The feasibility of formation and kinetics of NMR signal amplification by reversible exchange (SABRE) at high magnetic field (9.4 T).
Journal: 2014/November - Journal of the American Chemical Society
ISSN: 1520-5126
Abstract:
(1)H NMR signal amplification by reversible exchange (SABRE) was observed for pyridine and pyridine-d5 at 9.4 T, a field that is orders of magnitude higher than what is typically utilized to achieve the conventional low-field SABRE effect. In addition to emissive peaks for the hydrogen spins at the ortho positions of the pyridine substrate (both free and bound to the metal center), absorptive signals are observed from hyperpolarized orthohydrogen and Ir-complex dihydride. Real-time kinetics studies show that the polarization build-up rates for these three species are in close agreement with their respective (1)H T1 relaxation rates at 9.4 T. The results suggest that the mechanism of the substrate polarization involves cross-relaxation with hyperpolarized species in a manner similar to the spin-polarization induced nuclear Overhauser effect. Experiments utilizing pyridine-d5 as the substrate exhibited larger enhancements as well as partial H/D exchange for the hydrogen atom in the ortho position of pyridine and concomitant formation of HD molecules. While the mechanism of polarization enhancement does not explicitly require chemical exchange of hydrogen atoms of parahydrogen and the substrate, the partial chemical modification of the substrate via hydrogen exchange means that SABRE under these conditions cannot rigorously be referred to as a non-hydrogenative parahydrogen induced polarization process.
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Journal of the American Chemical Society. Mar/4/2014; 136(9): 3322-3325
Published online Feb/13/2014

The Feasibilityof Formation and Kinetics of NMR SignalAmplification by Reversible Exchange (SABRE) at High Magnetic Field(9.4 T)

+3 authors

Abstract

1H NMR signal amplification by reversible exchange (SABRE)was observed for pyridine and pyridine-d5 at 9.4 T, a field that is orders of magnitude higher than what istypically utilized to achieve the conventional low-field SABRE effect.In addition to emissive peaks for the hydrogen spins at the orthopositions of the pyridine substrate (both free and bound to the metalcenter), absorptive signals are observed from hyperpolarized orthohydrogenand Ir-complex dihydride. Real-time kinetics studies show that thepolarization build-up rates for these three species are in close agreementwith their respective 1H T1 relaxation rates at 9.4 T. The results suggest that the mechanismof the substrate polarization involves cross-relaxation with hyperpolarizedspecies in a manner similar to the spin-polarization induced nuclearOverhauser effect. Experiments utilizing pyridine-d5 as the substrate exhibited larger enhancements as wellas partial H/D exchange for the hydrogen atom in the ortho positionof pyridine and concomitant formation of HD molecules. While the mechanismof polarization enhancement does not explicitly require chemical exchangeof hydrogen atoms of parahydrogen and the substrate, the partial chemicalmodification of the substrate via hydrogen exchange means that SABREunder these conditions cannot rigorously be referred to as a non-hydrogenativeparahydrogen induced polarization process.

NMR hyperpolarization techniquesincrease nuclear spin polarization by several orders of magnitude,13 which leads to the corresponding increase in NMR signal enablingan array of applications including studies of catalytic processes4 and biomedical use of hyperpolarized substratesas MRI contrast agents.58 There are several hyperpolarization methods, including dynamic nuclearpolarization (DNP),9 spin-exchange opticalpumping,10 parahydrogen-induced polarization(PHIP)11 using parahydrogen and synthesisallow dramatically enhanced nuclear alignment (PASADENA),12 and others. One of the newest methods is signalamplification by reversible exchange (SABRE),13,14 with the experiments conducted by shaking the solutions of an Ircatalyst (i.e., Crabtree’s catalyst15 or N-heterocyclic carbene complex16)with parahydrogen and a polarizable substrate at a relatively lowmagnetic field of a few mT, followed by physical transfer of the sampleto the high-field NMR spectrometer. Alternatively, the insitu detection of SABRE effects at low magnetic fields hasbeen demonstrated.17 However, the latterapproach does not provide sufficient chemical shift resolution, andtherefore interpretation of the low-field NMR studies of SABRE oftenrelies on the previous reports of ex situ high-fielddetection.13,14,18 Furthermore, high-field SABRE is commonly thought to be unobservable,because of the expectation that canonical SABRE13 would be quenched by the fact that the J-coupling mediated flip-flops would no longer be energy conserving.The control experiments in the early SABRE studies seemingly confirmthese expectations.13 In addition, a commonmisconception is that SABRE is not a chemical process since the polarizedsubstrate appears to be chemically identical to its thermally polarizedcounterpart. Here, we show that generation of SABRE in high magneticfields is possible and that the substrate is clearly involved in achemical (hydrogen exchange) process while coordinated to a metalcenter. To the best of our knowledge, this is the first time the SABREeffects are generated and detected in situ in a highmagnetic field.

