Rhodium and Ruthenium

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

Important by-products are urea derivatives (ArNHC(0)NHAr) and azo compounds (Ar-N=N-Ar). The reaction is highly exothermic (—128kcalmol-1) and it is surprising that still such low rates are obtained (several hundred turnovers per hour) and high temperatures are required (130 °C and 60 bar of CO) to obtain acceptable conversions.533 Up to 2002, no commercial application of the new catalysts has been announced. Therefore, it seems important to study the mechanism of this reaction in detail aiming at a catalyst that is sufficiently stable, selective, and active. Three catalysts have received a great deal of attention those based on rhodium, ruthenium, and palladium. Many excellent reviews,534"537 have appeared and for the discussion of the mechanism and the older literature the reader is referred to those. Here we concentrate on the coordination compounds identified in relation to the catalytic studies.534-539... 

Intermolecular cyclopropanation of olefins poses two stereochemical problems enantioface selection and diastereoselection (trans-cis selection). In general, for stereochemical reasons, the formation of /ra ,v-cyclopropane is kinetically more favored than that of cis-cyclopropane, and the asymmetric cyclopropanation so far developed is mostly /ram-selective, except for a few examples. Copper, rhodium, ruthenium, and cobalt complexes have mainly been used as the catalysts for asymmetric intermolecular cyclopropanation. 

In the hydrogenation of alkenes, rhodium-, ruthenium- and iridium-phosphine catalysts are typically used [2-4]. Rhodium-phosphine complexes, such as Wilkinson s catalyst, are effective for obtaining alkanes under atmospheric pres-... 

Page, N.J., Cassard, D., Haffty, J. 1982a. Palladium, Platinum, Rhodium, Ruthenium, and Iridium in chromitites from the Massif du Sud and Tiebaghi Massif, New Caledonia. Economic Geology, 77, 1571-1577. 

Palladium, platinum, rhodium, ruthenium and iridium in chromite-rich rocks form the Semail ophiolite, Oman. Canadian Mineralogist, 20, 537-548. 

Although all of the above elements catalyze hydrogenation, only platinum, palladium, rhodium, ruthenium and nickel are currently used. In addition some other elements and compounds were found useful for catalytic hydrogenation copper (to a very limited extent), oxides of copper and zinc combined with chromium oxide, rhenium heptoxide, heptasulfide and heptaselen-ide, and sulfides of cobalt, molybdenum and tungsten. 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

Like other non-oxidic semiconductors in aqueous solutions, surface oxidized and photocorrosive InP is a poor photoelectrode for water decomposition [19,27,32,33], To enhance properties several efforts have focused on coupling of the semiconductor with discontinuous noble metal layers of island-like topology. For example, rhodium, ruthenium and platinum thin films, less than 10 nm in thickness, have been electrodeposited onto p-type InP followed by a brief etching treatment to achieve an island-like topology on the surface [27,28]. In combination with a Pt counter electrode, under AM 1.5 illumination of 87 mW/cm the metal (Pt, Rh, Ru) functionalized p-InP photocathodes [27] see a reduction in the threshold voltage for water electrolysis from 1.23 V to 0.64 V, and in aqueous HCl solution a photocurrent density of 24 mA/cm with a photoconversion efficiency of 12% [27]. 

The results observed in the hydrogenation of higher dialkylalkynes are in general accord with those observed with but-2-yne. Thus, for example, the selectivity and stereoselectivity observed in the liquid phase hydrogenation of pent-2-yne over carbon-supported palladium, rhodium, ruthenium and platinum and iridium—alumina [145], as shown in Table 22, show a similar pattern to that observed in but-2-yne hydrogenation. Similarly, the... 

Investigation of non-ferrous metal chelates that can efficiently absorb NO and provide stability toward oxidation. Many cobalt, copper, manganese, molybdenum, nickel, osmium, rhenium, rhodium, ruthenium and vanadium chelates have been demonstrated to be able to coordinate NO. Their application to flue gas scrubbing systems should be explored. 

The catalytic reduction of copper by hydrogen has been investigated on platinum, rhodium, ruthenium, and palladium. Thermodynamics show that Cu2+ in aqueous solution can be reduced by molecular hydrogen at room temperature. However, a copper solution under a hydrogen flow is perfectly stable because of the slow kinetics of the reaction. Metals such as platinum, palladium, and rhodium, which are able to activate hydrogen, can catalyze the reduction of copper. 

