Direct and indirect hyperpolarisation of amines using parahydrogen

Abstract

Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) are two widely used techniques for the study of molecules and materials. Hyperpolarisation methods, such as Signal Amplification By Reversible Exchange (SABRE), turn typically weak magnetic resonance responses into strong signals. In this article we detail how it is possible to hyperpolarise the ¹H, ¹³C and ¹⁵N nuclei of a range of amines. This involved showing how primary amines form stable but labile complexes of the type [Ir(H)₂(IMes)(amine)₃]Cl that allow parahydrogen to relay its latent polarisation into the amine. By optimising the temperature and parahydrogen pressure a 1000-fold per proton NH signal gain for deuterated benzylamine is achieved at 9.4 T. Additionally, we show that sterically hindered and electron poor amines that bind poorly to iridium can be hyperpolarised by either employing a co-ligand for complex stabilisation, or harnessing the fact that it is possible to exchange hyperpolarised protons between amines in a mixture, through the recently reported SABRE-RELAY method. These chemical refinements have significant potential to extend the classes of agent that can be hyperpolarised by readily accessible parahydrogen.

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Introduction

Hyperpolarisation methods are used to overcome the inherent insensitivity of Nuclear Magnetic Resonance (NMR) spectroscopy and Magnetic Resonance Imaging (MRI) where their use may lead to dramatic time and cost savings. One such hyperpolarisation method, Parahydrogen Induced Polarisation (PHIP),¹ produces the required non-Boltzmann nuclear spin distribution by the incorporation of parahydrogen (p-H₂), an example of a nuclear singlet, into a suitable substrate molecule. This effect was shown to yield an enhanced NMR signal in 1987 (ref. 2) and has been the subject of intense investigation.^1,3–6 A drawback of PHIP though, is the requirement for chemical change, caused by p-H₂ addition to an unsaturated centre such as an alkene. However, recently a p-H₂ technique that does not change the chemical identity of the sensitised molecule, called Signal Amplification By Reversible Exchange (SABRE), was reported.^7,8 In this process, p-H₂ is not directly incorporated into the substrate. Instead, polarisation is transferred via the J-coupling network that exists within a metal complex that co-locates p-H₂ derived hydride ligands and a weakly bound substrate (ligand).^9–11 Ligand exchange with excess unbound substrate and p-H₂ enables the build-up of a pool of polarised substrate molecules in solution in a catalytic fashion as shown in Scheme 1.¹² The SABRE polarisation of ¹H nuclei typically utilises a ⁴J_HH coupling between the catalysts hydride and substrate ligand protons. Tessari et al. have quantified these small spin–spin couplings to be ≈1.2 Hz.¹³ Alternatively, stronger ²J_HN couplings have now been used to achieve ¹⁵N polarisation transfer at micro-Tesla fields in a variant known as SABRE-SHEATH (SABRE-in shield enables alignment transfer to heteronuclei).^14,15 Intra-molecular spin–spin coupling networks within the substrate subsequently enables transfer to remote spins which do not exhibit direct coupling to the hydride ligands.¹⁶

Scheme 1 Route to SABRE hyperpolarisation of an amine, NH₂R.

One of the most effective precatalysts for this process is [IrCl(COD)(IMes)] (1) [where IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, COD = cis,cis-1,5-cyclooctadiene] which, after reaction with H₂ and an excess of substrate, typically forms [Ir(H)₂(IMes)(substrate)₃]Cl in protic solvents such as methanol.¹⁷ Neutral active catalysts of the type [Ir(H)₂(Cl)(IMes)(substrate)₂] have also been reported to achieve similar results.¹⁸ These metal based polarisation transfer catalysts have been shown to act on a range of substrates that contain multiple bonds to nitrogen, such as nicotinamide,^19,20 isoniazid,^21,22 metronidazole,²³ pyrazole,²⁴ imines,²⁵ diazirines²⁶ and nitriles,²⁷ and lead to polarised ¹H, ¹³C, ¹⁵N, ¹⁹F, ²⁹Si, ³¹P, and ¹²⁹Sn nuclei that yield substantially enhanced NMR responses in just a few seconds.^19,28–33 In fact, ¹H polarisations of 50% have been reported, while for ¹⁵N, values of over 20% have been achieved.^19,23

