Crystal structure, Hirshfeld surface analysis and computational study of bis(2-{[(2,6-dichlorobenzylidene)hydrazinylidene]methyl}phenolato)cobalt(II) and of the copper(II) analogue

Distinct coordination geometries are found in the crystals of the title CoII (trigonal bipyramidal) and CuII (square-planar) complexes, each defined by a N2S2 donor set derived from two chelating Schiff base anions.


Chemical context
Schiff base molecules are well-known ligands because of the ease of their formation and for their rich coordination chemistry with a wide range of metal ions. A prominent application of metal-Schiff base complexes is as catalysts in different chemical reactions (Patti et al., 2009). The Schiff base molecules themselves are of considerable interest as they display a broad range of biological activities such as antibacterial, anti-fungal, anti-viral, anti-malarial, anti-inflammatory, etc. (Guo et al., 2007;Przybylski et al., 2009;Annapoorani & Krishnan, 2013). A full range of metal complexes formed with these usually multidentate ligands often result in species with enhanced biological action (Bagihalli et al., 2008;Tian et ISSN 2056-9890 al., 2009, 2011Chohan et al., 2001). As part of our ongoing studies of Schiff base ligands and their metal complexes (Manawar, Gondaliya, Mamtora et al., 2019), the crystal and molecular structures, Hirshfeld surface analysis and computational study of homoleptic Co II (I) and Cu II (II) complexes derived from 2-{(1E)-[(E)-2-(2,6-dichlorobenzylidene)hydrazin-1-ylidene]methyl}phenol  are described herein.

Structural commentary
The cobalt complex (I), Fig. 1, lacks crystallographic symmetry and the metal ion is N,O-coordinated by two mono-anionic Schiff base ligands; selected geometric parameters are collated in Table 1. The N 2 O 2 donor set defines an approximate tetrahedron with the range of tetrahedral angles being over 30 . Thus, the narrowest angle of 94.06 (7) is found for O1-Co-N1 while the widest of 125.33 (8) is noted for O1-Co-O2. A geometric measure of a four-coordinate geometry is the value of 4 , which has values of 4 = 1.0 for an ideal tetrahedron and 4 = 0.0 for an ideal square-planar geometry (Yang et al., 2007). In (I), 4 = 0.82, indicating a geometry close to trigonal pyramidal. Each of the Schiff base ligands forms a sixmembered Co,O,C 3 ,N chelate ring. These adopt an envelope conformation with the Co atom lying 0.253 (3) Å out of the least-squares plane defined by the five remaining atoms of the O1-chelate ring (r.m.s. deviation = 0.0086 Å ); the equivalent values for the O2-chelate ring are 0.376 (3) and 0.0222 Å , respectively. The dihedral angle formed between the planar regions of the chelate rings is 86.44 (8) , consistent with a near to orthogonal relationship. For the O1-chelate ring, the dihedral angle between the five co-planar atoms and the fusedbenzene and pendent dichlorobenzene rings are 0.92 (13) and 7.34 (14) , respectively, and the dihedral angle between the benzene rings is 6.47 (15) , indicating a small deviation from planarity. The equivalent dihedral angles for the O2-chelate ring are 1.99 (14), 7.25 (12) and 5.58 (12) , respectively. These data are consistent with small twists about the N1-N2 [the C7-N1-N2-C8 torsion angle = 166.6 (2) ] and C16-C21 [C15-C16-C21-N3 = 6.4 (4)  Recently, the crystal structure of the precursor Schiff base ligand became available . Here, each imine bond has an E-configuration and the bond lengths of the imine bonds, i.e. 1.281 (2)  The molecular structure of (I) showing the atom-labelling scheme and displacement ellipsoids at the 35% probability level. A distinct coordination geometry is found in the Cu II complex, (II), Fig. 2 and Table 1. The Cu II atom lies on a crystallographic centre of inversion. As for (I), N,O-chelation is observed. From symmetry, the N 2 O 2 donor set is strictly planar. The Cu II atom lies 0.2582 (13) Å above the resultant square-plane. The chelate rings adopt an envelope conformation, as for (I), with the Cu atom lying 0.470 (2) Å above the plane through the remaining atoms of the chelate ring (r.m.s. deviation = 0.0129 Å ). The magnitude of the dihedral angle between the five co-planar atoms of the chelate ring and the fused-benzene ring [1.43 (13) ] resembles the situation in (I), but that formed with pendent dichlorobenzene ring is quite distinct, at 82.63 (8) , consistent with an orthogonal relationship. This is reflected in the C7-N1-N2-C8 torsion angle of 141.33 (19) . The different configuration arises to avoid steric hindrance within the square-planar environment. The Cu-O,N bond lengths span a wider range, i.e. 0.14 Å , c.f. 0.11 Å for the Co-O,N bond lengths in (II), with the Cu-O bond lengths being shorter than the Co-O bonds, and the Cu-N bonds being longer than the Co-N bonds. Comparable trends are seen in the configurations of the imine bonds, Table 1.

