Vol. 62
J. VONHOENE,R. G. CHARLES AND W. M. HICKAM
1098
TABLE I
0.14,Thioretio01
MOLECULAR WEIGHTOF COMBINED FRACTIONS
Run 4
Run 6
I
"n" value
Mol. wt.
Run 1
4 5 6 7 8 9 10
383 426 470 514 558
389 420 466 526 556
602 646
602
,
11
0 00
Number
Of
Oxyethyline
Units.
Fig. 2.
Fractions between the peak and the following trough when combined had the lower molecular weight, 605. It is probable that this difference is not significant and that our uncertainty in the molecular weight determination may be as high as 5%. Appreciable contamination of these fractions by the nearest neighbor compounds in the column would have led to the first group having a lower molecular weight. Figure 2 is a plot of mole fraction vs. number of oxyethylene units given by the Poisson distribution (points connected by a line which, of course,
690
-0bsd. valuesRun 4
Run 6
520
494
623-605 643 679
596 700
has no significance). Points obtained in two runs are plotted on the same graph and are in fair agreement with the theoretical values. The agreement is probably good enough for this chromatographic procedure to be useful in analytical applications. In recovering the compounds from solution it was found that the product was straw colored if the solvents were distilled off (the starting material was water white). Evaporation of solvents a t low temperature in a stream of nitrogen produced less color. Dissolving the higher n value samples in methanol showed that the samples contained a small amount of solid material, possibly silicic acid. This was removed by filtering the 5-10% solution through a fine glass frit and washing with two equal volumes of methanol. We plan to study some of the surface and solution properties of the pure compounds. We wish to thank Mr. W. Rinear and Dr. J. L. Rainey for the sample of pure OPE9.7 and Drs. I. Rosenthal and A. R. Weiss for instruction in the techniques of chromatography.
THERMAL DECOMPOSITION OF METAL ACETYLACETONATES MASS SPECTROMETER STUDIES BY JOANVONHOENE,ROBERT G. CHARLES^
AND
WILLIAMM. HICKAM
Westinghouse Research Laboratories, Pittsburgh 36, Pennsylvania Received M a y 9#1968
The heat stabilities of nine metal acetylacetonates, in the absence of oxygen, have been examined as a function of temperature. Those products from the decomposition which were gaseous a t room temperature were identified, and their quantities determined by means of a mass spectrometer. Acetone and carbon dioxide were found to be major decomposition products for all the acetylacetonates studied. For some of the chelates, acetylacetone and methane were also among the principal decomposition products. A number of other gases were found to be present also in smaller quantities. The order of decreasing heat stability for the acetylacetonates, based on the evolution of gaseous products, was found to be Na(1) > Cr(II1) > Al(II1) > Ni(I1) > Cu(I1) > Fe(II1) > Co(I1) > Co(II1) > Mn(II1).
An earlier paper described the relative heat stabilities of a series of metal acetylacetonates a t 191' in the absence of oxygen.2 I n the present work the decomposition of some of these compounds (formula I) has been examined in greater detail as a function of temperature. Those decomposition products, which are gaseous a t room temperature, have been identified and their quantities determined by means of a mass spectrometer. (1) Inquiries regarding this work ahould be addressed to this nuthor. (2) R. G. Charles and M a A. Pawlikowski, THISJOURNAL, 68, 440
( I 958).
Experimental Procedure The metal acetylacetonates used in this investigation were prepared, purified and dried as described reviously.2 Fifty-mg. samples of the compounds were sealecf a t a pressure of less than lo-* mm., into 10-cc. Pyrex glass containers
Sept., 1958
THERMAL DECOMPOSITION OF M E T A L
1099
ACETYLACETONATES
Methane Acetone
200
250
300
Acetylacetone
350
I50
200
250
300
350
0 400
Temperature of Pyrolysis ("C), Fig. 1.-Gaseous decomposition products from pyrolysis of metal acetylacetonates as a function,of pyrolysis temperature: solid lines, total products; dashed lines, principal components; measurements a t room temperature, duration of pyrolysis, 4 hr. (For clarity experimental points are given only for the total gas results.)