While SABRE-induced polarisation can also be achieved using in-field rf. transfer methods,^34–37 whose efficiency varies with pulse sequence,^37–39 spontaneous polarisation transfer occurs readily at low-field and it is this method we employ here. Moreover, as predicted,⁹ it has also been established that SABRE can be used to produce hyperpolarised singlet states⁴⁰ with long-lifetimes through transfer in ultra-low field, or after the implementation of rf. transfer.^41–46 Hence the diversity of applications found for this approach is growing and it clearly reflects not only a successful medium to test hyperpolarisation concepts but a potential route to transform the analytical potential of NMR.^47–50

In this article, we introduce a new class of substrate into the SABRE repertoire, the amine. This is achieved by the formation of iridium–amine complexes of type [Ir(H)₂(IMes)(RNH₂)₃]Cl (2, Scheme 1), whose kinetic behaviour is determined. Whilst the synthesis and use of iridium–amine complexes has been reported for catalytic transformations such as hydrogenation,^51–53 we use them here for polarisation transfer catalysis. We have recently shown a limited number of amines are amenable to SABRE.⁵⁴ Here, we start by detailing the hyperpolarisation of ammonia and benzylamine (BnNH₂) and its associated optimisation to achieve large NMR signal enhancements. We then show how hyperpolarisation can be achieved in a range of primary amines. Upon changing to sterically bulky primary amines, secondary amines or aromatic amines, we show that an active SABRE catalyst does not form upon reaction with 1. However, we exemplify co-ligand and relayed polarisation transfer protocols to overcome this limitation and hence expand further the range of amines amenable to polarisation by p-H₂.

Results and discussion

Direct ¹H hyperpolarisation of ammonia and BnNH₂ by SABRE

Our objective was to investigate the efficiency of the SABRE polarization of amines and ammonia and to determine their ligand exchange dynamics. A 5 mM solution of 1 in dry dichloromethane-d₂ containing an ≈6-fold excess of NH₃ relative to 1 at 298 K was therefore prepared. The aprotic solvent ensures that we maintain the necessary J-coupling network in [Ir(H)₂(IMes)(NH₃)₃]Cl (2-NH₃) during the study, as rapid ²H exchange results to form ND₃ in deuterated protic solvents. This complex yields a hydride signal at δ −23.8, alongside a broad response at δ 0.47 for free NH₃. The corresponding equatorial and axial NH₃ ligand ¹H NMR signals of 2-NH₃ appear at δ 2.19 and 2.88 respectively. 2D ¹H–¹⁵N HMQC measurements were subsequently used to locate the corresponding ¹⁵N signals for these ligands at d_axial −47.8 and d_equ −35.5. Full characterisation data for 2-NH₃ is available in the ESI.†⁵⁴ EXSY methods were then used to probe NH₃ and H₂ loss in 2-NH₃. At 298 K, the associated rate constant for NH₃ loss proved to be 1.64 s⁻¹ while that of H₂ loss is 0.32 s⁻¹. For comparison, the dissociation rate for pyridine in [Ir(H)₂(IMes)(py)₃]Cl is 13.2 s⁻¹ and suggests a higher stability for 2-NH₃ which agrees with the greater basicity of NH₃ relative to pyridine.⁵⁵

As 2-NH₃ undergoes both NH₃ and H₂ loss in solution, we sought to prove that it underwent SABRE catalysis. Thus, a 3 bar pressure of p-H₂ was introduced at 298 K and polarisation transfer was conducted at 60 G. A ¹H NMR spectrum at 9.4 T was then recorded which showed a 154-fold signal enhancement per proton for the free NH₃ response while the corresponding equatorial ligand signal, at δ 2.19, showed a 77-fold enhanced response (Fig. 1). Hence 2-NH₃ acts as a SABRE catalyst as it produces a hyperpolarised free ammonia response. In the presence of water, the observed signal enhancement of the protons in free NH₃ decreased to 40-fold per proton, matching that now observed for the equatorially bound NH₃ ligand This drop is reflected in the signal at δ 1.88, for what is a H₂O response, exhibiting a 75-fold signal gain per proton due to concomitant proton exchange; the ratio of 2-NH₃ : H₂O : NH₃ in this sample was 1 : 5 : 17.5. Under these conditions, the T₁ value for free NH₃ in the presence of the active SABRE catalyst was measured by inversion recovery to be 5.5 s.

