The crystal structures and Hirshfeld surface analyses of a cadmium(II) and a zinc(II) mononuclear complex of the new tetrakis-substituted pyrazine ligand N,N′,N′′,N′′′-[pyrazine-2,3,5,6-tetrayltetrakis(methylene)]tetrakis(N-methylaniline)

In the cadmium(II) and zinc(II) complexes of the tetrakis-substituted pyrazine ligand, N,N′,N′′,N′′′-[pyrazine-2,3,5,6-tetrayltetrakis(methylene)]tetrakis(N-methylaniline), the ligand coordinates in a mono-tridentate fashion, and both metal atoms have fivefold coordination spheres with distorted shapes.


Chemical context
The title ligand, N,N 0 ,N 00 ,N 000 -[pyrazine-2,3,5,6-tetrayltetrakis-(methylene)]tetrakis(N-methylaniline) (L), whose synthesis and crystal structure have been described in the preceding publication (Tesouro Vallina & Stoeckli-Evans, 2020), is a new tetrakis-substituted pyrazine derivative. It was designed to study its coordination behaviour with transition metals (Tesouro Vallina, 2001). The reaction of the ligand with CdI 2 and ZnCl 2 lead to the formation of the title mononuclear complexes I and II. Herein, we describe their syntheses, molecular and crystal structures and the analyses of their Hirshfeld surfaces.

Structural commentary
The molecular structure of the cadmium(II) complex, Cd(L)I 2 (I), of the ligand N,N 0 ,N 00 ,N 000 -[pyrazine-2,3,5,6-tetrayltetrakis(methylene)]tetrakis(N-methylaniline) (L), is illustrated in Fig. 1. Selected geometrical parameters are given in Table 1. The complex possesses twofold rotation symmetry, with the twofold axis bisecting the cadmium atom, Cd1, and the nitrogen atoms N1 and N4 of the pyrazine ring. The ligand coordinates in a mono-tridentate manner and the cadmium atom has a fivefold CdN 3 I 2 coordination environment with a distorted shape (see Fig. 2a). The 5 parameter for the fivefold coordination of atom Cd1 is 0.14 ( 5 = 0 for a perfect squarepyramidal geometry and = 1 for a trigonal-pyramidal geometry; Addison et al., 1984).
The molecular structure of the zinc(II) complex, Zn(L)Cl 2 Á0.6(CH 2 Cl 2 ) (II), is illustrated in Fig. 3. It crystallized as a partial dichloromethane solvate. Selected geometrical parameters are given in Table 3. The ligand L coordinates in a mono-tridentate manner and the zinc atom, Zn1, has a fivefold ZnN 3 Cl 2 coordination environment with a distorted shape (see Fig. 2b). The 5 parameter for atom Zn1 is 0.30.
A search of the CSD for a ZnN 3 Cl 2 coordination environment involving a pyrazine N atom yielded five relevant structures, which again involve the ligand TPPZ. They include two polymorphs of the mononuclear complex dichloro- [2,3,5,6-tetrakis(2-pyridyl) Table 2 Hydrogen-bond geometry (Å , ) for I.
The conformation of the ligand L differs in the two complexes (Fig. 4). The orientation of the phenyl rings with respect to the pyrazine ring and to each other is slightly different, and the various dihedral angles are compared in Table 5. It can be seen that the most significant difference, of 20.9 (2) , involves the orientation of ring D (ring B i in I) with respect to ring E (ring C i in I).

Supramolecular features
A partial view of the crystal packing of I is shown in Fig. 5 A view of the molecular structure of compound II, with atom labelling. Displacement ellipsoids are drawn at the 30% probability level. For clarity, H atoms have been omitted.  Hydrogen-bond geometry (Å , ) for II.

Figure 4
A comparison of the conformation of the ligand L in complexes I and II. For complex I, which possesses twofold rotation symmetry, ring D = B i , and ring E = C i [symmetry code: (i) Àx + 3 2 , y, Àz].

Table 5
A comparison of the conformation of the ligand (L) in complexes I and II. IÁ Á Á(pyrazine) contacts present, consolidating the chains propagating along the a-axis direction ( Fig. 6a and Table 2). This situation is similar to that observed in the crystal of the CdI 2 complex of TPPZ (GAHRIT; Saghatforoush, 2015). There, the IÁ Á Ácentroid(pyrazine ring) distance is 3.699 (1) Å with a Cd-IÁ Á Ácentroid angle of 175.92 (12) , compared to 3.9593 (12) Å and 155.19 (3) in complex I ( Fig. 6a and Table 2). In the crystal of II, molecules are linked by a series of C-HÁ Á Á interactions, forming layers lying parallel to the (111) plane; see Fig. 7 and Table 4. The dichloromethane molecules are linked across a center of symmetry with a short Cl4Á Á ÁCl4(Àx, Ày, Àz + 2) contact of 3.045 (5) Å and do not participate in any significant intermolecular interactions with the complex molecule. There are Zn-ClÁ Á Á(pyrazine) contacts present, which link inversion-related molecules, forming dimers ( Fig. 6b and Table 5). This arrangement is similar to that observed in the crystal structure of the ZnCl 2 complex of TPPZ (PAPCER; Hong et al., 2017). This compound crystallized with two independent molecules in the asymmetric unit. There, the ClÁ Á Ácentroid(pyrazine ring) distances are ca 3.087 and 3.167 Å , with the corresponding Zn-ClÁ Á Ácentroid angles being ca 152.62 and 141.76 . In the crystal structure of WIBVOS, a similar interaction is present with a ClÁ Á Ácentroid(pyrazine ring) distance of ca 3.987 Å and a Zn-ClÁ Á Ácentroid angle of ca. 170.96 . In complex II, the corresponding ClÁ Á Ácentroid(pyrazine ring) distance and Zn-ClÁ Á Ácentroid angle are 3.683 (2) Å and 155.96 (6) , respectively (Table 4).

