Effects of a Tridentate Pincer Ligand on Parahydrogen Induced Polarization

Abstract

The role of ligands in rhodium- and iridium-catalyzed Parahydrogen Induced Polarization (PHIP) and SABRE (signal amplification by reversible exchange) chemistry has been studied in the benchmark systems, [Rh(diene)(diphos)]+ and [Ir(NHC)(sub)3(H)2]+, and shown to have a great impact on the degree of hyperpolarization observed. Here, we examine the role of the flanking moieties in the electron-rich monoanionic bis(carbene) aryl pincer ligand, ArCCC (Ar=Dipp, 2,6-diisopropyl or Mes, 2,4,6-trimethylphenyl) on the cobalt-catalyzed PHIP and PHIP-IE (PHIP via Insertion and Elimination) chemistry that we have previously reported. The mesityl groups were exchanged for diisopropylphenyl groups to generate the (DippCCC)Co(N2) catalyst, which resulted in faster hydrogenation and up to 390-fold 1H signal enhancements, larger than that of the (MesCCC)Co-py (py=pyridine) catalyst. Additionally, the synthesis of the (DippCCC)Rh(N2) complex is reported and applied towards the hydrogenation of ethyl acrylate with parahydrogen to generate modest signal enhancements of both 1H and 13C nuclei. Lastly, the generation of two (MesCCC)Ir complexes is presented and applied towards SABRE and PHIP-IE chemistry to only yield small 1H signal enhancements of the partially hydrogenated product (PHIP) with no SABRE hyperpolarization.