Thermal decomposition kinetics of Zn 11 chelates of substituted chalcones

The thermal decomposition of Zn 11 complexes 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' -hydroxynaphthyl)·2-propen-1-one (MPHPO), 3-(3,4· dimethoxyphenyl).l-(2'·hydroxynaphthyl)·2·propen-l·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.


Results and discussion
All the complexes are coloured powders which are insoluble in water. The elemental analysis of the chelates showed zinc to ligand ratios of 1 : 2. The complexes were found to be stable in air and non-hygroscopic. The final pyrolysis product of all the complexes corresponds to ZnO. The thermal stability data (Table 1) of the chelates reveal that the introduction of a -OCH 3 group increases the thermal stability, where introduction of a chlorine atom decreases the thermal stability. The enhancement thermal stability by the presence of electron releasing -OCH 3 groups at position 3 and 4 was ascribed to the availability of higher electron density at the reactive centre. The lower thermal stability of CPHPO may be attributed to the electron withdrawing effect of a chlorine atom, leading to lower electron density at the reactive centre. The relative thermal stability of the chelates is Zn(CPHP0). 2 < Zn(PHP0) 2 < Zn(MPHP0) 2 < Zn(DMPHPOh. Mathematical analysis of the TG curves was carried out using the differential Freeman-Carroll equation, the integral method using the Coats-Redfern equation and the approximation method using the Horowitz-Metzger equation.

Freeman-Carroll equation :
Freeman-Carroll equation which may be written in the torm.
where W, = W~-W, W~ is the mass loss at the completion of reaction, W is the mass loss up to time t, Tis 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 kcal moJ-1. W, and T can be directly obtained from the TG traces. The temperature slopes dW/dT were converted into time slopes dW/dt, using the relation where lj> is the heating rate. The usual first-order rate law expression can be written in the following to1m using the terrns Wand W, dW -=kWr dt combining this with the Arrhenius equation as' (e.u.) c' (kcalmol-1 ) as' (c.u.) c' (kcal mol-1 ) where k is the Boltzmann constant, h is the Planck constant and T, is the peak temperature from DTG. The free energy of activation G* was calculated using the following equa-tion4

Coats-Redfem equation :
lo Assuming the decomposition of Zn 11 chalcone chelates to follow first order kinetics (n = I) a plot of log ( ln[(Wj(W~-W))!T 2 ]} against 1/Twas drawn which gave straight lines in all cases with a slope of -E'!2.303R from which the activation energy was calculated.

Decomposition kinetics :
The analysis of data using the Freeman-Carroll equation gives the order of the decomposition reaction near unity for these complexes. The values of activation energy E* obtained by all three methods for the zinc chelates are given in Table 2 The kinetic parameters show a somewhat different trend from that of thermal stability. This is due to the fact the decisive criteria in kinetics are often quite different from those which decide thermal stability. ·

Experimental
The chalcones were prepared by the condensation of 2· hydroxy-l-acetonaphthone with benzaldehyde, chlorobenzaldehyde, methoxybenzaldehyde and dimethoxybenzaldehyde employing the Claisen-Schmidt condensation reported earlier 5 • The zinc complexes of chalcones were prepared by retluxing a toluene solution of zinc acetate and the ligand in I : 2 molar ratio, in the presence of sodium acetate {pH 6-7) for 'f hours. The precipitates were filtered, washed with toluene and dried in a vacuum desiccator over fused caJc:um 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 6 K min-1 • Typicai TG curves are presented in Fig. I