Net Enhanced ¹H NMR Signals in Symmetric Molecules Produced via Heterogeneous Hydrogenation with Parahydrogen over Immobilized Metal Complexes

Heterogeneous parahydrogen-induced polarization (PHIP) offers a pathway to generate pure hyperpolarized molecular agents devoid of toxic catalysts. While conventional hydrogenative PHIP relies on hydrogenation with parahydrogen (p-H2) and breaking the equivalence of the p-H2-derived protons in the hydrogenation product, spontaneous net polarization is sometimes observed in symmetric systems. In this study, we explore a previously unreported case of net PHIP polarization in symmetric products of heterogeneous hydrogenation reactions in both gas and liquid phases. In particular, the effect was observed for ethylene and orthohydrogen (o-H2) produced in the heterogeneous hydrogenation reactions of acetylene and dimethyl acetylenedicarboxylate (DMAD), respectively, over an immobilized iridium-based catalyst complex. This polarized signal is amplified significantly under continuous-wave radiofrequency irradiation. By comparing these findings to a previously reported net polarization in dimethyl maleate obtained in a homogeneous hydrogenation reaction, we propose that asymmetric relaxation due to cross-correlation between chemical shift anisotropy (CSA) and dipolar coupling or between different dipolar couplings, which was suggested earlier for the homogeneous case, represents one of the possible mechanisms that could drive this phenomenon.