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.