Thermal Decomposition Kinetics of Ni(II) Chelates of Substituted Chalcones

CODEN ECJHAO E-Journal of Chemistry Vol. 1, No. 2, pp 105-109, April 2004 http://www.e-journals.net Thermal Decomposition Kinetics of Ni(II) Chelates of Substituted Chalcones K.G.MALLIKARJUN Department of chemistry, Jawahar Navodaya Vidyalaya Peddapuram – 533 437, East Godavari, (A.P.) India. Received 22 February 2004; Accepted 10 March 2004 Abstract The thermal decomposition of Ni(II) complexes of 3-(phenyl)-1-(2’hydroxy-naphthyl)-2-propen-1-one (PHPO), 3-(4-chlorophenyl)-1-(2’-hydroxynaphthyl)-2-propen-1-one(CPHPO), 3-(4-methoxyphenyl)-1-(2’-hydroxynapthyl)-2propen-1-one (MPHPO), 3-(3,4-dimethoxyphenyl)-1-(2’-hydroxynaphthyl)-2propen-1-one(DMPHPO) was studied by thermogravimetry. Mathematical analysis of the data has allowed us to determine various parameters using Freeman-Carroll equation, the integral method using the coats-Redfern equation and the approximation method using the Horowitz-Metzger equation. The trend of the kinetic parameters was found to be different from that of the thermal stability order. The low values of Z suggest the slow nature of the reaction. Keywords: Thermogravimetry, Chalcones, Nickel Compounds, Frequency factor, Introduction Very few systems have been reported1 showing the relationship between thermal stability of metal chelates and structure of the chelating agents. Wendlandt and co-workers2-5 and Hill and co-workers6,7 studied the thermal properties of metal chelates with different types of complexing ligands. Structural studies on several metal chelates of β-diketones and 2-hydroxycarbonyl compounds have been reviewed by Holm and O’ Connor.8 The metal chelates of divalent transition metals with some o-hydroxychalcones standout as a distinct class of o-hydroxycarbonyl compounds with low spin square-planar configuration, which do not easily form adduct. Extensive conjugation was found to be responsible for the strong field nature of the ligand.9 Chalcones usually exhibit germicidal10 bactericidal,11 fungicidal12 and carcinogenic activities.13 In continuation of our earlier work14-19 on thermal decomposition kinetics of metal chelates, the present investigation deals the thermal stability of the Nickel chelates of 3 - (phenyl) -1 - ( 2’hydroxynaphthyl ) – 2 – propen -1 –one (PHPO) , 3 - ( 4 – chlorophenyl ) - 1- (2’- hydroxynaphthyl ) 2 – propen -1 –one (CPHPO), 3 - ( 4 – methoxyphenyl ) – 1 - (2’- hydroxynapthyl ) - 2-propen -1- one (MPHPO), 3-(3,4-dimethoxyphenyl)-1-(2’-hydroxynaphthyl)-2-propen-1-one (DMPHPO) and evaluation of kinetic parameters employing the differential Freeman-Carroll equation20 the integral method using the Coats-Redfern equation21 and the approximation method using the HorowitzMetzger equation.22 106 K.G.MALLIKARJUN Experimental The chalcones were prepared by the condensation of 2-hydroxy-1-acetonaphthone with benzaldehyde, chlorobenzaldehyde, methoxybenzaldehyde and dimethoxybenzaldehyde employing the ClaisenSchmidt condensation reported earlier.23 The copper complexes of chalcones were prepared by refluxing a toluene solution of nickel acetate and the ligand in 1:2 molar ratios, in the presence of dilute ammonia (pH 8.5-9.0) for 1h. The precipitates were filtered, washed with toluene and dried in a vacuum desiccator over fused calcium chloride. The purity of the sample was checked by elemental analysis. The thermograms were recorded using a Perkin-Elmer TGS-2 thermo balance in ambient air and at a heating