Ethylene dimerization catalysis has, however, been more thoroughly investigated for the broader range of homogeneous catalysts. For example, active metal complexes containing titanium, nickel, iron, cobalt, rhodium, ruthenium, and palladium, are all known (133). Where possible, comparisons will be made with the relevant homogeneous catalyst systems. 

Freifelder obtained an 82% yield of benzylhydrazine by hydrogenating a freshly prepared hydrazone over Pd-C in ethanol at 0.3 MPa H2 in less than 30 min.83 However, when the hydrazone was allowed to stand for several days to a week, the yield dropped to 45-48%. In the hydrogenation of phenylacetone hydrazone, Biel et al. observed that the formation of large amounts of jV,iV -bis(l -phenyl-2-propylidene)hydrazine took place when hydrogenation proceeded slowly and incompletely with such catalysts as Pd-C, rhodium, ruthenium, and platinum oxide, and with solvents such as alcohol, water, ethyl acetate, tetrahydrofuran, and dioxane. The A(A%disubstituted hydrazine was obtained when the hydrogenation proceeded slowly to completion, as over platinum oxide in aqueous acetic acid. With Raney Ni in ethanol, the azine and l-pheny-2-propylamine were formed almost exclusively. 1-Phenyl-2-propylhy-drazine was obtained in acceptable yields of 55-70% by use of platinum oxide or supported platinum in alcoholic acetic acid at a pressure of 13.8 MPa H2. The products obtained over platinum oxide in various conditions are summarized in eq. 8.40.78... 

Platinum catalysts have been shown to be highly selective for the hydrogenation of halonitrobenzenes to haloanilines. A number of effective platinum catalysts or catalyst systems have been described in the literature, mostly in patents.96 Dovell and Greenfield found that the sulfides of the platinum metals and cobalt were highly selective in the hydrogenation of halo-substituted nitrobenzenes.117-119 There was no detectable dechlorination with the sulfides of palladium, platinum, rhodium, ruthenium, and cobalt no detectable debromination occurred with platinum sulfide trace debromination occurred with rhodium sulfide and cobalt sulfide and appreciable debromination occurred with palladium sulfide. Typical hydrogenations with 5% platinum sulfide on carbon catalyst are given in eqs. 9.52 and 9.53 with 2,5-dichloronitrobenzene and p-bromobenzene, respectively.118... 

Purification of Iridium.—In practice the purest iridium obtained by the foregoing process invariably contains small quantities of platinum, rhodium, ruthenium, and iron. In order to remove these, Matthey7 treats the metal as follows. The iridium, in a fine state of division, is fused with ten times its weight of lead, and kept in the molten condition... 

The solution from which the ammonium chlor-platinate has been separated still contains a little platinum with practically all the rhodium, ruthenium, and palladium. All are precipitated by metallic iron and dissolved in aqua regia. Addition of more ammonium chloride effects the precipitation of the remaining platinum, and the filtrate may be worked for the other metals (see p. 154). 

In an extension of this work, either zinc(II), palladium(II), rhodium(I) or copper(I) salts were immobilised in an ionic liquid film (SILP, vide supra) onto diatomic earth and the catalysts tested for activity in the reaction between phenylacetylene and 4-isopropyl-phenylaminc.1 391401 The supported rhodium, ruthenium and zinc complexes afford higher rates and selectivities relative to their use under homogenous reaction conditions. Lower rates are, however, observed with the copper salt, which is rationalised by strong complexation of the ionic liquid to the Cu(I) centre. 

As early as 1938, Roelen discovered the cobalt-catalyzed hydroformylation of olefins, then known as the oxo reaction, which allowed the synthesis of aldehydes by addition of carbon monoxide and hydrogen to alkenes. Not long after this discovery it was found that cobalt, rhodium, ruthenium and platinum are also suitable as catalysts. However, because of the considerable price advantage for large scale applications in industry, cobalt catalysts are mostly used. Rhodium complexes, however, are... 

In a series of papers, Holah et al. studied the donor character of TPPO, TPPS, and TPPSe towards rhodium, ruthenium, and rhenium. Treatment of TPPO with rhodium(III) chloride hydrate in ethanol causes reduction to Rh(I) and gives the dimeric complex (XXVI) in which TPPO is j -bonded to the metal. 

A range of metal catalysts have also been studied in aqueous solution for the transformation of carbon dioxide, including rhodium, ruthenium and iridium bipyridine or phenanthroline complexes.One of the most effective systems is the iridium complex shown in Figure 3.14. The ligand design concept used in this study is very clever. The catalytic activity of the complex and its solubility in aqueous solution can be tuned by the pH of the solution.Under acidic... 

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