Palladium, viii

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

Hydrogenation Catalysts. The key to catalytic hydrogenation is the catalyst, which promotes a reaction which otherwise would occur too slowly to be useful. Catalysts for the hydrogenation of nitro compounds and nitriles are generally based on one or more of the group VIII metals. The metals most commonly used are cobalt, nickel, palladium, platinum, rhodium, and mthenium, but others, including copper (16), iron (17), and tellurium... 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Catalysts. The methanation of CO and C02 is catalyzed by metals of Group VIII, by molybdenum (Group VI), and by silver (Group I). These catalysts were identified by Fischer, Tropsch, and Dilthey (18) who studied the methanation properties of various metals at temperatures up to 800°C. They found that methanation activity varied with the metal as follows ruthenium > iridium > rhodium > nickel > cobalt > osmium > platinum > iron > molybdenum > palladium > silver. 

The results used for a subsequent comparison of catalytic activity of all group VIII metals are related by Mann and Lien to palladium studied at a temperature of 148°C. At this temperature the appearance of the hydride phase and of the poisoning effect due to it would require a hydrogen pressure of at least 1 atm. Although the respective direct experimental data are lacking, one can assume rather that the authors did not perform their experiments under such a high pressure (the sum of the partial pressures of both substrates would be equal to 2 atm). It can thus be assumed that their comparison of catalytic activities involves the a-phase of the Pd-H system instead of palladium itself, but not in the least the hydride. 

We plan to make studies on palladium-copper, iridium-copper, and platinum-copper catalysts to extend our investigation of the effect of varying miscibility of the components on the structural features of the bimetallic clusters present. With these additional systems, the whole range from complete immiscibility to total miscibility of copper with the Group VIII metal will be encompassed. 

X-Ray studies confirm that platinum crystallites exist on carbon supports at least down to a metal content of about 0.03% (2). On the other hand, it has been claimed that nickel crystallites do not exist in nickel/carbon catalysts (50). This requires verification, but it does draw attention to the fact that carbon is not inert toward many metals which can form carbides or intercalation compounds with graphite. In general, it is only with the noble group VIII metals that one can feel reasonably confident that a substantial amount of the metal will be retained on the carbon surface in its elemental form. Judging from Moss s (35) electron micrographs of a reduced 5% platinum charcoal catalyst, the platinum crystallites appear to be at least as finely dispersed on charcoal as on silica or alumina, or possibly more so, but both platinum and palladium (51) supported on carbon appear to be very sensitive to sintering. 

Only one report mentions the cyclopropanation with diazodiphenylmethane in the presence of a group VIII metal catalyst. Remarkably enough, the selectivity of the reaction with 5-methylene-bicyclo[2.2.1]hept-2-ene (8) can be reversed completely. With Rh2(OAc)4 as catalyst, the exocyclie double bond is cyclopropanated exclusively (>100 1), whereas in the presence of bis(benzonitrile) palladium(II) chloride the endocyclic C=C bond is attacked with very high selectivity (>50 1)47). 

The dominant role of copper catalysts has been challenged by the introduction of powerful group VIII metal catalysts. From a systematic screening, palladium(II) and rhodium(II) derivatives, especially the respective carboxylates62)63)64-, have emerged as catalysts of choice. In addition, rhodium and ruthenium carbonyl clusters, Rh COJjg 65> and Ru3(CO)12 e6), seem to work well. Tables 3 and 4 present a comparison of the efficiency of different catalysts in cyclopropanation reactions with ethyl diazoacetate under standardized conditions. 

It is a matter of speculation as to whether or not the activity would pass through a significant maximum at a surface composition between 0 and 30% Rh. It is interesting to note in this connection that the magnetic susceptibility (156, 157) and the electronic specific heat coefficient (156) increase from low values at 60% Ag-Pd through pure palladium and reach a maximum at - 5% Rh-Pd, thereafter decreasing smoothly to pure rhodium. Activity maxima have also been reported for reduced mixed oxides and supported alloys of group VIII metal pairs. For example, in the... 

DMA in 500 ml ether mix rapidly with 270 ml 0.9 M phenyl-Li, boil fifteen hours and extract as for (VI) or as described previously to get 8 g oily 4-methoxy-indoline (or its 1-methyl derivative) (VII). Alternatively, add 36 g naphthalene to 300 ml tetrahydrofuran and add 11 g Na metal cut in small pieces. Reflux and stir three hours and add 18 g (VI) and 8 g DEA in 200 ml tetrahydrofuran rapidly and boil twelve hours. Evaporate in vacuum, dissolve the oily residue in 2N HCI and extract with ether. Proceed as described to get (VII). 4 g (VII) in 200 ml dry pyridine add to 6 g Cu chloride in 400 ml pyridine and reflux 1 xh hours. Pour on water and extract with ether. Wash extract with 4N HCI and then water and dry and evaporate in vacuum the ether to get 2 g of the indole (VIII). Alternatively, dissolve 4 g (VII) and 9.5 g cinnamic acid in 700 ml mesitylene, add 1 g 5% palladium-carbon and reflux five hours. Filter, wash with HCI and NaHC03 and dry and evaporate in vacuum the mesitylene to get the red, oily (VIII) (can chromatograph on alumina and elute with benzene-petroleum ether). 

