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

Abstract

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)2Z] and hetero-cyclopentanediyl [X(μ-NTer)2ZC(NDmp)] moieties (X,Z=P,As; Ter=2,6-Mes2−C6H3, Dmp=2,6-Me2−C6H3). The concerted mechanism of this reaction allowed observing strong nuclear spin hyperpolarization effects in 1H and 31P 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.