Crystal structure of tetrakis(μ2-(E)-2,4-dibromo-6-{[2-(pyridin-2-yl)ethyl]iminomethyl}phenolato)trizinc bis(perchlorate) acetonitrile disolvate

The structure is reported of a complex containing a trinuclear Zn cation lying on a crystallographic twofold axis. It consists of a tetrahedral ZnII atom bridging two six-coordinate ZnII atoms in which the two terminal ZnII cations adopt distorted octahedral geometries and the central ZnII cation adopts a distorted tetrahedral geometry.


Chemical context
Zinc(II)-derived metalloenzymes are among the most common found in biology. Some enzymes containing zinc(II) include carbonic anhydrase, carboxypeptidase, and phosphatase (Bertini et al., 1994;McCall et al., 2000). It is of interest to study zinc(II) complexes derived from tridentate Schiff base ligands because of the possibility of forming stable complex structures. Zinc(II) plays a structural role not only in enzymes but much progress has been made to incorporate it into metalorganic frameworks for drug storage and release, luminescence studies, and hydrogen-storage applications Bauer et al., 2007;Rosi et al., 2003).
Related complexes have been studied for their photoluminescent properties (Kundu et al., 2015;Chakraborty et al., ISSN 2056-9890 2013), drug therapeutic activity in DNA cleavage (Kumar et al., 2011), and phosphatase mimetic activity (Kumar et al., 2011;Gultneh et al., 1999). The coordination environment of the title compound, illustrated in Fig. 1, has been observed in zinc(II) complexes with tridentate N,N,O ligands (Hens & Rajak, 2015;Kim et al., 2015). Transition metal complexes of the related tridentate ligand, 1,3(2-pyridyliminomethyl)phenylenediamine, have been shown to form a variety of interesting complex structures (Kundu et al., 2015;Kumar et al., 2011;Bluhm et al., 2003;Souza et al., 2011;Sanyal et al., 2014;Okeke et al., 2017a,b;Okeke et al., 2018). The presence of a substituent on the aromatic group may change the geometry, coordination number, and consequently the reactivity of the resulting complexes especially because of its location on the aromatic ring that coordinates to the metal ion through the phenoxide oxygen atom.
In a continuation of our model studies of zinc complexes as Lewis acid center in zinc-containing hydrolytic enzymes (Gultneh et al., 1996;Gultneh et al., 1999;Okeke et al., 2017a,b) we report the structure of the title compound. This trinuclear zinc(II) complex has a 3:4 metal ion-to-ligand ratio. Since the title compound lies on a crystallographic twofold axis, the three zinc(II) ions form an angle of 180 o and thus are strictly linear. The central zinc atom is four coordinate and may serve as a suitable complex for various reactions because the Zn II Lewis acid metal center contains vacant coordination sites for coordination to a nucleophile. O2-Zn2 = 1.951 (2) Å ] and the bridging angles are Zn1-O1-Zn2 = 96.78 (8) and Zn2-O2-Zn3 = 93.73 (8) . The distortion from an octahedral geometry can be seen from the cis and trans angles which range from 77.49 (10) to 98.19 (9) and 160.47 (13) to 173.41 (12) , respectively. Since all three Zn atoms lie on the twofold axis, the Zn1-Zn2-Zn3 bond angle is exactly 180 . These metrical parameters are similar to those found in the most closely similar complex (Kim et al., 2015) where Zn-O distances for the terminal Zn atoms range from 2.126 (3) to 2.155 (4) Å while those for the central Zn atom range from 1.945 (3) to 1.965 (4) Å with Zn-O-Zn bridging angles ranging from 97.3 (1) to 98.7 (1) . The Zn-N imine and Zn-N py bond lengths range from 2.077 (4) to 2.117 (4) Å and 2.140 (4) to 2.176 (4) Å , respectively. In this complex there is no crystallographically imposed symmetry; however, the Zn-Zn-Zn bond angle is still close to 180 at 172.51 (3) . Symmetry codes: (i) Àx þ 1; y; Àz þ 1 2 ; (ii) x; Ày þ 1; z À 1 2 ; (iii) Àx þ 1 2 ; Ày þ 3 2 ; Àz.

Supramolecular features
In the cation there areinteractions between the dibromophenyl rings [centroid-centroid distance = 3.602 (2) Å ; CgIÁ Á Áperp = 3.344 (1) Å ; slippage = 1.319 (2) Å ] as well as halogen-bonding interactions [BrÁ Á ÁBr 3.6123 (5) Å ; C-BrÁ Á ÁBr, 129.08 (9) ] between the dibromophenyl rings in the cation, which stabilize its conformation. In addition there C-HÁ Á ÁO interactions between the anions and both the cations and solvent molecules as well as C-HÁ Á ÁN interactions between the cation and solvent molecules (Table 1). These interspecies interactions link the cations, anions and solvent molecules into a complex three-dimensional array as shown in Fig. 2.  (Kim et al., 2015). The major differences between this complex and 1 is a -CH 2 -link between the imine N and pyridine ring in the former instead of a -CH 2 -CH 2 -link in the latter, and different substituents on the phenyl ring.

Synthesis and crystallization
2-(2-Pyridyl)ethylamine (0.3023 g, 2.474 mmol) was dissolved in 50 mL of methanol. 3,5-Dibromosalicylaldehyde (0.6927 g, 2.474 mmol) was added to the solution and the mixture was refluxed for 5 h. The zinc(II) complex was prepared by reacting the ligand in 50 ml of methanol with Zn(ClO 4 ) 2 Á6H 2 O (1.3821 g, 3.712 mmol) with no added base. The mixture was stirred at room temperature overnight. The methanol was removed by rotary evaporation. The product was crystallized by slow evaporation of a solution in acetonitrile giving paleyellow to colorless crystals.

Tetrakis(µ 2 -(E)-2,4-dibromo-6-{[2-(pyridin-2-yl)ethyl]iminomethyl}phenolato)trizinc bis(perchlorate) acetonitrile disolvate
Crystal data 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.