Probing Mn Precatalyst Activation through Time-Resolved Spectroscopy: A Quantitative Evaluation of the Effects of CO and PPh3 as Coligands on Ultrafast Dynamics and C–C Bond Formation

An investigation into the effect of a phosphine coligand on the activation of precatalysts for manganese-catalyzed C–H bond functionalization is reported. Although simple precatalysts [MnBr(CO)5] and [Mn2(CO)10] are used extensively in these reactions, there is a dearth of alternate precatalyst structures, which has hindered the development of structure–activity relationships. In this work, the effect of substituting a carbonyl ligand for a phosphine ligand is reported. Investigation of the photochemical activation of the precatalyst fac-[Mn(inpy)(CO)3(PPh3)] (inpy = cyclometalated 1-(pyridin-2-yl)-1H-indole) 3 by time-resolved infrared spectroscopy (TRIR) reveals that light-induced dissociation of a CO ligand occurs preferentially over loss of the phosphine. The ultrafast dynamics of the initially formed solvent complex [Mn(inpy)(CO)2(toluene)(PPh3)] 9 are described, as is the slower substitution of the coordinated solvent by added pyridine to give [Mn(inpy)(CO)2(NC5H5)(PPh3)] 10. Replacing the pyridine with phenylacetylene again results in the substitution of the metal-bound toluene to give the alkyne complex [Mn(inpy)(η2-HC2Ph)(CO)2(PPh3)] 12. The alkyne undergoes a migratory insertion reaction into the Mn–C bond on a microsecond time scale with a very similar first-order rate constant to [Mn(inpy)(CO)4], 2, demonstrating that this key step in Mn-catalyzed reactions is not affected by the presence of the phosphine ligand.