Direct observation of reductive elimination of MeX (X = Cl, Br, I) from Rh^III complexes: Mechanistic insight and the importance of sterics

Rare cases of directly observed reductive elimination (RE) of methyl halides from Rh^III complexes are described. Treatment of the coordinatively unsaturated complexes [(^tBuPNP)Rh(CH ₃)X][BF₄] (1-3, X = I, Br, and Cl; ^tBuPNP = 2,6-bis-(di-tert-butylphosphinomethyl)pyridine) with coordinating and noncoordinating compounds results in the formation of the corresponding free methyl halides and Rh^I complexes. The rate increase of CH ₃I and CH₃Br RE in the presence of polar aprotic solvents argues in favor of an S_N2 RE mechanism. However, the RE of CH ₃Cl is faster in polar protic solvents, which argues in favor of a concerted C-Cl RE. The RE of methyl halides from complexes 1-3 is induced by steric factors, as treatment of the less bulky complexes [(ⁱPrPNP) Rh(CH₃)X][BF₄] (19-21; X = I, Br, Cl, respectively) with coordinating compounds leads to the formation of the adducts complexes rather than RE of the methyl halides. The accumulated evidence suggests that the RE process is nonassociative.