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.