Reductive Functionalization of a Rhodium(III)–Methyl Bond in Acidic Media: Key Step in the Electrophilic Functionalization of Methane

The reductive functionalization of RhIII–Me bonds in acids is a key step for RhI/III-based catalysts for methane functionalization. Heating electronic-rich [tBu3terpy]Rh(Me)(Cl)I (1; tBu3terpy = 4,4′,4″-tri-tert-butylterpyridine) in H2O, AcOD, or trifluoroacetic acid (HTFA) released a stoichiometric amount of methane. With use of a less donating ligand, [(NO2)3terpy]Rh(Me)(Cl)I (2; (NO2)3terpy = 4,4′,4″-trinitroterpyridine), a mixture of CH4 and CH3X (X = Cl, TFA, OAc, OH) was obtained. The selectivity between MeX and CH4 was found to be dependent on the halide present (I– or Cl–). In a key experiment with the removal of one halide from 2, {[(NO2)3terpy]Rh(Me)Cl}{BF4} (3) was completely protonated in acidic solvent. DFT calculations were employed to investigate the mechanism of reductive functionalization and protonation.