Crystal structures of bis[4-(dimethylamino)pyridinium] tetrakis(thiocyanato-κN)manganate(II) and tris[4-(dimethylamino)pyridinium] pentakis(thiocyanato-κN)manganate(II)

The crystal structures of the title salts consist of discrete anionic complexes, in which the MnII atom is either in a distorted tetrahedral or a trigonal–bipyramidal coordination environment by terminal N-bonding thiocyanate ligands. The complex anions are charge-balanced by two or three 4-(dimethylamino)pyridinium cations.


Chemical context
Thiocyanate anions are versatile ligands that can be coordinated to metal cations in different ways. The most prominent coordination modes include the terminal and the -1,3 coordination modes. The latter mode is of special importance for compounds showing cooperative magnetic phenomena (Palion-Gazda et al., 2015;Massoud et al., 2013;Mousavi et al., 2012). In this context, we have reported a number of compounds based on M(NCS) 2 moieties (M = Mn, Fe, Co and Ni) that show different magnetic properties including single-chain magnetism (Werner et al., 2015a,b;Rams et al., 2017a,b). In the majority of structures, the metal cations are linked by pairs of -1,3 bridging ligands into chains, but 2D networks are also realized in which the cations are linked by pairs and single anionic ligands into layers (Suckert et al., 2016;Wö hlert et al., 2012aWö hlert et al., , 2013. In some cases, compounds comprising bridging anionic ligands need to be prepared by thermal decomposition of precursors that consist of discrete octahedral complexes with terminal N-bonded thiocyanate anions. In this regard, we became interested in mixed crystals based on Mn II and Co II atoms with the strong N-donor co-ligand 4-dimethylaminopyridine that might be prepared by thermal decomposition of mixed crystals of the corresponding discrete precursor complexes. To prove mixed crystal formation, the X-ray diffraction powder pattern of all samples needs to be compared with physical mixtures with the same metal-to-metal ratio. We therefore attempted to prepare ISSN 2056-9890 [Mn(NCS) 2 (4-(dimethylamino)pyridine) 4 ], but in all cases obtained only the salt-like crystals 1 and 2, in which the Mn II atom is solely coordinated by thiocyanate ligands, either in a tetrahedral (1) or trigonal-bipyramidal (2) configuration, and charge-balanced by 4-(dimethylamino)pyridinium cations. The formation of these cations might be traced back to the fact that the neutral molecule is a strong base because of the electron-donating dimethylamino substituent and therefore can easily be protonated. It should be mentioned that neither of the two compounds could be prepared in larger amounts as a pure crystalline phase, because mixtures were always obtained. However, both compounds are of interest from a structural point of view, because negatively charged manganate complexes with a fivefold coordination by thiocyanate ligands are scarce. Moreover, a manganate(II) complex with 4-dimethylaminopyridine has already been reported in the literature (Wö hlert et al., 2012b;Fig. 1). In the structure of this compound, the Mn II atom is octahedrally coordinated to four terminal N-bonded thiocyanate anions and two neutral 4-(dimethylamino)pyridine ligands, and the twofold negative charge is compensated by two 4-(dimethylamino)pyridinium cations. Therefore, the crystal structures of the title compounds 1 and 2 supplement the coordination polyhedra realized for thiocyanatomanganate(II) complexes with 4-(dimethylamino)pyridinium as counter-cationic species.

Structural commentary
In the crystal structure of compound 1, the Mn II atom is surrounded by four terminal N-bonded thiocyanate ligands within a considerably distorted tetrahedral coordination sphere. The N-Mn-N bond angles vary from 93.83 (7) to 123.57 (7) (Fig. 2 and Table 1). The asymmetric unit of 1 comprises two cations and one complex anion. In contrast, the asymmetric unit of compound 2 comprises six cations and two anionic complexes, and the two Mn II atoms in 2 are fivefold coordinated to the thiocyanato anions. The resulting coordination polyhedra around the two central metal atoms can be described as distorted trigonal bipyramids ( Fig. 3 and Table 2). This is supported by calculation of the structural parameter 5 (Addison et al., 1984), which leads to a value of 0.85 for Mn1 and of 0.93 for Mn2 (ideal value for a trigonal-bipyramidal coordination is 1, that of an ideal square-pyramidal coordination is 0). The Mn-N bond lengths in both independent complexes are comparable, but in both of them the distances to the thiocyanate N atom in axial positions are significantly elongated, which might be the result of steric effects between the anionic ligands in the equatorial position (Tables 1 and 2). In the structure of 1, three Mn-N bond lengths are similar, whereas the fourth is significantly elongated by about 0.07 Å (Table 1). When comparing the Mn-N bond lengths of 1 and 2 with those of bis(4-(dimethylamino)pyridinium) [bis(4-(dimethylamino)-pyridine-N)tetrakis(thiocyanato-N)manganate (  View of the asymmetric unit of 1, with atomic labelling and displacement ellipsoids drawn at the 50% probability level.

