Reductive functionalization of a rhodium(III)-methyl bond by electronic modification of the supporting ligand

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 RhIII 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.