Direct Observation of Reductive Elimination of MeX (X = Cl, Br, I) from Rh^III Complexes: Mechanistic Insight and the Importance of Sterics

Abstract

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(CH₃)­X]­[BF₄] (<b>1</b>–<b>3</b>, X = I, Br, and Cl; ^tBuPNP = 2,6-bis-(di-<i>tert-</i>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 S_N2 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 <b>1</b>–<b>3</b> is induced by steric factors, as treatment of the less bulky complexes [(ⁱPrPNP)­Rh­(CH₃)­X]­[BF₄] (<b>19</b>–<b>21</b>; 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