Synthesis and Structural Studies of Co(II), Ni(II), Cu(II), Zn(II), Th(IV) and UO2(II) Complexes with 2-Acetylpyridine-4-methylbenzoyl hydrazone

A number of Co(II), Ni(II), Cu(II), Zn(II), Th(IV) and UO2(II) complexes with 2acetylpyridine-4-methylbenzoyl hydrazone (APMBH), obtained by the condensation of 2acetylpyridine with 4-methylbenzoyl hydrazine are reported. The reaction of metal chlorides with APMBH lead to isolation of complexes having the general formulas [M(APMBH)(H2O)nCl2] (n=1, M=Co(II), Ni(II); n=0 , M=Zn(II), UO2(II)), [Ni(APMBH-H)Cl(DMSO)]2, [Cu(APMBH-H)Cl]2 and [Th(APMBH)2Cl2]Cl2. The reaction of metal acetate with APMBH in 1:2 molar ratio yields the deprotonated bis-complexes [M(APMBH-H)2] ( M= Co(II), Ni(II), Cu(II), Zn(II) and UO2(II)). The complexes have been characterized by elemental analysis, spectral (IR, UV-Vis) magnetic and molar conductance measurements, which revealed that APMBH act as neutral tridentate NNOdonor ligand in the metal chloride complexes (1, 2, 5-7) and as a monobasic tridentate ligand in the dimeric complexes (3, 4). The ligand also acts as a monobasic NNO-tridentate in the deprotonated bis-complexes (8-12). Octahedral structure is suggested for Co(II), Ni(II), Cu(II) and Zn(II) complexes (1-3, 8-11), dimeric square pyramidal structure for Cu(II) complex (4) and trigonal bipyramidal structure for Zn(II) complex (5). Uranyl complexes 7 and 12 are expected to have pentagonal bipyramidal and hexagonal bipyramidal structures respectively. Eight coordinate structure is proposed for Th(IV) complexe (6). The antibacterial activity of Cu(II) and Zn(II) complexes (5 and 10) against Gram-positive and Gram-negative bacteria has been tested and show moderate and high activity respectively.

Hydrazones are considered an important class of ligands also because the processes pharmacological applications as antimicrobal, anticonvulsant, analgesic, anti-inflammatory, antiplatelet, antitubercular, and antitumoral agenst (Rollas and Kücükgüzel, 2007). Biological activities of hydrazone metal complexes are also reported for several transitional metals (Alageson et al., 2013 ;Sathyadevi et al., 2012). Moreover, it has recently been shown that hydrazones such as pyradoxal isonicotinoyl hydrazone analogs are effective iron chelators in vivo and in vitro and may be of value for the treatment of iron overload (Bernharat et al., 2008).
The coordination behavior of aroyl hydrazones is known to depend on the pH of the medium, the nature of the substituents and also on the position of hydrazino group relative to other moieties. Moreover, the deprotonation of the NH group which is readily achieved in complexation, results in the formation of tautomeric anionic species having new coordination properties (Al-Daher and Mustafa, 2013;Galic et al., 2012).
Hydrazones have also been proven to show potential applications as catalysts (Hosseini-Monfared et al., 2013), luminescent probes and molecular sensors (Guo et al., 2011;Sun et al., 2009). Many hydrazone complexes show an excellent catalytic activity in various reactions (Sadhukhan et al., 2011). They are also used as analytical reagents for spectrophotometric determination of several metal ions (Bale and Sawant, 2001).
In view of the above mentioned significant importance of hydrazone metal complexes in various fields and on continuation of our studies of hydrazone metal complexes (Al-Daher and Mustafa, 2013;Al-Daher and Hadi, 2013;Al-Daher and Al-Qassar, 2011), we were motivated to synthesis Co(II), Ni(II), Cu(II), Zn(II), Th(IV) and UO 2 (II) Complexes with 2-Acetylpyridine-4 'methylbenzoyl hydrazone (APMBH) (Scheme 1) and to describe their structural and antibacterial properties.

