Syntheses and crystal structures of bis(4-methylpyridine-κN)bis(selenocyanato-κN)zinc(II) and catena-poly[[bis(4-methylpyridine-κN)cadmium(II)]-di-μ-selenocyanato-κ2 N:Se;κ2 Se:N]

In the crystal structures of the title compounds, the Zn cations are tetrahedrally coordinated forming discrete complexes, whereas the Cd cations are octahedrally coordinated in an alternating cis–cis–trans and all-trans coordination and are linked into corrugated chains by pairs of μ-1,3-bridging selenocyanate anions.

The reactions of Zn(NO 3 ) 2 . 6H 2 O and Cd(NO 3 ) 2 . 4H 2 O with KSeCN and 4-methylpyridine (C 6 H 7 N; 4-picoline) lead to the formation of crystals of bis(4-methylpyridine-N)bis(selenocyanato-N)zinc(II), [Cd(NCSe) 2 (C 6 H 7 N) 2 ] (1), and catena-poly [[bis(4-methylpyridine-N)cadmium(II)]-di--selenocyanato-2 N:Se; 2 Se:N], [Cd(NCSe) 2 (C 6 H 7 N) 2 ] n (2), suitable for single-crystal X-ray diffraction. The asymmetric unit of compound 1 consists of one Zn cation that is located on a twofold rotation axis as well as one selenocyanate anion and one 4-methylpyridine ligand in general positions. The Zn cations are tetrahedrally coordinated by two terminal N-bonding thiocyanate anions and two 4-methylpyridine ligands, forming discrete complexes. The asymmetric unit of compound 2 consists of two crystallographically independent Cd cations, of which one is located on a twofold rotation axis and the second on a center of inversion, as well as two crystallographically independent selenocyanate anions and two crystallographically independent 4-methylpyridine ligands in general positions. The Cd cations are octahedrally coordinated by two N-and two Sbonding selenocyanate anions and two 4-methylpyridine ligands and are linked into chains by pairs of selenocyanate anions. Within the chains, the Cd cations show an alternating cis-cis-trans and all-trans coordination and therefore, the chains are corrugated. PXRD investigations prove that the Zn compound was obtained as a pure phase and that the Cd compound contains a very small amount of an additional and unknown phase. In the IR spectrum of 1, the CN stretching vibration of the selenocyanate anion is observed at 2072 cm À1 , whereas in the 2 it is shifted to 2094 cm À1 , in agreement with the crystal structures.