Figure 3
Molecular packing in the crystal of (I): (a) supramolecular layer sustained by C-HÁ Á ÁO, C-HÁ Á Á andinteractions shown as orange, blue and purple dashed lines, respectively, and (b) a view of the unit-cell contents in a projection down the c axis.

Hirshfeld surface analysis
The Hirshfeld surfaces were calculated for each of (I) and (II) employing Crystal Explorer 17 (Turner et al., 2017) and literature protocols (Tan et al., 2019). This study was undertaken in order to determine the influence of weak, noncovalent interactions upon the molecular packing in the absence of conventional hydrogen bonding.
On the Hirshfeld surface mapped over d norm for (I) in Fig. 5(a) and (b), the bright-red spots near the H27 atom of the (C23-C28) ring and the coordinating O1 atom are an indication of the C-HÁ Á ÁO interaction. Referring to Table 3, the presence of short interatomic contacts involving the Co II , chloride and chlorobenzene-hydrogen atoms and the atoms of the C1-C6 benzene ring are characterized as faint-red spots near the respective atoms on the d norm -mapped Hirshfeld surface. The blue bump near the H25 atom and the bright-   View of the Hirshfeld surface for (I) mapped (a) and (b) over d norm in the range À0.123 to + 1.343 arbitrary units and (c) with the shape-index property highlighting intermolecular C-HÁ Á Á/Á Á ÁH-C contacts. Table 3 Summary of short interatomic contacts (Å ) in (I) and (II) a .

Contact
Distance Symmetry operation 3.2858 (7) 1 + x, y, z orange spot about the C1-C6 ring on the Hirshfeld surface mapped with shape-index property in Fig. 5(c) correspond to the donor and acceptor of the C-HÁ Á Á contact. The absence of strong, directional interactions in the crystal structure of (II) is evident from its Hirshfeld surface mapped over d norm in Fig. 6, as the surface contains only some tiny, diffuse red spots near the atoms corresponding to short interatomic ClÁ Á ÁH and CÁ Á ÁC contacts listed in Table 4. On the Hirshfeld surfaces mapped over the electrostatic potential for (I) in Fig. 7(a), the donors and acceptors of the C-HÁ Á ÁO and C-HÁ Á Á contacts (Table 3) are viewed as blue and red regions near the respective atoms corresponding to positive and negative electrostatic potentials. The presence of a blue region near the Cu II atom and red region near the Cl2 atom in the corresponding surface for (II) in Fig. 7(b) is an indication of a short intermolecular CuÁ Á ÁCl2 interaction [3.2858 (7) Å ], as discussed further below, see Computational chemistry. The influence ofstacking interactions in each of the crystals of (I) and (II) is evident as the flat regions about the participating aromatic rings on the Hirshfeld surfaces mapped over curvedness illustrated in Fig. 8 Given the different coordination geometries in (I) and (II), it was thought of interest to calculate the Hirshfeld surfaces about the individual metal centres (Pinto et al., 2019). The different coordination geometries, approximately trigonal pyramidal for Co II , Fig. 9(a) and (b), and square-planar for Cu II in Fig. 9 A view of the Hirshfeld surface for (II) mapped over d norm in the range À0.016 to 1.528 arbitrary units.

