INTRODUCTION
Schiff base are capable of forming coordinate bonds with many of metal ions through both azomethine group and phenolic group or via its azomethine or phenolic groups.1-3 Infrared spectroscopy has been widely used as a powerful means of distinguishing between the three possible donation sites of Schiff bases. A large number of Schiff bases and their complexes are significant interest and attention because of their biological activity including anti-tumor, antibacterial, fungicidal and anti-carcinogenic properties. 4,5 Tetradentate Schiff bases are well known to coordinate with various metal ions forming stable compounds, and some of these complexes are recognized as oxygen carriers.6
To continue our investigation in the field of Schiff base complexes,7-11 we reported here preparation and characterization of the new complexes [Sn(acac-o-phdnH2)]Cl2, [Sn(acac-m-phdnH2)]Cl2 and [Sn(acac-p-phdnH2)]Cl2; (acac-o-phdnH2 (L1), acac-m-phdnH2 (L2) and acac-p-phdnH2 (L3) are N,N’-o-phenylenebis (acetylacetoneimine), N,N’-m-phenylenebis(acetylacetoneimine) and N,N’-p-phenylenebis(acetylacetoneimine, respectively). The solid products are well characterized through its elemental analysis, electronic spectra and thermal analysis, along with the assignment of their infrared spectra for most of the fundamental vibrations.
EXPERIMENTAL
All chemical used throughout this work were analytical reagent grade. The Schiff bases L1=acac-o-phdnH2 [N,N’-o-phenylene bis(acetylacetoneimine)], L2=acac-m-phdnH2 [N,N’-m-phenylene bis(acetylacetoneimine)] and L3=acac-p-phdnH2 [N,N’-p-phenylene bis(acetylacetoneimine)] were prepared according to published method12 from the condensation of o-phenylenediamine, m-phenylenediamine or p-phenylenediamine with acetylacetone in ethyl alcohol as a solvent. The separated precipitate was filtered, washed several times with minimum amount of ethyl alcohol and dried under vacuum at room temperature. The isolated three Schiff bases were characterized through its infrared and elemental analysis. The deep pink solid complex [Sn(acac-o-phdnH2)]Cl2 was prepared from the addition of tin(II) chloride (4 mmol) in acetone to a stoichiometric amounts of the acac-o-phdnH2, (1.088 g, 4 mmol) in acetone. The mixture was then stirred at room temperature for about 6-8 hrs. The separated solid product was filtered off, washed several times with acetone and dried under vacuum. The other two complexes [Sn(acac-m-phdnH2)]Cl2 (yellowish orange crystalline) and [Sn-(acac-p-phdnH2)]Cl2 (brownish black) were prepared in a similar way to that described above by the reaction of tin(II) chloride with acac-m-phdnH2 and acacp-phdnH2, respectively. The complexes formed were characterized through their elemental analysis, infrared spectra as well as thermogravimetric (TG) and differential thermal analysis (DTA). Analysis of the products obtained is summarized in Table 1. The percentage of tin was determined using atomic absorption method. An atomic absorption spectrometer PYE-UNICAM SP 1900 fitted with a tin lamp was used for this purpose. The infrared spectra of the Schiff bases and tin(II) Schiff base complexes were recorded from KBr discs using a Perkin-Elmer 1430 ratio recording infrared spectrophotometer. Thermogravimetric (TG) and differential thermal analysis (DTA) of the Schiff base complexes [Sn(acac-o-phdnH2)]Cl2, [Sn(acac-m-phdnH2)]Cl2 and [Sn(acac-p-phdnH2)]Cl2 were carried out using a Shimadzu computerized thermal analysis system DT-40. The system includes programs that process data from the analyzer with the chromotpac C-R 3A. The rate of heating of the samples was kept at 5℃ min–1. Accurate samples were analyzed under a N2 flow at 40 ml min–1 α-alumina powder was used as the DTA standard materials. Electronic spectra of the reactants and products were recorded at room temperature using Jenway 6405 Spectrophotometer with quartz cell of 1 cm path length.
Table 1.The elemental analysis data
RESULTS AND DISCUSSION
The three new complexes [Sn(acac-o-phdnH2)]Cl2, [Sn(acac-m-phdnH2)]Cl2 and [Sn(acac-p-phdnH2)]Cl2 formed in the reaction of SnCl2 with the Schiff bases acac-o-phdnH2, acac-m-phdnH2 and acac-p-phdnH2 are characterized through their elemental analysis, infrared spectra and through their thermogravimetric investigation. The results enable us to characterize the complexes and make an assessment of the bonding and structures inherent in them.
The structural formula of these three complexes was obtained according to the following facts. Elemental analysis of the three complexes under investigation shows the presence of chloride ions in the three complexes. Solutions of these complexes react with silver nitrate giving a white precipitate of silver chloride indicating the existence of chloride in the ionic form i.e. outside the coordination sphere of the tin ion.
However, the Schiff bases under study may be present in tautomeric equilibrium (forms I-III), which resemble the forms given by Dudek and Holm13 for similar Schiff bases.
