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

Tickner, Ben. J.; Живонитко, Владимир В.

doi:10.1039/d2sc00737a

Cited by 21 publications

(51 citation statements)

References 230 publications

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“…We believe that further development of metal-free imine hydrogenation can lead to much higher enhancements, ideally reaching the levels of metal-containing catalysts. Generally, this study shows that PHIP-assisted mechanistic studies provide important mechanistic information about metal-free hydrogenations, as proved many times similarly for metal-containing catalysts 19,20,45,46 and, in addition to this study, only once for a metal-free catalyst. 35…”

Section: Discussionsupporting

confidence: 77%

“…Although PHIP was mostly used to study reactions mediated by metal-containing catalysts, 19,20,27,28 it was reported that metal-free catalysts can activate parahydrogen, leading to PHIP effects. 29,30 Specifically, AABs were the first FLPs to show spin hyperpolarization effects in reactions with parahydrogen.…”

Section: Introductionmentioning

confidence: 99%

“…19 PHIP has demonstrated its power by proving the mechanisms of hydrogenation of various metal complex catalysts, especially in the detection of dihydride intermediates. 19,20 For example, it was used to study the kinetics of the formation of [IrCO(H) 2 I(PPh 3 ) 2 ] after the light-induced reductive elimination of hydrogen from that species; 21 to detect cationic Rh I intermediates with coordinated arene rings; 22 to determine the pathways of H 2 exchange in (µ-H) 2 Os 3 (CO) 10 clusters; 23 to understand ligand exchange in [IrCO(H) 2 Cl(PPh 3 ) 2 ] and [IrH 2 Cl(PPh 3 ) 3 ]; 24 and in many other interesting mechanistic investigations 19,20,25 including heterogeneous catalytic systems. 26 Although PHIP was mostly used to study reactions mediated by metal-containing catalysts, 19,20,27,28 it was reported that metal-free catalysts can activate parahydrogen, leading to PHIP effects.…”

Section: Introductionmentioning

confidence: 99%

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Parahydrogen-induced polarization study of imine hydrogenations mediated by a metal-free catalyst

et al. 2022

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Section: Discussionsupporting

confidence: 77%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Parahydrogen-induced polarization study of imine hydrogenations mediated by a metal-free catalyst

et al. 2022

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“…PHIP has been widely used to study mechanism in molecular homogeneous catalysis in solution, ,, and there is now a significant body of work dedicated to gas-phase PHIP using classical heterogeneous catalysts. , Our crystalline SMOM systems clearly establish a connection between molecular catalysis (rate, selectivity through precise ligand design, and atomic-level resolution of molecular structure) with heterogeneous systems (product/catalyst separation, stability, and recyclability). The highly active “single-site” nature of our SMOM system, and attendant high catalytic flux with retention of polarization in the products, also allows for the real-time interrogation of an evolving molecular heterogeneous catalyst system.…”

Section: Discussionmentioning

confidence: 99%

Mechanistic Insights into Molecular Crystalline Organometallic Heterogeneous Catalysis through Parahydrogen-Based Nuclear Magnetic Resonance Studies

et al. 2023

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The heterogeneous solid−gas reactions of crystals of [Rh(L 2 )(propene)] [BAr F 4 ] (1, L 2 = t Bu 2 PCH 2 CH 2 P t Bu 2 ) with H 2 and propene, 1-butene, propyne, or 1-butyne are explored by gas-phase nuclear magnetic resonance (NMR) spectroscopy under batch conditions at 25 °C. The temporal evolution of the resulting parahydrogen-induced polarization (PHIP) effects measures catalytic flux and thus interrogates the efficiency of catalytic pairwise para-H 2 transfer, speciation changes in the crystalline catalyst at the molecular level, and allows for high-quality singlescan 1 H, 13 C NMR gas-phase spectra for the products to be obtained, as well as 2D-measurements. Complex 4), as probed using EXAFS; meanwhile, a single-crystal of 1 equilibrates NMR silent para-H 2 with its NMR active ortho isomer, contemporaneously converting into 4, and 1 and 4 each convert para-H 2 into ortho-H 2 at different rates. Hydrogenation of propene using 1 and para-H 2 results in very high initial polarization levels in propane (>85%). Strong PHIP was also detected in the hydrogenation products of 1butene, propyne, and 1-butyne. With propyne, a competing cyclotrimerization deactivation process occurs to afford [Rh( t Bu 2 PCH 2 CH 2 P t Bu 2 )(1,3,4-Me 3 C 6 H 3 )][BAr F 4 ], while with 1-butyne, rapid isomerization of 1-butyne occurs to give a butadiene complex, which then reacts with H 2 more slowly to form catalytically active 4. Surprisingly, the high PHIP hydrogenation efficiencies allow hyperpolarization effects to be seen when H 2 is taken directly from a regular cylinder at 25 °C. Finally, changing the chelating phosphine to Cy 2 PCH 2 CH 2 PCy 2 results in initial high polarization efficiencies for propene hydrogenation, but rapid quenching of the catalyst competes to form the zwitterion [Rh(Cy 2 PCH 2 CH 2 PCy 2 ){η 6 -(CF 3 ) 2 (C 6 H 3 )}BAr F 3 ].

