The reaction of an iridium PNP complex with parahydrogen facilitates polarisation transfer without chemical change

The pincer complex [(C5H3N(CH2P(tBu)2)2)Ir(H)2(py)]BF4 is shown to be active for signal amplification by reversible exchange.


General conditions
All experimental procedures were performed under an atmosphere of nitrogen gas using standard Schlenk line techniques or in an M-Braun glove box. All solvents were dried using an M-Braun solvent system or distilled from the appropriated drying agent under N 2 . Deuterated d 4 -methanol, and h 5 -pyridine were obtained from Sigma-Aldrich, d 5 -pyrdine was obtained from Cambridge isotopes and used as supplied. [(C 5 H 3 N(CH 2 P(tBu) 2 ) 2 )IrH(C 8 H 11 )(NCCH 3 )]BF 4 (1) was prepared as accoring to publised methods 1 . Pentachloropyridine (4.58 g, 18.0mmol, 1.0 eq.), Pd(OAc) 2 (262mg, 1.17mmol, 6.5 mol%), 1,4bis(diphenylphosphino)butane (515mg, 1.21mol, 6.7 mol%), and Na 2 CO 3 (2.00 g, 18.9mmol, 1.05 eq.) in EtOH (50 mL) were sealed in a 300 mL pressure reactor and purged with N 2 .Then, the reactor was pressurised to 5bar with CO and the reactor was heated at 100 °C for 20 h. was added dropwise. The resulting suspension was stirred at rt for 5 h. Then, the suspension was filtered through Celite® and the filtrate was extracted with EtOAc (3 x 15 mL). The combined organic layers were dried (Na 2 SO 4 ) and concentrated under reduced pressure to give the crude product. Purification by flash column chromatography on silica using 8:2 -4:6 hexane-EtOAc as eluent gave d 3   SOCl 2 (150 µL, 2.06mmol, 3.0 eq.) was added dropwise to a stirred solution of d 7 -pyridine dimethanol (100 mg, 0.69mmol, 1.0 eq.) in THF (10 mL) at rt under N 2 . The resulting solution was stirred at 65 °C for 1h and then cooled to rt. The solution was poured into sat. NaHCO 3(aq) solution (15 mL) and the two layers were separated.

Sample preparation
For the polarisation measurements, 2.4 mg of the catalyst was dissolved in 0.6 mL of d 4 -methanol and 5 uL of h 5 -pyridine added. The solutions were then injected into a 5mm NMR tube fitted with a Young's valve.
A sample was prepared as described above and then pressurised with 3 bar of hydrogen gas before being allowed to stand for 1 hour so activate. [(C 5 H 3 N(CH 2 P( t Bu) 2 ) 2 )Ir(H) 2 (C 6 H 5 N)]BF 4 (4) was formed in this way. Samples were degassed and pressurised with 3 bar of parahydrogen gas as necessary.

Kinetic measurements of the exchange of H 2 and pyridine in 4
The activation parameters for H 2 elimination and pyridine exchange in 4 are present below in

Quantification of polarisation:
Polarization enhancement factors were calculation using the following equations: In a typical experiment, the thermal and polarised spectra where acquired using identical experimental parameters, where the thermal reference spectrum of the sample was only acquired once it has fully relaxed, (typical the sample was left for 10 minutes within the magnet) and before the sample was pressurised with parahydrogen gas. The raw integrals from the thermal and polarised measurement where used to calculate the enchantment level.

Effect of transfer field and temperature
In order to monitor the effect of the polarisation transfer field and the samples temperature a sample was prepared as described above and then pressurised with 3 bar of hydrogen gas before being allowed to stand for 1 hour to convert to 4. The sample was then degassed and pressurised with 3 bar of parahydrogen gas before being immersed in a water bath at the desired temperature. The sample was shaken for approximately10 seconds at either 0.5 or 65 G, and then rapidly inserted into the magnet for measurement. Parahydrogen was replenished after each measurement.
Samples that were polarized in the automated polarizer, used 12 mg of 1 and 0.31 mmol of pyridine in a methanol-d 4 volume of 3 ml. Parahydrogen was introduced into this solution via six inlet tubes at a pressure of 3 bar for 6 s. This solution was then transferred into the Bruker Avance III series 400 MHz spectrometer that was equipped with a TXO flow probe where the flow cell volume was 200 µl by a flow of nitrogen gas. Unless otherwise stated, a single transient was then recorded of the nucleus of interest. Once interrogated, the solution is returned to the polarizing chamber and this process repeated as required. A coil surrounds the reaction chamber such that a magnetic field can be generated in the z direction. This coil was designed to produce static DC fields in the range -150 to 150 x10 -4 T. Measurements can be made at either +ve or -ve G.

X-Ray crystal structure for 4
Crystals of 4 where grown from a concentrated solution of 4 in methanol. The sample was layered with benzene and the solvent allowed to slowly evaporate. A suitable crystal was selected and mounted on a SuperNova, Single source Eos diffractometer. The crystal was kept at 109.9 K. The structure was solved using Olex2 9 , and the olex2.solve 3 structure solution program using charge flipping and refined within the XL 10 refinement package using least squares minimization.