Thermal decomposition of cobalt ( III ) , nickel ( II ) , copper ( II ) , palladium ( II ) and platinum ( II ) complexes of N-allyl-N '-( 4 '-methylthiazol )-2 ylthiourea

Thermal decomposition of Co(III), Cu(II), Ni(II), Pd(II), and Pt(II) complexes of N-allyl-N'-(4'-methylthiazol)-2ylthiourea (AllMeTzTu), HL, have been studied by TG, DTG, and DTA curves. The complexes have the molecular formulae as CoL3.H2O, [Cu(HL)Cl2]0.5H2O and [CuL2(H2O)2] a square for ML2 (M = Ni II , Pd II and Pt II ), and [Pd(HL)Cl2]. The TG curves show that the four-coordinate complexes decompose in two stages to yield a free metal ,with exception [Pd(HL)Cl2] which gives PdS, as a residue, while the two six-coordinate complexes CoL3.H2O, and [CuL2(H2O)2] decompose in three stages to yield Co and Cu residues. The initial mass losses correspond to elimination of allylamine radical for all complexes in the same temperature range; and with MeTz when the temperature range extended beyond that range of the first stage for Pd(II) and Pt(II) complexes. Kinetic parameters (E # , n, ΔH # , ΔS # , ΔG # ) of the decomposition stages are determined and correlated with bonding and structural properties of the complexes.


Introduction
Thioureas are used as organocatalysts [1], vulcanization accelerators [2], pharmaceuticals [3], plant protection [4], silver polish [5], precipitation of heavy resins [8] electroplating corrosion inhibitors [9], and nylon and textile dyeing. However, increasing claims for probable human carcinogen upon inhalation, ingestion and dermal contact of thioureas or their toxic fumes (nitrogen oxides and sulfur oxides upon thermal decomposition may limit most of their applications in the near future. On the other hand, transition metal ions have proven advantageous in improving thermal stability, modifying thermal decomposition mechanism and products, clarity and the brightness of colour and effectiveness of organic ligands in their applications. The mechanisms of thermal decomposition of some thioureas and their metal complexes have been reported [10,11]. The present paper reports the thermal studies of Co(III), Ni(II), Cu(II), Pd(II) and Pt(II) complexes derived from N-allyl-N'-(4'-methylthiazol)-2ylthiourea. The kinetic parameters n, E # and A have been determined using Coats-Redfern method [12,13]. The other kinetic parameters ΔH # , ΔS # and ΔG # have been computed using standard equations.

Instrumentation
The TG, DTG, and DTA curves were obtained using a Shimadzu-50 thermal analyzer. The measurements were performed in dynamic nitrogen atmosphere at a heating rate of 10 C min -1 , using approximately 4-6 mg powdered samples contained in a platinum crucible. α-Al2O3 is used as a reference material.

Results and discussion
The thermal behavior of AllMeTzTu metal complexes was studied by using TG and DTA techniques from ambient temperature up to 800 ᵒ C in nitrogen flow, Figs. 1, 2, 3, 4, and 5. The stages of decomposition, temperature ranges, the temperature of the greatest rate of decomposition (DTGmax), the evolved products, as well as the found and calculated mass loss percentages of Ni(II), Pd(II), and Pt(II) complexes are given in Table 1.

NiL 2
[NiL2] is thermally stable up to 169 ᵒC and decomposes beyond this temperature in a two stages as indicated in the TG curve, Fig.1. The mass loss at 275 ᵒC corresponds to the formation of Ni(SCN)2 [15]. Beyond 275 ᵒC, a continuous mass loss in the TG curve has been observed up to 575 ᵒC which corresponds to the decomposition of Ni(SCN)2 to Ni. The DTA profile, Fig.8 , shows four endothermic peaks at 150, 166, 208 and 389 ᵒC and one strong exothermic one at 536 ᵒC. The two endothermic peaks at 150 and 166 ᵒC are in the temperature range of no mass loss on the TG curve and thus correspond to structure change and melting of the complex (M.p. of NiL2 in air = 161ᵒC), respectively. The latter two endothermic peaks at 208 and 389 ᵒC correspond to the decomposition of NiL2 to Ni(SCN)2 then to Ni, respectively. The strong exothermic peak at 536 ᵒC is due to dimerization of the evolved and intermediate species [15].

