Tetrakis(Triphenylphosphine)Palladium(0) & Wilkinson's Catalyst

2 PdCl₂ + 8 PPh₃ + 5 NH₂NH₂·H₂O → 2 Pd(PPh₃)₄ + 4 NH₂NH₂·HCl + N₂ + 5 H₂O

Preparations of phosphine and phosphite complexes of palladium(0) have been reported to result from reduction of palladium(II) complexes in the presence of the desired ligand^1-5. The products are generally formulated as PdL_4-n (where n = 0, 1), depending on the nature and amount of the ligand used. A related complex, [Pd(PPh₃)₂]_n has also been reported⁶.

Although this preparation is similar in concept to these previous ones, advantage is gained in being able to obtain a high yield of Pd(PPh₃)₄ in one step from palladium dichloride.

Procedure

A mixture of palladium dichloride (17.72 g, 0.10 mol), triphenylphosphine (131 g, 0.50 mol), and 1200 mL of dimethyl sulfoxide is placed in a single-necked, 2-L, round-bottomed flask equipped with a magnetic stirring bar and a dual-outlet adapter. A rubber septum and a vacuum nitrogen system are connected to the outlets. The system is then placed under nitrogen with provision made for pressure relief through a mercury bubbler. The yellow mixture is heated by means of an oil bath with stirring until complete solution occurs (~140°C). The bath is then taken away, and the solution is rapidly stirred for approximately 15 min. Hydrazine hydrate (20 g, 0.40 mol) is then rapidly added over approximately I min from a hypodermic syringe. A vigorous reaction takes place with evolution of nitrogen. The dark solution is then immediately cooled with a water bath; crystallization begins to occur at ~125°C. At this point the mixture is allowed to cool without external cooling. After the mixture has reached room temperature it is filtered under nitrogen on a coarse, sintered-glass funnel. The solid is washed successively with two 50-mL portions of ethanol and two 50-mL portions of diethyl ether. The product is dried by passing a slow stream of nitrogen through the funnel overnight. The resulting yellow crystalline product weighs 103.5-108.5 g (90-94% yield)^{Note 1}.

A melting point determination^{Note 2} on a sample in a sealed capillary tube under nitrogen gave a decomposition point of 116°C (uncorrected). This compares with a similar determination (115°C) performed on the product prepared by the method of Malatesta and Angoletta¹.

Properties

The Pd(PPh₃)₄ complex obtained by this procedure is a yellow, crystalline material possessing moderate solubilities in benzene (50 g L^-1), dichloromethane, and chloroform. The compound is less soluble in acetone, tetrahydrofuran and acetonitrile. Saturated hydrocarbon solvents give no evidence of solution. Although the complex may be handled in air, it is best stored under nitrogen to ensure its purity.

In benzene, molecular-weight measurements suggest substantial dissociation^1,4:

Pd(PPh₃)₄ Pd(PPh₃)_4-n + n PPh₃

Solutions of the complex in benzene rapidly absorb molecular oxygen giving an insoluble, green oxygen complex, Pd(PPh₃)₂O₂⁷. This oxygen complex has been implicated as an intermediate in a catalytic oxidation of phosphines^2,8.

Related displacements with acetylenes⁹ and electrophilic olefins⁶ have been reported to give complexes formulated as [Pd(PPh₃)₂ (olefin or acetylene)]. Also, oxidative additions of alkyl and aryl halides have been shown to occur giving palladium(II) complexes, Pd(PPh₃)₂(R)Cl¹⁰. As a catalyst, the complex has been shown capable of dimerizing butadiene to give 1,3,7-octatriene¹¹.

Notes

The checkers worked on one-third of the stated scale, obtaining a yield of 37.4 g (97%).
The checkers report that decomposition temperature does not appear to be a good criterion of identity or purity since it is not very reproducible.