Fig. 1 (a) The thermally polarised ¹H NMR SABRE spectrum (× 32 vertical expansion) recorded of 2-NH₃ (formed by reaction of 1 with NH₃ and H₂) in dichloromethane-d₂ at 298 K. (b) The corresponding SABRE polarised 9.4 T ¹H NMR spectrum after transfer under p-H₂ at 60 G. The hyperpolarised responses of free and of 2-NH₃ and residual H₂O are indicated.

The SABRE-induced hyperpolarisation of benzylamine (BnNH₂) was also investigated. A sample containing 1 (5 mM) and BnNH₂ (10 eq.) in dichloromethane-d₂ solution was exposed to 3 bar of H₂. The immediate formation of [Ir(H)₂(IMes)(BnNH₂)₃]Cl (2-BnNH₂) was observed. It gives a characteristic hydride resonance in the ¹H NMR spectrum at δ −23.97. Full characterisation data for this product is available in the ESI.† Interestingly, the ¹H NMR spectrum of 2-BnNH₂ showed that the BnNH₂ ligand that lies trans to hydride, yields inequivalent responses for its NH₂ protons at δ 4.92 and 2.30, and CH₂ protons at δ 3.60 and 3.18. This is due to hindered rotation around the Ir–N bond which results in an up/down distinction for the resonances of the equatorial ligand. In contrast, the axial ligand yields single responses which are equivalent at δ 4.24 (NH₂) and δ 3.83 (CH₂) due to free rotation on the NMR timescale about the Ir–N bond. The corresponding EXSY-derived rate constant for equatorial BnNH₂ loss from 2-BnNH₂ was 3.33 s⁻¹ while the rate of H₂ loss was 2.83 s⁻¹ at 298 K. Hence the rate of BnNH₂ loss is higher than that of NH₃ loss in 2-NH₃. This difference is due to NH₃ forming a stronger Ir–N bond as reflected in their relative pK_b values and suggests that it might perform better under SABRE that NH₃.

This was examined by p-H₂-based polarisation transfer at 60 G which resulted in hyperpolarised free BnNH₂ in solution. The signal enhancements were quantified to be 72-(NH₂), 56-(CH₂) and 194-fold (Ph) per proton as shown in Fig. 2a. However, by using d₇-BnNH₂ instead we were able to focus the SABRE polarisation into the two amino protons alone and this led to an improved signal enhancement of 916-fold per proton (Fig. 2b).

Fig. 2 (a) ¹H NMR spectra of BnNH₂, thermally polarised, top, and hyperpolarised, bottom. (b) ¹H NMR spectra for d₇-BnNH₂, thermally polarised, top, and hyperpolarised, bottom.

In order to investigate the T₁ contribution to this effect we determined values for BnNH₂ and d₇-BnNH₂ at 9.4 T. BnNH₂ proved to have effective T₁ values of 1.1 s (NH₂) and 4.7 s (CH₂) respectively while its ²H-labelled variant exhibited a similar 1.1 s T₁ value for the amino group in the presence of the active catalyst. Hence, the improved NH signal gain seen with d₇-BnNH₂ is due to a reduction in spin dilution which leads to more efficient SABRE transfer. The relaxation rates for BnNH₂ and d₇-BnNH₂ are both slower in the absence of the active SABRE catalyst in agreement with earlier reports that the catalyst plays a role in reducing relaxation times due to reversible binding. Consequently, BnNH₂ now shows T₁ values of 9.0 s (NH₂) and 11.0 s (CH₂), whereas d₇-BnNH₂ has a T₁ value of 10.1 s for its NH₂ group.

