41 research outputs found
Low-valent homobimetallic Rh complexes: influence of ligands on the structure and the intramolecular reactivity of RhâH intermediates
Supporting two metal binding sites by a tailored polydentate trop-based (trop - 5H-dibenzo[a,d] cyclohepten-5-yl) ligand yields highly unsymmetric homobimetallic rhodium(I) complexes. Their reaction with hydrogen rapidly forms Rh hydrides that undergo an intramolecular semihydrogenation of two CâĄC bonds of the trop ligand. This reaction is chemoselective and converts CâĄC bonds to a bridging carbene and an olefinic ligand in the first and the second semihydrogenation steps, respectively. Stabilization by a bridging diphosphine ligand allows characterization of a Rh hydride species by advanced NMR techniques and may provide insight into possible elementary steps of Hâ activation by interfacial sites of heterogeneous Rh/C catalysts
Parahydrogen-induced polarization in alkyne hydrogenation catalyzed by Pd nanoparticles embedded in a supported ionic liquid phase
Parahydrogen-induced polarization was observed in the gas phase heterogeneous hydrogenation of propyne catalyzed by Pd-0 nanoparticles embedded in an ionic liquid phase supported on activated carbon fibers (Pd-0/SILP/ACF). The results were markedly different from those obtained with a reference catalyst, Pd-0/ACF, demonstrating the important role of the ionic liquid
NMR Signal Enhancement for Hyperpolarized Fluids Continuously Generated in Hydrogenation Reactions with Parahydrogen
In the present study we analyze the
factors which can lower hyperpolarization
of fluids produced in a continuous flow regime by the parahydrogen-induced
polarization technique. We use the findings of this analysis to examine
the flow rate dependence of propane hyperpolarization produced in
the heterogeneous propylene hydrogenation by parahydrogen over Rh/TiO<sub>2</sub> catalyst. We have estimated the maximum attainable propane <sup>1</sup>H hyperpolarization yield and the corrected percentage of
pairwise hydrogen addition in heterogeneous hydrogenation, which was
found to be âŒ7%. The approach developed for polarization analysis
is useful for the optimization of experimental setup and reaction
conditions to obtain maximum hyperpolarization for parahydrogen-based
catalyst-free continuously generated fluids applicable in biomedical
magnetic resonance imaging
Gas Phase UTE MRI of Propane and Propene
Proton magnetic resonance imaging (1H MRI) of gases can potentially enable functional lung imaging to probe gas ventilation and other functions. Here, 1H MR images of hyperpolarized (HP) and thermally polarized propane gas were obtained using ultrashort echo time (UTE) pulse sequence. A 2-dimensional (2D) image of thermally polarized propane gas with âŒ0.9 Ă 0.9 mm2 spatial resolution was obtained in <2 seconds, showing that even non-HP hydrocarbon gases can be successfully used for conventional proton magnetic resonance imaging. The experiments were also performed with HP propane gas, and high-resolution multislice FLASH 2D images in âŒ510 seconds and non-slice-selective 2D UTE MRI images were acquired in âŒ2 seconds. The UTE approach adopted in this study can be potentially used for medical lung imaging. Furthermore, the possibility of combining UTE with selective suppression of 1H signals from 1 of the 2 gases in a mixture is shown in this MRI study. The latter can be useful for visualizing industrially important processes where several gases may be present, eg, gasâsolid catalytic reactions
Pairwise hydrogen addition in the selective semihydrogenation of alkynes on silica-supported Cu catalysts
Mechanistic insight into the semihydrogenation of 1-butyne and 2-butyne on Cu nanoparticles supported on partially dehydroxylated silica (Cu/SiO2-700) was obtained using parahydrogen. Hydrogenation of 1-butyne over Cu/SiO2-700 yielded 1-butene with â„97% selectivity. The surface modification of this catalyst with tricyclohexylphosphine (PCy3) increased the selectivity to 1-butene up to nearly 100%, although at the expense of reduced catalytic activity. Similar trends were observed in the hydrogenation of 2-butyne, where Cu/SiO2-700 provided a selectivity to 2-butene in the range of 72â100% depending on the reaction conditions, while the catalyst modified with PCy3 again demonstrated nearly 100% selectivity. Parahydrogen-induced polarization effects observed in hydrogenation reactions catalyzed by copper-based catalysts demonstrate the viability of pairwise hydrogen addition over these catalysts. Contribution of pairwise hydrogen addition to 1-butyne was estimated to be at least 0.2â0.6% for unmodified Cu/SiO2-700 and â„2.7% for Cu/SiO2-700 modified with PCy3, highlighting the effect of surface modification with the tricyclohexylphosphine ligand.ISSN:2041-6520ISSN:2041-653
Synthesis of Unsaturated Precursors for Parahydrogen-Induced Polarization and Molecular Imaging of 1-<sup>13</sup>CâAcetates and 1-<sup>13</sup>CâPyruvates via Side Arm Hydrogenation
Hyperpolarized forms of 1-<sup>13</sup>C-acetates and 1-<sup>13</sup>C-pyruvates are used as diagnostic
contrast agents for molecular
imaging of many diseases and disorders. Here, we report the synthetic
preparation of 1-<sup>13</sup>C isotopically enriched and pure from
solvent acetates and pyruvates derivatized with unsaturated ester
moiety. The reported unsaturated precursors can be employed for NMR
hyperpolarization of 1-<sup>13</sup>C-acetates and 1-<sup>13</sup>C-pyruvates via parahydrogen-induced polarization (PHIP). In this
PHIP variant, Side arm hydrogenation (SAH) of unsaturated ester moiety
is followed by the polarization transfer from nascent parahydrogen
protons to <sup>13</sup>C nucleus via magnetic field cycling procedure
to achieve hyperpolarization of <sup>13</sup>C nuclear spins. This
work reports the synthesis of PHIP-SAH precursors: vinyl 1-<sup>13</sup>C-acetate (55% yield), allyl 1-<sup>13</sup>C-acetate (70% yield),
propargyl 1-<sup>13</sup>C-acetate (45% yield), allyl 1-<sup>13</sup>C-pyruvate (60% yield), and propargyl 1-<sup>13</sup>C-pyruvate (35%
yield). Feasibility of PHIP-SAH <sup>13</sup>C hyperpolarization was
verified by <sup>13</sup>C NMR spectroscopy: hyperpolarized allyl
1-<sup>13</sup>C-pyruvate was produced from propargyl 1-<sup>13</sup>C-pyruvate with <sup>13</sup>C polarization of âŒ3.2% in CD<sub>3</sub>OD and âŒ0.7% in D<sub>2</sub>O. <sup>13</sup>C magnetic
resonance imaging is demonstrated with hyperpolarized 1-<sup>13</sup>C-pyruvate in aqueous medium
Chemical Reaction Monitoring Using Zero-Field Nuclear Magnetic Resonance Enables Study of Heterogeneous Samples in Metal Containers
We
demonstrate that heterogeneous/biphasic chemical reactions can be monitored with
high spectroscopic resolution using zero-field nuclear magnetic resonance. This
is possible because magnetic susceptibility broadening is insignificant at
ultralow magnetic fields. We show the two-step hydrogenation of dimethyl
acetylenedicarboxylate with para-enriched hydrogen gas in conventional
glass NMR tubes, as well as in a titanium tube. The low frequency zero-field
NMR signals ensure that there is no significant signal attenuation due to
shielding by the electrically conductive sample container. This method paves
the way for in situ monitoring of reactions in complex heterogeneous
multiphase systems and in reactors made from conductive materials without
magnetic susceptibility induced line broadening.</div