Figure 7
Views of the Hirshfeld surfaces mapped over the electrostatic potential (the red and blue regions represent negative and positive electrostatic potentials, respectively) for (a) (I) in the range À0.084 to +0.061 atomic units and (b) (II) in the range À0.095 to +0.163 atomic units.  Table 1.
The different coordination geometries about the metal centres are also reflected in the two-dimensional fingerprint plots shown in Fig. 10, only taking into account the Hirshfeld surface about the metal atom. The distribution of aligned red points from d e + d i $1.8 Å (lower portion) and d e + d i $2.0 Å (upper portion) for the Co-O and Co-N bonds, respectively, in (I) show different inclinations, Fig. 10(a), whereas the superimposed red points in the case of (II), Fig. 10(b), arise as a result of the symmetrical coordination geometry. For (I), the presence of short intramolecular CoÁ Á ÁH contacts formed with the chlorobenzene-H8 and H22 atoms (CoÁ Á ÁH8 = 2.64 Å and CoÁ Á ÁH22 = 2.55 Å ) result in dissymmetry in the Hirshfeld surface and are characterized as bright-orange spots on the shape-index-mapped surface in Fig. 9(a). The square-planar coordination geometry formed by the N 2 O 2 donor set in (II) results in an approximate cuboid Hirshfeld surface with rounded corners and edges.
In the fingerprint plot delineated into HÁ Á ÁH contacts of Fig. 11(b), the short interatomic contacts result in a peak at d e + d i $2.3 Å in the crystal of (I) while HÁ Á ÁH in (II) are at van der Waals separations or longer. The presence of the C-HÁ Á ÁO contact in (I) is recognized as the pair of spikes at d e + d i $2.2 Å in the fingerprint plot delineated into OÁ Á ÁH/ HÁ Á ÁO contacts of Fig. 11(c) whereas the comparatively small contribution from these contacts in (II), Table 4, show the points farther than sum of their van der Waals radii. The pair of forceps-like tips at d e + d i $2.8 Å in the fingerprint plots delineated into ClÁ Á ÁH/HÁ Á ÁCl contacts in Fig. 11 Views of the Hirshfeld surfaces calculated for the Co II (I) and Cu II (II) centres alone, highlighting the coordination geometries formed by the N 2 O 2 donor sets mapped over (a) the distance d e external to the surface in the range 0.922 to 2.221 Å for (I), (b) the shape-index (S) from À1.0 to +1.0 (arbitrary units) for (I), (c) the distance d e external to the surface in the range 0.919 to 2.114 Å for (II) and (d) the shape-index (S) from À1.0 to +1.0 (arbitrary units) for (II).

Figure 10
The two-dimensional fingerprint plots taking into account only the Hirshfeld surface about the metal centre in (a) (I) and (b) (II). crystals; for (II), these are beyond the sum of the van der Waals radii. From the fingerprint plot delineated into CÁ Á ÁH/ HÁ Á ÁC contacts, Fig. 11(e), the pair of short tips at d e + d i $2.7 Å indicate the presence of CÁ Á ÁH and C-HÁ Á Á contacts in (I), by contrast to only van der Waals contacts in (II). In the fingerprint plot delineated into CÁ Á ÁC contacts for (I) and (II), Fig. 11(f), the influence ofstacking interactions are characterized as the distribution of green points in the plot around d e = d i = 1.8 Å . Referring to Fig. 12(a), the distribution of points in the form of a thin line from d e + d i $3.7 Å in the fingerprint plot delineated into ClÁ Á ÁCl contacts for (I) is an indication of influence of these contacts on the packing of (I); this is confirmed in the next section, Computational study. The fingerprint plot delineated into CuÁ Á ÁCl/ClÁ Á ÁCu contacts of Fig. 12(b), with the small, i.e. 1.9%, but important contribution to the Hirshfeld surface of (II) is the result of a CuÁ Á ÁCl interaction prominent in its molecular packing, as justified from the interaction energy calculations described in the next section.

