Crystal structures and Hirshfeld surface analysis of trans-bis(thiocyanato-κN)bis{2,4,6-trimethyl-N-[(pyridin-2-yl)methylidene]aniline-κ2 N,N′}manganese(II) and trans-bis(thiocyanato-κN)bis{2,4,6-trimethyl-N-[(pyridin-2-yl)methylidene]aniline-κ 2 N,N′}nickel(II))

Each metal cation in the molecular structures of [Mn(NCS)2(PM-TMA)2] (I) and [Ni(NCS)2(PM-TMA)2] (II) (PM-TMA is 2,4,6-trimethyl-N-[(pyridin-2-yl)methylidene]aniline) is situated on an inversion centre and is in a distorted octahedral coordination by six N atoms.

Crystal structures and Hirshfeld surface analysis of trans-bis(thiocyanato-jN)bis{2,4,6-trimethyl-N-[(pyridin-2-yl)methylidene]aniline-j 2 N,N 0 0 0 }manganese(II) and trans-bis (thiocyanato-jN)bis{2,4,6trimethyl-N-[(pyridin-2-yl) Although the title compounds crystallize in different crystal systems [triclinic for (I) and monoclinic for (II)], both asymmetric units consist of one-half of the complex molecule, i.e. one metal(II) cation, one PM-TMA ligand, and one Nbound thiocyanate anion. In both complexes, the metal(II) cation is located on a centre of inversion and adopts a distorted octahedral coordination environment defined by four N atoms from two symmetry-related PM-TMA ligands in the equatorial plane and two N atoms from two symmetry-related NCS À anions in a trans axial arrangement. The trimethylbenzene and pyridine rings of the PM-TMA ligand are oriented at dihedral angles of 74.18 (7) and 77.70 (12) for (I) and (II), respectively. The subtle change in size of the central metal cations leads to a different crystal packing arrangement for (I) and (II) that is dominated by weak C-HÁ Á ÁS, C-HÁ Á Á, andinteractions. Hirshfeld surface analysis and two-dimensional fingerprint plots were used to quantify these intermolecular contacts, and indicate that the most significant contacts in packing are HÁ Á ÁH [48.1% for (I) and 54.9% for (II)], followed by HÁ Á ÁC/CÁ Á ÁH [24.1% for (I) and 15.7% for (II)], and HÁ Á ÁS/SÁ Á ÁH [21.1% for (I) and 21.1% for (II)].

Chemical context
Schiff bases are widely employed as ligands in the development of coordination chemistry (Liu & Hamon, 2019). Among them, derivatives of the Schiff base 2-pyridylmethanimine have been used as chelating ligands in the construction of discrete metal complexes exhibiting interesting luminescent properties (Basu Baul et al., 2013), magnetic spin-crossover behaviour (Lé tard et al., 1997;Capes et al., 2000), or biological and catalytic reactivities (Cozzi, 2004;Creaven et al., 2010). These ligands are also able to generate non-covalent interactions such as hydrogen bonding andstacking that aid in stabilizing the assembly and provide diversity to the architectures of the crystal structures. On the other hand, pseudohalides such as thiocyanate (NCS À ) and selenocyanate (NCSe À ) anions are a class of rigid ligands with either a terminal or a bridging coordination behaviour. They have been employed extensively with neutral N-donor co-ligands in the development of novel functional coordination materials, particularly in the field of molecular-based magnets (Suckert et al., 2016).

Structural commentary
The molecular structures of (I) and (II) are shown in Fig. 1 and 2. Although the title compounds crystallize in different space groups [P1 for (I) and P2 1 /n for (II)], in both cases the asymmetric unit consist of one-half of the molecule, i.e. one metal(II) cation, one PM-TMA ligand and one thiocyanate anion. Each metal(II) cation is located on a centre of inversion and adopts a distorted octahedral coordination environment with four N atoms from two symmetry-related PM-TMA ligands in the equatorial plane and two N atoms from symmetry-related NCS À anions in a trans axial arrangement. The M-N bond lengths [2.174 (2) to 2.312 (2) Å for (I) and 2.027 (3) to 2.184 (2) Å for (II)] and N-M-N bond angles [74.27 (6) to 180 for (I) and 78.4 (1) to 180 for (II)] for both complexes are all in the normal range for similar Schiff base complexes with Mn II (Chattopadhyay et al., 2002;Lucas et al., 2005) and Ni II (Guo, 2009;Layek et al., 2014) ions. Note that the Mn1-N1-C1 bond angle [164.3 (2) ] in (I) is somewhat more bent than the corresponding Ni1-N1-C1 bond angle [176.7 (3) ] in (II). The PM-TMA ligands are not co-planar, with the trimethylbenzene ring oriented to the pyridine ring at a dihedral angle of 74.18 (7) and 77.70 (12) for (I) and (II), respectively. An overlay of the complex molecules of (I) and (II) is illustrated in Fig. 3, showing the slight differences in orientation of the trimethylbenzene rings and thiocyanate anions. This impacts significantly upon the molecular packing as described in the next section. View of the structure overlay of (I) (red) and (II) (green).

