Iridium(III) Hydrido N-Heterocyclic Carbene–Phosphine Complexes as Catalysts in Magnetization Transfer Reactions

Abstract

The hyperpolarization (HP) method signal amplification by reversible exchange (SABRE) uses para-hydrogen to sensitize substrate detection by NMR. The catalyst systems [Ir(H)2(IMes)(MeCN)2(R)]BF4 and [Ir(H)2(IMes)(py)2(R)]BF4 [py = pyridine; R = PCy3 or PPh3; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene], which contain both an electron-donating N-heterocyclic carbene and a phosphine, are used here to catalyze SABRE. They react with acetonitrile and pyridine to produce [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4 and [Ir(H)2(NCMe)(py)(IMes)(PCy3)]BF4, complexes that undergo ligand exchange on a time scale commensurate with observation of the SABRE effect, which is illustrated here by the observation of both pyridine and acetonitrile HP. In this study, the required symmetry breaking that underpins SABRE is provided for by the use of chemical inequivalence rather than the previously reported magnetic inequivalence. As a consequence, we show that the ligand sphere of the polarization transfer catalyst itself becomes hyperpolarized and hence that the high-sensitivity detection of a number of reaction intermediates is possible. These species include [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4, [Ir(H)2(MeOH)(py)(IMes)(PPh3)]BF4, and [Ir(H)2(NCMe)(py)2(PPh3)]BF4. Studies are also described that employ the deuterium-labeled substrates CD3CN and C5D5N, and the labeled ligands P(C6D5)3 and IMes-d22, to demonstrate that dramatically improved levels of HP can be achieved as a consequence of reducing proton dilution and hence polarization wastage. By a combination of these studies with experiments in which the magnetic field experienced by the sample at the point of polarization transfer is varied, confirmation of the resonance assignments is achieved. Furthermore, when [Ir(H)2(pyridine-h5)(pyridine-d5)(IMes)(PPh3)]BF4 is examined, its hydride ligand signals are shown to become visible through para-hydrogen-induced polarization rather than SABRE.

Harnessing polarisation transfer to indazole and imidazole through signal amplification by reversible exchange to improve their NMR detectability

“…Upon replacing the protio form of these NHC ligands with their deuterated variants d 22 ‐IMes7, 22 and d 22 ‐SIMes, the levels of indazole and acetonitrile proton signal enhancements under SABRE changed as detailed in Table 1. For d 22 ‐IMes, the maximum indazole proton signal enhancement was now obtained at 90 G rather than 70 G, and after 35 s of contact with p‐ H 2 , rather than the 20 s used earlier.…”

“…The second form of catalyst, 3a , contains three indazole ligands, and now, polarisation transfer is facilitated by magnetic inequivalence effects. The mechanisms of ligand exchange in these types of complex have been examined previously when the substrate is pyridine and underpin the SABRE effect 7, 22. These studies have enabled the hyperpolarisation of acetonitrile and reveal that it is possible to improve the pyridine response when CD 3 CN is used.…”

Nuclear spin hyperpolarization of the solvent using signal amplification by reversible exchange (SABRE)

“…For both mechanisms, p-H 2 displaces the COD on the Ir-catalyst and the symmetry of p-H 2 is broken (Scheme 1). At low but non-zero field, such as the polarizing field used in these studies, the interplay between chemical shift evolution and scalar coupling between the hydride protons and the substrate protons create single spin longitudinal z-magnetization [21–25,28,29]. The chemical shift interaction between this z-magnetization and zero quantum states introduces a complex field dependency in the polarization of protons of the substrate.…”

“…Several reports of the participation of methanol (MeOH) in the catalytic complex have been reported. Fekete et al [29] confirmed the presence of [Ir(H) 2 (MeOH)(py)(IMes)(PPh 3 )]Cl (py = pyridine; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene) by multinuclear NMR. Eshius et al [32] proposed the presence of [Ir(IMes)(H) 2 (py) 2 (MeOH)]Cl to account for the reduced enhancement observed at trace concentrations while Lloyd et al[36] confirmed the presence of this complex as well as [Ir(IMes)(H) 2 (py) 2 (H 2 O)]Cl at low temperatures.…”

SABRE Hyperpolarization of Bipyridine Stabilized Ir-Complex at High, Low and Ultralow Magnetic Fields

“…Traditionally SABRE was a low-field method that was used to hyperpolarize various Ir-complexes and substrates [8,9,11,[28][29][30][31]. For this reason, first of all we would like to see whether the stabilized complex can be used for generating hyperpolarization in conventional low-field SABRE experiments (LF-SABRE).…”