J.C S. Faraahy I, 1980,76,152-161Thermal Decomposition of Solid Chromium(II1)Tris-N-benzo yl-N-phenylhy dr oxylamineBY JOS6 C. MACHADO," MAURO M. BRAGA, ANA M. P. R. DA LUZ ANDGILLES DUPLATRE~Departamento de Quimica, Universidade Federal de Minas Gerais,30.000 Belo-Horizonte, BrazilReceived 6th February, 1979The thermal decomposition of the CrIII tris-N-benzoyl-N-phenylhydroxylamine chelate,Cr(BPHA)3, has been investigated using radiometric methods by labelling the complex with 'Cr.The decomposition is found to occur in the temperature range 16O-18O0C, according to an auto-catalytic process. Kinetic models are discussed which should account for the experimental evidence.Although the coordination chemistry of a large number of metal complexes iswell documented, most thermal studies on coordination compounds have beenof a rather qualitative nature and very little is yet known about the precise kineticsand mechanism of thermal decomposition of chelates in the solid state.The presentpaper is the first of a series that has the purpose of studying the thermal decompositionof some metal complexes with the N-benzoyl-N-phenylhydroxylamine (BPHA)ligand. This ligand forms complexes with many metals and has been widely usedas a versatile organic analytical reagent.lm4 With C P , the Cr(BPHA)3 chelate isobtained, in which the central metal atom is hexacoordinated by the six oxygenatoms of the l i g a ~ ~ d . ~ The thermal stability of some BPHA metal chelates has beenstudied previously.6 In the present paper, the kinetics of thermal decomposition ofCr(BPHA)3 are reported and kinetic models are discussed in order to elucidate someaspects of the decomposition mechanism.EXPERIMENTALP R E PA R AT1 0 N 0 F cr-L A B E LLE D Cr(BPHA)3CrC13 .6Hz0 from Merck was irradiated with thermal neutrons in the Triga reactor of theNuclebras at Belo-Horizonte to obtain the 51Cr radioisotope (26 d). The irradiated chloridewas dissolved in deionised water and chromium hydroxide precipitated before centrifugingand washing with water. After the hydroxide was dissolved in dilute HCl aqueous solution,the pH was adjusted to 4.5k0.2 with ammonium acetate. A 10 % excess BPHA alcoholicsolution was then added under continuous stirring for 2 h. The finely-divided green precipi-tate was subsequently filtered, washed with hot water and dried.The compound wasfurther purified by chromatography through a cellulose column, with benzene as eluent.After the solvent was evaporated, the complex was vacuum-dried overnight and ground topass 300 mesh.The elemental analysis on an inactive sample prepared under the same conditions asgiven above led to the following results : calculated content per molecule of Cr(C13H1002N)3in wt %: Cr, 7.55; C, 68.02; H, 4.39; N, 6.10; found: Cry 8.10; C, 66.32; H, 4.55;N, 5.88.7 Permanent address : Laboratoire de Chimie NuclCaire, Centre des Recherches NuclCaires,B.P.20,67037 Strasbourg, Cedex, France.15MACHADO, BRAGA, D A L U Z A N D DUPLATRE 153THERMAL TREATMENTS, SEPARATION AND COUNTINGThe thermal decomposition experiments were carried out using an oil-bath with a thermalstability within +0.2"C.The samples ( M 15 mg) were introduced into 10 cm long, 0.5 cmdiameter thin-walled Pyrex tubes which were immersed in the oil-bath. After heating, thetubes were rapidly cooled in water, at room temperature, before analysis.The non-gaseous stable products of the thermal decomposition of Cr(BPHA)3 have beenseparated in a chromatographic column with cellulose as the stationary phase and mixturesof solvents of various polarities (cycloliexane, benzene, chloroform, acetone, methanol andacetic acid) as eluents. The stable products which have been characterized by infraredspectroscopy after the chromatographic separation were : benzanilide, chromium(1n)benzoate and a further unidentified chromium product, in agreement with previous resultse6For the chromium containing fractions, in the systematic experiments, thin layerchromatography was employed as the analytical technique, using a Merck microcrystallinecellulose layer E 1 mm thick applied on a 10 x 20 cm2 glass plate.After heating, the sampleswere dissolved in a small volume of chloroform, then deposited onto the plate. Purebenzene served as solvent and the material was eluted twice. The decomposed fractioncontaining the metal ion did not move significantly, while the fraction related to the un-decomposed chelate was eluted with Rf M 1. After elution, the plate was air dried and thecellulose film was cut into ten 