Chemical context
Thio-and selenocyanate anions are versatile ligands because of their variable coordination modes (Buckingham, 1994;Barnett et al., 2002;Werner et al., 2015a). The most common mode is the terminal coordination and -1,3-bridging mode, where the latter is more pronounced for chalcophilic metal cations, whereas the former dominates for less chalcophilic metal cations. For a given metal thio-or selenocyanate and a given mono-coordinating coligand, usually several compounds with a different ratio between the metal cation and the coligand are observed, for example M(NCX) 2 (L) 4 and M(NCX) 2 (L) 2 , or in very few cases also M(NCX) 2 (L) (M = +2 charge transition-metal cation, X = S, Se and L = neutral mono coordinating coligand). For compounds with the composition M(NCX) 2 (L) 4 and octahedrally coordinated metal cations mostly discrete complexes are observed and hundreds of them are reported in the literature. For ligand-deficient compounds with the composition M(NCS) 2 (L) 2 , the octahedral coordination still dominates, but some metal ions such as Co 2+ can show both octahedral and tetrahedral coordination (Mautner et al., 2018), whereas for Zn II , the tetrahedral coordination is found exclusively.
For simple geometrical considerations, compounds with the composition M(NCX) 2 (L) 2 and cations that shows an octahedral coordination must contain -1,3-bridging thio or selenocyanate anions, and in this case the structural variability is much larger. In practically all cases they consist of M(NCX) 2 chains or layers, but compared to chain compounds, layered structures are rare. In most of the layered compounds, the transition-metal cations are linked by single -1,3-bridging anionic ligands into layers (Werner et al., 2015b) or two metal cations are connected via pairs of anionic ligands into dinuclear units that condense into layers via single -1,3-bridging anions (Suckert et al., 2016). Moreover, for an octahedral coordination, in principle five different isomers exist, including the all-trans, the all-cis and three cis-cis-trans coordinations. The majority of chain compounds show an alltrans coordination in which the metal cations are linked by pairs of anionic ligands, leading to the formation of linear chains (Banerjee et al., 2005;Mautner et al., 2018;Werner et al., 2014;Rams et al., 2020). Linear chains are also observed in compounds where the coligands are still in the trans-position, whereas the thiocyanate N and S atoms are in the cis-position (Rams et al., 2017;Jochim et al., 2018), but there are very few examples where the coligands are in the cis-position, leading to the formation of corrugated chains (Banerjee et al., 2005;Shi, Chen & Liu, 2006;Makhlouf et al., 2022;Bö hme et al., 2020). Corrugated chains are also observed for an all-cis coordination, but only very few examples have been reported (Shi, Sun et al., 2006;Zhang et al., 2006;Marsh, 2009). However, all of the structure types mentioned above are well known for thiocyanate coordination compounds, whereas the structures of selenocyanate compounds are not as well explored and it has not been thoroughly investigated whether compounds with thio-or selenocyanate anions and the same metal:coligand ratio always show the same structures and are, for example, isotypic. This might partly be traced back to the fact that some of the selenocyanate compounds are not very stable and that compounds with bridging anionic ligands are more difficult to prepare if less chalcophilic metal cations are used (Wriedt & Nä ther, 2010).
To investigate this in more detail, we prepared compounds based on Zn(NCSe) 2 and Cd(NCSe) 2 , where the former metal ion prefers a tetrahedral and the latter an octahedral coordination. Cd II is also very chalcophilic, which means that compounds with bridging anionic ligands can easily be prepared. 4-Methylpyridine (C 6 H 7 N) was selected as coligand, for which the corresponding thiocyanate compounds have been reported, whereas compounds with selenocyanate are unknown.
With Cd(NCS) 2 , a solvate with the composition Cd(NCS) 2 (4-methylpyridine) 4 Á4-methylpyridineÁwater has been reported, in which the Cd cations are octahedrally coordinated by two terminal N-bonded selenocyanate anions and four 4-methylpyridine ligands [refcodes DEXYIO (Dyadin et al., 1984), DEXYIO10, (Pervukhina et al., 1986) and DEXYIO11 (Marsh, 1995)]. More importantly, two compounds with the composition Cd(NCS) 2 (4-methylpyridine) 2 are found that represent isomers. In one of these, the Cd cations are octahedrally coordinated by two terminal N-and S-bonded selenocyanate anions and two 4-methylpyridine ligands in an all-trans coordination. The Cd cations are linked by pairs of selenocyanate anions into chains, which because of the all-trans coordination are linear (FAPCOO02; Neumann et al., 2020). The second isomer was first reported in the triclinic space group P1 (FAPCOO; Taniguchi et al., 1986) but it was later pointed out that it is better described as monoclinic, in space group C2/c (FAPCOO01; Marsh, 1995). In this compound, the Cd cations are also octahedrally coordinated, linked into chains, but they are corrugated because an alternating all-trans and cis-cis-trans coordination is observed. The thermodynamic relations were determined for both isomers, indicating that they are related by monotropism with the isomer with corrugated chains as the thermodynamically stable phase (Neumann et al., 2020). Finally there is one 4-methylpyridine-deficient compound with the composition Cd(NCS) 2 (4-methylpyridine), in which the Cd cations are linked by pairs of anionic ligands into chains and each two of these chains are condensed into double chains via -1,1,3-(S,N,N)-bridging thiocyanate anions (refcode VUCBUT; Neumann et al., 2020).
To search for new compounds related to those noted above, Zn(NO 3 ) 2 Á6H 2 O and Cd(NO 3 ) 2 Á4H 2 O were reacted with KSeCN and 4-methylpyridine (4-picoline) 2 , which led to the formation of two compounds with the composition Zn(NCSe) 2 (4-methylpyridine) 2 (1) and Cd(NCeS) 2 (4methylpyridine) 2 (2). IR spectroscopic investigations revealed that the CN stretching vibration is located at 2072 cm À1 for 1 and at 2094 cm À1 for 2, indicating that compound 1 contains terminally coordinated anionic ligands, whereas in 2 this value is at the borderline between that expected for a terminal and a bridging coordination (Figs. S1 and S2 in the supporting information). For both compounds, single crystals were obtained and characterized by single-crystal X-ray diffraction. Based on the crystallographic data, PXRD patterns were calculated and compared with the experimental pattern, showing that compound 1 was obtained as a pure phase, whereas compound 2 is contaminated with a very small amount of an unknown phase (Figs. S3 and S4). It is noted that even if Cd(NO 3 ) 2 Á4H 2 O and KSeCN are used in excess in the synthesis, there are no hints of the formation of a 4-methylpyridine-deficient compound with the composition Cd(NCSe) 2 (4-methylpyridine), as observed with Cd(NCS) 2 (Neumann et al., 2020).

