The importance of understanding (pre)catalyst activation in versatile C-H bond functionalisations catalysed by...

Mn-catalysed reactions offer great potential in synthetic organic and organometallic chemistry and the success of Mn carbonyl complexes as (pre)catalysts hinges on their stabilisation by strong field ligands enabling Mn(i)-based, redox neutral, catalytic cycles. The mechanistic processes underpinning the activation of the ubiquitous Mn(0) (pre)catalyst [Mn₂(CO)₁₀] in C-H bond functionalisation reactions is now reported for the first time. By combining time-resolved infra-red (TRIR) spectroscopy on a ps-ms timescale and in operando studies using in situ infra-red spectroscopy, insight into the microscopic bond activation processes which lead to the catalytic activity of [Mn₂(CO)₁₀] has been gained. Using an exemplar system, based on the annulation between an imine, 1, and Ph₂C₂, 2, TRIR spectroscopy enabled the key intermediate [Mn₂(CO)₉(1)], formed by CO loss from [Mn₂(CO)₁₀], to be identified. In operando studies demonstrate that [Mn₂(CO)₉(1)] is also formed from [Mn₂(CO)₁₀] under the catalytic conditions and is converted into a mononuclear manganacycle, [Mn(CO)₄(C^N)] (C^N = cyclometallated imine), a second molecule of 1 acts as the oxidant which is, in turn, reduced to an amine. As [Mn(CO)₄(C^N)] complexes are catalytically competent, a direct route from [Mn₂(CO)₁₀] into the Mn(i) catalytic reaction coordinate has been determined. Critically, the mechanistic differences between [Mn₂(CO)₁₀] and Mn(i) (pre)catalysts have been delineated, informing future catalyst screening studies.