Gas-Phase Hydrogenation with Parahydrogen Over Immobilized Vaska's Complex

Generating parahydrogen-induced polarization (PHIP) of nuclear spins with immobilized transition metal complexes as hydrogenation catalysts allows one to produce pure hyperpolarized substances, which can open new revolutionary perspectives for PHIP applications. A major drawback of immobilized complexes is their low stability under reaction conditions. In the present work we studied an immobilized iridium complex, Ir/SiO2P, synthesized by a covalent anchoring of Vaska's complex on phospine-modified silica gel. This complex was used to obtain hyperpolarized gasses in the gas phase hydrogenation of propene, propyne and 1-butyne with parahydrogen in PASADENA and ALTADENA experiments. It was found that, in contrast to other immobilized complexes, Ir/SiO2P is stable under reaction conditions at up to 140 degrees C, and the reduction of iridium does not occur according to XPS analysis. Moreover, the application of Ir/SiO2P catalyst allowed us to generate continuous flow of hyperpolarized propene and 1-butene with (300-500)-fold NMR signal enhancement which is significantly higher than commonly observed for most supported metal catalysts. The shape of polarized propene signals in PASADENA experiment has indicated that parahydrogen addition to propyne occurs non-stereospecifically, i.e. PHIP was observed for all protons of the vinyl fragment of propene. The analysis of the polarized signals has shown that syn pairwise addition dominates, which was confirmed by spectra simulations. It was found that storage of Ir/SiO2P under Ar atmosphere leads to a decrease in PHIP amplitude and an increase in the activity of the catalyst. This observation is discussed in terms of the interaction of Ir/SiO2P with trace amounts of oxygen in Ar, leading to partial oxidation of triphenylphosphine ligand to triphenylphosphine oxide accompanied by the activation of the immobilized complex. It was also found that the interaction of Ir/SiO2P with alkenes likely leads to formation of stable monohydride complexes, decreasing the production of PHIP in hydrogenations. At the same time, stable substrate complexes are likely formed in alkyne hydrogenations, leading to a significant decrease in the monohydride complex formation and to an increased production of PHIP.