Volatile alkali metal β-ketoenolate compounds

J. inorg,nucl.Chem.,1969,Vol.31, pp. 625 to 631. PergamonPress. Printedin Great Britain

VOLATILE ALKALI METAL fl-KETOENOLATE COMPOUNDS R. BELCHER, A. W. L. D U D E N E Y * and W. I. STEPHEN

Chemistry Department, University of Birmingham, Birmingham

(Received 26July 1968) Abstract-Alkali metal compounds of a number of/3-ketoenols have been isolated and qualitative assessments of their volatility, thermal stability, hydrolytic stability, and extraction into organic solvents have been made. The influence of fluorine-containing and bulky substituents in the/3-ketoenols is discussed. The sodium /3-ketoenolate of 1-(undecafluorobicyclo[2.2.1]heptan-l-yl)-4,4,4trifluorobutan-2,4-dione, a tigand combining a bulky and fluorine-containing group is described. INTRODUCTION

As A RESULT of the special structure of their univalent cations [ 1], the alkali metals exhibit an unusual simplicity and mutual similarity in the properties of their compounds; moreover they show little tendency to form compounds of a covalent or water-insoluble nature. These properties combine to make the analytical chemistry of the metals complicated. It is, therefore, interesting to note the ability of a few types of alkali metal compound, e.g. organolithium compounds[2], alkoxides[3] and dipivaloylmethides[4], to exist in the vapour phase, because the separation of the metals from one another by gas-liquid chromatography (GLC), a technique which has been applied successfully to the separation and determination of some metals in the form of/3-ketoenolate complexes [5], becomes a possibility. The volatility of the dipivaloylmethides and the likelihood that the presence of fluorinated substituents in similar molecules would increase the volatility of the compounds led to the study of the series (I) which is reported here. R \ C~O

/ CR'

/

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*Present address: University College, P.O. Box 30197, Nairobi, Kenya. I. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Chap. 16. lnterscience, New York (1962). 2. A. Streitwieser, Jr. and J. H. Hammons, Progr. phys. org. Chem. 3, 74 (1965) and references

contained therein 3. M. S. Bains, Res. Bull. Punjab Univ. 15, 303 (1964); W. G. Bartley and W. Wardlaw, J. chem. Soc. 422 0958). 4. G. S. Hammond, D. C. Nonhebel and C. S. Wu, lnorg. Chem. 2, 73 (1963). 5. R. W. Moshier and R. E. Sievers, Gas Chromatography of Metal Chelates. Pergamon Press, Oxford (1965). 625

