Monomeric organometallic compounds and method of preparing same

Preparation of the novel monomeric compounds of the formula (t-Bu).sub.2 ME(t-Bu).sub.2, where M is Ga, Al, or In and E is As, P, Sb, or N, by reaction of (t-Bu).sub.2 MCl and LiE(t-Bu).sub.2. The resulting product (t-Bu).sub.2 ME(t-Bu).sub.2 can be pyrolyzed to form crystalline films of the metallic material, ME.

BACKGROUND OF THE INVENTION 
This invention relates to the production of organometallic compounds, and 
is particularly directed to the preparation of certain volatile monomeric 
III-V compounds which can be pyrolyzed to form films of the corresponding 
metallic compound. 
Single source organometallic precursors have been utilized to prepare 
epitaxial films of GaP, GaAs and InP. Although arsinogallanes have been 
known for over 25 years, monomeric arsinogallanes are rare. Arsinogallanes 
are usually found as dimers, trimers or adducts due to the proclivity of 
Ga(III) toward tetracoordination. Attempts to prepare monomeric 
arsinogallanes have focused on the use of bulky substituents. The first 
monomeric arsinogallane, [(Mesityl).sub.2 As].sub.3 Ga, was reported in 
1986 by Wells et al, Inorg. Chem., 1986, 25,2483. Cowley et al., reported 
a dimer of a t-Butyl derivative arsinogallane compound, [(t-Bu).sub.2 
AsGa(CH.sub.3).sub.2 ].sub.2, J. Chem. Soc., Chem. Commun., 1986, 1543; 
this compound is a solid having low vapor pressure. Only recently, 
Theopold et al., SCIENCE, 1988, 241, 334, reported the first 
mono(arsino)gallane monomer, (C.sub.5 (CH.sub.3).sub.5).sub.2 
GaAs(Si(CH.sub.3).sub.3).sub.2, and its conversion to amorphous GaAs 
powder Via reaction with alcohol; this compound is also a solid having 
negligible vapor pressure. 
One object of the invention is the provision of novel monomeric 
organometallic compounds. 
Another object is to provide a novel class of organometallic compounds 
which are volatile and can be readily converted to their respective 
metallic compounds. 
A still further object is the provision of procedure for preparing the 
above volatile organometallic compounds. 
Yet another object is to provide procedure for readily converting the above 
organometallic compounds to their respective metallic compounds, e.g., as 
films. 
SUMMARY OF THE INVENTION 
The above objects are achieved according to the invention by provision of 
the novel monomeric organometallic compounds having the formula 
(t-Bu).sub.2 ME(t-Bu).sub.2, where M is Ga, Al, or In and E is As, P, Sb, 
or N, by reacting (t-Bu).sub.2 MCl and LiE(t-Bu).sub.2, in a molar ratio 
of about 1:1, preferably in a non-polar solvent such as benzene. 
The resulting monomeric mono(III-V) compounds, (t-Bu).sub.2 ME(t-Bu).sub.2, 
are relatively volatile and can be pyrolyzed to form relatively pure 
crystalline III-V (metallic) deposits and films, at relatively low 
pyrolysis temperatures. 
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
According to the invention, monomeric organometallic compounds of the type 
(t-Bu).sub.2 ME(t-Bu).sub.2 can be prepared from the reaction of 
(t-Bu).sub.2 MCl and LiE(t-Bu).sub.2 according to the following reaction 
scheme: 
EQU (t-Bu).sub.2 MCl+LiE(t-Bu).sub.2 .fwdarw.(t-Bu).sub.2 ME(t-Bu).sub.2[+LiCl 
( 1) 
where 
M is Ga, Al, or In; and, 
E is As, P, Sb, or N. 
The molar ratio of the reactants (t-Bu).sub.2 MCl and LiE(t-Bu).sub.2 is 
1:1, but a small excess of the lithium-containing reactant, 
LiE(t-Bu).sub.2, is generally employed. 
The above reaction is carried out in a non-polar solvent such as pentane, 
hexane, benzene or toluene. The reaction is generally carried out at 
ambient temperature and under an inert atmosphere such as argon, but the 
temperature of the reaction can be varied. 
Following completion of the reaction, the solvent can be removed, e.g. 
under vacuum, and the product can be purified, as by sublimation, followed 
by recrystallization. 
The products of the above reaction are volatile monomeric mono(III-V) 
compounds such as mono(arsinogallane), (t-Bu).sub.2 GaAs(t-Bu).sub.2 and 
mono(phosphinogallane), (t-Bu).sub.2 GaP(t-Bu).sub.2. 
