Parahydrogen-Induced Polarization Study of the Silica-Supported Vanadium Oxo Organometallic Catalyst

Abstract

Parahydrogen can be used in catalytic hydrogenations to achieve substantial enhancement of NMR signals of the reaction products and in some cases of the reaction reagents as well. The corresponding nuclear spin hyperpolarization technique, known as parahydrogen-induced polarization (PHIP), has been applied to boost the sensitivity of NMR spectroscopy and magnetic resonance imaging by several orders of magnitude. The catalyst properties are of paramount importance for PHIP because the addition of parahydrogen to a substrate must be pairwise. This requirement significantly narrows down the range of the applicable catalysts. Herein, we study an efficient silica-supported vanadium oxo organometallic complex (VCAT) in hydrogenation and dehydrogenation reactions in terms of efficient PHIP production. This is the first example of group 5 catalyst used to produce PHIP. Hydrogenations of propene and propyne with parahydrogen over VCAT demonstrated production of hyperpolarized propane and propene, respectively. The achieved NMR signal enhancements were 200–300-fold in the case of propane and 1300-fold in the case of propene. Propane dehydrogenation in the presence of parahydrogen produced no hyperpolarized propane, but instead the hyperpolarized side-product 1-butene was detected. Test experiments of other group 5 (Ta) and group 4 (Zr) catalysts showed a much lower efficiency in PHIP as compared to that of VCAT. The results prove the general conclusion that vanadium-based catalysts and other group 4 and group 5 catalysts can be used to produce PHIP. The hydrogenation/dehydrogenation processes, however, are accompanied by side reactions leading, for example, to C4, C2, and C1 side products. Some of the side products like 1-butene and 2-butene were shown to appear hyperpolarized, demonstrating that the reaction mechanism includes pairwise parahydrogen addition in these cases as well.

Heterogeneous alkane dehydrogenation catalysts investigated via a surface organometallic chemistry approach

“…Studies using PHIP show that pairwise replacement of two hydrogens from propane with the ones originating from para-H 2 is not an efficient process over [(RSiO)V V (O)Mes 2 ], further highlighting the ambiguity in interpretation of the propane dehydrogenation mechanism in reaction conditions. 93 Detailed mechanistic studies of the microscopic reverse reaction (i.e. propene hydrogenation) show that by switching between hydrogenation and dehydrogenation conditions, the hydrogenation capacity of the catalyst 98 EXAFS data, consistent with the presence of a Co-O path of degeneracy 4 (1.94 Å), attributed to anionic oxygen directly bound to Co centre and adjacent siloxane oxygens interacting in a dative fashion, was used to establish the proposed surface structure.…”

“…This has been attributed to the formation of surface hydride species. 89 Furthermore, a more recent study indicates that a V III species is likely formed upon hydrogen treatment of [( SiO) 2 V V (O)Mes] at elevated temperatures (250–500 °C), 93 suggesting that in spite of the different precursors used, the active site structures and ‘activated’ materials are very similar. While various plausible pathways have been proposed, initiation likely starts with the C–H activation of propane to form mesitylene and a propyl vanadium species, followed by β-H elimination and formation of a propylene π-complex.…”

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

“…In many cases, a limiting factor for the practical applications of this technique is the presence of the toxic metal catalysts along with the hyperpolarized product in the reaction mixture, and their separation is not a trivial task. Several approaches have been tested to achieve separation, including hydrolysis/extraction of the hyperpolarized product in the case of homogeneous hydrogenation catalysts, 7 , 8 and the utilization of heterogeneous catalysts which, in addition to supported metals and metal oxides mentioned earlier, include surface-immobilized metal complexes 9 – 13 and catalytically active isolated surface sites and single-atom catalysts. 14 – 17 In this respect, the rather unusual metal-free activations of H 2 can be very interesting for providing metal-free hyperpolarized agents for in vivo use and other applications of NMR and MRI.…”

Single‐Site Heterogeneous Catalysts: From Synthesis to NMR Signal Enhancement

“…Based on the above observations, to achieve maximum polarization, the single‐site nature of the catalyst active centers should be preserved. Indeed, PHIP effects were successfully observed over immobilized Ir, Rh, V, Co, Cr, and Au complexes. The immobilization approach has shown its worth in PHIP experiments—the utilization of the Wilkinson's catalyst immobilized on silica made it possible to produce a continuous flow of hyperpolarized gas, which was used for the visualization of model objects, microreactors, and microfluidic devices …”