Catalytic activation of [Ir-(COD)(IMes)Cl] (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene; COD = cyclooctadiene), a common precursor complex of PTCs required for SABRE hyperpolarization, is shown in Fig. in a time series (30 min monitoring, temporal resolution of 20 s) of 1 H NMR spectra. Gas handling setup used in this work was described previously. The activation process studied involves hydrogenation of the COD ligand, which is an irreversible process that involves a conformation change from a square-planar Ir(I)-precursor complex into an octahedral Ir(III)-dihydride complex. The time scale for this activation process is on the order of minutes to hours, depending on substrate, solvent, and temperature. However, in this work, the generation of PTCs required for SABRE with ACN was elucidated and can be observed with enhanced NMR resonances at around -23 ppm using pH 2 bubbling prior NMR detection and benzylamine as a stabilizing Co-L (Fig. ). Moreover, generated intermediate Ir-hydride complexes (I-A, I-B, and I-C, structures shown in Fig. ) can be detected within a chemical shift range between -12 ppm and -18 ppm; the corresponding assignments are highlighted (note the increased NMR scale in the zoom-in).