Reactivity of Rh(I) Iminophosphine Carbonyl Complexes with Methyl Iodide

A series of Rh(I) iodocarbonyl complexes, [Rh(CO)I(PNAr)] (1a-g), has been prepared by the reactions of [Rh(CO)2I]2 with iminophosphine ligands, Ph2PC6H4-2-CH=NAr (PNAr; Ar = C6H5 (a), 2,6-Me2C6H3 (b), 2,6-iPr2C6H3 (c), 2-EtC6H4 (d), 2-MeOC6H4 (e), 4-MeOC6H4 (f), 3,5-(CF3)2C6H3 (g)). For 13CO labeled 1b, a relatively small 2JCP coupling (12 Hz) showed the CO ligand to be cis to the P-donor atom of the iminophosphine. Complexes 1a-g react with methyl iodide to give Rh(III) methyl or acetyl products. For complexes 1a,d,f,g the reactions result in equilibria between the methyl complexes [Rh(CO)(PNAr)I2-Me] (2) and acetyl complexes [Rh(PNAr)(COMe)I2] (3), whereas the reactions of 1b,c,e gave only the acetyl products [Rh(COMe)I2(PNAr)] (3b,c,e). Migratory CO insertion is promoted for systems in which the N-aryl group of the iminophosphine is bulky or contains an o-methoxy substituent. An X-ray structure of 3e reveals an interaction between the Rh center and the o-methoxy group (Rh-O ) 2.54 Å). Second order rate constants for MeI oxidative addition to 1a-g vary considerably, depending on the steric and electronic properties of the iminophosphine N-aryl substituents. The most reactive complex is 1e, and a mechanism is proposed in which the o-methoxy group interacts with the Rh center to promote both oxidative addition and CO insertion.