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 both in gas and liquid phases. In particular, the effect was observed for ethylene and o-H2 produced in the hydrogenation reactions of acetylene and dimethyl acetylenedicarboxylate (DMAD), respectively, over an immobilized iridium-based catalyst complex. This 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 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.