MODEL REACTIONS OF A CARBONYLATION CATALYST - PHOSPHITE INDUCED MIGRATORY CO INSERTION IN [MEIR(CO)(2)I-3](-)

Carbonylation of the anionic iridium(III) methyl complex, [MeIr(CO)(2) I-3](-) (1) is an important step in the new iridium-based process for acetic acid manufacture. A model study of the migratory insertion reac tions of 1 with P-donor ligands is reported. Complex 1 reacts with pho sphites to give neutral acetyl complexes, [Ir(COMe) (CO)I2L2] (L=P(OPh )(3) (2), P(OMe)(3) (3)). Complex 2 has been isolated and fully charac terised from the reaction of Ph4As [MeIr(CO)(2)I-3] with AgBF4 and P(O Ph)(3); comparison of spectroscopic properties suggests an analogous f ormulation far 3, IR and P-31 NMR spectroscopy indicate initial format ion of unstable isomers of 2 which isomerise to the thermodynamic prod uct with trans phosphite ligands, Kinetic measurements for the reactio ns of 1 with phosphites in CH,CI, show first order dependence on [1], only when the reactions are carried out in the presence of excess iodi de. The rates exhibit a saturation dependence on [L] and are inhibited by iodide. The reactions are accelerated by addition of alcohols (e.g . 18 x enhancement for L = P(OMe)(3) in 1:3 MeOH-CH2Cl2). A reaction m echanism is proposed which involves substitution of an iodide ligand b y phosphite, prior to migratory CO insertion. The observed rate consta nts fit well to a rate law derived from this mechanism. Analysis of th e kinetic data shows that k(1), the rate constant for iodide dissociat ion, is independent of L, but is increased by a factor of similar to 1 8 on adding 25% MeOH to CH2Cl2. Activation parameters for the k(1) ste p are Delta H-not equal=71(+/-3) kJ mol(-1), Delta S-not equal=-81(+/- 9) J mol(-1) K-1 in CH2Cl2 and Delta H-not equal=60(+/-4) kJ mol(-1), Delta S-not equal=-93(+/-12) J mol(-1) K-1 in 1:3 MeOH-CH2Cl2. Solvent assistance of the iodide dissociation step gives the observed rate en hancement in protic solvents. The mechanism is similar to that propose d for the carbonylation of 1. (C) 1998 Elsevier Science S.A.