Synthesis and X-Ray Crystal Structures of Trinuclear Nickel(II) Complexes Derived from Schiff Bases and Acetate Ligands with Biological Activity

The reactions of Ni(OAc)2 2H2O with Schiff base ligands 5-bromo-2-((cyclopentylimino)methyl)phenol (HL1) and 5-bromo-2-(((2-(isopropylamino)ethyl)imino)methyl)phenol (HL2) in methanol afforded two discrete trinuclear complexes [Ni3(L)2(μ2-η:η-OAc)2(DMF)2(BrSal)2] (1) and [Ni3(L)2(μ2-η:η-OAc)2(μ2-η:η-OAc)2] (2), where BrSal is the monoanionic form of 4-bromosalicylaldehyde. The complexes were characterized by elemental analysis, IR and UVVis spectroscopy. The crystal structures of the complexes have been determined by X-ray crystallography. In both complexes, the nickel atoms are in octahedral coordination geometries. The L1 ligand coordinates to the nickel atoms through the phenolate O and imino N atoms, and the L2 ligand coordinates to the nickel atoms through the phenolate O, imino N and amino N atoms. The antimicrobial activities of the complexes were assayed.


4. X-ray Crystallography
X-ray data for the complexes were collected on a Bruker SMART 1000 CCD single crystal diffractometer at 298(2) K, equipped with a graphite monochromator and a Mo Kα fine-focus sealed tube (λ = 0.71073 Å) by the ω scan method. Reflections were corrected for Lorentz and polarization effects, and for absorption by semi-empirical methods based on symmetry-equivalent and repeated reflections. The SMART software was used for data acquisition and the SAINT-PLUS software was used for data extraction. 6 The absorption corrections were performed with the help of SADABS program. 7 The structures were solved by direct methods and refined on F 2 by full-matrix leastsquares procedures. All non-hydrogen atoms were refined using anisotropic thermal parameters. Hydrogen atoms were included at idealized positions by using a riding model. The SHELX-97 programs were used for structure solution and refinement. 8

1. Materials and Physical Measurements
4-Bromosalicylaldehyde, cyclopentylamine, N-isopropylethane-1,2-diamine and nickel acetate were purchased from Sigma-Aldrich. All other chemicals were commercially available and used as received. Elemental analysis was carried out using model 2400 Perkin-Elmer CHN analyzer. Infrared spectra were collected by using KBr pellets on a Jasco-5300 FT-IR spectrophotometer. Electronic spectra were carried out with a Lambda 35 spectrometer.

3. Preparation of Complex 2
4-Bromosalicylaldehyde (0.20 g, 10 mmol) and N-isopropylethane-1,2-diamine (0.102 g, 10 mmol) were dissolved and stirred at ambient temperature in methanol (30 mL). Thirty minutes later, nickel acetate tetrahydrate (0.50 g, 20 mmol) was added to the solution. The color turned from yellow to deep green. The reaction mixture was stirred for about 1 h and filtered. The filtrate was al-

1. Chemistry
Both complexes were readily prepared by reacting nickel acetate tetrahydrate with Schiff base ligands. The elemental analysis data are consistent with the general molecular formulae of the complexes. In complex 1, the acetate ligands act as bidentate bridging groups. In complex 2, two acetate ligands act as bidentate bridging groups, and the other two act as chelating bridging groups. The complexes are soluble in common organic solvents like dimethyl sulphoxide, dimethylformamide, methanol, and ethanol.

2. Infrared and Electronic Spectra of the Complexes
In the spectra of the free Schiff bases HL 1 and HL 2 , the bands at 1650 and 1645 cm -1 , respectively, are assigned to the azomethine groups, ν C=N , 9 and the broad and weak absorptions at about 3430 cm -1 are assigned to the hydroxyl groups, ν O-H . In the complexes, the absence of the O-H stretching and bonding vibrations indicates coordination through the phenolate groups. The C=N stretches of the complexes are observed at lower frequencies (1643 and 1627 cm -1 ) when compared to the free Schiff bases. 10 The phenolic ν(Ar-O) in the free ligands exhibit bands at about 1270 cm -1 . However, in the complexes it appears at about 1283 cm -1 . The shift to higher wave number is an indication of C-O-M bond formation. The bands at about 1582 and 1408 cm -1 are assigned to the acetate ligands. 4a,5b The occurrence of new bands in the 450-670 cm -1 region confirms the presence of metal-nitrogen and metal-oxygen bonds, respectively. 11 In the electronic spectra of the free Schiff bases and the complexes, the absorption frequencies ascribed to the aromatic π-π* and n-π* transitions are located in the region 240-290 nm. 12 In the electronic spectra of the complexes, the absorption centered at 380 nm for complex 1 and 335 nm for complex 2 are assigned to ligand-to-metal charge transfer. 13

