Phosphine Complexes of the Platinum Group Metals

By B. W. Malerbi, M.A. Research Laboratories, Johnson Matthey & Co Limited

Complexes made by reacting the platinum group metals with alkyl or aryl phosphines are of comiderable potential interest. Many of them are soluble in aromatic or polar aliphatic solvents and are already Jinding applications as homogenous catalysts. In this paper the methods available for producing the phosphine complexes are reviewed, and details are given of a range of phosphine stabilised hydrido and phosphine stabilised carbonyl hydrido complexes of the metals, all of which have been produced under controlled conditions and are now available in research quantities.

Just over a hundred years ago Hofmann (I) consideration here, Pd (11), Pt (II), Rh (I) first reported that platinum and gold com- and Ir (I) complexes have the square-planar pounds could be reacted with triethylphos- form, and Ir (111), Rh (111), Ru (111) and phine in alcoholic solution to produce phos- 0s (11) have an octahedral configuration. phine co-ordination complexes. In 1870 Ru (11) and Rh (11) tend to give the bridged Cahours and Gal (2) prepared and character- structure of Fig. 4. ised complexes of gold, palladium and Most of the platinum metal phosphine platinum with tertiary phosphines and complexes known show the structures illus- arsines. Progress in this field was slow until trated in Figs I to 4. In phosphine complexes the thirties when Mann and Chatt (3) stimu- the ligand, shown in the diagram as R,P, may lated a new interest. This was heightened in consist of trialkyl (e.g. Et,P), triaryl (e.g. the forties and fifties by Hieber (4), Malatesta Ph,P), or mixed alkyl aryl tertiary phos- (5) and Vaska (6). Since then the expansion phines such as Et,PhP or EtPh,P. of effort has been almost explosive, with the Several general methods of preparing result that nearly every possible permutation phosphine complexes of platinum metals are of ligand and metal has now been prepared. known. The simplest, used by Mann among These phosphine complexes are part of a others, is to stir the metal halide with the much larger field of co-ordination complexes tertiary phosphine in ethanol either at room in which transition metals are linked to donor temperature, or with gentle heating up to atoms or groups of atoms by dative or co- boiling. Complexes such as PdCl,(PPh,), or ordinate bonds. Examples of such donor RhCl,(PPh,), result. If the boiling is groups of ligands are ammonia, amines, prolonged, especially if higher boiling alco- arsines, carbonyls, halogen ions, and olefins. hols such as ethanediol or 2-methoxyethanol In co-ordination complexes certain charac- are employed, reductive carbonylation or teristic configurations of atoms are common, hydride formation will result. As an example, for example the square planar (Fig. I) and the Vaska boiled IrCl, with excess PPh, in octahedral (Fig. 2). Less common are the 2-( P-methoxyethoxy) ethanol and obtained halogen-bridged binuclear species (Figs 3 IrCl(CO)(PPh,),. and 4) and the tetrahedral structure found in A different approach involves carbonylation some nickel complexes. Of the metals under of the metal halide, followed by addition of

Platinum Metals Rev., 1965, 9, (2), 47-50 47 the phosphine to the metal carbonyl halide. useful properties. The normal salts of these Heck (8) prepared [Rh(CO),Cl], from RhC1, metals are generally not appreciably soluble in by this method, and then added PPh, to give organic solvents other than alcohols, and for RhCl(CO)(PPh,),. The iridium analogue applications in the field of homogenous cataly- IrCl(CO)(PPh,),, prepared by Vaska, on sis this limits their usefulness. (An exception treatment with HCl gas yields the hydrido is the notable reactivity of rhodium trichloride complex IrHCl,(CO)(PPh,), . Presumably the trihydrate in ethanolic solutions; Harrod and analogous rhodium compound would react Chalk (9) showed that rhodium trichloride in similarly. In preparing a range of ruthenium ethanol readily catalysed the isomerisation of complexes Chatt (7) utilised the intermediate hex-I-ene to cis-hex-3-ene.) On the other [Ru,Cl,(PEt,Ph),]+Cl- (prepared by refluxing hand the tertiary phosphine complexes of the RuCl, with PEt,Ph in 2-methoxyethanol platinum metals are usually quite soluble in a under nitrogen), and reductively carbonylated wide range of organic solvents. The trialkyl- it by boiling it with ethanolic potassium hy- phosphine derivatives are especially soluble droxide to give RuHCl(CO)(PEt,Ph),. In but this causes difficulties in their preparation. another reaction with [Ru,Cl,(PEt,Ph),]+Cl-, A further advantage conferred by the phos- by shaking an ethanol solution with CO at phine ligands is the stabilisation of lower, 75°C and 50 atm, Chatt obtained the normally unstable, valency states of the dicarbonyl complex RuClz(CO),(PEt,Ph),. platinum metals. If the normal lower valent Complexing the salts of platinum group platinum metal salts, which are very reactive, metals with phosphines gives them certain were used as catalysts there is a danger of their

