Methylrhodium complex

Sometimes it is not the catalyst metal itself which is used but a neighbouring metal with similar properties. For example, the methylrhodium complex [Rh(Me)(CO)2l3] was widely accepted as an intermediate in the carbonylation of MeOH to AcOH involving [Rh(CO)2l2] and Mel even though it had never been observed. This was in part because the corresponding Ir complex was stable and well characterized. The rhodium species was subsequently observed by carrying out the reaction in a non polar solvent at a very high [Mel] which increased the rate of its formation and slowed down further reaction (see Section 4.2.4). 

The resulting methylrhodium complex (I) is kinetically unstable and rapidly isomerizes to the acetylrhodium complex (II). This then reacts with CO to form a labile six-coordinate complex (III) which, in the absence of methanol or water, undergoes reductive elimination to produce acetyl iodide and regenerate the catalyst. The catalytic cycle is then repeated via reactions (21) and (22) which produce acetic acid and regenerate the methyl iodide [14] ... 

The facially coordinating l,4,7-tiimethyl-l,4,7-triazacyclononane (Cn) ligand forms stable methylrhodium(III) complexes, such as [Rh(Me)3Cn], [Rh(Me)2Cn]OTf and [Rh(Me)Cn](OTf)2 (OTf=trifluoromethanesulfonate) and the latter two have rich aqueous chemistry. When dissolved in water, [Rh(Me)Cn] readily coordinates two water molecules to form the... 

The acetyl complex VII is in equilibrium in solution with a six-coordinate methylrhodium(III) complex VIII, analogous to the iridium complex VI. An interesting feature evident from variable-... 

Step (1) involves the formation of methyl iodide, which then reacts with the rhodium complex Rh(I)L by oxidative addition in a rate-determining step (2) to form a methylrhodium(III) complex. Carbon monoxide is incorporated into the coordination sphere in step (3) and via an insertion reaction a rhodium acyl complex is formed in step (4). The final step involves hydrolysis of the acyl complex to form acetic acid and regeneration of the original rhodium complex Rh(I)L and HI. Typical rhodium compounds which are active precursors for this reaction include RhCl3, Rh203, RhCl(CO)(PPh3)2, and Rh(CO)2Cl2. 

Oxidahve addihon of methyl iodide to a Rh(I) complex coordinated with iodide and carbonyl ligands affords methylrhodium(III) type complex where the methyl and the iodide hgands are situated in mutually trans positions (Eq. 1.2). 

The reactive methylrhodium(III) complex thus formed then undergoes CO insertion to give an acetylrhodium species as shown in Scheme 1.8. Reductive elimination of the acetyl and iodide ligands liberates acetyl iodide, which is hydrolyzed to produce acetic acid. The hydrolysis generates HI, which is recycled on reaction with methanol regenerating methyl iodide. The important elementary processes of CO insertion will be discussed in Chapter 7. 

---