Spontaneous Transfer of <i>Para</i>hydrogen Derived Spin Order to Pyridine at Low Magnetic Field

The cationic iridium complex [Ir(COD)(PCy3)(py)]BF4 (1) is shown to react with dihydrogen in the presence of pyridine (py) to form the dihydride complex fac,cis-[Ir(PCy3)(py)3(H)2]BF4 (2). Complex 2 undergoes rapid exchange of the two bound pyridine ligands which are trans to hydride with free pyridine; the activation parameters for this process in methanol are ΔH⧧ = 97.4 ± 9 kJ mol−1 and ΔS⧧ = 84 ± 31 J K−1 mol−1. When parahydrogen is employed as a source of nuclear spin polarization, spontaneous magnetization transfer proceeds in low magnetic field from the two nascent hydride ligands of 2 to its other NMR active nuclei. Upon interrogation by NMR spectroscopy in a second step, signal enhancements in excess of 100 fold are observed for the 1H, 13C and 15N resonances of free pyridine after ligand exchange. The degree of signal enhancement in the free substrate is increased by employing electronically rich and sterically encumbered phosphine ligands such as PCy3, PCy2Ph, or PiPr3 and by optimizing the strength of the magnetic field in which polarization transfer occurs.