key: cord-0711876-e9g9l4iz
authors: Rayner, Peter J.; Gillions, Joseph P.; Hannibal, Valentin D.; John, Richard O.; Duckett, Simon B.
title: Hyperpolarisation of weakly binding N-heterocycles using signal amplification by reversible exchange
date: 2021-03-23
journal: Chemical science
DOI: 10.1039/d0sc06907h
sha: a79be22befbf50de50a9abb8ae8ff44d11e69e5f
doc_id: 711876
cord_uid: e9g9l4iz

Signal Amplification by Reversible Exchange (SABRE) is a catalytic method for improving the detection of molecules by magnetic resonance spectroscopy. It achieves this by simultaneously binding the target substrate (sub) and para-hydrogen to a metal centre. To date, sterically large substrates are relatively inaccessible to SABRE due to their weak binding leading to catalyst destabilisation. We overcome this problem here through a simple co-ligand strategy that allows the hyperpolarisation of a range of weakly binding and sterically encumbered N-heterocycles. The resulting (1)H NMR signal size is increased by up to 1400 times relative to their more usual Boltzmann controlled levels at 400 MHz. Hence, a significant reduction in scan time is achieved. The SABRE catalyst in these systems takes the form [IrX(H)(2)(NHC)(sulfoxide)(sub)] where X = Cl, Br or I. These complexes are shown to undergo very rapid ligand exchange and lower temperatures dramatically improve the efficiency of these SABRE catalysts.

Signal Amplification by Reversible Exchange (SABRE) is a catalytic method for improving the detection of molecules by magnetic resonance spectroscopy. It achieves this by simultaneously binding the target substrate (sub) and para-hydrogen to a metal centre. To date, sterically large substrates are relatively inaccessible to SABRE due to their weak binding leading to catalyst destabilisation. We overcome this problem here through a simple co-ligand strategy that allows the hyperpolarisation of a range of weakly binding and sterically encumbered N-heterocycles. The resulting 1 H NMR signal size is increased by up to 1400 times relative to their more usual Boltzmann controlled levels at 400 MHz. Hence, a significant reduction in scan time is achieved. The SABRE catalyst in these systems takes the form [IrX(H) 2 (NHC)(sulfoxide)(sub)] where X ¼ Cl, Br or I. These complexes are shown to undergo very rapid ligand exchange and lower temperatures dramatically improve the efficiency of these SABRE catalysts.

Hyperpolarised magnetic resonance is receiving increasing attention from both the analytical science and medical communities due to its ability to create signals that are many orders of magnitude higher than those normally detected under Boltzmann control. [1] [2] [3] [4] [5] [6] The time and cost benets associated with this improvement have propelled this area of research forward over the past few decades. Two of the most prominent techniques used to create hyperpolarisation are dissolution Dynamic Nuclear Polarisation (d-DNP) and Para-Hydrogen Induced Polarisation (PHIP), 7, 8 which derive their non-Boltzmann spin energy level populations from interactions with unpaired electrons and para-hydrogen (p-H 2 , the singlet spin isomer of hydrogen), respectively. Both of these methods have been reviewed in detail. [3] [4] [5] 9, 10 Signal Amplication by Reversible Exchange (SABRE) is a PHIP method that does not involve the chemical incorporation of p-H 2 into the target substrate. 11, 12 Instead, under SABRE, spin order transfer proceeds catalytically through the temporary formation of a scalar coupling network between p-H 2 derived hydride ligands and the substrate's nuclei whilst they are located in a transient metal complex. The most common catalysts are of the type [Ir(H) 2 (NHC)(sub) 3 ]Cl (where NHC ¼ Nheterocyclic carbene and sub ¼ the substrate of interest, Fig. 1a) , 13, 14 although other variants are known. [15] [16] [17] For SABRE to be accomplished, the target substrate must be able to reversibly ligate to the metal centre and this limits the methods applicability; although several routes to overcome this have been reported. [18] [19] [20] Recently, the use of bidentate ancillary ligands such as NHC-phenolates 16 and phosphine-oxazoles 21 has been shown to expand the applicability of SABRE for a variety of different ligands and solvents (Fig. 1b) . For example, use of the PHOX ligand (PHOX ¼ (2-diphenylphosphanyl) phenyl-4,5-dihydrooxazole) gives 1 H NMR signal gains of up to 132-fold for 2-picoline; a substrate previously shown to be unpolarised under classic SABRE conditions. 22 The use of co-ligands to stabilise the active SABRE catalyst has proven successful for substrates that weakly associate to the catalyst (Fig. 1c) . Of particular note is the hyperpolarisation of sodium [1,2]-13 C 2 -pyruvate 23 and sodium 13 C-acetate 24 which could be used as in vivo metabolic probes. The importance of coligands in breaking the chemical symmetry of the SABRE catalyst is also well established and co-ligands such as acetonitrile, 25 sulfoxides, 23,26 1-methyl-1,2,3-triazole 27 and substrate isotopologues 28 have been employed.

