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 CO(2). 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(μ‐NTer)(2)Z] and hetero‐cyclopentanediyl [X(μ‐NTer)(2)ZC(NDmp)] moieties (X,Z=P,As; Ter=2,6‐Mes(2)−C(6)H(3), Dmp=2,6‐Me(2)−C(6)H(3)). 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 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.