)-2-AMINO-N ’-[ 1-( PYRIDIN-2-YL ) ETHYLIDENE ] BENZOHYDRAZIDE

The coordination chemistry towards the M(II) metal centre (M = Mn, Ni, Cu or Zn) of the hydrazone ligand (E)-2-amino-N’-[1-(pyridin-2-yl)ethylidene]benzohydrazide (H3L) has been explored and complexes having formulae [Mn(H2L)2] (1), {[Ni(H2L)2].DMF.0.4H2O} (2), {[Cu2(H2L)2(μ-Cl)2].DMF} (3) and [Zn(H2L)2] (4) have been isolated and characterized by IR, UV-Visible spectroscopy, elemental analysis and Xray crystal diffraction. Structural studies reveal that the mononuclear complexes (1), (2) and (4) adopt highly distorted octahedral geometries while the dinuclear complex (3) adopts a square pyramidal geometry around each copper(II) ion.


INTRODUCTION
Versatility coordination to metal [1,2], flexibility in assuming different conformations [3,4], possibility of tautomerism forms [5,6] and possibility of reacting in anionic or neutral forms [7,8], make the study of hydrazones chemistry extremely interesting.The chelating properties of this kind of ligands are diversified by their possible presence in neutral, protonated or deprotonated forms leading of various geometries upon coordination [9][10][11].
On continuing our work in the field of the synthesis of hydrazide ligand and the studies of their reactivities with transition metal, we describe a simple condensation of 2aminobenzoylhydrazide and 2-acetylpyridine to yield the title compound (E)-2-amino-N'-[1-(pyridin-2-yl)ethylidene]benzohydrazide (H 3 L).The chelating behaviors of this ligand have been investigated towards several transition metal ions.The synthesis and characterization of three mononuclear (Mn(II), Ni(II) and Zn(II)) and one dinuclear (Cu(II)) metal complexes with the potentially pentadentate Schiff base ligand H 3 L, having amine, hydrazone and pyridine moieties with potential atoms donor (Scheme 1).The X-ray crystal structure of Mn(II), Ni(II), Cu(II) and Zn(II) complexes are reported in this present work.

Materials and procedures
Methyl anthranilate, hydrazine hydrate, 2-acetylpyridine, MnCl 2 .4H 2 O, NiCl 2 .6H 2 O, CuCl 2 .2H 2 O as well as ZnCl 2 were commercial products (from Alfa and Aldrich) and were used without further purification.Solvents were of reagent grade and were purified by the usual methods.Elemental analyses were performed in a Carlo-Erba EA microanalyser.Infra-red spectra were recorded as KBr discs on a Bruker IFS-66V spectrophotometer.For the ligand, LSI-MS were recorded using a Micromass Autospec spectrometer with 3-nitrobenzyl alcohol as the matrix.The 1 H and 13 C NMR spectra of the Schiff base were recorded in CDCl 3 on a Bruker 500 MHz spectrometer at room temperature using TMS as an internal reference.The UV-Vis spectra were run on a Jenway 6505 UV-Spectrophotometer (1100-200 nm) on 10 -3 M DMF solutions of the complexes.The molar conductance of 10 -3 M solutions of the metal complexes in DMF was measured at 25 o C using a WTW LF-330 conductivity meter with a WTW conductivity cell.Room temperature magnetic susceptibilities of the powdered samples {calibrant Hg[Co(SCN) 4 ]} were measured using a Johnson Mattey scientific magnetic susceptibility balance.Melting points were recorded on a Büchi apparatus and are uncorrected.

X-ray data collection, structure determination, and refinement
The details of the X-ray crystal data, and the structure solution and refinement are given in Table 1.Diffraction data were collected using an ENRAF NONIUS Kappa CCD diffractometer with graphite monochromatized Mo K radiation ( = 0.71073 Å).All data were corrected for Lorentz and polarization effects and for absorption by semi empirical methods based on symmetry-equivalent and repeated reflections.Complex scattering factors were taken from the program package SHELXTL [12].The structures were solved by direct methods which revealed the position of all non-hydrogen atoms.All the structures were refined on F 2 by a full-matrix least-squares procedure using anisotropic displacement parameters for all non hydrogen atoms [13].The hydrogen atoms were located in their calculated positions and refined using a riding model.Molecular graphics were generated using ORTEP-3 [14].

