Reversible Hyperpolarization of Ketoisocaproate Using Sulfoxide‐containing Polarization Transfer Catalysts

Abstract The substrate scope of sulfoxide‐containing magnetisation transfer catalysts is extended to hyperpolarize α‐ketoisocaproate and α‐ketoisocaproate‐1‐[13C]. This is achieved by forming [Ir(H)2(κ 2‐ketoisocaproate)(N‐heterocyclic carbene)(sulfoxide)] which transfers latent magnetism from p‐H2 via the signal amplification by reversible exchange (SABRE) process. The effect of polarization transfer field on the formation of enhanced 13C magnetization is evaluated. Consequently, performing SABRE in a 0.5 μT field enabled most efficient magnetisation transfer. 13C NMR signals for α‐ketoisocaproate‐1‐[13C] in methanol‐d 4 are up to 985‐fold more intense than their traditional Boltzmann derived signal intensity (0.8 % 13C polarisation). Single crystal X‐ray diffraction reveals the formation of the novel catalyst decomposition products [Ir(μ‐H)(H)2(IMes)(SO(Ph)(Me)2)]2 and [(Ir(H)2(IMes)(SO(Me)2))2(μ‐S)] when the sulfoxides methylphenylsulfoxide and dimethylsulfoxide are used respectively.

Recently, these restrictions are being lifted through the use of relayed polarization transfer (SABRE-Relay) [37][38][39][40][41][42] which allows polarization of molecules such as alcohols, [38] sugars, [39] silanols [40] and others [41] without their direct ligation to iridium. Similarly, the hyperpolarization of weakly donating O-donor molecules such as pyruvate [43] and acetate [44] has been demonstrated. In the case of pyruvate, this proved to be possible by using sulfoxide containing iridium-NHC catalysts of the form [Ir (H) 2 (k 2 -pyruvate)(NHC)(sulfoxide)], as depicted in Scheme 1. [16,43] Mechanistic studies on these systems have reported that [IrCl (H) 2 (NHC)(sulfoxide) 2 ] plays a role in refreshing the p-H 2 spin order needed for SABRE catalysis. [43,45] Optimization has achieved 13 C NMR signal gains of up to 2135-fold (1.7 % 13 C polarisation) for sodium pyruvate-1,2-[ 13 C 2 ] in methanol-d 4 when the NHC is IMes (where IMes is 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene) and the sulfoxide is methylphenylsulfoxide. [16] These sulfoxide-containing catalysts have been applied to the hyperpolarization of molecules such as oxalate, although in this case, the formation of oxalate bridged iridium dimers of the form [(Ir(H) 2 (NHC)(sulfoxide)) 2 (μ,1k 2 ,2k 2oxalate)] was indicated which restricted the necessary ligand exchange processes required for SABRE. [45] In this work, we apply these novel sulfoxide-containing magnetization transfer catalysts to molecules with similar αketo acid moieties to pyruvate. We demonstrate that these catalysts can deliver significant 13 C NMR signal gains for ketoisocaproate (KIC), a common metabolic precursor for the amino acid leucine that has been used to monitor amino acid metabolism in vivo. [46][47][48] Our work begins by investigating if analogous polarization transfer catalysts of the form [Ir(H) 2 (k 2 -KIC)(NHC)(sulfoxide)] can be created. Therefore, a solution containing [IrCl(COD)(IMes)] (5 mM), ketoisocaproate (KIC) (35 mM) and dimethyl sulfoxide (22.5 mM) was activated with 3 bar H 2 in methanol-d 4 (0.6 mL). Thermal 1 H NMR measurements were then recorded to examine whether any hydride containing complexes were formed. A major product, with hydride ligands resonating at δ = À 27.09 and À 29.19 ppm, proved to form which is consistent with [Ir (H) 2 (k 2 -KIC)(dimethylsulfoxide)(IMes)] in which the hydride ligands and ligated KIC occupy the same plane ( Figure 1a). This solution was shaken for 10 seconds with p-H 2 (3 bar) at 6.5 mT before a single scan 1 H NMR spectrum was recorded at 9.4 T. The integral intensities of these two hydride ligand signals were found to be 700-times larger than their normal values (Figure 1c). Weakly hyperpolarized signals for [IrCl(H) 2 (dimethylsulfoxide) 2 (IMes)] [43,45] at δ = À 21.68 and À 15.73 ppm were also observed in addition to resonances at δ = À 15.09 and À 24.22 ppm. The latter signals are expected to correspond to the isomer of [Ir(H) 2 (k 2 -KIC)(dimethylsulfoxide)(IMes)] in which the KIC and hydride ligands are located as detailed in Figure 1. [43] Weakly hyperpolarized signals for the 1 H resonances of free KIC (< 6-fold per 1 H) are also observed in this NMR spectrum (see the Supporting Information).
