GB2268489A - Process for the preparation of DI-ALKYL compounds of group 2B metals - Google Patents

Abstract

Di-alkyl compounds of a Group 2b metal are prepared in a process in which a Group 2b metal is reacted with an alkyl halide in the presence of an alkali metal to obtain the di-alkyl compound of the Group 2b metal and alkali- metal halide, in which process the atomic ratio of alkali metal to Group 2b metal is above 1:1.

Description

PROCESS FOR THE PREPARATION OF DI-ALKYL COMPOUNDS OF GROUP 2B METALS The present invention relates to a process for the preparation of di-alkyl compounds of Group 2b metals, and to such compounds, in particular to di-alkyl zinc. In this specification by Group 2b metals are understood zinc, cadmium and mercury.

Di-alkyl compounds of Group 2b metals find increasing use in the electronics industry. Compounds and alloys containing elements such as zinc and cadmium may be deposited on substrates from volatile precursor compounds, such as their respective di-alkyl compounds, by thermal decomposition from the vapour phase to give a thin (semiconductor) layer. This technique is known in the industry as Metal Organic Chemical Vapour Deposition (MOCVD). When an epitaxial layer is grown the technique is better known as Metal Organic Vapour Phase Epitaxy (MOVPE). A process for the deposition of zinc sulphide films on a semiconductor substrate in which di-alkyl zinc is used in combination with hydrogen sulphide is described in European patent application No. 405,875. A method for the preparation of epitaxial layers of zinc sulphide and zinc selenide is described in UK patent application No. 2,221,924.

The presence of impurities in such semiconductor layers may have a tremendous effect on both their electrical and their optical properties. It is therefore desired that the precursor compounds, such as the di-alkyl compounds of the Group 2b metals, are very pure. For the production of p-type zinc selenide layers for use in opto-electronic devices, the iodine content in the zinc precursor is of the utmost importance. Iodine is an n-type dopant and hence, controlled p-type doping can only be achieved if the iodine content in the epitaxial layer, and therefore in the zinc and selenium precursors, is very low, preferably below 1 ppm (by weight). Hence, it is not surprising that strenuous efforts have been made to purify these precursors such as the di-alkyl compounds. In US patent specification No. 4,812,586 the purification of impure dimethylcadmium and dimethylzinc is described.According to this specification impure di-alkyl compounds are prepared by reacting alkyl halide with magnesium to yield a Grignard reagent. This reagent is subsequently reacted with cadmium halide or zinc halide to yield the respective impure di-alkyl metal compound. This compound was purified by adduct formation with specific amino compounds followed by removal of impurities from the adduct. The purified adduct was subsequently dissociated and subjected to distillation to yield the purified di-alkyl compound. It is evident that this process requires several steps.

In UK patent specification No. 1,242,789 a process is described in which metallic zinc is alkylated using alkyl halides in the presence of a minor amount of alkali metals. This process requires an excess of zinc in addition to that which is contained in the di-alkyl compound, because zinc is used as halogen acceptor.

So if the use of very pure zinc is required, as would be for MOCVD or MOVPE purposes, this process would involve the waste of considerable amounts of pure zinc.

The present process relates to a simple preparation of di-alkyl metal compounds with an efficient use of the Group 2b metal. Whereas in the prior art it is reported that the reaction of a Group 2b metal with alkali halide in the presence of an alkali metal requires the use of an alloy and the use of a mixture of alkyl iodide and bromide, it has now been found that excellent yields are attainable at relatively high alkali metal to Group 2b atomic ratios. The use of an alloy or a mixture of alkyl halides is then not necessary.

The present process therefore provides a process for the preparation of di-alkyl compounds of a Group 2b metal, comprising reacting a Group 2b metal with an alkyl halide in the presence of an alkali metal to obtain the di-alkyl compound of the Group 2b metal and alkali-metal halide, in which process the atomic ratio of alkali metal to Group 2b metal is above 1:1.

The Group 2b metal that can be used in the process according to the present invention includes zinc, cadmium and mercury.

Preferably, zinc or cadmium are used. In the most preferred embodiments zinc is used because zinc is most frequently used in MOCVD and MOVPE processes, and moreover use of zinc gives high yields.

The halogen moiety of the alkyl halide can be selected from chlorine, bromine, iodine or mixtures thereof. Especially alkyl bromides and/or alkyl iodides are advantageously used in the present process. Use of alkyl iodides tends to result in a product with the highest purity, so their use is especially preferred.

Although mixtures of halides may be used, such use is not required.