Materials and Measurements
All chemicals used were reagent grade from B.D.H or Fluka companies, used as supplied. Infrared spectra were recorded as KBr discs using Tensor 27 Brucker FT-IR spectrophotometer (4000-400 cm -1 ). Electronic absorption spectra were recorded in DMF 10 -2 solutions on Shimadzu U V-1650 PC spectrophotometer at room temperature using 1cm cell. Molar conductance was measured at ambient temperature using PMC3 Jeneway conductivity meter. Magnetic susceptibility measurements of the complexes were carried out using Bruker BM6 instrument at room temperature. 1 H-NMR spectra were recorded in DMF-d 7 solution using TMS as an internal standard on BRUKER : AVANCE (III) 400 MHz spectrophotometer. Melting points were obtained on Electro thermal 9300 melting point apparatus in open capillaries.
The metal content of the complexes was determined spectrophotometrically using Shimadzu (AA670) atomic absorption spectrophotometer, Zinc and Nickel were also determined by complexometric titration against standard EDTA solution (Vogel, 1989).Thorium and uranium were determined colourimetrically using thoron and arsinazo III as reagents respectively (Marczenko and Balcezak, 2000). Chloride content was estimated volumetrically using standard AgNO 3 solution (Vogel, 1989).

Preparation of ligand a. Preparation of the 4-methylbenzoyl hydrazine :
This was prepared according to literature (Furniss et al., 1989) by the reaction of excess hydrazine hydrate and ethyl toluate in ethanol yield 70%, mp. 116-117 o C (lit. mp.117 o C).

Antibacterial activity:
This was evaluated using agar diffusion method (Kethcum,1988). Gram-positive bacteria Staphylococcus aureus and Gram-negative bacteria Klebsiella pneumoniae were cultivated in nutrient agar on petri dishes. The test solution was prepared by dissolving (10 mg) of the tested substances in DMSO (1 mL). A 6 mm diameter filter paper discs were socked in the tested solutions. After 24h cultivation at 37 o C, diameters of zones of inhibition were determined. DMSO was inactive under applied conditions.

RESULTS AND DISCUSSION
The reaction of equimolar amounts of 4-methylbenzoyl hydrazine and 2-acetyl pyridine in ethanol gave the desired hydrazone ligand APMBH (Scheme 1) in excellent yield and purity. The analytical data and some of the physical properties of the prepared ligand and its complexes are shown in (Table 1)  The metal complexes are stable non-hygroscopic insoluble in most common organic solvents like chloroform, benzene, acetone, diethyl ether and slightly soluble in methanol or ethanol but more soluble in DMSO and DMF. Most of the complexes melt or decompose in the temperature range 237-295 o C and few melted above 300 o C Molar conductance data of 10 -3 M solutions of the complexes (Table 1) at room temperature show low values (1.66-14.92) ohm -1 mol -1 cm 2 indicating that they are non-electrolytes, except for Th(IV) complex (6) which behaves as 1:2 electrolyte (Geary, 1971).

Characterization of organic ligand
The main IR bands of the ligand with their tentative assignments in (Table 2) display a medium intensity band at 3338 cm -1 due to amide NH stretching vibration ν (N-H) of the hydrazone which may be involved in NH .… O hydrogen bonding. The strong bands at 1672 and 1610 cm -1 are assigned to the carbonyl group stretching vibration ν (C=O) and azomethine stretching vibration ν (C=N) respectively. The low energy of ν (C=N) could be attributed to conjugation with neighboring pyridine (Karbouj et al., 2010). The medium band at 623 cm -1 is assigned to the pyridine ring in plane deformation mode. The spectrum of the ligand displays also a weak band at 993 cm -1 due to N-N stretching vibration ν (N-N) (Despaigne et al., 2009 ;Singh, 2008). IR data suggest keto-form of the ligand in the solid state. 1 H-NMR spectrum of the free ligand in DMF-d 7 solution shows a weak signal at δ 10.79 ppm (s , 1H) assigned to NH proton. The aroyl protons appeared in their usual position at δ 7.32-7.86 ppm (m , 4H) and pyridine protons at δ 7.42-8.62 ppm (m , 4H). The spectrum exhibits additional signals at δ 2.46 ppm (s , 3H) and δ 2.39 ppm (s , 3H) characteristic of methyl groups (CH 3 -C=N) and (CH 3 -Ar) respectively (Karbouj et al., 2010;Jang et al., 2005). The number of protons assigned from 1 H-NMR is consistent with the formula.

