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1. Anomalously Large Antiphase Signals from Hyperpolarized Orthohydrogen Using a MOF‐Based SABRE Catalyst

   https://doi.org/10.1002/anie.202213581  

   Alam, Md Shahabuddin; Li, Xinlin; Brittin, Drew O.; Islam, Saiful; Deria, Pravas; Chekmenev, Eduard Y.; Goodson, Boyd M. (January 2023, Angewandte Chemie International Edition)

   Abstract Hyperpolarized orthohydrogen (o‐H₂) is a frequent product of parahydrogen‐based hyperpolarization approaches like signal amplification by reversible exchange (SABRE), where the hyperpolarizedo‐H₂signal is usually absorptive. We describe a novel manifestation of this effect wherein large antiphaseo‐H₂signals are observed, with¹H enhancements up to ≈500‐fold (effective polarizationP_H≈1.6 %). This anomalous effect is attained only when using an intact heterogeneous catalyst constructed using a metal–organic framework (MOF) and is qualitatively independent of substrate nature. This seemingly paradoxical observation is analogous to the “partial negative line” (PNL) effect recently explained in the context of Parahydrogen Induced Polarization (PHIP) by Ivanov and co‐workers. The two‐spin order of theo‐H₂resonance is manifested by a two‐fold higher Rabi frequency, and the lifetime of the antiphase HPo‐H₂resonance is extended by several‐fold. 

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2. Heterogeneous hydrogenation of phenylalkynes with parahydrogen: hyperpolarization, reaction selectivity, and kinetics

   https://doi.org/10.1039/c9cp02913c  

   Pokochueva, Ekaterina V.; Kovtunov, Kirill V.; Salnikov, Oleg G.; Gemeinhardt, Max E.; Kovtunova, Larisa M.; Bukhtiyarov, Valerii I.; Chekmenev, Eduard Y.; Goodson, Boyd M.; Koptyug, Igor V. (December 2019, Physical Chemistry Chemical Physics)

   Parahydrogen-induced polarization (PHIP) is a powerful technique for studying hydrogenation reactions in gas and liquid phases. Pairwise addition of parahydrogen to the hydrogenation substrate imparts nuclear spin order to reaction products, manifested as enhanced 1 H NMR signals from the nascent proton sites. Nanoscale metal catalysts immobilized on supports comprise a promising class of catalysts for producing PHIP effects; however, on such catalysts the percentage of substrates undergoing the pairwise addition route—a necessary condition for observing PHIP—is usually low. In this paper, we present a systematic study of several metal catalysts (Rh, Pt, Pd, and Ir) supported on TiO 2 in liquid-phase hydrogenation of different prototypical phenylalkynes (phenylacetylene, 1-phenyl-1-propyne, and 3-phenyl-1-propyne) with parahydrogen. Catalyst activity and selectivity were found to be affected by both the nature of the active metal and the percentage of metal loading. It was demonstrated that the optimal catalyst for production of hyperpolarized products is Rh/TiO 2 with 4 wt% metal loading, whereas Pd/TiO 2 provided the greatest selectivity for semihydrogenation of phenylalkynes. In a study of liquid-phase hydrogenation reaction kinetics, it was shown that reaction order with respect to hydrogen is nearly the same for pairwise and non-pairwise H 2 addition—consistent with a similar nature of the catalytically active sites for these reaction pathways. 

