Novel neutral iridium catalysts for effective hyperpolarisation and hydrogenation: a synthetic and mechanistic study

Since 2009, when the Signal Amplification By Reversible Exchange (SABRE) process, which transfers polarisation from parahydrogen to substrates in a non-hydrogenative manner, was first published, substantial efforts have been made in method optimisation and catalyst development. This work presents multi-step syntheses and studies on a series of novel neutral bidentate iridium carbene complexes, [Ir(COD)(R-NHC-O)]. The carbene ligands have a pendent phenolate group, incorporating the electron-withdrawing substituents, NO2, COOMe, Cl and H. The reactivity of each complex with substrate and hydrogen has enabled the identification of several intermediates, including dihydride COD complexes, [Ir(H)2(COD)(R-NHC-O)]. COD hydrogenation facilitates the formation of SABRE-active pyridyl dihydride species with structures dependent on the R substituent and solvent used. In DCM, the Ir-O bond is stable and complexes of [Ir(H)2(R-NHC-O)(py)2] form. However, in methanol, when the electron density is increased as R is changed to an H substituent, complete Ir-O bond cleavage occurs to form [Ir(H)2(H-NHC-O)−(py)3]+.
When studied with parahydrogen, these complexes act as efficient SABRE catalysts for a variety of substrates in a range of polarity solvents. They undergo the necessary substrate and hydrogen exchange processes, leading to the observation of enhanced NMR signals for pyridine, nicotinamide, nicotine and nicotinaldehyde.
A study into the parahydrogenation of phenylacetylene using the NO2-containing catalyst has also been completed. Although it was less reactive than Crabtree’s catalyst it proved more stable due to minimal degradation over time.