Scavenger-Based Immobilized Rh and Ir Complexes in Hydrogenation of Propyne and Propene with Parahydrogen

The immobilization of dimeric [M₂(COD)₂(μ–Cl)₂] complexes (M – Rh or Ir) by the interaction with -SH groups of 3-mercaptopropyl-functionalized silica gel leads to Rh_Cl–S–SiO₂ and Ir_Cl–S–SiO₂ catalysts active in hydrogenations of propene and propyne. Nuclear magnetic resonance enhancement in parahydrogen-induced polarization experiments was studied in a wide range of hydrogenations conditions (25–120 °C, 1.0–3.9 bar). The structural transformations were studied using ex situ X-ray photoelectron spectroscopy (XPS). It was established that Ir_Cl–S–SiO₂ demonstrated greater thermal stability in the hydrogenation of both propene and propyne in comparison with Rh_Cl–S–SiO₂. The beneficial effect of propyne was elucidated for thermal stability of studied catalysts and for the efficiency of the pairwise hydrogen addition. This can be explained by more efficient binding of the C≡C triple bond to an active center. The increase in reaction pressure typically leads to higher conversion in hydrogenations for both catalysts, but also decreases the temperatures sufficient for the reduction of anchored complexes with the formation of metal nanoparticles, which was confirmed by XPS.