1.5 cm wide strips, perpendicularly to the direction of elution.Each of the trips was taken off for radioactivity measurements.The countings were made on the 320 keV photopeak of 51Cr, using a NaI(T1) well-typescintillator coupled to a multichannel analyser.The experimental procedures were systematically carried out in the dark becauseCr(BPHA)3 is photosensitive, especially when in solution.The average error for a givenexperiment is estimated as k0.02 in terms of the decomposed fraction percentage, a. Theresults presented arise from a single batch of the compound, but preliminary experimentsshowed no significant change, viz. within experimental error, to appear in the a against Tcurves when using samples from various preparations.RESULTS AND DISCUSSIONPreliminary isochronal experiments with heating times of 20 and 40min havebeen conducted, in order to determine the temperature interval for Cr(BPHA),thermal unstability.The results are shown in fig. 1 in terms of the decomposedfraction, a. The decomposed fraction a at time t and for temperature Tis definedas the ratio of the radioactivity associated with the Cr-containing decomposedproducts to the total radioactivity of the sample. Examination of the curves infig. 1 led us to choose the 160-180°C interval as most suitable for the study of thethermal decomposition.A series of isothermal experiments in this temperature range was hence run andthe resulting curves are shown in fig. 2. The a against t plots display the well knownsigmoid-shape characteristic of many solid state thermal decompo~itions.~The isothermal curves were first analysed on the basis of a unimolecular law :-ln(l-a-) = kt+c.(1)A typical plot of -In (1 -a) against t is shown in fig. 3(a). Obviously, a singlefirst-order process cannot account for the decomposition process. Rather, twosubsequent first-order processes or some more complex reaction involving an inductionperiod are to be considered.Several reviews have been concerned with solid state de~omposition.~-~ O Theseshow that for almost all of the solids, nucleation and propagation of the nuclei havebeen invoked for the mechanism of decomposition.ll Owing to the presence of a1 54 THERMAL DECOMPOSITION OF CHROMIUM(III) COMPLEX2 JT("CRG. 1.-Isochronal variations of the decomposed fraction, a, as a function of temperature, T, forheating times of: 0 , 2 0 min ; ., 40 min.I0 20 40 60 80t/minFIG.2.4sothermal variations of the decomposed fraction, a, as a function of time, t, for tempera-turesof: +,161.5; *, 167.4; 0,170.5; A, 171.4; V,173.5; v, 175.0; X , 177.0; A, 179.0"CMACHADO, BRAGA, DA LUZ AND DUBLATRE 155induction time in the experimental isothermal curves, a process involving the growthof nuclei that would favour the decomposition of the chelate molecules is an a prioripossibility. Simple laws have been suggested for the early stages of decompositionon this basis,' sueh as :a = Ct", (2)where C is a constant at a given temperature and n an integer.t/rninFIG. 3.-Variations as a function of time, t, of (a) -In (1-a) at 175.OoC, (6) a* at: e, 167.4;0,170.5; v, 175.0; A, 179OC.Fig.3(b) displays some typical plots of af against t showing that eqn (2) wouldhold in the present case, with n = 3 and for values of a such as 0.03 < a < 0.5.The physical meaning of eqn (2) with n = 3 would be either a nucleation proceedingat a rather low rate with a two-dimensional growth of the nuclei or a rapid increaseof the nuclei number associated with a three-dimensional growth. Obviously,eqn (2) is only meaningful for small values of a. A further step was to use the Avramiequation based on random nucleation :a = 1-exp(-Kt)". (3)This equation was fitted to the experimental data using a least-square fitting procedurewith a resulting rz value of 3.9 kO.1 for all isothermal curves, except for the two lowertemperatures : n = 4.8 at 167.4"C and n = 2.1 at 161.5"C.However, for the lattertemperature the U-values are all below 0.12 and hence have a large relative error. Avalue of n = 4 is expected on the basis of the Avrami equation supposing a constantrate, k l , for spontaneous nucleation per unit transformed volume leading to a linea156 THERMAL DECOMPOSITION OF CHROMIUM(III) COMPLEXincrease with time of the number of nuclei and a cubic increase with time of the volumeof the growth nuclei at a linear rate k2. Fixing the n parameter value to 4 led to theK values given in table 1 and the resulting fitting curves were not significantly differentfrom the solid lines in fig. 2. The K parameter should be expressed as :K E klk$ (4)where both k , and k2 should obey the Arrhenius law.An Arrhenius plot of K isshown in fig. 4(a) and a least-square fitting led to :K(s-l) = (9.8 x 1015) exp (-165 kJ rnol-l/RT).