closed a t one end by a glass break-off tip. Final sealing of a sample under vacuum was accomplished by slowly collapsing the glass at the seal point during pumping. The sample remained a t room temperature during the pumping and sealing operations. The design of the sample containers used assured no sublimation of the acetylacetonates out of the hot zone duriGg the degradations. Similar sample containers constructed of Vycor glass were used for the degradations above 400". Each degradation run was carried out by placing the entire sample container into a preheated tube furnace, maintained a t constant temperature by means of an automatic controller utilizing a thermocouple sensing element. Although initially under vacuum, the samples a t the furnace temperatures were subjected to the pressures of the gaseous decomposition products. The pressures within some of the tubes, a t the higher temperatures, were 2 to 3 atmospheres. After the desired period of heating (usually four hours) the tube was removed and cooled rapidly to room temperature. Direct sealing of the sample container to the mass spectrometer vacuum system then permitted re-entry to the degradation products without contamination. This was accomplished by the dropping of a glass enclosed iron slug magnetically held above the break-off tip. The gaseous degradation products were, in this way, expanded into a known calibrated volume (usually 1000 ml.). The pressure of the expanded gas was determined by means of either a mercury manometer or the micromanometer associated with the ConsoIidated mass apectrometer, depending on the pressure. Considerable care was exercised to ensure proper mixing of the gases in order to obtain a representative sample. Empty sample containers processed in an identical manner to the true samples yielded a background of the order of only 0.1% of that obtained from any significant degradation of the metal acetylacetonates. The analyses of all samples
were carried out with the combined use of the Consolidated No. 21-103 mass spectrometer and the Datatron No. 205 electronic computer. Measurement of the pressure and volume of the expanded gas a t room temperature and the determination of the molecular composition of the gas mass spectrometrically, permitted the calculation of the total weight of each gaseous constituent. In the finaI analyses the results are expressed in terms of moles of gas evolved per mole of initial acetylacetonate taken.
Results and Discussion I n Fig. 1, An (the ratio of moles of gaseous decomposition product to moles of chelate taken) for the four-hour runs is plotted as a function of temperature for the principal decomposition products, and also for the total quantity of gas formed in each case. From Fig. 1 it will be noted that all the acetylacetonates studied decompose to some extent within the temperature range of 150 t o 400'. As a group, therefore, the acetylacetonates are less stable to heat than some other metal chelates that have been studied, for example, some of the phthalocyanines. The acetylacetonates are more similar in stabiIity to the Schiff base chelates reported by Marvel, Aspey and Dudley.5 It is interesting that
-
(3) Each point in Figs. 1 and 2 represents a separate experiment. (4) C. E . Dent and R. P. Linatead, J . Chem. Sac., 1027 (1934). (5) C.8. Marvel, S. A. Aaprey and E. A. Dudley, J. Am. Chem. Soc.,
'78, 4805 (1856).
