Mapping out the key carbon-carbon bond-forming steps in Mn-catalysed C-H functionalization

Detailed understanding of the mechanistic processes that underpin transition metal-catalysed reactions allows for the rational and de novo development of complexes with enhanced activity, efficacy and wider substrate scope. Directly observing bond-cleaving and -forming events underpinning a catalytic reaction is non-trivial as the species that facilitate these steps are frequently short-lived and present at low concentrations. Here, we describe how the photochemical activation of a manganese precatalyst, [Mn(ppy)(CO)(4)] (ppy = 2-phenylpyridine), results in selective loss of a carbonyl ligand simulating entry into the catalytic cycle for manganese-promoted C-H bond functionalization. Time-resolved infrared spectroscopy (on the ps-ms timescale) allows direct observation of the species responsible for the essential C-C bond formation step and an evaluation of the factors affecting its rate. This mechanistic information prompted the discovery of a new photochemically initiated manganese-promoted coupling of phenylacetylene with 2-phenylpyridine. This study provides unique insight into the mechanistic pathways underpinning catalysis by an Earth-abundant metal, manganese.