A Mechanistic Investigation of Oxidative Addition of Methyl Iodide to [Tp*Rh(CO)(L)]

Reaction of methyl iodide with square planar [κ2-Tp*Rh(CO)(PMe3)] 1a (Tp* = HB(3,5-Me2pz)3) at room temperature affords [κ3-Tp*Rh(CO)(PMe3)(Me)]I 2a, which was fully characterized by spectroscopy and X-ray crystallography. The pseudooctahedral geometry of cationic 2a, which contains a κ3-coordinated Tp* ligand, indicates a reaction mechanism in which nucleophilic attack by Rh on MeI is accompanied by coordination of the pendant pyrazolyl group. In solution 2a transforms slowly into a neutral (acetyl)(iodo) rhodium complex [κ3-Tp*Rh(PMe3)(COMe)I] 3a, for which an X-ray crystal structure is also reported. Kinetic studies on the reactions of [κ2-Tp*Rh(CO)(L)] (L = PMe3, PMe2Ph, PMePh2, PPh3, CO)] with MeI show second-order behavior with large negative activation entropies, consistent with an SN2 mechanism. The second-order rate constants correlate well with phosphine basicity. For L = CO, reaction with MeI gives an acetyl complex, [κ3-Tp*Rh(CO)(COMe)I]. The bis(pyrazolyl)borate complexes [κ2-Bp*Rh(CO)(L)] (L = PPh3, CO) are much less reactive toward MeI than the Tp* analogues, indicating the importance of the third pyrazolyl group and the accessibility of a κ3 coordination mode. The results strengthen the evidence in favor of an SN2 mechanism for oxidative addition of MeI to square planar d8 transition metal complexes.