Hyperpolarization Effects in Parahydrogen Activation with Pnictogen Biradicaloids: Metal-free PHIP and SABRE

Biradicaloids attract attention as a novel class of reagents that can activate small molecules such as H-2, ethylene and CO2. Herein, we study activation of parahydrogen (nuclear spin-0 isomer of H-2) by a number of 4- and 5-membered pnictogen biradicaloids based on hetero-cyclobutanediyl [X(mu-NTer)(2)Z] and hetero-cyclopentanediyl [X(mu-NTer)(2)ZC(NDmp)] moieties (X,Z=P,As; Ter=2,6-Mes(2)-C6H3, Dmp=2,6-Me-2-C6H3). The concerted mechanism of this reaction allowed observing strong nuclear spin hyperpolarization effects in H-1 and P-31 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-2 reversibly, leading to SABRE (signal amplification by reversible exchange) hyperpolarization of biradicaloids themselves and their H-2 adducts. In contrast, the 5-membered counterparts demonstrate rather irreversible parahydrogen activation resulting in hyperpolarized H-2 adducts only. Kinetic measurements provided parameters to support experimental observations.

Automated summary

The activation of parahydrogen by 4- and 5-membered pnictogen biradicaloids has been studied, allowing for observation of strong nuclear spin hyperpolarization effects in H-1 and P-31 NMR experiments. Reversible activation of H-2 was demonstrated by 4-membered biradicaloids, leading to SABRE hyperpolarization, while 5-membered biradicaloids showed irreversible parahydrogen activation.