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3. Nuclear Spin Hyperpolarization of NH ₂ ‐ and CH ₃ ‐Substituted Pyridine and Pyrimidine Moieties by SABRE

   https://doi.org/10.1002/cphc.202000483  

   Mandal, Ratnamala; Pham, Pierce; Hilty, Christian (September 2020, ChemPhysChem)

   Abstract Hyperpolarization of N‐heterocycles with signal amplification by reversible exchange (SABRE) induces NMR sensitivity gains for biological molecules. Substitutions with functional groups, in particular in theortho‐position of the heterocycle, however, result in low polarization using a typical Ir catalyst with a bis‐mesityl N‐heterocyclic carbene ligand for SABRE, presumably due to steric hindrance. With the addition of allylamine or acetonitrile as coligands to the precatalyst chloro(1,5‐cyclooctadiene)[4,5‐dimethyl‐1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene] iridium, the¹H signal enhancement increased in several substrates withorthoNH₂substitutions. For example, for a proton in 2,4‐diaminopyrimidine, the enhancement factors increased from −7±1 to −210±20 with allylamine or to −160±10 with acetonitrile. CH₃substituted molecules yielded maximum signal enhancements of −25±7 with acetonitrile addition, which is considerably less than the corresponding NH₂substituted molecules, despite exhibiting similar steric size. With the more electron‐donating NH₂substitution resulting in greater enhancement, it is concluded that steric hindrance is not the only dominant factor in determining the polarizability of the CH₃substituted compounds. The addition of allylamine increased the signal enhancement for the 290 Da trimethoprim, a molecule with a 2,4‐diaminopyrimidine moiety serving as an antibacterial agent, to −70. 

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4. “Direct” ¹³ C Hyperpolarization of ¹³ C‐Acetate by MicroTesla NMR Signal Amplification by Reversible Exchange (SABRE)

   https://doi.org/10.1002/anie.201910506  

   Gemeinhardt, Max E.; Limbach, Miranda N.; Gebhardt, Thomas R.; Eriksson, Clark W.; Eriksson, Shannon L.; Lindale, Jacob R.; Goodson, Elysia A.; Warren, Warren S.; Chekmenev, Eduard Y.; Goodson, Boyd M. (November 2019, Angewandte Chemie International Edition)

   Abstract Herein, we demonstrate “direct”¹³C hyperpolarization of¹³C‐acetate via signal amplification by reversible exchange (SABRE). The standard SABRE homogeneous catalyst [Ir‐IMes; [IrCl(COD)(IMes)], (IMes=1,3‐bis(2,4,6‐trimethylphenyl), imidazole‐2‐ylidene; COD=cyclooctadiene)] was first activated in the presence of an auxiliary substrate (pyridine) in alcohol. Following addition of sodium 1‐¹³C‐acetate, parahydrogen bubbling within a microtesla magnetic field (i.e. under conditions of SABRE in shield enables alignment transfer to heteronuclei, SABRE‐SHEATH) resulted in positive enhancements of up to ≈100‐fold in the¹³C NMR signal compared to thermal equilibrium at 9.4 T. The present results are consistent with a mechanism of “direct” transfer of spin order from parahydrogen to¹³C spins of acetate weakly bound to the catalyst, under conditions of fast exchange with respect to the¹³C acetate resonance, but we find that relaxation dynamics at microtesla fields alter the optimal matching from the traditional SABRE‐SHEATH picture. Further development of this approach could lead to new ways to rapidly, cheaply, and simply hyperpolarize a broad range of substrates (e.g. metabolites with carboxyl groups) for various applications, including biomedical NMR and MRI of cellular and in vivo metabolism. 

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5. Tunable iridium catalyst designs with bidentate N-heterocyclic carbene ligands for SABRE hyperpolarization of sterically hindered substrates

   https://doi.org/10.1039/D0CC06840C  

   Pham, Pierce; Hilty, Christian (December 2020, Chemical Communications)

   null (Ed.)

   A series of bidentate N-heterocyclic carbene (NHC) iridium catalysts, [Ir(κC,N-NHC)H 2 L 2 ]BPh 4 , are proposed for SABRE hyperpolarization. The steric and electronic properties of the NHCs are used to tune substrate affinity and thereby SABRE efficiency. The sterically hindered substrates 2,4-diaminopyrimidine and trimethoprim yielded maximum proton NMR signal enhancements of ∼300-fold and ∼150-fold, respectively. 

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