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“…Examples include MRI visualization of hydrogenation of propylene in a packed catalyst bed , or microchip reactors, hydrogenation of ethylene in a packed bed or in monolithic reactors, and CO 2 methanation . To minimize the distortion of the magnetic field, a glass tube model reactor with a thin layer of various catalytically active coatings was designed and implemented for spectroscopic and imaging experiments. , A significant improvement in NMR and MRI experiments can be achieved by using hyperpolarization of nuclear spins, which boosts signal intensity by up to several orders of magnitude − and, consequently, provides an increase in the signal-to-noise ratio (SNR). An efficient and inexpensive hyperpolarization technique particularly suited for studies in heterogeneous catalysis is parahydrogen-induced polarization (PHIP). − It is based on pairwise addition of the parahydrogen ( p -H 2 ) molecule to an unsaturated reactant (e.g., alkyne, alkene, etc.)…”

Section: Introductionmentioning

confidence: 99%

Getting the Most out of Parahydrogen-Induced Signal Enhancement for MRI of Reacting Heterogeneous Systems

et al. 2022

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Methods based on magnetic resonance imaging (MRI) can be used for operando studies of heterogeneous catalytic processes in the gas phase. MRI can provide a detailed understanding of the heterogeneous reactor operation based on the information about spatial distribution of reactants and reaction products. However, low spin density, fast diffusion, and short relaxation times of gases along with magnetic field inhomogeneities associated with heterogeneous catalytic systems complicate such studies and compromise achievable sensitivity. Spin hyperpolarization techniques in general and parahydrogen-induced polarization (PHIP) in particular provide a major increase in the intensity of nuclear magnetic resonance (NMR) signals. At the same time, an antiphase lineshape of NMR signals associated with PHIP in high magnetic fields is disadvantageous for MRI experiments. This is because magnetic field gradients (both intrinsic and applied) lead to mutual cancelation of the positive and negative parts of such signals so that, for instance, frequency-encoding gradients of an MRI pulse sequence can significantly diminish or even eliminate the useful signal. In this study, we explore the effects of an antiphase NMR signal shape on MR images. We first demonstrate these effects for a homogeneous solution with thermal polarization of nuclear spins. We then address MRI of heterogeneous catalytic hydrogenation of a gas (propylene) with parahydrogen in a high magnetic field. The results demonstrate the importance of antiphase-to-inphase signal shape conversion when MRI is applied in such studies to use the signal enhancement provided by hyperpolarization to the maximum possible extent. This approach, which is implemented for the first time in an MRI study of a heterogeneous object (catalyst beads in an operating model reactor), allowed us to detect MR images of the gaseous reaction product in a model reactor and achieve a 10-fold improvement in the signal-to-noise ratio compared to the conventional threedimensional MRI experiment performed on an antiphase NMR signal.

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

Abstract: Parahydrogen-induced polarisation (PHIP) is a nuclear spin hyperpolaristion technique employed to enhance NMR signals for a wide range of molecules. This is achieved by exploiting chemical reactions of parahydrogen (para-H2),...

Cited by 21 publications

References 230 publications

Parahydrogen-induced polarization study of imine hydrogenations mediated by a metal-free catalyst

Parahydrogen-induced polarization study of imine hydrogenations mediated by a metal-free catalyst

Mechanistic Insights into Molecular Crystalline Organometallic Heterogeneous Catalysis through Parahydrogen-Based Nuclear Magnetic Resonance Studies

Getting the Most out of Parahydrogen-Induced Signal Enhancement for MRI of Reacting Heterogeneous Systems

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