-10.0,-.y* -1.1.0- -d --1 2.02.20 2.24 2.2 8 2.32FIG. 4.-Arrhenius plots for rate constants : (a) K; (b) ks ; (c) kl.The treatment of the experimental data in terms of the Avrami equation thuspoints to a random nucleation followed by a three-dimensional growth of the nucleias representative of the thermal decomposition process of Cr(BPHA)3. But thenature of the nuclei is not disclosed by the kinetics. Isotropic growth of the nucleishould allow visual observation of the nuclei, at least in samples where the decom-position is well advanced.The observation of samples using a platinum heateMACHADO, BRAGA, D A LUZ A N D DUPLATRE 157coupled to a microscope showed the formation of small liquid regions, which diffuseslowly until the whole system is in the liquid phase. The formation of these smallliquid regions is initiated at E 161°C. Very strikingly, 161°C is the melting point ofbenzanilide,13 the major stable product of the composition of Cr(BPHA),. Thisexperimental observation suggests that the nuclei may consist in a second phasebuilt up by the molecules of benzanilide. The autocatalytic process of decompositionof Cr(BPHA), could hence be interpreted in terms of the formation of a liquid phase,in proportion as the chelate molecules are being decomposed in the initial solid phase,that would dissolve the complex thus giving way to a faster rate of decomposition.TABLE VALUES OF KINETIC PARAMETER K FROM THE AVRAMI EQUATION, FOR n = 4temperature K/lO-$ s-l temperature K/lO-$ s-l/"C /"C161 .-5 1.30 173.5 4.40167.4 2.33 175.0 5.43170.5 3.02 177.0 6.12171.4 3.45 179.0 7.37A simple reaction scheme would be :ksA, + B+C .. .kiA~ --, B ~ C . . .where A, and Al represent the original molecules A in the solid and in the liquidphase, respectively, and B and C are the products.Supposing product B is responsible for the creation of the liquid phase, the varia-tions of A with time may be expressed, as suggested by Bawn,' by making use of themolecular solubility s :where A and B are the numbers of molecules of A and B, respectively.symbols A(t) etc., have been avoided for the sake of clarity] :Thus [theNow : Al + A , + B = A,, the initial number of undecomposed molecules, so :dA- = - k,[A, - B(s + l)] - k1sB.dtDividing by A , and noting that a = B/Ao = 1 - (A/&) :(7)dadt - = k,+&[k,s- k,(s+ l)]158Integrating eqn (8) leads to :THERMAL DECOMPOSITION OF CHROMIWM(III) COMPLEXwith D = k,s-k,(s+ 1).(10)Obviously, eqn (9) is only valid for A, > 0 ; as soon as A, = 0, a11 molecules of Aare dissolved :4 A s = - = -B B'The limiting value of a for eqn (9) is thus :and the time needed for the complete dissolution of the chelate is given byFor a > alim, only reaction (11) is occurring and :a = [-k1(t-tdI} +slim* (13)Eqn (9) and (13) have been least-square fitted to the data and led to a mean value ofs = 1.05+0.1 for all curves.Fixing the value of s as 1 led to the parameters dis-played in table 2 and the corresponding calculated curves axe drawn as solid lines infig. 2 for each temperature. Although the chi-square value was lower using eqn (9)and (13) than using Avrami quation, the resulting calculated curves are not signifi-cantly different.TABLE 2.-mETIC PARAMETERS DEDUCED USING EQN (9) AND (13) FOR S = 1161.5167.4170.5171.4173.5175.0177.0179.00.631.071.331.752.022.443.263.860.491.131.331.451.932.252.563.09The least-square fitting of the Arrhenius plots of the k, and kl parameters givenin fig.4(b) and (c), respectively, led to :k,(s-l) = (4.4 x 1015) exp (- 174 kJ mol-l/RT)k,(s-l) = (9.6 x 10l6) exp (- 169 kJ rnol-l/RT)MACHADO, BRAGA, DA LUZ AND DUPLATRE 159Although not common behaviour for inorganic or metal chelate compounds,thermal decomposition accompanied by the formation of a liquid phase and aconsequent increase in the reaction rate is not unusual in organic solids, as discussedby Bawn in ref. (7). Preliminary experiments show that if a small amount ofbenzanilide is mixed with the Cr(BPHA), crystallites prior to heating, a pronouncedacceleratory effect of the thermal decomposition is promoted. Furthermore, iftwo samples consisting of the chromium chelate and of pure benzanilide, respectively,are deposited on the microscope plate close to each other, it is observed that themelting of the benzanilide is followed by the immediate dissolution of the