Catalytic Transfer of Magnetism Using a Neutral Iridium Phenoxide Complex

Abstract

A novel neutral iridium carbene complex Ir(κC,O-L)(COD) () [where COD = cyclooctadiene and L = 3-(2-methylene-4-nitrophenolate)-1-(2,4,6-trimethylphenyl) imidazolylidene] with a pendant alkoxide ligand has been prepared and characterized. It contains a strong Ir-O bond and X-ray analysis reveals a distorted square planar structure. NMR spectroscopy reveals dynamic solution state behavior commensurate with rapid seven-membered ring flipping. In CDCl solution, under hydrogen at low temperature, this complex dominates although it exists in equilibrium with a reactive iridium dihydride cyclooctadiene complex. reacts with pyridine and H to form neutral Ir(H)(κC,O-L)(py) which also exists in two conformers that differ according to the orientation of the seven-membered metallocycle and whilst its Ir-O bond remains intact, the complex undergoes both pyridine and H exchange. As a consequence, when placed under hydrogen, efficient polarization transfer catalysis (PTC) is observed via the Signal Amplification By Reversible Exchange (SABRE) approach. Due to the neutral character of this catalyst, good hyperpolarization activity is shown in a wide range of solvents for a number of substrates. These observations reflect a dramatic improvement in solvent tolerance of SABRE over that reported for the best PTC precursor IrCl(IMes)(COD). For THF, the associatedH NMR signal enhancement for the proton signal of pyridine shows an increase of 600-fold at 298 K. The level of signal enhancement can be increased further through warming or varying the magnetic field experienced by the sample at the point of catalytic magnetization transfer.

Iridium α‐Carboxyimine Complexes Hyperpolarized with para‐Hydrogen Exist in Nuclear Singlet States before Conversion into Iridium Carbonates

“…Increasing amine concentrations suppress the H 2 production pathway in accordance with a process that is preceded by dissociative amine loss. This type of behavior is typical for systems of this type . Furthermore, 4 major appears to be inert on this timescale.…”

Signal Amplification by Reversible Exchange (SABRE): From Discovery to Diagnosis

“…[18] Despite there being over 10 commercially available NHC ligands for the SABRE polarization of pyridine, [18,19] to date [IrCl(COD)(IMes)] remains the most impressive mediator of polarization transfer.T his optimum activity is common across ar ange of heterocyclic motifs, although rare examples exist for which this is not the case. [22] Other SABRE catalysts have been shown to undergo such non-hydrogenative transfer of polarization including the bidentate NHC-phenolate (1), [23] an iridium PNP-pincer (2), [14] and an iridium cyclooctene complex (3) [24] shown in Figure 2. [19] This reflects the complexity of the SABRE transfer step,although recently the p-accepting ability parameter [21] has been shown to offer better insight into how the NHC ligand influences the rate of pyridine exchange.…”

“…[19] This reflects the complexity of the SABRE transfer step,although recently the p-accepting ability parameter [21] has been shown to offer better insight into how the NHC ligand influences the rate of pyridine exchange. [22] Other SABRE catalysts have been shown to undergo such non-hydrogenative transfer of polarization including the bidentate NHC-phenolate (1), [23] an iridium PNP-pincer (2), [14] and an iridium cyclooctene complex (3) [24] shown in Figure 2. However,signal gains were reduced when compared to [IrCl(COD)(IMes)].…”

Competing Pathways in the Photochemistry of Ru(H)2(CO)(PPh3)3

“…The mechanism of this thermal exchange may involve loss of AsPh 3 or pyridine from 3 or 4, respectively, followed by formation of 2, intramolecular exchange of hydrogens, and recoordination of the ligand, as proposed for related iridium complexes. 58 Alternatively and more simply, 3 and 4 may be converted to dihydrogen isomers which undergo exchange with p-H 2 and regeneration of the initial structure. We have investigated the assignments further by use of Timney ligand effect constants 59 and by estimating the CO (2), the experiments seemed to conflict with our earlier TRIR experiments.…”