Zhivonitko Vladimir V, Beer Henrik, Zakharov Danila O, Bresien Jonas, Schulz Axel
NMR Research Unit, University of Oulu, P.O. Box 3000, 90014, Oulu, Finland.
Institute of Chemistry, University of Rostock, Albert-Einstein-Strasse 3a, 18059, Rostock, Germany.
Chemphyschem. 2021 May 5;22(9):813-817. doi: 10.1002/cphc.202100141. Epub 2021 Apr 7.
Biradicaloids attract attention as a novel class of reagents that can activate small molecules such as H , ethylene and CO . Herein, we study activation of parahydrogen (nuclear spin-0 isomer of H ) by a number of 4- and 5-membered pnictogen biradicaloids based on hetero-cyclobutanediyl [X(μ-NTer) Z] and hetero-cyclopentanediyl [X(μ-NTer) ZC(NDmp)] moieties (X,Z=P,As; Ter=2,6-Mes -C H , Dmp=2,6-Me -C H ). The concerted mechanism of this reaction allowed observing strong nuclear spin hyperpolarization effects in H and P NMR experiments. Signal enhancements from two to four orders of magnitude were detected at 9.4 T depending on the structure. It is demonstrated that 4-membered biradicaloids activate H reversibly, leading to SABRE (signal amplification by reversible exchange) hyperpolarization of biradicaloids themselves and their H adducts. In contrast, the 5-membered counterparts demonstrate rather irreversible parahydrogen activation resulting in hyperpolarized H adducts only. Kinetic measurements provided parameters to support experimental observations.
双自由基类化合物作为一类能够活化诸如氢气、乙烯和一氧化碳等小分子的新型试剂而备受关注。在此，我们研究了基于杂环丁二烯基[X(μ-NTer)Z]和杂环戊二烯基[X(μ-NTer)ZC(NDmp)]部分（X、Z = P、As；Ter = 2,6-二甲基苯基，Dmp = 2,6-二甲基吡啶）的多种四元和五元氮族双自由基类化合物对仲氢（氢气的核自旋为0的异构体）的活化作用。该反应的协同机制使得在氢核磁共振和磷核磁共振实验中能够观察到强烈的核自旋超极化效应。在9.4 T磁场下，根据结构不同，信号增强幅度在两到四个数量级之间。结果表明，四元双自由基类化合物能够可逆地活化氢气，导致双自由基类化合物自身及其氢加合物产生SABRE（通过可逆交换实现信号放大）超极化。相比之下，五元双自由基类化合物表现出相当不可逆的仲氢活化，仅产生超极化的氢加合物。动力学测量提供了支持实验观察结果的参数。