626

R. BELCHER, A. W. L. D U D E N E Y and W. I. S T E P H E N EXPERIMENTAL

Organic ligands. Some of the ligands shown in Table 1 were prepared by published methods [6-8]. l,l,l,5,5,5-Hexafiuoropentan-2,4-dione (hexafluoroacetylacetone) was obtained from KochLight Ltd. and those ligands containing a cyano group were provided by Dr. F. L. Rose. The rather elaborate synthesis of 1-(undecafluorobicycio[2.2.1]heptan-l-yl)-4,4,4-trifluorobutan-2,4-dione will be described in a separate publication. l,l,l-Trifluoro-5,5-dimethylhexan-2,4-dione. Under conditions analogous to those developed for the preparation of sterically hindered ligands by Hammond et al.[8], 71 g of ethyl trifluoroacetate and 53g of pinacolone in 11. of dimethoxyethane were condensed at 60 ° in the presence of 48g of a 50 per cent dispersion of sodium hydride in oil. The product was hydrolysed with 100 ml of concentrated hydrochloric acid with vigorous stirring and cooling, and poured into 21. of water. The upper layer was dissolved in 500 ml of n-hexane, and the hexane layer was separated, dried over magnesium sulphate and fractionally distilled through a 10-in. column at 38 torr to give l,l,l-trifiuoro-5,5dimethylhexan-2,4-dione, 34.8g (36%), b.p. 61.5-62"5 °. Anal. calcd, for CsH1102F.~: C, 49-0; H, 5.7%; Found: C, 48.6; H,6.1%. The fl-diketone gave a blue copper(ll) fl-ketoenolate, m.p. 97-99 °, upon shaking with aqueous copper(II) acetate solution, which was purified by sublimation at 100° and at 0.01 torr. Anal. calcd. for C16H~004FrCu: C, 42.3%; H, 4.44%; F, 25.1%. Found: C, 42.0%, H, 4.69%; F, 24.6%. Alkali metal fl-ketoenolates. These were prepared, generally in good yield, from about lg of the ligand by one of the undermentioned methods. The metal:ligand ratio was established by semimicro sulphated ash determination[9] although in some cases complete elemental analyses were obtained. Method A. Ligand, dissolved in approximately 50 per cent aqueous dimethoxyethane, was titrated with alkali solution, an E.1.L. pH meter (Model 23A) being used to follow the reaction. The solvent was removed under reduced pressure leaving a colourless solid which was purified by recrystallization from a suitable solvent (usually ethanol or ethyl acetate). Method B. An equivalent amount of M-alkali solution was added to the free ligand and the resulting solution was extracted several times with ether. The product was isolated by evaporation of the ether and the residue purified by sublimation at 0.01 torr. Method C. The ligand (in excess) and alkali carbonate were warmed together, and a suitable solvent (ether, hexone or ethyl acetate) was added periodically in small quantities to keep the products in solution. On cooling the solution the product precipitated. This was purified by sublimation at 0.01 torr. Method D. An excess of the ligand was added to a stirred suspension of the free metal (in the form of wire or powder) in ether or benzene, and the mixture was gently heated until all the metal had been consumed. In those cases where the product dissolved in the solvent, the latter was evaporated under reduced pressure until most of the product had precipitated. The collected solid was purified by sublimation at 0.01 torr or, in those cases where it was not volatile, by crystallization from ethanol or ethyl acetate. Solvent extraction experiments. Aqueous solutions (1.0 ml) of known concentrations of the B-ketoenolates were equilibrated for 20 min with 1-0 ml of the organic phase (ether or hexone). The phases were separated with a fine-tipped pipette and the concentration of compound in each phase was estimated by titrimetric analysis. The aqueous phases were titrated directly with standard hydrochloric acid to bromophenol blue or thymol blue; the organic phases were treated with standard acid and the excess of acid was then titrated with standard sodium hydroxide.

RESULTS A N D D I S C U S S I O N Some properties of the sodium/3-ketoenolates which have been studied are g i v e n i n T a b l e 1. A n u m b e r o f a n a l o g o u s c o m p o u n d s o f t h e o t h e r a l k a l i m e t a l s 6. R. Luin Belford, A. E. Martell and M. Calvin, J. inorg, nucl. Chem. 2, 11 (1956) and references contained therein. 7. J. Burdon and V. C. R. McLoughlin, Tetrahedron 20, 2163 (1964). 8. K. R. Kopecky, D. C. Nanhebel, G. Morris and G. S. Hammond, J. org. Chem. 27, 1036 (1962). 9. F. Pregl, Quantitative Organic Microanalysis (Edited by J. Grant), 4th Edn, p. 118. Churchill, London (1945).

Volatile alkali metal fl-ketoenolate compounds

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R. BELCHER, A. W. L. DUDENEY and W. I. STEPHEN Table 2. Sublimates of alkali metal/3-ketoenolates

Compound

Method of Melting point Sublimation preparation (°C) temperature

Metal analysis % Calcd. Found

(*C) 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

C~HO2F~Li" CsHO2FsNa CsHO2F6K C~HO2F6Rb C~HO2F6Cs CsH1002FaLi~ CsH100~F3Na CsHloO2F3K CsHloO2FzRb CsH1oOzFaCs ClxH1902Lic CllH1902Na C11Hj90~K