We report here on the use of co-ligands to allow the NMR hyperpolarisation of weakly binding N-heterocyclic derived substrates with functionality in the ortho-position that have proven to be routinely inaccessible to the SABRE technique (Fig. 1d ). 1 H signal gains of up to 1442 AE 84-fold were obtained for some of these substituted pyridines at 9.4 T and the expansion of this approach to 13 C and 15 N detection and other N-heterocyclic motifs is also exemplied.

Under standard SABRE conditions, ([IrCl(COD)(IMes)] (5 mM) and substrate (20 mM) in either methanol-d 4 or dichloromethane-d 2 (0.6 mL)), we conrm 22 that no 1 H based polarisation enhancements are observed aer polarisation transfer at 70 G under 3 bar p-H 2 for an array of 2-substituted pyridines including 2-picoline and 2,5-lutidine. Additionally, there is no evidence of any hydride containing complexes being formed under these conditions when probed by either PHIP or low temperature NMR experiments. Presumably, this is due to the substrates weak ligation ability due to steric constraints around the metal centre caused by the substituents that lie ortho to the nitrogen binding site.

To overcome these limitations, we hypothesised that using a weakly binding co-ligand could allow for the formation of stable yet active SABRE catalysts. The key requirement for a coligand being that it will bind to the metal centre with comparable affinity to the target substrate, in order for the target substrate itself not to be completely displaced. Previously, sulfoxide co-ligands have fullled this role 23, 26, 29 and, therefore, a sample was prepared that contained [IrCl(COD)(IMes)] (5 mM), 2,5-lutidine (20 mM) and DMSO-d 6 (30 mM) in methanold 4 and it was exposed to 3 bar H 2 for 1 h at room temperature. Aer this time, p-H 2 (3 bar) was added and the sample was shaken in a 70 G eld before being rapidly inserted into a 9.4 T spectrometer for NMR analysis. Pleasingly, a 90 AE 7-fold 1 H NMR signal enhancement was observed for the H6 resonance of 2,5-lutidine in the resulting spectrum when compared to the corresponding control spectrum obtained under Boltzmann conditions. The hydride region of this 1 H NMR spectrum now contained hydride resonances for multiple (>6, see ESI †) PHIP enhanced dihydride complexes; each of which could be responsible for the observed SABRE transfer.

When the same SABRE experiment was repeated using dichloromethane-d 2 as the solvent, the observed signal enhancement for 2,5-lutidine signicantly improves. Now, a 215 AE 13-fold signal gain was quantied for the H6 resonance of 2,5lutidine. Additionally, just two hydride-containing complexes are now present in solution (Fig. 2) . The rst complex exhibits hydride resonances at d À16.0 and À21.4 and is assigned to [IrCl(H) 2 (IMes)(DMSO-d 6 ) 2 ] on the basis of comparison to literature data. 29 The second complex has hydride resonances at d À23.0 and À23.5 and these are attributed to [IrCl(H) 2 (-IMes)(DMSO-d 6 )(2,5-lutidine)]. In this species, the 2,5-lutidine and chloride ligands lie cis to one another and trans to hydride. These two complexes exist in a ca. 1 : 2 ratio respectively when the initial ratio of DMSO to 2,5-lutidine is 3 in the equatorial plane and the 2,5-lutidine is trans to the NHC ligand. Overall, rapid ligand exchange in this catalytic system is supported by the fact that the free H 2 peak is broadened (30 Hz width at half height) and shied to 4.52 ppm which differs from the usual 4.64 ppm value.

