THE SYNTHESIS, PROPERTIES, AND CRYSTAL-STRUCTURE OF NBU4N[RH(CO)2(OAC)2], AND THE EXCHANGE OF ACETATE, CHLORIDE, AND IODIDE IN NBU4N[RH(CO)2(X)2]

Reaction of (n)Bu4N[Rh(CO)2Cl2], 1, with silver acetate under CO gave the diacetate salt (n)Bu4N[Rh(CO)2(OAc)2], 2. The complex has been characterised spectroscopically [nu(CO(term)) 1986, 2066; nu(CO2-monodentate) 1615 cm-1] and by a single-crystal X-ray determination. The anion is planar with monodentate acetates, Rh-O 2.074(6) and 2.053(7)-angstrom. Complex 2 reacted immediately with MeI to give MeOAc and [Rh(CO)2I2]-, 3, which then oxidatively added more MeI normally. It also reacted with HCl to give HOAc and [Rh(CO)2Cl2]-. Evidence was obtained for the formation of a labile mixed anion [Rh(CO)2I(OAc)]- on reaction of [Rh2(CO)4I2] with (n)Bu4N+ OAc-. It is unlikely that the promotional effect of acetate in methyl acetate carbonylation is due to the participation of acetato-rhodium complexes. Since the lithium acetate reacts readily with iodomethane to give lithium iodide, the observed promotion probably arises from the formation of LiI. MeI reacted with (n)Bu4N[Rh(CO)2Cl2], again via preliminary formation of (n)Bu4N[Rh(CO)2I2], which then added MeI normally. IR spectra showed that the equilibrium, 1 + 2I- reversible 3 + 2Cl-, favoured the chloride 1 in CH2Cl2, and the iodide 3 in methanol.