Synthesis of phosphorus and arsenic, halides and hydrides

Method for synthesizing alkyl halides of phosphorus, arsenic, or antimony from the corresponding phosphorus, arsenic, or antimony alkyl and phosphorus, arsenic, or antimony halide. An improved synthesis of alkyl phosphorus or arsenic hydrides from the corresponding alkyl phosphorus, arsenic, or antimony halides is also disclosed.

BACKGROUND 
The present invention relates to the synthesis of compounds having the 
formula: 
EQU R.sub.y MX.sub.(3-y) ( 1) 
wherein R is lower alkyl, M is arsenic or phosphorus, X is a halogen, and y 
is 1 or 2. (R, M, and X retain these definitions throughout this 
specification, unless the context indicates otherwise.) The invention also 
relates to methods for preparing compounds of the formula: 
EQU R.sub.y MH.sub.(3-y) ( 2) 
wherein H is hydride and the other substituents are like those of formula 
(1), from compounds of formula (1). 
Compounds of formula (2), have recently found favor as reactants for metal 
organic chemical vapor deposition of III/V compound semiconductor films 
for electronic, optical, and other technologies. The utility of such 
arsenic and phosphorus compounds for metal organic chemical vapor 
deposition is disclosed in U.S. Pat. No. 4,734,514, issued to Melas et al. 
on Mar. 29, 1988. That patent is hereby incorporated by reference herein 
in its entirety. 
While the alkyl hydrides of formula (2) are very useful compounds, their 
synthesis has been long and complicated, with a low yield. In Example 1 of 
the Melas patent previously incorporated by reference, the synthesis of 
diethylarsine using arsenic trichloride as a starting material requires a 
sequence of four reactions. The first three reactions are required to form 
diethylchloroarsine -- a compound according to formula (1). Thus, one 
problem facing the art has been how to form the compounds of formula (1) 
more directly and with higher yields. 
The arsenic or phosphorus compounds of formula (1) have been reported to be 
synthesized by directly reacting the corresponding alkyl halide with 
arsenic or phosphorus at 70.degree. C. in the presence of copper as a 
catalyst, according to the equation; 
##STR1## 
(L. Maier, Inorganic Synthesis 7:82 (1963).) (The antimony synthesis 
hasn't been reported.) Unfortunately, the yield of the halide has been 
reported to be quite low, particularly when is arsenic and the R group is 
ethyl. 
Another reaction scheme which provides formula (1) halides is found in 
Kharasch, et al., J. Org. Chem. 14, 429 (1949): 
EQU R.sub.4 Pb+2 AsCl.sub.3 .fwdarw.R.sub.2 PbCl.sub.2 +2 RAsCl.sub.2( 4) 
EQU R.sub.2 PbCl.sub.2 +AsCl.sub.3 .fwdarw.RAsCl.sub.2 +PbCl.sub.2 +RCl(5) 
This can be a one-step synthesis. The analogous synthesis for phosphorus 
is: 
EQU Et.sub.4 Pb+3PCl.sub.3 .fwdarw.3 EtPCl.sub.2 +PbCl.sub.2 +EtCl(6) 
However tetraalkyl lead compounds are toxic, and lead as an impurity might 
damage III/V films formed from the resulting product. 
Another multistep synthesis of formula (1) halides is found in Burton, J. 
Chem. Soc. 450 (1926) and Gibson, et al., J. Chem. Soc. 2518 (1931), as 
follows: 
##STR2## 
(In the formulas herein, "Ph" is phenyl). 
The yield of this reaction sequence is low. 
Other, less pertinent syntheses of the formula (1) halides are also known. 
(See Doak & Freeman, Oranometallic Compounds of Arsenic, Antimony, and 
Bismuth, John Wiley & Sons, Inc., 1970). 
The following two reactions are known for triphenylarsine (G. D. Parkes, R. 
J. Clarke and B. H. Thewlis. J. Chem. Soc. 429 (1947); A. G. Evans and E. 
Warhurst, Trans Faraday Soc., 44. 189 (198); H. D. N. Fitzpatrick, S. R. 
