Thermogravimetric studies of metal β-diketonates

J. lnorg, nu¢l. Chem.. 1967,Vol. 29. pp. 1931to 1936. PergamonPress Ltd. Printed in Northern Ireland

THERMOGRAVIMETRIC STUDIES METAL /~-DIKETONATES*

OF

KENT J. F~ISENTRAUT a n d ROBERT E. SIEVERS Aerospace Research Laboratories, ARC, Wright-Patterson Air Force Base, Ohio 45433

(First received 11 January 1967; in revised form 6 March 1967) Abstraet--Thermogravimetric analysis (TGA) is applied to the rapid determination of the relative volatilities of various metal fl-diketonate chelates. Differences are seen in the volatility of a series of thermally stable tris(2,2,6,6-tetramethyl-3,5-heptanedionato) rare earth chelates. The trend in volatility of the rare earth chelates is related to the size of the trivalent rare earth ion and is independent of chelate mass. The order of increasing volatility as seen from the thermograms of the volatile rare earth chelates parallels the order of increasing ease of elution of these chelates from non-polar gas chromatographic liquid partioning phases. Some of the chelates are considerably more volatile than alkane hydrocarbons of considerably lower carbon number. Volatilities of metal fl-diketonates of Cr(III), Fe(IID, Rh(III), Al(llI), Na(I), Zr(IV), Sc(III), Y(III), and the trivalent lanthanides are compared. Differences in the volatilities of metal chelates, arising from substitution in the chelate ligand, are illustrated for metal chelate derivatives of various fl-diketone ligands. INTRODUCTION

I t HAS not been generally appreciated that thermogravimetric analysis (TGA) can be a powerful tool in comparing the relative volatilities and thermal stabilities of many different metal fl-diketonate chelates. By examining the thermograms, it is possible to see volatility changes upon substitution of various chemical groups in the 1, 3, and 5 positions of fl-diketone ligands chelated with various metals. The TGA results can then be used to predict the relative gas chromatographic elution behaviour of some of the volatile metal chelates. In general, metal chelates that show greater volatility as indicated by their thermograms are eluted from gas chromatographic columns containing non-polar liquid partitioning phases prior to those chelates whose thermograms indicate lesser volatility. In the present study the TGA technique is used to determine the relative volatilities of a series of tris rare earth chelates of 2,2,6,6,-tetramethyl-3,5-heptanedione and a number of other volatile complexes. The thermal stability of the tris lanthanide dibenzoylmethide complexes,t1'2~ the divalentt3-6~ and trivalent ~5'61 metal 8-quinolinolate chelates, the tris lanthanide 8-quinolinolates, tTI divalent metal chelates of thiothenoyltrifluoroacetone,c8) metal benzohydroxamates,tg~ various metal acetylacetonate chelates, tl°'m and europium * Presented in part at the 19th Annual Summer Symposium on Analytical Chemistry, Edmonton, Alberta, Canada, June 24, 1966. ~1~R. G. CHARLESand A. PErtrtoTro, J. inorg, nucL Chem. 26, 373 (1964). ts~ R. G. Cr~RLES, J. inorg, nueL Chem. 26, 2195 (1964). c8~R. G. CHARLES,A. PERROTTOand M. A. DOLAN, J. inorg, nucL Chem. 25, 45 (1963). ~4~R. G. CHARLES, J. inorg, nucL Chem. 20, 211 (1961). ~6~1L G. CHARLESand A. LANGrR, J. phys. Chem. 63, 603 (1959). t~ R. G. CHARLES,Analytica chim. Acta 31, 405 (1964). m R. G. CHARLESand A. PERROTTO,Analytica chim. Acta 30, 131 (1964). ~8~E. W. BERG and K. P. REED, Analytica chim. Acta 36, 372 (1966). c9~ L. N. LAPAT~CK, J. F. HAZEL and W. H. McNAEB, Analytica chim. Acta 36, 366 (1966). ~10~R. G. Cr~RLES and M. A. PAWLIKOWSrd,J. phys. Chem. 62, 440 (1958). m~ W. W. WENDLANDT,J. L. BEAR and G. R. HORTON,J.phys. Chem. 64, 1289 (1960). 1931

