Crystal structure of bis{2-hydroxy-N′-[1-(pyrazin-2-yl)ethylidene]benzohydrazidato}cadmium(II)

The structure of a cadmium(II) aroylhydrazone complex, viz. bis{2-hydroxy-N′-[1-(pyrazin-2-yl)ethylidene]benzohydrazidato}cadmium(II) is described.


Chemical context
Aroylhydrazones are competent ligands for various functional coordination compounds. They have the ability of polydentate coordination and are often used as building units of polynuclear magnetic compounds (Huang et al., 2016;Zhang et al., 2010). Aroylhydrazones can exhibit keto-enol tautomerism, and the uncomplexed aroylhydrazone ligand is commonly found in its keto form (Kalinowski et al., 2008;. Metal complexes of deprotonated aroylhydrazones have been used in various catalytic and biological applications (Sutradhar et al., 2013;Yang et al., 2019;Yang, Chen et al., 2020). Aroylhydrazones synthesized from arylhydrazides and aromatic aldehydes/ketones with a nitrogen or oxygen atom in the ortho position can coordinate to metals in a tridentate chelating mode (Cindrić et al., 2017;Patel et al., 2018;You et al., 2018), and they have been used as probes and chemosensors for various metal ions. For example, the aroylhydrazone ligand containing a 4-(dimethylamino)phenylpropenyl or benzamide substituent specifically senses Al 3+ , Cd 2+ (Kar et al., 2015) and Ni 2+ ions (Manna et al., 2019) through significant changes in their absorption and emission spectroscopic behaviour after complexation with the metal ions. Here, we study the coordination attributes of an aroylhydrazone with cadmium.

Structural commentary
In the title complex, the Cd 2+ ion possesses a distorted octahedral N 4 O 2 coordination environment, which is generated by the two deprotonated ligands L (Fig. 1). The complex is bisected by a twofold crystallographic axis with the two ligands being equivalent by crystal symmetry. The complex is isomorphous to its Mn, Co, Ni, Cu and Zn counterparts . The O2-C7 and C7-N1 bond lengths in the title compound are 1.255 (5) Å and 1.355 (5) Å , respectively, indicating that the coordinated ligands are closer to the keto than the enol form, but are slightly more delocalized than in the purely keto tautomeric form as found in the free ligand form of similar aroylhydrazones. The free ligand L has not yet been structurally described, but the equivalent bond distances in e.g. 2-hydroxy-N 0 -[1-(3-methylpyrazin-2-yl)ethylidene]benzohydrazide, L 1 , with one more methyl group on pyrazine , were reported as 1.235 and 1.340 Å , respectively.
The ligand in the title complex is close to planar (the mean deviation from the average plane is 0.0763 Å ). The largest deviation from planarity is only 0.145 (3) Å , observed for atom C12 of the pyrazine ring. The Cd1 atom is nearly coplanar with each of the two ligands (deviation = 0.316 Å ). The dihedral angle between the two ligands is 78.705 (16) . The oxygen atom O1 of the phenolic group remains protonated, and forms an intramolecular hydrogen bond O1-H1Á Á ÁN1 [2.557 (4) Å , 146 (7) ].
The intramolecular hydrogen bond stabilizes the planar geometry of the ligand. The presence of the intramolecular hydrogen bond does also appear to affect the propensity of the metal complex towards crystallization. We found that when the hydroxyl group is in the meta or para position {3-hydroxy- where no intramolecular hydrogen bond can be formed, crystallization is substantially delayed and a much longer time is required for the complexes to crystallize.
In the title complex, the Cd1-N3, Cd1-N2 and Cd1-O2, bond lengths are 2.356 (3), 2.273 (3) and 2.277 (4) Å , respectively, which are close to typical for Cd 2+ complexes closely related to the title compound, such as bis{N 0 -[1-(pyridin-2yl)ethylidene]benzohydrazidato}cadmium(II) (Sen et al., 2005), bis{2-[2-(pyridin-2-ylmethylene)hydrazine-1-carbonyl]benzenesulfonamide}cadmium(II) (Sousa-Pedrares et al., 2008) and bis[N 0 -(2-hydroxybenzoyl)picolinohydrazonamide]cadmium(II) (Xu et al., 2014), bis{N 0 -[di(pyridin-2-yl)methylene]benzohydrazidato}cadmium(II) (Kuriakose et al., 2017)  The molecular structure of [Cd(C 13 H 11 N 4 O 2 ) 2 ] with displacement ellipsoids at the 30% probability level. Symmetry code: (xi) Àx + 1, Ày + 1, z. Table 1 Comparative analysis of ion radius and the bond lengths and bond angles of coordination polyhedra (Å , ).  (17) , while the cis angles vary between 69.83 (11) and 117.27 (11) . Bond distances and angles within the isomorphous series of the Mn, Co, Ni, Cu, Zn, and Cd complexes follow a trend consistent with the metal ion radius (Table 1). Bond lengths first decrease and then increase, with a minimum value for the Ni or Cu complexes, and a maximum for the title cadmium complex as a result of its substantially larger ion radius as the only 4d complex of the series. The trend of the N-M-O angle (within the same ligand) is opposite to that of the metal ion radius, and first increases and then decreases, with the maximum value appearing for the Ni complex (Brines et al., 2007;Reger et al., 2012;Sola et al., 1994;Database of Ionic Radii, 2020). The distortion from octahedral geometry increases with ion radius, and is most pronounced for the title cadmium complex, as can be seen for e.g. the N(mid)-M-N(mid) angles, which range from 172.30 to 174.46 for the 3d complexes, while the value for the 4d Cd complex is 170.63 (17) .

