Organorhodium compounds

An investigation of different organic solvents, buffer, surfactants, and organorhodium compounds established that the catalytic reduction of tetralin using [ Rh(l,5-hexadiene)Cl 2] proceeds with high efficiency at high substrate-to-catalyst ratios. The reaction occurs at r. t. and 1 atm. pressure in a biphasic mixture of hexane and an aqueous buffer containing a low concentration of a surfactant which stabilizes the catalysts.314... 

Although addition of organoiridium compounds to carbonyls and CN multiple bonds has been little investigated in comparison with those of organorhodium compounds, several characteristic reactions have been disclosed recently. 

CO Chemisorption on Clean Rh(lll) (23). The vibrational spectra of carbon monoxide chemisorbed on Rh(lll) at 300 K as a function of exposure are shown in Figure 2. At very low exposures (less than 0.1 L 1 L = 1 Langmuir 10-6 torr sec) only one peak at 1990 cm- - is observed in the C = 0 stretching region and no ordered LEED pattern is found. By comparison with the infrared spectra of relevant organorhodium compounds (50,... 

Rhodium(I) complexes are efficient catalysts for the valence isomerizations of cubane (23a equation 7) and its derivatives to the corresponding iy/j-tricyclo[4.2.0.CF ]octa-3,7-dienes. When a stoichiometric amount of [ RhQ(CO)2)2l is added to a chloroform solution of cubane, the organorhodium compound (24) is obtained in about yield. Treatment of (24) with a stoichiometric amount of triphenylphos-phine gives, in about 90% yield, the polycyclic ketone (25) together with a small amount (5-10%) of cy-clooctatetraene. A similar reaction is also reported for the methoxycarbonyl derivative (23b). 

Organorhodium compounds are found in oxidation states ranging fi-om +6 to -3. However, the most conunon oxidation states are +1 and +3. Rh(I) species exist as both a tetra-coordinated, square planar complex as well as a penta-coordinated trigonal bipyramidal complex. Rhodium, like platinum group metals, exhibits reversible oxidation states making a broad range of organic-catalytic transformations possible. 

Organorhodium compounds are prepared by similar methods to the preparation methods of organoruthenium compounds described in Chapter 16. 

Rhodiumcarbonyls such as Rh4(CO)i2 and Rh CCO) , form stable clusters as described below, and their reactivities are low compared to ironcarbonyl (Fe(CO>5) and rutheniumcarbonyl (Ru3(CO)i2). [RhCl(CO)2]2 is available as the raw material for the synthesis of organorhodium compounds. [RhCl(CO)2]2 is a stable red crystal. A highly pure crystal is easily obtained by sublimation. [RhCl(CO)2]2 reacts easily with various compounds as shown in Scheme 18.1 [7,19,20]. 

As the organorhodium compound of dipentadienylmetal compounds (CP2M), similar to ferrocene, is a 19-electron compound, it is similar to the cobalt compound (CP2C0), and is not stable. Hence, Cp2Rh is not available to be used as the raw material of the synthesis of organorhodium compounds. The representative cyclopentadienylrhodium compound is [Cp RhCl]2 which is prepared by the reaction of pentamethylcyclopentadiene with rhodium chloride as shown in eq. 

CpRh(CO)2 (Cp Rh(CO)2) and CpRh(PPh3)2 [29] are obtained from a Wilkinson complex as shown in eq. (18.17), and are also used as raw material for the synthesis of organorhodium compounds [7]. The reaction as shown in eq. (18.17) yields a benzene complex (CpRh(PPh3)2 C6H6). However, benzene is able to be removed by heating at 140°C under reduced pressure [29]. 

Figure 18.1 Organorhodium compounds coordinated by // -cyclopentadienide and diene or two olefins [30-33].

---