Carbonylative Rh

Binary carbonyls Rh Co4 (CO)i2 are available Rh2Co2(CO)i2 and RhCo3(CO)i2 are stable and useful in preparing different supported Rh-Co bimetallic catalysts. 

Ethylene hydroformylation was treated as a separate case, as difSculties arise from dramatic changes in the IR spectrum of dissolved ethylene as a function of its partial pressure. This was overcome using the method of band-target entropy minimisation (BTEM, see Chapter 4) to recover the pure component spectra of all observable species and their concentrations [72]. As well as the conventional acyl tetra-carbonyl, [Rh(C(0)Et)(C0)4], evidence was obtained for [Rh(C(0)Et)(C0)3(C2H4)], containing coordinated ethylene. The presence of this species indicates that ethylene can compete with H2 for the unsaturated [Rh(C(0)Et)(C0)3]. The ketone and polyketone side products of Rh-catalysed ethylene hydroformylation arise from insertion of coordinated ethylene into the Rh-acyl bond in [Rh(C(0)Et)(C0)3(C2H4) ... 

The reduction of metal ions in higher oxidation states by CO and H20 has been known for many years. Work on the reduction of Hg2+, Ag+, Ni2+, Cu2 +, and Pd2+ has been summarized recently (4). The reduction of these metal ions does not proceed via a stable intermediate carbonyl. Since a metal carbonyl must be an intermediate in this reaction, however, the coordinated carbonyl must be very susceptible to attack by water, reacting as soon as it is formed. The ability of a metal in a higher oxidation state to activate a coordinated carbonyl to attack by as weak a nucleophile as water was noted previously in the description of the work by James et al., on the reduction of rhodium(III) by carbon monoxide and water (62). Here a stable rhodium(III) carbonyl, Rh(CO)Cl2-, can be observed as the initial product of reaction of RhCl3 3HzO with CO. The Rh(III) is then efficiently reduced to the rhodium(I) anion [RhCl2(CO)2], even in nonaqueous solvents such as dimethylacetamide, where the only water available for reaction is the water of hydration of the starting rhodium chloride. 

Carbonylation Rh or Ru-acetate in HBF4 + phosphine Rh or Ru-acetate in Not specified Not specified 121... 

Other M—M bonds (substituted iron-group carbonyls, Rh or Ir carbonyls etc.,) may be expected to react with Ge—H and are worth study. 

Sulfated zirconia catalyst (S-Zr02) was also used for carbonylation of methanol (76) and DME (77). It has been shown that S-Zr02 exhibits low activity and selectivity in methanol carbonylation similar to pure acidic zeolite catalysts (see later under Zeolite Catalysts). In DME carbonylation, Rh-modified sulfated zirconia is less active and selective as compared to Rh-modified cesium salts of 12-tungstophosphoric acid (see later under Heteropoly Acid Catalysts). 

Scheme 6.9 Rh-catalyzed hydroformylation of styrene and hydrido-carbonyl Rh(I) and Ir(I) complexes...

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). 

There ate numerous alkyltitaniums, and many of their reactions resemble those of alkyllithiums and alkylmagnesium halides. They ate protolyzed by water and alcohols, R Ti(R )3 + HA — RH + A-Ti(R )3 they insert oxygen, R TiR + O2 — ROTiR and they add to a carbonyl group ... 

The use of CO is complicated by the fact that two forms of adsorption—linear and bridged—have been shown by infrared (IR) spectroscopy to occur on most metal surfaces. For both forms, the molecule usually remains intact (i.e., no dissociation occurs). In the linear form the carbon end is attached to one metal atom, while in the bridged form it is attached to two metal atoms. Hence, if independent IR studies on an identical catalyst, identically reduced, show that all of the CO is either in the linear or the bricked form, then the measurement of CO isotherms can be used to determine metal dispersions. A metal for which CO cannot be used is nickel, due to the rapid formation of nickel carbonyl on clean nickel surfaces. Although CO has a relatively low boiling point, at vet) low metal concentrations (e.g., 0.1% Rh) the amount of CO adsorbed on the support can be as much as 25% of that on the metal a procedure has been developed to accurately correct for this. Also, CO dissociates on some metal surfaces (e.g., W and Mo), on which the method cannot be used. 

Because they possess an odd number of valence electrons the elements of this group can only satisfy the 18-electron rule in their carbonyls if M-M bonds are present. In accord with this, mononuclear carbonyls are not formed. Instead [M2(CO)s], [M4(CO)i2] and [M6(CO)i6] are the principal binary carbonyls of these elements. But reduction of [Co2(CO)g] with, for instance, sodium amalgam in benzene yields the monomeric and tetrahedral, 18-electron ion, [Co(CO)4] , acidification of which gives the pale yellow hydride, [HCo(CO)4]. Reductions employing Na metal in liquid NH3 yield the super-reduced [M(CO)3] (M = Co, Rh, Ir) containing these elements in their lowest formal oxidation state. 

The structures are shown in Fig. 26.8c and d and differ in that, whereas the Ir compound consists of a tetrahedron of metal atoms held together solely by M-M bonds, the Rh and Co compounds each incorporate 3 bridging carbonyls. A similar difference was noted in the case of the trinuclear carbonyls of Fe, Ru and Os (p. 1104) and can be explained in a similar way. The M4 tetrahedra of Co and Rh are small enough to be accommodated in an icosahedral array of CO ligands whereas the larger Ir4 tetrahedron forces the adoption of the less dense cube octahedral array of ligands. 

Carbonyl hydrides and carbonylate anions are obtained by reducing neutral carbonyls, as mentioned above, and in addition to mononuclear metal anions, anionic species of very high nuclearity have been obtained, often by thermolysis. These are especially numerous for Rh and in certain Rh, Rh and Rhi5 anions have structures conveniently visualized either as polyhedra encapsulating further metal atoms, or alternatively as arrays of metal atoms forming portions of hexagonal close packed or body... 

Cases of the S-coordinated rhodium and iridium are quite scarce. To complete the picture, we next consider the possibilities of S-coordination using complicated derivatives of thiophene. 2,5-[Bis(2-diphenylphosphino)ethyl]thiophene is known to contain three potential donor sites, two phosphorus atoms and the sulfur heteroatom, the latter being a rather nucleophilic center (93IC5652). A more typical situation is coordination via the phosphorus sites. It is also observed in the product of the reaction of 2,5-bis[3-(diphenylphosphino)propyl]thiophene (L) with the species obtained after treatment of [(cod)Rh(acac)] with perchloric acid (95IC365). Carbonylation of [Rh(cod)L][C104]) thus prepared yields 237. Decarbonylation of 237 gives a mixture of 238 and the S-coordinated species 239. Complete decarbonylation gives 240, where the heterocycle is -coordinated. The cycle of carbonylation decarbonylation is reversible. 

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