B B B C C B B B C C .a _a .a

d300 d260 230d 200d 215-217 d250 230-232 204-205 159-162 186-187 256d 180d 180d

200 185 200 195d 190 190 185 195 180 190 200 200 170d

3.24 9.99 15.9 29"2 39.1 3.43 10.5 16"7 30"5 40"5 3"64 11.1 17-6

3.17 9.91 15.7 27.2 38.8 3-43 10.5 16"5 28"8 39"5 3"77 10.3 16"8

~The ligand is 12, Table 1. ~Fhe ligand is 11, Table 1. erhe ligand is 6, Table 1. aSee footnote bTable 1. have been characterised but these are only considered in detail here for the more promising ligands (Table 2); Table 1 suffices for a c o m p a r i s o n of the ligand properties of the fl-ketoenols. T h e methods of preparation and the properties of the fl-ketoenolates vary considerably but in each case it is considered that the acidic hydrogen a t o m of the enol t a u t o m e r of the ligand is replaced b y an alkali metal to give a c o m p o u n d of the type I. Ligands 1-5 and some similar ligands which have been studied[10] gave c o m p o u n d s which would not sublime at 0.01 torr. T h e c o m p o u n d s tended to d e c o m p o s e upon heating and where a clear melting point was exhibited, this was difficult to reproduce. Although the solubility in ether was low, recrystallization f r o m more polar organic solvents, e.g. ethyl acetate or ethanol, gave colourless crystalline products. Ligand 6, 2,2,6,6-tetramethylheptan-3,5-dione (dipivaloylmethane; D.P.M.) was the exception in that sublimates of the lithium, sodium and potassium compounds were obtained. This confirmed the findings of H a m m o n d e t a/.[5]. H o w ever, the p o o r analyses and irreproducible b e h a v i o u r of the sublimates upon heating (Table 2) despite their colourless appearance, indicated that partial thermal degradation occurred at the sublimation temperature. T h e marginal volatility of the fl-ketoenolates of this ligand is qualitatively understandable in terms of the ability of the bulky, enveloping, anion to " s c r e e n " the small cation. Each molecule appears to its neighbours as a neutral organic molecule rather than two oppositely charged ions and is therefore more easily r e m o v e d from the lattice. Bartley and Wardlaw[3] used a similar argument to account for the volatility of s o m e alkali metal alkoxides and they suggested that the metal-ligand bond was covalent in nature. 10. R. Belcher, B. Montford and W. I. Stephen, Unpublished results.

Volatile alkali metal/3-ketoenolatecompounds

629

Ligands 7-13 containing the trifluoromethyl group each gave sublimable/3ketoenolates. The increase in volatility of compounds affected by the introduction of the trifluoromethyl group has been discussed[11]. A further effect for alkali metal/3-ketoenolates is to stabilize the anion by attraction of the negative charge. This is evident in the increase in thermal stability with increasing fluorination (cf. ligands 1,7 and 12 in Table 1) and in the increased acidity of the ligands. The most favourable, i.e. the lowest, sublimation temperatures were exhibited by those fluorine-containing compounds which also contained an ester group (Ligands 8, 9, and 10). Unfortunately, the tendency to thermal decomposition allowed only a narrow temperature range for sublimation, and sublimates of the potassium compounds could not be obtained. Further, the sublimates melted irreproducibly at a low temperature, resolidified and melted again at a higher temperature; they dissolved readily in ether but tended to precipitate later as colourless solids. A change in structure may have been occurring. Colourless sublimates were obtained but the metal to ligand ratio decreased steadily upon successive sublimations, particularly in the case of ligand 9. The behaviour of the compounds of the fluorine-containing /3-diketones (ligands 7, 11, 12 and 13) was less complicated and a comparison of the properties o f ' t h e /3-ketoenolates of hexafluoroacetylacetone (HFA), 1,1,1-trifluoro-5,5dimethylhexan-2,4-dione (trifluoroacetylpivaloylmethane, TPM) and D P M is shown in Table 2. The lithium and sodium compounds of H F A and T P M sublimed readily'over a considerable range of temperature above 185° and without apparent decomposition. Colourless sublimates were obtained for the heavier metal compounds but the thermal stability tended to decrease with increasing atomic weight of the metal and these sublimates may not have been pure. In an effort to improve the volatility and thermal stability to a stage where G L C of all the alkali metals is a possibility, the preparation of a ligand combining fluorine-containing and bulky groups was considered. Although it should be possible to synthesise the fully fluorinated analogue of DPM, the practical difficulties of this synthesis were too great for this to be achieved during the present work, but the ligand, 1-(undecafluorobicyclo [2.2.1 ] heptan- 1-yl)-4,4,4-trifluorobutan-2,4-dione (I I), was successfully prepared. F2

