Probing Mn Precatalyst Activation through Time-Resolved Spectroscopy: A Quantitative Evaluation of the Effects of CO and PPh 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)] and [Mn(CO)] 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 -[Mn(inpy)(CO)(PPh)] (inpy = cyclometalated 1-(pyridin-2-yl)-1-indole) 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)(toluene)(PPh)] are described, as is the slower substitution of the coordinated solvent by added pyridine to give [Mn(inpy)(CO)(NCH)(PPh)] . Replacing the pyridine with phenylacetylene again results in the substitution of the metal-bound toluene to give the alkyne complex [Mn(inpy)(η-HCPh)(CO)(PPh)] . 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)], , demonstrating that this key step in Mn-catalyzed reactions is not affected by the presence of the phosphine ligand.