C. Hughes, and E. A. Moelwyn-Hughes, J. Chem. Soc. 3542 (1950)), and both 
monochloro and dichloroarsine derivatives can be made this way: 
EQU 2 (Ph).sub.3 As+AsCl.sub.3 .fwdarw.3 (Ph).sub.2 AsCl (9) 
EQU (Ph).sub.3 As+2 AsCl.sub.3 .fwdarw.3 (Ph)AsCl.sub.2 ( 10) 
However, these reactions are unknown for alkyl arsines, and 
chloroalkylarsines cannot be made in this manner. For example, the 
reaction of trimethylarsine with arsenic trichloride has been reported to 
form only the stable addition compound: 
EQU (CH.sub.3).sub.3 As.AsCl.sub.3 ( 11) 
(A. Valeur and P. Gaillot. Bull. Soc. Chim. Fr., 41. 1318 (1927). 
While not intending to be bound by this theory, the inventors believe that 
trialkylarsines have not previously been recognized as being reactive in 
the present context because triphenylarsine is far less basic than 
trialkylarsines. As a result, triphenylarsine does not form a stable 
adduct with arsenic trichloride and is readily reactive with additional 
arsenic trichloride or triphenylarsine to form products according to 
formula (1) above. On the other hand, attempts to form the products of 
formula (1) directly from the corresponding alkylarsine and trihaloarsine 
have failed because a stable adduct of these reactants forms and does not 
easily react to form the desired products. 
Redistribution reactions take place readily for both alkyl and aryl 
derivatives of bismuth. This perhaps may be attributed to the weak bismuth 
to carbon bonds in these compounds facilitating the exchange of R and X 
(halogen) groups. (A. Marguardte, Berichte 20 1516 (1887)). 
EQU Me.sub.3 Bi+2 BiBr.sub.3 .fwdarw.3 MeBiBr.sub.2 ( 12) 
EQU Et.sub.3 Bi+2 BiBr.sub.3 .fwdarw.3 EtBiBr.sub.2 ( 13) 
EQU 2 Ph.sub.3 Bi+BiCl.sub.3 .fwdarw.3 Ph.sub.2 BiCl (14) 
Both primary and secondary arsines of formula (2) are generally prepared by 
reducing a different arsenic compound with a reducing agent. Thus, 
alkylarsonic (or alkylarsinic) acids and alkylchloroarsines are common 
arsenic starting sources which can be reduced with zinc dust, zinc amalgam 
or zinc-copper couple in aqueous hydrochloric acid. For example; 
##STR3## 
See W. R. Cullen and W. R. Leeder, Can. J. Chem., 47 2137 (1969)) 
Lithium aluminum hydride has also been used, but the results are generally 
less satisfactory with poorer yields: 
##STR4## 
(See E. Wiberg and K. Modritzer, Z. Naturforsch, B, 11. 751 (1956) and B, 
12, 127 (1957)) 
Arsines made by the routes of equations (15) and (16) might be contaminated 
with zinc, mercury or copper and subsequently might damage the III-V films 
formed from these products. The water used in reaction (16) can produce 
oxygen-containing impurities in the films. 
SUMMARY OF THE INVENTION 
A first aspect of the invention is a method of synthesizing a compound 
having formula (1) above from the following starting materials: 
EQU R.sub.3 M and (17) 
EQU X.sub.3 M (18) 
wherein M, R and X are defined as before. The starting materials are 
reacted according to the following equation: 
EQU y R.sub.3 M (3-y) X.sub.3 M.fwdarw.3 R.sub.y MX.sub.(3-y) ( 19) 
A second aspect of the invention is a method for forming alkyl arsenic 
hydride (also called an alkylarsine or dialkylarsine) which is essentially 
free of water or harmful metallic impurities. In the present method, 
instead of adding water to the reaction mixture (which is described in 
Example 1 of the previously incorporated U.S. patent at column 15, line 62 
to column 16, line 10), the precursor of formula: 
EQU R.sub.y MH.sub.(3-y)AlH.sub.3 ( 20) 
is distilled under anhydrous conditions to directly isolate the 
corresponding dialkyl arsine. By using this procedure, which is contrary 
to the accepted practice, a product which is inherently free of water is 
produced.

DETAILED DESCRIPTION OF THE INVENTION 
For the first synthesis summarized above, one of the reactants is a Group V 
alkyl of formula (17) above. R is lower alkyl, which is defined herein as 
methyl, ethyl, propyl, or butyl, including all the isomers of propyl and 
butyl. M is selected from the group consisting of arsenic or phosphorus. 