Synthesis of the (E)-2-amino-N'-[1-(pyridine-2-yl)ethylidene]benzohydrazide ligand
Methylanthranilate (3 g, 20 mmol) and excess of hydrazine hydrate (8 mL, 160 mmol) were refluxed during 24 hours and stored overnight in a refrigerator.A white solid was separated, filtered off and recrystallized from ethanol to yield 2-aminobenzohydrazide (2.37 g, 15.69 mmol).The whole solid was dissolved in 40 mL of ethanol and 2-acetylpyridine (1.90 g, 15.69 mmol) was added.The resulting mixture was refluxed during 3 hours to yield a yellow solution.The solution was evaporated to dryness to give yellow solid which was washed with ether and dried in the air.Yield: 53.5% (2.13 g).Anal.calcd (found) for C

General characterization
One-pot synthesis of 1:1 molar ratio of the Schiff base (H 3 L) and M(II) metal salts in refluxing ethanol (M = Mn, Ni, Cu or Zn) afforded mononuclear or dinuclear complexes.The complexes gave analytically pure products and were characterized by elemental analysis, IR, UV-Vis spectroscopy and constant temperature magnetic measurements.Colors, magnetic moment, molar conductivity and spectral data of all the prepared Schiff base complexes are given in the experimental section.All complexes are paramagnetic in nature except the Zn(II) complex which is diamagnetic.The observed value of effective magnetic moment of the Mn(II) complex is 5.88 µ B .This value is close to the spin-only value (5.92 at room temperature and typical for high-spin Mn(II) complex with d 5 configuration (S = 5/2) and shows an octahedral environment in the vicinity of the manganese atom [15,16].For the {[Ni(H 2 L) 2 ] .DMF .0.4H 2 O} complex, the value of the magnetic moment of 2.81 µ B is above the expected value of 2.83 µ B for nickel(II) mononuclear complex in octahedral environment [17].{[CuCl(H 2 L)] 2 .(DMF)} complex shows µ eff value of 1.74 µ B . which is closer to the spin only value of 1.73 µ B .expected for copper complexes [18] with single unpaired electron located in an d x 2 -y 2 orbital.In fact no coupling phenomen exists in this complex at room temperature.
The IR spectrum of the H 3 L ligand display two sharp bands around 3450-3310 cm -1 assignable to  sym and  asym vibrations of the NH 2 group and a band around 3285 cm -1 due to (N-H) of the amide moiety.Strong bands at 1651 and 1615 cm -1 assignable to (C=O) and (C=N) respectively, and bands of medium intensity at 1583, 1567, 1512 and 1461 cm -1 attributable to (C=C) are also pointed.On complexation the values of (C=O) and (C=N) stretching vibrations are found to be lower (1626-1620 cm -1 and 1569-1467 cm -1 , respectively) than the values observed in spectrum of the ligand.This lower values of (C=O) and (C=N) stretching may be explained by the involvement of the carbonyl oxygen atom and the azomethine nitrogen atom in the coordination to the metal ions [19,20].The bands due to (NH 2 ) and (N-H) remain in the spectra of the complexes 1-4.All the complexes are soluble in DMF.The measurement of molar conductance gives  M values of 32, 30 and 36 S cm 2 mol -1 for complex 1, 3 and 4 respectively and 27 for complex 2 confirming that the three mononuclear (1, 2 and 4) complexes and the dinuclear complex 3 are neutral and non-electrolyte [21].The electronic spectra in DMF are recorded in the range 200-1000 nm.The intense absorption bands at short wavelengths, between 290 and 405 nm, may be assigned to ligand-metal ion charge transfer bands (LMCT).The octahedral geometry of the manganese(II) complex cannot be supported by the electronic spectrum of the complex.As expected, spin-forbidden d-d bands were not observed in the spectra of the high-spin d 5 Mn(II) complex studied in this work.The absorptions in the range 550-1000 nm may be assigned to d-d transitions of the nickel(II) complex.The octahedral geometry can be supported by the d-d transition bands revealed in electronic spectrum of the nickel(II) complex at 935, 729 and 401 nm.These three bands are assigned to  1 3 T 2g (F)← 3 A 2g (F),  2 3 T 1g (F)← 3 A 2g (F) and  3 3 T 1g (P)← 3 A 2g (F), in octahedral geometry of nickel(II) [22] complex.Electronic spectrum of Cu(II) complexes gave transitions, A 1g ←B 1g , B 2g ←B 1g and E g ←B 1g [22][23][24][25], but it is very difficult to resolve them into separate bands due to the very low energy difference between these bands.The observed band at 615 nm can be assigned to d-d transition [26].

