Synthesis and Thermal Degradation Kinetics of Co ( II ) , Ni ( II ) , Cd ( II ) , Zn ( II ) , Pd ( II ) , Rh ( III ) and Ru ( III ) Complexes with Methylquinolino [ 3 , 2-b ] benzodiazepine

A series of new complexes formed by the interaction of a new ligand Methylquinolino[3,2-b]benzodiazepine (L) with various transition metal ions have been isolated and characterized by elemental analysis and electronic, IR, magnetic moment and conductivity measurements. Thermogravimetric (TG) studies of the complexes have been performed in order to establish the mode of their thermal degradation. The thermal degradation was found to proceed in two steps. The kinetics and thermodynamic parameters were computed from the thermal decomposition data.


Introduction
In the last decade, much attention has been given to the organic ligands and transition metal complexes because of their biological relevance, interesting spectral and magnetic properties.The fused aromatic heterocyclic ligands and their metal complexes are being used extensively as pharmaceutical and chemotherapeutic agents [1][2][3][4] .On the other hand, quinoline and their derivatives form an interesting class of compounds which display attractive applications as pharmaceuticals [5][6][7][8] and are general synthetic building blocks, due to their chemical and biological relevance.Therefore, it was thought worthwhile to isolate and characterize some novel quinoline derivatives containing different donor atoms.

Physical measurements
All necessary precautions were taken to exclude oxygen and moisture during the synthesis of compounds.Analytical reagent grade chemicals were used as received for all the experiments.Infrared spectra of the ligand and its metal complexes in KBr pellets were recorded in the spectral range 4000-250 cm -1 range with SHIMADZU FTIR-8400S spectrometer.UV-Visible spectra were recorded on a SHIMADZU double beam spectrophotometer.The C, H, N and S content analyses were determined by using Carlo-Erba 1106 model 240 Perkin Elmer analyzer at University of Mysore, Mysore.Magnetic susceptibilities were measured on a Guoy Balance at room temperature using HgCo(NCS) 4 as calibrant. 1H NMR spectra were recorded in DMSO-d 6 solution on a JEOL 300 MH Z spectrometer and TMS was used as an internal reference.The molecular weight of the complexes was determined by Rast's method using biphenyl 9 .

Thermogravimetric analysis
The thermogravimetric (TG) curves of complexes were recorded in static air atmosphere.Dupont 9900 computerized thermal analyzer with 951 TG module thermo balance was used for recording TG curves.The temperature scale of the instrument was calibrated with high purity calcium oxalate [CaC 2 O 4 H 2 O].The operational range of instrument is from ambient to 970 o C.About 6-8 mg of pure sample was subjected for dynamic TGA scans at heating rate of 10 o C min -1 .The kinetic parameters for the degradation steps on TG curves were determined using the methods reported elsewhere [10][11][12] .

Synthesis of Ligand (L)
The mixture of 2-Chloro-6-methylquinoline-3-carboxaldehyde (1.569 g, 5 mmo1) dissolved in small amount of acetic acid and o-Phenylenediammine (0.541 g, 5 mmol) was taken in a 100 mL borosil beaker and a pinch of potassium iodide was then added.The whole mixture was made into slurry and was irradiated by placing the beaker in a microwave oven for about 10 minutes.The product obtained was poured into ice-cold water; the solid separated was filtered and dried.The product was separated on an alumina column (3x20 cm) using methylenechloride/acetonitrile (5:1) as an eluant (Scheme 1).

Synthesis of Cobalt(II) and Nickel(II) complexes
An alcoholic solution of L (2 mmol, 50 mL) and Metal chloride (1 mmol) was refluxed for 8 h and its volume was reduced in a rotary evaporator until a precipitate appeared.After cooling, the solid was filtered off, washed with water, methanol and ether, and then dried under reduced pressure at room temperature.

Synthesis of Cadmium(II) and Zinc(II) complexes
The Metal salt (10 mmol) was added to a solution of L (10 mmol) in dry ether (30 mL) with continuous stirring and stirring was continued for 1h at 0 o C and 18 h at room temperature.The resulting solution was concentrated to give a white compound.The compound was recrystallized by using chloroform/hexane (1: 1) mixture to give the desired complex.

Synthesis of Pd(II), Rh(III) and Ru(III) complexes
The complexes were prepared by mixing an ethanolic solution of PdCl

Results and Discussion
The complexes were microcrystalline coloured powder.They are stable at room temperature and soluble in DMSO and DMF.The elemental analyses agree well with a 1:1 metal-to-ligand stoichiometry for Cd(II), Zn(II) and Pd(II) and 1:2 for Co(II), Ni(II), Rh(III) and Ru(III) complexes (Table 1).The conductivity values measured in DMF at room temperature fell in the range 14.52-34.3mhos cm 2 mol -1 , which indicates the nonelectrolytic nature of all the complexes except Rh and Ru complexes, which show conductivity value of 47.04 and 46.05 respectively 13 .