Such monomeric organometallic products can be pyrolyzed in a vacuum or an 
inert atmosphere such as argon or helium, at a temperature significantly 
lower than possible with prior art compounds. The pyrolysis reaction 
scheme is as follows: 
##STR1## 
where M is as described above, and 
E is as described above. 
Alternatively, the above pyrolysis reaction can be carried out under a 
vacuum. 
The above pyrolysis reaction can be utilized for depositing III-V metallic 
films employed as semi-conductors in electronics. 
The above monomeric organometallic products of the invention can also be 
subjected to metal-organic chemical vapor deposition for depositing films. 
The advantages of the products, (t-Bu).sub.2 ME(t-Bu).sub.2, of the 
invention are: (1) the lower vapor pressure and reduced air-sensitivity 
reduces the toxicity (e.g.,AsH.sub.3) and safety hazards (e.g., 
flammability of (CH.sub.3).sub.3 Ga) of the materials presently used in 
film deposition by the metal-organic chemical vapor deposition (MOCVD) 
process; (2) the utilization of t-butyl groups, which can undergo 
beta-elimination reactions during pyrolysis, tends to result in lower 
carbon incorporation into the deposited metallic film; (3) highly 
crystalline films can be prepared by pyrolysis at less than 550.degree. 
C.; (4) electronic grade purity of the metallic deposits can be achieved 
through multiple sublimations; and (5) the correct stoichiometry of Group 
III to Group V materials is always maintained. 
In addition to the use of the materials produced according to the invention 
process as semiconductor films, such materials also have utility for 
infrared transparent domes of missiles, since these materials have good 
physical properties including good strength, thermal stability and thermal 
shock resistance, and have good optical or infrared transparency.

The following are examples of practice of the invention, which are 
understood as being illustrative and are not intended as limitative of the 
invention. 
EXAMPLE 1 
Benzene (20ml) was added to (t-Bu).sub.2 GaCl (0.66 g, 3.0 mmol) and 
LiAs(t-butyl).sub.2 (0.60 g, 3.1 mmol) at room temperature. After stirring 
for 2.5 days under argon, the yellow-orange solution was filtered using a 
fine frit and the solvent removed under vacuum to give crude (t-Bu).sub.2 
GaAs(t-Bu).sub.2 (1.1 g, 2.9 mmol, 97% yield) as a yellow orange liquid. 
The product, which began to sublime at 18.degree. C. at 10.sup.-3 torr, 
was purified by sublimation at 65.degree. C. at 10.sup.-3 torr. 
Recrystallization in pentane at -78.degree. C. gave yellow crystals 
suitable for x-ray analysis, mp 41.degree.-44.degree. C. See NMR Table 1. 
Two isopiestic molecular weight determinations in pentane gave molecular 
weights of 419 and 395, which are consistent with a monomeric structure 
(molecular weight of 373) in solution. The low melting point of 
41.degree.-44.degree. C., the mass spectrum and molecular weight 
determination of (t-Bu).sub.2 GaAs(t-Bu).sub.2 were all indicative of a 
monomeric structure in the solid, gas and liquid states. A single-crystal 
x-ray structure was performed to confirm the nature of the product. The 
stabilization of the monomeric unit is due to the fact that the Ga and As 
atoms are effectively shielded from intermolecular association by the 
bulky t-butyl substituents on both gallium and arsenic. 
EXAMPLE 2 
The process of Example 1 was substantially followed except LiP(t-Bu).sub.2 
was substituted for LiAs(t-Bu).sub.2 in the same molar proportions with 
respect to (t-Bu).sub.2 GaCl. 
The monomeric phosphinogallane, (t-Bu).sub.2 GaP(t-Bu).sub.2, was produced. 
Mp. 46.degree.-48.degree. C. .sup.- P NMR 23.9. See NMR Table 1. 
EXAMPLE 3 
The process of Example 1 was substantially followed except (t-Bu).sub.2 
AlCl was substituted for (t-Bu).sub.2 GaCl in the same molar proportions 
with respect to LiAs(t Bu).sub.2. 
The monomeric arsinoalane, (t-Bu).sub.2 AlAs(t-Bu).sub.2, was produced. See 
NMR Table 1. 
EXAMPLE 4 
The process of Example 1 was substantially followed except LiP(t-Bu).sub.2 
was substituted for LiAs(t-Bu).sub.2 in the same molar proportions with 
respect to (t-Bu).sub.2 AlCl which was substituted for (t-Bu).sub.2 GaCl. 
The monomeric phosphinoalane, (t-Bu).sub.2 AlP(t-Bu).sub.2, was produced. 
See NMR Table 1. 
