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. 2023 Apr 26;145(18):9970–9975. doi: 10.1021/jacs.3c01593

Benzoquinone Enhances Hyperpolarization of Surface Alcohols with Para-Hydrogen

Philip L Norcott 1,*
PMCID: PMC10176463  PMID: 37127286

Abstract

graphic file with name ja3c01593_0005.jpg

The nuclear singlet state of H2, para-hydrogen, can be used to increase the measurable signal-to-noise for magnetic resonance techniques—a form of hyperpolarization. Transfer of this polarization from para-hydrogen to alcohols through surface interactions rather than formal hydrogenation has only been demonstrated on heterogeneous catalysts tailored to minimize loss of spin order. Here, we find that a common platinum-on-carbon catalyst is capable of this interaction and that the addition of a benzoquinone significantly increases the signal output of hyperpolarized methanol or water.

Nuclear magnetic resonance (NMR) spectroscopy and imaging are indispensable but fundamentally insensitive techniques. The small population difference between energy levels for nuclei in an external magnetic field means only a minute fraction contribute toward a measurable signal. However, altering this typical distribution can result in dramatically stronger responses, called hyperpolarization.1 Para-hydrogen (p-H2) is the nuclear singlet state of dihydrogen; this pure spin state is simple and inexpensive to generate and store.2 While itself undetectable by NMR, upon reaction with another molecule this latent spin order can be transformed into detectable hyperpolarized magnetization. The resulting signal intensity of the product may be increased by orders of magnitude, allowing detection of trace analytes or intermediates, or dramatically reducing experiment time. This Para-Hydrogen Induced Polarization (PHIP, Figure 1A) is seen when p-H2 is added to alkenes or alkynes, for instance.3−5 This is most often achieved using homogeneous hydrogenation catalysts, but may also employ heterogeneous catalysts (so-called HET-PHIP).6 It has also been adapted to allow further transfer of hyperpolarized magnetization through proton exchange, by incorporating a pendant alcohol (PHIP-X, Figure 1B).7 The advantage of this proton exchange variant is the greatly expanded scope of potential target molecules, to reach any containing exchangeable protons, such as alcohols or water.

Figure 1.

Figure 1

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Methods of hyperpolarization from p-H2 involving hydrogenation (A) with corresponding relay of magnetization by proton exchange (B), or through reversible binding to a catalyst (C), with a proton-exchangeable mediator (D). Polarization to alcohols directly at a catalytic hydrogenation surface (E), and this work: a modification improving the capability of simple supported catalysts (F).

The complementary method known as Signal Amplification By Reversible Exchange (SABRE, Figure 1C) does not involve overall hydrogenation of a target, but rather involves reversible coordination of p-H2 with a metal complex which allows continual or repeated transfer of magnetization via scalar coupling.8,9 SABRE may similarly be modified to include targets with exchangeable protons, by utilizing a mediator molecule simultaneously capable of proton exchange and catalyst coordination (SABRE-Relay, Figure 1D).10

Recently, it has been shown that spin order from p-H2 may be transformed into hyperpolarized magnetization for water, methanol, and ethanol directly at certain surfaces, without hydrogenation or a mediator system. Bowers and Huang demonstrated this effect and called it Surface Waters Are Magnetized by Para-hydrogen (SWAMP, Figure 1E).11 Of key importance was the use of insoluble intermetallic nanoparticles based on platinum and tin, encapsulated in mesoporous silica. The incorporation of tin was shown to avoid contiguous active Pt sites and so restrict adatom diffusion, leading to relatively high levels of pairwise selectivity (that is, both incorporated hydrogen atoms derive from the same molecule of p-H2).12−14 In the absence of a hydrogenation reactant, bubbling p-H2 in D2O, methanol-d4, or ethanol-d6 produced hyperpolarized responses at the residual 1H signals of these solvents, arising from interactions at the surface at relatively high temperatures, between 80 and 120 °C. This hyperpolarization process could be achieved repeatedly due to the facile desorption and release of H2 and HD, allowing continual adsorption of fresh p-H2.

It would be expected that more reactive catalysts that activate hydrogen at milder temperatures would consequently suffer from increased adatom mobility and any correlation between p-H2 derived hydrogens, and in turn the SWAMP effect, would be lost. However, amid the many efforts to control pairwise addition under heterogeneous catalysis (for HET-PHIP), which has been found to greatly depend on catalyst composition, structure, and morphology,6 it has been shown that even simple supported metal catalysts are sometimes capable of pairwise p-H2 delivery.15,16 Accordingly, we set out to investigate if any simple, commercially available catalysts were also capable of exhibiting the SWAMP effect, and to explore strategies to improve their performance.