Platinum Metals Rev., 1965, 9, (2) 48 being reduced to the metal. When associated Pt"C,H5)3PJ4 wirh phosphines this is less likely. The This is made by reacting K,PtCl, with an phosphine ligand has been found to stabilise excess of triphenylphosphine in a mixture transition metal hydrides which are otherwise of ethanol and water at 55°C. unobtainable. Vaska (6) has prepared The compound consists of orange yellow Ir132Cl(l?Ph3)3and OsHCl(CO)(PPh,) 3, but crystals having a melting point of 120 to simple hydrides of iridium and osmium are 125'C. They are readily soluble in benzene unknown. The carbonyl ligand has a similar and sparingly so in hrxane. Benzene solutions stabilising effect but it is much weaker. The slowly decompose in air, and on standing in metal-carbon bond is also strengthened by the absence of air, they gradually deposit co-ordinated phosphines, as in the case of PtH,[(C,H,),P],. This compound can replace (ECJ')J't(CHd,. chlorine in CC1, by hydrogen. It has been reported by Wilkinson, et al.

(10)that the rhodium complex RhCl,(PPh,), PdChC(C8%)3PI 2 has the ability to promote hydrogenation and This is made by reacting (NH,),PdCI4 with hydroformylation (the simultaneous addition triphenylphosphine in ethanol at 2ooC. of CO and H,) of olefins and acetylenes under The compound consists of yellow crystals relatively mild conditions. Hydrogenation of which decompose at around 250°C. The hex-I-ene to n-hexane occurs at 20°C and crystals are soluble in benzene, toluene and less than I atmosphere pressure of hydrogen. chloroform and the resulting solutions are Hydroformylation of hex-I-ene is effected stable in air. after twelve hours at 55°C and go atmospheres pressure of a I :I mixture of carbon monoxide IWJ [(C$5)3PI 3 and hydrogen. The products are 20 per cent This is made by reacting IrC1,.3H20 with of 2-methylhexanoaldehyde and 70 per cent an excess of triphenylphosphine in a mixture of n-heptaldehyde. As an aside, it may be of ethanediol and water, boiling the mixture noted that Wilkinson (11) reports that for about four hours. ethanolic solutions of RhC1,.3H20 also The compound consists of white crystals catalyse the hydrogenation of olefins. having a melting point of 195 to 210'C. The The work of Harrod and Chalk, referred to crystals are insoluble in alcohol but soluble in earlier, suggests that a metal hydride is a carbon tetrachloride or CHC1,; but the likely intermediate in the case of isomerisation solutions are unstable. The solid slowly turns and hydrogenation reactions. The fact that green on exposure to light. some of these complexes are relatively labile in solution in the presence of air would imply that they may also catalyse organic oxidation IrHCIdCO) [(C,H,),PI 2 reactions under relatively mild conditions. This is made by passing dry HCI gas into It seems likely, on the basis of our present an ether suspension of IrC1(CO)[(C,H,),P]2. knowledge of homogeneous catalytic mech- The compound is a white crystalline solid anisms, that many applicationsof hydride com- having a melting point of 295 to 310°C. It is plexes of the platinum metals will soon emerge. very sparingly soluble in acetone, chloroform For these reasons the preparation of a OT benzene. number of these complexes has been surveyed in the Johnson Matthey Research Labora- IrCl(C0) [(C,H,),PI 2 tories, and details of seven of the compounds This is made by reacting IrC1,.3H20 with so far prepared are summarised below. All excess of triphenylphosphine in a mixture of these compounds are now available to poten- methyl digol and water. The mixture is tial users in research quantities. boiled for about two and a half hours.