These results suggest that a SABRE active polarization transfer catalyst has been created. This sample was then shaken at �1 μT in a mu metal shield for 10 seconds (SABRE-SHEATH conditions). [10,17] 13 C NMR signals for free KIC could readily be discerned at δ = 205.08 and 170.61 ppm which were enhanced by a factor of 160-fold per 13 C site (0.1 % 13 C polarisation) when recorded at 9.4 T (Figure 2a). A hyperpolarized 13 C signal, at δ = 160.99 ppm, is also visible which is attributed to the carbene of the IMes ligand in [Ir(H) 2 (k 2 -KIC)(NHC)(sulfoxide)]. [43] These results suggest that changing the pyruvate CH 3 group to a CH 2 (CH 3 ) 2 group does not restrict the ability to form a SABRE active complex and therefore extension to a wider range of molecules with structural variety at this position could be possible.
We now extend these measurements to the hyperpolarization of 13   [Ir(H) 2 (k 2 -KIC)(IMes)(sulfoxide)] in both cases. When these samples are shaken at �1 μT in the mu metal shield 13 C NMR signals are observed that arise from longitudinal single spin Zeeman magnetization of KIC-1-[ 13 C] and two spin singlet order for the KIC-1,2-[ 13 C 2 ] isotopologue which is present at 1.1 % abundance (Figure 2b-c). The formation of singlet order within KIC-1,2-[ 13 C 2 ] as a consequence of SABRE is expected to be independent of polarization transfer field. [43,49,50] In contrast, the creation of single spin Zeeman magnetization is expected to be highly dependent on polarization transfer field. [10,13,17,44] Therefore, the p-H 2 shaking process was increased to 20 seconds and repeated at various different magnetic fields (0.1 to 1.2 μT). These fields are achieved experimentally using a solenoid coil housed within a mu metal shield and has been described and used elsewhere. [13,43] Higher SABRE performance is achieved when the direction of the field in the solenoid is aligned with that of the 9.4 T spectrometer. We note that similar field profiles, of lower magnitude, can be achieved when these fields align antiparallel to the 9.4 T field, which we ascribe to the sample passing through a zero-field point during transfer between the shield and the spectrometer (see the Supporting Information).
