Effects of a Tridentate Pincer Ligand on Parahydrogen Induced Polarization.

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, Ar CCC (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 (Dipp CCC)Co(N2 ) catalyst, which resulted in faster hydrogenation and up to 390-fold 1 H signal enhancements, larger than that of the (Mes CCC)Co-py (py=pyridine) catalyst. Additionally, the synthesis of the (Dipp CCC)Rh(N2 ) complex is reported and applied towards the hydrogenation of ethyl acrylate with parahydrogen to generate modest signal enhancements of both 1 H and 13 C nuclei. Lastly, the generation of two (Mes CCC)Ir complexes is presented and applied towards SABRE and PHIP-IE chemistry to only yield small 1 H signal enhancements of the partially hydrogenated product (PHIP) with no SABRE hyperpolarization.