Rhodium complexes, equilibrium

The equilibrium concentrations of Rh2(CO)s and Rh4(CO)i2 were determined by infrared spectroscopy by monitoring the absorbance of the band at 1886.8 cm-1, which corresponds to the stretching of the bridging carbonyls of the tetrarhodium complex. More details of the experimental procedure can be found in the original papers. For our purpose, it is enough to say that the equilibrium concentrations of the rhodium complexes were quite low (< 10-3 mol dm-3), but the same was not true for the CO concentration ( 2 mol dm-3 see... 

The characterization of the rhodium complexes formed under hydro-formylation conditions by NMR techniques and in situ IR spectroscopy showed that there is a relationship between the structure of the [RhH(CO)2 (P-P)] (P-P = 4, 19-23) species and their enantiodiscriminating performance. In general, enantioselectivity was highest with ligands that have a strong bis-equatorial (ee) coordination preference, while an equilibrium of... 

The results do not prove that in the reaction conditions used the alkyl formation is not reversible, but only that, if it is reversible, the carbon monoxide insertion on both diastereomeric rhodium-alkyls must be much faster than the rhodium-alkyls decomposition. Restricting this analysis of the asymmetric induction phenomena to the rhodium-alkyl complexes formation, two 7r-olefin complexes are possible for each diastereomer of the catalytic rhodium complex (see Scheme 11). The induction can take place in the 7r-olefin complexes formation (I — II(S) or I — II(R)) or in the equilibrium between the diastereomeric 7r-olefin complexes (II(r) and... 

II(S)) and/or to a different reaction rate of the two diastereomeric 7r-olefin complexes to the corresponding diastereomeric alkyl-rhodium complexes (VI(s) and VI(R)). For diastereomeric cis- or trans-[a-methylbenzyl]-[vinyl olefin] -dichloroplatinum( II) complexes, the diastereomeric equilibrium is very rapidly achieved in the presence of an excess of olefin even at room temperature (40). Therefore, it seems probable that asymmetric induction in 7r-olefin complexes formation (I — II) cannot play a relevant role in determining the optical purity of the reaction products. On the other hand, both the free energy difference between the two 7r-olefin complexes (AG°II(S) — AG°n(R) = AG°) and the difference between the two free energies of activation for the isomerization of 7r-com-plexes II(S) and II(R) to the corresponding alkyl-rhodium complexes VI(s) and VI(R) (AG II(R) — AG n(S) = AAG ) can control the overall difference in activation energy for the formation of the diastereomeric rhodium-alkyl complexes and hence the sign and extent of asymmetric induction. 

Modification of Metal-Metal Bonding in Rhodium Complexes by a Bridging Pi phosphine. The yellow, planar complexes, (RNC)i,Rh+, undergo novel self-association reactions in concentrated solution to form the blue or violet dimers, (RNC)8Rh22+, via reaction (1) (1,2). The equilibrium constants for this reaction are strongly... 

Figure 22-7 Simplified catalytic cycle for hydroformylation using rhodium complexes. Note that the configurations of complexes are not known with certainty and that five-coordinate species are fluxional. Rhodium can be added as Rh(acac)(CO)(PPh3), HRh(CO)(PPh3)3, or similar complexes. The solvent in QH4 or CHjCH=CH2 hydroformylation is the aldehyde trimer which is in equilibrium with aldehyde.

Fig. 13 Dynamic equilibrium of a rhodium complex observed by NMR in [bmim]PF6 [63]...

Other rhodium complex also catalyzed the addition of the C-H bond in aldehyde to olefins [115-117]. The use of paraformaldehyde results in the formation of aldehydes [115]. Marder et al. proposed the reaction mechanism of CpRh(eth-ylene)2-catalyzed addition of C-H bond in aldehyde to ethylene by the use of isotope-labeling experiments [117]. They suggested that insertion of ethylene to the Rh-H bond must take place rapidly and reversibly, and this equilibrium must be established significantly faster than either aldehyde reductive elimination or product formation (Scheme 4). 

These catalytic reactions of dihydrosilanes make possible the use of asymmetric catalysts to produce chiral silicon compounds. Introduction of a chiral ligand L on the rhodium complex will not change the validity of the kinetic Scheme 12. However, in this case complexes 56 and 57 will be diastereomeric and their equilibrium concentrations will be different. The ratio of the substituted silanes will be close to k, [56] k2 [57]. 

In the hydroformylation of w-1-hexene the rhodium concentration was varied at a low P/Rh ratio of 20 1 to 40 1. By increasing the rhodium concentration from 50 to 400 ppm, the conversion rate rises from 33 to 44% under standard conditions. This relatively minor effect must be due to the fact that a high rhodium concentration implies a high concentration of Na-TPPTS, which has a negative effect on the solubility of 1-hexene in the aqueous phase (salt effect). On the other hand, the salt effect shifts the equilibrium of the rhodium complexes toward the phos-phine-rich complex 2. Hence, the n/iso ratio is improved substantially. 

Scheme 1 Equilibrium between chelated and nonchelated rhodium complexes in water.

Results for the separation of olefins are given in Table 6.2 [77]. It can be seen that the use of this rhodium complex permits a substantial increase in the selectivity of olefin separation compared with squalane. Also, the olefin equilibrium constants with the rhodium complex are usually much higher than those of the corresponding complexes with silver [62]. In fact, the difference between these complexes should be even greater, as the data for the complexes with rhodium were obtained at a temperature 10°C higher than those for the complexes with silver. Attention should be paid to the relationship between the olefin structure and the equilibrium constant of the complexes with rhodium. 

Examples are listed in Table 8.7 for various numbers of bonds (x) between the double bonds. For the compounds with x = 6, the formation of the 7-membered ring is the preferred reaction. For x >6, the polymer is the favoured product. For x = 4 there is a remarkable variation in behaviour with the catalyst no reaction is observed with the molybdenum carbene catalyst, but with the rhodium complex there is 86% conversion of substrate in 72 h to products consisting of about 5% of cyclic dimer , 4% of cyclic trimer and 91% of linear oligomers (M = 1815). In the early stages of reaction the products are mainly the cyclic species but these undergo ROMP once their equilibrium concentration has been exceeded. With the ruthenium complex as initiator the kinetics of ROMP are less favourable and the products after 72 h consist of 25% cyclic dimer, 17% cyclic trimer and 58 % of linear oligomers (Marciniec 1995a). 

This procedure resulted in an efficient and selective substrate conversion and it was established by spectrophotometry that there was no catalyst leaching to the apolar phase. However, a marked decrease in the catalytic activity was observed after the third cycle. This was probably caused by a continuous loss of free triphenylphosphine ligand present in equilibrium with the rhodium complex, ultimately generating inactive species. To overcome this problem the cationic rhodium complex (Rh(cod)(dppe)]PFg was tested. In the ternary mixture containing CH2CI2 this complex showed poor catalytic activity. Using methanol instead clearly increased the activity. This effect of increased activity in methanol is well known for rhodium complexes. 

The effect of iodide and acetate on the activity and stability of rhodium catalysts for the conversion of methanol into acetic acid have been studied. Iodide salts at low water concentrations (<2 M) promote the carbonylation of methanol and stabilize the catalyst. Alkali metal iodides react with methylacetate to give methyl iodide and metal acetate the acetate may coordinate to Rh and act as an activator by forming soluble rhodium complexes and by preventing the precipitation of Rhl3. A water-gas shift process may help to increase the steady-state concentration of Rh(I). The labile phosphine oxide complex (57) is in equilibrium with the very active methanol carbonylation catalyst (58) see equation (56). 

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