Preparation of bis(acetonitrile) palladium dichloride

A method for the production of tetrachloropalladic acid and bis(acetonitrile) palladium dichloride is described. Palladium metal is reacted with concentrated hydrochloric acid and chlorine to produce the acid which is converted to bis(acetonitrile) palladium dichloride by reaction with acetonitrile.

TECHNICAL FIELD 
This invention relates to the preparation of tetrachloropalladic acid and 
of bis(acetonitrile) palladium dichloride. 
BACKGROUND OF THE INVENTION 
bis(acetonitrile) palladium dichloride is a commercially important catalyst 
useful in a broad range of organic reactions. See, e.g., Heck, R. F. 
"Palladium Reagents in Organic Synthesis" (1985) Academic Press, New York; 
Tsuji, J. "Organic Synthesis With Palladium Compounds" (1980) 
Springer-Verlag, New York. 
Known methods for producing Bis(acetonitrile) palladium dichloride entail 
dissolution of the metal in aqua regia, conversion to tetrachloropalladic 
acid, conversion of the acid to one of several known salts, isolation of 
the salt and subsequent reaction to produce the desired product as shown 
by equations 1, 2 and 3: 
EQU Pd+HCl/HNO.sub.3 .fwdarw.H.sub.2 PdCl.sub.4 ! (aq) Equation 1: 
EQU H.sub.2 PdCl.sub.4 !+MCl.fwdarw.M.sub.2 PdCl.sub.4 ! M=K, Na, 
NH.sub.4Equation 2: 
EQU M.sub.2 PdCl.sub.4 !+CH.sub.3 CN.fwdarw.PdCl.sub.2 (CH.sub.3 
CN).sub.2Equation 3: 
See Kauffman, G. B., et al., Inorg. Synth. (1966) 8:234. 
Alternatively, bis(acetonitrile) palladium dichloride can be made by 
reaction of palladium dichloride with acetonitrile as shown by equation 4: 
EQU PdCl.sub.2 +2CH.sub.3 CN.fwdarw.PdCl.sub.2 (CH.sub.3 CN).sub.2Equation 4: 
See, Hartley, F. R., et al., Inorgan. Chem. (1979) 18:1394; Wayland, B. B., 
et al., Inorgan. Chem. (1969) 8:971; Ketley, German Patent No. 1,810,122 
(03 Jul. 1969); Anderson, G. K., et al., Inorgan. Synth. (1990) 28:60; 
Kharasch, M. S., et al., J. Am. Chem. Soc. (1938) 60:882. 
SUMMARY OF THE INVENTION 
Pursuant to this invention, tetrachloropalladic acid is formed by 
dissolving palladium in a mixture of concentrated hydrochloric acid and 
chlorine and expelling the excess HCl and Cl.sub.2 by distillation. This 
synthesis is illustrated by equations 5 and 6: 
##STR1## 
Pursuant to another aspect of the invention, direct addition of 
acetonitrile to an aqueous solution of tetrachloropalladic acid yields 
bis(acetonitrile) palladium dichloride in good yield which may exceed 90%. 
This reaction is illustrated by equation 7: 
EQU H.sub.2 PdCl.sub.4 ! (aq)+CH.sub.3 CN ).fwdarw.PdCl.sub.2 (CH.sub.3 
CN).sub.2 (&gt;90% overall) Equation 7: 
The three reactions shown by equations 5, 6 and 7 are appropriately run in 
a single vessel without isolation of any intermediates.

DETAILED DESCRIPTION OF THE INVENTION 
This invention provides novel methods for the preparation of 
tetrachloropalladic acid and of bis(acetonitrile) palladium dichloride. In 
the preferred practice of the invention, a reaction vessel fitted with a 
mechanical stirrer and a subsurface gas delivery tube is charged with 
palladium sponge. At least 1.0 molar, preferably 6.0 to 12.0 molar 
hydrochloric acid is added to the reaction vessel. Chlorine gas is added 
to the reaction vessel contents at a temperature of 0.degree. C. to 
120.degree. C. with stirring. Room temperature is appropriate. Higher 
temperatures may be employed if the reaction vessel is pressurized. 
Formation of a deep red-orange solution indicates the formation of 
tetrachloropalladic acid. The passage of chlorine gas through the reaction 
mixture is continued for a time period appropriate to produce a desired 
quantity of such acid. In general, the chlorine gas passes through the 
reaction mixture for a time period of five to ten hours. 
The product of this reaction is concentrated and excess hydrochloric acid 
and chlorine are expelled from it by distillation to provide an aqueous 
solution at least 0.1 molar, preferably 4.0 to 6.0 molar 
tetrachloropalladic acid. 
The concentrated acid solution, after filtration, is diluted with water to 
a molarity of at least 0.1 molar, preferably a molarity of 0.2 to 0.6. 
Acetonitrile is added with stirring to the diluted tetrachloropalladic acid 
solution in an amount stoichiometrically required to produce 
bis(acetonitrile) palladium dichloride pursuant to equation 7. The 
acetonitrile may be added in an amount more or less than the 
stoichiometric amount if it is desired to do so. The yield of this 
acetonitrile palladium dichloride is a function of the amount of 
acetonitrile added to the tetrachloropalladic acid solution. The reaction 
is appropriately stirred at room temperature for a reasonable time, e.g., 
one hour, and thereafter with ice cooling for an additional time, e.g., 
one hour. This reaction produces the desired bright orange 
bis(acetonitrile) palladium dichloride product. The product is collected, 
washed sequentially with water and acetonitrile and dried. 
EXEMPLIFICATION OF THE INVENTION 
A 3 liter 3-neck flask fitted with a mechanical stirrer and subsurface gas 
delivery tube was charged with palladium sponge (497.3 g). Concentrated 
hydrochloric acid (3,188 g) was added to the flask. Chlorine gas was 
slowly bubbled through the solution while stirring at room temperature for 
a total of eight hours. The deep red-orange solution was transferred to a 
5 liter 3-neck flask fitted with a mechanical stirrer, claissen adapter, 
still head, condenser and weighed receiver. The solution was concentrated 
to a total weight of 1,717 g by distillation. 
The concentrated solution was filtered through a Buchner funnel and divided 
equally between two 12 liter flasks. The solution in each flask was 
diluted with water to a total weight of 6,389 g. Each flask was fitted 
with a mechanical stirrer and while stirring at room temperature, 
acetonitrile (1,200 mL 99.9%) was added all at once. Each flask was 
stirred one hour at room temperature and then one hour with ice water 
cooling. 
The bright orange product was collected in a large Buchner funnel and 
washed with 2.times.1 liter of water and then with 1 liter of CH.sub.3 CN. 
The product was air dried to a constant weight. A total of 1,109 g (91.5% 
yield) of crystalline product was obtained. The material was identified by 
comparison of the IR spectrum with an authentic sample.