Arsenic is specifically contemplated due to its proven value for chemical 
vapor deposition processes. The R moieties of this material are selected 
according to the final compound desired. One particular reactant 
contemplated herein is triethyl arsine. Many trialkyl arsines are known. 
The other reactant used in the present process is a halide of formula (18) 
above. Assuming a single compound is to be synthesized, M is the same in 
formulas (17) and (18). The respective M moieties can be different, 
however, if a mixture of compounds is contemplated as the end product. In 
formula (18). X is a halide, preferably chloride, bromide, or iodide. 
Chloride is specifically contemplated for use herein. The reactants of 
formulas (17) and (18) undergo a redistribution reaction to form products 
according to formula (1). 
The following is an illustration of the present synthesis using 
triethylarsine and arsenic trichloride. As the first step of the 
synthesis, triethylarsine is added to arsenic trichloride (both are 
liquids at room temperature). A white, crystalline, solid, 1:1 adduct is 
formed immediately, regardless of the ratio of the reactants used (i.e., 
2:1, 1:2, or 1:1): 
EQU 2 Et.sub.3 As+2 AsCl.sub.3 .fwdarw.(Et.sub.3 As.AsCl.sub.3).sub.2(21) 
The adduct can be is isolated and the x-ray crystal structure has been 
determined. 
Once this adduct is formed, it will react further with more triethylarsine 
or arsenic trichloride in a second step according to one of the following 
equations: 
EQU (Et.sub.3 As.AsCl.sub.3).sub.2 +2 Et.sub.3 As.fwdarw.6 Et.sub.2 AsCl(22) 
EQU (Et.sub.3 As.AsCl.sub.3).sub.2 +2 AsCl.sub.3 .fwdarw.6 EtAsCl.sub.2(23) 
The reactions of the adduct with either triethylarsine or arsenic 
trichloride require long reaction times (about 30 hours) and vigorous 
heating. It is very likely that previous investigators failed to identify 
the proper experimental conditions for the present reaction in the past 
and only reported the formation of the adduct. 
The initial reaction is carried out in a solvent, e.g. hexane or petroleum 
ether. After the formation of the adduct the solvent must be removed 
because heating at a much higher temperature is required in the next step. 
Extreme care must be taken during solvent removal so the second reactant 
in equation (22 ) or (23) (depending on the desired end product) is 
conserved. Failing to do this will change the stoichiometry and thus 
drastically reduce the yield. As a precaution, the adduct cam be generated 
first by reacting triethylarsine and arsenic trichloride in a 1:1 ratio, 
and, after removal of solvent, an additional mole of triethylarsine (or 
arsenic trichloride) can be added (see Example 3). 
The alkyl halides made according to the present invention can be used as 
precursors to the corresponding alkyl hydrides. The latter compounds are 
directly useful for metal organic chemical vapor deposition. Several 
reactions may be used to exchange hydride groups for halide groups, one of 
which is described in the final part of Example 1 of the U.S. patent 
previously incorporated by reference. 
Another way to proceed from the halide to the corresponding hydride is as 
follows. Once the alkyl phosphorus, arsenic, or antimony halide is 
complexed with lithium aluminum hydride as described in Example 1 of U.S. 
Pat. No. 4,734,514, the mixture is distilled under anhydrous conditions, 
instead of adding water as has previously been done. The reaction thus 
proceeds as follows, starting from the addition of lithium aluminum 
hydride: 
EQU R.sub.y MH.sub.(3-y) +(3-y)LiAlH.sub.4 .fwdarw.R.sub.y 
MH.sub.(3-y).AlH.sub.3 +LiCl (24) 
##STR5## 
The resulting product is very free of water and other contaminants, down 
to a 1 or 2 parts per million level. 
EXAMPLE 1: SYNTHESIS OF DIETHYLARSENIC CHLORIDE 
196 grams (1.204 mol of triethylarsine diluted with 500 ml of petroleum 
ether at 40.degree.-60.degree. C. in a 1 liter flask, were reacted with 
109 grams (0.602 mol) of arsenic trichloride, diluted with 50-100 ml 
petroleum ether, by adding the arsenic trichloride solution dropwise at 
room temperature. There was almost no reaction heat. 