Computational chemistry
The pairwise interaction energies between the molecules in the crystals of each of (I) and (II) were calculated by summing up four energy components, comprising electrostatic (E ele ), polarization (E pol ), dispersion (E dis ) and exchange-repulsion (E rep ) as per the literature (Turner et al., 2017). In the present study, the energies were obtained by using the wave function calculated at the HF/3-21G level of theory. The nature and the strength of the energies for the key identified intermolecular interactions are quantitatively summarized in Tables 5 and 6 for (I) and (II), respectively.  Table 5 Summary of interaction energies (kJ mol À1 ) calculated for (I). Contact 9.60 À29.6 À7.6 À71.9 32.5 À73.5 Cg(C1-C6) Á Á ÁCg (C1-C6) Symmetry codes: (i) 1 + x, y, z; (ii) 1 À x, À y, 2 À z; (iii) 1 À x, À y, 1 À z; (iv) 2 À x, À y, 1 À z; (v) x, 1 + y, z; (vi) 2 À x, 1 À y, 1 À z. Table 6 Summary of interaction energies (kJ mol À1 ) calculated for (II). Symmetry codes: (i) À x, 1 À y, À z; (ii) 1 + x, y, z; (iii) 1 À x, 1 À y, Àz; (iv) 1 À x, À y, 1 À z.   The fingerprint plot delineated into (a) ClÁ Á ÁCl contacts for (I) and (b) CuÁ Á ÁCl/ClÁ Á ÁCu contacts for (II). Table 5, the atoms of (C23-C28) ring involved in the short interatomic CÁ Á ÁH/HÁ Á ÁC contacts, intermolecular C-HÁ Á Á andstacking interactions between the same pair of symmetryrelated molecules have maximum interaction energies. The dispersive component makes a major contribution to all of the intermolecular interactions in the crystal of (I). The low interaction energies for ClÁ Á ÁH and ClÁ Á ÁCl contacts are consistent with the small contributions from these contacts in the crystal. The presence of a CuÁ Á ÁCl2 contact in the crystal of (II) is an important feature of the packing. This interaction shows maximum interaction energy (Table 6) with significant contributions from the electrostatic component compared to stacking and other intermolecular interactions influential in the molecular packing.

For (I), among the intermolecular energies listed in
The magnitudes of intermolecular energies are represented graphically in the energy framework diagrams of Fig. 13(a)-(f). Here, the supramolecular architecture of each crystal is visualized through cylinders joining the centroids of molecular pairs using a red, green and blue colour scheme, representing the E ele , E disp and E tot components, respectively; the stronger the interaction, the thicker the cylinder.

Database survey
Schiff bases related to those reported in (I) and (II), i.e. having two imine functionalities and a single phenol/phenoxide atom/ ion on one ring only are quite rare. Thus, the only structure of an analogue available for comparison is a N,O-chelated dimethylaluminium compound with the ring bearing the phenoxide-oxygen also carrying t-butyl groups at the 3,5 positions and the second benzene ring bearing a chlorine atom in the 4position (UPEKEH; Hsu et al., 2017). By contrast, there are numerous examples of coordination complexes derived from Schiff base molecules with two 2-phenol substituents in each ring, LH 2 . In these instances, the dinegative Schiff base anion N,O-chelates two metal centres such as in binuclear Co 2 (L 1 ) 3 (JUKZOG; Liu et al., 2015), with a fac-N 3 O 3 donor set within an octahedral geometry, and Cu 2 (L 2 ) 3 (PPh 3 ) 2 (VOWBAM; Prakash et al., 2015) with tetrahedral NOP 2 donor sets; for the L 1 dianion, there are 3-ethoxy substituents in each ring and for L 2 , the are 4-chloro substituents.

Synthesis and crystallization
The title complexes (I) and (II) were synthesized according to the literature procedure (Manawar, Gondaliya, Mamtora et al., 2019). Briefly, the complexes were obtained by mixing the Schiff base, in ethanol, with an aqueous solution of the corresponding metal chloride in 1:1 and 1:2 molar ratios, respectively, in the presence of piperidine as basic catalyst for proton abstraction from the ligand molecules. The crystals in the form of red (I) and dark-brown (II) blocks were grown by slow evaporation from their respective chloroform solutions.

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 = 0.44 e Å −3 Δρ min = −0.46 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.