When the complex [IrCl(H) 2 (IMes)(DMSO-d 6 ) 2 ] is examined in the absence of 2,5-lutidine, the free H 2 peak appears at 4.62 ppm with a half-width of 12 Hz which suggests a slower rate of hydride/hydrogen exchange in this complex when isolated. The rate of hydride ligand loss from [IrCl(H) 2 (IMes)(DMSOd 6 )(2,5-lutidine)] to form H 2 was also quantied to be 8.78 AE 0.40 s À1 at 273 K. The corresponding rate of 2,5-lutidine loss was 9.64 AE 0.21 s À1 at 273 K. Ligand exchange at 298 K was too rapid to be accurately determined using EXSY methods.

Density functional theory (DFT) conrms that [IrCl(H) 2 (-IMes)(DMSO)(2,5-lutidine)] is the most energetically stable complex in solution, whilst [IrCl(H) 2 (IMes)(DMSO) 2 ] is predicted to be 12.2 kJ mol À1 higher in energy. Other potential complexes such as [Ir(H) 2 (IMes)(DMSO) 2 (2,5-lutidine)]Cl or the classical SABRE complex [Ir(H) 2 (IMes)(2,5-lutidine) 3 ]Cl are disfavoured with free energy values that are 71.4 and 74.7 kJ mol À1 higher than [IrCl(H) 2 (IMes)(DMSO)(2,5-lutidine)] respectively (see ESI †). Interestingly, we note that despite the fused bicyclic nature of quinoline, DFT suggests that [IrCl(H) 2 (-IMes)(DMSO)(quinoline)] is 9.0 kJ mol À1 more stable than its 2,5-lutidine analogue; this ligand is hyperpolarised later in this manuscript.

The identity of the sulfoxide co-ligand proved to exhibit a signicant impact on the level of the 2,5-lutidine NMR signal gains and the appearance of the free H 2 signal. It was found that the use of methyl phenyl sulfoxide increased the 1 H NMR signal gain for the H6 2,5-lutidine position to 319 AE 32 in an analogous dichloromethane-d 2 sample. Even higher signal enhancements of 723 AE 38-fold were achieved when diphenylsulfoxide (DPSO) was the co-ligand. No evidence was observed for the formation of the analogous bis-sulfoxide complex, [IrCl(H) 2 (-IMes)(DPSO) 2 ], was seen by either PHIP or low temperature NMR measurements. [IrCl(H) 2 (IMes)(DPSO)(2,5-lutidine)], which has hydride resonances at d À22.1 and À22.8, is the major product in this solution (see ESI † for full characterisation data). Now, the free H 2 signal appears at 4.25 ppm with a peak half-width of 190 Hz, which indicates more rapid ligand exchange than in the corresponding DMSO derivative at 298 K.

As a consequence of changing the sulfoxide co-ligand from DMSO-d 6 to DPSO, an increased concentration of the desired 2,5-lutidine containing SABRE catalyst results. Additionally, the rate of H 2 exchange is increased. These factors result in a higher polarisation transfer level to 2,5-lutidine in the presence of DPSO than DMSO-d 6 2 ] has increased to 21.8 kJ mol À1 ; this is reective of the cone angle of DPSO being 10% larger than that of DMSO. 30 This increase in the steric parameter for the sulfoxide ligand may in turn reduce its binding affinity for the metal centre. Consequently, 2,5-lutidine may be able to more effectively compete for the equatorial ligand site.

Employing the aliphatic sulfoxides, tetramethylene sulfoxide and dibutylsulfoxide, resulted in signal gains for H6 of 2,5lutidine of 131 AE 21 and 40 AE 8 fold respectively which are therefore similar to those achieved with DMSO-d 6 . Interestingly, no evidence of polarisation transfer into the undeuterated alkylsulfoxide co-ligands is observed in the 1 H NMR spectra recorded aer SABRE transfer at 70 G which may be due to short magnetic state lifetimes. In contrast, ca. 20-fold per proton signal gains were quantied for the CH resonances of the aryl rings in DPSO. This indicates that some polarisation transfer is occurring to the bound DPSO ligand. It is hypothesised therefore that deuteration of the sulfoxide co-ligand would further improve the efficiency of SABRE catalysis due to reduced spindilution. 28 SABRE polarisation transfer to DMSO from [IrCl(H) 2 (DMSO) 2 (IMes)] has previously been reported. 29 