Platinum Metals Rev., 1965, 9, (2) 49 The compound consists of lemon yellow 280’C. The crystals are soluble in alcohols and crystals having a melting point of 315 to 320’C. benzene, but attempts at recrystallisation They are soluble in high boiling alcohols result in loss of phosphine. and chloroform. References fRu&I,[(c~H&$],j)’ c1- I A. W. Hofmann, Ann. Chetn. Liebigs, 1857, This is made by reacting RuC1,.3H20 with 103,357 an excess of triphenylphosphine in boiling 2 A. Cahours and H. Gal, ibid. 1870, 155, 223, 355 methanol for twenty-four hours. 3 F. G. Mann and J. Chatt, J. Chem. SOL.1938, The compound consists of red-brown p. 1949,2086; 1939,p. 1622 crystals having a melting point of 176 to 4 W. Hieber and H. Heusinger,?. Znorg. Nucl. Chew., 1957, 4, 179 178°C. The crystals are sparingly soluble in 5 L. Malatesta et al,J. Chem. Sac., 1955, p, 3924; alcohols but readily soluble in acetone, 1958, 2323; 1963, 2080; J. Inorg. Nucl. Chem., 1958, 8, 561 chloroform and benzene. The solutions 6 L. Vaska et a1,J. Amer. Chem. Sac., 1960, 82, rapidly change in colour from red to green 1263; 196r, 831 756, 1262, 2784; 1962, 84, when exposed to air and the compound is not 679; 1964, 86, I944 7 (a) J. Chatt and R. G. Hayter,J. Chem. SOL., recry stallisable. 1961, 896 (b) J. Chatt and B. L. Shaw, Chem. Ind., 1960, 93 1 RhC’, [(C,H,),PI 3 (c) J. Chatt, B. L. Shaw, A. E. Field,J. Chem. This is made by reacting RhC13.3H,0 with Sac., 1964, 3466 a large excess of triphenyl phosphine in 8 R.F. Heck, J. Amer. Chem. Soc., 1964~86,2796 9 J. F. Harrod and A. J. Chalk,?. Amer. Chem. ethanol. The solution is brought just to the Sac., 1964, 86, 1776 boiling point and then allowed to cool. 10 J, A. Osborn, G. Wilkinson, J. F. Young, The compound consists of orange red Chem. Communications, 1965, 17 11 R. D. Gillard, J. A. Osborn, R. B. Stockwell, crystals having a melting point of 270 to G. Wilkinson, Proc. Chem. Soc., 1964, 284

Hydrogen Recovery by Palladium Diffusion AN INEXPENSIVE LARGE-SCALE PROCESS The big demand for pure hydrogen has 50 per cent hydrogen. The diffuser operates encouraged Union Carbide Corporation to at 350 to 400°C and produces hydrogen suit- develop a large-scale hydrogen diffusion pro- able for catalytic hydrogenation processes. cess using thin palladium films. Hydrogen An important consideration is the availa- 99.99 per cent pure is being produccd at bility of feed gases. The Union Carbide plant Corporation plants and the Olefins and Linde is designed for using off-gas streams from Divisions are prepared to manufacture units olefin plants and might also be used with for salc elsewhere. The first nine plants will coke oven gas or gas streams from oil refin- soon be operating, producing between them eries. Feed gases from such sources are low 34 million cubic feet of hydrogen per day. in cost and the reforming of other molecules R. B. McBride and D. L. McKinley have to generate fresh hydrogen is obviated. Only described these developments in a paper hydrogen sulphide seriously poisons the delivered at the 55th National Meeting of the palladium films and must be removed before American Institute of Chemical Engineers at the diffusion stage. Houston, Texas. Following experiments on Factors which merit favourable considera- the mechanisms of hydrogen diffusion through tion of diffusional recovery and purification palladium, a pilot plant was built which of hydrogen include high pressure and con- operated satisfactorily from 1961 onwards. centration of feed gas, absence of contami- Details were given of a 4 million cubic feet nants, and a requirement for high purity per day plant which uses a feed gas containing hydrogen at low pressure.

Platinum Metals Rev., 1965, 9, (2) 50