Structural commentary
The asymmetric unit of compound 1 consists of one selenocyanate anion and one 4-methylpyridine ligand in general positions, as well as one Zn II cation that is located on a twofold rotation axis (Fig. 1). The Zn cations are tetrahedrally coordinated by two symmetry-related terminal N-bonded selenocyanate anions and two symmetry-related 4-methylpyridine ligands (Fig. 1). The tetrahedra are slightly distorted with the N s -Zn-N s (s = selenocyanate) angle as the largest (Table 1). It is noted that compound 1 is isotypic to Zn(NCS) 2 (4methylpyridine) 2 reported by Lipkowski (1990).
The asymmetric unit of compound 2 consists of two crystallographically independent Cd cations, of which Cd1 is located on a twofold rotation axis whereas Cd2 is located on a center of inversion, as well as two crystallographically independent selenocyanate anions and two crystallographically independent 4-methylpyridine ligands (Fig. 2). Both Cd cations are octahedrally coordinated by two N-and two Sbonding selenocyanate anions and two 4-methylpyridine ligands but Cd1 is in a cis-cis-trans coordination with the pyridine N atoms of the 4-methylpyridine ligand in the cis position, whereas Cd2 is in an all-trans coordination (Fig. 2). Both octahedra are slightly distorted but Cd1 is more distorted than Cd2 (Table 2). The Cd cations are linked by pairs of selenocyanate anions into chains that show an alternating ciscis-trans and all-trans coordination. Because of the former, these chains are corrugated (Fig. 3).
Compound 2 is isotypic to the second isomer of Cd(NCS) 2 (4-methylpyridine) 2 that crystallizes in the monoclinic space group C2/c (Marsh, 1995). In this context, it is noted that two modifications are also known for the corresponding Fe compound Fe(NCS) 2 (4-methylpyridine) 2 (Neumann et al., 2020), of which form I is isotypic to compound 2 and the corrugated chain isomer of Cd(NCS) 2 (4-methylpyridine) 2 , whereas form II of the Fe compound is isotypic to the linear chain isomer. For the Fe isomers, the same thermodynamic relations were found as for the isomers with Cd(NCS) 2 with the corrugated chain isomer as the thermodynamically stable form (Neumann et al., 2020). Moreover, compound 2 is also isotypic to Cd(NCS) 2 (4-chloropyridine) 2 reported by Goher et al. (2003;refcode EMASIU). This can be traced back to the fact that the van der Waals radii of a methyl group and a chlorine atom are comparable, which is expressed by the so-called chloro-methyl exchange rule (Desiraju & Sarma, 1986 and references cited therein).
Finally, it is noted that some compounds with the general composition Cd(NCSe) 2 (L) 2 with L as a monocoordinating coligand are reported, in which the Cd cations are linked by pairs of anionic ligands into chains, but the majority of compounds show an all-trans coordination and the formation of linear chains. An overview is given in the database survey.

