Photochemical Pump, Benchtop NMR Probe Spectroscopy for Reaction Monitoring With para Hydrogen

ABSTRACT

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 through‐bore UV irradiation system. To overcome the inherent sensitivity limitations of benchtop NMR, para hydrogen‐induced polarisation (PHIP) was employed, providing significant signal enhancements for hydride‐containing products. Specifically, the addition of p H to trans ‐[IrCl(CO)(PPh)], that proceeds over 10 min, was monitored yielding a second‐order rate constant, 1.36 0.02 M s. To resolve faster catalytic processes, pump‐probe synchronisation between the UV pulse and NMR detection was implemented. A spin‐lock pulse was also added to preserve the singlet state of the p H‐derived hydride ligands in the reaction products, enabling the observation of coherent magnetic oscillations after reaction in the corresponding 2D pump‐probe NMR spectra. Following optimisation on diagnostic zero‐quantum correlations for cis ‐[Ru(H)(dppe)], 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 synchronised in situ irradiation makes benchtop NMR a powerful tool for the study of reactivity.