Iron-Catalyzed Parahydrogen Induced Polarization

Abstract

Parahydrogen induced polarization (PHIP) can address the low sensitivity problem intrinsic to nuclear magnetic resonance spectroscopy. Using a catalyst capable of reacting with parahydrogen and substrate in either a hydrogenative or nonhydrogenative manner can result in signal enhancement of the substrate. This work describes the development of a rare example of an iron catalyst capable of reacting with parahydrogen to hyperpolarize olefins. Complexes of the form (MesCCC)­Fe­(H)­(L)­(N2) (L = Py (Py = pyridine), PMe3, PPh3) were synthesized from the reaction of the parent complexes (MesCCC)­FeMes­(L) (Mes = mesityl) with H2. The isolated low-spin iron­(II) hydride compounds were characterized via multinuclear NMR spectroscopy, infrared spectroscopy, and single crystal X-ray diffraction. (MesCCC)­Fe­(H)­(Py)­(N2) is competent in the hydrogenation of olefins and demonstrated high activity toward the hydrogenation of monosubstituted terminal olefins. Reactions with p-H2 resulted in the first PHIP effect mediated by iron which requires diamagnetism throughout the reaction sequence. This work represents the development of a new PHIP catalyst featuring iron, unlocking potential to develop more PHIP catalysts based on first-row transition metals.

Iridium Pyridylpyrrolides Hyperpolarised by Para‐Hydrogen

“…Furthermore, we did not detect any significant H/D exchange on the COD ligand during the analogous reaction of compound 1 and pyridine under D 2 (refer to Supporting Information), which would be expected if a PHIP-IE type process were dominant. [11,12] It is worth noting that an emissive hyperpolarised o-H 2 signal can also be detected in Figure 2Bii (4.5 ppm); this has been observed in SABRE processes previously, and may be absorptive, [17] antiphase, [18] or emissive [19] based on the catalyst and hyperpolarisation conditions. The polarisation transfer field used here undoubtedly also plays a role in the extent to which this increased hyperpolarised magnetisation is observed on the different sites of each ligand.…”

“…Furthermore, we did not detect any significant H/D exchange on the COD ligand during the analogous reaction of compound 1 and pyridine under D 2 (refer to Supporting Information), which would be expected if a PHIP‐IE type process were dominant [11,12] …”

“…Of course this singular catalyst design is not prescriptive, with for instance COD‐free Ir(I) complexes shown to be effective catalysts for PHIP in aqueous systems [10] . Similarly, Fout and co‐workers recently showed that a cobalt complex was capable of hyperpolarising alkenes either by reversible exchange in a similar sense to SABRE, or through consecutive insertion‐elimination reactions (PHIP‐IE), [11] and remarkably were able to involve a diamagnetic iron(II) variant to achieve PHIP [12] . As such, there remain significant opportunities to develop alternative hyperpolarisation catalysts with different selectivity, properties, and applications to advance the field of NMR hyperpolarisation.…”