Dicarbonyl rhodium

The xanthene-backbone derived diphosphines (129) also led to well-defined rhodium dicarbonyl hydride complexes. They were used in one-phase catalysis and two-phase separation after careful acidification of the system.415... 

The catalyst precursor generally used for the reaction is rhodium dicarbonyl acetylacetonate. However, detailed infrared studies under the reaction conditions (ca. 1000 bar CO/H2 and 200°C) have shown both the [Rh(CO)4] and the [Rh12(CO)34 36]2 anions to be present in various concentrations at different stages of the reaction (62, 63). It is suggested that rhodium carbonyl clusters, characterized as having three intense infrared absorptions at 1868 10, 1838 10, and 1785 10 cm-1, are responsible for the catalysis (62), and it is believed that the reaction is dependent upon the existence of the following equilibria ... 

The infrared spectrum of a solution of rhodium dicarbonyl acetylacetonate with either 2-hydroxypyridine or piperidine (4 molar excess over rhodium) in tetraglyme at 210°C under a CO/H2 pressure of either 714 or 1225 bar contained bands consistent with the existence of the above equilibria [Eq. (12)]. The concentration of [Rh12(CO)34]2 was found to increase as the CO/H2 pressure was increased. In the absence of either 2-hydroxypyridine or piperidine, a higher pressure (ca. 1700 bar) was... 

The combination of rhodium dicarbonyl acetylacetonate complex (Rh(acac)(CO)2) and a diphosphite ligand, (2,2 -bis[(biphenyl-2,2 -dioxy)phosphinoxy]-3,3 -di-/i t/-butyl-5,5 -dimethoxy-l,T-biphenyl (BIPHEPHOS), is an excellent catalyst system for the linear-selective hydroformylation of a wide range of alkenes. This catalyst system has been successfully applied to the cyclohydrocarbonylation reactions of alkenamides and alkenylamines, which are employed as key steps for the syntheses of piperidine,indolizidine, and pyrrolizidine alkaloids. ... 

A sample of the polymer of step 1 product was hydroformylated by reacting it with a mixture of carbon monoxide and hydrogen under 690 kPa for 4 hours catalyzed by rhodium dicarbonyl acetylacetonate (10.2 mg) and tris(2,4-di-/-butylphenyl)pho-sphite (48.2 mg) dissolved in toluene. H-NMR spectrum of this polymer showed the presence of carbonyl functional groups in place of ethylenic unsaturation. 

Fig. 1. Model of rhodium dicarbonyl complex on dealuminated Y zeolite, as determined by IR and EXAFS spectroscopies and density functional theory. The Rh atom, near the upper center of the figure, has two CO ligands bonded to it, pointing upward, and two oxygen atoms of the zeolite lattice below. An Al atom is located between these two oxygen atoms. The dangling atoms of the cluster model of the zeolite are capped by hydrogen atoms for the calculation (Goellner, Gates, etal., 2000).

Goellner, J. F., Gates, B. G, Vayssilov, G., and Rosch, N., Structure and bonding of a site-isolated transition metal complex Rhodium dicarbonyl in highly dealuminated zeolite Y. J. Am. Chem. Soc. 122, 8056 (2000). 

Miessner, H., Burkhardt, I., Gutschick, D., Zecchina, A., Morterra, C., and Spoto, G., The formation of a well-defined rhodium dicarbonyl in highly dealuminated rhodium-exchanged zeolite-Y by interaction with CO. J. Chem. Soc., Faraday Trans. 185,2113 (1989). 

Whether the rhodium dicarbonyl was attached to the zeolite lattice or to an extra-framework anion such as OH, 0 or a labile ion, could be also decided upon using IR spectroscopy. Indeed lattice vibration between 1300 and 300 cm- characteristic of an NaY zeolite (16) are sensitive to the interaction of lattice oxide ions with cations. In particular, it was observed that an IR absorption band at 877 cm- grew simultaneously with the growth of CO absorptions at 2115-2048 characteristic of the dicarbonyl (13).This... 

Reaction pathways There is general agreement as to the nature of the catalyst precursor (26. 30). It is well admitted that even under CO low pressure Rh(III)-Y was reduced to the monovalent rhodium dicarbonyl attached to the zeolite framework via one or more oxide ions irrespective of the rhodium introduction procedure onto the zeolite. 

The following step could be the reductive elimination of the acetyl halide to react with methanol. The growth of IR bands in the 1710-1685 cm- domain might be interpreted as due to CH COI accumulation and possibly further reaction with substrates present in the medium. Nevertheless readdition of CO restored the monovalent rhodium dicarbonyl thus indicative that somehow CH COI was eliminated. 

Rhodium Y zeolites appear to be efficient at significantly lower temperature than rhodium X zeolites. This might be due to the higher polarization ability of the Y type zeolite which would favor the methyl addition onto the rhodium dicarbonyl which is the slow step in the case of rhodium. 

Rhodium Dicarbonyl-(l,l, 1-trifluor-2,4-pentan-dionato)- E15/I, 557 (aus Diketon)... 

Rhodium Dicarbonyl-[2-(dimethyl-amino-methyl)-phenyl-C,N]-XlII/9b, 310... 

Fig. 10. FT-RAIRS spectrum of rhodium dicarbonyl Rh(CO)2 on TiO2(H0) [56, 69, 70] following dissociative adsorption of Rh2C04Cl2 (a). The molecule is reformed after desorption of the CO at 450K following exposure to lOOL of CO (b). Reaction of Rh(CO)2 with hydrogen forms the monocarbonyl Rh(H)CO [71].

Some of the most thoroughly characterized supported metal complexes are zeolite-supported metal carbonyls. These have been prepared, for example, by the adsorption of Rh(CO)2(acac) on zeolites (e.g., the faujasite zeolite NaY [26] or dealuminated zeolite Y [27]) followed by CO treatment of the resultant material (Fig. 19.3). The IR spectra (not shown, but found in [26, 27]) of the rhodium dicarbonyl represented in Fig. 19.3 are consistent with a square-planar complex (formally Rh(I)) with the Rh atom bonded to two zeolite oxygen atoms. 

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