Characterization of the complexes Infrared spectra:
The FT-IR spectra of the complexes are recorded as KBr discs and the main IR bands with their tentative assignments are given in (Table 2), which shows that the band of ν (N-H) in metal chloride complexes (1, 2, 5-7) is either unchanged or shifted to higher frequencies indicating noninvolvement in coordination. The band due to carbonyl stretching vibration ν (C=O) is shifted to lower frequencies in these complexes by 31-71cm -1 indicating coordination through the oxygen atom of the carbonyl group (i.e. the hydrazone is coordinated in the keto-form) (El-Tabl et al., 2012).
The bands of ν (N-H) and ν (C=O) were absent in the spectra of the metal chloride 3 and 4) suggesting coordination of the enol-form of the ligand by deprotonation of the N-H proton, during the complexation. This behavior was also observed in the IR spectra of the deprotonated biscomplexes (8-12) in which the ν (N-H) and ν (C=O) disappeared also, due to the enolization of the carbonyl group on complex formation . The appearance of a new ν (C-O) band in these complexes in the range (1286-1301)cm -1 confirms the coordination of the hydrazone in the enolate form . This mode of bonding is supported by the appearance of another new band in the range (1579-1599)cm -1 corresponding to ν (N=C-O) . (El-Tabl et al., 2012;Hosseini-Mofared et al., 2013).
The azomethine stretching frequency ν (C=N) is shifted to lower frequencies by (15-52)cm -1 in the spectra of the complexes indicating coordination through azomethine group nitrogen atom (Ibrahim et al., 2009). At the same time, the band due to ν (N-N) was shifted to a higher frequency. The increase in the frequency of this band is a clear indication of the increase in the double bond character due to the loss of the electron density via electron donation to the metal ions and further confirmation of the coordination of the ligand via the azomethine group. (El-Tabl et al., 2012).
The pyridine in-plane deformation mode δ(py) at 623 cm -1 in the spectrum of the ligand shifts to higher frequencies 631-646 cm -1 in spectra of the complexes suggesting coordination of the pyridine nitrogen atom (Despaigen et al., 2009). The appearance of non-ligand bands in the spectra of the complexes at the ranges (519-580) cm -1 and (415-450) cm -1 which may be tentatively assigned to ν M-O and ν M-N respectively is another evidence for the bonding of the ligand with the metal ions (El-Tabl et al., 2012).
The spectra of the complexes (1 and 2) exhibit a broad band centered around 3400 cm -1 due to the symmetric and asymmetric stretching modes of coordinated water molecule. Furthermore, weak bands in the 887-899, 669-679 cm -1 ranges may represent the wagging and rocking modes of coordinated water (Singh, 2008).
Cobalt(II) complexes (1) and (8) show values in the 4.97-4.99 BM range indicating that they have three unpaired electrons. This reveals a high-spin octahedral geometry around Co(II) metal center . Higher magnetic moment value was observed for octahedral Co(II) complexes than the spin-only (3.87 BM) value may be due to a large orbital contribution. Nickel(II) complexes (2) and (9) show values in the range 2.92-3.13 BM which are consistent with two unpaired electron system of octahedral Nickel(II) complexes (El-Tabl et al., 2012). The slightly lower value of magnetic moment (2.75 BM) than the spin only value of nickel(II) complex (3) may be due to the proposed dimeric structure (Sadhukhan et al., 2011) (Fig. 1). Copper(II) complex shows a magnetic value that is equal to 1.84 BM which corresponds to one unpaired electron system in octahedral structure (El-Tabl et al., 2012). The magnetic moment of Cu(II) complex (4) was measured to be 1.49 BM which is below the theoretical value expected for Cu(II) centre S= 1 / 2 and indicative of net antiferromagnetic couple between the copper(II) ions through bridging units in the poly crystalline state, indicating dimeric structure (patel et al., 2013).

