Comparing the S-T ₀ Nuclear Spin Conversion in the Intermediates of Homogeneous and Heterogeneous Hydrogenations with Parahydrogen

In this work, we present an initial attempt to study the S-T₀ nuclear spin conversion in the pair of p-H₂-derived protons within the hydrogenation intermediates under heterogeneous conditions. The results differ significantly from those obtained under homogeneous conditions, where the S-T₀ conversion demonstrated significant sensitivity to external factors, including temperature, solvent, and catalyst. The singlet character of the p-H₂-derived protons transferred to the product molecule is also increased if strong continuous-wave (CW) irradiation is applied at the anticipated chemical shift of the hydrogenation intermediates during p-H₂ bubbling. In contrast, under heterogeneous hydrogenation conditions, there is no or only a subtle dependence of the apparent S-T₀ conversion in the hydrogenation intermediates on temperature and other external factors for all the catalysts tested. We have used the reaction of hydrogenation of 2-butyne with p-H₂ to study the S-T₀ conversion at the intermediates both in the gas phase and in the liquid phase. The three types of heterogeneous catalysts, monometallic (Rh/TiO₂), bimetallic (Rh-In/SiO₂), and immobilized complex (Ir_Cl-P-TiO₂), have demonstrated different results in terms of S-T₀ conversion. At the same time, the three immobilized complexes (Ir_L_'-L″-support) with different supports (SiO₂/TiO₂), as well as linkers (L″ = -P/-S) and coordination spheres (L' = OMe or Cl), demonstrated the same results. This indicates that the S-T₀ conversion depends mainly on the type of active species of the catalyst.