Cr(BPHA)3sample into the liquid phase.These facts lend support to the mechanism suggestedabove. However, the frequency factor related to kl, although certainly tainted with arather large error of very possibly more than an order of magnitude, would appear toohigh for a process in a liquid phase [but see Bawn in ref. (7)]. In addition, preliminaryresults on Fe(BPHA), indicate the formation of one molecule of benzanilide perdecomposed molecule of the chelate. If this were the case for Cr(BPHA)3, thevalue s = 1 found when fitting eqn (9) and (13) to the data would imply that a unitcell of the " liquid " phase would be defined by a single molecule of benzanilide.Inthis case, the definition of a " liquid " phase would hardly hold on a microscopicscale, at least in the initial stage of the decomposition. The physical situation mightrather be described as the molecule of benzanilide influencing the immediate sur-rounding undamaged molecules in such a way that one of the latter could decomposeat a higher rate than in a normal lattice site of the original solid.The important finding remains that some molecule(A)-molecule(B) interactionseems to be responsible for the acceleration of the decomposition as time proceeds.Supposjng the benmnilide molecules possess an important characteristic of liquidbehavisur, namely a high mobility, so that they can move freely in the lattice, theautocatalytic process may be described by the reaction scheme :Then :k3 A + B+C...k4 A+B 3 2BfC.. ..Dividing by A*, the initial number of undecomposed molecules, and noting thata = (B/A) = 1 -(A/&) leads toda - = k3(l -a)+k4Aoa(l -a). dtIntegrating eqn (15) gives :with CT = k3 +k4Ao.Fitting eqn (16) to the data led to the parameters recorded in table 3. Thecalculated curves were not very different from the solid lines in fig. 2, but the cor-responding chi-square values were higher than those found when using eqn (9) and(13). Further, the values at 167.4 and 1615°C for k3 and at 161.5"C for k4Ao ha160to be discarded in the Arrhenius plots (these are not reported). The least-squarefitting of these plots gave :k3(s-l) = (2.1 x 1O2O) exp (-200 kJ mol-l/BT)ksAo(s-l) = (8.4 x 1014) exp (- 149 kJ mol-lIRT).THBRM A L DE c o M P o SI TI o N o F c HR o MI UM ( I 11) c o MP LE xIn spite of the rather rough approximation made when supposing the benzanilidemolecules are free to move in the lattice even at the early stage of decomposition,the preceding treatment would tend to confirm the important role these moleculesplay in the acceleration of the decomposition rate.Owing to the vicinity of thebenzanilide fusion point in the temperature range where the decomposition occurs,the niolecules of benzanilide produced at the early stage of the decomposition mayprovide a larger free volume at the lattice site where they occur, thus favouring thedecomposition of neighbouring Cr(BPHA), molecules.TABLE 3.-KINETIC PARAMETERS DEDUCED USING EQN (16)ternperaturel'c k3/10-6 s-l k4Ao/10-3 s-l161.5167.4170.5171.4173.5175.0177.0179.05.922.055.137.857.909.9714.217.20.531.721.982.1 12.853.503.774.53Visual observatioii indicates that a genuine liquid phase is being formed.Sincethe latter two models used strongly suggest that the autocatalytic reaction proceedsvia a single benzanilide molecule, it is not excluded that this molecule does provide asufficient free volume for one neighbouring Cr(BPHA), molecule to decomposepractically at the same rate as it would in the genuine liquid phase. On this assump-tion, no change would be expected in the kinetics even if a liquid phase were indeedbeing built up by the benzanilide molecules.After all, this seems a likely possibilityin a system for which the molecular solubility would effectively be unity.Regarding the kinetics, the use of the concept of nucleation via the Avramiequation is undoubtedly quite attractive, owing to its large flexibility to account forthe variable kinetics that may be encountered with the various solids. However, inthe present case it seems that the nuclei effectively consist of molecules of one of theproducts of decomposition, the benzanilide.This investigation was supported by grants from Conselho de DesenvolvimentoCientifico e Tecnol6gico (CNPq) and Financiadora de Estudos e Projetos (FINEP).We thank both referees for their helpful remarks.S . C. Shome, Analyst, 1950,75,27.0. A. Vita, W. A. Levier and E. Litteral, Analyt. Chim. Ada, 1968, 42, 87.I. P. Alimarin, F. P. Sudakov and B. G. 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