References

L. Malatesta and M. Angoletta, J. Chem. Soc., 1186 (1957)
S. Takahashi, K. Sonogashira, and N. Hagihara, Nippon Kagaku Zasshi, 87, 610 (1966); Chem. Abstr., 65, 14485 (1966).
T. Krock and K. Baur, Angew. Chem., 77, 505 (1965)
E. O. Fischer and H. Werner, Chem. Ber., 95, 703 (1962)
J. Chatt, F. A. Hart, and H. R. Watson, J. Chem. Soc., 2537 (1962)
P. Fitton and J. E. McKeon, J. Chem. Soc. Chem. Commun., 4 (1968)
C. J. Nyman, C. T. Wymore, and G. Wilkinson, J. Chem. Soc. A, 561 (1968)
G. Wilke, H. Schott, and P. Heimbach, Angew. Chem. Int. Ed., Engl., 6, 92 (1967)
S. Takahashi and N. Hagihara, J. Chem. Soc. Jpn. (Pure Chem. Sec.), 88, 1306 (1967)
P. Foton, M. P. Johnson, and J. E. McKeon, J. Chem. Soc. Chem. Commun., 6 (1968)
S. Takahashi, T. Shibano, and N. Hagihara, Bull. Chem. Soc. Jpn., 41, 454 (1968)

The discovery of chlorotris(triphenylphosphine)rhodium¹ and its utility as the first practical homogeneous catalyst for the hydrogenation of double and triple carbon bonds opened up an enormous and still developing field of chemistry, not only in catalysis but in stoichiometric reactions².

Procedure

RhCl₃ + 4 PPh₃ -> RhCl(PPh₃)₃ + Cl₂PPh₃

Cl₂PPh₃ + H₂O -> OPPh₃ + 2 HCl

Rhodium(III) chloride trihydrate^§ (2 g) is dissolved in 70 mL of ethanol (95%) in a 500 mL round-bottomed flask fitted with gas inlet tube, reflux condenser, and gas exit bubbler. A solution of 12 g of triphenylphosphine (freshly crystallized from ethanol to remove triphenylphosphine oxide) in 350 mL of hot ethanol is added and the flask purged with nitrogen. The solution is refluxed for about 2 h, and the crystalline product that forms is collected from the hot solution on a Büchner funnel or sintered-glass filter. The product is washed with small portions of 50 mL of anhydrous ether; yield 6.25 g (88% based on Rh). This crystalline product is deep red in color. An isomeric species that is orange is obtained if the total volume of ethanol used is 200 mL or less and the solution is refluxed for a period of about 5 min. This substance often contains small amounts of the red product and, on continued refluxing, the orange crystals are slowly converted to the red form. The excess triphenylphosphine used in the preparation can be recovered by addition of water to the ethanol filtrates until precipitation begins. After allowing the solutions to stand 2 to 3 days in a stoppered flask, the triphenylphosphine crystallizes out. Recrystallization from ethanol and ethanol-benzene (1:1) removes triphenylphosphine oxide contaminant.

Properties

The burgundy red (mp 157°C) and orange polymorphic forms of RhCl(PPh₃)₃ have identical chemical properties. The complex is soluble in chloroform and dichloromethane to about 20 g L^-1 at 25°C. The solubility in benzene or toluene is about 2 g L^-1 at 25° but is very much lower in acetic acid, acetone, and other ketones, methanol, and lower aliphatic alcohols. In alkanes and cyclohexane, the complex is virtually insoluble. Donor solvents such as pyridine, dimethyl sulfoxide, or acetonitrile dissolve the complex with reaction, initially to give complexes of the type RhCl(PPh₃)₂L, but further reaction with displacement of phosphine may occur.

The solutions are very air sensitive, giving soluble dioxygen compounds. Both solid and solutions should be handled under oxygen-free dinitrogen or argon. On heating benzene, toluene, or best, methyl ethyl ketone solutions (or suspensions) of RhCl(PPh₃)₃, salmon-pink crystals of the chlorine-bridged dimer (Ph₃P)₂RhCl₂Rh(PPh₃)₂ are obtained essentially quantitatively. This dimer absorbs oxygen slowly even in the solid state. It may be reconverted to RhCl(PPh₃)₃by cleavage with triphenylphosphine in refluxing ethanol. The red solutions of RhCl(PPh₃)₃ absorb molecular hydrogen reversibly at 1 atm and 25°C, becoming pale yellow; these solutions are highly effective for the catalytic homogeneous hydrogenation of compounds with double and triple carbon bonds often selectivity depending on the nature of the substrate. Cationic species of the type [Rh(PPh₃)₂(sol)₂]⁺, where sol is a solvent molecule and many other similar species with chelating phosphines are also effective.

Note
§ The commercial product usually corresponds closely to RhCl₃·H₂O but small divergences from this stoichiometry are not significant in this preparation. The yield is calculated from the Rh content.