Synthesis , Spectral and Thermal Behavior of Some New Four Coordinated Complexes

A new symmetric bidentate Schiff base N, N' bis(4-fluorobenzaldehydene)-1,2-diaminoethane(L) and its complexes with general formula MLX2 (M= Zn(II), Cd(II) and Hg(II) and X=chloride, bromide, iodide, thiocyanate and azide) have been prepared. The ligand and complexes have been established by microanalysis, electronic, FT-IR, H and C NMR spectra, and by molar conductivity measurements. All compounds are non-electrolytes in chloroform or DMSO-d6. The thermal behavior of the complexes shows weight loss by decomposition of the anions and ligand segments in the subsequent steps. Some activation thermodynamic parameters such as E, ΔH, ΔS and ΔG were calculated from thermal analysis.


Introduction
Compounds containing azomethine group (RC=N-) were named Schiff base after Hugo Schiff 1 and are usually formed by the condensation of a primary amine with an active carbonyl compound.Schiff base metal complexes with Schiff base ligands have been prepared and presented in numerous reports ranging from the purely synthetic [2][3][4][5][6][7][8] to modern physicochemically such as catalytic application 9 , optical material 10 , dyes, luminophores 11 and polymer with conductance properties 12 to biochemically relevant studies of these complexes in the points of view such as biologically active substances, antitumor 13 , antivirial 14 and antifungal 15 .Because of these applications, synthesis of new Schiff bases and their metal complexes are still the aim of many recent investigations.
In continuation of our previous reports on Schiff base complexes [16][17][18] , the aim of this work is to prepare and investigate the structure of the chelates of zinc(II), cadmium(II) and mercury(II) with a new bidentate Schiff base ligand as N,N' bis(4-fluorobenzaldehydene)-1,2-diaminoethane(L) with nitrogen atoms as donor sites.The general formula of these complexes are MLX 2 in which L= bidentate Schiff base ligand Furthermore, thermal behavior of them was investigated and some activation thermodynamic parameters such as A, E * , ΔH * , ΔS * and ΔG * were calculated from thermal analysis.

Experimental
All the solvents used in the synthesis and analysis, 4-florobenzaldehyde, ethylenediamine, zinc, cadmium and mercury salts and other chemicals were purchased from Aldrich and Merck, and used without any further purification.The FT-IR spectra were recorded on the JASCO-680 model in the range of 400-4000 cm -1 using a KBr disk.Elemental analysis (C, N and H) was conducted on a CHNS-932(leco) elemental analyzer.UV-Vis spectra in the 200-800 nm range were recorded using a JASCO-V570 spectrometer at chloroform and DMSO. 1 H and 13 C NMR spectra were recorded using a Brucker DPX FT-NMR spectrometer at 500 MHz whit the samples dissolved in DMSO-d 6 using TMS as internal standard.Molar conductance of the Schiff base ligands and their transition metal complexes were determined in chloroform and DMSO (1.0×10 -3 M) at room temperature using Metrohm 712 conductometer.Thermogravimetric analyses have been obtained with PL-1500 TG Instruments.Synthesis of Schiff base ligand (L) Ehthylendiamine (0.060 g, 1 mmol) was added to 4-fluorobenzaldehyde (0.2482 g, 2 mmol) and severally stirred and grinded under solvent free conditions.The white product as Schiff base ligand was formed.Ligand was treated with cold water and was filtered.For more purification, ligand was recrystallized from methanol.

Physical data
The new Schiff base ligand (L) was prepare with the condensation reaction between ethylendiamine and 4-florobenzaldehyde.In continue, the reaction of bidentate Schiff base ligand (L) with MX 2 salts (X= chloride, bromide and iodide thiocyanate and azide, M= zinc and mercury and cadmium) in a equimolar ratio give complexes with general formula MLX 2 .Elemental analyses and other physical properties of the ligands and their complexes are summarized in Table 1.Elemental analysis are confirmed the stoichiometry of complexes.
The molar conductivity of complexes in chloroform or DMSO as solvent (1×10 -3 M) was measured at room temperature.The low molar conductivity for complexes shows them to be non-electrolytes [19][20][21] .

