Oxidative addition of methyl iodide to [Rh(CO)2I]2: synthesis, structure and reactivity of neutral rhodium acetyl complexes, [Rh(CO)(NCR)(COMe)I2]2

Reaction of [Rh(CO)2I]2 (1) with MeI in nitrile solvents gives the neutral acetyl complexes, [Rh(CO)(NCR)(COMe)I2]2 (R=Me, 3a; tBu, 3b; vinyl, 3c; allyl, 3d). Dimeric, iodide-bridged structures have been confirmed by X-ray crystallography for 3a and 3b. The complexes are centrosymmetric with approximate octahedral geometry about each Rh centre. The iodide bridges are asymmetric, with Rh–(μ-I) trans to acetyl longer than Rh–(μ-I) trans to terminal iodide. In coordinating solvents, 3a forms mononuclear complexes, [Rh(CO)(sol)2(COMe)I2] (sol=MeCN, MeOH). Complex 3a reacts with pyridine to give [Rh(CO)(py)(COMe)I2]2 and [Rh(CO)(py)2(COMe)I2] and with chelating diphosphines to give [Rh(Ph2P(CH2)nPPh2)(COMe)I2] (n=2, 3, 4). Addition of MeI to [Ir(CO)2(NCMe)I] is two orders of magnitude slower than to [Ir(CO)2I2]−. A mechanism for the reaction of 1 with MeI in MeCN is proposed, involving initial bridge cleavage by solvent to give [Rh(CO)2(NCMe)I] and participation of the anion [Rh(CO)2I2]− as a reactive intermediate. The possible role of neutral Rh(III) species in the mechanism of Rh-catalysed methanol carbonylation is discussed.