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

Abstract

Abstract 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.