462 research outputs found
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
Kinetics of recrystallization and grain growth in an ultra-fine grained CoCrFeNiMn-type high-entropy alloy
A novel high entropy alloy based on the CoCrFeMnNi system with substantial amounts of Al and C was studied. After cold rolling and annealing at 973-1273 K a duplex ultrafine grained structure composed of the recrystallized fcc grains and M23C6 and B2 particles was produced. Analysis of the coarsening behavior of grains and particles growth suggested that kinetics of both was controlled by volume diffusion. The apparent activation energy of structure coarsening during recrystallization was evaluate
Recrystallized microstructures and mechanical properties of a C-containing CoCrFeNiMn-type high-entropy alloy
The effect of cold rolling to 80% thickness reduction and annealing at 973-1373 K for 1h our on the microstructure and mechanical properties of a C-containing CoCrFeNiMn high-entropy alloy was studied. Cold rolling significantly strengthened the alloy to the yield strength of 1310 MPa. Annealing at 973 K or 1073 K resulted in incomplete recrystallization of an fcc matrix and M23C6-type carbide precipitations aligned with highly elongated grains/subgrains. Complete recrystallization occurred during annealing at 1173-1373
Effect of Al on structure and mechanical properties of Fe-Mn-Cr-Ni-Al non-equiatomic high entropy alloys with high Fe content
Microstructure and mechanical properties of the Fe-Mn-Cr-Ni-Al system non-equiatomic high entropy alloys with a different Al content (x =0-14 at.%) were studied in the present wor
Mechanical properties of a new high entropy alloy with a duplex ultra-fine grained structure
A new approach to increase the tensile performance of high entropy alloys (HEAs) by producing a duplex ultrafine- grained (UFG) structure was reported in this wor
Effect of thermomechanical processing on microstructure and mechanical properties of the carbon-containing CoCrFeNiMn high entropy alloy
Microstructural evolution during cold sheet rolling to 80% thickness strain and annealing at 600-1100° C for 30 min of the CoCrFeNiMn high entropy alloy doped with 1 at.% of C and resulting mechanical properties of the alloy are reporte
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