Pham, Pierce, and Christian Hilty. “Tunable Iridium Catalyst Designs with Bidentate N-Heterocyclic Carbene Ligands for SABRE Hyperpolarization of Sterically Hindered Substrates.” Chemical Communications 56, no. 98 (2020): 15466–69.
https://doi.org/10.1039/D0CC06840C.
A series of bidentate N-heterocyclic carbene (NHC) iridium catalysts, [Ir(κC,N-NHC)H2L2]BPh4, are proposed for SABRE hyperpolarization. The steric and electronic properties of the NHCs are used to tune substrate affinity and thereby SABRE efficiency. The sterically hindered substrates 2,4-diaminopyrimidine and trimethoprim yielded maximum proton NMR signal enhancements of ∼300-fold and ∼150-fold, respectively.