Promoting role of [PtI2(CO)]2 in the iridium-catalyzed methanol carbonylation to acetic acid and its interaction with involved iridium species

The catalytic activity of the iridium complexes involved in methanol carbonylation is significantly enhanced when the carbonyliodoplatinum dimer [PtI2(CO)](2) (10') is added to the reaction mixture. Under CO this complex readily affords the monomeric species [ PtI2(CO)(2)] (10). The turnover frequency value, which is 1450 h(-1) for iridium alone, reaches 2400 h(-1) for a Pt/Ir=3/7 molar ratio, under 30 bar of CO and at 190 degrees C. To get a deeper insight into the role of the platinum cocatalyst, model conditions ( dinitrogen, ambient temperature, CH2Cl2, PPN+ as counterion) have been adopted. [PtI2(CO)](2) ( 10') interacts with [PPN][IrI3(CH3)(CO)(2)] (4-PPN), affording the monoiodo- bridged anionic species [IrI2(CH3)(CO)(2)(mu- I)-PtI2(CO)](-) ( 11), which undergoes cleavage under CO to provide [IrI2(CH3)(CO)(3)] (6) and [PtI3(CO)](-)(9). Although we have to take into account the possible iodide dissociation from 4 in the polar reaction medium (CH3COOH, CH3OH, CH3I, HI, H2O), which can be scavenged by platinum to give 9, we should not discard the intermediacy of 11, even under working catalytic conditions. The crystal structures of [PPN][IrI3(COCH3)(CO)(2)] (8-PPN) and [PPN][PtI3(CO)] (9-PPN), which are both involved in the overall process, were determined by X-ray diffraction analysis. A catalytic cycle is herein proposed, in which the cooperative effect between the platinum promoter and the iridium catalyst is depicted.