Synthesis, crystal structure and thermal decomposition pathway of bis(isoselenocyanato-κN)tetrakis(pyridine-κN)manganese(II)

In the crystal structure of the title compound, discrete complexes are observed, in which the Ni cations are octahedrally coordinated by two terminal N-bonded selenocyanate anions and four pyridine coligands. Weak C—H⋯Se interactions occur in the extended structure.


Chemical context
Coordination compounds based on transition-metal thio-and selenocyanates can be divided into two major groups. In the first group, the anionic ligands are only terminally coordinated, which leads with monocoordinating ligands to discrete complexes that are of interest, for example, in the field of spincrossover materials (Gü tlich et al., 2000;Senthil Kumar & Ruben, 2017). In the second group, the anionic ligands act as bridging ligands, which is of structural interest, because a variety of structures with one-, two-or three-dimensional networks can form. Moreover, because these ligands can mediate magnetic exchange, they are also of interest in the field of molecular magnetism (Shurdha et al., 2013;Palion-Gazda et al., 2015;Prananto et al., 2017;Mekuimemba et al., 2018). In this context, compounds based on Co II cations are of special interest because of the strong magnetic anisotropy (Werner et al., 2014;Rams et al., 2020a,b;Mautner et al., 2018).
In the majority of cases, the synthesis of thio-and selenocyanate coordination compounds is performed in solution, that with less chalcophilic metal cations frequently leads to the formation of compounds with terminal anionic ligands. In contrast, the synthesis of compounds with a bridging coordination is more difficult to achieve, even if an excess of the metal salt is used in the synthesis. This behaviour is even more pronounced for selenocyanate compounds. In such cases, an alternative approach was developed, in which precursor complexes with terminal thio-or selenocyanate anions are thermally decomposed, leading to the removal of the coligands in separate steps (Wriedt & Nä ther, 2010;Wö hlert et al., 2012). In the course of this irreversible reaction, the desired compounds with bridging anionic ligands are obtained in quantitive yields. This approach is of special interest for the synthesis of selenocyanate coordination polymers because, on one hand, they are frequently difficult to prepare and, on the other hand, only a few such compounds with paramagnetic metal cations are reported in the literature. This method, however, can always be successfully applied for the synthesis of thiocyanates, whereas for selenocyanates sometimes the thermogravimetric (TG) curves are poorly resolved and in some cases poorly crystalline or even amorphous residues are obtained; the reason for this behavior is unknown (Wriedt & Nä ther, 2010). This is the case, for example, for compounds with the composition M(NCSe) 2 (C 5 H 5 N) 2 , with M = Fe, Co or Ni (C 5 H 5 N is pyridine). The Fe and Co compounds are isotypic to their thiocyanate analogs and consist of octahedrally coordinated metal cations that are linked by pairs of -1,3-bridging anionic ligands into chains Boeckmann et al., 2012). None of these compounds can be prepared from solution but the Fe compound is obtained as a pure crystalline material by thermal decomposition of its precursor, whereas the residue obtained for Co is amorphous. However, the Co compound can be prepared in crystalline form by thermal annealing below the decomposition temperature obtained from TG measurements . The Ni compound is also available by thermogravimetry, even if it is of low crystallinity, but comparison of the experimental powder X-ray diffraction (PXRD) pattern obtained after the first mass loss shows that Ni(NCSe) 2 -(C 5 H 5 N) 2 is not isotypic to M(NCSe) 2 (C 5 H 5 N) 2 , with M = Fe, Co and Cd (Nä ther & Boeckmann, 2023). Its crystal structure is still unknown and in this context it is mentioned that the occurence of different modifications, including polymorphs or isomers, is frequently observed for such thiocyanate coordination compounds (Werner et al., 2015). However, in this case, the question arose if the Mn compound is also available as a crystalline material and if it will adopt the structure type of its Fe, Co and Cd analogs or that of Ni(NCSe) 2 (C 5 H 5 N) 2 .
To answer this question, much effort was made to synthesize the desired compound Mn(NCSe) 2 (C 5 H 5 N) 2 in solution, but the pyridine-rich title compound Mn(NCSe) 2 (C 5 H 5 N) 4 was always obtained. Single-crystal X-ray diffraction proved that it The crystal structure of the title compound, showing the atom labelling and with displacement ellipsoids drawn at the 50% probability level.

Figure 1
Experimental (top) and calculated PXRD patterns (bottom) of the title compound.
is isotypic to its Cd, Zn, Fe, Co and Ni analogs (Boeckmann et al., , 2012Nä ther & Boeckmann, 2023) and comparison of the experimental PXRD pattern with that calculated from the single-crystal data proves that a pure phase was obtained ( Fig. 1). Therefore, this compound was used as a precursor for TG investigations to check if the pyridine-deficient compound Mn(NCSe) 2 -(C 5 H 5 N) 2 can be prepared and if this compound is isotypic to its Cd, Fe and Co analogs (see Section 4, Thermoanalytical investigations).