Effect of catalyst to substrate ratio on SABRE polarisation

Previous studies have shown that the SABRE effect is dependent upon the catalyst to substrate ratio as a consequence of kinetic and relaxation effects.^19,55 Therefore, we studied the effect of changing the ratio of BnNH₂ relative to 1 from 4-fold to 20-fold in a series of further experiments, undertaking the associated SABRE transfer studies at 60 G and 298 K. It was found that similar total polarisation levels result within experimental error during these experiments (see ESI†). Hence, we conclude that the observed signal enhancements under these conditions are essentially independent of ligand excess which suggests that slow exchange and fast relaxation within the catalyst restrict the maximum polarisation level.

Effect of p-H₂ pressure on SABRE polarisation of BnNH₂

As SABRE derives its polarisation from p-H₂, it could be the limiting reagent in this catalytic process and therefore affect the observed substrate polarisation level.¹⁹ Up until this point, we have been utilising 3 bar pressure of p-H₂ which reflects an ca. 6-fold excess when compared to the 50 mM substrate present in a 5 mm NMR tube. A sample containing 1 (5 mM), BnNH₂ (50 mM, 10 eq.) in dichloromethane-d₂ solution was therefore prepared and exposed to between 2 and 4 bar of p-H₂. The resulting signal gains, after polarisation transfer at 60 G, are shown in Fig. S14 (see ESI†) and a strong dependence on p-H₂ pressure is seen. This is consistent with the fact that H₂ exchange takes place after ligand dissociation and the remaining equatorially bound BnNH₂ ligand will experience a higher level of latent p-H₂ polarisation (see Scheme 1). When d₇-BnNH₂ is examined with 4 bar of p-H₂, the NH signal gain increases to 1079-fold per proton from the 916-fold signal gain achieved with 3 bar.

Effect of temperature on SABRE polarisation of BnNH₂

The temperature at which SABRE is conducted is also known to affect the efficiency of the polarisation transfer due to changes in the lifetime of the SABRE-active catalyst. We found here that cooling a dichloromethane-d₂ solution containing 1, BnNH₂ and 3 bar p-H₂ to 288 K results in a reduction in the level of signal enhancement when compared to 298 K data (Fig. S15, ESI†). Conversely 308 K gave an improved response with the overall polarisation level increasing by ∼40%. This fits with the observed rate constant for BnNH₂ dissociation increasing to 9.85 s⁻¹ from the 3.33 s⁻¹ value at 298 K. We therefore conclude the retained polarisation level in BnNH₂ is improved by the faster rate of substrate dissociation and shorter catalyst lifetime. For NH₃, a 251-fold ¹H signal gain per NH proton is observed at 308 K when compared to the 154-fold value at 298 K. This is consistent with the increase in the NH₃ dissociation rate constant to 10.42 s⁻¹ at 308 K when compared to 1.64 s⁻¹ at 298 K.

SABRE transfer to ¹³C and ¹⁵N

SABRE-induced hyperpolarisation of ¹³C was also observed for BnNH₂. Whilst polarisation transfer into the ortho phenyl carbon was readily observed using a standard ¹³C acquisition sequence after polarisation transfer 60 G under 4 bar p-H₂, the other ¹³C resonances had poor signal-to-noise ratios. We overcame this by using a ¹H–¹³C refocussed INEPT experiment that gave rise to a spectrum showing all 5 carbon environments after polarisation transfer at 60 G. We utilised long-range J-H–C-couplings to transfer this polarisation. ¹³C signal gains of up to 65-fold were achieved using this method (Fig. 3a). We further note that there is a very strong polarisation transfer field dependence on the BnNH₂¹³C signal intensities which is consistent with earlier reports on pyridine.³⁰

Fig. 3 (a) ¹H–¹³C refocussed INEPT NMR spectrum of hyperpolarised BnNH₂ (35 mM) achieved via2-BnNH₂ (5 mM) under SABRE in dichloromethane-d₂ solution after transfer at 60 G and 308 K; (b) ¹⁵N NMR spectrum of ¹⁵N-labelled Bn¹⁵NH₂ (35 mM) after SABRE transfer via2-BnNH₂ (5 mM) at 60 G and 308 K which gives rise to hyperpolarised resonances for free (δ 24.42) and equatorially bound (δ −5.59) substrate; (c) single scan thermally polarised ¹³C NMR spectrum in CD₂Cl₂ (15.66 M) and (d) 4096 scan thermally polarised ¹⁵N NMR spectrum of BnNH₂ (9.15 M).

