A New Ir‐NHC Catalyst for Signal Amplification by Reversible Exchange in D2O

Abstract NMR signal amplification by reversible exchange (SABRE) has been observed for pyridine, methyl nicotinate, N‐methylnicotinamide, and nicotinamide in D2O with the new catalyst [Ir(Cl)(IDEG)(COD)] (IDEG=1,3‐bis(3,4,5‐tris(diethyleneglycol)benzyl)imidazole‐2‐ylidene). During the activation and hyperpolarization steps, exclusively D2O was used, resulting in the first fully biocompatible SABRE system. Hyperpolarized 1H substrate signals were observed at 42.5 MHz upon pressurizing the solution with parahydrogen at close to the Earth's magnetic field, at concentrations yielding barely detectable thermal signals. Moreover, 42‐, 26‐, 22‐, and 9‐fold enhancements were observed for nicotinamide, pyridine, methyl nicotinate, and N‐methylnicotinamide, respectively, in conventional 300 MHz studies. This research opens up new opportunities in a field in which SABRE has hitherto primarily been conducted in CD3OD. This system uses simple hardware, leaves the substrate unaltered, and shows that SABRE is potentially suitable for clinical purposes.


General procedures
Materials 1,2-Dichloroethane was obtained as dry solvent from Sigma Aldrich at 99% purity. Ag 2 O was obtained from Sigma Aldrich at 99% purity. [Ir(Cl)(COD)] 2 (99% purity) was purchased at Strem Chemicals. D 2 O and CD 3 OD (Sigma Aldrich, 99.9%) were used as is, oxygen was removed by freeze-pump-thaw techniques in triplo. CDCl 3 was obtained from Cambridge Laboratory Isotopes, as 99.9% D grade, 0.05% V/V TMS. KI was purchased at Merck with 99.5% purity grade. CsF was acquired from Sigma Aldrich with 99% purity. Imidazole was obtained from Acros Organics, with 99% purity. Dry acetonitrile (MeCN, 99.9%) and tetrahydrofuran (THF, not stabilized, 99.8%) were purchased from Actual Chemicals, connected to the solvent purification system MB SPS-800. 1.0 M KOtBu in THF was obtained from Sigma Aldrich. CHCl 3 and MeOH were used as technical grade solvents.

Instrumentation and hyperpolarization procedures
Conventional NMR spectra were recorded at 298 K on a Bruker Avance II 500 MHz spectrometer equipped with a BBI probe in Nijmegen, The Netherlands. SABRE with complex 1 was performed on a Bruker Avance III 600 MHz spectrometer equipped with a cryo-cooled HCN probe in Nijmegen using 5 bar of 51% parahydrogen. SABRE with complex 2 was performed at the RWTH in Aachen, Germany, on a 42.5 MHz Magritek Spinsolve spectrometer and a Bruker Avance II 300 MHz spectrometer equipped with a BBO 300 MHz S1 5 mm probe. Measurements at 42.5 MHz involved samples in a middle wall Wilmad LabGlass 9-inch pressure valve NMR tube with 0.2 mL sample volume for pyridine, pressurized with 6 bar parahydrogen with 92% para isomer enrichment. For substrates methyl nicotinate, N-methyl nicotinamide and nicotinamide, a thin wall Wilmad LabGlass 9-inch NMR tube was used with 0.7 mL sample volume and 5.5 bar 92% parahydrogen. The hyperpolarization was achieved by manually shaking the sample for 5 seconds at a magnetic field of approximately 1 G and rapidly inserting into the bore. At 300 MHz, samples were also loaded with 0.7 mL sample volume, 92% parahydrogen at a pressure of 5.5 bar. The 92% parahydrogen was obtained from a Bruker parahydrogen generator, with a conversion temperature of 37 K. The reported chemical shifts (in ppm) are referenced to the residual solvent signals of CD 3 OD and D 2 O. NMR samples in high-field SABRE experiments were prepared using volumes of 0.7 mL in a thin wall 5 mm Wilmad quick pressure valve NMR tube of 7-inch length (See Figure S1). Parahydrogen used in SABRE experiments at 500 and 600 MHz was enriched to 51% para isomer following the procedure: [1] 4 bar of commercially available dihydrogen gas was stored for 2 hours over activated charcoal at 77 K using liquid nitrogen. The conversion of o-H 2 into p-H 2 is catalyzed by activated charcoal, which is otherwise symmetry forbidden. Prior to pressurization, the samples were quickly evacuated, before 5 or 5.5 bar was introduced over 15 seconds. The hyperpolarization was achieved by manually shaking the sample for either 5 or 20 seconds at a magnetic field of 65 G or of approximately 80 G and rapidly inserting into the bore. After insertion of the sample, a 90˚ rf pulse was applied immediately. Due to T1 relaxation, the observed polarization is optimal when the sample is entered smoothly and without time delay into the detector. Enhancement factors were obtained by dividing the absolute substrate integral in the hyperpolarized state by the absolute integral at the thermally polarized state. Both thermal and hyperpolarized spectra were recorded at identical experimental parameters, the thermally polarized signal was recorded after a three minute relaxation delay to ensure full relaxation to thermal equilibrium. High-resolution mass spectra were recorded on a JEOL AccuTOF (ESI). Elemental microanalyses were carried out by the Mikroanalytisches Laboratorium KOLBE, Mülheim an der Ruhr, Germany.