The alkyl groups in the alkyl halide compounds may be normal or branched. Although the present process can be carried out with a wide variety of alkyl halides, including those having long chain alkyl groups, the preparation of di-alkyl Group 2b metal compounds containing alkyl groups with more than 6 carbon atoms is not practical, because these di-alkyl compounds have a decreasing volatility and often a decreasing thermal stability. Therefore, the alkyl group in the alkyl halide has preferably from 1 to 4 carbon atoms. More preferably, the alkyl moieties are methyl or ethyl groups or mixtures thereof.

The reaction may be carried out under very mild conditions.

The pressure may be atmospheric, but also subatmospheric or superatmospheric pressures are feasible. Generally, the pressure is from 0.1 to 10 bar. For reasons of convenience it is preferred to carry out the process at atmospheric pressure. The di-alkyl compound is prepared under an inert atmosphere, e.g. nitrogen, argon or helium.

The reaction temperature may vary between wide ranges. Care is taken that the temperature does not exceed the decomposition temperature of the di-alkyl compound involved. Such decomposition is different for each di-alkyl compound. Taking the foregoing into account, the process is suitably carried out at a temperature from 20 to 170 OC. Since the reaction is exothermic, it is advantageous if the process is carried out in the presence of a solvent. Not only will the solvent ensure a homogeneous distribution of the reactants, but it also provides a convenient means for controlling the dissipation of the heat evolved. A wide variety of solvents may be used in the present process. Such solvents include aliphatic or aromatic hydrocarbons, such as pentane, hexane, heptane, benzene, toluene or xylene, and amides, such as dimethyl formamide.

Preferably, the solvent contains at least one moiety with electron donating properties. Examples of such a moieties contain a nitrogen or, particularly, an oxygen atom. Therefore, the solvent is preferably an ether. The ether may be cyclic, like tetrahydrofuran or dioxane, or non-cyclic, such as diethyl ether, di-(iso)propyl ether, diphenyl ether, di-isopentyl ether and mixture thereof.

Other suitable ethers are polyglycol ethers containing up to eight glycol moieties. Suitable examples of such polyglycol ethers include diglyme, triglyme and tetraglyme.

The process according to UK patent No. 1,242,789 employs a minor, almost solely catalytic, amount of alkali metal. Although it is possible to use a sub-stoichiometric amount of alkali metal in the present process, it is advantageous to employ at least a stoichiometric atomic ratio of alkali metal with respect to the Group 2b metal used. Therefore, the amount of alkali metal preferably ranges from 1.6 to 4.0 mole alkali metal per mole Group 2b metal. Surprisingly, it has been found that the purity of the product obtained is further enhanced if a relatively small excess of alkali metal is employed. Therefore the atomic ratio of alkali metal to Group 2b metal is more preferably from 2.0:1 to 2.5:1. The form in which the Group 2b metal and alkali metal are present in the reaction mixture is not critical. It is possible to use a physical mixture of alkali metal and the Group 2b metal involved. It is also feasible to employ an alloy of the metals. The relative amounts in the alloy or the mixture are suitably selected such that they correspond to the above molar ratios.

As alkali metal, lithium, sodium and potassium may be used.

The use of lithium is preferred because it is easy to handle, is available in relatively pure form and gives the highest yields in the process of the present invention.

In the process of UK patent No. 1,242,789 an excess of alkyl halide is used. In the present process suitably a substantially stoichiometric amount of alkyl halide is used. Variations are, of course, possible. The yields of the desired di-alkyl compound are enhanced if an excess of alkyl halide is used. However, in such cases the amount of unreacted reactant or by-products may contaminate the desired di-alkyl compound, thereby preventing the desired purity from being attained. Hence, it is feasible to have the process carried out at a molar ratio of alkyl halide to Group 2b metal of 0.8 to 4.0. However, if purity is paramount the process is preferably carried out at stoichiometric ratios. Therefore, the molar amount of alkyl halide is preferably substantially twice that of the Group 2b metal (in gramatom) so that no excess alkyl halide needs to be removed from the reaction mixture.

After completion of the reaction, the reaction mixture will contain the di-alkyl compound of the Group 2b metal, alkali metal halide, and, optionally, the employed solvent. When an excess of alkali metal is used, the reaction mixture will also contain unreacted alkali metal and/or alkali metal-alkyl. The di-alkyl compound therefore needs to be isolated from the reaction mixture.

All conventional techniques may be applied to achieve such separation. These techniques include filtration, decantation etc.

Conveniently, the di-alkyl compound is recovered by distillation.

