Crystal structure of poly[hexa-μ-bromido-bis{2-[1-(pyridin-2-yl)ethylideneamino]ethanolato}tetracopper(II)]

In the title Schiff base tetranuclear copper(II) complex, two discrete environments are present in the structure: CuN2OBr2 and CuBr4. Two copper(II) cations are situated in distorted square-based pyramidal environment, while two copper(II) cations are located in distorted tetrahedral geometry.


Chemical context
Schiff bases attract a great attention as ligands due to their simplicity of formation from amino and carbonyl derivatives.A rich coordination variability can be thus easily be attained and profited by following the introduction of other functional groups.Schiff base ligands are becoming increasingly important as they have interesting biological activities such as antibacterial, antitumor, insulin-mimetic and antifungi (Patil et al., 2012;Thompson & Orvig, 2001), and catalytical properties (Sutradhar et al., 2013).They are used in the preparation of photo-and pH-responsive sensors (Li et al., 2013), fluorescent receptors of metals (Chen et al., 2013), non-linear materials (Massue et al., 2013), nano-particles (Deng et al., 2013), hybrid inorganic-organic materials (Bhaumik et al., 2013), and even uranium complexes (Asadi et al., 2013), and ionic liquids (Ouadi et al., 2006).Many related tridentate Schiff base ligands have been successfully employed to build clusters of copper(II) ions bridged by halogen atoms (Wang et al., 2013;Sall et al., 2019;Sun et al., 2005).The incorporation of an amino alcohol fragment generally leads to the formation of [Cu 4 O 4 ] cubane-type clusters (Yan et al., 2009;Xie et al., 2002).In our present work, we have synthesized and characterized through X-ray diffraction analysis the title tetranuclear complex formulated as [Cu 4 Br 6 L 2 ] n , (HL = 2-{1-[(2-hydroxyethyl)imino]acetylpyridine}).

Structural commentary
The reaction of acetylpyridine and 2-aminoethanol in 1:1 ratio in ethanol yields the ligand 2-{1-[(2-hydroxyethyl)imino]-acetylpyridine} (HL).The reaction of the ligand HL with copper bromide yields a complex in which the ligand is reacted in its deprotonated form as L À .The coordination complex is formulated as [Cu 4 L 2 Br 6 ] n (I) (Fig. 1).
In the crystal of the tetranuclear complex, each of the two deprotonated ligands acts in a tridentate fashion, linking exclusively one copper(II) cation through its imino nitrogen atom, its pyridine nitrogen atom and its alcoholate oxygen atom.The two other Cu cations are only coordinated to bromide anions, which act as bridges.The metal centers present two different environments.According to the Addison index (Addison et al., 1984) � = (� À �)/60 (� and � are the largest values of the bond angles around the central atom) the coordination geometry around a pentacoordinated metal center can be discussed: � = 0 describes a perfect squarepyramidal while � = 1 describes a perfect trigonal-bipyramidal geometry.The geometries around the pentacoordinated Cu1 and Cu2 atoms are best described as distorted square-pyramidal, as shown by the Addison index: � = 0.0967 (Cu1) and � = 0.1517 (Cu2).For Cu1, the basal plane is occupied by O1, N1, N2 and Br1, the apical position being occupied by the Br2 atom.For Cu2, the basal plane is occupied by O2, N3, N4 and Br6, the apical position being occupied by the Br5 atom.Additionally, the sums of the angles subtended by the atoms in the basal plane, which are equal to 356.1 � (Cu1) and 356.3 � (Cu2), deviate severely from the ideal value of 360 � .For Cu1 and Cu2, the bond-angle values [92.51 (13)-108.25 ( 16) � ] between the atom occupying the apical position and the atoms in the basal plane also deviate considerably from the ideal value of 90 � .Additionally, the cissoid bond-angle values [80.5 (2)-98.12(13) � ] also deviate from the ideal value of 90 � .The coordination of the ligand to Cu1 or Cu2 results in the formation of two five-membered CuNCCN rings with biteangle values of 80.5 (2) � (Cu1) and 80.6 (2) � (Cu2) and CuNCCO rings with bite-angle values of 81.9 (2) � (Cu1) and 81.6 (2) � (Cu2).The geometry around the tetracoordinated atoms Cu3 and Cu4 was determined using the distortion index or the tetragonality parameter (Yang et al., 2007), which is stated as follows: � = (360 À � À �) / 141 (� and � are the two largest angles around the central atom).� = 0 designates a perfect square-planar geometry and � = 1 gives a perfect tetrahedron.The values of � = 0.88 (Cu3) and � = 0.86 (Cu4) are indicative of distorted tetrahedral geometries around the metal centers.In fact, the Br-Cu-Br bond-angle values [94.15 (4)-126.29 (6) � ] deviate severely from the ideal value of 109.5 � for a perfect tetrahedral geometry.

Supramolecular features
The crystal structure shows a three-dimensional polymer complex.The formation of this polymer was facilitated by A view of the title compound, showing the atom-labeling scheme.Displacement ellipsoids are plotted at the 30% probability level.

Table 1
Selected geometric parameters (A ˚, � ).bromide ions bridging copper(II) ions.The crystal packing of the complex is presented in Fig. 2. The polymer then develops as a band parallel to the bc plane (Fig. 3).Numerous intermolecular hydrogen bonds of the type C-H� � �Br (Table 2) connect adjacent units, resulting in a three-dimensional network.

Database survey
A search of the CSD (Version 5.42, November 2021 update; Groom et al., 2016) gave seven hits.One is a mononuclear Mo 5+ complex (BOFTOH; Jurowska et al., 2014) and two are coordination dinuclear complexes of Mn 2+ (JIKLIY and JIKLOE; Brooker & McKee, 1990).Similar Schiff ligands in which the methyl group is replaced by a phenyl group yielded three mononuclear Ni 2+ complexes (FOVBIE, FOVBOK, FOVBUQ; Chatterjee et al., 2019).Another similar ligand in which the alcohol group is replaced by a methoxy group yielded a Pd 2+ complex (PUYQUX; Nyamato et al., 2015).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were placed in idealized positions and refined using a riding model.The structure was refined considering a positional disorder for the following atoms: Cu1A, Br1A ,Cu3A, Br2A, Br6A, Cu2A, Br5A, Cu4A, with occupancy of ca 0.06-0.08.The packing in the crystal of the title complex, viewed along the c axis.(Bourhis et al., 2015), SHELXL2018/3 (Sheldrick, 2015), and OLEX2 (Dolomanov et al., 2009).

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.Refinement.Refined as a 2-component twin.

Figure 2
Figure 2Fragment of a [010] polymeric chain in the crystal structure of the title compound.

Table 3
Experimental details.