CoL 3 .H 2 O
The TG curve, Fig.2 , shows that the complex begins to decompose beyond 136 ᵒC in three steps. The first mass loss at 283 ᵒC corresponds to the formation of (MeTz)3Co(SCN)3. This is followed by two rapid mass losses in the temperature range 283-566 ᵒC, on the TG curve, collectively assigned to the thermal decomposition of (MeTz)3Co(SCN)3 to CoS(SCN) which decomposes beyond 715 ᵒC to CoS. The DTA curve, Fig.8, shows an endothermic peak at 143ᵒC corresponds to volatilization of uncoordinated water molecule and melting of the complex (M.p. of the complex in air = 142 ᵒC ); an exothermic peak at 248 ᵒC owing to thermal decomposition of CoL3 to (MeTz)3Co(SCN)3; an endothermic peak at 431 ᵒC due to decomposition of (MeTz)3Co(SCN)3 to CoS(SCN); and an exothermic peak at 592 ᵒC owing to decomposition of CoS(SCN) to CoS and dimerization of the evolved radicals [11,15].

[CuL 2 (H 2 O) 2 ]
The TG curve, Fig.4, indicates that the mass change begins at 138 ᵒC and continues up to 415 ᵒC. This corresponds to the formation of Cu2(C2N2)2(SCN)2 as an intermediate product. The next decomposition step occurs in the temperature range 415-673 ᵒC corresponding to the formation of Cu metal. The DTA curve, Fig.8, shows three endothermic and one exothermic peaks. The first endothermic peak at 201ᵒC corresponds to elimination of the two coordinated water molecules and the other two endothermic ones at 226 and 405 ᵒC are due to melting of CuL2 (M.p. of [CuL2(H2O)2] = 212ᵒC) and thermal decomposition of CuL2 to Cu2(C2N2)2(SCN)2, respectively. The strong exothermic peak at 527 ᵒC is due to decomposition of Cu2(C2N2)2(SCN)2 to Cu metal and dimerization of the evolved species.

[Pd(HL)Cl 2 ]
The TG curve, Fig.5, indicates only one step of decomposition beyond 175 up to 454 ᵒC. This corresponds to formation of PdS which remains stable beyond 454 ᵒC . The DTA curve, Fig.8, show three exothermic peaks at 317, 334.7 and 342454 ᵒC corresponding to decomposition of the complex and dimerization of the evolved radicals.

PdL 2
The TG curve, Fig.6, shows that the mass loss begins at 195 ᵒC in two steps. The mass loss at 283 ᵒC corresponds to the elimination of two allylamide and methyl radicals with the formation of an unstable intermediate formulated as

PtL 2
The complex is thermally stable up to 215 ᵒC, as indicated by the TG curve, Fig. 7, but beyond this temperature it decomposes in three steps. The first decomposition step is in the temperature range 215 -250 ᵒC which corresponds to the elimination of two allylamide and one methyl radicals with the formation of an unstable intermediate complex which decomposes in two successive steps (Table 1) eliminating TzSCN and MeTzCN radicals with the formation of Pt metal at 610 ᵒC. The DTA curve, Fig.8, shows two endothermic at 182 and 198 ᵒC with no weight loss in the TG curve, owing to crystal change and melting (M.p. in air =195 ᵒC) points of the complex; and one strong broad exothermic peak extended from 216 ᵒC to 270 ᵒC with a maximum at 234 ᵒC corresponding to the first decomposition step of the original complex and combination of the free radicals. Also, exothermic small peaks could be traced beyond this range due to thermal decomposition of the two successive steps.
In order to assess the influences of the mode of bonding of the ligand to the metal ions and the structural properties of the complexes on their thermal behavior, the order, n, and the heat of activation E # of the various decomposition stages were determined from the TG and DTG thermograms using the Coats-Redfern equations [12,13] in the following form: Where M = -E # /R and B = lnAR/Ф E # ; E # , R, A, and Ф are the heat of activation, universal gas constant, pre-exponential factor, and heating rate, respectively. The correlation coefficient, r, was computed using the least squares methods for different values of n, by plotting the left-hand side of the Eqs. 1 and 2 versus 1000/T. The n value which gave the best fit (r ≈ 1) was chosen as the order parameter for the decomposition stage of interest. From the intercept and linear slope of such stage, the A and E # values were determined. The other kinetic parameters, ΔH # , ΔS # and ΔG # were computed using the relationships: where k is the Boltzmann's constant and h is the Planck's constant. The kinetic parameters are collected in Table 2. The fol lowing remarks can be pointed out: The positive values of the activation entropies ΔS # indicate a less ordered activated complex than the reactants and the reactions are rapid [16].  are significantly different and consistent with charge -to radius ratio. However, the small difference of ΔG # values for the two complexes could be attributed to the structural rigidity of the remaining cobalt(III) complex versus structural flexibility of copper(II) compound at this stage of decomposition, as revealed by the low ΔS # value for copper(II) species (ΔS # =16.8 Jmol -1 ) and the high ΔS # value for cobalt(III) species(ΔS # = 285.6 Jmol -1 ).

(vi)
The reaction orders for all decomposition stages of all complexes are found to be nearly equal unity. It was emphasized that the order of a solid-state decomposition reaction has no intrinsic meaning, but is rather a mathematical smoothing parameter [17].