1932

K~rrr J. E[S~mRAtrr and ROBERT E. Smw~s

thenoyltrifluoroacetonate tm have been investigated using thermogravimetric techniques. Thermolysis curves have been reported for many other metal compounds, c13.x4~ In view of the fact that very little TGA work has been reported for a wide class of volatile metal fl-diketonate chelates, a study of this nature was undertaken. The effect of including different moieties in the 1, 3, and 5 positions of fl-diketonate complexes of a representative group of metals was investigated. In particular, the influence of fluorine substitution on the volatility of a series of metal fl-diketonates will be discussed. EXPERIMENTAL The following abbreviations have been used throughout this paper to specify the ligands: H(thd): 2,2,6,6-tetramethyl-3,5-heptanedione H(hfa): 1, I, 1,5,5,5-hexafluoro-2,4-pentanedione H(dfhd): I, 1,1,2,2,3,3,7,7,7-decafluoro-4,6-heptanedione H(fod): 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione H(ffa): 1,1,1 -trifluoro-2,4-p'entanedione H(acac): 2,4-pentanedione H(Br-acac): 3-bromo-2,4-pentanedione H(bzac): 1-phenyl-1,3-butanedione H(tpb): 4,4,4-trifluoro-1-phenyl-l,3-butanedione H(tta): 4,4,4-trifluoro- 1-(2-thienyl)-1,3-butanedione. All metal chelates studied herein were either synthesized by methods documented in the literature or were obtained from the indicated sources: Sc(thds), Y(thd)3, La(thd)3, Pr(thd)8,Nd(thd)3, Sm(thd)s, Eu(thd)a, Gd(thd)s, Tb(thd)a, Dy(thd)s, Ho(thd)a, Er(thd)s, Tm(thd)3, Yb(thd)a, and Lu(thd)3; cm Na(thd), Cr(thd)s, and Al(thd)3; as~ Zr(thd)4 ;txT~ Cr(hfa)a ;~18~Cr(dfhd)s;aa~ Cr(fod)a and Al(fod)s; t2°~ Cr(tfa)a, Fe(tfa)s, and Rh(tfa)s; c~1~ Cr(acac)~; ~2~ Cr(Br-acac)a; ~8~ Fe0afa)s and Al0ffa)a; nt'se~ Al(b_fa)~(acac) and Al(hfa)(acac)~; t ~ Al(tfa)3; t~s~ Al(acac)3; taT~ Rh(hfa)a; t28j Fe(tpb)8; 'a°~ were

~m R. G. CHARLES and 1L C. Om~,NN, J. inorg, nuel. Chem. 27, 255 (1965). a*~ C. DuvAL, Inorganic Thermogravimetric Analysis (2nd Edn) and references cited therein. Elsevier New York (1963). ~a,~W. W. W~-O~Ar,~or, Thermal Methods of Analysis and references cited therein. Interscience, New York, (1964) ~x~ K. J. EtS~NW~trr and R. E. SmVERS,J. Am. chem. Soc. 87, 5254 (1965). ~t6~G. S. HAMMOND,D. C. NONn~B~Land C. S. Wu, Inorg. Chem. 2, 73 (1963). tt~ R. E. SmwRs, K. J. Ezs~rcr~Atrr, D. W. MEE~ and C. S. SPRr~6~a, JR., Proc. 9th Int. Conf. Co-ord. Chem., St. Moritz, Switzerland. p. 479 (September 1966). ~s~ R. E. Smwm% R. W. Mosama and M. L. MORRIS,Inorg. Chem. 1,966 (1962). ~ R. W. MOSamR and R. E. SmWRS, Gas Chromatography of Metal Chelates p. 51. Pergamon Press, Oxford (1965). t~0~C. S. SPmNOE~, JR. Ph.D. Thesis, The Ohio State University (1967); C. S. SPRINGER, JR., D. W. MEEK and R. E. SmWRS, Inorg. Chem. 6, in press. ~ R. C. FAY and T. S. PIPER, J. Am. chem. Soc. 85, 500 (1963). t~> W. C. FER~r~LrOSand J. E. BLANCH,lnorg. Synth. (Edited by T. MOELL~a),Vol. V, p.130. McGrawHill, New York (1957). t~a~J. P. COLL~Ca~N,R. A. Moss, H MALTZ and C. C. H~X~D~L,J. Am. chem. Soc. 83, 531 (1961). ~'~ M. L. MORRIS, R. W. MOSHmRand R. E. Smwm, Inorg. Chem. 2, 411 (1963). t*~ R. G. LINCK and R. E. SmvEas, Inorg. Chem. 5, 806 (1966). t~6~M. L. MORRIS,R. W. Mosm~R and R. E. SmvEm, Inorg. Synth. (Edited by S. Y. T Y ~ ) , Vol. IX, p. 28. McGraw-Hill, New York (1967). t,~ R. C. YoLr~o, Inorg. Synth. (Edited by W. C. FERN~LrOS),Vol. II, p. 25. McGraw-Hill, New York (1946). t~s~j. p. COLLMAN,R. L. MARSHALL,W. L. YOUNG and S. D. GOLDnY,Inorg. Chem. 1, 704 (1962). tas~E. W. BERG and J. T. TR tmMV~R,J. phys. Chem. 64, 487 (1960); E. W. BERGand J. T. T~tmMrER, Analytica chim. Acta 32, 245 (1965).