IR spectra
The infrared spectra of ligand and its complexes were recorded in the region 4000-400 cm -1 .The most characteristic absorptions of the Schiff base ligand and its complexes are presented in experimental section.The absence of symmetric and asymmetric vibration related to (-NH 2 ) of amine and carbonyl stretching of aldehyde in the IR spectrum of ligand and appearance a new strong peak at 1651 cm -1 suggested that the amino and aldehyde functional groups have been converted to iminic group(C=N) as a new bound.IR spectra of the complexes showed obviously change in the location of absorbance band especially for ν(C=N) indicated that the Schiff base was coordinated to the metals atom.The band appearing at 1651 cm -1 due to the azomethine group (C=N) [22][23][24] was shifted to lower frequency by 4-14 cm -1 indicating participation of the azomethine nitrogen in the complexation.Schiff base ligand spectrum showed the stretching frequencies at 3035, (2927 and 2907) and 2848 cm -1 assigned to C-H of aromatic, aliphatic and iminic groups respectively.These bonds are shifted to higher frequency after its coordination to metal ions.The absorption band assigned to C=C bounds appeared at 1595 cm -1 that shifted to high frequency by 3-9 cm -1 after complexation.The stretching frequency at 1230 cm -1 can be attributed to the ν(C-F) smoothly affected by coordination of the ligand.The strong out of plane bending vibration of the aromatic C-H is present 25 at 845 cm -1 .This band is shifted to lower frequency by 2-16 cm -1 after binding of ligand to metal.Furthermore conclusive evidence that confirmed coordination of Schiff bases with the metal ions appearance of weak low-frequency new bands at 512-530 cm -1 assigned to the metal-nitrogen ν(M-N) [26][27] .The absorption at 2065 cm -1 in ZnL(N 3 ) 2 and the absorptions at 2064 and 2023 cm -1 in HgL(N 3 ) 2 is assigned to coordinated N 3 -28 and the absorption frequencies at 2068 cm -1 in ZnL(NCS) 2 is assigned to Ncoordinated SCN, the absorption at 2110 cm -1 in HgL(SCN) 2 and at 2122-2096 cm -1 in CdL(SCN) 2 are assigned to S-coordinated coordinated SCN -29-32 .This observation for azide and thiocynate complexes confirms well coordination of these anions (and somewhat halide ions in other complexes) as well as Schiff base ligand to metal centers.Electronic spectra Electronic spectra of the ligand and its complexes were recorded in DMF and or chloroform at room temperature as can see in experimental section.In the spectrum of ligand, an independent absorption band appeared and two shoulder bands appeared at 251, 276 and 287 nm regions that first two cases may be due to ‫*ת-ת‬ transition of aromatic rings and the third may be related to azomethine group.In the electronic spectra of most of complexes(entries 2,3,4,7 10 and 13), the bands assigned to aromatic rings and azomethine group are merged together and shifted to lower energies after coordination.In zinc thiocyanate and azide, mercury bromide, iodide and thicyanate complexes, the first band of ligand shift to higher wavelengths and a strong shoulder is appeared at 276 and 281 nm after coordination.Finally in cadmium thiocyanate and mercury chloride complexes two independent strong bands were detectable for ‫ת-ת‬ * transition of aromatic rings and azomethine group of ligand due to coordination.The d-d transition bands for complexes of these transition metals due to d 10 electron configuration won't be observed.Though metal to ligand charge transfer are predictable for this type of complexes but these transition were not observed.

