Analysis of Chemical Exchange in Iridium N-Heterocyclic Carbene Complexes Using Heteronuclear Parahydrogen-Enhanced NMR

Abstract

The signal amplification by reversible exchange process (SABRE) amplifies NMR signals by unlocking hidden polarization in parahydrogen through interactions with to-be-hyperpolarized substrate molecules when both are transiently bound to an Ir-based organometallic catalyst. Recent efforts have focused on optimizing the polarization transfer step from the parahydrogen-derived hydride ligands to the substrate in SABRE. However, this requires quantitative information on ligand exchange rates, which common NMR techniques struggle to provide. Here, we introduce an experimental spin order transfer sequence where readout occurs at 15N nuclei directly interacting with the catalyst. To overcome sensitivity challenges, enhanced 15N NMR signals are created, encoding discrete substrate dissociation rates. This methodology enables robust data fitting to proposed ligand exchange models, yielding substrate dissociation rate constants with higher precision than classical 1D and 2D 1H NMR approaches. This refinement provides enhanced accuracy for estimating the key activation enthalpy ΔH‡ and ΔS‡. Moreover, the higher chemical shift dispersion provided by signal-enhanced 15N NMR allows for the kinetics of substrate dissociation of both acetonitrile and metronidazole, previously inaccessible via 1H NMR due to small chemical shift differences between the resonances of free and Ir-bound molecules of these substrates.