Understanding Precatalyst Activation and Speciation in Manganese-Catalyzed C–H Bond Functionalization Reactions

An investigation
into species formed following precatalyst activation
in Mn-catalyzed C–H bond functionalization reactions is reported.
Time-resolved infrared spectroscopy demonstrates that light-induced
CO dissociation from precatalysts [Mn(C^N)(CO)4] (C^N =
cyclometalated 2-phenylpyridine (1a), cyclometalated
1,1-bis(4-methoxyphenyl)methanimine (1b)) in a toluene
solution of 2-phenylpyridine (2a) or 1,1-bis(4-methoxyphenyl)methanimine
(2b) results in the initial formation of solvent complexes fac-[Mn(C^N)(CO)3(toluene)]. Subsequent solvent
substitution on a nanosecond time scale then yields fac-[Mn(C^N)(CO)3(κ1-(N)-2a)] and fac-[Mn(C^N)(CO)3(κ1-(N)-2b)], respectively.
When the experiments are performed in the presence of phenylacetylene,
the initial formation of fac-[Mn(C^N)(CO)3(toluene)] is followed by a competitive substitution reaction to
give fac-[Mn(C^N)(CO)3(2)]
and fac-[Mn(C^N)(CO)3(η2-PhC2H)]. The fate of the reaction mixture depends on
the nature of the nitrogen-containing substrate used. In the case
of 2-phenylpyridine, migratory insertion of the alkyne into the Mn–C
bond occurs, and fac-[Mn(C^N)(CO)3(κ1-(N)-2a)] remains unchanged.
In contrast, when 2b is used, substitution of the η2-bound phenylacetylene by 2b occurs on a microsecond
time scale, and fac-[Mn(C^N)(CO)3(κ1-(N)-2b)] is the sole product
from the reaction. Calculations with density functional theory indicate
that this difference in behavior may be correlated with the different
affinities of 2a and 2b for the manganese.
This study therefore demonstrates that speciation immediately following
precatalyst activation is a kinetically controlled event. The most
dominant species in the reaction mixture (the solvent) initially binds
to the metal. The subsequent substitution of the metal-bound solvent
is also kinetically controlled (on a ns time scale) prior to the thermodynamic
distribution of products being obtained.