Rhodium ligand exchange

In another fluxional process involving ruthenium instead of rhodium, it has been shown that the rate-controlling step is the complex dissociation and that the ligand exchanges between the two annular nitrogen atoms by an intermolecular process. 

During the late 1960s, Homer et al. [13] and Knowles and Sabacky [14] independently found that a chiral monodentate tertiary phosphine, in the presence of a rhodium complex, could provide enantioselective induction for a hydrogenation, although the amount of induction was small [15-20]. The chiral phosphine ligand replaced the triphenylphosphine in a Wilkinson-type catalyst [10, 21, 22]. At about this time, it was also found that [Rh(COD)2]+ or [Rh(NBD)2]+ could be used as catalyst precursors, without the need to perform ligand exchange reactions [23]. 

The first application of a rhodium-ligand system was realized in the LPO-process (low pressure oxo Fig. 18). Huge stirred tank reactors are used, equipped with internal heat exchangers to control the heat of reaction. The solution of the catalyst recycle is simple but efficient. The catalyst remains in the reactor, products and unconverted propene are stripped by a huge excess of synthesis gas. Because of strong foaming, only a part of the reaction volume is used. After the gas has left the reactor, the products are removed by condensing, the big part of synthesis gas is separated from the liquid products and recycled via compressors. The liquid effluent of the gas-liquid separator... 

A mixture of rhodium II) acetate (228 mg, 0.516 mmol), the imidazolidinone (1.70 g, 6.15 mmol), and dry chlorobenzene (20 mL) is heated under reflux for 18 h in a flask fitted with a Soxhlet extraction apparatus into which a thimble is placed containing an oven-dried mixture of sodium carbonate and sand (2 1, 5 g). The progress of the ligand-exchange reaction can be monitored by HPLC (p-Bondapak-CN column, methanol). The resulting blue solution is concentrated under reduced pressure, and the residue is purified by column chromatography (reversed phase silica, Bakerbond Cyano 40 mm prep. LC packing, methanol). 

Species such as XXV, XXVI, or XXVII readily form coordination complexes when treated with AuCl, H20So(C0)j q, Idn(CO)3(r -C5Hj), Fe(C0)3(PhCH=CHC(0)CH3>, or [RhCl(CO)2]2 ( ) Tw results are of special interest. First, the skeletal nitrogen atoms in XXV-XXVII do not participate in the coordination process. Presumably, they are effectively shielded by the aryloxy units and are of low basicity. Second, coordinatlve crosslinking can occur when two phosphine residues bind to one metal atom. Ligand-exchange reactions were detected for the rhodium-bound species. The tri-osmium cluster adducts of XXV, XXVI, and XXVII are catalysts for the isomerization of 1-hexane to 2-hexene. 

Likewise, mononuclear complexes of rhodium and platinum containing only one meth-ylenecyclopropane ligand are prepared by ligand exchange reactions of the Feist s esters with (acac)Rh(CO)2 and trans-C 2(pyr)Pt(et hylene), giving complexes (acac)(CO)Rh(tF) and franj-Cl2(pyr)PtL (L = cF, tF), respectively (equation 311). 

P-31 NMR Studies of Equilibria and Ligand Exchange in Triphenylphosphine Rhodium Complex and Related Chelated Bisphosphine Rhodium Complex Hydroformylation Catalyst Systems... 

The tris(triphenylphosphine) rhodium carbonyl hydride complex also was used via ligand exchange to obtain known chelate complexes of bisphosphines... 

Figure I. Ligand exchange is a proposed key step in the mechanism of phosphine-rhodium complex catalyzed hydroformylation of olefins...

Axial ligand exchange at metal(III) porphyrins - In rhodium(III) porphyrins, the rather firmly bound chloride can be exchanged with hydroxide or rhodanide... 

Advances in Chemistry, Editors Alyea, E. C., and Meek, D. W., American Chemical Society, 1981, 196, 78, paper of "31p NMR Studies of Equilibria and Ligand Exchange in Triphenyl phosphine Rhodium Complex and Related Chelated Bis-Phosphine Rhodium Complex Hydroformylation Catalyst Systems," by Kastrup, R. V., Merola, J. S., and Oswald, A. A., in press. 

The preparations described here are developed from published work by Malatesta et al.5 and from more recent studies in the contributors own laboratory.2 The cobalt and nickel complexes are prepared by reduction of the corresponding metal nitrates with sodium tetrahydroborate in the presence of excess ligand, whereas the syntheses of the rhodium and platinum complexes involve simple ligand exchange processes. The preparative routes are suitable for use with triphenyl- or p-substituted triphenyl phosphites reactions involving o- or m-substituted triphenyl phosphites give much reduced yields of products which are difficult to crystallize and are very air-sensitive. These features probably reflect the unfavorable stereochemistry of the o- and m-substituted ligands. 

The case studies below will provide selected examples of calorimetric studies carried out on metal-ligand exchange reactions of phosphorous-based ligands (such as phosphines, phosphite, A -pyrrolyl-phosphines) and N-heterocyclic carbene ligands (NHCs) as well as addition reactions relevant to catalytic transformations, involving metals such as ruthenium, rhodium,platinum,and molybdenum. ... 

Continuing this line of argument, we find that the predominance of trigonal-bipyramidal complexes of rhodium(I) and iridium(I) could well mean that Fig. 2, the traditional reaction profile, applies to the little-studied ligand-exchange reactions of 4-coordinate complexes of these elements. However, such conclusions, unsupported by other evidence, must remain tentative,... 

N.m.r. spectra reveal that, while Rh6(CO)i6 is static at 343 K, rapid intramolecular exchange of the ligands over the six rhodium atoms of [Rh6(CO)i5] occurs at 203 [Rh7(CO)i6] is static at 203 K, but a partial ligand-exchange... 

The mechanism of the Rh-BINAP complex-promoted isomerization of N,N-di-ethylgeranylamine is briefly summarized, which is a digested abstract of literature [2,9,27,45,46]. The reaction starts from the simple nitrogen-coordinated cationic rhodium complex 73 generated by ligand exchange between the complex 17 and the substrate 45 (L denotes solvent molecules, THF, MeOH, CH2CI2, or BINAP), Eq. 15. 

The mechanism of the coupling was not explored in detail, but likely involves ligand exchange at the rhodium center and coordination by the pyridine substrate, directed cyclometalation, transmetalation of the organotin reagent, and reductive elimination to afford the ortho arylated products (Scheme 28). The nature of the reoxidation process is more equivocal. Trichloroethylene is generated during the reaction when 1,1,2,2-tetrachloroethane is chosen as solvent, which may be... 

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