Crystal structures of complexes
Crystals of 1-4 suitable for X-ray diffraction were obtained by slow recrystallization of the compound from dimethylformamide.The crystals structures of 1-4 are respectively shown in Figures 1-4 together with the atomic numbering scheme adopted and selected bond lengths and angles.Crystal data and structure refinement are given in Table 1.Compound 1, [Mn(H 2 L) 2 ] crystallizes in the centrosymmetric space group C2/c.The geometry around the manganese ion can be described as slightly distorted octahedral, with a N 4 O 2 core comprised by six donor atoms of two ligand molecules.Each ligand acts with two nitrogen and one oxygen atoms.The equatorial plane of the octahedron is formed by one hydrazone and two pyridine N atoms and one O atom from the carbonyl moiety, (rms mean deviation 0.043) with the Mn center 0.079 Å out of this plane.The axial positions are occupied by one oxygen atom and one hydrazino N atom, 141.24 (6) [O1-Mn-N3].The Mn-O1, 2.135(2) Å and Mn-N3, 2.332(2) Å distances correspond to the shortest and the longest bond distance to the manganese center respectively.The hydrazone N distances, Mn1-N2, 2.205(2) Å is smaller than those found in a similar complex [27].The coordination of each hydrazone to Mn results in the formation of two five membered chelating rings (MnNNCO and MnNCCN).The Mn-O is, thus, stronger than the Mn-N bond and correlates with the slightly less strained five-member ring defined by the MnN2N1C7O1 atoms, which has the N2-Mn-O1 angle of 71.48(6) o .The ring formed by MnN3C9C8N2 is slightly more rigid and has a N2-Mn-N3 angle of 69.83(7) o .
The complex 2, {[Ni(H 2 L) 2 ] .DMF .0.4H 2 O} crystallizes in the triclinic space group Pī.The structure is of a mononuclear nickel complex with the metal ion bound to two nitrogen and one oxygen atoms from each of the two molecules ligand in an octahedral environment.The arrangement around the nickel ion can be described as distorted octahedral, being the main distortion due to the tense five members ring [O1-Ni1-N2 76.44 (7) o ; N1-Ni1-N2 77.94 (7) o ; O2-Ni1-N6 76.25 (7) o ; N6-Ni1-N5 78.20 (7) o ].The axial positions of the distorted octahedral environment can be considered that occupied by one hydrazone nitrogen atom and one oxygen of the carbonyl, [O1-Ni-N1 154.32( 6) o ] and the equatorial plane is thus formed by one oxygen atom, two pyridyl nitrogen atoms and one nitrogen atom of the hydrazonic moiety.Interestingly, the distorted octahedral polyhedron can be viewed in two ways: firstly, the plane with axial position O1-Ni-N1 154.42(6) o and equatorial plane N2N5N6O2 (rms mean deviation of 0.034 and Ni1 0.025 away from that plane), secondly, the plane with axial position O2-Ni-N5 154.42(6) o and equatorial plane N2N1N6O1 (rms mean deviation of 0.023 and Ni1 0.002 away from that plane).These equatorial planes are roughly perpendicular (89.67(6) o ).The asymmetric unit of 2 contains one molecule of the complex with two lattice disordered solvent molecules: one dimethylformamide molecule disordered over two positions and two water molecules, which occupies each partially it site (16 and 26% respectively).Thus, the title complex is 0.4 water solvated.Intramolecular hydrogen bond involving the free amino group, and the uncoordinated nitrogen atom of the hydrazone moieties lead to the formation of six membered rings.No intermolecular hydrogen bonds were observed.The structure of {[CuCl(H 2 L)] 2 .DMF} 3 is centrosymmetric with space group C2/c and is consistent with dinuclear neutral units in which the two metal centers are linked by two ligand molecules and two chloride.The two copper atoms are pentacoordinated with a {N 2 OCl 2 } core formed by one monoanionic tridentated ligand molecule coordinated through the pyridine and the hydrazino nitrogen atoms and the oxygen of the carbonyl group.Two chloride ions shared by the two copper atoms complete the coordination sphere around each metal ion.The two chloride anions acts as bidentate bridging mode.The coordination of the hydrazone to Cu results in the formation of two five membered chelating rings (CuOCNN and two CuNCCN).The distortion of the coordination polyhedron from the square pyramid to the trigonal bipyramid is calculated by the Addison parameter  where  and  are the two largest donormetal-donor angles [28].A value of = 0 indicates an ideal square pyramid and when  = 1, the geometry around the metal center is a perfect trigonal bipyramid.The geometry around each copper(II) ion can be best described as a square pyramidal arrangement as indicated by the Addison's parameter: τ = 0.135 [28] 4.499 Å) [30].The DMF solvent molecule was found to be disordered around the twofold axis, in two positions with equal site occupancy (Figure 3b).
Intramolecular hydrogen bond involving the free amino group, and the uncoordinated nitrogen atom of the hydrazone moieties lead to the formation of six membered ring.In the crystal structure, a weak intermolecular H bond (N4 ….O2) is also observed, involving the free amino group, and the O atom of the DMF solvent molecule.
Compound 4, [Zn(H 2 L) 2 ] crystallizes in the centrosymmetric space group P2 1 /c.The geometry around the zinc ion can be described as slightly distorted octahedral, with a N 4 O 2 core comprised by six donor atoms of two ligand molecules.Each ligand acts with two nitrogen and one oxygen atom.The axial positions of the distorted octahedral environment can be considered that formed by one hydrazone nitrogen atom and one oxygen atom and the equatorial plane formed by one oxygen atom, one nitrogen atom from the hydrazone moiety and two pyridyl nitrogen atoms.Interestingly, the distorted octahedral polyhedron can be viewed in two ways: firstly, the plane with axial position O1-Ni-N7 148.84( 16) o and equatorial plane N3N6N8O2 (rms mean deviation of 0.054 and Zn -0.047 away from that plane), secondly, the plane with axial position O2-Ni-N6 147.93 (15) o and equatorial plane N8N7N3O1 (rms mean deviation of 0.035 and Zn -0.052 away from that plane).Two intramolecular hydrogen bond involving the free amino group and the uncoordinated nitrogen atom of the hydrazone moieties and one weak intermolecular H bond (N4 ….O2) are observed.