Hyperpolarisation through reversible interactions with parahydrogen

Abstract

Ir(COD)(NHC)Cl complexes provide significant insight into the catalytic processes underpinning SABRE hyperpolarization.

Delivering strong 1 H nuclear hyperpolarization levels and long magnetic lifetimes through signal amplification by reversible exchange

“…Furthermore, on comparing the d 2 -, d 22 -, and d 24 -IMes systems, there is an increase in both the bound T 1 and free substrate T 1 values. More importantly, when moving to 298 K where SABRE catalysis is conducted (38) further increases in the protons' T 1 values free substrates are observed. Consequently we reexamined the SABRE efficiency of 2b with these catalysts (Table 4).…”

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

“…With few exceptions, most SABRE substrates are aromatic nitrogen-containing compounds where the basic nitrogen acts as the electron donor to the metal, likely reflecting the preference of third row metal ions such as platinum and iridium for somewhat softer N-based ligands [21–23,25–35]. The SABRE experiment is carried out by dissolving the complex and substrate in a methanolic solution and bubbling or shaking the sample with several atmospheres of p-H 2 in the fringe field of the NMR magnet or in a polarizer equipped to generate fields on the order of several mT [36,37]. Recently, it has been shown that substrates can be enhanced using a catalytic complex soluble in an alcoholic water solution or attached to a solid support [26,34,38,39].…”

“…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. Drücker et al [35] observed a 2 to 5-fold enhancement of the methanol OH proton using the [Ir(COD)(PCy 3 )(py)] + catalyst and a pyrazole substrate while others have observed a NMR signal from H-D, likely produced by reaction of an Ir-hydride with an exchangeable deuterium ion donated to the catalytic complex by the solvent [31].…”

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

“…[1] The efficiency of the polarization transfer depends on the magnetic field in whicht he transfer occurs and on the lifetimeo ft he complex, [4] which in turn depends on the exchanger ates of p-H 2 and the substrate, and can be influenced by tuning the electronic and stericp arameters of the catalyst. [5][6][7] Generally,t he active complex 1 + + is formed upon hydrogenation of the cyclooctadiene ligand (cod) of the complex precursor [Ir(cod)(IMes)Cl] (6;I Mes = 1,3-dimesitylimidazol-2-ylidene) in the presenceo fa ne xcess (> 10 fold) of substrate (pyridine;F igure 1). At these high concentrations (mm)a nd optimized conditions, polarization levels of approx 5-10 %h ave been demonstrated.…”

“…The formation of 5 + + is normally monitored through the IMes proton signals, but due to the dynamic nature of the complex, we could not resolve the exact structure by NMR spectroscopy;r ecently, however, others have shownt hat complex 5 + + could be observed and characterized at low temperatures based on the hydride signals. [7] The resulting EXAFS was comparedw ith the experimental data of complex 1 + + ( Figure 7C)a nd found to be quite similare xcept for subtle variations in the relative intensities of the features between 4a nd 7 À1 and weakeri ntensities for the features belongingt op yridine in the Fourier transform. Although these differences are small, simulations still suggest that indeed one pyridine ligand is substituted by methanol, althoughi tc annotb ed etermined with certainty whether a cis or trans pyridine is substituted by methanol, or if it is at such al ong distance as indicated by our DFT computations for the gas-phase situation (Table 5; simulations and detailed analysis are provided in the Supporting information, Figure S5).…”