Computational (DFT) and Experimental (EXAFS) Study of the Interaction of [Ir(IMes)(H)₂(L)₃] with Substrates and Co‐substrates Relevant for SABRE in Dilute Systems

Abstract

Signal amplification by reversible exchange (SABRE) is an emerging hyperpolarization method in NMR spectroscopy, in which hyperpolarization is transferred through the scalar coupling network of para-hydrogen derived hydrides in a metal complex to a reversibly bound substrate. Substrates can even be hyperpolarized at concentrations below that of the metal complex by addition of a suitable co-substrate. Here we investigate the catalytic system used for trace detection in NMR spectroscopy with [Ir(IMes)(H)2 (L)3 ](+) (IMes=1,3-dimesitylimidazol-2-ylidene) as catalyst, pyridine as a substrate and 1-methyl-1,2,3-triazole as co-substrate in great detail. With density functional theory (DFT), validated by extended X-ray absorption fine structure (EXAFS) experiments, we provide explanations for the relative abundance of the observed metal complexes, as well as their contribution to SABRE. We have established that the interaction between iridium and ligands cis to IMes is weaker than that with the trans ligand, and that in mixed complexes with pyridine and triazole, the latter preferentially takes up the trans position.

Hierarchical Porous Organometallic Polymers Fabricated by Direct Knitting: Recyclable Single‐Site Catalysts with Enhanced Activity

“…The obtained good R‐factor of 0.012 suggested the existence of IrCl bond with small quantity within the polymer matrix. Furthermore, the IrC interaction at 2.03 Å with a coordination number of 4.1 observed with solid 1d is smaller than that at 2.08 Å with a coordination number of 5.6 found with complex bis‐NHC‐Ir 4a (Figure e) . Despite the slight change of coordination environment around Ir center, the bis‐chelating backbone of complex 4a were well preserved.…”

Heterogeneous Microtesla SABRE Enhancement of 15N NMR Signals

“…Figure shows successful enhancement of 15 N NMR signals from 15 N‐labeled py ( 15 N‐py) achieved by the so‐called SABRE‐SHEATH (signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei) approach. Here, 10 mg of the HET‐SABRE catalyst was added to a (protonated) methanol solution containing 100 m m 15 N‐py; the solvent was changed to MeOH because of mild concern that the quadrupolar 2 H nuclei in solvent molecules, which may transiently enter the inner sphere of the complex, could reduce polarization efficiency inside the magnetic shield . A strong polarized 15 N resonance was observed following 30 s bubbling of 50 % para‐H 2 inside a magnetic shield and rapid transfer to 9.4 T for detection (Figure d).…”

Aqueous NMR Signal Enhancement by Reversible Exchange in a Single Step Using Water-Soluble Catalysts

“…Signal amplification by reversible exchange (SABRE) is a relatively new hyperpolarization technique pioneered by Duckett, Green, and co-workers in 2009. , In SABRE, an organometallic catalyst is used to colocate a molecular substrate to be hyperpolarized and parahydrogen (pH 2 )a source of pure nuclear spin order. Like traditional PHIP, ,,− SABRE is of interest because it is cost-effective, potentially continuous, scalable, and rapid (achieving polarization enhancement in seconds). − ,− However, unlike traditional PHIP, SABRE does not require permanent alteration of the substrate to hyperpolarize it . Since its inception, considerable effort has been put forth to broaden the applicability of SABRE by investigating alternative catalyst structures, ,,− improving the nuclear spin polarization achieved for protons , and various heteronuclei ,,− (including through the application of variable applied DC and AC fields), demonstrating high-resolution imaging , (including at low magnetic field), widening the range of amenable substrate types, achieving enhancement in the limits of both low- , and high-concentration agents (including in complex mixtures), and demonstrating SABRE with (and separation/reuse of) heterogeneous microscale/nanoscale catalysts. , …”