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

[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-NC5 H4 )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.