Photochemical pump, benchtop NMR probe spectroscopy for reaction monitoring with parahydrogen

The ability to observe transient intermediates and determine precise kinetic parameters is fundamental to understanding catalysis. This study demonstrates a methodology for the in situ investigation of photochemical ligand exchange and oxidative addition using a benchtop NMR spectrometer equipped with a throughbore UV irradiation system. To overcome the inherent sensitivity limitations of benchtop NMR, parahydrogeninduced polarisation (PHIP) was employed, providing significant signal enhancements for hydride-containing products. Specifically, the addition of pH2 to trans- [IrCl(CO)(PPh3)2], that proceeds over 10 min, was monitored yielding a second-order rate constant, 1.36 ± 0.02 M−1 s−1. To resolve faster catalytic processes, pump-probe synchronization between the UV pulse and NMR detection was implemented. A spin-lock pulse was also added to preserve the singlet state of the pH2-derived hydride ligands in the reaction products, enabling the observation of coherent magnetic oscillations after reaction in the corresponding 2D pumpprobe NMR spectra. Following optimisation on diagnostic zero-quantum correlations for cis [Ru(H)2(dppe)2], a set of iodo-derivatives of Vaska’s complex were explored. For these systems two photochemical pathways were effectively mapped, enabling the identification of several ligand-exchange products. These results establish that the combination of hyperpolarisation and synchronized in situ irradiation makes benchtop NMR a powerful tool for the study of reactivity.