Indirect Detection of Short-Lived Hydride Intermediates of Iridium N‑Heterocyclic Carbene Complexes via Chemical Exchange Saturation Transfer Spectroscopy

For the first time, chemical exchange saturation transfer (CEST) nuclear magnetic resonance (NMR) is utilized to study short-lived hydride intermediates in the catalytic cycle of an organometallic complex [Ir­(IMes)­(Py)3(H)2]­Cl. These complexes are typically not observable by other NMR techniques because they are low concentrated and undergo reversible ligand exchange with the main complex. The intermediate complexes [Ir­(Cl)­(IMes)­(Py)2(H)2] and [Ir­(CD3OD)­(IMes)­(Py)2(H)2] are detected, assigned, and characterized in solution, in situ and at room temperature. Understanding the spin dynamics in these complexes is necessary for enhancing the performance of the nuclear spin hyperpolarization technique signal amplification by reversible exchange. By eliminating [Ir­(Cl)­(IMes)­(Py)2(H)2] and manipulating the spin system by radiofrequency irradiation, the nuclear spin singlet lifetime of the hydride protons was increased by more than an order of magnitude, from 2.2 ± 0.1 to 27.2 ± 1.2 s. Because of its simplicity and ability to unravel unobservable chemical species, the utilized CEST NMR approach has a large application potential for studying short-lived hydride intermediates in catalytic reactions.