Indian Journal of ChemistryVol. 20A, May 1981, pp. 453-455

Kinetics, Mechanism & Synthetic Applications of ThermalDecomposition of Cu(II) Chelates of Acetoacetanilide &

01- Chloroacetoace tanilideN. THANKARAlAN*. P. SREEMAN & T. D. RADHAKRISHNAN NAIR

Department of Chemistry. University of Calicut, Kerala, 673635

Received 17 June 1980; revised 22 September 1980; accepted 10 November 1980

Considerable difference has been observed in the thermal decompositions of Co(Il) chelates of acetoacetanilide,and e-chloroaeetoacetauilide, This difference has been explained on the basis of intramolecular N-H ... CI hydrogenbonding present in the latter complex. Both the intermediate and final products of thermal decomposition have beenidentified, and are in agreement with the proposed mode of decomposition. Controlled pyrolysis of Cu(II) chelatesof acetoacetanilides has been found to be an excellant method for preparing the corresponding phenyl isocyanates.Kinetic study has revealed that Mampel, and Avrami equations govern respectively the rates of decomposition ofacetoacetani- lide and cx-chIoroacetoacetaniIide chelates.

THERMAL analysis has been used to study thestability of metal complexes'. However,kinetics and mechanism of only a few of the

solid state reactions have been investigated=". Directcorrelation between thermal and solution stabilitieswas not observed for certain very stable metal chelates,presumably because their decomposition did notcommence with the scission of the metal-ligandbond(s)4'5. Although thermal studies can reveal thedifferences in strengths of metal-ligand bonds ofunsymmetrically constituted chelating ligands, nosuch study has been reported. An attractive aspectof thermogravimetry, namely its synthetic advantage,remains unexploited for metal chelates. The presentstudy is an attempt to examine all these aspects forCu(II) chelates of acetoacetanilide and a.-chloroaceto-acetanilide.

In our earlier studies on metal chelates of variouslysubstituted acetoacetanilides, we found that althoughstability in general decreased with increasing electron-withdrawing character of the substituent. the acetoacet-anilides containing a halogen on the «-carbon behavedanomalously". Intramolecular hydrogen bondingbetween the NH hydrogen and the ex-halogen seemedresponsible for this abnormal behaviour. Unless heldby the hydrogen bond, the NH hydrogen (structure I)would migrate tautomerically to the amide oxygen

(J)

(

(amide resonance would facilitate this transfer")thus weakening the amide oxygen-metal bond. Sincechelate resonance is; expected to be weak, the twometal-oxygen bonds of the same ligand moietywould anyway be of different strengths. It was ofinterest to examine whether thermal method couldindicate this difference in bis(acetoacetanilidato)-copper (II), [Cu(acacN)a]' and bis(ex-chloroacetoace-tanilidato )copper(II), [Cu(e<-ClacacN)£].

Materials and MethodsThe complexes were prepared as reported

elsewhere". Therrnograms were obtained (heating rate4 °C min-I) on a Stanton recording thermobalance,model TR-l. For preparing aryl isocyanates, thecomplexes were heated at 200-250 °C in a microdistillation apparatus, and the condensate collected.

Results and DiscussionWhereas decomposition of the acetoacetanilide

complex commenced at 205°C, that of the «-chloro-acetoacetanilide complex started only at 225°C, inair. In the decomposition of [CutacacNj.], twodistinct stages could be observed (Fig. 1). Throughindependent pyrolytic studies it was shown that inthe first stage of decomposition, phenyl isocyanate isremoved. The remainder of the complex decomposedbetween 390 and 500°C, leaving copper(II) oxideas the residue. In nitrogen atmosphere, decomposi-tion of the complex commenced at slightly highertemperatures (225 and 240°C respectively), butother characteristics of the thermo grams remainedpractically unchanged. It is thus evident that instatic air also it is pyrolysis and not oxidation thatoccurs.

On the basis; of the main contributing structure ofacetoacetanilide complex, (I), the features of thethermogram can be explained adequately as follows:The weaker (coordinate covalent) bond (further

453

INDIAN J. CHEM., VOL. 20A, MAY 1981

weakened by tautomerism attending amide resonance)breaks first followed by breakage of C~C singlebond to eliminate phenyl isocyanate". Migration ofthe NH hydrogen to IX-carbon in this process givesthe intermediate product (II). .

(Ill

which then decomposes at a higher temperature.The mass changes at the two stages agreed fully withthis explanation. The DT A curve showed twoexothermic peaks at 245 and 335°C. Infrared spect-rum of the crude intermediate product in KBr showeda weak band at 3080 cm? (olefinic vC-H), anotherweak band at 2910 orrr? (methyl vC-H), a strong andbroad band extending from 1620 to 1350 cm" (vC=C+oC-H), and two medium intensity bands at740 and 685 cnr? (coupled Cu~O~C vibration-").Thus, the infrared spectrum of the intermediateproduct is compatible with the proposed structure(II). It appears, therefore, that it is the enolate oxygenthat finally ends up as the oxide of copper. X-raydiffraction patterns of the final produts obtained inair and in nitrogen atmosphere were identical withthat of authentic copper (II) oxide sample. That thetwo copper-oxygen bonds of acetoacetanilide moietyare of different strengths, thus seems substantiated.

