Photochemical pump and NMR probe to monitor the formation and kinetics of hyperpolarized metal dihydrides

Abstract

On reaction of IrI(CO)(PPh₃)₂1 with para-hydrogen (p-H₂), Ir(H)₂I(CO)(PPh₃)₂2 is formed which exhibits strongly enhanced ¹H NMR signals for its hydride resonances. Complex 2 also shows similar enhancement of its NMR spectra when it is irradiated under p-H₂. We report the use of this photochemical reactivity to measure the kinetics of H₂ addition by laser-synchronized reactions in conjunction with NMR. The single laser pulse promotes the reductive elimination of H₂ from Ir(H)₂I(CO)(PPh₃)₂2 in C₆D₆ solution to form the 16-electron precursor 1, back reaction with p-H₂ then reforms 2 in a well-defined nuclear spin-state. The build up of this product can be followed by incrementing a precisely controlled delay (τ), in millisecond steps, between the laser and the NMR pulse. The resulting signal vs. time profile shows a dependence on p-H₂ pressure. The plot of k_obs against p-H₂ pressure is linear and yields the second order rate constant, k₂, for H₂ addition to 1 of (3.26 ± 0.42) × 10² M⁻¹ s⁻¹. Validation was achieved by transient-UV-vis absorption spectroscopy which gives k₂ of (3.06 ± 0.40) × 10² M⁻¹ s⁻¹. Furthermore, irradiation of a C₆D₆ solution of 2 with multiple laser shots, in conjunction with p-H₂ derived hyperpolarization, allows the detection and characterisation of two minor reaction products, 2a and 3, which are produced in such low yields that they are not detected without hyperpolarization. Complex 2a is a configurational isomer of 2, while 3 is formed by substitution of CO by PPh₃.