Distinction between the forms II and III is not large as a slight displacement of the hydrogen nucleus. It has been concluded from infrared evidences that similar Schiff bases are largely tautomerized into II and III with appreciable amount of each existing in the solid state and in solution.13, 14
Vibrational spectra
Infrared spectra of the Schiff bases under studies give a broad absorption band at about 3170 cm−1, which may be due to the stretching vibration of either an O-H group in accordance with formula II or an N-H group of the type indicated by the formula III. Evidence of that14 is found in the broadening of this band and shift from normal positions (3700-3400 cm−1 for free O-H and 3400-3300 cm−1 for free N-H). Also the free Schiff bases do not absorb in the region of carbonyl group, thus the formula I is eliminated and the ligand is believed to be a tautomeric equilibrium mixture of structures II and III.
The characteristic IR bands of [Sn(acac-o-phdnH2)]Cl2, [Sn(acac-m-phdnH2)]Cl2 and [Sn(acac-p-phdnH2)]Cl2 are shown in Table 2. Before explaining such assignments, the structures of these complexes must first be discussed. Bottcher et al.16 reported the crystal structure of some related Schiff base complexes of Co(III) and indicated that the tetradentate Schiff base ligand (ONNO) coordinates the cobalt atom in a planar fashion with a slightly irregular tetragon with Co(III). The values of the Co-O and Co-N bond lengths in the equatorial ligand plane are 1.881 and 1.892 A°, respectively.
The complexes under investigations indicate that the tin(II) is only four coordinated and may take the structures shown in Scheme IV. With such structures the complexes may belong to C2V symmetry and are expected to display 117 vibrational fundamental which are all monodegenerate. These are distributed between A1, A2, B1 and B2 motions; all are IR and Raman active, except for the A2 modes, which are only Raman active.
In the spectra of the complexes, medium strong bands in the region 3340-3217 cm−1 do exist, Fig. 1. These bands occur at lower frequency values known for O-H groups, therefore, the two bands found at 3279 and 3217 cm−1 for the complex [Sn(acac-o-phdnH2)]Cl2, and at 3320, 3200 cm−1 for [Sn(acac-m-phdnH2)]Cl2 and 3340 cm−1 for [Sn(acac-p-phdnH2)]Cl2 complexes, respectively, are assigned to coordinated N-H groups.15 Accordingly, the three complexes under study have coordinated N-H groups which indicated that, the Schiff bases under study coordinates with Sn(II) as a tetradentate through the two nitrogen and oxygen atoms as shown in Scheme IV and in the ketoamine form (III).
Table 2.(a): s=strong, w=weak, m=medium, sh=shoulder, v=very, br = broad. (b): ν, stretching; δ, bending.
The ν (C-H) for the phenyl group are assigned in the region 3150-3020 cm−1, Table 2, while those for the CH3 and =CH- groups are shown as expected at 2970, 2960, 2915, 2880 and 2860 cm−1. The carbonyl bands ν (C=O) of our complexes are shown at 1630 and 1605 cm−1 as a strong and medium strong band. The ν (C=C) stretching vibrations and phenyl breathing modes of the three complexes are observed in the range 1590-1490 cm−1. The bending vibrations, δ(-CH3) and (=CH) are observed in the region 1476-1310 cm−1. The description of their assignments follow the expression developed and used by others.17,18
The stretching vibrations of ν (C-O), ν (C-C) and ν (C-N) occur in the region 1285-1110 cm−1, Table 2. The tin-oxygen (O of Schiff base) stretching vibration, νas(Sn-O) are assigned for the three complexes at 514, 555 and 550 cm−1, while the corresponding νs(Sn-O) are assigned at 475, 490 and 505, 480 cm−1 for o-, m- and p- Schiff base complexes, respectively. The νas(Sn-N) are assigned at 345, 320 cm−1 and 325, 315 cm−1 and 350, 330 cm−1, for our study complexes, while νs(Sn-N) are assigned to the strong and weak infrared bands around 295 cm−1. The assignments of these metal-ligand stretches are made on the basis of their activities as well as the relative bond strengths shown from the crystal structure data known16 for some related Schiff base complexes, and also that the assigned bonds do not exist in the free ligand spectra. However, the ν(Sn-N) stretches were assigned at relatively low frequency values compared to ν(Sn-O). This is in agreement with the bond length values, which indicate that the Sn-N bond is the weakest with a large value of bond length.
Scheme IV
Fig. 1.Infrared spectra of (a): acac-o-phdnH2 Schiff base, (b): [Sn(acac-o-phdnH2)]Cl2, (c): [Sn(acac-m-phdnH2)]Cl2 and (d): [Sn(acac-p-phdnH2)]Cl2 complexes.
Thermal studies
Differential thermal analysis (DTA) and thermogravimetric (TG) were measured for the three complexes to investigate the mode of decomposition of these types of complexes, Fig. 2. The maximum temperature values Tmax /℃ together with the corresponding weight loss for each step of the decomposition are recorded in Table 3. The decomposition of the complex [Sn(acac-o-phdnH2)]Cl2 falls in one degradation stage. This stage of decomposition appears at two maxima 120 and 165 ℃ is associated with a weight loss of 54.96% corresponding to the loss of 5C2H2+2HCl+2NH3+H2O. The weight loss in this stage 54.96% is in full agreement with the calculated value of 55.23%. The thermal degradation for the other two complexes, [Sn(acac-m-phdnH2)]Cl2 and [Sn(acac-p-phdnH2)]Cl2, exhibit approximately one main decomposition step, Fig. 2, this stage of decomposition occurs at maximum temperatures 141, 259 and 109, 153 ℃, respectively, and accompanied by a weight loss of 47.11 and 66.16% correspond to the loss of 3C2H4+2HCl+2NH3+CO and 7C2H2+2HCl+N2+H2O+H2 in agreement with our predicted weight loss of 47.44 and 65.63% for [Sn(acac-m-phdnH2)]Cl2 and [Sn(acac-p-phdnH2)]Cl2 complexes, respectively. The above going conclu-sion was also supported by the fact that the infrared spectra of the final decomposition products for the complexes under investigation shows the absence of all bands associated with the Schiff base ligands and instead the characteristic spectrum of SnO is appeared. Accordingly, the following mechanisms are proposed for the thermal decomposition of the Schiff base complexes as follows:
Fig. 2.TGA and DTA curves of (A): [Sn(acac-o-phdnH2)]Cl2, (B): [Sn(acac-m-phdnH2)]Cl2 and (C): [Sn(acac-p-phdnH2)]Cl2 complexes.
Table 3.The maximum temperature, Tmax /℃, and weight loss values of the decomposition stages for the [Sn(acac-o-phdnH2)]Cl2, [Sn(acac-m-phdnH2)]Cl2 and [Sn(acac-p-phdnH2)]Cl2 complexes.
Electronic spectra
The electronic spectra of the SnCl2, acetylacetonate, schiff base (e.g, acac-o-phdnH2) and the complex, [Sn(acac-o-phdnH2)]Cl2 have been measured in methanol as a solvent, Fig. 3. The spectra of the uncomplexed ligand (acac-o-phdnH2) show a characteristic band at 275 nm which is assigned to n→π* transition to the chromophoric C=O group. This band shows a bathochromic shift in the tin(II) complexes of all the ligands. This is attributed to the involvement of carbonyl group in metal complexation as indicated by IR studies. The Schiff bases, acac-o-phdnH2, acac-m-phdnH2 and acac-p-phdnH2 were prepared in this study play a role of donor type (n-donor). The n-donor is neutral, even systems containing relatively an easily ionized atomic lone pair of electrons.19 The donated electrons come from a non-bonding molecular orbital occupied in the original donor by a lone pair of electrons as follows:
On the other side tin(II) chloride reacted as a vacant orbital acceptor19 which is neutral even system or even ions in which an orbital or orbitals of relatively high electron affinity are vacant. This type of acceptors combine especially with n-donors to form fairly stable compounds by partial dative acceptance of electron into vacant orbital as follows:
To make a comparison between reactants and products (Fig. 3), there are three electron spectra bands observed around 260 and 310 nm due to π→π* of acetylacetonate ring (C=O) and at around 379 nm as shoulder broad peak due to the metal-toligand charge transfer transition,20 between Schiff bases and SnCl2 (the color of schiff base widely change as soon as tin(II) chloride added from brownish yellow to pink color).
Fig. 3.Electronic spectra of (a): SnCl2, (b): Acetylacetonate, (c): acac-o-phdnH2, (d): acac+o-phenylenediamine, (e): Acetylacetonate+o- phenylenediamine / SnCl2 complex.
Kinetic calculation
From TGA curves, the order (n), activation energy (E*/kJ mol−1), and the pre-exponential factor, Z, for thermal reactions of the complexes have been elucidated by linearisation method of Horowitz-Metzger(HM) approximation method and Coats-Redfern integral method.21,22 The entropy of activation ΔS* in (JK−1mol−1) was calculated by using the equation: ΔS* = R ln(Ah/kB Ts), where kB is the Boltzmann constant, h is the Plank’s constant and Ts is the DTA peak temperature.23
Fig. 4.Kinetic parameter of (A): [Sn(acac-o-phdnH2)]Cl2, (B): [Sn(acac-m-phdnH2)]Cl2 and (C): [Sn(acac-p-phdnH2)]Cl2 complexes using Horowitz-Metzger (HM) approximation method and Coats-Redfern (CR) integral method.
Table 4.Kinetic parameters determined using the Coats-Redfern (CR) and Horowitz-Metzger (HM).
The enthalpy activation, ΔH*, and Gibbs free energy, ΔG*, were calculated from; ΔH*=E*–RT and ΔG*=ΔH* –T ΔS*, respectively. The data is presented in Fig. 4 and Table 4. The complexes with all the ligands start decomposing in the range 382-583 K. This shows that the complexes are anhydrous. All complexes decomposed via first order kinetics and energy of activation is 16-25 kJmol−1. The order of activation energy is: L3>L1>L2 for tin(II) complexes. The literatures contain various explanations of the relative order of thermal stability of complexes.24-26 The final product of the decomposition in all the complexes is SnO as confirmed by experimental and calculated weight losses.
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