Hirshfeld surface analysis and two-dimensional fingerprint plots
The   (Table 2; dashed red arrows), (b) a partial view along the a axis of the crystal packing of II, showing the Zn-ClÁ Á Á(pyrazine) interactions (Table 4; dashed red arrows). For clarity, the dichloromethane molecule has been omitted.

Figure 7
A view along the a axis of the crystal packing of compound II. The various C-HÁ Á Á interactions (Table 4; blue, red and green) are shown as dashed lines. The dichloromethane molecule has been omitted, and only the H atoms (blue, red and green) involved in the C-HÁ Á Á interactions have been included. Table 6 Summary of interatomic contacts (Å ) a , shorter than the sum of the van der Waals radii, in the crystal structures of I and II.
der Waals radii) through white to blue (distances longer than the sum of the van der Waals radii). A summary of the short intermolecular contacts in the crystal structures of I and II is given in Table 6. For complex I, the Hirshfeld surface (HS) mapped over d norm , and the two-dimensional fingerprint plots are given in Fig. 8. The red spots on the HS (Fig. 8a) correspond to the IÁ Á ÁH contacts, which give a pair of spikes in the fingerprint plot ( Fig. 8b) at d e + d i ' 3.0 Å , contributing 14.2% to the HS. The HÁ Á ÁH contacts contribute 63.4% and the CÁ Á ÁH contacts 18.0%. Any other atom-atom contacts contributed less than 2% and have not been included here.
For compound II, the Hirshfeld surface mapped over d norm , is shown in Fig. 9a, and that for the complex itself and the solvent molecule in Figs. 9b and 9c, respectively. The faint red spots correspond to the ClÁ Á ÁH contacts in the crystal. These give a pair of spikes in the fingerprint plots, at d e + d i ' 2.7 Å , contributing 22.7%, in the compound (Fig. 10a) and at d e + d i ' 2.7 Å , contributing 18.1%, in the complex (Fig. 10b). For the solvent molecule, a single sharp spike is observed (d e + d i ' 2.8 Å ) with a contribution of 59.6% to the HS (Fig. 10c). The HÁ Á ÁH contacts contribute 55.1, 59.4 and 25.2% to the Hirshfeld surfaces of the compound, the complex and the solvent molecule, respectively, while the CÁ Á ÁH contributions are 17.7, 18.8 and 6.8%, respectively. Any other atom-atom contacts contributed less than 2% and have not been included here.
Synthesis of the complex [Cd(L)I 2 ] (I): About 10 ml of a very dilute CH 2 Cl 2 solution of ligand L were introduced into a glass tube and layered with ca 2 ml of MeOH as a buffer zone. Then, 10 ml of a dilute methanolic solution of CdI 2 were added slowly to avoid possible mixing. The glass tube was sealed and left at room temperature. The colour of the interphase changed immediately to deep yellow and in hours to green. After a few days, green rod-like crystals were formed. IR (KBr pellet, cm    Synthesis of the complex [Zn(L)Cl 2 ]Á0.6(CH 2 Cl 2 ) (II): To a solution of ZnCl 2 (0.1 mmol, 0.014 g) in 5 ml of MeOH, a solution of L (0.05 mmol, 0.028 g, 5 ml CH 2 Cl 2 ) was added. The solution was stirred at RT for 2 h without any significant colour change. The clear light-green solution obtained was filtered to avoid any impurity and allowed to evaporate slowly. After a few days, yellow rod-like crystals were obtained. IR (KBr pellet, cm À1 ): 1599 (vs), 1507 (s), 1451 (m), 1363 (s), 1257 (m), 1171 (m), 1033 (m), 920 (m), 746 (s), 691 (s). Analysis for [Zn(C 36 H 40 N 6 )Cl 2 ]Á0.6CH 2 Cl 2 (743.99 g mol À1 ): calculated C 60.50, H 5.68, N 11.65%; found C 60.66, H 5.78, N 11.93%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 7. The C-bound H atoms were included in calculated positions and treated as riding on their parent C atom: C-H = 0.94-0.98 Å with U iso (H) = 1.5U eq (Cmethyl) and 1.2U eq (C) for other H atoms.
With the STOE IPDS I, a one-circle diffractometer, for the triclinic system often only 93% of the Ewald sphere is accessible. Hence, for compound II the _diffrn_reflns_Laue_mea-sured_fraction_full of 0.939 is below the required minimum of 0.95. For II, a small number of low-angle reflections, either in the shadow of the beam-stop or with bad agreement, were omitted during the final cycles of refinement.   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.15 e Å −3 Δρ min = −0.88 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq

Hydrogen-bond geometry (Å, º)
Cg3 is the centroid of the pyrazine ring N1/N4/C1/C2/C1 i /C2 i and Cg5 is the centroid of the C12-C17 ring.  (12) 155 ( 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 )