rate of 6K min-1. Kinetic data were evaluated from TG traces using the equations noted in Table-1. Table 1. Kinetic data Chelate Parameters From FreemanCarroll equation From CoatsRedfern equation From HorowitzMetzger equation Ni(PHPO)2 E* (kcal mol-1) Z(s-1) ∆S* (e.u.) G*(kcal mol-1) Kr (Su-1) 2.84 5.63 X 10-2 - 29.49 16.69 5.61 X 10-2 2.27 2.49 X 10-3 - 33.08 18.41 2.43 X 10-3 8.95 6.04 - 25.19 21.24 6.01 Ni(CPHPO)2 E* (kcal mol-1) Z (s-1) ∆S* (e.u.) G* (kcal mol-1) Kr (Su-1) 4.79 2.54 X 10-1 - 28.25 18.33 2.51 X 10-1 3.19 8.07 X 10-3 - 32.16 18.66 7.93 X 10-3 12.24 96.18 - 23.06 17.10 95.49 Ni(MPHPO)2 E* (kcal mol-1) Z (s-1) ∆S* (e.u.) G* (kcal mol-1) Kr (Su-1) 7.14 1.01 - 26.14 20.29 0.99 11.18 2.98 X10 - 23.75 23.13 28.70 13.95 3.76 X 102 - 21.92 24.98 3.69 X 102 Ni(DMPHPO)2 E* (kcal mol-1) Z (s-1) ∆S* (e.u.) G* (kcal mol-1) Kr (su-1) 8.69 3.16 - 25.37 21.89 3.13 14.06 4.12 X 102 - 21.22 25.06 4.09 X 102 16.04 44.57 X 102 - 18.92 25.82 44.35 X 102 Results and Discussion All the complexes are coloured powders which are insoluble in water. The elemental analysis of the chelates showed nickel to ligand ratios of 1:2. The complexes were found to be stable in air and nonhygroscopic. The final pyrolysis product of all the complexes corresponds to NiO. The relative thermal stability of the chelates is (Table 2 ) Ni(CPHPO)2 < Ni(PHPO)2 < Ni(MPHPO)2 < Cu(DMPHPO)2 . Mathematical analysis of the TG curves was carried out using the differential Freeman-Carroll equation, the integrate method using the Coats-Redfern equation and the approximation method using the Horowitz-Metzger equation. Thermal Decomposition Kinetics of Ni(II) Chelates 107 Table 2. Thermal decomposition data Chelate Decomposition Temp. (0C) Ni(PHPO)2 Ni(CPHPO)2 Ni(MPHPO)2 Ni(DMPHPO)2 215 208 230 244 Residue (Percentage) Order of reaction Theoretical Experiment Freeman-Carroll Metal Oxide Method 10.04 9.50 9.11 8.31 12.78 12.08 11.59 10.57 13.21 12.56 11.35 12.04 1.09 1.14 1.18 1.11 FREEMANN-CARROLL EQUATION Freeman-Carroll equation which may be written in the form. * −1 ∆ log ( dW / dt ) − ( E / 2.303R ) ∆ (T ) = +n ∆ log Wr ∆ log Wr Where Wr = Wα – W, Wα is the mass loss at the completion of reaction, W is the mass loss up to time t, T is the absolute temperature at time t, n is the order of reaction. R is the gas constant in calories and E* is the energy of activation in K cal mol-1. Wr and T can be directly obtained from the TG traces. The temperature slopes dW / dT were converted into time slopes dW / dt, using the relation23 dW dW dT  dW = ⋅ = dt dT dt  dt  φ  where φ is the heating rate. The usual first-order rate law expression dW = k (a − x) dt can be written in the following form using the terms W and Wr dW = kWr dt Combining this with the Arrhenius equation K = Z exp ( - E* / RT) We obtain  dW / dt  E* log  + log Z =− 2.303RT  Wr  Plot of log [ ( dw /dt) /Wr] against T -1 were drawn. They gave straight lines in all cases with slopes – E* /2.303R from which E*values were obtained. Z was calculated from the above equation and the entropy of activation ∆S* was obtained from the relation24 ∆S* = 2.3.3R log (Zh / kTs) Where k is the Boltzmann constant, h is the Planck constant and Ts is the peak temperature from DTG. The free energy of activation G* was calculated using the following equation 25 G* = E* - Ts ∆S* Kr = Z exp ( - E* / RTs) 108 K.G.MALLIKARJUN COATS-REDFERN EQUATION W∞    W −W   ZR  2 RT   E* log  ln ∞ 2  = log  1− − *  *  T E   2.303 RT   φ E (...truncated)