Electronic Spectra
The electronic spectra of the ligand and its metal complexes in the range 200 _ 1100 nm, were carried out in DMF. (Table 3) shows the electronic spectral bands of the current complexes.
The spectrum of the ligand in DMF solution exhibits two absorption bands at 32894 and 30674 cm -1 . The first band may be assigned to the π→π* transition in the benzenoid and pyridine moieties and intra ligand π→π* transition. The second probably due to n→π* of azomethine and carbonyl group (El-Tabl et al., 2012). These bands are shifted to lower wave numbers which may indicate the coordination of the hydrazone to the metal ions. The intense band observed in the spectra of all complexes at the region 28890-25125 cm -1 may be associated with the charge transfer transitions (LMCT).
The spectrum of copper(II) complex (10) showed a broad band centered at 14005 cm -1 . The position and the broadness of this band indicated that, Cu(II) ion distorted octahedral geometry. This broad band may consist of three super imposed transitions 2 B 1g → 2 E g , 2 B 1g → 2 B 2g and 2 B 1g → 2 A 1g This could be due to the Jahn-Teller effect that operates on the d 9 electronic ground state of six coordinate system (El-Tabl et al., 2012). The electronic spectrum of copper(II) complex (4) involves two ligand field bands appearing at 13623 cm -1 and 9823 cm -1 , corresponding to the transitions dz 2 → dx 2 -y 2 and dxz,dyz → dx 2 -y 2 respectively, as expected for distorted square pyramidal geometry (patel et al., 2013).
The diamagnetic uranyl complexes (7) and (12) and Th(IV) complex(6) have f 0 system and zinc(II) complexes (5) and (11) have d 10 system,so they do not show f→f or d→d transitions, respectively. The coordination number of these complexes was determined depending upon IR spectra which confirm the behaviour of hydrazone as a tridentate ligand, and molar conductance data, which reveal coordination number 7 and 8 for uranyl complexes (7) and (12) so pentagonal bipyramidal and hexagonal bipyramidal structures, respectively are suggested for these complexes due to linear O=U=O group, Thorium(IV) complex (6) is found to have coordination number 8, so either dodecahedral or square antiprismatic structure is expected (Cotton, 2006). Zinc(II) complexes (5) coordination number 5 and (11) coordination number 6 are proposed to have the more probable trigonal bipyramidal and octahedral structures respectively (Despaigen et al., 2009;Jang et al., 2005).

Antibacterial activity
The hydrazone, APMBH, ligand and its metal complexes (5) and (10) were evaluated for sensitive organisms (S.aureus and K.pneumoniae as Gram-positive and Gram-negative bacteria).
The hydrazone ligand and the tested complexes show low activity towards K.pneumoniae as Gram-negative bacteria. On the other hand, the ligand exhibits lower activity towards S.aureus as Gram-positive bacteria but Zn(II) complex (5) show high activity and Cu(II) complex (10) exhibit moderate activity against S.aureus as Gram-positive bacteria (Table 4).
Antimicrobal screening of the free ligand and two of its complexes (5) and (10) in vitro towards Gram-positive and Gram-negative bacteria showed moderate activity, in contrary to the lower activity of the free ligand.