Multi-Nuclear, High-pressure, Operando FlowNMR Spectroscopic Study of Rh/PPh3 - Catalysed Hydroformylation of 1-Hexene

The hydroformylation of 1-hexene with 12 bar of 1:1 H2/CO in the presence of the catalytic system [Rh(acac)(CO)2]/PPh3 was successfully studied by real-time multinuclear high-resolution FlowNMR spectroscopy at 50 °C. Quantitative reaction progress curves that yield rates as well chemo- and regioselectivities have been obtained with varying P/Rh loadings. Dissolved H2 can be monitored in solution to ensure true operando conditions without gas limitation. 31P{1H} and selective excitation 1H pulse sequences were periodically interleaved with 1H FlowNMR measurements to correlate the reaction progress with Rh-phosphine intermediates. Stopped-flow experiments in combination with diffusion measurements and 2D heteronuclear correlation experiments showed the known tris-phosphine complex [RhH(CO)(PPh3)3] to generate rapidly exchanging isomers of the bis-phosphine complex [Rh(CO)2(PPh3)2] under CO pressure that directly enter the catalytic cycle. A mono-phosphine acyl complex has been identified as a late stage in-cycle reaction intermediate.