H and 13 C NMR spectra
The results data of NMR spectra of the ligand and its metal complexes have been entered in experimental section. 1 H and 13 C NMR spectra were recorded at 500 MHz, using DMSO as solvent. 1 HNMR and 13 CNMR of ligand are depicted in Figure 2. Studies of 1 H and 13 C NMR spectra exhibit that the data have excellent correlation with pseudo-tetrahedral structure that proposed before for complexes.As a functional group signal, resonance of azomethine proton (H b and H b' ) appeared at 8.31 ppm 22 .These signals are red shifted to 8.46-8.63ppm with respect to TMS for entries 8, 9, 10, 11 and 13 or shifted to down filed 8.24-8.30ppm for entries 2, 3, 4, 5 and 6 and unchanged in spectra of entries 7 and 12, suggesting coordination by the azomethine nitrogens to metal ion.In the spectrum of ligand H d,d' appeared at 7.74 ppm as a doublet of triplet due to coupling with H e,e' and fluorine atom at benzene ring with coupling constants of 6.11, 5.98 and 2.05 Hz.In the spectra of complexes, the signals of H d,d' have shown a change in location, so that shifted to up fields (entries 7-13) or down fields (entries 2-6).Also the type of splitting of this signal changed due to coordination of ligand and appeared as doublet of doublet for entries 3, 5 and 6; as triplet for entries 9 and 10, as multiple for entries 7 and 8 and as broad singlet for entries 12 and 13.H e,e' due to coupling whit H d,d' and then fluorine atom are observed at 7.22 ppm as a triplet signal with coupling constants of 8.70 and 8.75 Hz.In the spectra of complexes, the signals of H e,e' are almost unchanged or shifted to upfields (entries 2, 4 and 5) or down fielded (entries 8-11 and 13).On the other hand, in the spectra of HgLBr 2 and HgLI 2 , these signals appeared as doublet of triplet and multiple signals respectively.For HgL(SCN) 2 and HgL(N 3 ) 2 these signals are seen as broad singlet that these chemical shifts and signal shapes confirm coordination of ligand to metal centers.The signal of H a,a' protons in the ligand are exhibited as a singlet at 3.84 ppm that are appeared as singlet in all complexes but are shifted to down fields after coordination.In the 13 C NMR spectrum of the ligand, two azomethine carbons C(5,5') resonances are observed as a single peak at 160.51 ppm [16][17][18] .These peaks are shifted to up field in all complexes, suggesting well coordination of the azomethine nitrogen to metal ion.Also, the 13 C NMR spectrum of the ligand showed a doublet peak at 163.40 ppm for C(1, 1') carbons due to coupling with fluorine atom with J = 985.50Hz that shifted to lower energies after coordination.C(2, 2') and C(3, 3') also are effected with fluorine atom and exhibit doublet peak at 115.50 and 129.87 ppm with coupling constants of 34.55 and 86.90 Hz.After coordination, these peaks are stayed as doublet but with a smoothly change in their chemical shifts.A singlet peak at 132.60 and 60.67 ppm is assigned to C(4, 4') and C(6, 6') respectively.The peaks of C(4, 4') is stayed singlet in all complexes except for entries 2, 3 and 9 that were transformed to doublet signals due to the coupling effect of fluorine atom.The signals related to C(6, 6') are being almost unchanged in all complexes.The carbon of thiocyanate group in ZnL(NCS) 2 , CdL(SCN) 2 and HgL(SCN) 2 are found at 135.45, 131.84 and 131.75 ppm respectively.
In final, based on the current evidences and according to previous report on this type of ligands 4,7 , the suggested structure for the complexes is pseudo-tetrahedral as drawn in Scheme 1.

Thermal investigation of Schiff base complexes
Thermal behavior (TG/DTG) of compounds was studied on PL-1500 TG instrument from room temperature to 600 o C at the heating rates of 10 degree per minute under oxygen atmosphere.The mass loss versus temperature data of the complexes extracted from the plots are seen in Table 2. Thermogravimetric investigations of the complexes indicate some information about the thermal stability of them.Complexes lose 81.2-100% of their weight during room temperature to 600 o C. The mass loss under 100 o C for some compounds indicates existence of a few lattice water molecules (6.99, 6.8, 3.4, 2.3% for Entries of 5-8 respectively).In other complexes weight loss was not observed until 200 o C indicating no water molecule (as lattice water or coordinated) in the compounds.
The thermodynamic activation parameters of decomposition processes of complexes including Arrhenius constant(A), activation energy (E * ), enthalpy (∆H * ), entropy (∆S * ) and Gibbs free energy of decomposition step (∆G * ) are evaluated based on plots using Coats-Redfern relation 31 .
The calculated data are collected in Table 3.The activation energies of decomposition are found to be in the range of 8-127kJ/mol.This range of values of the activation energies indicates relative thermal stability of the complexes.∆S * values are negative in most of investigated steps that states decomposition reactions proceed with lower rates than normal ones.∆H * and ∆G * values are positive and placed at ranges of 4.38-124 kJ/mol and 106-258 kJ/mol respectively.

Figure 1 .
Figure 1.1  HNMR and13  CNMR of ligand.In the13  C NMR spectrum of the ligand, two azomethine carbons C(5,5') resonances are observed as a single peak at 160.51 ppm[16][17][18] .These peaks are shifted to up field in all complexes, suggesting well coordination of the azomethine nitrogen to metal ion.Also, the

Table 1 .
Elemental analysis, color, %yield and molar conductivity of the ligand and its complexes.

Table 2 .
Thermal analysis data including temperature range, mass loss and proposed residue.

Table 3 .
Thermodynamic activation parameters of decomposition processes of complexes.
a refers to selected decomposition temperature range.