Parahydrogen-Induced Polarization by Pairwise Replacement Catalysis on Pt and Ir Nanoparticles

Abstract

Pairwise and random addition processes are ordinarily indistinguishable in hydrogenation reactions. The distinction becomes important only when the fate of spin correlation matters, such as in parahydrogen-induced polarization (PHIP). Supported metal catalysts were not expected to yield PHIP signals given the rapid diffusion of H atoms on the catalyst surface and in view of the sequential stepwise nature of the H atom addition in the Horiuti-Polanyi mechanism. Thus, it seems surprising that supported metal hydrogenation catalysts can yield detectable PHIP NMR signals. Even more remarkably, supported Pt and Ir nanoparticles are shown herein to catalyze pairwise replacement on propene and 3,3,3-trifluoropropene. By simply flowing a mixture of parahydrogen and alkene over the catalyst, the scalar symmetrization order of the former is incorporated into the latter without a change in molecular structure, producing intense PHIP NMR signals on the alkene. An important indicator of the mechanism of the pairwise replacement is its stereoselectivity, which is revealed with the aid of density matrix spectral simulations. PHIP by pairwise replacement has the potential to significantly diversify the substrates that can be hyperpolarized by PHIP for biomedical utilization.

Parahydrogen‐Based Hyperpolarization for Biomedicine

“…HET-PHIP effects were also successfully observed using heterogeneous catalysts comprising Pt and Pd nanoparticles supported on γ-Al 2 O 3 , [79] and later for many other supported metals (e.g., Rh, Ir, Co, Ni, Au, Cu) catalysts in gas, liquid and aqueous phase hydrogenations. [11–12, 25, 27, 78f, 78g, 80] Variation of the type of metal, metal particle size [80a, 80d, 81] and shape, [80e, 80h] type of support [80a, 80d, 80g, 80i] and reaction conditions were all shown to influence the magnitude of the PHIP effect. Polarization transfer from 1 H to heteronuclei ( 13 C, 15 N) during heterogeneous hydrogenation [9b, 62a, 62b] helps to increase the hyperpolarization lifetime significantly, and is particularly important for potential biomedical applications along with the use of aqueous media.…”

Toward production of pure 13C hyperpolarized metabolites using heterogeneous parahydrogen-induced polarization of ethyl[1-13C]acetate

“…30,31 The main disadvantage of this approach is the presence of the dissolved homogeneous catalyst, which is expensive and is challenging to separate from the hyperpolarized compounds. 32,33 Recently, however, PHIP was observed in the heterogeneous reactions in the gas 34 and in the liquid phases 35,36 over immobilized complexes, 37 supported metals 34,38 and ligand-capped nanoparticles. 39,40 Observation of 13 C HP resonance and therefore, production of 13 C-hyperpolarized molecules via heterogeneous hydrogenation with parahydrogen was so far shown only for 13 C HEP during hydrogenation of 2-hydroxyethyl acrylate over platinum nanoparticles capped with glutathione.…”

Strong Metal–Support Interactions for Palladium Supported on TiO2 Catalysts in the Heterogeneous Hydrogenation with Parahydrogen

“…Therefore, understanding the nature of active sites that can add two hydrogen atoms from the same hydrogen molecule as ap air to asubstrate molecule is ah ighly important task. [9][10][11] Recently,i tw as shown that the nature of the catalyst support can have as ignificant impact on the rate of the pairwise addition of hydrogen. [12,13] Remarkably,t itania-supported metal catalysts were shown to exhibit much higher levels of the pairwise addition of hydrogen (and larger PHIP effects) relative to that exhibited by metals on other supports.…”

“…[12,13] Remarkably,t itania-supported metal catalysts were shown to exhibit much higher levels of the pairwise addition of hydrogen (and larger PHIP effects) relative to that exhibited by metals on other supports. [9,14] Therefore, examination of the nature of the active sites for titania-supported nanoparticles in the contexto ft he pairwise addition of hydrogen may open up new possibilities for the rational preparation of catalysts that are able to maximize the observed PHIP effects and for the production of hyperpolarizedm olecular contrast agents potentially suitable for in vivo magnetic resonance imaging investigations. [14][15][16] In catalysis, it is well known that metals supported on titania exhibit the strong metal-support interaction (SMSI) effect.…”