X-Ray structure determination
For single-crystal X-ray diffraction, a single-crystal was cut to size and mounted on a Mitagen Microloop using high viscosity oil and shock frozen to 208K using liquid nitrogen. Intensity data were collected at 208K. The measurement was performed was performed on a Nonius KappaCCD single-crystal diffractometer (φ and ω scan mode) using graphite monochromated Mo Kα radiation. Diffraction images were integrated using Eval14. [2] Intensity data were corrected for Lorentz and polarization effects. A semiempirical multiscan absorption correction was applied (SADABS). [3] The structure was solved using SHELXT. [4] Refinement was performed with standard methods:refinement against F2 of all reflections with SHELXL-2014. All nonhydrogen atoms were refined with anisotropic temperature factors. The positions of the hydrogen atoms could initially be determined using a difference Fourier map. Hydrogens were subsequently, when possible, replaced by hydrogens at calculated positions and refined riding on the parent atoms.

Ligand and complex synthesis
The Itome ligand was synthesized as imidazolium.HCl salt, published elsewhere, [5] the Itome carbene was formed in-situ by the addition of Ag 2 O. 3,4,5-tris-(2-(2methoxyethoxy)ethoxy)benzyl chloride was synthesized according to a previously published method. [6] The IDEG ligand was synthesized as imidazolium .HI salt, the carbene was formed in a similar way is with Itome.
Itome.HCl ligand (1 equiv., 857 mg, 1.84 mmol) was mixed with Ag 2 O (0.5 equiv., 214 mg, 0.92 mmol) in 30 mL dry 1,2-dichloroethane under inert conditions in the dark and refluxed for 24 h. Subsequently, the mixture was cooled, and [Ir(Cl)(COD)] 2 (0.5 equiv., 619 mg, 0.92 mmol) was added and refluxed for additional 24 h. After cooling to room temperature, it was filtered over celite, the solvent was evaporated and the compound was additionally purified over flash column chromatography (neutral Al 2 O 3 , CHCl 3 :MeOH 1%), collecting the first fraction. Yield: 82% yellow powder (1.154 g, 1.51 mmol). Crystals suitable for X-Ray crystallography were obtained by diffusion of diethyl ether into a saturated solution of 1 in DCM. 1   The X-ray structure of 1 was elucidated, showing a square-planar complex, an Ircarbene distance of 2.026(3) Å and the NHC ligand has a buried volume (%V bur ) of 27.7%, a value close to the conventional SABRE-catalysts. [7] Full characterization of the X-ray is given in the .cif file. The structure has been deposited at the Cambridge Crystallographic Data Centre with deposition number CCDC 1442685.

Synthesis of selenoureas
The -acceptor strength of the ligands was determined by the synthesis of Se (selenourea) analogues of the Itome and IDEG complexes. The selenoureas were synthesized according to a previously published method.
(ref: DOI:10.1039/C4SC03264K.) Safety note: selenium and organoselenium compounds are highly toxic, and should be handled with care. The NHC salt (ca. 50 mg, weighed accurately), excess selenium (ca. 30 mg, pellets, grinded before use) and a stirring bar were added to a Schlenk tube and purged with nitrogen. Next, dry degassed THF (0.75 mL) and KOtBu (1.2 equiv., 1.0 M solution in THF) were added via the septum and the resulting suspension was stirred at room temperature overnight. The solvent was evaporated and the resulting residue was suspended in DCM (ca. 2 mL) and filtered through a pad of celite. The pad was washed with further DCM (ca. 2 mL). The DCM was evaporated and the residue was washed with pentane (3 x ca. 1 mL). The 77 Se chemical shift scales were calibrated to the 1 H spectrum using the unified Ξ scale according to Harris et al. [8] Synthesis of Se(Itome) According to the above described method, Se(Itome) was obtained as an brownish solid in 80% yield (49.0 mg). 1  The 77 Se chemical shifts in CDCl 3 of Si(Itome) and Se(IDEG) were 7.2 and 6.8 ppm (acceptor ability parameter, PAAP [9][10][11] ), respectively, which is lower than found for the ligand IMes in the best SABRE catalyst in CD 3 OD (31.6 ppm [11] ) but relatively close to it when the full PAAP range is considered. [10] SABRE of complex 1 in CD 3 OD at 600 MHz