When Bn¹⁵NH₂ is used instead of BnNH₂, the detection of a hyperpolarised ¹⁵N response is readily evident as shown in Fig. 3b. The ¹⁵N signal gain for the free material in solution proved to be ∼880-fold after polarisation transfer at 60 G and 308 K. The equatorially bound ¹⁵N resonance at δ −5.59, is 4 times larger than the free amine signal. As the ratio of free amine to equatorially bound Bn¹⁵NH₂ in solution is actually 7 : 2, the rate of Bn¹⁵NH₂ loss must be relatively slow, even at 308 K. Under this 60 G condition, polarisation transfer is likely to occur via the ³J_HH coupling between the Bn¹⁵NH₂ and the hydride ligands. To investigate the effect of using a ²J_HN coupling we repeated this measurement after polarization transfer within a μ-metal shield (ca. 350-fold shielding). Under these SABRE-SHEATH type conditions,^14,15 an ∼800-fold ¹⁵N-signal gain was observed and further optimisation may therefore be needed to maximise this response. The corresponding ¹H signal gains with this ¹⁵N labelled material after transfer at 60 G were now 33-(NH₂), 34-(CH₂) and 52-fold (Ph). These compare to the analogous values of 72-, 56- and 192-fold respectively with Bn¹⁴NH₂. Interestingly, the ¹H polarisation levels therefore decrease with ¹⁵N addition and we propose that this is an example of spin dilution.

Expanding the substrate range

In order to test the generality of amine polarisation via SABRE, we prepared a series of samples containing 1 (5 mM) and 10 eq. of the substrates shown in Fig. 4 in dichloromethane-d₂ solution. These substrates include a number of primary amines and each is successfully hyperpolarised after transfer at 60 G upon reaction with 1 and p-H₂. In fact, SABRE polarisation of phenylethylamine (PEA) and phenylpropylamine (PPA) results in strong signal enhancements and transfer is found to proceed across the corresponding C₂ and C₃ carbon chains into their phenyl rings. For PEA we found that the NH₂¹H signal gain is actually increased to 108-fold per proton compared to the 72-fold BnNH₂ value, and that the CH₂CH₂ bridge gave 50-fold (NCH̲₂) and 45-fold (CH̲₂) enhancements per proton. The 5-proton containing phenyl group gave a 92-fold gain per proton.

Fig. 4 Amine substrates polarised by SABRE using precatalyst 1 in dichloromethane-d₂ solution. Per proton signal gains are given for the indicated ¹H sites (* average across two sites due to peak overlap) observed at 9.4 T. Corresponding ¹H NMR spectra for thermally polarised and SABRE polarised experiments are given in the ESI.†

Spin-isolation of the phenyl group, by introducing an ether linkage, as in phenoxyethylamine (POEA) resulted in signal enhancements of 99-(NH̲₂), 47-(NCH̲₂), 147-(CH̲₂O) and as expected, just 8-fold (Ph) per proton for our test sample. We therefore conclude that polarisation transfer across the oxygen linker is inefficient at 60 G and a stronger aliphatic proton response results. The amines isobutylamine, allylamine and tryptamine were also studied as shown in Fig. 4. In all cases, the formation of [Ir(H)₂(IMes)(amine)₃]Cl was indicated (see ESI†) and polarisation transfer results.

