Tunable iridium catalyst designs with bidentate N-heterocyclic carbene ligands for SABRE hyperpolarization of sterically hindered substrates

A series of bidentate N-heterocyclic carbene (NHC) iridium catalysts, [Ir(kappa C,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 similar to 300-fold and similar to 150-fold, respectively.