After the addition was complete (in about 30 minutes), the mixture was 
refluxed with a hot water bath (no heating mantle) for 2-3 hours. The 
suspension or solution was transferred in two or three portions to a 500 
ml flask (to reduce volume and from that flask petroleum ether was slowly 
distilled at ambient pressure. The oil bath temperature never exceeded 
80.degree. C. 
After distillation of the petroleum ether, the temperature was slowly 
increase to a maximum of 140.degree. C. A white to faintly brownish 
(sometimes also faintly pink) colored precipitate formed. The precipitate 
melted sharply at 120.degree. C. and the liquid deposited black arsenic 
during the melting process. 
At this temperature a bubbler was placed on top of the flask and the 
mixture was heated with stirring for 20 hours. The result after that 
procedure was a black viscous liquid. The product was evaporated and 
condensed in vacuo at about 0.1 mm pressure from flask to flask via 
U-glass tubing using a heating mantle set to stage I, full power for about 
5 hours. A colorless liquid (sometimes contaminated with black arsenic due 
to splashing) having a boiling point of 151.degree.-152.degree. C. at 740 
mm pressure resulted. This product was recondensed for purity in the same 
manner (flask to flask). 
The diethylarsenic chloride obtained by this procedure was rather pure and 
did not colorize during reduction with LiAlH.sub.4. The combined yield of 
three of these runs was 620 g. of diethylarsenic chloride, corresponding 
to about 68% of theory. 
EXAMPLE 2: SYNTHESIS OF DIETHYLARSINE 
Lithium aluminum hydride 7.21 g = 0.190 mol) was suspended in diethyl ether 
ad diethylarsenic chloride made according to Example 1 (42.62 g = 0.253 
mol), diluted with diethyl ether, was added dropwise under an argon 
atmosphere. A vigorous exothermic reaction took place; the flask was 
cooled with ice water. The dropping rate was adjusted to allow gentle 
reflux of the ether solvent. The resulting diethylarsine was very volatile 
and extremely sensitive to oxygen, as indicated by immediate formation of 
white fumes. 
After adding the diethylarsenic chloride, the cooling bath was removed and 
reflux continued for 1 hour. 
All volatiles were then immediately condensed in vacuo. The reaction flask 
was heated with boiling water. After two hours, only traces of Product 
were still condensing into the collecting flask. 
The diethyl ether was removed at ambient pressure at an oil bath 
temperature of about 80.degree. C. The remaining liquid consisted of pure 
diethylarsine, a colorless liquid having a boiling point of 98 to 
100.degree. C. at 760 mm. pressure. The yield was 5.50 g., 75.2% of 
theory. 
By using twice as many mols of diethylarsenic chloride as of lithium 
aluminum hydride in a subsequent run, the yield was increased to 79% of 
theory. Thus, this ratio of ingredients is preferred. 
EXAMPLE 3 -- SYNTHESIS OF DIETHYLARSENIC CHLORIDE 
319 grams (1.97 mol) of triethylarsine diluted with 1.2 liters of hexane 
were reacted with 356 grams (1.96 mole) of arsenic trichloride as 
described in Example 1. The hexane was removed to leave behind the 1:1 
adduct shown in formula (21) above. 
Another 319 grams of triethylarsine were added to the adduct in the flask 
and the mixture was heated to a maximum of 140.degree. C. as in Example 1 
for about 30 hours. The product, diethylarsenic chloride, was isolated 
similarly by distillation. 
EXAMPLE 4 -- SYNTHESIS OF ETHYLARSENIC DICHLORIDE 
100 grams (0.61 mol) of triethylarsine and 225 grams (1.24 mol) of arsenic 
trichloride are reacted in 500 ml of hexane in a 1 liter flask. As in 
Example 1, the mixture is refluxed for 2-3 hours. The solvent is carefully 
removed and the temperature slowly raised to a maximum of 140.degree. C. 
After heating the reactants for about 30 hours the product, ethylarsenic 
dichloride, is isolated by distillation (boiling point 155.degree. C.). 
EXAMPLE 5 -- SYNTHESIS OF DI-BUTYLPHOSPHORUS CHLORIDE 
As in Example 1, tributylphosphine (1.0 mole) is reacted with phosphorus 
trichloride (0.5 mole) in petroleum ether, and di-butyl phosphorus 
chloride can be isolated by vacuum distillation.