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

Abstract

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 ].

Iron-Catalyzed Parahydrogen Induced Polarization

“…This constitutes a hurdle in the implementation of PHIP in biomedical applications . Heterogeneous catalytic systems for PHIP − and a recent example by Duckett and Weller reported impressive hyperpolarization at room temperature via para hydrogenation of gaseous alkenes using a solid-state molecular organometallic (SMOM) catalyst have been reported to address this impediment . Homogeneous catalysts based on more benign first-row transition metals are an attractive alternative but have remained comparatively underexplored in this area of research, mainly due to the difficulty of meeting the requirements for PHIP in such systems.…”

Developing Hyperpolarized Butane Gas for Ventilation Lung Imaging

“…However, it should be noted here that our experiments were not fully optimized: for example, the coil filling factor was unfavorable (small object in a large MRI coil), Figure c,d, and no ultrafast sequences were readily available. Moreover, the degree of polarization (estimated here at 0.5–1%) can also be substantially increased using more advanced and purpose-synthesized catalysts , indeed, we hope that this work will stimulate the development of new materials for producing butane gas with a higher level of hyperpolarization. It should also be noted that, even at a nominally lower P level, HP butane detection sensitivity is similar or even better than that of the HP 129 Xe gas contrast agent.…”

Photoinitiated Single-Crystal to Single-Crystal Redox Transformations of Titanium-Oxo Clusters

“…Titanium-oxo clusters have been extensively studied in recent years, , finding applications in photocatalysis, photochromism, and as precursors to, and molecular models for, titania-based materials. − These clusters may act as the secondary building units in photoactive Ti-based metal organic frameworks (MOFs), − which provides further inspiration for study of their photochemistry and reactivity. , Further to the assembly of linked 2D or 3D framework materials, there is also interest in directly crystallizing cluster species into functional crystalline materials, which may exhibit reversible sorption properties. , These concepts engage with the growing field of solid-state molecular inorganic chemistry, − which utilizes the stability and selectivity induced by a crystalline environment to enhance the properties of a molecular species, e.g., for selective catalysis. − When studying molecular chemistry in the crystalline state, an opportunity arises to follow reactivity via single-crystal to single-crystal (SC-SC) transformations. ,, If the size and molecular packing of the starting compound and product are similar, crystallinity can be maintained throughout a transformation. This allows the changing molecular structure to be monitored directly using in situ single-crystal diffraction techniques .…”

“… 20 , 21 These concepts engage with the growing field of solid-state molecular inorganic chemistry, 22 − 24 which utilizes the stability and selectivity induced by a crystalline environment to enhance the properties of a molecular species, e.g., for selective catalysis. 25 − 27 When studying molecular chemistry in the crystalline state, an opportunity arises to follow reactivity via single-crystal to single-crystal (SC-SC) transformations. 18 , 19 , 28 If the size and molecular packing of the starting compound and product are similar, crystallinity can be maintained throughout a transformation.…”