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 H₂ , ethylene and CO₂ . Herein, we study activation of parahydrogen (nuclear spin-0 isomer of H₂ ) by a number of 4- and 5-membered pnictogen biradicaloids based on hetero-cyclobutanediyl [X(μ-NTer)₂ Z] and hetero-cyclopentanediyl [X(μ-NTer)₂ ZC(NDmp)] moieties (X,Z=P,As; Ter=2,6-Mes₂ -C₆ H₃ , Dmp=2,6-Me₂ -C₆ H₃ ). The concerted mechanism of this reaction allowed observing strong nuclear spin hyperpolarization effects in ¹ H and ³¹ 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 H₂ reversibly, leading to SABRE (signal amplification by reversible exchange) hyperpolarization of biradicaloids themselves and their H₂ adducts. In contrast, the 5-membered counterparts demonstrate rather irreversible parahydrogen activation resulting in hyperpolarized H₂ adducts only. Kinetic measurements provided parameters to support experimental observations.