Catalytic Transfer of Magnetism Using a Neutral Iridium Phenoxide Complex

A novel
neutral iridium carbene complex Ir­(κC,O-L1)­(COD) (1) [where COD
= cyclooctadiene and L1 = 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 CD2Cl2 solution, under hydrogen
at low temperature, this complex dominates, although it exists in
equilibrium with a reactive iridium dihydride cyclooctadiene complex. 1 reacts with pyridine and H2 to form neutral Ir­(H)2(κC,O-L1)­(py)2, which also exists in two conformers that differ
according to the orientation of the seven-membered metallocycle, and
while its Ir–O bond remains intact, the complex undergoes both
pyridine and H2 exchange. As a consequence, when placed
under para-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 associated 1H NMR signal enhancement for the ortho 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.