The highest 13 C NMR signal gains are achieved using polarization transfer fields of 0.7 μT and 0.5 μT when dimethylsulfoxide or methylphenylsulfoxide are used respectively (Figure 3). In each case, the SABRE 13 C NMR signal gains are higher than those achieved by shaking at a �1 μT field for the same 20 second shaking time (955-fold compared to 155-fold and 985fold compared to 150-fold when dimethylsulfoxide or methylphenylsulfoxide are used respectively). This is consistent with the need for a SABRE-SHEATH matching condition to transfer polarization from p-H 2 derived hydride ligands directly to ligated 13 C sites. [10,13,17,44] While the maximum 13 C NMR signal enhancement of free KIC-1-[ 13 C] achieved using dimethylsulfoxide or methylphenylsulfoxide, at their optimal polarization transfer fields, are comparable (955-fold compared to 985-fold respectively), the 13 C NMR signal enhancement for KIC-1-[ 13 C] bound within the active polarization transfer catalyst is significantly higher when methylphenylsulfoxide is used (730-fold compared to 175-fold). This suggests that polarization transfer from p-H 2 into visible 13 C magnetisation on ligated KIC-1-[ 13 C] is more efficient within the active methylphenylsulfoxide-containing catalyst compared to the dimethylsulfoxide analogue. However, greater polarisation losses during ligand dissociation from the methylphenylsulfoxide-derived catalyst result in comparable 13 C signal enhancements of KIC-1-[ 13 C] free in solution for both catalysts. This difference is expected to be related to a range of factors including formation of a more efficient J-coupled network, [8] and faster relaxation within the active methylphenylsulfoxide-derived catalyst, although ligand exchange and catalyst lifetime are also expected to play an important role. [22,36] These NMR samples were stable over the time-period required to record the magnetic field profiles shown in Figure 3. However, when these methanol-d 4 solutions are left at room temperature for several hours, a significant drop in the resulting 13 C NMR signal enhancements is observed upon performing SABRE with fresh p-H 2 . Similar observations have been reported for pyruvate and related sulfoxide-based catalysts and this has been attributed to catalyst deactivation. [16] [16] When the solution containing methylphenylsulfoxde is left for a period of several weeks at 278 K, growth of single crystals is observed. X-ray diffraction of these reveals the presence of [Ir (μ-H)(H) 2 (IMes)(SO(Ph)(Me) 2 )] 2 (Figure 4a). Its structure is symmetric with both iridium centres exhibiting distorted octahedral geometries bridged by two hydride ligands. An iridium-iridium bond length of 2.729 Å is consistent with reported values for iridium-iridium dimers with metal-metal single bonds (2.607 Å-2.826 Å). [51][52][53][54] Shorter distances between iridium and the S sites of the sulfoxide (2.282 Å) support coordination through sulfur rather than oxygen (3.239 Å) and is consistent with related structures reported for similar systems. [55] π stacking between the phenyl rings of the IMes ligand and the sulfoxide is expected to be an important interaction in this crystal (see the Supporting Information).
The formation of single crystals is also observed when analogous solutions containing dimethylsulfoxde are left for a period of several weeks at 278 K. In this case, X-ray diffraction confirms the presence of [(Ir(H) 2 (IMes)(SO(Me) 2 )) 2 (μ-S)] (Figure 4b). This structure also contains two distorted octahedral iridium centres, but they are now bridged by an S 2À ligand. The longer iridium-iridium bond (2.908 Å) suggests a weaker metal-metal interaction. Decomposition of phenylvinylsulfoxde and dibenzylsulfoxide to form related sulfur-bridged iridium dimers has been observed in similar systems. [55] We show here that this transformation can occur more generally and expect that this decomposition process will play a role in reduced SABRE efficiency of such catalytic systems at long reaction times. Formation of oligomeric products from Ir-based catalysts, and their effect on reducing SABRE efficiency have been reported previously. [32,56] In conclusion, we demonstrate that the scope of sulfoxidecontaining SABRE polarization transfer catalysts can be extended to hyperpolarize molecules with similar α-keto acid motifs to pyruvate. This could lead to a larger range of potential applications of SABRE in monitoring chemical transformations. For example, KIC is a metabolic precursor to amino acids such as leucine and has been used as a hyperpolarized reporter to study this transformation in vivo. [46][47][48] The largest 13 C NMR signal gains achieved for KIC-1-[ 13 C] in this work are 985-fold (0.8 % polarization) which is lower than those achieved using alternative hyperpolarization techniques such as dissolution dynamic nuclear polarization (d-DNP) (15-32 % polarization). [46][47][48] In contrast, SABRE uses a simple, refreshable set-up and achieves these enhanced signals in a 20 second polarization step which is significantly shorter than the 1-1.5 hours required using d-DNP. [46] It is expected that these SABRE signal gains can be increased by further optimization of factors including temperature, sulfoxide, carbene ligand, p-H 2 pressure, temperature, and many others. [10,12,[16][17][18][19][20][21][22] We expect the types of molecules whose MR signals these catalysts can sensitize to increase in the future.