When secondary amines, such as dibenzylamine, were examined, no evidence for the formation of an active SABRE catalyst was observed. A similar result was observed for sterically hindered primary amines, such as isopropylamine and aromatic amines, such as aniline. Sterically demanding substrates, such as 2,6-lutidine, have been previously shown to be unable to be polarised using SABRE.⁵⁶ A full list of the amines probed in this study is available in the ESI.† We therefore postulate that sterically demanding or electron deficient amines fail to activate and form the necessary [Ir(H)₂(IMes)(amine)₃]Cl SABRE catalyst.

This problem could be overcome for aniline by the addition of the co-ligand 1-methyl 1,2,3-triazole (mtz) or CH₃CN. For the corresponding sample containing 1 (5 mM), aniline (10 eq.) and mtz (3 eq.) in dichloromethane-d₂ we achieved signal enhancements of 51-fold for the NH₂ group and 17-fold for the phenyl group, per proton. These signal gains are summarised in Fig. 5. When CH₃CN (8 eq.) is used instead of mtz, the polarisation levels increase to 306-(NH₂) and 193-fold (Ph) per proton. The active complex in this SABRE process was characterised as [Ir(H)₂(IMes)(aniline)₂(CH₃CN)]Cl and yields a distinctive hydride resonance at δ −24.78 (see ESI†). Utilisation of such a co-substrate strategy was however unsuccessful for the secondary amines as detailed in the ESI.†

Fig. 5 ¹H NMR signal gains per proton observed for the indicated aniline resonances when hyperpolarised by SABRE in the presence of the described co-ligand at 9.4 T.

Indirect hyperpolarisation of amines by SABRE-RELAY

As expected, substrate binding to the metal centre is needed for polarisation transfer to occur. We hypothesised that these amines might also be hyperpolarised indirectly. In this scenario, hyperpolarisation of a primary amine or ammonia is achieved and subsequent proton exchange, which may be mediated by residual water, allows for a polarised proton to be shuttled into the non-SABRE-active amine. Subsequent intra-substrate polarisation transfer then relays the signal gain more widely in this agent (Scheme 2).

Scheme 2 SABRE-RELAY polarisation of amines. (1) SABRE polarisation of an intermediary transfer agent, in this case a primary amine or ammonia. (2) Polarisation is then relayed into the target amine via proton exchange, either directly or via residual water present in the sample.

In order to test this hypothesis, a series of samples containing 1 (5 mM), target amine (10 eq.) and NH₃ (3–5 eq.) were prepared in dichloromethane-d₂ solution. 2-NH₃ formed in all cases as confirmed by the presence of a hydride resonance in the corresponding ¹H NMR spectra at δ −23.8. Polarisation transfer was then conducted at 60 G, and the resulting signal gains that were observed at 9.4 T are presented in Fig. 6.

Fig. 6 ¹H NMR signal gains observed per proton for the indicated amine resonances when hyperpolarised by SABRE-RELAY using 2-NH₃ at 9.4 T.

For isopropylamine (ⁱPrNH₂), the SABRE-RELAY polarised NH₂ signal showed a 220-fold signal gain while 27- and 150-fold enhancements were seen for the CH and CH₃ resonances respectively. This reflects a breakthrough as ⁱPrNH₂ was unable to be directly polarised by SABRE due to its steric bulk preventing adequate binding. Dibenzylamine (Bn₂NH) was also successfully polarised using this method, and yields ¹H signal gains of 274-(NH), 200-(CH₂) and 395-fold (Ph) per proton. Additionally, a ¹³C spectrum can be acquired in a single scan on these materials after polarisation transfer at 60 G such that a 475-fold signal gain for the C̲H₂ resonance is observed. Full NMR spectra are available in the ESI.† Furthermore, the aromatic amine, aniline, now exhibits a 150-fold NH₂ proton signal enhancement and a 9-fold signal gain for the phenyl ring under analogous conditions. We note that these signal gains are lower than those seen when CH₃CN is used as a co-ligand to achieve direct SABRE transfer as detailed in Fig. 5. We suggest that this difference in behaviour arises because a 60 G polarisation transfer field is no-longer optimal for intra-molecular polarisation transfer after proton exchange. This is clearly is not the case for transfer via directly bound aniline and the complexes scalar coupling network which is in fact commonly maximised for ¹H transfer at 60 G.

