni(NCS) 2 (4-benzoylpyridine) 2 (acetonitrile) 2 . This compound can be prepared pure and is a promising precursor to prepare an Ni II compound with bridging thiocyanate anions (Fig. S1 in the supporting information). Measurements using differential thermoanalysis and thermogravimetry (DTA-TG) prove that on heating two mass steps are observed that are accompanied by endothermic events in the DTA curve (Fig. 1). The experimental mass loss of 12.8% in the first step is in reasonable agreement with that calculated for the removal of two acetonitrile molecules of 13.1%, indicating the formation of a compound with the desired composition ( Fig. 1). If the X-ray powder diffraction pattern of the residue formed after the first mass loss is compared with that calculated for [Ni(NCS) 2 (4-benzoylpyridine) 2 ] reported in the literature, it is obvious that a crystalline phase has been formed ( Fig. S1 in the supporting information). This new form is also different from [Cd(NCS) 2 (4-benzoylpyridine) 2 ], indicating that a new isomeric or polymorphic form is obtained. The value of the CN stretching vibration of this form (2113 cm À1 ) is very different from that of the title compound (2080 cm À1 ) but comparable to that observed in the known modification of [Ni(NCS) 2 (4-benzoylpyridine) 2 ] (2121 cm À1 ) reported in the literature (Jochim et al., 2018), which indicates a similar thiocyanate coordination (Figs. S2, S3 and S4 in the supporting information). However, this powder pattern cannot be indexed and thus the structure of this new form is unknown.

Structural commentary
The asymmetric unit of the title compound consists of one Ni II ion that is located on a centre of inversion, as well as one thiocyanate anion, one 4-benzoylpyridine co-ligand and one acetonitrile ligand that occupy general positions (Fig. 2). The Ni II ions are sixfold coordinated by the N atoms of two terminal thiocyanate anions, two 4-benzoylpyridine and two DTG, TG and DTA curve of the title compound with the experimental mass loss in % and the peak temperatures in C. The calculated mass loss of two MeCN molecules amounts to 13.2% and the loss of two 4-benzoylpyridine ligands corresponds to 58.8%. Table 2 Hydrogen-bond geometry (Å , ).
acetonitrile ligands (Fig. 2). The Ni-NCS bond length to the negatively charged anionic ligands of 2.038 (3) Å is shorter than the Ni-N(pyridine) and Ni-NCMe bond lengths of 2.108 (2) and 2.108 (2) Å , respectively (Table 1). The bond angles deviate only slightly from ideal values, which shows that the octahedra are only slightly distorted (Table 1). This is also obvious from the octahedral angle variance of 0.71 and the quadratic elongation of 1.0006 calculated according to a procedure published by Robinson et al. (1971). The dihedral angle between the carbonyl plane (C13/C16/C17/O11) and that of the phenyl (C17-C22) ring is 22.2 (2) , and that between the planes of the pyridine ring (N11/C11-15) and the carbonyl group (C13/C16/C17/O11) is 33.7 (2) , which shows that the 4-benzoylpyridine ligand is not coplanar.

Supramolecular features
The discrete complexes are arranged into columns that proceed along the crystallographic a axis (Fig. 3). Along the b axis they are linked into chains by centrosymmetric pairs of weak C-HÁ Á ÁS hydrogen bonds between the acetonitrile H atoms and the thiocyanate S atoms ( Fig. 3 and Table 2).

Synthesis and crystallization
Ba(SCN) 2 Á3H 2 O and 4-benzoylpyridine were purchased from Alfa Aesar. Ni(SO 4 )Á6H 2 O was purchased from Merck. All solvents and reactants were used without further purification. Ni(NCS) 2 was prepared by the reaction of equimolar amounts of Ni(SO 4 )Á6H 2 O and Ba(SCN) 2 Á3H 2 O in water. The resulting white precipitate of BaSO 4 was filtered off, and the solvent was evaporated from the filtrate. The green solid was dried at room temperature.

Synthesis
Crystals of the title compound suitable for single-crystal X-ray diffraction were obtained by the reaction of Ni(NCS) 2 (26. Part of the crystal structure of the title compound, viewed along the crystallographic a axis, and with intermolecular C-HÁ Á ÁS hydrogen bonding shown as dashed lines.

Experimental details
Differential thermoanalysis and thermogravimetry (DTA-TG) were performed under a dynamic nitrogen atmosphere in Al 2 O 3 crucibles using an STA PT1600 thermobalance from Linseis. The XRPD measurements were performed using a Stoe Transmission Powder Diffraction System (STADI P) with Cu K radiation that was equipped with a linear positionsensitive MYTHEN detector from Stoe & Cie. The IR data were measured using a Bruker Alpha-P ATR-IR spectrometer.

Bis(acetonitrile-κN)bis(4-benzoylpyridine-κN)bis(isothiocyanato-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.