Reductive functionalization of a rhodium(iii)–methyl bond by electronic modification of the supporting ligand

Abstract

Net reductive elimination (RE) of MeX (X = halide or pseudo-halide: Cl⁻, CF₃CO₂⁻, HSO₄⁻, OH⁻) is an important step during Pt-catalyzed hydrocarbon functionalization. Developing Rh(I/III)-based catalysts for alkane functionalization is an attractive alternative to Pt-based systems, but very few examples of RE of alkyl halides and/or pseudo-halides from Rh^III complexes have been reported. Here, we compare the influence of the ligand donor strength on the thermodynamic potentials for oxidative addition and reductive functionalization using [^tBu₃terpy]RhCl (1) {^tBu₃terpy = 4,4′,4′′-tri-tert-butylpyridine} and [(NO₂)₃terpy]RhCl (2) {(NO₂)₃terpy = 4,4′,4′′-trinitroterpyridine}. Complex 1 oxidatively adds MeX {X = I⁻, Cl⁻, CF₃CO₂⁻ (TFA⁻)} to afford [^tBu₃terpy]RhMe(Cl)(X) {X = I⁻ (3), Cl⁻ (4), TFA⁻ (5)}. By having three electron-withdrawing NO₂ groups, complex 2 does not react with MeCl or MeTFA, but reacts with MeI to yield [(NO₂)₃terpy]RhMe(Cl)(I) (6). Heating 6 expels MeCl along with a small quantity of MeI. Repeating this experiment but with excess [Bu₄N]Cl exclusively yields MeCl, while adding [Bu₄N]TFA yields a mixture of MeTFA and MeCl. In contrast, 3 does not reductively eliminate MeX under similar conditions. DFT calculations successfully predict the reaction outcome by complexes 1 and 2. Calorimetric measurements of [^tBu₃terpy]RhI (7) and [^tBu₃terpy]RhMe(I)₂ (8) were used to corroborate computational models. Finally, the mechanism of MeCl RE from 6 was investigated via DFT calculations, which supports a nucleophilic attack by either I⁻ or Cl⁻ on the Rh–CH₃ bond of a five-coordinate Rh complex.