Parahydrogen-induced polarization with a metal-free P–P biradicaloid

Metal-free H2 activations are unusual but interesting for catalytic transformations, particularly in parahydrogen-based nuclear spin hyperpolarization techniques. We demonstrate that metal-free singlet phosphorus biradicaloid, [P(μ-NTer)]2, provides pronounced 1H and 31P hyperpolarization while activating the parahydrogen molecules. A brief analysis of the resulting NMR signals and the important kinetic parameters are presented

Water Reduction and Dihydrogen Addition in Aqueous Conditions With ansa-Phosphinoborane

Ortho -phenylene-bridged phosphinoborane (2,6-Cl 2 Ph) 2 B-C 6 H 4 -PCy 2 1 was synthesized in three steps from commercially available starting materials. 1 reacts with H 2 or H 2 O under mild conditions to form corresponding zwitterionic phosphonium borates 1-H 2 or 1-H 2 O . NMR studies revealed both reactions to be remarkably reversible. Thus, when exposed to H 2 , 1-H 2 O partially converts to 1-H 2 even in the presence of multiple equivalents of water in the solution. The addition of parahydrogen to 1 leads to nuclear spin hyperpolarization both in dry and hydrous solvents, confirming the dissociation of 1-H 2 O to free 1 . These observations were supported by computational studies indicating that the formation of 1-H 2 and 1-H 2 O from 1 are thermodynamically favored. Unexpectedly, 1-H 2 O can release molecular hydrogen to form phosphine oxide 1-O . Kinetic, mechanistic, and computational (DFT) studies were used to elucidate the unique ?umpolung? water reduction mechanism.Peer reviewe

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

Parahydrogen-induced polarization is a nuclear spin hyperpolarization technique that can provide strongly enhanced NMR signals for catalytic hydrogenation reaction products and intermediates. Among other matters, this can be employed to study the mechanisms of the corresponding chemical transformations. Commonly, noble metal complexes are used for reactions with parahydrogen. Herein, we present a PHIP study of metal-free imine hydrogenations catalyzed by the ansa-aminoborane catalyst QCAT. We discuss the reaction mechanism by showing the pairwise nature of the initial hydrogen activation step that leads to the formation of the negative net nuclear spin polarization of N-H hydrogen in the QCAT-H-2 intermediate, enabling the further transfer of parahydrogen-originating protons to the imine substrate with the accumulation of hyperpolarized amine products. Parahydrogen-induced polarization also demonstrates the reversibility of the catalytic cycle.Peer reviewe

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

Parahydrogen-induced polarization is a nuclear spin hyperpolarization technique that can provide strongly enhanced NMR signals for catalytic hydrogenation reaction products and intermediates. Among other matters, this can be employed to study the mechanisms of the corresponding chemical transformations. Commonly, noble metal complexes are used for reactions with parahydrogen. Herein, we present a PHIP study of metal-free imine hydrogenations catalyzed by the ansa-aminoborane catalyst QCAT. We discuss the reaction mechanism by showing the pairwise nature of the initial hydrogen activation step that leads to the formation of the negative net nuclear spin polarization of N-H hydrogen in the QCAT-H-2 intermediate, enabling the further transfer of parahydrogen-originating protons to the imine substrate with the accumulation of hyperpolarized amine products. Parahydrogen-induced polarization also demonstrates the reversibility of the catalytic cycle.Peer reviewe

Spontaneous N-15 Nuclear Spin Hyperpolarization in Metal-Free Activation of Parahydrogen by Molecular Tweezers

The ability of frustrated Lewis pairs (FLPs) to activate H-2 is of significant interest for metal-free catalysis. The activation of H-2 is also the key element of parahydrogen-induced polarization (PHIP), one of the nuclear spin hyper polarization techniques. It is demonstrated that o-phenylene-based ansa-aminoboranes (AABs) can produce H-1 nuclear spin hyperpolarization through a reversible interaction with parahydrogen at ambient temperatures. Heteronuclei are useful in NMR and MRI as well because they have a broad chemical shift range and long relaxation times and may act as background-free labels. We report spontaneous formation of N-15 hyperpolarization of the N-H site for a family of AABs. The process is efficient at the high magnetic field of an NMR magnet (7 T), and it provides up to 350-fold N-15 signal enhancements. Different hyperpolarization effects are observed with various AAB structures and in a broad temperature range. Spontaneous hyperpolarization, albeit an order of magnitude weaker than that for N-15, was also observed for B-11 nuclei.Peer reviewe

Parahydrogen-Induced Polarization in Hydrogenation Reactions Mediated by a Metal-Free Catalyst

