Long‐Term Generation of Longitudinal Spin Order Controlled by Ammonia Ligation Enables Rapid SABRE Hyperpolarized 2D NMR

Abstract

Symmetry breaking of parahydrogen using iridium catalysts converts singlet spin order into observable hyperpolarization. In this contribution, iridium catalysts are designed to exhibit asymmetry in their hydrides, regulated by in situ generation of deuterated ammonia governed by ammonium buffers. The concentrations of ammonia (N) and pyridine (P) provide a handle to generate a variety of stereo‐chemically asymmetric N‐heterocyclic carbene iridium complexes, ligating either [3xP], [2xP;N], [P;2xN] or [3xN] in an octahedral SABRE type configuration. The non‐equivalent hydride positions, in correspondence with the ammonium buffer solutions, enables to extend singlet‐triplet or S⟩→|T0⟩ mixing at high magnetic field and experimentally induce prolonged generation of non‐equilibrium longitudinal two‐spin order. This long‐lasting magnetization can be exploited in hyperpolarized 2D‐OPSY‐COSY experiments providing direct structural information on the catalyst using a single contact with parahydrogen. Separately, field cycling revealed hyperpolarization properties in low‐field conditions. Controlling catalyst stereochemistry by introducing small and deuterated ligands, such as deuterated ammonia, simplifies the spin‐system. This is shown to unify experimental and theoretically derived field‐sweep experiments for four‐spin systems.

Real-Time High-Sensitivity Reaction Monitoring of Important Nitrogen-Cycle Synthons by 15N Hyperpolarized Nuclear Magnetic Resonance

“…80,81,84−86 Additionally, ammonia and amines have been used as a coligand that leads to improved SABRE catalysis. 62,115 The 15 N polarization of benzylamine- 15 N (E-15 N) is reported to be ca. 800-fold at 9.4 T. 84 This involves the action of [Ir(H) 2 (IMes)-(E-15 N) 3 ]Cl in dichloromethane-d 2 solution.…”

“…The SABRE hyperpolarization of NH 3 and amines, such as benzylamine, and their use in SABRE-relay have been extensively reported. ,,− Additionally, ammonia and amines have been used as a coligand that leads to improved SABRE catalysis. , The 15 N polarization of benzylamine- 15 N ( E- 15 N ) is reported to be ca. 800-fold at 9.4 T .…”

Reversible Parahydrogen Induced Hyperpolarization of 15N in Unmodified Amino Acids Unraveled at High Magnetic Field

“…But in reality, the practicality of using it under experimental conditions can even be more important for the future development of hyperpolarization applications in academia and industry. [29,30] In this regard, parahydrogen (p-H 2 ), the singlet spin isomer of molecular hydrogen, stands out as a cost-effective and efficient hyperpolarization agent for enhancing spin order into interesting nuclei such as 15 N. [31][32][33][34] P-H 2 can be produced costeffectively on demand, [35] can provide a fast hyperpolarization build-up time in seconds, [36] and can be deployed under ambient experimental conditions. [37] Using transition metal catalysis, p-H 2 's undetectable singlet magnetization is unlocked.…”

Direct Production of a Hyperpolarized Metabolite on a Microfluidic Chip

“…Two methods have been shown to suppress ST mixing: spin locking on the 1 H hydride resonance during the chemical reaction ,,− and applying a hard π/2 purge pulse to deplete the | T 0 ⟩ state prior to the polarization transfer step. ,,− These two methods are illustrated in Figure d.…”