We discovered that when the common hydrogenation catalyst platinum-on-carbon (Pt/C, 3%w/w) was reacted with p-H2 in methanol-d4 at the Earth’s magnetic field, upon immediate transfer to a 1.4 T (60 MHz) NMR spectrometer (allowing equilibration of the sample at this field for 2 s) a weak but characteristic hyperpolarized emission response at the residual CD3OH resonance (∼4.8 ppm) was detected (Figure 2Ai). Further examination revealed the generation of a small amount of HD (see Supporting Information, Figure S2), which was a nonproductive pathway identified by Bowers and Huang.11 However, in our case refilling the NMR tube with fresh p-H2 gave no repeat of the hyperpolarized signal, suggesting the catalyst surface remains largely saturated with H2; the adsorbed hydrogens having lost the spin correlation originating from p-H2 evident at the beginning of the reaction, stalling the SWAMP effect.

Figure 2.

Figure 2

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(A) Thermally polarized and hyperpolarized single-scan 1H spectra of Pt/C in MeOD-d4, after successive additions of 2-methyl-1,4-benzoquinone (BQ). Resonances marked QAr and QMe correspond to quinone/hydroquinone aromatic and methyl resonances, HMDSO = hexamethyldisiloxane as internal standard. (B) Relative integrals of the hyperpolarized and thermal solvent residual OH resonances, normalized, and their difference for comparison.

We hypothesized that an additive may be a novel approach to this adatom diffusion problem, through the consumption of the surface-bound hydrogen to subsequently facilitate further hyperpolarization of methanol via SWAMP. We found that quinones are ideal candidates for this reactivity. These are an important class of redox-active molecules used in energy storage,17,18 catalysis,19,20 natural products,21,22 pharmacology,23 and biological processes,24−26 frequently utilizing the simple interconversion with their reduced hydroquinone form (Figure 1F). Benzoquinones are reactive to many radical or polar reductants, but are also susceptible to catalytic hydrogenation.27

Indeed, when 2-methyl-1,4-benzoquinone (BQ) was added to the earlier mixture of Pt/C in methanol-d4 under fresh p-H2, we were pleased to find the hyperpolarized CD3OH response returned. Furthermore, this signal was significantly stronger than the initial experiment (Figure 2Aii). Hyperpolarization was also evident on the residual CD2H– group of the solvent (see Supporting Information, Figure S12), while a small increase in the thermally polarized level of CD3OH was observed, in similarity with the original SWAMP system.11 Interestingly, closer examination of the H2 signal in this case showed no production of HD, perhaps suggesting that turnover by the benzoquinone is indeed more rapid than desorption of hydrogen gas. After complete consumption of the benzoquinone to the hydroquinone, no further hyperpolarization of methanol was possible. However, further additions of even more benzoquinone reactivated hyperpolarization of CD3OH for the second, third, fourth, and fifth time (Figure 2Aiii-vi). During these 5 cycles, the hyperpolarized response followed the same trend as the earlier reported SWAMP catalyst, when compared to the thermal signal. The latter slowly increased to eventually almost entirely destructively interfere with the hyperpolarized signal (Figure 2B).

To assess the effect of the amount of quinone additive on hyperpolarization level, a series of individual experiments were conducted with Pt/C suspended in methanol-d4, combined with varying amounts of quinone. The resulting 1H spectra were collected after reaction with a limited amount of p-H2 in a sealed NMR tube. The extent of quinone hydrogenation was ascertained from the benzoquinone/hydroquinone methyl resonances (see Figure S22). It was found that increasing amounts of benzoquinone linearly increased the level of CD3OH hyperpolarization, up to approximately 45-fold signal strength against the thermally polarized case when 10 equiv (w/w) were added (Figure 3). Beyond this, the quinone was only partially hydrogenated due to the constraints imposed by the reaction time, pressure, and volume of available p-H2, and the hyperpolarization levels gradually decreased. However, when such a system containing excess benzoquinone was replenished with fresh p-H2, it was found that the hyperpolarization could be reinitiated. The contribution from other factors, such as the effect on relaxation rates or catalyst poisoning at these higher concentrations, may well bear consideration in any future studies.

Figure 3.

Figure 3

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Difference between thermal and hyperpolarized spectra, normalized, as a function of the amount of 2-methyl-1,4-benzoquinone (BQ) added prior to p-H2, for (A) the CD3OH and (B) −CHD2 resonances of methanol; (C) amount of benzoquinone converted to the hydroquinone in each case.

The proportion of Pt on the support was also found to be significant; the equivalent 10%w/w catalyst gave only a very weak SWAMP effect in solvent alone under our experimental setup. However, addition of quinone again gave a much stronger response for the hyperpolarization of both sites of methanol (Figure 4A).

Figure 4.

Figure 4

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(A) Thermally polarized and hyperpolarized 1H spectra of methanol-d4, with either 10%w/w or 3%w/w Pt/C catalysts, each showing the effect of a benzoquinone (BQ) additive, and (B) analogous spectra in D2O; (C) spectra after bubbling p-H2 at different temperatures, (inset) benzoquinone (BQ) or hydroquinone (HQ) methyl resonances indicating reaction conversion.

The ability to hyperpolarize water (HDO) is of significant interest,28 in addition to alcohols (ROH). Pleasingly, it was found that the more effective Pt/C catalyst with benzoquinone could be used to hyperpolarize the residual solvent signal of D2O in an identical fashion (Figure 4B).

