Benchtop Proton NMR Study of Hyperpolarized Charged and Neutral Ir-IMes Hydride Intermediates in Acetonitrile

In this work, we utilize 1H benchtop nuclear magnetic resonance (NMR) spectrometer (proton frequency 44.7 MHz) for real-time monitoring of spin-transfer catalysis and hydrogen-assisted activation process starting from an iridium-based organometallic complex [Ir(IMes)(COD)Cl] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, COD = cyclooctadiene). We identify two distinct activation pathways: first, via the formation of neutral intermediate complex [Ir(IMes)(COD)(H2)Cl], and second, via the formation of charged complexes [Ir{+}(IMes) (COD)(H2)R]Cl{-} with bound ligands R = acetonitrile, ammonia, and benzylamine. These ligands originate from substrate choice in the activation process. We conclude that the pathway direction is dominated by solvent polarity. On the basis of this, we develop a protocol for obtaining stable and long-lasting SABRE (Signal Amplification by Reversible Exchange) polarization of an acetonitrile solvent in the presence of a benzylamine coligand. These results are important for better understanding of the chemical dynamics in SABRE systems as well as for the fundamental physics experiments that require large and long-lasting polarization of highly-concentrated molecules (e.g., those from the solvent itself).