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 [((t)BuPNP)Rh(CH3)X][BF4] (1-3, X = I, Br, and Cl; (t)BuPNP = 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 CH3I and CH3Br RE in the presence of polar aprotic solvents argues in favor of an SN2 RE mechanism. However, the RE of CH3Cl 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 [((i)PrPNP)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.