1100
J. VOKHOENE,R. G. CHARLES AND W. M. HICKAM
Cr(I1I) acetylacetonate has been reported to distil unchanged at 340°.6 It is apparent from Fig. 1 that considerable decomposition will occur unless the distillation is done quickly. The temperature at which gas evolution first becomes appreciable for the acetylacetonates is dependent on the metal present (Fig. 1). It is possible to arrange these compounds in the approximate order of decreasing heat stability based on the temperature required to produce significant amounts of gaseous decomposition products. The order obtained is Na(1) > Cr(II1) > Al(II1) > Ni(11) > Cu(I1) > Fe(II1) > Co(I1) > Co(II1) > Mn(III).’J The heat stability order obtained here is in general agreement with the results of the earlier study2where the relative heat stabilities of a series of acetylacetonates were compared at 191’ in a nitrogen atmosphere. Also in agreement with the results obtained in the earlier study12no general correlation was observed here between the above heat stability series and other properties of the metal acetylacetonates or with properties of the parent metal ions. Since the rate of decomposition for the acetylacetonates might reasonably be expected t o change on going from the solid to the liquid phase, it was of interest to compare the results plotted in Fig. 1 with the reported melting points of these compounds (Table I). The temperature at which decomposition for the Fe(II1) acetylacetonate first becomes appreciable was found to nearly coincide with the melting point for this compound. This was not found to be the case for the other acetylacetonates studied, however, nor was any break in the curves of Fig. l noted a t the melting point for any of the compounds. The principal gaseous decomposition products for many of the acetylacetonates are acetone and carbon dioxide (Fig. 1). For the Co(II), Co(III), AI(II1) and Fe(II1) acetylacetonates, these are the only products formed, in large amount, throughout the temperature range studied. In the case of Ni(I1) and Cr(II1) acetylacetonates, large amounts of methane are found, in addition, at the higher temperatures. 9 The Cu(I1) and Mn(II1) acetylacetonates differ from the other compounds studied in that acetylacetone is the principal decomposition product observed for the lower temperatures. This fact suggests that the mechanism for the decomposition of these chelates may be different from that for the (6) G. Urbain and A. Debierne, Compt. rend., 129, 302 (1899). (7) In determining this order, the relative positions of those compounds which begin to decompose a t about the same temperature were established by comparing the A n values a t temperatures above the initial decomposition temperature. ( 8 ) It is recognized that the order given might be somewhat different if the amount of undecomposed acetylacetonate were chosen as the criterion of stability. (9) I n addition to the gases indicated in Figs. 1 and 2, a number of other gaseous decomposition products were found t o be present in smaller amounts. These include mesityl oxide, methyl acetate, butene1, propylene, propane, hydrogen, mesitylene and water. Numerical tables of the quantities of these gases (together with the data plotted in Figs. 1 and 2) have been deposited as Document 6719 with the A.D.I. Auxiliary Publication Project, Photoduplication Service, Library of Congress, Washington, D. C. A copy may be secured by citing the Document number and by remitting $1.25 for photoprinta, or 11.26 for 36 mm microfilm in advance by check or money order payable to: Chief Photoduplication Service, Library of Congress.
1701. 62
decomposition of the other acetylacetonates mentioned. Significant amounts of carbon monoxide and acetic acid also were found for the Cu(I1) acetylacetonate at the higher temperatures. A detailed discussion of the mechanism of the decompositions studied probably is not justified. It i s not known whether the gaseous products show11 in Fig. 1 are the products initially formed in the degradations. It is interesting, however, that two of the major observed products, acetone and acetylacetone, involve very little rearrangement of the atoms from the manner in which they occur in the acetylacetonate molecules. It may be significant that the production of methane at the higher temperatures, for the Ni(I1) and Cr(II1) acetylacetonates (Fig. 1) is accompanied by a decrease in the acetone content of the gas mixture. Methane has been identified‘O as one of the thermal decomposition products of acetone, although a t higher temperatures than those used here. An inverse relationship also exists between the acetylacetone and acetone concentrations in the gas evolved by the Cu(I1) and Mn(II1) chelates. This suggests that acetylacetone is formed initially but then decomposes at the higher temperatures to form acetone. It is possible that acetylacetone may be an intermediate in the production of acetone for some of the other acetylacetonates also. Acetylacetone was, in fact, found (although only in small quantities) for most of the other acetylacetonates studied. It may be significant that two of the major decomposition products found here, acetone and carbon dioxide, are formed also by the pyrolysis of some metal acetates.’lJ2 The latter compounds have the same atomic grouping I1 found in the metal acetylacetonates. CHs-C=
7
(11)
For two of the acetylacetonates, Fe(II1) and AI(III), a brief study was made of the heat decomposition as a function of time, at constant temperature. Temperatures were chosen at which the decomposition was appreciable in four hours. In the case of Fe(II1) acetylacetonate, both the total amount of gas and the individual components were calculated in the same manner as for the four-hour runs. For the Al(II1) chelate only the total amount of gas was determined. Results are given in Fig. 2. For the Fe(II1) chelate in Fig. 2, An for the total gas appears to approach a limit with time. The curve obtained is very similar to that found at 191’ by a different method.2 The principal products observed for the Fe(II1) acetylacetonate decomposition were acetone and carbon dioxide over the period 0 to 20 hours (Fig. 2). The value of An for total gas in Fig. 2 for Al(II1) acetylacetonate increases with time but does not appear to approach a limit within 20 hours. The gaseous products of the decomposition were in this case not analyzed. (IO) C. D. Hurd, “The Pyrolysis of Carbon Compounds,” A.C.S. Monograph No. 50, (Chemical Catalog Co.) Reinhold Publishing Corp., New York, N. Y.,1829, p. 248. (11) W.Kronig, 2. Anpew. Chem., 37, 667 (1924). (12) Ref. 10, p. 481.
