Iridium(III) hydrido N-heterocyclic carbene-phosphine complexes as catalysts in magnetization transfer reactions

摘要

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)]BF_4 and [Ir(H)_2(IMes)(py)_2(R)]BF_4 [py = pyridine; R = PCy_3 or PPh_3; 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)(PPh _3)]BF_4 and [Ir(H)_2(NCMe)(py)(IMes)(PCy _3)]BF_4, 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)(PPh_3)] BF_4, [Ir(H)_2(MeOH)(py)(IMes)(PPh_3)]BF _4, and [Ir(H)_2(NCMe)(py)_2(PPh _3)]BF_4. Studies are also described that employ the deuterium-labeled substrates CD_3CN and C_5D_5N, and the labeled ligands P(C_6D_5)_3 and IMes-d_(22), 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-h_5)(pyridine-d_5)(IMes) (PPh_3)]BF_4 is examined, its hydride ligand signals are shown to become visible through para-hydrogen-induced polarization rather than SABRE.