Iridium Cyclooctene Complex That Forms a Hyperpolarization Transfer Catalyst before Converting to a Binuclear C–H Bond Activation Product Responsible for Hydrogen Isotope Exchange

Abstract

[IrCl(COE)2]2 (1) reacts with pyridine (py) and H2 to form crystallographically characterized IrCl(H)2(COE)(py)2 (2). 2 undergoes py loss to form 16-electron IrCl(H)2(COE)(py) (3), with equivalent hydride ligands. When this reaction is studied with parahydrogen, 1 efficiently achieves hyperpolarization of free py (and nicotinamide, nicotine, 5-aminopyrimidine, and 3,5-lutudine) via signal amplification by reversible exchange (SABRE) and hence reflects a simple and readily available precatayst for this process. 2 reacts further over 48 h at 298 K to form crystallographically characterized (Cl)(H)(py)(μ-Cl)(μ-H)(κ-μ-NC5H4)Ir(H)(py)2 (4). This dimer is active in the hydrogen isotope exchange process that is used in radiopharmaceutical preparations. Furthermore, while [Ir(H)2(COE)(py)3]PF6 (6) forms upon the addition of AgPF6 to 2, its stability precludes its efficient involvement in SABRE.

Signal Amplification by Reversible Exchange (SABRE): From Discovery to Diagnosis

“…[18] Despite there being over 10 commercially available NHC ligands for the SABRE polarization of pyridine, [18,19] to date [IrCl(COD)(IMes)] remains the most impressive mediator of polarization transfer.T his optimum activity is common across ar ange of heterocyclic motifs, although rare examples exist for which this is not the case. [22] Other SABRE catalysts have been shown to undergo such non-hydrogenative transfer of polarization including the bidentate NHC-phenolate (1), [23] an iridium PNP-pincer (2), [14] and an iridium cyclooctene complex (3) [24] shown in Figure 2. [19] This reflects the complexity of the SABRE transfer step,although recently the p-accepting ability parameter [21] has been shown to offer better insight into how the NHC ligand influences the rate of pyridine exchange.…”

“…[19] This reflects the complexity of the SABRE transfer step,although recently the p-accepting ability parameter [21] has been shown to offer better insight into how the NHC ligand influences the rate of pyridine exchange. [22] Other SABRE catalysts have been shown to undergo such non-hydrogenative transfer of polarization including the bidentate NHC-phenolate (1), [23] an iridium PNP-pincer (2), [14] and an iridium cyclooctene complex (3) [24] shown in Figure 2. However,signal gains were reduced when compared to [IrCl(COD)(IMes)].…”

“…Other SABRE catalysts have been shown to undergo such non‐hydrogenative transfer of polarization including the bidentate NHC‐phenolate ( 1 ), an iridium PNP‐pincer ( 2 ), and an iridium cyclooctene complex ( 3 ) shown in Figure . However, signal gains were reduced when compared to [IrCl(COD)(IMes)].…”

Perspective on the Hyperpolarisation Technique Signal Amplification by Reversible Exchange (SABRE) in NMR Spectroscopy and MR Imaging

“…Figure 17. Reaction scheme for the formation of the SABRE-inactive binuclear complex from the mononuclear SABRE active complex prepared by Duckett and co-workers [71].…”

Signalverstärkung durch reversiblen Austausch (SABRE): von der Entdeckung zur diagnostischen Anwendung

“…Other SABRE catalysts have been shown to undergo such non-hydrogenative transfer of polarization including the bidentate NHC-phenolate (1), [23] an iridium PNP-pincer (2) [14] and an iridium cyclooctene complex (3) [24] shown in Figure 2, however signal gains were reduced when compared to [IrCl(COD)(IMes)]. In summary, catalyst identity is critical for efficient SABRE and can be used to control the polarization outcome.…”