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.