In Table II are listed the neutral homonuclear carbonyls of the Group VIII metals that have been identified to date. Although palladium and platinum do not have any stable neutral carbonyls, a large... 

Manganese trichloride oxide, 4141 Mercury(I) oxide , 4613 Mercury(II) oxide, 4605 Molybdenum(IV) oxide, 4716 Molybdenum(VI) oxide, 4717 Nickel(II) oxide, 4821 Nickel(III) oxide, 4823 Nickel(IV) oxide, 4822 Niobium(V) oxide, 4818 Osmium(IV) oxide, 4833 Osmium(VIII) oxide, 4858 Palladium(II) oxide, 4825 Palladium(III) oxide, 4848 Palladium(IV) oxide, 4835... 

Palladium(II) oxide, 4825 Palladium(IV) oxide, 4835 Perchloric acid, 3998 Periodic acid, 4425 Permanganic acid, 4434 Peroxodisulfuric acid, 4482 Peroxodisulfuryl difluoride, 4328 Peroxomonosulfuric acid, 4481 Peroxytrifluoroacetic acid, 0666 Platinum hexafluoride, 4371 Platinum(IV) oxide, 4836 Plutonium hexafluoride, 4372 Potassium bromate, 0255 Potassium chlorate, 4017 Potassium dichromate, 4248 Potassium iodate, 4619 Potassium nitrate, 4650 Potassium nitrite, 4649 Potassium perchlorate, 4018 Potassium periodate, 4620 Potassium permanganate, 4647 Rhenium hexafluoride, 4373 Rubidium fluoroxysulfate, 4309 Ruthenium(VIII) oxide, 4862 Selenium dioxide, 4838 Selenium dioxide, 4838 Silver permanganate, 0021 Sodium chlorate, 4039 Sodium chlorite, 4038 Sodium dichromate, 4250 Sodium iodate, 4624 Sodium nitrate, 4721 Sodium nitrite, 4720... 

D. Isomerization with Palladium and Other Group VIII Metals. 38... 

Stern showed rather conclusively that the palladium does not depart to leave a carbonium ion but that both hydride migration and collapse to an aldehyde proceed simultaneously. The removal of the /3 hydrogen in a complexes by the heavier Group VIII metals has been documented. Thus Chatt and Shaw (63) showed that a platinum hydride complex could undergo the reversible addition of ethylene ... 

A double bond in the 7,8-(VI) or 8,9-(VII) position of a steroid which has the trans A/B ring configuration isomerizes to the 8,14 position (VIII) when treated with hydrogen and a palladium catalyst or platinum in the presence of acetic acid (69) (Fig. 12). Once the double bond reaches the 8,14 position it cannot be hydrogenated however it may be isomerized by treatment with HCl in chloroform to yield a isomer which can be reduced catalytically. 

The reaction is catalyzed by a group VIII metal species, particularly that of rhodium or palladium. The initial metal species may be any variety of complexes (e.g., PdCl2 Pd acetate, etc.). A source of halide is necessary iodide is especially effective. The most convenient source is methyl iodide, since it is likely a reaction intermediate. In addition, an organic promoter must be included for catalytic activity. These promoters are generally tertiary phosphines or amines. Also, chromium complexes were found to have an important promotional effect. 

Detailed investigations on the kinetics and mechanisms of reactions of square planar palladium (II) complexes are largely lacking. However, enough data exist to show that the reactions of palladium (II) complexes are much faster than those of platinum (II), and that the two systems react by the same type of mechanism. Some of the data available are given in Table VIII along with the same information on platinum (II) and nickel (II) for comparison (3). The results show an approximate relative order of reactivity for analogous complexes of the triad as follows ... 

In certain cases, distortion may be imposed by the crystal structure. Nickel and palladium(II) fluorides have the rutile structure (16, 18, 239) in the nickel salt the fluoride octahedron is slightly flattened, whereas in palladium(II) fluoride, it is elongated. The metal-fluorine distances are shown in Table VIII. The fluoride is, incidentally, the only paramagnetic... 

The examples in Table III, show that the hydrogen atoms occupy tetrahedral holes at the beginning of the transition series. As we move along the transition series, we observe the interstitial hydride shift toward octahedral holes and the hydrides of the heavier elements become progressively unstable. Palladium is exceptional since it is the only heavy element of group VIII that gives a simple hydride. Hydride formation is accompanied in most cases by a change in metallic lattice type and in all cases by a considerable increase in metal-metal distances. 

Most Group VIII metals adsorb carbon monoxide dissociatively, and, consequently, they are good Fischer-Tropsch catalysts.240 In contrast, Pd, Pt, Ir, and Cu do not dissociate carbon monoxide. Of these metals, copper and more recently palladium were found to be excellent methanol-forming catalysts. 

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