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

Marianna Fekete,Sam Hart,Adrian Whitwood,Simon B Duckett,Peter J Rayner,Ryan E Mewis,Oliver Bayfield,Natalie Pridmore,Oliver Bayfield,Oliver W Bayfield

doi:10.1021/ic401783c

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.

Highlights

Hyperpolarization (HP) methods are being used to improve the sensitivity of NMR and magnetic resonance imaging to substrate detection.[1]
The process does not involve a change in the chemical structure of the hyperpolarized substrate, as demonstrated in the more traditional p-H2induced polarization (PHIP) technique pioneered by Weitekamp and Eisenberg.[2−4] In PHIP, detection of a product that contains two nuclei that were originally located in a single p-H2 molecule is achieved and a simultaneous change in the chemical identity is required
We describe how the combination of an electron-donating N-heterocyclic carbenes (NHCs) and a phosphine produces a new set of high-activity catalysts for Signal amplification by reversible exchange (SABRE)

Summary

■ INTRODUCTION

Hyperpolarization (HP) methods are being used to improve the sensitivity of NMR and magnetic resonance imaging to substrate detection.[1]. It has previously been suggested that the HP efficiency of the SABRE catalyst is linked to both the lifetime of the metal complex and the strength of the magnetic field where polarization transfer occurs.[8] These concepts are further tested here by using a f low-polarization apparatus that has been previously described.[18,36] In particular, we explore the ligand-exchange processes and demonstrate how a number of new species are detectable, including characterization of a C−H bond activation product that acts as a resting state within the SABRE process In this case, it is products of the type [Ir(H)2(NCMe)(py)(IMes)(PR3)]BF4 that are dominant in the catalyst medium.

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES