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Abstract
Biradicaloids attract attention as a novel class of reagents that can activate small molecules such as H2, ethylene and CO2. Herein, we study activation of parahydrogen (nuclear spin‐0 isomer of H2) by a number of 4‐ and 5‐membered pnictogen biradicaloids based on hetero‐cyclobutanediyl [X(μ‐NTer)2Z] and hetero‐cyclopentanediyl [X(μ‐NTer)2ZC(NDmp)] moieties (X,Z=P,As; Ter=2,6‐Mes2−C6H3, Dmp=2,6‐Me2−C6H3). The concerted mechanism of this reaction allowed observing strong nuclear spin hyperpolarization effects in 1H and 31P NMR experiments. Signal enhancements from two to four orders of magnitude were detected at 9.4 T depending on the structure. It is demonstrated that 4‐membered biradicaloids activate H2 reversibly, leading to SABRE (signal amplification by reversible exchange) hyperpolarization of biradicaloids themselves and their H2 adducts. In contrast, the 5‐membered counterparts demonstrate rather irreversible parahydrogen activation resulting in hyperpolarized H2 adducts only. Kinetic measurements provided parameters to support experimental observations.
Highlights
Biradicaloids attract attention as a novel class of reagents that can activate small molecules such as H2, ethylene and CO2
Recent applications include photoswitching,[1] nonlinear optics,[2] molecular magnets[3] and activation of small molecules such as CO2, NH3 and H2.[4]. Among other matters, activation of molecular hydrogen is interesting for parahydrogen-based nuclear spin hyperpolarization methods such as PASADENA,[5] ALTADENA[6] and SABRE.[7]
The hyperpolarization requires a chemical activation of parahydrogen, the spin-0 nuclear spin isomer of H2.[8]. If the activation is pairwise, i. e., the two H atoms do not lose each other, a strong nuclear spin hyperpolarization and orders of magnitude NMR signal enhancements are observed due to the parahydrogen-induced polarization (PHIP).[8a,9] Typically, metalbased catalysts such as various Rh, Ir or other metal complexes are used for this purpose.[10]
Summary
Hyperpolarization Effects in Parahydrogen Activation with Pnictogen Biradicaloids: Metal-free PHIP and SABRE. The reversibility of activation was used to hyperpolarize biradicaloids themselves via a high-field SABRE effect, providing another example of SABRE in metal-free systems in addition to ansaaminoboranes.[11c] Symmetrical PÀ P and AsÀ As four-membered biradicaloid adducts (8-H2 and 10-H2) form symmetrical AA’XX’ spin systems (A = 1H, X = P,As) resulting in complex NMR multiplets having features of second-order spectra.[14] This can be viewed as a consequence of chemical equivalence of 1H and pnictogen spin pairs at the simultaneous magnetic inequivalence of spins in these pairs This allows one to observe unusual effects such as hyperpolarization of 31P without a need for polarization transfer radio-frequency pulses for 8-H2 or strongly enhanced 1H side band resonances in the case of 10-H2. We would expect that a rational design of biradicaloids for parahydrogen activation may lead to applications in these areas in the future
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