1933

Thermogravimetric studies of metal fl-diketonates

synthesized. Cr(bzac)8 was obtained from Distillation Products Industries. Fe(acac)s was obtained from MacKenzie Chemical Company. Rh(acac)s was a gift from Prof. J. P. Collman. Physical properties (elemental analysis, melting points, or spectra) were in agreement with literature reports or the anticipated elemental stoichiometry. The thermogravimetric data reported were obtained using a DuPont Model 950 Thermogravimetric Analyzer. The sample size was maintained as nearly constant as possible (approx. 10 mg) in order to minimize differences (induced by kinetic factors) involved in comparing thermograms obtained on samples of widely differing weights. Platinum sample pans were used in the measurements of all samples. The tliermograms were obtained using a programmed (10°C/rain) heating rate, and were observed in an atmosphere of helium gas at a measured flow rate of 60 ml/min, with the effluent being vented to a laboratory hood. Before use, helium was passed through a drying tower containing beds of molecular sieves and Drierite. l

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FIG. l.--Thermogravimetfic curves of volatile metal chelates of 2,2,6,6-tetramethyl3,5-heptanedione.

RESULTS AND DISCUSSION

The tris rare earth chelates of 2,2,6,6,-tetramethyl-3,5-heptanedione ¢15) have been

of considerable recent interest because these compounds are the first examples of fl-diketonate chelates of the lanthanides that are volatile at moderately low temperatures (100-200°C). These chelates are thermally and solvolytically stable and are anhydrous and unsolvated. They are monomeric in solvents such as chloroform, benzene, ethyl acetate, etc. The synthesis and physical properties of the rare earth chelates of 2,2,6,6,-tetramethyl-3,5-heptanedione, including their gas chromatographic elution behaviour, have been previously reported, c15) The gas chromatography elution data of the rare earth thd chelates, obtained with a column containing a high molecular weight hydrocarbon, showed that the chelates containing the rare earths of smaller ionic radius were eluted prior to those of rare earths of larger ionic radius. Various hypotheses were postulated to account for this behaviour, uS'x" Recently a similar trend in gas chromatographic elution behaviour was found for the same chelates from an SE-30 (polydimethylsiloxane) column, m'zm The gas chromatographic data indicated that there is a difference in the volatility of the rare earth thd ~so)R. E. StaYERS, K. J. EISENTRAtrr,D. W. MEEK and C. S. SPRINGER s JR., 152nd Nat. Mtg. Am. Chem. Soc., New York. (September 1966.)

1934

KENT J. EISENTRAUT and ROBERT E. StaYERS

chelates. This difference is further evidenced by the sublimation-vaporization curves shown in Fig. 1. Each of the rare earth thd chelates shows a smooth curve that approaches 100 per cent weight loss. The thermogram of the tetrakis chelate Zr(thd)4 is also shown in Fig. 1. It can be seen that Zr(thd)~ is volatile, however its volatility is less than the rare earth thd chelates. Shown also in Fig. 1 is the thermogram of Na(thd). In view of the paucity of volatile sodium compounds, it is noteworthy that Na(thd) undergoes sublimation. The correlation reported earlier, relating gas chromatographic data cls.17'3°~ with ionic radius of the trivalent rare earth ion, is parallel to the behaviour exhibited in the thermograms. Figure 1 confirms that as the radius of the trivalent rare earth ion decreases, the volatility of the chelate increases. It is interesting to notice that although the atomic weight of yttrium is only about one half that of erbium, Y(thd)a exhibits a TGA curve which lies in almost the same position as does the thermogram of Er(thd)a. Actually, although the mass of yttrium is about one half that of erbium, the ionic radius of yttrium is about the same as that of erbium. The ionic radii of the trivalent rare earth ions of co-ordination number 6 are given in Table 1.{Sx~ Other than Pm(III) and Ce(III) (which were not studied), TABLE 1.--IONIC RADII OF TRIVALENT RARE EARTH IONStSt) Rare earth ion

Sc(III)

Y(III)

Lu(III)

Yb(III) Tm(III) Er(III)

Ho(III) Dy(III)

Tb0II)

Ionic radius A.

0.68

0.88

0.848

0.858

0.894

0.923

0.869

0.881

0.908

Rare earth ion Gd(III) Eu(III) Sm(III) Pm(III) Nd(III) Pr0II) Ce(III) La(IlI) Ionic radius A.

