Cobalt-Catalyzed Hyperpolarization of Structurally Intact Olefins

In Situ Electrospinning MOF-Derived Highly Dispersed α-Cobalt Confined in Nitrogen-Doped Carbon Nanofibers Nanozyme for Biomolecule Monitoring

Designing a metal-organic framework (MOF)-derived nanozyme with highly dispersed active sites and high catalytic activity as well as robust structure for colorimetric biosensing of diverse biomolecules remains a substantial challenge. Here, an MOF-derived highly dispersed and pure α-cobalt confined in a nitrogen-doped carbon nanofiber (α-Co@NCNF) nanozyme with superior glucose oxidase (GOD)- and peroxidase (POD)-like activities was constructed for colorimetric assay of multiple biomolecules. Specifically, the α-Co@NCNF nanozyme was synthesized, utilizing in situ electrospinning Co-MOFs into polyacrylonitrile nanofiber (PAN) followed by a pyrolysis process. Taking advantage of the in situ electrospinning strategy, the α-Co nanoparticles were confined in continuous porous NCNF to restrict the growth and prevent the aggregation and oxidation during the pyrolysis process. The resulting special structure considerably improved the enzyme-like performance. A series of experiments validate that the enzyme-like activity of the α-Co@NCNF nanozyme was superior to that of Co@CoO@NCNF (derivatives from Co-MOFs grown on the surface of PAN nanofiber) and nature enzymes. Furthermore, α-Co@NCNF nanozyme-based colorimetric biosensing was developed for monitoring glucose, hydrogen peroxide (H2O2), and glutathione (GSH) and the corresponding linear ranges are 0.1-50 and 50-900 μM and 5-55 and 0.1-20 μM accompanied by the corresponding low detection of 0.03, 1.66, and 0.03 μM. The proposed method for the construction of α-Co@NCNF nanozyme with dual enzyme-like properties provides a new insight for designing novel nanozymes and has prospects for application in colorimetric biosensing.

Iron-Catalyzed Parahydrogen Induced Polarization

Parahydrogen induced polarization (PHIP) can address the low sensitivity problem intrinsic to nuclear magnetic resonance spectroscopy. Using a catalyst capable of reacting with parahydrogen and substrate in either a hydrogenative or nonhydrogenative manner can result in signal enhancement of the substrate. This work describes the development of a rare example of an iron catalyst capable of reacting with parahydrogen to hyperpolarize olefins. Complexes of the form (MesCCC)Fe(H)(L)(N2) (L = Py (Py = pyridine), PMe3, PPh3) were synthesized from the reaction of the parent complexes (MesCCC)FeMes(L) (Mes = mesityl) with H2. The isolated low-spin iron(II) hydride compounds were characterized via multinuclear NMR spectroscopy, infrared spectroscopy, and single crystal X-ray diffraction. (MesCCC)Fe(H)(Py)(N2) is competent in the hydrogenation of olefins and demonstrated high activity toward the hydrogenation of monosubstituted terminal olefins. Reactions with p-H2 resulted in the first PHIP effect mediated by iron which requires diamagnetism throughout the reaction sequence. This work represents the development of a new PHIP catalyst featuring iron, unlocking potential to develop more PHIP catalysts based on first-row transition metals.

Anomalously Large Antiphase Signals from Hyperpolarized Orthohydrogen Using a MOF-Based SABRE Catalyst

Hyperpolarized orthohydrogen (o-H₂ ) is a frequent product of parahydrogen-based hyperpolarization approaches like signal amplification by reversible exchange (SABRE), where the hyperpolarized o-H₂ signal is usually absorptive. We describe a novel manifestation of this effect wherein large antiphase o-H₂ signals are observed, with ¹ H enhancements up to ≈500-fold (effective polarization P_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 the o-H₂ resonance is manifested by a two-fold higher Rabi frequency, and the lifetime of the antiphase HP o-H₂ resonance is extended by several-fold.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Advancing homogeneous catalysis for parahydrogen-derived hyperpolarisation and its NMR applications