As well as its accessibility, another benefit of this type of catalyst is that the hydrogenation reactions were all able to be conducted at ambient temperature. Indeed, it interested us to ascertain the effect of different temperatures on the rate of benzoquinone reduction, and the associated effect on the detectable SWAMP response. As such, a sample containing Pt/C in methanol-d4 with 5 w/w equivalents of benzoquinone was submerged in a temperature-controlled bath and p-H2 was bubbled through the mixture at atmospheric pressure for 30 s, before transport to the spectrometer. As expected, lower temperatures slowed the hydrogenation (up to a point where the reaction was stalled and no SWAMP was detected), while higher temperatures allowed for higher turnover within the given time frame, which in turn produced higher levels of hyperpolarized magnetization (Figure 4C). Naturally these variables of pressure, temperature, reaction time, and concentration could all be further optimized, as has been undertaken for other hyperpolarization techniques such as SABRE.29,30 Ultimately, the results here support the relationship between consumption of benzoquinone and an enhanced signal response, in line with those results in Figure 3.

It must be emphasized that even when p-H2 was initially introduced to samples containing Pt/C and benzoquinone, no hyperpolarization of the C–H sites of the resulting hydroquinone was ever observed, which appears to rule out a PHIP-X mechanism of polarization transfer, where hydrogenation produces a hyperpolarized product capable of proton exchange to alcohols.7 There is evidence that catalytic hydrogenation of similar quinone substrates occurs via (possibly stepwise) reduction of the C=O double bond, or addition at each of the quinone oxygen atoms separately.31−33 In particular for the latter, it is unsurprising then that the pairwise addition for a HET-PHIP response is not seen, as this partial but not complete separation of p-H2 on the surface is unlikely. Reduction of the C=C double bond is evidently not the mechanism of this reaction, as we found no evidence of C–D incorporation when an analogous reduction was conducted under D2 gas (Supporting Information, Appendix S5).34

The associated color change during this reaction is easily discernible; the yellow of the benzoquinone disappeared within seconds under a hydrogen atmosphere, giving the colorless hydroquinone. Photochemically generated triplet excited states of benzoquinones have been studied in chemically induced dynamic nuclear polarization (CIDNP), a form of nuclear hyperpolarization originating from interactions with electron spin.35−37 To identify if ambient light played any similar role in generating the observed magnetization in our case, the reaction with p-H2 was repeated in the dark, but no significant change to the resulting spectrum was observed. Similarly, no hyperpolarization was detected when the reaction was exposed to light while using normal H2 rather than p-H2.

Furthermore, introducing p-H2 at high field (1.4 T) gave no discernible hyperpolarization. This is consistent with the observations of Bowers and Huang,11 to whose work we direct the reader for a more detailed discussion on the field effects of SWAMP and its mechanistic implications.

Given that one of the central advantages of the SWAMP approach to hyperpolarization is the simplicity of the reaction mixture—containing only heterogeneous catalyst and solvent—the potential treatment of the quinone additive merits consideration. Such compounds and their hydroquinone variants are representative of biologically amenable molecules, such as ubiquinone (coenzyme Q10), which suggests there may be less need for the rigorous separation that would be required for the platinum catalyst itself, if this technique were to be harnessed for the generation of a hyperpolarized mixture for clinical imaging.38,39 From a chemical efficiency perspective, an advantage of benzoquinone as an additive is the possibility for a facile reoxidation of the resulting hydroquinone, completing the catalytic cycle. Preliminary investigations in this regard, detailed in the Supporting Information (Appendix S7), prove reoxidation of the reaction mixture can be conducted cleanly, and the SWAMP experiment repeated, if the incorporation of such a recycling process were desired.

In conclusion, we have demonstrated that certain simple, accessible heterogeneous hydrogenation catalysts are capable of transferring magnetization to alcohols and water via the SWAMP effect at ambient temperatures. A benzoquinone additive assists in the turnover of fresh p-H2 on the surface, greatly increasing the level of signal output for methanol and water, but is itself not hyperpolarized and thus does not obscure adjoining signals. It would be of considerable interest in future work to extend this investigation to other quinones and other targets with exchangeable proton sites, as well as a wide screen of other supported metal catalysts. Ultimately, we anticipate that this work will assist in the uptake of SWAMP as an additional technique in the arsenal of methods to affect hyperpolarization from para-hydrogen, with applications ranging from chemistry to the clinic.

Acknowledgments

The author gratefully acknowledges the Australian Research Council for financial support (DE210100065).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c01593.

  • Experimental details and NMR spectra (PDF)

The author declares no competing financial interest.

Notes

Raw NMR data are available through the Australian National University Data Commons, located at https://datacommons.anu.edu.au (10.25911/edff-by92).

Supplementary Material

ja3c01593_si_001.pdf (7.7MB, pdf)

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Supplementary Materials

ja3c01593_si_001.pdf (7.7MB, pdf)

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