EXAMPLE 5 
The process of Example 1 was substantially followed except (t-Bu).sub.2 
InCl was substituted for (t-Bu).sub.2 GaCl in the same molar proportions 
with respect to LiAs(t-Bu).sub.2. 
The monomeric arsinoindane, (t-Bu).sub.2 InAs(t-Bu).sub.2, was produced. 
See NMR Table 1. 
EXAMPLE 6 
The process of Example 1 was substantially followed except LiP(t-Bu).sub.2 
was substituted for LiAs(t-Bu).sub.2 in the same molar proportions with 
respect to (t-Bu).sub.2 InCl which was substituted for (t-Bu).sub.2 GaCl. 
The monomeric phosphinoindane, (t-Bu).sub.2 InP(t-Bu).sub.2, was produced. 
EXAMPLE 7 
The process of Example 1 is substantially followed except LiSb(t-Bu).sub.2 
or LiN(t-Bu).sub.2 is substituted for LiAs(t-Bu).sub.2 in the same molar 
proportions with respect to (t-Bu).sub.2 GaCl, or to (t-Bu).sub.2 AlCl or 
(t-Bu).sub.2 InCl which is substituted for (t-Bu).sub.2 GaCl. 
The respective monomeric compound: (t-Bu).sub.2 GaSb(t-Bu).sub.2, 
(t-Bu).sub.2 GaN(t-Bu).sub.2, (t-Bu).sub.2 AlSb(t-Bu).sub.2, (t-Bu).sub.2 
AlN(t-Bu).sub.2, (t-Bu).sub.2 InSb(t-Bu).sub.2, or (t-Bu).sub.2 
InN(t-Bu).sub.2 is produced. 
EXAMPLE 8 
(t-Bu).sub.2 GaAs(t-Bu).sub.2 was heated to 150.degree. C. for 10 min 
without decomposition, but decomposed to red oligomers and/or polymers at 
188.degree.-190.degree. C. Pyrolysis of monomeric mono(arsino) gallane, 
(t-Bu).sub.2 GaAs(t-Bu).sub.2, under a Helium atmosphere with a cool 
yellow flame (400.degree. C.) resulted in crystalline GaAs and 
approximately a 1:1 mole ratio of 2-methylpropane and 2-methylpropene. The 
decomposition can be rationalized by either a beta-elimination followed by 
alkane elimination (reaction scheme 2), or a free radical mechanism or 
both. 
EXAMPLE 9 
(t-Bu).sub.2 GaP(t-Bu).sub.2 was pyrolyzed under substantially the same 
reaction conditions as noted in Example 8, resulting in the formation of 
crystalline GaP. 
EXAMPLE 10 
(t-Bu).sub.2 AlP(t-Bu).sub.2, (t-Bu).sub.2 AlAs(t-Bu).sub.2, (t-Bu).sub.2 
InP(t-Bu).sub.2, (t-Bu).sub.2 InAs(t-Bu).sub.2, (t-Bu).sub.2 
InSb(t-Bu).sub.2, (t-Bu).sub.2 InN(t-Bu).sub.2, (t-Bu).sub.2 
GaSb(t-Bu).sub.2, (t-Bu).sub.2 GaN(t-Bu).sub.2, (t-Bu).sub.2 
AlSb(t-Bu).sub.2, or (t-Bu).sub.2 AlN(t-Bu).sub.2 is pyrolyzed under a 
vacuum or inert atmosphere, e.g., helium, resulting in the formation of 
crystalline AlP, AlAs, InP, InAs, InSb, InN, GaSb, GaN, AlSb and AlN 
respectively. 
TABLE 1 
______________________________________ 
NMR of (t-Butyl).sub.2 M--E(t-Butyl).sub.2 
M E (t-Bu).sub.2 M 
E(t-Bu).sub.2 
______________________________________ 
Al P 1.27d(.sup.4 J.sub.PH =0.5Hz) 
1.53(.sup.3 J.sub.PH =11Hz) 
Ga P 1.27d(.sup.4 J.sub.PH =2.0Hz) 
1.30(.sup.3 J.sub.PH =9Hz) 
Al As 1.29s 1.57s 
Ga As 1.26s 1.40s 
In As 1.24s 1.40s 
______________________________________ 
From the foregoing, it is seen that the invention provides for the 
preparation of a novel class of volatile monomeric organometallic 
materials which can by pyrolyzed readily at relatively low temperatures, 
to form essentially pure electronic grade metallic deposits or films. 
Since various changes and modifications can be made in the invention 
without departing from the spirit of the invention, the invention is not 
to be taken as limited except by the scope of the appended claims.