Structure Description of Complex 1
The molecular structure of complex 1 is depicted in Fig. 1. Bond parameters associated with the metal atom are listed in Table 2. The molecular structure is centrosymmetric with Ni2 atom on an inversion center. The structure shows a trinuclear complex consisting of three Ni atoms in a linear array, which is bridged by two acetate ligands and four phenolate groups. The Ni···Ni distance is 3.053(2) Å. The Schiff base ligand coordinates to the Ni atoms through phenolate O and imino N atoms. The ligand 4-bromosalicylaldehyde coordinates to the Ni atoms through carbonyl O and phenolate O atoms. The acetate ligand acts as a bidentate bridging group. The Ni2 atom is located at the inversion centre of the complex in an octahedral geometry. This atom is coordinated by four phenolate O atoms from the Schiff base and 4-bromosalicylaldehyde ligands that form a plane and, near-perpendicularly to it, by two O atoms from two acetate bridging ligands that connect the central Ni atom to the outer Ni atoms. The greatest deviation of the bond angles from those expected for an ideal octahedral geometry is found for O1-Ni2-O3 with 79.  Table 2. Unlabeled atoms are related to the symmetry operation 1x, 1 -y, 1 -z.

4. Structure Description of Complex 2
The molecular structure of complex 2 is depicted in Fig. 2. Bond parameters associated with the metal atom are listed in Table 2. The molecular structure is centrosymmetric with Ni2 atom on an inversion center. The structure shows a trinuclear complex consisting of three Ni atoms in a linear array, which are bridged by four acetate ligands and two phenolate groups. The Ni···Ni distance is 3.043(2) Å. The Schiff base ligand coordinates to the Ni atoms through phenolate O, imino N and amino N atoms. Two of the acetate ligands act as bidentate bridging groups, and the other two act as chelating bridging groups. The Ni2 atom is located at the inversion centre of the complex in an octahedral geometry. This atom is coordinated by two phenolate O atoms from the Schiff base ligands and two O atoms from two bidentate bridging acetate groups that form a plane and, near-perpendicularly to it, by two O atoms from two chelating bridging acetate groups that connect the central Ni atom to the outer Ni atoms. The greatest deviation of the bond angles from those expected for an ideal octahedral geometry is found for O1-Ni2-O2 with 79.97(16)° and O1-Ni2-O2A with 100.03 (16) hibitory Concentration) method with three different concentrations of 100, 50 and 25 μg. 15 The activity was also assayed for the free Schiff bases, the pure solvent DMF and the standard gentamycine for each of antibacterial and flucanazole for antifungal cultures. Final adjustments were made using optical density measurement for bacteria (absorbance 0.05 at 580 nm). The zones of inhibition in millimeter for the compounds are presented in Table 3. The two complexes have similar activities against the bacteria and fungi, and they are more susceptible toward bacterial cells than fungicidal cells. In general, both complexes have better activities than the free Schiff bases. The trends are in   Table 2. Unlabeled atoms are related to the symmetry operation 1 -x, 1 -y, 1 -z.

5. Antimicrobial Activity
The complexes were assayed against the bacteria accordance with the literature that metal complexes are more active in the biological potential than the ligands used in the synthesis. 16 When comparing the antimicrobial activity of the studied complexes to those of reference antibiotics, the inhibitory ability is found to be good. For example, the two complexes have similar or even higher activity on the bacteria Escherichia coli and Salmonella typhi than gentamycine, and have effective activity on Aspergillus niger and Candida albicans, which is seldom seen in the literature. The results indicate that the two complexes can possibly be used in the treatment of diseases caused by the bacteria that were tested.

Conclusion
A pair of novel acetato-bridged nickel(II) complexes derived from the Schiff base ligands 5-bromo-2-((cyclopentylimino)methyl)phenol and 5-bromo-2-(((2-(isopropylamino)ethyl)imino)methyl)phenol were prepared. Single crystal structures of the complexes indicates that both complexes are trinuclear nickel(II) compounds. All of the nickel atoms in the complexes are in octahedral coordination geometries. The complexes have good antimicrobial activities against Escherichia coli, Salmonella typhi, Aspergillus niger and Candida albicans. CCDC Nos. 1974759 (1) and 1974760 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data request/cif. Crystal data and details of the data collection and refinement for the complexes are collected in Table 1. The concentration is 100 μg L -1 .