The thermogram of [Cu(IX-ClacacN)J is consider-ably different from that of [Cu(acacN)z] (Fig. 1).Although, decomposition started at a higher tem-perature, the Iigands were removed in a single acce-lerated step between 225 and 330°C. The presenceof electron-withdrawing substituent could havehastened the decomposition of the intermediateproduct, and hence given the semblance of a singlestep. However, since only one sharp exothermicpeak is observed in the DT A curve (at 275°C), whatappears more probable is that the IX-chlorosubstituenthas weakened the otherwise stronger enolate oxygen-metal bond, and strengthened the otherwise weakamide oxygen-copper bond, so that both the metal-oxygen bonds are now somewhat similar in strength,and hence the whole ligand moiety is removed in onestep.

Synthesis through pyrolysis ~ As mentioned earlier,controlled pyrolysis of the acetoacetanilide complexyielded phenyl isocyanate, which was identified fromits boiling point and conversion to diphenyl urea(m.p.P 241°C) The purity and high yield of theisocyanate obtained prompted us to investigatefurther the general applicability of preparing arylisocyanates by this method. Three substituted aceto-acetanilide complexes of copper(II) were examinedand the method was found to be successful. Theoperational simplicity which gives the product inhigh yield and purity (Table 1) adds to the syntheticimportance of the procedure. The product in each.case was identified from its physical constants-".

454

(

0-0 Copper acetoacetanilide c~\a~.tr-6 Copper oc_chloroaceto-

acetanilide chelate

100

90

80

70

0\E 60~a.E0 50U>

<i.,'" 400~

30

20

10

o tOo :WO 3'00 400 500 6'00 700TOc

Fig. 1 - Thermograms of copper acetoacetanilide and coppera:-cWoroacetoacetanilide chelates.

TABLE 1- PYROLYSIS OF ACETOACETANILIDE CHELATES OF Cu(II)

Ligand Product b.p.(yield, %) CC)

Acetoacetanilide Phenyl isocyanate 165(83)

Acetoacet-z-methyl- 2-Methylphenyl 85/27 mrnanilide isocyanate

(80)Acetoacet-4- 4-methylphenyl 84/24 mm

methylanilide isocyanate(85)

Acetoacet-4- 4-methoxyphenyl 72-73/20 mmmethoxyanilide isocyanate

(70)

Kinetics of thermal decomposition ~ The dynamicthermogravimetric data were subjected to analysisby the mechanism invoking Parabolic law, Jander,Ginstling-Brounshtein, Mampel and Avrami equa-tions''?", and the mechanism non-invoking Coats-Redfem-! equation. In the mechanism based ana-lysis, limited subsequently to Mampel and Avramiequations (since these alone gave reasonable para-meters), appropriate functional forms of fractionaldecomposition, g(lX), were substituted in theMacCallum-Tanner equation, as demanded by thespecific mechanism based equation. Values of logg(lX)were plotted against 103fT (Fig. 2) and thebest linear fits were examined further in the lightof the kinetic parameters obtained from mechanismnon-invoking general kinetic study to arrive at theprobable operating mechanism. These results inbrief are given in Table 2.

Activation energy for the first stage of decompo-sition of the acetoacetanilide chelate, is aboutdouble that for the single step decomposition of

\

THANKARAJAN et al. : THERMAL DECOMPOSITION OF Cu(TI) CHELATES

10~1.76 1-78 1·80 HI2 1-84 I.

-0.48-052

\\-0.56"'-0600> .2-0.64

-0.68-0.72-o.76L- -"-_--'

Fig. 2 - Plots of liT against log g(lX) and liT against

log [( In w:~ )fT2]

10~_5.219i"'8~-"T"-~-=-2.?"02•...•...-=-2.~04:!...,-..••2.~06"-r--"2.:;q08

54 COAT5-REDFERNPL-r--. - . for OT~ -5.6 Cu{acacN)2

i~-58i1~-6.0c2..-6.2

'"2

l.n-50

c~2' -56

10~1.98 2·0 202 2·04 2·06 2.08

-0-0.2

i!-0.4";;,-0.6

F-0.81.0

10Yr182 1-84 1-86 HI8 1-90

AVRAMI EQUATION[Cu(oc-ClacocNI2]

CompoundCoats-Redfern equation

TABLE 2 - KINETIC PARAMETERS OF THERMAL DECOMPOSITION

Mechanistic study

Ea {:;S*(kJ molr") (J deg 1mot=)

[Cu (acacN)21 (Stage 1)[Cu (IX-ClacacN)21

172.392.5

53.4-131.8

g( IX) = -In (1 - 11)for [Cu (acacN)21. and g( IX) = 1- (l-1X)1/3 for [Cu (IX-ClacacN)21

6.3 X io»1.5 X 106

z Ea(kJ mor ')

164.192.1

z Type ofequation

4.4 x loa5.2 x 105

MampelAvrami

cx-chloroacetoacetanilide chelate, although decom-position of the latter commences at a higher tempera-ture. This shows that there need not necessarily beany direct correlation between activation energy andthermal stability>, particularly when the mechanisticmodes also differ. The change of entropy from,....,50 J deg" mol " for the acetoacetanilide chelateto ,.., ~130 J deg? mol-1 for «-chloroacetoacetani-lide chelate would mean that the activated compJexof the latter has a far more ordered configuration thanthat of the former.

This investigation points also to the undesirabilityof inferring probable mechanism from formal simi-larity of metal chelates,

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I

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