The effect of the halide ligand on the SABRE performance was explicitly probed by the synthesis of the related bromide and iodide containing complexes, prior to studying their reaction with 2,5-lutidine in the presence of DPSO and 3 bar p-H 2 . When [IrBr(COD)(IMes)] is utilised, the 1 H NMR signal gain for the H6 proton of 2,5-lutidine was reduced to 570 AE 18 and a further reduction to a 171 AE 14-fold signal enhancement was quantied with the precatalyst [IrI(COD)(IMes)]. The reactivity of the corresponding dihydride products is readily apparent through their effects on the free H 2 signal, which shis to 4.19 and 4.04 ppm respectively with corresponding peak half-widths of 255 and 450 Hz. We deduce from these data that the rate of ligand exchange in [IrX(H) 2 (IMes)(DPSO)(2,5-lutidine)] increases for X ¼ Cl < Br < I. Hence, these data suggest the signal enhancements become limited by catalyst lifetime.

Our study so far shows that neutral catalysts with a bound halide ligand are responsible for polarisation transfer to 2,5lutidine in dichloromethane-d 2 . We wished to probe whether forcing the formation of a classical charged SABRE complex could further improve signal gains. To achieve this, we attempted to remove the halide ligand and replace it with a nonchelating counter ion. Therefore, the [Ir(COD)(IMes)(2,5-lutidine)]BF 4 pre-catalyst was synthesised from [IrCl(COD)(IMes)], 2,5-lutidine and AgBF 4 as detailed in the ESI. † Subsequently, this pre-catalyst was activated under the same conditions previously employed by exposure to 3 bar H 2 in the presence of 2,5-lutidine (15 mM) and DPSO (20 mM) in dichloromethane-d 2 for 1 h at room temperature. Aer SABRE transfer under 3 bar p-H 2 in a 70 G polarisation transfer eld a signicantly diminished signal enhancement of just 19 AE 8 fold was detected for H6 of 2,5-lutidine. No hydride resonances were seen in these 1 H NMR spectra and no discernible active SABRE complexes could be identied. However, the addition of excess Cl À to this system reformed the expected [IrCl(H) 2 (IMes)(DPSO)(2,5-lutidine)] product and this restored the SABRE effect. SABRE transfer using [Ir(COD)(IMes)(2,5-lutidine)]BF 4 was also attempted in methanol-d 4 , however, no polarisation transfer to 2,5-lutine was observed.

In a related experiment, using the [IrCl(COD)(IMes)] precatalyst in dichloromethane-d 2 the addition of excess chloride resulted in no discernible change in SABRE signal enhancement which indicates that the polarisation transfer efficiency is independent of chloride concentration once the SABRE catalyst of type [IrCl(H) 2 (IMes)(DPSO)(2,5-lutidine)] is formed (see ESI †).

The identity of the NHC has also been reported to inuence the level of polarisation transfer resulting under SABRE. 14,31,32 A series of six NHCs were therefore screened to probe their effect (a full summary is given in the ESI, Fig. S13 †) . Unlike in previous studies, deuteration of the IMes ligand resulted in no signicant change in the 1 H NMR signal enhancements seen for 2,5lutidine. 15, 28, 33, 34 Introducing chlorine atoms into the imidazole backbone instead of protons resulted in a comparable 756 AE 74 fold signal gain. Additionally, when the methyl groups located in the para-position of the aryl rings were replaced with chlorine or -CO 2 Me, reduced signal enhancements of 663 AE 31 and 619 AE 44 were quantied respectively. However, when the paramethyl groups of IMes were replaced by tert-butyl, a 891 AE 49fold signal enhancement for the H-6 resonance of 2,5-lutidine was quantied for DPSO as the co-ligand. This reects a 30% improvement when compared to the results obtained with the IMes ligand.