The in situ SABRE studies ofpyridine-h5 (Py-h5) and pyridine-d5 (Py-d5) at highfield (9.4 T) were performed using methanol-d4 solutions of N-heterocyclic carbene complex-based Ir catalyst,which shows the highest efficiency in low-field SABRE studies reportedto date.19 Parahydrogen gas (>90% para-state)20 was bubbled through ∼7 mM solutions of[IrCl(COD)(IMes)]16 (IMes =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene;COD = cyclooctadiene) in perdeuterated methanol, solution (1), using 100 mM concentration of substrate, Py-h5 or Py-d5 (see Supporting Information (SI) for details). The 1H NMR spectra were acquired immediately after the bubblingwas stopped (∼3 ± 2 s). This experimental approach allowedfor in situ detection of hyperpolarized species thatexist during the high-field SABRE process. All NMR spectra presentedhere were obtained using this approach unless otherwise noted.

Figure 1c shows the 1H NMR spectrumof Py-h5 in the catalyst solution (1) using the above experimental approach after 2 min of parahydrogenbubbling. For comparison, the canonical SABRE NMR spectrum of hyperpolarizedPy-h5, wherein the sample is first polarizedby bubbling parahydrogen in the NMR magnet’s fringe field priorto sample transfer to 9.4 T for signal acquisition, is shown in Figure 1b and demonstrates that all protons of Py-h5 are polarized. In contrast, only the signalsfrom the ortho-protons of Py-h5 exhibitsignificant enhancement in the in situ high-fieldexperiment. The observation of hyperpolarized ortho-H-Py is unexpected for two reasons: First, because SABRE was previouslyreported to be exclusively generated by parahydrogen exchange at lowmagnetic fields13,14 and explained by level anticrossingswhich should be quenched at high fields;21 and second, the strong selectivity for the ortho position of Pyis itself counter to previous observations. Moreover, NMR spectraacquired over a conventional range of proton chemical shifts (seeFigure 1c) also revealed other species withnonequilibrium polarizations: hyperpolarized orthohydrogen manifestingas a strong absorptive peak at ∼4.5 ppm and a weak emissivepeak at ∼2 ppm for the ortho methyl groups of the IMes moietyof the metal complex. While hyperpolarized H-D was recently reportedby Appelt et al.,17 observation of hyperpolarizedorthohydrogen in SABRE experiments is reported here for the firsttime. In retrospect, the absence of previous reports is not surprisingbecause T1 of dissolved orthohydrogen(see below) is only 2 s or less. Thus, most studies with exsitu detection would likely miss the presence of hyperpolarizedorthohydrogen as it would largely relax back to equilibrium levelduring sample transfer from the low polarizing field to the high detectionfield. Because of these concerns, the hydride spectral region wasalso investigated, and hyperpolarized signal of the dihydride complexat ∼ –23 ppm was detected with signal enhancementε ∼ 200 (calculated as the ratio of the hyperpolarizedsignal and the thermally induced signal) and T1 = 3.0 ± 0.3 s (Figure S2).In the previous reports, an antiphase hyperpolarization pattern wasobserved for the dihydride complex, but only when 15N-labeledpyridine was used to eliminate magnetic equivalence of the two hydrideligands.13,14 In contrast, here the hyperpolarized hydrideresonance is purely absorptive and is observed despite the fact thatthe two hydride ligands are essentially magnetically equivalent. Mostpublished SABRE studies present only the region of NMR spectra correspondingto substrate aromatic protons,18,21 which may obscure thepossible presence of other hyperpolarized species. Moreover, low-field in situ NMR studies lacking chemical shift resolution oftenassume that the entire signal is due to hyperpolarized substrate,while both orthohydrogen and Ir dihydride were polarized here by morethan 100-fold (corresponding to nuclear spin polarizations of >0.3%),suggesting that previously published low-field hyperpolarized spectramay have been significantly impacted by the presence of hyperpolarizeddihydride, orthohydrogen, or other species17,23 rather than detecting only hyperpolarized substrate molecules.24

Figure 1

(a) Schematics of the SABRE exchange process, with asterisktorepresent a potential intermediate state. (b) 1H NMR spectrumof catalyst solution (1) containing Py-h5 bubbled with parahydrogen in the fringe field of a 9.4T magnet followed by rapid transfer to 9.4 T for high-field NMR acquisition.(c) in situ NMR spectrum acquired after bubblingwith parahydrogen at 9.4 T, and (d) thermal NMR spectrum of solution(1) containing Py-h5 after2 min of parahydrogen bubbling. NMR peak assignments25 are also provided in SI.