F2

F

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II

II

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(II)

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This preparation, which is quite involved, will be reported separately. The sodium /3-ketoenolate of {II) was found to be remarkably thermally stable; it readily sublimed at 250° without apparent decomposition but did not become volatile below 200 ° at 0.01 torr. Thus, it is less volatile than the H F A analogue, a result which is not too surprising in view of the massive bulk of the bicyclic group. Solvent extraction experiments. An interesting effect of the trifluoromethyl 11. R. W. Moshier and R. E. Sievers, Gas Chromatography of Metal Chelates, p. 20. Pergamon Press, Oxford (1965).

630

R. BELCHER, A. W. L. D U D E N E Y and W. I. STEPHEN

group in the/3-ketoenolates is that their solubility in polar organic solvents is greatly increased. For the/3-ketoenolates of T P M and H F A , extraction from aqueous solution into such solvents occurs with increasing ease in the order shown. The small amounts of material available allowed only small-scale experiments of a semi-quantitative nature, but it was established that, for aqueous solutions of the /3-ketoenolates, the percentage of material extracted under comparable conditions into ether or hexone decreased with increase in the atomic weight of the metal and also decreased with decrease in the initial concentration of the flketoenolate. Thus, for 0.1 M solutions of the TPM compounds equilibrated with an equal volume of hexone, the percentages of alkali metal found in the organic phase for lithium, sodium, potassium, rubidium, and caesium were 82, 34, 15, 12 and 5 per cent respectively. For 1.0, 0.1, 0-003 and 0.0015 M-solutions of the lithium compounds of TPM, distribution ratios of 23, 6.8, 0.61 and 0.41 were found. H F A was a more efficient extractant than TPM. Thus, for 0.1 M-solutions of the sodium compounds of T P M and H F A , extractions of 37 and 60 per cent were obtained. A factor which affects the extraction of alkali metal/3-ketoenolates is their hydrolytic stability which may be expressed as follows: M L i g + H 2 0 ~ H L i g + MOH.

(1)

The more acidic the ligand, HLig, the less the hydrolysis according to equation (1) and the lower the pH at which extraction is possible. Comparative titrations under conditions similar to those of preparative method A showed that the practical pK values of the ligands decreased in the order TPM, H F A , ligand (II). T P M apparently gives a similar curve to that of l ,l,l-trifluorothenoylacetone (TTA), extraction data for which have been published[12]. The alkali metal compounds of T T A were extracted most efficiently at pH 8.7 which indicated that some excess of base was necessary to repress hydrolysis. T P M should behave similarly but the/3-ketoenolates of H F A and ligand (II) are hydrolysed to a negligible extent and should extract readily from neutral solution. Spectra. The mass spectra of most of the fl-ketoenolates were recorded on an Associated Electrical Industries M.S.9 instrument. In general the spectra are complicated, with polymeric species predominating, and are temperature-dependent. In each case the peak corresponding to the alkali metal is present but the peak corresponding to the 1 : l fl-ketoenolate is often weak. An exception is the spectrum of the sodium compound of ligand(lI) where the molecular ion is much stronger than any polymeric ion and a readily interpretable fragmentation pattern is shown. A detailed study of the mass, infrared, and ultraviolet spectra of the alkali metal fl-ketoenolates is in progress. Gas chromatography experiments. Because the heavier alkali metal /3ketoenolates would not sublime without partial decomposition, comprehensive G L C experiments were not carried out. An LKB 9000 gas chromatograph-mass spectrometer containing a column at 200 ° packed with celite with 1 per cent of silicone gum (S.E. 30) as the stationary phase was used in an attempt to elute 12. P. Crowther and F. L. Moore, A nalyt. Chem. 35,2081 (1963).

Volatile alkali metalfl-ketoenolate compounds

631

the lithium compound of TPM. No peak corresponding to this compound was obtained. Work is now continuing to prepare sufficient of (II) to carry out a comprehensive study of its ligand properties. Acknowledgements-We are grateful to Dr. F. L. Rose of I.C.1. Pharmaceutical Division for the gift of cyano-substituted compounds used in this study. One of us (A.W.L. D) thanks the Science Research Council for financial support.