Parahydrogen-induced polarisation (PHIP) is a nuclear spin hyperpolarisation technique employed to enhance NMR signals for a wide range of molecules. This is achieved by exploiting the chemical reactions of parahydrogen (para-H2), the spin-0 isomer of H2. These reactions break the molecular symmetry of para-H2 in a way that can produce dramatically enhanced NMR signals for reaction products, and are usually catalysed by a transition metal complex. In this review, we discuss recent advances in novel homogeneous catalysts that can produce hyperpolarised products upon reaction with para-H2. We also discuss hyperpolarisation attained in reversible reactions (termed signal amplification by reversible exchange, SABRE) and focus on catalyst developments in recent years that have allowed hyperpolarisation of a wider range of target molecules. In particular, recent examples of novel ruthenium catalysts for trans and geminal hydrogenation, metal-free catalysts, iridium sulfoxide-containing SABRE systems, and cobalt complexes for PHIP and SABRE are reviewed. Advances in this catalysis have expanded the types of molecules amenable to hyperpolarisation using PHIP and SABRE, and their applications in NMR reaction monitoring, mechanistic elucidation, biomedical imaging, and many other areas, are increasing.

Hyperpolarised NMR to aid molecular profiling of electronic cigarette aerosols

Signal amplification by reversible exchange (SABRE) hyperpolarisation is used to enhance the NMR signals of nicotine and acrolein in methanol-d4 solutions of electronic cigarette aerosols. Consequently, detection of 74 μM nicotine is possible in just a single scan 1H NMR spectrum. The first example of an aldehyde hyperpolarised using SABRE is demonstrated and we work towards novel real-world applications of SABRE-hyperpolarised NMR for chemical analysis.

Effects of a Tridentate Pincer Ligand on Parahydrogen Induced Polarization

The role of ligands in rhodium- and iridium-catalyzed Parahydrogen Induced Polarization (PHIP) and SABRE (signal amplification by reversible exchange) chemistry has been studied in the benchmark systems, [Rh(diene)(diphos)]⁺ and [Ir(NHC)(sub)₃ (H)₂ ]⁺ , and shown to have a great impact on the degree of hyperpolarization observed. Here, we examine the role of the flanking moieties in the electron-rich monoanionic bis(carbene) aryl pincer ligand, ^Ar CCC (Ar=Dipp, 2,6-diisopropyl or Mes, 2,4,6-trimethylphenyl) on the cobalt-catalyzed PHIP and PHIP-IE (PHIP via Insertion and Elimination) chemistry that we have previously reported. The mesityl groups were exchanged for diisopropylphenyl groups to generate the (^Dipp CCC)Co(N₂ ) catalyst, which resulted in faster hydrogenation and up to 390-fold ¹ H signal enhancements, larger than that of the (^Mes CCC)Co-py (py=pyridine) catalyst. Additionally, the synthesis of the (^Dipp CCC)Rh(N₂ ) complex is reported and applied towards the hydrogenation of ethyl acrylate with parahydrogen to generate modest signal enhancements of both ¹ H and ¹³ C nuclei. Lastly, the generation of two (^Mes CCC)Ir complexes is presented and applied towards SABRE and PHIP-IE chemistry to only yield small ¹ H signal enhancements of the partially hydrogenated product (PHIP) with no SABRE hyperpolarization.

Low-Cost High-Pressure Clinical-Scale 50% Parahydrogen Generator Using Liquid Nitrogen at 77 K

We report on a robust and low-cost parahydrogen generator design employing liquid nitrogen as a coolant. The core of the generator consists of catalyst-filled spiral copper tubing, which can be pressurized to 35 atm. Parahydrogen fraction >48% was obtained at 77 K with three nearly identical generators using paramagnetic hydrated iron oxide catalysts. Parahydrogen quantification was performed on the fly via benchtop NMR spectroscopy to monitor the signal from residual orthohydrogen-parahydrogen is NMR silent. This real-time quantification approach was also used to evaluate catalyst activation at up to 1.0 standard liter per minute flow rate. The reported inexpensive device can be employed for a wide range of studies employing parahydrogen as a source of nuclear spin hyperpolarization. To this end, we demonstrate the utility of this parahydrogen generator for hyperpolarization of concentrated sodium [1-13C]pyruvate, a metabolic contrast agent under investigation in numerous clinical trials. The reported pilot optimization of SABRE-SHEATH (signal amplification by reversible exchange-shield enables alignment transfer to heteronuclei) hyperpolarization yielded 13C signal enhancement of over 14,000-fold at a clinically relevant magnetic field of 1 T corresponding to approximately 1.2% 13C polarization-if near 100% parahydrogen would have been employed, the reported value would be tripled to 13C polarization of 3.5%.