Indirect Detection of Short-lived Hydride Intermediates of Iridium N-Heterocyclic Carbene Complexes via Chemical Exchange Saturation Transfer (CEST) Spectroscopy

For the first time chemical-exchange saturation transfer (CEST) 1H NMR is utilized for the study of short-lived hydride intermediates in the catalytic cycle of the Iridium-based organometallic complex [Ir(IMes)(Py)3(H)2]Cl, which are often not observable by other NMR techniques, since 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 situ and at room temperature in solution. Understanding the effects on the spin dynamics induced by these complexes is necessary for enhancing the performance of the nuclear spin hyperpolarization technique SABRE (Signal Amplification By Reversible Exchange). By eliminating [Ir(Cl)(IMes)(Py)2(H)2] and manipulating the spin-system by RF-irradiation, we were able to increase the nuclear spin singlet lifetime of the two protons in the main hydride complex by more than an order of magnitude, from 2.2±0.1 s to 27.2±1.2 s. The presented CEST NMR approach has a large application potential for studying short-lived hydride intermediates in catalytic reactions.