Cobalt and Rhodium

6 Cobalt and Rhodium Cobalticinium moieties derived from 

6 Cobalt and Rhodium The molecular structure of [(Pr 2PCH2)2)Rh((tl cat)-Beat ], HBcat - catecholatoborane, prepared by reaction of [RhQ(N2)(n 3)2] with catecholatoborane, has been reported. The crystal structure of [Rh(dppf)(ii -C5H5)BI%3], ri pf s diphenylphosphinofenocene, has also been described.  

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Propane, 1-propanol, and heavy ends (the last are made by aldol condensation) are minor by-products of the hydroformylation step. A number of transition-metal carbonyls (qv), eg, Co, Fe, Ni, Rh, and Ir, have been used to cataly2e the oxo reaction, but cobalt and rhodium are the only economically practical choices. In the United States, Texas Eastman, Union Carbide, and Hoechst Celanese make 1-propanol by oxo technology (11). Texas Eastman, which had used conventional cobalt oxo technology with an HCo(CO)4 catalyst, switched to a phosphine-modified Rh catalyst ia 1989 (11) (see Oxo process). In Europe, 1-propanol is made by Hoechst AG and BASE AG (12). 

Related substitution patterns are observed in tetranuclear cobalt and rhodium clusters. Thus, the small ligand, P(OMe) (L), occupies axial sites in [Co,(CO)1 L ] (x = 1,2) (17) whereas steric effects become important witfi (OPh) and the isomers shown in Fig. 2 are obtained with tetrarhodium derivatives. 

In the iron, ruthenium, osmium, cobalt, and rhodium complexes the xanthato ligands are isobidentate chelating. Selected examples are zra s-Ru(S2COEt)2(P-Me2Ph)2,265 cis- and zra s-Os(S2COMe)2(PPh3)2,266 Co(S2 COMe)3.267... 

VIII. Catalysts Other Than Cobalt and Rhodium. 53... 

The reaction rates of various types of olefins follow much the same pattern with both cobalt- and rhodium-catalyzed systems. Wender and co-workers (47) classified the nonfunctional substrates as straight-chain terminal, internal, branched terminal, branched internal, and cyclic olefins. The results they obtained are given in Table III. 

With cobalt catalysts, hydroformylation of ethyl cinnamate gave 91% of the hydrogenation product ethyl hydrocinnamate (15) and only 8% of the expected lactone, 16 (72). However, rhodium catalysis was effective in directing the reaction in favor of hydroformylation (70). The comparative results obtained with cobalt and rhodium are outlined in Table XXV. 

Another route to the diol monomer is provided by hydroformylation of allyl alcohol or allyl acetate. Allyl acetate can be produced easily by the palladium-catalyzed oxidation of propylene in the presence of acetic acid in a process similar to commercial vinyl acetate production. Both cobalt-and rhodium-catalyzed hydroformylations have received much attention in recent patent literature (83-86). Hydroformylation with cobalt carbonyl at 140°C and 180-200 atm H2/CO (83) gave a mixture of three aldehydes in 85-99% total yield. 

While cobalt and rhodium have been the focus of most research and are the metals of choice for commercial hydroformylation reactions, numerous other metals have been disclosed as catalysts in the patent literature. However, only some of the carbonyl-forming metals can be seriously considered. Even of these, a comparison of relative reactivity (118) based on cobalt as the standard indicates a decided preference for only two or three metals. This listing may be considered incomplete without the inclusion of platinum and copper, which have recently received significant attention (vide infra). 

The mechanism was similar to those involving cobalt and rhodium, and is depicted in Fig. 10. 

Of the three catalytic systems so far recognized as being capable of giving fast reaction rates for methanol carbonylation—namely, iodide-promoted cobalt, rhodium, and iridium—two are operated commercially on a large scale. The cobalt and rhodium processes manifest some marked differences in the reaction area (4) (see Table I). The lower reactivity of the cobalt system requires high reaction temperatures. Very high partial pressures of carbon monoxide are then required in the cobalt system to... 

Comparison of Cobalt- and Rhodium-Catalyzed Methanol Carbonylation Reactions... 

The synthesis of metalloporphyrins which contain a metal-carbon a-bond can be accomplished by a number of different methods(l,2). One common synthetic method involves reaction of a Grignardreagent or alkyl(aryl) lithium with (P)MX or (PMX)2 where P is the dianion of a porphyrin macrocycle and X is a halide or pseudohalide. Another common synthetic technique involves reaction of a chemically or electrochemically generated low valent metalloporphyrin with an alkyl or aryl halide. This latter technique is similar to methods described in this paper for electrosynthesis of cobalt and rhodium a-bonded complexes. However, the prevailing mechanisms and the chemical reactions... 

The reaction between alkenes and synthesis gas (syngas), an equimolar mixture of carbon monoxide and hydrogen, to form aldehydes was discovered in 1938 by Otto Roelen [1,2]. Originally called oxo-reaction , hydroformyla-tion is the term used today. This reflects the formal addition of formaldehyde to the olefinic double bond. Commercially, homogeneous metal complexes based on cobalt and rhodium are used as catalysts. With more than 10 million metric tons of oxo products per year, this reaction represents the most important use of homogeneous catalysis in the chemical industry. 

