Salt-promoted catalytic methanol carbonylation using iridium pincer-crown ether complexes

Abstract

Iridium complexes of pincer ligands containing aza-crown ether macrocycles are precatalysts for methanol carbonylation. Eleven different pincer complexes were examined in the presence of various potential external promoters under standard reaction conditions of 0.1 M CH₃I and 0.34 mM catalyst in methanol under 25 bar CO and heated to 150 °C for 3 hours. Methyl acetate is the major product under these conditions, with turnover numbers (TONs) for all acetyl-containing products ranging from 265 to 1950 based on the choice of metal salt (with LiCl and HfCl₄ giving highest TON) or tetrabutylammonium salts. While the pincer-crown ether complexes remain intact under conditions of low temperature and pressure, the precatalysts convert to new active species derived from pincer ligand methanolysis in high pressure reactors with excess methyl iodide.