After a first distillation a second fractional distillation may be employed. In the isolation of the di-alkyl compound of the Group 2b metal from the reaction mixture by distillation it may be advantageous to recover the first 1 to 10 per cent by volume of the product separately. In such case the main fraction which is then recovered as the desired product has an enhanced purity. The first fraction of the distilled product may be recycled by adding it to the original reaction mixture or to a subsequent batch of the same reaction, or may be discarded. In order to avoid any possible thermal decomposition of the di-alkyl compound, the distillation is advantageously carried out at a temperature below the decomposition temperature of the di-alkyl compound involved. For certain di-alkyl compounds it may thus be desirable to perform the distillation under subatmospheric pressure.Suitable pressures can, therefore, be selected from 1 bar to as low as less than 1 mbar.

Since the compounds according to the present invention are very suitable for use in MOCVD or MOVPE-applications, the present invention, moreover, relates to the use of the compounds of the present invention in MOCVD or MOVPE.

The invention is further illustrated by means of the following example.

Example Synthesis of dimethylzinc and diethylzinc In the following experiments lithium powder (325 mesh) or sodium powder (5-13 mesh) and zinc powder (40 mesh) were weighed into a three-necked round-bottomed flask in a glove box under an atmosphere of purified argon. The closed three-necked round-bottomed flask was subsequently evacuated and backfilled with purified argon several times. Solvent (30 ml) was added with a syringe through a septum. Subsequently, alkyl halide was added gradually through the septum using a syringe. The reaction mixture was allowed to reflux overnight and was subsequently filtered.

Subsequently, the reaction mixture was characterized by 1H-NMR and 13C-NMR. The yield of dimethylzinc/diethylzinc was determined from the NMR spectrum in combination with the weight of the zinc that could be recovered.

The nature and amounts of the reagents used and the results of the reactions are shown in the table below.

Experiment alkali zinc alkyl halide moles solvent yield No metal moles moles %a 1. Li 0.06 0.03 methyl iodide 0.065 diethyl ether 96 2. Li 0.037 0.18 methyl iodide 0.038 hexane 5 3. Li 0.038 0.015 methyl iodide 0.027 diethyl ether 68 4. Na 0.022 0.011 methyl iodide 0.023 diethyl ether 5b 5. Li 0.013 0.0068 ethyl iodide 0.014 diethyl ether 89 6. Li 0.018 0.007 ethyl bromide 0.015 diethyl ether 45 a Yield based on zinc originally present b Reaction is slow; after 40 hours yield had increased to 14%.

The above experiments indicate that the use of an ether is advantageous over the use of a hydrocarbon solvent. It is further apparent that lithium performs better than sodium. Although both iodides and bromides can be used, the use of iodides results in higher product yields. It appeared that in the product of Experiment No. 3, where a substantial excess of lithium was used, no residual iodide could be determined.

Claims (18)

1. Process for the preparation of di-alkyl compounds of a Group 2b metal, comprising reacting a Group 2b metal with an alkyl halide in the presence of an alkali metal to obtain the di-alkyl compound of the Group 2b metal and alkali-metal halide, in which process the atomic ratio of alkali metal to Group 2b metal is above 1:1.

2. Process according to claim 1, in which the Group 2b metal is zinc or cadmium.

3. Process according to claim 1 or 2, in which the alkyl halide is an alkyl bromide, an alkyl iodide or a mixture thereof.

4. Process according to any one of claims 1-3, in which the alkyl halide contains from 1 to 4 carbon atoms.

5. Process according to claim 4, in which the alkyl halide is a methyl halide or an ethyl halide.

6. Process according to any one of claims 1-5, which is carried out at a temperature ranging from 20 to 170 "C.

7. Process according to any one of claims 1-6, which is carried out in the presence of a solvent.

8. Process according to claim 7, in which the solvent is an ether.

9. Process according to claim 8, in which the ether is diethyl ether, di-(iso)propyl ether, di-isopentyl ether, diphenyl ether or mixtures thereof.

10. Process according to any one of claims 1-9, in which the amount of alkali metal ranges from 1.6 to 4.0 mole of alkali metal per mole Group 2b metal.

11. Process according to claim 10, in which the atomic ratio. of alkali metal to Group 2b metal ranges from 2.0:1 to 2.5:1.

12. Process according to any one of claims 1-11, in which the alkali metal is lithium.

13. Process according to any one of claims 1-12, in which the alkali metal is present in the form of a physical mixture with the Group 2b metal.

14. Process according to any one of claims 1-12, in which the alkali metal is present in the form of an alloy with the Group 2b metal.

15. Process according to any one of claims 1-14, in which the molar ratio of the Group 2b metal to the alkyl halide is in the range from 0.8:1 to 4.0:1.

16. Process according to claim 1, substantially as described hereinbefore with specific reference to the Examples.

17. Di-alkyl compound of a Group 2b metal, whenever prepared according to a process as claimed in any one of claims 1-15.

18. Use of a di-alkyl compound according to claim 17 in MOCVD or MOVPE.