A similar pattern has always been discussed for rhodium, with hydri-dotetracarbonylrhodium H-Rh(CO)4 as a real catalyst species. The equilibria between Rh4(CO)i2 and the extremely unstable Rh2(CO)s were measured by high pressure IR and compared to the respective equilibria of cobalt [15,16]. But it was only recently that the missing link in rhodium-catalyzed hydroformylation, the formation of the mononuclear hydridocomplex under high pressure conditions, has been proven. Even the equilibria with the precursor cluster Rh2(CO)8 could be determined quantitatively by special techniques [17]. Recent reviews on active cobalt and rhodium complexes, also ligand-modified, and on methods for the necessary spectroscopic in situ methods are given in [18,19]. 

A broad range of olefins, acetals, epoxides, alcohols, and chlorides were demonstrated effective alternative starting materials. Cobalt and rhodium carbonyls and bimetallic complexes were shown to catalyze the domino hydro-... 

In support of the previous statement that iridium has a lower predisposition to form bridging carbonyl bonds compared to cobalt and rhodium, in this case, only three of the fifteen carbonyl groups assume a doubly bridging disposition whereas all the others have a terminal arrangement. 

Scheme 6.27 considers other, formally confined, conformers of cycloocta-l,3,5,7-tetraene (COT) in complexes with metals. In the following text, M(l,5-COT) and M(l,3-COT) stand for the tube and chair structures, respectively. M(l,5-COT) is favored in neutral (18-electron) complexes with nickel, palladium, cobalt, or rhodium. One-electron reduction transforms these complexes into 19-electron forms, which we can identify as anion-radicals of metallocomplexes. Notably, the anion-radicals of the nickel and palladium complexes retain their M(l,5-COT) geometry in both the 18- and 19-electron forms. When the metal is cobalt or rhodium, transition in the 19-electron form causes quick conversion of M(l,5-COT) into M(l,3-COT) form (Shaw et al. 2004, reference therein). This difference should be connected with the manner of spin-charge distribution. The nickel and palladium complexes are essentially metal-based anion-radicals. In contrast, the SOMO is highly delocalized in the anion-radicals of cobalt and rhodium complexes, with at least half of the orbital residing in the COT ring. For this reason, cyclooctateraene flattens for a while and then acquires the conformation that is more favorable for the spatial structure of the whole complex, namely, M(l,3-COT) (see Schemes 6.1 and 6.27). 

Apparently real metallasilsesquioxanes of cobalt, rhodium or iridium have thus far not been reported in the literature. However, several silsesquioxane ligands containing donor-substituted side chains as well as related silsesquioxane dendri-mers have been found to form complexes with cobalt and rhodium complex frag-ments.i°3.ii4 ns... 

Table 8.5 Mono- and bimetallic cobalt- and rhodium-based catalysts prepared from carbonyl compounds and used in the CO hydrogenation and/or hydroformylation reactions.

Following the success with cobalt and rhodium, Shibata reported Ir(i)-based enantioselective catalytic reaction. Right after their observation that the efficiency of [IrCl(COD)]2-catalyzed PKR substantially increased by addition of a phosphane co-ligand, they moved directly to use chiral phosphanes and examined the enantioselectivity. " TON and TOE of the reaction were low and the number of examples was limited. Typically, the reaction required a fair amount of Ir(i) catalyst [IrCl(COD)]2 (0.1-0.15 equiv.) and (reaction time. However, this has remained as the best in terms of enantioselectivity to date. Moreover, this catalytic system provided the first asymmetric intermolecular reaction as well. 

Several differences between the cobalt- and rhodium-catalyzed processes are noteworthy with regard to mechanism. Although there is a strong dependence in the cobalt system of the ethylene glycol/methanol ratio on temperature, CO partial pressure, and H2 partial pressure, these dependences are much lower for the rhodium catalyst. Details of the product-forming steps are therefore perhaps quite different in the two systems. It is postulated for the cobalt system that the same catalyst produces all of the primary products, but there seems to be no indication of such behavior for the rhodium system. Indeed, the multiplicity of rhodium species possibly present during catalysis and the complex dependence on promoters make it... 

The nitro-pentammino-iridium salts resemble those of chromium, cobalt, and rhodium, and react similarly towards reagents, but they are more stable towards acids. They give on long heating with hydrochloric acid chloro-pentammino-derivatives, but are not decomposed by concentrated nitric acid nor by aqua-regia, and only slowly decompose on heating with concentrated sulphuric acid at 100° C. ... 

Although the halogenated chelates of chromium, cobalt, and rhodium would be difficult to prepare from the sensitive 3-halo-2,4-pentanediones, the copper (II) bromochelate was synthesized both from the bromodiketone and by direct bromination of copper acetylacetonate. The relatively labile copper chelates form much more rapidly than the kinetically stable chelates of chromium, cobalt, and rhodium. 

The acetylacetonates of chromium, cobalt, and rhodium were found to react with dimethylformamide in the presence of phosphorus oxychloride to yield formyl-substituted chelates (10). This is a well known technique for the introduction of an aldehyde group into reactive aromatic systems. The formylation of the chelate rings is a slow reaction, and by controlling the conditions it is possible to... 

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