Iridium α-Carboxyimine Complexes Hyperpolarized with para-Hydrogen Exist in Nuclear Singlet States before Conversion into Iridium Carbonates

The formation and hyperpolarization of an Ir(H)₂(amine)(IMes)(η²-imine)]Cl complex that can be created in a hyperpolarized nuclear singlet state is reported. These complexes are formed when an equilibrium mixture of pyruvate, amine (benzylamine or phenylethylamine), and the corresponding imine condensation product, react with preformed Ir(H)₂(amine)₃(IMes)]Cl. These iridium α-carboxyimine complexes exist as two regioisomers differentiated by the position of amine. When examined with para-hydrogen the hydride resonances of the isomer with amine trans to hydride become strongly hyperpolarized. The initial hydride singlet states readily transfer to the corresponding ¹³C₂ state in the labelled imine and exhibit magnetic state lifetimes of up to 11 seconds. Their ¹³C signals have been detected with up to 420 fold signal gains at 9.4 T. On a longer timescale, and in the absence of H₂, further reaction leads to the formation of neutral carbonate containing Ir(amine)(η²-CO₃)(IMes)(η²-imine)]. Complexes are characterized by, IR, MS, NMR and X-ray diffraction.