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

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

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

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

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

Diffusion measurements of hydrocarbons in H-MCM-41 extrudates with pulsed-field gradient nuclear magnetic resonance spectroscopy

Mesoporous materials are promising catalysts for production of biofuels. Herein, H-MCM-41 catalysts with different concentrations of the silica Bindzil binder (10-50 wt%) were prepared and characterized using pulsed-field gradient (PFG) NMR in the powder form and as extrudates. Effective diffusion coefficients (D-e) are measured in all cases. Diffusivities of n-hexadecane were found smaller for extrudates as compared to the powder catalysts. The estimates of diffusive tortuosity were also determined. PFG NMR data showed one major component that reveals diffusion in interconnected meso- and micropores and one other minor component (1-2%) that may correspond to more isolated pores or may represent complex effects of restricted diffusion. Therefore, several approaches including initial slope analysis of spin-echo attenuation curves, two-component fitting and Laplace inversion were used to discuss different aspects of diffusional transport in the studied H-MCM-41 materials. Correlations between D-e and the amount of Bindzil, the specific surface area, the micropore volume, the particle size, the total acid sites and the Lewis acid sites are discussed

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

Abstract 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