Supramolecular features
The crystal packing of (I) and (II) is dominated by weak C-HÁ Á ÁS, C-HÁ Á Á, andinteractions. In the crystal structure of (I), pairs of weak C-HÁ Á ÁS hydrogen bonds along with the C-HÁ Á Á interactions involving the methyl H atoms (H14A and H14B) and the trimethylbenzene ring or the midpoint of thiocyanate C N group, Table 1, link inversionrelated molecules into a chain running parallel to the a axis,  Table 2 Hydrogen-bond geometry (Å , ) for (II).

Figure 5
View of the three-dimensional network in (I) generated through C-HÁ Á Á andinteractions.

Figure 6
Formation of a two-dimensional supramolecular network in (II) generated through C-HÁ Á ÁS hydrogen bonds.

Figure 7
Crystal packing of (II) viewed along the c axis with aromaticstacking interactions bonds shown as dashed lines.
between the methyl H atoms and the trimethylbenzene rings, Fig. 5 (Table 1). For (II), adjacent molecules are linked together into a sheet extending parallel to (101), Fig. 6, by weak C-HÁ Á ÁS hydrogen bonds between the methine C-H groups and the thiocyanate S atoms, Table 2. On the other hand, as seen in Fig. 7, the packing in (II) also features weakstacking interactions arising from the pyridine rings and the trimethylbenzene rings [centroid-to-centroid distance = 4.147 (3) Å , dihedral angle = 17.41 (14) ]. There is an additional C-HÁ Á Á interaction between the methyl H atom and the midpoint of the thiocyanate C N group (Table 2). These interactions help to enhance the dimensionality into a threedimensional supramolecular architecture.

Hirshfeld surface analysis
The intermolecular interactions between the molecules in the crystal structures of (I) and (II) were quantified by Hirshfeld surface analysis (McKinnon et al., 2007) and two-dimensional fingerprint plots (Spackman & McKinnon, 2002)    Quantitative results of different intermolecular contacts contributing to the Hirshfeld surfaces of (I) and (II).

Synthesis and crystallization
All reagents were of analytical grade and were used as received without further purification. The bidentate Schiff base ligand, 2,4,6-trimethyl-N-[(pyridin-2-yl)methylidene]aniline (C 15 H 16 N 2 or PM-TMA) was synthesized according to a literature method (Theppitak et al., 2014). A solution of PM-TMA (89.7 mg, 0.4 mmol) in methanol (5 ml) was placed in a test tube. To a solution of Mn(ClO 4 ) 2 Á6H 2 O (50.8 mg, 0.2 mmol) in methanol (5 ml) was added KNCS (39.0 mg, 0.4 mmol), and the solution was stirred at room temperature for 30 min and then filtered to remove a white precipitate of KClO 4 . The solution was then carefully layered on the methanol solution of PM-TMA. After slow diffusion at room temperature for 3 d, yellow block-shaped crystals of (I) were obtained in 88% yield (44.7 mg) based on the manganese(II) source. Complex (II) was prepared following the procedure described above for (I), except that Ni(ClO 4 ) 2 Á6H 2 O (58.2 mg, 0.2 mmol) and KNCS (39.0 mg, 0.4 mmol) were used. Yellow block-shaped crystal of (II) were obtained in 82% yield (47.7 mg) based on the nickel(II) source.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All C-bound H atoms were placed in calculated positions and refined using a riding model with C-H = 0.93-0.97 Å and with U iso (H) = 1.2-1.5U eq (C).

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.