Supramolecular features
Two types of weak intermolecular interactions, C-HÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds andstacking and C-HÁ Á Á interactions, have a significant impact on the packing of the complexes in the solid state. Three intermolecular hydrogen bonds (  Table 2 Hydrogen-bond geometry (Å , ).

Database survey
There are also complexes of ligands L 4 and L 5 that are not isomorphous to the title complex: complex Cu 2 (L 4 ) 2 Cl 2 is binuclear, where each Cu centre has two -chlorine ligands along with a tridentate coordinated L 4 molecule, giving rise to a distorted square-pyramidal coordination environment. It belongs to the triclinic P1 space group (YELXAR; Tai  has a square-pyramidal coordination geometry in the mono-clinic P2 1 /n space group. Cu 2 (L 5 ) 2 Cl 2 (NICYOP; Mondal et al., 2013) is a binuclear complex and each Cu centre has a squarepyramidal coordination geometry. It is isomorphic to Cu 2 (L 4 ) 2 Cl 2 . A Zn complex, Zn(L 1 ) 2 ÁH 2 O (XIYNUP;  with the ligand L 1 with one more methyl group on pyrazine crystallizes in the monoclinic P2 1 /n space group. The planarity of the ligand is decreased compared to the title complex, and the Zn ion exhibits a distorted octahedral geometry. Also reported are five similar compounds featuring the ligands L 2 and L 3 with the hydroxyl group in the meta and para positions of the benzene ring, respectively. They crystallize as DMF solvates [M(L 2 ) 2 ]Á2(DMF) (DMF = dimethylformamide; M = Ni, Cu and Zn; CIZHIF, CIZGUQ and CIZJAZ) and [Cu(L 3 ) 2 ]Á2(DMF) (M = Ni and Cu; CIZHUR and COYWEV)  in the orthorhombic Pbcn space group. They also feature distorted octahedral structures and the planarity of the ligands is decreased compared to the title compound. All the complexes with the [M(Ligand) 2 ] core are distorted octahedral, and all metal centres have a mer geometry. All ligands L, L 1 , L 2 , L 3 , L 4 and L 5 are tridentate chelating.

Synthesis and crystallization
The title complex and ligand were synthesized according to literature procedures Yang et al., 2019). The complex was obtained by mixing a solution of the aroylhydrazone (0.02 mmol) in methanol (2 mL) and a solution of Cd(NO 3 ) 2 Á4H 2 O (0.01 mmol) in water (2 mL). After two weeks of static volatilization in a test tube at room temperature, clear light-yellow block-shaped crystals of Cd(L) 2 were obtained (

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 )