The remaining 1 H resonances of 2,5-lutidine also receive polarisation under these conditions with 635 AE 40, 868 AE 68, 240 AE 12 and 248 AE 8-fold signal gains for the H3, H4, ortho-CH 3 and meta-CH 3 sites respectively. When the SABRE sample containing the tert-butyl derived catalyst was cooled to 243 K, the presence of two minor hydride resonances at d À13.5 and À18.7 is revealed. These are attributed to the analogous bis-DPSO complex that was described previously for the IMes derived catalyst. The ratio of the active SABRE catalyst to the bis-DPSO complex was estimated to be 98 : 2. Additionally, the rate of H 2 exchange appears to be slower in the catalyst system formed when using the tert-butyl derived NHC catalyst. This is evidenced by the free H 2 peak in the NMR spectra appearing at d H 4.36 with a peak half width of 70 Hz. This is a downeld shi when compared to that seen with [IrCl(H) 2 (IMes)(DPSO)(2,5lutidine)], which exhibited a free H 2 peak at d H 4.25 and a peak half width of 190 Hz. This indicates a reduction in the rate of H 2 exchange which is consistent with an increase in catalyst lifetime; such observations would explain why higher signal enhancements are achieved if the complexes lifetimes are limiting.

Effect of temperature on the SABRE hyperpolarisation of 2,5lutidine As stated, the broad free H 2 and hydride signals are indicative of rapid ligand exchange on the active SABRE complexes at 298 K. Indeed, the rates of loss of 2,5-lutidine or hydride from the active catalyst were too fast to be accurately determined at 298 K using EXSY. 35 It was hypothesised that cooling the SABRE samples may increase the lifetime of the catalyst to be closer to that which has been shown both computationally 36 and experimentally 14 to be optimal. Therefore, a sample containing the tert-butyl derived catalyst ([IrCl(COD)(1,3-bis(4-tert-butyl-2,6dimethylphenyl)imidazole-2-ylidine)]), 2,5-lutidine and DPSO in dichloromethane-d 2 was cooled to the desired temperature for 1 minute prior to the SABRE transfer being conducted in a 70 G eld.

Aer cooling the sample to 283 K, a SABRE derived signal enhancement of 956 AE 38 was observed; a modest improvement on the 891 AE 49-fold signal gain previously detected at 298 K resulted. However, conducting the SABRE catalysis at 273 K yielded a signicantly improved 1442 AE 84-fold signal gain at 9.4 T, which corresponds to a ca. 4.5% polarisation level. Further cooling the sample to 263 K did not give further benets and a 934 AE 83-fold signal gain was quantied for 2,5-lutidine at this temperature. Therefore, we propose that conducting the SABRE hyperpolarisation of these weakly binding substrates at 273 K gives the optimum catalyst lifetime when using our coligand approach.

To probe this effect more precisely, the rate of 2,5-lutidine loss from the active catalyst was determined by EXSY. At 273 K the rate of this process was quantied to be 4.33 AE 0.02 s À1 which is extremely close to the ca. 4.5 s À1 which is predicted to be optimal by Barskiy et al. 36 At both 283 and 263 K, these values deviate away from optimum and rates of 14.84 AE 0.50 and 0.89 AE 0.02 s À1 are quantied respectively. Eyring-Polanyi analysis across a temperature range of 243-283 K reveals a DG s298 59.89 kJ mol À1 for this process. When probing the loss of axially bound DPSO from [IrCl(H) 2 (IMes)(DPSO)(2,5-lutidine)] at 273 K, no exchange could be detected on the timescale of SABRE using EXSY.

The generality of this method to sensitise the NMR detection of other weakly binding substrates with functional groups adjacent to their ligation site was also investigated. Fig. 3 shows the structures of the 11 other substrates examined in this study. They have been made amenable to SABRE by this co-ligand strategy. For clarity, their corresponding 1 H NMR signal enhancements are displayed with and without the DPSO coligand. At 298 K, 2-picoline, which until recently 21 has been previously reported as being unable to receive SABRE derived polarisation, 22 yields a 403 AE 49-fold ortho-proton signal enhancement at 9.4 T when examined using our co-ligand approach. When repeating the SABRE transfer at 273 K, a 885 AE 72-fold signal enhancement is observed at 9.4 T which equates to ca. 1.25% polarisation. This compares favourably to use of the bidentate PHOX ligand; 21 this yields 1 H SABRE NMR signal gains of 132-fold for the ortho-proton of 2-picoline at 8.5 T (ca. 0.45% polarisation).