Figure 3
View of the Mn coordination in 2, with atomic labelling and displacement ellipsoids drawn at the 50% probability level.

Supramolecular features
In the crystal structure of 1, the negatively charged tetrakis(thiocyanato)manganese(II) complex molecules are linked to the 4-(dimethylamino)pyridinium cations by intermolecular N-HÁ Á ÁS and C-HÁ Á ÁS hydrogen bonding between the pyridinium N-H group and C-H hydrogen atoms, and the thiocyanate S atoms into a three-dimensional network ( Fig. 5 and Table 3). There are two additional C-HÁ Á ÁN contacts between the pyridinium C-H hydrogen atoms and the thiocyanate N atom N4, which is exactly the N atom of the ligand that shows the elongated Mn-N bond length. In the crystal View of the six crystallographically independent 4-(dimethylamino)pyridinium cations in 2, with atomic labelling and displacement ellipsoids drawn at the 50% probability level.

Figure 5
Crystal structure of compound 1 in a view along the a axis. Intermolecular hydrogen bonding is shown as dashed lines.

Figure 6
Crystal structure of compound 2 in a view along the a axis. Intermolecular hydrogen bonding is shown as dashed lines. The polar character of this structure is emphasized by the same orientation of the complex anions relative to the b axis.
structure of 2, intermolecular N-HÁ Á ÁS, C-HÁ Á ÁS and C-HÁ Á Á N hydrogen bonding between the thiocyanate anions of the anionic complexes and the 4-(dimethylamino)pyridinium cations is also observed, leading likewise to a three-dimensional hydrogen-bonded network ( Fig. 6 and Table 4). The 4-(dimethylamino)pyridinium cations are stacked along the a axis into columns, but are slightly shifted one to the other within these columns. More importantly, the two pentakis-(thiocyanato)manganese(II) complexes point in the same direction relative to the crystallographic b axis, from which the polar and non-centrosymmetric arrangement becomes obvious (Fig. 6).

Synthesis and crystallization
MnSO 4 ÁH 2 O was obtained from Merck and Ba(NCS) 2 Á3H 2 O from Alfa Aesar. Equimolar amounts of both compounds were reacted in water. The resulting white precipitate of BaSO 4 was filtered off, and the filtrate was evaporated until complete dryness. The purity of the white residue of Mn(NCS) 2 was checked by X-ray powder diffraction (XRPD) and thermogravimetry. For the synthesis of complex 1, Mn(NCS) 2 (1.0 mmol, 170 mg) was reacted with 4-(dimethylamino)pyridine (0.5 mmol, 61.0 mg) in 1.0 ml of water. The precipitate was filtered off and the filtrate was allowed to stand under ambient conditions. After a few days, single crystals suitable for single-crystal X-ray diffraction had formed. For the synthesis of complex 2, Mn(NCS) 2 (1.0 mmol, 170 mg) was reacted with 4-(dimethylamino)pyridine (1.0 mmol, 122 mg) in 4.0 ml of water. Single crystals formed from the filtrate at room temperature in a closed test tube after a few days. XRPD measurements proved that mixtures were always obtained, sometimes consisting of compound 1 and 2 or one of these compounds contaminated with additional crystalline phases.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. The C-H and N-H hydrogen atoms were initially located in difference maps but were finally positioned with idealized geometry (methyl H atoms allowed to rotate but not to tip) and were refined with fixed isotropic displacement parameters U iso (H) = 1.2U eq (C, N) for aromatic and U iso (H) = 1.5U eq (C) for methyl H atoms. For 2, the Flack  Table 4 Hydrogen-bond geometry (Å , ) for 2.