Rhodium activations

A significant step was made by neutron irradiation of ruthenocene. It was found that 20—25% of the ruthenium activity was recoverable as ruthenocene, and that also considerable rhodium activity was sublimed along with the ruthenocene. The rhodium was identified as being rhodium dicyclopentadienide, produced in high yield by the decay of ruthenocene. 

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... 

Fig. 7 QM/MM partitions and methods used fra- modeling the rhodium active species and the corresponding difference in energy (in kcal mol ) between the lowest Pro-S and Pro-/f transition states in the hydrogenation of methyl-(N)-acetylaminoacrylate...

Mechanistically, the Monsanto process actually involves two interrelated catalytic cycles, one organometallic-based on rhodium ionic species, and an organic one where iodide should be considered as the true catalyst (Scheme 5). The two main components (rhodium and iodide) can be added in many forms. Under the reaction conditions Rhl3 is reduced by H2O and CO to monovalent rhodium active species 58, and methanol is converted to the iodo form, Mel. 

There is more to tire Wilkinson hydrogenation mechanism tlian tire cycle itself a number of species in tire cycle are drained away by reaction to fomi species outside tire cycle. Thus, for example, PPh (Ph is phenyl) drains rhodium from tire cycle and tlius it inliibits tire catalytic reaction (slows it down). However, PPh plays anotlier, essential role—it is part of tire catalytically active species and, as an electron-donor ligand, it affects tire reactivities of tire intemiediates in tire cycle in such a way tliat tliey react rapidly and lead to catalysis. Thus, tliere is a tradeoff tliat implies an optimum ratio of PPh to Rli. 

Similar activation takes place in the carbonylation of dimethyl ether to methyl acetate in superacidic solution. Whereas acetic acid and acetates are made nearly exclusively using Wilkinson s rhodium catalyst, a sensitive system necessitating carefully controlled conditions and use of large amounts of the expensive rhodium triphenylphosphine complex, ready superacidic carbonylation of dimethyl ether has significant advantages. 

Hydrogenation of benzene and other arenes is more difficult than hydrogenation of alkenes and alkynes Two of the more active catalysts are rhodium and platinum and it IS possible to hydrogenate arenes m the presence of these catalysts at room temperature and modest pressure Benzene consumes three molar equivalents of hydrogen to give cyclohexane... 

Nickel catalysts although less expensive than rhodium and platinum are also less active Hydrogenation of arenes m the presence of nickel requires high temperatures (100-200°C) and pressures (100 atm)... 

In 1968 a new methanol carbonylation process using rhodium promoted with iodide as catalyst was introduced by a modest letter (35). This catalyst possessed remarkable activity and selectivity for conversion to acetic acid. Nearly quantitative yields based on methanol were obtained at atmospheric pressure and a plant was built and operated in 1970 at Texas City, Tex. The effect on the world market has been exceptional (36). 

Acetyl chlotide is reduced by vatious organometaUic compounds, eg, LiAlH (18). / fZ-Butyl alcohol lessens the activity of LiAlH to form lithium tti-/-butoxyalumium hydtide [17476-04-9] C22H2gA102Li, which can convert acetyl chlotide to acetaldehyde [75-07-0] (19). Triphenyl tin hydtide also reduces acetyl chlotide (20). Acetyl chlotide in the presence of Pt(II) or Rh(I) complexes, can cleave tetrahydrofuran [109-99-9] C HgO, to form chlorobutyl acetate [13398-04-4] in about 72% yield (21). Although catalytic hydrogenation of acetyl chlotide in the Rosenmund reaction is not very satisfactory, it is catalyticaHy possible to reduce acetic anhydride to ethylidene diacetate [542-10-9] in the presence of acetyl chlotide over palladium complexes (22). Rhodium trichloride, methyl iodide, and ttiphenylphosphine combine into a complex that is active in reducing acetyl chlotide (23). 

Rhodium Ca.ta.lysts. Rhodium carbonyl catalysts for olefin hydroformylation are more active than cobalt carbonyls and can be appHed at lower temperatures and pressures (14). Rhodium hydrocarbonyl [75506-18-2] HRh(CO)4, results in lower -butyraldehyde [123-72-8] to isobutyraldehyde [78-84-2] ratios from propylene [115-07-17, C H, than does cobalt hydrocarbonyl, ie, 50/50 vs 80/20. Ligand-modified rhodium catalysts, HRh(CO)2L2 or HRh(CO)L2, afford /iso-ratios as high as 92/8 the ligand is generally a tertiary phosphine. The rhodium catalyst process was developed joindy by Union Carbide Chemicals, Johnson-Matthey, and Davy Powergas and has been Hcensed to several companies. It is particulady suited to propylene conversion to -butyraldehyde for 2-ethylhexanol production in that by-product isobutyraldehyde is minimized. 

Other Rhodium Processes. Unmodified rhodium catalysts, eg, 1 14(00)22 [19584-30-6] have high hydroformylation activity but low selectivity to normal aldehydes. 

Ruthenium. Ruthenium, as a hydroformylation catalyst (14), has an activity signiftcandy lower than that of rhodium and even cobalt (22). Monomeric mthenium carbonyl triphenylphosphine species (23) yield only modest normal to branched regioselectivities under relatively forcing conditions. For example, after 22 hours at 120°C, 10 MPa (1450 psi) of carbon monoxide and hydrogen, biscarbonyltristriphenylphosphine mthenium [61647-76-5] ... 

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). 

A related but distinct rhodium-catalyzed methyl acetate carbonylation to acetic anhydride (134) was commercialized by Eastman in 1983. Anhydrous conditions necessary to the Eastman acetic anhydride process require important modifications (24) to the process, including introduction of hydrogen to maintain the active [Rhl2(CO)2] catalyst and addition of lithium cation to activate the alkyl methyl group of methyl acetate toward nucleophilic attack by iodide. 

Efficient enantioselective asymmetric hydrogenation of prochiral ketones and olefins has been accompHshed under mild reaction conditions at low (0.01— 0.001 mol %) catalyst concentrations using rhodium catalysts containing chiral ligands (140,141). Practical synthesis of several optically active natural... 

The most common oxidation states, corresponding electronic configurations, and coordination geometries of iridium are +1 (t5 ) usually square plane although some five-coordinate complexes are known, and +3 (t7 ) and +4 (t5 ), both octahedral. Compounds ia every oxidation state between —1 and +6 (<5 ) are known. Iridium compounds are used primarily to model more active rhodium catalysts. 

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