More sterically demanding 2-ethyl pyridine and 2-isopropylpyridine also become hyperpolarised by SABRE but their ortho proton signal gains are now just 83 AE 6 and 1.4 AE 0.2-fold respectively aer SABRE transfer at 298 K. Interestingly, the decrease in signal enhancement is mirrored by a shi in the resonance of the free H 2 and an increase in peak half-width. For 2-picoline, the free H 2 resonance appears at 4.27 ppm with a peak half width of 150 Hz, whereas for 2-ethyl and 2-ispropyl pyridine the H 2 resonance is seen at 3.36 and 2.27 ppm, with peak half widths of ca. 550 and 700 Hz respectively. This highlights how both ligand exchange and SABRE enhancements remain sensitive to ligand binding effects even in related systems despite this co-ligand strategy. Pleasingly, by conducting the SABRE catalysis at 273 K, the signal gains for 2-ethyl pyridine and 2-isopropyl pyridine could be also improved to 570 AE 38 and 7 AE 1-fold respectively.

Additionally, we were able to transfer SABRE derived hyperpolarisation to 2,6-lutidine and a 153 AE 24-fold signal gain for the meta proton was quantied aer SABRE at 273 K. This sterically hindered and weakly binding substrate is reported to be unable to receive magnetisation under standard SABRE conditions 22 or when using the bidentate PHOX catalyst. 21 2-Hydroxy pyridine also gave improved signal enhancements when DPSO was utilised. The signal enhancement for its H6 resonance was 92 AE 13-fold at 9.4 T. Similarly, the pyrimidine motif showed substantial signal improvements could be achieved in a range of mono and bis-methylated derivatives. A signal gain of 1532 AE 88-fold was quantied for 2,4-dimethylpyrimidine (ca. 4.8% polarisation) which is signicant as no signal gain was recorded under standard SABRE conditions. 2-Methylpyrimidine also performs well using the optimised conditions and a 418 AE 54-fold signal enhancement is quanti-ed. Very recently the hyperpolarisation of 2-picoline has been reported to be up to 25 AE 7-fold using an acetonitrile co-ligand in methanol-d 4 . 37 The SABRE hyperpolarisation of quinoline is also signicantly improved to 2102 AE 105-fold when compared to just 4.4 AE 0.4-fold when no DPSO is present in dichloromethane-d 2 . The SABRE hyperpolarisation of quinoline in methanol-d 4 has previously been reported to be ca. 60-fold at 9.4 T. 38 Finally, chloroquine, an anti-viral agent that has recently gained signicant attention due to its use as a prospective COVID-19 treatment, 39-41 also showed 1 H SABRE polarisation. In the presence of DPSO, a 217 AE 57-fold signal gain was quantied, however, when it is absent no signal gain is observed. The appearance of the hydride region in these experiments is consistent with the formation of the corresponding highly reactive complex of type [IrCl(H) 2 (NHC)(DPSO)(substrate)] as the major product. The signicant enhancements seen for quinoline are consistent with the DFT ndings presented earlier which suggest greater binding affinity for quinoline than 2,5lutidine despite the bicyclic nature of the ring system.

As a nal demonstration of this co-ligand strategy, polarisation transfer to 13 C and 15 N nuclei was achieved. A sample containing [IrCl(COD)(1,3-bis(4-tert-butyl-2,6-dimethylphenyl) imidazole-2-ylidine)] (5 mM), DPSO (20 mM) and 2,5-lutidine (20 mM) in dichloromethane-d 2 was exposed to 3 bar p-H 2 at 273 K in a 0.5 G polarisation transfer eld. The subsequent 13 C NMR spectrum is displayed in Fig. 4 and shows a 1424 AE 204-fold enhancement of the C2 of 2,5-lutidine (d 155.2). Additionally, strong polarisation is seen throughout the carbon resonances with even the remote methyl groups being visible. Similarly, when a 15 N{ 1 H} 3-6,42 NMR spectrum is obtained aer the same sample is exposed to p-H 2 in a À3.5 mG eld, a 5048 AE 201-fold enhancement is seen at 9.4 T for the 15 N resonance of 2,5-lutidine at d 312.5.

