Promotion of Iridium-Catalyzed Methanol Carbonylation:  Mechanistic Studies of the Cativa Process

Abstract

The iridium/iodide-catalyzed carbonylation of methanol to acetic acid is promoted by carbonyl complexes of W, Re, Ru, and Os and simple iodides of Zn, Cd, Hg, Ga, and In. Iodide salts (LiI and Bu(4)NI) are catalyst poisons. In situ IR spectroscopy shows that the catalyst resting state (at H(2)O levels > or = 5% w/w) is fac,cis-[Ir(CO)(2)I(3)Me](-), 2. The stoichiometric carbonylation of 2 into [Ir(CO)(2)I(3)(COMe)](-), 6, is accelerated by substoichiometric amounts of neutral promoter species (e.g., [Ru(CO)(3)I(2)](2), [Ru(CO)(2)I(2)](n), InI(3), GaI(3), and ZnI(2)). The rate increase is approximately proportional to promoter concentration for promoter:Ir ratios of 0-0.2. By contrast anionic Ru complexes (e.g., [Ru(CO)(3)I(3)](-), [Ru(CO)(2)I(4)](2)(-)) do not promote carbonylation of 2 and Bu(4)NI is an inhibitor. Mechanistic studies indicate that the promoters accelerate carbonylation of 2 by abstracting an iodide ligand from the Ir center, allowing coordination of CO to give [Ir(CO)(3)I(2)Me], 4, identified by high-pressure IR and NMR spectroscopy. Migratory CO insertion is ca. 700 times faster for 4 than for 2 (85 degrees C, PhCl), representing a lowering of Delta G(++) by 20 kJ mol(-1). Ab initio calculations support a more facile methyl migration in 4, the principal factor being decreased pi-back-donation to the carbonyl ligands compared to 2. The fac,cis isomer of [Ir(CO)(2)I(3)(COMe)](-), 6a (as its Ph(4)As(+) salt), was characterized by X-ray crystallography. A catalytic mechanism is proposed in which the promoter [M(CO)(m)I(n)] (M = Ru, In; m = 3, 0; n = 2, 3) binds I(-) to form [M(CO)(m)I(n+1)](-)H(3)O(+) and catalyzes the reaction HI(aq) + MeOAc --> MeI + HOAc. This moderates the concentration of HI(aq) and so facilitates catalytic turnover via neutral 4.

Iridium‐Catalyzed Methanol Carbonylation

“…All of these observations are consistent with the ability of the promoter to remove an iodide ligand from the anionic [Ir(CH 3 )I 3 (CO) 2 ] − iridium intermediate and, indeed, that it is its key role [32,41,42,51] . Thus, under a CO atmosphere, [Ir(CH 3 )I 3 (CO) 2 ] − evolves rapidly towards [Ir(CH 3 )I 2 (CO) 3 ], before the methyl cismigration occurs to provide the neutral acetyl ˆ iridium species.…”

“…In this case, two isomers have been observed, with the kinetic product having been isolated as the fac,cis -[AsPh 4 ] [Ir(COCH 3 )I 3 (CO) 2 ] compound [32] . The thermodynamic product, the mer,trans - [34] .…”

“…Kinetic analyses of the effect of ruthenium on the rate of the carbonylation reaction [32] have shown that the addition of [Ru 2 ( µ -I) 2 I 2 (CO) 6 effect of ruthenium on the carbonylation rate is proportional to its concentration. All of these observations are consistent with the ability of the promoter to remove an iodide ligand from the anionic [Ir(CH 3 )I 3 (CO) 2 ] − iridium intermediate and, indeed, that it is its key role [32,41,42,51] .…”

Heterogeneous Partial (amm)Oxidation and Oxidative Dehydrogenation Catalysis on Mixed Metal Oxides

“…There are some alternate methods to producing methyl methacrylate. Among them is the oxidative dehydrogenation of methyl isobutyrate, (MIB) which is formed by the carbonylation of propene in the presence of methanol [39].…”

Orientation of trimethylolethane cyclic phosphite in rhodium complexes: Structure of [Rh(CH3COCHCOCH3)(CO)(P(OCH2)3CCH3)]

“…The Monsanto Company commercialised this process in 1970. Currently, the acetic acid manufacturing process has largely been substituted by the Cativa process which uses a similar, though more economical and environmentally friendly, iridium-based process, developed by BP Chemicals Ltd [4,5]. However, the former harsh conditions of the manufacturing process still spurred researchers on to search for new catalysts which work under milder conditions.…”