Mapping catalyst activation, turnover speciation and deactivation in Rh/PPh₃-catalysed olefin hydroformylation

We report new insights into the fate of the precious metal during hydroformylation catalysis of 1-hexene with Rh/PPh₃ complexes using multi-nuclear operando FlowNMR spectroscopy. By applying selectively excited ¹H and ³¹P{¹H} NMR pulse sequences we were able to characterise and quantify key hydrido-rhodium and acyl-rhodium intermediates formed during turnover as well as dormant dimeric carbonyl complexes. The quantitative catalyst distribution maps derived this way explain catalyst stability and activity across a range of reaction conditions, including why CO-lean conditions give faster hydroformylation catalysis through the suppression of dimer and cluster formation. The activation behaviour of five commonly used precursors and the thermal stability of the phosphine-hydrido complex [RhH(CO)(PPh₃)₃] have been investigated, and the benefits of applying controlled temperature gradients for quantitative FlowNMR spectroscopic reaction monitoring of dynamic catalyst systems are demonstrated.