In summary, we present results that demonstrate how a coligand strategy can be used to successfully hyperpolarise weakly binding substrates using the signal amplication by reversible exchange methodology. Each of the substrates have functional groups in their ortho position that hinder their binding ability. Signal enhancements up to 1442 AE 84-fold are achieved for 2,5-lutidine. This is all the more remarkable as such substrates have previously been shown to be inaccessible to the SABRE technique when the commonly used [IrCl(CO-D)(IMes)] pre-catalyst is employed. Typically, this is due to the inability to form an active SABRE catalyst of type [Ir(H) 2 (-IMes)(sub) 3 ]Cl due to steric congestion around the metal centre.

Our strategy hinges on the use of a sulfoxide co-ligand to allow the formation of an active SABRE catalyst of the type [IrCl(H) 2 (NHC)(sulfoxide)(substrate)]. When [IrCl(COD)(IMes)] is exposed to H 2 in the presence of dimethylsulfoxide-d 6 (DMSOd 6 ) and 2,5-luditine a 1 : 2 mixture of [IrCl(H) 2 (IMes)(DMSOd 6 )(2,5-lutidine)] and [IrCl(H) 2 (IMes)(DMSO-d 6 ) 2 ] were formed. When the sample was exposed to p-H 2 in a 70 G polarisation transfer eld polarisation transfer to 2,5-lutidine was observed. However, the presence of [IrCl(H) 2 (IMes)(DMSO-d 6 ) 2 ] in solution acts to rapidly consume p-H 2 which is evidenced by broadening of the resonances for its hydride ligands and free H 2 in the 1 H NMR spectrum. We found that increasing the size of the sulfoxide co-ligand inhibits the formation of the bissulfoxide complex. Therefore, when diphenylsulfoxide (DPSO) is utilised no evidence of the corresponding [IrCl(H) 2 (-IMes)(DPSO) 2 ] complex is observed and an increased signal enhancement for 2,5-lutidine was quantied.

The effect of the N-heterocyclic carbene (NHC) was also probed and a tert-butyl derived catalyst, [IrCl(COD)(1,3-bis(4tert-butyl-2,6-dimethylphenyl)imidazole-2-ylidine)], 14 was shown to yield a complex which gave the best SABRE polarisation transfer to 2,5-lutidine. Further improvements were also discovered by cooling the sample to 273 K and a signal enhancement of 1442 AE 84-fold was achieved for 2,5-lutidine at 9.4 T aer SABRE transfer at 70 G. The origin of this effect was determined to be due to the ameliorating the lifetime of the active catalyst. The rate of 2,5-lutidine loss at 273 K was determined to be 4.33 AE 0.02 s À1 which is extremely close to the predicted optimum of ca. 4.5 s À1 . 36 Additionally, SABRE hyperpolarised 13 C and 15 N NMR spectra were also recorded and signal enhancements of up to 1424 AE 204-fold and 5048 AE 201fold were quantied respectively.

The scope of this method has been proven by expansion to 11 other substrates. The method is easy to implement as the standard SABRE pre-catalysts simply need to be mixed with the sulfoxide and substrate. It is noteworthy that the highest reported SABRE polarisation levels of 2-picoline, which has previously be reported for use as in vivo pH sensors, 43, 44 is given under the conditions reported here.

The methodology presented here therefore greatly expands the range of substrates amenable to the SABRE and is expected to open up new applications for the technique in the future. For example, as signal to noise scales with the square root of the number of measurements, these hyperpolarisation levels reect remarkable measurement time reductions. Many active pharmaceuticals contain nitrogen heterocycles with sterically hindered nitrogen centres such as those in the diabetes treatment Januvia, 45 the cancer treatments Zytiga 46 and Sprycel, 47 the immunosuppressant Xeljanz 48 and the gastrointestinal drug Nexium. 49 The route we illustrate here demonstrates how the detection of key 1 H, 13 C or 15 N resonances in such materials can be achieved relatively simply in moments and work is being undertaken to render the SABRE method compatible for in vivo use. [50] [51] [52] [53] [54] [55] [56] 

Drugs of today

The authors would like to acknowledge the following for supporting this work: A Laidlaw Scholarship (J. G.), the Erasmus+ programme of the European Union (V. H.). The Wellcome Trust (Grants 092506 and 098335), the EPSRC (EP/R51181X/1), the MRC (MR/M008991/1) and the University of York.