Supramolecular features
In the crystal structure of compound 1, the discrete complexes are arranged into columns that propagate along the c-axis direction (Fig. 4). Within these columns, the selenocyanate anions and the 4-methylpyridine ligands always point in the same direction, from which the non-centrosymmetric arrangement is visible (Fig. 4). There are no directional intermolecular interactions between the complexes and nor is there any indication ofinteractions.
In compound 2, the chains are closely packed and propagate along the [101] direction (Fig. 5). As in compound 1, no pronounced intermolecular interactions are observed.

Database survey
According to a search in the Cambridge Structural Database (CSD Version 5.43, March 2022; Groom et al., 2016), no selenocyanate coordination compounds with 4-methylpyridine as anionic ligand have been reported but many compounds with the thiocyanate as anion can be found. Those with Zn(NCS) 2 and Cd(NCS) 2 were already mentioned in the Chemical context section (see above). Crystal structure of compound 1 viewed along the b-axis direction.   Symmetry codes: (i) Àx þ 1; y; Àz þ 1 2 ; (ii) Àx þ 3 2 ; Ày þ 1 2 ; Àz þ 1.

Figure 3
View of part of a chain in the crystal structure of compound 2 showing the alternating cis-cis-trans and all-trans coordination.
However, in this context it is noted that some selenocyanate compounds with pyridine as coligand are found, of which those with the composition M(NCSe) 2 (pyridine) 2 (M = Zn, Co, Ni, Cd) are of the most interest. The Zn compound crystallizes as discrete complexes with a tetrahedral coordination (OWOJEQ; Boeckmann, Reinert & Nä ther, 2011), wheres the compounds with Fe II , Co II and Cd II crystallize as linear chains with an all-trans coordination [CAQVIB , ITISUA ].

Synthesis
Zn(NO 3 ) 2 Á6H 2 O and Cd(NO 3 ) 2 Á4H 2 O were purchased from Sigma Aldrich and KSeCN was purchased from Alfa Aesar. All chemicals were used without any further purification.
Synthesis of compound 1. 0.5 mmol (143 mg) of Zn(NO 3 ) 2 Á6H 2 O and 1 mmol (144 mg) of KSeCN were reacted with 1 mmol (97.2 ml) of 4-methylpyridine in 2 ml of ethanol. The reaction mixture was stirred for 2 d and the colorless precipitate was filtered off, washed with a very small amount of ethanol and dried at room temperature. Single crystals were obtained from the filtrate by slow evaporation of the solvent.
Synthesis of compound 2. 0.5 mmol (154 mg) of Cd(NO 3 ) 2 Á4H 2 O and 1 mmol (144 mg) of KSeCN were reacted with 1 mmol (97.2 ml) of 4-methylpyridine in 2 ml of ethanol. The reaction mixture was stirred for 2 d and the colorless precipitate was filtered off, washed with a very small amount of ethanol and dried at room temperature. Single crystals were obtained from the filtrate by slow evaporation of the solvent.

Experimental details
The XRPD measurements were performed with a Stoe Transmission Powder Diffraction System (STADI P) equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator using Cu K 1 radiation ( = 1.540598 Å ).
The IR spectra were measured using an ATI Mattson Genesis Series FTIR Spectrometer, control software: WINFIRST, from ATI Mattson.
atmosphere in Al 2 O 3 crucibles using a STA-PT 1000 thermobalance from Linseis. The instrument was calibrated using standard reference materials.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. C-bound H atoms were positioned with idealized geometry (C-H = 0.93-0.96 Å ; methyl H atoms allowed to rotate but not to tip) and were refined isotropically with U iso (H) = 1.2U eq (C) (1.5 for methyl H atoms) using a riding model.

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 )
x y z U iso */U eq where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.74 e Å −3 Δρ min = −0.63 e Å −3 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.