0.938

0.950 0.964 0.979 0.995 1.013 1.034 1.061

all of the rare earth thd chelates gave thermograms similar to those shown in Fig. 1, that is, the position of each thermogram followed the ionic radii trend, but a few were deleted from the figure for the purpose of clarity. Similar data concerning the thermal analysis of the tris rare earth chelates of 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione, H(fod), also indicates the increase of chelate volatility with decreasing ionic radius of the trivalent rare earth ion.~0.a0~ Figure 2 shows the thermograms of some Cr(III) fl-diketonate chelates, where the R1 and R~ groups have been substituted as indicated. It is possible to see trends in volatility upon substitution. The fluorine substituted chelates show greater volatility than do the non-fluorine substituted species. The normal hydrocarbons, undecane and tetracosane (dotted curves), are included for comparison purposes. It is interesting to note the increase in volatility of the almost completely fluorinated species, Cr(hfa)a (Curve B), over that of the partially fluorinated species, Cr(ffa)a (Curve E), which is in turn greater than that of the non-fluorinated species, Cr(acac)s (Curve G). Furthermore, Cr(dfhd)a (Curve C) is more volatile than the less extensively fluorinated Cr(fod)a (Curve D) which in turn is more volatile than Cr(thd)a (Curve F). There is a large decrease in volatility of Cr(bzac)s (Curve J) compared to that indicated by the other Cr(III) chelate thermograms; however, this chelate does sublime at markedly higher temperatures. It is also interesting to notice the effect of including bromine in ~31~T. MOELImR, The Chemistry of the Lanthanides p. 20. Reinhold, New York (1963).

Thermogravimetric studies of metal ~-diketonates J

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FIG. 2.--Thermogravimetric curves of Cr(III) fl-diketonates.

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FIG. 3.--Thermogravimetric curves of some volatile AI(III) fl-diketonates. the 3-position of Cr(acac)s (Curve I); this tends to reduce the volatility of the chelate, and at about 200°C Cr(Br-acae)s decomposes instead of subliming. In the limited number of eases so far examined, it is noteworthy that substitution of a group other than H in the 3-position of the fl-diketone ligands tends to reduce thermal stability or substantially decreases the volatility of the resulting metal chelate. Figure 3 shows the thermograms of a series of tris aluminum chelates of a variety of/3-diketone ligands. It is again important to notice the increased volatility of the tris aluminum chelates with increased fluorine substitution in the chelate figand. The order of decreasing volatility is seen to be Al(hfa)3 > Al(hfa)2(acac ) > Al(hfa)(acac) 2 > Al(tfa)a > Al(fod)a -- Al(acac)~ > Al(thd)a. This is in general

1936

KENT J. EJSE~CrRAUT and ROBERT E. SmV£R$

agreement with the empirical rule that increasing the extent of fluorine substitution in the ligand shell increases the volatility of the metal complex. The differences in volatility of a group of iron (III) and rhodium (III) fl-diketonates are illustrated in Fig. 4. In all eases shown, the Fe(III) chelates are more volatile than the corresponding Rh(III) chelates containing the same ligand. Fe(tpb)s undergoes vaporization but it is less volatile than the other chelates shown, and requires a higher temperature. This observation and the low volatility exhibited by Cr(bzac)3 suggests that substitution of phenyl tings for methyl groups results in a decrease in the volatility of metal complexes.* I

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D. Rh (tfo) 3 E. Fe (ococ) 3

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300

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FIG. 4.--Thermogravimetdc curves of volatile Fe(III) ahd Rh(III) fl-diketonates. CONCLUSIONS

It has been demonstrated that the technique of thermogravimetric analysis, as applied to the determination of the relative volatility of a group of metal fl-diketonates, is rapid and effective. It can be used to predict the order of ease of gas chromatographic elution from columns containing representative non-polar liquid phases. The technique is also useful for the determination of the approximate temperature conditions required to effect gas chromatographic elution of a particular metal chelate. By obtaining thermogravimetric data prior to starting the gas chromatographic study, one can ascertain what column and injection port temperatures will be adequate and thereby avoid unnecessarily high temperatures that could cause thermal decomposition of the complex. TGA can also be used to detect volatility differences in complexes of the same ligand co-ordinated to different metals, such as the lanthanides, for which it was shown that the volatility of the chelate is related to the size of the metal ion.

Acknowledgement~This research was supported in part by the ARL In-House Independent Laboratory Research Funds, Office of Aerospace Kesearch, U.S. Air Force. * Our very recent thermogravimetric analysis of tris(1,3-diphenyl-l,3-propanediono)Cr(III) and tris(1,3-diphenyl-l,3-propanediono)AIClII) lend further credence to this suggestion. These chelates are volatile, but very high temperatures are required to induce volatilization (about 350-450°C).