SYNTHESIS AND CHARACTERIZATION OF COPPER(II) COMPLEX WITH 2,6-DIACETYLPYRIDINE- BIS(PHENYLHYDRAZONE)

The syntheses, physicochemical and structural properties of the novel Cu(II) complex with 2,6-diacetylpyridine bis(phenylhydrazone) (L), of the formula [CuL2]Br2 are presented. In the reaction of warm MeOH solutions of the ligand, 2,6-diacetylpyridine bis(phenylhydrazone) and CuBr2 in molar ratio 2:1 resulted in formation of black single crystals of the bis(ligand) complex. This is the first Cu(II) complex with this ligand that is characterized by SC-XRD. Two ligand molecules are coordinated in the usual tridentate mode, via nitrogen atoms of the pyridine ring, and two azomethine nitrogen atoms, forming distorted octahedral environments of the metal ion.

2,6-Diacetylpyridine bis(hydrazones) are especially interesting because of their versatile coordination chemistry, i.e., they can act as ligands of different denticity (from tridentate to heptadentate). Even though a large number of metal complexes with this ligand group is synthesized, the number of those that are structurally characterized is rather small (Groom et al., 2016). Since the presence of metal center could often enhance the properties of the organic ligand, even improve its biological activity, it is significant thoroughly to examine the coordination properties of this ligand class, and the structures of the obtained complexes.
Lately, one of these ligands, 2,6-diacetylpyridine bis(phenylhydrazone) (L) became the topic of our research, *Corresponding author: mirjana.lalovic@dh.uns.ac.rs thus the crystal structure of the first metal complex, [CoL 2 ]I 2 , was determined (Belošević et al., 2018). In this paper the synthesis, some physicochemical properties, as well as molecular, and crystal structure of copper complex with the same ligand, of the formula [CuL 2 ]Br 2 is reported.

Materials and methods
All chemicals used were commercial products of analytical reagent grade, except for the ligand, 2,6diacetylpyridine-bis(phenylhydrazone), which was synthesized as described previously (Belošević et al., 2018). Elemental analyses (C, H, N) of air-dried complexes were carried out by standard micromethods in the Center for Instrumental Analyses, ICTM in Belgrade. Molar conductivities of freshly prepared complexes solutions (c = 1×10 −3 mol dm −3 ) were measured on a Jenway 4010 conductivity meter. IR spectra were recorded using KBr pellets on a NEXUS 670 FTIR spectrophotometer (Thermo Nicolet) in the range of 4000-400 cm −1 . Melting points were measured on a Nagema melting point microscope Rapido.

Synthesis of complex
The mixture of 0.024 g CuBr 2 (0.1 mmol) and 0.068 g (0.2 mmol) of L was heated in 5 cm 3 of MeOH until complete dissolution. The obtained dark solution was left at the room temperature and evaporated to a small volume. After 50 h black prismatic single crystals were filtered and washed with MeOH. Yield: 0.033 g (31 %

Crystal structure determination
Diffraction data were collected on an Oxford Diffraction Gemini S diffractometer equipped with a Sapphire3 CCD area detector. Graphite monochromatized Mo Kα radiation was employed. Data reduction was performed with the CrysAlisPro software (Rigaku Oxford Diffraction, 2015). The crystal structure was solved by SHELXT (Sheldrick, 2015b), and refined with the SHELXL-2018 (Sheldrick, 2015a). The ShelXle (Hübschle et al., 2011) was used as graphical user interface for crystal structure refinement procedures. Additional calculations were performed the SHAPE (Llunell et al., 2013) and CrystalExplorer (Turner et al., 2017).
CCDC 2059113 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/.

Synthesis and physicochemical characterization of the complex
The reaction of warm methanolic solution of CuBr 2 and the ligand in molar ratio 1:2 yielded in formation of prismatic single crystals of the complex [CuL 2 ]Br 2 .
The obtained complex is stable in air and high temperatures. It is well soluble in DMF, and partially soluble in alcohols and H 2 O. Molar conductivity of its DMF solution has a value characteristic for 2:1 electrolyte type, thus is in concordance with the coordination formula (Geary, 1971).
In the IR spectrum of the complex the band found at 1520 cm -1 could be ascribed to ν(C=N) vibrations of the imine group. In the spectrum of the ligand, this band is found at higher wavenumbers (1563 cm -1 ), and due to coordination has suffered a negative shift (Curry et al., 1967;Kazak et al., 2009). The band at 3157 cm -1 corresponds to ν(NH) vibrations, while the other bands in the range above 2900 cm -1 originate from valent ν(CH) vibrations of CH 3 -groups and aromatic rings (pyridine and benzene). Bands at 1599 and 1494 cm -1 are the result of ν(C ••• C), ν(C ••• N) vibrations of the mentioned aromatic rings.

Crystal structure of the complex
The complex [CuL 2 ]Br 2 is isostructural with [CoL 2 ]I 2 (Belošević et al., 2018). Both complexes crystallize in C2/c space group, with very similar unit cell parameters. Complex cation [CuL 2 ] 2+ is situated on a two-fold rotation axis, so that the asymmetric unit of the unit cell is comprised of a ligand molecule, half of a copper atom, and a bromide ion (Figure 1). To facilitate structural comparisons of [CuL 2 ]Br 2 and [CoL 2 ]I 2 , pertinent structural parameters of both complexes are listed in Table 2. Coordination environment around Cu(II) is heavily distorted, and calculation of continuous shape measure (CShM) assigned the coordination polyhedron as heavily distorted octahedron, with CShM(OC-6)=5.911 (Alvarez et al., 2005). The coordination environment is formed by two meridionally arranged tridentate N 3 chelate ligands. Dihedral angle enclosed by planes through N1, N2A and N2B donor atoms of the symmetry-related ligands amounts 74.2°. Distortion of the octahedron may be seen through valance angles centered on Cu(II) atom, all of which deviate from ideal values. The largest deviations are observed by The transvalence angle N2A-Cu1-N2A and cis-valence angles N1-Cu1-N2A, N1-Cu1-N2B with values of 150.69(7)°, 74.14(7)°, and 76.56(8)°, respectively.
The bromide ion is involved in two hydrogen bonds, with very similar geometry (Table 3).

CONCLUSION
In this paper sythesis, structure, and some physicochemical properties of the novel complex with 2,6diacetylpyridine bis(phenylhydrazone), of the formula [CuL 2 ]Br 2 , are described. This is only the second structurally characterized metal complex with this ligand, and since this class of compounds is promising from the many point of views it is of a high importance to make some further research in this field.