I
THERMAL DECOMPCSITION OF METALACETYLACETONATES
Sept., 1958
1101
TABLE I APPEARANCE OF METALACETYLACETONATES HEATED4 HOURSin Vacuo M.p., "C. llt.
218
>230
Appearance of solid
Before heating
Dark blue crystalline powder Light green crystalline powder Dark purple crystalline powder
213
Dark green crystalline powder
194
Orangea crystals
184 216
Dark red crystals Maroon crystalline powder
184
Dark green powderl
After heating
Yellow-brown 250-275'; red-brown 300"; brown 350'; black 400' Brown and black crystals 251'; metallic Cu and brown residue 275-400' Unchanged 150-200'; amber 250"; charred 349-399' Amber 175-208"; charred, some green material 250-352" Charred, some pink material 175-220'; charred black mixed with green 250351" Unchanged 190'; amber 250-325'; brown 375" Unchanged 175"; amber and black 190-275' Unchanged 200'; maroon and charred 250350°, dark green liquid when hot; completely charred 399" Light brown 150-175'; dark brown, charred, 200-300 '
White crystalline powder
The color indicates a trace of iron(II1) acetylacetonate.
In Table I the appearances of the solid residues f roin decomposition are summarized. In general, the visual extent of decomposition parallels the amount of gas produced (Fig. l), decompositioii usually being accompanied by a darkening of the solid phase. For the Na(1) acetylacetonate, however, the appearance of the residues indicates that some decomposition has occurred in the temperature range 250-285' and t o a greater extent a t higher temperatures despite the fact that little gas is produced.13 This mould seem t o indicate that the principal products of decomposition are in this vase non-volatile. No attempt was made in the present studies to identify the components of the solid residues formed. What appeared to be metallic copper was visible from the decomposition of the Cu(I1) acetylacetonate. Metallic deposits of this type were not observed for the other compounds studied. Interesting color changes were observed for some of the residues. Cr(II1) acetylacetonate (or its tlocomposition products) was observed to give :L grceu liquid a t 250 to 350' which set to a maroon solid on cooling. The change was reversible a t 250O. Decomposition of the Co(I1) compound gave rise to some green material, perhaps indicating oxidation
*
(13) Significant amounts of tnethane were, however, produced at 4940.9
1.8-
-.
Hours Heated,
Fig. 2.--Gaseoris decomposition products from pyrolysis of metal acetylacetonates at constant temperature: solid lines, total products; dashed lines, principal components; measurements a t room temperature.
to Co(II1). Conversely, decompositioii of thc Co(II1) acetylacetonate gave 8ome pink material, possibly indicating some reductioii t o Co(I1). It is planned to study the residues obtained in more detail. Acknowledgments.-The writers are grateful to Mr. J. F. Zamaria, Miss I. Laminack, and Mrs. M. H. Loeffler, all of whom assisted in the experimental portion of this work.
