Abstract

The aim of this thesis is to study the metal-free activation of parahydrogen and the subsequent effects of parahydrogen-induced polarization (PHIP). PHIP is a nuclear spin hyperpolarization method that dramatically increases the sensitivity in NMR by enhancing signals in the spectrum. Typically, PHIP experiments involve catalysts based on metal complexes to activate parahydrogen. However, a recent discovery has shown that metal-free H2 activators can also lead to the spin hyperpolarization. At the same time, there were no reports about metal-free catalytic hydrogenations yielding in hyperpolarized products.

The first part of the thesis focuses on metal-free hydrogenations of alkynes mediated by the ansa-aminoborane-based catalyst HCAT. In this research, we performed hydrogenations of various alkynes in a high magnetic field using parahydrogen. During these reactions, HCAT interacted with alkynes to form HCAT-alkyne adducts, which activated the parahydrogen molecules and resulted in the corresponding alkenes. Consequently, we observed enhanced signals of HCAT-alkyne-H₂ intermediates, and most importantly, the alkene products. This represents the first example of PHIP effects on the hydrogenation products using a metal-free catalyst. Additionally, by applying PHIP, we studied and confirmed the proposed reaction mechanism.

Next, we proceeded to further investigate metal-free hydrogenations utilizing parahydrogen. In the second paper, we examined hydrogenation of imines facilitated by the catalyst QCAT. These experiments resulted in enhanced signals in amine products. Additionally, we demonstrated the reversibility of a catalytic cycle, wherein the amines could interact with QCAT and parahydrogen, showcasing hyperpolarization of the NH group hydrogens. This study highlights the significance of PHIP experiments in providing valuable mechanistic insights into metal-free hydrogenations.

Finally, we significantly expanded the range of biradicaloid-based metal-free parahydrogen activators. Namely, pnictogen biradicaloids represent another class of metal-free parahydrogen activators, following the ansa-aminoboranes, that exhibit PHIP effects. In our third paper, we employed four- and five-membered biradicaloids containing P or/and As radical centers. Significant findings were achieved with the four-membered As-P biradicaloid, which exhibited a high enhancement factor exceeding three orders of magnitude at 9.4 T, which is a particularly high level for metal-free systems. Furthermore, the reversibility of parahydrogen addition enabled the hyperpolarization of the biradicaloids themselves, thereby demonstrating the example of SABRE-like (Signal Amplification By Reversible Exchange) effect with metal-free catalytic systems.

Osajulkaisut / Original papers

Osajulkaisut eivät sisälly väitöskirjan elektroniseen versioon. / Original papers are not included in the electronic version of the dissertation.

1. Zakharov, D. O., Chernichenko, K., Sorochkina, K., Yang, S., Telkki, V.-V., Repo, T., & Zhivonitko, V. V. (2022). Parahydrogen‐induced polarization in hydrogenation reactions mediated by a metal‐free catalyst. Chemistry – A European Journal, 28(8), e202103501. https://doi.org/10.1002/chem.202103501

   Rinnakkaistallennettu versio / Self-archived version

2. Zakharov, D. O., Chernichenko, K., Sorochkina, K., Repo, T., & Zhivonitko, V. V. (2022). Parahydrogen-induced polarization study of imine hydrogenations mediated by a metal-free catalyst. Dalton Transactions, 51(36), 13606–13611. https://doi.org/10.1039/D2DT02178A

   Rinnakkaistallennettu versio / Self-archived version

3. Zhivonitko, V. V., Beer, H., Zakharov, D. O., Bresien, J., & Schulz, A. (2021). Hyperpolarization effects in parahydrogen activation with pnictogen biradicaloids: Metal‐free phip and sabre. ChemPhysChem, 22(9), 813–817. https://doi.org/10.1002/cphc.202100141

   Rinnakkaistallennettu versio / Self-archived version