From these results we can conclude that the SABRE-RELAY effect is able to polarise sterically hindered primary amines, secondary amines and aromatic amines that are not themselves accessible to SABRE. Thus, the scope of amine polarisation is vastly increased.

Conclusions

In summary, we have shown here how SABRE can be used to hyperpolarise a series of primary amines. This substrate extension opens up the SABRE approach to operate with a much wider range of analytes than was previously thought possible, as we extend beyond the original aromatic N-heterocycles, imines and nitriles. Activity is achieved by the formation of a series of complexes of the form [Ir(H)₂(IMes)(amine)₃]Cl. Relaxation studies, in conjunction with ligand dissociation rate measurements were used to demonstrate that the high relative stability of these complexes acts to limit the degree of SABRE signal gain. This hypothesis is consistent with the fact that increasing the p-H₂ pressure or reaction temperature leads to improved signal gains. Therefore, significant catalyst optimisation will be important if very high levels of hyperpolarisation are to be achieved by this route in the future.

Nonetheless, in the case of BnNH₂, ¹H NMR signal enhancement values of ∼100-fold per NH proton were achieved for benzylamine using [IrCl(COD)(IMes)]. Consequently, when d₇-benzylamine was used, the resulting focusing of the hyperpolarisation into the NH₂ resonance resulted in a 900-fold signal enhancement per proton at 9.4 T with a p-H₂ pressure of 3 bar. This value reduced to 33-fold for Bn¹⁵NH₂ after transfer at 60 G. Hence, we predict that further improvements can be made through a more detailed study of the effect of isotopic labelling.^18,19,57 We have also demonstrated transfer to ¹³C and ¹⁵N with diagnostic NMR spectra being collected at a 35 mM concentration in a single scan. We predict that application of high-field SABRE transfer techniques,^34–37,39 such as the LIGHT-SABRE³⁸ approach, might subsequently enable this process to work inside the magnet, but note that a rigorous study of the effect the polarisation transfer field plays on the resulting signal enhancement levels is justified.

In the course of these studies we found that sterically hindered primary amines, secondary amines and aromatic amines were unable to form an active SABRE catalyst of the type [Ir(H)₂(IMes)(amine)₃]Cl. This meant that direct polarisation transfer via such a complex was not possible. We found for aniline that the addition of a co-ligand such as CH₃CN overcame this problem via the formation of [Ir(H)₂(IMes)(aniline)₂(CH₃CN)]Cl such that signal enhancements of up to 306-fold per NH proton could be achieved.

An indirect route was described to overcome this limitation more generally, such that hindered primary amines, secondary amines and aromatic amines can be hyperpolarised by SABRE-RELAY.⁵⁴ Now, a SABRE-hyperpolarised intermediary, such as ammonia, is able to readily transfer polarisation into agents such as isopropylamine, benzylamine and aniline via either direct proton exchange or mediated by residual water present in the sample. This approach expands the range of amines that can be hyperpolarised without changing their chemical identity through interactions with p-H₂.

Given the increase in signal intensity that is observed for the amines in this study, we are now working towards their use as agents for mechanistic study^58–64 in transfer hydrogenation,^65,66 hydroamination,^67,68 and vitally important N₂ fixation reactions.^69–71 Additionally, since phenylethylamine is a naturally occurring monoamine based alkaloid that acts as a promoter of catecholamine (dopamine and norepinephrine) release in plants and animals we expect these observations to be of wide interest.^72,73 Furthermore, the SABRE-RELAY method⁵⁴ has recently been shown to offer a route to hyperpolarise an even larger range of hydrogen transfer acceptors using OH functional groups. Optimisation of the intermediaries NH polarisation level reflects a key part to optimisation of this technique and hence these results will be of interest to any potential developer.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by The Wellcome Trust (Grants 092506 and 098335), the EPSRC (EP/R51181X/1) and the University of York.

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Footnote

† Electronic supplementary information (ESI) available: Full experimental procedures, characterisation data and example spectra. See DOI: 10.1039/c8sc00526e

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