The in situ detectionenabled the measurementof relaxation parameters as well as the kinetics of hyperpolarizationbuild-up. The former was measured using a series of time-resolvedNMR spectra acquired using small angle excitation RF pulses (Figures 2b,d,f and 3b,d,f). The T1 values of hyperpolarized hydride and orthohydrogencould be overestimated, because residual dissolved parahydrogen maycontinue to react after the bubbling was stopped. The polarizationdynamics was studied by varying the bubbling time of parahydrogengas through solution (1) at 9.4 T (Figures 2c,e,g and 3c,e,g). The time constantsfor exponential polarization build-up for both Py-h5 and Py-d4-h26 are in good quantitative agreementwith their T1 values to within experimentalerror (Figures 2 and 3). The extrapolated values for ε (time→∞) ofhyperpolarized ortho protons of pyridine were −4.9 and −14.7for Py-h5 and Py-d4-h (Figures 2c and 3c, respectively). These results are in a qualitativeagreement with the significantly increased value for T1(Py-d4-h) compared to T1(Py-h5). It should also be noted that these T1 values significantly exceed the characteristic exchangetimes of Py and H2 with the Ir complex of ∼0.1 s.25 Taken together, these results are consistentwith substrate (Py) polarization mechanism that is different fromwhat is typically observed with canonical (low-field) SABRE—onethat instead relies on nuclear spin cross-relaxation with anotherspecies with highly nonequilibrium spin order in a manner akin tothe spin polarization-induced nuclear Overhauser effect (SPINOE).2,24,27 In addition to the fact thatthe enhanced signal of the substrate would be expected to grow witha time constant similar to its autorelaxation rate, the low valueexpected for nuclear spin cross-relaxation rates would explain themuch lower values of ε compared to low-field SABRE (Figure 1b,c). Moreover, when the decay process is reducedvia extending T1 (Py-d4-h vs Py-h5), the ε value increases significantly.

Motivated bythe previous report of H-D formation17 duringSABRE hyperpolarization and the lack of H-D signaturesin the present Py-h5 studies performedin methanol-d4 (Figure 2), Py-d5 was used as a SABRE substrate (Figure 3) as well as the substrate in H/D exchange studieswith “normal” (thermally equilibrated) hydrogen gas.During the latter experiments, a low-pressure NMR tube containingsolution (1) and Py-d5 (99.96%D) was allowed to react with “normal” hydrogen in theNMR magnet and monitored in situ with NMR spectroscopy.Although the Ir-catalyzed exchange is slow because there is no sampleagitation and the reaction is diffusion limited, H-D was formed asconfirmed by the observation of the characteristic splitting JHD = 42.8 Hz in the 1H NMR spectrum,Figure 4c.17,28 Moreover,the signal from the ortho-proton of Py-d4-h was increasing steadily during the extended reactionperiod (Figure 4b,d). Furthermore, the presenceof H-D was also detected in the thermal spectra after bubbling parahydrogenthrough solution (1) containing 100 mM Py-d5, Figure S3, but not in solution(1) containing 100 mM Py-h5, confirming that the source of deuterium in the formed H-D is indeedthe substrate Py-d5 rather than the deuteratedsolvent methanol-d4. This result is notconsistent with the observation by Appelt et al.,17 who concluded that H-D was formed with deuterium atomscoming from the solvent. This discrepancy is likely explained by theiruse of a different, i.e., Crabtree’s, catalyst17 compared to the [IrCl(COD)(IMes)]16 catalyst used here. Similar H/D exchange with H2/D229 and alcohols30 was described earlier.

Figure 2

In situ SABRE 1H NMR spectroscopy ofPy-h5 in catalyst solution (1) at 9.4 T. (a) NMR spectrum acquired after bubbling of parahydrogenat 9.4 T. (b,d,f) T1 measurements of hyperpolarized ortho-H of Py-h5, orthohydrogen,and ortho-CH3 of IMes respectively. (c,e,g)Hyperpolarization build-up curves of ortho-H of Py-h5, orthohydrogen, and ortho-CH3 of IMes respectively as a function of reaction/bubblingtime. ε values are calculated by comparing hyperpolarized spectralintegrals with those obtained with “normal” (thermallyequilibrated) signals.