We report nuclear spin hyperpolarization of various alkenes achieved in alkyne hydrogenations with parahydrogen over a metal-free hydroborane catalyst (HCAT). Being an intramolecular frustrated Lewis pair aminoborane, HCAT utilizes a non-pairwise mechanism of H-2 transfer to alkynes that normally prevents parahydrogen-induced polarization (PHIP) from being observed. Nevertheless, the specific spin dynamics in catalytic intermediates leads to the hyperpolarization of predominantly one hydrogen in alkene. PHIP enabled the detection of important HCAT-alkyne-H-2 intermediates through substantial H-1, B-11 and N-15 signal enhancement and allowed advanced characterization of the catalytic process.Peer reviewe

High-throughput continuous-flow system for SABRE hyperpolarization

Signal Amplification By Reversible Exchange (SABRE) is a versatile method for hyperpolarizing small organic molecules that helps to overcome the inherent low signal-to-noise ratio of nuclear magnetic resonance (NMR) measurements. It offers orders of magnitude enhanced signal strength, but the obtained nuclear polarization usually rapidly relaxes, requiring a quick transport of the sample to the spectrometer. Here we report a new design of a polarizing system, which can be used to prepare a continuous flow of SABRE-hyperpolarized sample with a considerable throughput of several millilitres per second and a rapid delivery into an NMR instrument. The polarizer performance under different conditions such as flow rate of the hydrogen or liquid sample is tested by measuring a series of NMR spectra and magnetic resonance images (MRI) of hyperpolarized pyridine in methanol. Results show a capability to continuously produce sample with dramatically enhanced signal over two orders of magnitude. The constant supply of hyperpolarized sample can be exploited, e.g., in experiments requiring multiple repetitions, such as 2D- and 3D-NMR or MRI measurements, and also naturally allows measurements of flow maps, including systems with high flow rates, for which the level of achievable thermal polarization might not be usable any more. In addition, the experiments can be viably carried out in a non-deuterated solvent, due to the effective suppression of the thermal polarization by the fast sample flow. The presented system opens the possibilities for SABRE experiments requiring a long-term, stable and high level of nuclear polarization

Ultrafast multidimensional Laplace NMR for a rapid and sensitive chemical analysis

Traditional nuclear magnetic resonance (NMR) spectroscopy relies on the versatile chemical information conveyed by spectra. To complement conventional NMR, Laplace NMR explores diffusion and relaxation phenomena to reveal details on molecular motions. Under a broad concept of ultrafast multidimensional Laplace NMR, here we introduce an ultrafast diffusion-relaxation correlation experiment enhancing the resolution and information content of corresponding 1D experiments as well as reducing the experiment time by one to two orders of magnitude or more as compared with its conventional 2D counterpart. We demonstrate that the method allows one to distinguish identical molecules in different physical environments and provides chemical resolution missing in NMR spectra. Although the sensitivity of the new method is reduced due to spatial encoding, the single-scan approach enables one to use hyperpolarized substances to boost the sensitivity by several orders of magnitude, significantly enhancing the overall sensitivity of multidimensional Laplace NMR

NMR Imaging of low pressure, gas-phase transport in packed beds using hyperpolarized xenon-129

Gas-phase magnetic resonance imaging (MRI) has been used to investigate heterogeneity in mass transport in a packed bed of commercial, alumina, catalyst supports. Hyperpolarized 129Xe MRI enables study of transient diffusion for micro- scopic porous systems using xenon chemical shift to selectively image gas within the pores, and, thence, permits study of low-density, gas-phase mass-transport, such that diffusion can be studied in the Knudsen regime, and not just the molecular regime, which is the limitation with other current techniques. Knudsen-regime diffusion is common in many industrial, catalytic processes. Significantly, larger spatial variability in mass transport rates across the packed bed was found compared to techniques using only molecular diffusion. It has thus been found that that these heterogeneities arise over length-scales much larger tha

Diffusion Measurements of Hydrocarbons in Zeolites with Pulse-Field Gradient Nuclear Magnetic Resonance Spectroscopy

Pulse field gradient NMR technique was used to determine self-diffusivity of heptane and pentadecane at room temperature for microporous catalysts, used both as powders and shaped with a binder extrudates. The results showed that diffusivities increased with increasing specific surface area, micro- and mesopore volume of the studied catalysts. The presence of Bindzil binder together with H-Beta-25 decreased hydrocarbon diffusivities. Self-diffusivities of heptane and pentadecane were smaller for extrudates than for the powder catalysts. The detailed information about mass transfer limitations is needed to further process optimization since effective diffusivity is directly correlated with self-diffusion coefficients. The estimates of the ratio of porosity and tortuosity were also determined. The diffusion measurements with relatively long observation times Delta (20 up to 1000 ms) and catalysts fully immersed in pentadecane revealed that a small portion of sites exhibits very small diffusivities in H-Beta-25-Bindzil extrudates, which is correlated with a low ratio of mesopore to micropore volumes of this material