Spectral study
The octahedral Co(II) complex exhibit three bands at 14380, 14766 and 16393 cm -1 , pertaining to 4 T 1g (F) → 4 T 2g (F) ( 1 ), 4 T 1g (F) → 4 A 2g (F) ( 2 ) and 4 T 1g (F) → 4 T 1g (P) ( 3 ) transitions, respectively.The absorption spectra of Ni(II) complex show two bands at 16240 cm -1 and 23251 cm -1 due to 3 A 2g(F) 3 T 1g(F) ( 2 ) and 3 A 2g(F) 3 T 1g(P) ( 3 ) transitions, respectively supporting the octahedral stereochemistry.The reflectance spectra of Zn(II) and Cd(II) complexes show no bands due to d-d transition.This phenomenon is natural as there is no possibility of transition due to non availability of empty d-orbital 16 .By considering the spectral data, the tetrahedral geometry for Cd(II) complex and square planar geometry for Zn(II) complex have been proposed [16][17][18] .The electronic bands observed at 16582, 21276 and 30284 cm -1 for Pd(II) complex ion are due to the transitions 1 A 1g 1 A 2g ( 1 ), 1 A 1g 1 B 1g ( 2 ) and 1 A 1g 1 E 1g ( 3 ) respectively in a square planar configuration.In the present investigation of Rh(III) complex, the observed electronic bands around 16580, 19370 and 22280 cm -1 are due to the transitions 1 A 1g 3 T 1g , 1 A 1g 1 T 1g and 1 A 1g 1 T 2g , respectively in an octahedral structure around Rh(III) ion 16 .The UV-Visible spectra of Ru(III) complex exhibit octahedral absorption band at 24560 cm -1 attributed to 1 A 1g 1 T 1g charge transfer transitions 19 .

IR Spectra
The IR spectral data of ligand and its metal complexes are presented in Table 2. IR spectroscopy can provide valuable information as to whether or not a reaction has occurred.The ligand L shows bands at 1662 cm -1 and 3332 cm -1 due to ν(C=N) and ν(NH) vibrations respectively 20 .These bands are shifting in the complexes indicates the coordination of nitrogen atom of quinoline and azepine moiety with the metal ions.On the basis of the above interpretation, it is concluded that the ligand behaves as a bidentate.

H NMR spectra
All the compounds show the 1 H NMR signals for different kinds of protons at their respective positions.The data are shown in Table 2.The 1 H NMR spectra of the ligand MQBD exhibit a singlets at 10.80 δ (s, N-H) and 8.6 δ (s, H-C=N).The 1H NMR spectra of complexes slightly changed compared to those of the corresponding ligand, and the signals appeared downfield, as expected, due to the coordination of nitrogen atoms to the metal ion [22][23][24] .

Thermal Analysis
The temperature of decomposition, the pyrolyzed products, the percentage weight loss of the ligand, and the percent ash are given in Table 3. TG curves of the complexes show two significant steps in the decomposition.In the first step, the loss of organic ligand moiety occurred in the range 190-450 o C with a mass loss of 61.97-80.26%.The decomposition temperature of second stage lies in the range 400-690 o C, which represents the loss of corresponding inorganic ligand with a mass loss in the range 7.89-14.65%.These experimental values are in agreement with the expected value.This observation suggests that these complexes do not have water molecule either inside or outside the coordination sphere.The ash from the complexes obtained in each case has been chemically identified as pure metal oxide.The experimental values of the ash content are in the expected range (8.57-30.58%).The thermograms obtained during TGA scans were analysed to give the percentage weight loss as a function of temperature.T 0 (temperature of onset of decomposition), T 10 (temperature for 10 % weight loss), T 20 (temperature for 20 % weight loss) and T max (temperature of maximum rate of degradation) are the main criteria to indicate the heat stability of the complexes.The higher the values of T 0, T 10, T 20 and T max , higher the heat stability.
Broido's method 10 was used to evaluate the kinetic parameters from TG curve.Using Broido's method, plots of ln[ln(1/y)] vs 1/T (where y is the fraction not yet decomposed) for different stages of the thermal degradation process of the complexes were made.The plots were linear over the conversion range of about 0.1 -0.9, supporting the assumption of first order kinetics.
In order to determine the thermal stability trend, the parameters T 0, T 10, T 20, T max, activation energy (E a ) and frequency factor (ln A), were evaluated and are given in Table 4.The kinetic parameters were obtained by applying the methods of Broid's for each step of transition.The activation energy E a and pre exponential factor ln A data reveal that the reactivities of all the systems differ significantly as shown from the different values of activation energy.All the complexes show the least activation energies in the first stage decomposition and maximum in second stage decomposition.The values of pre-exponential factor ln A of complexes indicate that the decomposition reaction of the complexes with the ligand can be classified as a slow reaction 26 .The thermodynamic parameters, enthalpy ( H), entropy ( S) and free energy ( G) of activation were calculated using standard equations and the values are given in Table 5.The present complexes show positive enthalpy values for two steps degradation.The first step enthalpy values compared with second step show that, in the first step the values are low, which may be due to the fact that the metal-organic bond is weaker than the inorganic ligand-metal ion bond.The entropy values obtained are negative for first step degradation (except Co(II) and Ru(III) complexes) and they become progressively positive for second step of degradation.This indicates that on decreasing the size of the group in the complex, gain rotational and transitional freedom decreases and hence entropy increases progressively.The free energies of complexes in both the steps are positive.The above results clearly show that the basic steps in the thermal degradation are similar for all new complexes.

Table 1 .
Physical constants of ligand and its complexes

Table 2 .
Some important IR and 1 H NMR data of ligand and its complexes

Table 4 .
Temperature characteristics, activation energies and frequency factors of decomposition process of complexes

Table 5 .
Thermodynamic parameters for the thermal degradation of the complexes