Figure 3

In situ SABRE 1H NMR spectroscopy ofPy-d5 (99.96% D) at 9.4 T using catalystsolution (1). (a) NMR spectrum acquired after bubblingof parahydrogen at 9.4 T; the inset shows a close-up of a portionof the main figure, along with that of a corresponding thermal spectrum.(b,d,f) T1 measurements of hyperpolarized ortho-H of Py-d4-h, orthohydrogen,and ortho-CH3 of IMes respectively. (c,e,g)Hyperpolarization build-up curves of ortho-H of Py-d4-h, orthohydrogen, and ortho-CH3 of IMes as a function of reaction time.

While the H/D exchange described here is unlikelyto contributesubstantially to the hyperpolarization process, it is happening concurrentlywith SABRE. These exchange studies (Figure 4) clearly demonstrate that the Py-d5 substrateis being modified as a result of this chemical exchange process. Therefore,this process cannot rigorously be referred to as an exclusively non-hydrogenative(NH)-PHIP,31 because at least some fractionof the substrate undergoes chemical modification when parahydrogenis used as a source of hyperpolarization. The contribution of thisprocess may crucially depend on the nature of the metal complex, thesubstrate, and experimental conditions and thus should not be dismissedwithout careful consideration.

Figure 4

Deuterium exchange studies using Py-d5 (99.96%D) at 1 atm. (a) Scheme of deuteriumexchange of Py-d5 and H2 (notethat only the ortho-position exhibits exchange). (b) The build-up curve of the ortho-Hsignal of Py-d4-h. (c)H2 region of NMR spectra of the Ir catalyst mixed withPy-d5 and “normal” H233 in a sealed NMR tube at roomtemperature at the start and finish of ∼56 h exchange processduring NMR experiments monitoring this exchange process. Note thecharacteristic JH-D = 42.8 Hz.28 (d) ortho-H-Py region of NMRspectra of the Ir catalyst mixed with Py-d5 and “normal” H2 in a sealed NMR tube atroom temperature at the start and finish of NMR experiments monitoringthe exchange process.

The observations of SABRE formed in a high field, hyperpolarized(absorptive) orthohydrogen and hydride signals, and the deuterium-protonexchange process differ with previously published studies.13,18 The discrepancy may be explained in part by the limitations of thedesign of the typical control experiments, for instance, where high-field in situ detection was not available or the deuterium exchangewas not studied thoroughly.

In conclusion, the formation ofhigh-field SABRE is reported allowingreal-time in situ studies of polarization kineticsto be performed. The effect is consistent with a SPINOE-type2,24,27 mechanism of nuclear spin cross-relaxationand polarization transfer to the Py substrate, although this conclusionis tentative and would certainly require further studies in the future.The mechanism is clearly different from that of the low-field SABRE.Furthermore, because at least a small fraction of the substrate undergoeschemical modification in the reaction with parahydrogen under ourconditions, this process cannot rigorously be referred to as an exclusivelynon-hydrogenative PHIP. While the magnitude of the enhancement factorsof high-field SABRE shown here is significantly smaller (up to 14.7for pyridine and >100 for orthohydrogen and metal dihydride) thanthose typically observed with low-field SABRE (e.g., Figure 1), it is still quite pronounced, and optimizationof this high-field effect may allow NMR signal enhancements withoutfield cycling for some applications. Finally, hyperpolarized orthohydrogenand Ir hydride are likely intimately involved in the high-field SABREmechanism; more generally, the presence of such highly polarized speciesin addition to the substrate should be accounted for in low-fielddetection, where chemical shift dispersion is negligible.17,32

Acknowledgments

This work wassupported by the RAS (5.1.1), RFBR (14-03-00374-a,14-03-31239-mol-a, 12-03-00403-a), SB RAS (57, 60, 61, 122), the Ministryof Education and Science of the Russian Federation, and the grantsMK-4391.2013.3, NIH ICMIC 5P50 CA128323-03, 5R00 CA134749-03, 3R00CA134749-02S1,DoD CDMRP W81XWH-12-1- 265 0159/BC112431.

Supporting Information Available

Ir complexpreparation andNMR spectra. This material is available free of charge via the Internetathttp://pubs.acs.org.

Supplementary Material

ja501052p_si_001.pdfja501052p_si_001.pdf
The authors declare nocompeting financial interest.

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