Structural commentary
The single-crystal structure determination proves that the title compound, Mn(NCSe) 2 (C 5 H 5 N) 4 , consists of discrete complexes. The asymmetric unit consists of one crystallographically independent Mn 2+ cation that is located on a centre of inversion, as well as one selenocyanate anion and two pyridine ligands in general positions. The Mn 2+ cation is octahedrally coordinated by four pyridine coligands and two terminally N-bonded thiocyanate anions that are in trans positions (Fig. 2); the Mn-N bonds to the anions are notably shorter than the bonds to the pyridine molecules (Table 1) and the bond lengths are similar to those in the corresponding Fe and Co compounds. The cis-N-Mn-N angles deviate from the ideal octahedral values by up to 2 , which shows that the octahedra are slightly distorted. It is noted that the title compound is isotypic to its Fe, Co, Zn and Cd analogues, with selenocyanate ligands, already described in the literature Boeckmann et al., , 2012. Moreover, the title compound is also isotypic to its thiocyanate analogs with Cu (Gary et al., 2004;Li & Zhang, 2004), Cd (Qu et al., 2004), Ni (Valach et al., 1984;Wang et al., 2006;Małecki et al., 2010), Fe (Søtofte & Rasmussen, 1967;Huang & Ogawa, 2006), Mn Małecki et al., 2011), Co (Hartl & Brü dgam, 1980;Deng et al., 2020), Mg (Lipkowski & Soldatov, 1993), Zn (Wu, 2004) and Co (Neumann et al., 2019) (see Section 5, Database survey).

Figure 4
DTG, TG and DTA curves, and the MS trend scan curve for the title compound measured at 4 C min À1 in helium. The experimental mass loss is given in % and the peak temperatures are given in C.
interactions. Within the structure, the Co(NCSe) 2 units form corrugated layers that lie parallel to the the ac plane (Fig. 3). Two C-HÁ Á ÁSe contacts are observed with C-HÁ Á ÁSe angles above 150 , which might indicate weak hydrogen-bonding interactions (Table 2). There are additional C-HÁ Á ÁSe and C-HÁ Á ÁN contacts, but from the distances and angles, it is concluded that they do not correspond to any significant interaction.

Thermoanalytical investigations
To investigate the thermal properties of the title compound, measurements using simultaneous differential thermoanalysis and thermogravimetry coupled to mass spectrometry (DTA-TG-MS) were performed. Upon heating, three mass losses are observed that are accompanied by endothermic events in the DTA curve (Fig. 4). The derivative thermogravimetric (DTG) curve shows that these events are reasonably resolved and from the MS trend scan curve it is obvious that in the first endothermic event, pyridine is removed. The experimental mass loss of 27.0% is in good agreement with that calculated for the removal of two pyridine ligands (Ám calc = À27.2%), whereas that in the second step is higher with a value of 40.5%, indicating that pyridine removal and the decomposition of Mn(NCSe) 2 occur simultaneously. The fact that even in the third step pyridine might be removed indicates that the reaction is more complex, except that the signal at m/z = 79 corresponds to some fragment formed during decomposition. However, to identify the product formed in the first mass loss this residue was isolated in a second TG measurement and investigated by IR and Raman spectroscopy, as well as PXRD. The CN stretching vibration of the selenocyanate anions is observed at 2099 cm À1 in the Raman and at 2090 cm À1 in the IR spectra, clearly proving that -1,3-bridging anionic ligands are present (Fig. S2 in the supporting information). The comparison of the experimental pattern with that calculated for Cd(NCSe) 2 (C 5 H 5 N) 2 , using single-crystal data retrieved from the literature, indicate that they are isotypic (Fig. S3). The pattern can easily be indexed, leading to a unit cell that is comparable to that of the Cd, Fe and Co compounds. Finally, a Pawley fit of the experimental pattern using the crystallographic data of the cadmium compound (with Mn replacing Cd) as starting model was performed, which supports all these findings. The refined lattice parameters are shown together with the difference plot in Fig. 5. As expected, due to the smaller ionic radii, the unit-cell volume is smaller for the Mn compound compared to its Cd anologue [1117.9 (1) versus 1145.6 (3) Å 3 ] . One mixed-metal compound with the composition HgSr(NCSe) 4 (C 5 H 5 N) 6 (CICLOP; Brodersen et al., 1984), a dinuclear complex with the composition [Fe(NCS) 2 ] 2 (C 5 H 5 N) 2 -[(3,5-bis(pyridin-2-yl)pyrazolyl] 2 (FIZYEU; Sy et al., 2014) and a complex with the composition Fe(NCSe) 2 (C 5 H 5 N) 2 (2- Pawley fit of Mn(NCSe) 2 (pyridine) 2 obtained by TG measurements of the title compound Mn(NCSe) 2 (pyridine) 4 . methyldipyrido[3,2-f:2 0 ,3 0 -h]quinoxaline) pyridine solvate (TISWOI; Tao et al., 2007) are also reported in the literature.

Synthesis and crystallization
MnCl 2 Á2H 2 O, KNCSe and pyridine were purchased from Alfa Aesar and used without any further purification.
6.1. Synthesis 0.25 mmol (49.7 mg) MnCl 2 Á2H 2 O and 0.5 mmol (72.0 mg) KNCSe were reacted with a mixture of 1.5 ml of pyridine and 1.5 ml of water. The mixture was stirred for 2 d at room temperature and the precipitate was filtered off, washed with very small amounts of water and dried in air. Single crystals were obtained under the same conditions but without stirring.
It is noted that even if a large excess of MnCl 2 Á2H 2 O and KNCSe was used in the synthesis, there are no hints for the formation of a pyridine-deficient compound with the composition Mn(NCSe) 2 (pyridine) 2 .

Experimental details
Single-crystal X-ray data were measured using an Image Plate Diffraction System (IPDS-2) from Stoe & Cie. 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 a skimmer coupling and a Balzer Quadrupol MS. The PXRD measurements were performed with a Stoe Transmis-sion Powder Diffraction System (STADI P) with Cu K 1 radiation equipped with a linear position-sensitive MYTHEN 1K detector from Stoe & Cie. 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
H atoms were positioned with idealized geometry (C-H = 0.95 Å ) and refined with U iso (H) = 1.2U eq (C) using a riding model. Crystal data, data collection and structure refinement details are summarized in Table 3.

Bis(isoselenocyanato-κN)tetrakis(pyridine-κN)manganese(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )