Synthesis, Characterization and Thermal Decomposition Studies of Cr(III), Mn(II) and Fe(III) Complexes of N, N'-Bis[1,3-benzodioxol-5- ylmethylene]butane-1,4-diamine

A bidentate Schiff base ligand namely, N,N'-bis-1,3-benzodioxol-5ylmethylene]butane-1,4-diamine was synthesised by condensing piperonal (3,4dioxymethylenebenzaldehyde) with butane-1,4-diamine. Cr(III), Mn(II), Fe(III) complexes of this chelating ligand were synthesised using acetates, chlorides, bromides, nitrates and perchlorates of these metals. The ligand and the complexes were characterised by elemental analysis, H NMR, UV-Vis and IR spectra, conductance and magnetic susceptibility measurements and thermogravimetric analysis. The thermograms of three complexes were analysed and the kinetic parameters for the different stages of decompositions were determined. Keywords; Schiff base, Diamine, Piperonal, Transition metal complexes, Thermal decomposition.


Introduction
The chelating Schiff base ligands derived from diamines and various carbonyl compounds, encompass a highly remarkable class of compounds having a wide range of applications in catalytic 1 , synthetic 2 , analytical 3,4 , clinical 5 and biochemical 6 areas and they possess considerable physiological activities 7,8 .Schiff base derivatives of 1,4-butanediamine and their metal complexes, also find a number of catalytic 9 , and biological 10 applications.A perusal of earlier work revealed that the coordinating possibility of 1,4-butanediamine is enhanced by condensing with a variety of carbonyl compounds [11][12][13] .But the literature survey showed that no work has been done on the transition metal complexes of the Schiff base derived from 1,4-butanediamine and piperonaldehyde.Therefore, a new Schiff base ligand derived from piperonal and 1,4-butanediamine and its transition metal complexes were synthesised and characterised.The ligand named as N,N'-bis-1,3-benzodioxol-5-ylmethylene]butane-1,4diamine (L) has two donor sites, two nitrogen atoms as azomethine groups (Figure 1).The use of piperonal as the carbonyl compound in these studies owed to the fact that several compounds containing the 3,4-methylenedioxy group possessed various biological activities [14][15][16] .Complexes of Cr(III), Mn(II) and Fe(III) were synthesised using acetates, chlorides, bromides, nitrates and perchlorates of these metals.Investigations on thermal decomposition behaviour of some of these complexes were also done.

Experimental
All chemicals viz.piperonaldehyde, butane-1,4-diamine, metal salts, solvents etc., used were of A R grade (E.Merck or B D H). Carbon, hydrogen and nitrogen analyses were carried out by using Hitachi CHN-O rapid analyzer at CDRI, Lucknow.The anions present in complexes were estimated by standard methods 17 . 1H NMR spectra of the ligand was recorded on a Varien-300 nuclear magnetic resonance instrument using DMSO-d 6 as solvent.Infrared spectra were measured in the range 4000-400 cm -1 on a Schimadzu FTIR-8101 spectrophotometer with KBr pellets.The solid state electronic spectra of complexes were recorded using a Schimadzu UV-1601 spectrophotometer.The magnetic measurements were made at room temperature by the Gouy method using Hg[Co(NCS) 4 ] as calibrant.The thermal decomposition behaviours of complexes were monitored using a Perkin Elmer TGA-7 Analyser.

Synthesis of the ligand and complexes
1,4-Butanediamine (20 m mol, 176 mg) solution in ethanol (50 mL) was mixed with a solution of piperonaldehyde (40 m mol, 604 mg) in ethanol (50 mL) in 1:2 molar ratio and was refluxed for about 1 h and then cooled.The white precipitate formed was filtered off, washed with water and a few ml of alcohol and then purified by recrystallising from ethanol (yield = 678 mg, 87%).
Complexes of Cr(III), Mn(II) and Fe(III) with this ligand were synthesised using acetate, chloride, bromide, nitrate and perchlorate salts of these metals.Solutions of the ligand and metal salts in methanol (10 m mol in 50 mL) (1:1 molar ratio) were refluxed for 2 to 3 h.Metal acetates were dissolved in methanol-water mixture and added to refluxing solutions of the ligand in methanol.The complexes, synthesised using metal chlorides, precipitated during refluxing and were filtered off.In other cases, the reaction mixtures were concentrated and the pasty mass obtained in each case was repeatedly washed with diethyl ether and/or petroleum ether and/or acetone to get the solid complexes separated.The complexes were filtered, washed with suitable solvents and then dried over anhydrous CaCl 2 .

Characterisation of ligand
In this paper we designed and synthesized a bidentate/tetrdentate Schiff base ligand, N,N'-bis-1,3-benzodioxol-5-ylmethylene]butane-1,4-diamine (L) by the condensation of 1,4-butanediamine with piperonal.The I H NMR spectrum of the ligand, was recorded in DMSO-d 6 and it showed a number of characteristic signals of the compound 18 .The peak observed at a δ value of 8.01 ppm was assigned to the azomethine protons in the molecule.
The signals due to the aromatic protons were observed in the range 7.30-6.83ppm.The singlet peak at 5.96 ppm was assigned to the methylinic protons of the dioxymethylene groups of the piperonal moieties present in the ligand.The inductive effect of the two oxygen atoms deshielded the methylenic protons and this resulted in the higher δ value for these protons.The peaks in the ranges of 3.71-3.65and1.95-1.91ppm were assigned to the methylinic protons of the butanediamine moiety of the ligand.The IR spectrum of the ligand showed bands at 3050 and 2983 cm -1 assigned to the C-H stretching of aromatic and methylene groups, respectively.The bands present at 1640 and 1255 cm -1 were assigned to the C=N and C-N stretchings, respectively 19 .Bands at 1191 and 1099 cm -1 were assigned to the in plane bending of the aromatic C-H and those at 872 and 816 cm -1 to the out of plane bending vibration of the aromatic C-H.The characteristic absorption frequency of the dioxymethylene group of piperonal moiety 20 was present at 926 cm -1 .The absence of the characteristic stretching frequency of C=O of the aromatic aldehyde group 19 , indicated that the condensation was complete.The elemental analysis and spectral data for L are consistent with the formula C 20 H 20 O 4 N 2 and the structure given in Figure 1.

Formulae and general properties of complexes
The reaction of the ligand (L) with different salts of Cr(III), Mn(II) and Fe(III), ions in 1:1 molar ratios gave metal complexes of the given formulae, as evidenced by the micro analytical and spectral data.The colours, magnetic susceptibilities and molar conductivities and melting points and the micro analytical data of the complexes are listed in Table 1.These air stable metal complexes were non hygroscopic, partially soluble in most organic solvents, but freely soluble in DMF and DMSO.The molar conductivities in DMF (10 -3 M) solution showed that all the complexes except one behaved as nonelectrolytes, indicating the coordinated nature of the anions, while the nitrato complex of Mn(II) behaved as a 1:2 electrolyte 21 .

IR spectra of complexes
Table 2 lists the most important IR spectral bands of the ligand and metal complexes.In the spectra of all the complexes the ν(C=N) was shifted to lower frequency, due to its involvement in coordination.Instead of the band at 1640 cm -1 present in the spectrum of the free ligand, new bands appeared in the ranges of 1608-1598, 1604-1591 and 1624-1599 cm -1 in the Cr(III), Mn(II) and Fe(III) complexes, respectively and were assigned to the coordinated azomethine groups [22][23][24] .
The characteristic absorption frequency of the dioxymethylyne group was found to be present in the spectra of all the complexes at the same frequency as it was observed in the ligand spectrum.This indicated the non-involvement of dioxymethylene groups in coordination in these complexes 20 .

Fe(III) complexes
The complexes of Fe(III) were orange in colour and gave weak bands in the range 421-402 nm, assigned to the spin-and parity forbidden 6 A 1g → 4 T 2g transitions of Fe(III) ion in an octahedral field 24,41 .The pale yellow or brown colours of the complexes also supported octahedral geometry.All the complexes of Fe(III), except the chloro complex, which were investigated here, showed magnetic moments in the range of 6.16-5.98B.M. indicating that they are high-spin with probable octahedral geometry 23,36 .

Thermogravimetric analysis
Thermograms of three complexes, viz, [CrLCl 3 (H 2 0)], [MnLCl 2 (H 2 O) 2 ]and [FeLCl 3 (H 2 0)] were analysed.They underwent dehydration reactions around 150 o C, losing two, four and two molecules of water, respectively, thus confirming the presence of coordinated water molecules and they gave Cr 2 O 3 , MnO 2 and Fe 2 O 3 , respectively, as the end products at temperatures around 600 o C. The decomposition patterns were in good agreement with the suggested formulae.By the analysis of the non-isothermal TG, using the integral method of Coats-Redfern, kinetic parameters, viz, order of reaction(n), activation energy(E a ), frequency factor(A) and entropy of activation(∆S * ) were calculated.The enthalpies and free energies of activation for various decomposition stages have also been calculated using the relations, ∆H * ═ E a -RT s and ∆G * ═ ∆H * -T s ∆S * where T s is the peak temperature of the decomposition stage investigated 42 .Figures 2, 3 and 4 give the TG-DTG traces of the complexes, the Table 4 gives the different stages of decomposition and the Table 5 gives the kinetic parameters.Based on inception temperature and activation energy, for the decomposition of complexes excluding the dehydration stage, stabilities of the complexes were found to be in the order, Cr > Mn > Fe. No

Table 1 .
Formulae, general properties and micro analytical data of ligand and complexes * AcO = CH3-COO -

Table 2 .
Significant IR spectral bands of ligand and Cr(III), Mn(II) and Fe(III) complexes.

Table 3 .
Electronic spectral bands of Cr(III), Mn(II) and Fe(III) complexes and their assignments.

Table 5 .
Kinetic parameters for the decomposition of Co(II), Ni(II) and Cu(II) Complexes.Structures suggested for different complexes are given in Figure 5 and 6.